Estuaries have always been important for mankind and therefore it is essential to have a good understanding of the flow and sediment dynamics there. The goal of this thesis is twofold. One objective is to use an existing model to gain a more thorough understanding of the flow dynamics in an estuary. The second objective is to extend the model such that it can also compute the suspended sediment concentration in a cross-section. The equations governing the flow dynamics are the shallow water equations. The advection-diffusion equation governs the sediment dynamics. To compute the flow and suspended sediment concentration in a cross-section, conditions are assumed to be uniform in the along-channel direction. To solve the equations a coordinate transformation is applied first. After the transformation, the cross-section of the channel is represented in the computational domain by a rectangle. In the vertical direction, an eigenfunction expansion is used with eigenfunctions derived from a special case of the Sturm-Liouville eigenvalue problem. In the horizontal direction, derivatives are approximated with a central finite difference scheme. In the frequency domain, variables are expressed as the sum of tidal components. The Galerkin method is applied in both the vertical direction and the frequency domain to optimise the weight functions for every location along the transect. The system obtained with the Galerkin method is solved using Newton-Raphson iterations and an LU-decomposition. To find the distribution of the erosion coefficient corresponding to a morphodynamic equilibrium, a time integration method is used. The effect of several parameters on the advective contribution to the cross-channel flow is systematically investigated. The results show that the steepness of the bottom slope affects the magnitude of the advective contribution to the residual lateral flow. For a steep bottom slope the contribution is large and for a gradual bottom slope the contribution is small. The curvature of the channel strongly affects the total cross-channel flow, depending on the magnitude of the radius of curvature, but hardly affects the flow caused by advection. The lateral density gradient can largely affect the \mbox{cross-channel} flow. Especially, the amplitude of the M$_2$ tidal component of the density gradient affects the advective contribution to the flow. Both the magnitude and characteristics of the advective contribution change when the amplitude of the M$_2$ tidal component of the density gradient is varied. The phase of the M$_2$ tidal component of the density gradient hardly affects the cross-channel flow and advective contribution of the flow. Measurement data of a cross-section of the Ems is compared with a simulation of this situation. The magnitude of the lateral flow is similar for the measurements and model results but there is a difference in the direction of the flow in the upper part of the water column. This deviation could be caused by the description of the free surface. In the measurements there is a time-varying thickness of the water column whereas the rigid lid assumption is applied in the model. However, other differences between the simulation and the actual situation could have contributed to a deviation between the measurements and model results as well. The results for the sediment module show that the model works as expected for a prescribed erosion coefficient and for computing the erosion coefficient in morphodynamic equilibrium for situations with only diffusive transport. For simple situations the analytical solution is approximated and for more complicated situations the results agree with the physical intuition. The main recommendation for further research is to investigate how the model can be extended such that it is also possible to compute the erosion coefficient in morphodynamic equilibrium for situations with both advective and diffusive sediment transport.

Using an idealized width-averaged model, the influence of spring-neap cycles on the transport and trapping of suspended fine sediment in tide-dominated estuaries is investigated. To this end we introduce a multiple time scale expansion. This provides a mathematically sound argument for treating the fast ebb-flood cycle and the slow spring-neap variations as independent time scales. With this expansion, semi-analytical approximations of the water motion and suspended sediment concentrations can be found as functions of the spring-neap time scale. The advantage of this is that the transport of sediment is then investigated in a tidally-averaged sense using analytically obtained temporal dependencies and model simulations based on the conditions in 2005 of the Ems-Dollard estuary. We found that sediment import is strongly enhanced during spring tide and that a combination of vanishing import processes and sediment export due to river discharge resulted in a net export of sediment during neap tide. Furthermore the spring-neap varying deposition of fine sediment in a bottom pool on the river bed is investigated. We found that the time needed for the system to adjust to new equilibrium conditions can not be neglected within a spring-neap cycle as temporal lag effects are clearly visible in the dynamic behaviour of the bottom pool and suspended sediment concentrations. These temporal lag effects are sensitive to the choice of the parameter governing erosion. Assuming different values of the erosion parameter we have shown that the long-term characteristics of the bottom pool are sensitive to these temporal lag effects. Choosing a low value of the erosion parameter results in the presence of a bottom pool throughout the spring-neap cycle. With a high value of the erosion no bottom pool is formed at any time in the spring-neap cycle.

Bathymetry strongly affects nearshore hydrodynamics that drive sediment transport. However, recent bed level information is not always available, especially not simultaneously resolved in space and time. Time series of radar backscatter images can be used to retrieve bed levels in the intertidal area with the so-called Temporal Waterline Approach (TWA) by Bell et al. [2016], yielding a two weeks average bathymetry at spatial resolutions in the order of a few meters. Bed levels are estimated for each radar cell individually by correlating temporal patterns of wet-dry transitions from the radar with hypothetical patterns found in the water level record for different tidal levels. In this study data recorded by a marine X-band radar located at Bunker Hill station (Sylt, German Wadden Sea island) is used to improve TWA, validate the improvement as well as to show its potential in application. A survey was conducted in May 2018 to provide ground truth. Two modifications are made, that improve robustness and reliability of TWA’s output by adding a second threshold and semi-automating the quality procedure. Root mean square error between radar-derived bathymetry (RDB)
and ground truth for the intertidal area is 43 cm (bias 12 cm) on a 7.5 m squared grid. Sensitivity analysis of four parameters (i.e. vertical resolution, inclusion of wave induced effects on water level, length and position of analysis window, thresholds in quality check) reveals, that TWA is most sensitive to the choice of thresholds. Sensitivity analysis is utilized to tune TWA to achieve full coverage of the intertidal area (zero coverage elsewhere) with sufficient accuracy. Additionally it is found that at costs of accuracy the period for which averaged bed levels are retrieved can be shortened from 14.5 to 8.3 days. This is promising, because RDB potentially is able to observe short-term changes in the order of a few days (e.g. storm events). To demonstrate the value of the improved TWA for bathymetry surveys in intertidal areas the method is applied to a 3.2 km stretch of beach in front of Bunker Hill station, capturing the beach state variations in the course of 2017. During wind sea dominated season (winter) a ridge-runnel type of beach is present, that changes to a skewed transverse bar and rip system during swell dominated season (summer). The method is sufficiently accurate and robust for application and proves its potential to reveal new insights, e.g. indications on how the transition between two beach states evolves. In coastal science RDB could serve as a tool to improve existing theories by providing detailed information, that reveals underlying processes. In coastal engineering it fills gaps in monitoring schemes, for example to evaluate effects of nourishment strategies.

Bed roughness is an important parameter for the prediction of sediment transport as well as calculating flow conditions near the bed. Numerical models are extensively used to make these predictions. In these models it is fundamental to have proper values for bed roughness. The bed roughness is being estimated using a dataset from an ongoing pilot project in the Dutch western Wadden Sea. This dataset consists of 40 days of velocity measurements using ADV (8 Hz) and ADCP (1 Hz) instruments and concentration of SPM measurements using OBS instruments.
To estimate the bed roughness, the dataset is analyzed using four methods for calculating the bed shear stress. The logarithmic profile, turbulent kinetic energy, vertical turbulent kinetic energy and the Reynolds stress method.

The data has been processed and averaged per tidal phase, so statistical analysis can be applied to it. From this analysis, it is found that the concentration of SPM increases at 6 cm with increasing wind speed. As a result of this increasing of SPM the bed roughness also increases. This leads to the hypothesis that suspended sediment makes the bed rougher, and is not primarily governed by horizontal advection but also local resuspension.
A 1DV numerical model is used in which horizontal advection is excluded to test this hypothesis.

Simulations are performed with stationary boundary conditions, using combinations of water depth (0.2 to 2.8 m) and velocities (5 cm/s to 70 cm/s). These simulations are imposed with an initial homogeneous concentration. For every combination, the concentration is increased gradually until the concentration profile becomes L-shaped.

Besides simulations with stationary conditions, timeseries of water depth and velocity are used to simulate one tidal cycle.
All simulations performed with this numerical model do not take wind and waves into account and water-bed exchange is excluded.
From the simulations with stationary boundary conditions the roughness is calculated using the LP method. It is found that bed roughness increases with increasing initial homogeneous concentration.

From the simulation of one tidal cycle, it is found that the roughness increases towards the turn of the tide. After the turning of the tide, the concentration profile becomes L-shaped, and the roughness is decreased. The concentration profile becomes homegeneous again after a certain threshold of velocity and waterdepth and at the same time the roughness increases again. During a tidal cycle, it is possible to have a collapsed concentration profile, which indicates a lower bed roughness and thus a smoother bed.

Intertidal flats and salt marshes can protect hinterland against wave energy and flooding by tides or storm surges. Thus, the development of tidal flats is important to foresee. The presence of specific small scale features could indicate a certain development and large scale morphological shape. The shape is a predictor for development and it is expected to influence small scale feature formation. Tidal flat management could benefit from technological process, e.g. in drones, if these indicators can be used. Hence, the aim of this research is to find indicators and gather sufficient knowledge to make them applicable.

The main research question is: "Can small scale morphological features on estuarine tidal flats be used as indicators for large scale morphological shape and development?”. The main question contains two aspects. The first facet is whether the small scale morphological features are indicators for large scale morphological shape and development. It focuses on detecting indicators. The second part is about gathering knowledge to properly use the indicators. It is investigated which mechanisms explain the found indicator roles of the small scale morphological features. The focus is on the identification of the mechanisms. Two sub-questions can be extracted from the above:
1. Are the small scale morphological features on estuarine tidal flats indicators for large scale morphological shape and development?
2. Which mechanisms explain the found indicator roles of small scale morphological features?

Indicators are detected with the analysis of aerial photographs and elevation measurements at transects in the Eastern and Western Scheldt. New and already in literature known features are identified and classified in this process. The mechanism that form and erase the small scale morphological features and their dependence on large scale morphological shape are investigated to explain the found indicators. The mechanisms are determined in various ways: Two 1D models are created, a field campaign is conducted, (subsequent) aerial photographs are looked into, elevation measurements are analyzed in detail and literature research is done.

Small scale morphological features on estuarine tidal flats can be used as indicators in the Western Scheldt. This research detects and explains features that indicate large scale morphological shape, development, hydrodynamic conditions and ecological activity of an area. However, the applicability of the indicators in other systems should be proven by extending this research to other situations. A specific sequence of small scale morphological feature categories between salt marsh and waterfront, a build-up, has been identified as indicator for convex large scale morphological shape and indirectly for accreting behavior. All the features in the build-up are indicators on their own. Megaripples, seen within build-up, indicate (future) convex profiles. Furthermore, all of the features indicate a certain relative height and slope when they are seen within the build-up. These individual indicator roles are clarified by elaborating on the hydrodynamic conditions that form and erase the features. Therefore, the prevailing conditions can also be determined with the indicators.

In this thesis the equilibrium cross-sectional areas of a tidal inlet are investigated. The most common used methods to study cross-sectional stability are the Escoffier method (Escoffier, 1940) and the Modified Escoffier method (Van de Kreeke, 2004). The Escoffier methods are mainly empirical and lack physical justification. Therefore, in this thesis a process-based model, as proposed by Hinwood et al. (2012), is used to show that in the long-term the inlet cross-sectional area approaches a stable equilibrium or closes. Additionally, the influence of higher harmonics on the equilibrium cross-sectional areas is studied. The stable equilibrium cross-sectional area of the Frisian inlet in the Dutch Wadden Sea is computed and the results are compared to earlier findings.

Flushing basins have been used at several locations in Germany, Belgium, and the Netherlands in the past centuries to maintain the navigational depths of harbour channels. These locations are not in use anymore or are only partly responsible for channel maintenance. This thesis encompasses a study to optimize the geometric parameters of a flushing basin system in a tidal back barrier basin. The addition of a flushing basin behind a tidal harbour channel is meant to affect the sediment regime in the channel from a flood-dominant towards an ebb-dominant regime. With a one-dimensional model of a schematized reference situation, the role of three parameters: the sluice width, the horizontal area of the flushing basin and the ratio between the tidal amplitude and the channel depth, in the tide-residual sediment transport is investigated for the flushing basin system. Three conditions need to be met simultaneously to create ebb dominance along the total length of the channel: a sufficiently large flushing basin area, a small sluice width, compared to the channel width and a relatively deep channel compared to the tidal amplitude.