Geomorphological Response of Montane Streams to Extreme Floods

Extreme flood events can cause significant geomorphic changes in river systems, including bank erosion, sediment deposition, river braiding and the formation of new channels, resulting in extensive damage to settlements, infrastructure, and floodplain structures. At the same time, information on the extent, nature and distribution of the geomorphologic impacts of floods is often not systematically collected and is therefore not available to assess the consequences of flood events and associated risks, or to provide a basis for efficient water management and flood protection.This study presents longitudinal research employing geomorphologic mapping and UAV photogrammetric reconstructions to track the intensity, type, and distribution of geomorphic changes to streams in the Opava River Basin, Jeseníky Mts., Central Europe, recurrently affected by devastating floods. The field survey was carried out repeatedly in the area after major floods in 1997, 2007, and 2024 using a consistent mapping methodology. This approach combined the mapping of geomorphologic flood effects with hydromorphological properties, including information on channel and floodplain modifications. The survey covered a contiguous stretch of river in the core flood zone over a length of 100 km, and the UAV campaigns following the 2024 flood focused on selected river segments representing hotspots of river dynamics.The large-scale mapping results allowed for an assessment of the spatial distribution of flood effects, the identification of critical elements and structures, and the analysis of relationships between stream modifications and the nature of geomorphic impacts. High-resolution models from UAV monitoring allowed us to quantify detailed geomorphic analysis and determine bank erosion rates, sediment volumes, and channel migration patterns.The analysis revealed substantial spatial variability in geomorphic responses, with particularly intense erosion and sediment deposition observed in narrow valley sections and areas of high flow velocity, as well as in relation to floodplain connectivity and channel modifications. The most significant geomorphological changes consistently reoccurred in the same locations, signaling the need for targeted river management and protection.The study highlights the importance of geomorphic mapping of flood impacts for understanding the risks posed by high-intensity floods, improving risk assessment, and efficient post-flood recovery efforts. Methodologically, it emphasizes the efficiency of UAV photogrammetry for detailed and rapid post-flood assessments, providing comprehensive information to better understand flood dynamics and target river management strategies.