Catastrophic Debris Streams (Sturzstroms) Generated by Rockfalls

Large rockfalls commonly generate fast-moving streams of debris that have been called “sturzstroms.” The geometry of sturzstrom deposits is similar to that of mudflows, lava flows, and glaciers. Sturzstroms can move along a flat course for unexpectedly large distances and may surge upward by the power of their momentum. A currently popular hypothesis to account for their excessive distance of transport suggests that sturzstroms slide on air cushions. Contrary to that hypothesis, evidence is herein presented to support Heim's contention that sturzstroms indeed flow.

The flow of a sturzstrom can be compared to flow of a mass of concentrated cohesionless grains in a fluid medium. Frictional resistance to such grain flow is, according to Bagnold, less than that for sliding of rigid bodies because of the buoyancy of an interstitial fluid which serves to reduce the effective normal pressure of the entrained grains. The presence of sturzstrom deposits on the Moon indicates that the interstitial fluid is not necessarily a compressed gas or a wet mud. The dispersion of fine debris and pulverized rock dust among the colliding blocks may have provided an uplifting stress during the motion of some terrestrial and lunar sturzstroms.

Scale models to provide kinematic simulation of sturzstroms may have practical application. Preliminary results suggest that a bentonite suspension of a certain consistency is a suitable material for scale models and that the flow of thixotropic liquids is kinematically similar to sturzstroms. The parameter “excessive travel distance” is introduced to replace the expression “equivalent coefficient of friction” as a measure of mobility of sturzstroms. There is, on the whole, a positive semilog correlation of the excessive travel distance to the size of the fallen mass. Exceptions to the rule include on the one extreme the unusual mobile Huascaran rockfall which gave rise to a sturzstrom with a dense interstitial mud and, on the other extreme, the least mobile Vaiont rockslide which remained a sliding block and failed altogether to generate a sturzstrom.