CORVALLIS, Ore. - The construction of jetties at the entrance to the Columbia River has dramatically changed the Pacific Ocean shoreline for long distances north and south of them, a new study concludes, in ways so profound that the land has yet to reach a stable equilibrium a century after they were built.
Contrary to older theories, these jetties have overwhelmed any changes caused by the construction of multiple dams on the Columbia River and their influence on sediment transport, researchers say. And even though much new land was created by jetty construction, some of it is already eroding and it's unclear exactly what the future may bring, before more stability is achieved.
Further complicating the picture is sea-level rise, increasing wave heights and the long-term potential for subsidence of land due to a subduction zone earthquake in this region.
Creation of the jetties in the late 1800s approximately doubled the rate of shoreline buildup compared to the period prior to their construction, the study concluded.
"It's not unusual for construction of a jetty to cause some changes in the beaches and shoreline near it," said Peter Ruggiero, an assistant professor of geosciences at Oregon State University. "But the impacts of the Columbia River jetties has just been amazing, the spatial scales of their influence are monstrous. I doubt when they were built anyone had a clue how significant their effect would be."
Findings of a new study were just published in Marine Geology, a professional journal, by researchers from OSU, the Washington State Department of Ecology, U.S. Geological Survey and UCLA. They examine what's called the Columbia River "littoral cell," a large coastal area about 100 miles long that's affected by its flow, sediment deposits and structures.
Construction began on the first jetty on the south side of the Columbia River in 1885, and the system was completed by 1917. The jetties have caused a major build-out and accumulation of new beach in both directions, and sediment is still accumulating further north of the river's mouth. But even as some new land is still being created, other parts are starting to erode.
"These sediment deposits can be pretty vulnerable, and we could see hundreds of feet of shoreline erosion in some areas just north of the Columbia River, the southern part of Long Beach," Ruggiero said. "Since sediment delivery from the river itself is just a shadow of what it once was, it's especially important to use dredged sediments wisely."
The whole issue of sediment dredging and placement has risen to the forefront just in recent years, Ruggiero said, as concerns about erosion have materialized. Dredged sediments were once dumped into the deep ocean because it was cheaper and more convenient, he said, but at least some of them are now being used strategically to help nourish beaches. Even though that sometimes raises concerns about cost, fisheries, crabbing grounds and other issues, he said, it would almost certainly be prudent to be using all dredged sediments for beach support as the science of this process more fully develops.
This coming summer, the U.S. Army Corps of Engineers is planning one of the biggest beach nourishment projects ever attempted near the mouth of the Columbia River. It's anticipated that about 250,000 yards of dredged sediment will be placed on beaches just north of the north jetty.
"As the coastal sediment supply from the river, inlets, shoals, and shore face continues to decrease, strategic management of material dredged from the inlets and estuaries will become increasingly important," the researchers wrote in their report conclusion.
In the meantime, multiple studies are under way to figure out how the beaches might change in the future, when an equilibrium might finally be reached so the land near the river stabilizes, and how to factor in anticipated sea-level rise and changing wave dynamics. The new research has also shown that sediment from deep water is also feeding beaches, a mechanism that had not been understood in the past.
When a major subduction zone earthquake eventually hits the region, it's expected that coastal land will subside somewhere between about one and eight feet, as it has in such events in the past. That will cause massive erosion of Pacific Northwest beaches of 1,000 feet or more in some places, researchers say, even though the same tectonic forces will begin pushing the land back upwards again over a period of decades and centuries.
The last subduction zone earthquake occurred in January, 1700. Based on historical analysis, the next event could occur almost anytime in the next 200 years or so, experts say.
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Peter Ruggiero, 541-737-1239