Swash flow due to an obliquely incident bore

Surfzone and swashzone water level fluctuations and cross‐shore fluid velocities observed at seven cross‐shore locations for 139 51‐min periods on a low‐sloped, fine‐grained sandy beach are shown to be modeled well by the depth‐averaged, nonlinear shallow water equations. The model, initialized with observations about 150 m from the shoreline in 300 cm water depth, accurately predicts the observed decrease in sea swell wave heights and orbital velocities with decreasing water depth across the surf zone and the observed maxima in infragravity orbital velocities and offshore‐directed mean flows near the outer edge of the swash zone. In the surf zone both the observed and predicted velocity fluctuations are dominated by sawtoothed‐shaped, pitched forward (asymmetrical) sea swell waves. In the swash zone, infragravity waves contribute substantially to the total velocity variance and asymmetry. Shoreward of the outer swash zone, observed and predicted total and infragravity velocity asymmetry decrease onshore, changing sign from positive to negative in the upper swash.

Field investigation on the results of non-linear shallow water equations in the Swash Zone

1. ABSTRACT The dynamics of waves after breaking is widely investigated because it controls several phenomena in the surf zone and swash zone. Several numerical models based essentially on non-linear shallow water equations (NLSWE) have been developed, but all of them fail to model turbulence in the bore. Many authors have measured fluid velocity in bores using Laser Doppler Velocimetry (LDV), Hot Wire and Hot Film anemometry, and Particle Image Velocimetry (PIV) with good results, though such studies possess several limitations imposed mainly by the presence of air bubbles. In order to overcome such limitations, a set of experiments was carried out in a flume using the Doppler Ultrasonic Technique for fluid velocity measurements. The instrument, a DOP1000, works essentially as a radar and utilizes ultrasound in the range 1 MHz-8 MHz as a carrier. This instrument is able to measure fluid velocity in several points along the US beam with negligible time delay. The maximum data rate obtained is ≈30 profiles per second for each probe, with three probes being employed and a maximum of 255 points per profile. The generated waves have a period of T=2.0, 2.5 and 3.0 s, and break as spilling on a 1:20 bottom. UDVP velocity profiles have been collected in three sections: one at the breaking point and two in the bore region after breaking.

Efficient computation of surf zone waves using the nonlinear shallow water equations with non-hydrostatic pressure

Infragravity waves, also known as surf beat, are important morphodynamic drivers in shallow water, especially inside the surf and swash zone where the short wave energy is dissipated due to breaking. In the past decades, great progress has been acquired in the understanding of surf beat and its implication in the coastal environments. However, many key features are still not fully understood, especially for complex natural systems. This thesis investigates infragravity wave dynamics in the surf and swash zone through a re-analysis of laboratory data, new numerical modelling and novel field measurements. The generation of infragravity waves in the surf zone is commonly associated with two individual mechanisms: release of second-order group-forced long waves and long waves generated by group-induced surf zone breakpoint oscillations. Both mechanisms are forced by radiation stress gradients, but due to their individual nature, different relationships between short and infragravity waves are expected. Determining these relationships, their effectiveness, and the governing hydrodynamic and morphodynamic conditions for each mechanism is complex. In the field, observations are still, to some extent, limited and generally restrained to small wave conditions. The first part of the thesis presents a comprehensive study of different infragravity wave generation mechanisms that includes a critical literature review, a re-analysis of previous laboratory data and an extensive numerical modelling investigation. This work provided new information about the implication of the different processes associated to bound wave shoaling, release and dissipation. In addition, key aspects related to the propagation patterns of infragravity waves have been identified. From the numerical investigation and the large amount of laboratory data re-analysed, clear and distinct relationships between the breakpoint and shoreline excursion have been established for each generation mechanism. The second part of the thesis presents a novel method to determine the dominant infragravity mechanism in the inner surf and swash zone in the field. In the field, the breakpoint oscillations and the shoreline motion are measured remotely via video and their relationship identified via cross-correlation. The identification of the dominant forcing mode, either bound wave or breakpoint, is interpreted based on the specific relationships previously determined. The results of thirteen field data sets collected from three different beaches indicate that, inside the surf zone, the dominance of bound wave or breakpoint forcing is strongly dependent on the surf zone width and the type of short wave breaking. Infragravity generation by bound wave release was stronger for conditions with relatively narrow surf zones and plunging waves; breakpoint forcing was dominant for wider surf zones and spilling breaker conditions, suggesting also that the bound waves remained forced inside the surf zone, being dissipated during short wave breaking. The numerical and laboratory results have also suggested a similar interpretation. This thesis has shown that the breakpoint and shoreline oscillations are relevant features to interpret the surf beat mechanics. The adopted methodology is based on commonly used techniques that can be easily implemented in remote sensing systems used for regular coastal monitoring, enabling easier data collection in more extreme wave conditions.

High intensity air bubbles generated in the surf zone and the thinning of swash flow make velocity measurements particularly challenging in coastal areas. These facts have led the need for a new measurement technique to quantify the surf and swash flow dynamics. Here, we tested infrared image techniques to measure the surface temperature and then to derive the velocity fields using cross-correlation algorithm for large-scale solitary waves breaking in the surf and swash zones. From the comparison with unspiked electromagnetic current meter (EMCM) data and previous validation, it is suggested that the infrared image velocimetry (IRIV) is satisfactory to quantify the surface turbulent flow in the surf and swash zones. The data obtained in the experiment provides a new description of surface thermal structure and kinematics for solitary breaking waves. Two-dimensional organized streaks of temperature structures are evident on the water surface behind the head of rebounding jet. Wavenumber spectrum analysis shows that the directionality of these thermal signatures evolves with time. Evolution of vorticity on the water surface during the run-up and run-down process of the solitary broken wave is discussed.

Laboratory observations of inner surf and swash-zone hydrodynamics on a steep slope

This study investigates the coupling field of grouped wind waves and their associated long waves in the surf and swash zones. Based on the calculated wave fields, the contributions of the wind waves and the long waves on the sediment transport efficiency axe discussed. Spatial variations of the incident grouped wind waves propagating over a plane slope are calculated based on time-dependent mild slope equation. Generation of the long waves is reproduced based on a time-varying breakpoint model proposed by Symonds et al. [1982]. In order to obtain the long wave solutions extending over the landward region from the still water shoreline, calculations using nonlinear shallow water equations are connected to the Symonds' model invoking a moving boundary treatment. The Shields parameters under composition of the grouped wind waves and the associated long waves are evaluated to assess the mobility of the bottom sediment. The results show that the long waves have greater sediment transport efficiency over the grouped wind waves in the swash zone.

This paper presents a numerical investigation of multiple identical swash events to study the swash–swash interaction processes and their impacts on beachface evolution. The numerical model, based on the Nonlinear Shallow Water Equations, is first calibrated/validated against two different single-event-based data-sets. Multiple swash events are generated by identical solitary waves separated by different time intervals, to achieve weak and strong wave-backwash interactions. After a small number of weak interaction events the main feature is erosion from lower and mid swash region and deposition seaward of the swash in a bed-step, created by a backwash bore, primarily due to bed-load. As the number of waves increases, the strength of this backwash bore reduces because of the reduced beach slope caused by the growing bed-step. This eventually leads to a net quasi-equilibrium between bed- and suspended-load per period in most of the swash and surf zones. For strong interaction, initial bed evolution per event is much slower, due to interactions, and is bed load dominated. A quasi-equilibrium is also established as the influence of suspended load grows. Overall bed change per period within the domain eventually converges in both cases. Final bed profiles (i.e. after the same elapsed time, but different numbers of waves) are fairly similar, both with an offshore swash bar. Both profiles continue to evolve on the offshore side of this bar. However, this evolution is driven by suspended load for the weak interactions and bed load for strong interactions. The implication is that similar swash morphological features can emerge from different swash processes, and also be maintained distinctly.

This study investigates the coupling field of grouped wind waves and their associated long waves in the surf and swash zones. Based on the calculated wave fields, the contributions of the wind waves and the long waves on the sediment mobility are discussed. Spatial variations of the incident grouped wind waves propagating over a plane slope are calculated based on time-dependent mild slope equations. Generation of the long waves is reproduced based on a time-varying breakpoint model proposed by Symonds et al. [1982]. In order to obtain the long wave solutions extending over the landward region from the still water shoreline, calculations using non-linear shallow water equations are connected to the Symonds' model invoking a moving boundary treatment. The Shields parameters under composition of the grouped wind waves and the associated long waves are evaluated to assess the mobility of the bottom sediment. The results show that the long waves have greater sediment transport ability over the grouped wind waves in the swash zone.

The effect of shallow water bathymetry on swash and surf zone modelled by SWASH

Rehman, K.; Park, K.-Y., and Cho, Y.-S., 2018. Experimental and Numerical Investigation of Solitary Wave Run-up Reduction. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 1111–1115. Coconut Creek (Florida), ISSN 0749-0208.Rising sea-levels and extreme wave events threaten coastal communities and stability of coastal regions. Accurate prediction of wave over-topping over coastal protection structures is challenging, but vital for effective hazard mitigation. Non-hydrostatic numerical modelling and laboratory experiments are used to assess magnitude of run-up over coastal protection structures under varying relative wave heights and structural features. The primary focus is the investigation of solitary wave impact with breakwaters, the consequent run-up and measures for its reduction. The experiments consisted of generating solitary waves in a 1.1 m high and 32.5 m long flume and observing its run-up for different heights of incident waves. A slope adjuster was used to vary the slope of a plywood plank for reproducing coastal features. Experimental observations were verified by proposing a numerical model based on non-linear shallow water equations (NLSWE) and solution is obtained by Godunov-type finite volume method. The NLSWE provide good approximation of shoaling, wave breaking, and wave reflection which arise due to wave overtopping in the swash and surf zones. The novel feature of the numerical model is the introduction of bed slope discretization technique – applicable on both structure and unstructured meshes- which offers well-balanced solution even for steep slopes encountered in case of breakwaters. Shock-capturing capabilities of Harten, Lax, and van Leer with contact wave restoration (HLLC) solver are utilized for accurate estimation of shocks and bore waves features during flow transitions. The proposed model gives excellentn agreement with experimental observations. The findings will further enhance the understanding of extreme wave propagation events over submerged coastal structures and related mitigation techniques.

The swash zone is that part of a beach over which the instantaneous shoreline moves back and forth as waves meet the shore. This zone is discussed using the nonlinear shallow water equations which are appropriate for gently sloping beaches. A weakly three-dimensional extension of the two-dimensional solution by Carrier & Greenspan (1958) of the shallow water equations for a wave reflecting on an inclined plane beach is developed and used to illustrate the ideas. Thereafter attention is given to integrated and averaged quantities. The mean shoreline might be defined in several ways, but for modelling purposes we find the lower boundary of the swash zone to be more useful. A set of equations obtained by integrating across the swash zone is investigated as a model for use as an alternative boundary condition for wave-resolving studies. Comparison with sample numerical computations illustrates that they are effective in modelling the dynamics of the swash zone and that a reasonable representation of swash zone flows may be obtained from the integrated variables. The longshore flow of water in the swash zone is in many ways similar to the Stokes’ drift of propagating water waves. Further averaging is made over short waves to obtain results suitable as boundary conditions for longer period motions including the effect of incident short waves. In order to clearly present the work a few simplifications are made. The main result is that in addition to the kinematic type of boundary condition that occurs on a simple, e.g. rigid, boundary two further conditions are found in order that both the changing position of the swash zone boundary and the longshore flow in the swash zone may be determined. Models of the short waves both outside and inside the swash zone are needed to complete a full wave-averaged model; only brief indication is given of such modelling.

A quantitative understanding of the migration of munitions and canonical objects in the nearshore is needed for the effective management of contaminated sites. Migrations of munitions with a density range of 2000 kg/m3 to 5720 kg/m3 were quantified in a large-scale wave flume. The forcing consisted of six cases of varying wave heights, periods, still water depths, and durations. The cross-shore profile, typical of natural sandy beaches, was sub-divided into swash, surf, and offshore zones. Overall, 2228 migration measurements were recorded with 16% and 84% of the migration observations classified as “motion” (net distance > 0.5 m) and “no motion” (net distance ≤ 0.5 m), respectively. The probability of munitions migration increased with proximity to the shoreline. There was a nearly equal probability of onshore or offshore migration in the swash zone. Migration in the surf zone tended to be offshore-directed (65%), while migration was onshore-dominant (65%) in the offshore zone. Migration in the offshore zone was preferentially onshore due to skewed waves over flat bathymetry. Less dense munitions in the offshore zone may have migrated offshore likely still related to the skewed nature of the wave profile causing transport in both directions through the majority of the wave phase. The largest migration distances occurred in the surf zone likely due to downslope gravity. Migration in the surf and swash zones is a balance between skewed/asymmetric forcing and downslope gravity, with downslope gravity tending to be pronounced provided the forcing is sufficient to initiate motion. An exception was sometimes observed in the swash zone where onshore forcing was sufficient to transport munitions to the seaward side of the berm where they became trapped in a bathymetric depression between the dune and berm. Relating overall migration (Lagrangian) to fixed hydrodynamic measurements (Eulerian) was ineffective. Parameters such as the Shields number, wave skewness, and wave asymmetry estimated from the closest measurement location were insufficient to predict migration. Large scatter in the migration data resulting from competing hydrodynamic, morphodynamic, and munitions response processes makes robust deterministic predictions with flow statistics and dimensionless numbers difficult.

The role of the swash zone in influencing the whole nearshore dynamics is reviewed with a focus on the interaction between surf and swash zone processes. Local and global hydromorphodynamic phenomena are discussed in detail, and a description of the overall swash zone operation is given. The effects of swash zone boundary conditions are highlighted, together with the importance of surf zone boundary conditions. Major emphasis is placed on illustrating the interactions of various hydrodynamic modes which, in turn, control the swash and surf zone morphology. Finally, methods to account for swash zone processes in coastal models with different temporal and spatial resolutions are proposed.