Assessing floodplain dynamics using radar interferometry in the East Ganga Plains, India

Abstract

Floodplains are essential elements of terrestrial water storage and perform various hydrogeomorphic, ecological, and socio-economic services. Ganga Plains are one of the world's largest and most populous plains. The East Ganga Plain (EGP), other than being densely populated, is also a fluvially highly dynamic region. This region hosts ‘hyperavulsive’ rivers and numerous wetlands, some of which are the largest wetlands of the Ganga Plains. Due to frequent flood hazards, all rivers of the region are embanked on both banks. We are investigating the surface dynamics of the region by calculating the surface displacement using InSAR (Interferometric Synthetic Aperture Radar) time series in ISCE-2 and Mintpy environments. We calculated the InSAR stack for the period Oct 2016 to May 2022 and built the connected network for the next three neighbours. We iteratively chose the multilooking value of azimuth 11 and range 78 to mitigate the low coherence issues due to the vegetation. We used ascending and descending tracks of Sentinel-1 to calculate the horizontal and vertical components of the velocity. Our results show that the entire area is tectonically subsiding. However, the spatial pattern of subsidence rate is varying – surfaces with seasonal water cover, such as active channel belts and wetlands, are exhibiting the highest subsidence rates with up to 7 cm/y and twice the subsidence rates of non-water surfaces. In many regions, the subsidence is accelerating.We are using satellite-based multispectral indices (MNDWI, NDVI) and in-situ measurements such as groundwater depth and rainfall and land use data to investigate the disparity in the subsidence rates in the region. The preliminary results suggest that the waterbodies are drying, vegetation cover and irrigation are increasing, and rivers are disconnected from their floodplains due to embankments. We emphasise the anthropogenic role in the acceleration of the subsidence due to river embankment, augmented by a high drawdown of groundwater for irrigation purposes.