Rapid reservoir erosion, hyperconcentrated flow, and downstream deposition triggered by breaching of 38 m tall Condit Dam, White Salmon River, Washington

Abstract

Condit Dam on the White Salmon River, Washington, a 38 m high dam impounding a large volume (1.8 million m 3 ) of fine-grained sediment (60% sand, 35% silt and clay, and 5% gravel), was rapidly breached in October 2011. This unique dam decommissioning produced dramatic upstream and downstream geomorphic responses in the hours and weeks following breaching. Blasting a 5 m wide hole into the base of the dam resulted in rapid reservoir drawdown, abruptly releasing~1.6 million m 3 of reservoir water, exposing reservoir sediment to erosion, and triggering mass failures of the thickly accumulated reservoir sediment. Within 90 min of breaching, the reservoir's water and~10% of its sediment had evacuated. At a gauging station 2.3 km downstream, flow increased briefly by 400 m 3 s À1 during passage of the initial pulse of released reservoir water, followed by a highly concentrated flow phase-up to 32% sediment by volume-as landslide-generated slurries from the reservoir moved downstream. This hyperconcentrated flow, analogous to those following volcanic eruptions or large landslides, draped the downstream river with predominantly fine sand. During the ensuing weeks, suspended-sediment concentration declined and sand and gravel bed load derived from continued reservoir erosion aggraded the channel by >1 m at the gauging station, after which the river incised back to near its initial elevation at this site. Within 15 weeks after breaching, over 1 million m 3 of suspended load is estimated to have passed the gauging station, consistent with estimates that >60% of the reservoir's sediment had eroded. This dam removal highlights the influence of interactions among reservoir erosion processes, sediment composition, and style of decommissioning on rate of reservoir erosion and consequent downstream behavior of released sediment.

Large-scale dam removal on the Elwha River, Washington, USA: River channel and floodplain geomorphic change

“…Measured peak suspended-sediment concentrations during instantaneous removal of Marmot Dam and Condit Dam were 3 times and 52 times greater, respectively, than those recorded on the Elwha River during the first two years of dam removal (49,000 mg/L at Marmot Dam and a hyperconcentrated flow of 850,000 mg/L at Condit Dam, compared to 16,300 mg/L from the Glines Canyon Dam removal on the Elwha River; Major et al, 2012;Wilcox et al, 2014;Magirl et al, 2014-in this volume), although sampling frequency was lower on the Elwha River than on those other rivers during instantaneous dam removal in this volume).…”

“…In the latter two cases, the sudden base level drop accompanying instantaneous dam removal led to rapid upstream propagation of major knickpoints or knickzones through the reservoir sediment, exerting a significant control on reservoir sediment erosion rate and thus sediment supply to the river below the dam sites. The reservoirs behind Marmot and Condit Dams had contained thick sediment deposits abutting the respective dams, facilitating rapid, large-magnitude knickpoint retreat upon dam removal Wilcox et al, 2014). Reservoir sediment erosion during the Condit Dam removal also largely proceeded by mass-movement processes in the dominantly finegrained deposit (60% sand, 35% silt and clay, 5% gravel), which contributed to the hyperconcentrated flow mentioned above .…”

Legacy effects on sediments in river corridors

“…At least 121 dams were removed across the United States during 1930-1999 and another 20 to 50 dams were decommissioned each year during the first decade of the 21st century . A common scenario following dam removal is incision of a channel into reservoir sediment, followed by channel widening, aggradation, and eventual stabilization in a manner that reflects the degree of base level lowering following dam removal, as well as particle size and cohesion of reservoir sediment (Stanley et al, 2002;Doyle et al, 2003;Lorang and Aggett, 2005;Cheng and Granata, 2007;Major et al, 2008;Wilcox et al, 2014). The distance over which sediment is dispersed downstream, and associated depositional patterns, depend on sediment supply versus river transport capacity and are likely to be highly variable across time and space as the channel gradually adjusts (Pizzuto, 2002).…”

Large-scale dam removal on the Elwha River, Washington, USA: Erosion of reservoir sediment

“…Most of the 1200 dams removed to date have been relatively small (b10 m) with modest sediment volumes that were less than the decadal sediment load of the channel (e.g., Cheng and Granata, 2007;Burroughs et al, 2009;Pearson et al, 2011;Major et al, 2012;Woelfle-Erskine et al, 2012;American Rivers, 2014). Quantitative data on reservoir and downstream river response to dam removal are limited to a few cases where detailed data have been collected (Doyle et al, 2002;Pizzuto, 2002;Major et al, 2012;Tullos and Wang, 2014;Wilcox et al, 2014). The rate and extent of reservoir sediment erosion and downstream release are relatively unknown for dam removals with large sediment volumes (N 10 times the annual sediment load), largely because so few have occurred.…”