A flood map from the tsunami risk presentation given to Oak Bay council shows how a two-metre sea level rise would affect Oak Bay. (Courtesy of the CRD)

A flood map from the tsunami risk presentation given to Oak Bay council shows how a two-metre sea level rise would affect Oak Bay. (Courtesy of the CRD)

Tsunami risk presentation exposes Oak Bay’s vulnerability

McNeill Bay, Oak Bay Marina shown as most vulnerable to flooding

A tsunami risk presentation to Oak Bay didn’t rock the boat much as council considered the local impact of the presented scenarios.

David Forde of Associated Engineering presented council, meeting as committee of the whole on May 16, with a summary of the CRD’s 2020 tsunami and flood risk assessment, which analyzed 11 scenarios and data from the Washington Geological Survey and the Oregon Department of Geology and Mineral Industry.

The focus rested on three tsunami scenarios: Cascadia Subduction Zone L1 (every 2,500 years), Cascadia Subduction Zone northern segment (every 500 to 600 years) and the Devil’s Mountain fault (every 2,000 years).

Mayor Kevin Murdoch, who said the Devil’s Mountain fault had the best name of the three, inquired how long it would take to reach Oak Bay. Forde said this scenario would need just five to 10 minutes to reach Oak Bay from across the Strait of Juan de Fuca, whereas the Cascadia Subduction Zone scenarios would take closer to an hour to arrive from their sources along coastal Washington.

“The Cascadia Subduction Zone is a much better understood seismic hazard, as well as tsunami source, relative to the Devil’s Mountain fault,” Forde noted.

Murdoch emphasized the Devil’s Mountain scenario is only expected once every 2,000 years, “just to put that into perspective.”

ALSO READ: Tsunami risk from Sooke to Sidney

In the case of flooding, Forde said it helps that Oak Bay is a relatively elevated municipality, above the six-metre contour.

The area most susceptible to flooding is what Forde called the McNeill Avenue corridor, which runs about 1.25 km between McNeill Bay and the Oak Bay Marina.

“The longstanding (regional) plan used to be based on a four-metre safe elevation,” Saanich Deputy Fire Chief Frank Macdonald said, adding it now falls on the district to inform the public about flood risk.

Macdonald said a digital letter of notice will be sent to residents living in what the presentation showed to be tsunami and flood-risk zones.

An educational brochure and interactive risk map of the CRD are also being crafted.

ALSO READ: New videos highlight southern Vancouver Island’s tsunami risk

Eileen Grant, Oak Bay’s emergency program manager, explained the nature of tsunamis and local protocol for them.

“They are quite unique and they are generally connected to earthquakes,” she said. “It doesn’t just happen because the ocean moves in a unique way.”

Grant said an earthquake capable of causing a tsunami could shake the area enough to knock people over. In this case, an evacuation order would already be announced and residents would want to move to safe zones, which include most of Oak Bay, besides the McNeill Avenue corridor. Residents of beachfront property would likely have to head to higher ground, she added.

“The big thing is we don’t want people to move unless they have to,” Grant said, noting the traffic and chaos it would cause.

In her 10 years as program manager, she said the only change in protocol has been that residents now must plan for a seven-day evacuation period, rather than the previous three-day plan.

“We need people to understand what could happen and what they need to do in these circumstances,” Grant said, adding it’s important for those living in higher-risk zones to have a grab-and-go kit ready at their door.

“We’re hoping to educate them so they know exactly what to do and whether they’re in a yellow or green zone.”

ALSO READ: Vancouver Island overdue for the big one, can also expect mega-thrust tsunami


 

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