· · · Institutional Repository

The first one who studied the problem of armour layer stability was Iribarren (1938). Iribarren derived stability criteria that were based on the acting physical processes on the slope of a structure. In the stability criteria of Iribarren only the influence of a few structural and hydraulic parameters were taken into account. In 1988 Van der Meer derived a stability relation in which the stability of armour layers was depending on several hydraulic and structural parameters. Van der Meer concluded that the stability of armour layers is strongly influenced by the composition of a structure. He implemented the influence of the composition of the structure in a permeability coefficient, P. This permeability coefficient is defined for four different structures. Vander Meer is a good tool for the design of armour layers, it has little physical background. The empirical character of the stability relation is found back in the definition of the permeability coefficient. If a different structure is designed than tested by Van der Meer, an estimation of this permeability coefficient has to be made. Since the permeability has large influence on the stability, new methods should be developed to calculate armour layer stability. Due to the irregularity of armour stones it would take a lot of effort to obtain a full analytical solution for the problem of armour layer stability. However, a good attempt into the direction of an analytical solution can be made, since good models are available that describe the water motion on and in coastal structures. An example of such a model is the model ODIFLOCS. ODIFLOCS stands for Qne dimensional flow Qn and in foastal structures. With ODIFLOCS it is possible to calculate the velocities on a coastal structure for certain hydraulic and structural parameters. With these velocities the hydrodynamic forces on the stones in the armour layer can be calculated. Iribarren (1938) proposed a model for armour layer stability. He assumed a hydrodynamic drag force that act on a stone, which is caused by the run-up or run-down, parallel to the slope of the structure. Two stability criteria for the stability of rock on a slope can be distinguished, which are upward and downward stability respectively. The drag force can be written as a function of the velocity on the slope of a breakwater. Iribarren was not able to calculate these velocities, since no models were available to calculate them, and estimated them by wave celerity in shallow water. Van den Berk (1999) was able to proceed the approach of Iribarren and calculated the velocities with the numerical model ODIFLOCS. He modelled homogeneous structures and calculated the armour layer stability for homogeneous structures for several hydraulic and structural conditions. He found that his results and the results ofVan der Meer were strongly correlated. In this research a next step will be made to see whether physical background can be given to the influence of structural permeability on armour layer stability. Structures with different permeabilities will be modelled and the velocities on the slope will be calculated for different hydraulic conditions. With the use of these velocities, which will be calculated with the numerical model ODIFLOCS, the stability can be determined. The test showed that the strongest increase in stability was found for gentle slopes in combination with low and high wave steepness. The test with steep slopes gave significant lower influence of structural permeability, compared with gentle slopes, on armour layer stability. This is contradicting with the findings of Van der Meer, who found a strong influence ofpenneability on the stability for high values of the surf similarity parameter. This shows that the penneabilities that are modelled in this research do not correspond to those tested by Van der Meer.

We model branching channel patterns in short tidal basins with two methods. A theoretical stability analysis leads to a relationship between the number of channels and physical parameters of the tidal system. The analysis reveals that width and spacing of the channels should decrease as the slope of the bottom profile and the Shields parameter increase and as the mean water depth decreases. In general, the channel depth should halve at every bifurcation. These theoretical results agree well with the field data from the Dutch Wadden Sea. A numerical model based on Delft3D, a software system of WL/Delft Hydraulics, is used to simulate the time evolution of a channel network in a geometrically simplified basin of similar dimensions as the Wadden Sea basins. The resulting channel network displays a three-times branching behavior, similar to the three- to four-times branching patterns observed in the Wadden Sea. The simulated channel pattern satisfies the relation derived from the theoretical analysis. The results of this pattern analysis provide for additional validation of two-dimensional/three-dimensional process-based morphodynamic models of tidal basins.

The Storm Surge Barrier in the New Waterway was built to protect the city and port of Rotterdam and the area of the lower Rhine against flooding during extreme conditions. This construction, consisting of two gigantic arc shaped barriers, is to be pivoted into the New Waterway and then lowered in case of an impeding emergency. The arc shape makes it a very efficient design against forces from the seaside, but if the level on the riverside surpasses the level on the seaside, a negative force will be exerted on the construction. Seiches contribute to this effect. The barrier only has a relatively small capacity to withstand a negative head difference. To accurately predict the maximum expected head difference a numerical model that handles seiches correctly is needed. In this thesis, the boundary conditions for such a numerical model are investigated. The program currently used to calculate the effects of seiches, RAS/FLOW predicts a head difference that exceeds the design specifications of the construction. However, the calculations done with Rasflow are not accurate with respect to the amplification of the seiches. The amplitude is overestimated significantly due to the use of an inaccurate boundary condition at the sea boundary of the model. The boundary condition at the channel entrance is very complex. Mendez Lorenzo (1997) studied a new boundary condition: the epsilon boundary. This boundary is a combination of a water level and a Riemann invariant with a factor epsilon. In the analytical case the results of this boundary condition match the analytical solution exactly. The step from the analytical boundary condition to a numerical boundary condition involves a set of derivations and simplifications that fixate the value for Epsilon. With a fixed value for epsilon, the amplification function obtained will only match one of the peaks in the spectrum: the peak for which the value of epsilon is set. In this thesis the addition of non-linear terms to the epsilon model can be found. The non-linear terms did not resolve the problem of the fixed epsilon. To reduce the complexity of the boundary condition a different approach to the problem is taken, namely a combination of a one and two-dimensional approach. In this model a two-dimensional sea area is attached to the onedimensional channel. Thus moving the complex boundary condition at the channel entrance to a simpler boundary condition on the open sea boundary. With this model it is possible to correctly model the amplification for more than one peak. The results obtained with this model are satisfactory and are recommended for a future implementation.

The effective thickness of the bedload layer over a river bed can be schematised as the total volume of bedload sediment that is in transport, divided by the surface. In practice, this layer is considered to have a constant thickness over time. Main question of this thesis is whether this assumption is valid. By analyzing the mass balance, it is possible to gain an approximation of the reality. By means of numerical approximations, it is possible to build a morphological model which imitates these equations. This imitation makes it possible to analyze the effect of specific parameters on the sediment transport and the bed surface elevation. In this way, the effect of neglecting the derivative over time of the effective thickness of the bedload layer can be investigated. The model of a river section of the Rhine between Emmerich am Rhein and Lobith results in a maximum influence of the derivative of the effective thickness of the bedload layer on the morphodynamic changes as the result of one flood event that is smaller than 4%. A parameter study makes it possible to test the obtained result on sensitivity. Because the morphodynamic model is based on a lot of input parameters, a well founded choice between the large variety of parameters has to be made. A rough sensitivity test shows that the at maximum 4% influence on the derivative is subject to a possible variation of approximately a factor 2, depending on the variation in the input parameters.

Weir is the construction widely used in hydraulic engineering. It is very important to investigate the behaviors of the flows over a weir. In general when water streams over a weir it behaves various flow patterns according to different flow conditions. Different flow patterns results in different inference on the water level profile and the distribution of recirculation zones at downstream of the weir. The simulation of these flows is of general practical interest to the design of hydraulic structures and the management of water resources. In this report, a view of how to simulation hydraulic jump over a weir is presented, from 1-D model, 2DV hydrostatic model, 2DV non-hydrostatic model using pressure gradients as unknowns, 2DV non-hydrostatic model using pressures as unknowns, to 2DV implicit non-hydrostatic model. A 2DV implicit non-hydrostatic numerical scheme is presented, which can simulate flows with steep water and bed level gradients. The numerical algorithm solve the Reynolds equations and The integrated continuity equation simultaneously, so that the water surface level is integrated into the system, and solved together with the pressure fields. The resulting algorithm is locally and globally mass conservative. Several numerical experiments illustrate the potential of the model, namely: the simulation of the water level and velocity fields of free, undular and submerged hydraulic jumps downstream of a weir. This model is proved to fairly accurately represent discontinuities in bottom topography and water surface profiles. The numerical model also has the ability to describe different flow regimes downstream of a weir, namely free flow, undulation, and fully submerged flow. The results from the numerical model show a fair agreement with the experimental data. The implicit non-hydrostatic model, provides correct wave heights, wave lengths, and predicts the upper and lower limit for the occurrence of undulations in a good manner.

The rapid development of Geographical Information Systems (GIS) has together with the inherent spatial nature of hydrological modelling led to an equally rapid development in the integration between GIS and hydrological models. The advantages of integration are particularly apparent in flood extent modelling. In this thesis, the integration of hydrological models and GIS is approached on the basis of performance, with performance taken as the balance of computational efficiency, flexibility of application, and most importantly the reliability of the integrated model. It is shown that predictive reliability is dominated by model uncertainties, particularly in model roughness parameters. These roughness parameters are found to be more conceptual than physical as they represent bulk momentum loss parameters at the reach scale. Limited data on spatial extent of flooding is available to constrain these uncertainties, and where such data is lacking the simplest numerical approach may be as reliable as more complex approaches. The overall performance of the simple approach is then higher as this is more easily integrated within GIS. Observations of flood extent from aerial photographs may help constrain uncertainties, though much more value is found from distributed water level observations in the floodplain. The lack of hydrological data also results in high resolution GIS data of elevation or land use being of limited value. As sufficient hydrological data is unavailable and perhaps impossible to acquire, model predictions made are recommended to be considered probabilistically, irrespective the level of integration with GIS.

Morphodynamics of a tidal inlet system on a micro-tidal coast in a tropical monsoon influenced region is modeled and discussed. Effects of tides, waves, river flows and system configuration on the inlet morphologies are investigated with the aid of process-based state-of-the-art numerical models. Seasonal and episodic behavior of the inlet system under the influence of the forcing processes is then described, modelled and explained.

In numerical modelling of water movement wetting and drying is a well known problem. The governing equations are not valid in the dry part of the computational domain which may result in problems with mass conservation, negative water depths and artificially enlarged gradients. A method is proposed that allows for the surface elevation to become negative while strict positivity is demanded from the water depth. In this way mass conservation is guaranteed. It is investigated whether this procedure is mass conservative, efficient and robust. To this end a simple finite element discretization of the inviscid shallow water equations (SWE) is derived. The resulting procedure is validated with several one- and two-dimensional analytical solutions for: 1) a one dimensional dam break, 2) flow over a long crested weir, 3) a one-dimensional oscillating water surface in a parabolic basin, 4) the runup of long waves on a beach, 4) a two-dimensional standing wave in a parabolic basin and 5) the spreading of a parabolic flood wave in two dimensions. In addition two laboratory experiments are used for validation. One experiment with solitary wave runup on a conical island and one experiment with a two dimensional dam break. In general the methods performance is good. However in two-dimensions it can be beneficial, in case of small gradients at the wet/dry interface, to use a lumped mass matrix at partially dry elements too. The wetting and drying iteration converges on average in 2 to 3 iteration steps. In some cases bifurcations and mass errors can occur. However mass errors are caused by rounding errors and can be resolved by using double precision. The occurrence of bifurcations is much less frequent in case of calculations in double precision and can be minimized by adjusting the BiCGSTAB settings.

In this thesis a non-hydrostatic two-layer 2DV model, based on the scheme presented by Stelling and Zijlema (2003), is developed. The model is non-hydrostatic; the velocities are corrected with a pressure gradient following from the requirement of a divergence free flow field per computational cell. The model is a two-layer model; only layered systems with two layers of different but constant density are considered and the mixing is left out of consideration. The divide between the between upper and lower layer is considered a sharp interface. The density is discontinuous at the interface and the fluids are completely separated by the interface. To guarantee an exact representation of the interface, a boundary following grid is used in the vertical. In several test cases the model has been validated. In a closed basin test the dispersion relation is shown to follow the linear dispersion relation for internal and external waves almost exactly when two layers are used. The model is also tested against analytical models with regard to dissipation of the waves by a viscous lower layer (mud) and showed comparable results. The model however performed less well for highly advective processes with large gradients such as the sex-change test. Considering differences in flow velocity between the layers, the stability of the model was substantially lower than according to linear theory. The limit for this new model has been found to depend on the thickness of one layer only in stead of the total depth. Finally the model was used to reproduce results of laboratory experiments found in literature. The steepening of internal waves and generation of solitons was reasonable well represented. The result of this study is a non-hydrostatic 2DV two-layer model, which has the same attractive properties as the model by Stelling and Zijlema regarding wave dispersion of both internal and ex-ternal waves.

Decline in injectivity due to suspended solids in injected water is a wide spread phenomenon in water injection projects. Reliable prediction of injectivity through experiments and modeling is very essential under such circumstances. A model for predicting the injectivity during internal filtration taking into account particle dispersion, retention kinetics, nonlinear filtration, permeability reduction and viscosity functions was proposed. Subsequently, the analytical model for external filtration was coupled with the numerical model for internal filtration using the concept of transition time to predict the overall decline in injectivity. Core flood experiments using hematite suspensions for various particle concentrations (1-5 ppm) were conducted in Bentheim sandstone cores to quantify the injectivity. Simultaneously, X-ray CT scanning was performed under dynamic conditions to obtain deposition profiles along the core at different times. From microscopic analyses and visual observations, it was found that surface deposition in the porous medium and face plugging at the inlet of the core were responsible for decline in injectivity. A good agreement was obtained between the modeled and experimental results showing the validity of the retention function. Further, the effect of various parameters (particle concentration, number of grids etc.) on injectivity was investigated. Finally, the results from the study help the operators in planning and design of water management strategy for improved oil recovery projects.

In this research, the initial formation and long-term evolution of channel-shoal patterns in schematised basins is simulated using a model based on the software package Delft3D. The resulting channel-shoal patterns are validated with field observations, among which the Western Scheldt estuary. This has proven that a complex model is able to simulate the emergence and evolution of nature-like patterns, on time scales from decades to centuries. Additionally, a comparison with other model types has given insight into the influence of different model assumptions and formulations and into the processes underlying the morphodynamic behaviour of channels and shoals in estuaries. Applying this new knowledge we are able to set up predictive models, which help us to manage our estuaries in a durable way, combining different model types and data, such that optimal use can be made of each research method.

This research aims to study the behaviour of the concrete-steel bond using numerical models, taking into account the effect of the different bar embedment length. Both plain and fiber reinforced concrete (FRC) are modeled. The interface bond stress as well as load-displacement response of the reinforcing steel is studied. The asociated cracking of the concrete around the slipping bars is analyzed and comparisons are made with the experiments.

The focus is on the numerical flow model TRIWAQ. It is developed as a hydrostatic free-surface flow model, which is currently being used by the KNMI and Rijkswaterstaat for predictions of water levels in the North Sea and Dutch estuaries. TRIWAQ has successfully been extended to the realm of non-hydrostatic modeling, TRIWAQ-NH, this allows the use of full Navier-Stokes equations. It has been validated to perform well for multiple processes such as dispersion and propagation. The goal is to assess the ability of TRIWAQ-NH for harbour problems. This has not been attempted before and poses a new challenge. Before an attempt is made to simulate a harbour, methods of imposing reflection are tested. For that matter, the thesis is split in two parts. The first part will investigate the ability of TRIWAQ-NH with respect to reflections. It will confine itself to four currently implemented open boundary conditions. It aims to provide an answer what condition is most suitable for future development into a partial reflecting boundary condition for harbour simulations. This is done by means of a literature survey, which inspects the background theory it stems from, its dependence on wave frequencies and limitations due to the angle of incidence. By means of 1 dimensional simulations the ability of each open boundary condition is tested when the non-hydrostatic method is used and will be referenced with a similar hydrostatic simulation. A monochromatic wave is used. The second part of the thesis focuses on validating 2 dimensional non-hydrostatic simulations with TRIWAQ-NH. This is done by modelling a simplified rectangular harbour basin of constant depth. A monochromatic wave is selected. From the 1 dimensional test cases, two conditions are selected and simulated, these are the Riemann and sponge layer condition. The last simulation has full reflective boundaries. Each is again referenced with a hydrostatic simulation. The current use of the model TRIWAQ-NH holds practical restrictions to the implementation of open boundary conditions. When such a definition spawns a length of more than grid cell, it needs to comply with the edges of the grid. For instance, a boundary under the angle of 77 degrees is only possible by creation of a multitude of smaller open boundaries, leading to human error. The thesis results in two recommendations. The first part covered reflections, from that segment of the thesis, it is recommended to further explore the option of a stronger and smaller grid sized sponge layer with either an open or closed boundary condition for future development as method for partial reflections. The second recommendation is of a more practical nature and does not emerge in the work. However, the current tools available for non-hydrostatic modelling are insufficient. The smaller wave lengths require smaller time frames then the current tools provide, which is in the order of hours, days and months.

As a good rule of thumb, one-line models have been in use for a wide range of coastal projects for the past 30 years to predict the beach morphological change and make the appropriate shoreline management plans. Though applied to many coastal sites with certain success, these models are still under further development so as to achieve better predictions. Hence, this research aims to extend a state-of-the-art one-line model (BEACHPLAN) in terms of longshore sediment transport rate by linking with a cross-shore profile model (COSMOS) through the OpenMI-based Pipistrelle platform. To couple these two existing models, three additional modules (i.e. Drift Interpolation module, Vector-To-Scalar module and Orientation Updating module) are developed. The existing two models and the three new modules work together as a whole composition (i.e. the coupled model) which aims to improve the one-line model in terms of longshore sediment transport and morphological response predictability. This coupled model calculates the longshore sediment transport rate based on both the modified CERC-formula and the Energetics approach. It is tested at an idealized straight coast and at Poole Bay, UK. This demonstrates that the model is numerically stable, effective in computing and able to give reasonable predictions of the shoreline morphological change. Hence, this dissertation shows that linking the existing models rather than developing completely new models is a potentially interesting research direction that is both economical and feasible to achieve. Some further developments of the model are also suggested. However, it is currently too early to reach the conclusion that the coupled model is better than the original one-line model since few validations have been made due to the limitations of field data and research time. Meanwhile, the working principle behind the coupled model needs further exploration and re-examination before a sound conclusion can be made. Therefore, the outcome of this research at this stage can be more precisely recognized as a proof of concept which demonstrates that it is theoretically reasonable and technically feasible to couple the one-line model and cross-shore profile model so as to realize practitioners’ certain requirements.

This paper focuses on experiments and simulations conducted in very sharp open-channel bends with flat and equilibrium bathymetry, corresponding to the initial and final phases of the erosion and deposition processes, respectively. The study of flow in curved open bends is relevant for flow in natural river configurations, as most river reaches are not straight. The configuration considered in the present work was designed as a test case in which the role of the cross-sectional flow is more severe than in meandering natural river reaches (radius of curvature of the channel is close to the channel width) and, thus, can serve for validation of numerical models used to predict flow and sediment transport in river engineering applications. This paper presents detailed new experimental data on the equilibrium bathymetry as well as depth-averaged distributions, vertical profiles, and cross-sectional patterns of the streamwise velocity, the cross-stream circulation, streamwise vorticity, and the turbulent kinetic energy at the initial and final stages of the erosion and deposition processes. The numerical simulations are performed using a three-dimensional nonhydrostatic RANS model for flow, sediment transport, and bathymetry, which employs fine meshes, accounts for the effect of small bed forms, and avoids the use of the law of the wall. The model predicts, rather accurately, the distribution of the streamwise velocity, the cross-stream circulation, and the turbulent kinetic energy in the simulations conducted with a fixed (flat and deformed bed corresponding to equilibrium conditions) prescribed bathymetry. In the case of a simulation conducted with loose bed, the model predicts satisfactorily the main features of the bathymetry at equilibrium conditions, despite the fact that including the interaction between the flow and the bathymetry increases the overall uncertainty in the model predictions. Results indicate that both improvements in the level of turbulence modeling and in the modeling of the sediment transport would allow further improvement in the predictive capabilities of morphodynamic models.