Hydrogeomorphic hazards and vulnerability assessment through basin morphometry in the climate change sensitive western Himalayan syntaxis: A case study of the Astore river basin.

This study presents a hydrogeomorphic hazard and vulnerability assessment of the Astore river basin in the western Himalayan syntaxis. Hydrogeomorphic hazards i.e. debris flows, debris floods, and fluvial floods, pose significant risks to local communities and infrastructure. Prior recognition of specific flow types is crucial because different flow types require specific remedial measures. To achieve this objective, the Astore river basin was initially divided into 65 sub-basins using field data and high-resolution satellite imageries, enabling the identification of sub-basins generating debris flow, debris flood, and fluvial flood. This field-based classification was validated through statistical analysis of selected morphometric parameters, including basin Melton's ratio, basin relief ratio, basin area, basin length, basin gradient, basin channel length, and fan gradient. Threshold values were established for all the morphometric parameters through cross-correlation plots to differentiate sub-basin-scale hydrogeomorphic hazard types. Morphometric analysis of parameters with >90% predictive accuracy revealed distinct signatures for each sub-basin and hazard type. Debris flow-generating sub-basins are typically small (basin area < 10 km2), steep (fan gradient > 10°), with high basin Melton's ratio (> 0.60) and basin relief ratio (>0.37). Conversely, fluvial flood-generating sub-basins are larger, less steep, having low basin Melton's ratio (< 0.30), low basin relief ratio (< 0.37), large basin area (> 10 km2), and gentle fan gradient (< 5°). Debris flood-generating sub-basins occupy an intermediate range. Integration of morphometric analysis outcomes with glacial lake and human settlement inventories and the spatial distribution of loose debris cover was assessed to produce comprehensive hazard and vulnerability maps at sub-basins level, providing critical insights for risk reduction and sustainable development planning in the tectonically active and climate-vulnerable Himalayan regions.