DEVELOPMENT OF A GREEN BUILD-OUT MODEL FOR WASHINGTON, DC: QUANTIFYING THE STORMWATER BENEFITS OF TREES AND GREEN ROOFS

The Green Build-out Model is an innovative and powerful planning tool that quantifies the cumulative stormwater management benefits of trees and green roofs across the District of Columbia. It calculates potential reductions in stormwater runoff within the municipal separate storm sewer system (MS4) and the combined sewer system (CSS) that contributes to water quality impairment in the Nation's capital. This is accomplished using a modified version of the hydrologic and hydraulic model of the District of Columbia coupled with GIS land cover data and approximations of the hydrologic processes associated with trees and green roofs.The study findings provide information by sewershed and watershed to target investments in trees and green roofs to yield the greatest return of stormwater benefits District-wide. The research also provides general hydrological relationships and modeling methodologies that are transferable to other municipalities. These findings show that trees and green roofs provide substantial overall reductions in stormwater runoff and discharge volumes in sewer systems District-wide. The greatest opportunity is at the sewershed level in the CSS area where the total reduction in discharge volume for all sewersheds was 22%. The study concluded that trees and green roofs provide stormwater controls in urban areas where options and space are limited. They show particular promise in the MS4 area where subsequent reductions in pollutant loadings could provide the District of Columbia an option to make progress toward meeting Total Maximum Daily Load (TMDL) requirements for its impaired waters.