City officials are keeping a close eye on all the factors that could lead to overland flooding during the spring runoff season, which stretches from mid-May to mid-July. In a briefing Tuesday, they said there are areas of potential concern, but they stopped short of sounding alarms. Herald reporter Reid Southwick examines what city hall is doing to prepare for spring runoff.

Monitoring snowpack

Snow levels in the Rocky Mountains are slightly higher than last year at some observation stations; in other spots, the snow pack is roughly at average thickness. What concerns city officials is heavy rain falling on the snow, which increases water levels heading downstream to Calgary. But snowpack alone cannot cause flooding. In 2012, mountain snow was a lot thicker than it is now, which created a welcome supply of drinking water during a hot summer.

Monitoring the weather

The single greatest cause of flooding in Calgary last June was an “incredible dump of rain in a short period of time, in the worst possible area it could fall,” said Rick Valdarchi, manager of city hall’s flood mitigation program. About 300 millimetres, roughly the same amount of rain that hits in Calgary in a typical year, fell in two days. About 10 to 20 millimetres of rain is forecast to hit the city Friday, which could slightly increase river levels but “nowhere near” what could trigger a flood, Valdarchi said. It could force pathways under bridges and overpasses to close temporarily.

Draining Glenmore Reservoir

The Elbow River flows into Calgary through the reservoir, which makes it a critical link in the flood-prevention story. City officials are starting to drain the basin, which they do every year, to make way for spring runoff. But they don’t want to drain the basin too much because it could cause water shortages through the summer. They also have to be careful because draining water too rapidly could destabilize river banks.

Monitoring river levels

City hall steps up its monitoring efforts every year during the runoff period, from mid-May to mid-July. Last year, there were two staffers rotating on day and night shifts. There are now two more workers on the team. Water typically flows through the Elbow River at eight to 10 cubic metres per second. Right now, the river is flowing faster — 19 or 20 cubic metres per second — because officials are draining the reservoir. During the flood, flows were at more than 700 cubic metres per second.

Getting the word out:

City hall plans to host two more open houses to share information on flood preparation and recovery efforts. The first will be held Wednesday, from 4:30 p.m. to 7:30 p.m., at the Ramsay Community Association hall, 1136 8 Street S.E. The second is scheduled for May 15, between 6 p.m. and 8 p.m., at Foothills Academy, 745 37 Street N.W. There will be no formal presentations, but residents can drop in to ask questions. More than 1,200 Calgarians attended open houses hosted by the city and province in March and April.

rsouthwick@calgaryherald.com

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