Prediction of flash flood hazard impact from Himalayan river profiles

To what extent can we treat topographic metrics such as river long profiles as a long-term record of multiple extreme geomorphic events and hence use them for hazard prediction? We demonstrate that in an area of rapid mountain erosion where the landscape is highly reactive to extreme events, channel steepness measured by integrating area over upstream distance (chi analysis) can be used as an indicator of geomorphic change during flash floods. We compare normalized channel steepness to the impact of devastating floods in the upper Ganga Basin in Uttarakhand, northern India, in June 2013. The pattern of sediment accumulation and erosion is broadly predictable from the distribution of normalized channel steepness; in reaches of high steepness, channel lowering up to 5m undercut buildings causing collapse; in low steepness reaches, channels aggraded up to 30m and widened causing flooding and burial by sediment. Normalized channel steepness provides a first-order prediction of the signal of geomorphic change during extreme flood events. Sediment aggradation in lower gradient reaches is a predictable characteristic of floods with a proportion of discharge fed by point sources such as glacial lakes.