GIS-Based Identification of Critical Points and Contributing Areas for Concentrated Runoff Hazard Screening

Flash floods generated by concentrated surface runoff represent a major natural hazard affecting small catchments and built-up areas. This study presents an updated national methodology for identifying critical points (CPs) and their contributing areas in the Czech Republic. The methodology integrates a LiDAR-derived digital terrain model, an updated delineation of built-up areas based on the national topographic database ZABAGED®, and hydrologically conditioned terrain models representing two limiting hydraulic-connectivity conditions. A new procedure was developed to delineate contributing areas across culverted stream sections and other drainage interruptions, enabling a more detailed representation of concentrated runoff pathways. Comparison with the original 2009 dataset demonstrated substantial changes in the number and spatial distribution of CPs resulting from updated input data and methodological refinements. The two hydraulic-connectivity scenarios produced similar national-scale patterns but locally altered runoff pathways, CP locations, and contributing-area boundaries. The resulting datasets provide a nationwide framework for flash-flood susceptibility screening and the spatial targeting of mitigation measures. Their integration into the operational Flash Flood Indicator of the Czech Hydrometeorological Institute further demonstrates their applicability to spatially targeted early warning. The CP approach is transferable to regions with suitable geospatial data, although adaptation to local conditions may be required.