Evaluating geomorphological changes and coastal flood vulnerability of the Nijhum Dwip Island using remote sensing techniques

This research critically assesses the spatiotemporal changes in morphology of the Nijhum Dwip Island caused by the dynamic wave action of the Bay of Bengal. Using image analysis techniques on Landsat Satellite Data (ETM+, OLI), the nature of shoreline alterations, erosion-accretion, and lob shifting from 2003 to 2021 has been identified. The findings reveal that since 2013, the island's morphology and geographical extent have undergone considerable changes, and it has gradually reached its current stable state in the Bay of Bengal. The fluvial sediments brought by the Ganges-Brahmaputra-Meghna (GBM) river system influenced by tide is mainly responsible for the sedimentary accretion in and around the island. Due to the complex wave action, the island has experienced alternating erosion and accretion domination scenarios. Compared with its extent in 2003, up until 2021, it has lost approximately 2.57 km2 area, erosion and accretion mostly occurring in the southern part, geometrically transforming the island from a rectangular to a subrounded shape. The depocenter shifting of the island was predominantly in the south-eastern direction during the whole period but has shifted to reverse direction in 2021. The minimum and maximum lob shifting occurred in 2006 and 2015, respectively, at approximately 600 m and 1800 m. ALOS PALSAR DEM (12.5m) in combination with secondary data from the Bangladesh Meteorological Department has been used to develop and simulate flood inundation scenarios under different surge heights. The projections illustrate that in the case of 2 m, 3 m, and 4 m surge heights, approximately 46.45%, 56.51%, and 61.42% of the total land will go underwater, respectively. In all scenarios, cropland and bare ground are the most affected classes.