PCSWMM 2-D Modelling for the Don Mills Channel Flood Control Facility and Culvert Upgrades

This paper documents a two-dimensional (2D) PCSWMM hydrodynamic modelling program supporting the Torbay Road Flood Control Facility (FCF) and staged culvert upgrades along the Don Mills Channel, Markham, Ontario. Objectives were to confirm storage required for a 5-year level of service (LOS), validate passive FCF operation governed by the Steelcase Road East constriction, and quantify system response to sequential culvert replacements at Steelcase Road West and Denison Street.

The EA-stage model was rebuilt with 2020 LiDAR, 2024 topographic and SUE surveys, and site SWM reports. Key 2D refinements re-established 1D–2D links, standardized hydraulic parameters, and regenerated meshes to match surveyed grades. Critical reaches at Steelcase Road West and Denison Street were converted from a 1D+2D dual representation to fully 2D domains to resolve channel–overbank exchanges. Event-based simulations applied the City’s 3-hour AES hyetographs for 2–100-year storms.

Results show the FCF provides enough storage, eliminates spill at Steelcase Road East for the 5-year LOS, and lowers hydraulic grade lines by 0.83m compared to existing conditions for the 5-year event. Upsizing Steelcase Road West and Denison Street removes roadway overtopping at Steelcase Road West up to the 100-year event while maintaining equal or lower peak HGLs and flows at the downstream MTO (Hwy 404) culvert. Increased local velocities from lower stages are mitigated with channel transitions and energy dissipation. The 2D routing framework in PCSWMM provides a defensible, design‑ready basis for the FCF and culvert upgrades by resolving storage–conveyance interactions and lateral exchanges, demonstrating local flood‑risk reduction without downstream impacts.