A Re-Examination of the Factors Controlling Mobility of Large Rock Avalanches

Hypermobility is one of the key identities of a large rock avalanche. It has been commonly recognized that large rock avalanche mobility (runout L and the H/L ratio) is controlled by both volume and fall height (H). This was supported by statistical analyses of previous studies using much of the data from prehistorical events. This raises three questions: (1) if the data from the prehistorical events are excluded, is mobility dependence on both volume and fall height still applicable? (2) Is there any other factor also controlling mobility? (3) Which factor does control mobility the most? To clarify these questions, the correlations between the rock avalanche mobility and initial volume, fall height, and toe drop height were re-examined using the data from 98 events with certainly rapid motion behavior. Toe drop height is defined as the maximum height between the toe of detached rock mass and that of the source slope. The results reveal that initial volume may have a much less control on mobility than fall height for most large rock avalanches except those with wet flows, unconfined flows, and rock-ice avalanches. Fall height has a vital control on mobility of all rock avalanches. Toe drop height is another factor controlling large rock avalanche mobility, whereas its control is weaker than fall height, but much stronger than the initial volume. Controlling roles of these factors on mobility of the large rock avalanches with different characteristics are different, implying that their mobility should also be attributed to other factors.