Flood — Juneau Borough, Alaska

2014-07-11 · near Douglas, Juneau Borough, Alaska

Event narrative

The annual glacier lake outburst flood that takes place on the Mendenhall Lake and River was a record event this year. Once the twelve billion gallons, that accumulated throughout the spring and early summer, in the basin began to drain on Thursday July 9 the lake rose significantly for next forty-two hours. There was a lot of publicity leading up to the event so this record event did not come by a surprise. The new record crest was eleven point eight feet on July 11 which was almost two feet above the moderate flood stage of 10.0 feet and broke the old record from 1994 of eleven point eighteen. The Mendenhall River crested at thirteen point fifty-two feet just under the record crest from 1995 of thirteen point seventy-five feet. After cresting both the Mendenhall Lake and River receded fast and was back at base-flow levels in 12 hours.

The record flows and stages from Mendenhall Lake and River damaged homes, campgrounds and roads. There was at least three feet of water that flooded the West Glacier spur road which borders the west side of Mendenhall Lake. The USDA Forecast Service closed the West Glacier Road and trail, Nugget Falls trail, Photo Point trail and restricted watercraft on the lake 24 hours before the flooding took place. They also closed the Mendenhall Lake Campground and evacuated 23 of the 70 sites that were occupied. Some campers moved to another campground while other moved to a designated area for them. There was significant flooding within the campground with 3-4 feet of water in places and 150 feet of road within the campground was damaged. The power was also cut by the City of Juneau to the campground.

The View Drive subdivision was cutoff with a one home sustaining significant damage. The City of Juneau told residents of View Drive, along the Mendenhall River, to shelter in place and they also cut their power once water was across the road and the subdivision was cutoff. The home that sustained significant damage was along the Mendenhall River and it had 43 inches in their garage and about 10 inches inside the house.

The City of Juneau closed the Brotherhood Bridge Trail which follows along the river. They also restricted access to all roads leading into Meander Way, from Rivercourt Drive to Killewich Ave along Riverside Drive but the closure was voluntary. The city was worried that the roads maybe impassible due to flooding from the River or water through the storm drain system.

There was a major bridge construction going on at near the mouth of the River. The construction company was pulling logs out of the river so it would not damage any of their work but logs were getting lodged and acting like a dam diverting water to the banks. This cause significant bank erosion but the bridge structure was not damaged by the trees and debris floating down the river.

Wider weather episode

The lake in Suicide Basin, dammed by the Mendenhall Glacier, had been filling all spring and into the early summer. After a warm and dry spring there was ample amount of snow melt from higher elevation into Suicide Basin. There was record amount of rain for the month of June in the Juneau area and with freezing levels above six thousand feet Suicide Basin filled rapidly as seen on the real-time lake gauge in the basin. The water level in the basin filled about two in a half feet per day on average and was past the previous height of sixty-two feet from the glacier lake outburst flooding event from 2011 by end the June. Water from the dammed lake in Suicide Basin started to drain into the Mendenhall Lake/River system on Wednesday July 9 after it crested at eighty-nine feet. The water level in Mendenhall Lake began to slowly rise 9 hours later early Thursday evening with the rise showing up on Mendenhall River twelve hours later. It was calculated that there was about twelve billion gallons of water in the basin before it started to drain. The rate of rise increased through the event and by Friday afternoon it was on a steep and fast rise upward.

Both the Mendenhall Lake and River rose significantly for the next forty-two hours and broke the record for stream flow and stage levels before it crested Friday afternoon July 11. The record flows and stages produced moderate flooding and damaged homes, campgrounds and roads in places along Mendenhall Lake and River.

View location on OpenStreetMap → (58.4357, -134.5660)

Source: NOAA Storm Events Database, event_id 538164. Narrative written by the NWS forecast office that issued the report.