Abstract:The transportation of pollutants from impervious surfaces during runoff events to receiving water bodies is a serious environmental problem. Summer runoff is also heated by impervious surfaces, causing thermal enrichment in receiving water body systems and degradation of coldwater aquatic ecosystems. End-of-pipe stormwater management facilities that are open to the environment can result in further elevated temperatures due to exposure to solar radiation. Receiving water systems that provide coldwater habitat require cool water temperatures to sustain healthy conditions for cold water flora and fauna (e.g., trout, dace). Underground Stormwater Detention Chambers (USDC) are a technology for the detention and treatment of stormwater runoff that can potentially solve the thermal issues associated with sun-exposed detention facilities while still providing an equivalent level of treatment services for stormwater pollutants. A field study of an USDC located in Southern Ontario was undertaken to characterize its treatment performance and effect on water temperature. The results were: the USDC was found to provide similar levels of stormwater treatment as wet detention ponds. On average, outlet maximum temperatures were 5˝C cooler than inlet maximum temperatures, and outlet water temperatures remained within the thermal regime for coldwater fish habitat throughout the evaluation period. There was little to no stratification of temperature, nor dissolved solids, but stratification of dissolved oxygen was observed mid-winter and into the spring.

In Ontario, underground stormwater detention chamber systems (USDC) are a relatively new alternative to wet detention ponds for the detention and treatment of urban stormwater runoff. Few tools are available for designers to predict the removal of contaminants from USDC. A conceptual model was developed to fill this void and make USDC a more approachable solution to stormwater management.The Model for Underground Detention Sedimentation (MUDS) predicts the removal efficiency of total suspended solids through sedimentation based on the hydraulic properties of USDC. As runoff enters the USDC waves of particles are released, the pathline of these particles are tracked along vertical and longitudinal axes as they move through the USDC to determine whether a given diameter of particle is removed by sedimentation. Based on the particle size distribution and the density of the particles, the mass removal efficiency is determined. The model was found to adequately predict the hydraulics and treatment efficiency of USDC.