A glacial outburst has despatched floodwaters dashing by means of the city of Juneau, Alaska, forcing residents to evacuate components of the state capital. The bizarre occasion, referred to as a glacial lake outburst flood, or GLOF, occurred as water spilled out of an ice-dammed lake and gushed downstream by means of melted tunnels within the underside of a big glacier.
The individuals of Juneau have skilled at the least one such flood each summer time for the final 15 years.
“It’s a narrative about glacier change,” says Jason Amundson, a glaciologist on the College of Alaska Southeast in Juneau, who’s monitoring the occasion. The warming local weather has induced glaciers right here to shrink and separate from each other. That’s left an empty valley alongside the sting of Mendenhall Glacier, which now fills with rain and meltwater every summer time. In some unspecified time in the future, the water collects deep sufficient that its stress forces a gap underneath the sting of the glacier — permitting it to flee.
The quantity of water that escapes is growing, Amundson says. CThe final couple of years, we’ve had a lot greater floods.”
Authorities are contemplating building initiatives which may sooner or later divert these floods, however Juneau residents are already taking issues into their very own palms — constructing levees of earth and rock as much as 4 meters excessive round their homes.
GLOFs have lengthy been identified to happen once in a while in locations like Alaska, Iceland, the Alps, the Andes and the Himalayas. They normally occur removed from human populations. In reality, prior to simply over a decade in the past, they didn’t occur in Juneau.
However one thing unusual occurred in July 2011. Water ranges within the Mendenhall River, which meanders by means of city, all of the sudden rose on a transparent, rainless day for no apparent motive.
Native scientists took a helicopter to seek for the supply of the flood — and eight kilometers from city, they discovered it: A naked mountain basin bigger than 100 soccer fields, that was hemmed in by Mendenhall Glacier on its decrease finish. This ice-dammed basin crammed with rain and meltwater throughout summer time — creating the right situations for a GLOF, says Eran Hood, a glacial hydrologist additionally on the College of Alaska Southeast who was on that helicopter flight.
“It was a thriller that ought to have been predicted forward of time,” he says, as a result of individuals knew the ice-dammed lake was there. As he peered down by means of the window, he realized that one thing new had occurred: The lake had misplaced a lot its water. It had in all probability spurted out underneath the glacier.
A staff together with Hood and Amundson now conduct periodic drone surveys to measure the lake because it grows throughout summer time. The U.S. Geological Survey displays the lake utilizing remote-operated cameras and a laser vary finder that measures the altering water top. They used this info, plus a number of native river stream gauges, to create a web based dashboard that gives updates on forecasts, water ranges and flood maps.
The floods have occurred each summer time since. However they didn’t develop a lot till 2023 and 2024, when the water quantity all of the sudden doubled in contrast with earlier years, reaching 60 million cubic meters — equal to about 24,000 Olympic-sized swimming swimming pools. These two episodes flooded a whole bunch of homes and eroded riverbanks — leaving some homes protruding dangerously over the dashing water.
Though these outburst floods are linked to warming and glacier retreat, GLOFs may also happen in different circumstances, Amundson says. If a glacier advances shortly, it could actually block off a facet valley, creating a brand new place the place water can accumulate in an ice-dammed lake.
This 12 months’s flood in Juneau started early on August 12. The river, which is often extra of a creek, rose by two meters in a single day, peaking by 7:15 a.m. native time at a report stream of practically 1,400 cubic meters per second — better than the typical stream of the Arkansas River.
Forward of this 12 months’s flood, the city ready greater than ever. Along with the everlasting levees that some individuals constructed round their houses, authorities put in momentary obstacles alongside the river, which largely held by means of the flood’s crest, defending many homes.
The U.S. Military Corps of Engineers is contemplating broader options that might take years to enact — reminiscent of a brand new dam or perhaps a gap drilled by means of the mountains to empty the water earlier than it bursts out underneath the glacier.
However for now, Amundson, Hood and colleagues have a extra instant objective. On the afternoon of August 13, they deliberate to board a helicopter, which is able to take them to a ridge over the lake, the place they are going to camp. As soon as the lake has absolutely drained, they’ll take pictures and measurements of the uncovered bowl, to map its dimension and depth, in hopes of higher estimating how a lot it could actually maintain — and the way giant a flood it would launch in future years.
That can assist individuals plan forward — whether or not it’s the Military Corps of Engineers, or native residents constructing dikes round their houses. “It’s loopy,” Hood says. It appears like individuals “reside in a citadel with a wall round it.”
“That is the brand new regular if that is the place you reside.”
