| 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. -- Jennings, et.al., 1981 |
