Author(s): Mario Klosch; Eddy J. Langendoen; Roman Dunst; Helmut Habersack
Keywords: River restoration; Channel widening; Bank erosion; Bar; Numerical modelling
Abstract: Originally implemented to gain cultivatable land and to protect from floods, river channelization has resulted to date in massive ecosystem deterioration, channel incision and often in severe flood risk. The situation aggravated due to deficits in the supply of sediment from the catchment, which became a limited resource as a consequence of hydropower plants and other transversal structures retaining sediment and hence disrupting the sediment connectivity. The removal of bank protections became a widely implemented measure to re-establish natural morphodynamics and the related riverine habitats, stabilize bed levels and to decelerate flow and attenuate flood peak discharges downstream. However, the space required by the restored river and the amount of sediment needed to build up the restored morphology remained unknown given the complex processes and interactions involved. In this context, especially the appearance of mid-channel bars and their interaction with bank erosion appear to play a central role. We introduce a model which considers the main processes of intra-event-scale bar-bank interactions, just as they were described in the conceptual model by Klösch et al. (2015). The model ensures mass and energy conservation between a cross section at the centre of the widening and adjacent cross sections. In the widening section, a mid-channel bar is represented by a plane elliptic bar top with a cosine-shaped margin, placed on top of an elliptic bar base which is linearly sloped in longitudinal direction. The bedload supply is defined by the transport capacity calculated in an independent cross section, which represents the channelized river upstream. Bank retreat and the related contribution of bank-derived gravel is controlled by a critical shear stress and an erodibility coefficient. The bedload output is determined by the transport in the widening section. At every time-step the resulting sediment balance is converted into an adjustment of bar morphology. Different geometric changes are applied to reconstruct the bar morphodynamics depending on: whether the bar is submerged or whether it diverts the flow into the surrounding branches; whether bank erosion occurred; and whether the sediment balance is negative or positive. As the bar grows, the shear stresses in the branches decrease and the width and accumulated sediment volume approach final values at a balanced budget, which may be used in the planning of river restoration measures. Moreover, effects of sediment retention in the river basin and of climate change on the morphodynamics may be tested. In a companion paper the model assumptions are verified with field data and the model is applied to a field site.