Development and Routing of Mudflow Resulting from Hypothetical Failure of Spirit Lake Debris Dam, Washington.

摘要

The explosive May 18, 1980, volcanic eruption of Mount St. Helens in southwestern Washington deposited nearly 4 billion cubic yards of debris in the upper 18 mi of the North Fork Toutle River valley between Spirit Lake and Camp Baker, a logging camp. The former outlet channel of Spirit Lake, located 5 mi north of the volcano, was dammed by these debris-avalanche materials to depths as great as 500 ft, leaving the lake without an outlet. The documentation of the technical assumptions that were made and the approaches that were used in the study by Swift and Kresch (1983) is the primary purpose of the report. The study area includes the entire lengths of the North Fork Toutle and Toutle Rivers, and the Coelitz from its mouth on the Columbia River to approximately 20 mi upstream from the mouth of the Toutle River. Study results are evaluated in terms of their sensitivities to the values of selected parameters from which they were computed. No refinements or re-evaluations have been made of the assumptions or approaches used to obtain the results presented in the study by Swift and Kresch (1983). All elevations in the report are referenced to sea level.