Utilizing geospatial tools for the assessment of river bank erosion and migration patterns in complex braided and meandering river systems

Abstract

Bank erosion along the Brahmaputra has serious environmental and socio-economic implications for the state of Assam, India. The lack of high-resolution satellite images to map erosion and deposition is one of the primary drawbacks of previous studies. This study investigates river bank erosion, deposition and migration of river banks during 2016–2022 using high-resolution Sentinel 2A/2B imagery. Multi-temporal post-monsoon images were analysed for three subperiods (2016–2018, 2018–2020 and 2020–2022) to automatically extract river banklines and morphodynamics using the normalized difference water index (NDWI) in Google Earth Engine (GEE). The Digital Shoreline Analysis System (DSAS) was employed to calculate the bankline migration rate. The study shows that the left bank of the Brahmaputra river experienced higher erosion and migration as compared to the right bank. The subperiod 2018–2020 revealed severe bank erosion and migration, potentially associated with intense flood events in 2020. Zone-wise analysis indicated that the left bank of Zone 1 experienced higher net erosion (8440 ha) and migration (−28.76 m/year). This may be attributed to scouring caused by the impact of flood water as it enters Assam with high erosive velocity. Similarly, Zone 3 of the right bank also experienced higher net erosion (7640 ha) and migration (−24.38 m/year). The accuracy assessment shows that there is almost a perfect agreement between the erosion/deposition and the reference data, with mean migrations within the confidence interval of 5% significance level. Given the above findings on erosion and bank migration, protective measures like agronomical and engineering measures should be implemented to protect the banks from getting eroded. This analysis provides valuable inputs for decision-makers in formulating targeted strategies to address the challenges posed by river bank erosion in the Brahmaputra river.