Vertical Current Velocity Research Articles

A Wetting and Drying Approach for a Mode-Nonsplit Discontinuous Galerkin Hydrodynamic Model with Application to Laizhou Bay

A wetting and drying treatment for a three-dimensional discontinuous Galerkin hydrodynamic model without mode splitting (external and internal modes) was developed. In this approach, computing elements are classified into wet, dry, and semidry elements, which are treated differently. In a Runge–Kutta time step, the reconstruction of the semidry elements and the combined utilization of two- and three-dimensional limiters help the model maintain stability. Numerical results show that the wetting and drying method can achieve a well-balanced property under the condition of still-water equilibrium and can reasonably describe the variation process of wetting and drying regions during a long wave run-up on a uniform slope and a tidal cycle in a basin with a variable slope. Analysis of the role of the limiters in the model indicated that the robustness of the three-dimensional hydrodynamic model can be effectively maintained when the two- and three-dimensional limiters are jointly applied for wetting and drying process simulation. A three-dimensional discontinuous Galerkin hydrodynamic model was applied with the presented wetting and drying method to simulate the tidal current evolution of a spring tidal cycle in southwestern Laizhou Bay in the Bohai Sea, in November 2003, and the simulated results of the water surface elevation and vertical layered current velocities agreed well with the measured data.

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.

Sediment settling process revealed by a bottom-tethered sediment trap at the east tip of the Shandong Peninsula

This paper investigates sediment settling process during summer seasons based on in-situ observation and sediment analysis at the east tip of the Shandong Peninsula, which is a pathway for sediments from the Bohai Sea and the North Yellow Sea to enter the South Yellow Sea in China. Bottom-tethered sediment traps, together with platforms equipped with current profilers/meters and temperature/salinity and turbidity sensors, were deployed for approximately 10 days in August 2017 over an elongated deposit off the coast of the Shandong Peninsula. A computed tomography (CT) scan and grain size and 210Pb analyses of trap sediment were carried out to determine the settling process of suspensions at the study site. Twenty-three couplets were found in the CT images of the trap, the time series of which was established using intervalometer disks. Quantitative characterization of rhythmites using a hounsfield unit indicates that the period of couplets is approximately quarter-diurnal, and the settling process could be divided into three stages during Aug. 21st – 27th. Current, wave, wind, and SSC data illustrate that the diversion of the bottom current leads to the formation of couplets, and the thickness of the couplet is controlled by the vertical current velocity and wind direction. Northerly winds enhance the difference in settlement between flood and ebb tides, whereas southerly winds smoothen the difference. The gale event inhibited the settlement of the suspended matter, and the source of the suspensions was converted from local to remote after the event, as evidenced by 210Pb data. No evident erosion can be detected in the sediment traps. The results are of significance to better understand the sedimentary dynamics near the Shandong clinoform and inspire the practical efficiency of bottom-tethered sediment traps in the settling process in coastal areas.

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.

Integrated marine monitoring for drilling discharges under mesotidal forcing

There is a range of hydrocarbon exploration and extraction industrial project activities taking place on the North West Shelf of the Australian Continent. The shelf is a diverse and important marine ecosystem, with a number of globally significant shallow coral reefs and marine protected areas, which are vital for survival of threatened and protected species. Some of the mentioned exploration and extraction activities, such as offshore drilling, may be permitted within few kilometres away from sensitive habitats extant near a project resource extraction area. The habitats may in some cases be presented by submerged shoal ecosystems of high biodiversity and thus high environmental/ecological value, due to the presence of either or both benthic biota and fish assemblages. As part of an environmental assessment program for the industrial activities, some of the offshore exploration operators examine the effects of drilling discharges on the marine ambience. This study describes such an assessment. For the assessment, a novel integrated marine monitoring approach was conceptually designed; the program was then executed in the field using bottom-mounted equipment and a remotely operated underwater vehicle with sensors mounted on it. This study focuses on assessing the wind conditions in the project area, sea level and current velocities, in situ water temperature, salinity, turbidity and acoustic echo intensity, over several stages of the drilling operations while the drilling cuttings and muds were discharged into the water column. The collected during the austral autumn months of April and May winds showed prevailing east-southeaserlies, which is aligned well with the general understanding of wind climatology of the area. Importantly, stronger than usual winds may have a defining impact on current speeds and directions, with the current speeds enhanced by such winds. On these occasions the water transporting a discharge material may pushed towards the sensitive habitat present in the area. Normally though the hydrodynamics in the project area are dominated by semi-diurnal tides. Both vertical current velocity and water temperature distributions indicated the presence of three layers within the water column over the project site. The vertical distribution of water salinity was rather homogenous throughout the entire water column. The monitored by an optical instrument turbidity and acoustic echo intensity from an ADCP (acoustic Doppler current profiler) showed that tracing discharge plumes with a remotely operated underwater vehicle is a viable technique, which could be implemented at other locations. Such independent parameters though need a thorough cross-calibration and inter-validation for the results to be fully understood.

Underwater docking of an under-actuated autonomous underwater vehicle: system design and control implementation

Underwater docking greatly facilitates and extends operation of an autonomous underwater vehicle (AUV) without the support of a surface vessel. Robust and accurate control is critically important for docking an AUV into a small underwater funnel-type dock station. In this paper, a docking system with an under-actuated AUV is presented, with special attention paid to control algorithm design and implementation. For an under-actuated AUV, the cross-track error can be controlled only via vehicle heading modulation, so both the cross-track error and heading error have to be constrained to achieve successful docking operations, while the control problem can be even more complicated in practical scenarios with the presence of unknown ocean currents. To cope with the above issues, a control scheme of a three-hierarchy structure of control loops is developed, which has been embedded with online current estimator/compensator and effective control parameter tuning. The current estimator can evaluate both horizontal and vertical current velocity components, based only on the measurement of AUV’s velocity relative to the ground; in contrast, most existing methods use the measurements of both AUV’s velocities respectively relative to the ground and the water column. In addition to numerical simulation, the proposed docking scheme is fully implemented in a prototype AUV using MOOS-IvP architecture. Simulation results show that the current estimator/compensator works well even in the presence of lateral current disturbance. Finally, a series of sea trials are conducted to validate the current estimator/compensator and the whole docking system. The sea trial results show that our control methods can drive the AUV into the dock station effectively and robustly.

Observations of a cycle of intense coastal upwelling and downwelling at the research site of the Shirshov Institute of Oceanology in the Black Sea

The paper presents the results of joint analysis of the response of vertical temperature and current velocity profile distributions in the coastal zone of the Gelendzhik region of the Black Sea to strong wind forcing in the third ten-day period of September 2013. This forcing was caused by the propagation of an atmospheric cyclone, which first initiated coastal upwelling that was later replaced by downwelling. We formulate a criterion for the development of full coastal upwelling and demonstrate its efficiency. We assume that frequent events of incomplete coastal upwelling and downwelling are associated with changes in the water dynamics (variations in the intensity and direction of the alongshore current) generally not related to local wind forcing.

Suspended sediment signatures induced by shallow water undulating bottom topography

Near-surface signatures of suspended sediment concentration (SSC) related to submerged bed forms displayed in visible band data gathered by space-borne imaging sensors are described. Detailed Acoustic Doppler Current Profiler (ADCP) data of vertical current velocity component, echo intensity, modulation of SSC and related oceanographic and meteorological observations in a tidal inlet of the North Sea have been analyzed. To understand the associated hydrodynamics of the optical imaging mechanism theoretical considerations based on the Navier–Stokes and sediment continuity equations have been carried out. As a first approximation these theoretical results explain signatures of enhanced SSC of in situ measurements and analyzed space-borne images. Strong currents flowing over steep bottom topography are able to stir up the sediments to form both a general continuum of SSC and localized pulses of higher SSC in the vicinity of the causative bed feature itself. The “strain rate” equivalent to the current gradient is an important parameter describing near water surface oceanographic and meteorological phenomena in the visible and microwave part of the electromagnetic spectrum observed by shore- and ship-based as well as air- and space-borne remote sensing systems. Significant disagreements between theoretical results of the “strain rate” generated by the surface current and in situ observations at the crests of submerged ridges and sand waves were identified. Possible explanations could be: first, the current velocity may not be barotropic near the crests of such bed forms and second, mass conservation may not only be maintained through an acceleration or deceleration of the flow. This implies that active up- and downwelling effects of the three-dimensional current field can also play a significant role for hydrodynamic interaction between current and underwater bed forms.

Assimilation of climatic data in a model of circulation of waters in the Black Sea with regard for the space and time behavior of the variances and cross-covariance functions of forecast errors

We propose an improvement of the algorithm of joint assimilation of the data on climatic temperature, salinity, and altimetric sea level in a model of circulation. Unlike the previous works, the variances of the forecast errors of temperature and salinity and the cross-covariance functions of of the forecast errors of salinity-level and temperature-level depend on the dynamics of waters. It is shown that the structure of the fields of cross-covariance functions in the upper mixed layer is formed by the vertical turbulent diffusion of the variances of forecast errors of temperature and salinity. At greater depths, these statistical characteristics are mainly determined by the vertical advection. We compared the results of calculations with and without taking into account the dynamics of the statistical characteristics. The analysis of the influence of the dynamics of these characteristics makes it possible to reconstruct the mutually adapted climatic fields of temperature, salinity, and horizontal and vertical current velocities in the Black Sea with the assimilation of data in the numerical model in each time step.

Numerical and experimental research on current generation in deepwater offshore basin

During the design and construction of the deepwater offshore basin, its current generation system is considered to be one of the key technologies. In this article, the state-of-the-art deepwater offshore basin at Shanghai Jiaotong University was examined as a case study. Different 3-D numerical models of the current generation system were developed and calculated. The uniformity of flow field in the basin, such as horizontal and vertical current velocity profiles, were calculated in detail. Besides, a 1:10 scaled model test was also carried out. The current velocities at different locations in the basin were measured and compared with the calculated results. It is concluded that satisfactory agreement can be resulted between the numerical simulation and model test. In addition, it is suggested that transition sections and turning vanes need to be set in inflow and outflow culverts to improve the uniformity of flow field in the basin. And the hydraulic performance of the deepwater current generation system can meet the requirements of model tests in deep water.

Cyclical variability of suspended sediment concentration over a low-energy tidal flat in Jiaozhou Bay, China: effect of shoaling on wave impact

During the spring-neap period of 17–24 August 2004, turbidity, horizontal and vertical current velocities and echo intensity were measured using OBS-3A and ADP-XR instruments over an intertidal flat within the semi-enclosed Jiaozhou Bay, China, to examine patterns in suspended sediment concentration (SSC) and possible control factors. SSC was found to be lower than 30 mg l−1 in most of the water column and for most of the tidal cycle. This is attributed mainly to the low hydrodynamic energy, in particular weak currents (near-bottom maximum 1- and 8-min-interval velocities were only 26.1 and 14.2 cm s−1, respectively), and limited fine-grained sediment supply by rivers. However, high SSC values ranging from 100 to >1,000 mg l−1 occurred over short periods at the beginning and the end of inundation. This phenomenon is attributed to the shoaling effect of frequent wind-generated waves, as a result of which near-bottom SSC fluctuations display a U-shaped trend during each tidal cycle.

Turbulence, watermass stratification and harmful algal blooms: an alternative view and frontal zones as “pelagic seed banks”

Watermass stratification has been considered the essential physical condition that dinoflagellates require to bloom because of their relative inability, unlike diatoms, to tolerate the elevated shear-stress associated with water-column mixing, turbulence and high velocity, coastal currents. The swimming speeds of 71 flagellate taxa, with a focus on dinoflagellates, are compared to the turbulence fields and vertical velocities that develop during representative wind conditions, upwelling and at frontal zones. The results suggest that the classical stratification–dinoflagellate bloom paradigm needs revision. Tolerance of turbulence, growth within well-mixed watermasses and survival and dispersal while entrained within current systems are well developed capacities among dinoflagellates. Their secretion of mucous, often copious during blooms, is suggested to be an environmental engineering strategy to dampen turbulence. Biophysical tolerance of turbulence by dinoflagellates is often accompanied by high swimming speeds. Motility speeds of many species exceed in situ vertical current velocities; this also allows diel migrational patterns and other motility-based behavior to persist. Species belonging to “mixing-drift” life-form assemblages can increase their swimming speeds through chain formation, which helps to compensate for the increased turbulence and vertical water-column velocities of their habitats. The ability of dinoflagellate species to tolerate the vertical velocities of offshore, frontal zones, where abundant populations often develop, suggests that fronts may serve as “pelagic seed banks”, occurring as pelagic analogues of nearshore seed beds, from which seed stock is dispersed. The different ecologies associated with the hypothesized, “pelagic seed banks” of vegetative cells and the “seed beds” of resting stage cells deposited onto sediments are discussed. There is a contradiction in the stratification–HAB paradigm: the quiescent conditions of a stratified watermass, with its characteristic nutrient-poor conditions are expected to promote stasis of the population, rather than growth and blooms. The analyses suggest that dinoflagellate blooms do not preponderate in stratified watermasses because the bloom species are biophysically intolerant of the higher velocities and turbulence of more mixed watermasses. The watermass stratification that often accompanies flagellate blooms is probably a secondary, parallel event and less essential than some other factor(s) in triggering the observed bloom.

Acoustic Doppler current profiler measurements in coastal and estuarine environments: examples from the Tay Estuary, Scotland

Acoustic Doppler current profilers (ADCPs) provide a means to measure the components of water current velocities in three dimensions. Such instruments have been used widely by the oil industry in deep offshore waters but their application to nearshore coastal and estuarine environments has been principally confined to the USA. Using examples of ADCP datasets acquired from the macrotidal Tay Estuary, eastern Scotland, the principles of field deployment, data acquisition and forms of output are critically summarised. It is shown, for the first time in the Tay Estuary, that vertical current velocities are significant and are particularly so in downwelling zones associated with the development and passage of axially convergent tidal fronts. The improved understanding of three-dimensional water and suspended sediment dynamics in coastal and estuarine waters is crucial to, inter alia, the sustainable management of effluent discharges and, in more general terms, it is predicted on the basis of the Tay case study, that ADCP measurements afford significant opportunities to refine understanding of geomorphological processes in a variety of aquatic environments worldwide.

Black Sea vertical circulation and extremes of the hydrochemical and hydrooptical parameters

A statistical evaluation of vertical current velocity profiles w(z) has been undertaken using data provided by current meter profilers. The calculation of profiles w(z) involves the estimation of the standard deviation of vertical current at each depth level, and the standard deviation of the statistical variability of that estimate. Profiles w(z) have been computed for the northwestern Black Sea, using current meter and CTD data from three surveys carried out during 1992–1994. The calculations show w(z) to have a two-layered structure, with zero values occurring in the main pycnocline. Such vertical current structures are consistent with a hydrodynamic model, whereby the current field is induced by buoyancy fluxes through the lateral basin boundaries. Existence of zero vertical current velocities at the pycnoclines yields the key to understanding the mechanisms responsible for the oxic-anoxic interface, and of the zones with steep vertical gradients in hydrochemical and hydrooptical characteristics.

Optimal statistical estimates of the vertical current velocity in the north-western Black Sea

The paper discusses a method for the statistical evaluation of the vertical current velocity componentV z(z) using data provided by profilers. The calculation of theV z(z) profiles involves the determination of the root-mean-square deviation of the error of determination of the vetical current velocity component at each level and the RMS deviation of the sample statistical variability of this quantity's estimates. ProfilesV z(z) have been computed for the north-western Black Sea using three surveys, with the involvement of an OLT profiler. The computations have shownV z(z) to have a two-layer structure, with zero being crossed in the main pycnocline. Such structure of the current velocity vertical component is consistent with the hydrodynamic model of the field of currents induced by buoyancy fluxes through the lateral boundaries of the basin. The availability of the zero values of the vertical current velocity in the pycnocline yields a tool for gaining insight into the mechanism controlling the emergence of the oxic/anoxic interface and areas with sharp vertical gradients of chemical and hydro-optical characteristics.

Dynamics of the hydrophysical probe's angular oscillations

A non-linear equation is derived for the hydrophysical probe's angular oscillations induced by tension fluctuations of the cable and by the influence of the horizontal and vertical current velocity components on the probe. The amplitude-frequency and phase-frequency characteristics of the angular oscillations are calculated for minor disturbances. Results of the analysis are shown to be consistent with the experimental data.

Ecological Impacts of the Construction of Dams in an Estuary

This paper deals with the ecological impacts of alternative methods of constructing dams in an estuary. The main options available were:using concrete blocks and rubbleusing only sand. The advantages of using sand are that it is available locally and it is cheaper than concrete blocks and rubble. However, the application of sand in a tidal basin requires low current velocities. During the final closing period of the dams, these velocities can be reduced by using the storm-surge barrier (to be built in the mouth of the Eastern Scheldt estuary) in two ways: either reducing the tidal amplitude or reducing the tidal frequency. The tidal reduction influences the vertical amplitude and current velocities, thereby decreasing the intertidal area, and the biomass and production of microphytobenthos and macrozoobenthos. A reduction of the horizontal tide leads to a reduction of the exchange of water between the North Sea and the estuary, with a decrease in current velocities. These phenomena affect the concentration of suspended and dissolved material; in particular, salt concentrations decrease with possible stratification. This paper describes the effects of different closing procedures on natural ecosystems and fisheries. The study resulted in a policy analysis and was presented to decision-making politicians.

Undersea Reactor Siting

An underwater siting guide for use in international and territorial waters must be created to evaluate the consequences of the release of fission products from a nuclear reactor sited in the ocean. This paper is an initial attempt to develop the basic equations for such a guide. A very conservative fission-product-release inventory to illustrate undersea application is developed consisting of 100% of the soluble and 1% of the insoluble fission products or ∼66% of the gross fission-product inventory. The ocean is divided into four distinct zones for which current velocity profiles and characteristic diffusion parameters are established. Based upon a three-dimensional diffusion model incorporating shear effect and with the assumptions of no current variance, zero mean vertical current velocity, and depletion of the inventory by radiological decay only, equations are presented which describe the physical transport and dispersion of the radioisotopes. A computer program, SEADIF, is applicable to a person immersed in the water and is used to determine, for both contained and uncontained systems, the distance factors and the radioisotope concentrations as a function of time and position. Representative results for a 10 MW(th) system are given.

ON THE MECHANISM OF LANGMUIR CIRCULATIONS AND THEIR ROLE IN EPILIMNION MIXING1

Six mechanisms have so far been proposed to explain Langmuir circulations and their associated wind streaks. One mechanism involving shearing instability and two requiring the action of a surface film are supported by the greatest amount of evidence. Studies at Lake George, New York, suggest that more than one mechanism may operate at one location. A particular mechanism may operate at a specific site because of typical conditions at that site, but a different mechanism may be more important at another site. Plots of thermal structure in the near‐surface layer of Lake George and measurements of vertical current velocities suggest that Langmuir circulations are the most important mixing process in the epilimnia of lakes.

On the frictionally induced transverse circulation of the Antarctic Circumpolar Current

The stationary distribution of the transverse and vertical current velocities were computed from the distribution of the down stream velocity component using the eddy viscosity concept.