Precipitation of 10mm to 15mm per hour for a duration of up to 15hours (about one fifth of SW Ontario’s annual precipitation) lead to extreme flow in surface waters, and is according to the Globe and Mail’s Article cause for turbidity and sediments in the water supply in the greater Vancouver area. Intense precipitation was one of the causes leading to the contaminated water supply in the town of Walkerton in 2001, where seven people died and thousands are still sick (see also CBC’s archives).
The article also talks about extreme weather conditions (storms with rain) in Toronto and the USA. Is this an example of “increasing occurence of extreme weather conditions” due to warmer average annual temperatures?
Update: See also CBC
Here is the text of the article:
Boil-water advisory hits two million in Vancouver Residents warned not to use tap water for drinking, brushing teeth and rinsing food after rain triggers mudlslides across region
With a report from Oliver Moore
VANCOUVER — Two million residents of Greater Vancouver have been advised to boil their cloudy water after heavy rainfall created mudslides in the region’s reservoirs, the widest water warning in Canadian history.
Health officials said tap water is unsafe for drinking, brushing teeth or washing fruits and vegetables after the mudslides increased the risk of bacteria and viruses infecting the water supply. The risks can’t be minimized by disinfectants such as chlorine — hospitals, seniors homes and schools have been told to use only bottled or boiled water.
“This is a precautionary measure,” said Dr. Patricia Daly of Vancouver Coastal Health. “We are telling people don’t drink tap water. Either boil it or drink bottle water.”
Dr. Perry Kendall, the province’s chief health officer, said one concern is that the water supply could contain viruses, such as the norovirus and the E. coli bacteria, both of which can lead to diarrhea.
“The turbidity is very high and when it washes the topsoil, it can get contaminated with a lot of different things that you don’t want contaminating your water, such as wild animal feces,” Dr. Kendall said.
The advisory could last for weeks until the water supply clears. In 2000, people living in the Ontario town of Walkerton had to boil or buy their water for seven months after their supply was found to be unsafe. At least seven people died and more than 2,300 were sickened by E. coli found in it.
In July of 1996, Quebec’s Saguenay region was hit by major flooding after a torrential rainfall and more than 100,000 people were put under a boil water advisory.
In British Columbia’s case, there was an intense period of heavy rainfall — 10 to 15 millimetres per hour for 15 hours straight — that caused a runoff into the three watersheds that supply water for about two million people from the North Shore to Vancouver to Langley.
“The very heavy storms we’ve experienced, for some unknown reason, concentrated a lot of rain in our watersheds,” said Paul Archibald, division manager of water supply for the Greater Vancouver Regional District.
He said 37 slides in 24 hours created the muddiness in the water, a mixture of sandy gravel and the more problematic clay, which has fine sediment that could take weeks to wash out.
Watershed officials are adding more chlorine to increase the amount of disinfectant in the water and are releasing the water supply into the reservoir to dilute its turbidity.
Extreme weather has also hit the eastern and southern United States in recent days, leaving numerous people dead.
At least eight people were killed in tornadoes in North Carolina. In other states, power lines were knocked down, mobile homes were wrecked and trees were toppled. About 80 kilometres offshore, a U.S. Coast Guard helicopter came to the rescue of a fishing boat taking on water, lowering a pump to the 10-metre vessel and escorting it back to land.
In Toronto, heavy precipitation raised urban waterways to near-flood levels. Yesterday afternoon, city officials considered closing a major commuter highway leading downtown. Earlier in the day, grim forecasts provoked a flood advisory and a warning that residents stay away from the city’s nine creeks and rivers.
In Surrey, B.C., 10,000 students had the day off, with some schools unable to open because they had no power, said Doug Strachan, a school district spokesman.
While the stormy weather conditions subsided yesterday, the cleanup was just beginning, especially in areas hit hard by the torrential downpour. Wind-whipped residents earned a brief respite with calmer conditions yesterday and prepared for yet another storm expected this weekend.
Homeowners in luxury townhouses in West Vancouver remained out of their homes for a second day yesterday after unstable trees from the cliffside threatened to crash down into the Seascapes development. Eighty-two people from 33 families evacuated the partially completed complex, where new units sell for more than $800,000.
“Our district crew is working throughout the region doing tree removal,” said Patricia Leslie, communications manager for the West Vancouver municipality. “Crews are in the area right now by the development assessing the structural and tree stability.”
Flood dangers remained for five rivers on Vancouver Island, with some areas marking 120 millimetres of rain in a 24-hour period.
Provincial emergency officials were eyeing the rainfall levels and the increased risk of melting snow on the island with warmer temperatures.
In Port Alberni, the main road was flooded and water pressure underground blew off manholes.
Al Wagar, who works at Southside Auto Parts in the central Vancouver Island resource town, said water was streaming off the mountains and pouring down the streets and from the sky.
“The water kept rising and rising. We had products up 15 inches off the ground and they still got damaged,” Mr. Wagar said. “We’re just left with a lot of mud right now.”