Background/Question/Methods Across the nation, there is increasing interest in low impact development/green infrastructure (LID/GI) techniques as a means of conserving, sustaining, or restoring the ecosystem goods and services provided by urban watersheds. In urban environments, the chemical, physical, and biological constituents of combined sewer overflows (CSOs) can significantly inhibit the ability of surface waters to support aquatic life and provide healthy recreational opportunities. CSOs occur when stormwater inflow to the collection system exceeds the sewer’s conveyance capacity, resulting in a surcharge or overflow. LID/GI strategies can reduce the frequency and volume of CSOs by mimicking pre-development hydrologic patterns through special design features incorporated into developed lots. These features promote infiltration, evapotranspiration, detention and other natural hydrologic processes with technologies such as rainwater harvesting systems, permeable pavements, and raingardens. While gaining more attention, LID/GI technologies are still relatively sparsely distributed in watersheds. As a result, no watershed scale empirical observations are available and hydrologic and hydraulic models are needed to simulate changes in watershed ecohydrology that might occur if widespread implementation of LID/GI technologies were achieved. While many different modeling techniques can be applied with reasonable effort and assumptions to small scale study areas (lots, blocks, etc), reliable modeling of LID/GI at larger scales (>10 acres) remains a challenge, and is the topic of this talk.

Results/Conclusions A two-step approach to evaluating the effectiveness of GI/LID as a CSO reduction measure in an urban watershed is presented. The first step applies the Low Impact Development Rapid Assessment (LIDRA) model to compare the cost-effectiveness of decentralized LID/GI approaches to centralized “grey” approaches that seek to increase sewer conveyance capacity. LIDRA considers life cycle cost considerations, spatial applicability constraints for various GI/LID technologies, and other relevant planning considerations. Applied to several CSO-sheds in New York City, the results indicate that under a wide range of cost and performance scenarios widespread implementation of LID/GI can reduce CSOs at costs comparable to or better than “grey” approaches. The second step uses more detailed hydrologic and hydraulic modeling of a two block study area in the Bronx, NY to demonstrate the effect that green roofs, rain gardens, and permeable pavements could have on the discharge hydrograph. The results show significant reductions in both the peak rate and total volume of discharge.