This interagency project provides modeling and mapping of inundation caused by glacial-outburst or jokulhlaup flooding on the Mendenhall River in Juneau, Alaska. Jokulhlaup, an Icelandic word for an event from a country that’s been dealing with this phenomenon for years, has recently created record-level floods in within Alaska that are predicted to continue. Recent updates to FEMA Flood Insurance Rate Maps did not include floods resulting from glacial outburst events. Therefore, the Alaska District worked closely with the City and Borough of Juneau (CBJ) and the hydrologist from the Alaska-Pacific River Forecast Center to conduct high water mark surveys, develop river hydraulic modeling, and determine flooding boundaries for the Mendenhall River.
While not regulatory in nature, these inundation maps enable the community and NOAA to provide more effective warnings of potentially hazardous areas as a result of jokulhlaup events. Greater warning times allow for flood-fighting activities and transportation of large valuables outside of flood areas. The maps also provide planning and transportation engineers with information to construct more resilient egress routes.
In 2014, there was a jokulhlaup event in the Juneau area that resulted in approximately thirty flooded private properties and significant flooding at Forest Service campgrounds, buildings, and the visitor’s center at Glacier National Park, all of which had to be closed because of the hazardous conditions. CBJ estimates approximately $1M in property losses. Of those private residences, several were within repetitive loss areas. The homeowners and public officials now have extreme estimate boundaries to provide needed guidance on new or reconstruction and maps that serve as a planning tool for CBJ when high flow events on the Mendenhall River are forecasted. City and Borough officials can identify areas most at risk and warn residents and businesses of potentially hazardous conditions. Flood-fighting activities can be planned and prioritized based on the inundation extents shown on the maps. The local and visiting public to the parks will be better informed with early flood and evacuation warnings.
CBJ has elected to pursue inclusion into the Advanced Hydrologic Prediction Service Flood Inundation Mapping web application that was developed and maintained by NOAA. The Mendenhall River will be the first stream in Alaska mapped through this system that is widely used by agency and public entities in the “lower-48” states.