Large-scale dam removal on the Elwha River, Washington, USA: Coastal geomorphic change

Abstract

Two dams on the Elwha River, Washington State, USA trapped over 20 million m 3 of mud, sand, and gravel since 1927, reducing downstream sediment fluxes and contributing to erosion of the river's coastal delta. The removal of the Elwha and Glines Canyon dams, initiated in September 2011, induced massive increases in river sediment supply and provided an unprecedented opportunity to examine the geomorphic response of a coastal delta to these increases. Detailed measurements of beach topography and nearshore bathymetry show that~2.5 million m 3 of sediment was deposited during the first two years of dam removal, which is~100 times greater than deposition rates measured prior to dam removal. The majority of the deposit was located in the intertidal and shallow subtidal region immediately offshore of the river mouth and was composed of sand and gravel. Additional areas of deposition include a secondary sandy deposit to the east of the river mouth and a muddy deposit west of the mouth. A comparison with fluvial sediment fluxes suggests that~70% of the sand and gravel and~6% of the mud supplied by the river was found in the survey area (within about 2 km of the mouth). A hydrodynamic and sediment transport model, validated with in-situ measurements, shows that tidal currents interacting with the larger relict submarine delta help disperse fine sediment large distances east and west of the river mouth. The model also suggests that waves and currents erode the primary deposit located near the river mouth and transport sandy sediment eastward to form the secondary deposit. Though most of the substrate of the larger relict submarine delta was unchanged during the first two years of dam removal, portions of the seafloor close to the river mouth became finer, modifying habitats for biological communities. These results show that river restoration, like natural changes in river sediment supply, can result in rapid and substantial coastal geomorphological responses.Published by Elsevier B.V.

New Insights on Coastal Foredune Growth: The Relative Contributions of Marine and Aeolian Processes

“…Bracketing the main SEDEX 2 period, topographic data were collected nominally monthly between June 2016 and June 2017 ( Figure 1d). There are vertical uncertainties of~± 0.07 m for the backpack-based topography (e.g., Ruggiero et al, 2005) and~± 0.13 m for the single-beam bathymetry (e.g., Gelfenbaum et al, 2015) surveys. These data are interpolated onto a cross-shore grid (dx = 0.1 m) in order to assess volumetric and contour changes.…”

Alongshore sediment bypassing as a control on river mouth morphodynamics

“…Several numerical models of river mouth dynamics have been developed, in some cases to estimate future river mouth geometries [ Tanaka , ] or in others to explain the physics behind observed river mouth morphologies [ Edmonds and Slingerland , ; Gelfenbaum et al , ]. Gelfenbaum et al .…”

Kelp, cobbles, and currents: Biologic reduction of coarse grain entrainment stress

“…In the near term, high turbidity and increased scour in the coastal waters around the river mouth will probably prevent the recruitment or growth of kelp (Duda et al, 2011). In the long term, a shift in substrate toward finer sediment size classes that are not utilized by kelp around the Elwha River delta may lead to a reduction in kelp density (Gelfenbaum et al, 2015). Thus, at this location, independent of current conditions, the maxi- Note: Species-specific, empirically determined parameters for Nereocystis luetkeana for kelp-assisted entrainment model .…”

“…The recent removals of the Elwha Dam and Glines Canyon Dam have garnered widespread attention due to the enormous volume of sediment liberated by these projects (East et al, 2015;Gelfenbaum et al, 2015). Consequently, a number of studies have focused on the transport and fate of sediment and the response of the coastal landscape to sediment loading (Duda et al, 2011;Miller et al, 2011).…”