Observations on the May 2019 Joffre Peak landslides, British Columbia

Pierre Friele,Marten Geertsema,Tom H Millard,Brian Menounos,Andrew Mitchell,John J Clague,Kate E Allstadt

doi:10.1007/s10346-019-01332-2

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 Mm3, with combined volume of 5 Mm3. 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 super-elevation and boulder sizes, was 5–10 m/s. The volume of debris deposited on the fan was ~ 104 m3, 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.

Highlights

On 13 and 16 May 2019, two large catastrophic rock avalanches occurred on the north slope of Joffre Peak, devastated the middle reach of Cerise Creek, and affected the alluvial fan
The north face of Joffre Peak is visible from British Columbia (BC) Highway 99 (Fig. 2), which connects the towns of Pemberton and Lillooet and is a scenic route through the Coast Mountains (Fig. 1)
Social media played a large role in rapidly alerting the geoscience community to the occurrence of the May 2019 Joffre Peak landslides, in southwest BC, allowing rapid photogrammetric data collection and hazard/risk assessment

Summary

Introduction

On 13 and 16 May 2019, two large catastrophic rock avalanches occurred on the north slope of Joffre Peak, devastated the middle reach of Cerise Creek, and affected the alluvial fan. On the afternoon of 13 May 2019 and the days thereafter, Brian Goldstone and Wayne Flann posted photos taken the previous week on the South Coast Touring Facebook page (https:// www.facebook.com/groups/southcoasttouring/) that indicated areas downslope from the recent scar were being affected by rockfall which was noticeably dirtying the slope This material was emanating from a wide vertical cleft on the left margin of the recent scar (Fig. 2b). Because LLLB had lower noise levels, the signals emerged earlier at that station than at WSLR, 10.0

50 Sclialtyy 40

13 May 2019 16 May 2019

Conclusions