Runout of dry granular avalanches

Laboratory experiments on granular avalanching of dry sands and gravels reveal a consistent pattern of runout distance varying with fall height, fall slope, and volume of material for volumes ranging from 0.1 to 1000 L. Data from the South Ashburton rock avalanche deposit show that its runout behaviour differs only slightly from that of the laboratory avalanches, extending the range of this behaviour to granular avalanches with volumes of about 100 000 m(3). By contrast, data from much larger rock avalanches (> 10(7) m(3)) depart significantly from the trends of the laboratory data; some factor not present in the laboratory, such as rock fragmentation or the presence of an erodible substrate, must influence the behaviour of these larger events. Travel angles as low as 13 degrees in the laboratory tests result from grain flow mechanisms with normal friction coefficients; they are not associated only with large-volume events and do not necessarily indicate unusual material mobility at any scale.