Mapping fluvial flood hazard at catchment and municipal scale: a case study from Slovakia

Fluvial floods occur when the water level in a watercourse rises and exceeds the capacity of the river banks, thus flooding the adjacent floodplain. The aim of this study is to map and assess fluvial flood hazard at catchment scale as well as municipal scale using the rainfall-runoff modeling, hydraulic modeling, and geographic information systems (GIS). As the study area, we selected the Gidra River Basin, which is located in western Slovakia. Moreover, we selected twelve municipalities from the studied basin based on the condition that urban area of the municipality is completely or partially located in the studied basin, i.e. can be significantly affected during a fluvial flood event. The Stochastic Rainfall Generator model was used to synthetically generate rainfall time series based on the observed annual maxima daily rainfall from the Častá and Cífer rainfall stations for the period 1990 – 2020. In order to calculate the design peak discharge with 100-year return period, we used the Continuous Simulation Model for Small and Ungauged Basins. The estimated design peak discharges were calculated for five cross-sections in the Gidra River Basin, which were considered independent for hydraulic modeling. Hydraulic modeling was performed with 1D steady-state flow conditions using the Hydrologic Engineering Center's River Analysis System. We modeled selected river sections of the main Gidra River and its tributary named Štefanovský potok. Both river sections were selected because they are listed as critical river sections for possible occurrence of fluvial floods in the last cycle of the Preliminary Flood Risk Assessment in Slovakia from 2018, which was elaborated under the Directive 2007/60/EC of the European Parliament and of the Council of 23 October 2007 on the assessment and management of flood risks. The obtained results from the catchment-scale hydraulic modeling, i.e. flood extent, flow depth, and flow velocity for Q₁₀₀ flood scenario, created the basis for the subsequent municipal-scale assessment. In order to distinguish the flood hazard at municipal scale, we calculated the fluvial flood hazard index (FFHI) using the flood extent, average flow depth, and average flow velocity in each municipality as indicators. First, we normalized the values of these indicators using the maximum method and then we used equal weighting of indicators to combine them to the final FFHI. Based on the obtained results, the highest fluvial flood hazard was recorded in the municipalities of Cífer, Budmerice, and Jablonec, which are located in the central part of the studied basin, but also in the municipalities of Píla and Častá at the upper part of the basin. The resulting FFHI at municipal level was compared with the number of previous fluvial floods in the studied municipalities in the period 1996 – 2024, where a very good agreement was achieved. Acknowledgment: Funded by the EU NextGenerationEU through the Recovery and Resilience Plan for Slovakia under the project No. 09I03-03-V03-00085.