A recent landslide at Mt. Breakenridge won’t cause a tsunami on Harrison Lake, but has prompted the Fraser Valley Regional District to take stock of its efforts to reduce the risk.
In January of this year, Tery Kozma captured footage of a landslide coming down the side of Mt. Breakenridge near Harrison Lake. The slide occurred in a geologically sensitive area, and FVRD staff sent the video along to emergency program coordinators in Kent and Harrison Hot Springs, as well as provincial stakeholders.
The scientists found that the landslides were happening in the surface soil on the lower part of the mountainside, and were simply running out into the lake. There was no immediate threat to the safety of those living around Harrison Lake.
This is good news — but there was initial cause for concern, as large landslides on Mt. Breakenridge could have the potential to cause a tsunami on Harrison Lake that could flood Harrison Hot Springs.
In the 1990s, scientists discovered that a large mass of rock was moving slowly down the mountainside. This rock mass, called a sackung, had the possibility to cause a massive landslide on Mt. Breakenridge. If enough material from a landslide landed in the lake at once, it could cause a tsunami that would impact First Nations, recreational properties and the Village of Harrison Hot Springs.
Studies from Emergency Management BC in 1990 showed that if 200 cubic metres of rock and debris came off the mountain, then waves as high as 25 m could be seen at Echo Island, 35 km away from the landslide and only five kilometres away from Harrison Hot Springs.
Although sediment from the Fraser River would help reduce the wave height near Harrison Hot Springs, it would still over-top the dikes by a few metres. The wave would pass through the village for around a minute and a half, with speeds of up to five metres a second.
It was only in 2020 that researchers found evidence that landslides around the lake had caused tsunamis in the past, as large amounts of sediment near Mt. Breakenridge, Mt. Douglas showed the possibility of displacement waves when they entered the lake.
Before, researchers assumed a major landslide at Mt. Breakenridge could happen less than once every 10,000 years. With the new data, that number goes down to once every 5,000 years, according to a report in SFU’s Centre for Natural Hazard Research newsletter.
The January landslide at Mt. Breakenridge didn’t mark the start of a potential major landslide and tsunami for Harrison Lake. But it did prompt discussions around how best to monitor Mt. Breakenridge and conduct evacuations in the event of an emergency, during the meeting between the FVRD, province and other stakeholders.
Evacuations for Harrison Hot Springs and Rockwell Drive would be the responsibility of the village and District of Kent, who share the Emergency Support Services team to provide things like food, clothing and lodging to evacuees. The FVRD would be responsible for evacuating other areas around the lake.
The report said that a tsunami-like event would require regional coordination, particularly as emergency management teams are quite small.
The FVRD report also noted that, from a scientific point of view, it would be part of best practice to monitor slope stability at the mountain. The province does not currently do this, and it’s unclear who should take responsibility for monitoring the area, since it is on rural Crown land. The report also noted there is a lack of options to bring in funding for this type of monitoring and associated early warning systems.