Flood following earthquake: Beware of a Cat soup in B.C.

Widespread flooding is possible following a major earthquake in B.C., a flood researcher warns.

Typically, earthquakes are known to give rise to fires, tsunamis and liquefaction (when the earth turns to liquid) following an earthquake. All of these perils can be excluded from earthquake policy coverage. But flood could also be an issue, says Jenny Koss, assistant program manager of flood management for the Fraser Basin Council.

A major earthquake could potentially damage dikes holding back water in B.C.’s Lower Mainland, Koss told brokers attending the Insurance Brokers Association of B.C. at its AGM and Leaders Conference last Thursday.

“It’s…important to think about how flood will interplay with different hazards [such as earthquakes],” she said. “There’s a lot that is holding back the coast here. High-consequence dikes are sites that have been rated as holding back a lot of risks. So, they have many assets behind them, and a large number of those dikes are located in the Lower Mainland.

Koss cited a study suggesting that “in a one-in-2,475-year earthquake [event], a lot of the high-consequence dikes would not hold up well.”

Koss prefaced her remarks about the potential impact of a large earthquake by observing that there are about 600 km of dikes in B.C.’s Lower Mainland. Home to more than three million people as of the 2021 census, the area includes Metro Vancouver and the Fraser Valley.

Richmond, B.C., a city of more than 200,000 people located in a coastal part of the Lower Mainland, is protected by a ‘superdike,’ Koss observed. The entire city is built in a floodplain.

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A 2015 study of 74 of the Lower Mainland dikes by the Fraser Basin Council found that 22 dikes had insufficient information to assign them a seismic risk rating. Of the remaining 52, only 53% are “seismically unstable,” the report found.

In 1948, B.C.’s second-worst flood ever caused dike failures, leading to the destruction of about 2,000 homes and the evacuation of 16,000 residents. Previous to that, B.C.’s worst ever flood happened in 1894.

“A 2015 study released by the Fraser Basin Council determined that a reoccurrence of the 1894 flood today could cause approximately $23 billion dollars in damages to the Lower Mainland,” the Government of B.C. posts on its website.

A 2014 report, Seismic Design Guidelines for Dykes, was prepared for B.C.’s Ministry of Forests, Lands and Natural Resource Operations Flood Safety Section. It estimates “the extent of damage resulting from large-scale flooding resulting from breaches to the different diking systems has been estimated to reach upwards of $50 billion (2013 dollars).” (That study was not specific to the Lower Mainland.)

Koss worked on a number of Fraser Basin Council studies since then, and showed brokers attending the AGM the results of research the council conducted on flood risk in 2019.

In a major earthquake, she said, “there would have been an estimated 50 centimeters of dike subsidence during a major earthquake like this…..And when you have dike subsidence, that’s how much the dikes are going to settle after all that earthquake shaking.

“50 centimetres? That’s half a metre. And that’s not even thinking about potential structural integrity issues that the dikes may have faced. And after a large event, it’s just important to note that it might take a long time to recover, which will leave us more susceptible [to future flooding events].”

 

Feature image courtesy of iStock.com/ronniechua