Top ten weather stories for 2007: story two
2. BC's Long Flood Threat
In early spring, experts from the British Columbia River Forecast Centre began bracing for and warning of the potential for the worst flooding in the province’s history. While most were focused on wild wind storms and falling trees, the mountain snowpack was accumulating at a record rate. It was great news for skiers and unprecedented numbers enjoyed the deep snow, but the growing snowpack was a threatening menace to those along swollen water courses, especially in north-coastal British Columbia and in the south along the Fraser River. Adding concern was the loss over years of huge tracts of lodgepole pine decimated by the pine beetle, reducing the watershed’s ability to store and slow release meltwater.
Snow water content ranged from a low of 83 per cent of normal in the Okanagan to 130 to 165 per cent of normal along the coast, in the north and in central regions. In the Fraser River, basin snowpack was amongst the largest measured since 1953, when detailed snow measurements began, and only slightly below that of 1972 when major flooding occurred. Officials began distributing close to five million sandbags to various “hot spots” and soldiers were ordered into the Lower Mainland in anticipation of rising waters.
In early April, unseasonably cool, wet weather not only delayed the melt but continued the build-up of snow. A month later, the snowpack had not diminished much and was ripening rapidly. Experts outlined a worst-case scenario: week-long heat followed by heavy rains before the usual peak river flow in June. Right on cue, hot weather arrived in late May with five days of +30°C temperatures. Scorching weather from Vancouver to Burns Lake smashed more than two dozen temperature records by 3 to 5°C, rapidly melting the mountain snowpack and swelling water courses in the Fraser River system and in the North-West. Behind the heat was more bad news – a widespread rain-bearing storm poised to move inland from the Pacific Ocean, escalating the flood risk for the Fraser River and the Lower Mainland. As it happened, the frontal system largely missed the Fraser watershed concentrating instead on the Bulkley and Skeena basins to the north.
Over the course of the spring, the British Columbia River Forecast Centre reported a number of significant flood events:
- The Nechako River peaked near a 50-year return period flow, and stayed at that extreme high for a five- to six-week period.
- The Bulkley River (at Smithers) exceeded its 100-year return period.
- The Skeena River experienced a 35-year return period event.
Along the banks of the Skeena, the City of Terrace was nearly marooned when rising waters and landslides forced the closure of several roads and highways and swamped rail lines. In the Bulkley-Nechako and Kitimat-Stikine basins, rising waters washed out secondary roads, shut down a bridge and cable ferry and led to the closure of a highway. Food and fuel hoarding started in Prince Rupert and Smithers, where flood waters rose to their highest levels in more than 80 years.
In the south, the long worry was over. By June 11, all monitored rivers had peaked or were falling. In the end, levels on the Fraser River never challenged the heights of near-flood years. The Fraser Valley avoided a catastrophe when cooler temperatures returned and a soppy storm diverted away at the last moment, sparing thousands of hectares of farm and residential land, avoiding the evacuation of tens of thousands of residents and saving an estimated cost-loss of $6 billion.
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