6 January 2020
A detailed analysis of the Joffre Peak landslides in Canada
In May 2019 two large landslides occurred on Joffre Peak in British Columbia in Canada. I described these events at the time, but a proper analysis has now been published (Friele et al. 2020) in the journal Landslides. The authors have found that these two landslides each had a volume in the range of 2 to 3 million cubic metres. Both started as large detachments on steep rock slopes, which then transitioned into rock avalanches before becoming debris flows and finally debris floods. The maximum extent of the primary debris flow deposit was about 4 km from the landslide source, whilst the debris flood reached the margin of the fan, about 6 km downstream.
The authors provide this composite image of the aftermath of the second of the Joffre Peak landslides:-
For me there are perhaps three interesting things to take from this excellent analysis:
1. The first Joffre Peak landslide followed precursory activity on the rock slope
Friele et al. (2020) note that photos of Joffre Peak in the two years prior to the first landslide indicate that during this period a large rockfall occurred on the slope that failed. The authors have used Sentinel Hub imagery to narrow this down to the period between 23 October and 7 November 2018. Photographs taken on 13 May 2019 and the days thereafter, and posted to the South Coast Touring Facebook page show that significant rockfall activity was occurring just before the collapse, suggesting that the first detachment was a progressive failure.
2. The collapse was probably associated with warming temperatures in the high mountain areas
The source of the landslide was probably an area of permafrost, given the elevation and the northerly aspect. Friele et al. (2020) note that “since 1970–1980, decadal average temperatures have risen incrementally, contributing to permafrost degradation. By 2019, permafrost in the landslide head scarp was likely severely degraded, and close to zero”. Whilst it cannot be proven, the authors suggest that the loss of permafrost conditions is likely to be the long-term cause of these landslides. This would fit with observations from elsewhere.
Meanwhile, the days prior to the first collapse were associated with warming conditions – Friele et al. (2020) observe that “Although temperatures fell slightly between the 13 and 16 May failures, weekly average temperatures were still 5–7 °C and the snowpack continued to melt. Interestingly, both landslides happened in the morning hours, perhaps reflecting a lag in the diurnal snowmelt pulse. Thus, snowmelt and high pore pressures may have contributed to the 16 May landslide as well.”
3. There is some residual hazard on Joffre Peak
The paper reports the observation made by Drew Brayshaw at the time that there is a further area of potential instability on Joffre Peak. Friele et al. (2020) suggest that whilst this is correct, a further collapse is not likely to be imminent, and that the risk associated with such a failure is likely to be low.
Friele, P., Millard, T.H., Mitchell, A. et al. 2020. Observations on the May 2019 Joffre Peak landslides, British Columbia. Landslides