Shoreline change assessment using geospatial tools: a study on the Ganges deltaic coast of Bangladesh

The Ganges deltaic coast of Bangladesh experiences an incessant movement over the time. Understanding the shoreline movement of this alluvial delta and a suitable method to calculate the rate of change poses a challenge for this highly dynamic coast having erosion and accretion. Using GIS and multi temporal LANDSAT images, the study investigated the positional change of the Ganges deltaic shoreline for the period of 1977-2017. LANDSAT images were radiometrically corrected and a spectral index i.e., normalized difference water index (NDWI) was applied to differentiate water and land features. A histogram based Otsu's Binary thresholding method along with image based visual interpretation was used to extract the shorelines. Net changes of shoreline position were statistically calculated using three different techniques, namely; End Point Rate (EPR), Linear Regression Rate (LRR) and Weighted Linear Regression (WLR). A comparison between the techniques was also made to choose and evaluate the suitable statistical technique to estimate the rate of shoreline change for this alluvial delta. Analyses showed that LRR technique had less positional uncertainty in compare to EPR and WLR, although at a particular transect the techniques were closely correlated. The EPR, WLR and LRR technique showed that the shoreline is experiencing landward movement (erosion) with an average rate of 0.62 m/yr, 0.96 m/yr and 0.27 m/yr respectively. Moreover, a high erosion rate of 5 m/yr at the mangrove forest area of the GDC is a great concern for the existence of the mangrove forest. During 1977-2017, an overall 6.29 sq. km land area has been lost although significant land depositions were observed at the river estuaries. This study revealed the spatiotemporal trend of shoreline position of the Ganges deltaic coast and that would be beneficial for the coastal management and planning of the region.