Last year’s record outburst flood took Juneau by surprise. As Suicide Basin refills, scientists are working to improve their forecasts.

a person stands at the lip of an empty basin
Researcher Eran Hood stands on the lip of an empty Suicide Basin just a few days after it drained to create a record-breaking glacial outburst flood in August 2023 (Anna Canny/KTOO)

Last summer’s record-breaking glacial outburst flood took everyone in Juneau by surprise. 

So this year, a team of local scientists and emergency managers are trying to improve flood forecasts. They talked about it during a Monday-night presentation at the University of Alaska Southeast.

Researcher Jamie Pierce with the U.S. Geological Survey says his agency is keeping a close eye on Suicide Basin, which has already started to fill again.

“We all know the anxiety when the basin is filling and not released yet. And we want to know the second it starts [releasing]” Pierce said. “We feel like we have enough in our favor now to be able to very confidently do that well.”

Each year, the basin — which is dammed by the Mendenhall Glacier — collects rain and meltwater. Eventually that water releases, draining first into Mendenhall Lake and then the river in what’s known as a glacial outburst flood, or a jökulhlaup.

Last year, 13 billion gallons of water burst out in a flood that was bigger and more damaging than ever before. So this year, scientists are using new technology to monitor the basin more closely. There’s also an improved warning system and a new set of flood maps to help homeowners figure out what to do when the water starts rising. 

Just last month, Pierce’s team installed new technology to understand how the basin fills and drains. There are sensors that measure water levels and a weather station to keep track of the rainstorms or hot-weather ice melt that add water. 

There are also two cameras that will take pictures of the basin four times a day. 

Meanwhile, researchers like Eran Hood at the University of Alaska Southeast are studying the shape of the basin, which is ever-evolving. 

Ice-dammed basins like Suicide Basin typically exist for a few decades, until a glacier retreats far enough that the dam no longer exists. That could take between 30 and 50 years. 

As the Mendenhall melts, the height of the ice dam gets lower. But at the same time, the face of the glacier is calving, which could make the basin wider. That means flood risk will continue to evolve too.  

“People always ask ‘Well is it gonna get bigger over time or smaller over time?,’” Hood said. “To answer that question accurately we need to be able to quantify how these competing processes are interacting with each other.”

They’ll do that by using frequent drone surveys to estimate the volume of the basin. There’s a brand new post-doc at the University who will do those surveys at least a half dozen times this summer.

Thanks to that technology, hydrologist Aaron Jacobs with the National Weather Service can get an idea of how much water is in the basin at any given time. 

“Currently, we are really, really low from where we were last year,” Jacobs said. “This time last year, we were 30 meters higher in ice elevation, water elevation than we are right now.”

But he says the challenge is forecasting how fast the water will drain. When it drains quickly, that creates a bigger flood peak. Last year, it basically all came out at once.

“Unfortunately there’s not really a confident way to say the size of this release. Is it going to be a full release? A half? A quarter? Or a certain percentage?,” Jacobs said. “That’s what we’re trying to get at.”

So going forward, flood forecasts will include that full release as the worst case scenario. From there, residents can consult an updated set of flood maps. 

City emergency manager Tom Mattice says the maps show what parts of town could flood under minor, moderate or severe flood scenarios.

“Everybody needs to have a personal plan,” Mattice said. “Where are you going to go? What are you going to do? Do you have a go-bag at your house?” 

The city also plans to introduce a new federal system called the Integrated Public Alert and Warning System, or IPAWS, which will allow them to send information about flooding or other emergencies directly to cell phones. 

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