Island area changes in the Sundarban region of the abandoned western Ganga-Brahmaputra-Meghna Delta, India and Bangladesh

Delta shapes are governed by relative dominance of wave, tidal, and fluvial processes, while their coastline changes primarily depend on activation and abandonment of distributary channels. The partly-reclaimed Sundarban Mangrove Wetlands occupy the fluvially abandoned western part of the macro-mesotidal Lower Ganga-Brahmaputra-Meghna Delta (GBMD) in India and Bangladesh. To ascertain the evolution of the planform of this 15,500-km² region over a century, maps and images pertaining to 1904-24 (Survey of India topographical maps), 1967 (Corona space photographs), 2001 (IRS-1D LISS-3 + Pan merged images), and 2015-16 (Resourcesat-2 LISS-4 fmx images) were digitally compared for documenting the changes in the areas of ~250 mangrove-covered and reclaimed tidal islands above the spring high water level. Area change of the individual islands was also studied based on their relative north-south and west-east positions.

The results indicated that while erosion of the estuary margins and the sea facing coastline -up to 40 m/yr-was continuing for decades in the southern islands, intervening channels between the northern islands were getting silted up, especially in the western sector, resulting in land gain. Area change in the central sector mostly tended to be small and erosional. Overall, the total island area & counts changed from 11,903 km² & 253 (1904-24), respectively, through 11,663 km² & 250 (1967), 11,506 km² & 244 (2001), and 11,455 km² & 251 (2015-16). The reduction rate of area, at -4.46 km²/yr, remained noticeably similar across all intervals of mapping / imaging years and projects that the region will lose 3.4 % of its present extent by 2100 if the observed tendencies continue. The trend of area reduction was 2.55-times higher in the western (Indian) segment of the region, than the eastern (Bangladeshi) section. At the level of individual islands, the trends of area change were classified into nine types involving linear and non-linear changes, with dominance of continuous (post-2001) erosion in 48.3 % (68.9 %) of the islands. Conversely, continuous (post-2001) accretion dominates in 15.8 % (31.1 %) of the islands.

The retrogradation of the southern Sundarban can be ascribed to sediment starvation of the western GBMD due to abandonment of its deltaic distributaries and shelf bypassing of sediments through the Swatch of No Ground submarine canyon. On the other hand, the accretion of the northern interior areas, especially in the west, was mostly related to sediment reworking in a floodtide dominated environment, intervened by reclamation efforts. Factors like relative sea level rise, locational shifts in landfalls of cyclonic storms, and reclamation-related deforestation had little detectable influence on island area changes. As the observed tendencies of area change are likely to continue, planning for the region must integrate the transformations into management and development initiatives.