Simulation of Low Impact Development Scenarios in SWMM

Low Impact Development (LID) is a relatively new concept, and has been increasingly popular in stormwater management. However, when it comes to engineering designs of LID, current guidance tends to rely on relatively simple approaches to evaluate LID designs (e.g. internal routing between subareas, manipulation of curve number, change of depression storage depth, etc.), and there are limited process-based tools and methods to evaluate the benefits from possible LID designs. The Stormwater Management Model (SWMM) is one of the most widely-used models in the stormwater management community, and the model has the capability of simulating traditional best management practices (BMPs) using built-in components (e.g. storage unit, outlet, weir, etc.). These features of the SWMM model provide opportunities to represent the hydrologic processes involved with LID practices.

This presentation investigates algorithms for simulating LIDs in SWMM. Simulation algorithms for three LIDs, namely the bio-retention area, porous pavement, and green roof, are proposed. The representation schemes are all based on existing components available in the SWMM model. Wherever applicable, the model representations mimic hydrologic processes in each LID, including infiltration, percolation, and ponding. The Horton Equation is used to simulate the infiltration processes in all three LIDs. With all input parameters acquired either from site conditions or from LID design requirements, the representation schemes provide a physical model that engineers may use to quantify LID scenario benefits.

To verify the SWMM representation, the modeling schemes are implemented in one watershed in Milford, Massachusetts for a ten-year continuous simulation. The predicted cumulative runoff volume reduction is compared with simulation results from the Best Management Practice Decision Support System (BMPDSS) program. The BMPDSS program was developed by Prince Georege’s County, Maryland to evaluate the LID and conventional stormwater BMPs. Comparison of simulation results indicates that the SWMM representation is approximate to the BMPDSS representation in runoff volume reduction for the three LIDs.