The work to protect lower Third Avenue from winter flooding is three-quarters of the way complete, according to city engineer Guy Cicon. The contract was award to Bowerman Excavating Ltd. earlier this year.
The Jade Restaurant, which the city purchased in July, was demolished on Aug. 29, making way for a new box culvert to be installed underneath the Restaurant Bridge at Third Avenue.
“That increases the cross-sectional area of the bridge to allow more flow to go through,” said Cicon.
“If you didn’t have that then you’d have a restriction. We have to increase the area to improve the flow.”
The restricted flow under the Restaurant Bridge (which was located beside the Jade) is part of the reason behind the repeated flooding of Third Avenue, Cicon said.
“That flow restriction is one of the causes of the flooding.”
Channel improvements along the entire length of Dry Creek, which flows underneath Restaurant Bridge, will deal with the other causes of the flooding.
“We’re doing channel improvements to improve the flow,” said Cicon.
This work also extends behind Smitty’s on the opposite side of Third Avenue and all the way back to approximately Bute Street.
“The existing channel cross-section isn’t big enough either so we’re enlarging the downstream channel.”
The width of the channel will increase from six to seven metres previously to 12 metres once construction is complete.
“It’s nearly doubling it.”
Widening the channel will include steepening the banks.
“We’re using gabion baskets so that allows steeper banks and more cross-sectional flow,” Cicon said.
Gabion baskets are riprap covered in wire or metal and designed to decrease erosion of stream banks.
It’s not just Port Alberni’s human residents that will benefit from the flood mitigation.
“We’re digging fish habitat pools,” said Cicon.
“They’re being installed upstream of Fourth Avenue [inside Dry Creek Park] for salmon habitat enhancement.”
The work is being supported by West Coast Aquatic.
According to Cicon, no modification of Dry Creek could be done without considering the salmon.
“You can’t do one without the other,” he added.
“A stream is inherently linked to the environment… engineering the flood protection isn’t independent of everything else. They go hand in hand.”
The grant funding that the city received (one-third each from the provincial and federal governments) required the city to show an environmental benefit as well.
“A project like this with grant funding must demonstrate an improvement to the environment,” Cicon said.
As part of that, there’s a low flow channel being carved into the bottom of the creek bed.
“It’s a carved ditch in the bottom of the ditch,” said Cicon.
“In very low flows, water is concentrated in a low flow channel so that fish can more easily move from one habitat pool to the other.”
A wider channel on the other hand means low water levels.
“If it was just a 12-metre wide bottom, the water would be really shallow.”
The low flow channel will be about one metre in width, Cicon added.
One of the problems encountered with this summer’s drought was fish pools drying up in city creeks. Sheena Falconer from West Coast Aquatic spent part of her summer relocating fish because of low flows. The low flow channel is designed to help alleviate this situation in future years.
Along with the salmon enhancement work, they will also do some general environmental restoration of Dry Creek’s banks.
“We’ll be doing some restoration work as well, including planting native vegetation like cedar, fir and salal,” Cicon said.
“It’s all about combining flood improvements with habitat improvements.”
The work should be completed by mid-October, before the winter storms and flooding it’s designed to mitigate.
Cicon hopes that the work to improve the creek’s flow will eliminate the
flooding that has plagued lower Third Avenue.
“The amount of water won’t change, it’s just how we’re managing that water.”