Iván Cáceres

Large-scale experiments on wave propagation over Posidonia oceanica

Posidonia oceanica, the most abundant seagrass species in the Mediterranean, supports a highly bio-diverse habitat and is crucial in protecting against coastal erosion. In this work, experiments in a large-scale facility have been performed, for the measurement of wave attenuation, transmission and energy dissipation over artificial Posidonia oceanica. The effects of submergence ratio corresponding to the seagrass height divided by water depth, and seagrass density as the number of stems per square metre on the above characteristics are investigated. Measurements of wave height at different locations along the vegetation meadow indicate the wave attenuation along the Posidonia oceanica for three different submergence ratios and two seagrass densities. Results are also analysed with regard to the wave-induced flow within the meadow, and the effects of the submergence ratio and the seagrass density on the mean flow characteristics, based on data of mean velocities taken at three locations within the seagrass.

Near‐bed hydrodynamics and turbulence below a large‐scale plunging breaking wave over a mobile barred bed profile

Detailed measurements are presented of velocities and turbulence under a large-scale regular plunging breaking wave in a wave flume. Measurements were obtained at 12 cross-shore locations around a mobile medium-sand breaker bar. They focused particularly on the dynamics of the wave bottom boundary layer (WBL) and near-bed turbulent kinetic energy (TKE), measured with an Acoustic Concentration and Velocity Profiler (ACVP). The breaking process and outer flow hydrodynamics are in agreement with previous laboratory and field observations of plunging waves, including a strong undertow in the bar trough region. The WBL thickness matches with previous studies at locations offshore from the bar crest, but it increases near the breaking-wave plunge point. This relates possibly to breaking-induced TKE or to the diverging flow at the shoreward slope of the bar. Outer flow TKE is dominated by wave breaking and exhibits strong spatial variation with largest TKE above the breaker bar crest. Below the plunge point, breaking-induced turbulence invades the WBL during both crest and trough half cycle. This results in an increase in the time-averaged TKE in the WBL (with a factor 3) and an increase in peak onshore and offshore near-bed Reynolds stresses (with a factor 2) from shoaling to breaking region. A fraction of locally produced TKE is advected offshore over a distance of a few meters to shoaling locations during the wave trough phase, and travels back onshore during the crest half cycle. The results imply that breaking-induced turbulence, for large-scale conditions, may significantly affect near-bed sediment transport processes.

Hybridization of the wave propagation model SWASH and the meshfree particle method SPH for real coastal applications

Two computational models, Simulating WAve till SHore (SWASH) and DualSPHysics, with different computational costs and capabilities have been hybridized in this work. SWASH is a time-domain wave model based on a finite difference method for simulating nonhydrostatic, free-surface and rotational flow while DualSPHysics is a Lagrangian mesh-less model based on the Smoothed Particle Hydrodynamics (SPH) technique. SWASH is a reliable model to generate and propagate waves in large domains, whereas DualSPHysics is normally used in areas close to the coastline to provide a detailed description of the interaction between sea waves and coastal structures. The presented technique is a one-way coupling, with a hybridization point where the information from SWASH is passed to DualSPHysics. SWASH is used to propagate waves along the fluid domain and to calculate velocities at different depths at the position of the hybridization point. Waves in DualSPHysics Eire generated by means of a moving boundary (MB) whose displacement in time is reconstructed using the velocities provided by SWASH. Each particle that forms the MB is displaced with its correspondent velocity that depends on its depth. The hybridization technique is validated with experimental data and the resulting model is proved to reproduce accurately wave heights and orbital velocities. Thus, the hybrid model preserves the flexibility and capabilities of DualSPHysics with important improvements in efficiency. In addition, it simulates wave propagation even more accurately than DualSPHysics taking advantage of SWASH strengths.

Large-scale laboratory study of breaking wave hydrodynamics over a fixed bar

A large-scale wave flume experiment has been carried out involving a T = 4 s regular wave with H = 0.85 m wave height plunging over a fixed barred beach profile. Velocity profiles were measured at 12 locations along the breaker bar using LDA and ADV. A strong undertow is generated reaching magnitudes of 0.8 m/s on the shoreward side of the breaker bar. A circulation pattern occurs between the breaking area and the inner surf zone. Time-averaged turbulent kinetic energy (TKE) is largest in the breaking area on the shoreward side of the bar where the plunging jet penetrates the water column. At this location, and on the bar crest, TKE generated at the water surface in the breaking process reaches the bottom boundary layer. In the breaking area, TKE does not reduce to zero within a wave cycle which leads to a high level of “residual” turbulence and therefore lower temporal variation in TKE compared to previous studies of breaking waves on plane beach slopes. It is argued that this residual turbulence results from the breaker bar-trough geometry, which enables larger length scales and time scales of breaking-generated vortices and which enhances turbulence production within the water column compared to plane beaches. Transport of TKE is dominated by the undertow-related flux, whereas the wave-related and turbulent fluxes are approximately an order of magnitude smaller. Turbulence production and dissipation are largest in the breaker zone and of similar magnitude, but in the shoaling zone and inner surf zone production is negligible and dissipation dominates.

COASTAL DYNAMICS AROUND A SUBMERGED BARRIER

Submerged barriers are one of the most promising kind of shoreline protecting structures. The advantages offered vs. the conventional emerged structures are somehow off set because they are much more difficult to project. This decreases their use and limits their application potential. The paper presents some results, obtained by using numerical models on the hydro-morphodynamic behaviour of such structural concept. They illustrate the observed interactions between the freeboard of the structure, its distance to the coast and the resulting wave field, induced circulation and sediment transport patterns.

Short communication: Numerical model for wave overtopping and transmission through permeable coastal structures

In this paper, an energetic wave propagation model that reproduces shoaling, refraction, diffraction, wave-current interaction, bottom friction and wave breaking is modified to simulate also the processes of overtopping and wave transmission over and through permeable coastal structures. This enhances the capabilities of the model and allows to obtain, at a low computational cost, a better reproduction of the wave field (and as a consequence currents and beach response) behind a coastal structure, especially if it is permeable and/or low crested. Model results are compared with laboratory data, showing a good agreement and the suitability of the followed approach.

Swash zone response under various wave regimes

The modelling of swash zone (SZ) sediment transport and the resulting morphodynamics have been areas of active research over the last decade. However, many details are still to be understood, whose knowledge will be greatly advanced by the collection of high-quality data under the controlled large-scale laboratory conditions. The research describes tests carried out in the large wave flume of the Maritime Engineering Laboratory at Catalonia University of Technology, to investigate the SZ under the storm conditions. Its main aim was to compare beach-profile responses for monochromatic waves, monochromatic waves plus free long waves, bi-chromatic waves and random waves. Both erosive and accretive conditions were considered. Results discussed here were derived from the analysis of only a part of the whole data set.

An analysis of nearshore profile and bar development under large scale erosive and accretive waves

ABSTRACT Typical morphodynamic laboratory tests have been carried out at small scales and without sufficient coverage or resolution. These limitations, applying particularly to accretive tests, have obstructed reliable observations and restricted modelling capabilities. Here we present experiments with erosive/accretive waves acting on a large scale flume bed profile. The paper reports a set of high-quality hydro-morphodynamic data. The analysis is focused on net transport rates and how these patterns change between different accretive conditions. The measured velocity and acceleration skewness are presented and discussed, linking the net transport over the bar to the measured hydrodynamics and sediment concentrations. The resulting profile behaviour is discussed as a function of hydro-morphodynamic settings to facilitate comparisons with other datasets.

Sand transport process measurements under large-scale braking waves

The effects of wave breaking on sediment transport are studied through a new series of mobile-bed experiments in a large-scale wave flume. During the campaign, one experiment involving detailed sand transport process measurements was repeated at 12 different cross-shore location. This procedure allows studying of the cross-shore variation of sand transport processes along the breaking zone. Starting from an initially 1:10 slope followed by a horizontal test section, a breaker bar developed in the breaking region as a result of onshore transport pre-breaking and offshore transport post-breaking. Near-bed suspended sediment fluxes were directed offshore along the complete test section, suggesting that the onshore transport pre-breaking is mainly attributed to bedload. The offshore suspended flux was the sum of an onshore wave-driven component and an offshore current-driven component. The wave-driven contribution to total suspended transport rates seems significant mainly before the breaking point where they account for ~30% of total suspended transport fluxes.

Near-Bed Turbulent Kinetic Energy Budget Under a Large-Scale Plunging Breaking Wave Over a Fixed Bar: TKE BUDGET UNDER BREAKING WAVES

Hydrodynamics under regular plunging breaking waves over a fixed breaker bar were studied in a large-scale wave flume. A previous paper reported on the outer flow hydrodynamics; the present paper focuses on the turbulence dynamics near the bed (up to 0.10 m from the bed). Velocities were measured with high spatial and temporal resolution using a two component laser Doppler anemometer. The results show that even at close distance from the bed (1 mm), the turbulent kinetic energy (TKE) increases by a factor five between the shoaling, and breaking regions because of invasion of wave breaking turbulence. The sign and phase behavior of the time-dependent Reynolds shear stresses at elevations up to approximately 0.02 m from the bed (roughly twice the elevation of the boundary layer overshoot) are mainly controlled by local bed-shear-generated turbulence, but at higher elevations Reynolds stresses are controlled by wave breaking turbulence. The measurements are subsequently analyzed to investigate the TKE budget at wave-averaged and intrawave time scales. Horizontal and vertical turbulence advection, production, and dissipation are the major terms. A two-dimensional wave-averaged circulation drives advection of wave breaking turbulence through the near-bed layer, resulting in a net downward influx in the bar trough region, followed by seaward advection along the bars shoreward slope, and an upward outflux above the bar crest. The strongly nonuniform flow across the bar combined with the presence of anisotropic turbulence enhances turbulent production rates near the bed.