Lower White River Channel Capacity and Flood Risk Communication

The White River drains from Mount Rainier and its near-infinite supply of sediment. The lower reaches of the river are highly dynamic, with rapidly changing sedimentation through developed areas that heavily influences the extent of flood risks. Cessation of dredging practices in the 1990’s has left the local communities with the prospect of containing flood flows with temporary measures and new levees. In January 2009, significant rainfall on top of a melting snowpack caused high flows in the White River in Western Washington, which spilled over its banks and inundated homes, businesses, and streets in the City of Pacific. The magnitude of this event, which was regulated by the upstream MMD (Mud Mountain Dam), previously was seen to occur without local flooding. Loss of channel capacity continues in present times, causing significant uncertainty in the appropriateness of the FEMA FIRMs (Flood Insurance Rate Maps), NWS (National Weather Service) flood stages, and general risk communication. Sedimentation, local levee projects, climate change, flood warnings, and MMD regulation all play into a system that can change from flood season to flood season and that has, in the past, lacked a clear picture of flood risk. A recent effort in 2021, sponsored by the USACE Silver Jackets program, brought together a group of local official and flood engineers to collaborate and develop better flood risk communication through maps and flood warnings. From this effort, a template was created that can be applied each flood season as the ongoing effort is made to estimate channel capacity and to predict sedimentation through use of HEC RAS 1d and 2d modeling. Each flood season, hydraulics are updated with new topo-bathy LiDAR, USGS gage data, and high-water marks to inform deployment of spotters, emergency management efforts, and reservoir regulation. From this effort, maps and flood warnings are updated annually for the Northwest River Forecast Center. Sediment modeling is being developed, in-parallel to hydraulics, to predict future conditions through the next 5-years. While sedimentation is another talk, predictions will be used to inform implementation of future flood protection measures for the Lower White River.