Installation of Impact Plates to Continuously Measure Bed Load: Elwha River, Washington, USA

Abstract

In 2008 and 2009, a series of bed load impact plates was installed across a channel spanning weir on the Elwha River, Washington. This is the first permanent installation of its kind in North America and one of the largest anywhere. The purpose of this system is to measure coarse bed load during and after the removal of Elwha and Glines Canyon Dams. It is estimated that 21 million m3 of sediment have accumulated behind both dams, with 45–50% of the total accumulated volume expected to be eroded through natural processes. The impact plate system consists of 72 plates installed at a diversion weir downstream from both dams and 5 km upstream from the river mouth. Of the 72 plates, 46 are instrumented with a geophone and 26 with an accelerometer. Collection of physical bed-load data for calibration of the geophone plates has begun, with additional measurements to be collected in the future. This paper describes the specifics of the Elwha impact plate system and the ongoing process to collect bed-load measurements for system calibration.