This study assesses the application of graph theory to examine the connectivity of aquatic habitat in the Sliedrechtse Biesbosch and preserve or improve the area’s ecological value. The study addresses the relation between hydrodynamics and ecology, and evaluates different definitions of connectivity. Graph theory provides a novel and promising approach to assess connectivity in aquatic environments. With increasing availability of data, more accurate and informative analyses can be carried out, resulting in more effective designs of restoration measures. The application of graph theory in such research has, as one of its main strengths, its visual accessibility to laypersons or others without expertise in interpreting numerical model results. Furthermore, graph theory can offer both a holistic system view of a water network by evaluating the system as a whole, as well as a local view which is possible by looking at the single nodes and edges in the graph. This enables investigation of the role of individual channels within the system and the identification of vulnerable spots within the network, limiting the availability of aquatic habitat.

Graph theory is applied in various fields of research. As numerous metrics exist to examine the properties of a graph, an introduction to the most important metrics is given in this study. Adequacy of metrics is dependent on the questions posed and parameters relevant for the specific topic to be investigated. It is shown that in aquatic habitat connectivity, metrics such as betweenness centrality and bridges are indicative to obtain a general view of the network. When temporal variations play a role, as is the case in a tidal area, metrics as the number of components (NOC), the order of the largest component and the length of connected pathways (LOCOP) of the largest component are suitable to determine the connectivity. Whereas these metrics show useful in studying aquatic habitat connectivity, they may be less appropriate in connectivity studies of the same water system aiming at other fields of application, e.g. sediment connectivity.

Results show that graph theory provides a useful instrument in analyzing the aquatic habitat connectivity of the Sliedrechtse Biesbosch, which is investigated in a case study. The ease at which key nodes and edges are identified offer great possibilities for the design of nature restoration measures. In the present layout of the study area, large variation of aquatic habitat connectivity occurs based on a flow velocity fragmentation threshold of 0.3 m/s, corresponding to the maximum tolerable flow velocity for the European flounder (Platichthys flesus). Due to tidal influences in the study area, flow velocities vary continuously and the threshold flow velocity is exceeded during part of the tidal cycle. Considering the available habitat of other species gives different results depending on the tolerable flow velocities of the specific species. As is shown in this research, a combination of graph theory and numerical modelling enables the design and simulation of different nature restoration measures and system layouts to improve the aquatic habitat connectivity of the area.

The method presented in this research can be particularly useful to ecologists investigating suitable habitats for specific fish species. Also for engineers and others involved in the design of nature restoration measures, the method can be helpful since the designs of restoration measures can be evaluated considering the effect on habitat availability. The research provides an informed basis for subsequent applications of graph theory and numerical modelling to aquatic habitat connectivity. By selecting the most suitable design parameters and improvements of the network schematization, a justified decision can be made on the most effective restoration measures concerning the improvement of available habitat. Especially considering a combination of habitat preferences, such as flow velocity, water depth and turbidity, can provide proper insight in the aquatic habitat connectivity of an area for specific species.

A dam-break wave can result in considerable damages and casualties. Events that can lead to such waves include dike and dam breaches, storm surges and impulse waves. Dutch history is familiar with a number of coastal dike breaches but also river dike breaches have occurred, resulting in dam-break waves reaching far inland. Beyond the Netherlands, propagating walls of water could occur when dams of water reservoirs collapse. More specific knowledge of the hydrodynamic behaviour of dam-break waves is necessary in the battle against water.
The impact of a dam-break wave can be reduced through a better understanding of the currently poorly understood link between bed roughness and wave hydrodynamic properties. The objective of the present study is therefore to study experimentally the effect of bed roughness on the hydrodynamic properties of dam-break waves, on both dry and wet beds. The approach was to generate dam-break waves through a lift-gate and a reservoir with a depth d0 = 0.4 m and to measure the water levels over time in the downstream channel. 7 different bed roughness configurations were made (range of ks = 0.6 to 48 mm) in addition to smooth bed reference test. Dry bed tests were performed, 5 wet bed tests with an initially still water level (h0) (range of h0/d0 = 0 to 0.125) were performed. The analysis focused on wavefront celerity, inundation depths and roller length which are used to characterise the hydrodynamic properties.
Visual observations showed substantial differences in wavefront behaviour between surges and bores as well as between smooth and rough beds. Validation of the generated waves was done by successfully comparing the reconstructed water levels over the flume length with a new approach.
Results showed that an increase in bed roughness resulted in a decrease in wavefront celerity for surges and that the initially still water level can work as a lubricant for bores. The initial still water level is dominant over the bed roughness when h0 >0.5ks.
The maximum water level increased with increasing bed roughness for surges, while bores showed an opposite behaviour. Increasing the initially still water level reduced the relative maximum water level for rough bed configurations.
In the present study, only some test configurations revealed the presence of a plateau height. If there was one, an increase in bed roughness showed a decrease in plateau height for bores with h0/d0 = 0.125. Three sections in the h0/d0 range with different plateau height behaviours were noticed.
The roller lengths were determined with the wavefront celerity and the plateau height. No trivial relations were found for the influence of bed roughness or initial still water level.
Overall, this project revealed some interesting behaviours that need to be further investigated in the future. It is recommended to conduct a full quantitative analysis once precise values of the bed roughness configurations are determined. Nevertheless, these results present some preliminary, yet interesting, findings that will contribute to a better understanding of the behaviour of dam-break waves over rough bed.

The Rhine-Meuse delta is characterized by the presence of many scour holes. Koopmans (2017) showed that these scour holes have very different growth rates, even if they are located within the same river branch. Most likely this is caused by either the heterogeneous subsoil, the presence of hydraulic structures, the geometry of the river or anthropogenic influences. Therefore, Koopmans (2017) advises to individually study scour holes in the delta since the local variations in conditions cause different shapes, sizes and growth rates, making every scour hole unique. One of these is the scour hole Beerenplaat, which is located at the confluence/bifurcation of the Oude Maas and the Spui. It is a large scour hole which recently gained much more attention due to the construction of a pilot nourishment by Rijkswaterstaat.

This thesis investigates the morphological development of the river bottom at the scour hole Beerenplaat and the location of the pilot nourishment, by analyzing measurements taken with Multibeam Echosounder. Due to the lack of knowledge on the composition of the heterogenous subsoil, it is tried to understand the observed morphological development based on the hydraulic conditions. At the scour hole Beerenplaat, the hydraulic conditions are characterized by two types of flow processes. The first type of flow process is caused by the confluence/bifurcation Oude Maas and Spui. With the help of Acoustic Doppler Current Profiler (ADCP) measurements and the output of a Delft3D model, the flow field at the confluence/bifurcation was investigated for four scenarios: peak ebb flow, flow changing from ebb to flood, peak flood flow and flow changing from flood to ebb. These scenarios are each paired with different flow features such as flow accelerations, flow stagnation and additional turbulence. It is expected that the second type of flow processes at the scour hole Beerenplaat are the 3D flow processes, which are flow contraction, a (curved) recirculation zone and the horseshoe vortex. Again, the output of the Delft3D model and the ADCP measurements are used to investigate them. Unfortunately, the limitations of both the measurement and the model and the use of an incorrect geometry of the scour hole Beerenplaat, made it not possible to succeed in this. Based on the actual profile of the scour hole, it is expected that the (curved) recirculation zone develops at the steep slope of the scour hole Beerenplaat during ebb flow.

By computing bed shear stresses and using literature, it was investigated how these flow processes attack the river bed during ebb flow, indicating areas vulnerable to scour. It was found that these areas were no match with the observed morphological development. Due to the incorrect geometry of the scour hole in the model, it was not possible to include the hydraulic forcing during flood flow. Most probably, it should be included to describe the expansion of the scour hole and growth towards the Spui. Despite, this research has given better understanding of the flow processes occurring at the scour hole Beerenplaat are shaped and affected by the tide.

Implemented in river landscapes, Nature-based Solutions (NbS) have the potential to reduce flood risk, while also playing an important role in restoring many of the ecosystem services that are lost as result of human interventions and global warming. A barrier to succesfull implementation of NbS is the lack of a global and common framework with guidelines for its implementation and evaluation. In response, the IUCN Global Standard for NbS was published. Even though the IUCN Standard has been designed to be applicable to NbS in all sectors and over the entire globe, knowledge on its applicability and usefulness for specific sectors remains limited at present. Therefore, this study aims to identify the challenges that occur in ex-post application of the IUCN Standard to river restoration projects with a focus on flood risk mitigation, and the added value that this application may provide.

At first, a literature study is conducted in which the content of the IUCN Standard is related to twenty-two other assessment frameworks for NbS and compared in-depth to the three most relevant frameworks. These comparisons indicate that that the IUCN Standard has a broad scope of application, provides limited flexibility in assessment to its users and is descriptive. Furthermore, the comparisons reveal that the IUCN Standard can be used as a tool to evaluate the extent to which the essential processes of a NbS, established by the IUCN, have been incorporated in the project (i.e., process-oriented framework). The standard can, however, not be used to evaluate project results, including biophysical and social results.

In the next part of the research, the IUCN Standard is applied to three case studies of river restoration projects with a focus on flood risk mitigation, of which at least two differ significantly in the surface area, position in the catchment, kinetic energy of the river, data accessibility, resources and the type of riverine NbS measures that were implemented. The selected case studies are the Eddleston Water Project, the “Room for the River” Deventer Project and the Missouri River Levee Setback Project. Despite facing challenges in the interpretation of indicators, data accessibility, a relatively coarse scale for evaluation, and tensions between the project objectives and the use of the IUCN Standard, the standard is successfully applied to all case studies. The case study results consist of (1) a total percentage match to the IUCN Standard and a statement on being in adherence to the standard, which may provide credibility to the project processes, (2) the strenghts and weaknesses of a project, which may be used to guide projects and strengthen (future) projects, and (3) a radar chart, which may provide possibilities to compare to and learn from other projects. Based on these results, it can be concluded that, despite of a few challenges, the IUCN Standard is applicable to river restoration projects with a focus on flood risk mitigation, and that application of the standard may provide added value in various ways, although restricted by the limited evaluation of flood risk mitigation.