NUMERICAL-SIMULATION OF SOME LAHARS FROM MOUNT-ST-HELENS

An unsteady, open-channel model is used to simulate lahars (discharge, flow depth, peak arrival time, etc.), under the restriction of negligible volume and solid fraction variations and of channelled flow. The model is able to deal with the propagation of steep fronts and transitions from a supercritical to a subcritical flow regime, and vice versa, conditions frequently occurring on account of the rapidity of lahar-triggering mechanisms (large water-debris volumes are often mobilized in a very short time) and of topographical characteristics. The model is based on equations governing balance of mass and momentum, while the energy dissipation term is expressed as a power function of the flow depth and discharge. Two past lahars at Mount St. Helens, the Pine Creek and Muddy River lahars of May 18, 1980, are simulated and the results obtained are discussed. For these lahars the triggering mechanism was the melting of snow and ice due to pyroclastic flows and surges. Many millions of cubic meters of water (mostly from the melting of ice and snow) and sediments (eroded material and pyroclastic products) were mobilized in only a few minutes. The simulation begins at some kilometers from the vent, where lahar volumes reach an almost constant value and the flow is channelled. The triggering mechanism plays a fundamental role in the downvalley destructive power of a lahar. Initial hydrographs characterized by steep slopes, such as those occurring as a consequence of pyroclastic flows and surges, rapidly flatten downvalley, as predicted by the model. Conversely, hydrographs with a prolonged peak discharge, even if the peak value is smaller, undergo little changes over distances of lens of kilometers, with disastrous consequences, as in the case of the 1980 North Fork lahar at Mount St. Helens, triggered by the seismic shaking of an avalanche debris.