Over 150 residents attend disaster lecture
Sylvie Paillard
[email protected]
Upgrade the area's dikes or face potentially catastrophic consequences, said an expert during a geological hazards lecture at the Sea to Sky Hotel Tuesday (Jan. 9).
More than 150 residents, including several members of District of Squamish council, attended the lecture, which detailed the potentiality of earthquakes, dike breaches and catastrophic flooding resulting from the collapse of the volcanic barrier holding back Garibaldi Lake.
"The dike breach flood hazard is high, and don't be surprised when the dike actually fails," said Pierre Friele, a geoscientist specializing in slope stability and flood protection.
By combining the frequency and the magnitude of Squamish hazards, Friele concluded that although a collapse of the Garibaldi Lake barrier would essentially wipe out the valley floor, the breaching of the municipality's dikes is of greater concern.
Dikes are built to sustain flood magnitudes seen every 200 years. But following a significant event in 1984 and another in 2003, dikes have become penetrable to 100-year events, according to Friele."During the 2003 flood, the peak water level was 55 cm below the crest of the [Government Road Eagleview] dike. It was that close to going over the bank," he said. "Later in 2004, a Ministry of the Environment inspector of dikes Neil Peters of Water Land and Air Protection, was giving a talk in Burnaby and he puts the caption on a slide [of the dike] saying 'Squamish river dike protects the District of Squamish'. My comment is 'Yes, but just barely.'"
Friele said a breach is also possible at the Eagleview dike due to upwelling of water on the inside of the dike creating a cavity that can build on itself and end up with a dike breach.
"During the flood they were bringing in truckload after truckload of gravel to try to keep that upwelling down and the dike didn't breach," he said.
A second dike in danger of breaching is at Judd Slough, said Friele, since the presence of piping creates weakness in its integrity.
Not as imminent, but of much higher impact, is a Garibaldi Lake barrier collapse. The barrier has collapsed before, said Friele, most notably in 1855 when 25 million cubic metres which layered the lower Cheakamus River with two metres of sedimentation.
"You can imagine if that happened today, how that would affect the flood hazard in Squamish," he said.
Today's danger also takes into account the Daisy Lake dam, which appears to be in the path of a section of the barrier that could collapse. BC Hydro's assessment of such a landslide shows that the resulting destruction of Daisy Lake dam would inundate the valley floor in water. Midnight Way would be inundated with 68 metres of water in 30 minutes, Brackendale with 16 metres of water within one and a half hours, the Garibaldi Estates would have 11 metres of water within two hours, and downtown Squamish would be under seven metres of water within three hours.
"So this is complete devastation," said Friele. "But what I have to point out is this is an extremely rare event."
Another extremely rare occurrence is a great earthquake. John Clague, professor at SFU specializing in geological and slope hazards, said Squamish has never had a damaging earthquake in a 150-recorded history. A so-called great earthquake is unlikely to damage Squamish since the two massive shifting tectonic plates that create the earth's vibrations are far west of the coastline and well into the Pacific Ocean.
Clague did say, however, that the type of soil on which much of Squamish rests is vulnerable to liquefaction, a phenomenon whereby disturbed sediment during a strong earthquake causes granular material to lose their strength, and a material once thought to be solid flows "like watery concrete".
"Strong shaking during earthquakes can cause the fronts of these submarine deltas to fail, to slide into the sea," said Clague. "The materials that underlie Squamish delta, in my mind, probably vulnerable to this, although that would have to be borne out by detailed geotechnical testing. But if it looks like a rose, smells like a rose, it's probably a rose."
The presentation ended on a positive note when Natural Resource of Canada representative Sonia Talwar took the podium to list action the federal is taking to mitigate the hazards.
Talwar said the government is in the midst of assessing hazards with District of Squamish in combination with a municipal growth strategy.
"We're trying to develop a series of different growth management scenarios for which we can then evaluate the risk and hazards associated with each strategy," she said.
A draft version of this methodology will be presented to council later this winter. Natural Resources Canada will also be presenting a series of hazards community workshops.
