News Release
| Release Number: | 97-033 |
| Dated: | 3/24/1997 |
| Contact: | Matt Rabe, 503-808-4510 |
Portland, Ore.-- The Portland District, U.S. Army Corps of Engineers, will close the lowest open row of outlet pipes at the Mount St. Helens Sediment Retention Structure (SRS) on Wednesday, March 26. This is the fourth row of pipes to be closed since the structure began collecting sediment ten years ago.
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 which allow the water to pass through the SRS and into a plunge pool and outlet channel. Each row contains five three-foot-wide pipes and each row is ten feet above the next lower row. The pipes are designed to be closed off a row at a time as the sediment level rises behind the SRS. The lowest three rows already have been permanently closed off. They were closed in 1991, 1994 and 1995, respectively.
A hydrographic survey conducted last November found the sediment level had almost reached the bottom of the lowest row of open pipes. Also, large amounts of debris brought downriver by last winter's high flows have partially plugged the pipes on the open rows. Corps engineers decided it was more cost-effective to close the lower row now rather than clearing the obstructions. The top two rows will be cleared of debris later this year.
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.