EXPANDING THE GREEN BUILD-OUT MODEL TO QUANTIFY STORMWATER REDUCTION BENEFITS IN WASHINGTON, DC

The original Green Build-out Model was developed as part of a collaborative effort between LimnoTech and Casey Trees under a grant from the U.S. Environmental Protection Agency. Its intent was to estimate the stormwater management benefits of certain low impact development (LID) practices, namely additional tree cover and green roof conversion, in the Nation's capital. The Green Build-out Model was developed by adding a “green component” to the combined sewer system (CSS) and stormwater planning model used in Washington, DC to support development of the Long Term Control Plan (LTCP) among other purposes. The model integrates standard hydrology and hydraulics with detailed GIS and the storage and infiltration attributes of green infrastructure. Application of the original model across Washington, DC showed promising stormwater reductions associated with moderate and intensive use of the modeled practices. A “greening ” of the city with trees planted along streets and in parking lots and some conversion of conventional roofs to green roofs would be expected to reduce the generation of storm water runoff by 5 to 10 percent on a city-wide basis. Reductions approaching 30 percent were projected to be possible in some sewersheds. These results showed great promise and the Advisory Team for the grant recommended expansion of the Green Build-out Model to incorporate a broader suite of green infrastructure practices.The work described in this paper reports on the expansion of the Green Build-out Model to include assessment of rain barrels, roof leader disconnection to lawns (or alternately to rain gardens), sidewalk bioretention planters, curb bump out bioretention planters, and permeable pavement. The methods of adapting a traditional hydrology model to mimic green infrastructure are described. The results show the potential for runoff reduction on a city-wide, watershed and sewershed basis; potential reductions in peak flow velocity related to stream bank erosion; and comparison of the opportunity for and effectiveness of the LID and green infrastructure practices applied.The results of this study have implications beyond the specific scenarios being modeled, particularly for urban areas in the northeastern U.S with similar rainfall patterns to Washington, DC. Implementation of total maximum daily loads (TMDLs) with quantified wasteload allocations for CSS and municipal separate storm sewer system (MS4) sources will require innovation and retrofitting of stormwater controls in already developed areas. Application of models such as the Green Build-out Model can provide a quantitative framework for the accounting of planned and realized load reductions that are required to meet TMDL implementation requirements.