OS24A-01 16:00h
The Bed State Storm Cycle
Results are presented from 70+ days of nearly continuous {\it in situ} acoustic imagery of the nearshore sandy seabed in about 3-m mean water depth. Five principal bed states were observed: irregular ripples, cross ripples, linear transition ripples, lunate megaripples, and flat bed. The linear transition and flat bed states were the most frequent, together accounting for 68% of the total time. Bed state occurrence was a strong function of incident wave energy, each bed state occurring within a relatively narrow range of incident wave energies. The bed response during the 12 major storm events spanned by the record was characterized by a repeatable bed state cycle, involving 4 of the 5 principal states (lunate megaripples did not appear repeatably, and thus may be a special case), and with no obvious dependence on prior bed state, or on 3rd-moment measures of wave non-linearity.
OS24A-02 16:15h
Bedform Migration and Bed Envelope Statistics
The generation and migration of bedforms (ripples, megaripples and sand bars) on sandy bottoms in the nearshore (0-8 m water depths) provides a mechanism whereby objects on the seafloor can become buried. As a bedform migrates past an object, the object will fall to the low point of the bedform trough and be buried by the passage of the following bedform crest. Existing data sets are being used to examine the temporal evolution of the bottom profile envelope at fixed locations. The maximum envelope thickness, D$_{max}$, is defined as the difference between the maximum and minimum extent of the bed during a given time window (4 day- to 12 year-windows are used in this study). D$_{max}$ is hypothesized to grow as an exponential taper (quickly at first, then slowing and approaching an asymptotic value) as the time window increases and this is observed. However, envelope thickness takes a long time, 2-6 years, to level off and approach an asymptotic value. Both megaripples and larger-scale morphological changes (eg, bar migration) affect D$_{max}$. Envelopes owing to megaripple migration grow quickly, reaching a maximum of $<$ 50 cm within about 8 days. Bar-scale erosion and accretion generates envelopes that are larger (1-3 m depending on cross-shore location) and grow slowly. To examine burial of objects on the bed, W is defined as the vertical scale of an object and is compared with the instantaneous elevation of the seafloor above envelope minimum. When D exceeds W, an object on the bed will be buried. The likelihood of burial is being examined at the end of set time windows. Percent burial is shown to increase with time and to decrease in deeper water.
OS24A-03 16:30h
Using a Navier-Stokes Solver to Drive a Discrete Particle Model for Sediment Transport in the Swash Zone
A volume of fluid Navier-Stokes solver for simulating inner surf and swash zone flows (RIPPLE) is used to drive a discrete particle model (DPM) for sediment transport. RIPPLE predicts the pressure gradient and fluid velocity in the horizontal and vertical directions providing the direct forcing for the hydrodynamics in the DPM. Coupling between RIPPLE and the DPM is one-way such that particle-particle and fluid-particle interactions do not provide direct feedback to the fluid phase. The fluid velocities are used to calculate the drag force on individual particles with the empirical drag law for spheres, while local pressure gradients also generate body forces on particles. Grain sizes used in simulations range from coarse sand to gravel. Cross-shore profiles of swash typically greater than 3 m are simulated. The number of particles used in simulations ranges from 10$^{3}$ to greater than 10$^{5}$. Flows with periods on the order of several seconds and wave heights up to 0.2 m are modeled. Preliminary simulations were performed with low particle concentrations and nearly-neutrally buoyant particles. These simulations represent tracers that interact with each other and the fixed-bed. Additional simulations with particles having more realistic sediment characteristics have been performed and demonstrate the suspension and advection of particles over several swash cycles. Implications of simulation results on the role of sediment advection on swash zone morphodynamics will be discussed.
OS24A-04 16:45h
Sediment transport in the inner-surf and swash zones
Sediment transport in the inner-surf and swash zones is modeled with two-phase equations [Hsu et al. 2004, Proc. Royal Soc. Lond. A, 460, p2223-2250] that have been extended to predict movement of fine sand. Comparisons of predicted velocity and sediment concentration profiles with observations collected in a U-tube suggest that given closures for fluid turbulence, particle intergranular forces, and fluid-particle interactions in the boundary layer, the two-phase model is capable of simulating sheet flow processes, including both bedload and suspended-load transport and the phase lag between the flow forcing and the bottom stress. Predictions of simpler models that neglect the suspended load transport or that assume a quasi-steady bottom stress with respect to the flow forcing are evaluated by comparison with the two-phase model predictions. Preliminary simulations driven with velocities measured during the SwashX experiment (Fall 2000) suggest that the phase lag between the flow forcing and the bottom stress is important to transport driven by sea-swell dominated swash, but is negligible for transport driven by infragravity-dominated swash. Predictions of suspended sediment transport are sensitive to the effects of breaking-wave-generated turbulence on the boundary layer. Funding of this research is provided by ONR and NSF.
OS24A-05 17:00h
Wave Induced Sediment Transport over Flat Beds
Traditional sediment transport models assume the incipient motion of sediment is a function of the shear stress applied to the bed. In these models, transport occurs when the vertical stress gradient applied to individual grains exceeds the horizontal resistive force of the grains resting on a fixed bed (Shields, 1936). When the Shields parameter exceeds a value of 0.8, a 10-100 grain diameter thick mobile sediment layer called sheet flow may occur. An alternative theory proposes that under certain free surface gravity waves, the horizontal pressure gradient may induce an instantaneous bed dilation and transport (Sleath, 1999). According to laboratory observations, plug flow occurs for Sleath parameters above 0.29 and can be several centimeters thick. In this presentation, we examine two sets of field observations of flow and suspended sediment over a flat bed. The first set of observations was obtained during the cooperative Duck94 Experiment. The orbital velocity, peak period, and water depth during the sampling period was 60 cm/s, 5 s, and 2.0 m, respectively. The Duck94 observations show that during large accelerations present under large wave crests, several centimeters of sediment are mobilized and transported onshore. The second set of observations was obtained during the cooperative SandyDuck Experiment. The orbital velocity, peak period, and water depth during the sampling period was 67 cm/s, 10 s, and 2.7 m, respectively. The SandyDuck observations also show suspension events associated with large wave crests, but show significantly smaller variations in the nearbed concentration. Numerical simulations of flow and sediment transport were performed with a 2-D k-$\omega$ bottom boundary layer model. The model assumes a no-slip bottom boundary condition and is forced with the observed free stream flow. The Duck94 observations exhibited lower bed stress and did not satisfy the no-slip bottom boundary condition required by the model. These observations were not consistent with sheet flow but were consistent with plug flow. The SandyDuck observations compare favorably with the wave bottom boundary layer model and are consistent with sheet flow theory. Although predictions of the Shields parameter are similar in both data sets, the larger accelerations present in the Duck94 observations yield significantly larger estimates of the instantaneous Sleath parameter and likely result in pressure gradient induced transport.
OS24A-06 17:15h
Characterization of Wave-Induced Boundary Layers Over a Rough Bottom
Field observations of the wave-induced flow over a fixed coral rough boundary were carried out in summer 2004 on the south shore of Oahu, Hawaii. Turbulent motions along the seabed generated by the oscillatory motion of surface waves are of great interest in coastal waters because wave boundary layers are the site of sediment entrainment, suspension and transport from the seabed as well as wave energy dissipation. The nature of this turbulent boundary layer over very rough bathymetry such as that characteristic of coral reef is not well-understood, although this type of inhomogeneous roughness is pervasive in natural settings. The observations were the first measurements carried out within a nearshore observatory presently under development that allows real-time characterization of the dynamic, tide and wave influenced reef environment. A downward-looking Bistatic Current Doppler Velocimeter (BCDV), mounted on an automated horizontal profiler, was used to obtain a 1cm vertical resolution, two-dimensional view of the wave-boundary layer over a wave orbital excursion ($\sim$2m). The bed morphology along the profile track was mapped using a scanning laser altimetry system. The objective of these measurements is to resolve the 2-D spatial structure of the oscillating boundary layer in a phase-averaged sense and to calculate spatial averages of Reynolds stresses and turbulent dissipation. The horizontal profiler allows integration of these quantities over the high spatial variability expected over very rough boundaries. Preliminary analysis of the field data set is presented here.
OS24A-07 17:30h
Multiple Frequencies of Waves on an Intertidal Mudflat, and Their Effect on Turbulence Statistics, Boundary Layer Structure, and Sediment Transport
On an intertidal mudflat in the Central San Francisco Bay, we conducted an experiment to measure the dynamics of waves, currents, turbulence, and sediment transport in shallow water (0-1.5 m depth). We obtained vertical profiles of velocity, sediment concentration, and density by deploying six Acoustic Doppler Velocimeters (ADVs), five Optical Backscatter sensors (OBS), and 3 Conductivity-Temperature (CT) probes between 1 cm and 35 cm for a period of 4 days in April 2003, logging nearly continuously at frequencies of up to 16 Hz. Multiple frequencies of waves are evident at this site, including a seiche with a period of ~ 500 seconds, ocean swell with a period of 8-15 seconds, and locally driven wind waves with a period of 1-3 seconds. These waves are irregular and intermittent, and interact nonlinearly with currents and potentially each other. Estimating turbulent kinetic energy (TKE), dissipation rate, and stresses are complicated by the multiple frequencies of motion. We use the Soulsby (1983) and Trowbridge (1998) methods to separate waves and TKE, and suggest a hybrid approach that includes components of each method. In order to model the various boundary layers, we separate the raw velocity data into vertical profiles of tidal, seiche, ocean swell, and wind wave velocities using a simple bandpass filter. The tidal and seiche velocity profiles are best described by a log-linear model in which the roughness z0, the friction velocity u*, and the linear parameter (z-z0)/L are optimized in a least-squares sense. However, the length scale `L' is not adequately predicted by a combination of an acceleration length scale (Soulsby-Dyer, 1981) and a stratification length scale (Monin-Yaglom, 1971). Instead, we find that the curvature in the seiche boundary layer is best described by a two equation TKE numerical model (GOTM, Burchard et al, 1999). The boundary layers of higher frequency waves such as ocean swell are well described by an analytical model such as the Smith (1977) model, which assumes a linear eddy viscosity. The superposition of the seiche motions with the tidally-driven mean velocity profile creates a pattern of alternating high and low mean stress between 0.05 Pa and 0.5 Pa. Sediment concentration and salinity can fluctuate at the same seiche frequency at up to 150 g/L and 0.5 ppt. However, sediment concentration bursts can be ~180 degrees out of phase with mean bed stress and velocity during an ebbing tide. This non-intuitive result highlights the need for detailed measurements of local wave climate, sediment concentrations, and their phasing in order to accurately model hydrodynamic and transport processes. In fact, this asymmetric phasing of sediment during the ebb accounts for as much as 20 percent of the residual shoreward sediment flux near the bed for a particular tidal period.
OS24A-08 17:45h
The Relative Effects of Wave Climatology and Tidal Currents on Beach Processes Adjacent to a Major Tidal Inlet, Ocean Beach, San Francisco, California
Identifying the processes that control the morphological evolution of beaches adjacent to tidal inlets is challenging due to the complex interactions between waves, currents, and bathymetry, each with high spatial and temporal variability. In the shadow of the large ebb tidal delta at the mouth of San Francisco Bay, CA, the wave refraction patterns at Ocean Beach are complex and the effects of the offshore wave climate on beach and nearshore morphology cannot be assessed simply by analyzing data from an offshore wave buoy. Instead, the United States Geological Survey has employed a multi-faceted approach that links wave data with numerical modeling, periodic three- dimensional topographic beach surveys, cross shore bathymetric surveys using personal watercraft, onshore grain-size analysis using a bed sediment camera, and a multi-beam survey covering the entire mouth of San Francisco Bay. Initial analyses demonstrate that the spatial distribution of wave energy and direction controls short-term (i.e. days to years) beach evolution, including the location of erosional "hot spots." These conclusions are supported by topographic LIDAR surveys that covered the study area in 1997, 1998 and 2002, bracketing the last major El Niño/ Southern Oscillation cycles. In this study, SWAN (Simulating WAves Nearshore) modeling is combined with high resolution bathymetry and high resolution beach surveys to quantify short-term morphological change and to provide links to nearshore processes. Initial SWAN results show a focusing of wave energy at the location of an erosional hot-spot on the southern end of Ocean Beach during the prevailing northwest swell. During El Niño winters, swell out of the west and southwest dominates the region, and although the wave energy is focused further to the north on Ocean Beach, the oblique wave approach sets up a strong northerly littoral drift, thereby starving the southern end of sediment, leaving it increasingly vulnerable to wave attack when the typical northwest swell returns. Over longer time periods (i.e. decades), tidal processes emerge as the dominant control on coastal evolution is this region, as changes in sediment supply and depositional patterns exert a strong influence on the ebb tidal delta volume and morphology. The tidal delta, in turn, strongly influences wave shielding, refraction, and focusing patterns on adjacent beaches. An accurate assessment of the interaction between wave and tidal processes is crucial for evaluating coastal management options in an area that includes the annual dredging and disposal of ship channel sediment and an erosional hot spot that is posing a major threat to local infrastructure.