Humber River bridges: a 2D flood model in HEC-RAS

Course project for CIV550 (Water Resources Engineering) at the University of Toronto. The site survey was on 28 September 2026 and the model was run in November 2026.

Map of the Humber River at the Old Mill Road bridge, seen from above. Pale particle traces run from the top to the bottom between two grey piers, and two red ellipses mark the slower water in the bays on both sides below the bridge. — Velocity and particle traces at the Old Mill Road bridge in the 100-year flood at 06:00, with the full shallow water equations (SWE-ELM). The red ellipses mark the two bays below the bridge. This crop has no colour scale. The aerial background is Bing Satellite imagery as shown in HEC-RAS RAS Mapper.

A 2D HEC-RAS model of 2.1 km of the lower Humber River in Toronto, testing how flood size, the equation set and valley roughness change the water levels at three bridges.

With the full shallow water equations the water levels at the bridges rise by 0.04 to 0.05 m, and the velocity under the Old Mill arches falls from 2.24 m/s to 1.57 m/s. Slow wakes form behind the piers and the water recirculates in the side bays. The run takes 4.8 times longer than the run with the diffusion wave.

Plans of the model. SWE-ELM is the full shallow water equations.
Plan	Peak inflow (m³/s)	Manning’s n	Equation set
Normal day	3.15	0.065	Diffusion wave
2-year flood	122.4	0.065	Diffusion wave
100-year flood	475.7	0.065	Diffusion wave
Future 100-year flood	590	0.065	Diffusion wave
Regional flood (Hurricane Hazel)	1702.1	0.065	Diffusion wave
Full momentum	475.7	0.065	SWE-ELM
Rougher valley	475.7	0.078	Diffusion wave
Smoother valley	475.7	0.052	Diffusion wave

The mesh has 213,349 cells of 5 m, refined to 1 m at the banks and 2 m at the 11 piers.

Shaded terrain map of the lower Humber River reach, coloured by ground elevation, with the mesh area, the Old Mill, TTC subway and Bloor Street bridges, and the inflow and outflow boundary lines labelled. — Ground elevation (m, CGVD2013) of the 2,081 m reach, with the 2D mesh (213,349 cells), the three bridges and the inflow and outflow boundary lines.

Two maps side by side over an aerial image of the valley, showing flow depth in blue in the 100-year flood on the left and the deeper, wider Regional flood on the right. — Flow depth (m) at 03:00 in (a) the 100-yr flood (475.7 m3/s) and (b) the Regional flood (1702.1 m3/s). The aerial background is Bing Satellite imagery as shown in HEC-RAS RAS Mapper.

Two maps side by side over an aerial image of the valley, showing maximum velocity from blue for slow to red for fast in the 100-year flood on the left and the Regional flood on the right. — Maximum velocity (m/s) from 00:00 to 06:00 in (a) the 100-yr flood and (b) the Regional flood; the red cells at the inflow line in (b) are a boundary effect. The aerial background is Bing Satellite imagery as shown in HEC-RAS RAS Mapper.

Line chart of water surface elevation against distance down the reach for each run, above the ground line and the three bridges, next to a small aerial map of the drawn channel centreline. — (a) Water surface along the channel centreline at the end of each run, with the ground, the bridge low chords and the S3 roughness band; (b) the centreline on an aerial image. The aerial background is Bing Satellite imagery as shown in HEC-RAS RAS Mapper.

Bar chart of the modelled headwater minus the TRCA level at the Old Mill, TTC subway and Bloor Street bridges, with four bars for each bridge. — Model headwater at 06:00 minus TRCA's 1D model level upstream of each bridge, for the 2-yr, 100-yr and Regional floods (n 0.065) and the 100-yr flood with n 0.078.

The 2-year, 100-year and Regional flows are from Civica (2018), and the structure data are from the Toronto and Region Conservation Authority (TRCA, 2018). The terrain is the Ontario lidar digital terrain model (package GTA2023-DTM-05), used under Ontario’s Open Government Licence. This is a course project and not a regulatory flood study.