Observations on the May 2019 Joffre Peak landslides, British Columbia

Abstract

Two catastrophic landslides occurred in quick succession on 13 and 16 May 2019, from the north face of Joffre Peak, Cerise Creek, southern Coast Mountains, British Columbia. With headscarps at 2560 m and 2690 m elevation, both began as rock avalanches, rapidly transforming into debris flows along middle Cerise Creek, and finally into debris floods affecting the fan. Beyond the fan margin, a flood surge on Cayoosh Creek reached bankfull and attenuated rapidly downstream; only fine sediment reached Duffey Lake. The toe of the main debris flow deposit reached 4 km from the headscarp, with a travel angle of 0.28, while the debris flood phase reached the fan margin 5.9 km downstream, with a travel angle of 0.22. Photogrammetry indicates the source volume of each event is 2-3 Mm 3 , with combined volume of 5 Mm 3 . Lidar differencing, used to assess deposit volume, yielded a similar total result, although error in the depth estimate introduced large volume error masking the expected increase due to dilation and entrainment. The average velocity of the rock avalanche-debris flow phases, from seismic analysis, was~25-30 m/s, and the velocity of the 16 May debris flood on the upper fan, from superelevation and boulder sizes, was 5-10 m/s. The volume of debris deposited on the fan was~10 4 m 3 , 2 orders of magnitude less than the avalanche/debris flow phases. Progressive glacier retreat and permafrost degradation were likely the conditioning factors; precursor rockfall activity was noted at least~6 months previous; thus, the mountain was primed to fail. The 13 May landslide was apparently triggered by rapid snowmelt, with debuttressing triggering the 16 May event.

Debris Flows, Boulders and Constrictions: A Simple Framework for Modeling Jamming, and Its Consequences on Outflow

“…The height of this initial log jam is a key unknown, and may vary between a few meters to more than 20 m in height depending on stochastic movements of woody debris based on observations in the field. This range is an expert estimate by the authors based on (a) the high height of the trees standing in the debris basin (see Figures 7b and 7c), as well as along the upstream confined debris flow channel (Jakob & Friele, 2010), and (b) evidence from field observations that large wood chunks (with sizes of several to more than ten m) are systematically transported at the front of large scale collapses transforming into debris flows (see pictures in Guthrie et al, 2012;Roberti et al, 2017;Friele et al, 2020). For jammed woody debris, evidence is lacking for a best estimate range, and even bounds are difficult to define.…”

Rock Avalanche-Generated Sediment Mass Flows: Definitions and Hazard

“…Zone 2 impacts P(E) 1 P(a Z1,1 1.3 × 10 6 m 2 ) 0.2 (0 < A Z1 ≤ 1.5 × 10 6 m 2 ) P(E Z2 ) 1 P(a Z2,1 4.3 × 10 4 m 2 | a Z1,1 ) 0.2 (0 < A Z2 ≤ 9.1 × 10 4 m 2 ) P(a Z2,1,1 ) 0.04 P(a Z2,2 2.8 × 10 5 m 2 | a Z1,1 ) 0.6 (9.1 × 10 4 m 2 < A Z2 ≤ 6.5 × 10 5 m 2 ) P(a Z2,1,2 ) 0.12 P(a Z2,3 9.4 × 10 5 m 2 | a Z1,1 ) 0.2 (6.5 × 10 5 m 2 <A Z2 ) P(a Z2,1,3 ) 0.04 P(a Z1,2 2.1 × 10 6 m 2 ) 0.6 (1.5 × 10 6 m 2 < A Z1 ≤ 2.9 × 10 6 m 2 ) P(E Z2 ) 1 P(a Z2,1 7.0 × 10 4 m 2 | a Z1,2 ) 0.2 (0 < A Z2 ≤ 1.5 × 10 5 m 2 ) P(a Z2,2,1 ) 0.12 P(a Z2,2 4.6 × 10 5 m 2 | a Z1,2 ) 0.6 (1.5 × 10 5 m 2 <A Z2 ≤ 1.1 × 10 6 m 2 ) P(a Z2,2,2 ) 0.36 P(a Z2,2 1.5 × 10 6 m 2 | a Z1,2 ) 0.2 (1.1 × 10 6 m 2 <A Z2 ) P(a Z2,2,3 ) 0.12 P(a Z1,3 3.5 × 10 6 m 2 ) 0.2 (2.9 × 10 6 m 2 < A Z1 ) P(E Z2 ) 1 P(a Z2,1 1.1 × 10 5 m 2 | a Z1,3 ) 0.2 (0 < A Z2 ≤ 2.5 × 10 5 m 2 ) P(a Z2,2,1 ) 0.04 P(a Z2,2,2 7.7 × 10 5 m 2 | a Z1, ) 0.6 (2.5 × 10 5 m 2 <A Z2 ≤ 1.8 × 10 6 m 2 ) P(a Z2,2,2 ) 0.12 P(a Z2,2,3 2.5 × 10 6 m 2 | a Z1,3 ) 0.2 (1.8 × 10 6 m 2 <A Z2 ) P(a Z2,2,3 ) 0.04 limitations to what can be seen from aerial or satellite imagery. This issue was highlighted by the 2019 Joffre Peak case described by Friele et al (2020), in which the extent of the Zone 2 area estimated from the post-event PlanetScope ortho imagery was different from that observed during field mapping. This discrepancy was because there were areas where the mass flow of saturated sediments and organic material did not entirely remove the trees present in the impact area, meaning the full impact could not be observed clearly in the satellite images.…”

INSPIRE standards as a framework for artificial intelligence applications: a landslide example

“…Moderate weather Sometimes (Friele, 2012;Segoni et al, 2018) Landslides can be triggered by intense rainfall (Segoni et al, 2018) or snowmelt. Rainfall thresholds for this study are derived from Friele (2012). Has land use Alpine Always (Evans and Clague, 1994) Landslides are common in the Alpine zone, especially under changing climatic conditions.…”

“…Moderate rainfall Sometimes (Friele, 2012;Segoni et al, 2018) Landslides can be triggered by intense rainfall (Segoni et al, 2018) or snowmelt. Rainfall thresholds for this study are derived from Friele (2012).…”