World’s largest dam removal reverses coastal erosion

Abstract

Coastal erosion outpaces land generation along many of the world’s deltas and a significant percentage of shorelines, and human-caused alterations to coastal sediment budgets can be important drivers of this erosion. For sediment-starved and erosion-prone coasts, large-scale enhancement of sediment supply may be an important, but poorly understood, management option. Here we provide new topographic measurements that show patterns and trends of beach accretion following the restoration of sediment supply from a massive dam removal project. River sediment was initially deposited in intertidal-to-subtidal deltaic lobes, and this sediment was reworked by ocean waves into subaerial river mouth bars over time scales of several months. These river mouth bars welded to the shoreline and then initiated waves of sediment accretion along adjacent upcoast and downcoast beaches. Although the downcoast shoreline has a high wave-angle setting, the sedimentation waves straightened the downcoast shoreline rather than forming self-organized quasi-periodic instabilities, which suggests that simple coastal evolution theory did not hold under these conditions. Combined with other mega-nourishment projects, these findings provide new understanding of littoral responses to the restoration of sediment supplies.

Littoral Sediment From Rivers: Patterns, Rates and Processes of River Mouth Morphodynamics

“…Removal of two dams on the Elwha River between 2011 and 2014 resulted in the exposure, erosion and transfer of tens of millions of cubic meters of sediment, roughly half of which was sand and gravel, from the reservoirs to the downstream river (Ritchie et al, 2018;Warrick et al, 2019). This resulted in sediment discharge to the coast that was one to two orders-of-magnitude greater than pre-dammed rates for several years during and following the dam removals, 5.4 Mt (or, 3.6 million m 3 ) of which was deposited in a newly expanding delta (Ritchie et al, 2018).…”

“…Significant wave heights at the river mouth are commonly less than 0.5 m, and wave events occasionally reach 1-2 m (Warrick et al, 2011b). Although the waves heights are generally small, the persistent northwest direction of the waves results in distinct patterns of longshore sediment transport, including transport divergence near the river mouth and strong eastward transport along the shoreline east of the river mouth (Miller et al, 2011;Warrick et al, 2011bWarrick et al, , 2019Miller and Warrick, 2012).…”

“…Evidence of the links between river supply and coastal sediment budgets abound, and the simple presence of river deltas and many littoral cells are direct results of river sediment Anthony, 2015;Nienhuis et al, 2020). For this reason, alterations of river sediment supplies can change the trajectory of deltas and littoral cells as shown by both reductions of sediment discharge (Anthony et al, 2014(Anthony et al, , 2015Pratellesi et al, 2018) and increases of sediment discharge (Kuenzi et al, 1979;Warrick et al, 2019;Nienhuis et al, 2020). Because river sediment is discharged infrequently and the movement of sediment in littoral cells is commonly more constant and driven by ocean waves, distinct stages arise in the evolution of river mouth sediment deposits (Kuenzi et al, 1979;Hicks and Inman, 1987;Cooper, 1993;; Barnard and Warrick, 2010;East et al, 2018;Measures et al, 2020).…”

“…To address these questions, this paper will examine the mouths of the Elwha River, Washington and the Santa Clara River, California (Barnard and Warrick, 2010;Warrick and Barnard, 2012;Warrick et al, 2019). Observations of both systems include large sediment discharge events, but they differ in the depositional forms and beach morphology at each site.…”

A model for the growth and development of wave‐dominated deltas fed by small mountainous rivers: Insights from the Elwha River delta, Washington

“…The removal of two dams beginning in September 2011 along the Elwha River in north‐west Washington introduced ca 8.2 million tonnes of sediment into the Elwha River over a two‐year period (Warrick et al ., ). This removal simulated a high‐magnitude river discharge event and the large increase in sediment discharge resulted in a historically unprecedented progradation of the Elwha River's wave‐dominated delta (Gelfenbaum et al ., ; Magirl et al ., ; Ritchie et al ., ; Warrick et al ., ). This progradation provides a natural laboratory for recording the evolution and resulting stratigraphy of swash‐bars within a small asymmetrical wave‐dominated delta.…”