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A basin full of icy meltwater perched above Juneau has broken through its glacial barrier and begun flooding down towards the Alaskan city, Juneau officials said today – the latest in what have become yearly outburst floods that hit this region.

As scientists flew by helicopter to assess the situation at what’s called Suicide Basin, Juneau officials warned residents to evacuate parts of the city that have been prone to repeated flooding. They predicted the peak floodwaters would arrive by tomorrow.

These floods, which began more than a decade ago, have become increasingly destructive in recent years and now pose a persistent summertime threat to residents who live along the Mendenhall River.

In preparation for this year’s torrent, city officials worked with the US Army Corps of Engineers and others to build a massive wall of Hesco barriers along the river in flood-prone areas to try to prevent the kind of destruction that hit neighbourhoods during the past two August floods.

Hesco barriers – cloth bags filled with dirt or sand and reinforced with a metal frame – were commonly used by the United States military in Iraq and Afghanistan to fortify their bases.

They have also long been used for flood control and act like giant sandbags.

The city has warned about 1000 households in Juneau about the need to evacuate, said Ryan O’Shaughnessy, emergency manager for the city and borough of Juneau, during a briefing about the flood.

The source of the floodwater is Suicide Basin, which abuts the Mendenhall Glacier that looms over Juneau and attracts hundreds of thousands of tourists each year.

The glacier acts as a dam for the jumble of icebergs and meltwater in the basin.

As it fills from ice melt and rainfall each summer, the pressure builds until the water reaches a tipping point, forcing its way out underneath the glacier and down towards the city below.

As the glacial flooding began this week, the Mendenhall River was already in a minor flood stage after heavy rains over the weekend and into yesterday.

That elevated level meant the Mendenhall River could surpass levels reached by previous floods, according to local officials.

“This will be a new record,” Nicole Ferrin, a meteorologist with the National Weather Service in Juneau, said during today’s briefing.

Ferrin said Mendenhall Lake – which sits below the glacier and releases water down to the river – could crest tomorrow at 5m, surpassing the record set last year when the lake reached 4.8m.

Authorities also warned residents to stay away from the Hesco barrier wall, as well as bridges and other spots along the riverbank where people have convened to watch the flood.

“This is a very dangerous situation,” O’Shaughnessy said.

Still, local officials expect the barriers to prevent the waters from inundating or destroying homes.

There are also several live-streaming cameras now set up allowing people to watch the river levels as well as a flood website that describes what flooding could happen if barriers weren’t in place.

City manager Katie Koester said in an interview that officials have analysed how the barriers – which stand 1.2m to 2.5m-tall and run for more than 3.2km – could fail and have reinforced them in some places.

She is confident they will protect residents – but still urged people to evacuate to avoid taking risks.

“This is a unique event and in the end, it’s an act of God and we don’t know what will happen,” Koester said.

These glacial outbursts have flooded from Suicide Basin more than 30 times since 2011 – and they vary in size.

The past two Augusts, the floods have been large and destructive. But the dynamic is always changing – as the Mendenhall Glacier recedes in the warming climate and the path of water out of the glacier also changes.

The need to find a solution has grown more urgent as the floods have become more destructive.

Residents and officials have debated whether to build a dam around Mendenhall Lake to catch floodwaters, to tap into Suicide Basin and slowly drain the water or to blow up the glacier with explosives.

The Hesco wall is a temporary solution devised after last year’s flood, Koester said, paid for by Juneau taxpayers, including a hefty bill of several thousand dollars assessed from residents who live in the flood zone. The Army Corps recommended the approach and provided the barriers.

“I really do feel like we are prepared this year,” Koester said. “I’m really hoping that we’ll have a success story by [Friday].”

Eran Hood, a hydrologist with the University of Alaska Southeast in Juneau who studies the glacier’s dynamics, flew up to it with other experts.

He’s been mapping the basin with drone flights every couple of weeks to help estimate the total volume that might escape in a full release.

He described the scene as “crazy”.

The basin is “just super full and it’s been raining for days,” he said. “It’s pretty stunning how much water is in there right now.”

The combination of an outburst flood coinciding with a rainstorm has long been a worst-case scenario for Juneau.

Since the rain has now stopped, these events are not stacked precisely, Hood said, but the level in the river is still higher than what he would like to see during this flood.

“It will definitely be a big test for the barriers,” he said.