The Spirit Lake dilemma: Engineering a solution for a lake with a problematic outlet

Abstract

The Mount St. Helens eruption on May 18, 1980, fundamentally transformed the surrounding landscape. The eruption triggered geophysical processes that are still unfolding. A debris avalanche caused by the eruption, for example, blocked the outlet from Spirit Lake. To prevent the rising lake level from breaching the blockage and potentially flooding communities downstream, the U.S. Army Corps of Engineers built an outlet tunnel to maintain safe lake levels. However, the tunnel must be periodically closed for repairs, during which time the lake can rise precariously high.In 2015, the Gifford Pinchot National Forest commissioned a study to assess risks associated with alternative outlet options. A team consisting of researchers from the U.S. Forest Service Pacific Northwest Research Station, the U.S. Geological Survey, and Oregon State University authored the study.At the team’s request, the U.S. Army Corps of Engineers conducted a damsafety risk-assessment of long-term solutions: maintaining the existing tunnel, rehabilitating the tunnel, creating an open channel across the blockage, or installing a buried conduit across the blockage. The assessment determined that there is no risk-free way to remove water from Spirit Lake, but the likelihood is generally low that these solutions will fail. With this information, the Forest Service is moving forward with developing a long-term solution to managing the Spirit Lake outlet.

Citation

Watts, Andrea; Grant, Gordon; Major, Jon. 2019. The Spirit Lake dilemma: Engineering a solution for a lake with a problematic outlet. Science Findings 218. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 5 p.