MULTIPLE-PULSED DEBRIS AVALANCHE EMPLACEMENT AT MOUNT ST-HELENS IN 1980 - EVIDENCE FROM NUMERICAL CONTINUUM FLOW SIMULATIONS

The complex 1980 Mount St. Helens debris avalanche is modeled numerica lly as a transient biviscous fluid flow. Several approaches are consid ered, including two-step rheologically-distinct models for avalanche I and combined avalanches II/III, and a composite flow model consisting of retrogressive slides of identical theology successively accreted t o the main avalanche flow. For the two-step situation, flow theologies are evaluated separately for the initial avalanche, comprising the de bris avalanche block facies, and an ensuing explosive-influenced flow. Strengths (normalized by density) as high as 250 m(2)/s(2) and appare nt Newtonian viscosities as much as 275 m(2)/s were established for th e block facies. These parameters for the explosively-influenced flow a re an order of magnitude lower. The distribution of stratigraphic unit s within flowing model debris, compared with field distributions, sugg ests that the higher-strength emplacement models are appropriate for d ebris deposited on Johnston Ridge and in the upper parts and flanks of the North Fork Toutle River valley. In general, models for which cons tant theology is assumed throughout the flow process provide lower-bou nd emplacement times, and excessive early velocities, as compared to t he prototype event. Because model calibration is based on matching run out by trial and error, it is therefore biased toward the theologic pa rameters essential to achieving that runout. These values characterize the flow in its latter stages, whereas the actual strength and viscos ity may have substantially decreased as a function of displacement. Tw o-dimensional models predict debris accumulation about twice as thick as that observed at the foot of Mount St. Helens, where flow divergenc e was significant. This discrepancy is lessened with a quasi-three-dim ensional modification of the flow model. Accretionary composite flow m odels with homogeneous theology simulate the overriding of early avala nche debris by later debris pulses. The accretionary composite model a lso predicts a thick flow snout down-valley. Since this feature is not indicated in the debris deposit, the model lends support to the conce pt of flow separation by theology. The two-step and accretionary compo site flow models complement one another and provide model-based suppor t for the Harry Glicken hypothesis for complex emplacement at Mount St . Helens.