Technical Memorandum Stream Erosion Methodology Table of Contents 1 Introduction ..................................................................................................................................................1 2 Purpose .........................................................................................................................................................1 3 Technical Approach .....................................................................................................................................1 4 Results and Discussion ............................................................................................................................... 8 References ...................................................................................................................................................... 9 Analysis Attachments ................................................................................................................................................. 10 Comprehensive Baseline Comprehensive – , 2014 , December 22 December Consolidated TMDL Implementation Plan Implementation TMDL Consolidated Technical Memorandum: Stream Erosion Methodology List of Tables Table 1: Measured Rate of In-stream Erosion in District Streams ................................................................. 3 Table 2: WTM In-Stream Erosion Relationships ............................................................................................ 4 Table 3: Comparison of In-Stream Erosion Load Calculation Methods for Hickey Run .............................. 6 Table 4: Proposed Scaling Factors .................................................................................................................... 6 Table 5: Comparison of in-stream erosion loads ............................................................................................. 7 List of Figures Figure 1: Empirical equation established by MDE to correlate percent watershed impervious and percent stream bank erosion .......................................................................................................................................... 5 Figure 2: SBE (as a percent of total TSS load) as a Function of Imperviousness and Stream Degradation Potential ............................................................................................................................................................. 7 ii | Page Technical Memorandum: Stream Erosion Methodology 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’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 the methodology for estimating pollutant loads associated with in- stream erosion 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 There are two primary generators of pollutant loads applicable to the District’s MS4 area: runoff loads associated with the build-up and wash-off of pollutants from the surrounding watershed, and in-stream loads associated with erosion of native bed and bank material and accumulated legacy sediments. Runoff loads are discussed further in separate technical memoranda. In-stream erosion loads are the topic of this Technical Memorandum. The purpose of this Technical Memorandum is to document the selection of the method used to calculate in-stream erosion loads. The Technical Approach employed includes: • A review of accounting for and calculating in-stream erosion loads in District TMDL development. • A literature review of in-stream erosion load calculation methods. • An evaluation of in-stream erosion load calculation methods and the selection of a method to apply in the IP Modeling Tool. • A review of applicable sediment delivery ratios. The Results section of this Technical Memorandum presents the selected in-stream erosion load calculation method and provides commentary on the rationale for the method’s selection and use in the IP. 3 Technical Approach 3.1 Review of In-Stream Erosion in the District TMDLs An evaluation of documentation from the various District TMDLs was conducted to determine if and how in-stream erosion was calculated in the development of each TMDL, and whether the in-stream erosion load is considered a point source or a non-point source. This evaluation was used to determine whether and how in-stream erosion should be accounted for in the baseline pollutant load modeling required to support development of the Consolidated TMDL IP. A full review of how in-stream erosion is handled in each of the TMDLs is presented in Attachment A. There are five TSS TMDLs for the District, and each addresses in-stream erosion to some extent. These include: 1 | Page Technical Memorandum: Stream Erosion Methodology 1. 2002 TMDL for TSS for the Upper and Lower Anacostia – This TMDL implicitly accounts for in- stream erosion by applying a high TSS EMC value for “open channel” tributaries (i.e., tributaries that do not have piped sections – which include Watts Branch, Popes Branch, Fort Dupont, and Nash Run) that are thought to have significant in-stream erosion. This high TSS EMC represents TSS contributions from in-stream erosion as well as from land-based sources. All stormwater loads, including loads from in-stream erosion, are considered a non-point source and are accounted for under the Load Allocation (LA). 2. 2003 TMDL for Total Suspended Solids in Watts Branch – This TMDL back-calculates the individual TSS contribution from land sources and in-stream erosion using the total Watts Branch TSS load estimated in the 2002 Anacostia TMDL. The contribution of TSS from in- stream erosion was estimated to be 52 tons/yr. The loads from in-stream erosion are considered a non-point source and are accounted for under the LA. 3. 2007 TMDL of Sediment/Total Suspended Solids for the Anacostia River Basin – This TMDL calculates in-stream erosion explicitly, but only for Watts Branch within the District (in-stream erosion is calculated for other water bodies in Maryland). The contribution of TSS from in- stream erosion in Watts Branch was estimated to be 67 tons/yr. The TMDL does not calculate in- stream erosion for other DC tributaries. The Watts Branch in-stream erosion load is considered a point source and is accounted for under the MS4 WLA. 4. 2010 Chesapeake Bay TSS TMDL – The Bay TMDL documentation implies that in-stream erosion is implicitly accounted for through model calibration. The documentation also implies that loads from in-stream erosion are considered a point source and are accounted for under the MS4 WLA. The inconsistency in accounting for in-stream erosion in the TMDLs is partly due to the fact that EPA, over time, has evolved a policy of specifying that in-stream sediment loads in urban areas are to be assigned to the MS4. This evolving policy in turn affected where it is accounted for in the TMDL IP baseline load modeling. Several factors ultimately informed the decision to include in-stream erosion in the MS4 baseline load modeling for addressing the Chesapeake Bay TMDL, and in the direct drainage baseline modeling for local TMDLs. These factors include the following: • The Chesapeake Bay Program (CBP) gives credit towards MS4 WLAs for stream restoration. A significant component of the credit accounting for stream restoration relates to the control of in- stream erosion (CWP/CSN 2014). This implies that the CBP links in-stream erosion at least in part to MS4 flows. Therefore, in-stream erosion will be included as part of the MS4 baseline load when accounting for the loads for the Chesapeake Bay TMDL. • The local TMDLs are inconsistent in allocating the loads from in-stream erosion to the MS4 or direct drainage areas. In addition, the local TMDLs do not calculate in-stream erosion for all DC tributaries even though all tributaries are known to have varying degrees of in-stream erosion. Because of these inconsistencies, in-stream erosion will be included as part of the direct drainage baseline load, until a future time when the TMDLs can revisit the issue of in-stream erosion. 3.2 Review of In-Stream Erosion Load Calculation Method The mechanisms of in-stream erosion are complex and often very location-specific. Stream erosion is dependent upon a number of variables, including extent and composition of development within the stream drainage area, channel geomorphology and geometry, presence and orientation of piped drainage, number of outfalls, and condition of riparian vegetation. In addition, eroded soils can be deposited at downstream “sinks”