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 Nieuwe Waterweg an ongoing pilot takes place in one of the groyne areas. This pilot is a part of the “De Groene Poort” project. The idea for this project is to create intertidal areas by modifying and nourishing groyne areas. These artificially formed intertidal areas will create a new habitat for wildlife and attract different vegetation. For this project, several modifications have been made to the groyne area: a dam has been placed in front of the groyne area, the groynes on either side have been partially heightened and a nourishment has taken place. To better understand the effects of these modifications to the groyne area, research has been done on the effects of the tide and waves caused by shipping on the flow and sediment behaviour inside the groyne area. Two different measurement campaigns have been carried out for this research. In the first measurement campaign six ADVs were placed in the groyne area between March 24 and April 12 ,2022. These ADVs measured the velocities and pressures with a frequency of 8 Hz. Also, sediment samples were taken during this campaign, which have been analyzed using laser diffraction. In the second campaign, ADCP measurements were carried out with a floating ADCP attached to a jet ski. With this ADCP the vertical velocity profiles have been obtained for several cross-sections at different stages of the tide. It was found that the flow in the groyne area has changed due to the dam and the nourishment. Typical flow in an emerged groyne field with the eddies is unrecognizable during lower water levels in the modified groyne area. The exchange of water between the main channel and the groyne area is through the gap in the dam, which removes the mixing layer that separates the groyne and the main channel during submerged conditions. The flow in this groyne area is lateral and divided into two main flows along either side of the nourished island. Rocks and a higher entrance in the downstream groyne compared to the entrance in the other groyne, gives the groyne area different flow velocities between ebb and flood. The neap and spring tidal cycle enhances Ebb and flood differences in the groyne area even more. These velocities differences also impact the sediment transport and its absolute direction in the groyne area. The dam also helps reduce the wave influence on the groyne area, although these waves still significantly influence the velocities. Waves also propagate in a lateral motion from groyne to groyne through the groyne area. The water can move material from the nourishment, but the spreading of this material is limited to a number of locations in the groyne area. For the largest part, the nourished material is still at the location where it is deposited.

In the Rhine-Meuse delta many scour holes are present. These scour holes form a potential risk on the surrounding bridges, levees, weirs, dikes and tunnels. In general, scour holes can develop when there is a local change in hydrodynamic conditions or erosive capacity. Extensive research has been done in the past on scour holes close to hydraulic structures in a homogeneous subsoil, however, a large amount of scour holes are not related to hydraulic structures. Additionally, in deltas, the subsoil mostly consists of alternating layers of soil with different erosive capacities. This experimental study focused on a simplified situation of scour holes in heterogeneous subsoil with a three-dimensional character. The bed of the flume consisted of a concrete layer with a hole in the centre in which sand was exposed to the flow. Water depth, flow velocity and the size of the geometry were systematically varied to investigate their influence on scour hole development. Additionally, general observations were performed on the flow structures and scour development in and around the scour hole. It was observed that the scour development is different compared to the results of earlier investigations as a result of the non-erodible top layer. The process of undermining is a limiting factor for sediment transport out of the scour hole. The water depth showed a significantly smaller influence on the scour depth compared to existing empirical formulas. Additionally, the size of the geometry seems to be the most important varied parameter in this study. Relative to the size of the geometry, the scour depth remained approximately equal with different experimental conditions. Compared to two-dimensional experiments, the scour holes became deeper, which suggests importance of three-dimensional flow for scour hole development.

This study assesses the application of graph theory to examine the connectivity of aquatic habitat in the Sliedrechtse Biesbosch and preserve or improve the area’s ecological value. The study addresses the relation between hydrodynamics and ecology, and evaluates different definitions of connectivity. Graph theory provides a novel and promising approach to assess connectivity in aquatic environments. With increasing availability of data, more accurate and informative analyses can be carried out, resulting in more effective designs of restoration measures. The application of graph theory in such research has, as one of its main strengths, its visual accessibility to laypersons or others without expertise in interpreting numerical model results. Furthermore, graph theory can offer both a holistic system view of a water network by evaluating the system as a whole, as well as a local view which is possible by looking at the single nodes and edges in the graph. This enables investigation of the role of individual channels within the system and the identification of vulnerable spots within the network, limiting the availability of aquatic habitat. Graph theory is applied in various fields of research. As numerous metrics exist to examine the properties of a graph, an introduction to the most important metrics is given in this study. Adequacy of metrics is dependent on the questions posed and parameters relevant for the specific topic to be investigated. It is shown that in aquatic habitat connectivity, metrics such as betweenness centrality and bridges are indicative to obtain a general view of the network. When temporal variations play a role, as is the case in a tidal area, metrics as the number of components (NOC), the order of the largest component and the length of connected pathways (LOCOP) of the largest component are suitable to determine the connectivity. Whereas these metrics show useful in studying aquatic habitat connectivity, they may be less appropriate in connectivity studies of the same water system aiming at other fields of application, e.g. sediment connectivity. Results show that graph theory provides a useful instrument in analyzing the aquatic habitat connectivity of the Sliedrechtse Biesbosch, which is investigated in a case study. The ease at which key nodes and edges are identified offer great possibilities for the design of nature restoration measures. In the present layout of the study area, large variation of aquatic habitat connectivity occurs based on a flow velocity fragmentation threshold of 0.3 m/s, corresponding to the maximum tolerable flow velocity for the European flounder (Platichthys flesus). Due to tidal influences in the study area, flow velocities vary continuously and the threshold flow velocity is exceeded during part of the tidal cycle. Considering the available habitat of other species gives different results depending on the tolerable flow velocities of the specific species. As is shown in this research, a combination of graph theory and numerical modelling enables the design and simulation of different nature restoration measures and system layouts to improve the aquatic habitat connectivity of the area. The method presented in this research can be particularly useful to ecologists investigating suitable habitats for specific fish species. Also for engineers and others involved in the design of nature restoration measures, the method can be helpful since the designs of restoration measures can be evaluated considering the effect on habitat availability. The research provides an informed basis for subsequent applications of graph theory and numerical modelling to aquatic habitat connectivity. By selecting the most suitable design parameters and improvements of the network schematization, a justified decision can be made on the most effective restoration measures concerning the improvement of available habitat. Especially considering a combination of habitat preferences, such as flow velocity, water depth and turbidity, can provide proper insight in the aquatic habitat connectivity of an area for specific species.

The seasonal Chindwin river (Myanmar) forms the central artery for transportation in the region. However, the dynamic river behaviour, archaic boat equipment and limited monitoring, yearly results in large numbers of grounding ships, causing injuries and economic losses. A pilot project involving CoVadem technology was initiated in the beginning of 2019 to benefit safer navigation on the Chindwin. CoVadem technology can chart the most up to date water depths by collecting under keel clearance measurements from the commercial fleet. The pilot project aims to increase safety on the Chindwin through sharing information about safe routes and forecasted water depths with captains sailing the river. The added value of using CoVadem technology, however, is vulnerable to the number of participating vessels. A combination of morphological changes and the typical spatial spread of CoVadem data limits the extent of the navigation channel that can be derived from the data. With only a small part of the navigation channel known it is unclear where passing of other vessels is possible, where speeds should be adjusted due to e.g., bottlenecks and where shorter routes are present. The objective of this research is to provide more information about the navigable area around CoVadem data in order to assist captains with navigation. During this research, two physics-based models have been developed, able to carry out this task. The ‘soil-model’ combines CoVadem data with assumptions about the maximum slope in the bed. The ‘axi-symmetric model’ utilises CoVadem data, the axi-symmetric solution and assumptions about sand dunes and river banks in a model to estimate a navigable area. Two ‘reliability indicators’ have been developed that can indicate areas of the river where the output of the axi-symmetric model is reliable in its estimate. One indicator is related to the channel stability, it is calculated from many years of satellite imagery. The other indicator is related to channel curvature. To measure the performance of the models and the reliability indicators, two dimensionless performance indicators have been developed: the safety score and the channel coverage score. The models and the reliability indicators were consecutively tested and evaluated for four study cases located along the Chindwin river. The results are promising. It follows from this research that navigation channel estimates around scarce CoVadem ship track data can substantially benefit from application of a physics-based model. The soil-model is very robust, but has limited added value for navigation. The axi-symmetric model increases the navigable width estimate around a single CoVadem trackline significantly O(100 m). The performance indicators can improve the reliability of the axi-symmetric model significantly. This research combines Big Data, physics-based models and remote sensing in a not early demonstrated way: with models tailored for navigable area estimates and with measured data as the starting point. The correlation between axi-symmetric model reliability and remote sensing/curvature is, moreover, something that has not been demonstrated before. Finally, the two developed performance indicators show great promise for the evaluation of navigable area estimates. As such, this research adds to current advancements in (open-access) cross-platform data accumulation and utilisation.

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.

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

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.

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.

In the Waal bed degradation occurs, which is mainly induced by a large number of river regulations measures done in the past. Rijkswaterstaat develops possible measures and new techniques to stop riverbed erosion and/or mitigate the negative effects. One of the possible new techniques to reduce riverbed erosion locally is groyne field nourishments. The objective for this research is to deepen insights how groyne field nourishments provide enough sediment to the river such that bed degradation locally does not develop further. The following research question is formulated for this: How can groyne field nourishments act as small feeder nourishment that release sediment slowly to the main channel? To answer the research question a literature study is done and a XBeach model is set-up. The literature study is done to obtain knowledge about the hydrodynamic and morphodynamic processes in a groyne field in case of emerged and submerged groynes. From this literature study it appears that the effect of navigation on the morphology in a groyne field is dominant over the effect of the river discharge when the groynes are emerged: net sediment is transported from the groyne field to the main channel mainly by suction. When the flow conditions increase and the groyne get submerged, the effect of navigation on the morphology in the groyne field decreases rapidly and the net sediment transport is from the main channel to the groyne field induced by the river discharge, mainly the large-scale eddies. Extra literature study is done to possible relevant coastal and river processes to get knowledge of the effects of waves and currents on the behaviour of nourishments in a groyne field since very limited research has been done before on groyne field nourishments. It appears that especially dune erosion and river bank erosion by waves and currents can be used to explain how nourishments erode by the primary and secondary ship wave system. After the literature study a XBeach model is set-up of for a series of emerged groynes on a straight section in the Waal. The model is calibrated and validated with data from Rijkswaterstaat and literature. In this model three possible nourishments with the same volume are applied at different locations in the groyne field: 1) one upstream in the groyne field; 2) One downstream in the groyne field; 3) And one in the middle of the groyne field. From this research it can be concluded that groyne field nourishment at all three locations can work as small feeder nourishment that release sediment slowly to the main channel in the case of emerged groynes for about 200 days per year. The nourishment placed in the centre of the groyne field appears to be the most effective nourishment, since 1) the waves and currents induced by navigation attack the nourishment along all its edges, 2) the centre nourishment causes in almost the whole groyne field an increase of sediment transport to the main channel and 3) the flow pattern in the groyne field is changed favourably for the sediment transport of already suspended sediment to the main channel. This research should be seen as first indication if nourishment could work at all. It is advised to research the material of the nourishment and to simulate the groyne field nourishments for a longer period, such that more flow conditions and more ship passage are included, to get a more complete picture of how groyne field nourishments act as small feeder nourishments. Furthermore a practical test with a centre nourishment is recommended to collected data of the hydro- and morphodynamics in a groyne field with nourishment.

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.

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: extit{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. 

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.

In the Rhine-Meuse Delta in the Netherlands, deep scour holes can develop very suddenly. The holes may form a potential risk for the stability of surrounding riverbanks, dikes, bridges, tunnels and buildings. A scour hole can mostly be the result of two conditions: local changes in hydrodynamical conditions or local changes in erosive capacity of the bed. Heterogeneity of the subsoil in the Rhine-Meuse Delta causes differences in erosive capacity, resulting the formation of deep scour holes. A distinct method to predict the development of scour holes in heterogeneous subsoil is not found yet. In this research a numerical model that simulates the hydrodynamics in a scour hole is used to gain more insight in the flow processes that are present in such a hole. The dependencies of the flow processes on the upstream water depth, upstream flow velocity and depth of the scour hole are investigated by changing these parameters systematically in the model simulations. The processes that are included in the analysis are the flow separation and recirculation zone, the contraction of flow, the horseshoe vortex and the vortices downstream of the hole. Changes in the depth of the scour hole showed some interesting phenomena. It is found that three-dimensional processes suppress the formation of the recirculation zone, as flow separation happened for milder slopes and shallower holes in two-dimensional simulations. The contraction of flow is not necessarily stronger for larger holes, which was hypothesized. The analysis of the bed shear stress in the model results shows a constant upstream slope angle, as the critical value for bed shear stress is not reached. In the downstream half of the hole, this value is reached and therefore erosion and sedimentation are expected. The bed shear stress in the model results shows strips of zero bed shear stress with large gradients at both sides of the hole. These strips have an angle of 1:8, which is also found in the laboratory experiments by Koopmans (2017). The downstream expansion of the scour hole with a poorly erodible top layer had the same widening angle.

Canal Emilio Mitre is a navigation channel that runs through the Rio de la Plata estuary in South America. It grants access to the second largest inland waterway system in South America, covering (parts of) Argentina, Uruguay, Brazil, Paraguay and Bolivia. The channel is a main bottleneck for navigation for two reasons; the channel has a continuous need for maintenance dredging and it is only navigable during a tidal window. Furthermore, the dredging effort shows high interannual variability, resulting in risks for the contractor and users. The current study identifies the dredging effort and strategy, and a conceptual model is made of what physical processes force the sedimentation. The IPCC reports are used to identify the effect of climate change and variability on the drivers of the sedimentation. The interannual variability is shown to be linked to climate variability, due to the ENSO phenomena. The drainage basin of the Rio Parana is affected by an increase in precipitation and sediment input during El Nino events as a result of the teleconnections of ENSO. A depth-averaged numerical model (Delft3D) is set-up to address the effect of climate, deepening and wind scenarios on the dredging effort and its variability. The numerical model was validated by comparing the reported dredged volumes in the system to the simulated sedimentation. Overall, the sedimentation in the system is found to be sensitive to highly energetic events due to a redistribution of the fine sediments over the estuary. The acquired knowledge on the physical processes of the system in combination with the outcomes of the scenarios are used to make recommendations on the future dredging strategy to reduce the risk due to variability. This study highlights the importance of taking into account the risks and chances due to climate variability in dredging projects in regions affected by teleconnections with a duration of around 10 years.

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.

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.

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.

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.

The La Plata Basin is the second largest drainage basin in South America. The Paraná River, the basin’s main river, discharges into the Río de la Plata estuary in Argentina. In the Lower Paraná Delta, near the deltaic front, a complex system of mostly natural and some man-made channels redistribute a part of the river flow. Like the river and the estuary, the channel network has been subject to sediment deposition, limiting the navigability needed for the small-scale transport of goods, leading to hindrance to tourism and other economic activities in the Lower Delta. Compared to the river system on the large scale, not much is known about the characteristics and the morphodynamic behaviour of the channel network. The objective of this research is to set a first step in providing insight on the relevant processes and human activities that influence morphology changes in the Lower Delta channel network. Making use of existing data, a literature study and data obtained during several field surveys, various hypotheses were formulated. Hypotheses included the potential influence of land use change in the catchment and climate change related trends in precipitation patterns, climatic variability associated with El Niño Southern Oscillation, the effect of artificially deepening one of the two main branches of the Paraná River and the excavation of artificial channels in the Lower Delta in the early twentieth century. The effects of said processes and human activities are tested with a scenario-based hydrodynamic modelling study, making use of Delft3D Flexible Mesh with sediment transport and morphology modules. Our analysis provided various insights in the behaviour of the Lower Delta channel network, and the influence of human activities and natural processes on this behaviour. A notable finding is that artificial deepening of the Paraná de Las Palmas by dredging its thalweg since 1990 has lead to a significant reduction in river flow entering channels supplied by the river branch and increased sedimentation rates in these channels. In addition, insight was gained on the spatial distribution of depositional and erosive patterns instigated by discharge and sediment input changes. This research has provided a first, global insight into the relevant processes governing channel morphodynamics. Some of the findings can serve as input for future dredging strategies, as results imply that the current strategy has lead to limitations to the navigability of an intensively navigated part of the channel network. Furthermore, the outcome of this research can serve as a starting point for research on one of the specific topics covered by the hypotheses, including but not limited to the influence of storm surges, the effect of a potential spatial heterogeneity in soil composition and the influence of tidal divides on morphological features in the channel network.