Homeowners warned of flood risk as lake starts to break through ice dam

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2 days ago

A huge basin of rainwater and snowmelt dammed by Alaska's Mendenhall glacier has started to release, and officials on Tuesday urged residents in some parts of Juneau to evacuate ahead of what could be a record surge of floodwater downstream.
Officials in recent days have been warning people in the flood zone to be ready to evacuate.
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On Tuesday morning, they confirmed water had started escaping the ice dam and flowing downstream, with flooding expected from late on Tuesday into Wednesday.
Flooding from the basin has become an annual concern and, in recent years, has swept away houses and swamped hundreds of homes.
A canoe, bottom right, glides on Mendenhall Lake, in front of the Mendenhall glacier, Juneau, Alaska (Becky Bohrer/AP)
Government agencies installed a temporary levy this year in hopes of guarding against widespread damage.
'This will be a new record, based on all of the information that we have,' Nicole Ferrin, a National Weather Service meteorologist, told a news conference on Tuesday.
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The Mendenhall glacier, a thinning, retreating glacier that is a major tourist attraction in south-east Alaska, acts as a dam for Suicide Basin, which fills each spring and summer with rainwater and snowmelt.
The basin itself was left behind when a smaller glacier nearby retreated.
Flood barriers to protect property against glacial outburst flooding have already been positioned (Marc Lester/Anchorage Daily News via AP)
When the water in the basin builds up enough pressure, it forces its way under or around the ice dam, entering Mendenhall Lake and eventually the Mendenhall River.
Before the basin reached the limit of its capacity and began overtopping over the weekend, the water level was rising rapidly – as much as 1.22 metres per day during especially sunny or rainy days, according to the National Weather Service.
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The threat of so-called glacier outburst flooding has troubled parts of Juneau since 2011. In some years, there has been limited flooding of streets or properties near the lake or river.
But 2023 and 2024 marked successive years of record flooding, with the river last August cresting at 4.9m – 30cm over the prior record set a year earlier, and flooding extending farther into the Mendenhall Valley.
This year's flooding was predicted to crest at between 4.96m and 5.12m.
Last year, nearly 300 residences were damaged.
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A large outburst can release some 15 billion gallons of water, according to the University of Alaska Southeast and Alaska Climate Adaptation Science Centre.
That's the equivalent of nearly 23,000 Olympic-size swimming pools. During last year's flood, the flow rate in the rushing Mendenhall River was about half that of Niagara Falls, the researchers said.
City officials responded to concerns from property owners this year by working with state, federal and tribal entities to install a temporary levee along roughly 2.5 miles of riverbank in an attempt to guard against widespread flooding.
The installation of about 10,000 1.2m barriers is intended to protect more than 460 properties from flood levels similar to last year, said Nate Rumsey, deputy director with the city's engineering and public works department.
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The US army corps of engineers is at the start of what is expected to be a years-long process of studying conditions in the region and examining options for a more permanent solution.
The timeline has angered some residents, who say it is unreasonable.
Outburst floods are expected to continue as long as the Mendenhall Glacier acts as an ice dam to seal off the basin, which could span another 25-60 years, according to the university and science centre researchers.