Since last spring, the hopper dredge Terrapin Island has been making passes along the mouth of the Columbia River, vacuuming up the slough from a shipping channel that handles about 46 million tons of cargo annually.
An estimated 10 million cubic yards of sediment is removed from the shipping channel each year and placed in the river, on islands and out at sea. The U.S. Army Corps of Engineers, charged with keeping the channel navigable, is drafting a new 20-year maintenance plan.
Clearing the mouth
Great Lakes, the largest dredging company in the nation, has a contract to remove 2 million cubic yards of sediment out of the mouth of the river. After five months of work, the contract is nearing completion.
“We’re basically excavating underwater,” said Shohei Ishikawa, a site manager for Great Lakes. “We are contracted by the Army Corps of Engineers. The purpose is to maintain navigable waterways.”
Between 3.5 million and 4.5 million cubic yards of sediment is removed each year from the mouth of the river, half a mile wide and 6 miles long. The mouth ends at river mile 3, near Jetty A and the entrance to Washington’s Baker Bay.
Like a giant catamaran, the Terrapin Island is made from two separate hulls connected at the top by hinges. On either side of the vessel are arms, like vacuum heads, lowered into the water and drug behind the vessel, excavating 9-foot-wide swaths of sediment down to between 55 and 60 feet. Pumps in the drag arms suck sediment into a central hopper, a giant wheelbarrow with an opening bottom that carries up to 3,000 cubic yards of sediment, equal to about 300 dump truck loads, gathered in about one hour of dredging. The bottom opens up, and the sediment drops out.
How the vessel dumps sediment is as complex as the dredging itself. Great Lakes has three designated dumping spots. The preferred site is a grid of deep water about 8 miles offshore in the Pacific Ocean. The dredge also releases sediment near Cape Disappointment’s North Jetty to reinforce the beaches underlying the rock structure. In foul weather, the dredge is allowed to dump in shallow water.
The Lower Columbia
A 600-foot-wide shipping channel is maintained at 43 feet deep 100 miles from the mouth upriver to Vancouver, Washington, where an additional 6 to 8 million cubic yards of sediment is taken out each year. About two-thirds is disposed of in the water, while another third is put on islands throughout the estuary and used for beach nourishment.
“We’ve got kind of a core engineering regulation that charges us with making sure we have capacity for dredge material placement,” said Jessica Stokke, the Army Corps’ project manager for Lower Columbia dredging.
Most of the material is course grain sand. The Army Corps finds places to put the material to use, such as helping create additional off-channel habitat in the Columbia National Wildlife Refuge or treating water for the city of Astoria.
“We place it along eroding shorelines, in the river,” Stokke said. “We try to place material near shore to support the jetties and shorelines north and south of the Columbia.”
Since 1890, the Army Corps estimates more than 1 billion cubic yards of sediment has been dredged between the mouth of the Columbia and Vancouver. The Army Corps does about half the annual dredging through its vessels the Essayons and Yaquina, while also contracting with companies.
About 15 of the 20 or so designated upland sites where dredge spoils are placed are at or nearing capacity, said Jeff Henon, a spokesman for the Army Corps.
The Army Corps is in the scoping stages of a new 20-year channel maintenance plan, gathering public comment on strategies to place the dredged materials off-channel, and on ways to reduce the need for dredging.
“We definitely already do this with pile dikes or wing dams,” Stokke said. “They direct river flow toward the channel, which keeps a faster flow. We have over 200 structures. We’ve been constructing them since the 1880s.”
Sponsoring the new 20-year plan with the Army Corps are the Port of Portland and Washington ports in Vancouver, Woodland, Kalama and Longview. The Ports each sponsored a project finished in 2010 to deepen the Columbia shipping channel to 43 feet.
The Army Corps is holding public hearings along the river to gather comments on how the channel should be maintained. On Oct. 17, they will be in Astoria.