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Squamish dykes below standard



Five years after floodwaters swept through Squamish, the dyke failures were presented to district council, along with three options to avoid future episodes.

The stretch of dyke by the eagle viewing area just outside Brackendale marked the scene of the most serious failure, according to geotechnician David Hill of Thurber Engineering Ltd.

“A lot of people assume that’s a problem point because the water was flowing straight towards (the dyke) because of the geometry of the river,” said acting mayor Greg Gardner.

But the problem was actually something called piping, according to Hill. The dyke — which, like the rest of the Squamish system, does not live up to current standards — is as narrow as 4.6 metres in some places in that area. That thickness, combined with seepage and pressure variances, causes a phenomenon known as piping. Piping is the creation of fast-flowing water conduits that result in critical erosion.

Hill put forward three options, ranging in price from $350,000 to $800,000. The options are a downstream berm, a mid-dyke barrier with a downstream berm or an upstream berm with a seepage barrier. The latter is the most costly, according to Thurber’s study, but the others involve significant property acquisition issues, which could ramp up their costs as well.

Council next heard from David Matsubara and Kerr Wood Leidal, who explained river behaviour in Squamish. According to their findings, peak flows are on a rising trend, with the 2003 flood, itself a result of rain and snowmelt, marking the highest watermark. According to their findings, removing sediment from the waterways is an insufficient strategy. The dykes will have to be elevated.

The province has downloaded such responsibilities to the municipalities. Squamish Director of Community Services Mick Gottardi pressed council to come to a swift decision so as to access provincial grant opportunities expiring Aug. 1. Council directed staff to choose one of the three options by deadline.