Horizontal Current Velocity Research Articles

The Role of Infragravity Waves in Near-Bed Cross-Shore Sediment Flux in the Breaker Zone

Results from a series of field experiments, conducted to investigate the influence of infragravity waves (from wave groups), ripple type and location relative to the breaker line on cross-shore suspended sediment flux close to the sea bed in nearshore environments, are presented. The field data were collected from Cable Beach (Broome) and Mullaloo Beach in Western Australia and Chilaw in Sri Lanka. These beaches experience different incident wave, tidal and morphological conditions, with Cable Beach having a 10-m spring tidal range, whilst the other two beaches have tidal ranges <1.0 m. Measurements included simultaneous records of surface elevation, two-dimensional horizontal current velocities and suspended sediment concentrations, together with half-hourly observations of the seabed topography. Although most of the data sets were obtained just outside of the surf zone, a few results from inside of the surf zone were also included. A significant correlation between wave groups and suspended sediment concentration was found at all of the measurement sites, either with or without bed ripples. The direction and magnitude of cross-shore suspended sediment flux varied with location with respect to the breaker line; however, other parameters, such as bed ripples and velocity skewness, could have influenced this result. In Broome, where the measurement location with respect to the breaker line varied with the tidal cycle, the cross-shore sediment flux due to swell waves was shoreward inside and just outside of the surf zone and seaward farther offshore of the breaker line. Further, sediment flux due to swell waves was onshore when the seabed was flat and offshore over post-vortex ripples. Sediment flux due to swell waves was onshore when the normalised velocity skewness towards the shore was high (positive); the flux was offshore when the skewness was lower, but positive, suggesting the influence of other parameters, such as ripples and grain size. The net cross-shore sediment flux was onshore when the Dean number was less than 1.67 and offshore when the Dean number was greater than 1.67. Nevertheless, the Dean number did not account for the influence of ripples or velocity skewness. The cross-shore sediment flux at the infragravity frequency was mainly offshore outside of the surf zone, whereas it varied between onshore and offshore inside of the surf zone.

Monthly variation of some parameters about internal solitary waves in the South China sea

In this paper, by non-dimensional analysis, it is found that finite-depth theory is more appropriate to the study of internal solitary waves (ISWs) in the South China Sea (SCS) than shallow-water theory. The 1-degree grid data of monthly mean temperature and salinity data at standard levels in the SCS are used to solve the linearized vertical eigenvalue problem. The nonlinear parameter and the wave phase speed are computed, then the nonlinear phase speed and the characteristic half-width of ISWs are calculated respectively by two different theories to investigate the difference between these two parameters in the SCS. The nonlinearity is the strongest near the continental slope of the SCS or islands where the bottom topography changes sharply, it is stronger in summer than that in winter; it increases (decreases) as pycnocline depth deepens (shallows), stratification strengthens (weakens) and pycnocline thickness thins (thickens). The nonlinear wave phase speed and the characteristic half-width are the largest in deep sea area, they then reduce peripherally in shallower water. The nonlinear wave phase speed in the SCS changes slightly with time, but the characteristic half-width changes somewhat larger with time. In most of the SCS basin, the nonlinear wave phase speed derived from shallow-water theory is very close to that derived from finite-depth theory, but the characteristic half-width derived from shallow-water theory is about 0.2–0.6 times larger than that derived from finite-depth theory. The ISW induced horizontal current velocity derived from shallow-water theory is larger than that derived from finite-depth theory. Some observed and numerical modeled ISW characteristic half-widths are compared with those derived from shallow-water and finite-depth theories, respectively. It is shown that, the characteristic half-widths derived from finite-depth theory agree better with observational and numerical modeled results than those derived from shallow-water theory in most cases, finite-depth theory is more applicable to the estimation of ISW characteristic half-widths in the northern SCS. It is also suggested that, to derive the precise ISW parameters in further study, the physical non-dimensional ratios which are related with ISW characteristic half-width, amplitude, thermocline and water depths should be calculated, so that an appropriate theory can be chosen for estimation.

Orientation Schemes Choice for Aquatic Inserting Piles Based on Gray Relation Analysis

In the process of rapid orientation device design for aquatic inserting piles, piles were fixed position using straight and steady function of the edge piles ahead. Piles were inserted to riverbed inside steel casing without horizontal current velocity. Three kinds of orientation schemes were put forward. To scientifically and accurately choose the best scheme, the evaluation index system was established in the way of expert's mark. The gray relation analysis method was adopted to analyze each scheme, synthesize each factor, and finally educe the optimal scheme.

Satellite observations of surface manifestations of internal waves in the Caspian Sea

Results of satellite observations of surface manifestations of internal waves in the Caspian Sea are presented. It is proposed that the possible cause of generation of the revealed internal waves is uninodal seiches with a nodal line located in the vicinity of the Apsheron Sill. The basic parameters of internal waves in the Caspian Sea, having the form of classical soliton trains, are determined. Seasonal variability of surface manifestations of the internal waves is revealed. The horizontal current velocity of the solitons is assessed. According to the estimation, velocity is about 0.2 m/s, which is sufficient for wind ripple modulation.

Two repeat crossings of Drake Passage in austral summer 2006: Short-term variations and evidence for considerable ventilation of intermediate and deep waters

A high-resolution, full-depth, hydrographic section across Drake Passage with tracers and horizontal velocity measurements was repeated within three weeks in austral summer 2006. In-situ data (temperature, salinity, dissolved-oxygen and horizontal current velocities) are used to determine the location of the fronts and eddies of the Antarctic Circumpolar Current (ACC) inferred by Barré et al. (2011) from 7-day composite dynamic topography maps along the transect. Frontal displacements are about 40 km (2 stations) along the section. Considerable differences in properties between the two sections are observed throughout the whole water column with values as high as 0.2 °C in temperature, 0.01 in salinity, 0.03 kg m −3 in neutral density and 10 μmol kg −1 in dissolved-oxygen concentration found below a depth of 3000 m. Only part of the differences is attributable to frontal or eddy displacements along the section. The other part results from the spatial heterogeneity of water properties upstream the section and the funnelling of the flow due to the topographic constraints of the Shackleton Fracture Zone (SFZ). The considerable short-term differences in water properties in rather large-scale structures that cannot be accounted for by frontal motions along the section call for caution when interpreting differences in hydrographical properties obtained by oceanographic cruises that are years apart in terms of climatic signals. The two sections show efficient ventilation of Antarctic Intermediate Water (AAIW) by Winter Water (WW) in the Yaghan Basin, with an estimated eddy-enhanced subduction rate of over 170 m year −1. The SFZ constitutes a barrier, causing the two Southern ACC Fronts (SACCF) branches to separate by about 400 km and creating sheltered conditions in partial isolation from the ACC, while promoting an active recirculation region in the Ona Basin. This active recirculation, marked by cyclonic eddies carrying cold, fresh and oxygenated water from south of the Southern Boundary of the ACC, causes effective ventilation of the whole Circumpolar Deep Water (CDW) density range. The differences in small-scale structures between the two sections deserve further analyses.

The Wave Glider: enabling a new approach to persistent ocean observation and research

The Wave GliderTM wave-powered unmanned maritime vehicle represents a novel and unique approach to persistent ocean presence. Wave Glider is a hybrid sea-surface and underwater vehicle in that it is comprised of a submerged “glider” attached via a tether to a surface float. The Wave Glider vehicle is propelled by the purely mechanical conversion of ocean wave energy into forward thrust, independent of wave direction. In this paper, we provide an overview of the design of this new platform and present results from the engineering sea trials conducted with several prototype and production versions of the vehicle. In addition to the Wave Glider technology, we present results from ongoing scientific demonstration programs. Results from test deployments of a conductivity–temperature–depth sensor and its applicability to oceanography are discussed. An evaluation of horizontal current profiles and current velocity estimation from a Wave Glider operating in Monterey Bay (California, USA) are presented.

Sensitivity study of the effects of wave-induced vertical mixing on vertical exchange processes

The hydrodynamic model COHERENS-SED, developed by the present authors through introducing wave-enhanced bottom stress, wave dependent surface drag coefficient, wave-induced surface mixing, SWAN to COHERENS, is modified to account for wave-induced vertical mixing. The COHERENS-SED model can also be used for one-dimensional, two-dimensional, three-dimensional current and salinity calculations. One-dimensional model and three-dimensional model are used to study the effects of the wave-induced vertical mixing. The horizontal current velocity profiles obtained by the model are in good agreement with the analytical velocity profiles under the same input conditions. Numerical results show that higher wave height would generally generate larger vertical eddy viscosity and lower horizontal velocity. The results for fresh water in Yellow River Delta show that the wave-induced vertical mixing increases the momentum of fresh water transferring ability downwards to seabed and salt water's mixing with upper fresh water. Fresh water flume length is compressed considerably.

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.

Vertical swimming behavior influences the dispersal of simulated oyster larvae in a coupled particle-tracking and hydrodynamic model of Chesapeake Bay

Because planktonic organisms have swimming speeds that are orders of magnitude lower than horizontal current velocities, it is unclear whether behavior of weak-swimming bivalve larvae could influence dispersal distance, encounters with suitable habitat, or subpopulation connec- tivity. We used a numerical approach to investigate whether these processes could be affected by species-specific differences in larval vertical swimming behavior of 2 oyster species (Crassostrea vir- ginica and C. ariakensis) in Chesapeake Bay, a partially mixed estuary. A coupled particle-tracking and hydrodynamic model was forced with observed winds and freshwater flow and included the best available estimate of present-day oyster habitat. Model scenarios were conducted with hydrody- namic predictions from June to September, 1995 to 1999, to simulate a range of environmental con- ditions. Simple larval swimming behaviors were parameterized for the 2 oyster species with results from preliminary laboratory experiments and literature. To isolate the effect of circulation, settlement habitat, and larval behavior on the spatial trajectories of particles, vertical swimming velocity was the only biological process represented in the model; egg production and larval growth were not included. Differences in larval swimming behavior had significant consequences for particle trans- port in Chesapeake Bay by influencing dispersal distances, transport success, and the degree of con- nectivity between 'subpopulations' in different tributaries. Most particles (>96%) did not return to the same reef on which they were released, and there were behavior-dependent differences in spa- tial patterns of the 'source' and 'sink' characteristics of oyster reefs. Simulated larval behavior had greater influence on spatial patterns of transport success than did interannual differences in circula- tion patterns. These model results have implications for fisheries management and oyster restoration activities.

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.

Variability in horizontal current velocities in the central and eastern equatorial Atlantic in 2002

Near‐surface current velocity data from the Pilot Research Moored Array in the Tropical Atlantic at 23°W in the year 2002 were re‐examined and compared to simultaneous unpublished velocity data at 10°W, near the surface. Strong 7‐day‐period fluctuations were observed in the zonal velocity component and in temperature records at 23°W. The large temperature fluctuations near the thermocline depth suggest a vertical velocity component at the same 7‐day period. The 7‐day‐period signals occurred in June/July simultaneously with the seasonal upwelling. The meridional velocity component presented similar spectral contents at 23°W and at 10°W. Two distinguishable quasiperiodic signals were observed in the meridional component at both locations: The first signal occurred in the spring, had a periodicity of about 14 days, was subsurface intensified, and had the characteristics of a mixed Rossby‐gravity wave. The second signal occurred three times in the year (once in winter 2001–2002, another in summer, and another during fall 2002), had a periodicity of about 20 days, was strongest close to the surface, and was the result of the passage of tropical instability waves (TIWs). Satellite sea surface temperature (SST) data show the same periodicities as the meridional velocity records, and present a spatial structure in accordance with the TIW‐SST signature. The summer of 2002 showed different TIW‐SST patterns depending upon location, suggesting the existence of different types of TIWs. Overall, variability in the current meter data for the year 2002 was mainly due to current instabilities, except around boreal spring, where intraseasonal variability in surface winds was strongest.

Eulerian-Lagrangian Measurements for Estimating the Sediment Transport Parameters in Estuary

The novel instrument based on a Lagrangian Acoustic drogue (LAD) has developed for estimating the sediment transport parameters in the estuary. This instrument consists of GPS, ADCP and Nephiometer with Lagrangian drogue. The field experiments using LAD was conducted in Brisbane River, Australia on Dec. 7, 2006. Time-space distributions of the turbidity, horizontal current velocity and direction were obtained. The settling velocity is estimated by analyzing the temporal and spatial changes of the turbidity contour lines. The linear correlation was found to be existed between suspended sediment concentrations (SSC) and settling velocity. An Approximated equation using the method of least means squares agrees well with the results by Van Leussen (1999).

Estimation of Vertical Mixing Based on Water Current Monitoring in the Hypolimnion of Lake Biwa

Using continuous monitoring data over last two years in the North Basin of Lake Biwa, we showed the time change of water currents and temperature from May 2003 to May 2005. Two acoustic current meters (ACMs) were deployed at 1.8m and 54m height from lake bottom respectively, at the observation point with the depth of 90m near the deepest place (104m depth). We measured horizontal current velocity, current direction, and water temperature at every 20 minutes, and estimated the bottom stress with horizontal current velocity and water temperature. Average water current velocity near the lake bottom through the observation was weak as reported before, however, maximum current velocity due to strong typhoon in October 2004 reached almost 30cm/s. Moreover, comparatively strong water current velocity was observed from January to February in each winter. The estimation of the bottom stress with current velocity suggested us that strong currents could re-suspend bottom sediments.

Solitons Northeast of Tung-Sha Island During the ASIAEX Pilot Studies

In a recent study, satellite images have shown that internal solitons are active in the northern South China Sea (SCS). During the Asian Seas International Acoustic Experiment (ASIAEX) pilot studies, current profiler and thermistor chain moorings were deployed in the spring of 1999 and 2000 to investigate internal solitons northeast of Tung-Sha Island on the continental slope of the northern SCS. Most of the observed internal solitons were first baroclinic mode depression waves. The largest horizontal current velocity, vertical displacement, and temperature variation induced by the internal solitons were around 240 cm/s, 106 m, and 11/spl deg/C, respectively, while the estimated nonlinear phase speed was primarily westward at 152 /spl plusmn/ 4 cm/s. The observed internal solitons could be categorized as four types. The first type is the incoming wave from deep water and can be described reasonably well with the KdV equation. The second and third types are in the transition zone before and close to the turning point (where the upper and lower layer depths are equal), respectively. These two types of solitons were generally near the wave-breaking stage. The fourth type of soliton is a second baroclinic mode and probably was locally generated. The time evolutions are asymmetric, especially at the middle depths. A temperature kink following the main pulse of the soliton is often seen. Higher order nonlinear and shallow topographic effects could be the primary cause for these features. The appearance/disappearance of internal solitons coincides mostly with spring/neap tide. The internal soliton is irregularly seen during the neap tide period and its amplitude is generally small. The time interval between two leading solitons is generally around 12 h. The first baroclinic mode of the semidiurnal tide has a larger amplitude than the diurnal tide and could redistribute its energy into the soliton.

Biogeochemical response to physical forcing in the water column of a warm monomictic lake

Based on microstructure measurements of temperature and horizontalcurrent velocity the physical structure in the water column of Lake Kinneretwascharacterized as a five layer system consisting of a surface mixed layer, lowerepilimnion, metalimnion, upper hypolimnion and benthic boundary layer. Usingoxygen and hydrogen sulfide as natural chemical tracers, the time scale ofchemical change was identified in relation to advection, mixing and biologicalprocesses. Rapid changes due to advection that took place on an hourly timescale were removed by referring the data back to the temperature of the water.Biological activity dominated the hydrochemical changes observed in the meta-and upper hypolimnion. These were expressed by DO depletion rates of 2.0 and0.4g m−2 d−1, respectively.Verticaland horizontal mixing were shown to occur on a seasonal time scale. Once thechemical stratification process was completed the slow mixing through thebenthic boundary layer became the limiting factor for subsequent reactions inthe water column.

Transport of molluscan larvae through a shallow estuary

Dispersal of invertebrate larvae is determined by larval swimming behavior, the length of planktonic development and the hydrodynamic regime. Larvae of estuarine invertebrates must refrain from export or invade an estuary after development in the ocean. This study investigates retention patterns of estuarine molluscs by measuring time series of larval abundance in relation to hydrodynamic processes. Previous investigations of larval dynamics have generally focused on larger estuarine systems that are often stratified and have relatively long hydraulic residence times. The estuary studied in this investigation supports dense populations of infaunal clams yet has a water depth to tidal amplitude ratio near unity. To access processes affecting larval retention, the circulation patterns of the estuary were measured with time series of salinity, temperature, pressure and hori- zontal velocity. Transport rates of larvae between ocean and estuary, and within the estuary proper, were calculated from velocity and larval concentration time series. The daily residence time of the estuary was determined for the summer spawning period. The results demonstrate that molluscan larvae were routinely transported between the estuary and nearshore zone in tidal flows. Based on the magnitude of the horizontal current velocities, passive transport of larvae predominates during most of the tidal cycle in the estuary. Residence time calculations suggest that the ability of larvae to remain in the estuary through larval development is unlikely, and there was no evidence of selective retention of mature bivalve larvae in the estuary. Rather, larvae are exported rapidly from the estuary and undergo development in the coastal ocean. Mesoscale physical processes in the coastal ocean probably control variation in the delivery of larvae back to estuarine systems. Recruitment to this and similar estuaries must therefore be dependent on invasion.

Discussion

In the present work, the potential of correlation methods applied to acoustic signals backscattered from suspended sediments, is examined with regard to the measurement of near-bed vertical profiles of the horizontal current velocity. The technique uses the cross-correlation of the backscattered signal between pairs of downward looking horizontally separated transducers, to measure the time taken for suspension structures to advect from one transducer to another. This approach requires a degree of spatial coherence in the suspension field, and advantage is taken of this coherence to measure the flow. To investigate the technique, backscattered data collected using an array of transducers has been examined. The array was deployed along stream in an estuarine environment, which was subject to strong rectilinear currents, and high levels of suspended sediment concentration. Using pairs of transducers with separations between 0.25–5.42 m, the structure of the suspension field has been assessed, the implication of these observations for measuring flow discussed, and nearbed measurements of current profiles obtained. The results show that in the estuarine environment investigated, if the transducer pair separation was of the order of a metre or less, and currents were sufficient to retain sediments in suspension, vertical profiles of the horizontal current could be measured. To assess the results conventional electromagnetic and impeller current meter measurements were collected at four heights above the bed. Comparison of the conventional reference measurements with the cross-correlation velocities show that the correlation method has the potential to provide reliable measurements of near-bed current profiles.

Investigation into the application of cross-correlation analysis on acoustic backscattered signals from suspended sediments to measure nearbed current profiles

In the present work, the potential of correlation methods applied to acoustic signals backscattered from suspended sediments, is examined with regard to the measurement of near-bed vertical profiles of the horizontal current velocity. The technique uses the cross-correlation of the backscattered signal between pairs of downward looking horizontally separated transducers, to measure the time taken for suspension structures to advect from one transducer to another. This approach requires a degree of spatial coherence in the suspension field, and advantage is taken of this coherence to measure the flow. To investigate the technique, backscattered data collected using an array of transducers has been examined. The array was deployed along stream in an estuarine environment, which was subject to strong rectilinear currents, and high levels of suspended sediment concentration. Using pairs of transducers with separations between 0.25–5.42 m, the structure of the suspension field has been assessed, the implication of these observations for measuring flow discussed, and nearbed measurements of current profiles obtained. The results show that in the estuarine environment investigated, if the transducer pair separation was of the order of a metre or less, and currents were sufficient to retain sediments in suspension, vertical profiles of the horizontal current could be measured. To assess the results conventional electromagnetic and impeller current meter measurements were collected at four heights above the bed. Comparison of the conventional reference measurements with the cross-correlation velocities show that the correlation method has the potential to provide reliable measurements of near-bed current profiles.

A two‐dimensional numerical model which simulates the temperature, salinity and velocity fields in knight inlet, British Columbia

A two‐dimensional laterally integrated model of Knight Inlet, British Columbia is used to simulate the time‐varying temperature, salinity and velocity fields. The model includes the influence of the wind, tides, solar heating and freshwater inflow. The level 2.5 turbulent closure scheme of Mellor and Yamada is used to parametrize the diffusion terms in the model. In this paper, two turbulent kinetic energy boundary conditions at the air‐water interface are compared. One boundary condition is for an enhanced surface turbulent layer due to the flux of wind energy while the other is based on the Law of the Wall approach. The model is compared to data collected in the summer of 1989 from four moored vertical arrays. Salinity, temperature, horizontal current velocity and wind velocity were measured for about thirty days. Measurements were made throughout the water column so that the thin surface layer and deep water flows could be resolved. A moored array near the mouth of the inlet was used to provide open boundary data for the model, while the remaining data were compared to the simulated data. The modelled salinity fields were very realistic particularly when the enhanced turbulent surface layer boundary condition was used. The temperature fields were well modelled as long as the surface temperature was prescribed. The simulated velocities were very similar to those produced by a numerical model which simulated density (with the pressure effect removed) directly, instead of solving for temperature and salinity and then determining density via an equation of state.

The method of calculating the current velocity's verticla component in problems of fluid dynamics in reservoirs

A new algorithm for calculating the vertical velocity is suggested. Horizontal current velocity components are expressed through level inclination and wind/temperature forcing. The acquired relations are given in the difference form, with the accuracy being sufficiently high. The algorithm is tested using cases where a numerical solution is matched up with an exact one. The derived data are subsequently analysed.