A glacial outburst has despatched floodwaters speeding via 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 via melted tunnels within the underside of a giant glacier.
The folks of Juneau have skilled at the very least one such flood each summer season 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 prompted 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 season. In some unspecified time in the future, the water collects deep sufficient that its strain forces a gap beneath the sting of the glacier — permitting it to flee.
The quantity of water that escapes is rising, Amundson says. CThe final couple of years, we’ve had a lot larger floods.”
Authorities are contemplating development tasks 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 infrequently in locations like Alaska, Iceland, the Alps, the Andes and the Himalayas. They normally occur removed from human populations. In actual fact, prior to only 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 via city, abruptly rose on a transparent, rainless day for no apparent cause.
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 season — creating the proper circumstances 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 folks knew the ice-dammed lake was there. As he peered down via 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 beneath the glacier.
A workforce together with Hood and Amundson now conduct periodic drone surveys to measure the lake because it grows throughout summer season. The U.S. Geological Survey screens the lake utilizing remote-operated cameras and a laser vary finder that measures the altering water top. They used this data, plus a number of native river circulate gauges, to create a web-based dashboard that gives updates on forecasts, water ranges and flood maps.
The floods have occurred each summer season since. However they didn’t develop a lot till 2023 and 2024, when the water quantity abruptly 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 tons of of homes and eroded riverbanks — leaving some homes protruding dangerously over the speeding water.
Though these outburst floods are linked to warming and glacier retreat, GLOFs also can happen in different circumstances, Amundson says. If a glacier advances rapidly, it will probably 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 circulate of almost 1,400 cubic meters per second — higher than the common circulate of the Arkansas River.
Forward of this 12 months’s flood, the city ready greater than ever. Along with the everlasting levees that some folks constructed round their properties, authorities put in momentary boundaries alongside the river, which largely held via the flood’s crest, defending many homes.
The U.S. Military Corps of Engineers is contemplating broader options that will take years to enact — akin to a brand new dam or perhaps a gap drilled via the mountains to empty the water earlier than it bursts out beneath the glacier.
However for now, Amundson, Hood and colleagues have a extra quick purpose. On the afternoon of August 13, they deliberate to board a helicopter, which can take them to a ridge over the lake, the place they may camp. As soon as the lake has totally drained, they’ll take pictures and measurements of the uncovered bowl, to map its measurement and depth, in hopes of higher estimating how a lot it will probably maintain — and the way massive a flood it’d launch in future years.
That can assist folks plan forward — whether or not it’s the Military Corps of Engineers, or native residents constructing dikes round their properties. “It’s loopy,” Hood says. It appears like folks “reside in a fort with a wall round it.”
“That is the brand new regular if that is the place you reside.”
