An integrated approach for investigating geomorphic changes due to flash flooding in two small stream channels (Western Polish Carpathians)

Study region: Poland, Carpathians, Gorce Mountains, Jamne and Jaszcze catchments. Study focus: The hydrological response to, and geomorphic impacts of, a 18 July 2018 flood involving two small catchments (each covering less than 12 km2) that were not hydrologically controlled were examined. A peak flood discharge was simulated with two hydrological models: SCS-CN and SCS-UH. Hydrological modelling was performed in order to complement the post-flooding investigations and to verify the hydrological data via cross-check comparison. Geodetic measurements of the channels and ALS and TLS data were used to analyse the geomorphological changes. In addition, the flood magnitude recorded for the studied streams was compared with the other local flash floods in the Carpathians using the unit K flood index (a non-dimensional measure). New hydrological insights for the region: The maximum unit flow (4.1–4.8 m3 s−1 km-2) placed this flood event among the 10 largest flash floods in the Polish Carpathians (based on the K index). The peak discharge simulated with the SCS-CN and SCS-UH hydrological models was similar to the data obtained during the post-flooding investigation, indicating that this method can be used for catchments in the Polish Carpathians that are not managed hydrologically. The high unit stream powers estimated for the analysed channel cross sections (>750 W m-2) were associated with narrow channels and steep slopes. The most significant geomorphological changes were observed in unmanaged channel reaches. Moreover, the channel transformations in the Jamne catchment were greater than those in the Jaszcze catchment (in a more forested area). This analysis can be useful in the evaluation of flood risk management processes associated with flash floods in small, unmanaged catchments.