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This thesis is about the hydrodynamic processes that occur when a nearshore sandbar migrates towards the shore driven by the waves. These processes are relevant because process-based morphological models have difficulty simulating coastal accretion events in contrast to erosion events. Simulating accretion events is important for engineering purposes, for example, to optimize shoreface nourishment for the conservation of beach and dunes of the Netherlands. During onshore sandbar migration, sediment is dominantly transported at the bed where the orbital wave motion applies friction on the bed. This transport, known as bedload, may result in a cross-shore net transport due to various hydrodynamic processes. First off, the orbital motion under waves in the nearshore are not sinusoidal but asymmetric as waves have steep fronts and flat backs. The wave shape is reflected back in the orbital motion which has two consequences, first, the largest flow velocities are found under the wave crests, and second, the largest horizontal pressure gradients are found under the wave fronts. Numerical modeling results have shown that in the wave bottom boundary layer the shape of the orbital motion is altered due to flow deceleration. However, there have not been any observations to validate this. Another set of hydrodynamic processes are the time-averaged currents generated by the waves. In the wave bottom boundary layer these processes are ‘progressive wave streaming’ and ‘wave shape streaming’. In addition, mass flux by waves (Stokes drift) and wave breaking processes also generate currents that have an affect on the time-averaged bottom shear stress. Although the time-averaged bed shear stress is relatively small compared to the time-varying component it may have a large impact on the net sediment transport. The accumulative effect of the aforementioned processes on the time-averaged bed shear stress is not well understood., Coastal Engineering

This thesis is about the hydrodynamic processes that occur when a nearshore sandbar migrates towards the shore driven by the waves. These processes are relevant because process-based morphological models have difficulty simulating coastal accretion events in contrast to erosion events. Simulating accretion events is important for engineering purposes, for example, to optimize shoreface nourishment for the conservation of beach and dunes of the Netherlands. During onshore sandbar migration, sediment is dominantly transported at the bed where the orbital wave motion applies friction on the bed. This transport, known as bedload, may result in a cross-shore net transport due to various hydrodynamic processes. First off, the orbital motion under waves in the nearshore are not sinusoidal but asymmetric as waves have steep fronts and flat backs. The wave shape is reflected back in the orbital motion which has two consequences, first, the largest flow velocities are found under the wave crests, and second, the largest horizontal pressure gradients are found under the wave fronts. Numerical modeling results have shown that in the wave bottom boundary layer the shape of the orbital motion is altered due to flow deceleration. However, there have not been any observations to validate this. Another set of hydrodynamic processes are the time-averaged currents generated by the waves. In the wave bottom boundary layer these processes are ‘progressive wave streaming’ and ‘wave shape streaming’. In addition, mass flux by waves (Stokes drift) and wave breaking processes also generate currents that have an affect on the time-averaged bottom shear stress. Although the time-averaged bed shear stress is relatively small compared to the time-varying component it may have a large impact on the net sediment transport. The accumulative effect of the aforementioned processes on the time-averaged bed shear stress is not well understood., Coastal Engineering

Wave-supported gravity flows (WSGFs) generate rates of sediment flux far exceeding other cross-shelf transport processes, contributing disproportionately to shelf morphology and net cross-shelf fluxes of sediment in many regions worldwide. However, the conditions deemed necessary for the formation of WSGF limit them to a narrow set of shelf conditions; they have been observed exclusively in regions where the seabed consists of very fine-grained sediment and typically co-occur with nearby river flood events. Here we document the occurrence of a WSGF event on a predominantly sandy seabed and in the absence of a preceding river flood. Our measurements confirm that the dynamics are governed by the friction-buoyancy balance observed in other WSGF and that WSGF can form in mixed grain-size environments and transport high concentrations of sand. The occurrence of WSGF on a predominantly sandy seabed suggests that they may occur under a much wider range of conditions and, given the global prevalence of sandy shelves, they may be a more frequent and more ubiquitous feature of shelf dynamics than previously thought., Environmental Fluid Mechanics, Coastal Engineering

The Sand Engine is a 21.5 million m3 experimental mega-nourishment project that was built in 2011 along the Dutch coast. This intervention created a discontinuity in the previous straight sandy coastline, altering the local hydrodynamics in a region that is influenced by the buoyant plume generated by the Rhine River. This work investigates the response of the cross-shore stratified tidal flow to the coastal protrusion created by the Sand Engine emplacement by using a 13 h velocity and density survey. Observations document the development of strong baroclinic-induced cross-shore exchange currents dictated by the intrusion of the river plume fronts as well as the classic tidal straining which are found to extend further into the nearshore (from 12 to 6m depth), otherwise believed to be a mixed zone. Estimates of the centrifugal acceleration directly after construction of the Sand Engine showed that the curvature effects were approximately 2 times stronger, suggesting that the Sand Engine might have played a role in controlling the cross-shore exchange currents during the first three years after the completion of the nourishment. Presently, the curvature effects are minute., Coastal Engineering, Environmental Fluid Mechanics

The Sand Engine is a 21.5 million m3 experimental mega-nourishment project that was built in 2011 along the Dutch coast. This intervention created a discontinuity in the previous straight sandy coastline, altering the local hydrodynamics in a region that is influenced by the buoyant plume generated by the Rhine River. This work investigates the response of the cross-shore stratified tidal flow to the coastal protrusion created by the Sand Engine emplacement by using a 13 h velocity and density survey. Observations document the development of strong baroclinic-induced cross-shore exchange currents dictated by the intrusion of the river plume fronts as well as the classic tidal straining which are found to extend further into the nearshore (from 12 to 6m depth), otherwise believed to be a mixed zone. Estimates of the centrifugal acceleration directly after construction of the Sand Engine showed that the curvature effects were approximately 2 times stronger, suggesting that the Sand Engine might have played a role in controlling the cross-shore exchange currents during the first three years after the completion of the nourishment. Presently, the curvature effects are minute., Coastal Engineering, Environmental Fluid Mechanics

Wave-supported gravity flows (WSGFs) generate rates of sediment flux far exceeding other cross-shelf transport processes, contributing disproportionately to shelf morphology and net cross-shelf fluxes of sediment in many regions worldwide. However, the conditions deemed necessary for the formation of WSGF limit them to a narrow set of shelf conditions; they have been observed exclusively in regions where the seabed consists of very fine-grained sediment and typically co-occur with nearby river flood events. Here we document the occurrence of a WSGF event on a predominantly sandy seabed and in the absence of a preceding river flood. Our measurements confirm that the dynamics are governed by the friction-buoyancy balance observed in other WSGF and that WSGF can form in mixed grain-size environments and transport high concentrations of sand. The occurrence of WSGF on a predominantly sandy seabed suggests that they may occur under a much wider range of conditions and, given the global prevalence of sandy shelves, they may be a more frequent and more ubiquitous feature of shelf dynamics than previously thought., Environmental Fluid Mechanics, Coastal Engineering

We present a new mechanism for cross-shore transport of fine sediment from the nearshore to the inner shelf resulting from the onshore propagation of river plume fronts. Onshore frontal propagation is observed in moorings and radar images, which show that fronts penetrate onshore through the nearshore and surf zone, almost to the waterline. During frontal passage a two-layer counterrotating velocity field characteristic of tidal straining is immediately set up, generating a net offshore flow beneath the plume. The seaward flow at depth carries with it high suspended sediment concentrations, which appear to have been generated by wave resuspension in the nearshore region. These observations describe a mechanism by which vertical density stratification can drive exchange of material between the nearshore region and the inner shelf. To our knowledge these are the first observations of this frontal pumping mechanism, which is expected to play an important role in sediment transport near river mouths., Environmental Fluid Mechanics, Coastal Engineering

We present a new mechanism for cross-shore transport of fine sediment from the nearshore to the inner shelf resulting from the onshore propagation of river plume fronts. Onshore frontal propagation is observed in moorings and radar images, which show that fronts penetrate onshore through the nearshore and surf zone, almost to the waterline. During frontal passage a two-layer counterrotating velocity field characteristic of tidal straining is immediately set up, generating a net offshore flow beneath the plume. The seaward flow at depth carries with it high suspended sediment concentrations, which appear to have been generated by wave resuspension in the nearshore region. These observations describe a mechanism by which vertical density stratification can drive exchange of material between the nearshore region and the inner shelf. To our knowledge these are the first observations of this frontal pumping mechanism, which is expected to play an important role in sediment transport near river mouths., Environmental Fluid Mechanics, Coastal Engineering

The South-Holland coast of the Netherlands undergoes the influence of the Rhine river plume released from the Rotterdam waterways. An experiment, STRAINS, was conducted to study the impact of the fresh water on the nearshore hydrodynamics and sand transport. As part of the experiment, an instrumented bottom frame measured the near-bed hydrodynamics at 12 m depth. The flow was decomposed in the tidal, wave and turbulent component. During moderate energetic wave conditions the cross-shore tidal flow was of similar magnitude as the wave orbital flow. The cross-shore tidal flow was asymmetric and larger in the seaward direction. The cross-shore tidal component may be generated by tidal straining due to the river plume., Coastal Engineering, Environmental Fluid Mechanics

The South-Holland coast of the Netherlands undergoes the influence of the Rhine river plume released from the Rotterdam waterways. An experiment, STRAINS, was conducted to study the impact of the fresh water on the nearshore hydrodynamics and sand transport. As part of the experiment, an instrumented bottom frame measured the near-bed hydrodynamics at 12 m depth. The flow was decomposed in the tidal, wave and turbulent component. During moderate energetic wave conditions the cross-shore tidal flow was of similar magnitude as the wave orbital flow. The cross-shore tidal flow was asymmetric and larger in the seaward direction. The cross-shore tidal component may be generated by tidal straining due to the river plume., Hydraulic Engineering, Civil Engineering and Geosciences

The South-Holland coast of the Netherlands undergoes the influence of the Rhine river plume released from the Rotterdam waterways. An experiment, STRAINS, was conducted to study the impact of the fresh water on the nearshore hydrodynamics and sand transport. As part of the experiment, an instrumented bottom frame measured the near-bed hydrodynamics at 12 m depth. The flow was decomposed in the tidal, wave and turbulent component. During moderate energetic wave conditions the cross-shore tidal flow was of similar magnitude as the wave orbital flow. The cross-shore tidal flow was asymmetric and larger in the seaward direction. The cross-shore tidal component may be generated by tidal straining due to the river plume., Coastal Engineering, Environmental Fluid Mechanics

The South-Holland coast of the Netherlands undergoes the influence of the Rhine river plume released from the Rotterdam waterways. An experiment, STRAINS, was conducted to study the impact of the fresh water on the nearshore hydrodynamics and sand transport. As part of the experiment, an instrumented bottom frame measured the near-bed hydrodynamics at 12 m depth. The flow was decomposed in the tidal, wave and turbulent component. During moderate energetic wave conditions the cross-shore tidal flow was of similar magnitude as the wave orbital flow. The cross-shore tidal flow was asymmetric and larger in the seaward direction. The cross-shore tidal component may be generated by tidal straining due to the river plume., Hydraulic Engineering, Civil Engineering and Geosciences

The Sand Engine is a mega-nourishment constructed in 2011 with the purpose of replenishing the surrounding southern Dutch coast by exploiting the strength of local alongshore currents for the next 20 years. Long term monitoring of the Sand Engine depends upon remote sensing coupled with in-situ measurements due to both its large spatial and temporal scales and variability. Herein, emphasis is made upon quantitatively and qualitatively observing the effects of this mega-nourishment upon its leeward side in the direction of the predominant alongshore current. One of the first applications of cBathy along the Dutch coast shows promising results for this remote technique to capture morphodynamics within the area over the coming years. cBathy is an algorithm developed to make estimations of nearshore bathymetry based upon the celerity of the propagating wave field extracted from Argus coastal images. So far, analysis shows that the Dutch wave climate variability influences hourly cBathy depth approximations, however running average depth estimations yield small deviation compared with a measured ground truth bathymetry of the area., Hydraulic Engineering, Civil Engineering and Geosciences

The Sand Engine is a mega-nourishment constructed in 2011 with the purpose of replenishing the surrounding southern Dutch coast by exploiting the strength of local alongshore currents for the next 20 years. Long term monitoring of the Sand Engine depends upon remote sensing coupled with in-situ measurements due to both its large spatial and temporal scales and variability. Herein, emphasis is made upon quantitatively and qualitatively observing the effects of this mega-nourishment upon its leeward side in the direction of the predominant alongshore current. One of the first applications of cBathy along the Dutch coast shows promising results for this remote technique to capture morphodynamics within the area over the coming years. cBathy is an algorithm developed to make estimations of nearshore bathymetry based upon the celerity of the propagating wave field extracted from Argus coastal images. So far, analysis shows that the Dutch wave climate variability influences hourly cBathy depth approximations, however running average depth estimations yield small deviation compared with a measured ground truth bathymetry of the area., Hydraulic Engineering, Civil Engineering and Geosciences

The Netherlands’ strategy to combat coastal erosion since 1990 has been through nourishment, initially as beach nourishments but more and more as shoreface nourishments. In the light of sea level rise projections the yearly nourishment magnitudes continue to increase. In view of this an innovative soft engineering intervention, comprising an unprecedented 21 Mm3 sand nourishment known as the Sand Engine, has recently been implemented in the Netherlands. The Sand Engine nourishment is a pilot project to test the effectiveness and efficiency of a local meganourishment as a measure to account for the anticipated increased coastal recession in this century. The proposed concept, a single mega-nourishment, once every 20 years, is expected to be more efficient and effective in the long term than traditional beach and shoreface nourishments, presently being used at the Dutch coast with typically a three to five year interval. While the judgement is still out on this globally unique intervention, if proven successful, it may well become a generic solution for combating sea level rise driven coastal recession on open and vulnerable coasts., Hydraulic Engineering, Civil Engineering and Geosciences

Observations of the spatially dependent velocity field over movable bed forms subjected to slightly skewed and asymmetric regular wave forcing were collected. The dynamics between the ripple elements is dominated by coherent vortices, characterized by the swirling strength, and evidenced in the temporal and spectral characterization. Within the boundary layer, spectral energy in the second harmonic (3f0) is amplified at the ripple slopes and is consistent with the location of the expected strongest pressure gradients. First-moment and second-moment velocity statistics were used to address the spatial variability of the intra-ripple hydrodynamics. Estimates of displacement and momentum thicknesses (?* and ?mom) are smaller than suggested by the higher harmonics, but consistently highlight areas of adverse and favorable pressure gradients. Shear stress and roughness estimates were inferred by fitting a logarithmic model to first-moment and second-moment statistics of the velocity field. The maximum Shields parameter was observed to peak at the stoss slope of asymmetric ripples during the strongest and shorter half-wave period (onshore). First-moment roughness estimates are similar in magnitude to bed load parameterizations provided by Li et al. (1997), and about a factor of 3 larger than second-moment estimates. Assessment of the vertical transfer of horizontal momentum derived using a Reynolds decomposition suggests that stresses inferred from the logarithmic law using first-moment velocity statistics appropriately reproduce the mean momentum transfer for the longer and weaker offshore half-wave period., Hydraulic Engineering, Civil Engineering and Geosciences

The Sand Engine is a mega-nourishment constructed in 2011 with the purpose of replenishing the surrounding southern Dutch coast by exploiting the strength of local alongshore currents for the next 20 years. Long term monitoring of the Sand Engine depends upon remote sensing coupled with in-situ measurements due to both its large spatial and temporal scales and variability. Herein, emphasis is made upon quantitatively and qualitatively observing the effects of this mega-nourishment upon its leeward side in the direction of the predominant alongshore current. One of the first applications of cBathy along the Dutch coast shows promising results for this remote technique to capture morphodynamics within the area over the coming years. cBathy is an algorithm developed to make estimations of nearshore bathymetry based upon the celerity of the propagating wave field extracted from Argus coastal images. So far, analysis shows that the Dutch wave climate variability influences hourly cBathy depth approximations, however running average depth estimations yield small deviation compared with a measured ground truth bathymetry of the area., Hydraulic Engineering, Civil Engineering and Geosciences

Observations of the spatially dependent velocity field over movable bed forms subjected to slightly skewed and asymmetric regular wave forcing were collected. The dynamics between the ripple elements is dominated by coherent vortices, characterized by the swirling strength, and evidenced in the temporal and spectral characterization. Within the boundary layer, spectral energy in the second harmonic (3f0) is amplified at the ripple slopes and is consistent with the location of the expected strongest pressure gradients. First-moment and second-moment velocity statistics were used to address the spatial variability of the intra-ripple hydrodynamics. Estimates of displacement and momentum thicknesses (?* and ?mom) are smaller than suggested by the higher harmonics, but consistently highlight areas of adverse and favorable pressure gradients. Shear stress and roughness estimates were inferred by fitting a logarithmic model to first-moment and second-moment statistics of the velocity field. The maximum Shields parameter was observed to peak at the stoss slope of asymmetric ripples during the strongest and shorter half-wave period (onshore). First-moment roughness estimates are similar in magnitude to bed load parameterizations provided by Li et al. (1997), and about a factor of 3 larger than second-moment estimates. Assessment of the vertical transfer of horizontal momentum derived using a Reynolds decomposition suggests that stresses inferred from the logarithmic law using first-moment velocity statistics appropriately reproduce the mean momentum transfer for the longer and weaker offshore half-wave period., Hydraulic Engineering, Civil Engineering and Geosciences

The Netherlands’ strategy to combat coastal erosion since 1990 has been through nourishment, initially as beach nourishments but more and more as shoreface nourishments. In the light of sea level rise projections the yearly nourishment magnitudes continue to increase. In view of this an innovative soft engineering intervention, comprising an unprecedented 21 Mm3 sand nourishment known as the Sand Engine, has recently been implemented in the Netherlands. The Sand Engine nourishment is a pilot project to test the effectiveness and efficiency of a local meganourishment as a measure to account for the anticipated increased coastal recession in this century. The proposed concept, a single mega-nourishment, once every 20 years, is expected to be more efficient and effective in the long term than traditional beach and shoreface nourishments, presently being used at the Dutch coast with typically a three to five year interval. While the judgement is still out on this globally unique intervention, if proven successful, it may well become a generic solution for combating sea level rise driven coastal recession on open and vulnerable coasts., Hydraulic Engineering, Civil Engineering and Geosciences

The cause of sediment suspension events during flow reversal under waves in the nearshore is not well understood. Vortex tubes and horizontal pressure gradients have been suggested to be the cause of the suspension events. A medium sized wave flume experiment has been conducted to give insight in the hydrodynamics of the wave bottom boundary layer over a fixed single-barred profile. Flow measurements were made with PIV and the swirling strength of the velocity fields were analyzed. Around flow reversal vortex tubes were identified. The vortex tubes had similar size and swirling strength as vortices generated by vortex shedding over a rippled bed. Therefore, vortex tubes under waves in the nearshore could explain the sediment suspension events around flow reversal., Water Management, Civil Engineering and Geosciences

The cause of sediment suspension events during flow reversal under waves in the nearshore is not well understood. Vortex tubes and horizontal pressure gradients have been suggested to be the cause of the suspension events. A medium sized wave flume experiment has been conducted to give insight in the hydrodynamics of the wave bottom boundary layer over a fixed single-barred profile. Flow measurements were made with PIV and the swirling strength of the velocity fields were analyzed. Around flow reversal vortex tubes were identified. The vortex tubes had similar size and swirling strength as vortices generated by vortex shedding over a rippled bed. Therefore, vortex tubes under waves in the nearshore could explain the sediment suspension events around flow reversal., Water Management, Civil Engineering and Geosciences

To study onshore bar migration and the accompanying intra-wave sediment transport a wave flume experiment was conducted. The wave flume had a rigid bottom with a single bar profile. The focus of the experiment was to measure the hydrodynamics in the wave bottom boundary layer. The results show that the skewness of bottom stress is not only related to wave skewness but also to wave asymmetry., Hydraulic Engineering, Civil Engineering and Geosciences

To study onshore bar migration and the accompanying intra-wave sediment transport a wave flume experiment was conducted. The wave flume had a rigid bottom with a single bar profile. The focus of the experiment was to measure the hydrodynamics in the wave bottom boundary layer. The results show that the skewness of bottom stress is not only related to wave skewness but also to wave asymmetry., Hydraulic Engineering, Civil Engineering and Geosciences

The retention of floating matter within the surf zone on a rip-channeled beach is examined with a combination of detailed field observations obtained during the Rip Current Experiment and a three-dimensional (3-D) wave and flow model. The acoustic Doppler current profiler–observed hourly vertical cross-shore velocity structure variability over a period of 3 days with normally incident swell is well reproduced by the computations, although the strong vertical attenuation of the subsurface rip current velocities at the most offshore location outside the surf zone in 4 m water depth is not well predicted. Corresponding mean alongshore velocities are less well predicted with errors on the order of 10 cm/s for the most offshore sensors. Model calculations of very low frequency motions (VLFs) with O(10) min timescales typically explain over 60% of the observed variability, both inside and outside of the surf zone. The model calculations also match the mean rip-current surface flow field inferred from GPS-equipped drifter trajectories. Seeding the surf zone with a large number of equally spaced virtual drifters, the computed instantaneous surface velocity fields are used to calculate the hourly drifter trajectories. Collecting the hourly drifter exits, good agreement with the observed surf zone retention is obtained provided that both Stokes drift and VLF motions are accounted for in the modeling of the computed drifter trajectories. Without Stokes drift, the estimated number of virtual drifter exits is O(80)%, almost an order of magnitude larger than the O(20)% of observed exits during the drifter deployments. Conversely, when excluding the VLF motions instead, the number of calculated drifter exits is less than 5%, thus significantly underestimating the number of observed exits., Hydraulic Engineering, Civil Engineering and Geosciences

In this paper a new wave pool concept is presented using ship hulls as wave generators. To evaluate the potential of such a concept a towing tank experiment is conducted to investigate ship wave generation in a confined space. In the towing tank experiment various hull sizes and velocities are examined to maximize the generated wave height. The laboratory data shows that when blocking of the cross section of the pool by the hull is large, the large return flow significantly influences the wave height. This is in contrast to the open water case where the return flow is much smaller and therefore is not incorporated in wave height predictions. Based on the results of the physical model tests an optimum hull size and velocity are derived to maximize the potential use for surfing and a preliminary design of the wave pool is proposed., Hydraulic Engineering, Civil Engineering and Geosciences

Scale relations are necessary to choose proper sediment for physical models. The use of lightweight artificial sediment makes it possible to preserve the Reynolds number, Shields number and relative settling velocity altogether. There is also the possibility to preserve the Shields number in combination with the Dean number which would be more suitable in the case of significant wave breaking. Both cases have significant scale effects. A physical model test showed that the scale relations are useful under restricted conditions., Civil Engineering and Geosciences

The retention of floating matter within the surf zone on a rip-channeled beach is examined with a combination of detailed field observations obtained during the Rip Current Experiment and a three-dimensional (3-D) wave and flow model. The acoustic Doppler current profiler–observed hourly vertical cross-shore velocity structure variability over a period of 3 days with normally incident swell is well reproduced by the computations, although the strong vertical attenuation of the subsurface rip current velocities at the most offshore location outside the surf zone in 4 m water depth is not well predicted. Corresponding mean alongshore velocities are less well predicted with errors on the order of 10 cm/s for the most offshore sensors. Model calculations of very low frequency motions (VLFs) with O(10) min timescales typically explain over 60% of the observed variability, both inside and outside of the surf zone. The model calculations also match the mean rip-current surface flow field inferred from GPS-equipped drifter trajectories. Seeding the surf zone with a large number of equally spaced virtual drifters, the computed instantaneous surface velocity fields are used to calculate the hourly drifter trajectories. Collecting the hourly drifter exits, good agreement with the observed surf zone retention is obtained provided that both Stokes drift and VLF motions are accounted for in the modeling of the computed drifter trajectories. Without Stokes drift, the estimated number of virtual drifter exits is O(80)%, almost an order of magnitude larger than the O(20)% of observed exits during the drifter deployments. Conversely, when excluding the VLF motions instead, the number of calculated drifter exits is less than 5%, thus significantly underestimating the number of observed exits., Hydraulic Engineering, Civil Engineering and Geosciences

In this paper a new wave pool concept is presented using ship hulls as wave generators. To evaluate the potential of such a concept a towing tank experiment is conducted to investigate ship wave generation in a confined space. In the towing tank experiment various hull sizes and velocities are examined to maximize the generated wave height. The laboratory data shows that when blocking of the cross section of the pool by the hull is large, the large return flow significantly influences the wave height. This is in contrast to the open water case where the return flow is much smaller and therefore is not incorporated in wave height predictions. Based on the results of the physical model tests an optimum hull size and velocity are derived to maximize the potential use for surfing and a preliminary design of the wave pool is proposed., Hydraulic Engineering, Civil Engineering and Geosciences

Scale relations are necessary to choose proper sediment for physical models. The use of lightweight artificial sediment makes it possible to preserve the Reynolds number, Shields number and relative settling velocity altogether. There is also the possibility to preserve the Shields number in combination with the Dean number which would be more suitable in the case of significant wave breaking. Both cases have significant scale effects. A physical model test showed that the scale relations are useful under restricted conditions., Civil Engineering and Geosciences

Artificial Surf Reefs (ASR) are built to improve the surf conditions of the waves. Three ASR have been realized over the last decade and many more are planned to be constructed within a period of ten years. Even though a hot topic, ASR design is often done fairly ad hoc and there remains great uncertainty as to what the optimal dimensions of the ASR should be. Through combination of numerical and experimental modeling efforts, this study aims at: 1) Improving our general understanding of how ASR design affects the actual surfability; 2) Designing an optimal reef shape; 3) Obtaining measurements of the surfability parameters to verify the predicted surfability by the numerical wave model. The quality of a surf break is expressed in four measurable parameters: breaker height, breaker shape, peel angle and the attendant currents. Usually a surf break differs from other places along the coast by the ability to make waves peel along their crest. Therefore the peel angle is studied thoroughly. By considering idealized topographies, the important reef properties that affect the peel angle become clear. The model applied in the numerical study is a state-of-the-art, deterministic amplitude evolution model that takes into account quadratic interactions (triad interactions). Optimal shape dimensions were estimated in the numerical study for varying reef shapes and wave conditions. The resulting ASR design was constructed in a wave basin in the laboratory of Fluid Mechanics of the Faculty of Civil Engineering. Unidirectional incident waves (regular and bichromatic) were generated by three piston type wave makers with second-order wave board control. The laboratory observations were used to verify three parameters that were predicted with the numerical study: breaker height, breaker shape and the peel angle. The combined numerical and experimental study shows that: Two reef properties are needed to obtain surfable peel angles. First there must be a certain angle enclosed betwee, Civil Engineering and Geosciences

Artificial Surf Reefs (ASR) are built to improve the surf conditions of the waves. Three ASR have been realized over the last decade and many more are planned to be constructed within a period of ten years. Even though a hot topic, ASR design is often done fairly ad hoc and there remains great uncertainty as to what the optimal dimensions of the ASR should be. Through combination of numerical and experimental modeling efforts, this study aims at: 1) Improving our general understanding of how ASR design affects the actual surfability; 2) Designing an optimal reef shape; 3) Obtaining measurements of the surfability parameters to verify the predicted surfability by the numerical wave model. The quality of a surf break is expressed in four measurable parameters: breaker height, breaker shape, peel angle and the attendant currents. Usually a surf break differs from other places along the coast by the ability to make waves peel along their crest. Therefore the peel angle is studied thoroughly. By considering idealized topographies, the important reef properties that affect the peel angle become clear. The model applied in the numerical study is a state-of-the-art, deterministic amplitude evolution model that takes into account quadratic interactions (triad interactions). Optimal shape dimensions were estimated in the numerical study for varying reef shapes and wave conditions. The resulting ASR design was constructed in a wave basin in the laboratory of Fluid Mechanics of the Faculty of Civil Engineering. Unidirectional incident waves (regular and bichromatic) were generated by three piston type wave makers with second-order wave board control. The laboratory observations were used to verify three parameters that were predicted with the numerical study: breaker height, breaker shape and the peel angle. The combined numerical and experimental study shows that: Two reef properties are needed to obtain surfable peel angles. First there must be a certain angle enclosed betwee, Civil Engineering and Geosciences