Figure 6
From: Reducing risk from lahar hazards: concepts, case studies, and roles for scientists
Lake-level stabilization to prevent failure of a natural debris dam and a subsequent lahar. At Mount St. Helens (USA) a tunnel was bored through a mountain ridge to divert water from Spirit Lake into an adjacent drainage basin. In this case debris-avalanche and pyroclastic-flow deposits formed the potentially unstable natural dam. This geologic cross section shows the 2.5-km-long outlet tunnel, which stabilizes the lake by keeping the water surface at a safe level below the dam crest (from Sager and Budai [1989]).