The Sediment Retention Structure
The Sediment Retention Structure (SRS) on the North Fork Toutle River in southwest Washington is one element of the unprecedented three-part solution to stop the debris avalanche from moving downstream and causing navigation problems. Levee modifications at Kelso, Wash., and dredging in the Toutle and Cowlitz rivers are parts two and three of the long-term plan.
A conventional dam can stop the flow of water completely. The Sediment Retention Structure (SRS) does not stop water; it works with nature to stop sediment by slowing the flow of water so that the sediment drops out. Instead of traveling downstream and settling in the river channels where it can cause flooding and impede navigation, sediment builds up behind the SRS in a single large manageable deposit.
The SRS consists of an 1800-foot-long embankment rising 184 feet above the post-eruption streambed, with concrete outlet works and an unlined spillway at one end. The embankment is made of fractured rock with a tapered core of impervious clay taken from the site. The structure rests on ancient river gravels, and water passes underneath and rises inside the embankment when the lake level behind it is high. Drainage pipes set into the embankment face, between layers of roller-compacted concrete, allow this water to run back into the lake when the level recedes.
Upstream from the SRS, where the wide, braided channel of the North Fork Toutle enters the lake, the stilling action of the impounded water caused the sediment to drop to the bottom. The sandbar can be seen is the natural collection point of the SRS. This bar will gradually build downstream toward the embankment as the lake fills with about 258 million cubic yards of sand, gravel and sediment over the next 50-years.
The large block or center monolith of the outlet works contains six rows of five outlet pipes or conducts through which water and fish can pass into the plunge pool and outlet channels below. These pipes can be closed off permanently as the sediment level rises behind the structure. By about the year 2035, when the sediment has reached the 400-foot-wide spillway at an elevation of 125 feet above the post-eruption streambed, all the conduits will be closed and the river will flow continuously over the spillway.
The entire site was blasted out of solid rock, with most of the excavated material put to use in building the project. Rock bolts hold the rock face under compression to a depth of 20 feet, through a four-inch of shotcrete. The course of the river was changed three times during construction: first to the river while a diversion pipe was buried at the south side of the valley; then through the pipe while the structure was being built over it; and then north again through the outlet works. At that point, in November 1987, the SRS formed a lake and began retaining sediment. Final construction work on the embankment and spillway was completed in December 1989.
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<< See also SRS Project Specs & the Closing of the Outfall Pipes >>