District of Hope council got an update on flood mitigation at the Jan. 24 meeting. There is a lot to be done and it’ll cost an estimated $11 million.
Construction crews have been hard at work the last few months around Fraser Canyon Hospital, where the raging Coquihalla River badly damaged the riverbank.
“The water roared right through here and was actually taking one-metre diametre rock and displacing it,” said Brian LaCas from Vancouver-based LCI Engineering Group. “Once that rock was removed it became very vulnerable because the sediment behind the rock is so fine.”
Crews have since built a terraced road along the riverbank and placed rock armour (riprap),” LaCas explained.
To lower the water level and allow construction to happen, a flood canal was built on the other side of the river. The canal was built over just six days and LaCas said it will be kept as a permanent structure to help with future flooding. In the meantime, it’s being used as new fish habitat.
“Our team of biologists came out, and they directed machinery to put in wood and rock so we didn’t just leave a flood channel,” LaCas said.
While the hospital area is just about finished, there are other spots needing attention.
LaCas said Gardner Drive is the highest priority spot needing attention. Flooding eroded the riverbank behind houses all the way to backyard fencelines.
Second on the list is building a flood barrier along Old Hope Princeton Way. Third is building a campground flood channel upstream of the Kawkawa Lake Road bridge and fourth is getting a dike built at Rotary Park.
The $11 million estimate covers construction, engineering, environmental and other costs.
LaCas said engineering plans are complete for all of those projects, and all that’s needed is funding.
A total of 277.6 millimetres of rain fell on Nov. 14-15, with LaCas observing that “just under a foot of rain is pretty extreme in our business.”
But for as much havoc as it caused, it could have been worse, and almost certainly will be in the future. LaCas classified November’s flooding as something that would happen every decade or two. He noted that November’s event saw the Coquihalla River peak at a flow of 686 cubic metres per second, which is significant. But, he said the type of flooding that would happen every 200 years would bring nearly twice that.
“Our modeling, with climate change built in, showed that the 200-year flood would be 1238 cubic metres per second. That’s what we’re designing for,” he said. “Many people have been saying that this was the great flood, and it wasn’t.”
LaCas showed council a 2D model of flood risk that presented a chilling scenario.
If a 200-year flood happened with current flood mitigation in place, he suggested the entire Robertson subdivision would be under water, and water would cross through Old Hope Princeton Way, running past Starbucks and McDonalds before making a turn back into the Fraser River.
“There are no adequate barriers to prevent the flood from running down that corridor,” he said.
He said that even though River Parade is protected by a dike, it would be vulnerable to water coming in from behind through the Coquihalla campground, and homes on 7th Avenue would also be at risk. A dike at Rotary Park would protect those homes.
LaCas said the work done to protect Fraser Canyon Hospital is designed to withstand a 200-year flood, and other flood mitigation projects are being planned with that in mind.
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