The Pannerdense Kop is a bifurcation point where the Bovenrijn splits into the Waal and the Pannerden Canal. In recent decades, the discharge partitioning changed to more discharge in the Waal at the expense of the Pannerden Canal, which negatively impacts navigation, flood safety and freshwater availability in the downstream river area. This recent change of the river system is linked to changes in bed level that may have started as a result of peak flows in the 1990s. An overview of peak flow impacts on the bed is missing whilst peak flows will occur more frequent and increase in magnitude due to climate change.

This thesis investigates the morphological response of peak flows on the river bed at the Pannerdense Kop, using field measurements and numerical models. Five sources of field data are used, all based on bed level measurements. A 1D Sobek model and a 2D Delft3D model are analysed to determine the peak flow response in these existing morphological models. The model results are compared with the field data.

Results indicate that peak flows seem to cause deposition at the upstream end of the Waal and to a lesser extent at the Pannerden Canal. Additionally, several patches of erosion and deposition over peak flows are related to floodplain outflows, groyne fields, and bends which are not captured in the 1D model because of width smoothing. Changes on this scale are captured in the 2D model although improvement is possible. On a smaller scale, results indicate that river dunes are present during peak flows and disappear in the weeks after the peak. These small-scale changes are not in the models as a result of the grid size exceeding the dune length. One source of field data shows that peak flows may also lead to a large-scale erosion adjustment wave in the Waal, although further research is required to determine whether such large-scale morphological changes occur, to characterize the nature of these changes, and to assess the extent to which they are represented in existing numerical models. These insights improve the understanding on the morphological response to peak flows at the Pannerdense Kop, and the possibilities and limitations of current morphological models.
... Read more

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.
... Read more

Climate change is causing the global sea level to rise. Research and discussion of the effects of sea level rise are often focused along coastlines. However, the effects of higher water level and changing morphodynamics can reach far inland via rivers. This study uses a one-dimensional numerical model to analyze bed level response to sea level rise. The model simulates 100 years of steady sea level rise on a 1000 km long, fixed width channel. Sea level rise creates a backwater curve which grows in the upstream and vertical directions. The transient response of the channel bed is an aggradation wave the grows in the upstream and vertical directions. We studied different cases which vary in sediment flux, flow discharge, grain size, and rate of sea level rise and found changes in the rate of growth of the aggradation in both directions. All runs start in an equilibrium state and run for 100 years.

This study finds a close relationship between the equilibrium slope and depth of the channel and the shape of the backwater curve. This relationship drives the aggradation patterns. For example, for the same amount of sea level rise, a flat, deep channel has a longer backwater curve with a smaller relative increase in depth than that of a steeper, shallower channel. The backwater curve drives the aggradation patters, such that the flat, deep channel then has aggradation over a longer reach, and a smaller increase in bed level than the steeper shallower channel.

With the cases modeled in this study, three general trends in aggradation rates emerge: (1) an aggradation mound that grows quickly upstream, with slower increase in bed level as found in flatter, deeper channels; (2) a faster increase in bed level with slower upstream growth, found in cases with steeper slope and shallower depths; and (3) faster growth in both bed level and upstream direction caused by an increased rate of sea level rise.

Since natural channels are often complex with multiple sources of discharge inputs, a tributary case is also
included. Starting from equilibrium state and applying sea level rise to the downstream boundary, the model
shows aggradation waves in the regions downstream and upstream of the confluence, starting from the downstream boundary and the confluence. There is also degradation just downstream of the confluence. The scour hole grows at first, in depth and the downstream direction then reduces. The aggradation moving upstream intersects with the degradation moving downstream and fills in the scour hole. In some cases, we see the scour fills in within the 100 year time frame, resulting in a net increase in bed level. In a tributary system, the risk of scour is greatest for tributaries with high flow discharge or low sediment flux.

The transient response of the bed level to sea level rise is aggradation. As the water level continues to rise, the bed level is expected to do the same, but at a lower rate. In this study, the nominal rate of sea level rise is 10 mm/yr. The fastest rate of bed level rise in the model results is less than 6mm/yr, creating an ever increasing water depth. This is beneficial for shipping and navigation in the channel, which would not require dredging of the aggradated material. However, the reduction in bankfull volume with rising water levels is dangerous for flood control.
... Read more

The natural development of the morphology of rivers in time is important due to the functions rivers fulfil, such as navigability and drinking water supply. Rivers adjust their morphology when there is a change in the river controls, which are mainly the water discharge, sediment discharge and downstream water level. The morphology adjustments continue until a river has reached its equilibrium state (i.e. equilibrium bed slope and equilibrium bed surface sand fraction), which is when all sediment that is supplied upstream of the river can be transported downstream. Even though an assessment of how the river changes towards its equilibrium state is important for the performance of the functions of a river, it is often overlooked. This can for example lead to increased maintenance costs and dangerous situations. A quick method to calculate the equilibrium state of a river can help prevent these negative impacts. The analytical relations by Blom et al. (2017) are a promising method to predict how a river will change, since they give useful and rapid insight in the equilibrium river state. However, the degree of uncertainty of the equilibrium river state as a result of the uncertainty in the input parameters is not yet known. It is recommended to examine this uncertainty before interpreting how a river will change. The aim of this research is therefore to investigate what the range of uncertainty of calculated equilibrium river states is, and to execute a sensitivity analysis to examine which input parameters contribute the most to this uncertainty. The river Waal is chosen as a case study for these analyses. The calculated equilibrium state of the river Waal, including the range of uncertainty, is compared to river state measurements to assess how well an equilibrium state can be identified. A model is created based on the analytical relations, and is used to perform three different analyses; a deterministic analysis that results in a spatially varying equilibrium state without uncertainties, a sensitivity analysis that assesses the influence of each input parameter on the equilibrium state, and an uncertainty analysis that results in a bandwidth of possible equilibrium states. The result of the sensitivity analysis is that the mean gravel content of the total sediment supply and the total sediment supply itself have the largest impact on the uncertainty of the equilibrium state. The result of the uncertainty analysis is that the equilibrium bed slope can be approximately 1.8E-5 smaller or larger than the mean bed slope, with an uncertainty bandwidth of approximately 3.7E-5, and that the equilibrium bed surface sand fraction can be approximately 10% to 20% smaller and 10% to 35% larger than the deterministic bed surface sand fraction, with an uncertainty bandwidth between 20% to 50%, depending on which sediment transport relation has been used. From the comparison between the calculated equilibrium state, including uncertainties, and the measured river state, it follows that the current bed slope is steeper than the equilibrium bed slope, and that the current bed surface sand fraction is either larger than or within the uncertainty bandwidth of the equilibrium bed surface sand fraction. The found milder equilibrium bed slope is as predicted. However, the found equilibrium sand fraction does not match the prediction that the bed surface sand fraction is smaller in the equilibrium state. From the comparison and the results of the uncertainty analysis follows that an equilibrium state cannot yet be identified, because 1) the uncertainty range of the calculated bed surface sand fractions is considered to be too large to predict the equilibrium state of the river Waal, and 2) the equilibrium bed surface sand fraction predictions do not match the expectation. To improve the equilibrium state identification of the river Waal, it is recommended to 1) assess why there is a mismatch between the expected and calculated equilibrium bed surface sand fractions, 2) perform research to reduce the input uncertainty, and 3) validate the equilibrium state predictions. ... Read more

This master thesis is about the positive effects of river system behaviour along the river IJssel. Due to failure of the flood defences along the IJssel, the protected areas will flood and water will flow out of the river. The discharge in the river will decrease and thus the water level at the downstream ends of the river will also decrease. This will have an impact on the required dimensions of the flood defences downstream as the loads will be lowered due to the decrease in water level. The design of the dimensions will change and the costs of the flood defences downstream are lower. In this research, flood simulations are done in the dike-rings along the river IJssel due to overtopping with and without breach formation. The simulations are done in a developed HEC-RAS model and eventually the decrease in water level is generated within the model computations. The tool OKADER is used for the calculations of the required dimensions of the flood defences in 2050 and the associated costs when changes in water level is incorporated. At the end the interest is to quantify the positive effects of system behaviour in terms of differences in required dimensions of the flood defences along the river IJssel and the amount of saved costs. ... Read more

Due to climate change, a new water management policy is considered in the Netherlands. The shift towards greater extremes in both wet periods and dry periods requires a more dynamic policy. It is investigated what effect the placement of a permanent water barrier in the Rijnmond has on the water levels in the Rhine branches, and if this can be an opportunity to help the drought prone area in the east of the Netherlands.This study shows that replacing the Maeslantkering with a permanent barrier has no influence on the water levels upstream of Nijmegen in the Waal at average to low Rhine discharges (2000 - 600 m^3/s), and thus has no effect on the water distribution at the Pannerdensche Kop. If the fresh water demand to stop the salt wedge is eliminated, a different water distribution over the Rhine branches is possible, but structural changes at the bifurcation point have to be made in order to achieve this. River water can be used to supply the east of the Netherlands with water. In this study, three options to do so are examined. The most promising options of these is placing a pumping station in the Rhine nearby Lobith and supplying the streams in the area with water, many of which fall dry in summer. From there, the water can be used for irrigation, infiltration, and other fresh water demands, relieving the stress on the groundwater during dry spells. Building a permanent storm surge barrier is not a requirement, but because it increases the navigability of the Waal river, water can be extracted more often with a permanent barrier. A solution to combat drought in the east of the Netherlands seems to be more urgent than a solution to mitigate sea level rise. However, a permanent barrier can help in mitigating the effect of drought on groundwater levels in the east of the Netherlands by allowing for more flexibility in using river water extraction. ... Read more

The objective of this thesis is to assess the effects of climate change on the initial, transient and equilibrium response of mixed-sediment river. Climate change will cause changes to hydrographs, which in turn affects sediment transport capacity and thereby the river bed profile. In order to create a better understanding of the processes in play we analyse a theoretical situation using the Lower Rhine as reference. The river Rhine is a heavily engineered river. The exact effects of climate change are therefore difficult to predict, as human intervention is significant. A simplified version of the Rhine can however give an insight into river response to climate change.

First an analyses of historical discharge and future discharge of the Lower Rhine was made, which gave an insight in the changes to the hydrograph that can be expected in the future. Next a model was made representing a theoretical river reach. For this river reach the upstream hydrodynamic boundary is varied and the results of these variations are analysed. Using the results of this model and the hydrograph expectations it was discussed what can be expected to happen to a river bed under these circumstances.
... Read more

Sediment nourishments have become an increasingly attractive alternative to deal with continuous bed degradation problems in the river system. When supplied to the river, sediment nourishments induce a sediment wave that propagates through the system causing changes in bed level and bed surface texture. In the present study we use a one-dimensional numerical model to analyse how the propagation of these sediment waves varies under different conditions. We find that the propagation of the sediment wave tends to be predominantly dispersive for large spatial variations of the flow, which is the dominant case when the height of the wave is large in comparison to the flow depth, or when the sediment is dispersed over a large enough reach to lead to a backwater effect. In artificial sediment nourishments the height of the sediment wave is usually limited to prevent creating an additional obstacle to navigation, and it is then that the temporal variations in the flow, due to high discharge events for example, determine the propagation of the wave. We study the morphodynamic response in a mixed-size sediment river system to a sediment nourishment, and identify the physical mechanisms that impact the propagation of mixed-sized sediment waves. In order to prevent it from being easily flushed downstream, the grain size distribution of the nourished sediment is typically selected to be coarser than the river bed. This reduces sediment mobility and enhances sediment deposition over the nourished reach, which in turn causes a deficit in sediment supply to the downstream reach and leads to the formation of a degradational wave. The propagation celerity of the degradational wave is found to be significantly faster than that of the sediment wave, which means that the effects of the nourishments in a river system can be observed significantly further downstream than from where the front of the sediment wave is found. We find that the celerities of both the sediment wave and the degradational wave are predominantly affected by the adjustments in surface grain size in the system. These adjustments migrate in the river system as a fining wave, which both accelerates the sediment wave, and decelerates the degradational wave. We learn that higher content of the finer fractions in the nourished sediment accelerate the evolution of the wave and lead to a predominantly translational behaviour. It also affects the magnitude of the incision depth, the closer the composition of the nourished sediment is to that of the bed surface, the less severe the scour. And even though it does not affect the propagation celerity of the degradational wave, it does impact how far downstream the wave travels, affecting a larger river reach the coarser the grain size composition of the sediment wave is. Finally, we analyse the results from a nourishment pilot project carried out in the Bovenrijn from 2016-2019. We find that the propagation of the sediment wave associated with the nourishment is only observed during high discharge events. The wave is predominantly dispersive, and only during a prolonged extreme discharge event did the wave show slight translational behaviour. We observe that lateral sorting mechanisms caused the tracer sediment to have a different trajectory from the sediment wave, which highlights the limitations of using a one-dimensional model. We also observe that the changes in bed level are related to the changes in surface grain size, with sediment deposition over the nourished reach and bed degradation just downstream. ... Read more

Ships in rivers create waves and these can have a negative impact on fish habitats along the river banks. A modelling study is carried out to investigate how these ship-induced waves can be damped in groyne fields by making structural modifications to the groynes. For this purpose, different types of openings (notches) are applied to the groynes. Next, hydro-ecologic indicators are used to assess the impact of notching of groynes on fish habitat suitability. The results suggest that relatively simple modifications can significantly improve the ecological value of the groyne fields. ... Read more