News Release
| Release Number: | 98-033 |
| Dated: | 4/20/1998 |
| Contact: | Matt Rabe, 503-808-4510 |
Portland, Ore. – A little more than 10 years after it began collecting sediment, the last row of outlet pipes at the Mount St. Helens Sediment Retention Structure will be closed Wednesday. But area residents shouldn’t be too concerned – there is still room to store an additional 190 million cubic yards of sediment behind the structure.
There is currently an estimated 68 mcy of volcanic sediment trapped behind the SRS.
Maintenance workers from the U.S. Army Corps of Engineers’ Portland District will close the five-pipe row. Unlike the five previous rows, which were permanently sealed between 1991 and 1997, the top row is an operating row that can be reopened, as needed.
The National Marine Fisheries Service and the Washington Department of Fish and Wildlife support the closure. Experts from the two agencies said migrating juvenile fish are better off traveling over the spillway.
The SRS, located 22 miles east of Castle Rock, Wash., slows the flow of the North Fork Toutle River. The slower flows allow volcanic sediment to drop out of the water and settle above the SRS. The structure was designed with six rows of outlet pipes that allow the water to pass through the SRS and into a plunge pool and outlet channel. Each row contains five 3-foot-diameter pipes and each row is 10 feet above the next lower row. The pipes were designed to be closed off one row at a time as the sediment level rose behind the SRS. After the top row is closed, the river will flow only over the structure’s spillway. If future construction-related work is required within the spillway, the operating row could be temporarily reopened and the spillway blocked off.
Sediment will continue to be trapped behind the structure and hydraulic engineers expect a gradual gradient, or slope, to continue to develop as more materials deposit up-river of the SRS. Sediments that may pass by the structure are expected to flush through the system.
The SRS, built in the late 1980s following the May 18, 1980, eruption of Mount St. Helens, collects sediment that would otherwise settle in the channels of the Toutle, Cowlitz and Columbia rivers where it could cause flooding and impede navigation.
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Sediment Structure Timeline
Fall 1985Plans for SRS presented
Summer 1986SRS construction funds authorized
Winter 1986SRS construction begins
Fall 1987Sediment begins collecting behind structure
Winter 1989Construction completed
- First row of pipes permanently closed
- Second row of pipes permanently closed
- Third row of pipes permanently closed
March 1997Fourth row pipes permanently closed
September 1997Fifth row of pipes permanently closed
April 22, 1998Top row of pipes closed (can be reopened, as needed)