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The Hazards of Glacier Lake Outburst Floods: The 28 Nov 2020 Elliot Creek, Canada Event

The Hazards of Glacier Lake Outburst Floods: The 28 Nov 2020 Elliot Creek, Canada Event

 Paul Somerville, Chief Geoscientist, Risk Frontiers
 

Glaciers that once blanketed and held mountain slopes together are melting rapidly and retreating due to climate change, leaving mountain slopes unstable and vulnerable to sudden failure.

Glacier lake outburst floods are rapid releases of large amounts of water from a glacier. On November 28, 2020, in the southern Coast Mountains of British Columbia, about 18 million cubic metres of rock descended 1,000 m from a steep valley wall near the West Grenville glacier and traveled across the toe of the glacier before entering a 0.6 square km glacier lake, producing a tsunami run-up over 100m high.

UBC (2022) quoted Professor Marten Geertsema as describing the event as “imagine a landslide with a mass equal to all of the automobiles in Canada travelling with a velocity of about 140 kilometres an hour when it runs into a large lake.” .

The rockfall generated a magnitude 5 earthquake that was recorded around the world. There are no reported fatalities from this event.
Water overtopped the lake outlet and scoured a 10-km long channel before depositing debris on a 2 square km fan below the lake outlet. Displaced water destroyed salmon-spawning habitat over a distance of 8.5 km and created a plume of sediment and organic matter more than 60 km from the head of the fjord into which the floodwaters discharged to Bute Inlet. Floodwater, organic debris, and fine sediment entered the fjord where it produced a 60 km long sediment plume and altered turbidity, water temperature, and water chemistry for weeks.

Figure 1. Slopes exposed by the retreating glacier at the head of Elliott Creek. Source: The Canadian Press/Ho-Hakai.
Canyon created by the scouring of Elliot Creek bed by the rockfall. Source: Bastian Fleury / 49 North Helicopters.

Global Impact of Glacier Lake Outburst Floods

More than 1,000 glacier lake outburst floods from alpine lakes have been reported globally since 1900, resulting in more than 12,500 fatalities, with large amounts of damage to infrastructure and farmland, and disruptions of transportation and communication (Carrivick and Tweed, 2016).

Not all glacial lake outburst flows are triggered by landslides or avalanches, but most, like the Elliot Creek event, involve complex down valley flows varying in time and space from clearwater floods through hyperconcentrated and debris flows (Clague and Evans, 2000; Clague and O’Connor, 2021).

In that sense, they are examples of hazard cascades, in which a triggering event produces an outflow that commonly evolves and causes secondary effects as it propagates down valley.

An assessment of the hazard posed by landslides and avalanches into alpine lakes requires lake inventories and modeling that can help predict where and why these lakes will form and grow in the future (Haeberli and Drenkhan, 2022).

Glacier outburst floods are a pervasive natural hazard worldwide. They have an association with climate primarily via glacier mass balance and their impacts on society partly depend on population pressure and land use.

Given the ongoing changes in climate and land use and population distributions there is therefore an urgent need to identify the spatio-temporal patterns of glacier outburst floods and their impacts. Carrivick and Tweed (2016) present data compiled from 20 countries and comprising 1348 glacier floods spanning 10 centuries. Societal impacts were assessed using a relative damage index based on recorded deaths, evacuations, and property and infrastructure destruction and disruption. These floods originated from 332 sites; 70% were from ice-dammed lakes and 36% had recorded societal impact.

Glacier floods have directly caused at least: 7 deaths in Iceland, 393 deaths in the European Alps, 5745 deaths in South America, and 6300 deaths in central Asia.

Peru, Nepal and India have experienced fewer floods but higher levels of damage.

One in five sites in the European Alps has produced floods that have damaged farmland, destroyed homes and damaged bridges.

Ten percent of sites in South America have produced glacier floods that have killed people and damaged infrastructure.

Fifteen percent of sites in central Asia have produced floods that have inundated farmland, destroyed homes, damaged roads and damaged infrastructure.

Overall, Bhutan and Nepal have the greatest national-level economic consequences of glacier flood impacts.

Carrivick and Tweed (2016) recommend that accurate, full and standardised monitoring, recording and reporting of glacier floods is needed for improved understanding of spatio-temporal patterns in glacier flood occurrence, magnitude and societal impact. They point out that future modelling of the global impact of glacier floods cannot be based on the assumption that current trends will continue, and will need to consider combining land-use change with probability distributions of geomorphological responses to climate change and to human activity.

The Carrivick and Tweed (2016) data span the time range from 1525 to the present, with the European Alps and Iceland having records that extend furthest back in time. They found that floods are occurring progressively earlier in the year at two thirds of sites that have produced more than 5 floods (32 sites). The cumulative number of events shows an increase in slope at about 1860 that continues to increase before apparently decreasing after the mid-1990s in all major world regions. However, they do not address the completeness of their data in previous centuries, nor do they raise the question of whether their data show the effects of climate change.

The 1958 Lituya Bay Earthquake, Landslide and Tsunami

The largest known tsunami generated by a landslide was caused by a landslide into a bay, not into a glacial lake.

The 9 July 1958 magnitude 7.8 Lituya Bay earthquake was a strike-slip earthquake on the Fairweather Fault in Alaska. It triggered a rockslide of 30 million cubic meters (about 90 million tons) into the narrow inlet of Lituya Bay, Alaska (Pararas-Carayannis,1999). The impact was heard 80 km away, and the sudden displacement of water resulted in a tsunami that washed out trees to a maximum elevation of 524 metres (1724 feet) at the entrance of Gilbert Inlet.

This was a tsunami of very limited extent, but it is the highest tsunami ever recorded.

The locations of the Lituya Bay and Elliot Creek slides are shown in Figure 3.

Figure 2. Location map of the rockfall and forest trimline in Lituya Bay. Bottom: Forrest trimline in Lituya Bay due to scour from the landslide-generated tsunami. Source: Pararas-Carayannis (1999).
Figure 3, Locations of the Lituya Bay slide (northwest star, northwest of Juneau, Alaska) and Elliot Creek slide (southeastern star, northwest of Vancouver, British Columbia and Seattle, Washington), with glaciers shown in white.

References

Carrivick, J. L., and Tweed, F. S. (2016). A global assessment of the societal impacts of glacier outburst floods. Global and Planetary Change, 144, 1–16. https://doi.org/10.1016/j.gloplacha.2016.07.001

Clague, J. J., and Evans, S. G. (2000). A review of catastrophic drainage of moraine-dammed lakes in British Columbia. Quaternary Science Reviews, 19(17–18), 1763–1783. https://doi.org/10.1016/s0277-3791(00)00090-1

Clague, J. J., and O’Connor, J. E. (2021). Glacier-related outburst floods. In W. Haeberli & C. Whiteman (Eds.), Snow and ice-related hazards, risks, and disasters (pp. 467–499). Elsevier. https://doi.org/10.1016/b978-0-12-817129-5.00019-6

Geertsema, M., Menounos, B., Bullard, G., Carrivick, J. L., Clague, J. J., Dai, C., et al. (2021). The 28 November 2020 landslide, tsunami, and outburst flood – A hazard cascade associated with rapid deglaciation at Elliot Creek, British Columbia, Canada. Geophysical Research Letters, 49, e2021GL096716. https://doi. org/10.1029/2021GL096716

Haeberli, W., and Drenkhan, F. (2022). Future lake development in deglaciating mountain ranges. Oxford Research Encyclopedia on Natural Hazard Science.

Pararas-Carayannis, George (1999). The Mega-Tsunami of July 9, 1958 in Lituya Bay, Alaska. http://www.drgeorgepc.com/Tsunami1958LituyaB.html

University of British Columbia (2022). Scientists expose causes and effects of massive B.C. landslide. https://www2.unbc.ca/newsroom/unbc-stories/scientists-expose-causes-and-effects-massive-bc-landslide.

About the author/s
Paul Somerville
Chief Geoscientist at Risk Frontiers | Other Posts

Paul is Chief Geoscientist at Risk Frontiers. He has a PhD in Geophysics, and has 45 years experience as an engineering seismologist, including 15 years with Risk Frontiers. He has had first hand experience of damaging earthquakes in California, Japan, Taiwan and New Zealand. He works with Valentina Koschatzky in the development of QuakeAUS and QuakeNZ.