Braided rivers are among the most dynamic natural Earth systems, with a rapid and complex morphological evolution. Limited understanding and inadequate algorithm implementation of specific processes affect the accuracy of physics-based models. This leads to uncertainties that complicate the effective design of interventions and protection measures. In recent years, artificial intelligence techniques rapidly advanced and the availability of satellite imagery products increased. This research sets a novel attempt to predict the planform evolution of braided rivers with deep learning using satellite images. The study focuses on the middle and lower reaches of the Brahmaputra-Jamuna River in India and Bangladesh. We developed JamUNet, a U-Net-based convolutional neural network (CNN). The model is trained with the Global Surface Water Dataset (GSWD) to classify each pixel as "Non-water" or "Water". Four images from the same month over four consecutive years were used as input. The fifth year image served as target. JamUNet demonstrates a general capability in capturing the planform evolution. Morphological processes like meander migration, channel abandonment, and confluence and bifurcation development are often well captured. However, temporal patterns are lacking. More complex phenomena, like channel formation and channel shifting, remain unpredicted. JamUNet also tends to underpredict the total areas of erosion and deposition. Overall, JamUNet achieves a 5-6% improvement compared to the benchmark method for which no morphological change occurs in metrics such as precision, recall, F1-score, and critical success index (CSI). Among these, recall is the most meaningful metric for evaluating the model performance. JamUNet can serve as a preliminary tool for water management authorities in India and Bangladesh. It can assist in prioritising bank protection in erosion-prone areas and support land reclamation projects and inland navigation. Caution is always advised due to the model tendency to underpredict erosion. More research is required to improve the current model. Nonetheless, deep-learning modelling seems a promising field of research. Testing alternative model architectures and incorporating additional data, such as water levels or river discharge, could improve the model performance. ... Read more

Understanding morphodynamic processes and structures is essential for effective river management and enhancing our knowledge of river systems. River bars can be investigated through theoretical analyses, field measurements, experimental studies, or numerical modelling. While numerical modelling offers accuracy, it is computationally intensive, making multiple simulations or parameter calibrations both time-consuming and impractical. The numerical simulations follow physical laws and equations which make the runtime significantly high making instantaneous results in times of emergencies unfeasible.
Convolutional neural networks (CNNs), a type of data-driven modelling, have been employed to study the physical parameters defined by linear stability analysis. This study utilizes Delft3D simulations to generate diverse datasets, facilitating easier access and variability. A specific type of riverbar pattern, and alternate bars were chosen for simplicity. The CNN model takes initial bed levels as inputs and provides predictions for the next step of bed level or a time series, with velocity included an additional parameter to assess its influences on the model performance. The model is able to predict the bar behaviour with R2 being 0.99. The model can predict bar suppression or migration solely based on the initial bed levels provided. The model performance did not improve with an additional input parameter although this possibility can be explored with other architectures. However, the model currently lacks accuracy in making one-step-ahead predictions, potentially due to boundary issues within the numerical model or the CNN itself. Further optimization and exploration of additional methods are necessary. The integration of physical parameters into the training process may improve prediction accuracy. Strong conclusions cannot be drawn until additional research is conducted. ... Read more

The Netherlands, Germany, and Belgium were hit by heavy and prolonged precipitation in July 2021. As time passed, weather warnings escalated, leading to evacuations due to predicted floods, including in the Rur catchment. It was difficult to forecast the flooding of the Rur, raising the question of which elements are crucial in a flood predictionmodel for the Rur river. This question is addressed by addressing both a hindcast of the 2021 flood event and creating forecasts based on the weather forecast of July 13, 2021.

The Rur river basin is characterised by topographic and geological variations, with the steep Eifel responding differently than the flat lowlands, and human intervention in the form of reservoirs and lignite mines. A hydrological Wflow_SBM model has been derived for the Rur river basin, encompassing these characteristics, along with a hydrodynamic ProMaIDes model for the downstream reach of the Rur. These models were compared to investigate various aspects: river routing, floodplain flow, tirbutary interactions, the influence of reservoirs, and the impact of reduced groundwater levels.

The results of the 2021 floods indicate that modelling flows in floodplains is crucial to shaping the flood wave, both in tributaries and the Rur itself. Additionally, the reservoir played a significant role in attenuating the flood wave, with the increase in the outflow of the reservoir primarily affecting the tail of the wave. The reduced groundwater level was simulated by adding a leakage termto the saturated subsurface zone, whose indirect effect is significantly greater than the leakage termitself. Moreover, the tributaries Worm and Inde, particularly, are influential in the Rur’s discharge. These characteristics are also evident in the simulated forecasts, although the spatial and temporal resolution is significantly lower for these meteorological predictions.

Finally, the characteristic response of the Rur demonstrates that not everymodel type is equally practical for flood forecasting. The dominant flow from the reservoirs is highly regulated and is unlikely to induce inundations downstream. Complex flow patterns in floodplains only become relevant in the Dutch Rur, which makes two-dimensional modelling particularly valuable here. Therefore, it is recommended to use a one-dimensional discharge model, incorporating delay effects from winter bed flows. When predicted discharges at the Stah station are exceeded, two-dimensional simulations may provide a solution, the model area reduced to the Dutch Rur, focussing on predictions where a critical value related to floodplain capacity (Qlimit = 300 m^3/s) is exceeded. ... Read more

The Rhine-Meuse estuary is part of the Southwestern Delta area in the Netherlands. The nature present in the estuary suffered a great impact due to urbanization, as a natural delta transformed into an industrialized river, mainly due to port activities. Various parties decided to collaborate in order to return the tidal nature in the Rhine Meuse estuary through constructing tidal parks. Although the idea of reintroducing tidal nature into an industrialized estuary seems promising, problems arise when the projects are being executed. Human interventions necessary to complete the design of the tidal parks often lead to undesired morphological changes. These either consisted of an excessive amount of cohesive sediment import, or hardly any import. Consequently, maintenance in form of dredging or nourishing activities is required. When introducing tidal nature, little maintenance is desired, both from a financial perspective and from an ecological perspective. The undesired morphological changes result from limited knowledge on the impact of human interventions on the hydro and morphodynamics in tidal parks. This research therefore primarily focuses on gaining new knowledge on the hydrodynamics and morphodynamics inside tidal parks and assessing the effect of different elements such as groynes and longitudinal walls on the latter. This is done through a case study, the Groene Poort, a collective name for the tidal parks located in the Nieuwe Waterweg.

A depth-averaged (2DH) Delft3D model is used to research the hydro and morphodynamics of the tidal park. A constant tidal forcing and a constant fresh water discharge are used as boundary conditions. The effects of wind, waves and density driven currents are neglected. The effects of changes in hydrodynamic forcing, including a morphological tide, a spring tide and a spring tide in combination with a surge on the morphodynamics are determined. This research showed that the tidal flow results in relatively small velocities inside the studied tidal park with mean velocities around 5 cm/s. The critical velocities for transport of noncohesive sediment transport are only reached at the west entrance of the studied tidal park. At this location the sediment deposits, further transport towards the river bank is not possible resulting from the magnitude of velocities. A spring tide is responsible for an increased import of noncohesive sediment at the river bank area.

This research provides a guide on the effect of relatively small adaptations in geometry which will enhance or decrease the natural processes of sedimentation and erosion in the tidal parks. Tables are included providing the important parameters both with respect to the hydro and morphodynamics of various adaptations in geometry. These tables can be used in future design and adaptations of already constructed tidal parks. However, the effect of the surrounding area including the location in the river greatly impact the hydrodynamics and morphological development of the tidal parks.
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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. ... Read more

Large-scale measures within the river programmes such as Room for the River, Natura2000 and Water Framework Directive have increased the biodiversity in Dutch rivers and have achieved a more natural landscape. However, recent river programmes have shown conflicts between safety against flooding and riverine nature rehabilitation with maintaining navigational water depths. Therefore, an integrated approach is called out upon to improve the river management within these programmes. Conventional groynes are typically used to improve functions within the river programmes, whose primary function is to maintain navigational water depth. Groynes are transverse structures in which large turbulent structures are observed around the groynes. Consequently, significant bed shear stresses are developed, leading to significant local scour. Investigations for improving alternative groyne configurations are often studied for optimizing the groyne structure. The flexible groyne is considered an optimization of the conventional groyne structure. The flexible groyne consists of steep slopes and has a permeable characteristic. Although a fair amount of research has already been carried out on various groyne configurations, detailed flow characteristics around and through permeable, sloped groynes are limited. Hence, this research aims to numerically quantify the effects of permeability and head steepness of groynes. A literature study is executed to investigate the critical processes required for capturing within the numerical model to answer the research question. From the literature study, it appears that the most important processes are the non-hydrostatic effects, adapting large turbulent structures and implementing the porous zone using a non-linear function. Multiple software packages have been analyzed. Fluent is chosen, which is analyzed to capture the required processes adequately.
A new numerical model has been set up for investigating the research question by simulating flow around groynes. The model is validated against three experimental studies for various characteristics. The important mean flow characteristics have been validated within an acceptable range. Still, it appears that the numerical model tends to underestimate the mean streamwise flow velocities, overestimate the Reynolds shear stresses and shift the peak values of the Reynolds shear stresses downstream. Four configurations are identified for the simulations of varying head steepness and porosity. It appears that the increase of the porosity reduces the large turbulent structures and bed shear stresses close to the groyne and shifts the peak values of the Reynolds shear stresses, and bed shear stresses further downstream. The porosity reduces the maximum Reynolds shear stresses and bed shear stresses compared with the Reynolds shear stresses, and bed shear stresses for an impermeable groyne. These reductions are because of the momentum exchange between the free flow region and the flow through the porous structure, which reduces the mean flow velocity in the free flow region. For decreasing steepness, the large turbulent structures and bed shear stresses are observed close to the groyne due to increasing deflection. The flow appears to follow the geometry of the sloped groyne more smoothly. This research is seen as a first approach for modelling a porous, sloped groyne. Further improving and analyzing numerical modelling for porous, head sloped groynes are advised to increase the model's accuracy. Furthermore, more simulations for varying the head steepness and porosity is suggested to improve the relations between the increase of porosity and head steepness for the flow characteristics. In addition, including sediment transport models within the model is expected to increase the understanding of the hydrodynamics and morphology around these specific groynes. ... Read more

The Gulf of Mottama, located in the southwest of Myanmar, is home to morphologically interesting processes. An entrance of around 100 km wide narrows into a funnel­shaped bay towards the Sittaung River in the north. The entire estuary region is subject to strong dynamic morphological activity which is alleged to be driven by the large tidal energy and sediment inputs. The dynamics of the tidal channels in the Sittaung estuary have resulted in rapid bank erosion of up to 3 km/y. The Sittaung estuary harbours a unique set of characteristics. This has made the question of which processes and mechanisms cause the bank erosion a complex one for which no clear cut answer is currently available. The knowledge of the main processes and mechanisms is valuable to improve the understanding of morphological functioning in dynamic estuaries. The main research question has thus been formulated as: • What is the effect of the large incoming tide and storm events on the bank erosion of the Sittaung estuary and to what extent can this erosion be simulated with a numerical model? The research objectives have been achieved through a literature analysis, system analysis, satellite analysis and modelling simulations. Main conclusions of the research have been as follows: The satellite analysis has shown little to no correlation between the wet season discharge increase and the bank erosion rates. It also showed large differences between erosion rates at different heights of the estuary. Modelling results indicate a prominent role but cannot substantiate a quantitative impact of the tidal forcing and associated tidal bore. River discharge fluctuations were shown to have little effect on the bank erosion. • The results of both the satellite and numerical modelling analysis have shown no additional implications on the dynamic morphology caused by large incidental storm events. Long­term simulations with respect to the bank erosion resulted in several hindrances. Heightened levels of channel incision occurred through a fault in the numerical schematization. The representation of bank erosion in the model has also been subject to stalling. ... Read more

This experimental study is about the bed-load transport over inlet sills of longitudinal training walls. The focus is on the flow structures around the inlet: the shape, the processes and the effect on sediment transport. Groynes have been replaced by longitudinal training walls in an 11-km long pilot project to optimize training of the river Waal in the Netherlands. These training walls have many advantages, making it worth exploring the performance of these new river training structures further. The inlet sill affects the discharge and sediment transport distribution and hence the morphological stability of the system. However, currently operational morphodynamic models cannot reliably compute the sediment flux from the main channel towards the sheltered side channel behind the longitudinal training wall. Therefore, the research aim was to develop a description of the sediment transport over the inlet sill of these longitudinal walls for implementation in two-dimensional depth-averaged river models. Since knowledge on the flow structures and validation data lacked, a series of flume experiments with the inlet area has been performed. This study shows that the blocking effect due to the sill that functions as obstacle is dominant at the upstream side of the inlet and helical flow due to curvature at the downstream side of the inlet are the dominant three-dimensional flow structures at the upstream side of the sill. Further research is needed for the identification of the shape of the flow structure due to blocking. ... Read more

Like many rivers around the world, the Loire river in France has a history of human interventions in order to facilitate navigation and port development. Next to affecting bed levels directly, the heavy modification of the river-estuary has induced significant changes in the hydrodynamic and morphodynamic behaviour of the Loire. As a result, the Loire river is experiencing an ongoing bed degradation. In the estuary, the interventions have led to an amplification of the tidal amplitude, a shift of the tidal and salt intrusion limit in upstream direction and a larger tidal asymmetry. According to Winterwerp and Wang (2013), this tidal deformation leads to an increased import of fine sediment in the estuary and a reduced hydraulic drag. This in turn enhances tidal deformation further. Due to this positive feedback mechanism the Loire estuary has evolved into a hyper-turbid state, associated with large suspended sediment concentrations and the formation of fluid mud.

The behaviour of the river-estuary on large temporal and spatial scales and its response to historical and possible future interventions is not yet sufficiently understood. Therefore, an idealized, computationally efficient, three-dimensional morphodynamic model of the river-estuary is developed with the FLOW module of the Delft3D software suite, in which the main processes influencing the morphology of the Loire river-estuary are included.

The current estuarine geometry, lacking the presence of large intertidal areas, induces a flood-dominant tidal signal. In the mouth of the estuary, the baroclinic pressure gradient introduces a mean landward-directed velocity near the bed and amplifies the mean seaward-directed velocity near the surface. This gravitational circulation causes the formation of an Estuarine Turbidity Maximum at the tip of the salt wedge, which is strengthened further by the flood-dominance of the tide. Model results confirm the feedback mechanism between tidal deformation, the import of fine sediment and the effective hydraulic drag as described above. However, next to tidal deformation, strengthening of the gravitational circulation also plays a large role in this mechanism for the Loire.

To mitigate bed degradation, several measures are simulated that decrease the transport capacity of the flow, increase the sediment supply or do both. Over the first 10 years, sediment nourishments are the most effective according to the model. However, after 100 years of simulation the removal of all groynes present within the domain leads to the most sedimentation relative to the reference scenario. To decrease the import of mud into the estuary, two measures have been investigated. Increasing bed levels in the main channel of the estuary mainly leads to a decrease of the tidal range, whereas restoring tidal flats turns the tide from flood- into ebb-dominant. In both cases, a decrease of the salt intrusion length and a significant weakening of the gravitational circulation occurs, leading to a tidally averaged export of fine sediment and very low mud concentrations.
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The goal of this study is to examine the feasibility of this solution, from a water quality and hydraulic point of view. Firstly, the current state of the three water bodies was investigated and a stakeholder analysis was conducted to look into the social and political context. Secondly, the effect of the solution on the water quality in the WL and TLR was researched. The most important water quality parameters were qualitatively discussed and after that, the quantitatively changes in the WL were modelled. A convection-diffusion model was set up for different parameter concentrations in R. The initial parameter concentrations were gathered by field measurements and extensive online research. The water quality assessment shows that the Biochemical Oxygen Demand (BOD), Chemical Oxygen Demand (COD), turbidity and Total Suspended Solids (TSS) concentrations in the RR have a better value than in the WL. The model itself shows for every parameter that after 100 days of mixing the water is not completely mixed in the WL. The water in the TLR is flushed with WL water and its quality is therefore more or less equal to WL water. It is concluded that the proposed solution improves the water quality in the WL and the TLR. However, long and frequent mixing is necessary for the WL water to reach the RR water quality level. Thirdly, a hydraulic analysis was carried out by investigating the hydrological and geometrical characteristics of the three water bodies. Thereafter, the hydraulic impact was examined by comparing six different flushing scenarios.The flow, water depth and sediment transport rates for different time intervals were modelled in R over the distance of the TLR. A Multi Criteria Analysis was done to interpret the results on the consequences in the TLR. The outcome of the optimum scenario is when the gate has an opening height of 5%. The total transported sediment volume is significantly larger in this scenario, which is beneficial. From a hydraulic perspective the proposed solution is feasible for all water bodies. However, more extensive research on for example the impact on the hydraulics in the TLR is needed to get more conclusive results. As a spin-off, our project (co-)developed educational tools that can be used within the HUNRE curriculum and as awareness raising activities in Hanoi with citizens, schools and the like. Fieldwork with Vietnamese students was conducted to start with building a data base on water quality of the water bodies in Hanoi. The tools that are created are manuals, instruction videos and an introduction lecture. Furthermore, the database, that is built, can be extended with more fieldwork in the future. The extent to which the educational tools and the database integrate in the study program of HUNRE remains unsure, however the awareness among Vietnamese students on the importance of water quality has increased. Furthermore, the OKP project strives to realize the integration of the educational tools in the future. Therefore, this research objective is expected to be achieved. ... Read more

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.
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This thesis presents a new methodology for representing the effect of spiral flow on the bed shear stress magnitude and direction, using a low-vertical-resolution three-dimensional model. Furthermore, it discusses the performance of the new methodology in comparison with conventional high-vertical-resolution three-dimensional and depth-averaged models. ... Read more

The city of Guayaquil suffers from regular floods. During the wet season, typically from late December until late April or early May, multiple floods per week can occur. Mainly the excessive rainfall in combination with high tide penetrating into the city results in a high flood risk, but some flood-prone areas can also flood in case of spring tide only.
The main objective of this research is to investigate the possibility of reducing pluvial and coastal flooding in urban areas by constructing a (semi-permanent) barrier in a sea branch, which retains the incoming tide and creates storage for excessive rainfall. In addition, local storage areas spread over the city are considered to delay stormwater runoff into the sea branches. Based on a system analysis and by numerical modelling, several closure locations and their effects are assessed.
Temporary storage of stormwater behind a barrier in a sea branch is a suitable solution to prevent both coastal and pluvial flooding. Based on the results of this research and possible locations of the barriers, a combination of three selected barriers is most opportune, because all catchment areas adjacent to a sea branch can drain their stormwater in a closed-off part behind one of these barriers. In order for these barriers to be effective, they must be closed during low tide prior to heavy rainfall. All three barriers are able to withhold the stormwater volume from their corresponding catchment areas during a 10-year design rainfall event. Even in the event of the highest possible water level during low tide, being neap tide in combination with the storm surge of El Niño, the storage capacities are sufficiently large. Besides the large-scale and small-scale solutions that are currently considered by the local authorities, they are advised to also consider the intermediate-scale solution presented in this study.
Local stormwater storage in the form of water squares in parks and playgrounds is a small-scale solution to reduce pluvial flooding. The storage capacity of these areas is much smaller than the storage capacity behind a barrier, but it is a solution for low-lying urban areas that are not adjacent to a sea branch or river. When the storage capacity of parks and playgrounds in some catchment areas is not sufficient, underground storage basins can also be considered as local storage areas.
The local authorities are advised to set up regulations on return periods for designing flood risk-reducing structures and to assess the economic losses of floods in urban areas, in order to be able to estimate the acceptable cost of these structures.
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Following from the programme Room for the River, side channels are created in the Netherlands. For their maintenance, alternatives to dredging vessels are needed because of nature development, such as surface screens, that use the flow itself to generate erosion.
A field experiment using surface screens was executed at the small side channel in the Welsummerwaard along the river IJssel for this study.
It turned out the screen has a positive effect by reducing the sedimentation in the channel. This effect is the largest when using a large angle of attack and a large penetration. Also, applying the screen at the bank instead of in the middle of the channel is profitable.
Recommendations are given for Rijkswaterstaat to further develop the concept, including increasing the influence of the screen, for example by working with batteries of screens at each bank. Eventually, this concept could be used for the maintenance of all side channels in the Netherlands. ... Read more

The dynamic nature of braided rivers can lead to river bank erosion, bankline shifts and navigational hindrance. River training measures can be implemented in order to mitigate these problems. An alternative to conventional river training works is the closure of a channel within the braided network. Until recent there has been no systematic research on channel closure and guidelines or recommendations were absent. Recent simulations with numerical models on sand-bed braided rivers investigated both the effects of channel closure around the island and on the braided network. The results of these studies have their limitations and validation with physical models or pilot tests are necessary. Therefore, the objective of this thesis is to obtain a better understanding of the local morphodynamic effects of interventions aiming at channel closure in braided rivers.

This has been done by conducting flume experiments with a simplified section of a braided river with gravel-bed similarity. At first the laboratory experiment was designed such that a stable and reproducible planform developed. A section, consisting of an upstream fixed Y-shaped confluence followed by an alluvial island surrounded by two channels and fixed outer banks, was used to systematically study the local effects of channel closure. The obtained results were analyzed and compared with the numerical studies of Ostanek Jurina (2017) and Schuurman et al. (2016).
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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. ... Read more

The reach of the Magdalena River around the city of Barrancabermeja experiences large issues concerning its navigability since the construction of the Yondó Bridge. The purpose of this study is to investigate possible causes of this poor navigability and to come up with an improvement to the current situation. The influence of bars and scour holes around bridge piles are seen as hypothetical problem causes. A bar mode analysis shows that, for the examined river section, the river contains one alternate bar over the years. It is plausible that the construction of the bridge has induced changes in flow conditions in such way that the original alternate bar started to erode and eventually totally disappeared. Therefore, it is highly possible that the construction of the bridge forms the main reason for the shift of the thalweg. The scour holes around the piers of the Yondó Bridge that were approximated by the empirical method of Melville and Coleman. These scour holes are incorporated in the Delft3D-model to assess their influence by adapting the initial bed elevation profile. From the model simulation it turns out that the presence of the holes does result in more erosion in the vicinity of the bridge.

Three possible solutions (null-solution, groynes and guide bunds) have been weighed using Multi-Criteria Analysis. Based on this analysis the guide bunds appeared to be the most suitable solution. The structure was implemented in the Delft3D-model and some additional simulations proved that the effect of this structure on the hydro and morphodynamic conditions in the river is twofold. First, the guide bunds improve the distribution of the flow over the cross section of the river. More flow is forced through the right side and indeed the flow velocities turn out to be higher at that location. Moreover, the flow velocities on the left side decrease, as expected. However, the structure has an opposing effect on the cumulative erosion and sedimentation. More sedimentation takes place at the right side of the channel, whereas the left side of the channel gets deeper. It can be concluded that the best way to improve the situation in Barrancabermeja, is the construction of a guide bund structure in the vicinity of the Yondó Bridge. However, more detailed (physical) model tests should be performed to gain better insight in the effect of the guide bunds.
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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. ... Read more

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. ... Read more

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. ... Read more