Enhancement of the Green Build-Out Model to Quantify Stormwater Reduction Benefits in Washington, DC

The Enhanced Green Build-out Model is a planning tool that quantifies the cumulative stormwater management benefits of various green infrastructure practices intensively applied across the District of Columbia. The Enhanced Green Build-out Model calculates potential reductions in stormwater runoff within the municipal separate storm sewer system (MS4) and the combined sewer system (CSS) that contribute to water quality impairment in the Nation's capital. The green infrastructure component of the Enhanced Green Build-out Model was added to an existing hydrologic and hydraulic model of the District called Mike Urban. Mike Urban was developed and applied by the DC Water and Sewer Authority (WASA) to support development of the Long Term Control Plan for the CSS. The MS4 areas were added to the model so that all of the municipal sewer systems were included in one planning tool. In its original application, the Green Build-out Model was designed to simulate tree canopy expansion and green roof conversion and progression (Casey Trees and LimnoTech, 2007). The Enhanced Green Build-out Model includes the green roof and tree coverage from the original model and builds upon them by quantifying wet weather flow reductions for a new set of practices including rain barrels, roof leader disconnection to rain gardens, sidewalk bioretention planters, curb bump-out bioretention planters, and permeable pavement. Green infrastructure was integrated into the model by utilizing detailed GIS coverages and mimicking the hydrologic processes inherent with each green infrastructure practice. The model was run with average annual rainfall conditions. Reductions in runoff volume and CSO discharge volume and frequency were calculated for both the CSS and MS4 areas and for the Anacostia, Potomac, and Rock Creek watersheds within the District.