Mapping coastal dynamics induced land use change in Sandwip Island, Bangladesh

Evaluating satellite water extraction indices, particularly for coastal environments, guarantees that satellite-derived water maps are as accurate and functional as possible, notwithstanding the unique complexities these areas present. Variability in salinity levels, intricate land-water boundaries, dynamic sediment loads, and tidal fluctuations often complicate coastal water mapping. Sandwip Island in Bangladesh is one of the most complex and dynamic coastal environments in the world and is our area of focus. Six water information extraction indices were evaluated: normalized-difference vegetation index (NDVI), modified normalized difference water index (MNDWI), automated water extraction index for built-up areas (AWEInsh) and shadows (AWEIsh), multi-band water index (MBWI), and normalized difference water index (NDWI), using Sandwip Island’s satellite Landsat imagery acquired in February 1990, 2000, 2010, and 2020. The results showed that NDWI performed the best based on the total area obtained and classification accuracy. NDWI was then used to assess the erosion and accretion dynamics of the island for the study period (1990–2020). In the period 1990–2000, the island saw significant erosion and accretion along its coastlines in all parts, while the 2000–2010 period indicated that the island eroded on all sides. However, the situation was totally opposite during 2010–2020. The results illustrated the best performance of the NDWI algorithm in mapping surface water in the complex and dynamic Sandwip coastal environment. Also, erosion and accretion change temporally and spatially on the island. While this study is confined to Sandwip Island in Bangladesh, the findings hold the potential for broader applicability in regions with comparable characteristics.