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Estimating the morphologically relevant sediment flux in the free-flowing section of the Rhine

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To understand the functioning of a river system it is crucial to have information on the sediment transport. Estimates of long-term sediment fluxes in the Rhine at the German-Dutch border are required to understand the morphological evolution of the Dutch Rhine Delta. Especially information about the long-term flux of bed material load is of interest because this flux is morphologically relevant. Bed material load interacts continuously with the river bed and it influences the bed surface texture and river slope and width.

The objective of this research is to gain more insight in the amount of bed material load at Lobith (at the German-Dutch border) and in the free-flowing Rhine, the region upstream from the border. To accomplish this, a literature study on the definitions of wash load and bed material load is carried out and different methods to make a distinction between the two are discussed.

This study shows that the wash load cut-off size – the sediment grain size that marks the boundary between wash load and bed material load – is a dynamic variable that depends on the local flow and/or bed characteristics. Therefore, the wash load cut-off size is different for every river and can also change within a river system. It is suggested to set the cut-off size for a specific location equal to the diameter for which 1% of the river bed grains at that location are smaller (D1).

D1 as a cut-off size and sediment transport data based on direct measurements (Frings et al., 2014a,b) give an average annual flux of bed material load of 0.5 Mt/a or 200,000 m³/a at Lobith for the period 1991-2010. It should be noted that there are large uncertainties in these numbers. First of all there are large uncertainties in the used sediment data. Second, multiple assumptions have been made in the process of distinguishing between wash load and bed material load.

The last part of this research contains a comparison between the results of this study and the sediment transport determined by three existing computer models. The model results did not differ that much from the results of this study. The results of this study were especially close to the results of the model for which the input was almost identical to the conditions under which the measurements – on which the results of this study are based - took place. It can be concluded that at least the order of magnitude of the bed material load estimated at the German-Dutch border is probably correct.