Modeling Retrofitted Extended-Detention Wet Ponds and Wetland Pockets
The restoration of stressed watercourses in urbanized watersheds can best be achieved by taking advantage of opportunities such as the retrofitting or improvement of existing best management practices (BMPs). Within the Sixteen Mile Creek and Emery Creek Watersheds, two existing on-line stormwater management (SWM) ponds were recently retrofitted for water quality enhancement: the Ninth Line SWM facility (Mississauga) and the Pine Valley SWM pond (Vaughan). These ponds were originally constructed in the 1980's for flood control, but without the pollutant removal capabilities of today's extended-detention wet pond and wetland/wet pocket systems.
Each component of these retrofit BMPs was designed to employ a variety of pollutant removal pathways to increase the efficiency of sediment removal and improve the quality of water discharging downstream. Typical pollutant removal pathways, in order of dominance, are sedimentation, adsorption by vegetation, physical filtration, and nutrient uptake by the wet pond and wetland/wet pocket compartments plants and algae. The original flood control and riparian storage functions were preserved in each retrofit design.
In order to quantify the sediment removal capabilities of each retrofit, the QUALHYMO and STORM computer programs were used with water budget models to design the forebay, wet pond, wet pocket, and extended-detention storage features. Annual sediment loadings and evaporation characteristics, during periods of minimal precipitation, were then used to develop a maintenance schedule for the new retrofit features.
A dynamic-wave routing analysis was also necessary for the Ninth Line SWM facility. This analysis used the OTTHYMO, QUALHYMO and BOSS-DAMBRK computer programs and confirmed that the relatively flat gradient of the receiving watercourse and two undersized culverts downstream will not cause a backwater effect significant enough to submerge the new outlet structure during the water quality design storm. This analysis also determined the realtime drawdown operation of the on-line facility, for both continuous and less frequent event storm simulations, as well as the submergence problems affecting the original flood control outlet structure.
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