Integrating flood and landslide susceptibility for multi-hazard assessment in the Langat Catchment, Malaysia

Abstract Background Floods and landslides frequently affect the same catchment systems, yet they are usually assessed in isolation, which limits the ability of local authorities to identify areas exposed to their combined spatial influence. An integrated multi-hazard assessment is therefore needed to provide a more comprehensive understanding of spatial hazard susceptibility. This study develops an integrated flood–landslide multi-hazard susceptibility map for the Langat Catchment, Selangor, Malaysia, focusing on three districts: Hulu Langat, Sepang, and Kuala Langat. A geographical information system (GIS)-based frequency ratio (FR) method was applied to generate the individual flood and landslide susceptibility maps, using conditioning factors that comprised elevation, slope, curvature, aspect, rainfall, distance to river, distance to road, soil, geology, fault line, land use, and the topographic wetness index (TWI). Unlike separate single-hazard assessments, the proposed approach integrates the two susceptibility surfaces within a common 10 m spatial framework using hazard-frequency moderation factors adapted to the Malaysian disaster context. Results Flood susceptibility is concentrated in the lower catchment areas of Sepang and Kuala Langat, where 28.8% of the area is classified as high and 9.7% as very high susceptibility. Landslide susceptibility is concentrated in the hilly upstream terrain of Hulu Langat, where 15.5% of the area falls within the very high susceptibility class. In the integrated multi-hazard map, 31.8% of the area is classified as moderate, 11.5% as high, and 30.9% as very high susceptibility. Validation using the area under the receiver operating characteristic curve (AUC-ROC) yielded an AUC of 0.803 and overall classification accuracy of 63.3%, indicating good predictive performance. Conclusion The findings demonstrate that the integrated framework captures the contrasting spatial dominance of flooding in the lower catchment and of landslide susceptibility in the upper catchment. The resulting multi-hazard map provides a transparent spatial decision-support tool for land-use planning, mitigation prioritization, infrastructure protection, and emergency preparedness in tropical catchments exposed to multiple geohazards.