The dam that is not quite a dam will have its worthiness tested long before Canmore spends $40 million in Cougar Creek.
The debris flood retention structure, which resembles a dam but is designed to hold sediment as well as water during debris flood events on the mountain creek, will be put to the test in Vienna this year at the Institute of Mountain Risk Engineering.
Canmore’s manager of engineering Andy Esarte said it is one of only several laboratories worldwide that can undertake this particular type of modelling for steep creek mitigation.
“It is a big apparatus that they use there and their facility allows for testing with sediment laden waters, so they will scale down the sediments as well, based on all the geotechnical information we have,” Esarte said. “They can create a scaled down sediment concentration in the water that will help accurately represent the conditions we can expect in Cougar Creek.”
With each project the unique conditions of the location and the creek variables are different, so the model is constructed specifically to match the creek. In this case, it will be a 30 to one scale model, which will be a metre high and several metres wide.
“What we talked about is creating a scale model with moveable components, so they can test things like the gradient in the tunnel, the width of the tunnel and the geometry of the inlet to the tunnel,” Esarte said. “They can test it for different flow rates and sediment concentrations to model the return periods.
“They can change all the variables and see how the structure reacts; they can also change the spacing of the inlet grill so they can play with different geometries.”
The inlet is key, and is the factor that differentiates the structure from a dam. The inlet allows water and certain sediments through and Esarte said the modelling process will help refine its size, which will have an important effect on annual maintenance costs.
“The part that will be informed through this testing are all the details of how the water flows to the inlet through the structure,” he said. “What that will all translate into is a much better idea of how much sediment we can expect to deposit behind the structure.”
Long-term maintenance costs of the retention structure and who is responsible for it – the municipality or the province – has been an issue of concern for some members of council. Esarte said Canmore has been putting money into maintaining Cougar Creek on an annual basis already, without a debris structure.
“Maintenance started the day we put infrastructure on an alluvial fan and when you start developing out on a fan like that your maintenance infrastructure costs start at day one, whether you understand them or capture them,” he said.
