[1] We simulated the erosion and accretion of a natural beach using a wave-resolving eddy-diffusive model of water and suspended sediment motion in the bottom boundary layer. Nonlinear advection was included in this one-dimensional (vertical profile) model by assuming that waves propagated almost without change of form. Flows were forced by fluctuating pressure gradients chosen to reproduce the velocity time series measured during the Duck94 field experiment. The cross-shore flux of suspended sediment beneath each field-deployed current meter was estimated, and beach erosion (accretion) was calculated from the divergence (convergence) of this flux. Horizontal pressure forces on sediment particles were neglected. The model successfully predicted two bar migration events (one shoreward bar migration and one seaward) but failed to predict a third (seaward migration) event. Simulated seaward sediment transport was due to seaward mean currents. Simulated shoreward sediment transport was due to covariance between wave-frequency fluctuations in velocity and sediment concentration and was mostly confined to the wave boundary layer. Predicted seaward (shoreward) bar migration was driven by a maximum in the current-generated (wave-generated) flux over the sandbar. A wave-generated downward flux of shoreward momentum into the wave boundary layer contributed to shoreward sediment transport and often had a local maximum over the bar crest. Second-order nonlinear advection of sediment, mostly representing shoreward advection by the Stokes drift, also often had a local maximum over the bar crest. Together, wave-generated momentum fluxes and the Stokes drift substantially increased shoreward transport and were essential to predictions of shoreward bar migration. INDEX TERMS:4546 Oceanography: Physical: Nearshore processes; 4255 Oceanography: General: Numerical modeling; 4558 Oceanography: Physical: Sediment transport; 3022 Marine Geology and Geophysics: Marine sedimentsprocesses and transport; 3020 Marine Geology and Geophysics: Littoral processes; KEYWORDS: sediment transport, benthic boundary layers, beach erosion Citation: Henderson, S. M., J. S. Allen, and P. A. Newberger (2004), Nearshore sandbar migration predicted by an eddy-diffusive boundary layer model,

[1] Nonlinear energy transfers with sea and swell (frequencies 0.05-0.40 Hz) were responsible for much of the generation and loss of infragravity wave energy (frequencies 0.005-0.050 Hz) observed under moderate-and low-energy conditions on a natural beach. Cases with energetic shear waves were excluded, and mean currents, a likely shear wave energy source, were neglected. Within 150 m of the shore, estimated nonlinear energy transfers to (or from) the infragravity band roughly balanced the divergence (or convergence) of the infragravity energy flux, consistent with a conservative energy equation. Addition of significant dissipation (requiring a bottom drag coefficient exceeding about 10 À2 ) degraded the energy balance.

[1] We used a simple energy balance equation, and estimates of the cross-shore energy flux carried by progressive surf beat, to calculate the rate of net surf beat forcing (or dissipation) on a beach near Duck, North Carolina. Far inside the surf zone, surf beat dissipation exceeded forcing. Outside the surf zone, surf beat forcing exceeded dissipation. When incident waves were large, surf beat dissipation inside the surf zone and forcing just outside the surf zone were both very strong (the surf beat energy dissipated in the surf zone in a single beat period was of the same order as the total amount of surf beat energy stored in the surf zone). During storms, shoreward propagation of surf beat maintained surf beat energy in the surf zone. Net surf beat dissipation in the surf zone scaled as predicted by a simple bottom stress parameterization. The inferred dissipation factor for surf beat was 0.08, within the range of wave dissipation factors usually observed in the field and 27-80 times larger than drag coefficients appropriate for the mean longshore current. The observed rapid forcing, rapid dissipation, and shoreward propagation of surf beat are not simulated by existing models of surf beat dynamics.

We derive an analytical model for the wave‐forced movement of single‐stem vegetation and test the model against observed vegetation motion in a natural salt marsh. Solutions for constant diameter and tapered stems are expanded using normal mode solutions to the Euler‐Bernoulli problem for a cantilevered beam. These solutions are compared with motion of water and of the sedge Schoenoplectus americanus observed (using synchronized current meters and video) in a shallow salt marsh (depth < 1 m). Consistent with theory, sedge motion led water motion, with the phase decreasing (from 90 to 0 degrees) with increasing wave frequency. After tuning of a single free parameter (Young's modulus), the theory successfully predicted the transfer function between measured water and stem motion. Formulae predicting frequency‐dependent wave dissipation by flexible vegetation are derived. For the moderately flexible stems observed, the model predicted total dissipation was about 30% of the dissipation for equivalent rigid stems.

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In a small lake, where flows were dominated by internal waves with 10–32-h period, slow but persistent mean transport of water over many wave periods was examined. Acoustic Doppler profilers (ADPs) and a vertical string of temperature loggers were deployed where the lower thermocline intersected the sloping lakebed. Near (&lt;1 m above) the bed, internal waves, coherent with a lakewide seiche, propagated upslope at ~0.023 m s−1. Near-bed wave-induced water velocity fluctuations had a standard deviation of &lt;0.02 m s−1. Near the surface, velocity fluctuations had similar magnitude, but lateral wave propagation was unclear. Averaged over many wave periods, the near-bed Eulerian velocity flowed downslope at ~0.01 m s−1, and was roughly cancelled by an upslope internal-wave Stokes drift (estimated by assuming that weakly nonlinear waves propagated without change of form). To examine net transport, while relaxing approximations used to estimate the Stokes drift, the observed temperature range (9°–25°C) was divided into 0.5°C increments, and the depth-integrated, wave-averaged flux of water in each temperature class was calculated. The coldest (near-bed) water was slowly transported onshore, opposite the Eulerian mean velocity. Onshore flux of warm near-surface water was comparable to an Eulerian-mean flux, indicating minimal near-surface Stokes drift. Intermediate water, from the middle of the water column and the outer boundary layer, was transported offshore by an offshore Stokes drift. The downslope near-bed Eulerian mean velocity, together with intensification of mean stratification within 0.4 m of the bed, may enhance boundary layer mixing.

[1] Internal waves with diurnal period dominated velocities measured by an Acoustic Doppler Profiler (ADP) in a small lake (main basin 3000 m by 400 m by 18 m). ADP profiles and an along-lake temperature section indicate that the observed waves, like seiches, had horizontal wavelengths exceeding the metalimnion length. However, unlike non-dissipative seiches, the observed waves propagated vertically, carrying energy to the lakebed where waves were absorbed, rather than being strongly reflected. This absorption is predicted by a standard parameterization of boundary layer dissipation. The absence of upward-propagating energy precludes seiche resonance, limits focusing of waves toward attractors, and suggests that hypolimnion dissipation was limited by the supply of downward-propagating energy. Vertical wavelengths were less than the lake depth. Simplified calculations suggest that vertically-propagating waves, as opposed to vertically standing seiches, are most likely where vertical wavelengths are short, near-bed stratification is strong, and lakes are short and deep.

We study water flows and wave dissipation within near-bed pneumatophore canopies at the wave-exposed fringe of a mangrove forest on Cù Lao Dung Island, in the Mekong Delta. To evaluate canopy drag, the three-dimensional geometry of pneumatophore stems growing upward from the buried lateral roots of Sonneratia caseolaris mangroves was reconstructed from photogrammetric surveys. In cases where hydrodynamic measurements were obtained, up to 84 stems per square meter were observed, with stem heights < 0.6 m, and basal diameters 0.01-0.02 m. The parameter a = (frontal area of pneumatophores blocking the flow)/(canopy volume) ranged from zero to 1.8 m −1. Within-canopy water velocity displayed a phase lead and slight attenuation relative to above-canopy flows. The phase lead was frequency-dependent, ranging from 0 to 30 degrees at the frequencies of energetic waves (> 0.1 Hz), and up to 90 degrees at lower frequencies. A model is developed for wave-induced flows within the vertically variable canopy. Scaling suggests that acceleration-induced forces and vertical mixing were negligible at wave frequencies. Consistent with theory, drag-induced vertical variability in velocity scaled with Λ = T w /(2πT f), where T w = wave period, T f = 2/(C D a|u|) is the frictional time scale, C D ≈ 2 is the drag coefficient, and |u| is a typical flow speed. For fixed wave conditions (|u| and T w), theory predicts increasing dissipation with increasing vegetation density (i.e. increasing a), until a maximum is reached for order-one Λ. For larger Λ, within-canopy flow is so inhibited by drag that further increases in a reduce within-canopy dissipation. For observed cases, Λ ≤ 0.38 at energetic wave frequencies, so wave dissipation near the forest edge is expected to