Concentrated surface run-off caused by torrential rain leads to the transport of sediments from soil erosion processes within catchment areas of critical points, which represents a basic component of flood risks. Clear identification of such critical points offers a basis for a suitable new strategy of threat mitigation, via both organizational and structural measures in catchment areas. Critical points are determined in places where generated paths of concentrated surface run-off cross given boundaries of built-up areas. The threshold values for the catchment area of a critical point were derived on the basis of hydrological calculations, field research, and the evaluation of hundreds of specific manifestations of damage in built-up areas for particular critical profiles. The characteristics were determined as follows: contributing area between 0.3 and 10 km², average slope more than 3.5%, and percentage of arable land more than 40%. Indicator F was determined for the distribution of the territory of the Czech Republic according to the risk of flooding. Knowledge of the existence of critical points enables the implementation of preventive measures, the evaluation of flood risk associated with the transport of sediment due to erosion processes, and the improvement of management measures in respective catchment areas, even before an event occurs. The proposed procedure outputs shall be reflected in spatial planning documentation, land consolidation, and catchment area management plans. Incorporation of critical points into open-access public web-maps can help with assessing the semi-quantitative expression of risk to built-up areas arising from the threat of local flooding.

Since the year 2012, nationwide monitoring of erosion on agricultural land has been carried out in the Czech Republic with more than 2,300 cases of erosion events recorded so far. The evaluation of the relationships between the actual manifestation of erosion and the characteristics of the 5600 erosion enclosed areas (EEAs), on which surface runoff develops after erosion-forming rainfall, are presented in this contribution. Most frequently grown crops during an erosion event include maize (49.8% of cases), rapeseed (13.7%), potatoes (8.1%) and sugar beets (2.1%). The distributions of affected erosion enclosed areas (EEAs) in terms of shares of soils with low susceptibility and medium susceptibility to compaction, drainage systems and contributing areas of critical points are bimodal; the low percentage (90%). The percentage of hydrologic soil group B in EEA has the high share dominant over the low one. Unsurprisingly, erosion events have been recorded predominantly in EEAs with low forest cover and on arable land (share >90%). The distribution of EEAs across altitudes corresponds with the relief of the country. Obtained results will be used to validate the implementation of erosion-monitoring systems and for the adaptation of soil erosion prevention and mitigation measures in the sustainable land use policy.

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.