Green infrastructure is increasingly being considered for application in urban stormwater management designs. Many municipalities, regulatory agencies, and advocacy groups promote use of low impact development (LID) to reduce runoff and increase infiltration. To show LID benefits, engineers must quantify advantages of green versus traditional grey infrastructure. To assist engineers in quantifying benefits of green infrastructure, the United States Environmental Protection Agency updated its Storm Water Management Model (SWMM 5) with explicit LID controls in 2009. SWMM now simulates five LID devices: bio-retention cells, infiltration trenches, porous pavements, rain barrels, and vegetated swales.
There is limited documentation regarding modeling techniques using the LID controls, although the 2009 SWMM Applications Manual discusses modeling LID using SWMM features available before the LID controls were added. This paper evaluates different SWMM modeling approaches using SWMM LID controls. No field measurements were performed; the discussion is limited to evaluation of the SWMM LID modeling controls.
An example from the Applications Manual was chosen as the base case for this study. The specific example was chosen because it contains multiple sub-catchments, each five acres or less. The example was modified from a developed condition without green infrastructure to include LID devices. Five sub-catchments were altered to include LID controls. A sixth sub-catchment was left as a control sub-catchment. A seventh fully pervious sub-catchment was also left as-is. Several simulations were conducted to assess effectiveness of the SWMM LID controls.
Each LID device was modeled as its own sub-catchment and then as part of the existing sub-catchment. Simulation results were inconsistent. In several simulations, the LID control increased peak and total runoff. Perplexingly, only one parameter affected results according to sensitivity analyses. Simply routing impervious areas to pervious areas before sending runoff to an outlet sufficiently modeled the effect of green infrastructure.