Flood susceptibility mapping in the Alappuzha Coastal Lowland, Kerala, India using GIS and multi-criteria decision analysis

Coastal lowland environments are highly vulnerable to recurrent flooding owing to their unique hydrological constraints and the limited ability of conventional assessment approaches to capture flood dynamics in low gradient terrains. This study assesses flood susceptibility in Alappuzha District, Kerala. A geographic Information System (GIS) based Multi-Criteria Decision Analysis (MCDA) framework was developed that integrates seven physical and environmental indicators: rainfall, elevation, slope, drainage density, proximity to the river & canal network, land use/land cover, and proximity to road infrastructure. Indicators were standardised and combined using a literature-informed weighted linear combination to generate a composite Physical Vulnerability Index (PVI). The resulting index was classified into five vulnerability classes using Jenks’ natural breaks optimisation, revealing extensive high- and very-high-vulnerability zones concentrated within the central and southern floodplains, particularly in Kuttanad, Ambalappuzha, and Cherthala. Quantitative validation against the observed 2018 flood extent, derived from Sentinel-1 SAR imagery for 92 villages and aggregated at the taluk level, demonstrates a strong positive correlation (Spearman ρ = 0.771), with correct identification of the most and least flood-prone administrative units. The framework explicitly addresses physical susceptibility only; social, economic, and institutional dimensions of vulnerability lie beyond its present scope and are identified as priorities for future work.