Evaluation of Bank Erosion in the Ganges River, Bangladesh: An Integrated Geological, Geotechnical, and Hydrogeological Study

Abstract

The purpose of the research was to integrateally characterize the erosional behavior of the Ganges riverbanks and understand the geological, geotechnical, and hydrological influence on bank erosion. The current study has used multi-temporal Landsat images spanning from 1973–2020 for planform analysis using Geographic Information System (GIS), geological fieldwork data, six standard penetration test (SPT) data, laboratory analysis, and slope stability model using SLIDE to understand geological-geotechnical factors and the influence of seasonal hydrological fluctuations on the bank erosion. GIS-based planform analysis identified the erosion-accretion rate and vulnerable areas for erosion. Results suggest that the intensity of bank erosion is higher at the outer meander bend, and the right bank is more erosion-prone than the left bank. Geological and geotechnical studies suggest that bank material types and their degree of cohesion control bank erosion. In the convex outer bend of the river, the helical flow erodes the non-cohesive sandy materials at the toe of the bank, hence, the upper clayey layer loses its strength and collapses, which is the main cause of the cantilever-type bank failure in this river. Results also suggest that the Factor of Safety (FS) is proportional to the difference between river water level (RWL) and groundwater level (GWL) in the uniformly graded high permeable soil. When the water level gradient shifts from GWL toward RWL, the FS of the bank slope decreases and collapses after reaching its critical point.