Rehabilitating Stormwater Management Facilities in Twickenham Park

The City of Richmond Hill will conduct a Schedule B Class Environmental Assessment to look at downsizing an aging system of four contiguous online stormwater management facilities (SWMFs) and bringing the design of the remaining facilities in line with current municipal and provincial criteria for water quality, water quantity and erosion control. The four SWMFs are located on the Upper East Don River and accept drainage from a 106-hectare catchment area. Initially, the city had identified only one SWMF as being in need of repairs, but it concluded that it would be more beneficial to take a systems approach and include three other SWMFs located just downstream in the study. The current design of these facilities provides 80% of total suspended solids (TSS) removal for only approximately 12 hectares of the total 42 hectares of development. The intent of this feasibility study is to examine options to provide 80% TSS removal for all of the developed portions of the study area by considering different conveyance and end-of-pipe controls. The study will look at redesigning the remaining facilities to detain 25 mm rainfall events for a minimum of 48 hours. The slower release of this volume of water will help reduce erosion in the downstream system, which will mitigate negative impacts to fish habitat, bridges and existing infrastructure. Natural sediment transport processes will be brought back closer to their original, pre-development state with a new offline storm pond configuration and the re-establishment of a natural channel upstream that was removed in the 1980s. Natural landscaping will increase vegetation coverage, which will minimize solar radiation on the open water. This an important feature, as the discharge of cooler water will enhance habitat conditions for the endangered redside dace just downstream. The proposed study aligns with the city’s Official Plan and its Environmental Strategy. Both of these documents include specific objectives related to watershed and stormwater management, such as protecting and restoring surface and stormwater quality to ensure protection of aquatic life and ecological health. Innovative aspect(s): • By looking at interconnected SWMFs as a system rather than on an individual basis, the municipality will have a wider range of options available to meet overall project objectives and provide greater benefits at a lower cost • The municipality is proposing to connect with community stakeholders during the study and use this opportunity to discuss the benefits of implementing stormwater management measures at the residential level, including downspout disconnections, pervious surface driveways and patios, rain barrels and rain gardens Replicability: • Considering the entire stormwater management system when rehabilitating SWMFs could provide a useful methodology to other municipalities with aging stormwater management infrastructure (Project description from original funding application)