Under the Dutch Flood Protection Programme (HWBP), extensive reinforcement of primary flood defences is planned by 2050. However, progress is slow and costs are exceeding expectations. Current reinforcement strategies face significant challenges: inward reinforcement is constrained by spatial limitations, while construction-based solutions are costly and lack flexibility in light of climate policy uncertainty. Outward dike reinforcement (ODR) offers an alternative, but its application is currently restricted by regulations under the Rivierkundig Beoordelingskader (RBK). In particular, the ‘1-mm rule’ requires that any hydraulic impact be mitigated to within 1 mm, often necessitating substantial mitigation works that render ODR disproportionately complex and expensive, and thus rarely applied despite its promise.

This thesis investigates whether ODR can be considered a technically feasible and cost-effective alternative under a relaxed RBK framework, in which its hydraulic impact is absorbed by the flood defence system, eliminating the need for mitigation measures. In doing so, it aims to support engineering practice through preliminary design insights, while also identifying regulatory elements that may warrant reconsideration to facilitate broader applicability of ODR.

Four self-designed conceptual dike variants are created to assess the hydraulic impact, technical feasibility, and cost-effectiveness of ODR under varying conditions. The designs are developed at a preliminary level and represent inward, construction-based, and two outward variants with 5-metre (Tuimeldijk) and 20-metre expansions, applied to a hypothetical reinforcement of the southern Waal banks. The outward variants define a bandwidth for evaluating the effects of expansion, providing a framework for generalising ODR performance. The hydraulic impact is analysed using a simplified 1D compound channel model to identify influential parameters and general trends, complemented by detailed D-Hydro Suite simulations for site-specific effects. These results inform the feasibility assessment, which uses exceedance-based limit-state formulations of the main failure mechanisms to evaluate dike performance and potential functional lifetime reduction (FLR). Cost-effectiveness is assessed through short-term design comparison and long-term net present value (NPV), using a life cycle cost (LCC) analysis that incorporates adaptability and feasibility outcomes.

The expected water level difference (WLD) due to ODR in the Waal River ranges from 0 to 4 cm, depending on the application and floodplain characteristics. Two design graphs are presented, offering first-order estimates of WLD for 5-metre and 20-metre expansions. The relationship between expansion and WLD is observed to be approximately linear, enabling interpolation across the design graphs. Bottlenecks in the river system show the highest WLD sensitivity, as narrow and smooth floodplains amplify hydraulic impact. Continuous reinforcement results in higher WLD than discrete interventions, although short interventions are penalised by steep local water level gradients and cause larger WLD per unit of intervention length. Adaptation lengths required to absorb WLD effects span several tens of kilometres, but depend strongly on endpoint definition.

ODR is technically feasible considering the performance of affected dikes. Of the main failure mechanisms, only overflow and piping are influenced by the hydraulic impact of ODR, resulting in an estimated FLR of approximately two years if applied in the Waal. Dike stability is never compromised while WLD remains below 10 centimetres, and effects on overtopping can be neglected. FLR is primarily governed by the annual climate-change-induced increase in hydraulic load, the magnitude of WLD, dike subsidence (for overflow), and the additional robustness provided by the blanket layer or sheet pile (for piping). If these structural elements include a buffer equal to or half the WLD, respectively, FLR due to piping is unlikely. Two strategies are considered to address FLR: adding an asphalt layer to all affected dikes or accepting the reduced lifetime.

The cost-effectiveness of an optimised ODR design is comparable to, and under certain scenarios, proves more cost-effective than construction-based reinforcement over a 100-year horizon, while offering greater adaptability where inward reinforcement is either unfeasible or prohibitively expensive. This is particularly true when recycled soil is used and in light of uncertainties regarding spatial constraints on the inner slope, sheet pile prices, and functional lifetime. The optimised design should minimise outward extent and material use, in line with minimal designs such as the Tuimeldijk variant, while ensuring sufficient resistance against piping without requiring sheet piles. It should be applied over stretches exceeding 10 kilometres, as cost-effectiveness increases with length. Choosing FLR proves to be the more cost-effective strategy of the two, as uncertainties associated with FLR have limited impact on overall cost-effectiveness. In short-term cost comparisons, ODR is only cost-effective if the design closely resembles the Tuimeldijk variant or incorporates recycled soil. Otherwise, the excessive soil volumes make it less competitive than non-outward alternatives.

These findings demonstrate that ODR, where the flood defence system absorbs the hydraulic impact, can be a technically feasible and cost-effective alternative to conventional reinforcement strategies, provided that RBK regulations are relaxed. It is therefore recommended to integrate this variant of ODR as the final step in the current line of reasoning for riverward reinforcement upheld by the HWBP.
To enable practical application, it is proposed to relax the hydraulic limit in the RBK to 2 centimetres. This adjustment would allow a broader range of ODR configurations to be assessed using the presented design graphs, while maintaining cost-effectiveness and feasibility.
Additionally, the 1-mm rule should serve as a threshold for defining adaptation length, provided a clear backwater formulation is integrated into the RBK. Further research is required to confirm that other regulatory conditions within the RBK are upheld under this revised hydraulic limit. This would allow ODR to serve as an additional alternative in complex situations, helping to reduce project complexity today while preserving flexibility for future challenges.

Groynes are hydraulic structures commonly applied in the Dutch rivers. Depending on the water depth, groynes can be emerged or submerged. When a groyne is emerged, it has two main functions: maintaining the water depth in the main channel and preventing the river from eroding its bank. When the discharge in the river increases, the groynes become submerged. A submerged groyne acts as an obstacle to the flow, resulting in additional resistance and an increase in the upstream water level.

To increase the flow capacity of a river during extreme conditions, a couple of measures can be considered: increasing the height of the dikes and embankments, reducing the overall resistance of the river cross-section and increasing the area of a river cross-section.

This thesis focuses on reducing the overall resistance of the river cross-section, and more specifically, on investigating the effect that streamlining the downstream slope of a groyne has on the groyne-induced resistance. Hypothetically, streamlining the groyne by decreasing its downstream slope would decrease the intensity of flow processes like downstream flow separation, which could increase the discharge capacity of the groyne, and thus of the river cross-section, as a result of the decrease in groyne-induced resistance. An increase in discharge capacity would decrease the flood risk during extreme conditions.

To investigate the effect of streamlining a groyne by decreasing its downstream slope, a physical model experiment was set up in the 5 x 40 m^2 flume at the Hydraulic Engineering Laboratory of Delft University of Technology. The physical model represents part of a river cross-section. From the total 5 m width, the main channel and groyne field both took up 2 m and the floodplain took up 1 m. In streamwise direction, the flume contained six groynes and five full groyne fields. The groyne fields had transverse bed slope of 1:25. Electromagnetic flow meters (EMF meters) were used to measure the flow velocity at many points in both streamwise and transverse direction. The water level was measured by laser altimeters.

Decreasing the downstream slope of a groyne to 1:8 compared to a reference situation with a downstream slope of 1:3 resulted in a 4 % increase of the discharge capacity over the crest of the groyne. This increase of discharge capacity was not uniform over the transverse axis. Near the groyne tip, this increase of discharge capacity was more than 6 %, whereas at the end of the groyne, the discharge capacity decreased slightly compared to the reference situation.

A more detailed analysis of the data gave insight in why decreasing the downstream slope of a groyne increases the discharge capacity of a groyne. Streamlining the groynes resulted in a significant decrease of the relative turbulence intensity along the downstream slope of the groyne. This was consistently around 50 % lower for the streamlined groynes compared to the reference situation, indicating less intense turbulent structures. Moreover, auto-correlation functions of the flow velocity signals showed a shorter correlation length in conjunction with lower amplitudes and smaller periods of the fluctuations for the streamlined groynes. This indicates a signal with less correlation and higher frequencies. These results explain why the discharge capacity of a streamlined groyne is larger than of a reference groyne.

In the Nieuwe Waterweg an ongoing pilot takes place in one of the groyne areas. This pilot is a part of the “De Groene Poort” project. The idea for this project is to create intertidal areas by modifying and nourishing groyne areas. These artificially formed intertidal areas will create a new habitat for wildlife and attract different vegetation. For this project, several modifications have been made to the groyne area: a dam has been placed in front of the groyne area, the groynes on either side have been partially heightened and a nourishment has taken place. To better understand the effects of these modifications to the groyne area, research has been done on the effects of the tide and waves caused by shipping on the flow and sediment behaviour inside the groyne area.

Two different measurement campaigns have been carried out for this research. In the first measurement campaign six ADVs were placed in the groyne area between March 24 and April 12 ,2022. These ADVs measured the velocities and pressures with a frequency of 8 Hz. Also, sediment samples were taken during this campaign, which have been analyzed using laser diffraction. In the second campaign, ADCP measurements were carried out with a floating ADCP attached to a jet ski. With this ADCP the vertical velocity profiles have been obtained for several cross-sections at different stages of the tide.

It was found that the flow in the groyne area has changed due to the dam and the nourishment. Typical flow in an emerged groyne field with the eddies is unrecognizable during lower water levels in the modified groyne area. The exchange of water between the main channel and the groyne area is through the gap in the dam, which removes the mixing layer that separates the groyne and the main channel during submerged conditions. The flow in this groyne area is lateral and divided into two main flows along either side of the nourished island. Rocks and a higher entrance in the downstream groyne compared to the entrance in the other groyne, gives the groyne area different flow velocities between ebb and flood. The neap and spring tidal cycle enhances Ebb and flood differences in the groyne area even more. These velocities differences also impact the sediment transport and its absolute direction in the groyne area. The dam also helps reduce the wave influence on the groyne area, although these waves still significantly influence the velocities. Waves also propagate in a lateral motion from groyne to groyne through the groyne area. The water can move material from the nourishment, but the spreading of this material is limited to a number of locations in the groyne area. For the largest part, the nourished material is still at the location where it is deposited.

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.

For the past 2000 years river training works have been performed on the Dutch rivers. These training works have shaped the river system of today. Another result of the river training works, especially over the last two centuries, is a decrease in biodiversity. To protect the biodiversity some floodplains in the Netherlands have been classified as Natura2000 areas. Construction works on these floodplains are not allowed, unless these measures are a last resort. Also, if this is the case, compensating measures have to be taken elsewhere.

Before 2050 about 1500 kilometres of dikes and 500 sluices and pumping stations need reinforcements. Dike reinforcements could be executed by only adding soil to the dike. Another option is to add structural elements to the dike. A soil-based approach is preferred because there is more experience and a higher level of security of the reliability for a soil-based structure.

One such soil-based approach is a longitudinal mound. A longitudinal mound is a body of soil which is parallel with the dike, with the goal to reduce the wave height at the dike itself. As a result of the wave height reduction the necessary dike crest level will be reduced as well. Therefore, a reinforcement of the dike itself is not needed. The crest of this longitudinal mound is lower than the crest height of the dike. The longitudinal mound will be submerged during design conditions and will act like a submerged wave breaker.

Costs, emissions and construction time could potentially be reduced by using local soil. This local soil can be obtained in two different ways. Firstly, it is possible to use the surplus of soil of another local project for the longitudinal mound. Secondly, the soil for the longitudinal mound could be taken from the floodplain itself.

However, only little is known about the hydrodynamic effects of a longitudinal mound on the floodplain. This thesis research is done to find possible locations for a longitudinal mound, the hydrodynamic effects and the differences between a simple and more complex model of the longitudinal mound. This is done with a multicriteria analysis for the location study and with a conceptual model and a 2D D-Flow FM model for the hydrodynamic effects.

In the multicriteria analysis the studied criteria are the size of the floodplain, structures on the floodplain and inside the dike, the availability of clay on the floodplain, the habitats on the floodplain and the wave height at the dike.

The multicriteria analysis has been performed from the point of view from multiple stakeholders. For all locations a compromise is necessary. Different locations for a longitudinal mound are preferred depending of the point of view of the stakeholders.

In the conceptual model three design parameters for the longitudinal mound are taken into account, the crest height, the crest width and the slope. For each combination of these three parameters the conceptual model calculates the new equilibrium water level and the transmitted wave height from the longitudinal mound towards the dike. The transmitted wave height is calculated with the best empirical fit on multiple datasets by Friebel and Harris in 2003.

With the Van der Meer overtopping formula the freeboard of the dike above the water level can be determined. This is done for the original situation without longitudinal mound and subsequently for the situation with all combinations of the longitudinal mound. From these calculations it can be concluded that the necessary dike crest height decreases when a longitudinal mound is present. However, more soil is needed than for a traditional dike reinforcement.

Also the conceptual model does not include a backwater effect. The water level does not immediately jump to the new equilibrium water level, so the water level increase should be smaller than calculated in the conceptual model. On the other hand, in the conceptual model all waves are assumed to be perpendicular to the dike. If waves are not perpendicular the necessary freeboard is smaller. The absolute dike crest height reduction with a longitudinal mound is therefore smaller for non-perpendicular waves than for perpendicular waves.

The 2D D-Flow FM model has been supplied by Deltares. The grid consists of cells of 20 by 10 square metres on the main river channel and 20 by 20 square metres on the floodplain. To model the longitudinal mound with a higher accuracy the grid on the floodplain has been refined to 5 by 5 square metres. On this refined grid three different variants have been modelled. All variants have a crest height of about half a metre below design water level and their alignment is identical. For Variant 2 a connection of half the longitudinal mound height has been made with the dike. For Variant 3 the same volume of soil needed for the longitudinal mound has been removed from the floodplain by lowering it by 0.3 metres.

There are only small differences between the three variants. Compared to the original situation there was only a difference in the order of millimetres of water level at the main river channel. The main differences are found between the dike and the longitudinal mound. In this area the Bernoulli effect is found, at locations of increased flow velocity lower water levels are found and vice versa. The subsequent difference in water level is about 5 to 10 centimetres.

The flow velocity depends on the difference of flow area in longitudinal direction between the longitudinal mound and the dike, following the Bernoulli principle. So, the main contributor to the water level change on the floodplain is the alignment of the longitudinal mound. Therefore, the alignment of the longitudinal mound is an important design parameter and can be used to find a trade-off between increased water levels and increased flow velocity.

As this process is not incorporated in the current version of the conceptual model the results between the conceptual model and the 2D D-Flow FM model are different. Therefore, it is recommended that the water levels between the longitudinal mound and dike are calculated separately in the conceptual model. To do this the area between the dike and longitudinal mound can be split into multiple segments. With energy and momentum balances the water levels in these segments can be calculated.

It is also recommended that the 2D D-Flow FM model is used at a smaller floodplain as well to see if the effect on the main river channel is similarly small. Next, it could be helpful to try different alignments for the longitudinal mound to see how these influence the water levels and flow velocities.

Finally, in this research only the flow has been modelled in 2D. However, the wave reduction is also of importance. The next step is to add a wave model to the 2D model to as well. With this addition it would be possible to make the comparison between the wave height reduction in the conceptual model relative to a 2D model as well as for the water level.

The flooding of the River Geul in July 2021 was an eyeopener for the society, the government and many others involved. Besides the major structural damage as a consequence of the flooding, the emotional damage was and still is massive. Therefore, the need for measures to reduce the impact of these extreme events is bigger than ever. Many people had to temporarily leave their house and companies had to shut down. One thing became clear: the impact of comparable or even worse flooding has to be lowered, and one way to achieve that is by implementing measures in the River Geul which reduce the flood risk. The main goal of this research is to find out the impact that certain measures have on the consequences of flooding of the River Geul. Therefore, the main research question is: What is the impact of hydraulic or hydrological measures on the consequences of extreme future flooding of the River Geul? An important aspect is to consider the multidimensional impact of each measure. This can be either social, economic, or ecological impact due to flooding and the implementation of the measure itself.

The evaluation of the impact of measures is done by using different methods. First of all, an existing SOBEK model is used to simulate two extreme events: July 2021 (recurrence interval ≈ 1/100 - 1/1000 per year) and a stress test (recurrence interval ≈ 1/10.000 per year) which simulates 60% larger discharge and precipitation amounts than July 2021. Based on the bottlenecks in the River Geul, which are either bridges, weirs, culverts or orifices, measures are defined and simulated in SOBEK. A distinction is made between possible measures down- and upstream of Valkenburg and in Valkenburg, dependent on the expected effect of the measure. All the measures are simulated for both July 2021 and the stress test. The measures that significantly reduce the flooding depth and local water level nearby urban areas, which follows from the longitudinal cross-sections of the maximum simulated water level, are rated as effective. The effective measures are also evaluated with a damage victim module (SSM2017) to calculate the reduction of damage and victims, but no cost-benefit analysis is performed. Finally, expert judgement is used to estimate the costs of the effective measures. Social impact and the impact on Natura 2000 area is also examined here.

Eleven measures were considered, and four of them showed effective results and therefore their impact is compared. The effective measures are: widen the river downstream Valkenburg, bypass through Valkenburg performed as a tunnel (in-situ or drilled) or channel, remove obstacles in the River Geul along Valkenburg and enlarge the current basins in the River Geul. For the flooding of July 2021, the optimization of current basins in the River Geul scored by far the worst on damage reduction and costs. River widening and removing obstacles both reduce the amount of damage by 6 percent, while the costs are at least a factor of four lower compared to the bypass (dependent on the type of bypass). However, the bypass reduces the damage by at least 34 percent. The most expensive bypass is the drilled tunnel, but it comes with less social impact, because barely any buildings have to be removed in the city centre of Valkenburg (assuming that the subsoil in Valkenburg is suitable for a drilled tunnel). For the stress test, the damage reduction of the effective measures was not significant compared to July 2021. Thus, for extremer events the measures have no longer a considerable impact. This is not an argument to not implement measures, because the probability that a similar event as the stress test recurs is at least a factor of ten lower compared to July 2021.

Depending on the preferences of decision makers the choice for a measure can be made. Based on this research and looking at the flooding of July 2021, a bypass in Valkenburg reduces the damage along the River Geul the most with approximately one-third. For a bypass performed as a drilled tunnel the social impact is limited as maximum five buildings have to be removed but a cost estimate of 112 - 207 million euros is associated with this choice. River widening downstream Valkenburg or removing obstacles in Valkenburg comes with lower costs of 4 - 23 million euros in combination with no social impact but this results in a damage reduction of only 6 percent.

Currently multiple projects investigate the effect of lowering the crest of groynes in the Dutch rivers with the aim to lower the backwater effect of groynes under high water levels. There is a major uncertainty however in the modelling of groynes, and therefore in the prediction of the effect of groyne lowering. Currently submerged groynes are modelled subgrid as weirs. The flow processes differ however between groynes and weirs as discharge over a groyne is not purely defined by the contraction and expansion over the groyne, but also by lateral shear and interaction with the surrounding main channel and floodplain. The result is a lower resistance of the cross section than expected from weir modelling. For this reason a numerical investigation was started, comparing the modelling of groynes as different subgrid weirs in a 2D model and the modelling of groynes included in the bed topography in a 3D (non-)hydrostatic model. There were large differences between all different methods of groyne modelling. Furthermore the available experimental data did not suffice to explain the differences.
For that reason a new physical model has been set up in the Delft University of Technology, at the hydraulic laboratory of the Faculty of Civil Engineering and Geoscience. The experiment includes a 1:30 scale model of a representative transect of the Waal river. The model is 5 m wide and 30 m long and includes 6 groynes and 5 groyne fields. It further includes a part of the main channel at one side and a part of the floodplain at the other side of the groyne fields. Gravel is fixed to the bed to ensure hydraulically rough flow conditions. In the experiment many measurements were done. The first aim of the measurements was to obtain a good spread of points to validate numerical models on. The second aim was to gain insight in the different flow processes in order to describe the differences between groyne and weir flow and quantify the resistance groynes have on a flow.
An important observation was the existence of a region of low flow, at the tip of the groyne. The observation indicates a complex three-dimensional flow which effectively redistributes discharge over the transverse. The contribution of this flow to the two-dimensional momentum balance is then directly as a secondary circulation and indirectly by alternating the distribution of discharge. Complex flow furthermore invalidates the hydrostatic pressure assumption. The observed flow needs further investigation with either more accurate measurement devices or in a 3D non-hydrostatic model. Another observation includes the contraction and expansion of flow over the groyne crest. The expansion of flow over the main body of the groyne seems comparable to the expansion of flow over a weir, which can be predicted as Carnot head losses. The observation of expansion losses is however clouded by other flow processes, such as bed shear stress, the lateral flow around the groyne and possible three-dimensional processes induced by the groyne tip, so that the observed head loss over the groyne is only partially explained by weir-like expansion losses.
The comparability between groyne flow and weir flow does not seem to hold at the groyne tip, where no separation of flow is observed. Measurements were performed 10 cm away from the transition between the groyne tip and main body of the groyne. This means that at full scale there is a region of 3 m on either side of this geometrical transition where the transition lies between weir-like overflow and non-weir-like overflow.
With the here performed simulations it is possible to adapt weir head loss formulas on the observed head losses over groyne. It should be possible to validate a three-dimensional non-hydrostatic model, which can further quantify the effect of three-dimensional flow and differentiate the different contributions to groyne head loss. Based on these models proper tuning parameters can be chosen to represent the complex flow in a two-dimensional model and separately model the weir-like expansion.

One of the preferred features of plastic is its high durability. This merit is its main disadvantage in the natural environment. When poorly disposed of, plastic waste can enter rivers from surface waters and subsequently end up in the ocean. Here, plastic spreads out over the entire ocean and ultimately ends in ocean gyres. Plastic material in the ocean causes environmental damage. Marine animals can get entangled in plastic debris, plastic accumulation occurs when consuming seafood and the material spreads toxins in the seawater. It has also caused economic damage: marine plastic accumulation led to €5 to 18 billion to key economic sectors for 87 coastal countries in 2020 (Deloitte, 2021). Attempts have been made to capture buoyant plastics from the ocean with interception devices, with mixed success.

 

The placement of an interception device in a river is thought to be a better approach to the problem. A standard interception design is that of a floating boom. Plastic debris is transported by the river flow and these objects guide buoyant plastics into a collection tray. Currently, these interception designs are still in their infancy. A difficulty in the optimisation of these interception devices lies in the limited knowledge of the trajectories of plastic waste. Computational fluid dynamics (CFD) modelling can help to create a better understanding of these aspects. This tool can provide a rapid assessment of different flow conditions with different types of plastic debris modelled. Further knowledge into this field can ultimately help into better retention of plastics in rivers.

 

Plastic modelling for buoyant particles has been performed in the marine environment (Van Utenhove, 2019) and in the river (Van Welsenes, 2019) with a Lagrangian study. However, both works did not perform a validation study. The combination of a CFD model and the results of physical model tests can be valuable in removing uncertainties related to plastic trajectories and the modelling of these.  One physical model test performed on plastics is the release of plastic particles in a flume (Zaat, 2020) Small films were released in a 2DV physical model, representing plastic bags. It was found that these particles followed an inverted Rouse profile for spherical particles times a shape factor.

The study by Zaat (2020) was replicated in the CFD modelling environment of OpenFOAM in this thesis. Subsequently, the retention of plastic particles was investigated. Three hydrodynamic conditions were investigated, with uniform inlet flow velocities of 0.10, 0.55 and 0.90 m/s. The $k-\epsilon$ turbulence model was used. An Euler-Euler approach was applied with a discrete and continuous phase. This modelling approach is computationally less expensive compared to a Euler-Lagrangian framework and can be easily established for different cases. The drag models Gibilaro, Wen-Yu and Syamlal-O'Brien were tested to investigate which concentration profile best fits an inverted Rouse profile for rising particles. The interception device was modelled as a square obstruction of 10\% of the water depth and the model was assumed as a rigid-lid. The retention of particles by the system was explored for neutrally buoyant particles, light material and high-density polyethylene (HDPE) and polypropylene (PP) films.

 

This study found relative differences in near-surface flow and the analytical approach of 3 to 4%. The Gibilaro model was best applicable and model coefficients for the virtual mass, lift and turbulent dispersion of 0.5, 1.6 and 1.0 were found. The concentration profiles followed the inverted Rouse profile and that for films closely for the medium and high flow cases, but not for the low flow case. A lower retention of plastics was observed for increasing flow velocities. In the extreme case of low flow and light particles a build-up of particles near-surface can be observed and runs against the streamwise direction.

Several limitations were present in this study. Only two parameters of the plastic particles were adjustable, which were the particle diameter and density. In reality, plastic debris has different shapes will experience drag differently, especially the friction drag. The rigid-lid model is valid only if the velocity head is smaller than that of 10% of the water depth, which is not the case for the high flow velocity case.

 

The results show that on the basis of a rise velocity and a density a representative diameter can be determined from Stokes’ law. The Euler-Euler modelling approach provides an accurate assessment and gives a proper representation of the particles for the Rouse distributions. This is done without handling the particles individually and thus results can be found more quickly. Both for HDPE and PP inverted Rouse profiles were found that followed the theoretical profile with an underestimation of 10\%-20\% near-surface. The retention was influenced mainly by the size of the wake. It is evident that the interception system can work for buoyant particles. The object can be placed during low flow conditions for the highest efficiency. For high discharge in rivers, more particles will be in suspension and thus further away from the water surface, which makes this design less preferred during high flow conditions.

 

Recommendations for further research are to perform more physical model tests, implement a non-spherical drag model applicable for plastic material and to apply a three-phase model. This removes uncertainties related to the model and to the current knowledge in particle dynamics. The addition of physical model tests would benefit the current knowledge the greatest. It is only possible to expand the complexity of the CFD model with a larger availability of these physical model tests.

Backward erosion piping is a form of internal erosion where small pipes are formed below a dike. These pipes are formed in a direction opposite to the flow that transports sand particles. Piping is a very important failure mechanism in the protection of dikes, which can be unpredictable due to the different soil characteristics. Piping is well managed in the Netherlands, but there is a lot to be discovered regarding theerosion processes inside the pipe. For example, how fast the pipe develops and what this is dependent on. The objective of this thesis is to monitor and study the development of the pipe, to extend the knowledge about the different processes that influence the progression rate of the pipe. The main research question is: How do the different parameters and processes influence the progression and the sediment transport rate in laboratory experiments of backward erosion piping? To monitor the development of the pipe, different small-scale laboratory experiments were performed to obtain new data regarding piping and to study the influences of different parameters. The piping experiments were performed in three series: (1) configuration of the setup, (2) effect of grain size and (3) hydraulic loading. These experiments were performed in the previously developed setup of Vera van Beek (Van Beek, 2015). This setup was modified to measure the pore pressures and to guide the pipe through the middle of the setup. While conducting the experiments, different measurements were performed. This included measuring of the pipe length and geometry, collecting the sand boil and a dye injection to follow the flow. The literature study performed for this thesis has shown that in the past many experiments were performed regarding piping, but these studies did not focus on the different processes and the sediment transport rate of the pipe. By focussing on the progression phase of the pipe (continuous transport), the experimental data is compared to existing models (primary and secondary erosion). Sellmeijer’s model (Förster et al., 2012) is the current rule that is applied in the Netherlands for dike safety, but this model does not include time-dependency. This research showed that the development of the pipe is not a stationary process but depends on various conditions, such as
soil characteristics. The hypothesis formed at the beginning of this thesis listed several soil parameters which influence the progression rate of the pipe. Concerning the sediment transport rate of the pipe, Cheng’s model for bedload transport (Cheng, 2004) is evaluated and compared with the measured results. The analysis showed that the adapted formula of Cheng (Equation 2.24) overestimates the sediment transport rate in the pipe. From the analysis of the experimental results, it can be concluded that two parameters play an important role in the progression and sediment transport rate of the pipe: the particle diameter and hydraulic permeability. These parameters showed an influence on the progression rate which can be used to study piping on a larger scale. The most interesting result is the fact that experiments with a larger particle diameter have an overall larger progression rate.

Rivers provide many of our day-to-day needs. They allow, for instance, the production of drinking water and the movement of goods via inland waterways. However, river systems experience disturbances, such as droughts and pollution plumes. These disturbances can impair the functioning of river systems and their ability to provide the desired functions and services. Since some of these disturbances have very little impact while other cause many problems, it is important to determine how a river system handles different disturbances. Resilience is often used to indicate and assess the behaviour of systems with regard to disturbances. Using the concept of resilience with regard to river systems can help determine how well river systems handle disturbances. In order to determine how resilient a river system is, a method to assess the resilience of river systems is needed. Research into the resilience of river systems has only been done with regard to flood resilience. This resilience does not consider other disturbances, such as droughts, and therefore does not represent the full resilience of the system. Furthermore, this research often resulted in suggestions and pointers to create and improve resilience, but not in a clear method to help assess it. Hence, the objective of this study was to develop a method to assess the resilience of river systems. In order to create this method several questions need to answered. First, a clear definition of resilience of river systems is needed. For this it should first be determined what resilience is. This is done using a literature study. The first part of this literature study focussed on general and field-specific definitions of resilience. This resulted in the definition of resilience of river systems as used in this study: the resilience of a river system is the ability of the system to continue functioning during a disturbance. Where continuation of functioning is defined as the continued achievement of the goal of a certain function, such as the continued production of crops for the function of agriculture, even if this only occurs to a smaller extent, and the continued shipping of goods for the function of shipping. The disturbances considered are short-term disturbances, such as droughts and pollution plumes. After the determination of the definition of resilience of river systems several remaining questions need to be answered. The impact of disturbances on the different river system functions has to be determined and visualised. Next, the impact of these disturbances has to be classified. Finally, the resilience of river systems should be assessed using the classification for the impact of these disturbances. The second part of the literature study, focussing on the assessment of resilience, is used to help determine, visualise and classify the impact of disturbances. The development of the final method is an iterative process between application to a case study and the amendment of the method based on this application. Using the results of the literature study, a first design of the resilience assessment approach, the method for the assessment of resilience of river systems, was made. This approach consists of four steps with several sub steps. In the first step a system overview is created. This step is based on the ideas of flood resilience theory. It states how a system overview should be created, what external factors, characteristics and functions should be included and how to categorise them. The second step consists of the creation of disturbance-impact graphs for the different functions and disturbances. These graphs, based on the idea of stressor-response graphs of system robustness analyses, are used to visualise the impact of the disturbances. The third step gives an assessment of the resilience of a function based on the disturbance-impact graphs and a resilience classification with associated scoring system, which is based on the ideas of a current impact classification used for droughts. The fourth and final step consists of the assessment of the resilience of the entire river system based on the assessment of the individual functions. The approach was applied to a case study, the Meuse river system, to test it. This case study focussed on low discharges caused by droughts. The approach was applied to the whole system, thus to all its characteristics (water quality, water quantity, vegetation and fauna and sediment) and all its functions (shipping, electricity, industry, drinking water production, agriculture, flood protection and recreation), to create a system overview (the first step of the approach). The application focussed specifically on the functions shipping and drinking water production. This application was used to check the content and applicability of the sub steps of the first step of the approach, using the system overview, and to check the content, applicability and order of the sub steps of the second step of the approach, using the functions shipping and drinking water production. The application of the system overview led to a clear distinction between external factors, characteristics and functions, which has been adopted in step 1. Furthermore, the application to the specific functions resulted in a clearer definition of system boundaries and a clearer link between the external factors and the disturbance. Finally, the application led to an additional sub step. This sub step consists of a check to determine whether the chosen disturbance is actually relevant for that specific function. The application of the approach showed that both shipping and drinking water production of the Meuse river system appear to be very resilient with regard to low discharges due to droughts. This is likely due to fact that both functions have already been adapted to this disturbance. For shipping this is likely largely due to the fact that the water level in the Julianakanaal is managed to ensure continued shipping possibilities during discharge fluctuations. Pumping facilities have been constructed in order to maintain water levels even when very small discharges occur. The large resilience of the drinking water production by Waterleiding Maatschappij Limburg is likely largely due to the fact that the intake point is located in the Lateraalkanaal. The water level of this canal is managed using weir and sluices for shipping and drinking water production. Only the first two steps of the approach have been tested using the case study. The final version of these steps, as found in this report, can be used in practice. The tested steps have shown to be applicable to the tested functions and it assumed that these steps are also applicable to the other functions. The third and fourth step should be tested before they are used in practice. Especially step 3a (the assignment of the different levels of the resilience classification to the disturbance-impact graphs) might require additional adaptation.

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.

Concerns regarding plastic pollution arise as large quantities of plastic enter the ocean and affect wildlife and possibly human health. Rivers form a dominant pathway for macro plastic particles into the ocean. Plastic pollution in rivers can result in blockage, where it affects water quality and ecology. This increases risks of flooding and provides health concerns. To prevent further dispersion of plastics into the ocean and reduce health and flood risks, reduction of plastic concentrations in rivers is desired. Buoyant macro plastics are a dominant polluter and understanding their cross-sectional distribution in rivers could promote areas for efficient extraction or monitoring. As field measurements are expensive and time consuming it is desired to accelerate research with modelling techniques. To improve modelling of buoyant macro plastics and translate cross-sectional distributions it is necessary to model relative motion of buoyant macro plastic particles to the flow and one another. Previous research has shown that the understanding of macro plastic forces and transport is limited. The range of macro plastic properties complicates modelling approaches to macro plastic transport. To simplify, a classification of macro plastics is introduced according to cornerstones in behaviour. A basic force model is constructed (derived from riverine wood transport studies) to provide an understanding of the effect of size, density, wind and discharge on transport of different macro plastic types. This new understanding is applied to a variety of flow features induced by a range of river elements (river bend, channel widening, river confluence and groyne) to develop a hypotheses on the cross-sectional distribution of the different macro plastic objects in these environments. These hypotheses are tested in a numerical transport model. A theoretical background study on particle response times and Stokes numbers is performed to identify a relation between the particle response time and relative particle dispersion (compared to the flow). This provides a possibility for translating the transport of the different macro plastic objects into the numerical model. To define numerical modelling input of different buoyant macro plastic objects, particle response times are calculated and translated to horizontal particle dispersion coefficients in D-WAQ PART (software package of choice). Each of the previously formulated river elements is modelled as well in the numerical model. The modelling results show that river bends cause an increase in particle concentrations along the river bend. Channel widening increases the concentration along the boundaries. A river confluence increases the concentration along the tributary side of the channel. A groyne increases the macro plastic concentration along the groyne-free side of the channel. These results show that particle response times and their relation to Stokes number and particle dispersion propose an interesting tool for the modelling of relative particle motion towards the flow and the different macro plastic objects. This study identifies the significance, diversity and complexity of macro plastic particles in riverine environments. It highlights the need for classification and simplification to facilitate modelling and still be able to obtain relevant results. Describing macro plastics according to cornerstones in behaviour provides an efficient tool to simplify modelling. Coupling of different macro plastic classes to a range of riverine environments and understanding the differences in their cross-sectional distribution provides practical use.

Along river banks, on the transition zone between the river and see, tidal marshes can develop. The tidal marshes accommodate multiple plant and fish species. As a result of the stressful conditions in tidal marshes, unique intertidal ecology develops. However, due to the densifying of the river banks, tidal marshes are disappearing.
Restoration of tidal marshes can reintroduce the unique intertidal ecology on several locations in the estuary. To obtain restoration of tidal marshes, constructed tidal marshes come into play, which can be built at designated places along the river.
However, constructed tidal marshes are not necessarily built to restore the unique tidal nature, but can also have other functions as recreation and contribution in green city area.

Nevertheless, is it still unknown how constructed tidal marshes behave and what the optimal design is. Namely, a mismatch exists between the policymakers and designers on the one side and academic knowledge on the other side. To fill up this gap, accessible knowledge from the experts should be available for designers.
Therefore, this research provides guidelines for designers and gains more insight into the behaviour of constructed tidal marshes. This thesis focusses on extracting general knowledge from the results of a numerical model, applied on a case study.

Simulations of a numerical 1D Sobek model applied on the case study achieves the influence of model configurations on accretion. Therefore, tidal forcing, marsh design and system adjustments are divided into multiple components. The model simulations of the separate components give the influence on bed shear stress and potential sedimentation.
Furthermore, executed fieldwork calibrates the model on measured flow velocities. Besides, the Manning coefficient is estimated, and cross-sections are obtained by gps measurements. The parameters are subsequently used as model input.

From the simulations, it can be concluded that tidal asymmetry determines the duration of stagnant water and thus the settling of fine particles. A considerable increase in sedimentation is reached when flats are participating. However, an increasing flat area encourages ebb-dominance and can even lead to erosion.
Next, when the width of the cross-section is large compared to the depth, more sedimentation is predicted. With flood-dominant bed shear stresses, a broad cross-section leads to accretion.
In the case study, the presence of sand particles is not expected, as sand is deposited close to the inlet, where the bed shear stress is ebb-dominant. In contrast, silt settles throughout the system. Especially at low energetic conditions, such as bends, silt settles. As the occurring bed shear stress is mostly flood-directed, it is likely that sediment entering the system, does not leave the system anymore, and accretion of the bends is presumed.
The placement of gate culverts in flood direction leads to higher accretion rates due to the longer slack duration. However, at the exact locations of structures, higher bed shear stresses can develop, and erosion is expected.

In submerged conditions groynes add resistance to a river, thereby increasing the water levels for a particular discharge. Groynes are modelled in two ways, either they are included in the bed topography when the grid resolution is sufficiently high or with a sub-grid parametrisation when the grid cell-size is larger than the geometry of the groyne. To investigate the flow patterns over and around groynes, data from a groyne flume experiment, as carried out by the BAW, is analysed. This to investigate to what extent a groyne behaves as a weir. The experiment is modelled in 2DH and in 3D to compare the performance of the sub-grid parametrisation with full 3D non-hydrostatic simulations.

In 2015, the Dutch government initiated the ‘longitudinal training dams pilot project’. With the help of longitudinal dams, a two-channel system is created in which the river is divided into a main and a side channel. A combination of an inlet and several openings between the dams allows water and sediment to be exchanged between the two channels. This is expected to influence the discharge (and sediment) distribution between the main and side channel. Understanding the hydraulic conditions and the impact of the regulatory inlet and opening crest heights is expected to improve the control strategy and future design of river interventions using the concept of longitudinal training dams. A one-dimensional (Matlab) and a two-dimensional (Delft3D-Flow) model are created for this research. Both schematize the two channels with dams and openings. The influence of several mechanisms, factors and dimensional design parameters on the discharge distribution is assessed. Additionally, using Delft3D-Flow, the detailed flow patterns around the inlet and openings are evaluated and the schematizing of the inlet as part of the local bed topography or by using subgrid weirs is compared. From the several model runs, the discharge distribution shows to be mainly controlled by differences in longitudinal water level slopes in both channels, caused by the combination of too short inlet and opening lengths. It is not influenced significantly by the placement of the longitudinal training dams in a river bend. For increased crest heights the flow conditions change from submerged to free-flowing. In submerged conditions the velocity magnitudes and flow angles atop the inlet and opening crests are shown to increase in downstream direction while for free-flowing conditions they remain nearly constant. The free-flowing conditions are well described by empirical relationships, while for submerged conditions an additional angle submergence coefficient is suggested to parameterize flow angles. After some modifications, which will be included in the new Delft3D-Flow software release, schematization as part of the bed topography and subgrid weirs both predict similar discharges (< 7% relative difference). If interest is only given to water levels and discharge distributions, the use of subgrid weirs is recommended in Delft3D-Flow. For models assessing other river functions requiring local flow patterns or sediment transport, schematization as part of the bed topography is essential. However, by including the suggested parameterization for flow angles, the errors in flow angles for subgrid weir can be reduced significantly. From this research, the adjustable inlet and opening crest heights do not show a large influence on the discharge distribution (5-10% difference). It mostly impacts the tipping point for submerged to free-flow conditions. In submerged conditions, the inlet crest height does reduce the specific discharge in the side channel, limits the transverse velocities in the main channel, and greatly influence the flow angles atop the crest. Besides the crest heights, the channel width and inlet length should also be included as adjustable parameters in the design or control strategy of new systems of longitudinal training dams.