Scientists issue warning about landslide looming above Boston Bar

The discovery of a massive historic landslide on the mountainside directly above Boston Bar has prompted a preliminary warning about its potential impact on the small Fraser Canyon community.

The evidence of the slide comes in the form of new imagery showing instability and previous movement on the steep hillside above Boston Bar. It’s not yet clear how much of a threat the slide poses today to the community below, but the scale and location of the “landslide complex” was enough to prompt the scientists who discovered the slide to warn regional officials that they should be wary of approving new development in the community until they know more.

Public safety concern

Last year, geoscientists analyzing new LIDAR imagery of the slopes along the Fraser Canyon noticed concerning patterns on the west-facing mountainside looming over Boston Bar.

LIDAR is a radar-like technology that provides a three-dimensional look at an area’s topography. It allows geoscientists to see through forests and vegetation and closely inspect the surface of a mountainside, river bottom, or valley bottom.

The geoscientists were analyzing LIDAR data collected by the federal government as part of work being done for local First Nations. And when they inspected, or “interpreted” the imagery, they found the characteristics of an ongoing long-lasting landslide. The toe of the slide directly abutted the Boston Bar townsite.

In essence, they found the entire mountainside could be unstable.

More research is needed to determine what, if any, danger the slide poses to the small community at its base. Dozens, if not hundreds, of similar historical landslides exist around British Columbia. Some move a little bit each year. Others are static. Some are unlikely to ever pose a problem for nearby settlements while others could carve a swath of destruction with little warning. A slow-moving landslide may pose little daily threat to human life while still impacting the viability and stability of buildings and infrastructure.

The behaviour of each landslide depends on a multitude of factors, including from the pitch of the slope, the composition of soil and underlying rock, climactic conditions, vegetation, nearby human activity including logging, and seismic activity. Many of those factors are themselves uncertain or subject to variations over time.

The most memorable landslides tend to be those that take place in a series or minutes or hours, when a chunk of hillside gives way and threatens the lives of anyone in its path. But BC also has many slides that involve land moving very slowly and steadily downhill. Those may not kill or maim people, but can still cost landowners and governments millions of dollars—and alter the future of entire communities..

In Chilliwack, after originally allowing homes to be built on what turned out to be a slow-moving landslide in its Eastern Hillsides and Marble Hill area, the municipal government spent more than $21 million to buy out homes on much of the land. Many of the homes are still standing and rented out because the landslide is not considered to pose a major safety threat, but future development has been banned over a large swath of land.

Further north in the Fraser Canyon, past Lillooet, the provincial government had to spend more than $80 million to stabilize a 300-metre section of Highway 99 that was built on an ongoing, slow-moving slide.

Engineers still don’t know the danger posed by the slide above Boston Bar. But a report by staff at the Fraser Valley Regional District said the engineering firm that made the discovery “felt compelled to proactively share the information … in the broader interest of public safety.”

The firm, BGC Engineering, was working for local First Nations at the time of the discovery, though they have also regularly done work for the FVRD, including on landslides in the Chilliwack River Valley. They recommended that the FVRD further analyze the slide to determine its threat to Boston Bar. (Boston Bar is not incorporated and land use planning and other services are provided by the regional district.) An investigation won’t come cheap, and BGC suggested the FVRD apply for grants from the province and the Union of BC Municipalities.

But until the landslide is studied further, FVRD officials say the landslide will need to be factored into plans for the future of Boston Bar.

“Without further assessment the uncertainty around public safety will persist,” FVRD planning manager Katelyn Hipwell wrote in a report delivered last week to the regional district’s electoral area services committee.

She suggested that the slide might need to be factored into how and when the regional district allows residents to build new structures.

“Future community planning decisions would require an extremely cautious and conservative approach,” Hipwell wrote. “Site specific considerations for development, such as a building permit for a new dwelling, would need to be made under difficult circumstances that would put the burden of characterizing the known landslide hazard on individual property owners.”

The FVRD is now preparing an application to the province for funds to study the slide, according to spokesperson Samantha Piper. Until the study is complete, Piper said existing frameworks that guide development in geohazard areas will apply.

Piper noted that the FVRD has a database of more than 1,000 geohazard reports across the region. They can be publicly accessed via an online webmap. (Users need to click the layer icon in the top-right corner and select “geohazard reports” under “hazards.” Reports can be found by selecting a property and then clicking the rightward-facing arrow until a link to the report becomes visible.)

As that database suggests, there is no shortage of land that has the potential to do a little—or a lot—of damage in the Fraser Valley and Canyon. Few threats, however, pose a potential threat to an entire community the way the landslide above Boston Bar does.

The only comparable might be the landslide potential in the Chilliwack River Valley. We’ll have a story on that threat in the coming weeks.