Hydraulics of the August 7,1980, pyroclastic flow at Mount St. Helens, Washington

Open-channel hydraulic theory for flow of an inviscid fluid down a channel of changing slope and width yields a longitudinal velocity profile that matches measured velocities of the August 7, 1980, pyroclastic flow at Mount St. Helens. The model is useful for prediction of zones of erosion, deposition, and hazards. It requires only topographic data, estimates of volumetric flow rate, and a Manning coefficient appropriate for the channel.