Design Standards Affect Urban Flood Risk in Urban Areas - Toronto Historical Flooding From Flood Plain to Flood Drain

Urban drainage design standards and flood plain management standards have evolved over several decades, reducing flood risks related to sewer back-up, and overland (pluvial) and riverine flooding in Ontario. The following figure illustrates how sanitary sewer systems were exposed to high inflow and infiltration risks from rooftops and foundation drains (weeping tiles), how properties were exposed to inflows from limited lot grading and overland design, or at a larger scale, from flood plain surface flooding risks.

New subdivisions built post 1980's, have separated sanitary sewer systems with lower exposure to inflows and infiltration that surcharge older systems, major/minor drainage system design to keep overland flow on right of ways, away from buildings, or safely conveyed to valleys along easements.

Flood risks exist across a range of spatial scales from large river flood plain to local infrastructure floor drain. Spatial analysis of overland flow paths has correlated the risk of surface flooding along flow paths and their spread with observed basement back-up flooding. The map below illustrates how overland flow paths extend beyond regulated valley limits onto development areas, and shows how clusters of sanitary sewer back-up often lie within the overland flow spread.

Mapping and analysis above had been completed by CityFloodMap.Com. A detailed assessment of the analysis is available here.