Alternative Strategies for a Permanent Outlet for Spirit Lake Near Mount St. Helens, Washington

摘要

The natural outlet of Spirit Lake, located approximately 4 miles north of Mount St. Helens, Washington, was blocked by a debris avalanche created by the May 18, 1980, eruption of the mountain. The water level in Spirit Lake is being maintained at an elevation of approximately 3460 feet by a temporary pumping facility. However, the blockage could fail, causing catastrophic flooding and mudflows downstream on the Toutle and Cowlitz Rivers. The Army Engineer Portland District, Portland, is examining six alternatives for a permanent outlet for Spirit Lake to prevent failure of the blockage: (1) an open channel across the blockage to the North Fork Toutle River; (2) a gravity-fed conduit buried in a trench through the blockage to the North Fort Toutle River; (3) a permanent pumping facility with a buried pipe across the blockage to the North Fork Toutle River; (4) a tunnel excavated through rock to the North Fork Toutle River; (5) a tunnel excavated through rock to South Coldwater Creek, a tributary to North Fork Toutle River (the preferred alternative); and (6) a tunnel through rock to Smith Creek in the Lewis River drainage basin. The 'no action' alternative is also being considered. All of the alternatives being considered would affect the Mount St. Helens National Volcanic Monument. Originator-supplied keywords include: Outlet works, Water tunnels, Flood control, Landslide dams, Dam failure, and Environmental impact statement.