Geomorphic controls of the shape and mobility of rock avalanches
A graphical and statistical treatment of data taken from a sample of 40 rock avalanches selected mainly from the literature enables us to distinguish three ways in which the local morphology controls the shape and motion of such landslides. The three different situations are characterized as follows: (1) channeling of the debris mass, (2) unobstructed spreading of the debris mass, and (3) right-angle or almost right-angle impact against an opposite slope. Mobility of rock avalanches decreases going from situation 1 to situation 3. As presumably local morphology exerts its action ("geomorphic control") by influencing the rate of dissipation of the total mechanical energy involved in the process, the three cases are respectively defined as low-energy-, moderate-energy-, and high-energy-dissipative. As secondary results, we find the following. (1) By using the properties of the ratio of "excessive travel distance" to "travel distance," the Hsu (1975) runout-prediction model can be modified so as to avoid the difficult problem of estimating the landslide volume prior to failure. (2) If landslide volume and type of geomorphic control can be preliminarily estimated, then a maximum possible runout can also be computed corresponding to each of the three classes of geomorphic control.