A local weather specialist says Environment Canada failed to send out rainfall warnings until after most of the heaviest rains had fallen on May 28, flooding the city’s Atlantic Avenue Water Pollution Control Plant and much of the city.
However, the engineering consultant hired to investigate the flooding of the plant said even if a warning had come out there was little plant workers could have done to prevent the damage.
“In this case there really wasn’t any warning to do anything on. And to be very honest, if there was one there was nothing they could have done at the plant to have stopped it anyway,” CIMA’s Troy Briggs said, adding the plant failed due to overhelmed hydraulic equipment.
With water flows entering the plant at a rate 30 per cent above peak capacity, the plant was simply couldn't handle it.
“There wasn’t any equipment they could have brought on. There really weren’t any more tools available to them to address this event, due to the flows that we saw.”
Briggs is recommending the city implement a “quick and dirty fix” and temporarily remove one of four filtration screens to allow a faster flow of water. A long-term fix includes the creation of an emergency bypass that would divert water and increase screen-cleaning rates to prevent clogging and the slowdown of water flow.
Total cost would be in the $1.45-million range and could take up to 15 months to complete.
The Atlantic Avenue facility was designed to handle a maximum of 766 megalitres of water a day, averaging about 75. At the peak of the storm water flow was well over 1,100 megalitres, or 1.1 billion litres.
“Basically the flows were higher than the plant could handle,” Briggs told a media briefing prior to Monday night’s Committee of the Whole meeting at city hall.
While preventative measures like these are in place at more recently-built facilities in other communities, Briggs said it’s not uncommon for older plants, like Thunder Bay’s 1970s-era system to be without.
“It’s really just the past 10 years we’re starting to see it put into more facilities as standard practice, I would guess. And it’s because plants have experienced these problems and that’s why they’re doing it.”
With a capacity rate 10 times higher than the average expectancy, Briggs added the Thunder Bay plant is more equipped to handle heavy precipitation events than most other cities in Ontario.
“It’s significantly higher than I would see in other plants,” he said. “That 10:1 peak flow to average flow is well beyond what you would see. It’s designed for peak flow much greater than I would see at any other plant of this size,” he said. “And yet we experienced flows beyond that flow during this event. Most plants I would typically see a 4:1 peak factor.”
All told, 91.3 milliletres of rain fell in a 24-hour span, adding to more than 66 millilitres combined that fell in the preceding five days. The additional rainfall separated the 2012 storm from other big storms, including a 1977 deluge that dropped 131 millilitres on the city, nearly wiping out Fort William Historical Park.
“What was unusual about this storm was that it stalled over Thunder Bay,” climatogolist Graham Saunders told council.
“For whatever reasons the stalled over Thunder Bay, so we had at least two hours of very intense rainfall,” said climatologist Graham Saunders, adding the timing was a bit of bad luck.
“Realistically it should have been caught earlier than it was,” he said, adding it might have been prudent for Environment Canada to release a weather warning sooner, noting at that time of day the storm began, not many would have seen it.
Briggs said his findings show the first alarm at the treatment plant, run by automation at night, went out at 1:09 a.m. on May 28, a little more than an hour after the rain began. The first responder was on scene within 24 minutes. Eleven minutes later, at 1:44 a.m. the first alarm in the tunnels in the screen and grip building sounded. By 2:35 a.m. the pump station dry well had flooded and by 3:44 a.m. all pumps at the station were under water and not functioning.
City officials say Briggs’s recommendations, which council received as a first report on Monday night, pertain only to the sewage treatment plant, and not the entire sewer system, which would still have been overwhelmed even if the preventative measures were in place at the treatment plant.
Reports on the entire sewer system are expected before council next year.
