Specific problems in connection with the sediment regime have arisen in the granite and gneiss area in the northern part of Austria due to the increased sediment supply of coarse sand and fine gravel as a product of rock weathering. One of the physical characteristics of these sediments is their high mobility in form of ripple bedforms along the surface. In these rivers, the microtopographical structures of the bed surface are hydraulically active, especially in the initial stages when ripples have not overtopped the largest elements of the bed's roughness elements. There is a lack of detailed studies of the impacts of these microtopographical structures on ripple dynamics. Thus, the aim of the present study was to investigate the impact of various microtopographic roughness elements on ripple movement. This study was performed using a 1:1-scaled physical laboratory experiment. The results showed, in all repetitions of the experiment with variable grain sizes, that the higher the microtopographical variability of the river substrate and turbulence was, the higher the measured transport velocities of the ripples were. The expected hidden impacts on sediment transport were not documented. Important implications for sediment management were discussed, as river sections with high accumulation rates of coarse sand and fine gravel are prone to additional degradation as downstream transport decreases over smooth ripple-dune morphologies. Additionally, structural elements such as exposed rocks may increase the downstream transport due to the increased variability in turbulence.
Zusammenfassung Der naturnahe Wasserbau besitzt aufgrund seiner Begrifflichkeit bereits den Anspruch, natürliche und naturnahe Rahmenbedingungen und Prozesse in Planungen von wasserbaulichen Maßnahmen wie Hochwasserschutz oder Ufersicherungen miteinzubeziehen. Hier spielt in einer mittel-bis längerfristigen Betrachtung der Feststoffhaushalt eines Fließgewässers eine entscheidende Rolle. In Bezug auf natürliche oder anthropogene Veränderungen können Defizit und Überschuss zu entscheidenden und nachhaltigen Veränderungen der Lebensraumqualität führen. Vor allem in der Region der Böhmischen Masse ist ein Überschuss von Grobsand-und Feinkiesfraktionen (Korndurchmesser 1 bis 10 mm) verantwortlich für regionale Probleme sowohl in Bezug auf den Hochwasserschutz als auch für die Gewässerökologie, wie z. B. hinsichtlich des Erhalts von Kieslaichplätzen und Flussperlmuschelpopulationen. Im Forschungsprojekt "Feststoffmanagement im Mühlviertel und im Bayerischen
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