Low impact development strategies for wetland modeling

Maintaining wetland hydroperiods is vital to protecting the water balance and overall health of the ecosystem. Development activities proximal to a wetland can alter the hydroperiod causing flooding or water deficiencies, as well as having other adverse effects on the wetland. A hydrologic water balance was conducted for several wetland complexes in Southern Ontario to assess the impact of future development. Existing and proposed conditions were modeled using the US Environmental Protection Agency (EPA) Stormwater Management Model (SWMM) engine in PCSWMM. Four wetland complexes located in Southern Ontario were investigated using Toronto and Region Conservation Authority (TRCA) guidelines. This presentation will only discuss one of these complexes; the Southwest Wetland. Results for the proposed Southwest Wetland showed a decrease in average annual runoff of 14% due to development when no mitigation was implemented. Each wetland required different mitigation strategies to reduce impacts from the nearby land development. Low impact development (LID) controls were investigated as a mitigation solution. Mitigation measures are expected to mimic the natural timing, flows, and volumes of the ephemeral drainage features that will be altered during development. The Southwest Wetland model showed a large deficit in both infiltration and runoff in the post-development scenario. Since post-development conditions indicate a water budget change greater than 10%, mitigation measures will be required to increase runoff and infiltration. Model results suggest that 1.2 ha of additional rooftop area should be routed to a vegetative swale LID in the Southwest Wetland to maintain the wetland water budget and mimic existing conditions.