How to save a resort town from future floods

Harrison must raise its dikes by at least one metre if it wants to protect residents from a future Harrison Lake flood.

That’s one of the main takeaways from a new report on how vulnerable Harrison is to future flooding events from the nearby lake. The report was delivered to Harrison council earlier this month.

Currently, the village of Harrison Hot Springs and its approximately 2,000 residents are protected by a one-and-a-half kilometre dike stretching from Harrison Hot Springs Resort in the west, across the waterfront lagoon, to Rockwell Drive in the east. The dike was built after the historic 1948 flood, and was built tall enough to withstand what was then considered a significant flood.

In the 70 years since then, models have changed. The Harrison dike was built to withstand the 1948 flood, when water levels reached 13.4 metres in the lake, and is tall enough to withstand what is now considered a one-in-500-year flood (or one that has a 0.2% chance of happening each year).

A larger dike might be considered overkill.

But waves would overtop the dike, and that could potentially cause a breach along its length. And new models suggest that Harrison should be prepared for even higher water levels—and that would mean raising the dike by just over a metre, from 13.9 metres up to 15 metres.

Due to climate change, hydraulic experts predict that what is now a one-in-500-year flood will become more than twice as likely over the next 30 years. By 2100, that exceptionally rare one-in-500-year flood could be expected to occur 10 times more often, every 50 years—or one with a 2% chance of happening each year.

By raising the dike, the Harrison report suggests, the village will remain protected from big floods until the end of the century. Of course, it won’t be cheap. The report suggests it will cost roughly $5 million dollars. And since Harrison Hot Springs is the government responsible for its dike, it would historically need to source that funding through grants or through its roughly 2,000 residents.

(The boat launch that intersects the dike also poses problems. The report suggests using temporary solutions in that one location during a flood, such as sandbagging, or raising the dike level slightly and installing gates that can close off the launch during high waterflood events.)

In a meeting in early June, council agreed to apply for a federal-provincial grant to cover the cost of raising the dike. (It will also apply for an additional $1 million to raise the height of the road to the wastewater treatment plant, so it isn’t severely damaged in future floods.) The funding would come from the Canada Community-Building Fund, formerly known as the Gas Tax.

The grant application is due at the end of June, and Northwest Hydraulics Consultant is now working on detailed designs for the application.

After last year’s Sumas Prairie disaster, the province promised to overhaul how it funds and manages flood protections across BC. Those changes, coupled with a new flood strategy, could affect the chances of success for the grant application, either by creating new funding streams, or by determining the priority level of improving the Harrison dike, which, unlike many across the province, may currently meet provincial standards.

The same report presented to council also looked at the challenges associated with a potential tsunami on Harrison Lake—something The Current reported on in October of last year.

Although it’s not clear if a landslide on Mt. Breakenridge would create a community-swamping tidal wave across the lake, a slide there could still cause potential challenges. The report said preventing a landslide is prohibitively expensive, but it recommended that different organizations work together to monitor the mountain’s slopes. It also suggested that Harrison establish another emergency route out of the village—something Harrison, the District of Kent, and the Seabird Island Band are currently working to develop.