The effect of wave breaking on surf-zone turbulence and alongshore currents : a modeling study

Author Posting. © American Meteorological Society, 2005. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 35 (2005): 2187–2203, doi:10.1175/JPO2800.1.The effect of breaking-wave-generated turbulence on the mean circulation, turbulence, and bottom stress in the surf zone is poorly understood. A one-dimensional vertical coupled turbulence (k–ε) and mean-flow model is developed that incorporates the effect of wave breaking with a time-dependent surface turbulence flux and uses existing (published) model closures. No model parameters are tuned to optimize model–data agreement. The model qualitatively reproduces the mean dissipation and production during the most energetic breaking-wave conditions in 4.5-m water depth off of a sandy beach and slightly underpredicts the mean alongshore current. By modeling a cross-shore transect case example from the Duck94 field experiment, the observed surf-zone dissipation depth scaling and the observed mean alongshore current (although slightly underpredicted) are generally reproduced. Wave breaking significantly reduces the modeled vertical shear, suggesting that surf-zone bottom stress cannot be estimated by fitting a logarithmic current profile to alongshore current observations. Model-inferred drag coefficients follow parameterizations (Manning–Strickler) that depend on the bed roughness and inversely on the water depth, although the inverse depth dependence is likely a proxy for some other effect such as wave breaking. Variations in the bed roughness and the percentage of breaking-wave energy entering the water column have a comparable effect on the mean alongshore current and drag coefficient. However, covarying the wave height, forcing, and dissipation and bed roughness separately results in an alongshore current (drag coefficient) only weakly (strongly) dependent on the bed roughness because of the competing effects of increased turbulence, wave forcing, and orbital wave velocities.This work was funded by NSF, ONR, and NOPP

Evaluation of the Acoustic Doppler Velocimeter (ADV) for Turbulence Measurements

Accuracy of the acoustic Doppler velocimeter (ADV) is evaluated in this paper. Simultaneous measurements of open-channel flow were undertaken in a 17-m flume using an ADV and a laser Doppler velocimeter. Flow velocity records obtained by both instruments are used for estimating the true (‘‘ground truth’’) flow characteristics and the noise variances encountered during the experimental runs. The measured values are compared with estimates of the true flow characteristics and values of variance (^u92&, ^w92&) and covariance (^u9w9&) predicted by semiempirical models for open-channel flow. The analysis showed that the ADV sensor can measure mean velocity and Reynolds stress within 1% of the estimated true value. Mean velocities can be obtained at distances less than 1 cm from the boundary, whereas Reynolds stress values obtained at elevations greater than 3 cm above the bottom exhibit a variation that is in agreement with the predictions of the semiempirical models. Closer to the boundary, the measured Reynolds stresses deviate from those predicted by the model, probably due to the size of the ADV sample volume. Turbulence spectra computed using the ADV records agree with theoretical spectra after corrections are applied for the spatial averaging due to the size of the sample volume and a noise floor. The noise variance in ADV velocity records consists of two terms. One is related to the electronic circuitry of the sensor and its ability to resolve phase differences, whereas the second is flow related. The latter noise component dominates at rapid flows. The error in flow measurements due to the former noise term depends on sensor velocity range setting and ranges from 60.95 to 63.0 mm s21. Noise due to shear within the sample volume and to Doppler broadening is primarily a function of the turbulence dissipation parameter. Noise variances calculated using spectral analysis and the results of the ground truthing technique are compared with theoretical estimates of noise

Coupled dynamics of interfacial waves and bed forms in fluid muds over erodible seabeds in oscillatory flows

© The Author(s), 2015. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Journal of Geophysical Research: Oceans 120 (2015): 5698–5709, doi:10.1002/2015JC010872.Recent field investigations of the damping of ocean surface waves over fluid muds have revealed waves on the interface between the thin layer of fluid mud and the overlying much thicker column of clear water, accompanied by bed forms on the erodible seabed beneath the fluid mud. The frequencies and wavelengths of the observed interfacial waves are qualitatively consistent with the linear dispersion relationship for long interfacial waves, but the forcing mechanism is not known. To understand the forcing, a linear model is proposed, based on the layer-averaged hydrostatic equations for the fluid mud, together with the Meyer-Peter-Mueller equation for the sediment transport within the underlying seabed, both subject to oscillatory forcing by the surface waves. If the underlying seabed is nonerodible and flat, the model indicates parametric instability to interfacial waves, but the threshold for instability is not met by the observations. If the underlying seabed is erodible, the model indicates that perturbations to the seabed elevation in the presence of the oscillatory forcing create interfacial waves, which in turn produce stresses within the fluid mud that force a net transport of sediment within the seabed toward the bed form crests, thus causing growth of both bed forms and interfacial waves. The frequencies, wavelengths, and growth rates are in qualitative agreement with the observations. A competition between mixing created by the interfacial waves and gravitational settling might control the thickness, density, and viscosity of the fluid muds during periods of strong forcing.This study was supported by the Coastal Geodynamics Program at the Office of Naval Research and by the Physical Oceanography Program at the National Science Foundation

Using Moored Arrays and Hyperspectral Aerial Imagery to Develop Nutrient Criteria for New Hampshire\u27s Estuaries

Increasing nitrogen concentrations and declining eelgrass beds in Great Bay, NH are clear indicators of impending problems for the state’s estuaries. A workgroup established in 2005 by the NH Department of Environmental Services and the NH Estuaries Project (NHEP) adopted eelgrass survival as the water quality target for nutrient criteria development for NH’s estuaries. In 2007, the NHEP received a grant from the U.S. Environmental Protection Agency to collect water quality information including that from moored sensors and hyper-spectral imagery data of the Great Bay Estuary. Data from the Great Bay Coastal Buoy, part of the regional Integrated Ocean Observing System (IOOS), were used to derive a multivariate model of water clarity with phytoplankton, Colored Dissolved Organic Matter (CDOM), and non-algal particles. Non-algal particles include both inorganic and organic matter. Most of the temporal variability in the diffuse attenuation coefficient of Photosynthetically Available Radiation (PAR) was associated with non-algal particles. However, on a mean daily basis non-algal particles and CDOM contributed a similar fraction (~30 %) to the attenuation of light. The contribution of phytoplankton was about a third of the other two optically important constituents. CDOM concentrations varied with salinity and magnitude of riverine inputs demonstrating its terrestrial origin. Non-algal particle concentration also varied with river flow but also wind driven resuspension. Twelve of the NHEP estuarine assessment zones were observed with the hyperspectral aerial imagery on August 29 and October 17. A concurrent in situ effort included buoy measurements, continuous along-track sampling, discrete water grab samples, and vertical profiles of light attenuation. PAR effective attenuation coefficients retrieved from deep water regions in the imagery agreed well with in-situ observations. Water clarity was lower and optically important constituent concentrations were higher in the tributaries. Eelgrass survival depth, estimated as the depth at which 22% of surface light was available, ranged from less than half a meter to over two meters. The best water clarity was found in the Great Bay (GB), Little Bay (LB), and Lower Piscataqua River (LPR) assessment zones. Absence of eelgrass from these zones would indicate controlling factors other than water clarity

Turbulence in the coastal environment during HYCODE

A tall tripod equipped with two acoustic Doppler velocimeters (ADVs) was deployed at a water depth of 15 m off the coast of New Jersey near the LEO-15 site. Sensors were co-located near the bottom to provide good estimates of Reynolds stress. Thermistors were located within several centimeters of the velocity sample volume to provide simultaneously sampled estimates of turbulent temperature variance and vertical temperature flux. One of the ADVs was equipped with a pressure and a temperature sensor. A wave/tide gauge was placed at 4 meters above bottom. The instruments were deployed late July through early December of 2000 and late June through early August of 2001. For the 2001 deployment, a single beam acoustic Doppler velocity sensor (DopBeam) was added to measure high frequency vertical velocity variance and echo intensity within the bottom boundary layer. A second tripod was deployed nearby and was equipped with an array of LISST sensors and an MSCAT. The purpose of this report is to document the instrumentation and deployment of the tripods and to document the tall tripod data by providing a description of the processing and data formats, time-series summaries of the burst averaged data along with preliminary analyses.Funding was provided by the Office of Naval Research under Contract No. N00014-99-1-0213

Mechanisms of surface wave energy dissipation over a high-concentration sediment suspension

Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 120 (2015): 1638–1681, doi:10.1002/2014JC010245.Field observations from the spring of 2008 on the Louisiana shelf were used to elucidate the mechanisms of wave energy dissipation over a muddy seafloor. After a period of high discharge from the Atchafalaya River, acoustic measurements showed the presence of 20 cm thick mobile fluid-mud layers during and after wave events. While total wave energy dissipation (D) was greatest during the high energy periods, these periods had relatively low normalized attenuation rates (κ = Dissipation/Energy Flux). During declining wave-energy conditions, as the fluid-mud layer settled, the attenuation process became more efficient with high κ and low D. The transition from high D and low κ to high κ and low D was caused by a transition from turbulent to laminar flow in the fluid-mud layer as measured by a Pulse-coherent Doppler profiler. Measurements of the oscillatory boundary layer velocity profile in the fluid-mud layer during laminar flow reveal a very thick wave boundary layer with curvature filling the entire fluid-mud layer, suggesting a kinematic viscosity 2–3 orders of magnitude greater than that of clear water. This high viscosity is also consistent with a high wave-attenuation rates measured by across-shelf energy flux differences. The transition to turbulence was forced by instabilities on the lutocline, with wavelengths consistent with the dispersion relation for this two-layer system. The measurements also provide new insight into the dynamics of wave-supported turbidity flows during the transition from a laminar to turbulent fluid-mud layer.This work was supported by Office of Naval Research Award N00014-06-1–0718, which was part of the ONR Multidisciplinary University Research Initiative (MUD-MURI): entitled ‘‘Mechanisms of Fluid-Mud Interactions Under Waves.’’ Additional support was provided by National Science Foundation grant 1059914.2015-09-1

Controls on Floc Size in a Continental Shelf Bottom Boundary Layer

Simultaneous in situ observations of floc size, waves, and currents in a continental shelf bottom boundary layer do not support generally accepted functional relationships between turbulence and floc size in the sea. In September and October 1996 and January 1997, two tripods were deployed in 70 m of water on the continental shelf south of Woods Hole, Massachusetts. On one a camera photographed particles in suspension 1.2 m above the bottom that had equivalent circular diameters larger than 250 um, and on the other, three horizontally displaced acoustic current meters measured flow velocity 0.35 m above the bottom. The tripods were separated by ~ 150 m. Typically, maximal floc diameter stayed relatively constant, around 1 mm, and it showed a dependence on turbulence parameters that was significantly weaker than that predicted by any model that assumes that turbulence-induced stresses limit floc size. Occasionally, when waves and currents generated intense near-bed turbulence, flocs were destroyed. These precipitous decreases in maximal floc size also were not predicted by conventional models. The correlation in time between episodes of floc destruction and elevated combined wave current stresses provides the first quantitative support for the hypothesis that floc size throughout bottom boundary layers can be controlled by breakup in the intensely sheared near-bed region. These observations demand a reassessment of the forces limiting floc size in the sea, and they indicate the potential for significant simplifying assumptions in models of floc dynamics

Turbulence in the shallow nearshore environment during SANDYDUCK '97

An array of five acoustic Doppler velocimeters (ADV), which produce high quality measurements of the three-dimensional velocity vector in a sample volume with a scale of one centimeter, was deployed from late August through late November of 1997 at a water depth of approximately 4.5 m off Duck, North Carolina. The sensors were deployed near the sea floor but above the centimeters-thick wave boundary layer, and the sampling scheme was designed to resolve turbulence statistics averaged over tens of minutes, much longer than typical wave periods but shorter than time scales associated with variablity of energetic wind-driven and wave-driven alongshore flows.Funding was provided by the National Science Foundation under Grant No. OCE-9810609, the Mellon Foundation and Rinehart Coastal Research Center

Spatial variability of bottom turbulence over a linear sand ridge mooring deployment and AUTOSUB AUV survey cruise report R/V RRS Challenger, cruise number 146 Broken Bank, North Sea, U.K., 17 – 28 August 1999 cruise report

Two successful AUTOSUB deployments were carried out during August 1999 as part of the AUTOSUB Thematic Program project titled “Spatial Variability of Bottom Turbulence over a Linear Sand Ridge,” funded by the Natural Environment Research Council (NERC), U.K. The AUTOSUB Autonomous Underwater Vehicle (AUV) was deployed and used to survey flow patterns at a location near the Broken Bank, southern North Sea, U.K. The AUV was equipped with acoustic flow and turbulence sensors and its surveys aimed at mapping the spatial variation of flow and turbulence near the bed and over topographic features. Three instrumented bottom mounted frames were also deployed, around the AUV survey area, for a period of approximately 5 days. The purpose of this array was to gather information on the temporal variability of the flow and turbulence near the seabed and to identify the important terms that drive circulation around the bank. Additional data were gathered including CTD casts, seabed samples and acoustic images of the seabed (side-scan sonar). The purpose of this data collection was to help identify the flow patterns around ridges and to understand the mechanisms controlling the maintenance and evolution of such features. This report describes the operations carried out by researchers from the University of South Carolina, Woods Hole Oceanographic Institution, Southampton Oceanography Centre and the AUTOSUB Team on the R.V. RRS Challenger during the period 17th –28th August 1999.Funding was provided by the Office of Naval Research under Contract No. N00014-01-10255 and the Natural Environment Research Council, UK Award GST/02/2155 to the University of Southampton