Calgary roads plagued by sinkholes after severe flooding

Calgary roads are recovering after a busy summer that saw city crews repairing about 200 sinkholes, which occurred after the June 20 flood.

“We probably see in Calgary around 10 or 12 sinkholes throughout the course of the year,” said city roads spokesperson Sean Somers.

“Most of those would be caused by watermain breaks.”

The city roads department saw an increase in the number of sinkholes after the rising Bow River waters caused widespread flooding in many parts of downtown in June.

“After the flooding, the downtown core north of 25th Ave., east from 14th Street and west of Macleod Trail was under water,” Somers said.

“About 80 per cent of the sink holes in Calgary happened in the confines of that area.”

Sinkholes occur when there’s a watermain break or excessive groundwater washes away sand, rock and gravel under city roads, leaving no support under pavement.

“What happened was, because all of that land was underwater, as the water began to recede, so did some of the gravel,” Somers explained.

There’s no way to predict where these craters will occur as they’re underground and not exposed until pavement gives way.

The biggest sinkhole the roads department has seen was two metres wide and two metres deep.

“It really depends on the circumstance, but generally in Calgary, they’re relatively small,” he said.

Somers said he hopes the occurrence of sinkholes in the city will taper off.

“After the big spike in repairs which happened the week after the flood, since then, we’ve only had one big one and it ended up not being that big in the end,” he said.

“The one on the corner of 10th Avenue and Macleod Trail, we were able to get there and have that repaired within a day,” he said.

Fixing Calgary’s sinkholes involves backfilling the hole, compacting the gravel and earth, before repaving the area.

Sinkholes as big as the recent one in Florida, which swallowed parts of a resort, and in Louisiana, which swallowed an area of swamp and continues to grow in size, won’t occur in Calgary.

“The sandstone that the city’s downtown is constructed upon would likely be stable, it is consolidated rock,” said Stephen Hubbard, geologist and associate professor at the University of Calgary.

“That bedrock is overlain by alluvium – river and glacial outwash deposits.

“It is more unconsolidated and is more prone to washing out if high water flow establishes beneath the surface. Most large structures would be anchored into the sandstone bedrock.”