Shades of Green: Using SWMM LID Controls to Simulate Green Infrastructure
The application of green infrastructure in urban storm water management planning continues to expand. Many municipalities, regulatory agencies, and advocacy groups encourage use of low impact development (LID) principles to limit runoff and increase infiltration. For LID to be a viable storm water management strategy, engineers must be able to quantify the benefits and limitations of green infrastructure and how it is successfully and optimally integrated with grey infrastructure. In 2009, the United States Environmental Protection Agency updated its Storm Water Management Model (SWMM) with explicit LID Controls capable of simulating five LID devices: bio-retention cells, infiltration trenches, porous pavements, rain barrels, and vegetated swales.
A 2010 examination (McCutcheon, et al., 2012) of SWMM’s LID Controls (version 5.0.21) showed no significant advantage to using the green infrastructure modeling tools instead of routing runoff to an impervious area with parameters mimicking green infrastructure. The 2010 evaluation identified needed corrections for one control, which has since been implemented (SWMM 5.0.22).
This study evaluates the LID Controls in SWMM 5.0.22. This evaluation compares measured field data to SWMM output. Field data from a study conducted by United States Geological Survey and University of Wisconsin in Madison, Wisconsin was used for comparison (Selbig and Balster, 2010). The field study measured and evaluated performance of turf-grass and prairie-vegetated rain gardens in clay and sand soils. SWMM LID Controls were used to attempt to reproduce results of the Madison study. The field data were compared against simulated results for a bio-retention cell. Regardless of the LID Control selected for simulation, the model parameters for soil properties, storage volumes, surface characteristics, and underdrains are essentially the same among green infrastructure devices. Although exact matches between model output and field data were not achieved, the modeling results replicate the reported performance data within the ranges of uncertainty for the field measurements.
While this study only examined one of the five SWMM LID Controls, its results are still significant. Findings from this study can be summarized as follows:
- The bio-retention cell LID Control reasonably mimics measured performance over a single event, as well as a long-term simulation period.
-SWMM’s LID Controls can be calibrated to measured performance and used with confidence to evaluate green infrastructure.
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