Flood — Juneau Borough, Alaska

2024-08-05 to 2024-08-06 · near Mendenhall Valley, Juneau Borough, Alaska

Event narrative

The level of Mendenhall Lake was slightly below normal levels of 5.0 feet before this event started in the early morning hours of Sunday August 4th. Twenty four hours into the flooding event, by noon on Monday, water levels had risen 5 feet and were over the moderate flood stage of 10 feet. By this time, areas near Skaters Cabin Rd and the Mendenhall Glacier Visitor Center were experiencing moderate flooding impacts with the USFS Mendenhall Lake Campground being evacuated. As water levels continued to rise, homes along View Drive were cut off with flood waters from 1 to 3 feet deep. Areas below the Mendenhall Back Loop Road bridge started to see flooding of yards between 12 and 13 feet but other locations that saw flooding during last years major event when the river level was near 13 to 13.5 feet did not see flooding at that level due to storm sewer one way values the City and Bough of Juneau installed prior to this event. Also the bank stabilization and armoring that residents installed on the right and left side of the river below the Mendenhall Back Loop Road bridge after last years event remained in tact and protected those residences from flooding/erosion impacts.

From Monday morning to the crest early Tuesday morning, the rate of rise on the river was around 0.4 FT per hour. The rate of rise was faster this year than last year, which could be due to a more efficient subglacial conduit drainage system, primed by a previous atmospheric river. The river continued to rise on this fast pace through the Monday evening hours and reached major flood stage of 14 feet at 2015AKST, when most of the flooding of homes and roads below the Mendenhall Back Loop bridge started to take place. By 2245AKST on August 5th the Mendenhall River went over the previous record stage set last year of 14.97 feet. The river would rise for another 3 and a half hours and crested at a new record crest of 15.99 feet (preliminary crest from the USGS) at 0315AKST on August 6th. The Mendenhall Lake and River were back below minor flood stage of 9.0 feet by noon on Tuesday, August 6th. The river system was above flood stage for 28 hours.

Through an extensive storm survey, it was determined that water came over the banks of the Mendenhall River along Marion Dr, Riverside Dr, Killewich Rd and Meander Way and produced major flooding to homes west of Riverside Drive and made roads impassable during the peak of the event. The local electric company cut power to those residences due to the flood waters. The amount of water that flowed over Killewich Drive was much higher than last year, causing water to move like a strong river down Riverside Drive and Killewich Dr into Emily Way, Sharon Street, Gee Street, Long Run, and Northland St - areas that had not seen such flooding before. Residents in those areas saw water not just coming from the river to the west, but also from the north, as water made its way through the community from the breach at Killewich Dr. These flood waters produced major impacts, with 1 to 4 FT of water in homes, causing significant damage. There were some areas that reported as much as 6 feet of water in their homes. As the floodwaters moved through homes west of Riverside Drive, the water flowed down Rivercourt Way and returned to the Mendenhall River. After the Mendenhall River breached the banks around Marion Drive, Riverside Drive, Killewich Drive, and Meander Way, water entered the storm sewer drains but could not flow into the Mendenhall River due to the high velocity and pressure from the flood flow. This caused water pressure to build up in the storm sewer system, leading to flooding in areas around the Mendenhall Valley Library and other nearby locations as water emerged from manhole covers. This also occurred near the back of the Safeway by the Mendenhall River at Brotherhood Bridge.

There were changes to the sections of the river above the Back Loop Bridge along View Drive after last years record event which was able to hold more water within its banks. This area did not see moderate flooding impacts until the Mendenhall Lake gauge read above 11.5 feet. This was due to large amounts of 'river scouring' from last year's event. Even through there was major flooding along View Dr, the events high water marks along the south side of View Drive were about 6 inches lower than last year (which saved one house from flood damage). Meanwhile, residents on the north side of View Drive experienced more water flowing across their property, causing increased road and home damage with reports of 4 to 7 feet of water in homes and other erosion issues.

In the end, there were about 300 homes majorly affected from this significant flood. This was another historic event as it was the second year in a row there was major flooding impacts to an area that historically has not seen any flooding. The area affected this year expanded from last year due and increase of water released from Suicide Basin in a single event. This year, there was approximately 16 billion gallons of water that came out of the basin and into the Mendenhall Lake and River system which was 2 billion more than last year.

Wider weather episode

Since 2011, Suicide Basin, a side valley that is dammed by the Mendenhall Glacier, fills with water from snowmelt and rainfall then drains during the summer as a glacial outburst flood. This year, the glacier dammed lake in Suicide Basin, like in 4 other years (2018, 2019, 2020 and 2023), was full to the top of the Mendenhall Glacier (top of the ice dam) and started to flow down a side drainage channel on July 31st. After about 4 days, Suicide Basin began to drain sub-glacially as indicated by the USGS laser sensor by the increasing water level drop Sunday night into Monday August 5th. USGS Mendenhall Lake gauge started to rise from the additional water from the basin early Monday morning August 5th and visual reports from the USGS webcam in the basin confirmed that the basin had started to drain. The Mendenhall Lake gauge steadily rose through Monday, then the rate of rise continued to increase exponentially as the inflow to the lake from Suicide Basin was getting larger. Over the course of this glacier dam outburst flooding event, Mendenhall Lake and River rose an impressive eleven feet over less than two days. This event broke all previous records, even last years record event by one foot, and caused significant impacts. This flooding event was historic due to areas seeing flooding that had never flooded before and impacting around 300 residents. There were moderate to major flooding impacts in the Mendenhall Valley, where a majority of Juneau's population resides, and historically does not flood.

View location on OpenStreetMap → (58.3756, -134.5878)

Source: NOAA Storm Events Database, event_id 1210851. Narrative written by the NWS forecast office that issued the report.