Vertical Current Profiles Research Articles

Characteristic Analysis of Vertical Tidal Profile Parameters at Tidal Current Energy Site

Many mathematical models have been proposed to estimate vertical tidal current profiles. However, as previous studies have shown that tidal current energy sites have different characteristics in their vertical tidal current profiles, it is necessary to estimate the profiles from field-measured data for practical purposes. In this study, we measured layered tidal currents over two months using an acoustic Doppler current profiler (ADCP) to analyze the characteristics of vertical tidal current profiles at the Jangjuk Strait, a candidate site for tidal current energy. As a result, the power law parameter α and bed roughness β were estimated as 4.51–12.41 and 0.38–0.42, respectively. Additionally, the maximum roughness length representing seabed roughness in the logarithmic profile was estimated as 0.221 m, and the estimated friction velocity was 0.038–0.194 m/s. Furthermore, a high correlation was observed between the depth-averaged tidal current velocity and friction velocities at all sites during flood and ebb tide conditions. A high correlation was also found between the bed roughness, roughness length, and power law exponent at relatively deeper sites. Tidal current energy sites display distinct characteristics compared to other sea areas. Therefore, it is essential to account for field conditions when conducting numerical modeling and design.

Dependency of the drag coefficient on boundary layer stability beneath drifting sea ice in the central Arctic Ocean

The ice-ocean drag coefficient Cw\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$C_{w}$$\\end{document} and turning angle θw\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ heta_{w}$$\\end{document} are crucial parameters in ice-ocean coupled simulations, determining the transfer of momentum between the two media. These parameters are often treated as constants regardless of the static stability at the ice-ocean interface. This study investigates the variability of Cw\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$C_{w}$$\\end{document} and θw\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ heta_{w}$$\\end{document} based on direct observations of thermal and kinetic energy balance. The observations were conducted beneath multiyear ice packs widely across the central Arctic during a period transitioning from ablation to refreezing, indicating significant variability of Cw\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$C_{w}$$\\end{document} = 1–130 ×\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ imes$$\\end{document} 10−3 and θw\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ heta_{w}$$\\end{document} = − 19–1° at 5 m depth. Comparing different stations, the observations suggest a pronounced dependence of Cw\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$C_{w}$$\\end{document} on the stability parameter (μ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mu$$\\end{document}) resulting from mechanical and buoyant forcing. Cw\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$C_{w}$$\\end{document} rapidly decays with increasing μ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mu$$\\end{document}, indicating that the ice-to-ocean momentum transfer is enhanced for neutral or unstable conditions, while it is weakened for stable conditions. In addition, observed vertical profiles of currents revealed that |θw|\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$|\ heta_{w}|$$\\end{document} tends to be smaller for unstable and larger for stable conditions. We suggest that numerical simulations using constant values could result in an underestimate of large-scale near-surface currents during the ice growing period.

Implementation of additional spectral wave field exchanges in a three-dimensional wave–current coupled WAVEWATCH-III (version 6.07) and CROCO (version 1.2) configuration: assessment of their implications for macro-tidal coastal hydrodynamics

Abstract. An advanced coupling between a three-dimensional ocean circulation model (CROCO) and a spectral wave model (WAVEWATCH-III) is presented to better represent the interactions of macro-tidal currents with winds and waves. In the previous implementation of the coupled interface between these two models, some of the wave-induced terms in the ocean dynamic equations were computed from their monochromatic approximations (e.g. Stokes drift, Bernoulli head, near-bottom wave orbital velocity, wave-to-ocean energy flux). In the present study, the exchanges of these fields computed from the spectral wave model are implemented and evaluated. A set of numerical experiments for a coastal configuration of the macro-tidal circulation off the Bay of Somme (France) is designed. The impact of the spectral versus monochromatic computation of wave-induced terms has a notable effect on the macro-tidal hydrodynamics, particularly in scenarios involving storm waves and opposing winds to tidal flows. This effect manifests as a reduction in the wave-induced deceleration of the vertical profile of tidal currents. The new implementation provides current magnitudes closer to measurements than those predicted using monochromatic formulations, particularly at the free surface. The spectral-surface Stokes drift and the near-bottom wave orbital velocity are found to be the spectral fields with the most impact, respectively increasing advection towards the free surface and shifting the profile close to the seabed. In the particular case of the Bay of Somme, the approximation of these spectral terms with their monochromatic counterparts ultimately results in an underestimation of ocean surface currents. Our model developments thus provide a better description of the competing effects of tides, winds, and waves on the circulation off macro-tidal bays, with implications for the study of air–sea interactions and sediment transport processes.

An Investigation of Wave-Driven Current Characteristics across Fringing Reefs under Monochromatic Waves

The aim of this study is to better understand cross-reef wave-driven current characteristics, which are crucial to biological, ecological, and geomorphological processes within coral reefs. This study reports a set of new wave flume measurements to assess flow along the water depth and across a fringing reef profile under the action of a plunging breaker. Laboratory results are presented in view of cross-reef variations in both the wave height and the mean water level (MWL); the vertical profiles of wave-averaged mean currents below the wave trough and along the reef are also presented. To resolve the two-dimensional vertical (2DV) flow characteristics across the reef, Reynolds-Averaged Navier–Stokes (RANS) equations were solved using k-ω SST closure, modified to improve stability, and a Volume of Fluid (VOF) approach was used to capture the water surface. This numerical model was first validated via experimental measurements in view of waves and flows. It was then used to analyze the cross-reef distributions of the mean flow field, turbulent kinetic energy (TKE), and Reynolds shear stress across the reef.

Inverse estimation of a vertical current velocity profile using motions of an FPSO and artificial neural network

This study utilizes artificial neural network (ANN) models to inversely estimate the near-real-time 3D ocean vertical current profile (magnitudes and directions) using floater-mounted motion sensors with (or without) two bi-axial inclinometers on the top portion of a riser. A coupled time-domain dynamics model that takes into account the interaction between the FPSO (Floating Production Storage and Offloading platform), mooring lines, risers, and cables is established to provide inputs and outputs for ANN models. The dynamics simulations are performed using the 2-year-long measured environmental (wind, wave, current) conditions. Then, three ANN models with different combinations of input variables are evaluated, and the process of hyperparameter selection is conducted to increase the prediction accuracy compared to the actually measured current profiles. The root-mean-square errors (RMSEs) and R-squared of current speed, the contour plots of measured and estimated current speed and direction, representative motions of the FPSO, and inclination and azimuthal angles of a riser are systematically presented. The results demonstrate that the proposed ANN model adequately estimates the 3D current profile from FPSO motions while the accuracy especially at deeper depth is improved by considering bi-axial inclinometers on the riser as additional inputs.

Meltwater Lenses Over the Chukchi and the Beaufort Seas During Summer 2019: From In Situ to Synoptic View

AbstractWe investigate the Chukchi and the Beaufort seas in the Arctic Ocean, where salty and warm Pacific Water flows in through the Bering Strait and interacts with the sea ice, contributing to its summer melt. Thanks to in situ measurements recorded by two saildrones deployed during summer 2019 and to refined sea ice filtering in satellite L‐band radiometric data, we demonstrate the ability of satellite sea surface salinity (SSS) observed by Soil Moisture and Ocean Salinity and Soil Moisture Active and Passive to capture SSS freshening induced by sea ice melt. We refer to these freshening events as meltwater lenses (MWL). The largest MWL observed by the saildrones during this period occupied a large part of the Chukchi shelf, with a SSS freshening reaching −5 practical salinity scale, persisting for up to 1 month. This MWL restricted the transfer of air‐sea momentum to the upper ocean, as illustrated by measured wind speed and vertical profiles of currents. With satellite‐based sea surface temperature, satellite SSS provides a monitoring of the different water masses encountered in the region during summer 2019. Using sea ice concentration and estimated Ekman transport, we analyze the spatial variability of sea surface properties after the sea ice edge retreat over the Chukchi and the Beaufort seas. The two MWL captured by the saildrones and the satellite measurements resulted from different dynamics. Over the Beaufort Sea, the MWL evolution followed the meridional sea ice retreat whereas, in the Chukchi Sea, a large persisting MWL was generated by advection and subsequent melting of a sea ice filament.

The residual circulation profile of the Bons Sinais Estuary in central Mozambique - potential implications for larval dispersal and fisheries

The residual circulation in estuaries determines the net exchange of water, heat, salt, fine sediments and drifting biological organisms between estuarine and nearshore marine waters. The Bons Sinais Estuary in central Mozambique is ~ 30 km long with the city of Quelimane and an industrial port on the northern bank of its upper reaches. To investigate residual circulation in the estuary, seasonal (wet, dry and transition season) CTD profiling data were collected at 11 fixed stations between the upper estuary and mouth, and vertical current profiles were measured over a full tidal cycle at a fixed mid-estuary station. Strong longitudinal gradients in salinity and density indicated that the estuary was river-dominated during the wet season and tide-dominated during the dry season, but the water column remained partially mixed. Tidally averaged vertical profiles from the mid-estuary station revealed: uniform vertical temperatures, warmest during the wet season; depth-stratified salinity during the wet season, but uniform profiles during the dry and transition seasons with highest salinity during the dry season when the density was also highest. The density was uniform and somewhat lower in the transition season, and in the wet season the density was even lower, but stratified. The vertical velocity profile showed a classical two-layer circulation model, with downstream flow intensifying at the surface, and upstream flow at the bottom, during the wet and transitory seasons, when freshwater discharges into the estuary. The flow velocities obtained from a calibrated Hansen and Rattray model fitted the observed data well, confirming that a simplified modelling approach is adequate to describe the residual flow of the Bons Sinais Estuary. The residual circulation model provides insights useful for fisheries research and predicting the spread of water-borne pollutants in the estuary.

Impact of along-track altimeter sea surface height anomaly assimilation on surface and sub-surface currents in the Bay of Bengal

In this study, altimeter based sea surface height anomaly (SSHA) observations from the Jason-3 altimeter are assimilated in a high resolution Indian Ocean model and the impact of assimilation is analysed in the Bay of Bengal region. The study uses a 3-D primitive equation numerical ocean model forced with analysed atmospheric data to generate ocean state for a particular period, in this case from the year 2018. To study the impact of SSHA assimilation, two sets of model outputs are generated, one in which no SSHA is assimilated (Ctrl-R) and the other in which along-track SSHA from the Jason-3 satellite is assimilated (Assim-R) using Ensemble Optimum Interpolation technique. The impact of assimilation is primarily analysed on ocean surface currents. For this purpose, satellite-derived sea surface currents and buoy based vertical current profiles are used to evaluate Ctrl-R and Assim-R. It is found that there is a profound improvement in ocean current simulation after assimilation of SSHA. Mesoscale eddies that were weak and misplaced in Ctrl-R have now improved in Assim-R in terms of both strength and position. Sub-surface current simulation from Assim-R exhibits observed variability in most of the evaluation period, which was absent in the Ctrl-R. Computed statistics show much improved (15%–35%) ocean current simulations in Assim-R at all the depths. Particle trajectory analysis has been performed and it is seen that assimilated model velocity fields faithfully simulate the observed trajectory of tracers. Power spectra of simulated SSHA from both Assim-R and Ctrl-R were compared with spectra from satellite SSHA and it is found that after assimilation the slopes of the simulated spectra become much closer to slope of spectra from the observations. These results indicate that the along track assimilation of SSHA improves the model analysis and thereby provides accurate initial conditions for model forecast.

Introducing multi-round clustering on capturing the multidirectional extreme current profile for deep-water ROV operation

ROVs (Remotely Operated Vehicles) are playing an important role in installation, construction, and maintenance of subsea systems. During operation, ROVs are usually connected to surface facilities through umbilical lines where the entire umbilical-ROV system is affected by vessel motions, wave, and current induced loads. During the design process, it is commonly assumed that the current profile that leads to the extreme response of an offshore structural system has a typical profile with a uniform direction. However, in deep water, the variation of current direction along water depth becomes more apparent. Hence, the effect of direction variation cannot be neglected when establishing the extreme current profile for deep-water ROV operation.The study focuses on how to efficiently acquire the multidirectional extreme current profiles through a new multi-round clustering method. Firstly, static analysis of a typical deep-water umbilical-ROV system was performed under thousands of multidirectional current profiles extracted from a database of one-year continuously recorded ocean current. Different responses of the system such as ROV offset, umbilical lateral displacement, and maximum tension in the umbilical were investigated and set as a benchmark for further process. Then, a multi-round clustering procedure using a modified maximum dissimilarity algorithm for ROV operation was introduced and applied to extract the extreme multidirectional current profile more efficiently.It is shown that the proposed multi-round clustering methodology can be implemented efficiently and accurately in capturing the extreme vertical current profiles from a current database for ROV operation in different working depths. The study also showed that the effect of bottom current on deep-water ROV operation is more significant than the surface current. Therefore, for ROV operation in deep waters, more precautions shall be made on the bottom current profiles.

Alternative configurations to optimize tension in the umbilical of a work class ROV performing ultra-deep-water operation

The utilization of ROV (Remotely Operated Vehicle) for subsea operation has been increasing in the past few years alongside the increasing use of subsea technologies in deep-water oil and gas activities and other maritime operations. While survey class ROVs have reached the deepest depth in the ocean, the work class ROVs are mostly limited to 3 km depth. For a work class ROV operating in deep waters, the excessive static tension due to self-weight of the ROV and its umbilical system leaves small room before the dynamic tension exceeds the safe working load of the umbilical. Snap loads can also exist in the low-tension area of the umbilical when the ROV system is under dynamic motions which can lead to a large tensile load in the line. Therefore, there is a need for a practical solution to ensure that the safe working load is not exceeded during ultra-deep-water operation.This study focuses in reducing the tension in the umbilical hence increasing the operational depth of a work class ROV from deep to ultra-deep waters. Three different configurations, namely the hook, hook & top floaters, and mid-depth hook are numerically investigated and compared with the conventional free-flying ROV configuration. Firstly, deep-water operation of a 3 km ROV system is simulated under combination of a real extreme vertical current profile and various irregular wave conditions. This shows negligible effect of the current profile on umbilical maximum tension.The study also shows that for 3 km ROV operation, the suggested hook configuration is sufficient to effectively reduce the tension load as well as reducing the possibility of snap in the umbilical. Using hook configuration in the umbilical has significant benefits in minimizing ROV motion and also filters out the destructive snap induced high frequency tension fluctuations in the umbilical. For 6 km operation, either hook & top floaters or mid-depth hook can be utilized whereas the mid-depth hook configuration provides a broader environmental operating window for ROV operation. However, installation of floaters on the umbilical to produce the mid-depth hook configuration increases the ROV offset.

A laboratory experiment of rip currents between the ends of breaking wave crests

abstract In order to investigate the rip current formed between the ends of breaking wave crests, a laboratory experiment was conducted over a bathymetry with an equilibrium profile. For the experiment, pseudo intersecting wave trains were produced in a wave basin, where regular waves were generated by running two parts of wave maker out of phase. The pseudo intersecting wave trains were compared with the intersecting wave trains, which are formed by the interference of two wave trains with slightly different directions and are the cause of the rip current, by conducting numerical simulations using FUNWAVE. The rip current developments between the ends of breaking wave crests through their node regions were confirmed similarly in both numerical simulations. In the hydraulic experiment using the pseudo intersecting wave trains, the rip current flow was observed by utilizing Large Scale Particle Image Velocimetry and Acoustic Doppler Velocimetry, and the wave height and the mean surface were measured. From the observations, it was confirmed that the rip current was developed through the node region with relatively low wave height, and it was found that its strength was varied by the refraction and dissipation of the ends of wave crests due to the wave-current interaction. The horizontal and vertical velocity profiles of the rip current were presented in the present experiment. The comparison between the measured results and the numerical results showed a reasonable agreement, however, also confirmed the limitation of the numerical model on breaking and mixing process. Using particle trajectories, it was observed that vortices were developed at the ends of breaking wave crests, which played a role of generating and accelerating the rip current.

Seamount effect on circulation and distribution of ocean taxa in the vicinity of La Pérouse, a shallow seamount in the southwestern Indian Ocean

The La Pérouse seamount (60 m depth) has so far been poorly studied despite it being a short distance (160 km) from Réunion Island. As part of the MADRidge project, a multidisciplinary cruise was conducted to evaluate the effect of this shallow seamount on the local hydrology and ecology. Current measurements, temperature and chlorophyll-a profiles, and mesozooplankton and micronekton samples were collected between the summit and 35 km away. Micronekton data were supplemented with stomach content of pelagic top predators as well as fisheries statistics from the domestic longline fleet operating from Réunion. Vertical current profiles revealed distinct patterns between the offshore and seamount-flanked stations, giving evidence of topographical induced flow instabilities, notably on its leeward side (west) relative to the east flank. Distinct patterns in temperature and chlorophyll-a vertical profiles suggest the formation of convergent and divergent circulation cells as a result of the irregular and crescent-like summit topography. Spatial differences in zooplankton abundance were detected with higher biovolumes on the leeward flank. The overall acoustic backscatter for micronekton over the summit was weaker than offshore, but highly concentrated in the upper layer. Albacore tuna and swordfish dominate the longline catch west of Réunion, seemingly in association with a deep (900 m) topographic feature. Yet the largest catch is not directly associated with La Pérouse which would be too shallow for top predators to aggregate around in the long term. Enhanced levels of phytoplankton or zooplankton enrichment at La Pérouse were not demonstrated in this study, nor was there notable diversity of micronekton species. This might explain the relatively limited importance of this seamount to the tuna fisheries in this region.

Features of Profiles for Currents, Momentum Flux, and a Turbulence Dissipation Rate in Wind-Wave Channel

Vertical profiles of the mean horizontal currents U(z), the vertical momentum fluxes τ(z), and the turbulence kinetic-energy dissipation rates (TKE dissipation rate) e(z) in the upper water layer (UWL) are considered and their joint analysis is carried out. For this purpose, data from laboratory measurements performed in the wind-wave channel of the Institute of Applied Physics, Russian Academy of Sciences (RAS) [1, 2], are used. They correspond to conditions of strong wind and breaking wind waves. The profiles of the currents and momentum fluxes are estimated for x and z components of the velocity at five horizons in the UWL at four various wind values. The empirical estimates of e(z) obtained from the same data in the previous work [3] are used in the joint analysis. It is established that (a) velocity of currents U(z) increases noticeably when compared to the values of U(z) in the absence of waves, (b) the momentum flux in the water τw(z) decreases noticeably when compared to that in the air τa(z), and (c) τw(z) significantly attenuates with depth according to ratio τw(z) ~ 1/z2. The mentioned anomalies of profiles U(z) and τ(z) in the UWL are analyzed together with the previously determined pattern of TKE dissipation-rate falloff with depth according to ratio e(z) ~ 1/z2 in order to search for an interpretation of the results.

Operational fatigue loading on tidal turbine blades using computational fluid dynamics

As the world moves to a greater reliance on renewable energy, a vital component will be the predictability and dependability of the energy source; tidal energy provides such a solution. Horizontal axis tidal turbines are the most mature technology of all the marine renewable energy devices currently under development, as full-scale porotypes are already being tested and operated. As the industry develops and strives for commercial viability, it is becoming increasingly vital to develop a robust understanding of the complex interaction between the tidal flow, turbine blades and the support structure. This study uses an advanced computational fluid dynamics model to explore the operational fatigue loadings induced on tidal turbine blades. Two factors are considered, the presence of a support structure and varying vertical velocity profile of the tidal current. In order to perform the investigation, a model of a concept 16 m diameter horizontal axis tidal turbine with a monopile support structure is created. An investigation of the operational fatigue loadings due to variations in the positioning and the diameter of the support structure, the tidal turbine blade loads were found to varying by up to 43% of the maximum total thrust force.

Statistical modelling of extreme ocean current velocity profiles

Knowledge about extreme ocean currents and their vertical structure is important when designing offshore structures. We propose a method for statistical modelling of extreme vertical current velocity profiles, accounting for factors such as directionality, spatial and temporal dependence, and non-stationarity due to the tide. We first pre-process the data by resolving the observed (vector) currents at each of several water depths into orthogonal major and minor axis components by principal component analysis, and use harmonic analysis to decompose the total (observed) current into the sum of (deterministic) tidal and (stochastic) residual currents. A complete marginal model is then constructed for all residual current components, and the dependence structure between the components is characterized using the conditional extremes model by Heffernan and Tawn (2004). By simulating under this model, estimates of various extremal statistics can be acquired. A simple approach for deriving design current velocity profiles is also proposed. The method is tested using measured current profiles at two coastal locations in Norway, covering a period of 2.5 and 1.5 years. It is demonstrated that the method provides good extrapolations at both locations, and the estimated 10-year design current velocity profiles appear realistic compared to the most extreme velocity profiles observed in the measurements.

Validation of HF radar sea surface currents in the Malta-Sicily Channel

A network of High-Frequency radar (HFR) stations runs operationally in the Malta-Sicily Channel (MSC), Central Mediterranean Sea, providing sea surface current maps with high temporal (1 h) and spatial (3 × 3 km) resolutions since August 2012. Comparisons with surface drifter data and near-surface Acoustic Doppler Current Profiler (ADCP) observations, as well as radar site-to-site baseline analyses, provide quantitative assessments of HFR velocities accuracy. Twenty-two drifters were deployed within the HFR domain of coverage between December 2012 and October 2013. Additionally, six ADCP vertical current profiles were collected at selected positions during a dedicated field survey. External radio-frequency interferences lead to significant gaps in the HFR coverage, which were minimized by an interpolation technique applied to the surface current fields. The validation of HFR velocities was first performed for each individual station in the network, using data generated with both the ideal and the measured antenna patterns. The validation was then repeated with pairs of HFR stations using the site-to-site baseline error analysis. The analysis confirms that the baseline error estimations and the full validation metrics for this HFR frequency band are consistent, and in some aspects superior, when compared to results from similar studies in other regions. The validation with drifter tracks suggests that the adopted interpolation procedure does not in general impact on the comparison metrics, but may introduce biases when used to extrapolate currents in regions where radar geometrical constraints are suboptimal. The analysis of the percentage of contribution of individual HFR stations shown that the quality of HFR data is influenced by the network geometry.

Application of power law to vertical distribution of longshore currents

The vertical profiles of longshore currents have been examined experimentally over plane and barred beaches. In most cases, the vertical profiles of longshore currents are expressed by the logarithmic law. The power law is not commonly used to describe the profile of longshore currents. In this paper, however, a power-type formula is proposed to describe the vertical profiles of longshore currents. The formula has two parameters: the power law index (a) and the depth-averaged velocity. Based on previous studies, power law indices were set as a = 1/10 and a = 1/7. Depth-averaged velocity can be obtained through measurement. The fitting of the measured velocity profiles to a = 1/10 and a = 1/7 was assessed for the vertical longshore profiles. The vertical profile of longshore currents is well described by the power-type formula with a = 1/10 for a plane beach. However, for a barred beach, different values of a needed to be used for different regions. For the region from the bar trough to the offshore side of the bar crest, the vertical profiles of longshore currents given by the power-type formula with a = 1/10 and a = 1/7 fit the data well. However, the fit was slightly better with a = 1/10 than that with a = 1/7. For the data over the trough region of cross-shore distribution of the depth-averaged longshore currents, the power formula with a = 1/3 provided a good fit. The formulas with a = 1/10 and a = 1/7 were further examined using published data from four sources covering laboratory and field experiments. The results indicate that the power-type formula fits the data well for the laboratory and field data with a = 1/10.

Estimation of Potential Energy Generated From Tidal Stream in Different Depth Layer at East Flores Waters Measured by ADCP

Tidal works on the currents occur in each layer with significant variations. Variations of types of tidal media are very appropriate to the phase of the tides. Measurement of currents in each layer is necessary to assess the potential that can be used in determining and studying the placement of energy sources as a source of energy to support renewable energy in East Flores.A 15day measurement with the Acoustic Doppler Current Profile (ADCP) instrument can record the speed profile in the study area which has tidal current variabilities and power density variations in the verticalcurrent profiles. The analysis showed that there were two layers of depth with very optimum potential energy: at -3.5 – -5.5 meters with the highests speed of tidal current 3.69m/s and -7.5 – -9.5 meters with the highests speed of tidal current 3.68m/s from the Lowest Astronomical Tide (LAT). Each layers has different character, the upper layer has a maximum-high kinetic power density of 25.71 kW/m2 at layer 10 and 25.43 kW/m2 at layer 9 and the bottom layer had an average kinetic power density of 11.64kW/m2. This bottom layer had the potential with a lower capacity, it shows that this site is also applicable for seabed mounted tidal turbines.

Fine scale depth regulation of invertebrate larvae around coastal fronts

AbstractVertical migrations of zooplankters have been widely described, but their active movements through shallow, highly dynamic water columns within the inner shelf may be more complex and difficult to characterize. In this study, invertebrate larvae, currents, and hydrographic variables were sampled at different depths during and after the presence of fronts on three different cruises off the southern coast of South Africa. Internal wave dynamics were observed in the hydrographic data set but also through satellite imagery, although strong surface convergent currents were absent and thermal stratification was weak. During the first two cruises, fronts were more conspicuous and they preceded strong onshore currents at depth which developed with the rising tide. Vertical distributions of larvae changed accordingly, with higher abundances at these deep layers once the front disappeared. The third cruise was carried out during slack tides, the front was not conspicuous, deep strong onshore currents did not occur afterward and larval distributions did not change consistently through time. Overall, the vertical distributions of many larval taxa matched the vertical profiles of shoreward currents and multivariate analyses revealed that these flows structured the larval community, which was neither influenced by temperature nor chlorophyll. Thus, the ability to regulate active vertical positioning may enhance shoreward advection and determine nearshore larval distributions.

Гидроакустическое оборудование автономных подводных профилирующих аппаратов

Autonomous underwater profilers are designed for measurements of vertical profiles of ocean currents and marine environmental parameters. Such device moves along a vertically stretched cable at the buoy stations. Generally, a profiler comprises convenient platform for mounting of hydroacoustic equipment. A suit of sensors installed at the profiler carries out the measurements in the water column from the near-surface layer to the bottom allowing a researcher to obtain uniform data with the same accuracy. During the survey, the device usually maintains a stable orientation relative to the direction of flow, which is important for acoustic Doppler measurements of the speed and direction of ocean currents. This paper deals with applications of the moored automatic mobile profiler Aqualog for underwater communication and data transmission, acoustic measurements of the flow velocity and small-scale inhomogeneities.