Mission’s steep incline may protect it from the devastating floods that have rocked its Fraser Valley neighbours, but landslides are another story.
While dozens of properties have been flooded by elevated waters in Hatzic Lake, the larger threat to Mission and its surrounding communities stem from over-saturated soil collapsing on infrastructure and property, according to Trevor Regehr, vice president of Mission Search and Rescue (MSAR).
He said the Fraser Valley Regional District’s (FVRD) evacuation alert for 1,600 properties in the Hatzic Valley was issued after a landslide forced MSAR to rescue a family by helicopter.
“At any point, you could have a big slide come through the valley,” Regehr said, noting the family’s property was adjacent to a steep mountain. “We haven’t had any really big ones, but it could happen.”
Since the first atmospheric river struck B.C. on Nov. 14, MSAR has responded to several slides in the Mission area.
A mudslide on Nov. 28 shut down all traffic on Sylvester Road, which is one of only two streets leading into the isolated northern communities of the valley such as Durieu and Miracle Valley.
— Patrick Penner (@portmoodypigeon) November 28, 2021
Regehr said he knows of at least a half-dozen spots on the road that are prone to collapsing along its eastern edge.
Stave Lake Street, the other access route into these two communities, was also in danger of being cut off from a culvert blockage, Regehr said. “Everyone could have been stuck.”
He said that landslides could also potentially cause a “bridge blowout” if a log were to become jammed – a concern for a few bridges close to Cascade Falls.
The FVRD said in an email their concern is over the “unpredictability of slide events, and the potential for road access to be cut off.”
Slides are the result of both overflowing creeks and streams and soaked soil collapsing under its weight.
The city attempted to update their tree bylaw (but failed to pass it in 2020), partly to help manage slope stability in the face of the developers clear cutting of large properties.
Mission’s Chief Administration Officer Mike Younie said the second atmospheric-river event caused several small landslides in Mission proper.
“All it takes is a bit of a bank slough along the creek and trees fall in and block a culvert, and then you’ve got bridges being eroded,” Younie said.
There are several areas in Mission with a history of slides that city staff are monitoring, Younie said, including along Frog Hill on Stave Lake Street (north of Richards Avenue), and Edwards Ravine in north Hatzic to name a few.
In comparison, the flooding caused by the high water levels of Hatzic Lake will be “nothing as catastrophic” as the flooding damage to Abbotsford and Hope, Regehr said, adding it typically dissipates a few hours after the rain stops.
The heavy rainfall caused a series of overland flooding and mudslides in Mission on Nov. 30. One mudslide, just south of Hayward Dam, took out a fence and damaged the a water storage for over a dozen homes on Wilson Road, forcing the city to take the system offline.