Geomorphic Effects of Monsoon Floods on Indian Rivers

Abstract

The southwest summer monsoon contributes the bulk of India’s rainfall. Consequently, almost all the geomorphic work by the rivers is carried out during the monsoon season in general and the monsoon floods in particular. Indian rivers are characterized by high average flood discharges and large temporal variability. There is also significant spatial variation in the magnitude, frequency and power of floods, on account of regional variations in monsoon rainfall, basin characteristics and channel geometry. As a result, the channel responses and the geomorphic effects also vary spatially. This paper describes the hydrological and geomorphological aspects, as well as the geomorphic effects of monsoon floods in the Indian rivers. The geomorphic effects of floods are most impressive only in certain areas — the Himalaya, the Thar Desert, and the Indus-Ganga-Brahmaputra Plains. There are numerous instances of flood-induced changes in the channel dimension, position and pattern in these areas. In the Ganga-Brahmaputra Plains, the annual floods appear to be geomorphologically more effective than the occasional large floods. In comparison, the rivers of the Indian Peninsula are, by and large, stable and the geomorphic effects of floods are modest. Only large-magnitude floods that occur at an interval of several years to decades are competent to modify the channel morphology in a significant way. A synthesis of the various case studies available from the Indian region indicates that often the absolute magnitude of a flood is not as important with respect to the geomorphic effects as the flow stress and competence.