A new study that examines sediment transport adjacent to the Columbia River Mouth incorporates both physical observations and modeling techniques to track how strategically placed dredged material can resupply sand to nearby beaches, thereby addressing erosion.
New research informs dredging efforts at Columbia River mouth, conserving valuable resource
Navigating the “Graveyard of the Pacific”
Major rivers such as the Columbia-Snake River system are ancient conduits for commerce, carrying millions of tons of international trade each year. The Columbia River also carries a lot of sediment from the interior: As North America's largest river by volume flowing into the Pacific Ocean, every year the Columbia transports an estimated 5 million tons of sediment downstream.
At unaltered river deltas, processes such as wind, waves, and currents distribute this sediment across adjacent coastal areas—some will be deposited onshore and in nearshore environments, while some is transported further offshore.
But the Mouth of the Columbia River (MCR), like most major inlets around the world, is engineered for shipping traffic, with rock jetties that concentrate its flow into navigable channels. Before the inlet was improved for navigation in the early 20th century, passage from the river mouth to the ocean was so treacherous that the area was known to mariners as the “Graveyard of the Pacific”.
Since 1891, the MCR has been dredged to facilitate safe passage of large commercial vessels. Today, the U.S Army Corps of Engineers (USACE) dredges between 1.7 and 3.5 million tons (between 2 and 4 million cubic meters) of sediment from the six-mile-long entrance channel at the river mouth each year.
Historically, some dredged sediment was disposed of offshore in deep water. This process can have significant impacts on adjacent beaches over time by depriving them of sand. And as rising sea levels accelerate coastal erosion, a steady supply of quality beach sand is critical to keeping coastal areas above water.
Directly north of the MCR, for example, at Benson Beach, erosion has been problematic for decades. Bathymetric and topographic surveys performed annually by the U.S. Geological Survey (USGS), Washington State Department of Ecology, and Oregon State university revealed that over 2 million cubic meters of sediment eroded from Benson Beach between 2014 and 2019.
Dredging, Nearshore Habitats, and Erosion at the River Mouth
Studying the complex interactions of dredging, sediment supply, and coastal processes at the MCR to inform sediment management required understanding the concerns of a diverse set of stakeholders. For years, the Lower Columbia Solutions Group and other State, federal, and community constituents met regularly to address sediment management at the MCR. A top priority identified by the group was to address coastal erosion impacts adjacent to the inlet.
With many competing interests, finding solutions to coastal erosion can be problematic. One way to combat coastal erosion is by applying dredged sand from a nearby inlet directly on the beach, a process known as beach nourishment. This can be expensive, though, and may restrict public access to beaches.
New research provides evidence that strategically depositing dredged sediments nearshore, where coastal processes can then redistribute them, is a viable alternative to direct beach nourishment.
The study, published in Coastal Engineering, is a collaboration between researchers from USGS, USACE, Washington State Department of Ecology, Oregon State University, Deltares, and Delft University of Technology.
“This work is the culmination of more than 20 years of research, experimentation, and, ultimately, collaboration among local regulators, industry stakeholders, and state and federal agencies to do the right thing, based on what the science was telling us,” said Hans R. Moritz, USACE Hydraulic Engineer and co-author of the study.
Prior to passage of the National Environmental Policy Act (NEPA) in 1970, dredged material at the MCR was, for example, deposited in ways that can negatively impact Dungeness crab habitat, prompting outcries from the shellfish industry.
Under NEPA, USACE was directed to deposit dredged material further offshore and limit the size of the placement sites to protect crab habitat. This led to the unintended formation of sandy shoals at the river mouth, creating hazards for ships. Other research revealed that sediments placed far offshore could not be transported shoreward to accumulate in nearshore environments—they were, in effect, lost to sea.
Sediment lost to sea is not only a wasted resource, but a valuable and limited one. Just as sediment supply dwindles from the Lower Columbia River estuary, the need for sediment grows: erosion is undermining the stability of the entrance jetties, as well as the loss of public beaches and dune areas.
A Viable Path Forward
Local regulators, industry stakeholders, and State and federal agencies thus wanted to limit the amount of sediment lost to deeper water offshore, while still maintaining navigable channels and healthy nearshore habitats at the river mouth. However, achieving these goals in a region as dynamic as the mouth of the Columbia River, where the outflow of the river meets ocean swells of the Pacific, is challenging.
“We wanted to be able to predict where dredged sand goes after it is placed, how quickly it disperses, and how much moves toward the coast to increase the sand supply to beaches,” said Andrew Stevens, USGS Oceanographer and lead author of the study.
In the study, approximately 216,000 cubic meters of sediment dredged from the MCR was experimentally deposited nearby as a submerged, low-relief berm. Using multibeam bathymetry, the team repeatedly monitored the changing morphology of the berm, tracking its dispersion to nearshore habitats. From these observations, they tested a hydrodynamic and sediment transport model to accurately predict the observed changes and quantify the amount of sediment delivered toward the coast.
“Instead of removing sediment from the littoral system, this approach allows for natural processes to transport a portion of the sediment from the placement site to desired onshore locations, replenishing beaches in a more natural way,” said George Kaminsky, Coastal Engineer with the Washington State Department of Ecology and a co-author of the study. “To change the practice of how a system like this is managed takes coordination, research, consensus building, and adaptive management techniques. By applying rigorous science, presenting it to policy makers and soliciting their feedback, this study represents an approach to solving complex coastal management issues that could be applied to other large river systems.”
“Results from this study improve our understanding of how natural processes re-work dredged sand placed within nearshore waters at the river mouth, enabling us to improve beneficial use of dredged material without causing ecological harm,” said Moritz. “The sustainability of the inlet and its 100-year-old jetty system is dependent on how we beneficially use dredged material to maintain the inlet’s tidal shoal morphology. Without the tidal shoals on which the jetties were built, there can be no engineered inlet at the river mouth.”
Read the related press release about this study.