1.4 Flooding on the Mendenhall River from a Jokulhlaup: How to forecast a moving target

Monday, 11 January 2016: 4:30 PM
Room 242 ( New Orleans Ernest N. Morial Convention Center)
Scott D. Lindsey, NWS/Alaska Pacific River Forecast Center, Anchorage, AK; and E. H. Moran, E. Hood, and A. Jacobs

In July of 2011, an unexplained rise began on the lake level of Mendenhall Lake near Juneau, Alaska that ended up causing moderate flooding on the Mendenhall River and reaching the second highest level in the gage record (46 years). That flooding was traced to an ice-covered lake in the Juneau ice field known as a Jokulhlaup that had filled over time and released quickly without much visible evidence to indicate where the water originated.

Since that time, the area of the glacier field causing this flooding has been pinpointed as the Suicide basin and some monitoring has been installed to help with advance warning. A pressure transducer transmitting water level data in real-time was installed in the Suicide basin lake in the spring of 2014. A sharp drop in early July indicated a release in progress that resulted in the flood of record on the Mendenhall. A second release occurred a month later and reached the 5th highest stage on record. The rapidity with which this lake has filled and released and the rapid thinning of the ice covering the lake (allowing for a greater volume of water), has caused scientists, forecasters and emergency managers to seek answers on what has changed in the glacier structure that has caused this release to occur on a regular basis.

Then in early June of 2015, the first release of the year occurred at a much lower level than has ever occurred before according to the installed pressure transducer. While the river crested below flood stage, the hydrograph for the Mendenhall Lake gage shows at least 9 distinct outburst floods from the beginning of June to the end of July 2015. Researchers at the University of Alaska Southeast and hydrologists at the Alaska Pacific River Forecast Center have developed some rudimentary tools to forecast the timing and crests of these releases, but some fundamental changes have evidently occurred in the glacier structure that have caused these relationships to change rapidly with time. This presentation will document the history of these releases, discuss the monitoring of the glacier dammed lake level, and share the modeling efforts to forecast the releases in order to provide better decision support and forecasts for the residents who live along the Mendenhall River in Juneau.

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