Debris flood mitigation on Three Sisters Creek in Canmore is expected to cost $3.8 million to undertake.
When council was presented with plans and a conceptual design in February, a price estimate for the work was not yet available, but Town manager of engineering Andy Esarte updated council on the cost during its March committee of the whole meeting.
Esarte said meetings with Alberta Parks, Environment and Sustainable Resource Development, Alberta Transportation and TSMV developers have seen the plans positively received.
“We met with Alberta Transportation and have a general agreement on the concept at the parkway that they are responsible for and the proposed mitigation there,” he said.
Esarte said the municipality would be responsible for $540,000 of the overall budget, which still requires funding approval from the Province through the Alberta Community Resiliency Program grant program.
“We are now getting into the phase with creek mitigation where it will likely take the province time to work through the priorities province-wide and to work through business cases put forward by municipalities,” he said. “This makes a really strong business case for Canmore and gets us in the queue for funding.”
Three Sisters Creek mitigations are fundamentally different from Cougar Creek and one of the main reasons is that only half of the alluvial fan it drains into is developed.
Like Cougar Creek, however, Three Sisters experienced a debris flood, which is a very rapid surging flow of water heavily charged with debris in a steep channel. While Cougar Creek saw 90,000 cubic metres of sediment wash down into the fan, Three Sisters had 20,000 cubic metres of debris.
Much of the debris was captured in a pond on the undeveloped golf course property. Basins like the pond can act as debris flood containment structures and it resulted in less sediment travelling further downstream and affecting culverts under Three Sisters Parkway.
The conceptual design proposed putting a berm near the apex of the fan to deflect debris floods away from the residential area for future floods.
A consultant was hired to develop mitigation options and several different elements were recommended, including the deflection berm to cut off the flow of the creek at the apex and direct it to the west. Additional work includes “hardening” the channel bed with vegetation to give it stability, looking at changes to the outlet of the pond and stabilizing the banks, especially near the parkway.
The potential hazard for the creek is not the same as Cougar Creek in terms of intensity and magnitude, according to the risk hazard. The potential debris flood for a 100- to 300-year event would be a shallow flow about a metre deep.
That means risk to loss of life is less than what was found on Cougar Creek, however, loss of economic value in terms of flood damage to houses in that area is high because of the property values. The risk assessment found 461 parcels that could be flooded were valued at $321 million.
