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HomeMy WebLinkAboutWestchester County Stormwater Management Planning Manual Westchester govcom Andrew J. Spano, Westchester County Executive WESTCHESTER COUNTY STORMWATER MANAGEMENT PLANNING MANUAL II fig} s . { A & , • I c � r q . 1. .� • to .464 Prepared By: coM For Westchester County Department of Planning Gerard E. Mulligan, AICP, Commissioner • e'vcbm Contents • Section 1 - Introduction 1.1 Introduction 1-1 1.2 Structure of the Planning Manual 1-2 1.3 Planning Approach 1-2 1.3.1 Initial Problem Definition and Planning Objectives 1-2 1.3.2 Data Collection 1-3 1.3.3 Stream Assessment 1-3 1.3.4 Stormwater Modeling 1-3 1.3.5 Data Analysis 1-4 1.3.6 Problem Definition 1-4 1.3.7 Planning Objective Consensus 1-5 1.3.8 Management Alternatives/SMP Development and Evaluation 1-5 1.3.9 Stormwater Management Practices 1-6 Section 2- Problems and Planning Objectives 2.1 Preliminary Problems and Objectives 2-1 2.2 Stakeholder Interaction 2-2 Technical Memorandum 2.1 - Stakeholder Participation Section 3- Data Collection 3.1 Geographical Information Systems (GIS) 3-2 i 3.1.1 Data Layers 3-2 3.1.2 Watershed Delineations 3-3 3.1.3 Land Use Data 3-3 3.1.4 Land Surface Imperviousness 3-5 3.1.5 Soils 3-5 3.2 Meteorological Data 3-6 3.2.1 Precipitation 3-6 3.2.2 Temperature 3-6 3.3 Stream Flow 3-6 3.4 Water Quality Data 3-7 3.5 Event Mean Concentrations (EMCs) 3-7 Technical Memorandum 3.1 - GIS Data Layers Relevant for Stormwater Management Planning Introduction TM3.1-1 Land Use Data TM3.1-2 Impervious Surface TM3.1-3 • Westchester County Stormwater Management Planning Manual "estclhestcr Table of Contents g"`"`""' Section 4 - Fluvial Geomorphic Assessment 4.1 Introduction 4-1 4.2 Measured Reach Stream Survey 4-1 41 4.2.1 Reach Delineation 4-2 4.2.2 Data Collection for Rosgen I, II,and partial III Analyses 4-2 4.2.2.1 Bankfull Elevation 4-3 4.2.2.2 Condition of Stream and Channel Stability 4-4 4.2.2.3 Bank Stability(Rosgen BEHI and Shear Stress Ratio) 4-7 4.3 Stream Assessment Findings 4-9 Technical Memorandum 4.1 - The Rosgen Stream Classification Method Data Collection for Rosgen I, II, and partial II Analyses TM4.1-1 Stream Assessment Findings TM4.1-8 Section 5 -Stormwater Modeling 5.1 Purpose and Application 5-1 5.2 Model Data Needs 5-3 5.3 Model Results 5-3 5.4 Use of Model Results 5-4 Section 6 - Data Analysis 6.1 Impervious Surface 6-1 6.2 Land Use 6-5 4 6.3 Historical Stream Flow Data 6-7 6.4 Historical Water Quality Data 6-7 6.5 Simulated Pollutant Loadings 6-5 6.5.1 Runoff 6-6 6.5.2 Channel Flow 6-7 6.5.3 Pollutant Loading 6-8 6.6 Flooding Analysis 6-8 6.7 Stream Channel Stability 6-9 Section 7- Problem Definition and Consensus on Planning Objectives Section 8 - Management Alternatives/SMP Selection 8.1 Prioritization Process 8-1 8.1.1 Potential Prioritization Criteria 8-2 8.1.2 Criteria Weighting 8-3 Technical Memorandum 8.1 - EVAMIX Description of EVAMIX TM8.1-1 Selection of Evaluation Criteria TM8.1-1 Potential Prioritization Criteria TM8.1-2 Criteria Weighting TM8.1-5 ii • Westchester County Stormwater Management Planning Manual Table of Contents Example EVAMIX Application on the Hallocks Mill Brook Watershed. TM8.1-10 Section 9- Stormwater Management Practices 9.1 Applicable SMPs 9-1 9.1.1 SMPs to Prevent Future Stormwater Impacts 9-2 9.1.2 SMPs to Remediate Impacted Watersheds 9-5 Technical Memorandum 9.1 - Overview of the New York State Stormwater Design Manual Technical Memorandum 9.2 - General Guidance for Improved Site Design M Appendices Appendix A-Hallocks Mill Brook Conceptual Design Memorandum Appendix B -Hallocks Mill Brook Fluvial Geomorphic Assessment Data Appendix C-Hallocks Mill Brook Estimated Pollutant Loadings Appendix D-Croton Watershed Pollutant Loading Screening Model iii 0 W r in Section 1 Introduction Key Points • This Stormwater Management Planning Manual is a guidance document for municipalities • The Manual presents a general planning approach to stormwater management on a watershed basis • This section highlights the major planning steps 1.1 Introduction The Westchester County Stormwater Management Planning Manual(Planning Manual)is the result of a project commissioned by the Westchester County Department of Planning.The Planning Manual's intent is to provide a planning framework for municipalities with which to manage stormwater on a watershed basis. In this way,stormwater management planning will occur within the parameters that hydrology and watersheds impose. By developing a watershed-wide planning approach,it will become easier to develop land use plans and zoning ordinances that encourage proper stormwater management,and that protect the natural hydrologic cycle and stream habitat from being degraded by improper development.The Planning Manual is intended as a complement to the New York State Stormwater Management Design Manual,which can be downloaded from the NYSDEC website (http://www.dec.state.ny.us). The Planning Manual is written as a guidance document for local officials on the municipal level,not as a set of required procedures. It reflects many of the recent developments in improved stormwater management that are being encouraged in many neighboring states. New York State's Stormwater Management Design Manual focuses on site specific stormwater management practices(SMPs)for new developments and presents very specific design criteria that will be required as part of a permit application.The Planning Manual has a broader focus: to help local officials understand the watershed management planning process. It guides them through the data collection, data analysis,and stormwater modeling steps that are vital to developing a watershed-based stormwater management plan. It presents an approach to assessing the current and future condition of the watershed,and provides guidance on selecting SMPs to meet two watersheds goals: • the prevention of deterioration of surface water quality and damage to the streams and riparian habitats,and • the restoration or repair of already impacted stream systems. As part of the development of the Planning Manual, one subwatershed was singled out to demonstrate how the planning approach can be applied. The County selected Hallocks Mill Brook as the demonstration subwatershed.Thus,in addition to a 1-1 Westchester County Storm water Management Planning Manual 11 �-rrm Section 1 -Introduction �= description of the overall planning process,various elements of the planning process, 0 as carried out in the demonstration project,are included in the Planning Manual. For example,a stormwater model was developed for Hallocks Mill Brook to simulate stormwater runoff and runoff related pollutant loading to the stream. 1.2 Structure of the Planning Manual a To help the reader better understand the planning process,an illustrative example of the Hallocks Mill Brook watershed is presented.This is done by dividing the planning manual into a right and left side. The right side of the manual is designed to be a complete stand-alone document that details the stormwater planning approach. The left side is designed to clarify the concepts presented on the right side and is not 41 written as an independent document.Rather,the left side is designed to supplement the right side of the manual with text,graphics,tables,and charts. The illustrative example is not relevant throughout the document,and is only presented where appropriate. The Planning Manual has nine chapters.Chapter 1 is the introduction,and gives a general overview of the stormwater management planning process. Chapter 2 discusses the development of an initial set of planning objectives. Chapter 3 discusses data collection needs. Chapter 4 introduces one of the key field studies required to better understand the watershed:Fluvial Geomorphic Assessment. Chapter 5 is devoted to the development of a stormwater model of the watershed for both 4 estimates of runoff rates and volumes,as well as for estimates of pollutant loading. Chapter 6 discusses several key aspects of data analysis. Chapter 7 describes the process of defining the problems to be addressed within the watershed through modeling and data analysis,as well as the process of adjusting and finalizing planning objectives. Chapter 8 discusses the prioritization process used to identify 4 highly impaired areas of a watershed.Chapter 9 briefly touches on the selection of Stormwater Management Practices(SMPs)and the conceptual design of SMPs,but refers to the New York State Stormwater Management Design Manual as the primary guidance document for design. Technical memorandums were placed at the end of selected chapters to provide greater detail on specific methods,tools,and techniques that can be used in the planning process. The intent of these "Tech-Memos" is to provide valuable information for engineers,scientists and planners without complicating the manual for the broader target audience. 1.3 Planning Approach Stormwater management planning is similar to most planning efforts,and benefits greatly from an organized,step-by-step approach that emphasizes both technical analysis and stakeholder interaction. For this manual,the planning process has been broken down into a number of steps,each of which is briefly described below. 1-2 • Westchester County Storm water Management Planning Manual ""tc ver com Section 1 -Introduction • 1.3.1 Initial Problem Definition and Planning Objectives The importance of planning on a watershed basis is now generally recognized,and many states are reorganizing their permitting and planning requirements to reflect natural watershed boundaries rather than political boundaries. This does,however, make the planning process more difficult to organize.The first step in the planning process is to gather information from the major stakeholders within the watershed on the problems that they perceive within the watershed,and the planning objectives that they wish to achieve to address the perceived problems.This is important because there are usually multiple stakeholders,and they often have different reasons for wanting stormwater planning and management to occur. In many cases,the various objectives will overlap or complement each other,but there are often conflicting objectives as well.Fortunately,stakeholder groups and a stakeholder process are already well established within the Croton Watershed through prior programs, and this step has,in many cases,already been accomplished. 1.3.2 Data Collection The next step in the process is data collection. Based on the preliminary planning objectives,as well as the generally recognized categories of basic data required for effective stormwater management planning,a data collection program should be established. This can include collection of available data from existing sources,as well as field studies. The data collection program should be carefully planned to gather the data needed for the development of stormwater models,for the development of planning scenarios,and for the assessment of the present condition of the streams and stream habitats. In most cases,the data will need to be put into a database and a Geographic Information System(GIS) that allow the project team to effectively analyze the data. 1.3.3 Stream Assessment Stream assessments provide a powerful means of determining existing and potential watershed problems influencing water quality.Walking the individual streams and stream networks reveal subtle,and at times,severe changes occurring throughout the watershed. As a direct expression of the watershed,streams reflect the local physiography, geology,climate,and land use. Stream assessments involve walking the streams within the watershed,recording key information,and collecting data essential for characterization and assessment of the health of the watershed and stream system. The process provides information on channel stability,riparian and aquatic habitat, sediment supply,the amount of water the channel can convey,areas of erosion and sedimentation, and sources of impacts to the stream channels.This information,when coupled with stormwater runoff modeling,forms the heart of the data collection and analysis effort required for watershed based stormwater management planning. 1-3 Westchester County Stormwater Management Planning Manual `M stchester Section 1 -Introduction g"`corn 1.3.4 Stormwater Modeling • A key component of watershed planning is the development of a stormwater management model. Stormwater models take rainfall data and calculate stormwater runoff reaching the stream from the entire watershed. The model should be capable of using actual rainfall data from a rain gage on a daily,hourly, or even shorter measurement interval over a representative period of time, e.g. 5 or 10 years. The 1111 model tracks how much water evaporates,infiltrates into the ground,or runs off into the stream. It should also provide the ability to simulate non-point source contaminant loading for water quality parameters of concern. There are numerous models available,including the USEPA Storm Water Management Model (SWMM). A simpler approach to estimating pollutant loads from non-point sources is provided in Appendix A of the NYS Stormwater Management Design Manual. This "simple method" provides approximate estimates of annual average pollutant loads,and can be used as a first step prior to more complex watershed modeling. 1.3.5 Data Analysis Once the data collection has occurred and modeling results are available, the data and information must be analyzed. The primary components of the data analysis step include: • Characterization of water quality, including comparison with applicable water quality standards • Characterization of stream flow • Comparison of stream flows with channel carrying capacities 1 • Analysis of land use patterns and trends to identify potential non-point sources of pollution • Identification and analysis of potential point sources of pollution • Identification of areas of erosion,flooding, sedimentation, and aquatic habitat degradation 1.3.6 Problem Definition In many planning studies, problem definition is considered the first step in the planning process, and is one of the primary functions of the stakeholder committee. Once accomplished,the list of defined problems guides the activities of the project team and provides the focus for the implementation of the plan. For the Croton Watershed,the existing stakeholder process will usually already have identified numerous concerns and problems relating to stormwater, the health of the stream system, and the water quality within the stream. It is important to remember, however, that in most cases, these concerns were expressed without the benefit of some of the detailed data collection, modeling,and data analysis accomplished under 1-4 • Westchester County Storm water Management Planning Manual �Y -r Section 1 -Introduction iV00m • the stormwater management planning process.The water quality and flow data analysis,analysis of land use trends,stream assessment,and stormwater modeling all provide critical data and insight to sharpen and clearly define the problems within the watershed. For this reason,problem definition should not be thought of as an"up-front" stakeholder activity.Problem definition benefits greatly from a more continuous and adaptable approach,consisting of: • Initial listing of problems and concerns by stakeholders • Data analysis by the project team • Presentation of technical analysis results to the stakeholders • Adjustment of problem priorities based on results of the technical analyses through stakeholder consensus building The adjustment of the list of perceived watershed problems from the already existing stakeholder process is accomplished by confronting initial perceived problems with the problems established through data analysis and modeling. This process often reveals the existence of problems not identified by stakeholders. It may also provide evidence that some of the perceived problems that have been identified may not,in fact,be relevant to the watershed. Using the additional insight gained from the results of the various analyses, stakeholders should be given the opportunity to develop a final list of prioritized problems that reflect the results of the analyses balanced with the concerns of the stakeholders. 1.3.7 Planning Objective Consensus Once the problems have been identified and prioritized,clearly defined planning objectives must be established.The distinction between problems and objectives is often blurry. Problems are what is presently wrong within the watershed,or what changes might occur in the watershed that will result in degradation of the water quality or stream habitat. Objectives are measurable,clearly defined planning targets to solve the problems or prevent degradation. For example,a problem might be erosion of a stretch of stream,or flooding at a particular intersection.The objectives might be to achieve a stable channel configuration,or to limit flooding at the intersection to storms of 25-year frequency. The establishment of planning objectives from the finalized list of problems must be done through the stakeholder process to achieve as much consensus as possible. 1-5 Westchester County Stormwater Management Planning Manual Westd .goarm Section 1 -Introduction 1.3.8 Management Alternatives/SMP Selection With the development of consensus on the planning objectives,specific management alternatives can be developed which represent the recommendations of the plan. The selection of projects for implementation should follow a carefully designed evaluation process to ensure that the highest priority and most cost effective projects are implemented first. 1.3.9 Stormwater Management Practices It is often helpful to think of management alternatives as consisting of two types of activities.The first type of activity consists of specific measures to prevent further degradation of stream habitat and water quality through implementation of SMPs for 4 new developments,public education,or changes to zoning and stormwater ordinances.Management alternatives and SMPs for the purposes of preventing further degradation to the watershed must be designed and implemented. Some general site design recommendations are provided in this Planning Manual,however, the reader is referred to the New York State Stormwater Management Design Manual as the primary guidance document for design of SMPs.The reader is also reminded that additional design criteria may be applicable in areas subject to New York City's Watershed Rules and Regulations(WRR). The second type of activity consists of specific measures to restore or improve already degraded stream habitat and water quality through specific stormwater projects such as channel restoration,bank stabilization,wetland creation, and urban retrofits of SMPs. Once the watershed planning has been completed,management alternatives and SMPs for the purposes of watershed restoration must also be designed and implemented. This phase is outside the scope of this Planning Manual. The reader is referred to the New York State Stormwater Management Design Manual as the primary guidance document for design of SMPs.The Planning Manual does, however,contain detailed information on prioritizing sites for restoration. In addition,Appendix A contains conceptual designs for several sites in the Hallocks Mill Brook Watershed that illustrate several options for tackling variable site conditions. Following the selection,design and implementation of SMPs,a monitoring and evaluation program should be established to ensure that the SMPs are achieving the objectives for which they were intended.This ongoing process also will help to ensure that the planning process adapts to emerging technologies,changes in environment and changes in objectives 1-6 • *Om • Section 2 Problems and Planning Objectives Key Points IIIt • Preliminary planning objectives are those objectives identified by the existing stakeholder process or by a newly formed stakeholder group at the beginning of the planning process. • In the Croton Watershed,stakeholders may already have identified many of the issues and concerns through existing stakeholder groups • Stakeholder input is needed to provide local knowledge and to establish planning priorities. • Initial planning objectives should be tested against the results of the technical analyses The planning process should start with the identification of preliminary planning objectives. This can only be done through stakeholder participation. A successful stormwater plan requires consensus from all parties with an interest in the future of the area's water resources. This requires stakeholder participation throughout the planning process. Usually,it is easier to work with a representative group of stakeholders,organized into a Technical Advisory Committee (TAC). 0 Within the Croton watershed,there have already been planning and stormwater related studies carried out in the past. Many of these studies involved stakeholder participation. The first step,therefore,is to identify any existing stakeholder groups and to identify planning objectives that already exist from prior work within the watershed. If a more local,focused watershed TAC is planned,the makeup of the TAC should be as broad as possible,and should combine interest groups,lay people,professional, and government regulators.Combining stakeholders into one group allow the lay members to better understand the technical issues. It also allows the professional members to become more familiar with the"layman's" concerns and perceptions,and it helps to reconcile differing points of view through mutual education and compromise. 2.1 Preliminary Problems and Objectives A proven approach to actively engage the TAC in the process of planning is to identify,discuss,and catalogue all technical and stakeholder issues during a preliminary stakeholder meeting. It is not the intent of the preliminary stakeholder meeting to reach agreement on problems and objectives,but to catalogue and acknowledge all issues.Consensus will be sought later in the process once the technical work has been'completed. This initial meeting,often called a"visioning process",should result in a complete list of perceived problems and concerns. 2-1 110 Westchester County Storm water Management Planning Manual �.;� tester Section 2—Problems&Planning Objectives gnvcom 2.2 Stakeholder Interaction • Recognition of differences and compromise are the keys to a successful stakeholder process,but this will not happen at an initial meeting.Compromise and consensus are goals that can only be achieved through the learning process as the technical analyses are completed and presented. The TAC should meet as important results from the technical analyses become available,to go over the results and to discuss the implications. This process of educating the TAC and the project team is critical to reaching consensus later in the project. During subsequent meetings,the problems and issues identified during the preliminary meeting are continually tested against the results of the various technical analyses (stormwater modeling,water quality and quantity trend analyses etc.)as they become available. It will often be found that many of the perceived problems initially identified by the TAC are less critical than were originally thought,and in some cases,are not even realistic concerns. Others will be supported by the data,and some problems not initially flagged by the stakeholders will be uncovered by the data analysis. Working towards consensus on problems and objectives later in the planning process, with the support of sound,scientific analysis,works very well in moving most stakeholders toward a workable plan that all can agree to implement. The final set of issues,supported by data analysis,then becomes the focus for the development of the final planning objectives. 1 2-2 • Westchester gov.com Tech Memo 2.1 Stakeholder Participation A complete stakeholder program has many elements. A well-run stakeholder involvement process should: • provide representation of all key stakeholder groups, • inspire participation of stakeholder representatives at stakeholder committee meetings, • allow for development of specific program goals, • use the meetings as a forum where consensus is built around the project goals and appropriate methods to achieve these goals, • provide stakeholder representatives with information necessary to support informed decision-making by the broader stakeholder group, It • establish a mechanism for future decision making and problem resolution. Frequently used approaches to stakeholder participation and public education include: • Public meetings for information exchange • Periodic meetings and active participation in the planning process of stakeholder groups • Information dissemination using newsletters,websites, and local libraries and schools • Workshops,field trips,stream cleanup days, and other participatory activities. This memorandum focuses only on the stakeholder role during the stormwater planning process. The first step is to identify the major stakeholders within the watershed. This is important because there are usually multiple stakeholders, and they often have different reasons for wanting stormwater planning and management to occur. In many cases,the various perspectives of stakeholders will overlap or complement each other,but there are often conflicting objectives as well. A list of potential stakeholders should be developed,and an active and representative stakeholder group formed. This is often called the"Technical Advisory Committee", or TAC.Stakeholders commonly included in a TAC are: • local and state government staff(e.g. NYSDEC,NYCDEP Bureau of Water Supply,Westchester County Department of Planning,municipal government zoning or environmental boards,and Town Engineers), TM 2.1-1 • Westchester County Storm water Management Planning Manual Westchester Technical Memorandum 2.1 gO1'CO11' • members of the economic development community (e.g. Chamber of Commerce,the homebuilders association,major developers), • significant users of surface water (e.g. industry,large agricultural tracts,public water suppliers, and even fire departments), 41 • larger discharges to the watershed(wastewater treatment plants,permitted industrial dischargers) • natural resource management organizations and scientific groups (e.g. USGS and Water Resources Research Institutes), • concerned institutional landowners (e.g. golf courses, corporate centers, universities) • agricultural organizations • natural resource conservation services and cooperative extension services • watershed organizations and environmental organizations • sport fishing associations i Once stakeholders have been identified and a representative TAC assembled, a process of stakeholder participation must also be established. The following steps can be followed to establish initial stakeholder participation within the stormwater planning process: • early identification of existing stakeholder groups and other potential stakeholders, • appointment of a representative of each key stakeholder group to the TAC • preliminary identification of the issues related to the watershed perceived as critical by stakeholders, including resource protection, stormwater management,flooding,water quality, land development, planning policy issues, and shared or fragmented responsibility for stormwater management • periodic meetings of the TAC for presentation of technical analysis results The project team should hold initial meetings with the TAC to list all the perceived problems related to stormwater and the health of the watershed streams,and to establish initial project objectives. These objectives will help define the concerns of the community about the watershed,and also help to focus the data collection and data analysis efforts. It is important,however, to recognize that these are perceived problems and initial planning objectives. After the data collection and analysis portions of the planning process have occurred, additional TAC meetings should be held to present TM 2.1-2 a Westchester County Storm water Management Planning Manual •r Technical Memorandum 2.1 - _ ='UODm the results. Once the TAC is fully informed of these results,the problems and objectives must once again be examined for relevance and applicability in light of the knowledge gained from the results of the field studies, data analysis,and modeling. The TAC should then provide priorities for the final list of problems and planning objectives to guide the project team towards an implementation plan. I l TM 2.1-3 • Data Collection A data collection program was established as the first step to developing a stormwater management plan for the Hallocks Mill Brook. The program initially ! focused upon compiling all available existing data, and then assessing additional data needs. A field data collection program was then established to acquire important missing data. Data required for developing a stormwater management plan was compiled from a • number of sources. These included: • Westchester County Geographic Information Systems (GIS) • National Climatic Data Center(NCDC) a • United States Geological Survey(USGS) • US Census • New York City Department of Environmental Protection (NYCDEP) � • Westchester County Department of Planning • Local Municipalities • Interviews with Municipal Officials 1 • Field Visits The field data collection program included interviews with municipal officials, site visits, and stream assessments. • \I stthester Section 3 • Data Collection Key Points • A carefully planned and well-organized data collection program is a critical early step in the planning process. • In the Croton Watershed, there is a considerable amount of data available for stormwater management. • The data is used to understand the characteristics of the watershed. • This data can also be used to develop stormwater models and to analyze non-point sources of pollution. A carefully planned and well-organized data collection program sets the stage for the stormwater management planning process. It is recommended that all existing data be collected and analyzed prior to developing a field data collection program.Once gaps in existing data are identified,an efficient approach to data collection in the field can be developed. Data collection is expensive, and the data collection program should be focused to gather only the information needed for the planning approach. It may be organized in a Geographic Information System(GIS)and database so that it can be efficiently accessed and analyzed. There are a number of basic data categories for which data must be assembled for developing a stormwater management plan.These include: • Precipitation data(preferably hourly) • Stream flow data (preferably daily flows) • Water Quality data • Land use data • Information on the storm sewers and stormwater management structures (basins,culverts,constructed wetlands,stormwater discharge points,etc.) • Soil data(needed to estimate runoff) • Stream morphology data(stream channel type and shape,slope,flood plain characteristics,etc.) • Projections of future population or development In general,most data can be stored in a database linked to a GIS. The software used is a matter of personal preference,and can range from simple spreadsheets to a fully designed,user-friendly database system. 3-1 • Geographic Information Systems(GIS) The ESRI suite of Arc products was used to develop a Geographic Information System (GIS) database for the Hallocks Mill Brook Watershed. The data was compiled, O organized,viewed,and analyzed using the GIS. The majority of GIS data was obtained from the Westchester County GIS website. Additional data was obtained from the NYCDEP and the Town of Yorktown,one of two municipalities encompassing the Hallocks Mill Brook subwatershed. Efforts were II made to obtain data from the Town of Somers,but no GIS data were available. Data from all sources were compared for accuracy and completeness. Final data sets were selected from a single source or a combination of sources. Information obtained through field visits was used to update existing GIS data layers, , or to create new data layers. Digital aerial photography obtained from the New York State GIS Clearinghouse website and digital USGS quadrangles were incorporated into the GIS for background, data enhancement,and data verification. The working GIS dataset was used to analyze and manipulate data necessary for watershed planning and stormwater modeling. A series of base maps was produced / depicting relevant watershed planning attributes, such as the hydrologic network and land use. GIS was used to develop tables of information necessary to develop the stormwater model,and then to display the results of the model. Data Layers I Using GIS, a series of base maps were produced to illustrate the Hallocks Mill Brook Watershed. Physical settings,land use, degree of impervious surface,hydrologic features (e.g., streams and wetlands),transportation systems, drainage basins, and other relevant watershed planning aspects were all depicted in the base maps. The following figures demonstrate the mapping: I Figure HMB 3.1-Westchester County Drainage Divides Figure HMB 3.2-Hallocks Mill Brook 7 AI. ,1 Municipal Boundary p' 4 Hallocks Mill Brook ���� �,, 4 Stream / �4 Wetland i"'�� -Lake 1 • 1,71011 �d' itild0 l - ova • i©Hallocks I 0,, Mill Brook 1 1 �� Bronx ! f It =Basinii, A : ss,,i I Q Croton River Basin =Lower Hudson y River Basin �� �'� Q Lower long • Island Sound 11"" Basin �d �.., y Upper Long if IslandSound �1 } t .11 , asi� �Upper HudsonkAtIiiii.,,,,,tt fLver Basin 0 , Mks 5 0 5 Mies ,a Subwatershed 9 Westchester County Storm water Management Planning Manual ,st,,chesterm Section 3—Data Collection • 3.1 Geographic Information Systems (GIS) A GIS is an important tool for stormwater planning. Collected data can be easily organized and later analyzed with the aid of GIS.Westchester County maintains an extensive library of GIS data.The data is available for download,free of charge,from http://giswww.co.westchester.ny.usi. In addition to the county data,municipalities may be able to provide GIS data. Municipal data can sometimes be more detailed or current,and should be acquired if possible for comparison with existing data. When multiple sources of data are available,an effort should be made to obtain the most comprehensive and accurate available data,either from a single source or from a combination of sources. I A working GIS data set provides both a visual and analytical tool for watershed management. The GIS allows the production of graphical representation of a variety of watershed properties.For example,GIS maps can be prepared showing such features as geologic properties,water quality analysis results,or the results of stream assessments. Analyzing and manipulating data contained in GIS data layers also generates key information and tables required to adequately construct a stormwater model. Model output data can be incorporated back into the GIS data set to generate visual aids for analyzing model results,to assess current watershed conditions,and to develop planning scenarios. 3.1.1 Data Layers The first step in developing an efficient operating GIS is gathering and organizing available data layers. Attention should be given to those data layers that will provide information relevant to watershed management planning. Examples of relevant data layers include: • Watershed Drainage Divides(i.e.,watershed boundaries) • Storm or Combined Sewers • Land Use • Hydrologic Features A comprehensive list of data layers useful for watershed management is provided in Technical Memorandum 3.1. More detailed information is provided for some of the more important layers specific to more narrowly defined,stormwater management planning,which can be considered a part of comprehensive watershed management. The maps and data layers should be put into the GIS system. If they are not currently available,then the need for developing the maps and data layers will depend upon the precise objectives of the watershed planning effort. Topographic maps and aerial 3-2 9 Watershed Delineation Figure HMB 3.3 Delineated Hallocks Mill Brook With the aid of GIS,the county USGS Digital • Delineated Watershed StreamElevation Model was used to delineate the Lake Hallocks Mill Brook watershed into 56 smaller tell oiol drainage units.This was done based upon natural ihY drainage patterns due to land surface elevation and to include desired analysis points. These ! drainage units will eventuallybecome the �nl�l� 1^ g pil1511 j,�1 foundation for developing a stormwater model �� `' toltigiand identifying potential stormwater problem iti,A I i I fli ��� areas within the subwatershed. ilititia ip � �,igit Land Use Data 4 The Hallocks Mill Brook watershed is composed A of a wide variety of land use types. Within the Mlles Town of Yorktown,the Hallocks Mill Brook watershed is comprised of mostly residential land use with some open space and parkland.Within the Town of Somers,agricultural land is present as well. A land use coverage for the Hallocks Mill Brook watershed was developed first using the 1995 county land use coverage. This was updated using the more detailed information contained in the county agricultural coverage,the county wetlands coverage and the county water coverage. Figure HMB 3.4 Development of Land Use Layer The primary use of land use information was for 0 Hallocks Mill Brook , stormwater modeling and pollutant loading D Wetland O modeling. To simplify this,the land use layer -Lake ►'•%� I I Agriculture District .�^a I provided by the County website was consolidated Land Use OResidential into 4 land use classifications based upon n Non Residential 'i 0t Zn I ��IUndeveloped uniqueness of pollutant loading factors, �i infiltration parameters and runoff parameters.The agricultural layer was classified according to the ��!!li �� '1 f st/ rima agricultural land use as either fertilized co 'lit �' �1.o 1 P rY g ��� —�yt Il�tor pasture so that different pollutant loading 1 '�', _,event mean concentrations could be a lied to 4` li�_,�modeled runoff.With the additions of water A '�� y�,,i wetlands,8 land use classifications were used for ► � iy�;;. modeling. /.� g A ,o, 1 1 0 1 Miles .,�� Westchester County Stormwater Management Planning Manual fester Section 3—Data Collection '-"corn • photography can also be included in the GIS to provide a visual backing for the GIS layers. 3.1.2 Watershed Delineation One of the primary GIS maps for stormwater management shows the boundaries of ! watersheds and subwatersheds. The Croton Watershed is one of six major drainage divides within Westchester County (here called watershed).There are forty-eight minor drainage divides within the Croton Watershed (here called subwatershed). Detailed watershed planning and stormwater modeling of subwatersheds often requires that the subwatersheds be further divided into even smaller units. This delineation can be performed manually using mapped contours or digitally with the aid of GIS. 3.1.3 Land Use Data The response of a watershed to rainfall is highly influenced by land use. In general, a watershed in its natural land use state will have reached an equilibrium. This equilibrium is evident in healthy streams and habitats. Increasing urbanization without proper stormwater management practices disrupts this equilibrium and increases stormwater flows,causes peak flows to increase,and negatively impacts the health of a watershed. Because increasing impervious surfaces through development increases precipitation runoff quantity and flow rates,the effects of these increases will eventually be seen in higher flooding frequencies,stream erosion and sedimentation, and habitat impairment. Stormwater runoff is being seen as an increasingly significant nonpoint pollution source. "By the mid-1970's, stormwater runoff generally was considered to contribute as much as half of the total pollutant loading discharged to the nation's surface waters from all sources" (Smullen,Shallcross,Cave,1999). Estimating pollutant loading to streams from stormwater runoff is a function of land use classification,and requires a good understanding of land use patterns within the study area. Table 3.1 summarizes non-point source pollutants typically found in stormwater. 3-3 r Figure HMB 3.5 Finalized Land Use Map Hallocks MIII Brook d Stream �. - The final land use coverage was updated using I.Agriculture Fallow re41 population data from the 2000 census. Areas with I I Agriculture Pasture !y,V Forest populations greater than one person per acre were =Gott Course Mlxed Use '��` µ��/ changed to residential land use. Km Residential 'rJ l J g Water 1 ®Wetland �� �� /4,, . .t�" *ALAI/A \ � A Pik 1 0 1 Mles i Mr Table HMB 3.1-Land Use Consolidation I Original County Land Use Classification Consolidated Land Use Classification Agricultural- Fertilized Agricultural Agricultural- Pasture Cemetery Nature Preserve Private Recreation Public Park Active Forested Public Park Passive Undeveloped Water Supply Commercial I Institutional Manufacturing Mixed Mixed Commercial& Residential Office Transportation / Utility High Density Residential Medium Density Residential Low Density Residential Residential Very Low Density Residential Water Body Water - Wetlands 2 Notes: 1. Agricultural land use was expanded using the agriculutural district GIS coverage 2. Wetlands land use was developed using the wetlands GIS coverage Westchester County Storm water Management Planning Manual 1 -r Section 3—Data Collectiongovcom Table 3.1. Listing of Typical Nonpoint Source(NPS)Pollutants Found in Runoff from Developed Areas (Terrene Institute 1994) Common Nonpoint Impacts of Pollutants Source(NPS)Pollutants Sediment Fills in ponds and reservoirs with mud.Contributes to the decline of submerged aquatic vegetation(SAV)by increasing turbidity and reducing the light available for photosynthesis.Acts as a sink for nutrients and toxicants and as a source when disturbed and re-suspended.Fills in and eliminates habitat space between rocks and other features that are otherwise used as habitat by fish and aquatic species. Total Phosphorous(TP) A contributing factor to eutrophication(over-enrichment of nutrients)in water bodies that creates subsequent algal blooms.Algal blooms contribute to the decline of SAV and aquatic species by reducing the light available for photosynthesis,and decreasing the level of dissolved oxygen(DO)available to support aquatic species,and may cause changes in the composition of plankton and fish species. Total Nitrogen(TN) Like phosphorus,contributes to eutrophication and algal blooms,and subsequent decreased photosynthesis and decreased DO. Chemical Oxygen Decreases the concentration of DO.Low DO concentration and anaerobic conditions Demand(COD) (complete absence of DO)can lead to fish kills and unpleasant odors.Primarily released as organic matter in the"first flush"of urban runoff after a storm. Bacteria High concentrations can prevent human contact sports and recreation such as swimming,boating,etc.Also contributes to increased need for treatment of raw water withdrawn for public water supplies. Zinc Most commonly found toxic metal in the mod-Atlantic region;chronically exceeds EPA water quality criteria.Primary cultural source is the weathering and abrasion of galvanized iron and steel. Copper Chronically exceeds EPA water quality criteria.Primary cultural source is a component of anti-fouling paints on boat hulls and from leaching and abrasion of copper pipes and brass fittings.An important trace nutrient,it can be bio-accumulated and,thereby,create toxic health hazards within the food chain and increase long-term ecosystem stress. Lead Lead from gasoline burning in automobiles is less a problem today because of unleaded gasoline.However,lead from scraping and painting bridges and overpasses remains. Chronically exceeds EPA water quality criteria.Attaches readily to fine particulates that can be bio-accumulated by bacteria and benthic organisms while feeding.Lead has adverse health hazards when consumed by humans. Oil and Grease Toxicity contributes to the decline of zooplankton and benthic organisms.Accumulates in tissues of benthic organisms;a threat to humans when consumed directly or when passed through the food chain.Primary cultural source is automobile oil and lubricants. Arsenic An important trace metal,it can be bio-accumulated and,thereby,create toxic health hazards within the food chain and increase long-term ecosystem stress.Increased toxicity for spawning and juvenile fish.Primary cultural source is fossil fuel consumption. Cadmium Primary cultural source is metal electroplating and pigments in paints.It can be bio- accumulated and,thereby,create toxic health hazards within the food chain and increase long-term ecosystem stress. Chromium An essential trace nutrient.Primary cultural source is metal electroplating and as a component of paint pigments.It can be bio-accumulated and,thereby,create toxic health hazards within the food chain and increase long-term ecosystem stress. Pesticides Primary cultural source is agricultural farmlands,residential and institutional lawns, large turf management areas(golf courses,etc.).It can be bio-accumulated and,thereby, create toxic health hazards within the food chain and increase long-term ecosystem stress. Land use information for watershed management planning is available in the form of GIS maps. Land use data can be enhanced by combining multiple data layers to obtain additional information.For example,agricultural data layers containing information on the type of agricultural land use can be used to further define a generic agricultural category so that more accurate estimates of pollutant loading can be applied.Other sources of land use information include aerial photography,zoning, 3-4 Westchester County Storm water Management Planning Manual Hallocks Mill Demonstrative a Land Surface Imperviousness Table HMB 3.2 Estimates of%Impervious Surface by Land Use Land Use Classification Estimated"/o Impervious Percent impervious land cover Agriculture Fertilized 2% 5% was estimated using literature values for each land use Agricultural-Pasture 2% -5% classification. Residential Forest 2% -5% land-use imperviousness was Mixed Urban 85% updated based upon 2000 US Residential Based Upon Population 1 Census population Wetlands 2% -5% information (See Tech Memo Water 0% 3.1 for further detail). Notes: 1.5% minimum Soils Soils data obtained from the county website was categorized according to texture into the following basic categories: • Complex • Muck • Water • Fine Sandy Loam • Rock • Unknown • Gravel Loam Sand • Silt Loam • Loam • Very Stony Loam Figure 3-6 Soils A description of each soil type code was obtained pHalloaksMill Brook from the Soil Survey of Putnam and Westchester Stream d Texture Counties, New York. Complex Rne Sandy Loam Gravel Loam Sand Very Stony Loam The generalized soils coverage was merged with Silt Loam Loa Muck the sub-watershed-landuse coverage to obtain the Rock ggis soil breakdown for each modeled watershed. A "a°a` table with the area of each soil type within each landuse subshed was developed for the model. N COM A - 0 1 MOs HMB 3-5 Document Code • Westchester County Storm water Management Planning Manual Westchester Section 3—Data Collection 4' `�1DV00m 11 and field visits.With the aid of GIS,the distribution of land use types within each subwatershed can be calculated. 3.1.4 Land Surface Imperviousness Impervious surfaces are those surfaces that don't allow rain to infiltration into the soil. Examples included paved roads,roofs,and parking lots. Impervious surfaces are a determining factor in the response of a watershed to rainfall,and are significant in stormwater runoff and pollutant loading.Estimates of percent impervious surface within a subwatershed are based upon land use and can be developed from established criteria or customized to a particular area. Estimates of existing impervious surface cover can be made in several ways: • Using literature values associated with land use • Direct measurement using aerial photography or tax maps 1 • Correlation with population density estimates 3.1.5 Soils Soils data,like land use,is a critical factor in determining the response of watersheds to rainfall. In most non-urban watersheds,impervious surfaces (paved areas) are a much smaller percentage of the total area of the watershed than are pervious surfaces. Tight,relatively impervious soils will cause more runoff than loose,sandy soils in all the undeveloped areas of the watershed. Soil data also provide essential information for watershed modeling.The soil type and classification defines the infiltration parameters used in stormwater models that describe the infiltration properties of the pervious land cover areas. Descriptions of soil types and the necessary modeling infiltration parameters may be obtained from the USDA-Natural Resources Conservation Services Soil Survey of Putnam and Westchester Counties,1994. 3-5 0 Westchester County Storm water Management Planning Manual Hallocks Mill Brook Demonstrative Example Meteorological Data Precipitation The National Climatic Data Center maintains data for 9 hourly precipitation gages within Westchester County. It was determined,however,that none of these gages presented the desired length of record or accuracy to perform long-term stormwater modeling. Instead,precipitation data was obtained from a station located in a Bridgeport,CT,approximately 40 miles southeast of the watershed. The station was selected based upon the length of record and the accuracy of the data. Calibration with stream gages indicated that there were some discrepancies during brief storm events where rainfall at the gage and within the watershed were different,most likely during thunderstorms,but that overall the precipitation between the two areas was 11 consistent. Hourly data from 1948 through 2000 was obtained. Temperature Temperature data,daily minimum and maximum,was compiled from two sets of NCDC data. Records from Yorktown were used for the years 1967 through 2000. II White Plains was used to complete the data set from 1948-1966. Sporadic missing periods of record were synthesized using similar time periods from other years within the dataset. Streamflow 1 Limited stream flow data exists within the Hallocks Mill Brook minor watershed. Four gages were identified in the Croton,two of which were not suited for calibratin purposes because flows were influenced by dams or controlled releases. I 1 CDM HMB 3-6 Document Code • Westchester County Stormwater Management Planning Manual Wtchester Section 3—Data Collection a`y...govcom • 3.2 Meteorological Data In addition to spatial data contained in GIS maps,another important category of data for stormwater management is meteorological data. Within the general category of meteorological data,precipitation and temperature data are both important,and are used in developing stormwater models and in understanding stream flow patterns. 3.2.1 Precipitation Rainfall is one input parameter required in all stormwater models. Generally,each model requires some pre-processing of the readily available weather data. Hourly precipitation records,typically beginning in the 1940s,are available to order for most first-order National Weather Service stations at the National Climatic Data Center website,http://1wf.ncdc.noaa.gov/oa/ncdc.html.Fifteen-minute precipitation records may also be available for selected stations,but the period of record typically begins in the 1970s. When determining which precipitation gage record to select,consideration should be given to the period of record,quality of data, gage location and altitude. Attention should be paid to missing periods within a precipitation record and erroneous data. There are several public record precipitation stations located within the Croton Watershed.An analysis of the quality of these gages has shown that the long-term record for many of these stations suffer from extended periods of missing data. Therefore,care should be taken in collecting and applying precipitation data for stormwater modeling. 3.2.2 Temperature Temperature data is a basic input parameter to many stormwater models. It is important to calculate evaporation and snow melt rates. Like rainfall data, temperature data is available for downloading from the National Climatic Data Center. 3.3 Stream Flow Stream flow is made up of two basic components. One is called base flow,and is the flow in streams during most of the year when it is not raining. The source of water for base flow is continuous discharge of groundwater into the stream.The other component is stormwater runoff over the land surface during and immediately after a rain event. Stormwater models are designed to estimate runoff from a variety of rainfall events. They generally are not set up to simulate base flow. Calibration of a stormwater runoff model requires known stream flow data.This data is usually available from USGS for local stream gages located within or near a drainage area. Ideally,stream flow data will be available within the study area.When local gages are not available,gages located on nearby streams may be used and correlated to the study area. Special attention should be given to differences in precipitation history between a non-local gage and the study area. Also,differences in 3-6 • Westchester County Stormwater Management Planning Manual "' r Section 3—Data Collection xoom • elevation,aquifer characteristics,prevalent land use,stormwater drainage area and groundwater drainage area can further reduce the effectiveness of using a non-local gage.Whether local or non-local,gages affected by dam controls or other unnatural circumstances should be discarded. 3.4 Water Quality Data Although the focus of stormwater management planning has traditionally been on water quantity(flooding,impacts of stormwater on stream channel erosion and sedimentation),the water quality aspects of stormwater have been receiving more and more attention. Most of the water quality monitoring and analysis that currently takes place is in response to Section 303(d)regulations of the Clean Water Act. This section requires states to list all impaired waters which would not support the stream's designated use even after appropriate and required water pollution control technologies have been applied(primarily to point sources).The states must then determine the conditions that would restore the water to the quality that meets the water quality standards.As a follow-up,the states must then develop Total Maximum Daily Loads(TMDLs)for each water body on the list. Although the TMDL program is not the main focus of this Stormwater manual,water quality should not be considered as completely separate from stormwater planning because of the significant impact of stormwater on the quality of the streams. For this reason,stormwater pollutant loading modeling is a critical aspect of stormwater management programs. To identify potential water quality problems and assess the importance of stormwater as a source of pollution in the stream,basic water quality data should be collected. This is often a difficult task,and data are often insufficient for a complete analysis. Good sources of in-stream water quality are: • Intakes for public water supplies • USGS gages • Project related water quality sampling(e.g. special habitat studies, studies related to hazardous waste sites) • Data collected as part of State Section 303(d)listing of impaired water bodies. • New York City DEP 3.5 Event Mean Concentrations Stormwater runoff is a known major pollutant source. Unfortunately, sampling stormwater is a time-consuming,expensive,and difficult field study to carry out. In most cases,watershed or stormwater management planning will be accomplished without local stormwater quality data collected from the study area. Without site 3-7 • Westchester County Stormwater Management Planning Manualr Section 3—Data Collection g°• 0 specific sampling,quantification of runoff pollutant loading is difficult. The usual approach makes use of data collected elsewhere and published as an Event Mean Concentrations (EMC).An EMC provides the mass of a chemical per volume of runoff as an average value for the entire storm. EMCs are dependent on the land use from which the stormwater originates. For example,nutrient concentrations are higher in agricultural area runoff than they are from forested areas. Table 3.1 presents EMC values for a variety of pollutant parameters from multiple land use types. As the table shows,EMCs vary significantly between land use types for each parameter. Study's that have developed EMC estimates are typically specific to a particular land use type. Compilation of EMC data for application on a watershed level requires multiple data sources. Typically,the data is highly variable and is better viewed as a range of estimated loading. If water quality data is available,EMCs can be varied during model calibration to better match existing,in stream water quality. 3-8 • Westchester County Storm water Management Planning Manual `?Ne r 1 Section 3-Data Collection 0 Table 3.1 EMCs Event Mean Concentrations (EMCs) I Agriculture IIII Parameter Water Forest Urban Pasture Crop Wetlands BOD mg/L No Data 1 14.1 27.7 20 No Data Standard Deviation 10.0 13.8 10 COD 1 mg/L 171 113 100 159 220 171 Standard Deviation 100 62 25 89 100 100 TSS mg/L No Data 40 78.4 410 500 No Data Standard Deviation, 21 81.1 295 500 TP mg/L 0.064 0.15 0.315 0.75 2.72 0.1 Standard Deviation 0.044 0.07 0.218 0.4 0.83 0.1 TN 1 mg/L 1.6 0.75 2.4 4.2 9.86 0.5 Standard Deviation 1.1 0.52 1.6 1.7 0.8 0.5 Pb mg/L 0.00266 0.016 0.0675 0.018 0.076 0.00266 Standard Deviation 0.0023 0.059328 0.03 0.04 0.0023 Cu mg/L 0.0022 No Data 0.0135 0.005 0.053 0.0022 Standard Deviation 0.0015 0.009345 0.009 0.031 0.0015 Zn mg/L 0.0652 0.072 0.162 0.15 0.23 0.0652 Standard Deviation 0.0495 0.02 0.123 0.12 0.31 0.0495 References: 1 EPA 1982-Chesapeake Bay Program 2 Metropolitan Washington Water Resources Planning Board 1978-Occoquan/Four Mile Run Non-Point Source Correlation Study 3 Updating the U.S.Nationwide Urban Runoff Quality Database;James T.Smullen,Amy L.Shallcross and Kelly A.Cave 1999 3-9 • Westchester govcom Technical Memorandum 3.1 • GIS Data Layers Relevant for Stormwater Management Planning Stormwater management planning requires the collection of GIS data. The more important data layers are summarized below. ➢ Watershed Drainage Divides o Major Divides-the Croton Watershed is one of 6 major drainage divides within Westchester County o Minor Divides-watershed management is best performed at this level. There are 48 minor drainage divides within the Croton Watershed,one of which is the Hallocks Mill Brook > Governmental Borders and Boundaries-this includes municipal,county, state, and congressional boundaries > Roads-provide impervious surfaces to a watershed,and can alter the natural drainage system of the watershed through catchments and sewers > Storm or Combined Sewers Lines-important to understand the drainage system of a watershed,and to identify discharge points to streams and water bodies > Railroads-used for geographical and land use purposes > Land Use-critical watershed management component to determine runoff characteristics and pollutant loadings > Zoning-used to enhance existing land use information and to provide insight into possible changes to land use > Population Blocks-used to provide information on residential impervious surface,population and housing density,and water use estimates ➢ Soils-provides infiltration characteristics of the watershed needed for stormwater modeling ➢ Shallow geologic formations-provides further information on infiltration and baseflow characteristics ➢ Hydrologic Features-provides the natural stormwater conveyance system of a watershed. o Streams and Rivers o Water Bodies o Wetlands TM 3.1-1 • Westchester County Stomiwater Management Planning Manual WestdxsCOter m Technical Memorandum 3.1 g°` • ➢ Stream/River Gauging Stations-used to identify the locations of gauging stations that may be used to provide historical flow data ➢ Dams-used to identify flow restriction points > Land Elevation(contours)-used to define overland drainage patterns ➢ Location of major water users (wells,stream intakes),and maps of the water supply and sewer service areas within the watershed ➢ Potential point sources of contamination(discharge permits,locations of storage tanks,hazardous waste sites,major industrial sites etc.) > Sensitive or protected natural features Additional GIS data can also be useful for graphical presentation of the watershed. Aerial photographs and Unites States Geological Survey(USGS) topographic quadrangles(quads)are useful as a background tool for the GIS dataset. Digital 1- meter infrared orthoimagery is available for the entire State of New York from the New York State GIS Clearinghouse website,http://www.nysgis.state.ny.us/,free of charge. These photos were taken between 1994 and 1999.Higher resolution aerial photography of Westchester County was performed in 2000 as part of a statewide base mapping program,but was not available at the time this manual was developed. Topographical data can be obtained through multiple subscription services. Information is available from the USGS website,http://mapping.usgs.gov/. Land Use Data Land use data can be compiled from a wide variety of sources.Selecting the appropriate level of detail of land use data for stormwater management planning can be somewhat difficult. For example,there is little known about the different response of deciduous forest versus evergreen forest to rainfall,and the distinction is not useful within a stormwater model. For these reasons,it is often more efficient to consolidate land uses to create simpler maps with fewer land use classifications. Land use categories that do not possess independent estimates for percent imperviousness surface,stormwater model runoff characteristics,or pollutant EMC's should be consolidated. The benefit of consolidation will be seen in shorter stormwater model run times and a reduction in the effort required for data analysis. When creating a land use map, the most current data should be used. Recent development may not be present on existing land use maps,and should be incorporated into a land use map that is going to be used for stormwater management planning.Field visits,building permits,zoning regulations,and interviews can all be used to help determine recent changes to land use. Land use classifications can also be updated to reflect recent development by applying the most recent population data (Angell,Clement,Smullen,1998). Minimum population thresholds can be used to TM 3.1-2 • Westchester County Storm water Management Planning Manual Technical Memorandum 3.1 v.com rn • identify recent conversion of vacant or agricultural land to residential land use.The population thresholds should be based upon local population statistics and zoning so as to not overestimate recent development. Impervious Surface Impervious ground cover within residential land use has been shown to be a function of population density. Angell,Clement and Smullen(1998)presented the application of the Stankowski,(1974)and Manning(1977)impervious cover-population density correlations to estimating impervious cover in residential areas.The correlations are expressed as: Figure TM3.1 Residential Land Use Impervious Cover % Impervious vs. Population Density 100 ....... - 90- iiiiii:"is: :::..... :.�iiiiiiiii 80- Stankowski 70_ Manning 0 60 ,AALLL - - 35 ppa Q 50- ' 0 40- 0 30 r,' 20- 10- 0 0 20 40 60 80 100 120 Population Density(#/acre) Stankoski,I = 0.177 D 0.792-0.039logD Manning,I=104.95-81.27(0.974)PD I= % Impervious Surface PD=Population Density(per acre) D= Population Density (per square mile) Using these relationships,population density can be used to refine estimates of impervious cover within residential areas,and can also be used to identify areas that have been developed subsequent to the mapping of the land use available for the study. The formulas given above help to distinguish areas of high-density residential development from those of lower density. TM 3.1-3 • �V .voom Section 4 • Stream Assessment Key Points • A stream channel assessment involves walking the stream, recording key information, and collecting data to characterize the stream channel • It is a means of determining existing and potential watershed problems related to water quality and habitat degradation • Stream assessments provide the data needed to link stormwater flows to the current and projected condition of the stream channel • Results are critical for establishing priorities relating to planning objectives, to locate sites for stream restoration, and to prioritize implementation of stormwater management practices 4.1 Introduction Following each significant rainfall,stormwater runoff eventually finds its way to streams and water bodies. This can occur naturally as overland runoff from vegetated areas,or it can occur as direct discharges from paved-over areas through storm sewers. The amount of stormwater and the quality of that stormwater reaching the stream can have a significant effect on the health of the stream. For example, increased stormwater flows and peak flows can cause flooding,scour stream channels,and widen or shift stream channels.Stormwater can also increase sediment loads, and carry bacteria,organic matter,metals,and other pollutants to the stream. Changes in the stream flow patterns and water quality can subsequently reduce the diversity of aquatic insects and fish within the stream. Stream assessment provides a powerful means of determining existing and potential watershed problems influencing water quality,as individual streams and stream networks reveal subtle,and at times,severe changes occurring throughout the watershed. As a direct expression of the watershed,streams reflect the local topography, geology,climate,and land use. Over time,streams reach a condition of stability where the channel cross-section and slope may remain constant, despite some natural lateral shifts in the location of the channel. If accelerated changes in the channel dimensions are noted,it often indicates a response to direct modifications of the channel,such as channelization,or a change in the nature of the watershed. The degradation of stream channels may also lead to declining water quality.These changes can only be discovered through stream assessments. A stream assessment involves walking the stream,recording key information and collecting data essential for the characterization and assessment of the stream channel. Field personnel make specific observations related to channel stability,riparian and aquatic habitat,sediment supply,and sources of impact to the stream channels.One widely used stream assessment method was developed by Rosgen(Rosgen,1996). 4-1 I Westchester County Stormwater Management Planning Manual Hallocks Mill Brook Demonstrative Example Stream Survey Fifteen miles of stream within the Hallocks Mill Brook watershed were assessed using the Rosgen Classification Method.Key information and essential data for characterization and assessment of channel conditions(Rosgen Level I and Level II 411 methodology,see Tech Memo 4.1)was collected during the survey. Existing perennial channels were divided into reaches that generally did not exceed 1,000 feet in length. For each channel reach,a cross-section survey was completed at a representative location. Measurements included the stream invert,edge of water,maximum depth, bankfull depth,and flood prone level(Rosgen,1996). Using this information,each perennial stream reach within the Hallocks Mill Brook watershed was classified with the Rosgen Classification method. Figure HMB 4.1 Cross Section Locations 4 * cross Sections A' Hallocks Mill Brook N Stream 1 Wetland Lake I IS 4;c4 I o 4 4 Nf r4 7 0 1 Mlm 1 . 1 HMB 4-1 • Westchester County Storm water Management Planning Manual r Section 4—Stream Assessment �ucom The approach used to do stream assessment is described in general terms below. 4.2 Stream Survey Data collection involves a team of two to three team members walking the streams and collecting data,sketching topographic cross-sections of the stream at key locations or reaches,and rating the stability and condition of stream channels and stream banks.Stream stability is often defined as the ability of the stream to maintain, over time,its dimension,pattern,and profile in such a manner that it is neither aggrading nor degrading. Stable and self-maintaining stream channels whose physical and biological functions are at the optimum level are said to be operating at their full potential. The loss of stream stability results in degraded water quality and biological health. Stream surveys provide information to determine how the current stream condition either meets or differs from its full potential. A number of factors can change the condition and stability of streams. The factor with the most rapidly observed effects on stream stability is land use. Altering watershed land use (such as removing trees for agriculture or paving open space for parking lots)changes the local hydrology,sediment supply,and transport.The other important change that affects stream stability is a direct disturbance to the stream channel(e.g.,channelization,culverts,and bridges). The ability to understand and predict these responses and the associated effects is the goal of the stream classification that follows from the stream survey.Once completed,the information can be used to: • assess past impacts, • anticipate consequences of proposed management strategies, • evaluate the potential for recovery without intervention, • better understand the processes of channel adjustment, • determine the feasibility and prioritization of restoration,and • develop restoration designs to replicate natural stable tendencies. Examples of the type of data collected for each stream reach include: • Channel dimensions • Channel materials (sand,gravel,silt,etc.) • Evidence of direct human channel impacts • Physical constrictions(culverts,bridges,etc.) 4-2 • Westchester County Stormwater Management Planning Manual Hallocks Mill Brook Demonstrative Example a Figure HMBtt4.2 Sample Stream Survey Channel Reach Photo and Cross-Section–Rosgen Type E6 I. f 11 it , . ' . �1 Existing Ground ii, 1 "E°f 14 ,t 4,` t�1 -- ---BankfullWidth .r°1` ` ' t- x •11• it ',' —Flood Prone Elevation • 9 '� • -i 1 .; 305.00 41 a• ,�..4 N .� v H ::: -x , z ) aa) 290.00 q.� { w m 0 10 20 30 40 50 60 " � l 7 ' Distance from an arbitrary datum [ft] Figure HMB 4.3 Sample Stream Survey Channel Reach Photo and Cross Section–Rosgen Type C4 4 i, ,II i 'i I # Existing Ground A 1- i '�� tit. ; - ---Bankfull Width - ; , , ,,, m F 305.00 ;,, a� " I _. —1- 300.00 -- . r 00 si o 295.00 y. l • •.- T T. sY, 290.00 , c • • ,� -. -� Ft*% , . w m 0 10 20 30 40 50 60 f _ _ Distance from an arbitrary datum [ft] 1 HMB 4-2 Westchester County Storm water Management Planning Manual MT to. r Section 4—Stream Assessment ;gov.com • Measures of stream bank stability and degree of erosion • The condition and width of the riparian buffer Another piece of information collected at each reach location is the stream"bankfull elevation". This is the elevation where the stream channel is flowing at full capacity. 111 This bankfull flow event has been found to be the discharge that moves the most sediment over time,and is therefore important for understanding erosion and sediment related water quality concerns. The field team also collects information on a number of other factors that influence the overall"state" or condition of a stream in addition to channel characteristics. These factors include hydrologic,biological, ecological, and human factors. 4.3 Survey Results One important result of the detailed stream assessments is the classification of all the 1 stream channels in the watershed.This is often done using the Rosgen stream classification system. This classification system is useful to infer general stability, stream bank erosion potential,recovery potential,sensitivity to disturbance,and the importance of vegetation in maintaining/influencing channel shape.The stream channel assessments provide the foundation for identifying existing and potential stream problem areas. In addition,this information can also be used for the identification of reference reach areas to be used as model areas during the development of stream restoration projects. When used in combination with stormwater models,assessment data can be used toward the development of management measures that will ameliorate water quality problems and flooding within the watershed. See Technical Memorandum 4.1 for further details 4-3 • Westchester County Stormwater Management Planning Manual "'este!rester Technical Memorandum 4.1 °v`-011 • Technical Memorandum 4.1 The Rosgen Stream Classification Method One of the most widely used stream classification methods is based on the approach developed by Dave Rosgen (Rosgen, 1996). Rosgeri s system uses a hierarchy of assessment levels to classify a stream reach according morphological conditions of similar streams the he has encountered during his years of field study. The number of assessment levels used to classify a stream determines the detail of the classification. • Level I-course geomorphic assessment of basin relief,landform, and valley morphology • Level II -more detailed geomorphic assessment of channel dimensions, patterns, and materials • Level III - assessment of the existing condition and stability of the stream • Level IV-verification of classifications obtained in Levels I, II and III through field measurements. Used to develop predictive relationships Data Collection for Rosgen I, II, and partial III Analyses Listed below are the specific parameters that are evaluated during the measured reach survey, including the Rosgen Level I, II,and partial III analyses. Channel Morphology Channel Disturbance • Bankfull width • Altered hydrology • Bankfull depth • Direct human channel impacts • Bankfull width/depth ratio • Culvert(type, shape, estimated span,edge of pool conditions) • Flood prone area width • Utilities • Entrenchment ratio range Channel Habitat • Bankfull cross-sectional area IN Fish barriers (chronic) • Water surface slope range Canopy cover(over channel) • Channel materials • Stream classification type (Rosgen, 1996) TM 4.1-1 • Westchester County Storm water Management Planning Manual r Technical Memorandum 4.1 gOUcom Channel Stability • Depositional features • Woody debris Riparian Condition ■ Sediment supply • Riparian composition • Reach bed stability • Riparian width (aggradation/degradation and boundaries) • Percentage of canopy cover(over channel) • Degree of bank erosion • Extent of vertical raw bank For each identified measured reach,one cross-sectional survey is made using a stretched tape measure,survey rod,and site level. Cross section surveys are done at representative crossover locations. Measurements include the stream invert,edge of water,maximum depth,bankfull depth,and flood prone level(Rosgen,1996). The location can be noted using a global positioning system. The assessment also indudes periodic pebble counts to quantify channel material categories (silt/clay,sand, gravel,cobble,boulders)found in the watershed. Bankfull Elevation Once channel reaches are identified,a bankfull elevation at each cross section is ascertained using field indicators,including depositional features (point bars), changes in sediment size distribution,changes in bank slope,flow lines,vegetation changes,and stain lines on boulders. The bankfull elevation corresponds to the flow that occurs once every 1 to 2 years. This bankfull flow event has been found to be the discharge that moves the most sediment over time. Therefore,it is widely considered to be the channel forming flow for undisturbed,dynamically stable streams. However,in disturbed streams,the bankfull elevation can be difficult to determine. Since disturbed streams are in the process of adjusting to a changed flow regime and an altered sediment supply,bankfull indicators may be misleading,weak or completely absent. In these circumstances,the determination of bankfull is extremely objective and may lead to ambiguous or highly variable results. Calibrating Bankfull Elevation If a gage is located on the stream,it can be used to estimate bankfull flow by correlating flows to the elevations found. If no gage station is available to calibrate the bankfull flow,the bankfull width, depth and cross sectional area of measured reaches in the watershed are compared to previously developed regional curves(plots of flow vs. drainage area). In the Croton watershed,the Eastern Catskills regional curve developed by New York City Department of Environmental Protection and the TM 4.1-2 • Westchester County Storm water Management Planning Manual -rm Technical Memorandum 4.1 Southeast Pennsylvania regional curve developed by Dunne and Leopold(1978) are the most appropriate curves to date. Condition of Stream(Rosgen partial Level III)and Channel Stability Stream morphology,as defined by Level II criteria,serves as the basic physical stream categorization.There are a number of other factors that influence the overall"state" or condition of a stream in addition to channel morphology. These factors include hydrologic,biological,ecological,and human factors. Partial Level III analyses incorporate these other factors in addition to the morphological template to further describe the existing stream condition of the stream channel. Stream stability is often defined as the ability of the stream to maintain,over time,its dimension,pattern,and profile in such a manner that it is neither aggrading nor degrading. Stable and self-maintaining stream channels whose physical and biological functions are at the optimum level are said to be operating at their full potential. The loss of stream stability results in degraded water quality and biological health. Stream assessment provides information to determine how the current stream condition either meets or differs from its full potential. A number of factors can change the condition and stability of streams. The factor with the most rapidly observed effects on stream stability is land use. Altering watershed land use (such as removing trees for agriculture or paving open space for parking lots) changes the local hydrology,sediment supply,and transport. Another important factor is a direct disturbance to a stream channel(e.g.,channelization, culverts,and bridges). The influence of natural or imposed disturbances varies by Rosgen stream type as shown in Table 1. The ability to understand and predict these responses,and the associated effects by stream type is important to (1) assess past impacts, (2)anticipate consequences of proposed management strategies, (3) evaluate the potential for recovery without intervention, (4)better understand the processes of channel adjustment, (5) determine the feasibility and prioritization of restoration,and (6) develop restoration designs to replicate natural stable tendencies. The partial Level III field inventory includes visual observation of parameters beyond the fundamental Level II morphologic assessment. The approach specifically includes assessment of the following stream characteristics: (1)presence of woody debris, (2) sediment supply, (3)reach bed stability, (4) degree of bank erosion, (5) extent of vertical raw bank,(6) evidence of altered hydrology, (7) direct human channel impacts, (8)culverts,(9)utilities, (10)fish barriers, (11)riparian composition, (12) riparian width, (13)canopy cover,and(14)reference reach potential. These parameters all influence the current stream condition and potential for recovery to an optimum condition. As such,the Rosgen Partial Level III analysis helps describe channel conditions as they relate to stream stability,potential,and function. The Partial Level III analysis can,in part,support the following objectives (Rosgen, 1996): TM4.1-3 Westchester County Storm water Management Planning Manual lr Technical Memorandum 4.1 go�com • Develop a quantitative basis for comparing streams having similar morphologies, but with different states or conditions. • Describe the potential natural stability of a stream,as contrasted with its existing condition. • Determine the departure of a stream's existing condition from a reference condition. • Provide guidelines for documenting and evaluating additional field parameters that influence stream state(e.g.,flow regime,stream size,sediment supply,channel stability,bank erodibility, and direct channel disturbance). • Provide a framework for integrating companion studies (e.g.,fish habitat indices, and composition and density of riparian vegetation). • Develop and/or refine channel stability prediction methods. • Provide the basis for efficient Level IV validation sampling and data analysis. The field data results in the assignment of a stream type for the reach in question. Table TM4.1.1 shows the various stream types,and the watershed management interpretations for each of a number of criteria. These criteria are: Riparian Vegetation Controlling Influence on Erosion: riparian vegetation has a significant influence on the stability of certain stream types. If vegetation has a high influence, riparian SMPs have a high priority. Streambank Erosion Potential: Bank erosion can be accelerated by changes to the watershed or to the channel itself.The Rosgen Bank Erosion Hazard Index(BEHI) provides a numerical indication of the erosion potential based on the stream classification recorded during the fieldwork. Sediment Supply: streams vary in the supply of sediment available to cause water quality impairments.For those streams with high sediment supply from channel sources and stream adjacent slopes,watershed SMPs and stream restoration have a high priority. Recovery Potential: once the cause of the instability is corrected,this criterion assesses the natural recovery ability of the stream to regain stability. Sensitivity to Disturbance:each stream type can be classified by the degree to which is sensitivity to human disturbance. TM4.1-4 Table TM4.1.1 Management interpretations of various stream types(Source:Rosgen, 1996) CG Cn CuC ~ n m g ci, et, ..r y m Oro rt z ¢, Ort .! 0 '3 aq pa �� m O� C SD 2 ti 0., Ortrrl . "J' CJq CD n C' .fir C� "t ""R' ".S `n O_ ,-, fu C. rt C 'L3 5 "5' 4•fp 0 " • g p w 5. 0 m C O x' 'P dQ r� to C O N T 'P Q Z Al Very low , Excellent Very low Very low Negligible D6 High Poor High High Moderate A2 Very low Excellent Very low Very low Negligible Da4 Moderate Good Very low Low Very high A3 Very high Very poor Very high Very high Negligible DA5 Moderate Good Low Low Very high A4 Extreme Very poor Very high Very high Negligible DA6 Moderate Good Very low Very low Very high A5 Extreme Very poor Very high Very high Negligible E3 High Good Low Moderate Very high A6 High Poor High High Negligible E4 Very high Good Moderate High Very high B1 Very low Excellent Very low Very low Negligible E5 Very high Good Moderate High Very high B2 Very low Excellent Very low Very low Negligible E6 Very high Good Low Moderate Very high B3 Low Excellent Low Low Moderate Fl Low Fair Low Moderate Low B4 Moderate Excellent Moderate Low Moderate F2 Low Fair Moderate Moderate Low B5 Moderate Excellent Moderate Moderate Moderate F3 Moderate Poor Very high Very high Moderate B6 Moderate Excellent Moderate Low Moderate F4 Extreme Poor Very high Very high Moderate Cl Low Very good Very low Low Moderate F5 Very high Poor Very high Very high Moderate C2 Low Very good Low Low Moderate F6 Very high', Fair High Very high Moderate C3 Moderate Good Moderate Moderate Very high G1 Low Good Low Low Low C4 Very high Good High Very high Very high G2 Moderate Fair Moderate Moderate Low C5 Very high Fair Very high Very high Very high G3 Very highPoor Very high Very high High C6 Very high Good High High Very high G4 Extreme Very poor Very high', Very high High D3 Very high Poor Very high Very high Moderate G5 Extreme Very poor Very high Very high High D4 Very high Poor Very high Very high Moderate G6 Very high Poor High High High D5 Very high Poor Very high Very high Moderate Notes: 1. Includes increases in streamflow magnitude and timing and/or sediment increases. 2. Assumes natural recovery once cause of instability is corrected. 3. Includes suspended and bedload from channel-derived sources and/or from stream adjacent slopes. 4. Vegetation that influences width/depth ratio stability. Westchester County Stormwater Management Planning Manual t` •r Technical Memorandum 4.1a:wcom Bank Stability(Rosgen Bank Erosion Hazard Index Rating and Shear Stress Ratio) It is now commonly thought that the contribution of streambank erosion to total sediment yield has traditionally been underestimated. Streambank erosion is a natural process that occurs along rivers and streams. Stream channel bed and bank erosion can be accelerated when the hydrology of a watershed is significantly altered and when local variables controlling bank erosion processes are altered. Natural bank erosion occurs as a result of a number of processes,such as mass wasting,surface erosion,fluvial entrainment,freeze-thaw,liquefaction,and ice scour. As described in Applied River Morphology(Rosgen,1996),there are several factors that influence the ability of a streambank to resist erosion including the following: • Ratio of stream bank height to bankfull stage; • Ratio of riparian vegetation rooting depth to stream bank height; • Degree of rooting density; • Composition of stream bank materials; • Stream bank angle (i.e. slope); • Bank material stratigraphy and presence of soil lenses;and • Bank surface protection afforded by debris and vegetation. An important part of the stream assessment should include a measure to quantify existing bank erosion. One such measure is the Rosgen Bank Erodibility Hazard Index (BEHI) rating. This can be calculated for each measured reach. Table TM4.1.2 shows Rosgen's BEHI rating procedure,which characterizes various streambank conditions into numerical indices of bank erosion.Each column provides a numerical score for several bank measurement ratios,and observations about such factors as vegetation root density and the slope of the banks. The final column totals the numbers,and shows the range of scores associated with very low to extreme bank erosion potential. TM4.1-6 Westchester County Stom►water Management Planning Manual Technical Memorandum 4.1 go`.com Table TM4.1.2 Bank Erodibility Hazard Rating Guide(From Rosgen, 1996). BANK EROSION POTENTIAL Bank Ht/ Root Root Bank Angle Surface Bkf Ht Depth/ Density (Degrees) Prot.(%) TOTALS Bank Ht (%) Very Value 1.0-1.1 1.0-0.9 100-80 0-20 100-80 Low Index 1.0-1.9 1.0-1.9 1.0-1.9 1.0-1.9 1.0-1.9 5-9.5 Low Value 1.1-1.19 0.89-0.50 79-55 21-60 79-55 y _ Index 2.0-3.9 2.0-3.9 2.0-3.9 2.0-3.9 2.0-3.9 10-19.5 Moderate Value 1.2-1.5 0.49-0.30 54-30 61-80 _ 54-30 Index 4.0-5.9 4.0-5.9 4.0-5.9 4.0-5.9 4.0-5.9 20-29.5 High Value 1.6-2.0 0.29-0.15 29-15 81-90 29-15 Index 6.0-7.9 6.0-7.9 6.0-7.9 6.0-7.9 6.0-7.9 30-39.5 Very I Value 2.1-2.8 0.14-0.05 14-5 91-119 14-10 High Index 8.0-9.0 8.0-9.0 8.0-9.0 8.0-9.0 8.0-9.0 40-45 Extreme Value >2.8 <0.05 <5.0 >119 <10 Index 10 10 10 10 10 46-50 Numerical Adjustments: Bank Materials: Bedrock: bank erosion potential always very low Boulders: bank erosoin potential low Cobble:decrease by one category unless mixture of gravel/sand is over 50%,then no adjustmen Gravel:adjust values up by 5-10 points depending on composition of sand Sand:adjust values up by 10 points Silt/clay: no adjustment Stratification: 5-10 points(upward)depending on position of unstable layers in relation to bankfull: As an additional tool to evaluate bank erosion risks,the shear stress ratio developed by Rosgen(1986)can also used for each measured reach. The ratio is intended to highlight elevated bank erosion risk for those banks where near-bank shear stresses are significantly greater than the overall channel,such as along the outer banks of meander bends. The shear stress ratio,therefore,considers hydraulic (and ultimately planform pattern)factors,while the BEHI focuses more on detailed bank attributes. The value of near-bank shear stress ratio is expressed as: tib/tic= (Db XSbXY)/(DcXScXY) where: Db = mean depth of the third of the cross section nearest the evaluated bank, Dc = mean depth of entire cross section, Sb = slope at the third of the cross section nearest the evaluated bank, TM4.1-7 Westchester County Storm water Management Planning Manual q ter Technical Memorandum 4.1 xgov.com Sc = slope at the entire cross section, y = specific weight of water. Following Rosgen(1986),the erosion risk ratings associated with calculated shear stress ratios are shown in Table TM4.1.3 below. Table TM4.1.3 Bank Erosion Risk Associated With Shear Stress Ratios Following Rosgen(1996) Shear Stress Ratio Bank Erosion Risk Rating Less than 0.8 Very Low 0.8 - 1.05 Low 1.06 - 1.14 Moderate 1.15 - 1.19 High 1.20 - 1.60 Very High Greater than 1.60 Extreme Stream Assessment Findings This classification system is most useful when evaluated using the associated management interpretations as presented above to infer general stability,stream bank erosion potential,recovery potential,sensitivity to disturbance, the importance of vegetation in maintaining/influencing channel morphology,etc. In addition,the Partial Level III parameters provide valuable insight when combined with the stream classifications within a particular watershed as to general stream condition or state, and provide some insight into the sources of channel condition,particularly with regard to channel instability and active adjustment. The BEHI ratings are valuable in identifying areas subject to and/or sensitive to accelerated bank erosion,and predicting areas with potential for high sediment yield. The stream channel assessments provide the foundation for identifying existing and potential stream problem areas. In addition,this information can also be used for the identification of reference reach areas to be used as model areas during the development of stream restoration projects. When used in combination with stormwater modeling results,assessment data can be used toward the development of management measures that will ameliorate water quality and flooding within the watershed. TM4.1-8 Westchester County Storm water Management Planning Manual Hallocks Mill Brook Demonstrative Example Stormwater Modeling The USEPA Storm Water Management Model(SWMM)was selected to simulate the relationship between precipitation and stormwater runoff within the Hallocks Mill Brook watershed.Continuous simulation runoff modeling was performed using over 50 years of precipitation and temperature records. Overland runoff volumes,peak runoff flow rates, and estimated annual pollutant loadings were all estimated using SWMM. Figure HMB 5.1 Stormwater Runoff rJ Snowpack ' , f , r'> 'Precipitation- - Tributaries Ridge r�"'` ,n' - rr- 1 Agriculture 1 f Basin '', ---N‘,00. f:-'''' f Lake" j-..t '"Zone _ y err ,Forestry ;tv •.0.11!'.!..t.::,''' . .." ti ' .1i"wn ! 'Town i A r A. '. . '' • " WetlandWatcr:hcd ' • .'k' ar�raf n .� ,'�":Ir W f45elciatlorr. aaroundwatel '�'?,S .s" y;,jn z: aquifer) se.- ii. .a....,4`,::-tia; r '., - Prnda<ad by lane Cooncll Di Governomntr Reference:http://www.epa.gov/owow/watershed/whatis.html HMB 5-1 W .gm Section 5 Stormwater Modeling Key Points • Stormwater models are computer models that simulate the runoff of water during storms • Models provide a way of estimating total stormwater flows and peak flows across an entire watershed.Models can also provide estimates of contaminant loads carried to streams by stormwater. • Models are the only tools that can be used to project the impacts of future changes to land use on stormwater flows • Stormwater models provide the flow estimates that help to explain the impacts to streams identified during the stream assessment • Stream assessment data and stormwater model results together provide the information and tools necessary to develop an effective stormwater management program. 5.1 Purpose and Applications A key tool in watershed management planning is a stormwater model. A stormwater model is a computer-based simulation of the runoff of water caused by storms,and provides a means to assess the impacts of stormwater runoff quantity and quality anywhere within the study area. Detailed stormwater models can provide critical information that can be used to assess the current conditions of a watershed,as well as assist in developing planning recommendations and making implementation decisions.Stormwater models are the only tool that can project impacts of future changes to land use on stormwater quantity and quality. Combining stormwater model results with information obtained through stream assessment provides the information and tools necessary to develop an effective stormwater management program. The results of stormwater runoff quantities and peak flows quantified with a stormwater model are often seen downstream in eroded and unhealthy stream channels. This type of information can be useful in determining where stormwater remediation projects would be necessary,as well as most effective. Planning decisions can be made by using the stormwater model to simulate the effects of land use changes on stormwater runoff,and relating that to the conditions and properties of the stream channels that would be affected.For example, it may not be desirable to significantly increase impervious cover and runoff upstream of sensitive channels where erosion may be a concern. A range of models is available to serve many different requirements of watershed management,as summarized in Table 5-1. Table 5-1 is an updated and amended listing of available models from a recent USEPA Office of Research and Development Urban Watershed Management Branch compendium of models for Total Maximum Daily Load development. 5-1 Westchester County Stormwater Management Planning Manual R ,� re, .1... r Section 5—Stormwater Modeling - ' '0°m Table 5-1—Preliminary List of Available Watershed Models Available Watershed Models Code Name Caretaker Applicablility Level ACTMO Agricultural Chemical Transport Model USDA-ARS A M AGNPS Agricultural Nonpoint Source Pollution USDA-EFS A M Model ANSWERS Areal,Nonpoint Source WS NC State/EPA A/R D Environmental Response Sim.Model ARM Agricultural Runoff Model EPA A D AUT-Q/ Illinois Urban Drainage Area Simulator Ill Water Surv. U M ILLUDA (quantity) FHWA Federal Highway Administration FHA Highways S Model FLUX Tributary Mass Discharge ACOE-WES (all) S GL/CREAMS Chemicals,Runoff,and Erosion from USDA-ARS A D Ag Management Systems GWLF Generalized Watershed Loading Cornell U/R/A S/M Functions HEC-5Q Water Quality Simulation Model ACOE-HEC N/A D HSPF Hydrologic Simulation Program- EPA U D Fortran MIKE 11 Microprocessor-based Modeling DHI N/A D Systems for Rivers and Channels MIKE 11 Res Reservoir Mgt.,Water Quality DHI N/A D Simulation&Impact Assess MUNP Management of Urban Nonpoint U Md U M Pollution Model NPS Nonpoint Source Model EPA U/A/F D PRMS Precipitation-Runoff Modeling System USGS F D P8 P8-Urban Catchment Model Narr.Bay Proj. U M SIMPTM Simplified Particle Transport Model OTAK Inc. U M SITEMAP SW Interceptor&Trtmt Eval Model for (NPSMAP) Analysis and Planning Fetrow Eng. U/M/R M SLAMM Source Loading and Management WI DNR U M Model STORM Storage,Treatment,Overflow Runoff HEC U M Model SWMM Stormwater Management Model EPA U/M/R D SWRRB Simulation for Water Resources in USDA R/A D Rural Basins WASP4 Water Quality Analysis Simulation Program EPA N/A D WEPP Water Erosion Prediction Project Many A M WMM Watershed Management Model CDM U/R/A/F S WRENS Water Resource Evaluation of Nonpoint USFS F S Silvicultural Sources. Applicability:U=urban;A=agricultural;R=rural;M=mixed,F forest Level or Ease of Use:S=simple;M=moderate;D=detailed 5-2 Westchester County Storm water Management Planning Manual Hallocks Mill Brook Demonstrative Example Model Data Needs The data described in Section 3 was used to construct the stormwater model of the Hallocks Mill Brook. The modeling was performed on the delineated subwatersheds shown in Figure 3.3. Hourly precipitation and daily temperature data were used to perform the runoff simulation of the Hallocks Mill Brook watershed. Figure 5.1 Sample Precipitation Data 2000 Hourly Precipitation Record 0.5 0.4 ...-_ 7 0 7/3 0) 0.3 C C p 0.2 H C C 0.1 , e��Je6 �rPQM\ �a� )owe �J�� s)c3 �oec G�ooec ,eeec Ge��� Figure 5-2 Sample Temperature Data December 2000 Daily Temperature Record 70 60 50- 40- c O Min P2 r 30- ■ LL III Max � 20- 10- 0 1111111111111i 11111111 111111 1 i 11 1 , (1111r V V V V V V V V C) C) V C) C) C) V V V V V V C) C) C) C) C) C) C) V C) V 01d 0) d 0) 0) N 0) d 0) d N CD d 01 d N N 0) d 01 d N N 01 01 0) 01 d 01 01 9 0000�00000000000000CI000000000 r-N A 4 6 6)�O 6 O r N A a ti)CO OTO<N A 4 In CD A 00 O)0 N N N N N N N N N NVf Ch HMB 5-2 Westchester County Stormwater Management Planning ManualIest�chester Section 5—Stormwater Modeling ```go%.com 5.2 Model Data Needs All stormwater models require a basic level of data to perform a simulation.The type and complexity of data required will depend upon the specific model used and the desired output. The quality and level of detail of the input data will directly determine the detail and accuracy of the model results. Precipitation data is one of the primary input parameters of a stormwater model.In order to perform detailed stormwater modeling,the selected model should be capable of processing long-term precipitation data from actual precipitation stations located within or near the study area. Other common data needs include: • Land use • Meteorological data o Precipitation o Temperature o Wind Speed • Soil characteristics(infiltration parameters) • Impervious surface cover estimates • Drainage units(watersheds) • Overland drainage area • Overland drainage slope • Stream network&stormwater drainage system maps Potential data sources for most of these parameters are provided in Section 3 of this manual. 5.3 Model Results Stormwater models are capable of providing different types of results. The complexity and capabilities of the chosen model will determine the types of results and level of detail of the model output. Some typical results are: • Estimates of runoff volumes,either on a storm-by storm,seasonal,or annual basis.This data can easily be extracted from most models. 5-3 Westchester County Storm water Management Planning Manual Hallocks Mill Brook Demonstrative Example Model Results A sample of the stormwater modeling results for the Hallocks Mill Brook watershed are presented in the following graphics: Figure HMB 5.3 Average Annual Runoff Volume Amual Runoff Volume-Existing(in r �<10 105 0 215 - 111 >20 4,4 fig Al% 1 0 1 Miles Figure HM8 5.4 Average Annual BOD Loading Annual BOD Loading(161acre) of —0-5 5 20 X20-35 104435-50 50+ i r� G1 HMB 5-3 Westchester County Stormwater Management Planning Manual \\ ,tchester Section 5—Stormwater Modeling g°VODm • The spatial distribution of runoff across the watershed,or the amount of runoff being produced in different parts of the watershed. This data is slightly more complex and may be somewhat more difficult to obtain from different models. • Peak flows,or the peak rate of runoff. • Estimates of nonpoint pollutant loadings produced by stormwater. • Flooding or surcharging of the stormwater conveyance system 5.4 Use of Model Results An accurate stormwater model is an essential tool for watershed management planning.The information obtained through stormwater modeling will provide a better understanding of the dynamics of the watershed as it exists in its current state, as well as a predictive tool to assess potential changes to the watershed. Effectively assessing the results of a stormwater model is a critical step in the planning process. One of the more straightforward uses of model results is the sizing of stormwater controls,pipes,culverts,and basins. The average and peak runoff volumes estimated through the stormwater model can be used to provide the data necessary to adequately design such structures so that they will handle current and future stormwater flows.Without a stormwater model,it is nearly impossible to accurately assess these potential flow rates. Another use of model results is to estimate non-point sources of pollution within a watershed.Stormwater runoff is often a significant pollution source to streams and surface water bodies. The degree of runoff pollutant loadings for different pollutant parameters is dependent on the land use type over which the water flows,and degree of impervious cover.The pollutant loading estimates provided by a stormwater model can be used in conjunction with land use data and trends to identify current sources of non-point pollutant loading,as well as provide planning information to prevent or control future pollutant sources.This type of information is invaluable for the protection of drinking and recreational waters,where excessive pollutant loadings could have the greatest impact. When used in conjunction with information obtained though a stream assessment, stormwater models can be used to relate the flows at specific points along a stream with the health of the stream indicated by the assessment.This type of information can be used to assist in planning decisions that will best protect habitats and prevent erosion. Stormwater models provide the results that are critical to identifying appropriate stormwater management practices (SMPs)within a watershed. For example,a watershed that is suffering from excessive nutrient loading may be well served by 5-4 Westchester County Stormwater Management Planning Manual ., . er Section 5—Stormwater Modeling g° m implementing a more efficient nutrient program for local farmers. Identifying those areas within that watershed that are actually contributing the nutrient loading is necessary so that the efforts are not misdirected. As a planning decision tool, stormwater models can be used to predict the effects of proposed land use changes on a watershed.For example,the effects of increased urbanization within an area of the watershed on the stream channels within the watershed can best be assessed by determining the increased flow volumes and flow rates that will be experienced by the channels.This information,coupled with the stream assessment,can be used to ensure that development considerations anticipate potential impacts to streams and mitigate these impacts through stormwater management practices. 5-5 W.-1 r sw>agovcom Section 6 Data Analysis Key Points • Data analysis should include the analysis of all data collected, including stream flow, impervious surfaces, land use, water quality, runoff pollutant loadings, runoff volumes, and flooding. • The goal is to increase our understanding of the watershed, adequately describe existing stream conditions, and understand the connection between changes in the watershed and stream response. • Data Analysis is the key link between understanding the problems and developing the solutions. Adequate analysis of the available data provides the critical technical input to the planning process. Data analysis for stormwater planning requires the analysis of a wide variety of data: • Impervious surface • Land use • Historical stream flow data • Historical water quality data • Simulated pollutant loadings • Simulated runoff volumes • Simulated channel flows • Flooding patterns • Regulatory structure related to water,stormwater,and land use • Stormwater model output An important first step in data analysis is to perform a quality assurance/quality control(QA/QC)check of the data. Data obtained from outside sources should not be relied upon for accuracy until the information has been verified from complimentary sources or field investigations. For example,land use data can be checked for accuracy through spot field checks and a review of aerial photography. 6.1 Impervious Surface One of the primary indicators of watershed"health" is the amount of impervious surface cover in the watershed. Based on numerous research efforts, studies and observations,a general 6-1 Westchester County Storm water Management Planning Manual Hallocks Mill Brook Demonstrative Example Impervious Surface Fi.ure HMB 6.1 Estimated Im.ervious Surface Cover Impervious Surface Cover(%) The Hallocks Mill Brook watershed is comprised <20%25% of a wide variety of land use types, from 30% 35% >35k agricultural to commercial to residential. This land use information was used to develop estimates of impervious ground surface throughout the watershed. These impervious surface estimates are Abased upon the capacity for precipitation to 1IIAiinfiltration the ground in the stormwater model. Ivir A 1 0 1 Miles Figure HMB 6.2. Estimated Impervious Surface Cover of Build-Out Conditions Impervious Surface Cover Build-out(%0 Q<20% 20%-25% 25%-30% A build-out analysis (discussed further below)was 30-35% >35% *al performed on the watershed. The result of urbanization and development can be easily seen in the potential impervious cover. it%F... •tot igiarc *Flip 1 0 1 Miles HMB 6-1 Westchester County Stomiwater Management Planning Manual Wotchester Section 6—Data Analysis -govcom categorization of watersheds has been widely applied to watershed management based on percent impervious cover(Schueler, 1995).This is summarized in table 6-1. Table 6.1 Impervious Surface as an Indicator of Stream Health Characteristic Sensitive Degrading Non-Supporting Percent Impervious Cover 0%to 10% 11%to 25% 26% to 100% Channel Stability Stable Unstable Highly Unstable Water Quality Good to Excellent Fair to Good Fair to Poor Stream Biodiversity Good to Excellent Fair to Good Poor Sediment and temperature Also nutrients and Pollutants of Concern only metals Also bacteria Source:Schueler,1995 Analysis of impervious cover consists primarily of estimating percent impervious cover based on land use data and population density data. It is primarily done using GIS. Once impervious cover percentages have been calculated, an initial impression of the health of the watershed can be gained using Table 6-2, which summarizes several of the impacts of traditional development on streams and watersheds. Figures 6-3 and 6-4 illustrate the changes to the volume and duration of runoff,as well as potential changes to the physical stream channel before and after development.Figure 6-3 also illustrates the benefits of using various SMP's and low impervious techniques to managing stormwater. As Figure 6-4 depicts,traditional development within a watershed may raise the elevation of the floodplain and reduce summer low flows when compared to predevelopment conditions. Figure 6-3 Comparison of volume and duration of stormwater runoff before and after land development, and reductions in runoff from SMP's. iter Development Atter Development Lower Imperviousness Bioretennon Before Development a tossed Swales 141 With Conventional „ Stormwater .: • Before Development Management y; lime Time (Prince George's County Department of Environmental Resources et.al.,undated) 6-2 Westchester County Storm water Management Planning Manual ` tester Section 6—Data Analysis `0011 Figure 6-4 Potential impacts of development on stream flow and flooding Response of Stream Geometry Predevelopment % 1+ ._ j �� t4.0.414%,„ NI'S.* IP' I .. iik, 0.;,.. ,.....• ; , — — _ Flood�lan Limite __ • Summer Low Row Level LI , A, Post Development ! ` f ii di . iv '''4:, 10 It AL. s�. � Flnodptain Limit _ rr� ,r In — - --- -� �~ � iii. mr. Summer Low Flow Level --....-- (Schueler 1995(a),and Schueler 1987) Table 6.2 Impacts of Traditional Development on Watershed Resources Changes in Stream Hydrology Changes in Stream Morphology • Increased magnitude/frequncy of severe • Channel widening and downcutting floods • Streambank erosion • Increased frequency of erosive bankfull and • Channel scour sub-bankfull floods • Shifting bars of course sediments • Reduced ground water recharge • Imbedding of stream substrate • High flow velocities during storm events • Loss of pool/riffle structure • Stream enclosure or channelization Changes in Stream Water Quality Changes in Stream Ecology • Instream pulse of sediment during • Reduced or eliminated riparian buffer construction • Shift in external produciton to internal • Nutrient loads promote stream and lake production algae growth • Reduced diversity of aquatic insects • Bacteria contamination during dry and wet • Reduced diversity of fish weather • Creation of barriers to fish migration • Higher loads of organic matter • Degradation of wetlands,riparian zones and • Higher concentrations of metals, springs hydrocarbons,and priority pollutants • Decline in amphibian populations • Stream warming • Trash and debris jams Schueler 1995 It is important to note that not all development necessarily creates these impacts. Cluster developments can significantly reduce the amount of impervious cover added to a watershed, 6-3 Westchester County Stomiwater Management Planning Manual t Section 6—Data Analysis '=v.com and thus substantially reduce the volume of stormwater generated. Clustering and other conservation development design techniques can contribute to reducing infrastructure expansion,reducing impacts to infiltration and ground water recharge, and reducing the generation of stormwater.Additional storm water management practices(SMPs)can be engineered into the overall development design to minimize or eliminate the impacts of runoff. Avoiding the generation of unnecessary stormwater and maximizing the infiltration of rainfall and stormwater into pervious areas are two primary strategies for minimizing stormwater impacts. The vast majority of impervious cover is created to accommodate vehicles. As population grows in an area, so does the number of automobiles and vehicles, and the number of acres of paved areas for driving,parking,loading/unloading,and storing vehicles.As an example, for a subdivision with a 36 foot wide street,2 sidewalks that are 5 feet wide on either side of the street, a 40-foot radius cul-de-sac,and 15 driveways that are 75 feet long,the relative portion of total street-related impervious cover components is: Street 49% Driveways 31% Sidewalks 13% Cul-de-sac/Turnaround 7%. Many techniques and alternative designs are now widely used in nearby areas to reduce the amount of impervious cover required to safely meet the needs of vehicles and pedestrian movement. Large commercial parking areas cause particular concern.Table 6-3 compares the runoff expected to be generated from a traditional parking lot versus a meadow.This is included here to illustrate why traditional impervious cover(without SMPs)creates more rapid runoff of stormwater(i.e., decreased time of concentration), more volume of stormwater(i.e., runoff volume),higher runoff rates(i.e.,runoff velocity), and increased pollutant loadings (i.e., annual pollutant loads)than an open meadow of equal size and slope and with the same amount of rainfall. 6-4 Westchester County Storm water Management Planning Manual Hallocks Mill Brook Demonstrative Example Land Use Residential land use makes up the majority of the Hallocks Mill Brook watershed, covering 61% of the watershed area. Mixed urban land use,comprised of commercial, industrial and institutional land uses,contributes another 6%. Approximately 70% of the total watershed area is developed,or suburban land. Forested (open space) land use is the second most prevalent,with 16% of the total area. Agricultural lands are 8% of the total area (5% pasture,3% fertilized). The remaining land use in the watershed is comprised of wetlands and water (7% and 2% respectively. Figure HMB 6.3 Land Use Distribution Hallocks Mill Brook Watershed-Land Use Distribution Residential 61% Water 2% Mixed Urban Wetland 6% Forested 7% 16% Agricultural- Agricultural-Pasture Fertilized 5% 3% This land use distribution plays a critical role in determining how the watershed will react to precipitation, and the degree of non-point pollutant loadings to the receiving waterways. As the watershed continues to develop, the land use distribution will become more urbanized. In order to assess these potential build out conditions, a build out analysis was performed on the Hallocks Mill Brook watershed. The build out was performed based upon zoning regulations for those areas zoned as residential. Areas that are not readily subject to development(steep slopes,wetlands, preserved lands)were not included in the build out scenario. HMB 6-2 Westchester County Stormwater Management Planning Manual Westchester Section 6—Data Analysis "CO11 Table 6.3 Comparison of One Acre Parking Lot vs.One Acre Meadow in Good Condition Parameter Parking Lot Meadow Runoff Curve Number 98 58 Runoff Coefficient 0.95 0.06 Time of Concentration(minutes) 4.8 14.4 Peak Discharge Rate(cfs)2yr/24 hr 4.3 0.4 Peak Discharge Rate(cfs)100 yr 12.6 3.1 Runoff Volume from 1"Storm(ft3) 3450 218 Runoff Velocity of 2yr Storm(ft/sec) 8 1.8 Annual Phosphorus Load(lb/ac/yr) 2 0.5 Annual Nitrogen Load(lb/ac/yr) 15.4 2 from Center for Watershed Protection 1994 Although development can result in the deterioration of ground water, streams and riparian resources,there are many design and protection practices that can be easily incorporated into the planning and design process to reduce or eliminate these impacts. Many of these techniques are discussed in the New York State Stormwater Management Design Manual. There are many publications and information resources readily available to decision makers for information on design alternatives and SMPs that can improve the runoff characteristics of developed lands.The"References"section of this manual contains sources for many of those resources. 6.2 Land Use Another primary indicator of watershed"health" is the distribution of land use types across the watershed. Runoff pollutant loadings,volumes,and peak flow rates are directly connected to the land use within a watershed. Deforestation and urbanization reduce a watershed's natural ability to capture and retain stormwater through infiltration and evaporation. The effects of the reduced time of concentration(the amount of time it takes precipitation to reach a stream or storm sewer),and increased peak flow rates can be seen in downstream flooding,erosion,habitat destruction,and pollutant loadings. Agricultural land, depending on the farming practices used,can also generate large amounts of rainfall runoff and reduce infiltration.A major concern for agricultural land use is the application of chemicals to the land(namely fertilizers and pesticides). When applied in quantities greater than what is taken up by the target plants,these chemicals become suspended in runoff from rainfall events and become"pollutants" as they are carried into streams. Both the quantities and locations of different land use types can have an impact on the "health" of a watershed.Deforestation and urbanization along waterways are more likely to cause erosion and habitat destruction than similar land use types that are not directly connected to the waterways. This type of information is critical to make future planning decisions that will best protect the watershed 6-5 Westchester County Stormwater Management Planning Manual Hallocks Mill Brook Demonstrative Example Figure HMB 6.4 Build-Out Land Distribution Hallocks Mill Brook Watershed-Land Use Distribution-Build-Out Conditions Residential 70% Water ^mom 2% Wetland 7% Agricultural-Fertilized Mixed Urban 1% 6%n Forested 13% Agricultural-Pasture 1% As demonstrated in the Figures HMB 6.2 and 6.3,build out under current zoning regulations in the Hallocks Mill Brook watershed would result in an increase of residential land use and a decrease of agricultural and forested land. The increase in residential land use under this build out scenario is approximately 650 acres. HMB 6-3 Westchester County Storm water Management Planning Manual • •r Section 6—Data Analysis #gov.com Analysis of land use data occurs as part of the development of the stormwater model. The key results are: • Estimates of the area of each land use type for the entire watershed,and for each subwatershed • Estimates of percent impervious surface for each land use type • Projections of expected changes in land use within the watershed over the planning period In order to develop a projection of future land use,several approaches are possible. Which one is selected depends on the objectives of the study and the recommendations of the stakeholder task force. 1. Build-out Approach If GIS maps of zoning are available,one frequently applied approach is to assume that the entire watershed will be developed according to the existing zoning. This provides a"worst case" development scenario. This approach can be refined by restricting projected full development by eliminating areas of steep slopes, sensitive environments,wetlands,etc.,or by assuming that only a percentage of full build-out can or will occur due to physical constraints. 2. Population Based Approach Full build-out according to zoning rarely occurs. Another approach is to take population projections for a municipality for the future date used in the plan(for example,25-years in the future). The difference between the projected future population and the present population represents growth that requires further development within the watershed. Existing zoning densities or existing land use densities can be used to project the number of acres required to provide homes for the increased population. By simply calculating the current commercial acreage within the watershed, divided by the current population,a factor of"acres of commercial land use per person" can be established. This factor,multiplied by the expected population growth,provides an estimate of additional commercial development that will be needed to serve the increased population. The additional residential and commercial development must then be distributed within the watershed,replacing either unprotected open space or agricultural land. 3. Vision Based Approach A third approach is not based on projections,but rather on a set of planning objectives. In this case,future development scenarios are created to meet certain land use planning goals established by the county or by the municipality.These 6-6 Westchester County Stormwater Management Planning Manual W stehester Section 6—Data Analysis g°Vcom scenarios are then simulated by the stormwater model to provide vital information about the expected impacts of each scenario. 6.3 Historical Stream Flow Data The analysis of stream flow data consists primarily of developing a basic,statistical understanding of streamflow.This requires that there be a gage with a reasonable period of record of daily flows located on the stream itself. For a USGS operated gage,basic flow statistics will usually be available,providing minimum,mean,and maximum flows. If a reasonable period of record is available,it is important to develop additional statistics of stream flow.Examples of additional statistics include: • Calculation of flow return frequencies or flow duration curves. These are curves that provide information on how often a particular flow will be experienced over the course of a normal year,as well as the return period of large flood events. For example,the curves may help to determine that flow rates of 1000 cubic feet per second occur once every 100 years. • Calculation of particular drought indicators.For example,the Q7-10 flow is an indicator that is frequently used to assess severe drought conditions. It is defined as the lowest mean flow over a consecutive 7-day period that is likely to occur with a frequency of once every 10 years. • Calculation of base flow and stormwater runoff on a monthly and annual basis. • Calculation of base flow,runoff,and total streamflow as a percentage of precipitation over the watershed. 6.4 Historical Water Quality Data Analysis of existing water quality data is a key task in the stormwater management planning process.Comparing existing water quality with current Federal and State water quality guidelines identifies existing water quality problems. Once pollutants of concern have been identified,potential sources of contaminants should be identified.Although an analysis of point sources of contamination to the streams is not the focus of a stormwater management plan,it is important to recognize those situations where point sources may be a significant contributor to total pollutant loads,along with nonpoint sources (primarily stormwater). Otherwise, the emphasis on improving water quality may be incorrectly placed on improvement of point sources over nonpoint sources,or vice-versa. A variety of parameters can be used to measure the health of streams and rivers,as well as the suitability of these surface waters as a source of potable water supply. 6-7 Westchester County Storm water Management Planning Manual Hallocks Mill Brook Demonstrative Example Simulated Pollutant Loading Estimates of simulated average annual runoff pollutant loadings were determined for common pollutants found in stormwater runoff. Analysis was performed on existing conditions and under the build-out scenerio. Additional pollutant loading figures are provided in Appendix C. Figure HMB 6.5—COD Existing Conditions Figure HMB 6.6—COD Build-Out Conditions Annual COD Loading(lb/acre) Annual COD Loading liblacrel / 71550-150 —75-150 300111 —150300 _ 300 450- _30050 IV 450500 �� 450-600lal�0+ �� 600+ li A1411101 � lid& I II KAU itiltlii 1. gel* tilt miks vl Alp k Ott /NW Wital‘— Al I OIL Or 1 0Miles A I 0 1 Miles Figure HMB 6.7 TSS Existing Conditions Figure HMB 6.8 TSS Build-Out Conditions Annual TSS Loading(lb/acre) Annual TS5 Loading(Ib/acre) 0-50 0-so50-100 —so-100 —100-150 100-150 5 122; 250 `� 2 -250 ��� 250+ 01,4 Its 0i,g04ap -4 . tott,4„,,i, �� .11,,t,Al‘ortoir AI%s1 or*A 1 0Miles A 1 0 I Miles ISII HMB 6-4 Westchester County Storm water Management Planning Manual Section 6—Data Analysis a;'' .com Because most of the streams in the Croton watershed contribute flow to drinking water reservoirs,stormwater runoff and nonpoint source pollutant runoff are significant concerns.Table 6.5 summarizes some specific water quality parameters that are important indicators of the quality of raw water to a drinking water supplier. If data are available,statistical summaries of water quality parameters should be developed(minimum,maximum,mean values)and compared to the expected loading of pollutants simulated by the stormwater model. This type of analysis can be used to adjust stormwater runoff pollutant loading estimates to local conditions. Table 6.4 Water Quality Parameters of Importance to Drinking Water pH Apparent Color Physical Parameters Alkalinity Hardness Total Dissolved Solids Conductivity Turbidity TSS Particulates& Total coliform Microbial Contaminants E. coli Giardia Cryptosporidium TOC DBP Precursors UVAbs@254nm (Organic Compounds&Bromide) SUVA Bromide Ammonia Inorganic Compounds Nitrite (Nutrients) Nitrate D.Orthophosphate T.Phosphate Inorganic Compounds Iron (Metals) Manganese Arsenic Inorganic Compounds Sulfate (Secondary Contaminants) Chloride Sodium 6.5 Simulated Pollutant Loading Stormwater runoff is a known major pollution source. Unlike point sources,which may be easily identified,quantifying stormwater runoff pollutant loading is a complicated task. Identifying potential sources of pollutant loading requires information gathered in multiple facets of the stormwater planning process. 6-8 Westchester County Stormwater Management Planning Manual Hallocks Mill Brook Demonstrative Example Simulated Runoff Volume Figure HMB 6.9—Simulated Annual Runoff Volume Amual Runoff Volume-Existing(in r--J<10 1105:15 1- z0 Average annual runoff volumes were calculated >20 Oa using SWMM. The analysis was done using units of inches so that subwatersheds of different sizes could be compared. Subwatersheds with extensive ��� amounts of wetlands or water are simulated as I�� ��� having high amounts of annual runoff volumes. I t A t og Al%**It 1 0 1 Miles ' Figure HMB 6.9—Simulated Annual Runoff Volumes of Build-Out Conditions Annual Runoff Volume-Build-Out(in) <110 15-10 15-20 >p 11. taiUnder build-out conditions, the amount of annual runoff is increased throughout the watershed. This additional runoff has the potential to erode streams, destroy habitats, and cause flooding. �� �► Its 0f� I' �4 l��l► AV 'i 1 0 1 Miles HMB 6-5 Westchester County Storm water Management Planning Manual Section 6—Data Analysis ucom The stormwater model is a key technical tool that can be used to help identify potential sources. Runoff pollutant concentrations have been found to be mostly a function of land use. Runoff pollutant loading can therefore be seen as a function of the amount of stormwater that runs off a particular type of land use multiplied by the concentration for each pollutant for that land use.The data derived from a stormwater model provides the necessary information to develop estimates of pollutant loading. Estimates of pollutant loading for each subwatershed,usually on an annual basis,are good relative indicators of pollution sources. Once highly contributing subwatersheds have been identified,the appropriate SMPs and planning policies can be instituted that address factors specific to those subwatersheds. For example,if sedimentation is discovered to be a major concern within a watershed,it may be necessary to identify the potential sources of sediment supply. If relevant water quality data is available, sediment concentrations within a stream during precipitation events can be compared with concentrations during dry periods. If concentrations during dry periods are within a tolerable range,the source of the sediment load can likely be attributed to the total suspended solids(TSS) concentration of overland runoff during precipitation or stream erosion caused by excessive stormwater flows. TSS loading estimates from the stormwater model,field observations from the stream assessment,and water quality data can then all be used to quantify the sediment supply being contributed by overland surface runoff.The estimated TSS loading of each of the upstream subwatersheds could then be compared to determine which upstream areas are causing the elevated TSS levels. The land use and impervious surface characteristics of those subwatersheds can then be assessed to determine applicable SMPs or planning policies specific to those subwatersheds that would help to alleviate the TSS loadings. Besides identifying existing pollutant sources within a watershed,simulated pollutant loading estimates are a powerful planning tool to assess the impacts of potential land use changes. As described earlier in this manual,deforestation and urbanization tend to increase overall runoff pollutant loading. Estimates of potential runoff pollutant loading are critical to protect drinking water supplies,recreational waters,natural habitats, and other sensitive areas.This type of information can be used to assist in implementing stormwater management practices,regulate zoning,and protect natural land uses. 6.6 Simulated Runoff Volumes The volume of runoff generated within a watershed can be another indicator of the "health" of a watershed.Without the proper stormwater controls,highly urbanized/impervious areas tend to generate large amounts of runoff. These increased runoff volumes and rates typically have a negative impact upon the health of a watershed. 6-9 Westchester County Storm water Management Planning Manual Hallocks Mill Brook Demonstrative Example Simulated Channel Flow Figure HMB 6.10 Sample Channel Flow CDF Stormwater runoff and base Subwatershed#25-Channel Flow CDF --- -c-0025 Bankfull flow estimates 100 -- were analyzed 90 - on a continuous 80 - 1 - basis for each of 70 60 - the � - 3 50 ----------- subwatersheds w 40 within the 30 Hallocks Mill 20 - - Brook 10 watershed. The ° 0 0.1 0.2 total flow rate %of Observations At or Below the Stated Value within each channel was compared to the bankfull discharge identified during the stream survey to estimate the occurrence of bankfull exceedence. Channels experiencing frequent bankfull exceedence were usually found to concur with unstable channels prone to erosion and sedimentation. HMB 6-6 Westchester County Stommwater Management Planning Manual Westchester Section 6—Data Analysis °vcoi" Annual runoff volumes simulated by the stormwater model are a good tool to assess the runoff volumes within a watershed. The runoff volumes generated by a subwatershed can best be thought of as a depth of runoff covering that entire subwatershed. In this way,the runoff generated between subwatersheds can be compared,in spite of differences in area.Subwatersheds that provide the highest relative amounts of runoff are more likely to be experiencing or causing stormwater problems. 6.7 Simulated Channel Flow The conveyance network is crucial in relating upstream environments to downstream conditions.The consequences of stormwater runoff are typically manifested in channel erosion,sedimentation,habitat destruction, and flooding.While simulated runoff volumes provide a good indicator for the potential sources of such problems, their cumulative effects can only be assessed by analyzing the flow experienced in downstream channels. Relating modeled stream conditions to a stream survey is an excellent tool for watershed management and planning. The stream survey will provide information on current stream conditions and probable channel morphology.Stormwater modeling results can be used to determine if a channel is experiencing flows in excess of its natural condition and the potential impacts of the excessive flow. One indicator of elevated flow conditions is the exceedence of bankfull stream conditions.The bankfull discharge (described in Section 4 of this manual)is associated with the flow that performs the greatest amount of"work" on the channel geometry,and under stable conditions is expected to occur with a frequency of once every 1 to 2 years.When bankfull conditions are achieved more frequently than the expected recurrence interval(1 to 2 years),channels tend to become unstable,with increased erosion and sedimentation the result. By comparing bankfull discharges identified during the stream survey with simulated channel flow rates obtained through stormwater modeling,the frequency and annual duration of bankfull exceedence can be obtained. 6.8 Flooding Analysis Flood prone areas are obvious stormwater trouble spots and can usually be identified from stakeholder information. However,most of the identified flooding areas will likely be the result of undersized pipes,undersized culverts,or poor drainage systems. The flooding of streams can be less obvious,and therefore harder to identify. Interviews with municipal officials and highway officials should be carried out to collect any information on flooding that might exist.Locations of flooding should then be mapped for comparison with stormwater modeling results and the results of the stream channel stability analysis. 6-10 Westchester County Storm water Management Planning Manual Section 6—Data Analysis °°m Identifying the source and cause of flooding can be difficult. Information obtained during the data collection,stream assessment,and stormwater modeling phases of the stormwater planning process can be used identify probable flooding causes,as described in the sections above. Increases in impervious ground cover and inadequately sized stormwater drainage systems are frequently the source of flooding problems within a watershed. Increased flow intensities and quantities to streams can produce frequent flooding conditions. The source of such flooding conditions can at times be attributed to recent development,but can also result from the cumulative effects of years of changes in land use. In some cases,flooding is caused simply by poor maintenance of existing stormwater structures such as culverts,drains,catch basins,inlets,and outlets. If the cause of flooding is primarily due to development,the percent of impervious ground cover within a subwatershed is a good indicator for potential flooding sources. The impacts of highly impervious surfaces can be seen in urban flooding conditions where the stormwater drainage system cannot handle large or peak flows, or in downstream stormwater elements where channels or pipes are overburdened. In areas where impervious ground cover is producing frequent flood events,additional development and imperviousness could severely impact a watershed. Years of erosion and sedimentation also can contribute to flooding events. Channels or other hydrologic features that become burdened with sediment loads can suffer a reduction in maximum discharge capacity. This reduced flow capacity will be seen in increased flooding events. The stream assessment provides vital information where such sedimentation is occurring or is likely to occur. 6.9 Regulatory Structure It is important to understand the regulatory structure influencing stormwater management within the study area. There are numerous,Federal Statutes,State Regulations,and local ordinances that may directly or indirectly affect the implementation of the stormwater plan. Part of developing a good stormwater management plan is to summarize the regulatory structure on the Federal,State, County, and municipal level in order to be able to answer the following questions: • Does the regulation directly affect stormwater planning?Examples might include an ordinance that requires the capture of runoff from the 25-year storm,or a Federal Act requiring inventory and control of all major stormwater outfalls within the watershed. • Does the regulation provide a potential source of funding?There are numerous State and Federal programs that provide funding for stormwater planning,agricultural nutrient and runoff control measures,habitat restoration,etc. 6-11 Westchester County Stormwater Management Planning Manualmeter Section 6—Data Analysis 1pV00m • Does the regulation indirectly affect stormwater planning?Many regulations are not directed at stormwater planning,but may indirectly have a great deal of influence on the way stormwater must be dealt with. For example,Federal regulations pertaining to the Clean Water Act or the Safe Drinking Water Act may require substantial investments in the control and treatment of stormwater,even though the latter is primarily directed at improving water quality for drinking water protection. The Westchester County Non-Point Source Pollution Program contains a thorough review of the regulatory structure in Westchester County. 6-12 Section 7 Problem Definition and Consensus on Planning Objectives Key Points • This step is designed to integrate the results of the technical analysis with the insights of the stakeholders. • It is important to confront perceived problems obtained from the stakeholders with problems identified and supported by data analysis. • This step also helps to educate stakeholders and build a consensus on possible solutions. The stormwater management planning approach outlined here is designed to provide the technical analysis required to assess perceived stormwater issues within a watershed,identify any issues that may have gone unnoticed,and then to develop planning alternatives that address these issues.Information obtained through data collection,stormwater modeling,and stream assessment will lead to a better understanding of actual stormwater issues within a watershed,and provide concrete evidence to support(or refute)stakeholder concerns. Achieving stakeholder consensus on the definition of problems and on the objectives of stormwater planning requires a carefully designed stakeholder participation process. In order to achieve consensus,the following approach is recommended.The approach assumes that an active stakeholder Technical Advisory Committee (TAC) has been established to work with the project team throughout the planning process. • Establish an initial set of perceived problems during a"project kickoff" workshop. Do not attempt to assess the accuracy of the problems presented. This should be a complete list of problems and issues relating to stormwater management. • Hold frequent stakeholder meetings throughout the period of technical analysis. Only hold meetings when there is important information to convey. This usually means that planning assumptions or modeling assumptions are being conveyed,or that results from an analysis have become available. Provide full information about the findings during the meetings,and use the stakeholder TAC to make interim decisions as needed to proceed with the technical studies.These meetings serve to educate the TAC on the findings, as well as to educate the project team on the priorities of the various members of the TAC. • Once all the technical studies are complete and results have been presented and discussed with the TAC,a second workshop should be held. In this workshop,the goal should be to achieve consensus on the high priority problems,to agree on which perceived problems no longer appear to be of 7-1 Westchester County Stormwater Management Planning Manual "cstcl)cstc r Section 7—Problem Definition `.`"I" concern, and to establish a set of planning objectives to solve the high priority problems. The final set of high priority problems must be based both on the priorities of the TAC as well as on the technical analysis portion of the study(modeling,stream assessment, data analysis,etc.).The set of planning objectives should relate directly to the problems uncovered by the various technical analyses performed,but also can relate to gaps in the local or State regulatory structure. The planning objectives form the blueprint for subsequent implementation of the stormwater plan recommendations. 1 7-2 (S' Section 8 Management Alternatives/SMP Selection Key Points • In this step, the problems within the watershed are prioritized using technical information and stakeholder input. • Prioritizing focuses efforts where they are most needed, helps to phase implementation, and results in a more effective program. • This step is the link between the watershed characterization study and an implementation plan. 8.1 Prioritization Process Once the technical portions of the watershed management planning process (modeling,stream assessment,data analysis, etc.)are complete and consensus on planning objectives has been reached,the information must be assessed to identify the high priority stormwater problem areas within the watershed. Assessing quantitative data,such as pollutant loadings,along with qualitative data,such as stream erosion, requires that the overall impairment level of the subwatersheds be considered. Prioritizing the most highly impaired subwatersheds ensures that management efforts are focused where they are most needed.Once the highly impaired subwatersheds have been identified,planning objectives can be developed to address the problems specific to each.Preventing further degradation within these high priority areas of the watershed could provide the most beneficial stormwater impact,and remediation efforts can be targeted to reduce the current stormwater impairments. Prioritizing subwatersheds requires developing a process through which they can be ranked. Although most stormwater impairment issues are common to all watersheds, the significance of each is specific to the watershed,and should be determined by local stakeholders. While pollutant loadings may be of great concern in a particular watershed,flooding may be the main concern in another. It is important,however,to rely on all of the information obtained and not to focus upon one parameter. One approach is to use a multi-criteria evaluation program to rank the subwatersheds in ascending order of priorities.Which subwatersheds receive the highest priority is a matter of planning objectives.For example,those with the most problems could be considered first,or those with the highest beneficial use. Alternatively,high priority could be attached to those subwatersheds with the greatest potential for improvement. The prioritization process can be used as a convincing planning tool for watershed management.High priority areas within a watershed are likely to be poor choices for further development.Also,potential build out and land use scenarios can be analyzed using the prioritization process to assess the potential impact caused by changes in stormwater characteristics.This information could then be used to determine how best to protect a watershed from further urbanization,either through preventing 8-1 Westchester County Stormwater Management Planning Manual com Section 8—Management Alternatives further development or using planning standards that will protect the"health" of the watershed. In order to use a multi-criteria evaluation program(there are several commercially available),a set of evaluation criteria must be developed,and "scores" for each of the alternatives must be developed. The selection of criteria will depend on the planning objectives as well as the availability of data and information to score the watersheds. A list of potential criteria is presented below.Most multi-criteria evaluation techniques require the development of a two dimensional matrix consisting of the options to be evaluated(columns) and a set of evaluation criteria(rows). In this case, the options being evaluated are the subwatersheds. For every combination of subwatershed and criterion,a score is assigned.The choice of the criteria is governed, in part,by the need for the scoring to be as objective as possible. By objective,we mean that the scores should represent impartial data and information useful in making decisions. The criteria must be clear and unambiguously defined. See Tech Memo 8.1 for a detailed description of Evamix, a multi-criteria evaluation program. 8.1.1 Potential Prioritization Criteria Data and information obtained throughout the stormwater management planning process should be used to rank subwatersheds. The purpose of ranking subwatersheds should be to identify those areas within a watershed that are experiencing the most negative stormwater impacts,those that are causing the greatest impacts,and those that are most sensitive. Examples of possible criteria that can be effectively used for the prioritization of subwatersheds include: > Stormwater Model o The depth of runoff generated by each subwatershed,as compared with the average volume generated across the entire watershed o The number of hours per year that channel flow exceeds bankfull conditions o Pollutant loadings (i.e.,BOD,COD,TSS...) ➢ Stream Assessment o The potential for stream bank erosion o The sensitivity of a channel to human disturbance o The recovery potential of a channel o The availability of sediment supply to cause water quality impairments 8-2 Westchester County Storm water Management Planning Manual `\esti�sterm Section 8—Management Alternatives vco o The riparian controlling influence on erosion o Observed direct human channel impacts ➢ Other Data Analysis o Reported flooding problems o Percent impervious surface cover 8.1.2. Criteria Weighting The other input variable required for the prioritization procedure is the selection of weighting factors for each of the criteria.While the scoring process strives to be as objective as possible and is carried out by the project team,the selection of weights is inherently subjective and should be done by the decision-makers,planners,or stakeholders. Unlike the matrix of scores,numerous possible weight sets are possible, and all are equally"valid". In general,criteria weights should be assigned by the stakeholder task force,and can be done during a workshop. This process also provides an excellent opportunity for actively engaging the stakeholders in the planning process. 8-3 :,. •r •v.com Technical Memorandum 8.1 EVAMIX Evaluation of alternative Stormwater Management Practices (SMPs)is an important first step when moving from the planning phase into design and implementation of stormwater management alternatives. Because there are often numerous options for achieving the same result,it is often helpful to use decision support software to help with the evaluation process. In using an evaluation program to help prioritize subwatersheds,the objective of prioritization should be clear from the outset. For example,severely degraded subwatersheds would be ranked as a high priority for restoration.They might be of low priority for protecting and preserving natural resources,where only relatively pristine stream sections would be targeted for protection and/or preservation. Usually multiple prioritization are appropriate, • depending on the intent.Examples might include prioritizing stormwater mitigation, stream restoration, habitat protection,or any other number of objectives. There are numerous approaches possible that fall under the category of"multi-criteria evaluation". Some are simple spreadsheets of alternatives and evaluation alternatives,with rankings of high,medium and low. More sophisticated evaluation software exists,however,and this technical memorandum illustrates the use of a multi-criteria evaluation program called EVAMIX. Description of EVAMIX EVAMIX is a decision support software program that can be used to aid in the prioritization process. It is a matrix based,multi-criteria evaluation program. EVAMIX makes use of both quantitative and qualitative criteria within the same evaluation,regardless of the units of measure. This feature gives the program much greater flexibility than most other matrix based evaluation programs,and allows the evaluation team to make use of all data available to them in its original form. The following are required for the EVAMIX evaluation: > A distinct set of alternatives to evaluate.For the stormwater management planning approach,each of subwatersheds becomes one of the"alternatives" being evaluated and prioritized. > A set of clearly defined criteria used to compare the subwatersheds on the basis of water quality and stormwater needs. > A separate matrix of subwatersheds and criteria for each of the prioritization exercises.Scores assigned to every watershed for each criterion. > An explanation of the scores.The explanation should include data used,or provide the rationale for the score. > Weighting factors assigned to each criterion.These weights should represent the relative importance of each criterion. TM 8.1-1 Westchester County Stormwater Management Planning Manual Westchester m Technical Memorandum 8.1 Selection of Evaluation Criteria The use of EVAMIX requires the development of a two-dimensional matrix consisting of the subwatersheds being evaluated(columns)and a set of evaluation criteria (rows).An example of an evaluation matrix can be found in the Hallocks Mill Brook example in section 8. For every combination of subwatershed and criterion,a score is assigned.The choice of the criteria is governed,in part,by the need for the scoring to be as objective as possible. In other words,the scores should represent impartial data and information useful in making decisions.The criteria must be clear and unambiguously defined,and can be set up as either quantitative criteria(e.g. estimated annual TSS loading within the subwatershed,annual hours of bankfull flow exceedence),or qualitative criteria(e.g. stream recovery potential,flooding problems). The choice of whether to define a criterion as quantitative or qualitative depends on the feasibility of describing the impact with numbers,the availability of data to assign scores to each option,and the reliability of the data.For example,one aspect of the evaluation for water quality may be the importance of the stream to cultural or recreational resources within the watershed.This criterion can clearly be defined in quantitative terms,with the units defined as the number of such resources found in the watershed,if available data on the number and location of such resources exist. If gaining hard data within the time frame of the analysis is difficult,or the data are unreliable,it may not be appropriate to assign a quantitative number to this criterion. In this case,the number of impacted cultural/recreational resources could simply be defined as"high","medium",or"low",and the scores assigned as qualitative scores. Potential Prioritization Criteria Possible criteria that can be effectively used for the prioritization of subwatersheds are described below. In general,if stormwater management is the goal,those streams that rank high are usually more highly impacted,more prone to flooding,and have higher pollutant loads than those that rank lower. High ranking subwatersheds are also those whose recovery potential is high, Stormwater Model Generated Flow Criteria These criteria represent provide an indication of the expected runoff from the watershed and its relationship to the streams carrying capacity. Percentage of Runoff Above Mean Runoff for Watershed:this is a criterion that provides a comparison between subwatershed runoff and the entire watershed runoff. If the percentage is above the mean for the watershed,that subwatershed is likely to be urbanized,or have highly impervious soils causing increased runoff. If the percentage is low,it is likely to be less developed. Number of Hours per Year Flow Exceeds Bankfull Estimate:a criterion measuring the frequency of flooding.The model results are compared with the estimated bankfull discharge of each subwatershed's channel to compute this number. TM 8.1-2 Westchester County Stormwater Management Planning Manual Technical Memorandum 8.1 ucom Stormwater Model Contaminant Loading Criteria Water quality criteria can form an important part of the evaluation. In most cases, actual sample data will not be available,and model generated loading can be used as a substitute. Care should be taken to use the best available land use data to develop the model,and review of land use data by a TAC(what is that?)is advisable. The criteria used for Hallocks Mill Brook were taken from model simulation results from other areas of the country(?). Theyindicate the simulated intensity of contaminant loading coming from stormwater based on land use characteristics in the subwatershed. BOD Loading:a criterion measuring the load of BOD (Biological Oxygen Demand) in lbs/ acre/ year. COD Loading:a criterion measuring the load of COD (Chemical Oxygen Demand) in lbs / acre/ year. TSS Loading:a criterion measuring the load of TSS (Total Suspended Solids)in lbs/acre/year. TP Loading:a criterion measuring the load of TP(Total Phosphorus) in lbs/acre/year. TN Loading:a criterion measuring the load of TN(Total Nitrogen) in lbs/acre/year. Pb Loading:a criterion measuring the load of Pb(Lead) in lbs/acre/year. Cu Loading:a criterion measuring the load of CU (Copper)in lbs/acre/year. Zn Loading:a criterion measuring the load of Zn(Zinc) in lbs/acre/year. Stream Channel Characteristics These criteria result from the stream classification work. They measure the streams current state,with more degraded streams receiving higher priority for SMPs or restoration activities. In most cases,the entire stream length within a watershed will not have been assessed.Some method will therefore be needed to assign a single score to the subwatershed based on scores taken from several reaches within a subwatershed. In the Hallocks Mill Brook subwatershed,the dominant or most representative score was sought. Other approaches could average the results,or take the worst or best score,depending on the intent of the evaluation. Streambank Erosion Potential: bank erosion can be accelerated by changes to the watershed or to the channel itself.The Rosgen Bank Erosion Hazard Index (BEHI) provides a numerical indication of the erosion potential based on the stream classification recorded during the fieldwork. The score range is numeric: TM 8.1-3 Westchester County Stormwater Management Planning Manual Westchester Technical Memorandum 8.1 `""' Very low (5-9.5) Low (10-19.5) Moderate (20-29.5) High (30-39.5) Very High(40-45) Extreme (46-50) Sensitivity to Disturbance:each stream type can be classified by the degree to which it is sensitive to human disturbance. A stream has a higher priority for restoration or protection using watershed SMPs if it is sensitive to disturbance. The score range is: Very low(1) Low (2) Moderate (3) High (4) Very High (5) Extreme (6) Recovery Potential:once the cause of the instability is corrected, this criterion assesses the natural recovery ability of the stream to regain stability. A stream has a higher priority for restoration if it is likely to recover slowly. The score range is: Very Poor (6) Poor (5) Fair (4) Good (3) Very Good (2) Excellent(1) Sediment Supply:streams vary in the supply of sediment available to cause water quality impairments. For those streams with high sediment supply from channel sources and adjacent stream slopes,watershed SMPs and stream restoration have a high priority. The score range is: TM 8.1-4 Westchester County Stormwater Management Planning Manual t • :arm Technical Memorandum 8.1 ;- Very low(1) Low(2) Moderate (3) High(4) Very High(5) Riparian Vegetation Controlling Influence on Erosion:riparian vegetation has a significant influence on the stability of certain stream types.This is measured on a 5-point scale. If vegetation has a high influence,riparian SMPs have a high priority. Negligible (1) Low(2) Moderate (3) High(4) Very High(5) Current Impact Criteria These criteria assess the current state of the stream based on observation and reported flooding. Direct Human Channel Impacts:a qualitative assessment based on observation from the field team.Those areas where impacts are high receive a higher priority for restoration or watershed SMPs.The score range is: Low (1) Moderate (2) High(3) Reported Flooding Problems:a qualitative criterion that indicates if flooding has been reported to Town officials or related by Town officials to the project team.The score range is: 1 if flooding has been reported 0 if no flooding has been reported TM 8.1-5 Westchester County Storm water Management Planning Manual Westchester Technical Memorandum 8.1 gm"'"' Percent Impervious of Watershed: a quantitative criterion that is an indicator of the degree of urbanization of a watershed. In general,more highly urbanized watersheds show the greatest impact, and are a higher priority for SMPs and/ or stream restoration. The percent impervious is estimated from the land use distribution within each subwatershed. Criteria Weighting The other input variable required for the prioritization procedure is the selection of weighting factors for each of the criteria.While the scoring process strives to be as objective as possible and is carried out by the project team, the selection of weights is inherently subjective and should be done by the decision-makers, planners, or stakeholders. Unlike the matrix of scores, numerous possible weight sets are possible, and all are equally "valid". In general,criteria weights should be assigned by the stakeholder task force, and can be done during a workshop. This process also provides an excellent opportunity for actively engaging the stakeholders in the planning process. Example of EVAMIX Application on the Hallocks Mill Brook Watershed EVAMIX, the decision support multi-criteria evaluation program described above, was used to determine high priority stormwater management areas within the Hallocks Mill Brook watershed. The intention was to identify the most highly impacted areas of the watershed, and then to develop potential stormwater management practices (SMPs) that could be implemented to alleviate the stormwater issues specific to that area of the watershed. Prioritization Criteria The data gathered throughout the demonstration of the watershed planning approach for the Hallocks Mill Brook watershed was used as EVAMIX prioritization criteria. The analysis included information gathered from field investigation,stormwater modeling,and the stream survey. Because the prioritization for the Hallocks Mill Brook watershed was a demonstration of the process,criteria weighting using a local stakeholder group was not performed. Stakeholder input and a further knowledge of local watershed issues would be required to develop meaningful criteria weighting. Instead,each of the parameters discussed below were used to perform the prioritization,with equal weighting. The prioritization criteria are presented in tabular format in Table 1. TM 8.1-6 Westchester County Stormwater Management Planning ManualWestchester Technical Memorandum 8.1 -1Cr'g°Ecom Stormwater Model Generated Flow Criteria Percentage of Runoff Above Mean Runoff for Watershed:the hourly runoff volumes %Hours Above Watershed fwersge generated from the stormwater model for each o1 10% 10%-15% subwatershed were compared against the mean 15o-25 0 `� runoff volume generated across the entire >25% �t� Hallocks Mill Brook watershed.The results of the analysis were used as a qualitative prioritization parameter. The subwatershed with the greatest ��� �` percentage of hours of mean runoff volume in exceedence of the watershed average received the ���� highest priority ranking. 111,1144k Allik prq A 1 0 1 M les Number of Hours per Year Flow Exceeds Bankfull Estimate:the hourly channel flow AnnHual HovsofBanfullExceedence volumes generated from the stormwater model 5 510 were compared against the estimated bankfull 5-20� flow volumes determined during the stream 20 5 survey.The results of the analysis were used as a qualitative prioritization parameter. The `� subwatershed with the greatest number of annual ��� `� hours of bankfull exceedence received the highest lkWi4;4'4� priority ranking. ,�f A.1600.A 1 0 1 Mles Stormwater Model Contaminant Loading Criteria Estimates of runoff pollutant loadings were used as qualitative parameters. The subwatershed with the highest estimated annual loadings of each particular pollutant TM 8.1-7 Westchester County Stormwater Management Planning Manual Westchester Technical Memorandum 8.1 giDiV00m received the highest priority ranking. Pollutant loading figures are provided in Appendix C of the manual. Stream Channel Characteristics Stream channel characteristics obtained during the stream survey were used as qualitative prioritization parameters.When multiple stream reaches were located within the same subwatershed,it was necessary to select one set of characteristics to perform the evaluation on the subwatershed basis.Typically,the stream reach characteristics of the stream reach that dominated a particular subwatershed were assigned to that subwatershed. The criteria and scoring were applied as described above. Current Impact Criteria Field observations of direct human channel impacts and reported flooding problems were used as qualitative prioritization parameters. Subwatersheds exhibiting more direct human channel impacts received high priority ranking,as did subwatersheds with reported flooding problems. Percent Impervious of Watershed:the degree of impervious surface cover was estimated for each of the subwatersheds and used as a quantitative prioritization parameter. Details of the impervious surface cover are discussed in Section 6 of the manual. The subwatershed with the highest percentage of impervious surface cover received the highest priority ranking. TM 8.1-8 Westchester County Stormwater Management Planning Manual ! , r Technical Memorandum 8.1 ��= go coo Table 1-EVAMIX Prioritization Criteria Annual Hours %of Hours of Bankfull Annual Pollutant Loading flbs/acre) Erosion Sensitivity to Recovery Sediment Riparian Reported Percent Subwatershed Above Mean Exceedence BOD COD TSS TP TN Pb Cu Zn Potential Disturbance Potential Supply Control Flooding Impervious 02 19.37% 6.7 22.45 248.65 186.27 0.59 4.01 0.08 0.02 0.19 10 2 1 2 3 0 14% 03 20.67% 14.7 18.22 132.39 177.01 0.46 3.03 0.06 0.01 0.13 10 3 1 3 3 0 11% 05 13.59% 0.9 22.93 162.64 127.51 0.51 3.90 0.11 0.02 0.20 30 5 3 4 5 0 19% 06 6.29% 0.3 12.32 74.74 183.33 0.34 1.89 0.01 0.00 0.05 10 3 1 3 3 0 5% 07 14.83% 4.0 27.23 193.11 151.39 0.61 4.63 0.13 0.03 0.24 30 5 3 4 5 0 22% 09 4.42% 1.9 28.24 200.30 157.03 0.63 4.81 0.14 0.03 0.25 10 3 1 3 3 0 21% 10 16.10% 0.4 26.99 207.24 150.50 0.61 4.64 0.13 0.03 0.24 40 5 3 4 5 0 22% 11 7.05% 0.6 28.50 202.10 158.45 0.64 4.85 0.14 0.03 0.25 10 2 1 2 3 0 22% 12 30.15% 43.1 13.27 272.05 163.13 0.77 3.79 _ 0.05 0.02 0.17 10 3 1 3 3 1 8% 13 15.36% 1.4 24.95 176.96 138.73 0.56 4.25 0.12 0.02 0.22 10 2 1 2 3 0 20% 14 22.21% 8.4 17.10 403.20 95.11 0.55 3.74 0.09 0.02 0.23 40 5 3 4 5 0 14% 15 29.78% 1.0 17.13 361.99 101.36 0.49 5.09 0.09 0.02 0.22 0 0 0 0 0 0 15% 16 22.24% 0.6 24.58 17775 136.65 0.55 4.22 0.12 0.02 0.22 40 5 3 4 5 0 20% 17 24.22% 122.7 2259 160.22 125.61 0.50 3.85 0.11 0.02 0.20 40 5 4 5 5 0 18% 18 28.53% 1.4 11.21 207.12 89.24 0.45 2.64 0.05 0.01 0.14 30 5 3 3 5 0 10% 19 17.68% 8.9 24.08 184.83 133.86 0.54 4.23 0.12 0.02 0.21 10 3 1 3 3 0 20% 20 22.54% 1.6 28.01 199.49 156.00 0.63 4.77 0.13 0.03 0.24 10 3 1 3 3 0 23% 21 11.76% 0.9 27.53 195.23 153.06 0.61 4.69 0.13 0.03 0.24 30 5 3 4 5 0 22% _ 22 5.98% 0.2 29.47 208.99 163.85 0.66 5.02 0.14 0.03 0.26 30 5 3 4 5 0 24% 23 22.75% 38.9 24.02 238.32 133.74 0.58 4.32 0.12 0.02 0.23 10 3 1 3 3 1 19% 24 17.26% 1.3 28.37 201.19 157.73 0.63 4.83 0.14 0.03 0.25 30 5 3 4 5 0 23% 25 18.42% 5.1 23.28 173.75 129.46 0.52 4.04 0.11 0.02 0.21 10 2 1 2 3 1 19% 26 3.13% 0.4 19.73 160.32 116.32 0.47 3.47 0.10 0.02 0.17 20 3 3 3 5 1 17% 27 723% 0.3 26.87 190.57 149.41 0.60 4.57 0.13 0.03 0.23 30 5 3 4 5 0 22% 28 12.80% 2.0 27.90 197.87 155.13 0.62 4.75 0.13 0.03 0.24 10 3 1 3 3 0 23% 30 1.90% 0.9 26.48 187.78 147.22 0.59 4.51 0.13 0.03 0.23 10 3 1 3 3 0 22% 31 24.98% 1.3 21.75 154.28 120.96 0.49 3.70 0.10 0.02 0.19 40 5 3 4 5 0 18% 32 3.85% 0.7 20.53 145.64 114.18 0.46 3.50 0.10 0.02 0.18 10 3 1 3 3 0 17% TM 8.1-9 Westchester County St0rmwater Management Planning Manual Wt,tdiattQ'rni Technical Memorandum 8.1 Table 1(continued)-EVAMIX Prioritization Criteria Annual Hours %of Hours of Bankfull Annual Pollutant Loading(lbs/acre) Erosion Sensitivity to Recovery Sediment Riparian Reported Percent Subwatershed Above Mean Exceedence BOD COD TSS TP TN Pb Cu Zn Potential Disturbance Potential Supply Control Flooding Impervious 34 14.06% 0.4 17.62 132.78 97.99 0.40 3.02 0.08 0.02 0.16 30 5 3 4 5 0 14% 35 12.04% 1.6 25.47 180.66 141.64 0.57 4.34 0.12 0.02 0.22 40 6 5 5 3 0 21% 36 7.89% 4.0 15.13 120.94 88.55 0.35 2.65 0.07 0.01 0.13 30 5 3 4 5 0 14% 37 10.75% 1.0 14.81 113.69 85.17 0.34 2.57 0.07 0.01 0.13 30 5 3 4 5 0 12% 38 3.85% 1.0 78.53 586.95 437.64 1.77 13.64 0.38 0.08 0.69 0 0 0 0 0 1 65% 39 48.54% 53.4 16.04 172.48 89.17 0.39 2.90 0.08 0.02 0.16 10 3 1 3 3 0 13% 40 21.73% 0.7 14.14 100.30 78.63 0.32 2.41 0.07 0.01 0.12 30 5 3 3 5 0 12% 41 7.49% 0.5 19.74 139.99 109.75 0.44 3.36 0.09 0.02 0.17 30 5 3 4 5 0 16% 42 26.49% 7.5 14.38 338.75 79.95 0.46 3.14 0.07 0.02 0.19 40 5 3 4 5 0 12% 43 9.91% 2.4 16.78 243.79 106.94 0.47 3.40 0.09 0.02 0.17 10 3 1 3 3 0 17% 44 921% 0.3 11.85 84.13 65.93 0.26 2.02 0.06 0.01 0.10 30 5 3 4 5 0 10% 45 21.89% 3.9 28.63 222.19 164.88 0.66 4.98 0.14 0.03 0.25 10 2 1 2 3 0 25% 46 34.84% 11.0 0.47 346.40 14.84 0.2.3 1.18 0.01 0.00 0.10 10 3 1 3 3 0 4% 47 16.48% 0.9 17.73 182.90 98.56 0.42 3.55 0.09 0.02 0.17 40 5 3 4 5 0 15% 48 25.92% 75.7 15.05 437.76 8937 0.54 3.57 0.08 0.02 0.22 10 3 1 3 3 0 14% 49 15.13% 238.4 15.10 158.73 83.% 0.36 2.85 0.07 0.02 0.15 10 3 1 3 3 0 12% 50 11.12% 1.4 1.06 202.81 22.34 0.17 0.88 0.01 0.00 0.06 30 5 3 4 5 0 5% 51 28.66% 0.4 23.54 166.92 130.87 0.53 4.01 0.11 0.02 021 40 5 3 4 5 0 19% 52 39.93% 18.6 84.53 619.34 476.47 1.91 14.50 0.41 0.08 0.74 40 5 3 4 5 1 71% 53 43.89% 0.1 16.16 495.41 92.98 0.59 3.89 0.08 0.02 0.25 40 5 3 4 5 1 14% 54 41.84% 13.3 40.02 290.83 224.80 0.90 6.85 0.19 0.04 0.35 40 5 3 4 5 0 33% 55 36.21% 48.6 20.55 145.73 114.24 0.46 3.50 0.10 0.02 0.18 40 6 5 5 3 0 17% 56 1.90% 2.0 9.90 226.79 112.32 0.62 3.01 0.05 0.02 0.14 30 5 3 4 5 0 9% 57 24.98% 1.0 7.58 501.63 49.51 0.41 4.10 0.04 0.01 0.19 0 0 0 0 0 0 7% 58 3.85% 1.0 4.12 442.% 30.83 0.29 3.87 0.03 0.01 0.15 0 0 0 0 0 0 5% 60 14.06% 1.0 35.54 260.17 222.72 0.94 6.48 0.17 0.04 0.32 0 0 0 0 0 0 29% 61 12.04% 1.4 3293 251.29 184.47 0.75 5.75 0.16 0.03 0.29 40 6 5 5 3 0 26% 63 7.89% 172 14.23 248.57 ::.39 0.42 3.00 0.07 0.01 0.16 40 5 3 4 5 0 14% TM 8.1-10 Westchester County Storm water Management Planning Manual Westchester Technical Memorandum 8.1 go\.(om EVAMIX Results The results of the EVAMIX prioritization process are presented in the figure below. The high priority subwatersheds were evaluated for the potential of implementing stormwater management practices that would alleviate the stormwater issues specific to that subwatershed. The highest priority watersheds (1 - 10) are shown in the darkest shading in the figure below. Prioritization Ranking ==_ <= 10 H11 - 20 21 - 30 05 02 I—I31 - 40 > 40 07 03 I• 06 10 13 14 $ 26 2- 16 2 21 ore15 4 3 24 30 28 18 32 34 36littab‘ 39 42 41 40 46 48 49 50 ‘1110110‘54107 45 56 58 57 60 61 A __ - = 63 0.5 0 0.5 1 Miles TM 8.1-11 Westchester govcom Section 9 Stormwater Management Practices Key Points • A Stormwater Management Practice (SMP) is a device,practice, or method for controlling stormwater runoff or reducing its impacts • Selection and implementation of Stormwater Management Practices occur as a result of the planning process • Stormwater Management Practices can be focused on improving impacted areas and preventing future impacts This section is designed to provide a framework and recommendations for municipalities to implement consistent strategies and practices to more effectively protect and manage their natural water resources while accommodating planned growth. Two approaches are highlighted: improving impacted areas and preventing future impacts. Protecting watersheds from stormwater impacts requires the use of Stormwater Management Practices(SMPs). A SMP is a device,practice,or method for removing, reducing,retarding,or preventing targeted stormwater runoff constituents, pollutants,and contaminants from reaching receiving waters.Various techniques exist to accomplish the purpose of conservation design.Several techniques are presented in this report to assist municipal officials and others to understand how to improve the development design process. The most effective opportunities to improve and protect water resources occur when new land development is being planned. Many best management practices can be incorporated into new development plans that will improve the recharge and runoff characteristics of the land parcel.Lands that serve important hydrologic functions can be identified and protected from disturbance or their functions restored. Lands that are not undergoing new development should also be looked to for opportunities to reduce their impacts to water resources. Those areas that are suffering from or responsible for negative stormwater impacts should be examined to determine the potential to implement SMPs that would improve the health of the watershed.SMPs can also be implemented that will prevent future degradation of watershed health. 9.1 Applicable SMPs Once planning objectives have been developed and the highest priority subwatersheds have been identified,appropriate stormwater management practices should be selected that will work towards achieving those objectives. SMPs can be focused on remediating currently impacted watersheds. Examples include structural and non-structural practices such as stream restoration,stormwater detention basins, or changes in agricultural practices.SMPs can also be focused on preventing the 9-1 Westchester County Stormwater Management Planning Manual Westchester Section 9—Stormwater Management Practices go\cot t future degradation of a watershed through such SMPs as land preservation,changes in building code, or stormwater retention requirements for future development. The intention of establishing or constructing a SMP should be to achieve a specific goal. Simply reducing pollutant loading within a subwatershed is not an adequate design criterion,but reducing total suspended solids loadings by 25% is. Without specific design goals,the effectiveness of implementing stormwater BMPs cannot be evaluated. Some non-structural SMPs, such as a public education program,may not have measurable goals,but they can be an indispensable component of an overall stormwater management plan. 9.1.1 SMPs to Prevent Future Stormwater Impacts One goal of the process outlined in the Planning Manual is to provide municipalities and stakeholders with an understanding of the dynamics of a watershed. The stream assessment, stormwater modeling, and data analysis can then be used as planning tools to protect the health of a watershed and determine potential impacts caused by changes in land use or development. While it may not be possible or practical to completely prevent development in a watershed, adequate planning can be used to prevent or reduce its impacts. The New York State Stormwater Design Manual provides guidance on some acceptable SMPs for new development (table 9.1). These include ponds,constructed wetlands,infiltration systems,filtration systems,and open channel drainage. In addition to those SMPs provided in the table 9.1, the New York State Stormwater Management Design Manual lists the following SMPS as suitable for pretreatment or supplemental practices,but not as stand-alone practices: ➢ Catch basin inserts ➢ Dry ponds ➢ Underground vaults (designed for flood control) ➢ Oil/grit separators and hydrodynamic structures ➢ Filter strips ➢ Grass channels (includes ditches designed primarily for conveyance as well as modified practices that can achieve some pollutant removal) ➢ Deep sump catch basins ➢ On-line storage in the storm drain network ➢ Porous pavement 9-2 Westchester County Stormwater Management Planning Manual Westchester Section 9—Stormwater Management Practices gcn.uxn The Design Manual provides further information on these SMPs. Additional SMPs that can be established to prevent the further degradation of the health of watershed include: ➢ Zoning restrictions-current zoning laws should be reviewed and, if necessary,modified to prevent inappropriate development from occurring. ➢ Building codes-standards within building codes, such as minimum lots size or set backs,can be established to control development and prevent further stormwater impacts. ➢ Land acquisition/protection-many areas have adopted programs to obtain land that is critical to the health of a watershed. This land can then be preserved and protected from future development. Note also that New York City has developed Watershed Rules and Regulations (WRR) that apply to the Croton watershed. These WRRs might impose special considerations for pollutant loading or stormwater management, and should be consulted as well. See Technical Memorandum 9.1 - Overview of The New York State Stormwater Design Manual 9-3 Westchester County Stormwater Management Planning Manual \V 4chester go Section 9—Stormwater Management Practices Table 9.1 Stormwater Management Practices Acceptable for Water Quality Group Practice Description Micropool Extended Pond that treats the majority of the water quality volume through extended Detention Pond detention,and incorporates a micropool at the outlet of the pond to prevent sediment resuspension. Wet Pond Pond the provides storage for the entire water quality volume in the permanent pool. Pond Wet Extended Pond that treats a portion of the water quality volume by detaining storm Detention Pond flows above a permanent pool for a specified minimum detention time. Multiple Pond System A group of ponds that collectively treat the water quality volume. A stormwater wetland design adapted for the treatment of runoff form small Pocket Pond drainage and which has little or no baseflow available to maintain water elevations and relies on ground water to maintain a permanent pool. Shallow Wetland A wetland that provides water quality treatment entirely in a wet shallow marsh. Extended Detention A wetland system that provides some fraction of the water quality volume by Wetland detaining storm flows above the marsh surface. Wetland A wetland system that provides a portion of the water quality volume in the Pond/Wetland System permanent pool of a wet pond that precedes the marsh for a specified minimum detention time. A shallow wetland design adapted for the treatment of runoff from small Pocket Wetland drainage and which has variable water levels and relies on groundwater for its permanent pool. Infiltration Trench An infiltration practice that stores the water quality volume in the void spaces of a gravel trench before it is infiltrated into the ground. Infiltration An infiltration practice that stores the water quality volume in a shallow Infiltration Basin depression,before it is infiltrated into the ground. Dry Well Surface Sand Filter A filtering practice that treats stormwater by settling out larger particles in a sediment chamber,and then filtering stormwater through a sand matrix. Udnerground Sand A filtering practice that treats stormwater as it flows through underground Filtering Filter settling and filtering chambers. Practicies Perimeter Sand Filter A filtering practice that incorporates a sediment chamber and filter bed as parallel vaults adjacent to a parking lot. Organic Filter A filtering practice that used an organic medium such as compost in the filter,in the place of sand. Bioretention A shallow depression that treats stormwater as it lows though a soil matrix and is returned to the storm drain system. Dry Swale An open drainage channel or depression explicitly designed to detain and Open promote the filtration of stormwater runoff into the soil media. Channels An open drainage channel or depression designed to retain water or intercept Wet Swale groundwater for water quality treatment. Table 5.1 in the NYS Stormwater Design Manual 9-4 Westchester County Stormwater Management Planning Manual Westchester Section 9—Stormwater Management Practices Agoveom 9.1.2 SMPs to Remediate Impacted Watersheds Watersheds that have already been impacted by stormwater may require remediation efforts to restore the"health" of the watershed.Once a watershed has been damaged, it can be difficult to reverse the impacts. Remediation efforts should focus on achieving the planning objectives established for the watershed,and should be implemented or constructed where they will provide the most overall benefit for the watershed. High priority areas within the watershed should be examined first to determine if opportunities exist for SMPs. The New York State Stormwater Design Manual provides guidance on some acceptable SMPs,and the requirements for the acceptance of additional SMPs. That manual,however,is focused upon SMPs applicable for future development. Identifying opportunities to remediate existing stormwater problems can be more difficult due the inherent restrictions on space in areas that have already been developed. The SMPs implemented for remediation efforts will likely not be able to achieve the reductions to stormwater flow and pollutant loadings outlined in that manual. Restoring the"health" of an impaired watershed will likely require the implementation of multiple SMPs. The projects could be required in the impaired subwatershed or upstream in subwatersheds that are responsible for the problems. Efforts can include restoring natural habitats,restoring damaged stream channels, reducing sediment loadings,preventing erosion,reducing pollutant loadings,etc. The chosen SMPs should have measurable and obtainable goals. Table 9.1,taken from the New York State Stormwater Design Manual,provides SMPs that meet the water quality/water volume specifications established in that document. The Design Manual also lists additional SMPs (provided in Section 9.1 of this Manual) that are not currently effective for stand-alone application to meet the standards for new development established by that document. Those SMPs may however be appropriate for retrofit applications.Some examples of additional SMPs that might be used to restore watershed"health" include: ➢ Riparian buffer enhancement-this includes restoring or creating a natural vegetative buffer along stream corridors. Riparian buffers can improve water quality by removing sediment and pollutants before they are discharged to a waterway.The increased resistance to runoff will reduce runoff peak flow rates. The reintroduction of native vegetative species and the removal of invasive species will also help to restore natural habitats and natural stream processes. > Stream restoration-examples include bank stabilization,channel realignment,and step-pools. 9-5 Westchester County Stormwater Management Planning Manual Westchester Section 9—Stormwater Management Practices gxn.com ➢ Reducing directly connected impervious surfaces- this includes eliminating directly connected rooftop drainage. The reduced directly connected impervious surfaces will result in greater opportunities for infiltration, reductions in runoff volumes,and reductions in stormwater peak flow rates. ➢ Public awareness and education-informing local residents of local stormwater issues and ways to improve stormwater conditions. ➢ Initiating changes in fertilizer and chemical application-this can include the types,amounts,and application methods for chemicals used in agricultural practices to control pests and promote vegetative growth. Improved practices can greatly reduce nonpoint source stormwater pollutant loadings. See Technical Memorandum 9.2 -General Guidance for Improved Site Design 9-6 Technical Memorandum 9.1 Overview of the New York State Stormwater Design Manual A New York State Stormwater Design Manual (NYS Design Manual)has been prepared and is currently available from the NYSDEC website. The purpose of the manual is described as follows: • To protect the waters of the State of New York from the adverse impacts of urban stormwater runoff • To provide design guidance on the most effective stormwater management practices(SMPs)for new development sites • To improve the quality of SMPs constructed in the State,specifically in regard to their performance,longevity, safety,ease of maintenance,community acceptance,and environmental benefit. The NYS Design Manual provides basic information on why stormwater impacts need to be controlled,discusses stormwater permits and other stream related permits, and provides unified stormwater sizing criteria.The rest of the manual focuses on the selection and design of SMPs,and provides guidance on practices acceptable within New York State. The material found in the official New York State Stormwater Design Manual will not be reproduced here;the reader is referred to that document for guidance.Only the unified stormwater sizing criteria will be summarized here,because these criteria are pertinent to the overall planning approach discussed in this planning manual. ("New York State Stormwater Management Design Manual",NYS Department of Environmental Conservation,October 2001.) Unified Stormwater Sizing Criteria for New York State Chapter 4 of the NYS Design Manual presents guidelines for sizing stormwater management practices(SMPs)to meet pollutant removal goals,reduce channel erosion,prevent overbank flooding,and help control extreme floods. Each of these goals is met by specific criteria. The criteria require a combination of SMPs,and many of the SMPs listed can be designed to partially or completely meet one or more of the criteria. Thus,it is not the intent that each criterion requires a separate SMP,but that the design,as a whole,is tested against each of the criteria. Each of the sizing criteria is summarized below. Water Quality(WQ,) The water quality criterion is designed to capture and treat 90% of the average annual stormwater runoff volume.The manual indicates that capturing all storms 1.3 inches TM 9.1-1 Westchester County Stormwater Management Planning Manual Westchester gox.com Technical Memorandum 9.1 or less will achieve this goal. The following equation is recommended to test the design of new facilities for meeting the water quality objective. WQ„= (P)(Rv)(A)/12 Where: WQ„ =water quality volume in acre-feet P = 90% Rainfall Event Number (1.3 inches for most of Westchester County) Rv = 0.05+ 0.009(I),where I is the percent impervious cover of the site A = site area in acres The equation indicates that a minimum impervious cover of 5% is assumed, and that all other impervious cover (paved or gravel roads,sidewalks,parking lots,buildings, patios, pools, etc.) is considered to be directly connected to the storm drains with only about 10% lost to initial abstraction. Stream Channel Protection Volume Requirements (Cp„) Stream channel protection volume requirements are designed to protect stream channels from erosion. The requirement is met by providing 24-hour extended detention of the one-year,24-hour storm event(2.8 inches in Westchester County). In this way,peak flows from these relatively frequent storms would be attenuated. Overbank Flood Control Criteria (Q,) The primary purpose of the overbank flood control sizing criterion is to prevent an increase in the frequency and magnitude of out-of-bank flooding. The required approach is to provide storage to attenuate the post development 10-year,24-hour peak discharge rate Qp to predevelopment rates. This would require storage of a 5 inch storm in Westchester County. Extreme Flood Control Criteria(Qi) The intent of the extreme flood criterion is to prevent the increased risk of flood damage from large storm events,maintain the boundaries of the predevelopment 100- year floodplain,and to protect the stormwater management facilities that have been installed. To do this,the criterion requires that peak flows be held to predevelopment rates for up to the 100-year storm through storage and controlled release. In Westchester County,this means storage for a 7.5 inch storm. TM 9.1-2 Westchester County Stonnwater Management Planning Manual r Technical Memorandum 9.1 'vcom All of the above criteria require some degree of storage,and thus should be met by a combination of SMPs and/or site design that minimizes impervious cover and attempts to disperse and infiltrate stormwater rather than concentrate it and treat it. The NYS Design Manual also discusses conveyance criteria as well as the need for downstream analysis. Downstream analysis may allow for the channel protection, overbank, and extreme flood requirements to be waived if it can be shown that upstream and downstream flows can be better managed through other measures that have been analyzed on a wider scale. Finally,the manual sets out special provisions for designing SMPs for"stormwater hotspots". A stormwater hotspot is defined as a land use or activity that generates higher concentrations of hydrocarbons,trace metals,or toxicants than are found in typical stormwater runoff.These sites require a higher level of treatment,and infiltration may need to be avoided. The following land uses are deemed stormwater hotspots: • Vehicle salvage yards and recycling facilities • Vehicle fueling stations • Vehicle service and maintenance facilities • Vehicle and equipment cleaning facilities • Fleet storage areas • Industrial sites • Marinas • Outdoor liquid container storage • Outdoor loading and unloading facilities • Public works storage areas • Facilities that generate or store hazardous materials • Commercial container nurseries • Other land uses and activities as designated by an appropriate review authority TM 9.1-3 , go/4ccm Technical Memorandum 9.2 General Guidance for Improved Site Design One way to reduce the impacts of development on a watershed is through improved site design. BMP development design includes first, evaluating each site to determine the conservation design principles to be achieved,and second,design techniques to apply to the actual design and layout of the proposed development,and its stormwater and other infrastructure systems to achieve those conservation design principles. Each of these is discussed below. The goal is to produce a design which balances the demands of human use(scale, pattern,autonomy,privacy,views,etc.)with the requirements for a sustainable landscape (reduction in land fragmentation and use conflicts,preservation of watershed hydrology,protection of wildlife corridors and species diversity, conservation of natural resources,etc.). The following is a suggested series of steps that landowners, developers,and municipalities can take to achieve conservation design goals and work together in a more effective process.This process may seem to increase the time necessary to receive final project approvals,but will strengthen support for the plan and significantly reduce the time needed for preliminary and final plan review and approval. Determine Your Goals • Define what is driving the decision to develop the property. Consider the"site context". • Clearly identify the goals to work towards--these are your design goals for the project.Goals could be economic and/or personal/family related,as well as visual,ecological,agricultural,historical,and educational. • Consider your time schedule and that of the municipal review process. Conduct a Resources Inventory This step involves identifying and mapping the natural,scenic,historic and other resources and land use patterns associated with the property. Resources to consider in the inventory include: • Site context-regional,local and site characteristics of land ownership,visual patterns,cultural patterns,roadways,vegetation,wildlife habitat,topography, etc. -consider possibilities for linking other landscapes,stream corridors, critical farmland,distinctive woodland patterns;identify or established wildlife or recreational trail corridors,etc. • Current land use(agriculture,wooded lot,vacant,etc.) • Wind patterns and micro-climate TM 9.2-1 Westchester County Stormwater Management Planning Manual Westchester Technical Memorandum 9.2 govcom • Slopes, topography • Hydrology, drainage (wetlands,floodplains, streams, etc.) • Scenic viewsheds (interior and exterior) • Historic resources • Soil patterns(hydric soils,prime agricultural soils, etc.) • Vegetation patterns (landscape texture and patterns) • Zoning • Land fragmentation(agricultural,natural habitat,human use,viewsheds) • Endangered/threatened species • Unusual habitats,critical natural areas,etc. Undertake a Site Analysis • Compare/overlay/combine the natural/scenic/historic resource and land use pattern information to create a general understanding of the site's opportunities and constraints,particularly as they relate to your design goals. Some initial constraints could present opportunities. • Prepare a site analysis map that outlines the most important opportunities and constraints. The site analysis should identify both the unbuildable areas (wetlands,floodprone,or steep slope) and the most outstanding aspects of the remaining land (such as scenic vistas,meadows,hedgerows,mature woodlands,historic buildings or other structures,stone walls, etc.). Create Conceptual Designs or Sketch Plans • Use the site analysis to create conceptual designs (sketch plans). List opportunities and constraints of each plan. This component involves four steps: o Delineate proposed conservation areas (based on the findings of the site analysis)and potential development areas. o Locate desired/permitted structures (housing units,buildings, etc.) on the property (as they relate to the design goals). o Connect house sites with streets (logical alignment) and trails(where appropriate). o Draw in lot lines for the house sites. • Meet with municipal officials and critique plans--what is liked,not liked, and why. TM 9.2-2 Westchester County Storm water Management Planning Manual ` e0... ter Technical Memorandum 9.2 ^k`govcom • Identify a direction for final design. Formulate A Final Design (or Master Sketch Plan)As the Basis for An Engineered Site Plan • Synthesize discussion of conceptual designs(sketch plans) and finalize design. • Develop legal instruments necessary to realize plan goals, e.g.,conservation easements,deed restrictions,homeowners association,estate planning,etc. Obtain Approvals(Follow-up) • Obtain government approval of master sketch plan,and • Proceed to Preliminary and/or Final Engineered Plan approvals. TM 9.2-3 Appendix A Hallocks Mill Brook Watershed Conceptual Design Memorandum \V -g errm Conceptual Design Memorandum Introduction Potential stormwater management practice(SMP)projects within the Hallocks Mill Brook watershed have been identified using a ranking system incorporating modeled flow and pollutant loadings along with the results of a stream survey conducted throughout the watershed.Those subwatersheds within the Hallocks Mill Brook watershed that were identified as highly impaired using the EVAMIX prioritization process (see Technical Memorandum 8.1)were reviewed using aerial photographs and site visits to determine stormwater management projects that could be developed to address impairments specific to that subwatershed. Eight potential SMP project locations were selected from the highly ranked subwatersheds.From these eight projects,three were selected for conceptual design. Selected Conceptual SMP Designs Conceptual Design Project #1: Junior Lake Figure D.1 Junior Lake .' Pool i. . •• 't .? 1C S. } -, R '.1' 110Ur, +fir`` ., ,. 6 ` . Junior Lake �s 4♦ r�1�y1 ' Wetland . fig' Pie!,it .1 I'll , . 11, i 1 . 104 'I', iril +IAA A"il: • ' `, 4 , _ ir .. i Hyland Road 1,114 • 1/., . , ,,, 6.,- ' . F., ",: rte sil Upstream Channel' • Pew . Ike 0 Background Description Junior Lake Watershed contains a mix of developed and undeveloped land. The subwatershed was ranked as a high priority watershed due to the combination of stream survey results and estimated runoff pollutant loadings.The channels located in this subwatershed are some of the most impaired in the entire Hallocks Mill Brook Watershed. The Yorktown Memorial Town Park was identified during field visits as a good location for SMP projects. The park provides an excellent opportunity for visible stormwater control implementation along with a series of potential projects.The park A-1 Westchester County Storm water Management Planning Manual Westchester gov.com Appendix A— Conceptual Design Memorandum contains a playground,a community swimming pool,a small lake and open space. Through conversations with Yorktown,it was discovered that the lake is filled with sediment,leaving depths of only 1'-2'.In January 2002,a proposal to dredge the lake was cancelled when sampling showed high levels of contaminants. Flow enters the park from the south through a highly eroded and entrenched channel. Heavy sediment deposits were also observed in the channel. The channel feeds into Junior Lake and then continues north to Hallocks Mill Brook. Conceptual Design • SMP-Channel Restoration Stream classifications performed during the stream survey indicate that the channels in this area are extremely sensitive to disturbance and have a poor recovery potential. The channel upstream of Junior Lake is exhibiting the severe erosion and heavy sedimentation characterized by this stream type.A low recovery potential indicates that the stream is not likely to develop an equilibrium in the current land use and hydrology regimes. In this type of situation, channel restoration is recommended as a remedial action. Channel restoration would include bank stabilization and riparian buffer enhancement. "Hard"bank stabilization measures would include armoring channel banks with boulders or stone riprap,especially at the point where upstream culverts discharge to the stream. Banks can also be protected using rootwads, rock "j" vanes (both for meanders), rock cross vanes(good for straight sections and for creating a pool, also good for controlling grade), log vanes and live branch layers. Root and rock vanes could be employed to reduce shear stresses along the banks and to create pools."Soft" bank stablization measures, also called bioengineering practices, would include armoring stream banks and channels with coconut fiber blankets and logs. Re- foresting and re-vegetating channel banks would then permanently stabilize the banks to reduce erosion along the channel.During the fluvial geomorphic assessment, this channel reach was classified as C4 -G4c.A stable configuration for this channel would likely be C. The result of channel restoration should help to reduce the sediment yield that is plaguing Junior Lake. Figure D 2 View of Upstream Channel, Left Bank Figure D.3 View of Upstream Channel, Right Bank rr s ` f.. A-2 Westchester County Stonnwater Management Planning Manual :. •r Appendix A—Conceptual Design Memorandum . ` .com • SMP-Wetland Restoration Downstream of the Haylan Road Bridge, wetlands are dominated by invasive species. Invasive species present in the wetland area include purple loosestrife and common reed,although red-osier dogwood and cattail are also present in significant numbers. These invasive species limit the presence of indigenous plants and impair the habitat of aquatic life.Invasive species are to be removed by hand or targeted herbicide applications and replaced with native species such as winterberry, viburnum, sweet pepperbush,sedges, bulrushes, and rushes. Restoration of the wetland upstream of Junior Lake would help to curb sedimentation and pollutant problems in the lake by filtering and treating the stormwater before it enters the water body. Figure D.4 Downstream of Haylan Road Bridge . ► • SMP-Lake Dredging Through conversations with Yorktown, it was learned that Junior Lake is loaded with sediment. The lake is approximately 1'-2'deep.A previous proposal to dredge the lake was cancelled because of high pollutant concentrations found in the sediment. Dredging the lake, however, would increase hydraulic storage and remove the contaminated deposits. The contaminated sediment would have to be treated as hazardous materials and disposed of appropriately, in accordance with an approved operation and maintenance plan. Summary of Subwatershed Junior Lake Park provides an excellent opportunity to develop and construct stormwater management projects in a highly visible environment.The channels in this area are some of the most sensitive and impaired in all of the Hallocks Mill Brook watershed and they are already exhibiting the effects of the surrounding urbanization.The past issues with contaminants and sediments in the lake indicate that stormwater control bmp projects are greatly needed in this area. Also,the bridge on Hylan Road was destroyed during Hurricane Floyd and is barricaded from traffic. As an added project to beautify the area,this bridge and abutmentscould be removed altogether and the stream banks restored. A-3 II Westchester County Stormwater Management Planning Manual Westchester Appendix A—Conceptual Design Memorandum "A'COm Water quality and quantity issues are usually the cumulative result of stormwater management problems upstream of a location,and not just those immediately found at a location.Some of the subwatersheds upstream or tributary to Junior Lake are highly developed,compared to the rest of the Hallocks Mill Brook watershed.The impacts of these highly impervious areas are being felt downstream. Estimated Costs Channel Restoration-$82,500 (550 linear feet @$150 per linear foot) Wetland Restoration-$50,000 (approximately 1 acre) Lake Dredging-$150,000(approximately 4,000 cubic yards of sediment) Conceptual Design Project #2: Yorktown High/Middle School Figure D-5 Yorktown Hi.h/Middle School Sl ,.� ..!►, riall i ! ,� . F ' ;tit t c � ' ) 17 kt 'l..rfi'l,Zr,? if =Jai. .. Upstream Wetland Hi•h School , f ir ya ' Parking Area �t , �, r 4 ' y '� ,1Pond + ;K'r Outfall • - ' . ► • I . M .r,‘..... .1 i .‘t 4 - .'.* N• (; . . ck :-"'"" . Middle School r Background Description Two channels enter the campus of Yorktown High/Middle School on the northeast corner. The western channel is piped under Crompond Road and emerges in a small daylight(surface) exposure(approximately 6'wide) approximately 100'-150'from the roadway.The land between this exposure and the roadway is landscaped grass along with a small existing wetland. The eastern channel enters the campus through an adjacent wetland. The channel is piped under the school entrance roadway and emerges along with the western channel. The two channels converge into one pipe at the outlet of the exposure and travel beneath the school parking lot. A-4 Westchester County Stonnwater Management Planning Manual t •r Appendix A—Conceptual Design Memorandum .gc com Once beyond the parking lot,the channel again emerges.Flow initially is conveyed though a 150'-200'channel and then enters a pond.The channel segment is eroded and in need of bank stabilization and restoration.Additional outfalls enter the pond, but little flow was observed. The pond is approximately 200' x 100' in size and is surrounded by landscaped grass areas.The surrounding area contains picnic tables and a few trees.Outflow from the pond is controlled with an overflow weir.Once over the weir,flow is again piped. Two pipes carry the flow under athletic fields with eventual discharge on the western side of the campus. The pipes discharge into a small ponded area within a southerly flowing channel. This discharge occurs on the edge of a state-designated wetland. The banks of the wetland channel are strewn with trash and litter. The implementation of SMPs on school grounds would provide high community visibility and also potential educational value.However,issues may arise with concerns over the introduction of standing water.The potential does exist for multiple stormwater BMPs on the school campus, either as solo projects or in conjunction. Conceptual Design • BMP-Parking Lot Sand Filter A sand filter could be constructed to treat the runoff from the school parking lot. Adequate area was available for a surface sand filter since changes are planned for the downstream pond and channel (see below). The parking lot is approximately 400'x 100'. The parking lot total drainage area is approximately 2.5 acres, the site is 100% impervious cover and the water quality volume for the sand filter is 0.26 acre/ft. A sand filter plan is shown on drawing G-3, detail calculations are attached in Appendix A. • BMP-Channel Restoration and Pond Buffer The channel between the parking lot area and the pond is eroding and in need of bank stabilization. The situation could be partially addressed with the construction of a wetland upstream to reduce peak flow rates. The addition of a vegetative buffer along the banks of the channel also would help reduce flow rates to the channel and reduce erosion. The addition of a vegetative buffer around the pond also could help reduce peak flows and treat runoff. The channel length is approximately 150 feet and the pond is approximately 200'x 100'. Summary of Subwatershed The campus of Yorktown High/Middle School presents a good opportunity to implement stormwater best management practices in an educational and highly visible location. The subwatershed,made up almost entirely of the campus,ranked second on the priority list developed from stormwater modeling and field investigations. A combination of stormwater best management projects at this location could showcase the effectiveness of stormwater management controls.The A-5 I Westchester County Storm water Management Planning Manual •r Appendix A—Conceptual Design Memorandum =iV00m potential also exists for hands-on participation from students in developing some of the projects along with wetland dean-ups. Construction is currently taking place on the campus.The addition of buildings on the grounds would lead to more impervious cover and increased runoff potential. Also,if this construction involves the alteration of the existing parking facilities or the addition of new parking areas,sand filters could be installed at a reduced inconvenience to the school. Estimated Costs Sand Filter -$38,500 Channel Restoration -$30,000(150 linear feet Q$200 per linear foot) Conceptual Design Project #3: Yorktown Area Fi.ure D-6 Downtown Commercial Area ..4 ,t Irk° :lig —4 400tb 41 tr / i'7%, . L"• Crompond Road g Ilo. !II .." . LI Y � ' , - • j 1 Probabl- Vortex o•r t'• '4 iti - L i., , .. . . ' • f K . ./k wil . Commerce Street . s z "milL N 4114)t.001. 1. Saw Mill River Road [ 1 . .` A r . ' .J.- • °� K `' �, J , y' Background Description Downtown Yorktown is comprised of the most densely developed land use-within the Hallocks Mill Brook watershed. The area is primarily commercial,consisting of shopping centers,offices and other businesses.When only runoff was considered,this subwatershed was ranked as a high priority because of relatively high pollutant loading and a high degree of impervious cover. A-6 Westchester County Storm water Management Planning Manual Appendix A—Conceptual Design Memorandum `gym Conceptual Design • SMP— Vortex Separators The commercial area in Yorktown possesses the most heavily developed land use within the Hallocks Mill Brook watershed. Because of the dense commercial development, the streets in this area experience high vehicular traffic. The area is serviced by a storm sewer network that eventually discharges downstream of the subwatershed. Total supspended solids loadings within this subwatershed were estimated to be among the highest within the Hallocks Mill Brook watershed. The pollutant loading from the street runoff could be reduced with the installation of vortex separator(s). Vortex separators are pollution control devices that remove oil and sediment from stormwater and stores them for periodic removal. The controls would reduce suspended solids loadings along with oils and greases. The efficiency and, therefore, value of these systems are dependent on periodic cleaning of the systems. Without this maintenance, these systems will not properly perform. The Vortechnics stormwater system can provide a peak treatment capacity is 25 ft3/sec. The 10-year storm event peak flow rate produced in SWMM was approximately 45 ft3/sec.In order to treat storm events of this magnitude, two side-by- side systems would need to be installed. This configuration is costly and requires a large area. A more efficient installation would be one unit with a bypass for excessive flows, with a total suspended solids annual removal efficiency of approximately 77%. Installation of the single vortex separator unit is presented on drawing G-4. Estimated Costs Vortechs Model 16000 -$40,000 (includes delivery,system manhole frames and covers) Installation Costs -$20,000 A-7 Westchester County Storm water Management Planning Manual Westcl icstcr Appendix A—Conceptual Design Memorandum `v`""' Conceptual SMP Design Projects Not Selected Project #1 -Wetland Development at Yorktown High School/Middle School Background Description A description of the potential project site is provided above in Conceptual Design Project#2. Potential SMP • Wetland Development The development of a wetland area on the northeast side of the campus would filter and treat stormwater flows prior to entering the downstream pond. Flow currently piped on the northeastern portion of the campus would be excavated and introduced to daylight. The existing wetland area could then be expanded. There is approximately 200' x 100' of available development space. The resulting wetland would be bordered to the west by tennis courts, to the north by a small hill leading to an open area south of Crompond Road, to the East by the school entrance road, and the south by a parking lot. Flow would exit the wetland and be conveyed to the existing outfall pipes. The existing wetlands in the surrounding areas indicate that this would be a good candidate for wetland development and expansion. Projects #2 and #3 - Subwatershed #31 Background Description Subwatershed#31 was not identified with a high priority during the ranking process. However, the stream survey identified severe erosion and sediment deposits in this area and indicated that there was relatively new development with no apparent stormwater controls. A follow up site visit confirmed the heavy erosion and sedimentation,but the root of the damage more likely occurs from upstream conditions. Subwatershed #31 is located downstream of the confluence of a first order and a second order channel. At the headwaters of the first order channel is Sparkle Lake. The second order channel has tributaries through relatively high-density residential areas and wetlands. Erosion and sediment problems were not reported as severe upstream of the confluence on either channel. A-8 Westchester County Storm water Management Planning Manual tcl�ester Appendix A—Conceptual Design Memorandum ==ogo�'com The developments located along the channel within subwatershed#31 possess cul-de- sac streets,with approximately six large homes on each.Each cul-de-sac street has a small bridge passing over the channel.Stormwater is apparently captured in storm drains and discharged directly to the channel, or allowed to run off overland into the channel. The high imperviousness of the streets and the landscape of the pervious areas allows for little infiltration and capture of stormwater runoff.While problems are exhibited in the channel upstream of these developments,the direct runoff created is surely compounding the effects. Potential SMP • Channel Restoration Stream classifications performed during the stream survey indicate that the channels in this area are highly sensitive to disturbance and have a good recovery potential. This type of classification allows for either in channel remediation or out of channel SMPs. Erosion Upstream of Developments Erosion Downstream of First Development • . r A '!r. =3 •r g,. zl `.� 6e i. . ... , . 14r... * t ' _ v- Y,, . } yam, r 11j. s + r l: ,li &4 14° ilI [� �4 z b Sediment Deposits Development , ��„i. . , 'atom; r ' F At-' 3- _yam° , r \s4 • T , A-9 II Westchester County Stormwater Management Planning Manual + V L,%a •r Appendix A— Conceptual Design Memorandum eucom In-channel remediation within subshed #31 would include bank stabilization and regrading.Additionally,in-channel structures may be employed to reduce the shear stresses and peak flow rates. Potential SMP • Wetland Development The erosion and sediment problems being experienced in subshed#31 are likely the result of upstream conditions. The creation of a pond-wetland system would provide filtering of stormwater and attenuation of peak flows. The reduction in peak flows would reduce shear stresses along the channel banks and hence reduce sediment supply to the channel. Sporadic wetland areas already exist in the northern portion of subshed 31 that could be expanded and enhanced with a pond system. Optional designs could include a shallow wetland,extended detention shallow pond/wetland system or a pocket wetland. The actual development of the wetland system would be dependent upon land ownership, which has not been determined at this time. Potential Wetland Area L ' 4 I o•«s neva • << yy Project #4- Subwatersheds #52 and #54 Background Description Subsheds#52 and#54 are located on the downstream end of the Hallocks Mill Brook Watershed,and experience heavy flow volumes.Results of the stream survey indicated that the channels within these two subsheds were eroded and experiencing some heavy sediment deposition.Subshed#54 is located immediately downstream of subshed#52. The Yorktown Water Pollution Control Plant was identified from aerial photographs as a good location to evaluate and address the channel conditions.The Yorktown Water Pollution Control Plant is located on the border of the two subsheds. A follow-up field visit to the plant confirmed the results of the stream survey. Potential Projects Potential SMP • Channel Restoration Stream classification performed during the stream survey indicate that the channel near the WPCP has a high sensitivity to disturbance and a good recovery potential. A-10 Westchester County Storm water Management Planning Manual � � rrn Appendix A—Conceptual Design Memorandum 1.4 ,«> This type of classification allows for either in channel remediation or out of channel stormwater burps. In channel measures could include bank stabilization, expansion of the riparian buffer and regrading. It was also observed during the field visit that the bridge on Greenwood Street,just upstream of the plant,had experienced washout, apparently from the channel exceeding its banks. Further investigation would be required to determine the frequency of this event and possible remediation actions. Erosion is occurring upstream and downstream of the WPCP. Sediment Loading Downstream of the WPCP Erosion Downstream of the WPCP Outfall ;''''t R y� t 9 _ - i ,li :4.1:" ',:z0,1 w ^ ,.., �` 4j" Project#5— Subwatershed#60 Background Description A description of the potential project site is provided above in Conceptual Design Project#3 Potential SMP • Sand Filter An area with a large parking lot was identified in an area adjacent to subshed #61 in subshed#60.A field visit revealed that this location was a shopping center.Stormwater from this area is collected and piped to outfalls along the channel in subshed#55. Potentially,an underground sand filter could be constructed to treat the runoff from the parking lot and discharged to the storm sewer system A-11 WESTCHESTER COUNTY STORMWATER MANAGEMENT PLANNING MANUAL HALLOCKS MILL BROOK WATERSHED CONCEPTUAL STORMWATER MANAGEMENT PRACTICE DESIGNS HALLOCKS MILL BROOK WATERSHED pUTNAM COUNT`SN `-- - --- 1 North Salem Somers Peekskill LIST OF DRAWINGS Lewisboro SHEET NO. TITLE (J GENERAL - COVER SHEET C Ortla ndt� ✓ 0-1 HALLOCKS MILL BROOK WATERSHED PLAN 1 - 0-2 CONCEPTUAL DESIGN I I:JUNIOR LAKE Croton-on\ co CONCEPTUAL DESIGN 12:YORKTOWN HIGH SCHOOL Hudson Yorktown Bedford G-4 CONCEPTUAL DESIGN#3:YORKTOWN COMMERCIAL AREA 4---'--- Mount Kisco c_---LNew Castle LOCATION MAP NTS WESTCHESTER OFTPLANNINGORK M Air Ill )11.2 rAu► l ? jill Pr 1 \ • ii'q , MI Iiiii l 9 0 bilk i 70 t I: ,� ��� OR W�H SCH0011 a I ji Air 4 ,er - 1 f '‘.. ''''''''' t 7 PROJ:CT 3 -� YO'S+WN 4OMMERC.Teir t i . , , , . viiii►6ri,,,i4 a 11 va ..... 644---=, aye,in it iii 1 '1/t WI' 0 lir ` HALLOCKS MILL ROOK r `` WATERSHED BO DARY I ' I PLAN SCALE: DESIGNED BY: D. KSYNIAK WESTCHESTER COUNTY. NEW YORK PROJEC NO. 5053-33211 DRAWN BY: I. OLIVER CDM DEPARTMENT OF PLANNING FILE NAME: GHMPL002.DWG SHEET CHK'D BY: L. SANTOS CROSS CHK'D BY: L. SANTOS HALLOCKS MILL BROOK WATERSHED PLAN SHEET NO. APPROVED BY: T. SCHOETTLE CROTON STORMWATER PLANNING MANUAL G-1 DATE: MAY 2002 P / EXISTING LAKE BOTTOM WATER SURFACE WETLAND RESTORATIONROpO RATION PROPOSED \O'NS WEIR /W -2 (SEE 1) BUFFERVEGETAS DEPTH RP IMMING (1' ') NOTE •0 LAKE DREDGING — /� (SEE SCHEMATIC 2) / si, l C� �•� Ni, INFLOW Elf Y JUNIORHISTORIC LAKE v\ei 4 / YYY 1110111111____ (0)UT—.-FLOW LAKE '.."---I POOL BOTTOM (TYP) �V Y yY L HOUSE �o�, Y YYYYy JUNIOR LAKE L GRASS Lu EXISTING CONTAMINATED �� y YY YY Yy WETLAND N SEDIMENT ---V ce TO BE REMOVED PARKING ± LOT 3 / GRASS W )= / JUNIOR LAKE DREDGING SCHEMATIC-a PLANcz.) I NTS I PLAYGROUND FLOOD CONTROL iii / OUTLET L_ EMBANKMENT X WETLAND RESTORATION (SEE SCHEMATIC 4) _ HAYLAN ROAD PROPOSED VEGETATION -— —_A,_ — INFLOW FLOOD CONTROL STAGE N I I 0 WATER DUALITY STORAGE EXISTING BRIDGE ��IOUTFLOW ru p TO BE REMOVED *I f `:,. A• 'L__ a L L k RIPRAP 6'TO 14' cr cc ix k DEEP SHALLOW SEDIMENT TO ORIFICE a ca v) k >I AQUATIC VEGETATION BE REMOVED PLATE N J o CHANNEL RESTORATION Y U -d a < PROPOSED SECTION N z o (SEE SCHEMATIC 3) 8 w \ VEGETATION o C' o m k (�' WETLAND RESTORATION SCHEMATIC-4 1 m INFLOW x NTS VI = ; NOTES: o BANK BOULDER] 1.INVASIVE SPECIES TO BE REMOVED IN SHALLOW AREA 0 PLACEMENT INCLUDING PURPLE LOOSE STRIFE AND COMMON REED. SITE RHILL AVE. SPECIES CHANNEL RESTORATION SCHEMATIC-3 (E:WINTERBERRY.SWEET PEP ERBUSH,SEDGES AND RUSHES.) JUNIOR LAKE PROJECT SCHEMATIC-1 NTS c� NOTES: NTS NOTES. 1.APPROXIMATELY 550-FT OF CHANNEL RESTORATION 1.CURRENT STREAM TYPE C4 >G4c N 2.STRUCTURES SHOWN FOR REFERENCE ONLY. K) DESIGNED BY: D.KSYNIAK PROJEC NO. 5053-33211 CDM WESTCHESTER COUNTY. NEW YORK ro DRAWN BY: L OLIVER DEPARTMENT OF PLANNING FILE NAME: GCDJL003.DWG `nSHEET CHK'D BY: L.SANTOS DESIGN#1: SHEET NO. j CROSS CHK'D BY: L.SANTOS JUNIOR LAZE APPROVED BY: T.SCHOETTLE CROTON 8PORMWATER PLANNING MANUAL 0-2 DATE: MAY 2002 NDERDRAIN FLOW DIVERSION COLLECTION SYSTEM FILTER BED vmspigivaado INFLOW p❑jAd k;. I 1 1 1 , OUTFLOW0 0 /A p. I I I I RPROPOSED PROPOSED �.��.�I�._.-.� VEGETATION lirVEGETATION PRETREATMENT . SEDIMENTATION CHAMBER BANK BOULDER PLACEMENT CHANNEL RESTORATION SCHEMATIC - 6 NTS FLOW DIVERSION r PERFORATED STANDPIPE NOTES: 1 WATER QUALITY VOLUME 1. CURRENT STREAM TYPE C4->G4c INFLOW FILTERED BED o r,-`'""'"-1 ,. �__. OUTFLOW N N F Z D a EE 7 0 x UNDERDRAIN Z COLLECTION SYSTEM CROMPOND RD.�' o� O ATHLETIC C�gti� 0 . FIELD TF TOPSOILS Ct W i SpR/4C � M FILTER FABRIC 4 S SAND Via.^; =r TENNIS COURTS inillAyi 'PO FILTER _x,"=•`• "` WETLAND OUTLET •4'�µz ~s~s PARKING AREA PIPE •'';-•'"_ =*' HIGH CHANNEL FILTER FABRIC SCHOOL ••(•• giriii SAND FILTER ��� ..•�. LOCATION PERFORATED PIPE/GRAVEL Atiwo/lis.— POND UNDERDRAIN SYSTEM MIDDLE GRASS SCHOOL SAND FILTER SCHEMATIC - 5 ATHLETIC NTS FIELDS DESIGNED BY: D.KSYNIAKWESTCHESTER COUNTY. NEW YORK PROJEC NO. 5053-33211 CDM DRAWN BY: I.OLIVER DEPARTMENT OF PLANNING FILE NAME: GCDYHO04.DWG SHEET CHK'D BY: L.SANTOS CONCEPTUAL DESIGN 2r SHEET NO. CROSS CHK'D BY: L.SANTOS YORKTOWN HIGH SCHOOL APPROVED BY: _ T.SCHOETTLE CROTON STORMWATER PLANNING MANUAL 04 DATE: MAY 2002 1 y /,-4 I ALUMINGEUMWffH ANGLE SEALANT / �/ i FIIN ✓% NEOPRENE GASKET �� ,- I .„ i .,,. 1 i 1' IA :r „1r Mr.wi VwbsYmM. , ---__ 4 , _WE- I BYPASS L.L1111 T°GUITALL \ A . .1„ - • I .,,,,,...41 II ll SEALANT FLOW CONTROL e\ ,,1 \\ �� PLAN B — B EWALLYRIE "au �,,,\ \ \\ MIM MATCH FINIS4E TO LPPFIRNal FSiAATF 1 1 N' _ eME ELEVATIONS \ \ CONCRETE GRADE SIDES ,", , �m i�. PACKAGE \ '�, "�.l —( ,ry VORTEX SEPARATOR 1, II / 11\ Lcr wvr�' egic'_.au rr ... 1 % y — \/ Lasinuj [ . c. 'ctr't2nA'Zr'c7 ;tlfttr L. -c hittrit2' MINIMDM 6' ^ �COMPACTW CRI Road - SECTION A — A wV Storm Sewer(approx.location) No o do MO Feet Hydrologic Feature STORM SEWER SYSTEM: STORM SEWER SYSTEM: YORKTOWN PROJECT SITE SCHEMATIC 7 VORTEX SEPARATOR DETAIL SCHEMATIC 8 BYPASS LAYOUT SCHEMATIC 9 NTS NTS NTS NOTES: NOTES: 1.PEAK TREATMENT CAPACITY:25 CFS 1.FLOWS GREATER THAN 25 CFS WILL BYPASS THE SYSTEM. 2.TSS ANNUAL REMOVAL EFFICENCY OF 77x. DESIGNED BY: D.KSYNIAK WESTCHESTER COUNTY. NEW YORK PROJEC NO. 5053-33211 �D� DRAWN BY: L OLIVER DEPARTMENT OF PLANNING FILE NAME: GCDYT005.DWG SHEET CHK'D BY: L.SANTOS CONCEPTUAL DESIGN#3: SHEET NO. CROSS CHK'D BY: L.SANTOS YORKTOWN AREA APPROVED BY: T.SCHOETTLE CROTON STORMWATER PLANNING MANUAL a4 DATE: MAY 2002 Appendix B Hallocks Mill Brook Fluvial Geomorphic Assessment Stream Reach Locations N •tlr A-1 1 A �Mth 1.3 A York: 1 11 k 2 Springs � : niESprin• ,ft III 1 �� nE' ork:- 4 1.6 cSprirgs A-19 A-1.8 •:9s A-1.7 •' �5 - ESpr Ings �AWWW7ark' .6 - ESprings•-21 k`� ' Qan .pr kgs A �� : r bg sB-22 1011.18::' -Spr gs&2.1 L= k�='8 •1 BrooIto •7Z f alb ksMiIIHM-t Brook- -8-26 i,,��ybcksMiIIHM-2 hi, ✓TYalbcksMtl HM-3 •2 �6 •ksitle 0-2 •.•• sitle _ •ksbe B roo Rode G1.6 _. N BraoksiyeG 4 Broo -tee.C-1.3 :.•ksitl B,G1 2 + Brooksi gC-1.1 <<!h 2500 0 2500 Feet .•._.� Westchester County Stormwater Management Planning Manual Appendix B-Hallocks Mill BrookFluvial Geomorphic Assessment Data Brookside A 2.1 Brookside A 2.2 310.00 305.00 -Existing x -Existing Ground E Ground m m 305.00 a 300.00 -f-Bankfull --tF-Bankfull a Width m Width m` c m m m "Do 0 m 300.00 , -a- Flood Prone 2 295.00 -a- Flood Prone `o __ __ Elevation c1 Elevation a w w 295.00 - 290.00 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 0 5 10 15 20 25 30 35 40 45 50 55 60 Distance from an arbitrary datum[ft] Distance from an arbitrary datum[ft] Sample Profile --Thalweg Sample Profile --Thalweg _: L. 304.00 ------ -M-Bankfull Elevation Z 304.00- --Bankfull Elevation c a 302.00- ••1 302.00- o m 300.00- o c 300.00- ro o 298.00 m i; 298.00 w� w� `0 296.00 I 1---1--ii m 296.00 it 0.00 50.00 100.00 150.00 200.00 250.00 300.00 0.00 50.00 100.00 150.00 200.00 250.00 300.00 Distance,in feet,from an arbitrary datum Distance,in feet,from an arbitrary datum Rosaen Stream Tyne Classification Flow Calculations Rosqen Stream Type Classification Flow Calculations Entrenchment 2.85 Bankfull Depth 0.36[ft] Entrenchment 3.10 Bankfull Depth 1.04[ft] Width:Depth 61.78 Area 8.02[sq.ft] Width:Depth 15.79 Area 16.93[sq.ft] Sinousity NC Manning's n 0.0380 Sinousity NC Manning's n 0.0350 Slope 0.0130 Velocity 2.22[ft/s] Slope 0.0100 Velocity 4.24[ft/s] D50 NC [mm] Discharge 17.76[cfs] D50 NC [mm] Discharge 71.78[cfs] Stream Type D5 Shear Stress 0.28[Ib/sq.ft] Stream Type C4 Shear Stress 0.62[Ib/sq.ft] Project Name: Croton Watershed Study Project Name: Croton Watershed Study Project Number: 1027.01 Project Number: 1027.01 Surveyed: 1/22/97 By: CAS Surveyed: 1/22/97 By: CAS B-2 Westchester County Stormwater Management Planning Manual Appendix B-Hallocks Mill BrookFluvial Geomorphic Assessment Data Brookside B 2.1 Brookside B 2.2 305.00 - 305.00 . -Existing x -Existing E Ground E Ground m in v v '300.00 2 300.00 • 1n -f-Bankfull m -f-Bankfull -2 Width 2 ------- Width al co c c N co > > O O m 295.00 -A- Flood Prone m 295.00 --6- Flood Prone o Elevation o Elevation .i w w 290.00 -- 290.00 0 5 10 15 20 25 30 35 40 45 50 55 60 65 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 Distance from an arbitrary datum[ft] Distance from an arbitrary datum[ft] Sample Profile Thalweg Sample Profile Thalweg . 304.00- t Bankfull Elevation `; Z,304.00 -- --- t Bankfull Elevation T.,m 302.00- , .L°302.00- m 300.00- c m 300.00- ` ° in 298.00- 2 m 298.00- TO ri, m 8 296.00- . m n 296.00 w m 294.00 • I I w m 294.00 I + i 0.00 50.00 100.00 150.00 200.00 250.00 300.00 0.00 50.00 100.00 150.00 200.00 250.00 300.00 Distance,in feet,from an arbitrary datum Distance,in feet,from an arbitrary datum Rosgen Stream Type Classification Flow Calculations Rosgen Stream Type Classification Flow Calculations Entrenchment 2.77 Bankfull Depth 1.72[ft] Entrenchment 2.22 Bankfull Depth 1.06[ft] Width:Depth 11.23 Area 33.18[sq.ft] Width:Depth 19.55 Area 21.80[sq.ft] Sinousity NC Manning's n 0.0300 Sinousity NC Manning's n 0.0400 Slope 0.0014 Velocity 2.57[ft/s] Slope 0.0070 Velocity 3.17[ft/s] D50 NC [mm] Discharge 85.43[cfs] D50 NC [mm] Discharge 69.06[cfs] Stream Type E6 Shear Stress 0.14[Ib/sq.ft] Stream Type C3Shear Stress 0.45[Ib/sq.ft] Project Name: Croton Watershed Study Project Name: Croton Watershed Study Project Number: 1027.01 Project Number: 1027.01 Surveyed: 1/22/97 By: CAS Surveyed: 1/22/97 By: CAS B-3 Westchester County Stormwater Management Planning Manual Appendix B-Hallocks Mill BrookFluvial Geomorphic Assessment Data Brookside B 2.3 Brookside B 2.4 305.00 305.00 Existing , Existing E Ground j Ground I O v a300.00 300.00 1.8 f Bankfull --f--Bankfull ° Width f 0 10 Width m o 0 m 295.00 -A- Flood Prone m 295.00 o Elevation o -.0- Flood Prone Elevation w w 290.00 290.00 0 10 20 30 40 50 60 70 80 90 100 110 0 20 40 60 80 100 120 140 160 Distance from an arbitrary datum[ft] Distance from an arbitrary datum[61 Sample Profile -0-Thalweg -0-Thalweg Profile tThalweg 2 304.00 - -N-Bankfull Elevation - 2,400.00 - -N-Bankfull Elevation c302.00 c 300.00- - is 300.00- ° `m 298.00- o c 200.00- > >To cs 0 > 100. 00- m o 296.00 _ w •m 294.00 • I I I i • I w m 0.00 I I i I I 0.00 50.00 100.00 150.00 200.00 250.00 300.00 0.00 50.00 100.00 150.00 200.00 250.00 300.00 Distance,in feet,from an arbitrary datum Distance,in feet,from an arbitrary datum Rosgen Stream Type Classification Flow Calculations Rosgen Stream Type Classification Flow Calculations Entrenchment 1.50 Bankfull Depth 1.20[ft] Entrenchment 3.61 Bankfull Depth 1.93[ft] Width:Depth 28.16 Area 40.33[sq.ft] Width:Depth 17.55 Area 65.34[sq.ft] Sinousity NC Manning's n 0.0400 Sinousity NC Manning's n 0.0400 Slope 0.0037 Velocity 2.50[ft/s] Slope 0.0050 Velocity 4.02[ft/s] D50 NC [mm] Discharge 100.71 [cfs] D50 NC [mm] Discharge 262.62[cfs] Stream Type C4->F4 Shear Stress 0.27[Ib/sq.ft] Stream Type C4 Shear Stress 0.59[Ib/sq.ft] Project Name: Croton Watershed Study Project Name: Croton Watershed Study Project Number: 1027.01 Project Number: 1027.01 Surveyed: 1/22/97 By: CAS Surveyed: 1/22/97 By: CAS B-4 Westchester County Stormwater Management Planning Manual Appendix B-Hallocks Mill BrookFluvial Geomorphic Assessment Data Brookside B 2.5 Brookside B 2.6 305.00 305.00 -Existing -Existing Ground , Ground m A---- -A 1n' V A ro300.00 300.00 41 -Bankfulli. f Bankfull m Width m Width c c m@ N j O O m 295.00- m 295.00 -6-- Flood Prone -A- Flood Prone o Elevation 2tO Elevation d d w w 290.00 290.00 0 20 40 60 80 0 20 40 60 80 100 Distance from an arbitrary datum[ft] Distance from an arbitrary datum[ft] Sample Profile - Thalweg Sample Profile --4-Thalweg e,400.00 Bankfull - Z,400.00 -IN- - m m c 300.00- i-- 300.00- o m 200.00- `o 811 200.00- at Ts e• '• o 100.00- m 0 100.00- w 16 0.00 I I i I I w 1° 0.00 I i I I 0.00 50.00 100.00 150.00 200.00 250.00 300.00 0.00 50.00 100.00 150.00 200.00 250.00 300.00 Distance,in feet,from an arbitrary datum Distance,in feet,from an arbitrary datum Rosgen Stream Type Classification Flow Calculations Rosgen Stream Type Classification Flow Calculations Entrenchment >2.2(Incomplete data) Bankfull Depth 2.23[ft] Entrenchment>2.2(data not taken) Bankfull Depth 1.79[ft] Width:Depth 10.94 Area 54.63[sq.ft] Width:Depth 23.09 Area 73.66[sq.ft] Sinousity NC Manning's n 0.0400 Sinousity NC Manning's n 0.0400 Slope 0.0050 Velocity 4.31 [ft/s] Slope 0.0050 Velocity 3.80[ft/s] D50 NC [mm] Discharge 235.46]cfs] D50 NC [mm] Discharge 280.07[cfs] Stream Type C4 Shear Stress 0.65]Ib/sq.ft] Stream Type C4 Shear Stress 0.54[Ib/sq.ft] Project Name: Croton Watershed Study Project Name: Croton Watershed Study Project Number: 1027.01 Project Number: 1027.01 Surveyed: 1/22/97 By: CAS Surveyed: 1/22/97 By: CAS B-5 Westchester County Stormwater Management Planning Manual Appendix B-Hallocks Mill BrookFluvial Geomorphic Assessment Data Brookside B 2.7 Brookside B 2.8 305.00 305.00 - -Existing K -Existing E Ground E Ground is v 10 Z'300.00- 300.00 :e f Bankfull e t Bankfull m Width m c Width m c CD O O a m 295.00- 2 295.00 -6- Flood Prone -A- Flood Prone ° Elevation o 05 Elevation Ili ii,a a w w 290.00 290.00 0 20 40 60 0 10 20 30 40 50 Distance from an arbitrary datum[ft] Distance from an arbitrary datum[ft] Sample Profile -e-Thalweg Sample Profile -40-Thalweg - i'_, 306.00 f Bankfull Elevation - a,304.00 - - t Bankfull Elevation 15 8 304.00- w 302.00 c{ 302.00- a 300.00- a 300.00- o 2 298.00- ° m 298.00- m 296.00- 1." > '2 294.00- a o 296.00 W 1292.00 I I I I I wm 294.00 I I I I I 0.00 50.00 100.00 150.00 200.00 250.00 300.00 0.00 50.00 100.00 150.00 200.00 250.00 300.00 Distance,in feet,from an arbitrary datum Distance,in feet,from an arbitrary datum Rosgen Stream Type Classification Flow Calculations Rosgen Stream Type Classification Flow Calculations Entrenchment 1.24 Bankfull Depth 0.94[ft] Entrenchment 2.26 Bankfull Depth 1.41 [ft] Width:Depth 18.68 Area 16.38[sq.ft] Width:Depth 10.15 Area 20.21 [sq.ft] Sinousity NC Manning's n 0.0400 Sinousity NC Manning's n 0.0350 Slope 0.0200 Velocity 4.89[ft/s] Slope 0.0040 Velocity 3.26[ft/s] D50 NC [mm] Discharge 80.19[cfs] D50 NC [mm] Discharge 65.77[cfs] Stream Type F4b Shear Stress 1.12[Ib/sq.ft] Stream Type E4 Straightened Shear Stress 0.33[Ib/sq.ft] Project Name: Croton Watershed Study Project Name: Croton Watershed Study Project Number: 1027.01 Project Number: 1027.01 Surveyed: 1/22/97 By: CAS Surveyed: 1/22/97 By: CAS B-6 Westchester County Stormwater Management Planning Manual Appendix B-Hallocks Mill BrookFluvial Geomorphic Assessment Data Brookside C 1.1 Brookside C 1.2 305.00 305.00 • F Existing Existing E Ground E Ground m m v � m 300.00 300.00 • --A • :e -f-Bankfull f f Bankfull Width '" Width c C m m O > m 295.00 B 295.00 c Flood Prone c -A- Flood Prone 4Elevation 2 Elevation m d L11 w 290.00 - - 290.00 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 Distance from an arbitrary datum[ft] Distance from an arbitrary datum[ft] Sample Profile --Thalweg Sample Profile -0-Thalweg ,ii _ i', 304.00- f Bankfull Elevation .. , 304.00 f Bankfull Elevation co J G a 302.00- C 2 302.00- `m • m c c 300.00- c c 300.00- o m o m m '0 m'298.00 a 298.00- • • • • w m 296.00 I I i I I w m 296.00I I t 0.00 50.00 100.00 150.00 200.00 250.00 300.00 0.00 50.00 100.00 150.00 200.00 250.00 300.00 Distance,in feet,from an arbitrary datum Distance,in feet,from an arbitrary datum Rosgen Stream Type Classification Flow Calculations Rosgen Stream Tvpe Classification Flow Calculations Entrenchment 15.16 Bankfull Depth 0.66[ft] Entrenchment >6.6 Bankfull Depth 0.59[ft] Width:Depth 7.16 Area 3.16[sq.ft] Width:Depth 14.59 Area 5.10[sq.ft] Sinousity NC Manning's n 0.0300 Sinousity NC Manning's n 0.0300 Slope 0.0033 Velocity 2.01 [ft/s] Slope 0.0010 Velocity 1.08[ft/s] D50 NC [mm] Discharge 6.37[cfs] D50 NC [mm] Discharge 5.51 [cfs] Stream T .e E6 Shear Stress 0.12[Ib/sq.ft] Stream Type C5 Shear Stress 0.04[Ib/sq.ft] Project Name: Croton Watershed Study Project Name: Croton Watershed Study Project Number: 1027.01 Project Number: 1027.01 Surveyed: 1/22/97 By: CAS Surveyed: 1/22/97 By: CAS B-7 Westchester County Stormwater Management Planning Manual Appendix B-Hallocks Mill BrookFluvial Geomorphic Assessment Data Brookside C 1.3 Brookside C 1.4 305.00 305.00 F E Existing E Existing GroundGround v =v 300.00 m 300.00 a M- Bankfull -. -f--Bankfull Width c c 295.00 Width E a)o o ' m 295.00- c -A- Flood Prone co -A Flood Prone 2 o Elevation 2 290.00 Elevation d 32 w w 290.00 285.00 - 0 10 20 30 40 50 60 70 80 0 10 20 30 40 50 60 70 80 Distance from an arbitrary datum[ft] Distance from an arbitrary datum[ft] Sample Profile Thalweg Sample Profile _}Thalweg a.,304.00 -MI-Bankfull Elevation . 2, 305.00- (Bankfull Elevation 302.00 715 w I0 300.00- a c m 300.00- c a 295.00- .2 `m 29876 a) .00- ° m 290tr3 cu .00 s • 296.00 8 a 285.00- • w m 294.00 I I I I I w f0 280.00I I I 0.00 50.00 100.00 150.00 200.00 250.00 300.00 0.00 50.00 100.00 150.00 200.00 250.00 300.00 Distance,in feet,from an arbitrary datum Distance,in feet,from an arbitrary datum Rosgen Stream Type Classification Flow Calculations Rosgen Stream Type Classification Flow Calculations Entrenchment 3.43 Bankfull Depth 0.74[ft] Entrenchment 1.08 Bankfull Depth 0.74[ft] Width:Depth 22.61 Area 12.30[sq.ft] Width:Depth 23.17 Area 12.55[sq.ft] Sinousity NC Manning's n 0.0300 Sinousity NC Manning's n 0.0400 Slope 0.0040 Velocity 2.53[ft/s] Slope 0.0190 Velocity 4.12[ft/s] D50 NC [mm] Discharge 31.14[cfs] D50 NC [mm] Discharge 51.74[cfs] Stream Type C4 Shear Stress 0.18[Ib/sq.ft] Stream Type F3 Shear Stress 0.85[Ib/sq.ft] Project Name: Croton Watershed Study Project Name: Croton Watershed Study Project Number: 1027.01 Project Number: 1027.01 Surveyed: 1/22/97 By: CAS Surveyed: 1/22/97 By: CAS 8-8 Westchester County Stormwater Management Planning Manual Appendix B-Hallocks Mill BrookFluvial Geomorphic Assessment Data Brookside C 1.5 Brookside C 1.6 305.00 305.00 -Existing -Existing E Ground E Ground m m v 'o 300.00 m 300.00 41 -f-Bankfull - -Bankfull a Width Width c c a a a a > 0 m• 295.00 m 295.00 -,� Flood Prone a-- Flood Prone o Elevation 2 Elevation w w 290.00 290.00 0 10 20 30 40 50 60 0 10 20 30 40 Distance from an arbitrary datum[ft] Distance from an arbitrary datum[ft] Sample Profile -0-Thalweg -0-Profile -0-Thalweg -�Bankfull Elevation 304.00 f Bankfull Elevation _ Z,304.00 - Z• m 302.00 A- 302.00- G-2 300.00- c a m a 300.00- o• m 298.00- ° m 298.00- -j296.00 m aA a 294.00- m�'0 296.00- • • • w 0 292.00 w m 294.00 I I I I I 0.00 50.00 100.00 150.00 200.00 250.00 300.00 0.00 50.00 100.00 150.00 200.00 250.00 300.00 Distance,in feet,from an arbitrary datum Distance,in feet,from an arbitrary datum Rosgen Stream Type Classification Flow Calculations Rosgen Stream Type Classification Flow Calculations Entrenchment 2.09 Bankfull Depth 1.06[ft] Entrenchment 1.57 Bankfull Depth 1.44[ft] Width:Depth 15.42 Area 17.32[sq.ft] Width:Depth 10.41 Area 21.68[sq.ft] Sinousity NC Mannings n 0.0380 Sinousity NC Manning's n 0.0300 Slope 0.0107 Velocity 4.13[ft/s] Slope 0.0039 Velocity 3.73[ft/s] D50 NC [mm] Discharge 71.50[cfs] D50 NC [mm] Discharge 80.80[cfs] Stream Type C4 Shear Stress 0.69[Ib/sq.ft] Stream Type C4->G4c Shear Stress 0.32[Ib/sq.ft] Project Name: Croton Watershed Study Project Name: Croton Watershed Study Project Number: 1027.01 Project Number: 1027.01 Surveyed: 1/22/97 By: CAS Surveyed: 1/22/97 By: CAS B-9 Westchester County Stormwater Management Planning Manual Appendix B-Hallocks Mill BrookFluvial Geomorphic Assessment Data Brookside C 1.7 Granite Springs A1.1 305.00 305.00 F E -Existing -Existing m 4 Ground EGround m 300.00 300.00 f f Bankfull --U--Bankfull 03 Width Width co > co' o > 2 295.00 -- Flood Prone A 295.00- --A- Flood Prone iElevation o Elevation m > w w 290.00 290.00 0 20 40 60 80 100 120 140 0 10 20 30 40 50 60 70 80 Distance from an arbitrary datum[ft] Distance from an arbitrary datum[ft] Sample Profile -0-Thalweg -4-Profile tThalweg Z T; m 305.00 ---- tBankfull Elevation - 2,304.00- tBankfullElevation w::--- 302.00- e 300.00- c m m 300.00- .5 ami 295.00- ° m 298.00 to co m 2 • * tito' 296.00 w 10 290.00 I I I I w m 294.00 I I I I I 0.00 50.00 100.00 150.00 200.00 250.00 300.00 0.00 50.00 100.00 150.00 200.00 250.00 300.00 Distance,in feet,from an arbitrary datum Distance,in feet,from an arbitrary datum Rosen Stream Type Classification Flow Calculations Rosgen Stream Type Classification Flow Calculations Entrenchment 1.20 Bankfull Depth 1.15[ft] Entrenchment 4.54 Bankfull Depth 0.63[ft] Width:Depth 14.69 Area 19.34[sq.ft] Width:Depth 14.44 Area 5.70[sq.ft] Sinousity NC 1 Manning's n 0.0350 Sinousity NC Manning's n 0.0350 Slope 0.0106 Velocity 4.68[ft/s] Slope 0.0190 Velocity 4.01 [ft/s] D50 60[mm] Discharge 90.51 [cfs] D50 NC [mm] Discharge 22.83[cfs] Stream Type F4 t Shear Stress 0.73[Ib/sq.ft] Stream Type C4->G4c Shear Stress 0.67[Ib/sq.ft] Project Name: Croton Watershed Study Project Name: Croton Watershed Study Project Number: 1027.01 Project Number: 1027.01 Surveyed: 1/22/97 By: CAS Surveyed: 1/22/97 By: CAS B-10 Westchester County Stormwater Management Planning Manual Appendix B-Hallocks Mill BrookFluvial Geomorphic Assessment Data Granite Springs A 1.2 Granite Springs A 1.3 305.00 305.00 -Existing -Existing E Ground E Ground 1' 1s 300.00 300.00 • - • -Bankfull -f-Bankfull m Width 63 ---- Width c c m t6 a E m ° m m 295.00 295.00 -A- Flood Prone c -A- Flood Prone o Elevation 2 Elevation - I w w 290.00 _._ ___ - 290.00 0 10 20 30 40 50 60 70 0 10 20 30 40 50 60 Distance from an arbitrary datum[ft] Distance from an arbitrary datum[ft] Sample Profile Sample Profile +Thalwe +Thalweg g - z,400.00 t Bankfull Elevation - a,304.00- f Bankfull Elevation t @ w T 302.00- - 300.00- @ o 300.00 o 2 200.00- ° m 298.00- 0 100.00 m 0 296.00 un' 0 0.00 i I I 1 1 w m 294.00 i I I I I I 0.00 50.00 100.00 150.00 200.00 250.00 300.00 0.00 50.00 100.00 150.00 200.00 250.00 300.00 Distance,in feet,from an arbitrary datum Distance,in feet,from an arbitrary datum Rosen Stream Type Classification Flow Calculations Rosgen Stream Type Classification Flow Calculations Entrenchment 5.04 Bankfull Depth 0.81 [ft] Entrenchment>4.5 Bankfull Depth 0.57[ft] Width:Depth 9.45 Area 6.17[sq.ft] Width:Depth 20.54 Area 6.76[sq.ft] Sinousity NC Manning's n 0.0350 Sinousity NC Manning's n 0.0350 Slope 0.0100 estimate Velocity 3.25[ft/s] Slope 0.0230 Velocity 4.33[ft/s] D50 NC [mm] Discharge 20.04[cfs] D50 NC [mm] Discharge 29.27[cfs] Stream Type E4 Shear Stress 0.42[Ib/sq.ft] Stream Type C4 Shear Stress 0.79[Ib/sq.ft] Project Name: Croton Watershed Study Project Name: Croton Watershed Study Project Number: 1027.01 Project Number: 1027.01 Surveyed: 1/22/97 By: CAS Surveyed: 1/22/97 By: CAS B-11 Westchester County Stormwater Management Planning Manual Appendix B-Hallocks Mill BrookFluvial Geomorphic Assessment Data Granite Springs A 1.4 Granite Springs A 1.5 305.00 305.00 Existing Existing E §§ Ground a it v v m 300.00 m 300.00 e -f-Bankfull e -f-Bankfull Width Width c N N N d O O m• 295.00 m 295.00- c -� Flood Prone c -A- Flood Prone 4 Elevation o Elevation T> d m w w 290.00 290.00 0 10 20 30 40 50 60 0 10 20 30 40 50 60 70 Distance from an arbitrary datum[ft] Distance from an arbitrary datum[ft] Sample Profile -0-Thalweg Sample Profile -0-Thalweg - 2,304.00-- -M-Bankfull Elevation t.304.00- -El-Bankfull Elevation lii,.. 302.00- d E 302.00- m 300.00- m 300.00- ° m 298.00- 9- m 298.00TO 0 - TO 0 m a 296.00 i e m 296.00- W 10 294.00 I I I I I I • W m 294.00- I I I I I 0.00 50.00 100.00 150.00 200.00 250.00 300.00 350.00 0.00 50.00 100.00 150.00 200.00 250.00 300.00 Distance,in feet,from an arbitrary datum Distance,in feet,from an arbitrary datum , - Rosgen Stream Type Classification Flow Calculations Rosaen Stream Type Classification Flow Calculations Entrenchment 2.28 Bankfull Depth 0.47[ft] Entrenchment 1.70 Bankfull Depth 0.67[ft] Width:Depth 44.54 Area 10.00[sq.ft] Width:Depth 24.58 Area 11.06[sq.ft] Sinousity NC Manning's n 0.0300 Sinousity NC Manning's n 0.0380 Slope 0.0026 Velocity 1.52[ft/s] Slope 0.0190 Velocity 4.10[ft/s] D50 NC [mm] Discharge 15.23[cfs] D50 NC [mm] Discharge 45.31 [cfs] Stream Type C5 Shear Stress 0.08[Ib/sq.ft] Stream Type B3 Shear Stress 0.78[Ib/sq.ft] Project Name: Croton Watershed Study Project Name: Croton Watershed Study Project Number: 1027.01 Project Number: 1027.01 Surveyed: 1/22/97 By: CAS Surveyed: 1/22/97 By: CAS B-12 Westchester County Stormwater Management Planning Manual Appendix B-Hallocks Mill BrookFluvial Geomorphic Assessment Data Granite Springs A 1.6 Granite Springs A 1.7 305.00 305.00 Existing Existing E Ground § Ground 9 b' -o v m 300.00 m 300.00 -f-Bankfull :e -f-Bankfull cWidth c Width o o o0 o 0 m 295.00 m 295.00 -6- Flood Prone -A- Flood Prone Elevation .2 Elevation r73 lb' w Low 290.00 ------- 290.00 0 10 20 30 0 10 20 30 40 50 60 70 80 90 Distance from an arbitrary datum[ft] Distance from an arbitrary datum[ft] Sample Profile -Thalweg Sample Profile -Thalweg _ a., 305.00 t Bankfull Elevation _ . 305.00 f Bankfull Elevation II 300.00 E@ cc 2 295.00- omrj o y 'o 290.00 I I I I I w m 290.00 I I I 1 I 0.00 50.00 100.00 150.00 200.00 250.00 300.00 0.00 50.00 100.00 150.00 200.00 250.00 300.00 Distance,in feet,from an arbitrary datum Distance,in feet,from an arbitrary datum Rosgen Stream Type Classification Flow Calculations Rosgen Stream Type Classification Flow Calculations Entrenchment 1.19 Bankfull Depth 0.64[ft] Entrenchment 1.23 Bankfull Depth 0.84[ft] Width:Depth 28.50 Area 11.84[sq.ft] Width:Depth 20.97 Area 14.82[sq.ft] Sinousity NC Manning's n 0.0420 Sinousity NC Manning's n 0.0400 Slope 0.0420 Velocity 5.34[ft/s] Slope 0.0170 Velocity 4.27[ftfs] D50 NC [mm] Discharge 63.21 [cfs] D50 NC [mm] Discharge 63.25[cfs] Stream T .e F3b Shear Stress 1.65[Ib/sq.ft] Stream Type F3 Shear Stress 0.87[Ib/sq.ft] Project Name: Croton Watershed Study Project Name: Croton Watershed Study Project Number: 1027.01 Project Number: 1027.01 Surveyed: 1/22/97 By: CAS Surveyed: 1/22/97 By: CAS B-13 Westchester County Stormwater Management Planning Manual Appendix B-Hallocks Mill BrookFluvial Geomorphic Assessment Data Granite Springs A 1.9 Granite Springs A 2.1 305.00 - 305.00 Existing -Existing Ground E Ground 1n 300.00 m 300.00 t -f-Bankfullf-Bankfull Width m `c m Width a0 0 'm 295.00 m 295.00 c -es Flood Prone c -es- Flood Prone C CS Elevation O Elevation w w 290.00 290.00 0 10 20 30 40 50 60 70 80 0 10 20 30 40 50 60 70 80 90 100 Distance from an arbitrary datum[ft] Distance from an arbitrary datum[ft] Sample Profile -0-Thalweg -0-Profile - Thalweg a.,304.00 - t Bankfull Elevationz 304.00 -N-Bankfull Elevation m 302.00- w 2 302.00- - cIP c-e 300.00- m 300.00- - m c c cc 298.00- ° e 298.00- ° e md296.00 � mm _m'moo' 296.00 d 294.00- • • W 0' 294.00 1 I I I w 0 292.00, I I I I 0.00 50.00 100.00 150.00 200.00 250.00 300.00 0.00 50.00 100.00 150.00 200.00 250.00 300.00 Distance,in feet,from an arbitrary datum Distance,in feet,from an arbitrary datum Rosgen Stream Type Classification Flow Calculations Rosgen Stream Type Classification Flow Calculations Entrenchment 4.09 Bankfull Depth 1.09[ft] Entrenchment 2.65 Bankfull Depth 1.13[ft] Width:Depth 14.00 Area 16.52[sq.ft] Width:Depth 26.48 Area 33.71 [sq.ft] Sinousity NC Manning's n 0.0400 Sinousity NC Mannings n 0.0350 Slope 0.0100 Velocity 3.78[ft/s] Slope 0.0065 Velocity 3.69[ft/s] D50 NC [mm] Discharge 62.47[cfs] D50 NC [mm] Discharge 124.49[cfs] Stream Type C3 Shear Stress 0.64[Ib/sq.ft] Stream Type B4c Shear Stress 0.45[Ib/sq.ft] Project Name: Croton Watershed Study Project Name: Croton Watershed Study Project Number: 1027.01 Project Number: 1027.01 Surveyed: 1/22/97 By: CAS Surveyed: 1/22/97 By: CAS B-14 Westchester County Stormwater Management Planning Manual Appendix B-Hallocks Mill BrookFluvial Geomorphic Assessment Data Granite Springs A 2.2 Granite Springs A 2.3 305.00 305.00 Existing Existing Ground § Ground 1'o A ---A m o C;300.00- a- 300.00- e f Bankfull -i--Bankfull Width Width m m `mI o > i o o v' 295.00 m 295.00- -A- Flood Prone -A- Flood Prone, g Elevation o Elevation Ia, j w w 290.00 - 290.00 0 10 20 30 40 50 60 70 0 10 20 30 40 50 60 70 Distance from an arbitrary datum[ft] Distance from an arbitrary datum[ft] Sample Profile Thalweg Sample Profile ♦Thalweg - a, 304.00 f Bankfull Elevation - Z, 306.00 f Bankfull Elevation ][ 2 302.00- w s 304.00- E a 300.00 c6 302.00- 29800 .0 c 300.00- 4 m . g m 298.00- > > 1 296.00 • i i 296.00 5 A)a° 294.00- • °'a° 294.00- w t0 292.00 I I I I I w m 292.00 I I I • I I • , 0.00 50.00 100.00 150.00 200.00 250,00 300.00 0.00 50.00 100.00 150.00 200.00 250.00 300.00 Distance,in feet,from an arbitrary datum Distance,in feet,from an arbitrary datum Rosgen Stream Type Classification Flow Calculations Rosgen Stream Type Classification I Flow Calculations Entrenchment >2.2 extensive floodplain Bankfull Depth 1.57[ft] Entrenchment 2.68 extensive floodplain Bankfull Depth 1.41 [ft] Width:Depth 14.96 Area 36.79[sq.ft] Width:Depth 13.83 Area 27.59[sq.ft] Sinousity NC Manning's n 0.0350 Sinousity NC Manning's n 0.0350 Slope 0.0037 Velocity 3.30[ft/s] Slope 0.0086 Velocity 4.72[ft/s] D50 NC [mm] Discharge 121.33[cfs] D50 NC [mm] Discharge 130.13[cfs] Stream Type C4 Shear Stress 0.33[lb/sq.ft] Stream Type C4 Shear Stress 0.70[Ib/sq.ft] Project Name: Croton Watershed Study Project Name: Croton Watershed Study Project Number: 1027.01 Project Number: 1027.01 Surveyed: 1/22/97 By: CAS Surveyed: 1/22/97 By: CAS 8-15 Westchester County Stormwater Management Planning Manual Appendix B-Hallocks Mill BrookFluvial Geomorphic Assessment Data Granite Springs B 2.1 Granite Springs B 2.2 305.00 305.00 Existing -Existing E Ground E Ground v v m 300.00 a300.00 r2 e -Bankfull --f--Bankfull Width . c c Width in in ---- m 295.00 m 295.00 --6- Flood Prone Elevation o Elevation 17i ' ' d d w w 290.00 290.00 - 0 10 20 30 40 50 60 0 10 20 30 40 50 60 Distance from an arbitrary datum[ft] Distance from an arbitrary datum[ft] Sample Profile -•-Thalweg Sample Profile Thalweg _ 2 304.00- (-Bankfull Elevation _ Z, 306.00 f Bankfull Elevation Ii co 302.00- . w 304.00- cc.e 302.00- `m 300.00- - a 300.00- o m 298.00 4 (Ti 298.00- m o 296.00 iu a o' 296.00- 294.00- w - 294.00- I I I I I W f0 292.00 II I 1 0.00 50.00 100.00 150.00 200.00 250.00 300.00 0.00 50.00 100.00 150.00 200.00 250.00 300.00 Distance,in feet,from an arbitrary datum Distance,in feet,from an arbitrary datum Rosqen Stream Type Classification Flow Calculations Rosqen Stream Type Classification Flow Calculations Entrenchment 2.00 Bankfull Depth 0.84[ft] Entrenchment 1.60 Bankfull Depth 0.73[ft] Width:Depth 13.43 Area 9.58[sq.ft] Width:Depth 22.40 Area 11.89[sq.ft] Sinousity NC Manning's n 0.0370 Sinousity NC Manning's n 0.0380 Slope 0.0146 Velocity 4.15[Ws] Slope 0.0158 Velocity 3.93[ft/s] D50 NC [mm] Discharge 39.79[cfs] D50 NC [mm] Discharge 46.67[cfs] Stream Type B4c Shear Stress 0.72[Ib/sq.ft] Stream Type B4c Shear Stress 0.70[Ib/sq.ft] Project Name: Croton Watershed Study Project Name: Croton Watershed Study Project Number: 1027.01 Project Number: 1027.01 Surveyed: 1/22/97 By: CAS Surveyed: 1/22/97 By: CAS B-16 Westchester County Stormwater Management Planning Manual Appendix B-Hallocks Mill BrookFluvial Geomorphic Assessment Data Granite Springs B 2.3 Granite Springs B 2.4 305.00 305.00 -Existing -Existing Ground § Ground to' 15 v v 2300.00 2 300.00- lli -f--Bankfull :e -f-Bankfull c Width c t Width m o N N 0 0 2 295.00 2 295.00- \........,,..,,1 -d- Flood Prone -A- Flood Prone Elevation Elevation iii 'T m m w w I 290.00 290.00 0 10 20 30 40 50 60 0 10 20 30 40 50 60 70 Distance from an arbitrary datum[ft] Distance from an arbitrary datum[ft] Sample Profile --Thalweg Sample Profile --Thalweg - a-304.00- --Bankfull Elevation - a., 305.00- -o-Bankfull Elevation 1 r2 302.00- -D 5 e 300.00- •c e 300.00- g c 298.00- g c 295.00- ii > 296.00• g 1O A,a 294.00- •• m o 290.00 u' m 292.00 I i I I I I w m 285.00- I 1 - I I I I 0.00 50.00 100.00 150.00 200.00 250.00 300.00 350.00 0.00 50.00 100.00 150.00 200.00 250.00 300.00 350.00 Distance,in feet,from an arbitrary datum Distance,in feet,from an arbitrary datum I Rosqen Stream Type Classification Flow Calculations ' Rosgen Stream Type Classification Flow Calculations Entrenchment 1.58 Bankfull Depth 0.94[ft] Entrenchment 1.11 Bankfull Depth 0.60[ft] Width:Depth 20.94 Area 18.42[sq.ft] Width:Depth 33.65 Area 12.00[sq.ft] Sinousity NC Manning's n 0.0380 Sinousity NC Manning's n 0.0380 Slope 0.0053 Velocity 2.63[ft/s] Slope 0.0130 Velocity 3.14[ft/s] D50 NC [mm] Discharge 48.44[cfs] D50 NC [mm] Discharge 37.66[cfs] Stream T .e B4c Shear Stress 0.29[Ib/sq.ft] Stream Type F4 Shear Stress 0.48[Ib/sq.ft] Project Name: Croton Watershed Study Project Name: Croton Watershed Study Project Number: 1027.01 Project Number: 1027.01 Surveyed: 1/22/97 By: CAS Surveyed: 1/22/97 By: CAS B-17 Westchester County Stormwater Management Planning Manual Appendix B-Hallocks Mill BrookFluvial Geomorphic Assessment Data Granite Springs B 3.1 Hallocks Mill XS-1 305.00 - I 305.00- Existing E Ground § -Existing Ground . e m 300.00 m 300.00 { -f-Bankfull :e a Width a --0- -Bankfull m m Width a a > > 0 0 f 295.00 m 295.00- ---A- Flood Prone 0 Elevation 0 -A- Flood Prone > > Elevation m m w w 290.00 290.00 - 0 10 20 30 40 50 60 0 20 40 60 80 100 120 140 160 Distance from an arbitrary datum[ft] Distance from an arbitrary datum[ft] Sample Profile -40-Thalweg Sample Profile +Thalweg a., 304.00 f Bankfull Elevation - r 305.00- f Bankfull Elevation ZU mi c a 302.00- c 300.00- a 300.00- • oc a 295.00- ° m 298.00- tTi° 10 _o' o 296.00- > o' 290.00 W a 294. 00 I I I I I W m 285.00 I I I I 0.00 50.00 100.00 150.00 200.00 250.00 300.00 350.00 0.00 50.00 100.00 150.00 200.00 250.00 300.00 350.00 Distance,in feet,from an arbitrary datum Distance,in feet,from an arbitrary datum s Rosgen Stream Type Classification Flow Calculations Rosgen Stream Type Classification Flow Calculations Entrenchment >2.2 extensive floodplain Bankfull Depth 1.06[ft] Entrenchment 4.07 Bankfull Depth 1.59[ft] Width:Depth 28.46 Area 32.22[sq.ft] Width:Depth 21.42 Area 53.88[sq.ft] Sinousity NC Manning's n 0.0380 Sinousity NC Manning's n 0.0400 Slope 0.0140 Velocity 4.73[fUs] Slope 0.0100 Velocity 4.95[ft/s] D50 NC [mm] Discharge 152.28[cfs] D50 56[mm] Discharge 266.50[cfs] Stream Type C4 Shear Stress 0.90[Ib/sq.ft] Stream Type C4 Shear Stress 0.95[Ib/sq.ft] Project Name: Croton Watershed Study Project Name: Croton Watershed Study Project Number: 1027.01 Project Number: 1027.01 Surveyed: 1/22/97 By: CAS Surveyed: 1/22/97 By: CAS B-18 Westchester County Stormwater Management Planning Manual Appendix B-Hallocks Mill BrookFluvial Geomorphic Assessment Data Hallocks Mill XS-2 Hallocks Mill XS-3 305.00 305.00 F x E -Existing E Existing m Ground m Ground m 300.00 300.00 02 -f-Bankfull m --M- o -Bankfull c Width c Width m o 0 0 m 295.00 . o 295.00 o -A- Flood Prone o -A- Flood Prone Elevation Elevation 0 w - w 290.00 - 290.00 0 10 20 30 40 50 60 70 80 0 10 20 30 40 50 60 70 80 90 Distance from an arbitrary datum[ft] Distance from an arbitrary datum[ft] Sample Profile - Thalweg Sample Profile t-Thalweg _ 2•400.00 -.-Bankfull Elevation - i',400.00 ----- a� Bankfull Elevation e m a 300.00- c n 300.00- `m `m c c 200.00- c c 200.00- o m o m 0 100.00- 1,, Tc, 100.00- w 0 0.00 i I I I I I w m 0.00 I I I I I I 0.00 50.00 100.00 150.00 200.00 250.00 300.00 350.00 0.00 50.00 100.00 150.00 200.00 250.00 300.00 350.00 Distance,in feet,from an arbitrary datum Distance,in feet,from an arbitrary datum Rosaen Stream Type Classification Flow Calculations Rosgen Stream Type Classification Flow Calculations Entrenchment 1.11 Bankfull Depth 1.46[ft] Entrenchment 2.23 Bankfull Depth 2.00[ft] Width:Depth 32.59 Area 69.52[sq.ft] Width:Depth 17.04 Area 67.91 [sq.ft] Sinousity NC Manning's n 0.0380 Sinousity NC Manning's n 0.0450 steps Slope 0.0050 Velocity 3.52[Ws] Slope 0.0150 estimate Velocity 6.32[ft/s] D50 NC [mm] Discharge 244.38[cfs] D50 NC [mm] Discharge 428.86[cfs] Stream Type F4 Shear Stress 0.45[Ib/sq.ft] Stream Type B3c Shear Stress 1.82[Ib/sq.ft] Project Name: Croton Watershed Study Project Name: Croton Watershed Study Project Number: 1027.01 Project Number: 1027.01 Surveyed: 1/22/97 By: CAS Surveyed: 1/22/97 By: CAS B-19 Westchester County Stormwater Management Planning Manual Appendix B-Hallocks Mill BrookFluvial Geomorphic Assessment Data Hallocks Mill XS-4 Hallocks Mill XS-5 305.00 305.00 F x E Existing E Existing m Ground Ground V V m 300.00 ' m 300.00- -i--Bankfull --t-Bankfull CO @ Width 0 Width > °' 0 0 m 295.00 m 295.00- a Flood Prone 5 -6- Flood Prone 5 Elevation 5 Elevation m d W w 290.00 290.00- - 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 Distance from an arbitrary datum[ft] Distance from an arbitrary datum[ft] Sample Profile --Thalweg Sample Profile -0-Thalweg a.,400.00 - --M-Bankfull Elevation _ L,400.00- -Of-Bankfull Elevation 12 a 300.00- 12 a 300.00- 2. `o c 200.00- o m 200.00- 0 100.00- > > 100.00- w 10 0.00 1 1 I I I I w ' 0.00 , I I I I I I 0.00 50.00 100.00 150.00 200.00 250.00 300.00 350.00 0.00 50.00 100.00 150.00 200.00 250.00 300.00 350.00 Distance,in feet,from an arbitrary datum Distance,in feet,from an arbitrary datum Rosaen Stream Tvoe Classification Flow Calculations Rosaen Stream Tyne Classification Flow Calculations Entrenchment>2.87 Bankfull Depth 2.45[ft] Entrenchment 1.10 Bankfull Depth 1.38[ft] Width:Depth 11.15 Area 67.10[sq.ft] Width:Depth 31.71 Area 59.95[sq.ft] Sinousity NC Manning's n 0.0400 Sinousity NC Manning's n 0.0400 steps Slope 0.0125 estimate Velocity 7.20[ft/s] Slope 0.0150 estimate Velocity 5.57[ft/s] D50 NC [mm] Discharge 483.31 [cfs] D50 NC [mm] Discharge 333.90[cfs] Stream Type B3c Shear Stress 1.77[Ib/sq.ft] Stream T .e F3 Shear Stress 1.26[Ib/sq.ft] Project Name: Croton Watershed Study Project Name: Croton Watershed Study Project Number: 1027.01 Project Number: 1027.01 Surveyed: 1/22/97 By: CAS Surveyed: 1/22/97 By: CAS B-20 Westchester County Stormwater Management Planning Manual Appendix B-Hallocks Mill BrookFluvial Geomorphic Assessment Data North A-1.1 North A-1.3 310.00 310.00 F Existing x Existing E Ground E Ground v 305.00 v 305.00 it a. ee -f-Bankfull -f-Bankfull m Width It Width `c 300.00 c 300.00 m -A- Flood Prone m -6- Flood Prone o Elevation H 295.00 Elevation • 295.00 '- 10 a a w w 290.00 290.00 0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35 40 Distance from an arbitrary datum[ft] Distance from an arbitrary datum[ft] Sample Profile --Thalweg Sample Profile --Thalweg f Bankfull Elevation - a• 306.00- -la-Bankfull Elevation - 2,305.00- • 6 304.00- 1 a. c 302.00- c e 300. 00- - a 300.00- c . 03298.00 I �6.00- 295.00• 4.00 ;I2.00 I I I I `u 290.00 1 I I I 1 0.00 50.00 100.00 150.00 200.00 250.00 300.00 0.00 50.00 100.00 150.00 200.00 250.00 300.00 Distance,in feet,from an arbitrary datum Distance,in feet,from an arbitrary datum Rosgen Stream Type Classification Flow Calculations Rosgen Stream Type Classification Flow Calculations Entrenchment 1.43 Bankfull Depth 0.41 [ft] Entrenchment 1.95 Bankfull Depth 0.60[ft] Width:Depth 31.20 Area 5.22[sq.ft] Width:Depth 23.79 Area 8.64[sq.ft] Sinousity NC Manning's n 0.0380 Sinousity NC Manning's n 0.0380 Slope 0.0270 Velocity 3.52[ft/s] Slope 0.0233 Velocity 4.18[ft/s] D50 NC [mm] Discharge 18.37[cfs] D50 21 [mm] Discharge 36.14[cfs] Stream T .e B3 Shear Stress 0.68[Ib/sq.ft] Stream Type B4 Shear Stress 0.85[Ib/sq.ft] Project Name: Croton Watershed Study Project Name: Croton Watershed Study Project Number: 1027.01 Project Number: 1027.01 Surveyed: 1/22/97 By: MEW Surveyed: 1/22/97 By: MEW B-21 Westchester County Stormwater Management Planning Manual Appendix B-Hallocks Mill BrookFluvial Geomorphic Assessment Data Yorktown A-1.1 Yorktown A-1.2 310.00 310.00 F Existing Existing E Ground E Ground 1'n 8 v v 2305.00- m 305.00- - U -Bankfull -f-Bankfull m Width m Width c c l0 co i pa o m 300.00- -it- Flood Prone m 300.00- -e- Flood Prone o Elevation o A- -A Elevation ud w w 295.00- 295.00 -- 0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 Distance from an arbitrary datum[ft] Distance from an arbitrary datum[ft] Sample Profile +Thalweg Sample Profile ♦Thalweg 2 304.00 f Bankfull Elevation - z, 304.00• t Bankfull Elevation c �° 302.00- a 302.00- a aa 300.00- .c. `m d c c c 300.00- ° m 298.00 9 m is at 8 8 296.00- 298.00-. • • � w 10 294.00 I I I I I w f0 296.00 I 1 I I I 0.00 50.00 100.00 150.00 200.00 250.00 300.00 0.00 50.00 100.00 150.00 200.00 250.00 300.00 Distance,in feet,from an arbitrary datum Distance,in feet,from an arbitrary datum Rosgen Stream Type Classification Flow Calculations Rosgen Stream Type Classification I Flow Calculations Entrenchment 1.67 Bankfull Depth 0.30[ft] Entrenchment 11.35 extended floodplain Bankfull Depth 0.52[ft] Width:Depth 24.85 Area 2.30[sq.ft] Width:Depth 11.09 Area 2.95[sq.ft] Sinousity NC Manning's n 0.0380 Sinousity NC Manning's n 0.0300 Slope 0.0300 Velocity 3.05[fUs] Slope 0.0015 Velocity 1.17[fUs] D50 NC [mm] Discharge 6.99[cfs] D50 NC [mm] Discharge 3.44[cfs] Stream Type B4 Shear Stress 0.56[Ib/sq.ft] Stream Type E5 Shear Stress 0.04[Ib/sq.ft] Project Name: Croton Watershed Study Project Name: Croton Watershed Study Project Number: 1027.01 Project Number: 1027.01 Surveyed: 1/22/97 By: MEW Surveyed: 1/22/97 By: MEW 8-22 Westchester County Stormwater Management Planning Manual Appendix B-Hallocks Mill BrookFluvial Geomorphic Assessment Data Yorktown A-1.3 Yorktown B-1.1 310.00 310.00 -Existing : -Existing E Ground E Ground 1nm v v 2' 305.00 z' 305.00 2 -f-Bankfull -0 Width -e --f-Bankfull a a Width c c m a a o > > O O m 300.00 -A- Flood Prone m 300.00 c Elevation c -e Flood Prone o o -- Elevation N w w N 295.00 295.00 0 5 10 15 20 25 30 35 40 45 50 55 0 20 40 60 80 100 Distance from an arbitrary datum[ft] Distance from an arbitrary datum[ft] Sample Profile Thalweg Sample Profile -e-Thalweg -f-Bankfull Elevation .= 304.00 -E-Bankfull Elevation z 304.00- -_---___-_.... -_ a, j 302.00- ,°'�, E 302.00- a m 300.00- = a 300.00- c c c 298.00-9- N 298.00- ° m0 8 a 296.00- a>> >0 296.00- m a 294.00- w 6 294.00 w m 292.00 r 1 I I I • 0.00 50.00 100.00 150.00 200.00 250.00 300.00 0.00 50.00 100.00 150.00 200.00 250.00 300.00 Distance,in feet,from an arbitrary datum Distance,in feet,from an arbitrary datum Rosqen Stream Tvpe Classification Flow Calculations Roscien Stream Type Classification Flow Calculations Entrenchment 1.41 Bankfull Depth 0.46[ft] Entrenchment 8.36 Bankfull Depth 0.50[ft] Width:Depth 25.59 Area 5.43[sq.ft] Width:Depth 21.41 Area 5.28[sq.ft] Sinousity NC Manning's n 0.0400 Sinousity NC Manning's n 0.0380 Slope 0.0300 Velocity 3.82[ft/s] Slope 0.0185 Velocity 3.31 [ft/s] D50 NC [mm] Discharge 20.75[cfs] D50 NC [mm] Discharge 17.49[cfs] Stream Type B4 Shear Stress 0.85[Ib/sq.ft] Stream Type C4 Shear Stress 0.57[Ib/sq.ft] Project Name: Croton Watershed Study Project Name: Croton Watershed Study Project Number: 1027.01 Project Number: 1027.01 Surveyed: 1/22/97 By: MEW Surveyed: 1/22/97 By: MEW B-23 Westchester County Stormwater Management Planning Manual Appendix B-Hallocks Mill BrookFluvial Geomorphic Assessment Data Yorktown B-1.2 Yorktown B-1.4 310.00 310.00 F -Existing I -Existing E Ground E Ground v v 2 305.00 305.00- -f-Bankfull --f-Bankfull le- Width m Width c c 0 t0 N O O m 300.00 -A- Flood Prone m 300.00 / Flood Prone o --- Elevation o A r Elevation m m w w 295.00 -- .--- 295.00 0 5 10 15 20 25 30 35 40 45 50 0 5 10 15 20 25 30 35 40 Distance from an arbitrary datum[ft] Distance from an arbitrary datum[ft] Sample Profile +Thalweg Sample Profile -•-Thalweg 2, 304.00 ----- -E-Bankfull Elevation 2.304.00- -s-Bankfull Elevation mcg dm 2a 302.00- c 302.00- o c 300.00- `o c 300.00- 0 298.00• d o 298.00 1 • • W a 296.00 I I I W m 296.00 I I I I I 0.00 50.00 100.00 150.00 200.00 250.00 300.00 0.00 50.00 100.00 150.00 200.00 250.00 300.00 Distance,in feet,from an arbitrary datum Distance,in feet,from an arbitrary datum - Rosgen Stream Type Classification Flow Calculations Rosgen Stream Type Classification I Flow Calculations Entrenchment 4.65 Bankfull Depth 0.65[ft] Entrenchment 4.10 extended floodplain Bankfull Depth 0.45[ft] Width:Depth 11.23 Area 4.78[sq.ft] Width:Depth 17.23 Area 3.43[sq.ft] Sinousity NC Manning's n 0.0300 Sinousity NC Manning's n 0.0380 Slope 0.0094 Velocity 3.50[ft/s] Slope 0.0010 Velocity 0.71 [ft/s] D50 NC [mm] Discharge 16.72[cfs] D50 NC [mm] Discharge 2.44[cfs] Stream Type E5 Shear Stress 0.36[Ib/sq.ft] Stream Type C5 Shear Stress 0.03[Ib/sq.ft] Project Name: Croton Watershed Study Project Name: Croton Watershed Study Project Number: 1027.01 Project Number: 1027.01 Surveyed: 1/22/97 By: MEW Surveyed: 1/22/97 By: MEW B-24 Westchester County Stormwater Management Planning Manual Appendix B-Hallocks Mill BrookFluvial Geomorphic Assessment Data Yorktown B-1.5 Yorktown B-1.6 310.00 310.00 • Existing Existing E Ground E Ground > > v Z•305.00 ' 305.00 --t-Bankfull --f--Bankfull m Width m Width c c ix 0 m 0 0 m 300.00 -A- Flood Prone m 300.00 -� Flood Prone o A Elevation o Elevation w w 295.00 295.00 0 5 10 15 20 25 30 35 40 45 0 5 10 15 20 25 30 35 40 45 50 Distance from an arbitrary datum[ft] Distance from an arbitrary datum[ft] Sample Profile -*-Thalweg --0-Profile -}Thalweg 304.00 --E-Bankfull Elevation z 304.00- -E-Bankfull Elevation w 03 w 03 - 302.00 302.00- c 0 m ' m 300.00- 0 c300.00- _ c o 298.00- > > 298.00 > > d�o d�0 296.00�� • Lu m 296.00 I 1_4_......4___1 w m 294.00 I I i I I 0.00 50.00 100.00 150.00 200.00 250.00 300.00 0.00 50.00 100.00 150.00 200.00 250.00 300.00 Distance,in feet,from an arbitrary datum Distance,in feet,from an arbitrary datum Rosgen Stream Type Classification I Flow Calculations Rosoen Stream Type Classification Flow Calculations Entrenchment >3 Extended Floodplain Bankfull Depth 0.48[ft] Entrenchment 2.18 Bankfull Depth 0.80[ft] Width:Depth 22.77 Area 5.32[sq.ft] Width:Depth 13.92 Area 8.84[sq.ft] Sinousity NC Manning's n 0.0380 Sinousity NC Manning's n 0.0310 Slope 0.0170 Velocity 3.12[ft/s] Slope 0.0057 Velocity 2.99[ft/s] D50 NC [mm] Discharge 16.56[cfs] D50 12[mm] Discharge 26.42[cfs] Stream Type C4 Shear Stress 0.50[Ib/sq.ft] Stream Type C4 Shear Stress 0.27[Ib/sq.ft] Project Name: Croton Watershed Study Project Name: Croton Watershed Study Project Number: 1027.01 Project Number: 1027.01 Surveyed: 1/22/97 By: MEW Surveyed: 1/22/97 By: MEW B-25 Westchester County Stormwater Management Planning Manual Appendix B-Hallocks Mill BrookFluvial Geomorphic Assessment Data Yorktown B-1.7 Yorktown B-1.8 310.00 310.00 Existing Existing 0 Ground E Ground Iv, -o m v f2 305.00- a 305.00 1.6 -f-Bankfull --F-Bankfull Co Width m Width c c 0 N N O N O m 300.00- Flood Prone o 300.00 -- Flood Prone o Elevation o Elevation 'di a N 9 m w w 295.00 295.00 0 5 10 15 20 25 30 35 40 45 50 55 60 65 0 5 10 15 20 25 30 35 40 45 50 55 60 Distance from an arbitrary datum[ft] Distance from an arbitrary datum[ft] Sample Profile -40-Thalweg -4-Thalweg Profile ♦Thalweg _ 2. 306.00 -N-Bankfull Elevation _ a.304.00- -i-Bankfull Elevation wa 304.00- mm - - - c 2i 302.00- c mt 302.00- g m 298.00 0 300.00 296.00 > > 298.00- °-'�° 294.00 m o w m 292.00 I I I I w m 296.00 1 1 I 0.00 50.00 100.00 150.00 200.00 250.00 300.00 0.00 50.00 100.00 150.00 200.00 250.00 300.00 Distance,in feet,from an arbitrary datum Distance,in feet,from an arbitrary datum Rosgen Stream Type Classification Flow Calculations Rosuen Stream Type Classification Flow Calculations Entrenchment 1.72 Bankfull Depth 0.63[ft] Entrenchment 2.16 Bankfull Depth 0.77[ft] Width:Depth 31.07 Area 12.36[sq.ft] Width:Depth 28.67 Area 17.01 [sq.ft] Sinousity NC Manning's n 0.0420 Sinousity NC Manning's n 0.0400 Slope 0.0130 Velocity 2.94[ft/s] Slope 0.0150 Velocity 3.74[ft/s] D50 26[mm] Discharge 36.35[cfs] D50 NC (mm] Discharge 63.56[cfs] Stream Type B4c Shear Stress 0.50[Ib/sq.ft] Stream Type B4c Shear Stress 0.69[Ib/sq.ft] Project Name: Croton Watershed Study Project Name: Croton Watershed Study Project Number: 1027.01 Project Number: 1027.01 Surveyed: 1/22/97 By: MEW Surveyed: 1/22/97 By: MEW B-26 Westchester County Stormwater Management Planning Manual Appendix B-Hallocks Mill BrookFluvial Geomorphic Assessment Data Yorktown C-1.1 Yorktown C-1.2 310.00 - 310.00 F Existing -Existing E Ground E Ground v• 305.00 v 305.00 ✓ a 2 -f-Bankfull m -f-Bankfull It Width le Width c 300.00 c 300.00 a m E a 0 o a- Flood Prone m -- -A- Flood Prone c Elevation c Elevation 2 295.00 2 295.00 - w w 290.00 ---- 290.00 0 5 10 15 20 25 30 35 40 45 0 5 10 15 20 25 30 35 40 45 50 55 Distance from an arbitrary datum[ft] Distance from an arbitrary datum[ft] Sample Profile -Thalweg Sample Profile --Thalweg a, 305.00- t Bankfull Elevation 305.00 - f Bankfull Elevation � � 300.00 .e c-e c£ 300.00 - mIi; 29500- c c i m 29500-29000 >m o 285.00 I I W m 290.00 1 i 1 I I 0.00 50.00 100.00 150.00 200.00 250.00 300.00 0.00 50.00 100.00 150.00 200.00 250.00 300.00 Distance,in feet,from an arbitrary datum Distance,in feet,from an arbitrary datum Rosgen Stream Type Classification Flow Calculations Rosgen Stream Type Classification Flow Calculations Entrenchment 1.65 Bankfull Depth 0.63[ft] Entrenchment 1.28 Bankfull Depth 0.71 [ft] Width:Depth 19.81 Area 7.82[sq.ft] Width:Depth 13.53 Area 6.73[sq.ft] Sinousity NC Manning's n 0.0380 Sinousity NC Manning's n 0.0400 Slope 0.0211 Velocity 4.11 [ft/s] Slope 0.0220 Velocity 4.22[ft/s] D50 NC [mm] Discharge 32.15[cfs] D50 NC [mm] Discharge 28.38[cfs] Stream Type B4 Shear Stress 0.81 [Ib/sq.ft] Stream Type F4b Shear Stress 0.92[Ib/sq.ft] Project Name: Croton Watershed Study Project Name: Croton Watershed Study Project Number: 1027.01 Project Number: 1027.01 Surveyed: 1/22/97 By: MEW Surveyed: 1/22/97 By: MEW B-27 Westchester County Stormwater Management Planning Manual Appendix B-Hallocks Mill BrookFluvial Geomorphic Assessment Data Yorktown C-1.3 Yorktown C-1.4 310.00 310.00 E' Existing -Existing E Ground E Ground m is v L^305.00 a305.00- -f-Bankfull --M--Bankfull ea Width a Width a c c a 5 i o 0a f 300.00 -6- Flood Prone a 300.00 A __- _ -A -a- Flood Prone oElevation o - Elevation tTi m > ., w w 295.00 -------- 295.00 - 0 5 10 15 20 25 0 5 10 15 20 25 30 35 40 45 50 Distance from an arbitrary datum[ft] Distance from an arbitrary datum[ft] Sample Profile Sample Profile t Thalweg [77t Thalweg _ >. 304,00-- - -M-Bankfull Elevation _ a.,304.00- - -M-Bankfull Elevation c a 302.00- c a 302.00- o m 300.00- o m 300.00- 0 298.00- m 0 298.00 w m 296.00 I I I I { w m 296.00 I I I 1 I 0.00 50.00 100.00 150.00 200.00 250.00 300.00 0.00 50.00 100.00 150.00 200.00 250.00 300.00 Distance,in feet,from an arbitrary datum Distance,in feet,from an arbitrary datum Rosgen Stream Type Classification Flow Calculations Roscien Stream Type Classification Flow Calculations Entrenchment 1.39 Bankfull Depth 0.77[ft] Entrenchment >2.2 Bankfull Depth 0.44[ft] Width:Depth 12.23 Area 7.29[sq.ft] Width:Depth 49.22 Area 9.45[sq.ft] Sinousity NC Manning's n 0.0380 Sinousity NC Manning's n 0.0380 Slope 0.0170 Velocity 4.17[ft/s] Slope 0.0050 Velocity 1.58[ft/s] D50 NC [mm] Discharge 30.43[cfs] D50 NC [mm] Discharge 14.95[cfs] Stream Type B4c Shear Stress 0.78[Ib/sq.ft] Stream T .e C6 Shear Stress 0.13[Ib/sq.ft] Project Name: Croton Watershed Study Project Name: Croton Watershed Study Project Number: 1027.01 Project Number: 1027.01 Surveyed: 1/22/97 By: MEW Surveyed: 1/22/97 By: MEW B-28 Appendix C Hallocks Mill Brook Watershed Estimated Annual Runoff Pollutant Loadings Generated from SWMM Analysis Westchester County Stormwater Management Planning Manual Appendix C—Hallocks Mill Brook Watershed Estimated Annual Runoff Pollutant Loadings Annual BOD Loading(lb/acre) iv 0-5 —5.20 H20-35- 011111111)14 35 50 * � Estimated Annual BOD Loading Current Conditions AllitItVi 4*1 svliim 1 0 1 Miles Annual BOD Loading (lb/acre) I 10- 5 1 15-20 I20- 35 0 ET 35- 50 Estimated Annual BOD Loading I-150+ �} Build-Out Conditions iii#114 tote ,... AI% 4,11P A 1 0 1 Miles I C-1 Westchester County Stormwater Management Planning Manual Appendix C—Hallocks Mill Brook Watershed Estimated Annual Runoff Pollutant Loadings Annual TSS Loading(Ib/acre) of —0-50 X50-100 — 100-150 tllli ®2-1M:250 50+150-250 ilEstimated Annual TSS Loading Current Conditions 4110a„vox atts- A 1 0 1 MN les Annual TSS Loading(lb/acre) 0-50 if 50- 100 Estimated Annual TSS Loading 100- 150 Build-Out Conditions 150 250 i�� 250+ Wiliill Vii itillifil, wir Ilyib iirib Auk 1st N A 1 0 1 Miles I C-2 Westchester County Storm water Management Planning Manual Appendix C-Hallocks Mill Brook Watershed Estimated Annual Runoff Pollutant Loadings Annual COD Loading(Ib/acre) iv 75-150 - 150-300 —300-450 All Haso600456000+-600oo Estimated Annual COD Loading Current Conditions 11114�rOSli "VI& arN A IF 1 0 1 Mmes Annual COD Loading (Ib/acre) 75- 150 `– 150-300 Li — 300-450 ate Estimated Annual COD Loading _ 450-600 Build-Out Conditionsr 600+ ali Ali a Itiiittilletf0)P4o AllipArilit A A I 1 0 1 Miles 1 C-3 Westchester County Storm water Management Planning Manual Appendix C–Hallocks Mill Brook Watershed Estimated Annual Runoff Pollutant Loadings Annual TP Loading(lb/acre) iv 0-0.25 —0.25-0.50 -0.50-1.00 .4' 11.00So+-1.50 '434Estimated Annual TP Loading Current Conditions 1 4 AO% #trir JI, A 1 0 1 Miles Annual TP Loading (lb/acre) 0- 0 - — 0.25 0.50Ili 0.50- 1.00 �� Estimated Annual TP Loading _ 1.50+ 1.50- 1.50 . Build-Out Conditions to a4,.... ,4,_,,,,4 A ....,,ult N A 1 0 1 Miles C-4 Westchester County Storm water Management Planning Manual Appendix C—Hallocks Mill Brook Watershed Estimated Annual Runoff Pollutant Loadings Annual TN Loading(lb/acre) if 0-3 —3-6 —6 9 014 12 H 9 12+- Estimated Annual TN Loading Current Conditions iIPJW4 ,4tA 1 0 Miles Annual TN Loading (lb/acre) 1 13-6 1 16- 9 � 12 12 + 1 �� Estimated Annual TN Loading + IV Build-Out Conditions tiV Allif 1111 jilt ittriftAtik A M4111 1 1 0 1 Miles 11 C-5 Westchester County Stormwater Management Planning Manual Appendix C–Hallocks Mill Brook Watershed Estimated Annual Runoff Pollutant Loadings Annual Cu Loading(lb/acre) ii 0-0.01 —0.01 -0.03 0.03-0.04 611 H 0o.. 405-*0.05 Estimated Annual Cu Loading Current Conditions aA al',Aar A Air 1 0 I Miles Annual Cu Loading (lb/acre) FT0- �0.011 - 0.03 1 X0.03-0.04 1111 ET0.04- 0.05 i Estimated Annual Cu Loading 0.05+ 'kV Build-Out Conditions dittli 111Ns4ATAmi 1,4114* M# gi I ti A 1 0 1 Miles 1 i C-6 Westchester County Stormwater Management Planning Manual Appendix C-Hallocks Mill Brook Watershed Estimated Annual Runoff Pollutant Loadings Annual Pb Loading(lb/acre) of 0-0.05 —0.05-0.10 —0.10-0.20 014 H00.40+20-0.40 Estimated Annual Pb Loading Current Conditions 144���. 06 N A Allr 1 0 1 Niles ISI Annual Pb Loading(lb/acre) 0- 0 n -0.05 0.10 I—I 0.10-0.20 ni0.20-0.40 �4 0.40+ � 0 Estimated Annual Pb Loading Build-Out Conditions itijelit Zifilolit 11116P 11°C. igtri AI% A 1 0 1 Miles I C-7 Westchester County Stormwater Management Planning Manual Appendix C-Hallocks Mill Brook Watershed Estimated Annual Runoff Pollutant Loadings Annual Zn Loading(lb/acre) if 0-0.1 —0.1 -0.2 —0.2-0.4 0 -0.6o.s+ 11111114 Estimated Annual COD Loading Current Conditions It% ri1114.4 fire4at N N IF 4/1 A1111°1° 1 1 0 I Miles I Annual Zn Loading (lb/acre) I 10-0.1 1 10.1 -0.2 ri0.2-0.4 111 rn0.4-0.6 �� Estimated Annual Zn Loading 0.6 + � Build-Out Conditions 10/1 110 1114—t vq.1 � 1404 arA 1 0 1 Miles i C-8 Westchester County Stormwater Management Planning Manual Appendix D—Croton Watershed Pollutant Loading Screening Model Appendix D Croton Watershed Pollutant Loading Screening Model Westchester County Stormwater Management Planning Manual Appendix D—Croton Watershed Pollutant Loading Screening Model Technical Memorandum Section 1 Introduction A watershed management model(WMM)was created for the Croton River Basin(Croton) using the USEPA Storm Water Management Model(SWMM). The focus of the model was to develop stormwater runoff pollutant loading estimates for the sub-watersheds within the Croton. These pollutant loading estimates could then serve as guidance for focusing watershed management efforts within the Croton River Basin on those sub-watersheds exhibiting the most severe pollutant loadings. Once high priority sub-watersheds have been identified,a more detailed analysis of impairments specific to that sub-watershed is required. A recommended sub-watershed level stormwater management planning approach is presented in the Croton Stormwater Planning Manual(Planning Manual). In the Planning Manual,this approach is applied to the Hallocks Mill Brook Watershed,one of the sub-watersheds within the Croton River Basin. This document serves as a supplement to the Planning Manual. Section 2 Physical Setting The Croton is one of six major drainage basins located within Westchester County (Figure 1). While the majority of the basin is located in Westchester,parts of the basin extend into Putnam County and the State of Connecticut(Figure 2). The basin is approximately 212 square miles, and consists of 46 minor watersheds. Each of the 46 minor watersheds eventually drains to the Croton Reservoir.The Croton Reservoir provides potable water for Westchester County and New York City. Section 3 Pollutant Loading Screening Model 3.1 GIS Data Collection and Development The majority of data required to develop the SWMM was collected from the Westchester County Geographical Information Systems (GIS)web page, http://giswww.westchestergov.com/. The data was organized and analyzed using the ESRI suite of Arc products. Additional data was obtained from the New York City Department of Environmental Protection(NYCDEP)for areas located outside of Westchester County. Watershed Delineation The minor watershed divides provided in the Westchester County GIS coverage were used to define the drainage units within Westchester. Those sub-watersheds that extended beyond the county border were delineated using data obtained from the NYCDEP and manually using USGS quadrangles. Each sub-watershed was assigned a 3-digit numerical model identification number(Figure 3) D-1 Westchester County Storm water Management Planning Manual Appendix D—Croton Watershed Pollutant Loading Screening Model Land Use Generalized land use information(1996)obtained from the Westchester County GIS coverage was consolidated into the following land use categories: Agricultural Fertilized Residential Agricultural Pasture Water Forested Wetlands Mixed Urban The 1996 land use data was updated using 2000 Census population blocks by overlaying the land use data with the population data. Those areas not classified as residential in the 1996 land use data that showed population densities greater than 1 person per acre were changed to residential land use. The updated land use data was superimposed upon the sub-watersheds to determine the breakdown of land use within each sub-watershed. The total area of each land use within a sub-watershed was then assigned a 5-digit model identification number;the first 3 digits representing sub-watershed and the last 2 representing land use. Impervious Ground Cover Percent impervious land cover was estimated using previously established criteria based upon land use classification. Residential land use imperviousness was estimated based upon 2000 US Census population information using relationships between population density and impervious ground cover developed by Stanakowski(1974) and Manning(1977) as presented by Angell,Clement and Smullen(1998). Impervious cover estimates for the remaining land use types are presented in Table 1. D-2 Westchester County Storm water Management Planning Manual Appendix D—Croton Watershed Pollutant Loading Screening Model Table 1—Impervious Cover Estimates Land Use Classification Estimated '/) Impervious Agricultural 5% Forested 5% Residential 5% 1 Mixed Commercial/Industrial 85% Wetlands 2% - 5% Water 0% Notes: 1. Provided as a minimum% impervious.Actual estimate based upon population density 2. Dependent upon soil saturation conditions 3. Precipitation occuring over water bodies is directly connected to the stormw ater conveyance system Soils The 1986 Soil Conservation Service(SCS)Soil Survey for Putnam and Westchester Counties was used to develop infiltration properties across the study area.Soil types were categorized according to texture into the following basic categories: Complex Rock Fine Sandy Loam Silt Loam Gravel Loam Sand Very Stony Loam Loam Water Muck Unknown The GIS soils coverage was superimposed upon the land use to determine the soils composition for each land use sub-watershed within the study area. A weighted average for each modeling infiltration parameter was then developed for each of the land use sub- watersheds based upon the information presented in Table 2. D-3 Westchester County Stormwater Management Planning Manual Appendix D—Croton Watershed Pollutant Loading Screening Model Table 2—Estimation of Green-Ampt Infiltration Parameters(SWMM RUNOFF Variables) SUCT HYDCON SMDMAX USDA Soil Texture Avg. Capillary Suction Saturated Hydraulic Initial Moisture Deficit Classification Conductivity for Soil (in) (in/hr) (vol. of air/vol. of voids) Sand 1.95 9.27 0.346 Loamy Sand 2.41 2.35 0.312 Sandy Loam 4.33 0.86 0.246 Loam 3.50 0.52 0.193 Silt Loam 6.57 0.27 0.171 Sandy Clay Loam 8.60 0.12 0.143 Clay Loam 8.22 0.08 0.146 Silty Clay Loam 10.75 0.08 0.105 Sandy Clay 9.41 0.05 0.091 Silty Clay 11.50 0.04 0.092 Clay 12.45 0.02 0.079 Notes: 1. Synthesized from Handbook of Hydrology, D.R. Maidment, Editor in Chief, McGraw Hill, Inc. 1993, pp 5.1-5.39. 3.2 Stormwater Model The USEPA Storm Water Management Model (SWMM)was used to develop the screening level pollutant loading model for the Croton River Basin. The model accepts actual precipitation and temperature records and simulates runoff while considering infiltration, evaporation and storage on both impervious and pervious surfaces. Meteorological Data Precipitation records were obtained from the National Climactic Data Center (NCDC) for the Bridgeport,CT precipitation station. This station,located approximately 40 miles southeast of the study area,was selected based upon the length of record and the accuracy of the data. Calibration with stream gages indicated that there were some discrepancies during brief storm events where rainfall at the gage and within the Croton Watershed were different,most likely during thunderstorms,but that overall the precipitation between the two areas was consistent. Hourly data from 1948 through 2000 was obtained. Precipitation data was converted from an hourly to 15-minute time step using Rainmaster, a statistical storm analysis tool developed by Mitch Heineman (CDM). The 15-minute precipitation interval allows for a more detailed evaluation of infiltration and evaporation D-4 Westchester County Storm water Management Planning Manual Appendix D—Croton Watershed Pollutant Loading Screening Model Temperature data, daily minimum and maximum,was compiled from two sets of NCDC data. Records from Yorktown were used for the years 1967 through 2000.White Plains was used to complete the data set from 1948-1966.Sporadic missing periods of record were synthesized using similar time periods from other years within the dataset. Streamflow Observed stream flow data is required to calibrate the model. Four USGS stream flow gages were identified in the Croton, two of which were not suited for calibration purposes because flows were influenced by dams or controlled releases. Data was collected for USGS gages 01374976,"Angle Fly Brook,Whitehall Corners,NY" and 01374987,"Kisco River,Mount Kisco,NY". Daily flow data from 1996 through September 2000 was collected for each gage. Event Mean Concentrations(EMCs) Pollutant loading EMCs were assembled from multiple data sources. These EMCs were applied to simulated annual stormwater runoff volumes for each minor watershed to estimate annual runoff pollutant loadings. D-5 Westchester County Stormwater Management Planning Manual Appendix D-Croton Watershed Pollutant Loading Screening Model Table 3-Estimated Pollutant Loading Event Mean Concentrations by Land Use Event Mean Concentrations(EMCs) Ag Ag Water Forest Urban Pasture Crop Wetlands BOD mg/L No Data 1 14.1 27.7 20 No Data Std Dev 10.0 13.8 10 COD mg/L 171 113 100 159 220 171 Std Dev 100 62 25 89 100 100 TSS mg/L No Data 40 78.4 410 500 No Data Std Dev 21 81.1 295 500 TP mg/L 0.064 0.15 0.315 0.75 2.72 0.1 Std Dev 0.044 0.07 0.218 0.4 0.83 0.1 TN mg/L 1.6 0.75 2.4 4.2 9.86 0.5 Std Dev 1.1 0.52 1.6 1.7 0.8 0.5 Pb mg/L 0.00266 0.016 0.0675 0.018 0.076 0.00266 Std Dev 0.0023 0.059328 0.03 0.04 0.0023 Cu mg/L 0.0022 No Data 0.0135 0.005 0.053 0.0022 Std Dev 0.0015 0.009345 0.009 0.031 0.0015 Zn mg/L 0.0652 0.072 0.162 0.15 0.23 0.0652 Std Dev 0.0495 0.02 0.123 0.12 0.31 0.0495 References: 1 EPA 1982-Chesapeake Bay Program 2 Metropolitan Washington Water Resources Planning Board 1978-Occoquan/Four Mile Run Non-Point Source Correlation Study 3 Updating the U.S.Nationwide Urban Runoff Quality Database;James T.Smullen,Amy L. Shallcross and Kelly A.Cave 1999 Model Results Estimated annual pollutant loadings obtained by applying EMCs to the average annual runoff simulated with SWMM are presented in the following figures. D-6 Westchester County Stormwater Management Planning Manual Appendix D—Croton Watershed Pollutant Loading Screening Model Figure 1— Estimated Annual BOD Loadings 015 036 01 4 021 i) 042 030 1. 014 b . 02, 025 022 002 047 010 03 0 i �c a 018 ,} 01• .+nr•41 016 If 011 043 03 ► 03 i 004 if111.' 041 020 03 �� 033 Amual BOD Loading(lbs/acre) 039 _0-5 0 5-10 05 10-15 015-20 _20-25 _25-30 Figure 2—Estimated Annual COD Loadings 015 034 036 01 111111111 0214 042 030 ,11 014 16-. 02, 025 022 002 047 4 010 03 10°A.04/ 016 011 01• ~nr• 4 u 016043 ,� / '' �r...,� � 007 03 jr,•� 03f 034 041 20 03 027111%1)1, 033 Annual COD Loading(lbs/acn 039 0-0 -100 100-200 005 200-300 300-400 400-500 >500 D-7 Westchester County Stormwater Management Planning Manual Appendix D—Croton Watershed Pollutant Loading Screening Model Figure 3—Estimated Annual Total Nitrogen Loadings 015 034 036 01 021 .14 042 +030 1011111 Z j 014 02 025 022 002 1!' 047 . 4T 03 4s. 010 03 111141°4? 018 011 #"1.41riAgra. 043 01 It 016 fahlt i 007 03 03 f 004 1 1 041 11• 020 -.1141r1:1111P' 03 027 033 039 Annual TN Leading(lbs/acre) <1 005 3 5 5-7 7-10 >10 Figure 4—Estimated Annual Total Suspended Solids Loadings 015 034 036 01 021 042 +030 ' ,A 101114, � i 014 02, 025 022 002 47 � �� 1 1 010 � 04. It•• 018 } 011 r,SII `�• 043 01' / 016 11. I 1• 1.44_(11;1117. h. 007 03rib 03 004 041 20 01114111112S0* 03 033 039 Annual TN Loading(lbs/acre) 005 11<31_5 5-7 7-10 —>10 D-8 Westchester County Stormwater Management Planning Manual Appendix D—Croton Watershed Pollutant Loading Screening Model Figure 5—Estimated Annual Total Phosphorous Loading 015 034 036 01- 021 j 042 +030 � i ' 014 ���}}4 11• 02• 025 022 1011111 02 1 047 401. 03 03l'�� } I010 04. u ' 011 043 '��• +3 016 01• w1 �c f..,,,,, 1� 007 a3 03 I 004 �'� , 041 03 ��• 0200 i,... 027 033 Annual TP Loading(lbs/acre) 039 0-0.25 005 0.25-0.5 0.5-0.75 0.75-1 _ 1-1.25 Figure 6—Estimated Annual Lead Loading 015 034 036 01 021 `l 042 ,030 ill. T J 014 �, 025 022 002311. 44047 03 010 JI/47)04it 018 ;�r } 011 043 01 •rf ■ u 016 '"� � 007 03 I'004 I 041 03 �� 020 03 027 #: 033 039 Amual Pb Loading(lbs/acre) 005 00.030.-030.06 0 0.03-.03 0.06 0.06-0.09 0.09-0.12 =0.12-0.14 D-9 Westchester County Storm water Management Planning Manual Appendix D—Croton Watershed Pollutant Loading Screening Model Figure 7—Estimated Annual Copper Loading 015 036 01 .411 021 Jiii 042 030 414 02 025 022 biaT i 014 1 . 411047 40, .. 03 } y 010 03 111111?' Nfld. O18 011 .�di. 016 043 01.0,14 74 007 03 jr► , ; �► co r 004 ' )00, 041 ,�• 020 03 f 027 ,�: 033 039 Amual Cu Loading Obs/acre) 005 e°O- 0-0.01 _0.01-0.02 MN0.02-0.03 Figure 8—Estimated Annual Zinc Loading 015 034 036 01 021 042 030 ,,44114 100110 Z J 014 *. 02. 025 022 002 0f. 47 A1;011411 o,o 03 018 011� ��trIler4°44.11110.1.adhahh. 016043 lif, 07 Ir, ,,,, 03 r 03 041 020 03 027 033 ' 039 Amual Zn Loafing Obs/acre) 005 00.10-.01.2 - 0.1-0.2 0.22 0.3 0.3-0.4 X0.4-0.6 1 D-10 Westchester County Storm water Management Planning Manual Appendix D-Croton Watershed Pollutant Loading Screening Model Table 4-Estimated Annual Pollutant Loadings BOD COD TSS TP TN Pb Cu Zn I , d a� oil a ai oil or: a 2 c b° e c u � e u c c u � c u � c u � c u a w w a m w c m c a - a > A i w > .E .Le a > w > m > a 9 > 0 m o 8 0 �' 8m oa 8m o 8 0 � 3m oa at o M J.... HD JG � f/1O Jr. yt] Jr, tlJD J ... N� JG W D JC to s002 8.29 5.22 128.19 57.02 77.37 62.58 0.27 0.16 1.76 1.14 0.04 0.03 0.01 0.00 0.13 0.08 s003 1.77 0.60 111.43 59.39 41.68 24.26 0.16 0.08 0.85 0.58 0.02 0.00 0.00 0.00 0.08 0.03 s004 9.78 5.84 200.67 96.09 105.66 80.28 0.37 0.22 2.40 1.56 0.05 0.03 0.01 0.01 0.17 0.10 s005 18.21 11.67 175.10 63.57 209.34 207.13 1.02 0.43 4.90 1.78 0.08 0.06 0.03 0.02 0.22 0.20 s006 13.37 8.95 178.86 71.55 -100.50 88.46 0.40 0.25_2• .75 1.84 0.07 0.05 0.01 0.01 0.20 0.13 s007 25.93 16.92 235.03 85.93 231.32 213.15 0.88 0.48_5• .26 2.75 0.11 0.09 0.03 0.02 0.30 0.23 s008 9.38 6.06 182.36 82.94 80.92 66.67 0.33 0.20 2.26 1.53 0.05 0.04 0.01 0.01 0.17 0.10 s009 18.49 12.95 165.06 52.23 110.84 109.86 0.45 0.30 3.32 2.22 0.09 0.08 0.02 0.01 0.23 0.17 s010 15.95 10.38 242.72 107.01 136.60 118.82 0.53 0.31 3.66 2.27 0.08 0.06 0.01 0.01 0.24 0.16 s011 7.60 4.55 184.34 89.74 82.36 62.06 0.31 1 0.18 1.98 1.33 0.04 0.03 0.01 0.00 0.15 0.08 s012 13.55 9.33 139.04 48.47 88.85 83.89 0.35 0.23 2.55 1.70 0.07 0.06 0.01 0.01 0.18 0.12 s013 11.70 7.52 199.50 87.03 102.57 83.87 0.40 0.24 2.63 1.77 0.07 0.04 0.01 0.01 0.20 0.11 s014 13.28 8.20 164.08 71.88 129.03 103.86 0.40 0.24 2.59 1.59 0.05 0.04 0.01 0.01 0.17 0.11 s015 12.72 7.87 218.24 99.60 139.13 119.57 0.58 0.29 3.39 1.81 0.07 0.04 0.01 0.01 0.20 0.13 s016 17.88 12.22 156.00 50.59 127.65 123.28 0.51 0.31 3.41 2.04 0.08 0.07 0.02 0.01 0.22 0.16 8017 18.83 12.68 221.08 83.80 143.34 131.76 0.59 0.35 3.87 2.42 0.10 0.07 0.02 0.01 0.26 0.18 x018 11.55 6.03 103.90 52.66 167.72 127.47 0.42 0.21 2.23 0.93 0.02 0.02 0.01 0.01 0.10 0.07 s019 4.68 2.53 152.83 76.38 64.07 43.75 0.25 0.13 1.46 0.99 0.03 0.02 0.00 0.00 0.12 0.05 5020 19.96 13.74 167.14 53.55 129.89 124.37 0.48 0.32 3.49 2.27 0.09 0.08 0.02 0.01 0.23 0.17 s021 28.35 18.51 335.48 132.23 280.06 268.86 1.30 0.62 6• .99 3.28 0.14 0.11 0.03 0.02 0.39 0.30 s022 23.51 15.63 234.62 87.97 188.94 174.78 0.72 0.42 4.65 2• .69 0.10 0.09 0.02 0.02 0.29 0.21 $023 6.67 3.64 113.54 55.66 91.48 69.40 0.29 0.15 1.61 0• .85 0.03 0.02 0.00 0.00 0.10 0.06 $024 10.61 7.07 145.46 58.77 81.21 70.85 0.32 0.20_2.20 1.47 0.06 0.04 0.01 0.01 0.16 0.10 5025 13.37 8.87 142.40 53.55 109.83 100.89 0.44 0.25 2.73 1.59 0.06 0.05 0.01 0.01 0.17 0.12 $026 8.94 5.26 239.02 115.95 102.30 74.68 0.40 0.23 2.54 1.72 0.06 0.03 0.01 0.01 0.20 0.10 x027 17.17 11.42 188.86 72.03 141.39 130.96 0.58 0.33 3.57 2.05 0.08 0.07 0.02 0.01 0.23 0.16 $028 9.80 6.00 130.44 57.59 98.14 77.70 0.31 0.18 1.98 1.23 0.04 0.03 0.01 0.01 0.13 0.08 $029 2.88 1.57 90.55 45.00 38.64 26.57 0.15 0.08 0.88 0.60 0.02 0.01 0.00 0.00 0.07 0.03 $030 21.98 14.70 221.72 83.54 166.10 151.88 0.60 0.37 4.27 2.63 0.10 0.08 0.02 0.01 0.27 0.20 s031 10.55 5.86 112.11 53.64 133.92 101.72 0.35 0.19 2.01 1.03 0.03 0.02 0.01 0.00 0.11 0.07 s032 1.00 0.19 86.62 47.03 30.75 16.90 0.12 0.06 0.61 0.42 0.01 0.00 0.00 0.00 0.06 0.02 s033 0.88 0.21 1,281.41 746.28 24.99 13.95 0.55 0.36 11.84 7.87 0.03 0.02 0.02 0.01 0.51 0.37 s034 21.49 12.71 349.75 170.28 248.63 202.51 0.84 0.45 5.37 2.89 0.08 0.06 0.02 0.01 0.30 0.21 s035 5.79 3.36 125.88 59.73 78.06 64.76 0.34 0.16 1.76 0.89 0.03 0.02 0.01 0.00 0.11 0.07 s036 20.50 14.20 192.82 63.79 134.01 130.66 0.55 0.35 3.87 2.47 0.10 0.08 0.02 0.01 0.26 0.19 s037 3.98 2.05 127.03 64.76 59.16 39.81 0.21 0.11 1.23 0.81 0.03 0.01 0.00 0.00 0.10 0.04 $038 19.45 13.05 214.32 80.08 161.71 155.57 0.72 0.39 4.26 2.31 0.10 0.08 0.02 0.01 0.26 0.19 s039 10.87 6.81 169.39 74.83 103.21 82.57 0.36 0.21 2.35 1.52 0.06 0.04 0.01 0.01 0.17 0.10 s040 24.16 15.75 314.72 132.33 198.57 170.00 0.70 0.44 4.85 3.14 0.11 0.09 0.02 0.01 0.33 0.22 $041 12.52 7.93 134.84 55.43 118.11 101.55 0.40 0.23 2.50 1.41 0.05 0.04 0.01 0.01 0.15 0.11 $042 14.90 8.45 177.12 83.62 196.00 160.42 0.84 0.31 3.45 1.53_ 0.05 0.04 0.01 0.01 0.17 0.13 s043 8.50 5.67 111.45 44.28 65.68 58.12 0.26 0.16 1.76 1.15 0.05 0.03 0.01 0.01 0.13 0.08 5044 8.17 3.99 132.59 66.26 174.19 155.24 0.79 0.28 3.18 0.71 0.03 0.02 0.01 0.01 0.11 0.10 s045 9.65 5.99 144.27 64.53 93.42 74.60 0.31 0.19 2.07 1.32 0.05 0.03 0.01 0.01 0.14 0.09 5046 7.29 4.78 112.88 47.76 59.40 50.28 0.23 0.14 1.57 1.06 0.04 0.03 0.01 0.00 0.12 0.07 5047 12.24 7.75 180.23 79.82 116.26 98.82 0.43 0.25 2.75 1.63 0.06 0.04 0.01 0.01 0.18 0.12 *Based on event mean concentrations(emc's)applied to the average annual runoff volume for each subshed obtained from a 50 year SWMM run(1950-1999) D-11 Westchester County Storm water Management Planning Manual Appendix D—Croton Watershed Pollutant Loading Screening Model Figure 7—Estimated Annual Runoff Volume 015 036 01 0424A 030 .A 014 02 025 022 CO2 1 047 � 1 010 03 4,1°.J 04. �a 018 011 043 �„ ��•.4)74 01• •�c 016 ii,;2:4" 03 ' illt#I6 03 004041 41• 020 CO 0111111111211171111111401111J1110, 27 J 033 039 Annual Runoff Volumes(inches) 005 3-5 5-7 7-10 10-15 >15 D-12 References Center for Watershed Protection, New York State Stormwater Management Design Manual,October 2001 New York City Department of Environmental Protection New York State Department of Environmental Conservation Prince George's County Department of Environmental Resources&Maryland Department of natural Resources, Low Impact Development,Prince George's County, MD,undated. Rosgen,D. 1996. Applied River Morphology. Wildland Hydrology,Pagosa Springs, CO. Schueler,T., Environmental Land Planning Series: Site Planning for Urban Stream Protection,for Metropolitan Washington Council of Governments,Washington D.C., December 1995. Pub. No.95708. Schueler,T., Controlling Urban Runoff-a Practical Manual for Planning and Designing Urban Best Management Practices. Metropolitan Washington Council of Government, DC. 202 pp.,1987. Smullen J.T.,Shallcross A.I.,Kelly A.C., Updating the U.S. Nationwide Urban Runoff Quality Data Base,Water Science Technology,Vol. 39,No. 12, 1999,Elsevier Science Ltd,Great Britain, pp. 9-16. United States Department of Agriculture Soil Conservation Service. 1974.Soil Survey Westchester and Putnam County, NY. United States Department of Commerce. National Oceanic and Atmospheric Administration. 1987. The National Pollutant Discharge Inventory. United States Department of Environmental Protection. 1982. "Chesapeake Bay Program Technical Studies: A Synthesis". White,K. and Sloto,R. 1990. "Base-Flow-Frequency Characteristics of Selected Pennsylvania Streams. Unites States Geological Survey". WRIR-90-4161 Westchester County Website http://giswww.westchestergov.com/ US Department of Commerce National Oceanic and Atmospheric Administration Website http://www.noaa.gov/ USGS Department of Water Website http://waterdata.usgs.gov