Chilliwack and B.C. no stranger to mudslide risk

Landslides and land instability profile the Chilliwack region with the potential for dire consequences.

The fatal mudslide in Snohomish County, Washington, came without warning and took no more than one minute to rip out homes and kill at least 24 people. At the time of writing, some 90 people remain unaccounted for.

The devastating slide has all but obliterated the tiny town of Oso, 88 kilometres north of Seattle. One geologist reported that the slide was over 1,300 metres wide and that in places debris was over 10 metres thick. Nearly 50 structures have been destroyed.

Horror has given way to despair as desperate survivors wait for word of missing loved ones while the grim tally mounts. They cling to hope, praying for miracles, as rescuers struggle through the quagmire of dangerously unstable mud pushed up into berms of clay and quicksand. Navigating over that terrain is both agonizingly slow and treacherous. Rescuers were helping rescuers to stay safe.

The cause for this tragedy has been excessive rains which saturated the land making it extremely unstable. A smaller slide happened in 2006 and a past report allegedly warned of the potential for catastrophic failure. When the slope let go, it released mud, rock, trees, and debris that blew down the hillside at a staggering 20 metres a second. Anyone in its path didn’t stand a chance.

Landslides are no stranger to people in B.C. In January 2005 a mudslide in North Vancouver destroyed two homes and killed a woman sleeping in her bed. In 2008 the Sea to Sky highway near Whistler was closed for five days after a massive rockslide almost took out a passenger bus. It happened again in 2011 when a rockslide near Lions Bay destroyed a vehicle and released rocks the size of cars down the slope.

In July 2010 an aging dam gave way near Oliver, releasing mud, water, trees and debris down the hillside and destroying five homes and several farms. A month later, a two-kilometre wide landslide in the Pemberton River Valley put one thousand residents on flood alert. In July 2012 four people died when a massive landslide destroyed several homes in the Kootenay community of Johnson’s Landing. And just two months ago, a rockslide on Highway 3 near Keremeos closed the route completely.

Landslides and land instability profile the Chilliwack region with the potential for dire consequences. In June 2011 a landslide east of Chilliwack roared across four lanes of the highway and onto the railway track. It slightly injured one woman whose car was rolled by the debris.

“There is a history of small landslides and debris flows in the Chilliwack River valley between Slesse Creek and the mouth of the valley,” said Dr. John Clague, Canada Research Chair in Natural Hazards Research, Simon Fraser University. “Although none has been fatal, a few have caused property damage. Property owners are exposed to a not well-documented risk from landslides and debris flows from the valley walls; these steep slopes are developed in glacial sediments. There is also the possibility that a landslide in these sediments could block or impede the flow of Chilliwack River, with downstream consequences.”

Many slides are triggered by excessive rains that saturate and destabilize the ground but earthquakes, volcanoes, and extremes of alternate freezing and thawing are also triggers. In the Fraser Valley, as in western Washington, we’ve had an excessive amount of rain this month. As at March 25, Chilliwack had received 247 mm, well in excess of the March average of 176.l mm.

“Because the degree of risk (in the Chilliwack valley) is ill-defined, and in light of the landslide at Oso, Washington, it is my opinion that the Fraser Valley Regional District should commission a geotechnical study to evaluate the hazard,” said Clague.

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