Glacial lake outburst floods (GLOFs).
Mendenhall Glacier, Juneau, Alaska (USA)
On Saturday 5th August 2023, state officials in Juneau, Alaska, declared an emergency as a glacier lake outburst flood brought havoc near the state capital city. Structures including houses and bridges were destroyed. The flooding was caused by an ice-dam break in Suicide Basin, an offshoot on the Mendenhall Glacier, located about 12 miles north of the city [Figure 1]. The Mendenhall Lake level rose about 2.5m on Saturday at 11:15 p.m. local time, well above the previous record of almost 2m in July 2016. Significant flooding was reported in areas that previously have not seen flooding, and there has been significant erosion along the riverbanks, according to the National Weather Service.
You can see videos of some of the damage the flood caused (from ABC and BBC News) here:
Alaska: Moment house collapses into river during flood - BBC News
A timelapse produced by the USGS can be seen here:
USGS HIVIS (Hydrologic Imagery Visualization and Information System)
Figure 1. Satellite image (2018)
GLOFs
Thousands of people live in areas at direct risk from glacial lake outburst floods (GLOFs). GLOFs occur when large meltwater lakes form behind unstable natural dams. Collapse of the natural dam can cause dramatic flooding downstream. The threat is growing because glaciers around the world are melting rapidly. GLOFs are one of the most important hazards linked to climate change in glaciated areas.
How do outburst floods occur?
Outburst floods occur when the barrier enclosing a glacial lake is breached or overtopped and the water exits in huge flows at great speed. GLOFs have an exceptionally flashy discharge regime, with a sudden rise to a very high flood peak. Short warning times and extreme peak
flows make downstream communities and infrastructure highly vulnerable.
Outburst floods occur in three main ways.
· Dam breach is the failure of the dam itself. Moraines formed from loose sediment are vulnerable to erosion. Streams erode loose debris; ‘piping’ removes fine sediment as water seeps through the dam; the moraine ridge may gradually weaken over time or collapse without warning in an earthquake tremor.
· Landslides and avalanches: glacial lakes are often ringed by steep mountain slopes, and snow avalanches pose a hazard. Large landslides or avalanches falling into glacial lakes create huge displacement waves which can overtop the dam and cause flooding downstream.
· Jökulhlaups are outburst floods from subglacial lakes. This Icelandic term meaning ‘glacier run’ describes the sudden release of water melted by geothermal heating or volcanic eruptions beneath glaciers. Subglacial lakes can drain in dramatic fashion. If the lake is deep enough, then water will drain as the ice begins to float. Flowing meltwater is slightly warmer than ice, which allows a small channel to expand rapidly.
Figure 2. Aerial view - Mendenhall Glacier, July 2021
Photo: Salwan Georges/The Washington Post via Getty Images.
I was fortunate to visit the Mendenhall Glacier and meltwater lake ahead of it in September 2017. It was eerie, beautiful place. [Figure 3]
Figure 3. Ground view - Mendenhall Glacier (September 2017)
Photo. David Redfern