Climate Change Implications for Rainwater Conveyance Infrastructure
Short sharp bursts of rain are increasing in both frequency and intensity and will lead to even massive mudslides and floods across southern British Columbia, according to a recent article in the Vancouver Sun reporting on the work of researchers at the University of British Columbia.
The newspaper article notes that the UBC analysis “projects plenty more severe weather to come and that could mean trouble as engineers design storm drains, road beds and hillside communities without taking into account recent changes in weather patterns attributed by most scientists to global warming.”
The controversial research, originally done for a 2001 master's thesis, was published last summer in the Journal of the American Water Resources Association, The model is based on measurements taken at the District of North Vancouver municipal hall rain gauge and studies of rainfall and runoff patterns from around the world.
“If we are experiencing climate change … and we are getting increasing rainfall intensities, then [engineers] are using old data to design for future conditions that may not be valid,” Robert Millar, a professor of civil engineering at UBC, told the Vancouver Sun in an interview..
When the thesis results came to the attention of drainage engineers at the Greater Vancouver Regional District (GVRD), a study was commissioned into the impacts of global warming on the region, according to Robert Hicks, a senior engineer with the GVRD.
“The research, led by Matthias Jakob of BGC Engineering, was at odds with what UBC was saying,” Hicks said, “The study by Jakob, published in the Canadian Water Resources Journal, says that a weather pattern similar to El Nino with a 50- to 70-year temperature and precipitation cycle could account for the short-term changes attributed to the greenhouse effect in the UBC research.”
Jakob's study analysed data from rain gauges throughout the Lower Mainland operated by the GVRD and the Meteorological Service of Canada dating back to the late '50s and early '60s. Published in 2003, the journal article predicts lower-than-normal rainfall intensity as the cycle enters a cooler phase.
It also notes: “A long-term rise in the magnitude of high intensity rainfall events could . . . necessitate the replacement of the storm water and sewerage drainage, which would be associated with very high costs.”
According to the Vancouver Sun article, Hicks said the records used for UBC's analysis are simply too short to be meaningful. “Depending on how one treats the records statistically, one can get different results.”
The Vancouver Sun reports that Millar disagrees on two counts. Firstly, because the region's municipalities replace pipe and drains regularly, he claims it would be cheap and easy to increase capacity gradually. Secondly, he says the UBC projections have been re-run using several mathematical techniques and come out the same. “We stand by these results,” the Sun quotes Millar as saying..
The severe weather trend is easy to miss using standard analysis, which assumes that past conditions are a good indicator of future conditions, Millar told the Sun.
Total annual rainfall figures from Vancouver International Airport show a 15- to 20-per-cent increase over the past 40 years. But five-minute bursts of rainfall have doubled in intensity to a rate of over 60 millimetres an hour in 2001, up from about 25 millimetres an hour in the mid-60s.
“In North Vancouver, over the past 30 years, we observe an increase in intensity of 40 per cent in two-hour high intensity rain bursts,” he said. “That's a huge difference for someone designing infrastructure.”
Storm sewers are designed to last about 50 years, “so we argue that it would be prudent to begin to accommodate these increases,” Millar said. (City of Surrey engineers estimate the cost of replacing one block of storm sewers at about $200,000.)
Most municipal sewers installed over the past 30 years would be designed for flows that would overwhelm the system about once in 10 years and cause “nuisance flooding.”
Millar and his associates, Catherine Denault and Barbara Lence, believe those one-in-10-year events could soon be happening every year.
Short periods of intense rain tend to destabilize hillsides and result in mudslides of the type that have plagued the area around Hope and that caused the death of a woman in North Vancouver in 2005.
“We know that landslides tend to be triggered by high intensity short duration rainfall and if the frequency and intensity of rainfall is increasing, then we are likely to see more landslides,” Millar explained.
The projections cited in the UBC article were first completed in 2001 for Denault's master's thesis, but the trio reworked the data last year to account for the most recent weather trends and their conclusions remain the same.
“What I see happening in recent years, flooding in the Squamish River, flooding in the Chilliwack River, very significant landslides, these are all consistent with our analysis,” Millar said.
Extracted from an article orginally published in the November 8, 2006 issue of the Vancouver Sun newspaper.