Spatial Heterogeneity of Flood–Salinity WASH Vulnerability: An Integrated FS-WVI and CatBoost–SHAP Assessment across Eight Unions of Subarnachar Upazila, Noakhali, Bangladesh

<title>Abstract</title> <p>Coastal Bangladesh is increasingly affected by the combined impacts of flooding and salinity, creating serious challenges for water, sanitation, and hygiene (WASH), particularly in low-lying coastal communities. The unexpected flash flood of August 2024 further demonstrated the need for integrated assessments that consider both environmental hazards and household vulnerability to support effective adaptation planning. This study developed a Flood–Salinity WASH Vulnerability Index (FS-WVI) by integrating remote sensing observations with household survey data to evaluate multidimensional WASH vulnerability in Subarnachar Upazila, Noakhali. The findings revealed pronounced spatial variation in vulnerability among the eight unions. Charaman Ullah 0.652, Mohammadpur 0.651, and Char Jubilee 0.647 exhibited the highest FS-WVI values, whereas Char Clark 0.297 showed the lowest vulnerability level. The results further demonstrated that WASH vulnerability was not determined by hazard exposure alone. Instead, different combinations of exposure, household sensitivity, and adaptive capacity deficit shaped the final vulnerability pattern across the study area. Exposure was the dominant vulnerability component in Char Jubilee, Charaman Ullah, and Mohammadpur, while Char Clark, Char Wapda, East Char Bata, and Char Bata were primarily influenced by household sensitivity. Char Jabbar was mainly affected by adaptive capacity deficit. The predictive analysis achieved 67.5% classification accuracy with a ROC–AUC of 0.721, while feature ranking consistently identified tubewell depth and the Compound Hazard Index as the strongest factors associated with household drinking-water salinity. The explainability analysis further confirmed that the influence of compound hazards varied according to groundwater access and household conditions, reinforcing that WASH vulnerability results from the interaction of environmental and socio-economic factors rather than hazard intensity alone. The proposed framework provides a practical and transferable approach for evidence-based WASH planning, climate adaptation, and targeted intervention in coastal Bangladesh.</p>