Coastal Erosion, Soil Mechanics, and Sediment Dynamics in the Bengal Delta: A Case Study from Kalapara, Bangladesh

Shorelines in Bangladesh are among the most vulnerable regions to the compounded impacts of climate change, sea-level rise, and intensified natural hazards. Although sediment deposition in Bengal Delta’s promotes accretion in certain areas, many coastal zones are experiencing severe erosion, driving large scale population displacement and heightening socioeconomic vulnerability. This study examines shoreline change dynamics and geotechnical soil properties along the Kalapara coast in Patuakhali District over a 31-year period (1989–2020) through the integration of multi-temporal Landsat imagery and field-based geotechnical analysis. A total of 1,533 transects were evaluated using the Digital Shoreline Analysis System (DSAS), revealing that 77.9% of the shoreline experienced erosion, while only 22.1% showed accretion. Dhulasar union exhibited the most extreme rates of both erosion (33.57 m/yr) and accretion (22.88 m/yr), while Nilganj remained comparatively stable. Geotechnical testing of thirty shoreline soil samples from six unions indicated low specific gravity values (2.21–2.68), suggesting poor compaction and high susceptibility to erosion. The plasticity index ranged from -26.77 to 39.1, with Mohipur, Lalua, and Champapur dominated by non-plastic to low-plastic soils, and Dhulasar and Nilganj characterized by high plasticity and cohesive soils. Particle size analyses showed that most soils comprisedmedium to fine sand (up to 98.34%), with minimal silt and clay fractions; however, Nilganj, Latachapli, and Dhulasar contained higher proportions of silty-clayey material, contributing to enhanced slope stability and erosion resistance. These findings emphasize the critical role of soil mechanical properties in governing shoreline evolution and highlight the urgent need for integrated coastal zone management strategies that couple geotechnical stabilization with continuous remote sensing-based monitoring to protect vulnerable areas like Kuakata beach from irreversible coastal degradation.