Bore collapse and wave run-up on a sandy beach

Wave run-up on beaches and coastal structures is initiated and driven by collapsing incident bores, this process is often considered to define the seaward limit of the swash zone. It is hence a key feature in nearshore wave processes as extreme run-up can lead to structure overtopping and coastal inundation during storm conditions. In addition, the turbulent nature of incident bores and their collapse suspends and advects sediment, resulting in a highly morphologically dynamic swash zone. The cross shore bore collapse location varies from wave to wave and the process is very limited in both spatial and temporal extent, making direct measurement problematic. This paper presents high spatial-temporal resolution LiDAR field measurements of the evolving free-surface in the surf and swash zone which enable the bore collapse detection for 166 waves. These measurements are used to investigate the link between broken wave properties at bore collapse and wave run-up. Incident bores are identified at the seaward boundary of the LiDAR profiles and tracked through the inner surf and swash zones to the run-up limit. It is found that the vertical run-up height exceeds that which would be expected for a perfect conversion of potential to kinetic energy during bore collapse for 24% of the bores measured. By returning to an existing ballistic-type model to describe the run-up of individual waves, we show that wave run-up can be divided into three components: the bore collapse, terminal bore celerity and their non-linear interaction. For the present dataset, the contribution of the bore collapse and terminal bore celerity is 26% and 27% respectively, while non-linear interactions between the two dominates and account for 47% of the measured run-up. By including the terminal bore celerity, the ability to predict run-up is increased by 30% with the determination coefficient r increasing from 0.573 to 0.785. Likewise, the RMS-error for the wave run-up shows an approximately 10% reduction from 0.325 to 0.295 m

Bore collapse and wave run-up on a sandy beach

Wave run-up on beaches and coastal structures is initiated and driven by collapsing incident bores, this process is often considered to define the seaward limit of the swash zone. It is hence a key feature in nearshore wave processes as extreme run-up can lead to structure overtopping and coastal inundation during storm conditions. In addition, the turbulent nature of incident bores and their collapse suspends and advects sediment, resulting in a highly morphologically dynamic swash zone. The cross shore bore collapse location varies from wave to wave and the process is very limited in both spatial and temporal extent, making direct measurement problematic. This paper presents high spatial-temporal resolution LiDAR field measurements of the evolving free-surface in the surf and swash zone which enable the bore collapse detection for 166 waves. These measurements are used to investigate the link between broken wave properties at bore collapse and wave run-up. Incident bores are identified at the seaward boundary of the LiDAR profiles and tracked through the inner surf and swash zones to the run-up limit. It is found that the vertical run-up height exceeds that which would be expected for a perfect conversion of potential to kinetic energy during bore collapse for 24 % of the bores measured. By returning to an existing ballistic-type model to describe the run-up of individual waves, we show that wave run-up can be divided into three components: the bore collapse, terminal bore celerity and their non-linear interaction. For the present dataset, the contribution of the bore collapse and terminal bore celerity is 26 % and 27 % respectively, while non-linear interactions between the two dominates and account for 47% of the measured run-up. By including the terminal bore celerity, the ability to predict run-up is increased by 30 % with the determination coefficient r increasing from 0.573 to 0.785. Likewise, the RMS-error for the wave run-up shows an approximately 10 % reduction from 0.325 to 0.295 m.</p

A 3D parallel Particle-In-Cell solver for extreme wave interaction with floating bodies

Floating structures are widely used for vessels, offshore platforms, and recently considered for deep water floating offshore wind system and wave energy devices. However, modelling complex wave interactions with floating structures, particularly under extreme conditions, remains an important challenge. Following the three-dimensional (3D) parallel particle-in-cell (PIC) model developed for simulating wave interaction with fixed bodies, this paper further extends the methodology and develops a new 3D parallel PIC model for applications to floating bodies. The PIC model uses both Lagrangian particles and Eulerian grid to solve the incompressible Navier-Stokes equations, attempting to combine both the Lagrangian flexibility for handling large free-surface deformations and Eulerian efficiency in terms of CPU cost. The wave-structure interaction is resolved via inclusion of a Cartesian cut cell method based two-way strong fluid-solid coupling algorithm that is both stable and efficient. The numerical model is validated against 3D experiments of focused wave interaction with a floating moored buoy. Good agreement between the numerical and experimental results has been achieved for the motion of the buoy and the mooring force. Additionally, the PIC model achieves a CPU efficiency of the same magnitude as that of the state-of-the-art OpenFOAM ® model for an extreme wave-structure interaction scenario

Behavioural modification of local hydrodynamics by asteroids enhances reproductive success

The reproduction of apex species, such as sea stars, is important for sustaining many marine ecosystems. Many sea star species reproduce externally, introducing gametes in the turbulent benthic boundary layer. Sea stars often aggregate and adopt characteristic behaviour, such as arched posturing, while spawning. Here we quantify, for the first time, the hydrodynamic advantages of postural changes and the extent to which they enhance the efficiency of external reproduction. Hydrodynamic and fertilisation kinetic theoretical modelling were used to provide context and comparison. The arched posture was clearly important in the downstream advection of gametes. Digital particle image velocimetry, acoustic doppler velocimetry and dye release experiments indicated reduced wake and lower shear stresses downstream of arched sea stars, which increased downstream transport of gametes compared to those in the flat position. In all cases, sperm concentration decay rates of two orders-of-magnitude over distances < 20 cm were inferred from fluorometry, confirming the requirement for close aggregation. The level of turbulence and hence downstream gamete dilution was increased by greater current speeds and a rougher seabed. Both an arched posture and hydrodynamic conditions may improve external reproduction efficiency, with behavioural mechanisms providing the primary contribution

A new approach for scaling beach profile evolution and sediment transport rates in distorted laboratory models

Laboratory wave flume experiments in coastal engineering and physical oceanography are widely used to provide an improved understanding of morphodynamic processes. Wave flume facilities around the world vary greatly in their physical dimensions and differences in the resulting distortion of the modelled processes are reconciled using scaling laws. However, it is known that perfect model-prototype scaling of all hydro and morphodynamic processes is rarely possible and there is a lack of understanding to what extent distorted models can be used for direct morphological comparison. To address this issue, distorted scale laboratory flume experiments were undertaken in three different facilities, with the aim to measure and compare beach profile evolution under erosive waves and increasing water levels. A novel approach was developed to transform and scale the different experimental geometries into dimensionless coordinates, which enabled a direct quantitative comparison of the beach profile evolution and sediment transport rates between the differing distorted experimental scales. Comparing results from the three experiments revealed that the dimensionless scaled morphological behaviour was similar after the same number of waves – despite very different degrees of model distortion. The distorted profiles appeared to be suitable for comparison as long as a modified version of the Dean number is maintained between them. The new method was then validated with two further published datasets, and showed good agreement for both dimensionless profile shape, dimensionless sediment transport and morphodynamics parameters. The new approach scales the sediment transport by the square of the runup, proportional to HL, rather than H2, and yields good agreement between the datasets. It is further shown that the new scaling method is also applicable for comparing distorted profile evolution under water level increase, as long as the water level is raised in a similar way between the experiments and by the same total increment relative to the significant wave height (Δh/Hs).</p

The influence of swash-based reflection on surf zone hydrodynamics:a wave-by-wave approach

A detailed understanding of the behaviour of waves in the nearshore is essential for coastal engineers as these waves cause beach erosion, coastal flooding and damage to coastal structures. Significantly, the influence of reflected waves is often neglected in surf zone studies, although they are known to influence wave properties and circulation in the nearshore. In this paper, a phase-resolving model is rigorously applied to model conditions from the prototype-scale BARDEXII experiment in order to examine and assess the influence of swash-based reflection on surf zone hydrodynamics at both the individual wave and time-averaged timescales. Surface elevation is separated into incoming and outgoing signals using the Radon Transform and a crest tracking algorithm is used to extract incident and reflected wave properties. It is found that on steep beaches (tan beta > 1: 9) the swash-based reflection - the reflection generated in the swash during the backwash contributes significantly to the intrawave variability of individual wave properties such as the wave height to water depth ratio gamma, through the generation of quasi-nodes/antinodes system. For gamma expressed with individual wave heights, variations up to 25% and 40% are obtained for the modelled regular and irregular wave tests, whereas it reaches 15% when it is based on the significant wave height. The outgoing wave field-induced hydrodynamics is also found to affect time-averaged parameters: undertow and horizontal velocity skewness. The undertow is mainly strengthened, particularly in the shoaling region where the outgoing component dominates over the contribution from the incoming wave field. Offshore of the bar, an onshore-directed flow streaming close to the bed is also generated under the outgoing wave field, and is suspected to help in stabilising the bar position. This, along with the influence of the outgoing wave field on the horizontal velocity skewness and the presence of quasi-standing waves, suggests a complex contribution of the hydrodynamics induced by swash-based reflection into sediment transport rates and nearshore bar generation/migration

Energy dissipation in the inner surf zone: new insights from LiDAR-based roller geometry measurements

The spatial and temporal variation of energy dissipation rates in breaking waves controls the mean circulation of the surf zone. As this circulation plays an important role in the morphodynamics of beaches, it is vital to develop better understanding of the energy dissipation processes in breaking and broken waves. In this paper, we present the first direct field measurements of roller geometry extracted from a LiDAR data set of broken waves to obtain new insights into wave energy dissipation in the inner surf zone. We use a roller model to show that most existing roller area formulations in the literature lead to considerable overestimation of the wave energy dissipation, which is found to be close to, but smaller than, the energy dissipation in a hydraulic jump of the same height. The role of the roller density is also investigated, and we propose that it should be incorporated into modified roller area formulations until better knowledge of the roller area and its link with the mean roller density is acquired. Finally, using previously published results from deepwater wave breaking studies, we propose a scaling law for energy dissipation in the inner surf zone, which achieves satisfactory results at both the time‐averaged and wave‐by‐wave scales