Possibilities and limitations for considering critical flows and associated erosion risks in hazard zone planning

Scientific studies in recent years regarding morphologically active flood events have repeatedly demonstrated the phenomenon of "supercritical flows" (Froude number > 1), associated with massive erosion and overbank scouring, thus supporting Grant's (1997) thesis: "...that natural rivers never experience supercritical flows over long distances and over long periods of time" - "...and morphological changes lead back to the sub-critical state" (Hauer & Habersack, 2009; Hauer et al., 2021; Hauer et al., 2025a, Hauer et al., 2025b). Supercritical flows have always been considered in the construction of hydropower plants, and evidence of targeted and controlled energy dissipation downstream of the plant must be provided. While this safety aspect has also been addressed in river engineering with regard to artificial steps, the potential occurrence in free-flowing sections (steep and/or smooth channels, hydraulic constrictions) has not yet been systematically included in hazard zone planning. The aim of this study was to determine the possibilities and limitations of the extent to which existing data from hazard zone planning (maps of flow velocity and water depth), combined with information on the soil composition/use of the floodplains, can be used to determine the erosion risk caused by supercritical flows(Froude number > 1) and hydraulic jumps, and the associated high erosion energies.