With rising seas, Squamish faces 'unacceptable' risk from flood 

Proposed super dikes won’t be enough to stop risk of deaths from flood in 2100

click to enlarge FILE PHOTO
  • File photo

Should sea levels rise by a metre, even with proposed ‘super dikes’ built, Squamish residents could face an unacceptably high risk of death in a major flooding event, according to a new report.

“Squamish isn’t alone,” said David Roulston, manager of municipal infrastructure. “These are some pretty scary results, but Squamish is one of many floodplain communities in B.C. and in the world. There are many areas of the world that have unacceptable risk — New Orleans, Netherlands.”

A report authored by consultants Kerr Wood Leidal Associates Ltd. that was presented to council on Feb. 26 said there’s an “unacceptable” risk to life with current dike structures.

This risk may remain unacceptable even after upgrades recommended by the Integrated Flood Hazard Management Plan are complete, the report continues.

This prediction was made assuming Year 2100 conditions in which — among other things — sea levels rise by a metre.

That’s the standard the province has urged communities to use as a planning blueprint.

It’s also the metric used in the District’s Integrated Flood Hazard Management Plan, which is a document that serves as the stepping stone for this new report, Quantitative Risk Assessment for Squamish River Floodplain.

The chance of death for the most vulnerable individuals in a flood event was greater than 0.01 per cent in many scenarios, which exceeds the District’s risk tolerance criteria.

Vulnerable individuals were defined as those who spend all their time in the hazard area, which, in this study, were places at risk of being flooded by the Squamish River, Mamquam River and Howe Sound.

The District’s 0.01 per cent standard has been adopted elsewhere in the world, such as Australia, Hong Kong and the United Kingdom.

The idea is that people’s annual chances of getting killed by a natural hazard shouldn’t be greater than everyday risks, such as driving and participating in recreational activities.

There were three models used to examine the possible death toll.

The first, the Netherlands model, was based off flooding events in that country. The second, the New Orleans model, draws from the Hurricane Katrina disaster. The final one, called the RCEM, is based off a model developed by the U.S. Bureau of Reclamation and is used for assessing dam safety.

Each model has its own set of assumptions.

For example, the Netherlands model assumes that everyone caught in the breach zone of the dike will die.

In that model, the risk of death in Squamish will exceed the District’s acceptable standard regardless of whether the town upgrades its dikes to provincial standards, or even the coveted ideal, which is a “super dike” recommended in its flood management plan.

A super dike would theoretically be able to withstand a one-in-500 year disaster.

There are two exceptions, though. Building a super dike will be able to reduce the chances of death to acceptable levels in areas of the Squamish floodplain south of the Mamquam River.

Constructing the proposed sea dike identified in the flood management plan will also reduce risk of death to acceptable levels in areas closest to Howe Sound.

In the RCEM model, regardless of what kind of dike is built — be it the existing one, one built to provincial standard, or the super dike, the risk of death is higher than acceptable standards.

However, under this model, there were no results for coastal areas closest to Howe Sound.

Under the New Orleans model, however, the town scored much more favourably.

In all areas it scored within the acceptable death risk with the existing dikes.

There was one exception, and this was the coastal area. The existing sea dike isn’t enough to mitigate death risk — the one proposed by the flood management plan must be built to drive down risk to acceptable levels.

“There are probably many communities that have unacceptable risk and don’t know it. So we’re in the unique position of having this information, and so because we have this information, we can plan our mitigation,” Roulston said.

He said the District has poured $20 million in the past 12 years into flood protection measures.

The town plans to spend a minimum $2 million a year from municipal sources in the current financial plan.

Among other things, there was also an analysis of how cost-effective it would be to spend money on dikes.

The report found for every dollar spent on a super dike upgrade north of the Mamquam River, there’d be two dollars saved as a result of avoiding water damage.

South of the Mamquam River, this jumped to $9.60 for every dollar spent. On the coast, upgrading the sea dike to the standard proposed in the flood plan jumped it up to greater than $100 saved per dollar spent.

“You’re actually at an unprecedented level,” said David Roche of Kerr Wood.

“What that’s basically telling you is that there’s no way that Squamish would see a metre of sea level rise and not take action to protect downtown. It just makes too much sense. The writing’s on the wall.”

The full report can be viewed here.

This article originally appeared in The Squamish Chief on Feb. 28.

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