Snowpack at highest mid-May level in decade, raising flooding fears
It all depends on the weather now. Over a few cool weeks, B.C.'s mountain snowpack leaped to its highest level for May 15 in a decade, increasing the flood risk across the province, according to the latest snow survey bulletin from the River Forecast Centre.
A prolonged heat wave — similar to last year's heat dome, which settled over B.C. at the end of June — could cause major flooding, says the bulletin, released Friday:
“A combination of several days of intense heat directly followed by heavy rain is a worst-case scenario.”
In an average year, about 18 per cent of B.C.'s mountain snowpack has melted by May 15. This year, it's estimated that only 2.6 per cent has melted. As a result, the flood risk across the province has risen, with the average snowpack at 128 per cent of normal.
Weather will determine if it melts slowly, steadily, or quickly, which would cause rivers to overflow and flood.
The bulletin notes that the major 1948 and 1894 floods on the Fraser River were likely caused by a similar scenario to the one feared this spring: “A sudden switch from the persistent cooler spring temperatures to an intense heat wave lasting at least five days.”
A secondary high-risk scenario could involve “widespread heavy rainfall” that happens when rivers are flowing at their highest levels from snowmelt, similar to the 1972 floods that devastated parts of Kamloops.
The bulletin says temperatures over the next two weeks are expected to remain cool. Short periods of warmer weather — like the sunny skies forecast for the long weekend — will allow some snow to melt, possibly easing the risk. But it's unclear what kind of weather June will bring.
While B.C.'s flood defences have been improved since past floods, many dikes are inadequate to face a flood the size of the 1894 Fraser River flood.
From Hope to the Salish Sea, the majority of dikes on the lower Fraser River don't meet current design standards, according to a Fraser Basin Council report from 2016. An assessment found 71 per cent of the region's dikes are “vulnerable to failure by overtopping” during a major Fraser River or coastal flood, and only four per cent of assessed dike segments meet provincial standards for dike crest height, which is set to protect against a 500-year freshet flood, similar to 1894, or a 200-year winter coastal storm surge flood.
On other rivers across the province, it's a similar story. As part of a four-month investigation, Postmedia News examined flood mitigation efforts in 75 B.C. communities and found that more than two-thirds don't have a detailed, costed flood plan, have only parts of a plan or have just started work to create a plan.
As a result of climate change, experts believe, what is now considered a 500-year flood could become more frequent. And what were once 100-year floods could happen every 10 or 20 years, or even annually in some places.
It's a reality organic farmer Krystine McInnes has been living. The owner of Grown Here Farms in Cawston has been evacuated four times in the past four years — twice due to wildfires on the hills above her farm and twice due to flooding.