Transition of blue space in the urban area of khulna city corporation using geospatial and machine learning techniques

Urban blue-space loss and grey-space expansion have significantly altered the surface water systems of the Khulna City Corporation, increasing the environmental degradation and hydrological vulnerability of this climate-sensitive coastal city. This study used multi-temporal Landsat imagery, including Landsat 5 TM data from 1994 to 2004 and Landsat 8 OLI data from 2014 to 2024. The analysis integrated the Normalized Difference Vegetation Index, Normalized Difference Built-up Index, and Normalized Difference Water Index with Random Forest classification and post-classification change detection to quantify long-term land-use/land-cover transitions and urban blue-space transformation. Post-classification comparison generated directional transition matrices documenting conversion pathways among land-cover categories. The results show that surface water bodies have decreased from 4.20 km2 in 1994 to 2.27 km2 in 2024, while the built-up area has increased from 25.9 to 53.3% of the area studied. The accuracy assessment resulted in an overall accuracy of 84.5 to 89.3%, with Kappa coefficients ranging from 0.78 to 0.86. Water body classifications have remained reliable throughout all periods, which has supported the use of classifiable maps for long-term migration analysis. Temporal analysis revealed two main phases: an early phase characterized by mixed land reclamation and later phases dominated by conversion of water bodies into built-up areas. Spatial analysis identified critical hotspots of canal infill, wetland loss, and riverbank modification, mainly in central and southern municipal zones. These findings highlight the need to treat urban blue spaces as critical infrastructure and to promote integrated management, natural drainage solutions and regular remote sensing monitoring. The study frames the loss of blue space as a process of urban transformation and advocates better monitoring of urban water systems in coastal secondary coastal cities.