Sediment aggradation rates in Himalayan rivers revealed through the InSAR differential residual topographic phase

Abstract

Using Interferometric Synthetic Aperture Radar (InSAR), we quantify sediment aggradation rates in the proximal gravel-rich portions of rivers draining from the Himalayan mountain front onto the Gangetic Plains. We develop a novel approach based on the differential residual topographic phase (DRTP) by implementing the small baseline subset (SBAS) InSAR method on Sentinel-1 C-band InSAR images. With this approach, we measure millimetre-scale relative elevation changes in four river channels over approximately 15 km of their length from the Himalayan mountain front downstream to the gravel-sand transition. This study is the first to apply differential residual topographic phase mapping to seasonally dry (ephemeral) rivers. These measurements record the changes that result from sediment deposition during the summer monsoon floods from 2016 to 2021. Results indicate sediment aggradation in river channels during the wet monsoon, with rates reaching up to approximately 20 mm yr-1 (i.e. per monsoon) near the mountain front and decreasing to near zero downstream of the gravel-sand transition. Meanwhile, the floodplain in the basin is subsiding at varying rates that average ĝ1/4 15 mm yr-1. These findings enable a temporal understanding of sediment aggradation rates that impact river avulsion and flood risk in the plains, particularly for the rapidly growing rural communities in Nepal and Bihar, India. Our study demonstrates the feasibility of the InSAR technique for geomorphological monitoring that can act as input into flood risk modelling and management in the Gangetic Plains.