USACE monitors Chena River basin breakup conditions for flood risk after record snowpack

Posted on May 18, 2022

When days become longer and temperatures get warmer in the interior region of the state near Fairbanks, the U.S. Army Corps of Engineers – Alaska District uses snow survey data to forecast potential flood conditions on the Chena River during the spring breakup season.

The organization partners with the U.S. Department of Agriculture’s Natural Resources Conservation Service to measure winter snowfall levels and collect additional data from nine monitoring stations in the Chena River basin. The agencies analyze this information to estimate the volume of runoff when the snowpack melts in the mountains. In turn, USACE can anticipate possible high-water events and the need to regulate streamflow by operating Moose Creek Dam at the Chena River Lakes Flood Control Project in North Pole.

The interagency team traveled twice by helicopter on March 30 and again April 30 to assess the latest snow conditions at the remote sites. Spread out over about 1,500 square miles, the water contained in the melting snowpack, known as snow water equivalent, will eventually flow into the Chena River and pass-through downtown Fairbanks.

Based on this data, officials are predicting a heavy spring runoff that will require USACE personnel to be prepared to act should the river rise to significant levels.

“Snow surveys measured about double the normal SWE in the basin, which is the highest recorded since measurements started in 1980,” said Nathan Epps, chief of the Hydraulics and Hydrology Section.

This past winter, the greater Fairbanks area experienced record snowfall and a freezing rain event that contributed to the unusually large amount of water seen in the snowpack at some of the telemetry stations.

“The freezing rain event in late December left an ice layer within the snowpack, which was not found in the higher elevations of the upper Chena basin,” said Rosie Duncan, a USACE employee who participated in the snow survey work. “If freezing rain has fallen within the basin, the [monitoring stations] would still register that and report it as part of the snow water equivalent.”

In other words, a lot of snow has accumulated, and it has a higher density than what is typical for the Interior, Duncan said. According to the National Weather Service’s “Spring Breakup Outlook,” the flood potential for the Chena and Tanana rivers is above average. However, the rate at which this snow melts will influence whether dam operations are necessary.

“Ideally, there will be a gradual increase in temperatures to just above freezing with no additional precipitation or ice jams, resulting in a longer period over which snowmelt is added to the watershed,” she said. “However, we have had below-average temperatures in April, so the current ice jam potential is above average for the Tanana and Chena basins.”

During normal operations, Moose Creek Dam regulates the flow of the Chena River at a rate of no more than 12,000 cubic feet per second in downtown Fairbanks. Downstream flood effects along the river also depend on conditions in the Tanana and Little Chena rivers as well as local drainages. Low-lying areas near the Chena River may experience minor flooding, while elevated groundwater can occur for several thousand feet downstream of the dam.

Although the floodplain behind the dam remains dry for most of the year, USACE officials may impound water when the river level runs high because of significant melting snow, ice jams or heavy rains. After the 1967 flood of Fairbanks that caused about $80 million in damages, the Chena Project was constructed in the 1970s to protect the city and Fort Wainwright from future disasters. Since then, the 7 ½ mile earthen dam has operated 30 times to keep local communities safe and prevent an estimated $418 million in flood damages.

“Moose Creek Dam is a valuable asset for the Fairbanks North Star Borough,” said Mark DeRocchi, chief of the Engineering, Construction and Operations Division. “Over 20 years, it has prevented potentially catastrophic floods in the region.”

Meanwhile, construction will begin this spring to reinforce the structure. Referred to as a “mega project” and funded by the recently enacted Infrastructure Investment and Jobs Act, the Bauer Foundation Corp. of Florida was awarded a $75.5 million contract to establish a mix-in-place concrete barrier wall at the base of dam that spans 6,200 linear feet at depths of up to 65 feet.

The project stems from a 2017 modification study that recommended reinforcement of the dam to extend its life and enhance protection of the greater Fairbanks region for many years to come. Construction is anticipated to be complete by January 2026. The dam will continue to operate and regulate the flow of the Chena River as needed while work is underway.

“The successful completion of this modification project will allow us to address risks associated with aging infrastructure and deliver upgraded infrastructure that is built to last,” DeRocchi said.

The public is encouraged to stay informed of weather and flood conditions by monitoring news reports and social media posts. It also is recommended that people remove belongings from low-lying areas, such as basements and crawl spaces, to protect these items from potential flood damage.

As USACE prepares for a busy spring that may involve operating Moose Creek Dam to reduce flood risks and making safety improvements to the structure itself, local citizens can be assured that the team has their best interest in mind.

“Public safety is always our top priority,” DeRocchi said.

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