Flood hazards created by natural dams on the Toutle and Cowlitz River systems formed from rock and debris deposits derived from the May 18, 1980, eruption of Mount St. Helens were analyzed and evaluated. The post-eruption structures investigated include the dam impounding Spirit Lake and the dam at Elk Rock on the North Fork Toutle River. Working under emergency conditions, efforts were made to adapt modeling programs to the unique drainage conditions confronted. Dam-break and flood-routing models utilized in the study are described.
The Elk Rock dam failed after the assessment was conducted. Computed peak discharge values were approximately the same as those measured or estimated during and following the failure.
The studies illustrate that useful dam-break and flood-hazard estimates are possible with existing computer models, even though data may be only approximate and incomplete.
Mount St. Helens, in southwestern Washington, erupted at 8:32 a.m., P.s.t., on May 18, 1980. The north side of the volcano's cone was blown off by a powerful lateral blast, and a huge volume of debris buried land and water features in the drainage area immediately north and northwest of the mountain. Major geomorphic and hydrologic changes extended downstream along the Toutle and Cowlitz Rivers, with mudflows and hydraulic effects reaching the Columbia River (fig. 1). Details of these post-eruption events are described in other reports of this volume.
This report deals with emergency flood hazard situations created by the massive debris avalanche deposit in the North Fork Toutle River valley and by downstream sediment deposition. The debris deposit is up to 600 ft thick; it formed a dam approximately 500 ft high at the outlet of Spirit Lake, creating a set of conditions conducive to overtopping and subsequent downstream flooding. To date (March 1981), this has not occurred. However, Geological Survey hydrologists witnessed and recorded the overtopping and failure of a similar but smaller dam in the North Fork Toutle River near Elk Rock. A flood-hazard assessment made the day before the dam failure proved to be reasonably accurate and useful for the choice of advance protective measures.
The report contains (1) a brief description of the computer models employed to make the dam-failure and flood-threat analyses; (2) analyses of the dam-failure potential at Spirit Lake and at Elk Rock; and (3) hypothetical flood routings from both of these sites.
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