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WMOST v3 Case Study: Cabin John Creek, Maryland
Citation:
Detenbeck, N., T. Stagnitta, J. White, S. McKenrick, A. Le, A. Brown, A. Piscopo, AND M. Ten Brink. WMOST v3 Case Study: Cabin John Creek, Maryland. U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-19/185, 2019.
Impact/Purpose:
This case study provides an example application of EPA ORD’s Watershed Management Optimization Support Tool (WMOST) v3.01 to Cabin John Creek, a tributary to the Potomac River Basin within the Chesapeake Bay. Cabin John Creek (CJC) is a highly urbanized watershed with requirements to meet both suspended sediment Total Maximum Daily Load targets to protect the biological communities within CJC and nitrogen and phosphorus loading targets to protect the biological integrity of the downstream Chesapeake Bay. The U.S. Environmental Protection Agency (US EPA) established the Chesapeake Bay (CB) TMDL to restore conditions in the Bay as well as upstream waters by 2025 (US EPA 2010). The TMDL identifies the necessary pollution reductions from major sources of total nitrogen (TN), total phosphorus (TP) and total suspended sediment (TSS) across the Bay jurisdictions and sets pollution limits necessary to meet water quality standards. Many of the planned reductions in TN and TP in the TMDL Phase 1 Watershed Implementation Plans (WIPS) were achieved through point source controls on discharges from wastewater treatment plants (WWTP) and, to a lesser extent, through agricultural best management practices (BMPs). Planned reductions in the Phase II (post-2014) and Phase III WIPs rely much more heavily on reduction of urban runoff through stormwater control measures, requiring an assessment of the most cost-effective practices to meet these reductions. The results of this analysis demonstrated that the most cost-effective strategies to meet the CJC sediment loading targets would exceed load reductions required for the Chesapeake Bay TMDL. Meeting sediment load reduction targets will require greater implementation of green infrastructure stormwater control measures, particularly those with enhanced infiltration. Modelling results show limited effectiveness for gray infrastructure (extended detention basins) in meeting water quality goals.
Description:
A case study application of EPA’s Watershed Management Optimization Support Tool (WMOST v3) was carried out to inform management options for the heavily urbanized watershed of Cabin John Creek (CJC), MD, a tributary to the Potomac River, for the 10-year period 2014 - 2025. CJC was chosen as a representative case study of a watershed required to meet loading targets for both the Chesapeake Bay Total Maximum Daily Load (TMDL) as well as a Maryland nontidal stream TMDL for total suspended solids (TSS), but without water supply constraints. WMOST is an application designed to facilitate cost-effective integrated water resource management at the scale of 12- to 10-digit Hydrologic Units (HUC12 – HUC10; 10,000 – 250,000 acres). Optimizations were performed to meet the single least-cost objective, while meeting one of the constraints imposed by total nitrogen and total phosphorus load reduction targets (4% TN, 5% TP) associated with the Chesapeake Bay TMDL or the TSS loading reduction targets (21%) for a CJC non-tidal sediment TMDL, implemented either as a TSS load constraint or as an associated flow target. Alternative approaches to deriving TSS loading targets were explored: reduction of 1) maximum modelled daily load for a given year, 2) confidence limits for maximum modelled daily load based on historic temporal variability, 3) peak flow targets based on simulation of pre-development conditions (1-year 24-hour event), and 4) peak flow targets associated with predicted TSS load based on sediment rating curves. Decision variables included the type and implementation level of upland management (seven structural stormwater control measures (SCMs), nonstructural urban tree canopy planting), riparian buffer restoration), instream measures (outfall enhancement, stream restoration), and programmatic measures (street sweeping). Baseline unit runoff, recharge, and loading time series were derived from the Beta 3 version of the Phase 6 Chesapeake Bay Watershed Model, adjusted to distribute loads between two hydrologic soil groups (A + B, C + D) for all developed hydrologic response units. Costs included annualized capital costs for design and construction of SCMs or other BMPs (based on actual cost data from Montgomery County, MD, the city of Rockville, and the MD State Highway Administration) plus operation and maintenance costs based on default values from EPA’s System for Urban Stormwater Treatment and Analysis IntegratioN (SUSTAIN) tool. Robustness of solutions was tested by comparing results for a year with above-average annual precipitation (2003) with results for 2014, a year with 100-year average annual precipitation but one large event.Peak flow targets based on application of sediment rating curves (499 cfs in 2014, 535 cfs in 2003) were also not achievable. The minimum achievable targets based on incremental scenarios of steadily decreasing peak flow targets were 700 cfs for the 2014 weather regime and 525 cfs for the 2003 weather regime. These flow targets required implementation of a combination of sand filters, infiltration basins, and porous pavement for both 2003 and 2014 weather regimes. The least-cost solutions chosen to achieve a 21% reduction in annual and maximum daily TSS loads included implementation of infiltration basins, dry pond to wet pond conversions, and sand filters for both 2003 and 2014 weather regimes. Implementation of least-cost solutions to meet TSS loading targets would yield load reductions for total N (17.4 – 17.9%) and total P (16.6 – 19.9%) that far exceed load targets for those nutrients (5% TN, 4% TP). Meeting sediment load reduction targets will require greater implementation of green infrastructure stormwater control measures, particularly those with enhanced infiltration.
URLs/Downloads:
WMOST_V3_CASESTUDY_CABINJOHNCREEK_ 508_FINAL3.PDF (PDF, NA pp, 14342.881 KB, about PDF)Appendix B
Appendix E1
Appendix E2
Appendix F
Appendix G
Appendix H
Appendix I
APPENDIX J1
APPENDIX J2
APPENDIX J3
APPENDIX J4