The impact of extreme rainfall and flash floods on the flood risk management process and geomorphological changes in small Carpathian catchments: a case study of the Kasiniczanka river (Outer Carpathians, Poland)

Short-duration extreme rainfall and flash floods are the major natural hazards in small Carpathian catchments. Quantifying forcing rainfall, hydrological responses and geomorphological impacts is the key to mitigating the negative impacts of flash floods. This article focuses on the hydrometeorological aspects of a flood event, the geomorphological changes of hillslopes and the river valley, in the Kasiniczanka catchment—48 km2 (Outer Carpathians, Poland). Results revealed that the flood in 2014 was generated by 6-h rainstorm with a total of 95.2 mm, and the mean intensity ranging from 7.1 to 31.3 mm h−1. The flood peak ranged from 60 to 171 m3 s−1, and it was approximately two times higher than an 0.1% flood. The unit peak flow ranged between 3.6 and 4.6 m3 s−1 km−2, and the K index (which is non-dimensional measure and allows the comparison of flood magnitudes in catchments of differing size), ranged from 3.9 to 4.1. These two measures revealed that this flood was among the worst, recorded in catchments ranging from 13 to 48 km2 in the Carpathians as a whole. The most significant geomorphological changes were observed in unmanaged channel reaches (the upper and middle parts of the catchment), contrary to lower part, where the river channel was protected by a hydrotechnical infrastructure. Flood analysis enabled the evaluation of the flood risk management process, related to flash floods in small catchments. In this context, some proposals to reduce flood risk level are presented and discussed.