The District needed to investigate ways to reduce its volume of wet weather runoff to ease the burden on existing sewer infrastructure, decrease the number and volume of CSOs, reduce streambank erosion in urban streams, and improve water quality. The District wanted to consider green infrastructure practices to help achieve these goals. However, before making a significant investment, the District needed to understand the extent of the potential benefits and opportunities for implementation.
Findings from this study demonstrated the benefit of green infrastructure practices applied on a city-wide scale, and showed the value of including green infrastructure in the District’s long-term water management plans. The study results indicated that reductions in average annual CSO volume discharges could be as much as 43% or nearly 1 billion gallons, city-wide, under aggressive green build-out planning scenarios. This is significant, given the estimated $2.2 billion cost for CSO controls in the District. Reductions in MS4 discharge volumes could be as much as 26% and could also be important to TMDL implementation. Peak stormwater velocities would also be greatly reduced, which is important because of the high rates of streambank erosion observed in urban streams in the District.
The model provides an innovative and powerful planning tool for stormwater management. LimnoTech has used the analysis results to guide a three-year green infrastructure retrofit demonstration study being undertaken with a coalition of government agencies and nonprofits. The goal of this follow-on effort is to provide further proof of the stormwater management benefits of green infrastructure and to investigate the logistics required to make large-scale green investments across the District. To date, the model has formed the scientific basis for DC Water and the District to pursue green infrastructure as an integral part of their combined sewer and stormwater planning efforts.