THE LANDSLIDE HAZARD IN THE HIMALAYAS - GEOLOGICAL CONTROL AND HUMAN ACTION

Mountains are high-energy environments characterised by instability and variability. Much of this instability is the result of a variety of landsliding processes; mudflows, rockfalls and debris slides/avalanches being especially important. This landsliding constitutes a major hazard in the Himalayas. Early work has suggested that certain rock types are more predisposed to failure than others. It also seems possible that specific types of failure are related to certain rock types and properties. This analysis suggests that these relationships can be employed in a hazard assessment. Phyllite rocks are the most susceptible to landsliding followed by shales, schists, poorly cemented sandstones, gneiss, granites and quartzite. This order is also appropriate for slope failures within weathered material although weathered gneiss appears to be more susceptible to rill and gully erosion. Clearly rock type exerts a major influence on landsliding, but assessing the role of rock type in comparison to the effect of human activity is difficult. Human activity is often influenced by the nature of slopes, including the type and depth of material on the slopes. As these are affected by rock type, it is difficult to differentiate the contributing factors. Because of this it would be dangerous to assume that landsliding in the Himalayas can be completely explained and predicted by the nature of the rock type.