Rivers are essential for sustaining human life, preserving ecosystems, providing clean water, supporting energy production, offering recreation, and enabling transportation. Human interventions have led to the creation of so-called ‘engineered rivers’, such as the Dutch Rhine, which has undergone significant interventions over the past two centuries including straightening, dam construction, and the addition of fixed beds.

Fixed beds are found in the Dutch Rhine, such as in Nijmegen, St. Andries, and Spijk. Composed of non-erodible materials, they are strategically placed on the outer bends of rivers to enhance navigation by causing erosion in the inner bends, widening the river. Similar features worldwide include sediment nourishments and natural bedrock reaches.

This study investigates the large-scale morphodynamic effects of fixed beds, focusing on their influence on river slopes and sediment trapping. The research begins by examining the initial response of a fixed bed. A fixed bed results in (1) a sill-effect, (2) increased roughness, and (3) decreased mobility, and these effects are separated and treated individually. Conceptual models based on river dynamics theory are used to understand and predict how these effects contribute differently to the morphodynamic responses.

Following that, the study continues by looking into the transient and long-term response of a fixed bed using both conceptual and numerical models. These numerical models are created using the model system SOBEK-RE. The fixed bed-related effects are still considered separately with reference models created first and the effects integrated after. The reference model focuses on the transient state due to narrowing, where the slope decreases and the bed level increases. By doing this a comparison can be made of the fixed bed related effects with and without it. A similar process is repeated for a model run where the effects are all combined to assess their relative importance and the overall combined effect. The models reveal that all three effects contribute significantly to the fixed bed.

The model's key findings indicate that over a 50-year period, natural narrowing of the river reduces the slope by 4%. Introducing a fixed bed amplifies this effect: the upstream slope decreases by 3% and the downstream slope by 7%. This demonstrates that fixed beds alter the riverbed's slope, decreasing it downstream and increasing it upstream. At the upstream side of the fixed bed, it traps sediment caused by an M1-backwater curve. The height up to which this upstream sediment trapping continues is determined by two important parameters: the sill length and the sill height. However, the study acknowledges uncertainties related to model dimensions, sediment uniformity, discharge, and parameter choices. Real-world effects depend on the fixed bed's width, length, and protrusion relative to water level.

It is vital for water management authorities to recognize the importance of fixed bed structures, especially in extensively engineered rivers. This is because the fixed beds can result in significant and long-lasting changes to the riverbed.
... Read more

In the last few years, the climate crisis has been accelerating at a dizzying pace and poses an emergency threat to our planet. Rainfalls have transformed into intense downpours, and flash flooding, combined with the sea level rise, leads to a higher risk of inundation of densely populated coastal cities. Moreover, the melting of the permafrost can lead to significant landslides, triggering the generation of mega-tsunami waves. The latter can have a catastrophic impact, not only on the infrastructure but also on human life. Man-induced climate change is responsible for the increase in frequency and intensity of extreme natural events, such as tsunamis, floods, and storm surges. Recent research indicates that almost one-fourth of the world population lives at high-risk locations to at least 0.15m of inundation depths with a return period of 1 in 100 years. Therefore,
the urge of implementing protection measures against unsteady flows is imperative.

Undoubtedly, the involvement of engineers can play a pivotal role in order to analyze these flows in the built environment and provide sufficient coastal and building plans to ensure safety and reduce reconstruction
costs. The behavior of the unsteady flow around a structure is not a well-understood topic and results in a lack of accuracy and reliability. More insights are required into the fluid-structure interactions to come up
with a safe building design.

In the present study, unsteady flows are generated using the dam-break technique in line with previous research. The Thesis aims to model, validate, and implement a simplified approach to analyze the complexity of the hydrodynamic behavior of unsteady flows around impervious buildings, with different orientations
and blockage ratios. To do so, the research introduces a numerical simulation method of a dam-break wave, using the two-dimensional, and non-rotational shallow water equations. The Galerkin finite-element model is applied for the discretization of the solution on a limited domain.

Initially, the flow of the dam-break wave was validated in the absence of the structure, using a dam-break experimental work for comparison. Then, in order to insert a structure in the domain, a second experiment with a structure is used, which generates tsunami-like waves using the vertical release technique. The gate, which represents the dam, is located at x=0 and opens instantaneously at t=0s. Behind the gate, the reservoir maintains an amount of water that flows, after the opening of the gate, into the channel generating the shock
wave. At the channel downstream of the gate, initial water levels are considered and the behavior of a bore propagation is simulated. The building is located on the downstream side and different impervious building configurations were studied in terms of orientation and shape to investigate the impact of the unsteady flow.
To analyze the complex hydrodynamic processes of flooding, four fixed points around the structure are set to measure the action of the bore on different impervious building configurations. The water elevations and the averaged velocity profiles in time were derived at each point. Moreover, the horizontal forces in the x and y directions are calculated by the model, integrating the stresses over the wet surface of the buildings.

The general behavior of the fluid-structure interactions is captured well, especially upstream of the building, and insights are gained regarding the behavior of the fluid around the structure and the parameters of
influence. Results showed that orientation changes completely the impact on the building configurations. The separation of the flow and the blockage ratio are the main parameters that are influenced by the angle of rotation and change the behavior of the loading process at the initial impulsive phase, when the wave arrives at the structure, and at the hydrodynamic phase, where the flow has a quasi-steady behavior. Overall, a good agreement is achieved with the experimental data, although the numerical model overestimates the loads
acting on the different building configurations. Results proved that the orientation of the building with respect
to the flow facilitates the flow around the structure and contributes to lower water levels, and to a better distribution of the horizontal loads on the surface. The best results were achieved for an angle of rotation of 45°, where symmetrical separation of the flow is also playing an important role.

... 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