City of Toronto Wet Weather Flow Management Master Plan

Abstract

Introduction Development and urban growth within the City of Toronto and surrounding regions has resulted in very intense pressures on the ecosystem, and the alteration of the hydrologic cycle and natural environment. Urban development has two major adverse effects on wet weather flows; increased stormwater runoff quantity and degradation of stormwater runoff quality. The increase in impervious area, changes in surface grading and high surface runoff associated with urban development all act to reduce stormwater infiltration into the ground and increase the volume and peak flow rates of stormwater runoff to receiving waters. Wet weather flow also results in combined sewer overflows (CSOs), storm sewer discharges, and infiltration and inflow (I/I) into the sanitary sewer system causing treatment plant by-passes all of which contribute to degraded water quality in area watercourses and the waterfront. The consequences of these negative impacts include increased flooding and erosion, physical destruction of terrestrial and aquatic habitat, reduced stream base flow, postings of recreational beaches, nutrient enrichment, contaminated sediments, stressed aquatic communities and degradation of the overall environment. More recently the impacts, summarized through the Metropolitan Toronto and Region Remedial Action Plan Stage 1: Environmental Conditions and Problem Definition report [1], have contributed to Toronto’s designation as one of 42 polluted areas of concern within the Great Lakes basin. In the absence of a legislative requirement for wet weather flow management for existing urban areas, previous wet weather flow control initiatives were driven in large part by the need to address local flooding problems and impacts on recreational beach areas. Although source control options have been considered and implemented to varying degrees, to date, the problems have largely been addressed through the construction of infrastructure and end-of-pipe treatment facilities. Although these initiatives represented significant efforts and provided local environmental improvements, it was recognised that a watershed based strategy was required to provide a more comprehensive and consistent approach to mitigating the impacts of wet weather flows.