World\u2019s largest dam removal reverses coastal erosion

Jonathan A Warrick,Shawn R Harrison,Andrew C Ritchie,Guy Gelfenbaum,Andrew W Stevens,Ian M Miller

doi:10.1038/s41598-019-50387-7

Jonathan A Warrick, Shawn R Harrison + Show 4 more

Open Access

https://doi.org/10.1038/s41598-019-50387-7

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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.

Highlights

Coastal erosion and flooding hazards along the world’s shorelines pose risks to human settlements, infrastructure and natural resources[1,2,3]
We investigated the timing of the transition from either stable or erosional to accretional for the study area by evaluating time series of the cross-shore positions of the mean high water (MHW) elevation, the standard datum for shoreline position assessments in the U.S and a recognized local proxy for the shoreline[26]
Deltas throughout the world are increasingly vulnerable to erosion and flooding from human activities that have reduced sediment inputs, compacted existing sediments, and increased sea levels[6,15,44]

Summary

Introduction

Coastal erosion and flooding hazards along the world’s shorelines pose risks to human settlements, infrastructure and natural resources[1,2,3]. Because river mouths and deltas are inherently dynamic landforms from the variability of river sediment supply with time as well as other hydrologic and oceanographic factors[7,9,10], erosion of these systems can be pronounced where river sediment is intercepted by dams, such as documented for the Nile, Yangtze, Mekong, Ebro and other Mediterranean deltas[7,11,12,13,14,15] Given these coastal challenges, there is growing interest in supplementing or restoring sediment inputs to coastal systems to reduce erosion and flooding. Easting (km) river delta, reduced sediment supplies resulted in shoreline erosion that averaged 0.6 m/yr during the latter 20th century and increased significantly over this time[26]

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Conclusion