OS12B-01 INVITED
Modeling of Waves with Smoothed Particle Hydrodynamics on the GPU
Providing an accurate representation of breaking waves is extremely difficult due to the complexity of the free surface, splash-up, and the induced vortical flows in the water. Monaghan (1994) and Dalrymple and Rogers (2004) are examples of using the numerical method Smoothed Particle Hydrodynamics to model these breaking waves. Both of these studies accurately show the plunging jet and the splash-up of breaking plungers. However, full details of the flow requires highly resolved 3-D calculations. SPH is computational intensive, involving large numbers of computational particles and very small time steps. Recently Herault (2008) has shown that very high resolution and significant speed-ups in model calculation occurs by computing on the graphics card (GPU), rather than the CPU of computers. This use of the GPU is an on-going paradigm shift, which will be shown. Examples of breaking waves, along with a number of example free surface flows, will be shown.
OS12B-02 INVITED
ECORS Truc Vert'08: a Multi-Institutional International Nearshore Field Experiment
A large multi-institutional international field experiment (ECORS Truc Vert'08) was conducted Feb-April 2008 on the southern part of the French Atlantic coastline. More than 120 scientists, students and technicians participated to this effort coming from 3 continents and 6 countries : Australia (University of New South Wales), France (SHOM, University Bordeaux I, University Pau et Pays de l'Adour, University Sud-Toulon Var, University Joseph Fourier, University Perpignan, BRGM, University Lyon 1), Great Britain (Plymouth University), New Zealand (NIWA), The Netherlands (Delft University of Technology, University of Utrecht) and USA (Naval Postgraduate School, University of Miami, Franklin and Marshall College). Truc Vert beach is a high-energy, dynamic, macrotidal, double-barred beach representative of most of the beaches on this 250 km long coastline. The inner bar can go through all the states within the intermediate classification and usually exhibits a transverse bar and rip morphology (380 m alongshore wavelength). The outer bar is changeable from linear to crescentic (720 m alongshore wavelength). The goals were to measure the hydrodynamic processes, sedimentary processes and morphologic responses on a macrotidal beach during energetic wave conditions and covering a large spectrum of spatial and temporal scales. This dataset will facilitate the validation of surf zone wave, hydrodynamic and morphodynamic models, it will lend insight into the morphodynamic evolution of three dimensional beaches and it will fill the gaps in previous nearshore data sets. A wide range of unique instrumentation was used including continuously sampled 2Hz high-resolution surfzone video cameras, daily topographic surveys, bathymetric surveys from the French naval vessels and personal watercrafts, high frequency velocity and pressure sensors, acoustic Doppler current profilers, sediment transport devices, sand porosity and grain size devices, and position-tracking drifters. Measurements were taken during various wave conditions including short-period sea waves (Hs=1m, Tp=7s) and energetic long-period swell waves (Hs=8.2m, Tp=18s). In particular, 4 consecutive storms with significant wave height greater than 5 m including a 10-year storm were measured. Tidal ranges varied between 1.8m (neap tide) and 5m (spring tide). Observations include several cycles of crescentic bar development and destruction, cross-shore and alongshore migration of the bar, diurnal berm destruction and development. For instance, the outer bar migrated 350 m alongshore during the experiment. The large storms generated intensive alongshore currents (averaged velocity greater than 1.5m/s) and transport, resulting in rapid migration of the inner bar and the reshaping of the outer crescentic bar into a linear bar.
OS12B-03
Grain Size and Morphological Variability
Grain size on natural beaches has traditionally been assumed to be uniform and modeling efforts assume a single mean grain size for an entire beach environment. Many recent studies contradict this assumption and suggest that sediment grain size on a beach is not homogeneous and that variations in sediment size and supply are important in sediment transport and morphodynamics at all scales. Unfortunately, measuring grain size is difficult, tedious and time consuming. Therefore, in spite of the evidence pointing to the importance of grain size in sediment transport and morphodynamics, many previous studies have been based on only a few field samples. Rubin (2004) introduced a technique for measuring surface grain size in situ in rivers and deeper coastal waters, using a digital camera and auto-correlation of digital images. Using this technique, information about the surface grain size distribution can be obtained quickly and inexpensively. Following Rubin (2004), we have developed a mobile digital imaging system (DIS) for surveying grain size on beaches. The DIS was used during two experiments: RCEX, a rip current experiment in Monterey, CA in April 2007 and Truc Vert '08, a multi-institutional, international experiment, on the Atlantic coast of France in March 2008. Preliminary results suggest that grain size varies spatially with the morphology of beach features and temporally with changes in tide level, wave energy, and morphodynamics. These data are being used to examine the relationship between morphological, sedimentological and hydrodynamic variability.
OS12B-04
Nearbed Sediment Transport in the Swash Zone of Laboratory Beaches
Rapid flow velocities and substantial sediment transport gradients in the swash zone can result in large morphological changes. Since swash flows are complex owing to asymmetry, turbulence and infiltration processes, it is difficult to understand and quantify sediment mobility and transport patterns. Nearly all studies of instantaneous swash-zone sediment transport have focused on the suspended load component even though time-averaged measurements have shown the bedload component can be a dominant transport mode. Our work involves testing newly fabricated conductivity concentration probes to resolve the time- dependent sediment concentration profile in the nearbed transport layer of the swash-zone. The probe is tested in concert with more frequently-used Conductivity Concentration Meters (CCMs) from Deltares to provide validation for obtained data. Preliminary data from the swash zone of a laboratory beach will be discussed.
OS12B-05
Rip Current Retention
Here, we show for the first time spatially synoptic estimates of rip current flow patterns and Lagrangian transport behavior using a fleet of 30 position-tracking surfzone drifters over multiple rip current systems in Sand City, Monterey Bay, CA. The rotational characteristics of the rip current resulted in a high number of Lagrangian observations that temporally and spatially iterated. Only 10% of the drifters that entered a rip current exited the surf zone resulting in high surfzone retention with maxima occurring in the center of the rip current vortices. Rip current diffusivities in the cross-shore have a periodic response, which modulates at ~300s, the time required for a drifter to complete one revolution around a rip current cell, before decreasing to an asymptotic limit. This suggests that material initially (t<90 s; κxx = 4.9 - 6.1 m2/s) diffuses, then re-collects, reaching a lower asymptote (t>200 s; κxx = 0.9 - 2.2 m2/s). The alongshore diffusivity is also periodic, but its asymptotic limit is larger (κyy = 2.8 - 3.9 m2/s), as the drifters spread to neighboring rip currents, whereas the cross-shore offshore limited by the surf zone width, reducing material transport. Asymptotic rip current diffusivities are similar to other asymptotic diffusivities of surf zones that support non-rip current circulation patterns. New thoughts of rip current behaviors are presented suggesting that rip currents retain more floatsom material within the surf zone as opposed to transporting floatsom material offshore.
OS12B-06
Three-dimensional currents in the outer nearshore zone
Cross-shore flows on the continental shelf are primarily wind-driven and are affected by the Earth's rotation (Coriolis force). In contrast, surf zone flows are primarily wave-driven and exist at scales that are too small to be affected by rotational effects. There is a transition zone between the continental shelf and the surf zone (e.g. the "inner shelf" or the "outer nearshore" zone) that had, until recently, been relatively poorly studied. However, recent studies suggest that in this area the wind-driven transport (dominant on the continental shelf) shuts down (Kirincich et al., JGR, 2005), yet the wave-driven transport (dominant in the surf zone) is not yet fully established. Hence, phenomena that usually exert small forcing (compared to wind or wave forcing effects) and are therefore routinely neglected can become important. Indeed, recent observations (Fewings et al., JPO, 2008) suggest that cross-shore wind stress, usually small in comparison to the Coriolis force due to alongshore flow, can be significant in forcing cross-shore flow on the inner shelf. Similarly, Lentz et al. (JPO, 2008) suggest that wave-induced forcing, however small outside the surf zone, can still have an effect on offshore directed undertow velocities on the shelf. The modeling of flows in this transition region needs to consider wind forcing, wave forcing, Coriolis effects and 3D effects, and a shelf circulation model that incorporates 3D wave forcing effects should be most appropriate. One particular example of such a model is the Princeton Ocean Model POM that has recently been adapted to the prediction of surf zone currents (Newberger and Allen, JGR, 2007a, 2007b). POM is already well-tested on the continental shelf; however its applicability to the transition region, while promising, is unproven. Herein, we apply POM to this region and compare results with velocity observations from the Sandyduck experiment that included 6 upward-looking Sontek/SI Acoustic Doppler Profilers (ADP's) positioned in approximately 5m through 12m of water depth. Similar observations from the Duck94 experiment should also prove useful. Model results are then analyzed to shed light on the dominant dynamics of the flows in the transition region between the surf zone and the continental shelf.
OS12B-07
Nonlinear wave statistics in a focal zone
In a statistical description of ocean surface waves, the occurrence of extremely large waves is expected, but with low probability. In narrow-band, unidirectional waves it has been shown that the random-wave equivalent of the Benjamin-Feir instability results in an increased likelihood of extreme events (high kurtosis values) through near-resonant cubic interactions. Recent findings indicate however that freely developing directionally-spread waves do not develop such non-Gaussian features. Instead, the wave field returns to a near-Gaussian state, which is also what is invariably observed in field observations. However, ocean waves, and in particular on the continental shelf and near the coast, are not necessarily freely developing. To investigate the effects of medium inhomogeneities due to currents and topography on the nonlinear wave statistics, we consider the concomitant effect of refractive focusing and nonlinearity. Thereto we develop a spectral nonlinear evolution model and perform Monte-Carlo simulations of waves traversing a refractive focal zone. Simulations show that narrow-band directionally spread waves can develop instabilities when traversing a refractive zone, resulting in strongly non-Gaussian features (high kurtosis) locally. We will present our modeling approach and principal simulation results, and discuss the implications for ocean wave statistics. This research is supported by the Office of Naval Research (Coastal Geosciences and Physical Oceanography).
OS12B-08
VERTICAL STRUCTURE OF THE TURBULENT DISSIPATION RATE IN THE SURFZONE
Surfzone turbulence is generated at the surface by breaking surface gravity waves and near the bed from the vertical shear of strong mean currents. Surfzone turbulence vertically mixes momentum, sediment, tracers, and biota. However, the vertical structure of surfzone turbulence and the relative contributions of these two sources is not well understood. Turbulence is often quantified using the turbulent dissipation rate. Previously, the vertical structure of the dissipation rate has been indirectly examined using a single fixed Acoustic Doppler Velocimeter (ADV) along with tidal changes in water level and bathymetry evolution. In October 2008, a vertical array of 3 ADVs will be deployed for 2 weeks in the surfzone to study the vertical structure of the dissipation. The rapidly-sampled ADVs together with an algorithm that converts from frequency to wavenumber spectra is used to estimate the dissipation. Breaking-wave and bottom-boundary-layer dissipation scalings will be examined to determine the relative importance of surface and bottom turbulence sources under different wave, tide, and current conditions. Supported by ONR and NSF