Wave Orbital Velocity Research Articles

Tropical storms influence the movement behavior of a demersal oceanic fish species

Extreme weather events strongly influence marine, freshwater, and estuarine ecosystems in myriad ways. We quantified movements of a demersal oceanic fish species (gray triggerfish Balistes capriscus; N = 30) before, during, and after two hurricanes in 2017 using fine-scale acoustic telemetry at a 37-m deep study site in North Carolina, USA. During storms, gray triggerfish movement and emigration rates were 100% and 2550% higher, respectively, than on days with no storms. We found that increased movement rates were much more strongly correlated with wave orbital velocity (i.e., wave-generated oscillatory flow at the seabed) than either barometric pressure or bottom water temperature, two covariates that have been demonstrated to be important for organisms in shallower water. Higher movement rates during storms were due to increased mobility at night, and emigrations typically occurred at night in the direction of deeper water. Overall, we found significant storm effects on the movement behavior of a demersal fish species in the open ocean, despite our study occurring in deeper water than previous studies that have examined storm effects on animal movement. We conclude that tropical storms are a driving force behind the structure of marine ecosystems, in part by influencing movements of mobile animals.

Modeling of transient hydrodynamic lifting forces of sailing yachts and study of their effect on maneuvering in waves

The calculation of hydrodynamic lifting forces sustained by a sailing yacht's hull performing maneuvers in regular incident waves is reported. The mathematical model involves the application of the lifting surface theory for transient flows, implemented through the numerical scheme of the vortex lattice method and corrected for linear thickness effects of the simulated body geometry. The mathematical influence of the waves appears through the derivation of the wave potential and it is employed in a kinematic sense via the introduction of an additional inflow velocity component on the lifting surface. Validation with data derived from experiments in calm water showed very good agreement. Additionally, the variation of the side force coefficient while sailing in waves has been quantified in terms of the amplitude of oscillation due to the wave orbital velocities. The formulation is intended to upgrade the respective computational component in a mathematical model previously developed by the authors, for the maneuvering motions of sailing yachts. Parametric studies of maneuvering and course-keeping demonstrate a sensitivity of the yacht to wave steepness and heading angles, with a deficit in sustaining capacity appearing for lower wind speed.

Wind-wave and Tidally Driven Sediment Resuspension in a Macrotidal Basin

A coupled hydrodynamic-wave-sediment model is used to simulate the broad-scale tidal circulation, surface waves, and suspended sediment concentrations (SSC) in Minas Basin, a 70-km long tidal estuary in the Bay of Fundy, during winter and summer periods. The model hydrodynamics are validated using acoustic-Doppler current profile observations, the surface SSC predictions are compared to satellite observations, and model results indicate that strong seasonal signals in SSC can be explained in part by seasonal changes in fetch-limited surface waves generated by local winds over the basin. The spatial and temporal variability of SSC is evaluated in this study by focussing on different forcing conditions from waves and tidal currents, the two primary physical process that influence the response of sediments in suspension. Model predictions in the intertidal areas indicate that surface waves can increase the bed shear stress from tidal currents alone by up to 1–5 Nm− 2, causing excess bed shear stresses to be higher and result in higher SSC by 100–200 gm− 3 particularly during wind events that are stronger and more frequent in winter months. Resuspension of sediments on tidal flats is driven by the combination of shear stresses from near-bed wave orbital velocities and tidal currents, and transport of the suspended materials over deeper areas of the basin is driven by advection from the strong tidal currents.

Group Line Energy in Phase-Resolved Ocean Surface Wave Orbital Velocity Reconstructions from X-band Doppler Radar Measurements of the Sea Surface

The wavenumber-frequency spectra of many radar measurements of the sea surface contain a linear feature at frequencies lower than the first order dispersion relationship commonly referred to as the “group line”. Plant and Farquharson, showed numerically that the group line is at least partially caused by wave interference-induced breaking of steep short gravity waves. This paper uses two wave retrieval techniques, proper orthogonal decomposition (POD) and FFT-based dispersion curve filtering, to examine two X-band radar datasets, and compare wave orbital velocity reconstructions to ground truth wave buoy measurements within the field of view of the radar. POD allows group line energy to be retained in the reconstruction, while dispersion curve filtering removes all energy not associated with the first order dispersion relationship. Results show that when group line energy is higher or comparable to dispersion curve energy, the inclusion of this group line energy in phase-resolved orbital velocity reconstructions increases the accuracy of the reconstruction. This increased accuracy is demonstrated by higher correlations between POD reconstructed time series with buoy ground truth measurements than dispersion curve filtered reconstructions. When energy lying on the dispersion relationship is much higher than the group line energy, the FFT and POD reconstruction methods perform comparably.

Anomalous Diffusion of Sand Tracer Particles Under Waves

A model is presented for the horizontal and vertical diffusion of sand tracer particles by waves and is verified using a reanalysis of the experiments of Miller and Komar (1979, https://doi.org/10.1086/628455). In the model, sand particles take random jumps at a rate associated with the wave period but also can bury one another resulting in intermittent long rest times between jumps. The particle jump length between burials is modeled by considering a vortex entrainment and advection mechanism, resulting in a probability distribution as a function of wave orbital excursion amplitude and particle settling velocity. The model is formalized using the theory of continuous time random walks to obtain closed‐form expressions for the horizontal particle spreading rate, and plume shape, without tunable parameters. Importantly, the model predicts subdiffusive particle spreading, a previously unconsidered phenomenon that is apparent in the experimental data. The plume horizontal extent versus time is also well predicted compared to experimental data.

Field Observations of Turbulence, Sand Suspension, and Cross‐Shore Transport Under Spilling and Plunging Breakers

AbstractMeasurements of wave orbital velocity, near‐bed turbulence levels, and sediment suspension were obtained under plunging and spilling breakers in the outer surf zone on the beach at Vejers, Denmark. For the same range of relative wave heights and indicators of wave nonlinearity, we observed significantly larger suspended sediment concentrations and onshore‐directed rates of suspended sediment transport under (long period) plunging breakers, compared to (short period) spilling breakers. This is consistent with the long‐held understanding that, for a given beach slope, onshore transport and beach accretion are associated with longer‐period waves and offshore transport and erosion are associated with shorter‐period waves. An intra‐wave analysis of hydrodynamics and sediment suspension revealed that the main reason for the larger suspended sediment transport rates under plunging waves was (i) larger time‐averaged suspended sediment loads under plunging waves, and (ii) a larger difference in cumulated sediment load under the wave crest phase compared to the wave trough phase for plunging breakers. This latter difference was due to an earlier arrival at the seabed of higher levels of turbulent kinetic energy under plunging waves compared to spilling waves. Hence, both magnitude and timing within the wave cycle of turbulent kinetic energy production are important in a quantification of sediment transport under breaking waves. For ensemble‐averaged intense suspension events, contributing roughly 50% of the total sediment suspension during individual records, we found that instantaneous near‐bed sediment load was linearly related to instantaneous levels of Froude‐scaled turbulent kinetic energy.

On estimation of coastal wave parameters and wave‐induced shear stresses

AbstractWave parameters, e.g., wave height, near‐bed wave orbital velocity, and wave‐induced shear stresses, are important hydrodynamic parameters for sediment processes in coastal oceans. Wave orbital velocity is particularly critical in sediment resuspension. Several algorithms to calculate wave orbital velocity have been proposed, including linear wave theory, spectrum, and Joint North Sea Wave Project methods, but the validity of these algorithms in relatively shallow waters is not well understood. In this study, we compared the wave parameters obtained by different instruments and algorithms at four sites, one within the intertidal zone with a mean depth of 1 m and the remainder three in deeper offshore water with mean depths of 15–30 m. We found a high consistency of the estimated wave height, peak wave period, and wave orbital velocity among different datasets and different algorithms at the offshore sites, while there were significant discrepancies at the shoreline site. Using Ursell number, our study suggests that it is reliable to apply any of the three algorithms and different instruments (acoustic Doppler velocimeter and buoy) in deeper water. However, for very shallow water, it is recommended to use the measured high‐frequency velocity and spectrum method to calculate wave orbital velocity, and use wave gauge instrument or zero‐crossing algorithm to obtain wave height and period information. Finally, the effect of turbulence and bedform morphology on wave‐induced shear stress is discussed: without removing the turbulence or taking onto account bedforms (e.g., ripples), the orbital velocity will be remarkably over‐estimated or under‐estimated.

Ocean Wave Measurement Using Short-Range K-Band Narrow Beam Continuous Wave Radar

We describe a technique to measure ocean wave period, height and direction. The technique is based on the characteristics of transmission and backscattering of short-range K-band narrow beam continuous wave radar at the sea surface. The short-range K-band radar transmits and receives continuous signals close to the sea surface at a low-grazing angle. By sensing the motions of a dominant facet at the sea surface that strongly scatters signals back and is located directly in front of the radar, the wave orbital velocity can be measured from the Doppler shift of the received radar signal. The period, height and direction of ocean wave are determined from the relationships among wave orbital velocity, ocean wave characteristics and the Doppler shift. Numerical simulations were performed to validate that the dominant facet exists and ocean waves are measured by sensing its motion. Validation experiments were conducted in a wave tank to verify the feasibility of the proposed ocean wave measurement method. The results of simulations and experiments demonstrate the effectiveness of the short-range K-band narrow beam continuous wave radar for the measurement of ocean waves.

A Wave-Resolving Simulation of Langmuir Circulations with a Nonhydrostatic Free-Surface Model: Comparison with Craik–Leibovich Theory and an Alternative Eulerian View of the Driving Mechanism

AbstractThe present study performs a wave-resolving simulation of wind-driven currents under monochromatic surface gravity waves using the latest nonhydrostatic free-surface numerical model. Here, phase speed of the waves is set much greater than the current speed. Roll structures very similar to observed Langmuir circulations (LCs) appear in the simulation only when both waves and down-wave surface currents are present, demonstrating that the rolls are driven by the wave–current interaction. A vorticity analysis of simulated mean flow reveals that the rolls are driven by the torque associated with wave motion, which arises from a correlation between wave-induced vorticity fluctuation and the wave motion itself. Furthermore, it is confirmed that the wave-induced torque is very well represented by the curl of the vortex force (VF), that is, the vector product of mean vorticity and Stokes drift velocity. Therefore, it is concluded that the simulated rolls are LCs and that the wave effects are well represented by the VF expression in the present simulation. The present study further revisits the scaling assumptions made by previous studies that derived VF formulation and shows that there is disagreement among the previous studies regarding the applicability of VF formulation when the wave orbital velocity (proportional to the amplitude times the frequency) is much smaller than the mean flow velocity. The result from the present simulation shows that the VF expression is still valid even with such small wave amplitudes, as long as phase speed of the waves is much greater than the current speed.

Wave-current interactions in the continental shelf bottom boundary layer of the Australian North West Shelf during tropical cyclone conditions

The distant passage of an intense and large Category 3 Tropical Cyclone (TC) generated highly energetic surface waves in a large region across the Australian North West Shelf (NWS). Two sites on the continental shelf experienced maximum significant wave heights of up to 10 m with near-bed wave orbital velocities up to 0.7 m s−1 at 40 and 74 m depth. Concurrent current profiles were measured between 0.5 and 8.5 m above the seabed at these sites and therefore the observations presented in this study allowed a detailed analysis of the wave-current interactions in the continental shelf bottom boundary layer during highly energetic tropical cyclone conditions. These observations revealed a strong modification of the current profiles during the TC with an increase of the apparent bottom roughness by up to 2 orders of magnitude compared to the typical tide-dominated current conditions. This modification was predominantly due to wave-current interactions; however, the results also suggest that changes in bed forms during the TC conditions likely acted as a secondary mechanism contributing to the enhanced bottom roughness experienced by the mean currents. A comparison of the total mean bed shear stresses as a function of the waves and currents showed a somewhat larger enhancement due to the nonlinear wave-current interactions than predicted by common semi-empirical models. Overall, the enhancement of the mean bed shear stresses during the TC significantly reduced the mean current velocities in the bottom boundary layer, highlighting the importance of including wave-current interactions within ocean circulation models to accurately predict shelf currents during TC conditions.

Coral Reef Drag Coefficients—Surface Gravity Wave Enhancement

AbstractA primary challenge in modeling flow over shallow coral reefs is accurately characterizing the bottom drag. Previous studies over continental shelves and sandy beaches suggest surface gravity waves should enhance the drag on the circulation over coral reefs. The influence of surface gravity waves on drag over four platform reefs in the Red Sea is examined using observations from 6-month deployments of current and pressure sensors burst sampling at 1 Hz for 4–5 min. Depth-average current fluctuations U′ within each burst are dominated by wave orbital velocities uw that account for 80%–90% of the burst variance and have a magnitude of order 10 cm s−1, similar to the lower-frequency depth-average current Uavg. Previous studies have shown that the cross-reef bottom stress balances the pressure gradient over these reefs. A bottom stress estimate that neglects the waves (ρCdaUavg|Uavg|, where ρ is water density and Cda is a drag coefficient) balances the observed pressure gradient when uw is smaller than Uavg but underestimates the pressure gradient when uw is larger than Uavg (by a factor of 3–5 when uw = 2Uavg), indicating the neglected waves enhance the bottom stress. In contrast, a bottom stress estimate that includes the waves [ρCda(Uavg + U′)|Uavg + U′|)] balances the observed pressure gradient independent of the relative size of uw and Uavg, indicating that this estimate accounts for the wave enhancement of the bottom stress. A parameterization proposed by Wright and Thompson provides a reasonable estimate of the total bottom stress (including the waves) given the burst-averaged current and the wave orbital velocity.

The role of hydrodynamic forcing, sediment transport processes and bottom substratum in the shoreward development of Posidonia oceanica meadow

This paper investigates the combined role of hydrodynamic forcing, sediment transport processes and sea bottom features in determining the location of the meadow upper limit of the endemic Mediterranean seagrass Posidonia oceanica. For this purpose, an approach including extreme wave analysis and numerical modelling is applied to two sandy beach systems located in southern Sardinia (Italy). Information about bathymetry, P. oceanica meadow extension and bottom features has been made available by previous published studies. Based on the 30-year long NOAA hindcast dataset, a wave climate analysis is carried out to identify the incoming storm wave conditions (1 year return period) to be simulated with the Delft3D model package. The simulation results extend the current knowledge highlighting the importance of wave-induced hydrodynamic parameters as limiting factors for the survival of P. oceanica meadows. In particular, the results suggest that on sandy substrata the meadow upper limit lies well offshore of the surf zone, in areas with little morphological activity, where the wave orbital velocities associated to storms are on the order of 0.8 m/s and the mean current magnitude does not exceed 0.5 m/s. On rocky bottoms, the P. oceanica meadow can extend up to the outer surf zone of storms and is thus able to colonize shallow areas subject to stronger hydrodynamic forcings than those observed on sand. This difference in the plant tolerance to wave forcing depending on the substratum type shows how both hydrodynamic and geological factors play a key role in defining the environmental conditions for the development of seagrass meadows.

Material Transport under a Wave Train in Interaction with Constant Wind: A Eulerian RANS Approach Combined with a Lagrangian Particle Dispersion Model

The interaction of a developed train of gravity deep water waves with suddenly applied winds is investigated in this manuscript. The direction of the wind is the same as that of the wave train (i.e., following) and its imposed surface shear stress is constant and steady. The focus of this study is on a micro-scale water wave field where the time scale is on the order of ten wave periods and the length scale is on the order of ten wave lengths. Accurate 2D Reynolds-averaged Navier–Stokes (RANS) multi-phase simulations of Navier–Stokes equations are performed in a Eulerian framework to capture the flow features, induced by the wave field and the surface wind. The sufficiently large spatial domain in the horizontal direction, combined with the sufficiently long simulation time, permits the development of surface currents and the consequent formation of a near-surface shear layer. The interaction of surface currents with the wave orbital velocity field results in the generation of spilling breaking waves. Downstream of the domain, vertical turbulent structures are observed as the result of such breaking waves. Lagrangian particle tracking is performed, using the RANS simulation velocity and eddy diffusivity data. A second order random acceleration particle tracking method is applied with the vitally important spatial gradients of the eddy diffusivity (Journal of Marine Science and Engineering, 2018, 6, 10.3390/jmse6010007.) also included in calculations. The spatial gradients of the eddy diffusivity were proved to be a key factor in material transport simulations. Our particle tracking results exhibit strong vertical mixing downstream of the domain and by means of visualizing the spiral trajectory of neutrally buoyant particles. Such enhanced vertical mixing (caused by horizontal winds) is the result of the strong near-surface advection (induced by currents) and the turbulence (induced by breaking waves). The objectives of this paper are twofold. Firstly, a numerical approach to simulate wind breaking waves is proposed based on: using K − ϵ RANS model to capture turbulence features, employing the Volume of Fluid Method (VOF) to model the free surface flow, and applying a constant shear stress body force at the interfacial cells to simulate the wind force. Such treatment of the winds eliminates the need for fully resolving the air phase. The computed eddy viscosity profiles are in good agreement with the experimental profiles reported in the literature ( ν t = − κ u * z , Journal of Physical Oceanography, 1977, 7, pp. 248–255; Journal of Physical Oceanography, 1984, 14, pp. 855–863). Secondly, effects of the horizontally applied wind on the vertical mixing and eddy viscosity profiles on the water column are studied. It is observed that, away from the surface and outside the shear layer, the negative horizontal gradient of eddy diffusivity (induced by the dampening effect of breaking surface waves), combined with the downwards advection velocities (induced by breaking waves), results in an enhanced vertical mixing and reduced horizontal drift of transported material.

Critical Depths Derived for Three Distinct Modes of Coastal Sediment Transport

Shi, H.Y.; You, Z.J.; Yin, B.S., and Bai, Y.C., 2018. Critical Depths Derived for Three Distinct Modes of Coastal Sediment Transport. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 271–275. Coconut Creek (Florida), ISSN 0749-0208.A unified criterion is developed for three distinct modes of coastal sediment transport: incipient movement of sediment (only bedload), initiation of bedripple formation (bedload and suspended load), and inception of bed ripple disappearance or sheet flow (mainly bed load) based on three unique laboratory datasets collected on a horizontal bed of sediment under regular waves. The onset velocity criterion on the three distinct sediment transport modesis derived to be of the same simple form, U0 = 2πC (1 − T0/T), where U0 is the maximum on set wave orbital velocity close to the bed, T is the wave period, and C and T0 are the coefficients dependent of sediment property only. The newly developed criterion is then extended to calculate critical depths for the distinct modes of sediment transport under coastal waves by replacing regular wave height H and period T with statistical wave height Hrms and period Trms according to the recently developed “zero-crossing and energy balance” wave analysis method and linear wave theory. The critical depth calculated from this unified criterion is shown to be a function of sediment grain size and density, and wave height and period, but more sensitive to wave period. The critical depth estimated for the initiation of bed sediment ripple formation is found to be much larger than the depth of closure derived for beach-fill designs.

On Estimating Turbulent Reynolds Stress in Wavy Aquatic Environment

AbstractSeveral methods have been developed for the estimation of the turbulent Reynolds stress in wavy aquatic environment. They are based on different physical assumptions and often give discrepant results. It is practically difficult to quantify the uncertainties in these estimations. Using high‐resolution velocity measurements of acoustic Doppler velocimeter (ADV) from a coastal benthic layer subject to moderate wave influence (the ratio of rms wave orbital velocity to current magnitude was 0.23–0.92), this study tests a Synchrosqueezed Wavelet Transform (SWT)‐based method and three existing methods (i.e., the Coherence, Cospectra, and Ensemble Empirical Mode Decomposition [EEMD] methods) for wave‐turbulence decomposition. In particular, we evaluate the performance of different methods for objective estimation of the turbulent Reynolds stress. Power spectra and cospectra analysis is conducted to quantify the uncertainties in the estimations. The results suggest that the Coherence method tends to overestimate the Reynolds stress due to incomplete removal of wave motions from the observed velocity records; the Cospectra method performs poorly because the empirical model does not fit the observed cospectra well; both the EEMD and SWT methods underestimate the Reynolds stress, as they tend to attribute turbulent fluctuations at frequencies in the vicinity of the wave frequencies to wave motions. In general, the SWT method performs best inducing lowest uncertainty in the Reynolds stress estimation. For the data set analyzed in this study, the estimations with the Coherence, Cospectra, EEMD, and SWT methods account for 70%, 50%, 51%, and 60% of the total covariance of horizontal and vertical velocities, respectively.

In situ observations of wave pumping of sediments in the Yellow River Delta with a newly developed benthic chamber

A specially designed benthic chamber for the field observation of sediment resuspension that is caused by the wave-induced oscillatory seepage effect (i.e., the wave pumping of sediments) is newly developed. Observational results from the first sea trial prove that the geometry design and skillful instrumentation of the chamber well realize the goal of monitoring the wave pumping of sediments (WPS) continuously. Based on this field dataset, the quantitative contribution of the WPS to the total sediment resuspension is estimated to be 20–60% merely under the continuous action of normal waves (Hs ≤ 1.5 m) in the subaqueous Yellow River Delta (YRD). Such a large contribution invalidates a commonly held opinion that sediments are purely eroded from the seabed surface by the horizontal “shearing effect” from the wave orbital or current velocities. In fact, a considerable amount of sediments could originate from the shallow subsurface of seabed driven by the vertical “pumping effect” of the wave-generated seepage flows during wavy periods. According to the new findings, an improved conceptual model for the resuspension mechanisms of silty sediments under various hydrodynamics is proposed for the first time.

Longitudinal Pipeline Scour Propagation Induced by Wave-Current Interaction For the South Sumatra-West Java Submarine Pipeline

Scouring process around subsea pipelines could reduce the soil bearing capacity which affected to the pipe stability. Scouring initial process against time should be known to discover scouring propagation. This paper aims to analyze the time scale calculation of 32 inch diameter in-trench pipe, until meet the maximum-scouring-depth stage. Embedment (e/D) variation is given to know the impact to the scour propagation. Wave and current condition presented to meet the real condition. Wave orbital particle velocity (Uw) is calculated to obtain the non-dimensional factors (Uc/(Uc+Uw)) and KC. The results showed according to the deeper pipe embedment, it takes longer time to reach the maximum scouring depth.

DualSPHysics: A numerical tool to simulate real breakwaters

The open-source code DualSPHysics is used in this work to compute the wave run-up in an existing dike in the Chinese coast using realistic dimensions, bathymetry and wave conditions. The GPU computing power of the DualSPHysics allows simulating real-engineering problems that involve complex geometries with a high resolution in a reasonable computational time. The code is first validated by comparing the numerical free-surface elevation, the wave orbital velocities and the time series of the run-up with physical data in a wave flume. Those experiments include a smooth dike and an armored dike with two layers of cubic blocks. After validation, the code is applied to a real case to obtain the wave run-up under different incident wave conditions. In order to simulate the real open sea, the spurious reflections from the wavemaker are removed by using an active wave absorption technique.

A synthetic map of the north-west European Shelf sedimentary environment for applications in marine science

Abstract. Seabed sediment mapping is important for a wide range of marine policy, planning and scientific issues, and there has been considerable national and international investment around the world in the collation and synthesis of sediment datasets. However, in Europe at least, much of this effort has been directed towards seabed classification and mapping of discrete habitats. Scientific users often have to resort to reverse engineering these classifications to recover continuous variables, such as mud content and median grain size, that are required for many ecological and biophysical studies. Here we present a new set of 0.125∘ by 0.125∘ resolution synthetic maps of continuous properties of the north-west European sedimentary environment, extending from the Bay of Biscay to the northern limits of the North Sea and the Faroe Islands. The maps are a blend of gridded survey data, statistically modelled values based on distributions of bed shear stress due to tidal currents and waves, and bathymetric properties. Recent work has shown that statistical models can predict sediment composition in British waters and the North Sea with high accuracy, and here we extend this to the entire shelf and to the mapping of other key seabed parameters. The maps include percentage compositions of mud, sand and gravel; porosity and permeability; median grain size of the whole sediment and of the sand and the gravel fractions; carbon and nitrogen content of sediments; percentage of seabed area covered by rock; mean and maximum depth-averaged tidal velocity and wave orbital velocity at the seabed; and mean monthly natural disturbance rates. A number of applications for these maps exist, including species distribution modelling and the more accurate representation of sea-floor biogeochemistry in ecosystem models. The data products are available from https://doi.org/10.15129/1e27b806-1eae-494d-83b5-a5f4792c46fc.

Correcting Surface Wave Bias in Structure Function Estimates of Turbulent Kinetic Energy Dissipation Rate

AbstractThe combination of acoustic Doppler current profilers and the structure function methodology provides an attractive approach to making extended time series measurements of oceanic turbulence (the rate of turbulent kinetic energy dissipation ε) from moorings. However, this study shows that for deployments in the upper part of the water column, estimates of ε will be biased by the vertical gradient in wave orbital velocities. To remove this bias, a modified structure function methodology is developed that exploits the differing length scale dependencies of the contributions to the structure function resulting from turbulent and wave orbital motions. The success of the modified method is demonstrated through a comparison of ε estimates based on data from instruments at three depths over a 3-month period under a wide range of conditions, with appropriate scalings for wind stress and convective forcing.