Dual drainage modelling of the Cooksville Creek watershed, Ontario

Under different storm conditions urban drainage systems may undergo various flow regimes including backwater, surcharging, reverse flow and surface ponding. Evaluation of the performance of a stormwater management system under these different storm conditions requires detailed hydraulic-grade line analysis of both the minor and major drainage systems.

Computational Hydraulics International, the Credit Valley Conservation Authority and the City of Mississauga have worked in collaboration to develop a high resolution hydrologic/hydraulic dual drainage model for the highly urbanized and flood vulnerable Cooksville Creek watershed, using PCSWMM/SWMM5. The objectives of this modelling project were to determine hydraulic grade lines and capacities of existing storm sewers and overland flow routes, as well as to identify areas at risk of flooding. The completed model consists of more than 4000 subcatchments covering the 33 km2 area and 8000 conduits representing more than 500 km of drainage network including storm sewer pipes, major system flow paths, ditches and the creek itself.

This paper presents the framework of the modelling project and includes discussions on QA/QC procedures, high resolution subcatchment discretization, automated major system development, calibration, validation and historical event analysis. One of the outcomes of this model was to identify specific neighbourhoods for further detailed studies where system deficiencies and alternative solutions can be evaluated. Considering the drainage issues (surface and basement flooding) experienced in the watershed due to recent large storm events, the next phase of this study is to use the dual drainage model in combination with a sanitary sewer model to evaluate basement flooding potential and propose mitigation strategies.