Morphodynamic trends in the freely flowing Rhine

Abstract

The river Rhine is subject to a multitude of controls imposed by nature and humans. Natural controls refer to climate and tectonics, while human activity is represented by a series of interventions aimed at flood protection, navigation, land reclamation and exploitation of river supplied resources such as hydro energy and sediment. These controls can drastically change the longitudinal profile of the river as well as the composition of its bed sediment. When analyzing field data taken for bed elevation and grain size distribution with respect to temporal changes, although being possible to lead towards useful findings and conclusions, it is not trivial to explicitly indicate the causes. This is because, especially in heavily engineered rivers as the Rhine, the aforementioned controls are existent at different time periods and act upon different sections of the river but their respective effects on river morphodynamics tend to overlap since they demonstrate temporal variability that is characterized by shorter and longer time scales. In the present literature study, the focus is on the freely flowing reach of the Rhine, that initiates from the most downstream impoundment at Iffezheim and reaches up to the North Sea. For this section of the Rhine, temporal trends of the main imposed controls are presented using available field data and findings from previous studies. Then temporal trends are then similarly presented for the morphodynamic response of the river to the imposed controls, while their relation is discussed. The main natural controls on the freely flowing Rhine that are considered in this study are water discharge and base level (due to tectonics and sea level). For the first, data were analyzed from 10 gauging stations well distributed along the study area. A periodicity suggested to be linked with atmospheric oscillations by previous studies was identified after plotting the probability density functions of discharge for different time intervals, that yet varies between different gauging stations. Considering larger temporal scales, two main base controls are recognized. The first has shaped the low gradient Mainz basin and follows directly from differential movements of adjacent tectonic units. The second is found at the mouth of the Rhine and corresponds to a relative sea rise of roughly 30 cm in 100 years. With respect to human activity, nourishment and dredging as well as training works were considered. Narrowing, straightening and impoundments of the main reach and tributaries were the main human controls in the past. Removal dredging that was also extensive in the past was followed by re-allocation dredging. Various strategies of sediment nourishments are combined with the latter to counteract bed degradation in recent times. The morphodynamic response of the river to the controls stated above as revealed by bed elevation measurements, is a general incision of the bed to lower levels that is also followed by a lowering of water levels. Degradation was most prominent in different time periods at Oberrhein below the Iffezheim dam, at lower Niederrhein and at the upstream Dutch Rhine branches. Cumulative incision reaching up to 2 meters is observed at these reaches relative to 1934 measurements. At recent times the incision rates are largely decreased especially for the German reaches where nourishments are carried out. At the Dutch Rhine, degradation continues at the upstream Waal reach and Pannerdensch Kanaal. Nevertheless the first demonstrates higher rates of bed lowering reversing the former trend and hence changing the implications for the bifurcation stability. At the German-Dutch border area, bed degradation has left Niederrhein and Waal with steeper and milder slope respectively. Finally, locations of finer exposed historical deposits coincide with locations of ongoing bed incision in otherwise stabilized reaches. A general coarsening of the bed surface texture is also revealed by field data. This coarsening is a result of bed degradation (due to depletion of the finer material from the bed and exposure of coarse historical deposits) but also of nourishments with coarse -relative to bed- sediment. An example of the first case is demonstrated at the IJssel reach during the intense degradation of 1980s. For the latter an example can be drawn by the lower Niederrhein where the strong coarsening revealed at available measurements corresponds in time with strongly decreased degradation rates and nourishments of very coarse material. Recent field data of sediment transport rates demonstrate large scatter. Nevertheless, previous studies that consider geological time scales suggest a more or less constant input of sediment load to the Dutch Rhine that yet comes with a strong increase in grain size. In conclusion, the present study demonstrates that the (ongoing) adjustment of the river profile and bed texture originates from a multitude of controls, the effects of which strongly overlap in time and space. Data analysis has limitations in linking causes and effects but can still provide insights when combined with numerical modelling. The results presented here can thus be considered in a following study that will use mathematical models to reproduce the adjustment of the freely-flowing Rhine.