New study highlights findings and predictions for Juneau glacial outburst flood

an ice-filled basin dammed by a glacier

paper published last week in Frontiers in Earth Science explores the evolving dynamics of Suicide Basin, a glacier-dammed lake in Juneau that releases annual outburst floods, causing inundation and erosion in the Mendenhall Valley.

Outburst floods, which occur when water dammed by a glacier suddenly releases and floods downstream areas, have occurred from Suicide Basin almost every year since 2011, when the glacier-dammed lake first began forming during the summer months. The largest outburst flood event to date occurred in July 2016, when over 16,000 cubic feet per second of water flowed through the Mendenhall River, impacting homes and other infrastructure.

A collaborative team of researchers from the University of Alaska Southeast (UAS), the City and Borough of Juneau, the National Weather Service, the U.S. Geological Survey, the Alaska Division of Geological and Geophysical Surveys, and the Alaska Climate Adaptation Science Center, which provided the primary funding for the study, have been monitoring the basin for several years to better understand and predict flooding events.

Suicide Basin is dammed by the Mendenhall Glacier in Juneau. Figure courtesy of Christian Kienholz.

“We expect GLOFs from Suicide Basin to pose a threat over the next decade, hence we plan to continue the monitoring over the years to come,” notes lead author Christian Kienholz. Managing this risk into the future is a priority for the agencies involved to ensure continued success in keeping people and infrastructure safe.

Multiple factors influence the magnitude of future floods from Suicide Basin, making it difficult to assess what the future holds. The thinning of the ice dam, which is currently dropping by 5 meters annually, is lessening the capacity of the basin to store water. However, floating ice in the basin is melting away rapidly, providing a larger reservoir to store water behind the dam. The area of the basin that can store water is also increasing slowly as Mendenhall Glacier retreats.

The observation of several unforeseen mechanisms during the 2018 and 2019 outburst floods reinforced the research team’s knowledge that ice dynamics in the basin are evolving quickly. Over the course of several days, the water level at Suicide Basin dropped 50 meters (higher than Juneau’s tallest building) as the floods occurred. Due to the lower elevation of the ice dam as it melts, the drainage events in both seasons began with a previously unobserved flow of water overtop Mendenhall Glacier before the basin drained beneath the glacier. In 2018, the outburst flood stopped short of completely draining the basin, leaving close to forty percent of the water in Suicide Basin. In 2019, drainage slowed prematurely before beginning again, causing two smaller flood peaks rather than one.

“Tracking these changes in the basin gives scientists and city managers an idea of the volume and potential drainage scenarios of future events,” said UAS scientist Eran Hood.

If only taking into account the melt rate of the ice dam and the previous storage capacity of the basin, the researchers predict we may see a slightly smaller drainage event in 2020. But, Hood stressed, that’s just one potential scenario; the basin could begin draining before filling to the level of previous years, producing a substantially smaller outburst flood, or series of low floods. Alternatively, the basin could fill to historic levels and then drain more completely or rapidly than it has in the past, creating a larger flood.

Should an outburst flood coincide with a period of high snow or ice melt, an already elevated flood stage could rise much higher than what we’ve seen before, according to National Weather Service Senior Service Hydrologist Aaron Jacobs. “If we get an outburst flood that coincides with higher than normal streamflows at Mendenhall lake and river, we would see flooding in places we haven’t seen flooding before,” said Jacobs.

These possible outcomes highlight the need for continued monitoring of Suicide Basin, and data to help understand the drainage mechanisms that drive floods. Continuing efforts of previous years, the research team intends to conduct drone surveys of the basin using high-resolution fixed-wing drones to map the ice elevation and underlying topography of Suicide Basin. AK CASC scientist Gabriel Wolken was featured in a case study by drone company Wingtra for his work producing digital elevation models of the basin.

The multi-agency response is a model of how a collaborative approach can work to address the evolving challenges posed by environmental hazards. The findings and methods from this research will improve forecasting capabilities at other outburst flood areas around the state, which may grow in number as climate change causes rapid glacial retreat and the formation of new ice-dammed basins.

“If it wasn’t for all these agencies coming together and working as one, it would have been a lot more difficult to put together this monitoring program and information database that we’ve gathered throughout these past ten years,” said Jacobs.

For the latest information on outburst flooding in Juneau, the National Weather Service Suicide Basin webpage provides near-real-time updates on the water level in the basin and at Mendenhall Lake and River.