A major flood from Suicide Basin now appears highly unlikely with colder weather setting in, since at the current rate of water fill it would take 145 days for the ice dam to be full, an expert told Juneau Assembly members during a special meeting Thursday.
Regardless, Assembly unanimously agreed to proceed with its latest emergency measures including acquiring 60,000 sandbags from the U.S. Army Corps of Engineers to distribute to residents in flood-vulnerable areas. Assembly members also approved an emergency ordinance establishing a cooperative agreement with the Corps to begin work on a four-mile levee built from large boxy barricades known as Hesco barriers.
The basin is about half full with a water level of about 1,222 feet and a full glacial outburst flood today would cause the Mendenhall River to rise to about 10.5 feet, Aaron Jacobs, a hydrologist for the National Weather Service Juneau, said during the online midday meeting. That is considered “moderate flood status,” but the impacts would be limited to areas near the Mendenhall Glacier such as Mendenhall Campground, and be well below the levels of 15 to 16 feet that caused major damage the past two summers.
“We are seeing about a foot rise per day as our freezing levels have been dropping,” Jacobs said. “And so to put that into context, to see a full basin that we know of right now that would be 145 feet that we would have to fill.”
Also, the Mendenhall River is “fairly low right now and we should continue to see the base flow of the Mendenhall River river slowly drop as those freezing levels continue to stay low,” he said.
The possibility of a major flood in October was raised in early September by city leaders who said it was important to not be overly alarmist, yet to be prepared for the worst just in case. The announcement came about a month after the Mendenhall River reached a record level of 15.99 feet, exceeding the worst-case scenario of forecasters and damaging about 300 homes — many in areas not initially expected to be affected.
Jacobs said more than one flood from Suicide Basin during a year is common, but secondary floods haven’t been severe and the latest in the year a flood has occurred is late October in 2022. The water level in the basin at that time was 1,312 feet, considered a moderate flooding level.
A wintertime release is not impossible, but has never happened between November and January, he said. If the basin is unusually full over the winter one possibility is an early-season release next May or June.
The actions taken by the Assembly during the meeting are intended to help individual homeowners safeguard their properties with the common-size sandbags, while constructing a semi-permanent levee along the developed side of the Mendenhall River.
While the Corps of Engineers is providing the sandbags and Hesco barriers — the latter of which have a three-foot-square footprint, are four feet tall and filled with sand by a commercial loader — the Assembly is being asked to approve up to $2 million at its next regular meeting to pay for the local costs of the first phase of the project stretching from Marion Drive to Killewich Drive to Meander Way.
Because many of the barriers would be on private property, City Manager Katie Koester said there are a number of risks and complications — such as heavy equipment damaging yards and homeowners resisting the barriers — that will need to be taken into account.
• Contact Mark Sabbatini at mark.sabbatini@juneauempire.com or (907) 957-2306.
