Abstract
The Pannerdense kop bifurcation is the most upstream bifurcation in the Dutch Rhine-Meuse delta and is constructed around 1709. The elevation of the bifurcation and connected channels decreases. More important, the bifurcation is in disequilibrium, which is in dangerous contrast with the stability perceived by society and Rijkswaterstaat. The bed
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of the smaller downstream channel, Pannerdensch Kanaal, lowers faster than the bed of the larger downstream channel, the Waal. Existing models are not able to reproduce this observed trend in bed elevation difference. However, those models consider only a uniform sediment, while the bed of the channels have a diverse sediment mixture, different for each channel. Furthermore, bed armouring has been observed in the channels. Therefore, an existing one dimensional river model is adapted in order to incorporate processes related to non-uniform sediment. The hypothesis is that these improvements, which are a more realistic approach of the sediment household around the bifurcation, leads to a model that can reproduce the observed trends in bed elevation difference.
The sediment transport in the non-uniform sediment model is calculated using the Van Rijn sediment predictor and corrected for the hiding exposure effect. At the bifurcation, the sediment is dived over the two downstream channels using a point relation which encounters transverse bed lope caused by an upstream meander bend. In the non-uniform sediment model, the mixture of the sediment input of the two downstream channels is based upon the mixture of the sediment output of the upstream channel and the observed mixture of each downstream river bed.
Results of the uniform sediment model show that the model is sensitivity to the mean grain size of the three channels connected to the bifurcation. Furthermore, it shows the bed of Waal is lowering faster than the bed of the Pannerdensch Kanaal which is the opposite of the observed trend. Given a mixture of 3 sediment sizes, the non-uniform sediment model shows the opposite, which is in accordance with the observations. However, the model is highly sensitive to parameter choices that reflect our lack of understanding of fundamental sediment transport processes.
Observations have shown that the bed of the Pannerdensch Kanaal is lowering faster than the Waal. When this instability of the bifurcation continues to grow, it will become increasingly more difficult to maintain the wanted discharge ratio. The non-uniform sediment model shows that processes related to the non-uniformity of the sediment mixture on the riverbed play a key role in the morphological process at the Pannerdensch Kop bifurcation. Improving river models which encounter those processes will help to understand, and maybe slow down or stop, the instability of bifurcations like the Pannerdense Kop.
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