Elwha River Restoration: Sediment Modeling

Abstract

The National Park Service, with technical support from the Bureau of Reclamation, is in the process of removing Elwha and Glines Canyon Dams on the Elwha River near Port Angeles, Washington to restore anadromous fish and the natural ecosystem and areas of cultural significance to the Lower Elwha Klallam Tribe. Elwha Dam was completed in 1913 and forms Lake Aldwell. Glines Canyon Dam was completed upstream in 1927 and forms Lake Mills. These two dams are the largest ever removed. These dams are being removed concurrently in controlled increments over a three-year period, which began on September 17, 2011. As of July 2010, reservoir sedimentation for the two lakes was estimated to be 24 million yd 3 , of which 20 million yd 3 are stored in Lake Mills. Reservoir sediment is being eroded and redistributed by the river as the dams are removed and the reservoirs are drawn down. This paper describes a mass balance numerical model that is being used as part of the sediment adaptive management and monitoring program. This program is presently being implemented to compare measured effects with predictions and recommend corrective actions if necessary. Facilities have been constructed for water quality and flood protection to mitigate for sediment effects, including water treatment plants, new wells, a new surface water intake, raising the height of existing levees, and the construction of new levees. The sediment effects of dam removal have been predicted based on a drawdown experiment of Lake Mills, numerical modeling, and physical laboratory modeling. A numerical mass balance model was developed for use during the adaptive management program to provide up-to-date predictions based on changing hydrology and dam removal schedules. In addition, the numerical model is also being used to help guide monitoring activities and synthesize monitoring data.