Geomorphic and hydrologic aspects of monsoon floods on the Narmada and Tapi Rivers in central India

Abstract

High-magnitude floods during the monsoon season are considered to be India's recurring and leading natural disaster. Such large floods are extremely important events, not only in terms of human impact, but also from the standpoint of geomorphic effectiveness and geomorphic work. The Narmada and Tapi Rivers in central India are characterized by one of the most intense flood regimes in the seasonal tropics. Modern, historical and palaeoflood records indicate that large floods are relatively common hydrologic and geomorphic events on the Narmada and Tapi Rivers. A catastrophic flood on Tapi in July 1991, that occurred from heavy rains and a dam break, represents one of the highest unit discharges yet recorded in any part of central India. Flood competence studies indicate that the flood power of the two rivers is comparable with some of the largest historic and prehistoric floods reported from other parts of the world. The rivers have preserved geomorphic and sedimentologic records of palaeofloods for the last two millennium, and in Choral River a 5000 year old flood record has been located. Examination of the synoptic conditions associated with the flood generating low-pressure systems reveals that all but a few of them are the result of Bay of Bengal depressions. The flooding in such large rivers profoundly challenges flood-hazard management, because of limited instrumental records and large spatio-temporal variation. It is therefore imperative that the design engineering community in India incorporates the geomorphic, sedimentologic and botanic information in the planning and design of water resource development projects.