Natural and human-induced landsliding in the Garhwal Himalaya of northern India

After the March 28, 1999, Garhwal earthquake, 338 active landslides, including 56 earthquake- induced landslides, were mapped in a 226-km(2)-study area in the Garhwal Himalaya, northern India. These landslides mainly comprised shallow failures in regolith and highly weathered bedrock involving avalanches, slides, and flows. The total volume of active landslide debris in the region was estimated to be similar to1.3 million m(3) including 0.02 million m(3) (<2% of the total volume) moved during and within a few days of the earthquake. The denudation produced by the active landsliding within the study area is equivalent to a maximum landscape lowering of similar to5.7 mm. If active landsliding persists for a duration of between similar to1 and 10 years, then denudation due to landsliding is in the order of similar to0.6-6 mm a(-1). Approximately, two-thirds of the landslides in this region were initiated or accelerated by human activity, mostly by the removal of slope toes at road cuts, suggesting that human activity is accelerating denudation in this region. Three ancient catastrophic landslides, each involving >1 million m(3) of debris, were identified and two were dated to the early-middle Holocene using cosmogenic radionuclide Be-10 and Al-26. Cosmogenic radionuclide Be-10 and Al-26 were also used to date strath terraces along the Alaknanda River in lower Garhwal Himalaya to provide an estimate of similar to4 mm a(-1) for the rate of regional denudation throughout the Holocene. Natural landsliding, therefore, contributes similar to5-50% of the overall denudation in this region and is important as a formative process in shaping the landscape. (C) 2001 Elsevier Science B.V. All rights reserved.