Author(s): Julian Binder; Martin Glas; Christoph Hauer; Marcel Liedermann; Helmut Habersack; Michael Tritthart
Keywords: Gravel structures; 3D numerical modelling; Sediment transport; Danube River
Abstract: Artificial gravel structures can increase the ecological function of a river, sustainably providing habitats for both aquatic and terrestrial species. However, their application as hydraulic structures with effects on hydrodynamics and morphology of a river’s main channel needs to be further investigated. Aim of the present study is to assess the suitability of different variants of gravel structures for low flow regulation in the context of navigation, while simultaneously analyzing potential effects for flood protection. For this purpose, different geometrical variations of gravel structures under different hydrologic conditions in the Austrian Danube River were analyzed using the hydrodynamic model RSim-3D and the sediment transport model iSed. In particular, the gravel structures were modified with respect to their width and height as well as their position in the river reach; a complete removal of the structure was also studied. The investigated discharge scenarios ranged from low flow conditions to a 100-year flood. The gravel structures abiotic impacts on the river’s main channel were derived by calculating parameter differences of the variants to the status quo focusing on water surface elevation, depth averaged flow velocity and bed shear stress. Moreover, differences of bed level changes after 30 days of mean flow conditions and 2 days of highest navigable discharge conditions were compared. Artificial gravel structures showed similar effects as groynes with respect to the hydro- and morphodynamics of a river’s main channel. Concerning water surface elevation, gravel structures caused a local decrease at the position of the structure and a local increase upstream. Both depth averaged flow velocities and bed shear stresses were higher along the structure. Relative erosion tendencies occurred in the main channel parallel to the gravel structure, giving rise to relative sedimentation tendencies downstream. These hydrodynamic and morphodynamic effects varied depending on the modelled discharge scenario. The highest differences of parameters compared to the status quo were evident during mean flow conditions and highest navigable discharge conditions. The obtained results clarify that artificial gravel structures can be used as an effective alternative to groynes for low flow regulation. Thus, the frequency of maintenance dredging events for providing safe navigation conditions can be reduced. In addition to their river engineering function, they also serve as bedload depots and provide ecologically valuable habitats in a regulating context through the ongoing changes in morphology. Furthermore, no adverse effects for navigation and flood protection occurred in the investigated areas. However, the magnitude and spatial extent of the impact of gravel structures on a river’s’ main channel indicate large influence that needs to be considered in the planning process of these hydraulic structures. Therefore, the outcomes of this study can be incorporated into future research and planning of artificial gravel structures.