Storyline Claudia

Natural Dynamics and Human intervention

Key message:

For the Lower Rhine River, including the German Niederrhein and the Dutch Bovenrijn and Waal, the response of the river to human interventions in the past has been studied. These interventions include the construction of levees and groynes, and dredging and sediment mining. Due to their fixed planform and width, most engineered rivers can only respond to interventions by changing (1) channel slope, through channel bed erosion or sedimentation, and (2) bed surface grain size.

Human interventions in the lower Rhine River have mainly resulted in a reduction of the channel width along most of its course. By narrowing its channel, a river’s flow velocity increases, and so thus its sediment transport capacity. A smaller channel slope then suffices to transport the sediment flux downstream, which is achieved through erosion of the bed. Bedrock near Bonn stops erosion at that point, however. As a result, the channel slope has become steeper just downstream of Bonn.

The incision rate of the river bed has reduced in time. In the long run, this rate will continue to reduce until a new equilibrium slope has been reached. Adjustments to human interventions generally take centuries. In addition, the grain size composition of the bed surface has coarsened over the past decades; the upstream part of the Waal has transformed from a sand-bed river into a gravel-bed river. This coarsening of the bed surface may be (partly) related to gravel nourishments upstream.

Future Developments

Key message:

Between now and 2100, the Lower Rhine River – the German Niederrhein and the Dutch Bovenrijn and Waal – will keep adjusting to human interventions in the same way as it did in the past. In addition, the consequences of climate change – sea level rise and a change of the river’s discharge regime – and changes in the river’s sediment flux will have an impact on this adjustment.

According to model calculations, climate change may enhance this response by up to 50%. An increase in the moderate to high river discharges, in particular, is projected to enhance the incision and increase the concavity of the longitudinal profile of the river bed. Sea level rise will probably contribute – in a relatively small way – to this concavity increase by increasing the sedimentation at the downstream end.

Little is known about possible changes in the river’s sediment flux. This flux may decrease, increase, or change towards a higher contribution of gravel. Higher sediment fluxes may reduce river bed incision by 0-50%, and lower fluxes may increase it by 0-40%.

Sources

• Ylla Arbós, C., Blom, A., Viparelli, E., Reneerkens, M., Frings, R.M. and R.M.J. Schielen, 2021. River response to anthropogenic modification: Channel steepening and gravel front fading in an incising river. Geophysical Research Letters 48, e2020GL091338, https://doi. org/10.1029/2020GL091338.
• Ylla Arbós, C., Blom, A., Sloff, C.J. and R.M.J. Schielen. Climate change vs. human intervention: 21st century drivers of channel response in an engineered river. Geophysical Research Letters (submitted)