Characterizing the 2D shape complexity dynamics of the islands of Sundarbans, Bangladesh: a fractal dimension approach

Fractal dimension index (FDI) and shape index (SI) of the islands of Bangladesh Sundarbans situated at the estuarine part of the Ganges–Brahmaputra–Meghna (GBM) delta were estimated with the aid of remote sensing techniques between the years 1989 and 2010. The main objective was to correlate the temporal change in these two indices within two decades with the possible geomorphic processes regulating these patterns. The Sundarban region is composed of almost 200 islands having self-similar coastlines with FDI and SI ranged from 1.002 to 1.202 and 1.016 to 7.546, respectively. Both FDI and SI values along with their temporal change showed that the complexity and irregularity in the island shape increased in the northern and north-eastern parts of the Sundarbans, while in the southern end it exhibited an opposite trend. Apart from a few exceptions, the longer serrated coastlines of the north (FDI > 1.18) became more serrated and rough due to the predominant tidal effect (particularly ebb tidal flow) within the two decades. The comparatively smoother coastlines of the southern islands (FDI < 1.18) became more compact and circular due to strong sea wave action during the same course of time. The spatial variation in FDI was explained in terms of the relative elevation of the islands, which was found to be lower in the northern part, their differential response to rising sea level and the amplified tidal action in this funnel shaped estuary. Apart from changes in fractal dimension, a net land loss of ≈ 60 km² was observed within this island system during the study period. These observations imply that with an increasing sea level rise, the northern and southern parts of Sundarbans Island system will respond differentially, resulting in inundation in northern area (water encroachment in the low-lying islands) and land loss in the southern part (erosion in the coastal boundary) causing a severe loss of ecosystem services provided by the mangrove forest.