Field Evidence of Inverse Energy Cascades in the Surfzone

Low-frequency currents and eddies transport sediment, pathogens, larvae, and heat along the coast and between the shoreline and deeper water. Here, low-frequency currents (between 0.1 and 4.0 mHz) observed in shallow surfzone waters for 120 days during a wide range of wave conditions are compared with theories for generation by instabilities of alongshore currents, by ocean-wave-induced sea surface modulations, and by a nonlinear transfer of energy from breaking waves to low-frequency motions via a two-dimensional inverse energy cascade. For these data, the low-frequency currents are not strongly correlated with shear of the alongshore current, with the strength of the alongshore current, or with wave-group statistics. In contrast, on many occasions, the low-frequency currents are consistent with an inverse energy cascade from breaking waves. The energy of the low-frequency surfzone currents increases with the directional spread of the wave field, consistent with vorticity injection by short-crested breaking waves, and structure functions increase with spatial lags, consistent with a cascade of energy from few-meter-scale vortices to larger-scale motions. These results include the first field evidence for the inverse energy cascade in the surfzone and suggest that breaking waves and nonlinear energy transfers should be considered when estimating nearshore transport processes across and along the coast.

Observations of Diurnal Coastal-Trapped Waves with a Thermocline-Intensified Velocity Field

Using 18 days of field observations, we investigate the diurnal (D1) frequency wave dynamics on the Tasmanian eastern continental shelf. At this latitude, the D1 frequency is subinertial and separable from the highly energetic near-inertial motion. We use a linear coastal-trapped wave (CTW) solution with the observed background current, stratification, and shelf bathymetry to determine the modal structure of the first three resonant CTWs. We associate the observed D1 velocity with a superimposed mode-zero and mode-one CTW, with mode one dominating mode zero. Both the observed and mode-one D1 velocity was intensified near the thermocline, with stronger velocities occurring when the thermocline stratification was stronger and/or the thermocline was deeper (up to the shelfbreak depth). The CTW modal structure and amplitude varied with the background stratification and alongshore current, with no spring–neap relationship evident for the observed 18 days. Within the surface and bottom Ekman layers on the shelf, the observed velocity phase changed in the cross-shelf and/or vertical directions, inconsistent with an alongshore propagating CTW. In the near-surface and near-bottom regions, the linear CTW solution also did not match the observed velocity, particularly within the bottom Ekman layer. Boundary layer processes were likely causing this observed inconsistency with linear CTW theory. As linear CTW solutions have an idealized representation of boundary dynamics, they should be cautiously applied on the shelf.

The Vertical Structure of Low-Frequency Motions in the Nearshore. Part I: Observations

Field observations of oscillating currents in the surfzone of a natural beach show significant vertical structure in energy, phase, and rotation at low frequencies around 0.005 Hz, where most of the energy is associated with vorticity motions. Energy levels in the cross-shore component of the flow seaward of the sandbar decay near the bottom. Shoreward of the bar crest, the flow decays nearly linearly over the water column. Conversely, a weaker alongshore component of the flow increases near the bottom seaward of the sandbar and is roughly depth-uniform inside the bar crest. Near this 0.005-Hz frequency band, the coherence between the uppermost and successive vertically separated sensors drops off quickly, with as much as a 70%–80% coherence drop over the water column (ranging from 2.5 to 4 m). The phase relative to the uppermost sensor shifts approximately linearly over depth, with as much as 50° phase lag at the bottom that can lag or lead the surface. Rotary coefficients also vary across the surfzone and are generally nonzero with rotational directions (cyclonic or anticyclonic) and orientation that depend on sensor position relative to the sandbar and alongshore current profile. The rotary coefficients are generally not uniform with depth and can change sign in the vertical. The observed behavior is qualitatively predicted by boundary layer theory (discussed in the companion paper by Lippmann and Bowen). The nonuniform vertical structure has implications to the interpretation of field data and horizontal nearshore mixing.

Coastal-Trapped Waves in the East China Sea Observed by a Mooring Array in Winter 2006

Using five mooring array observations in the coastal water of the East China Sea (ECS) in winter 2006, the authors identify three kinds of low-frequency waves using the ensemble empirical mode decomposition (EEMD) method. The analysis indicates that the periods of the waves varied from 2 to 10 days, which are consistent with coastal-trapped wave (CTW) modes: the Kelvin wave (KW) mode, the first shelf wave (SW1) mode, and the second shelf wave (SW2) mode. An analytical model is established and the dispersion relation of the waves from the analytical method agrees well with the observations. The wind-forced, coastal-trapped wave theory is then applied. The calculation shows that over a wide shelf, the forcing term of wind stress curl plays an important role in shaping the CTW. Numerical solutions reproduce the sea level variation and the alongshore current. The results show that the sea level variation mainly resulted from the KW mode, but the alongshore current resulted from both the KW and SW1 modes.

The Generation of Surfzone Eddies in a Strong Alongshore Current

The surfzone contains energetic two-dimensional horizontal eddies with length scale larger than the water depth. Yet, the dominant eddy generation mechanism is not understood. The wave-resolving model funwaveC is used to simulate surfzone eddies in four case examples, from the SandyDuck field experiment, that had alongshore uniform bathymetry. The funwaveC model is initialized with the observed bathymetry and the incident wave field in 8-m depth and reproduces the observed cross-shore structure of significant wave height and mean alongshore current. Within the surfzone, the wave-resolving funwaveC-modeled E(f, ky) spectra and the bulk (frequency and ky integrated) rotational velocities are consistent with the observations below the sea–swell band (<0.05 Hz), demonstrating that the model can be used to diagnose surfzone eddy generation mechanisms. In the mean-squared perturbation vorticity budget, the breaking wave vorticity forcing term is orders of magnitude larger than the shear instability generation term. Thus, surfzone eddies (vorticity) generally are not generated through a shear instability, with possible exceptions for very narrow banded in frequency and direction and highly obliquely large incident waves. The alongshore wavenumber spectra of breaking wave vorticity forcing is broad with the majority (>80%) of vorticity forcing occurring at short alongshore scales <20 m. However, the alongshore wavenumber spectra of vorticity is red, which may be due to a 2D turbulence inverse energy cascade bringing energy to longer wavelengths or may result from an amplified vorticity response to direct forcing at smaller ky.

Numerical Simulation on Sudden Siltation Induced by Winner Storm and Large Waves in Outer Channel of Huanghua Port, China

A multinest storm surge-wave-sediment model is applied to simulate strong storm surge caused by winter storm in Bohai Bay in Oct, 2003. The fields of current and sediment of for- and post- regulating structure are calculated to analysis the effect of regulation. The results show that the alongshore current caused by storm surge strengthen the cross-channel component and that provided the impetus conditions for sediment transport. The siltation in the outer channel of Huanghua port in the second stage is very severe. For example, the thickness of slitting sediment in the area, 3~19 kilometers (away from the entrance of port) is over 2.0 meters. While the third stage dike for debris barrier is completed, the hyper-concentrated silt-laden flow offshore could be eluded, and the maximum deposition area goes out thus the sediment siltation is reduced.

Nearshore Currents along the Karnataka Coast, West Coast of India

Measured current data at 7 locations and tide data at 3 locations during the pre-summer monsoon period along the west coast of India is used in the study. The surface currents during March showed a predominant northward trend and during April it was towards south. Estimated tidal currents were upto 25 cm s−1 with an average value of 8 cm s−1. Current tidal form number varied from 0.56 to 1 at different locations indicating currents are mixed. M2 and S2 tidal current constituents rotated clock wise at all location. Near surface, the alongshore current was 2.6 to 5.9% of the alongshore wind and near bottom it was 1.9 to 3.6% of the alongshore wind.