Kinematic Properties of Mudflows on Mt. St. Helens

Abstract

Summarized are findings of a computer‐based study of the mud‐flows on Mt. St. Helens following its eruption on May 18, 1980. Analyses were made of the start‐up dynamics and flow of mud down various drainages of the mountain, and of the surge wave generation in Swift Reservoir when the mudflow down East Pine Creek entered the reservoir. Viscosities of mudflows in the Muddy Creek and Pine Creek drainages were evaluated in order to establish a basis for general analysis of mudflow phenomena. Starting with estimates of snow deposition on the mountain, and incorporating the characteristics of the pyroclastic blast of the eruption, initiation of flow was approximated. Subsequent flow down different drainages was modeled, and time sequencing of the flows into Swift Reservoir was compared with measured reservoir elevations recorded on the morning following the eruption. Additional kinematics of the flows were compared with corresponding field measured data where possible.