Latest / DEEP INSIGHT / Researchers say this B.C. lake faces a tsunami threat here's why

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Researchers say this B.C. lake faces a tsunami threat here's why · DEEP INSIGHT

• 0:01
  Speaker 1: When you think about tsunamis lakes probably aren't the first
• 0:05
  Speaker 1: thing to come to mind. But here in British Columbia,
• 0:08
  Speaker 1: the narrow and deep Harrison Lake is surrounded by towering peaks,
• 0:13
  Speaker 1: and that has had researchers pondering an unsettling threat for decades.
• 0:17
  Speaker 1: A lake tsunami about an hour boat ride from the
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  Speaker 1: resort community of Harrison Hot Springs. Now the threat largely
• 0:25
  Speaker 1: centers around these mountains here, like Mount Breckenridge. Researchers say
• 0:30
  Speaker 1: that a large landslide could cause a tsunami on Harrison Lake.
• 0:34
  Speaker 1: But how likely is that threat and how could it
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  Speaker 1: play out?
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  Speaker 2: The mountains on the east side are made of a
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  Speaker 2: weak rock called the Settler shist, which is oriented and
• 0:44
  Speaker 2: embedded in such a way that it moves if it
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  Speaker 2: were to fail all at once and slide into the
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  Speaker 2: lake with the hundreds of millions of cubic meters of debris.
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  Speaker 3: The light brown looking patches. Those are areas where with
• 0:59
  Speaker 3: recent movement on them.
• 1:02
  Speaker 1: In the late nineteen eighties, The province says Mount Breckenridge
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  Speaker 1: was identified as an unstable slope an initial report in
• 1:09
  Speaker 1: nineteen ninety. It took its best guess at a catastrophic failure,
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  Speaker 1: It estimated a two hundred million cubic meter landslide could
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  Speaker 1: trigger a roughly twenty seven meters tsunami at the source.
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  Speaker 1: As that wave travels, it could approach the beach at
• 1:24
  Speaker 1: about fifteen point five meters high and eventually overtop the
• 1:28
  Speaker 1: Harrison Beach Dike by around one point five meters. Water
• 1:32
  Speaker 1: would top the dike for about eighty seconds, traveling inland
• 1:35
  Speaker 1: at five meters per second.
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  Speaker 2: The Lake's not a simple situation. Depending where slides started
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  Speaker 2: where it entered the lake, the wave height would be
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  Speaker 2: radically different in different places.
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  Speaker 1: Now, researchers say that more recent studies suggest the mountain
• 1:50
  Speaker 1: is crumbling in smaller bits rather than large masses, providing
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  Speaker 1: scientists some relief. Holy In twenty twenty, the province says
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  Speaker 1: this this mountain experienced a high number of rock falls.
• 2:03
  Speaker 1: There was a temporary evacuation order for a nearby hydroelectric camp,
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  Speaker 1: but it was lifted in a short time.
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  Speaker 2: And there's two slides down here.
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  Speaker 1: Drew Brayshaw says, beyond Mount Breckinridge there are at least
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  Speaker 1: six areas along the shore of Harrison Lake where landslides
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  Speaker 1: could occur. Some researchers have also been looking under the
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  Speaker 1: lake for more answers and have found evidence of what
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  Speaker 1: they believe to be tsunami generating landslides.
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  Speaker 3: Here we have right below us, we have a deposit
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  Speaker 3: of about one point three million cubic meters.
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  Speaker 1: We met with Martin Gertzma, a researcher with the province
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  Speaker 1: of British Columbia. So far, his team has found evidence
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  Speaker 1: of three landslide deposits in Harrison Lake, one right here
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  Speaker 1: under Mount Breckinridge.
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  Speaker 3: That would have generated a modest tsunami locally, but not
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  Speaker 3: all over the lake.
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  Speaker 1: They found the largest opposit but Mount Douglas, equivalent to
• 2:59
  Speaker 1: about two hundred fifty thousand dump truck loads, and the
• 3:02
  Speaker 1: third by Silver Mountain.
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  Speaker 3: It's possible that we're underestimating the number, but that's all
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  Speaker 3: we've seen so far. So from that point of view,
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  Speaker 3: three fairly significant events in this lake over thousands of
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  Speaker 3: years is very quite a low landslide probability.
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  Speaker 1: Now, all three of these landslides were far smaller than
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  Speaker 1: what was studied in the nineties. In that case, again
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  Speaker 1: they looked at the possibility of a two hundred million
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  Speaker 1: cubic meter landslide.
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  Speaker 3: If there was something that large, then that would definitely
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  Speaker 3: reach the town of Harrison Hot Springs, and you would
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  Speaker 3: have perhaps twenty minutes twenty five minutes warning before the
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  Speaker 3: waves reached that We continue to oscillate around the lake.
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  Speaker 3: The likelihood of something like that, we think.
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  Speaker 4: Is very low.
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  Speaker 1: Now, this isn't the only lake that researchers say faces
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  Speaker 1: a tsunami threat. Not far from Harrison, a lake tsunami
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  Speaker 1: actually happened in two thousand and seven at Shahala's Lake.
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  Speaker 1: My colleague Johanna Wagstaff has been looking into that.
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  Speaker 5: That impact sent a wave thirty eight meters up the
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  Speaker 5: opposite shore right here, equal to roughly a twelve story building.
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  Speaker 5: Then it raised the entire eight kilometers down the length
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  Speaker 5: of the lake. No one was here that winter day,
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  Speaker 5: but scientists later piece the story together from the strip
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  Speaker 5: trees you can see behind me, bare rock and sonar
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  Speaker 5: scans at the lake bottom. The trigger for all of
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  Speaker 5: this heavy rain on snow that saturated the already fractured
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  Speaker 5: bedrock until the slope gave way. Researchers are using what
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  Speaker 5: they've learned here at Shahalis to identify other BC lakes
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  Speaker 5: at risk, especially as wetter, unstable slopes become more common.
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  Speaker 4: With climate change.
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  Speaker 1: Back here at Harrison Lake, so yes, there is path
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  Speaker 1: research raising concerns of lake tsunamis. There is evidence of landslides,
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  Speaker 1: but there are still lots of unanswered questions. The mayor
• 5:00
  Speaker 1: of Harrison Hot Springs says it's important to know the risks,
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  Speaker 1: even if they are challenging.
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  Speaker 4: To prepare for the lake tsunami is is hopefully something
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  Speaker 4: that never happens. Difficult from what I've read to really plan.
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  Speaker 4: I'm hoping the province is monitoring it and that if
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  Speaker 4: Mount Breckenridge looks like it's starting to shift or some
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  Speaker 4: event might occur, that we'll get some advanced notice and
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  Speaker 4: we can take some precautionary measures.
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  Speaker 1: Mayor Talen says the village is designing higher dikes to
• 5:27
  Speaker 1: better protect against more likely threats like flooding from rising
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  Speaker 1: water levels. And as for locals here, we ask some
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  Speaker 1: about the lake tsunami threat.
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  Speaker 4: No, it's not a big concern me.
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  Speaker 3: I just don't think that it will happen, you know,
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  Speaker 3: Like I mean, we don't know. It surprises me.
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  Speaker 2: I didn't know There was.
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  Speaker 5: A brief while when I was like, I was a
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  Speaker 5: little bit concerned, but since Ana, I've heard conflicting things
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  Speaker 5: about it.
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  Speaker 3: It'll be a very low risk for this location, but
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  Speaker 3: high consequence.
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  Speaker 2: Just like the fact that we have potential for huge
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  Speaker 2: magnitude nine earthquakes in the Pacific Northwest, we have potential
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  Speaker 2: for landslides to enter lakes and make waves, and that's
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  Speaker 2: the sort of level of concern that it needs to be.
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  Speaker 1: While researchers continue to question the threats of lakes, Tsunamis,
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  Speaker 1: Harrison Lake, nestled in these mountains is a reminder of
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  Speaker 1: the risks and uncertainties of BC's powerful and dynamic landscapes.
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  Speaker 1: Beneath Breach CBC News Harrison Hot Springs