Engineering answers for Spirit Lake

An aerial view to the south of Mount St. Helens in 1982 as another lahar—melted snow and volcanic rock (think wet cement)—occurred. When the lahar encountered the debris blockage from 1980, part of it flowed into Spirit Lake (bottom left), while the rest flowed west into the Loowit Creek drainage that flows into the upper North Fork Toutle River. USGS photo by Tom Casadevall.

In Science Findings #218, “The Spirit Lake Dilemma: Engineering a Solution for a Lake with a Problematic Outlet,” USDA Forest Service – Pacific Northwest Research Station writers explore new research into the future repair or replacement of an outflow tunnel at Spirit Lake, on Mount St. Helens.

The eruption of Mount St. Helens on May 18, 1980 fundamentally transformed the surrounding landscape, triggering geophysical processes that are still unfolding.

Spirit Lake, with Mount St. Helens, Washington, in the background (2015). A debris avalanche triggered by a volcanic eruption on May 18, 1980, blocked the lake’s natural outlet. A tunnel was built to safely remove water from the lake and minimize the risk of catastrophic flooding to communities downstream. Maintaining the tunnel is expensive, so long-term solutions are being explored. USDA Forest Service photo by Rhonda Mazza.

Among them was a debris avalanche caused by the eruption, that blocked the outlet from Spirit Lake to the North Fork Toutle River.

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 level rises.

Prolonged closures, combined with increased volume from melting rainfall and snow in the spring, could allow the water level to rise high enough to breach the natural dam.

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 dam safety 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.


Source information: Rhonda Mazza is a public affairs specialist for the USDA Forest Service – Pacific Northwest Research Station, which publishes Science Findings. Find past Science Findings at: https://www.fs.usda.gov/treesearch/.

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