Top ten weather stories for 2012: story three
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3. BC Flooding … Larger, Longer and Lethal
The month of April started with a warning of potential serious flooding across the province by river forecasters in British Columbia. In many areas, spring surveys showed snowcover to be extremely deep – between 120 and 135 per cent of normal across numerous watersheds, including the entire length of the Fraser River. Snowpack was among the deepest measured in years with the fifth highest on record in the Fraser River basin and the second highest ever on the Skeena-Nass. At Roger’s Pass, snowfall in March totaled 324cm - 172% above average and the greatest in 47years of records. Adding concern was the loss over years of huge tracts of lodgepole pine decimated by the pine beetle, reducing a watershed’s ability to store and slow the release of meltwater. A brief hot spell, augmented by rainfall, led to flooding in late April in the Okanagan and Similkameen regions. With cooler spring weather through April and May, the spring freshet stalled across the province and became unusually long and late. A month later, mountain snowpacks hadn’t diminished much but were ripening quickly. And there was more bad news. Widespread June rains and violent thunderstorms enhanced the snowmelt, escalating the flood risk in the Kootenays, Okanagan, along the Fraser River and elsewhere. Floods were being fought on several fronts across the province. Emergency Management BC and municipalities opened 19 local emergency centres and the River Forecast Centre issued high water advisories for at least a dozen rivers from one end of the province to the other. Late in June, a moist weather system anchored off Oregon’s coast began soaking much of the province with 25 to 50 mm of rain triggering a new round of flooding. Some communities got as much rain in one day as they would normally see in the entire month, with several reporting double to triple normal amounts and new monthly and yearly records.
Along the mighty Fraser River the threat of breached dikes, endangered livestock, and damage to homes and property loomed large over the entire 600-kilometre stretch between Prince George and the Fraser Canyon. Thousands of worried residents were put on flood alert. In some places, rivers and lakes reached levels not seen in decades, forcing hundreds of residents from their homes, shuttering businesses, and collapsing and closing roadways. Sections of asphalt gave way leaving gaping holes that swallowed dozens of vehicles. Just south of Salmon Arm flash floods on a number of creeks near Sicamous knocked houses off their foundations and crumbled roadbeds, forcing the closure of the Trans-Canada Highway and Highway 97A along Mara Lake. Among those adversely affected, marinas suffered the worst during their high season. Near Nelson, one man drowned when a bridge washed away.
Other weather-related fatalities followed on the morning of July 12 when residents of Johnson’s Landing on the scenic shores of Kootenay Lake heard a loud rumbling and felt the ground shake. Forty-five seconds later a massive landslide engulfed the community destroying six homes and killing four people who were buried under tonnes of debris. The 4-m deep field of mud, rock and broken trees extended the length of several football fields cutting a massive swath through the tiny community. The tragedy was triggered by torrential June rains and a late-melting snowpack that also swelled Kootenay Lake to its highest level in 40 years. June rains were monsoonal, with a record 228 mm falling at Nelson and Castlegar just southwest of Johnson’s Landing. Prior to the landslide, it had rained on 12 of 13 days and for 7 straight days following. What caused the landslide was both geo-technical and hydrometeorological in nature: a debris flow and flooding across an unstable slope initiated by persistent rains on a deep snowpack that was delayed in melting by weeks of cool weather and a sudden rush of melt waters from higher elevations. Two days after the landslide, emergency officials rushed to Fairmont Hot Springs, 45 km away, after a mudslide forced the evacuation of hundreds of vacationers. At that site, a nearby creek overflowed sending tonnes of mud, boulders and rocks downhill.
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