Managing Wet Weather Flow to Reduce Basement Flooding and Stormwater Runoff Pollution

The City of Toronto has embarked on a program to reduce basement and surface flooding in 32 chronic areas in the City. The reduction of basement flooding was one of 12 objectives in City’s Wet Weather Flow Master Plan (WWFMP) in 2003. In addressing the basement flooding issue, the City has also considered to improve the stormwater runoff quality in the same study by treating the runoff prior to discharge to receiving bodies of water via the various outfalls. Both basement and surface flooding and stormwater runoff quality improvements were considered in the recently completed study for Basement Flooding Areas 13, 15 and 22, of which the outfalls from the first two study areas discharge to Black Creek, a tributary of Humber River East, and the latter study area discharges to Don River East.

In the analysis undertaken, basement flooding control was considered for major storm events up to the 100-year design storm event while stormwater runoff quality control was considered for the frequent storm events. Although flows at both ends of the scale were considered in the study, it was found that sewer and surface water levels flooding control measures still provided benefits in the reduction of pollutants during frequent storm events. However, to improve the stormwater runoff quality for the frequent storm events, a hierarchy of control measures beginning with source control, then if required to achieve the water quality objectives, progressing to conveyance control, then end-of-pipe controls were evaluated in accordance with those established in the WWFMP.

To evaluate the impact of all remedial measures on the quantity of the stormwater, the InfoWorks CS computer model parameters were adjusted to reflect the effect of each measure. Subsequently the model was applied using the 1991 average year rainfall from April to October to simulate flows for the control of water quality. To evaluate the impact on the stormwater quality, the Event Mean Concentration (EMC) approach developed in the WWFMP was applied. The reduction of total loadings of four pollutants due to the application of the remedial measures was estimated.