Monday, April 9, 2018

How snowpack affects river flood risk

Bow river snow pack, Centre Street Bridge
While skiers and snowboarders celebrated the season’s higher-than-average snowfall, some Calgarians have been less enthusiastic, concerned that all this snowfall – once it melts – may increase our risk of river flooding. That risk is what The City of Calgary’s River Engineering Team monitors.

“We track weather and river conditions year-round,” says Sandy Davis, planning engineer. “We work with Alberta Environment’s River Forecast group, who also monitors conditions and river flow forecasts on an ongoing basis.”

And while the snowpack within the watershed that feeds the Elbow and Bow Rivers is higher than average for this time of year (approximately 120% of our average snowpack), Davis explains it is not too unusual, nor does it significantly increase our risk of river flooding.

“The important thing to remember is that, in Calgary, a high snowpack alone does not cause our rivers to flood,” says Davis. “There are many factors that influence our flood risk, the most important one being heavy rainfall upstream of Calgary.”

The data Davis’s group collects shows many other years in which similar high snowpacks did not result in river flooding. For example, in 2017, a high snowpack and a warm spring led to a quick melt and high river flows. While that prompted a temporary boating advisory on the Bow River, there was no flooding.

“The main driver of river flooding is heavy rainfall events, which are challenging to forecast because we are so close to the mountains,” explains Davis. “We can usually see large rain events about five to seven days out, but they may change course and may not hit our area. At the same time, once a large rainfall event is on its way, we may only have 24 hours or less to fully understand its scale.”

These kinds of rainfall events are most likely to occur mid-May through mid-July, so it is important for citizens to be aware of their risks and, if they live, work or commute in flood-prone areas, to be prepared to respond on short notice, if required.

For more information on how to understand, prepare and stay informed about flood risk, visit calgary.ca/floodinfo and sign up to receive our e-newsletter, which will be distributed biweekly throughout flood season.

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