Riverbank erosion in the Ganges reach of Bangladesh: influence of geotechnical factors and seasonal water level fluctuations

This study aims to characterize the erosional behaviour of the Ganges riverbanks by investigating the geotechnical properties and seasonal hydrological fluctuation in Bangladesh. Multi-temporal satellite images (1973–2020) were analysed using Geographic Information System (GIS) to select study locations for detailed analysis. The GIS-based analysis identified erosion-accretion patterns and highlighted erosion-prone and stable riverbank locations. Further, field investigation, geotechnical laboratory analyses, and slope stability model using SLIDE software were used to assess the targeted study factors from the selected locations. Field investigation and geotechnical assessments revealed that bank material cohesion and composition strongly influence erosion. River flow at the outer bends erodes non-cohesive sandy materials at the bank toe, destabilizing the upper cohesive clayey layer and triggering cantilever-type bank failures. Furthermore, results from SLIDE-based simulations show that, for uniformly graded high-permeable soil, the factor of safety (FS) of the bank slope is directly related to the difference between the river water level (RWL) and the groundwater level (GWL). As the water level gradient shifts from GWL toward RWL, the FS of the bank slope decreases, ultimately leading to collapse once a critical point is reached. The study emphasises the critical interplay between RWL, GWL, and soil properties in determining erosion dynamics. These findings may support the design of targeted riverbank protection measures, guide the timing of engineering interventions under fluctuating hydrological conditions, and assist river managers and planners in mitigating erosion risks along vulnerable reaches of large alluvial rivers.