LONGVIEW – Despite a breakdown and a shortage of big winter storms so far, the U.S. Army Corps of Engineers said it's already learning lessons from its experimental sediment-control structures on the Toutle River.
“We can’t say how effective they are at this point,” said Tim Kuhn, Cowlitz-Toutle project coordinator for the corps’ Portland District. “But we are where we want to be at this point to collect data. We’ve learned a lot already.”
Last week, the corps hired a contract helicopter to dump 1,300 tons of rock into the north fork of the Toutle River to stop it from cutting a channel under the Corps’ “cross valley structure.” The CVS, as it’s called, is a long labyrinth of pilings and weirs the Corps built last summer and fall to slow the river’s current and force it to deposit sediment washing downstream from Mount St. Helens.
The erosion started in the early December rainstorms. Had it been allowed to continue, it would have confined the river to a narrow channel and defeated the purpose of the corral-like structure – to spread the river out and force it to deposit silt, according to the corps.
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So here’s lesson one: If the Corps is to build more weirs in its ongoing effort to control volcanic silt, it will have to use more rock to protect them, Kuhn said.
Lesson learned: The river flows freely through the CVS, so salmon and steelhead shouldn’t have any difficulty swimming through, Kuhn said.
In addition to the CVS, the Corps last year built 14 “island-creating structures,” which are horseshoe-shaped weirs that will create eddies that are expected to cause sediment to accumulate on their downstream sides, just as islands form in rivers below a log jam. The Corps already learned something about them, too: They should have been spaced differently to improve their sedimenttrapping performance, Kuhn said.
The island-creating and CVS projects are near the old Camp Baker, a Weyerhaeuser Co. logging camp destroyed by the eruption of Mount St. Helens on May 18, 1980.
The weirs are considered experimental. If they work, the Corps might build more and bigger structures as a long-term strategy to prevent the Toutle from washing millions of tons of volcanic silt into the Cowlitz River. Unchecked, the silt buildup could increase the risk of flooding along the Cowlitz.
Now that the erosion problem on the CVS has been repaired, “we’re in a position to be getting the full breadth of information we want” from the project, Kuhn said.
He acknowledged that so far no major storms have hit the Toutle watershed, so the Corps has yet to see how effective the structures will perform when they’re swamped by large volumes of silt, logs and other debris. But it’s only midwinter, and “spring an be a pretty dramatic time” weatherwise, Kuhn added.
“A big (storm) event would be nice. We could see how the system holds up.”
The structures cost $3.5 million, which the Corps paid for with federal economic stimulus funds. It spent another $400,000 last week on the erosion-control work.
The Corps is looking for a low-cost alternative to raising the 125-foot sediment-retaining dam it built on the Toutle’s north fork in the mid 1980s. The dam has trapped 100 million cubic yards – a stack 9 miles high on a football field – but its trapping efficiency has diminished, allowing silt to pass downstream. Raising it would cost hundreds of millions of dollars and cause great fish-passage problems.
Geologists predict the Toutle will continue washing large amounts of volcanic silt downstream for several decades to come.