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Model Selection and Justification Technical Memorandum

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Technical Memorandum Model Selection and Justification Table of Contents 1. Introduction ............................................................................................................................................1 2. Purpose ....................................................................................................................................................1 3. Technical Approach................................................................................................................................ 2 4. Results and Discussion .........................................................................................................................15 Analysis References .................................................................................................................................................. …17 Attachments .................................................................................................................................................. 19 Comprehensive Baseline Comprehensive – , 2014 , December 22 December Consolidated TMDL Implementation Plan Implementation TMDL Consolidated Technical Memorandum: Model Selection and Justification List of Tables Table 1: Modeling Approach used in Mainstem Waterbodies for MS4 Areas ............................................... 4 Table 2: Modeling Approach used in Tributary Waterbodies for MS4 Areas ................................................ 4 Table 3: Modeling Approach used in Other Waterbodies for MS4 Areas ...................................................... 5 Table 4: Results of Statistical Analysis of Precipitation Data at DCA, 1948-2013 ....................................... 12 Table 5: Typical Precipitation Depths (inches) at DCA ................................................................................. 12 Table 7: Reference Runoff Coefficients from Schueler (1987) ...................................................................... 13 List of Figures Figure 1: Main Categories of District TMDL Waterbodies .............................................................................. 3 Figure 2: Model Framework Diagram .............................................................................................................. 9 Figure 3: DCA Annual Rainfall ........................................................................................................................ 11 ii | Page Technical Memorandum: Model Selection and Justification 1. Introduction The District Department of Environment (DDOE) is required to develop a Consolidated Total Maximum Daily Load (TMDL) Implementation Plan (IP) as established in the District of Columbia’s (District’s) Municipal Separate Storm Sewer System (MS4) National Pollutant Discharge Elimination System (NPDES) permit (U. S. EPA 2011 and U. S. EPA 2012). The IP will define and organize a multi-year process centered on reducing pollutant loads originating within the District MS4. The level of pollutant control will be based on past TMDL studies performed to protect impaired water bodies in the District. The IP will include a summary of the regulatory compliance strategy to satisfy TMDL-related permit requirements, a summary of data and methods used to develop the IP, specific prioritized recommendations for stormwater control measures, a schedule for implementation and attainment of Waste Load Allocations (WLAs), and a method for tracking progress. Substantial public involvement will be sought in plan development. This Technical Memorandum on Model Selection and Justification is one in a series of technical memoranda that provide detailed information on research, analysis, programs and procedures that support development of the Consolidated TMDL IP. 2. Purpose Pollutant load estimation models are used extensively to develop TMDLs and to support municipal stormwater management programs. A variety of models have been used in the District for these purposes. These models use mathematical calculations to simulate rainfall generated runoff across a sewershed or watershed. Pollutant mass or load is subsequently produced by multiplying the runoff flow volume by a pollutant concentration, usually an event mean concentration (EMC). Pollution reduction is achieved by taking into consideration the effect that various best management practices (BMPs) and non-structural practices have on runoff generation or pollutant concentration. The requirement to develop a Consolidated TMDL IP for the District includes a provision to identify and apply a model to support pollutant load estimation and pollutant reduction, and to track progress in achieving WLAs. In particular, the main requirements for developing a modeling tool specify that the model will: • estimate baseline and current pollutant loads; • tabulate loads on an annual basis; • estimate pollutant load reductions achievable via various BMP implementation scenarios; and • be able to represent the daily expression of the TMDL. This Technical Memorandum documents the selection, justification, and description of the model (henceforth called the “IP Modeling Tool” or “IPMT”) that will be used to help develop the Consolidated TMDL IP. This Technical Memorandum also provides information on the IP Modeling Tool inputs and comparison to other models, and includes a Technical Approach that addresses: • review of previous modeling studies of the MS4 Area; • review of publically available modeling tools and calculators; • model needs; • model selection and justification; • model description and requirements; • model comparisons; and • model limitations. 1 | Page Technical Memorandum: Model Selection and Justification It also includes a Results and Discussion section that presents and discusses the model selection and model comparisons in the context of the Consolidated TMDL IP. 3. Technical Approach 3.1 Review of Previous Modeling Studies of the MS4 Area The District has completed 26 TMDL studies for various 303(d)-list impaired waterbodies. TMDL studies typically consist of multiple related individual TMDLs, such as TMDLs for related pollutants in a single waterbody (e.g., TMDLs for multiple metal species in a waterbody) or TMDLs for related waterbodies (e.g., a TMDL for a specific pollutant for a mainstem waterbody and its tributaries). The 26 TMDL studies vary in complexity with respect to both the modeling performed to establish loads and also in the assignment of MS4 WLAs. This section provides an overview of the modeling approaches used in the TMDL studies. It summarizes the key differences between the various TMDL models with respect to how loads are developed and describes how the MS4 WLAs are calculated and expressed for each TMDL. This section also summarizes the information used to delineate the MS4 drainage areas and describes the data and methods used to compute runoff from these areas, because this information is integral to the modeling of MS4 loads. A full explanation of all the models and model inputs that were used to develop the TMDLs is provided in Attachment A.1. 3.1.a District Waterbody Characterization for TMDL Modeling As described above, there are three major types of waterbodies in the District: mainstem waterbodies; tributary waterbodies; and other waterbodies that are connected to a mainstem but are not tributaries, such as the Tidal Basin (see Figure 1). The following sections describe each waterbody type and provide some basic information on how the waterbodies are modeled. 2 | Page Technical Memorandum: Model Selection and Justification Figure 1: Main Categories of District TMDL Waterbodies 3.1.b Mainstem TMDL Models There are 12 TMDL studies for the mainstem waterbodies in the District. Table A-1 below shows the list of TMDL studies for mainstem waterbodies and the main modeling approach used to calculate runoff from the respective MS4 drainage areas in each study. Modeling approaches for the mainstem included use of an HSPF model originally developed for Watts Branch, and the MOUSE model used for the CSO LTCP. The Watts Branch HSPF model was originally developed by ICPRB in 2000 to help provide flow inputs for other Anacostia models because Watts Branch is the only stream in the District with a long term record of stream discharge. In the Watts Branch HSPF model framework, all land areas are categorized into one of three land use types: Impervious, Urban Pervious, and Forested Pervious. For each land use type, the model predicts the daily flow volume per unit area of base flow and surface runoff (storm flow) during a simulation period. Because many of the small water and sewersheds in the District were assumed to be hydrologically similar to Watts Branch, the Watts Branch model was applied to these sub-drainage areas to calculate runoff by first categorizing the land use types in the sub-drainage areas according to land use types, and then by using the runoff calculations in the model. Several TMDLs also use the land model based on DHI’s MOUSE that was developed for the Long Term Control Plan (LTCP). The District of Columbia Water and Sewer Authority (DC Water) built a complex hydrologic and hydraulic model of the DC sewer system as part of its development of the LTCP for the combined sewer system. The MOUSE (now known as Mike Urban) software was used to develop the LTCP model. As part of the LTCP process, the separate storm sewer area was also studied to characterize the storm runoff generated throughout the city. The LTCP model was calibrated for flow at various key points within the
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