Stormwater Low Impact Development (LID) controls are sustainable development practices designed to capture surface runoff using some combination of detention, infiltration, and evapotranspiration. Recently, stormwater and wastewater utilities have started to embrace the use of LID controls as a viable means of managing stormwater runoff from new and existing development. However, the benefit that this type of sustainable infrastructure can play in the reduction of combined sewer overflow (CSO) and sanitary sewer overflow (SSO) discharges is not well understood and is often overlooked. It appears that using green infrastructure, it should be possible to eliminate overflows resulting from the small and more frequent rainfall events. It is being postulated now that green Infrastructure can provide sustainable sewer overflow control, as it can be designed to remove stormwater runoff from over 90% of storm events from entering the sewer collection system. LID measures should also be effective at extenuating runoff so peak flow rates are lowered, which should reduce the occurrence of rainfall induced CSO and SSO events. Some LID techniques are also known to improve stormwater quality.
In September 2010, U.S. Environmental Protection Agency (EPA) released a new version (5.0.021) of Storm Water Management Model Version 5 (SWMM5) that offers LID modeling capability for the first time. Five types of LIDs can be modeled: bio-retention cells (rain gardens), infiltration trenches, porous pavement, cisterns (rain barrels), and vegetative swales. Using this new software release and real world examples, this paper will present SWMM5’s LID features, input data requirements, modeling procedures, and output results for quantifying the LID impacts on sewer overflows. The modeling results should help us quantify the LID impacts on sewer overflows and allow the sustainable developers answer questions like how much rainfall can be captured in a typical design year using a certain type of LID, or how many rain gardens are needed in a sewershed to capture a certain amount of rainfall.