Many dikes too low for high water
Expert warns of serious problems if flooding rises to historic levels
Many of the dikes across the Lower Mainland are as much as one metre too low to withstand historic flood levels and few meet current standards for design and stability, according to provincial government documents.
Every spring is like rolling the dice that hundreds of kilometres of barriers and pumps will withstand the waters of a melting snowpack, said John Clague, a professor of earth sciences at Simon Fraser University.
All it takes is a combination of heavy snowpack, warm spring temperatures and heavy rain to create dangerously high waters. Just last May, thousands of people were evacuated from Grand Forks, while low-lying areas along the Fraser River were inundated.
“We certainly saw that when the Fraser peaked in the first and second week of May, which has never happened before,” said Clague.
“Normally, we have predictable high flows that ramp up over the course of the spring and peak in mid to late June.”
Studies commissioned by the province and local authorities suggest that the “frequency and extent of flooding is likely to increase” due to temperatures and precipitation driven by climate change, according to a recent assessment of Lower Mainland dikes.
About 290 dikes in B.C. are managed by dozens of diking authorities, many of which cannot bear the expense of significant upgrades.
“There are going to be some areas where crest levels are going to be a serious problem,” said Clague.
“Given that diking is the responsibility of municipalities or regional districts, it’s going to be really variable,” said Clague.
Among the impediments to addressing the issue is that no one knows for sure which dikes need to be raised, because crest height was not included in the last provincial assessment.
Most of the dikes in the Lower Mainland were built or upgraded to a design standard set in 1969, but more recent hydraulic modelling has shown that could be up to one metre too low.
The ministry of forests, lands, natural resource operations and rural development last week announced it will seek a survey of nearly 1,000 kilometres of dikes, along with their crest heights, across the entire province.
“The lack of dike crest information (in conjunction with lack of design flood modelling) makes flood risk planning extremely challenging in these communities,” reads the ministry’s request for proposal.
The report will be funded by the federal Natural Disaster Mitigation Fund.
That “snapshot” of B.C.’s dikes will help with efforts to create infrastructure that is more resilient to climate change, said Valerie Cameron, a manager with the ministry’s water management branch.
“Models are projecting much greater extremes in floods and droughts, and B.C. is such a dynamic environment that we really do rely on those dikes,” she said.
“Once we know where those risks are, we can prioritize how we can mitigate those risks.”
Because dikes tend to settle and erode over time, crest height estimates could be more than 40 years out of date, she said.
In 2015, a desk-top study of Lower Mainland dikes found that none of the Lower Mainland’s dikes meet current provincial standards.
While no field work was done for that report, records showed that more than half of the dikes are too low to withstand “design flood levels,” based on water levels on the Fraser River during the flood of 1894, the highest on record, and 1948, the second highest.
A rapid snowmelt in May of 1894 caused the river to rise, triggering flooding from Harrison to Richmond and devastated parts of Chilliwack.
Only four per cent of the dike segments are high enough to contain a co-called “design event” with at least 60 centimetres to spare, the report says. More than 70 per cent of Lower Mainland dikes would fail by overtopping if those conditions occurred today.
A study by the Fraser Basin Council put the cost of such a flood at $23 billion.
“We know that a flood of that magnitude has happened in the past and we know it could happen again,” said Steve Litke, watersheds program manager for the council. “There is also work underway to examine the effects of climate change and the hydrology associated with rising sea levels.”
The provincial government is assuming that sea levels will rise by one metre by the end of this century. Design criteria for sea dikes have not yet been updated to account for such a change.
Communities near the mouth of the Fraser will feel the combined effects of higher seas, rising river levels and violent weather.
“Coastal storm surges are creating higher water levels as far as the Alex Fraser Bridge, so that is a pretty significant flood risk,” said Litke. “If the sea level rose a metre, that transition point would move even further up-river.”