Increasing rain the biggest factor in flooding

An engineering firm studying the Tsolum River concludes the amount of rainfall, especially extreme rain, is the most important factor in whether flooding will occur.

And it looks like we've entered a period of increasingly heavy rainfall in this region, Graham Hill added Tuesday in a report to the Comox Valley Regional District.

An engineering firm studying the Tsolum River concludes the amount of rainfall, especially extreme rain, is the most important factor in whether flooding will occur.

And it looks like we’ve entered a period of increasingly heavy rainfall in this region, Graham Hill added Tuesday in a report to the Comox Valley Regional District.

In the wake of serious flooding in East Courtenay on Nov. 16, 2009 and Dec. 24, 2010, the CVRD commissioned Northwest Hydraulic Consultants to study and report back.

Many rivers on the east side of Vancouver Island from Duncan to Port Hardy — including the Tsolum, Puntledge and Oyster —had their largest and/or second-largest recorded flood in the past five years, noted Hill, Northwest’s project engineer.

Peak rainfall days have been above average for the past decade, Hill said.

“We’re seeing more intense rainfall. We’re seeing more intense flooding,” he commented.

“Floods in the town of Courtenay are governed by the magnitude and timing of discharge into the Courtenay River,” which Hill noted is only about three kilometres long and fed mainly by the Puntledge and Tsolum.

He said the Puntledge watershed is more than twice the size of the 40-kilometre-long Tsolum’s.

BC Hydro and clearcut logging in the rivers’ watersheds have been blamed by some for flooding, but Hill said statistics do not support those accusations, at least as major contributors.

High tide and storm surges can contribute to Courtenay River flooding. Hill produced graphs to show that BC Hydro has timed the Puntledge flow well using its power-generating dam high on the river to avoid high flows when the tide is high.

As for logging, Hill said there’s no overall consensus about its effects on flooding.

Hill said a study eight years ago concluded logging’s contributions to flooding were overwhelmed by the rainfall factor. He added that aerial photographs showed there is less clearcut land in the affected watersheds than in the 1960s.

Other factors such as climate change, a rise in sea level and huge cyclical weather patterns such as El Niño indicate we could be entering a period when flooding is more common, Hill stated.

“In the future, a 100-year event may be a 10-year event,” he responded to questioning by CVRD board members.

The report recommends Courtenay and the regional district work together to update floodplain mapping, which Hill estimated should be done every 20 years.

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