OREGON STATE UNIVERSITY

Seashore North of Columbia River Faces Major Erosion

07/24/2007

PORTLAND, Ore. – Some of the most dramatic beach erosion in the Pacific Northwest during the next 20 years may take place just north of the Columbia River jetty, where a century of shore building has ended and a major shift toward erosion has been identified.

This new trend, which scientists now believe began a few decades ago and is one of the more extreme examples of the dynamic and constantly changing Pacific Northwest seashore, was described today by a geoscientist from Oregon State University at the Coastal Zone 2007 conference in Portland.

Researchers predict that by the year 2020, the shore areas for about six miles north of North Head – a large outcropping south of Seaview and Long Beach – may retreat between 100 and 300 meters, depending on several variables. Large areas of land that have been created in the past 100 years are now going to be reclaimed by the ocean.

“The Pacific Northwest ocean beaches are spectacular systems, with constant change, new sand dunes forming and disappearing, so on one level this is not all that unusual,” said Peter Ruggiero, an OSU assistant professor of geosciences. “In this case, we can trace the forces at work directly to the creation of the Columbia River jetties about a century ago.”

Those jetties were built beginning in the 1890s and designed to create a more workable channel for navigation, and have in fact helped address many problems on what was once considered the “Graveyard of the Pacific,” an area of the Columbia River bar that has claimed more than 2,000 vessels and 700 lives. But in the process, the jetties and multiple dams on the river have vastly changed sediment transport processes in ways that are still being worked out.

In this case, the original construction of the jetties cut in half an “ebb tidal delta,” a vast shoal of sand off the mouth of the Columbia River. The changed interaction of river and tidal currents, ocean waves and sediment transport resulted in decades of shore accretion both north and south of the jetties, sometimes adding as much as 10-20 yards a year of new land.

Now, that process has reversed in some areas, and one of those particularly hard hit will be the region just north of the jetty, the very southwestern tip of Washington state. Erosion projections have already caused some development plans to be reconsidered, Ruggiero said, and many public and policy agencies need to be made more fully aware of just how extreme the erosion may be within the next 15 years.

The erosion will affect the first few miles just north of the jetty, Ruggiero said, and studies suggest that remaining shore areas on the Long Beach peninsula will continue to build outward, rather than erode.

Benson Beach between the Columbia River north jetty and North Head, part of Fort Canby State Park, began eroding a few decades ago, Ruggiero said. This has had a significant impact on park campgrounds and new development plans based on the anticipated erosion have been developed.

Beginning in the 1990s, erosion continued past North Head, and for the first time is occurring in front of the coastal communities of Seaview and Long Beach. The newest OSU projections indicate this trend will continue northward, but by 2020 will still only be affecting the first six miles north of the jetty. Erosion may continue beyond that date, Ruggiero said, but existing computer models are not precise enough to make such projections.

“On average we’re predicting about 200 meters of inland erosion in this area, but there are a lot of variables,” he said. “These projections could change based on climate change and variability, increases in wave height, even the dredging and placement of sand done by the Army Corps of Engineers.”

Ultimately, the erosion of the area north of the Columbia River north jetty is linked to erosion of a submerged sand body named Peacock Spit. This submerged spit, a remnant of the ebb tidal delta, has been lowering for decades after the construction of the jetty. Eventually, the lowering of the offshore seabed allowed larger waves to hit closer to shore, eroding the beach. There is also simply less material available to feed beaches to the north.

Since before human settlement, Ruggiero said, the Columbia River has been a driving force that shapes the beaches of both Oregon and Washington. Before the era of dam construction, seasonal floods would dump 5-10 million cubic yards of sand a year into the system, and lower sea levels at times in the past helped Columbia River sediments to form the basis for much of the Pacific Northwest coastal zone.

Although tamed by dams, stored sediments and reduced floods, peak river flow still determines how much sand is transported to the river mouth and adjacent ocean beaches. Researchers are still learning how much of 4.5 million cubic yards of sand dredged each year from the river mouth is due to the erosion of shoals fed by floods of long ago, and how much comes from the modern estuary.

What researchers do know is that the amount dredged is a comparatively large amount of sand, and they are urging constructive use of 100 percent of the material dredged from the river and bar, instead of dumping portions of it in the deeper ocean.

The OSU presentation at the Portland conference represents ongoing work that is one part of a national effort, largely supported by the U.S. Geological Survey, to better understand coastal hazards and changes on all United States seashores. Previous reports have already been finished on East Coast and Gulf of Mexico beaches, Ruggiero said. In the Pacific Northwest, scientists are trying to track changes since the late 1880s, and also devoting special attention to changes in the past few decades.

Coastal changes can be driven by storms, hurricanes, seasonal cycles, waves, ocean currents, sediment deposition, and many other factors. Long-term climate change and more temporary phenomena such as El Nino and La Nina can also play important roles. In general, Oregon coastal areas are significantly affected by major storm events, while Washington beach changes appear more linked to long-term trends. But the discussion, Ruggiero said, also always seems to come back to the Columbia River and the management operations under way on it.

“Some important changes in using dredged sediments, for instance, were made in the 1970s, but there’s still more we can do,” Ruggiero said. “Part of what’s interesting just lately, and very positive, is to see all the parties at the same table, really working out these issues, people from state and federal governments, agency managers, coastal communities, scientists, the fishing industry.”

“Not long ago after a discussion at one meeting I saw a government official laughing and slapping high fives with a crab fisherman,” Ruggiero said. “That’s not something you see real often.”