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Gifts and Perils of Landslides

Catastrophic rockslides and related landscape developments are an integral part of human settlement along upper Indus streams

Kenneth Hewitt

Rock Slope Failure and Avalanches

A particular class of landslides dominates the Karakoram—events that combine catastrophic rock slope failure and rock avalanches. They are catastrophic in having sudden occurrence, great size and high speed. They are restricted mainly to the world’s more rugged mountains, because they require large collapses on steep rock walls and a descent of hundreds of meters. Average volume for the Karakoram events is around 200 million cubic meters; at least 32 rank as megaslides (more than a cubic kilometer), and the largest exceed 40 cubic kilometers. The crushing forces involved are so great that, in less than a minute, huge volumes of solid bedrock are reduced to rubble, sand and dust. This transforms the collapsing mass into a rock avalanche—a high speed run out of broken and crushed rock that appears to flow, plunge and surge forward, creating immense dust clouds like the more familiar snow avalanches. Their velocity generally exceeds 100 kilometers per hour and may reach 250 kilometers per hour. When movement falls below such high speeds flow halts abruptly, but within two or three minutes, large areas are buried by rubble and dust.

2010-09HewittF2.jpgClick to Enlarge ImageThe fresh or undisturbed rock avalanche surface usually consists of large boulders. However, in the main body of material, revealed where erosion cuts through, a dense matrix of crushed and pulverized rock envelops the larger clasts. On relatively open and level valley floors the debris spreads to a sheet a few meters thick, lobate in plan and with only minor surface irregularities. However, in the rugged terrain of the Karakoram, local topography complicates matters. Rock avalanches traveling directly across a valley may stall against the opposing wall and remain very thick. The fast-moving debris can climb 500 meters or more up opposing slopes. When moving down narrow canyons it responds to curves and valley-side spurs by swinging from side to side or rising and falling in caroming flow. At valley junctions the debris can split, sending separate lobes far up or down the valleys, or blocking tributaries. A great diversity of plan forms and surface features results (see figure 5).

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