Canadian Consulting Engineer

Dual Cascade Structures

Where the Windermere North Storm Outfall released into the North Saskatchewan River southwest of Edmonton, the drop from the top of the bank into the river was a full 50 metres. Because of this steep ...

June 1, 2009   By Stantec Consulting

Where the Windermere North Storm Outfall released into the North Saskatchewan River southwest of Edmonton, the drop from the top of the bank into the river was a full 50 metres. Because of this steep drop, the velocity in the flow of the stormwater could exceed 6 metres a second, and it could cause a great deal of damage to the receiving water body in terms of scouring the stream bed and eroding the downstream channel. In order to mitigate the damage, the energy in the flow had to be less than 1.0 metres/second.

Stantec’s Urban Land and Buildings Group, led by Keith Shillington, P. Eng. project manager, came up with a solution to dissipate the energy in the flow with a series of dual cascade-drop concrete structures. Each structure incorporates two rectangular reinforced concrete chambers with inside dimensions of 2,440 x 3,050 mm. These two chambers are connected by a common 300 mm thick wall but are separated by a vertical gap of 1390 mm. Each structure has a capacity of 1.80 cu. m per second.

The solution was both simple but technically advanced at the same time. During the design stages, Stantec asked the University of Alberta to help test the hydraulic flows. Several experimental runs were done under different flow conditions on a 1:4 scale model. The results were then extrapolated to the design discharge of 1:80 cu. m/sec. The research, which was done in the Department of Hydraulic Engineering, showed that the energy was dissipated mainly by the impact of the jet at the base of the chamber and the front wall. The energy dissipation at the design flow rate is estimated at 54%.

A total of four structures were installed down the bank at the outflow and the results are being monitored. The system saved the client 60 per cent in reduced costs compared to a conventional drop structure and tunnel.

The project won a 2009 Showcase Award from Consulting Engineers of Alberta.

Prime consultant/urban planning, design, engineering & public consultation: Stantec Consulting (Keith Shillington, P. Eng.) Testing: University of Alberta Geotechnical: J. R. Paine & Associates Client: Windermere Lands Contractor: Sureway Construction


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