Record rainfall, snowmelt caused one-in-12,000-year fatal B.C. landslide: report - Macleans.ca
 

Record rainfall, snowmelt caused one-in-12,000-year fatal B.C. landslide: report


 

VANCOUVER – A massive landslide that ripped through a small hamlet in southeastern British Columbia last year, killing four people, was caused by a deluge of rain and a late spring snowmelt that triggered the largest slide to hit the region in at least 12,000 years, a report into the disaster concludes.

The report, released Thursday by the Regional District of Central Kootenay, makes a number of recommendations to ensure the disaster that struck Johnsons Landing last July doesn’t repeat itself, but it also suggests the slide was unprecedented in size and nearly impossible to predict.

The slide struck in the morning of July 12, 2012, sending 320,000 cubic metres of soil, trees and rock down an area known as the Gar Creek channel, hitting several homes at speeds of up to 150 kilometres per hour.

Valentine Webber, his 17- and 22-year-old daughters Rachel and Diana, and German woman Petra Frehse were buried in the debris, prompting a frantic rescue operation that was hampered by harsh weather and unstable conditions. All four died.

“Record June rainfall and late snowmelt saturated the soils on the slope above the community and triggered the landslide,” says the regional district’s report.

“A landslide of similar size has not occurred in this area since deglaciation (last 12,000 years).”

The debris destroyed five homes and left another damaged. Many other residents of Johnsons Landing, a community of about 35 people more than 200 kilometres southwest of Calgary, were forced to flee their homes, some of which remain under an evacuation order.

The report identifies 18 properties that remain at risk of another potentially deadly slide, including five where the risk is considered “very high.” That means the risk of dying for someone living on those properties due to a slide in any given year would be as high as one in 10.

Nine properties are in an area considered to be at a high risk, while four more are in a zone where the risk is considered moderate.

The report recommends restricting land and house development on those 18 properties unless geotechnical research indicates those areas are safe, but it also says such work would likely be too expensive to conduct for some of the properties.

The document says previous assessments of the area did not raise the possibility of such a massive landslide.

That included terrain mapping and site assessments in 1983, 1994, 2001 and 2002 — none of which identified “the possible occurrence of a landslide large enough to travel onto the Johnsons Landing bench,” the report says.

The report notes, however, that local residents observed high water levels in local creeks, increasing amounts of sediment in the creeks, and debris blockages in the days and weeks leading up to the slide.

The regional district’s report includes a number of recommendations, including:

— Establishing uniform and consistent criteria for assessing landslide risks.

— Creating a landslide monitoring program, including collecting data on slope movements, rainfall and snowpack accumulation and melt.

— Launching a public awareness campaign about how to recognize unusual creek activity and slope instability.

— Creating communication plans and protocols to update residents and visitors of local conditions when there is an increased risk of landslides.

— Improving emergency communications by ensuring consistent protocols for all emergency personnel responding to landslides.

The regional district said in a news release it would review the recommendations to determine what steps it will take.

“We have already spoken with those residents in the highest hazard zones so they understand the level of risk to their particular properties and to themselves,” Dawn Attorp, the regional district’s manager of corporate administration, said in a statement.

“We still have a considerable process ahead of us before we can lift the evacuation order for the landslide area and take the next steps in supporting the community in its recovery.”

Calls to the provincial government were directed to Emergency Management BC, which could not be reached for comment.


 
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Record rainfall, snowmelt caused one-in-12,000-year fatal B.C. landslide: report

  1. The Johnson’s Landing bench was formed by sediment coming down off the mountain and filling up the shore of the lake.

    Much of the land thereabouts is steep, while the sediment deposition zones are relatively flat.

    So people built on the flatter areas, exactly the areas susceptible to debris flow because they’re MADE of debris of previous landslides.