An Evaluation of Modeling Green Infrastructure Using LID Controls
Green infrastructure is increasingly being considered for application in urban stormwater management designs. Many municipalities, regulatory agencies and advocacy groups promote the use of low impact development (LID) to reduce runoff and increase infiltration. To show LID benefits, engineers must quantify the advantages of green versus traditional grey infrastructure.
The United States Environmental Protection Agency (USEPA) updated its storm water management model (SWMM5) with explicit LID controls in 2009 to assist engineers in quantifying the benefits of green infrastructure. SWMM5 now simulates five LID devices: bio-retention cells, infiltration trenches, porous pavements, rain barrels and vegetated swales.
Limited documentation is available regarding modeling techniques using the LID controls, although the SWMM Applications Manual (Gironas et al., 2009) discusses modeling LID using SWMM5 features available before the LID controls were added. This study represents an initial evaluation of different SWMM5 modeling approaches using SWMM5 LID controls. No field measurements were performed. The discussion is limited to a modeling evaluation of the SWMM5 LID controls.
An example from the SWMM Applications Manual was chosen as the base case for this study. The specific example was chosen because it contains multiple sub catchments, each approximately five acres (2.03 ha) or less.
The example was modified from a developed condition without green infrastructure to include LID devices. Five subcatchments were altered to include LID controls. A sixth subcatchment was left as a control subcatchment. A seventh fully pervious subcatchment was also left as is. Several simulations were conducted to assess the effectiveness of the SWMM5 LID controls.
Each LID device was modeled as its own subcatchment and then as part of the existing subcatchment. Simulation results were inconsistent. In several simulations, the LID control increased peak and total runoff. Also, a simple sensitivity analysis indicated that only one parameter affected results. A third approach of simply routing impervious areas to pervious areas before sending runoff to an outlet was also modeled and produced results that showed a decrease in peak runoff values which indicate that the approach may sufficiently model the effect of green infrastructure.
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