The Dry Creek flood mitigation is only a piece of a long-term puzzle in flood-proofing lower Third Avenue, said city engineer Guy Cicon.
“The flooding we received on Third and Fourth Avenue (on Saturday) is independent of the work that we did on Dry Creek,” said Cicon.
City crews were forced to close a section of Third Avenue between Bute and Burde streets Saturday night as standing water became a hazard.
The deficiency lies with the city’s aging sewer infrastructure, said Cicon.
“When we reached 5 millimetres per hour (of rain), the rainfall exceeded the capacity of the underground drainage system,” Cicon said. Precipitation reached that rate for four or five hours during the afternoon and early evening of Saturday, Dec. 5.
“This was a very intense rainstorm event.”
While city crews cleaned out storm water pipes over the summer in preparation for possible winter storms, Cicon said that the system just isn’t sufficient for hours of 5 mm per hour rains.
“Despite the storm drainage maintenance that we did this summer, the volume of water exceeded the capacity of the system.”
In order to head off more flooding, the city is considering installing another storm water drainage outlet into Dry Creek.
“We are looking into installing another storm water outlet into Dry Creek at Bute Street to relieve the hydraulic pressure in the catchment area,” said Cicon. The city already has one drainage outlet in the area.
The catchment area’s inability to keep up with the storm water during heavy rainfall is why the manhole at Burde Street and Fourth Avenue always blows.
“That’s called surcharging and it’s when the system is at its capacity and water escapes through the manhole. We had an inch of rain over four hours—we just don’t have the capacity for that.”
The storm water outlet could be installed in the coming months, Cicon said.
“We have to do some analysis and engineering on the project and if we can we will do it in the next two or three months.”
In the long term, Cicon said that the city will continue along its storm water management plan.
“Our long-term storm water management plan is to separate our storm water from our combined sewer system,” he said.
Separating storm water from sanitary sewers has been a decades-long project for the city, Cicon said, adding that the more separation there is in the city’s sewer system, the better its storm water management will be.
“We’ll separate that in the Bute Street catchment area.”