Officials and residents in Alaska’s capital city are preparing for what has become an annual summer glacial outburst flooding event that threatens the densely-populated Mendenhall Valley. Similar to past years, flooding is expected in early August.
Federal, state, tribal and local agencies are closely monitoring the Mendehall Glacier, where an ice-dammed lake known as Suicide Basin fills annually with glacial melt that eventually bursts and releases billions of gallons of flood water down the valley through the Mendenhall River.
The City and Borough of Juneau is partnering with the Central Council of Tlingit and Haida Indian Tribes of Alaska, as well as state and federal partners in preparations, then monitoring and responding to the flooding event.
Last year, the river flooded to a record-breaking 16.65 feet on Aug. 13, but nearby neighborhoods were mostly spared and damage was minimal due to close monitoring, early warnings and semipermanent flood barriers installed along the riverfront.
This year, crews with the U.S. Army Corps of Engineers and the City and Borough of Juneau have fortified and built up the flood barriers to withstand a flood about 30% larger than last summer. Crews are also installing industrial pumps in the next few weeks to pump out water that seeps through the barriers and to prevent flooding through city storm drains.
Officials estimate the basin will be full sometime between Aug. 1 through 9. Historically, there have been one to six days from the time the water level in the basin peaks to when it begins to release, officials said.
Updates on the filling Suicide Basin, forecasting, flood levels and evacuation maps will be available on a public Juneau Glacial Flood Dashboard. Officials are asking Mendenhall Valley residents to make a plan to voluntarily evacuate. A wireless cell phone alert will be issued to all area residents when the flooding begins.
Officials have planned for an evacuation advisory for roughly double the number of homes advised last year — roughly 1,900 residences compared to 1,000 residences, as well as businesses, schools and transportation corridors.
Ryan O’Shaughnessy, the emergency program manager with the City and Borough of Juneau said in an online media briefing on Thursday the evacuation advisory is to prepare for a potential scenario where flood waters breach the flood barriers, known as HESCO barriers.
“We wanted to include areas that may not be directly inundated, but where access or travel, including by first responders, may be compromised. So, in other words, if we wouldn’t be able to get to you during a breach scenario, or if you may not be able to get to safety, we are recommending that you evacuate,” he said.
O’Shaughnessy said officials have modeled many scenarios at different flood levels, and there will be frequent public alerts as flood waters rise.
“We have a very high level of confidence in the HESCO barriers, but this is a dynamic event,” he said. “It’s a natural disaster, and we can’t predict every element of it, and to help us protect the rest of the community, our first responders, and everyone involved, we’d really like to ask the community to evacuate.”
Evacuation will be advised and not mandatory, unless Gov. Mike Dunleavy authorizes mandatory evacuation, O’Shaughnessy said.
He said the City and Borough of Juneau have requested the governor issue a preemptive disaster declaration, like last year, and Dunleavy’s office is currently reviewing the request.
Public flood preparedness events are taking place every Saturday in July, where residents can review flood maps, get emergency tips, and collect free sandbags to fortify their homes against flooding. The next event is July 11 from 11 a.m. to 3 p.m. at Dimond Park Field House.
Residents can monitor Suicide Basin through a National Weather Service monitoring page. Officials recommend signing up for emergency alerts and reviewing Mendenhall River inundation maps. KTOO Public Media has set up several livestreams to monitor the flooding event. An emergency shelter hosted by the Red Cross will be set up, like last year, at the former Floyd Dryden Middle School site.
