Reconstruction of a major storm event from its geomorphic signature: The Ladakh floods, 6 August 2010

Extreme precipitation events in arid mountain regions determine flood, debris flow, and landslide hazards, and are principal drivers of geomorphic change. Increased frequency of intense precipitation in many regions of the world over the past few decades increases the importance of documenting and understanding such events when they occur. We use the geomorphic record to reconstruct the distribution and magnitude of river discharge from a devastating convective storm event in the interior of the western Himalaya. We use this record to estimate the rates, magnitudes, and distribution of precipitation and magnitude of debris flows generated by the same event. The storm occurred on 6 August 2010 over the Ladakh Range in the northwest Indian Himalaya, killing ∼600 people and devastating more than 60 villages. It triggered numerous debris flows and significantly widened the lower reaches of the main tributary valleys. We demonstrate that rainfall was concentrated in a 6-km-wide, range-parallel band centered 4.5 km up from the range front, and that within this band precipitation intensity was ∼30 times that above it. At least 75 mm of rain fell in 30 min during the heaviest part of the storm, which is at least 1.5 times the highest 24 h maximum for the previous 100 yr. The results demonstrate that the geomorphic record can be used to constrain the intensity of rainfall without meteorological stations, and also record its distribution better than could be achieved with normal densities of instrumentation.