Role of Meteorology and Local Orography on a Flood Event in the Lower Subansiri Basin and Post-Flood Changes to Land Use and Land Cover

Extreme rainfall in the Arunachal Himalaya has the potential to create floods in the downstream regions of Assam, with the rivers in the region exceeding their capacity and competence. High-intensity rainfall also leads to higher sediment generation in the sub- Himalayan catchment. Though floods have been incurring huge losses to both life and property frequently in Assam, there are only a few studies on the meteorological and orographic dynamics of such extreme rainfall events and their potential to create a flood. The present study highlights the pre-flood and post-flood scenario in the Lower Subansiri Basin (LSB) in Brahmaputra valley, Assam, through satellite data analysis and ground field surveys to establish linkages between extreme rainfall events and a subsequent major flood event that occurred during September 2012. We observed that the flood was mostly triggered by the extreme rainfall induced by orographic lifting of moisture-laden winds from the south. In addition to submerging an area of ~1900 km2, the flood also brought along fragile Neogene clastics that increased barren soil over the floodplains by ~47% compared to the pre-flood period, thus disrupting the agrarian economy of the region for several cropping seasons. These findings demonstrate the need for a reliable meteorological forecast for extreme rainfall as a prerequisite for developing effective flood-forecasting models in the Brahmaputra valley, which will positively contribute towards flood hazard management in the region.