Belgeo (Mar 2012)
Landslide causes: Human impacts on a Himalayan landslide swarm
Abstract
Geographers are commonly called upon to diagnose the causes of natural disasters and provide guidance for policy makers. This case study concerns one of the landslide and flood disasters that afflicted the Himalaya in the later Monsoon of 2010, the Almora District (Uttarakhand), landslide swarm, which affected thousands of kilometres of roadway and caused damages >$ 125,000,000. Causes were sought on a 7.4km reach of roadway that had been part of a previous study. Individual landslides were surveyed along with a suite of potential environmental indicators. These results show that the numbers of landslides on the roadbed were not unusual, so research switched to exploring factors that may have enhanced their volumes. Local opinion considers that the major causes were human impacts, especially recent urban development, or geological weaknesses. However, results show that human impacts such as building construction, deforestation, and the collapse of agricultural terraces were associated with smaller landslides, while geological factors showed no significant correlations with landslide volume. Road engineering factors such as the location of another road upslope and the collapse of roadcut retaining wall also proved unimportant. However, the height of the roadcut, channel incision undermining the roadcut and, especially, the steepness of the hillside upslope proved significant. Despite the absence of any direct link between human activity and landslide volumes in Almora, a comparison of the 2010 results from this survey and that of another reach of hill road, which was also part of the 1985 survey, showed important differences in landslide numbers.In 1985, the Kilbury Road, which cuts through reserved forest, carried 153 landslides to the Almora Lower Mall’s 88. In 2010, there were only 9 landslides on the Kilbury Road but 108 on the suburban Almora Lower Mall. The conclusion is that human impacts in Almora have combined to prevent the landslides healing as they have in the undisturbed forest along the Kilbury Road. However, past surveys confirm that landslide numbers on the Almora Lower Mall have remained steady since 1985. Since, the impacts of this rainfall event were manifest only as increased landslide outfall, not increased numbers, the chief influences of outfall volume remain hillslope angle, height of the initial roadcut, undermining by channel incision and the fact that these hillsides exist close to criticality, their limits of stability.
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