The U.S. Geological Survey says that dredged sediments placed in waters near the mouth of the Columbia River, where waves and coastal currents can redistribute them, helps to reduce erosion along nearby beaches, according to new research published this month.
New data collected and models developed during the study can help resource managers optimize strategies to manage the sand, a valuable resource, at the river mouth. The models developed for this study provide critical information to resource managers and stakeholders about where and how quickly the dredged sediment disperses after it is placed on the seabed.
The Columbia-Snake River System is a conduit for commerce, carrying millions of tons of international trade by large container vessels each year. The river also moves large amounts of sediment eroded from the landscape, depositing it into the vast river system.
As the largest river by volume flowing from the Americas into the Pacific Ocean, the Columbia transports an estimated 5 million tons of sediment downstream toward its mouth each year. The U.S. Army Corps of Engineers (USACE) dredges between 2 and 4 million cubic meters (about 800-1600 Olympic swimming pools worth) of sediment from the six-mile-long entrance channel at the river mouth each year to maintain safe passage for large vessels.
“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.
This creates a secondary management challenge of how to balance dredging needed for large vessel safety with the desire to maintain beach sand.
“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 was dredged from the river mouth and experimentally deposited nearby as a submerged, relatively nearshore, low-relief berm.
Using multibeam bathymetry, scientists repeatedly monitored the changing shape of the berm, tracking its dispersion toward nearby beaches. From these observations, scientists tested a hydrodynamic and sediment transport model to accurately predict the observed changes and quantify the amount of sediment delivered toward the coast, as well as optimal locations for the dredge sand berms.
Dredged material management strategies have adapted over time based on input from stakeholders concerned with impacts to coastal erosion, navigation safety and benthic habitats. Historically, dredged sediment has been disposed in deep water offshore, which can have significant impacts on adjacent beaches over time by depriving them of sand.
To address these challenges, local regulators, industry stakeholders, and state and federal agencies work together to limit the amount of sediment lost to deeper water offshore, while still maintaining navigable channels and healthy nearshore habitats at the river mouth.
For example, placement of some dredged sand in small areas near the river mouth created mounds on the seabed that inadvertently impacted Dungeness crab habitat or created navigation hazards to mariners. More recently, new placement techniques spread the dredged sand in thin layers over large nearshore areas, limiting impacts to sensitive habitats and allowing natural processes to transport the sand toward the coast.
