Corps exploring several options for managing tons of sediment from Mount St. Helens

Corps exploring several options for managing tons of sediment from Mount St. Helens

Corps exploring several options for managing tons of sediment from Mount St. Helens
Corps exploring several options for managing tons of sediment from Mount St. Helens

In its long, continuing effort to protect Cowlitz River communities from Mount St. Helens flood threats, the U.S. Army Corps of Engineers is asking an intriguing question. Is it possible, with a little help from an upstream dam in Lewis County, to get the Cowlitz to sweep itself free of the tons of volcanic debris expected to wash into it during the next three decades?

The idea is to send pulses of clear water out of 600-foot Mossyrock Dam, the state’s tallest, to flush the sand and gravel the Toutle River keeps disgorging into the Cowlitz.

“The river is a good dredge,” said Jeremy Britton, the technical lead for a corps team in Portland evaluating the idea.

As improbable as this option might seem, it is one of 16 measures the corps is evaluating as a long-term solution to the volcano sediment problem. A report recommending a preferred solution is expected late this year.

With no action, the corps estimates that 20 million to 70 million additional cubic yards of sediment will clog the Cowlitz channel by 2035. To put those numbers in perspective, about 40 million cubic yards of muck clotted the Cowlitz after the volcano’s mud flows roared through it on May 18, 1980, triggering the corps’ most intense dredging effort ever.

The corps has not yet calculated how much the silt buildup would add to flooding odds, but it certainly would put lower river communities at an elevated risk.

Most of the sediment washing into the Cowlitz comes off the so-called debris avalanche deposit, which is the remains of Mount St. Helens’ old north flank. It buried the upper Toutle Valley up to 600 feet deep in places, and the north fork of the Toutle continues to eat it away at a fantastic rate.

This will mark the second time the corps has developed a long-range solution to the problem. In the mid 1980s, it built a 125-foot high dam on the north fork of the Toutle to trap the sediment. The corps considered building the dam up to 250 feet high, but engineers settled for the lower structure given the difficulty of predicting how much erosion would occur. At the time, engineers figured they’d have to revisit the issue again in the future.

That future is now. The sediment dam has been full for several years and is allowing silt to pass downstream. Giant sand bars again are appearing in the Cowlitz.

The corps isn’t tipping its hand about what measure or measures it will pick, emphasizing that it’s a complex engineering and hydrological problem. Other considerations are costs (no estimates have been developed yet) and environmental factors — such as endangered salmon and the possibility that smelt may become a federally listed species, too.

It’s likely that a combination of measures, rather than one single action, will be recommended, said Tim Kuhn, the corps Cowlitz-Toutle project coordinator.

“We’re looking for the most cost-effective approach to doing this. We’re not going to implement something that is good for fish but doesn’t do anything for flood (protection),” Kuhn said.

Finally, the corps must choose a flexible plan, because erosion estimates still are built around many assumptions and unknowns.

“Adaptability is one of our main evaluation factors,” Britton said. “The plan has to be adaptable because there are a lot of uncertainties.”

Dredge Toutle river at “LT-1” sump

Description: Resume periodic dredging at the county owned dredge spoils site off Toutle Park Road, This site was used extensively in the 1980 to intercept silt before it reached the Cowlitz.

Pro: Highly flexible to adjust to sediment flow.

Con: Marginally reliable because major storms could overwhelm it and flush large volumes of silt into the Cowlitz; limited dredging time because of fish concerns; relatively high costs because the nearest haul sites for dredge material already are filled in.

Multiple sediment-retaining dams

Description: Build a series of small (10-20 foot tall) earthen dams upstream of the existing sediment-retaining dam. The idea would be to “terrace” the valley and create a series of permanent sediment storage traps.

Pro: Flexibility. Structures could be added or raised to adjust to the flow of sediment.

Con: More streams above the existing dam could be blocked to fish passage. Major storms could breach these structures, as they did the old “N-1” dam the corps built across the valley at Camp Baker in 1980.

Raise entire sediment retaining dam

Description: Raising the entire structure, now 125 feet tall, to increase sediment-storage capacity.

Pro: Highly effective; one-time cost.

Con: Likely high costs, low flexibility to adjust if sediment flow is lower than forecast (so taxpayers would have wasted money “overbuilding” the structure; would worsen fish-passage problems and further inundate Toutle tributary streams behind the structure.

Sediment plain 'sump'

Description: Dredge out part of the sediment storage area behind the existing sediment-retaining dam, creating a “sump” or low point to catch more sediment, restoring dam’s sediment-catching ability.

Pro: Flexible, few environmental effects.

Con: Sump could be overwhelmed by sediment from a major storm, as occurred at old N-1 dam in 1981. Debris dredged from the sump might get eroded back into the river. Would need ongoing maintenance and funding.

Use Mossyrock Dam to ‘flush’ sediment from Cowlitz

Description: Timing large water releases from the state’s highest dam to help flush sediment out of the Cowlitz and into the Columbia River. May be considered in conjunction with steps to straighten out the river to accelerate its flow, and thus its scouring action.

Pro: Few environmental concerns, and would help keep silt in the Columbia River system, an environmental benefit that, for example, would help decrease erosion of coastal headlands.

Con: Not sure if it would work. Costs unknown, but potentially high to compensate dam owner, Tacoma City Light, for any loss of hydroelectric power production from Mossyrock Dam; moves the sediment problem from one river — the Cowlitz — to the Columbia.

Cowlitz River dredging

Description: Periodic dredging of the Cowlitz channel, similar to but more definitively planned than stopgap dredging near the river’s mouth in 2007-08.

Pro: Highly flexible to adjust to changes in sediment flow; relatively low costs.

Con: Dredging windows of opportunity limited by fish concerns, and might be further restricted if smelt are listed as threatened or endangered; marginally effective; space to dispose of dredge spoils limited in Longview-Kelso-Lexington areas; needs ongoing funding.

Long shots: Things considered but have very little chance of being adopted

• Elk Rock sediment retaining dam: Building another sediment retaining dam across the north fork of the Toutle where its valley narrows. It would have to be very tall, likely very costly, and it would have a backwater effect well into the Mount St. Helens National Volcanic Monument, which is supposed to be kept in as natural a condition as possible.

• Debris avalanche stabilization: Stabilizing the debris avalanche deposit by grading, seeding and riverbank protection. An enormous undertake of dubious value, in that the debris avalanche is about 17 miles long, more than 600 feet deep in places and is located in one of the most unstable river drainages in North America.

• Protecting river banks along Toutle and Cowlitz River: Does not stop the sediment flow — most of which comes from the upper Toutle Basin — and would be very costly.

• Raising levees: Costly due to land acquisition costs (dikes need to be widened as their raised); doesn’t protect undiked areas or bridges over the river.

• Straightening the Cowlitz River: Eliminating the big “oxbow” turn in the river known as Horseshoe Bend just north of Lexington; cutting a new channel near the river’s mouth from about the Lakeside Industries asphalt plant through the old Long-Bell Log pond. The idea, intended to increases the river’s flow and flushing action, would have huge costs and environmental impacts.

• Raise spillway of existing sediment retaining dam: This would increase the sediment-storage area behind the dam, but the maximum the spillway can be raised withou t jeopardizing the rest of the dam (unless it, too, were raised) is 20 feet. So this option doesn’t gain much sediment-control.


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