This study is an analysis of sand mining in the Vietnamese Mekong Delta (VMD) with the use of the optical satellite data set PlanetScope. This is done with a detection and classification model of sand mining vessels in the VMD. The classification model is based on machine-learning and it is trained with three classes: sand mining vessels, other vessels, and background. This study shows that it is possible to distinguish vessel types with a 3 metre spatial resolution and it shows great potential for a vessel classification model based on machine learning.

The hydropower development in the Mekong Basin affects the silt and clay concentrations in the river which has ecological as well as socio-economic effects on the Mekong Delta and its inhabitants. Therefore, the first objective of this research was to obtain a better understanding of the effect of hydropower development on the silt and clay load in the Mekong Delta. Subsequently, the objective was to determine possible sediment management strategies to mitigate the impacts on the silt and clay load in the Mekong Delta.

A conceptual model was developed and applied to analyse these impacts. By means of a sediment balance model with distributed sediment yield and individual trapping efficiencies of dams the annual sediment load has been determined.

This research has shown that the annual silt and clay load downstream has decreased from 140 Megaton ± 40 [Mt] before hydropower development to 26 ± 9 Mt in 2020. Including future hydropower dam projects, this could further decrease to 4-8 ± 3-5 Mt in 2040. Approximately 4% of this sediment load will deposit in the Mekong Delta and therefore it is predicted that the clay and silt deposition will be close to zero in the future.

By 2040, approximately 75% of the total trapped sediment load is trapped in mainstream dams even though they account for 16% of the total dams. The majority (50%) will get trapped in Chinese dams that account for 9% of the total number of dams. This means that almost all sediment coming from China is trapped and will not reach the Mekong Delta. Around 80-85% of the future sediment load that will reach the delta has its source in the 3S region.

This reduction has implications for the sustainability of ecosystems and the productivity of the Mekong Delta. Clay and silt are nutrient rich alluvial sediments that support the ecosystems in the floodplains and delta. These nutrients are crucial for the soil fertility vital for rice cultivation and for various biological processes for fish. Today, nearly 80% of the 20 million people living in the Mekong Delta depend on the river system for their livelihood. However, the rice cultivation is expected to decrease significantly and by 2040, the loss of fisheries could cost close to $23 billion.

Thus, the computed reduction displayed that sediment management strategies are inevitable to maintain the productivity of the Mekong Delta in the future. Therefore, two sediment management strategies were reviewed in this research: cancelling planned dams and reservoir sluicing. Reservoir sluicing is passing incoming sediment-laden water through the reservoir by discharging high flows to enable sediment to move past the dam without depositing.

The analysis of these two strategies showed that the maximum sediment load increase is 1-3 Mt as a result of cancelling planned dams and 2-3 Mt due to reservoir sluicing. This means a very small increase for the deposition in the Mekong Delta (4% deposits). Therefore, the sediment load increase was considered unsatisfactory, suggesting that these strategies are not providing the solution for the Mekong Basin.

The results indicate that effective sediment management strategies are urgently needed and that more research is needed into the effects of other strategies on the silt and clay load in the Mekong Delta.

The port of Rotterdam is one of the most important ports in Western Europe and via the 'Nieuwe Waterweg' it provides maritime trading between the large hinterland and the rest of the world. To provide port access for the large vessels, the channel and port basins need to be dredged regularly. At the same time, in other locations, sediment is required to fill erosion holes, develop natural habitats and improve water quality. Different stakeholders joined forces to acquire more knowledge about managing the sediment in the system. This resulted in the project called 'Proeftuin Sediment Rijnmond'(PSR). One of the pilot projects of PSR is located in the ’Nieuwe Waterweg’ where a longitudinal dam is constructed between groynes to study the effects on nourished sediment. The location used for this pilot is called Groyne Field 9. At the same time, the area around Groyne Field 9 is involved in another project called the 'Groene Poort'. This project focuses on natural development in the areas between groynes. The research in this thesis contributes to knowledge acquirement for the various projects. It studies the effect primary vessel waves have on the hydrodynamic behaviour in the semi-closed Groyne Field 9.

The study answers the research question: How can primary vessel waves impact the hydrodynamics in the semi-closed Groyne Field 9 in the 'Nieuwe Waterweg' and how can the hydrodynamic conditions be modified with different structural layout designs? Two different approaches are used to answer the question. First, a data analysis is conducted using 2 relevant data sets and secondly, a numerical model is developed to be able to study the topic in a more controlled environment.

The First part of the research analyses two data sets simultaneously. Water level and velocity measurement data obtained inside the groyne field is combined with Automated Identification System data (AIS data), which contains velocity, location and more information about sailing vessels. Combining these data sets enables the possibility to study the effect of the vessels' characteristics on the water level in Groyne Field 9. A regression analysis is carried out which results in an equation that shows the correlations of all parameters. The most significant parameter influencing the severity of the primary vessel wave is the sailing velocity. Other important parameters that cause water level drawdown are the length, width and draught of the vessel.

In the second part of the research, a numerical model of sailing vessels in the 'Nieuwe Waterweg' is developed in Delft3D Flexible Mesh software. This can be used to analyse the hydrodynamic effect on the groyne field in a controlled way. To simulate vessels passing through the waterway, a local atmospheric pressure field is used. This pressure field simulates the vessels’ hull and by moving the pressure field through the numerical domain, a primary wave is modelled. To research the effect of primary vessel waves on the semi-closed groyne field, various simulations are conducted and analysed which can be distinguished by two sets of simulations...

Hydropower plants have proven to generate electricity reliably and predictably, store water to meet water demands during dry periods, improve navigability and reduce flood risk. The reservoirs of these hydropower plants face ongoing sedimentation, negatively effecting the electricity generation, water storage volume, navigability and flood risk. Obtaining an equilibrium of the in- and outgoing sediment flux reduces these negative effects.

The incentive for this report is the planned run-of-river hydropower dam cascade in the main stream Mekong river in Lao PDR. The Mekong river is characterised by the distinct low and high flow seasons and a unique ecosystem. At the location of the planned cascade, the river is located between mountains and the river bed consists of bedrock with alluvial bed forms at the banks and between rock outcrops.

Long, narrow and relatively shallow reservoirs are created due to the construction of the planned dams in the main stream Mekong. Flushing operations have proven to be an effective method to recover reservoir volume from rivers with a high season flow with long, narrow and shallow reservoirs. The Mekong river in Lao PDR has all those properties.

This report attempts to determine operational rules for a run-of-river hydropower dam cascade to increase the sediment flux by flushing operations at the dams, whilst still maximizing the electricity generation.

To do so, two steps are taken. Firstly, a set of experiments on the Mekong river and flushing events at a dam is performed in a Delft3D-FM software with the Real Time Control module. The output of these experiments are used as input for a bucket model with volume conservation to test different operational rules.

Using the Delft3D-FM software with the Real Time Control module, a one-dimensional representation of the Mekong river is made. This model is used for two purposes. First, the reduction of the sediment flux is determined by comparing the sediment flux during a representative year for the situation pre-dam and post-dam construction. Secondly, this model is used to test the influence of the draw down rate, water level set point and duration of flushing operations on the cumulative sediment flux through a dam for four different discharges. The results of the flushing experiments will then be used as input for the second step of the research, the bucket model.

Different simulations are performed using operational rules with different threshold discharges for initiating flushing events, different draw down rates and different frequencies of flushing a reservoir. Using random sampling the influence of these three variables is tested. Additionally, two predetermined strategies are tested and compared to the operational rules tested using random sampling. In the first strategy the dams in the cascade are flushing progressively from the most upstream dam towards the downstream dam. The second operational strategy flushes the reservoirs progressively starting from the downstream dam towards the most upstream dam.


Obtaining free flowing conditions during a flushing event increases the sediment flux significantly compared to flushing events that do not reach free flowing conditions, especially for lower discharges. Increasing the draw down rate proves to have limited influence on the sediment flux further upstream in the reservoir, while the flood wave released at the dam increases. Extending the duration of free flowing conditions after draw down proves to be an effective method to increase the sediment flux at the lowest reducing in electricity generation.

Performing a flushing event with 48 hours at free flowing conditions during the flood season increases the yearly cumulative sediment flux on average by a factor three. The cumulative electricity generation is reduced by just 2.5 %. Extending the duration of the free flowing conditions in a flushing event increases the sediment flux per lost GWh electricity generation.

Current operational rules at the planned cascade in the main stream Mekong do not consider flushing at all. It is recommended to include flushing events in the operational rules which are initiated if the discharge is above the design discharge of the turbines and below extreme discharges to increase the sediment flux and minimize electricity generation loss.

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.

Due to river training in the past centuries, the bed level of the Waal started and is still decreasing by several centimetres a year. Erosion of the riverbed causes problems for objects underneath the riverbed like cables, pipelines and foundations of structures. Furthermore, some parts of the river are eroding at a higher velocity compared to other parts due to a difference in the composition of the bed material. This can result in a jump in the bed level while the water level cannot follow this jump. This results in less available draught for inland vessels. If no mitigation measures are applied and the bed level keeps decreasing, more problems are likely to occur resulting in less transport over the Waal. A possible solution to reduce bed erosion inside the Waal locally is the use of groyne field nourishments.
The objective of this research is to obtain more knowledge about the flow in a real-life groyne field and the effect of vessels on this flow. The research question formulated for this research is, therefore: \textit{what is the influence of vessels on the flow properties inside a groyne field and in what way do the characteristics of the vessels influence this?} To answer this research question a literature study is done first. Secondly, a measurement campaign is performed and finally, the results are analysed to investigate the effect of the characteristics of the vessels.
The literature study is done to obtain more knowledge about the flow properties inside a groyne field. The distinction is made between emerged and submerged groynes and the scenario with and without vessels. The situation where the groynes are emerged is dominant for the erosion inside the groyne field. In this scenario, the flow inside the groyne field consists out of one or two circulation patterns. The first circulation pattern is a large eddy in the downstream part of the groyne field, referred to as the primary eddy. When the groyne field is long enough, a second eddy is present in the upstream part of the groyne field, the secondary eddy. This eddy has a flow direction opposite to the primary eddy and a smaller flow velocity. When a vessel passes a groyne field, the flow pattern inside the groyne field changes due to the primary waves created by the vessel. Firstly, the water level inside the groyne field is raised due to the bow wave. Secondly, due to depression in the water level caused by the primary wave the water level inside the groyne field is lowered. Finally, the water level inside the groyne field is increased again due to the stern wave.
To investigate the effect of vessels on a groyne field, a measurement campaign is performed. The flow direction, flow velocity and water level inside the groyne field are measured with the use of several measurement instruments for two weeks. The obtained data by the measurements is analysed and visualised. The results show that the primary eddy remains partly intact when a vessel sails past the groyne field. Since the primary eddy remains partly intact, the water is guided towards the upstream part of the groyne field. In the upstream part of the groyne field, the secondary eddy does disappear and the flow is directed out of the groyne field. This results in the water, and sediment, flowing out of the groyne field mainly in the upstream part of the groyne field. This has resulted in a scour hole present in the upstream part of the groyne field.
In this research the effect of multiple vessel characteristics on the water level and flow velocity inside the groyne field is investigated. The vessel characteristics investigated are the draught, the sailing speed, the vessel length, the vessel width and the distance of the vessel towards the measurement instrument. The effect of vessels on the water level and flow velocity inside the groyne field differs for each vessel. Information about the vessels has been obtained by using AIS (Automatic Identification System) data. During the processing of the AIS-data, irregularities and errors were found and mostly removed from the AIS-data. According to the final results, the draught of a vessel does not influence the water level and flow velocity inside the groyne field. An increase in the sailing speed of a vessel and a decrease in the distance of a vessel towards the groyne field does have an enlarging effect on the water level difference and the flow velocity inside a groyne field. The combined effect of increasing the length and the width of a vessel also has an enlarging effect on the water level difference and the flow velocity inside the groyne field. It should be noted that the results show a large variance and it is impossible to predict the effect of a single vessel based on the vessel characteristics.
The measurement campaign showed that a constant water level fluctuation is present inside the groyne field even when no vessel is affecting the flow inside the groyne field. Multiple explanations for this fluctuation are given with transverse oscillations between both riverbanks being the most likely.
The measured flow pattern inside the groyne field without vessel is according to the literature. When the flow is affected by a vessel, the flow pattern differs from the flow pattern described in the literature. Furthermore, the effect of a vessel is likely not only depending on the characteristics of a vessel but also on the flow mechanism inside the river system such as the helical flow, the angle of the groyne field with respect to the main channel and the constant water level fluctuation inside the groyne field.
This research adds to the already existing knowledge about the flow in groyne fields. To investigate the effectiveness of groyne field nourishments, a pilot is planned where the actual nourishment will take place at the same location as the measurement campaign. Therefore, this research can be used to better prepare the planned nourishment pilot to investigate the effectiveness of groyne field nourishments. Furthermore, this research contains data about the flow pattern inside the groyne field without nourishment which can be compared to the situation during and after the nourishment pilot. In the end, this research can be a part of the answer to whether groyne field nourishments can reduce or stop the erosion inside the Waal river. 

River bars are large-scale bedforms formed by the local deposition of sediments, the length of which scales with the channel width and the height with the water depth. They create suitable habitats for aquatic fauna and riparian vegetation, making them useful in river restoration projects. Understanding the dynamics of river bar development is required for a proper design of the restoration project and for developing a sustainable management scheme. The channel half-width-to-depth ratio is the key controlling parameter for the formation of alternate bars. Existing stability analyses define a resonance point and critical width-to-depth ratio, marking the transition between the bar regimes. Seasonal variations in the river discharge and the propagation of flood waves make the half-width-to-depth ratio a time-dependent parameter, varying over periods of days to months. This research aims to create a better understanding of the development of alternate bars in varying discharge conditions. For that, insight into the timescale of development of the bars is necessary to link this with the timescales associated with natural discharge variability. A straight river channel is modelled in Delft3D, with non-erodible banks to which a fixed perturbation is added, to trigger the formation of free bars and to which hybrid bars become fixed. In the first set of simulations, the upstream boundary condition is a constant discharge, with magnitudes between 50 and 1250 m$^3$/s, corresponding to a half-width-to-depth ratio between 77 and 8. The different types of alternate bars and the time to develop the bars are analysed from this constant-discharge analysis. The damping length and wavelength of the hybrid bars showed good agreement with the values determined from the theory of Struiksma et al. (1985). Free bars became suppressed when a hybrid bar pattern developed, showing that hybrid and free bars do not coexist locally. The resonance point is clearly obtained and assigned to a half-width-to-depth ratio of 37-38. Contrary to existing stability analyses, the critical half-width-to-depth ratio is marked by a gradual transition at half-width-to-depth ratios from 11 to 20 instead of a single value. In this transition, free bars develop with intervals of a few months. A clear difference in the development of hybrid bars and free bars results in a shorter timescale of development for free bars than for hybrid bars, which is in line with laboratory experiments (Fujita and Muramoto 1985; Crosato et al 2010). Also, in the mathematical derivations for describing the two types of bars, the distinction between an initial response (free bars, Colombini, Seminara, and Tubino (1987)) and a final stable bed (hybrid bars, Struiksma et al. (1985)) is considered. The numerical results show that free bars develop fastly when the bed becomes unstable, in the case of a relatively flat bed. The associated timescale shows little variation over the different simulations and stays around 20 days, which is in the same order of magnitude as the timescale related to seasonal variation. A pattern of hybrid bars develops in downstream direction, starting at the fixed perturbation. Based on the definition of the damping length by Struiksma et al. (1985), a theoretical expression is defined, which gives a good fit to the time at which a pattern of hybrid bars has developed. The transition of alternate bars around resonant conditions is modelled using a steadily increasing and decreasing discharge. The distinction between the timescales of hybrid and free bars results in a different response of the bars to a varying discharge. The first response of the bed in the transition from subresonant to superresonant conditions is the adaptation of free bars towards the new bar regime. A pattern of hybrid bars follows, developing in downstream direction. The reversed transition is governed by a migration of the hybrid bar pattern, making room for the damped bar pattern. The difference in the transitions between rising and falling discharge in combination with a bed adaptation after a temporal in- or decreased discharge led to a type of hysteresis in the transition around the riverbed's resonance point, which was not earlier detected. Hysteresis is ascribed to the increased bed stability when a hybrid bar pattern has developed, suppressing the formation of free bars in the transition from superresonant to subresonant conditions and to the time-lags associated with lower discharge conditions. This thesis assesses the timescales of alternate bars and the transition around the resonance point. The numerical model is able to assess both free and hybrid bars and therefore gives valuable information in the difference of their development. The difference is linked to the (linear) theories describing the bars and applied for answering the research question, showing the development of free bars should be largely influenced by discharge variation due to seasonal variation. The varying discharge simulations provided insights into the response and transition of alternate bars around resonance conditions, leading to a type of hysteresis which was not detected earlier. Recommendations are made towards increasing the river geometry complexity and discharge variation and the assessment of the transition around critical conditions to verify analyses around critical conditions.

The research focuses on sedimentation in reservoirs. Subjects include settling distribution in a reservoir, dredging and flushing of sediment in reservoirs, impact of sedimentation on reservoir performance and financial effects of long term sedimentation in reservoirs.

Flexible river training structures such as tetrahedron frames (`porcupines') can be attractive for control of braided river channel networks in regions where permanent control structures (e.g. groynes or dams) are too expensive or potentially inefficient, such as systems with highly dynamic flow regimes and morphology. Porcupines provide hydraulic resistance, generating energy loss and reducing velocities that may further encourage sediment deposition. Porcupine systems have seen increasing implementation, especially for bed or bank protection, but were also recently implemented in a channel-control pilot project on the Ayeyarwady River in Myanmar. Many porcupine systems to date are designed by trial and error due to lack of quantitative design criteria. The 2019 pilot project used large-scale numerical modelling of resistance areas to evaluate the potential impacts of porcupine structures; however, how to best incorporate the impacts of porcupine structures on flow and sediment transport in numerical models has not been systematically evaluated. Improved models or better estimates of uncertainties could facilitate future porcupine system design. Therefore, this study examines methods for incorporating porcupine fields into numerical models and interpreting those models to make informed design choices. Data and observations from the 2019 pilot project implementation, a 2018 porcupine flume experiment, and extensive literature review have been used to examine the expected hydrodynamic and morphodynamic impacts of porcupine systems that need to be accurately captured in large-scale numerical models. A 2DV model using two resistance formulations representing porcupines was elaborated and tested against experimental porcupine performance. The Uittenbogaard (2003) rigid cylinder model was found to replicate well porcupine behavior for less dense systems under limited flow conditions; however further verification is needed for more dense systems, variable flow regimes, wider boundary conditions and mobile beds. The 2DV model was collapsed to 1DH, using two different resistance formulations, to examine the strengths and weaknesses of large-scale models in predicting porcupine performance. The Baptist (2005) resistance model was found to best represent porcupines; however, parameterization was not straightforward from porcupine geometry, indicating that further studies will be needed to confidently translate porcupines into `rigid cylinders' for input into the model. In addition, neither the 2DV nor 1DH models were able to capture the hydrodynamics at the leading and trailing edges of porcupine fields, which were found to be critical for design considerations from the pilot study data. Therefore, these important limitations need to be taken into account when interpreting model results and examining how to improve them in the future.

In the past years, climate change has caused an increasing number of extreme weather events. In the Netherlands this is, for instance, reflected by the growing amount of extreme rainfall events. This creates big issues for water management authorities like water boards. Water boards are often responsible for local streams, that cannot handle the peak discharges caused by extreme rainfall. This raises the question whether measures have to be taken, for instance by creating additional storage areas to decrease discharge peaks. For questions like these computational model studies can be used to gain insight in the distribution of discharges and water levels.

During these model studies often one-dimensional models are used. Recent developments made two-dimensional modelling, including for instance the simulation of inundation, possible for regular users like water boards. Two-dimensional modelling is proved to be more accurate for rivers, compared to one-dimensional modelling. Water boards, however, often deal with water systems of smaller scale. This raises the question whether it is worth for authorities like water boards to invest their resources in this new modelling approach. Several physical system properties are of influence on the answer to this question. Therefore, the main question of this research is:

Which physical processes are of influence to the choice of a modelling approach, one or two dimensional, for the modelling of stream systems in particular situations?

The methodology of answering this question consists of three major parts:

1) identify possible differences in the model properties of 1D versus 2D modelling;
2) analyze the output of test models to see whether the differences in model properties actually influence the effects of physical processes; and
3) analyze a case area to see whether these processes are important in practice.

The research scope consists of the hydrodynamics of systems with a length scale of 10^3 – 10^5 m and a temporal scale of hours to a few days. This range corresponds with a range in discharges of a few cubic meters per second up to a few thousands of cubic meters per second. The choice for this range has been made to be able to analyze the effects between the scales of streams and rivers, as until now mostly rivers have been modelled using 2D modelling approaches.

The starting point of this research has been the analysis of differences in model properties. The used flow equations, numerical methods, computational grids and bathymetry implementation are analyzed and the differences are discussed. Concluding, it can be stated that there are differences, which may be of influence to the model output. The main differences are the number of dimensions on which the flow equations are calculated and the way the bathymetry is schematized by the computational grid.

Using these differences, physical parameters have been specified that were varied in a range of test models. These are mainly the parameters that cause 2D effects, like meandering and varying roughness coefficients. The modelling of flood waves could also cause differences due to the fact that the 1D model is not able to model storage with the used default settings. Lastly, the way the bathymetry is used in the simulations is of great importance, due to the way the bathymetry is discretized in 2D models. The goal of varying these physical properties is to analyze whether these variations would influence the quantity of the effects the differences in model properties are causing. Comparing the output of both a 1D and 2D model, it appeared that there are significant differences in the observed water levels. Generally speaking, the following observations were made:

1. The smaller the simulated spatial scale, the larger the 1D-2D differences are
2. The higher the meandering intensity, the larger the 1D-2D differences are
3. The lower the summer bed roughness, the larger the 1D-2D differences are
4. The larger the 2D grid cell size, the larger the 1D-2D differences are
5. The smaller the flood wave duration, the larger the 1D-2D differences are

The underlying physical processes have been analyzed to see how their influence varies in different situations and for different spatial scales. In the end, six processes were found to be influencing the 1D-2D differences:

1. The effects of grid size on bathymetry discretization
2. The water level and velocity difference in cross-sectional direction for varying roughness
3. The water level and velocity difference in cross-sectional direction for varying meandering intensities
4. The interaction between summer bed and floodplains
5. The Boussinesq approach and consequences for the summer bed – floodplain flow area ratio
6. The effects of storage

When combining the findings of the test models and the analysis of a small scale stream system, it was found that the above processes are of the most influence on the following situations:

1. In situations with a complex bathymetry the process of grid discretization is having the most influence.
2. In situations with a high meandering intensity two physical processes are playing a role: high water level and velocity gradients in lateral direction, and a high summer bed – floodplain interaction.
3. In situations with a very low roughness compared to the surrounding area two physical processes are playing a role: high water level and velocity gradients in lateral direction, and a high summer bed – floodplain interaction.
4. In situations with a lot of elevation changes and open water bodies, a lot of potential storage is present.

In these four situations a 2D approach is preferred, as the physical processes that are present in these situations are all processes that are poorly implemented or represented by a 1D model. Of course, there are also some more practical considerations to be made about the modelling choice. The most important one is the research goal; whether inundation modelling is desired and what the required model output is. Another important consideration is the available time for running the simulations; 2D models have a significantly longer simulation time compared to 1D models.

All things considered, this research provides an overview of the differences in model properties between 1D and 2D modelling approaches, an analysis of the important physical processes that determine the choice of a modelling approach and some recommendations about practical issues one could encounter when performing a modelling study.

In the proximity of the Eastern Scheldt storm surge barrier (ES-SSB), one of the largest hydraulic structures of the Dutch Delta Works, scour holes have developed that potentially threaten the stability of the structure. Although, these scour holes were anticipated in the design phase recent bathymetry measurements show that they are growing deeper than initially predicted. Uncertainties in the predictions of the equilibrium dimensions partially arise from the presence, thickness and erodibility of clay layers, but first and foremost from a lack of understanding of the flow conditions in the vicinity of the scour holes. High flow velocities are still observed near the bed in the scour holes, that can be associated with high bed shear stresses. This implies that an equilibrium state has not yet been reached and that the scour holes still become deeper. The presence of these high flow velocities is not fully understood, making predictions on the development of the scour holes and mitigation strategies difficult. As a result, the risk of geotechnical instabilities increases, which may potentially endanger the stability of the ES-SSB.

The objective of this master thesis is to obtain more fundamental understanding of the flow patterns and turbulence structure in the vicinity of tidal barriers, such as the Eastern Scheldt storm surge barrier, and how these may contribute to ongoing scour. In this thesis, we experimentally investigate the development of a shallow jet that experiences a streamwise increase in depth, with and without the effect of grid turbulence. The aim of this thesis is to obtain additional insights into the fundamental processes in scour holes using experimental results, that can contribute to the improvement of equations and models that estimate scour near hydraulic structures.

Braiding rivers are characterised as highly dynamic, and experience annual morphological changes in plan- form. This dynamic nature of the river leads to navigational hindrance and risk of unstable bifurcation points where discharge distributions might switch. In pilot studies, porcupines have shown promising results in re- tarding the flow and cause sediment deposition near river banks to prevent bank erosion. However the aim now becomes to apply the porcupines on a much larger scale to increase the channel roughness and influ- ence discharge distributions of bifurcation points such that the flow is mainly diverted to the channel where the highest discharge is required. This way the main channel receives the largest discharge and therefore sedimentation in these channels is prevented. Currently it is not known how porcupines should be modelled in a numerical model. It is simply assumed that porcupines can be modelled similar to vegetation which is schematised as rigid cylinders with a certain density, drag coefficient and resulting representative roughness. No measurements are available to validate the assumed roughness of porcupines and if the hydro- and mor- phodynamic behaviour, represented by the model, is true.

In this thesis laboratory experiments are conducted to assess the near-field hydro- and morphodynamic ef- fect of porcupines and generate more knowledge about their behaviour. Experiments with a concrete bottom give a detailed insight in the hydrodynamic behaviour of porcupines without the interference of bedforms and morphological developments. Experiments with a sediment bottom give more insight in the morpho- logical development and flow patterns over time which clearly influenced the initial hydraulic behaviour. Experiments are conducted in a 12 metre long and 0.8 metre wide flume with a recirculating pump. The wa- ter level, discharge, density of the porcupine field and configuration of the field are systematically varied to identify the dependences on the drag and sedimentation/erosion volumes in the near-field domain of the porcupines. Additionally, general observations are performed, describing the flow structures and sedimenta- tion patterns in and around the porcupine field.

From fixed-bed experiments it is observed how the flow is retarded by the presence of porcupines. Flow is pushed around the field in both transverse and vertical direction. Behind the porcupines, longitudinal flow vectors are downward directed and the flow velocity near the bed is significantly reduced. It is observed that staggered porcupine grids help to retard the flow stronger and captures sediment behind the field in wider strokes. Non-staggered grids only work effectively in the line of porcupines. Between those lines barely any retardation is observed and therefore only narrow strokes of sedimentation are observed behind the lines of porcupines. The reduction in flow velocity behind the porcupines is similar to the velocity reductions ob- served in experiments with vegetation. The velocity retardation is gradually restored in longitudinal direction where the effect of porcupines gradually diminishes. For different experiments this deceleration of the flow has been observed and it follows a linear trend line, that by means of extrapolation can be used to quantify the effective retardation length in longitudinal direction. Water level differences over the porcupine field are observed indicating loss of energy, and pushing up the water level upstream. Porcupines can effectively in- fluence bifurcation points in braiding rivers this way. The local water level gradient over the porcupine field, combined with the velocity measurements, is used to determine the drag and representative roughness of the porcupines by using the equations of Baptist. The obtained values for the roughness are validated by sim- ulating the flume in SOBEK with the corresponding roughness coefficients. Comparing the measured water levels with the computed water levels by the model gave a relatively good fit indicating that porcupines can be schematised by the equations of Baptist with a few adjustments. The Reynolds stresses give an indication of the height of the bed shear stresses. Measurements show that the shear stress in the near-bed region behind the porcupines is lower or the same compared to the undisturbed velocity profile. Lower bed shear stresses indicate a reduction in sediment transport and is therefore an important mechanism to reduce the bedload sediment transport.

Mobile-bed experiments show clear erosion patterns inside the porcupine field due to the increased turbu- lence intensity generated by the porcupines themselves. For the experiment with low water levels and high flow velocities erosion is observed to be most severe, whereas in experiments with lower field density an

overall sedimentation is observed inside the porcupine field. Due to the erosion porcupines sink into the bed significantly reducing their effectiveness on retarding the flow. To prevent scour larger spacing between the porcupines seems beneficial. A growing sedimentation ridge behind the porcupines is observed that influ- ences the flow retardation even stronger, enhancing further sedimentation. Once maximum sedimentation height is reached, the ridge will migrate downstream as a growing sedimentation bar gradually sloping down towards its initial bed level. This length scale of the sedimentation bar is comparable with the retardation length scale. Combined with the migration rate of the sedimentation bar an estimation of the time scales can be obtained. Based on the mobile-bed experiments it is concluded that least erosion within the field is beneficial and therefore low water levels and high flow velocities should be avoided.

Although initial results show that porcupines show similar behaviour compared to vegetation and that the roughness can be estimated by using the equations of Baptist it has become clear that there are still major differences between the behaviour of vegetation and porcupines. Therefore further research is required to improve schematisations of porcupine behaviour, especially to improve the schematisation of the sediment transport around porcupines since no descriptions are yet available.

Despite the many positive effects of dams, they can also have several negative impacts on natural systems. One of these impacts is the decrease in sediment inflow towards the coast which can lead to coastal erosion.
A technique that can be applied to improve the situation downstream is modifying the discharge through the dam. Unfortunately, there have not been many studies done yet on such a re-operation regime. This thesis tried to get more understanding of the possible consequences of a modified discharge regime concerning restoration of pre-dam sediment regimes based on the Volta Delta in Ghana as a case study.
In this thesis, the effect of modifying dam operations on both the upstream part of the river (the reservoir) and the downstream part have been investigated. For the upstream section, a simulation was made for a simplified model for a reservoir with similar characteristics as the Volta Lake. According to this model, the modified dam operations are not useful for reservoirs like the Volta Lake. The technique might be useful for shallower reservoirs.
A modified dam operation scenario aims to reintroduce flood pulses to the river. For the downstream part of the river, three aspects of the operation scenarios have been investigated: the discharge peak height during an operation, the duration of such a peak and the extra sediment load that might be brought to the river from the reservoir. The response of the river downstream was measured by two aspects: the sediment load to the ocean and the bed level changes in the river.
To show the response of the river, several simulations were made with different aspects of the operation scenario. At each simulation, only one parameter was changed, e.g., the peak discharge is changed from 1500 to 5000 m3/s in different simulations. By doing so, graphs could be made to show a correlation between the three operation aspects and the two aspects of the river's behavior.
It turns out that by increasing the discharge, more sediment will be transported to the ocean (with a maximum of 16% of pre-dam sediment load). There will be more non-cohesive material carried to the sea than cohesive material. When there is no extra sediment coming to the river, the river bed will degrade. This will happen at equal rates all along the river which will not change the bed slope.
By increasing the sediment load from upstream of the dam without introducing flood pulses, there will be more sediment transported to the ocean. However, most of the sediment will settle at the river bed causing accretion. The accretion of the river bed will not happen at equal rates along the river so the bed slope will change.
Finally, the change in peak duration will affect neither the transport of sediment nor the river bed. The reason for this might be that the river bed adapts to the different flow conditions in such a way that the same amount of sediments is transported towards the coast.

In this research two problem statements are defined, namely (1) the validity of the assumption that groynes and weirs are equivalent in hydraulic models, and (2) the difficulty of modelling groynes and weirs in the 3D numerical solver package OpenFOAM. Using the incompressible 3D Navier-Stokes equations solver for multiphase phase flows, the so-called interFoam solver un- der OpenFOAM framework, two different models are set up for tackling the two problem statements.

The interFoam evaluation model addressed the issues of computational mesh types which affect the simulation results, and the methods for regulation of water level at the downstream boundary. The two types of the meshes which are used in the research are structured non-orthogonal mesh and unstructured more-orthogonal mesh. The benchmark test is the flow over the weir in the flume. The use of a non-orthogonal mesh could simulate the flow separation which is found downstream of the weir. However the unstructured meshes which are composed of more orthogonal parts and less non-orthogonal parts, could not reproduce the flow separation in 2D simulations correctly for high and small specific discharges. For the 3D simulations, the unstructured meshes apparently simulate the correct flow profile for low specific discharges. In this interFoam evaluation model the control structure or gate is used for maintaining the water level at the downstream boundary. In other words, it uses the physical method of water level boundary condition.

The weir-groyne comparison model is developed with different settings from the former model for the assessment of the differences between weirs and groynes in open-channel flumes. The water level is regulated by a trial-and-error discharge adjustment method making use of inlet and outlet tanks. The mesh type used is the unstructured more-orthogonal mesh in 3D simulations since the severely non-orthogonal meshes introduce numerical artifacts. A variety of weirs and groynes with different slopes are simulated for the comparison of energy losses and streamlines patterns. From the simulation results, the streamlines of the flow over the weir and the groyne are found to be different and the energy head losses also differ between these two structures.

For recommendations, to evaluate the skill of the interFoam solver the 3D simulations for unstructured meshes should be carried out to make sure that they can reproduce the flow sep- aration correctly. The use of a mathematical boundary condition for defining the water level is recommended to avoid the extra computational power for the control structure. To acquire the knowledge of flow around groynes and weirs, the experimental set-ups are recommended, and well-proved non-linear k − ε and k − ω turbulence closure models should be applied in the numerical modellings.

The Ayeyarwady River is the most important river in Myanmar, connecting the cities Mandalay and Yangon, making inland water transport economically attractive. The Ayeyarwady River has a dynamic character, caused by the significant discharge difference between the dry and rainy season, and the related suspended sediment transport. Point bar formation in inner bends causes flow deflection towards the opposite riverbank. This causes local instability with as a result the banks to retreat. Vegetation contributes to the strength of riverbanks, by increasing the shear strength of the soil through roots and increasing hydraulic resistance, reducing flow velocities near the bank. However, to what extent the vegetation on the Ayeyarwady riverbanks contributes to this stability is debatable. Therefore it was the objective of this thesis to obtain a better understanding of the effects of vegetation on riverbank stability. By means of a fieldwork photo material of the river reach between Mandalay and Pakokku was collected, which was used for the NDVI validation. NDVI is the Normalised Difference Vegetation Index and is a remote sensing indicator for the ‘greenness’ of vegetation on satellite imagery. More vegetation means a higher NDVI value, hence it provides information about the livelihood of the vegetation present on the subsoil. In combination with determined bank retreat rates from a yearly comparison of satellite images in Google Earth, it was examined whether higher NDVI values, obtained with the Google Earth Engine, resulted in reduced bank retreat rates and therefore if NDVI can be used as a bank stability parameter.
Nine regions of bank retreat were both altogether and separately examined, but the graphs showed no definite answer of retreat rates being dependent on NDVI. Therefore, the results of the riverbank retreat analysis were categorised based on location, erosion mechanism, the slope, and vegetation classes. The areas where fluvial entrainment was the primary erosion mechanism showed a clear trend. When NDVI was smaller than 0.2, maximum bank retreat rates appeared to be 200 meters per year. When NDVI was higher than 0.2, bank retreat rates did not exceed 80 meters per year. On satellite images, vegetation was not observed in these areas, so the influence of vegetation remained questionable. In areas where mass failure caused bank retreat, no reduction in bank retreat was found. The results showed considerable scatter, although much more vegetation was present on these banks. Water level variability played a crucial role in the evaluation of the net effects of vegetation on riverbank stability. During low water, vegetation cannot provide the positive impacts, especially on steep river banks.
It is not possible to identify vegetation types from NDVI records only. NDVI also does not show which erosion mechanism takes place. This makes riverbank stability difficult to predict by using NDVI only, and therefore, NDVI does not seem to be an appropriate estimator for the additional effects of vegetation on riverbank stability. However, it can be used in combination with other remote sensing techniques to identify healthy vegetation areas and to make roughness estimations in river planform analyses.

The Ayeyarwady River (also called Irrawaddy River) is the most important river of Myanmar and due to the country’s rapid development it is expected to become even more important. The river flows roughly from north to south through Myanmar and is very dynamic and mostly unregulated. With a length of 2170 km and an over the year average (highly seasonally varying) discharge of 13’000 m3/s into the Andaman Sea (Bhardwaj, Owen, & Leinbach, 2012), the Ayeyarwady is one of the bigger rivers in Asia. To more than before take into account the interests of different stakeholders, as well as ecological aspects, sustainable management of the river is needed. Understanding the key aspects of the river flow can be a first step to sustainable river management (Richter et al., 2003). Pollution due to a large variety of activities of different nature make that water quality monitoring is of high importance (Thanda Thatoe Nwe Win, Bogaard, & Van de Giesen, 2015). For monitoring and modelling the water quality, information about the mixing of tracers trough the river is needed, which can be quantified with the use of dispersion coefficients. Little research has been done about the Ayeyarwady River in general considered its size and importance. Very limited data about the mixing of tracers and the parameters needed to estimate the mixing of tracers was available. This research focuses on the situation around the Ayeyarwady-Chindwin confluence in the first week of February 2017 (dry season). Hence, there is a very different situation during for example wet season. For the water quality, mainly the mixing in the longitudinal direction (direction of the main river flow) is of interest, which can be quantified by a longitudinal dispersion coefficient (Kx). First relevant parameters for estimating Kx were identified based on the theory. This appeared to be the discharge, roughness and bathymetry. Besides, Kx has to be calibrated by floater experiments. To get better insight into the magnitude of these parameters, flow velocity and depth measurements (needed for estimating the discharge, roughness and bathymetry) and floater experiments have been done during a week of fieldwork in the area. Due to loss, theft and destruction of floaters, less data was collected than planed. To get further insight in the mixing of tracers, a numerical model was made in the software Delft3D based on data collected during the fieldwork. Based on the combined results of the theory, measurements done during the fieldwork and the Delft3D model, it is expected that the magnitude of Kx in the Ayeyarwady River is somewhere in between 50-500 m2/s (best estimate: Kx~300 m2/s), although this has to be confirmed by further research. When the found value is compared with values found for other bigger rivers this value for Kx appears to be somewhat on the low side. From the Delft3D model runs follows that the longitudinal dispersion coefficient in the Chindwin River is higher than in the Ayeyarwady, possibly even a factor 10. Besides, insight in the effect of the different parameters on the dispersion was obtained, contributing to a better understanding of processes causing the mixing of tracers in the Ayeyarwady and Chindwin rivers. Estimating the highly sensitive longitudinal dispersion coefficient (Kx) appeared to be challenging, mostly due to the remote and highly dynamic character of the area. To make a better estimate of Kx, the uncertainty in the parameters needed (discharge, roughness, bathymetry and spreading of floaters for calibration) has to be reduced. Although some modelling options in Delft3D could be tried to narrow the range of these parameters, the best option to reduce this uncertainty is collecting more (high quality) data in the field.

Rapid hydropower development is taking place in the Mekong river, which is expected to change the natural river regime. Because of the ecological and economical importance of the Mekong delta, the effects of these changes could have large consequences in the delta area. This research focuses on the hydrodynamical processes and assesses potential risks of changes to these processes, caused by hydropower development, could have on the ecology and socio-economy. Furthermore, a number of mitigation measures in the delta have been proposed to reduce the potential risks.

Worldwide, dam reservoirs loose over one percent of their storage capacity every year due to sedimentation. Proper management is therefore urgent. While drawdown flushing is considered an effective method to remove sediments from the reservoir, the increased sediment flux has an impact on the downstream environment. On the short term, several species suffer high mortality rates due to increased turbidity. Also on the long term the impacts might be significant. The deposition of fine sediments can change the habitat suitability and thereby jeopardize the conditions for recovery.

The aim of this research is to evaluate the effect of different flushing operation scenarios on the physical habitats at the ecologically relevant meso-scale. An idealised, depth-averaged hydro-morphodynamic model is set up in Delft3D-Flow, a modelling software package developed by Deltares. The model represents a reach of the Avisio, a river situated in the Eastern Italian Alps. Across this river, the Pezzè dam was built in 1952, trapping all incoming sediments. Every three years the dam is flushed, which gained more public attention over the last years. A specific reach was chosen as a reference case as it consists of a channel bar topography where fine sediments accumulate up to ten times more during a flushing event than in other, more channelised reaches. It is therefore considered the most affected by the flushing event. The reach has a length of roughly a kilometre and is situated 10 kilometres downstream of the dam.

The hydro-morphodynamic model assumes a non-erodible bed, as the coarsened river bed is not expected to move significantly during a flushing event. This bed has the shape of an alternate bar topography. The simulation of the flushing event is simplified as an influx of bed load transport with a fraction size of one millimetre, and a magnitude close to the equilibrium transport capacity. Deposition occurs upstream and to a higher extent downstream of the bars, where flow velocities are low and secondary flow aids the movement of sediments into these areas. Subsequently, a clean water peak is imposed to investigate its effectiveness in removing the fine sediments from the reach. The hydrograph of this peak is varied in shape, duration and magnitude to simulate different operation scenarios and natural rainfall runoff events.

By applying the Mesohabitat Evaluation Model (MEM), a habitat suitability model developed for the meso-scale, it was possible to divide the reach into classes based on flow velocity, flow depth and shear stress. Such classification highly depends on the governing discharge. The classes are mainly distinctive by the division between high and low energy classes and in this way show a high correlation to the deposition of fines. The model suggests that fine sediments remain in the system only when low energy classes are present. Although the MEM classification aims at the meso-scale, it follows a micro-scale approach and therefore undermines the advantages of assessment at the ecologically more relevant meso-scale. It is recommended to develop the MEM-procedure by accounting for neighbouring computational cells.

The MEM-procedure gives an useful indication of the spatial variety in deposition and erosion patterns. It however does not provide insight into the implications of sedimentation to the ecology, without the coupling with a biological model. Such a biological model describes the suitability of the physical habitat for a specific organism and thereby incorporates a functional goal.

As such a biological model could, due to time restrictions, not be applied in this research it was chosen to perform a micro-scale based suitability study. This illustrates the potential of morphodynamic modelling as a tool for habitat suitability modelling. Simplified preference curves were derived for spawning trout, of which the physical habitat requirements are sensitive to the deposition of fine sediments. It was found that the deposition of fine sediments hardly effects this habitat. When required nonetheless, any considered peak flow recovers a substantial amount of suitable habitat. This implies that, if a clean water peak of a sufficient magnitude follows the flushing operation, the impact on the spawning habitat, and probably any habitat, is minimal.

Whether such a clean water peak occurs, can be partly controlled by the dam operation, but also depends on the hydrology of the catchment. A ten-year hydrological time series of the Avisio river, measured upstream of the dam, shows that the catchment of this reservoir does not provide sufficient water to guarantee a clean water peak during any season. However, with the significant contribution of two tributaries that flow into the Avisio river between the dam and the reference reach, it is highly probable that a peak flow of sufficient magnitude occurs during high flow season. To ensure the benefits of a clean water peak, it is recommended to plan the flushing event at the start of the high water season, which lasts from May to July. For other rivers, it might be possible to adopt such a clean water peak as part of the flushing operation strategy, providing a higher level of control. Even though this study suggests that the implications of the flushing event to the physical habitats are minimal, and of no comparison to the potential direct impacts, this method of evaluation shows potential to assess other morphological relevant events and even long term morphological changes.

The use of rivers for navigation and the increased human activity along their banks generally requires river control and improvement measures. Most rivers have a natural tendency for continuous change of alignment, e.g. meandering and braiding rivers. Construction of bridges, towns, berths, etc. have required fixation of the river alignment at many places, changing the natural morphology of the rivers. This might lead to bank erosion, erosion around bridge pillars, sedimentation of navigation channels, etc. Adequate measures against this requires a reliable prediction of the morphological changes.
For simulating the morphological effects in bends there are two different major factors involved that have been described by several scientists: bed slope effects and spiral flow. For modelling morphological development of a river bend several tests have been done on two different cases that have been researched in a laboratory flume (Delft Hydraulics Laboratory (DHL) and Laboratory of Fluid Mechanics (LFM)) in the 80s. In this research the effects of the major characteristics on the bed development of the bend are examined. This has been done by varying the different input parameters that have influence on the secondary flow and the bed slope effects. Subsequently the varied input files are used to model the same bend with Delft3D 4 Suite, D-Flow FM with an unstructured grid and D-Flow FM with a structured grid. In this way the differences are shown between the different kinds of modelling of the same input parameters.

The parameters that have been tuned are Ash, Bsh and Csh for the bedload transport factor that is influencing the bed slope effect. The other parameter is Espir that influence the amount of spiral flow in a bend. The last is αcal that is a multiplication factor in the sediment transport formula from Engelund-Hansen in Delft3D 4 Suite. After optimising these parameters it was not possible to reproduce the flume experiments. Reason for this is probably a simplification in the numerical calculation because with similar parameters of Struiksmas modulation [6] in 1985, which reproduce the flume well, it was still not possible.
To improve the reliability of the model it is recommended to study the following aspects:
• Improvement of the inflow boundary conditions, to improve the way in which water and sediment flows into the system.
• Improvement of the numerical modelling, to create a model that can simulate the characteristics of the river bed in a better way.
• Look for test cases which are close to reality to see if the updates in the model are truly simulating the reality.

By improving these points, the morphological changes might be predicted in a better way than it is in the current situation. Also it will be possible to have less crashes during the run of simulations.

Vegetation on river banks and bars contribute to the stability of these morphological features; however, the exact processes that contribute to this stabilization are still subject to research. This study has looked into the flow phenomena at the streamwise interface between a vegetated and non-vegetated river bed. Flow velocities in vegetated flows are lower than in non-vegetated flows under otherwise similar conditions due to the increased drag by the plants. Therefore, a shear layer develops across the interface. Within this shear layer, Kelvin-Helmholz like vortices develop, inducing exchange of mass and momentum through the interface. These vortices are present in the water column, but their strength near the bed governs sediment transport mechanisms, which are the basis of morphological activity. The objective of this study was therefore: to obtain a better understanding of the coherent flow structures in the interface between vegetated and non-vegetated river beds and the effect of these structures on sediment transport mechanisms. This has been done in a series of flume experiments with a compound channel.