Sediment aggradation rates for Himalayan Rivers revealed through SAR remote sensing

Abstract. This study uses Synthetic Aperture Radar (SAR) to quantify sediment aggradation rates in the proximal gravel-rich portions of the rivers that drain out of the Himalayan Mountain Front onto the Gangetic Plains. Implementing the Small Baseline Subset (SBAS) InSAR (Interferometric SAR) method on Sentinel-1 C-band InSAR residual topographic phase, we measure millimeter-scale elevation changes during the period from 2016 to 2021 covering ~15 km reaches of four rivers from the mountain front downstream to the gravel-sand transition. This is the first study to apply differential residual topographic phase mapping seasonally dry (ephemeral) rivers. Results indicate sediment aggradation in river channels that accumulates during the wet monsoon, with rates reaching up to approximately 20 mm/yr (i.e., per monsoon) near the mountain front, decreasing to nearer zero downstream of the gravel-sand transition. Meanwhile, the floodplain in the basin is subsiding at varying rates that average ~15 mm/yr. These findings enable a temporal understanding of sediment aggradation rates that impact river avulsion and flood risk, particularly for the rapidly growing rural communities in Nepal and Bihar, India. Our study demonstrates the feasibility of InSAR techniques in geomorphological monitoring that can act as input into flood risk modelling and management in the Gangetic Plains.