Mendenhall Glacier located outside Juneau, Alaska. Photo / Getty Images
The record-breaking surge of water that burst from an Alaskan glacier this week was mostly contained by the temporary levee built in Juneau to prevent the type of flood damage that blasted the city during the past two summers, officials said today.
Floodwaters began pouring out of an area known as Suicide Basin earlier this week, swelling the Mendenhall River to an all-time high, faster than scientists had predicted.
But the United States Army Corps of Engineers helped erect 3.2km of earth-filled Hesco barriers that blocked much of that overflow from inundating neighbourhoods along the river.
The barriers are cloth sacks in metal frames that are filled with sand or dirt.
These glacial outburst floods have been periodically pouring out from beneath the Mendenhall Glacier since 2011, but in the past two years they have grown in size and destructive force.
Two years ago, the swollen river sheared away large swathes of forested riverbank, taking with it houses and parts of a three-storey condominium complex. Last year, the surge of glacial melt flooded some 300 homes.
But this year, despite facing a flood larger than any before, the fortified wall largely held.
Juneau city manager Katie Koester expressed relief from the podium today.
“I know we’re not entirely out of the woods, but the Hesco barriers really have protected our community,” Koester said during the briefing.
“If it weren’t for them, we would have hundreds and hundreds of flooded homes that we would be all planning to deal with now.”
Authorities at a command centre in Juneau monitored the rising waters overnight using drones to fly along the new levee. They plan to do a more extensive assessment once the waters recede further.
Floodwaters did seep into some areas, inundating roads, trails, parking lots and possibly some homes, according to Juneau authorities.
A portion of the abutment of a bridge over the river was damaged and has been closed for repairs.
A tree carried by the floodwaters rammed into the Hesco barriers, which needed to be fortified.
The floodwaters come from Suicide Basin, a rock depression that is sealed off on one side by the Mendenhall Glacier, which is receding amid the warming climate.
Each summer, ice melt and rainfall fill the basin. When it is full, the pressure eventually forces itself out beneath the glacier and down towards Alaska’s capital city.
The basin is closely monitored throughout the year with cameras, lasers and regular drone flights to map it and estimate the volume of water it contains. There have been larger floods pulsing out in recent summers as more icebergs melt and more water accumulates in the basin.
This time, the flood began when the basin contained about 60 billion litres of water, said Aaron Jacobs, senior service hydrologist of the National Weather Service in Juneau.
It released just after a multiday storm dropped some 18cm of rain on the area, pushing the Mendenhall River into a minor flood before the basin burst.
The level of Mendenhall Lake - which sits below the glacier and releases water down to the river - reached a high point of 5m, surpassing the record set last year of 4.8m.
Juneau officials consider the Hesco barrier a temporary solution while they debate a longer-term fix to what’s now an annual threat.
Some residents opposed the barriers.
Property owners in the flood zone were asked to pay more than US$6000 each over the next decade for the defences, which cut through people’s yards and obstructed some views.
The Army Corps recommended the approach and provided 37,800 linear feet of Hesco barriers along with more than 100,000 sandbags to Juneau, Brigadier General Joseph C. “Clete” Goetz of the Army Corps’ Pacific Ocean division said during a news conference today. The corps helped repair the tree strike and is studying how the barriers hold up during the flood, he said.
“There is still flooding that has happened. There are still people who will be looking to us to assist them for recovery,” said Roald Helgesen, chief operating officer of the Central Council of the Tlingit and Haida Indian Tribes of Alaska.
“But we are a far cry from where we were last year.”