THE GENESIS AND BEHAVIOUR OF MASS FLOWS INVOLVING LARGE-SCALE ENTRAINMENT ALONG THEIR PATHS

Entrainment of saturated material along their paths by rockslides and rock avalanches has been recognized as an important mechanism in long run-out landslides and in transforming initial rockslope failures into mobile mass flows. The process was a key element in the development of four catastrophic landslides in Tajikistan (1949), Peru (1962, 1970), and the Philippines (2006); the total death toll in these events is estimated at nearly 9,000 people. In the 1949 Khait, Tajikistan case a rockslide, triggered by a M7.4 earthquake, was transformed into a highly mobile rockslide-loess flow by the incorporation of a significant volume of saturated loess from the valley floor and steep valley sides of the Obidara River. The landslide (total vol. > 100 Mm³) ran out a total distance of 11 km over a vertical distance of only 1500 km, burying the towns of Khait and Kusurak. In the Cordillera Blanca, Peru, falls of ice and rock from the North Peak of Huascaran in 1962 and 1970 were transformed into high-velocity debris flows by the incorporation of snow from the surface of Glacier 511, and the massive entrainment of surficial materials from an area downslope from the glacier margin, which significantly increased initial failure volumes. In 1962 the initial fall had a volume of 3 Mm³ whereas the final volume of the destructive highly-fluid debris flow which overwhelmed Ranrahirca was in the order of 13 Mm³. In 1970, the initial fall from the same North Peak, triggered by a M7.9 earthquake, had a volume of about 7.5 Mm³. The total volume of the highly destructive flow which split into two lobes (one of which overwhelmed the town of Yungay) and swept into the Rio Santa was in the order of 50 Mm³. The velocity of the event was extraordinary, with a mean velocity estimated to be 75 m/s. Both the 1962 and 1970 events continued downstream as very rapid distal debris flows/debris floods, that in 1970 reached the Pacific Ocean, 180 km downstream. Lastly, in the Leyte Island, Philippines case entrainment of saturated colluvium increased the mobility of an initial rockslide as it travelled over a flat valley floor. The four case histories illustrate the role of entrainment in transforming initial rockfalls/rockslides into high-velocity, highly destructive mass flows which have the potential of overwhelming communities at some distance from the source instability.