Dikes require big-ticket upgrades to hold back sea

Report flags seismic risks, possible 'retreat' from some areas

Diagram showing latest design standard changes for sea dikes.

Diagram showing latest design standard changes for sea dikes.

Rising sea levels will force major upgrades to the Lower Mainland’s expansive network of dikes in the coming decades, according to a provincial government report.

The newly released findings estimate dike upgrades could cost $9.5 billion over the next 90 to 100 years to prepare the Metro Vancouver area to withstand a significant increase in ocean levels as a result of climate change.

The report, titled Cost of Adaptation – Sea Dikes and Alternative Strategies, examined 250 kilometres of ocean shoreline from West Vancouver to White Rock and on the Fraser River as far upstream as the Port Mann Bridge.

It’s a follow-up to a 2011 report that predicted a one-metre rise in sea level along the B.C. coast by the end of the century.

Actual construction of larger dikes or other protective structures would be a small $880-million slice of the overall cost estimate.

A larger chunk is $1.6 billion for property acquisition, much of it in areas where taller dikes will require larger footprints, encroaching on adjacent land.

But the biggest cost component is an estimated $3.25 billion for seismic upgrades – more than half of the $6.3-billion overall cost before a 50 per cent contingency is added.

One of the risks flagged in the report is that an earthquake could cause some soil layers underneath dikes to liquefy, threatening their integrity.

The proposed changes take into account not just the sea level rise but land subsidence, maximum high tide, storm surge, wave effects and the need for freeboard.

Not all areas of low-lying land threatened by the ocean would be defended.

The report calls for a strategy in some flood-prone areas of “managed retreat” where currently developed areas are decommissioned over time and returned to a natural or low-value state that can flood periodically.

“Managed retreat may be a viable option at Mud Bay [in Surrey],” it says.

“However, the decision to retreat is complicated and would have to be made with extensive stakeholder input and economic analyses.”

Also contemplated are $10-million sea gates at the mouths of the Serpentine and Nicomekl Rivers, as well as a $25 million sea gate that could be closed during storms to protect Vancouver’s False Creek area.

Steveston’s densely developed waterfront of historic buildings might be protected by using Shady Island and a new sea gate as a storm surge barrier.

The report also considered the value of property and buildings in areas of Metro Vancouver – as well as social and environmental factors – as part of deciding whether expensive flood-protection works are justified on a cost-benefit basis.

The report also suggests a Regional Flood Protection Plan be drawn up with the province, municipalities and other agencies participating.

Developers are already recognizing the need for change.

Norm Shearing, vice-president of Parklane Homes, told a Metro Vancouver forum last month plans for the new River District residential area in Vancouver along the Fraser River were revised at huge expense to meet higher flood plain requirements.

“It was a thing we had to do to protect ourselves from the future,” Shearing said.

“The implications of sea level rise on the region and the engineering solutions that are going to have to occur is huge, it’s enormous.”

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