Wind-induced Currents Research Articles

Spatial variability of sedimentological properties in a large Siberian lake

The analysis of spatial variability of lake sediment properties in Lake Lama, Central Siberia is presented. The aims were to characterize the spatial structure in lake sediment composition, to determine major spatial patterns of sediment properties by calculating estimates at locations where no samples were available, and to assess the main processes determining these patterns. Sediment properties were measured at 71 spatially distributed locations in the lake, comprising particle size distribution, biogeochemical properties such as total organic carbon, total nitrogen or stable carbon isotopes, as well as geochemical distribution of major and trace elements. Spatial analysis consisted of a principal components analysis and, subsequently, calculation and modelling of variograms to describe the spatial structure in the data as well as determination of spatial estimates using block-kriging to map spatial structure. The results showed that particle size, biogenic silica, CaO, δ13Corg, Zr and TOC/TN ratio were a major source of variability in sediment properties and, thus, dominate the sediment structure in Lake Lama. Major anomalies occurred near river inlets, depending on river size and its position as well as slope of the river bed. Other anomalies were associated with water depths, morphology of the lake basins, and wind-induced currents and re-suspension in the shallow part of the western basin. The spatial structure in sediment properties indicate that several processes act at different spatial scales. Moreover, there was a considerable amount of smallscale variability that could not be quantified due to sampling design. The results showed that heterogeneity in lake sediment composition is a main characteristic of large lake systems, and must be taken into account, especially in paleolimnological and environmental applications.

Modelling of wind-induced currents in water basins

This paper presents a numerical modelling study into wind-induced currents in shallow water basins. A three-dimensional, semi-implicit, finite difference numerical model is described, incorporating non-hydrostatic pressure, based on a sigma-transformed system in the vertical direction. The numerical model was first validated for sloshing in a rectangular tank; excellent agreement was obtained against analytical solutions, provided the non-hydrostatic pressure is incorporated. The model was further verified against experimental data on wind-induced circulation in shallow rectangular tanks. It was then used to simulate wind-induced, three-dimensional flow fields in idealised rectangular tanks. A parameter study was carried out for wind-induced flows in Esthwaite Water, and the predictions compared against field data. It is shown that the numerical model is capable of accurately simulating the wind-induced circulation in shallow enclosed water bodies and that the topography and wind stress are of primary importance. The non-hydrostatic pressure does not have a significant effect on the wind-induced flow fields considered herein.

Modelling of wind-induced currents in water basins

Modelling of wind-induced currents in water basins

Three-dimensional numerical modelling of wind-driven circulation in a homogeneous lake

A three-dimensional numerical model has been developed to study wind-induced circulation patterns in a shallow homogeneous lake with a complex bathymetry. The governing equations are the unsteady Reynolds-Averaged Navier–Stokes equations in which the non-hydrostatic pressure distribution has been included. The model was tested against analytical solutions and laboratory data for wind-induced currents and then applied to Esthwaite Water, a small lake in Cumbria, UK. The model was used to study the main model parameters and to generate typical circulation patterns for a variety of conditions in the lake. Simulations showed that a non-hydrostatic pressure distribution did not have any noticeable influence on the overall circulation pattern in the lake. However, comparisons with field data at some measurement stations in the near-shore region with sharply varying bottom topography showed that the hydrodynamic pressure component had some influence on the vertical velocity profile.

Sea-Ice Drift on the Northeastern Shelf of Sakhalin Island

Abstract Ice-drift velocity records from coastal radar stations, combined with data from moored current meters and coastal wind stations, are used to examine sea-ice motion off the northeastern coast of Sakhalin Island in the Sea of Okhotsk. Ice motion is shown to be governed primarily by diurnal tidal currents and wind-induced drift, which explain 92%–95% of the total ice-drift variance. Diurnal tidal motions predominate off the northern Sakhalin coast, accounting for 65%–80% of the variance, while low-frequency wind-induced motions prevail off the south-central coast, accounting for over 91% of the ice-drift variance. Maximum diurnal tidal ice-drift velocities range from 90–110 cm s−1 on the north coast to 10–15 cm s−1 on the south coast, in good agreement with the barotropic model of Kowalik and Polyakov. The presence of diurnal shelf waves accounts for the strong diurnal currents on the steeply sloping northern Sakhalin shelf, while the absence of such waves explains the weak diurnal currents on the more gently sloping south-central shelf. Using a vector regression model, the authors show that wind-induced ice-drift “response ellipses” (the current velocity response to a unity wind-velocity forcing) are consistent with a predominantly alongshore response to the wind, with wind-induced currents most pronounced off the south-central coast where water depths are relatively shallow. Time–frequency analysis of wind and ice-drift series reveals that, in winter, when sea ice is most extensive and internally cohesive, the ice response is almost entirely aligned with the alongshore component of the wind; in spring, when sea ice is broken and patchy, the ice responds to both the cross- and alongshore components of the wind.

Case Study: Modeling of Sediment Transport and Wind-Wave Impact in Lake Okeechobee

There are increasing demands for reliable engineering tools for sediment modeling and water resources management. The Lake Okeechobee environmental model (LOEM), which was calibrated and verified to simulate sediment resuspension and transport in Lake Okeechobee, Florida, is a dependable tool to meet those demands. The LOEM contains 2,126 horizontal grid cells and 5 vertical layers. The primary hydrodynamic and sediment transport driving forces are wind waves, surface wind stresses, and inflows/outflows. The LOEM was calibrated and verified, using two sets of observed data from May 16 to June 13, 1989 and January 17 to March 3, 2000, respectively. The model results indicate that sediment solids are resuspended primarily by wind–wave action and transported by lake circulation. The strong relationship between significant wave height and suspended sediment concentration in the lake indicates that sediment resuspension is primarily driven by wind-induced waves. To simulate this sediment resuspension, the processes of wind–wave- and current-induced bottom shear stresses on the lake bed were added to the LOEM. Once resuspended, the suspended sediment is transported to different areas of the lake by wind-induced currents. The importance of wind–wave, currents, and their interactions to sediment transport is included and discussed. By using the comprehensive data set for model calibration and verification, the LOEM model is proven to be a useful tool to water sources management in the lake.

Development of the Volga Delta in Response to Caspian Sea-Level Fluctuation during Last 100 Years

The Volga Delta developed in the closed Caspian basin can be classified as a river-dominated type. The subaqueous part of the delta (avandelta) is characterized by an extremely gentle slope and may be subdivided into shallow and deep parts. The very low-angle slope of the shallow avandelta is a major factor to mitigate impacts of Caspian sealevel (CSL) changes on the Volga Delta during the last 100 years. The CSL has fluctuated 3.46 m in the last 100 years, with a high of −25.55 m relative to the Baku Datum (BD) in 1903 and a low of −29.01 m BD in 1977. Sea-level changes can be grouped into three stages: (1) slowly falling at an average rate of 1.1 cm yr−1 in 1900–1929; (2) rapidly falling at an average rate of 6.3 cm yr−1 in 1930–1977; (3) rapidly rising at an average rate of 8.8 cm yr−1 in 1978–1999. During the rapid fall stage of sea level, the Volga delta-plain expanded at a rate of 2.3 km2 yr−1 to 180 km2 yr−1 because of alluvial accumulation and exposure of the shallow avandelta. Flourishing subaqueous vegetation promotes this expansion by trapping suspended sediments on the avan-delta. The shallow avandelta is primarily covered by freshwater and its geomorphic and sedimentary features are controlled more by the river flow than by the CSL changes, waves or wind-induced currents. The shallow avandelta decelerated delta progradation during sea-level fall and delayed delta retreat during sea-level rise. Significant changes in the delta-plain area have not occurred in the rapid CSL rise stage. Moreover, the distributaries and their distal underwater channels underwent erosion by the river flow at least until the end of the last century. The CSL has risen with an average annual rate of 8.8 cm during the last 20 years, yet coastal erosion, coastal flooding, wetland loss and saltwater intrusion, observed in other deltas, have not occurred here. This may have resulted from the existence of the shallow avandelta and show that the world's deltas respond to sea-level rise in different ways.

Modelling the depth of penetration of the wind's momentum at super-inertial frequencies due to non-linear effects in frontal regions

[1] A numerical model with a coast and density front is used to examine the influence of the front upon the depth of penetration of the wind's momentum. We find that non-linear processes associated with spatial variations in vorticity play an important role in driving wind-induced currents at depth. Internal wave generation and propagation also occur in the frontal region. For a bottom front, the downward propagation of internal waves in the frontal region is inhibited by the presence of sloping isotherms and a region of enhanced current magnitude develops at depth at the front's edge on the stratified side. For a surface front the presence of enhanced surface vorticity on the well mixed side of the front, means that the wind's energy is trapped in the surface layer. These calculations help to explaining the large spatial variability of wind forced energy found in observations.

Development of rudist lithosomes in the Coniacian–Lower Campanian carbonate shelves of central-southern Italy: high-energy vs low-energy settings

The Late Cretaceous shallow-water depositional areas of southern Tethys were complexes of unprotected shelves occupied by foramol assemblages that produced loose, diagenetically stable bioclastic debris not involved in significant in situ cementation processes. Both storm- and wind-induced currents and waves exercised a strong control on the distribution of the shifting biogenic sediments which covered the open sea-floor, constituting large coalescing sheets of winnowed fine to coarse skeletal sands. Rudists spread over all shelf sectors, from more open and external areas to more internal ones, occupying different substrata and furnishing the bulk of the skeletal component by means of bioerosion processes. They colonised mobile sediments giving rise to complex bodies with peculiar characteristics related to environmental parameters of the different sectors of the shelf. On the basis of detailed sedimentological, taphonomic and palaeontological data, we recognised two main rudist-rich depositional settings (‘end members’) in the southern Italy Senonian rudist-bearing successions. In successions pertaining to hypothesised marginal shelf sectors, characterised by high-energy regime deposits, rudist lithosomes are metric in thickness and lateral extent and lens-like in morphology, rich in bioerosion-derived skeletal sand and silt. Rudists are highly diversified. Large elongated cylindro-conical hippuritids (mostly pertaining to the genera Hippurites and Vaccinites), thick-shelled radiolitids and plagioptychids largely dominate. Rudists clustered in life position are subordinate; they often form small bouquets. More commonly these organisms appear fallen but only barely reworked. The rudist-rich bodies laterally pass into clean bioclastic grainstone in which sedimentary structures, related to current and/or storm erosional action, are common. No evidence of significant original relief of the rudist bodies in respect of the neighbouring sediment can be recognised. The submarine erosion and/or the high-energy processes operating presumably inhibited the aggradation of the tidal sediments above the marginal ones. As a consequence the vertical facies organisation shows widespread subtidal cycles, as commonly recognised in open shelves with ramp-like morphologies. In successions pertaining to more internal and/or low-energy sectors, rudist-rich beds rhythmically alternate with finer-grained foraminiferal limestones. Small elevator radiolitids with oligospecific diversity are dominant, mostly concentrated in clumps. Rudists in growth position are abundant, although a large quantity of shells appear toppled with little reworking. They may form laterally continuous biostromal shell beds. Sedimentary structures such as cross-lamination and gradation are only occasionally present. The resulting facies are commonly arranged in peritidal/shallow subtidal cycles in which evidence of subaerial (up to pedogenic modifications on a large and small scale) and, less frequently, submarine exposure is common. Intermediate successions have been recognised, characterised by deposits of silty-sand plains, which present intercalations of graded, bioclastic, storm-related beds. Sedimentological characteristics seem to document more open conditions in which submarine erosion was intermittently prevalent. In these successions rudist species that are commonly found both in high-energy and low-energy assemblages coexist.

Processes influencing wind-induced current profiles in near coastal stratified regions

Cross sectional and single point models of wind-induced currents in a stratified sea coastal region are used to determine the role of internal waves due to various wind periods upon mixing and current profiles. The limitations of a point model in reproducing current profiles are also examined. Calculations are performed with both fixed vertical diffusion coefficients and those derived from a turbulence closure model. Wind forcing at sub-inertial, inertial and super-inertial frequencies together with a wind impulse are considered. Results show that the offshore extent of the coastal boundary layer where mixing occurs depends upon the frequency of the wind forcing. The extent of this region and offshore propagation of internal waves influences the offshore variability of current profiles and the extent to which they can be reproduced by a single point model. Calculations with periodic wind forcing show that within the coastal boundary layer a point model cannot reproduce the current profile due to the internal pressure gradients associated with the internal wave field. However outside this region, current profiles from a point model are in good agreement with those computed with the cross section model when a barotropic pressure gradient forcing proportional to the local wind forcing is included. In the case of transient wind forcing, the inertial period dominates and determines the offshore extent of the coastal boundary layer. The transient nature of the response is such that although the single point model, outside the coastal region can reproduce the surface current due to direct wind forcing, the current at depth is not reproduced. The offshore extent of the coastal boundary layer computed with the turbulence energy model is comparable to that found with fixed diffusion coefficients although in some cases there are important differences which influence current profiles.

A model study on variations in larval supply: are populations of the polychaete Owenia fusiformis in the English Channel open or closed?

The polychaete Owenia fusiformis is one of the most ecologically important species in the muddy fine sand sediments in the English Channel where it is distributed in geographically separated populations. A vertically averaged Lagrangian hydrodynamic model integrating tidal residual currents and wind-induced currents was used to drive an advection–diffusion model for investigating the variability of larval transport in order to assess the self-seeding capabilities and the degree of connectivity between local populations. Three different types of environmental forcing (i.e. tidal forcing alone, tidal forcing coupled with either NE winds or SW winds) were applied to 19 distinct populations. Without wind influence, self-seeding is the principal mechanism involved in the renewal of most populations. However, larval retention ranged from under 1% up to 81% in relation to the adult habitat size and the mean velocity of tidal residual currents. Wind forcing had a strong influence on larval dispersal patterns by modifying the origin and densities of settlers as well as the degree of connectivity between populations. As a consequence, larval supply from distant populations generally exceeded local supply and the inter-annual variability of wind forcing induced large year-to-year variations in larval settlement rates. Larval exchanges occurred mainly between neighbouring populations and three groups of interconnected local populations were thereby identified. Within each group, settlement patterns were related to inter-annual variations in the direction and magnitude of larval exchanges.

Metal Mine Tailings and Sludge CO-Deposition in a Tailings Pond

In the Lower Cell of the Heath Steele mine tailings arealocated 50 km north of the city of Miramichi, New Brunswick,Canada, tailings and lime treatment sludge were co-depositedand the effluent occasionally exceeded regulatory dischargelimits for total suspended solids (TSS), especially on verywindy days. A combined field and laboratory investigationwas undertaken during 1998-1999 to identify the cause of thehigh TSS. The methodology included field measurement of windvelocity and sampling of bed tailings, sludge,suspended sediment, and the water cover. The samples werethen subjected to various laboratory examinations using x-raydiffraction, scanning electron microscopy interfacedwith energy dispersion x-ray spectroscopy, critical shearstress measurements, and water chemistry analysis. Thesuspended sediment was found to be composed primarily ofcalcite and metal hydroxides derived from the bed sludge inthe cell. Only a very small amount of tailings (less than5%) was detected in the suspended sediment. The sludge,which covered the tailings in the shallow western section ofthe Lower Cell at depths less than 1 m, was a loose, low-densitymaterial with a low critical shear stress (approximately 0.058 Pa). In shallow water cover (less than 1 m), calculated bed shearstress mobilized by wind-induced waves and return currents exceeded the critical shear on a number of occasions, resultingin resuspension of the sludge and hence high TSS. Although occasional elevated zinc concentrations appeared to follow a similar pattern to high TSS, there was no evidence that thesuspended sludge sediments would release metals into the watercover, due to the high pH of the Lower Cell water cover. Since the tailings did not resuspend significantly, it was clear thatat water cover depth less than 1 m, sludge was eroded instead oftailings, and thus provided a barrier against tailings resuspension. As part of the final closure scheme for the HeathSteele tailings area, sludge and tailings were dredged and relocated from areas where the water cover was < 1 m to deeperwater cover areas to ensure that the effluent met required totalsuspended solids discharge criteria.

Lagrangian Transport by Currents Variable in Time in the Model of Wind-Induced Circulation of the Black Sea

We construct a simple model of seasonal variability of circulation in the Black Sea, which satisfactorily explains the data of altimetric satellite observations. The model is used to describe the nonstationary wind-induced currents in a round basin and study the trajectories of water particles in the surface layer of the sea. It is shown that, for nonstationary currents, these trajectories can be quite complicated. It is also assumed that Lagrangian mixing can be caused by the complex trajectories of water particles.

Morphodynamique d'une dune sous-marine du détroit du Pas de Calais. Morphodynamics of an undersea sandwave of the Dover Straits

This paper investigates the dynamics of sandwaves in relationship with tidal currents and weather conditions. The studied sandwaves are located in the Dover Straits and are covered by megaripples. These megaripples have migration speeds of and slopes of 34°, suggesting the potential for avalanches to occur along the flanks of the sandwave. Tidal cycles without reversing currents were observed during stormy weather. Wind-induced currents lead to a unidirectional migration of megaripples and sandwaves. Well-defined areas without megaripples were observed and correlated with an increase in sandwave height. We propose interpretations for understanding sandwave saturation and migration

Morphodynamique d'une dune sous-marine du détroit du pas de Calais

This paper investigates the dynamics of sandwaves in relationship with tidal currents and weather conditions. The studied sandwaves are located in the Dover Straits and are covered by megaripples. These megaripples have migration speeds of and slopes of 34°, suggesting the potential for avalanches to occur along the flanks of the sandwave. Tidal cycles without reversing currents were observed during stormy weather. Wind-induced currents lead to a unidirectional migration of megaripples and sandwaves. Well-defined areas without megaripples were observed and correlated with an increase in sandwave height. We propose interpretations for understanding sandwave saturation and migration. To cite this article: D. Idier et al., C. R. Geoscience 334 (2002) 1079–1085.

Horizontal distribution of vendace larvae in Lake Pyhäjärvi (SW Finland): effect of wind-induced currents

(2002). Horizontal distribution of vendace larvae in Lake Pyhajarvi (SW Finland): effect of wind-induced currents. SIL Proceedings, 1922-2010: Vol. 28, No. 2, pp. 726-730.

Analysis of time-varying wind-induced currents in the North Channel of the Irish Sea, using empirical orthogonal functions and harmonic decomposition

The response of the currents in the North Channel of the Irish Sea to time-dependent wind and open boundary forcing is examined using a three-dimensional model of the region. For wind-induced motion, we primarily consider currents produced by a clockwise rotating wind field with periods above, at, and below the inertial period. With the longer period wind forcing a calculation is performed with an anticlockwise rotating wind. Flow fields from the model, in particular currents in the centre of the Channel and at points in shallower water, are analysed using the harmonic method in time and an expansion of empirical orthogonal functions (EOFs) in the vertical. Results from the analysis show that with wind forcing at the inertial period, currents are produced throughout the water column, although with longer and shorter period forcing the depth of penetration of the wind's momentum is confined to a near surface layer. In the case of an anticlockwise rotating wind, for a given wind period (excluding the inertial), the thickness of the near surface layer is less than that for the clockwise wind. In the case of short period forcing, in deep water the wind stress and elevation gradient terms, are balanced by the acceleration term whereas at longer periods the acceleration term is small, and the balance is between rotational and elevation gradient terms and the wind stress. In shallow water, the bottom stress contributes to the balance of terms. Also the bottom Ekman layer occupies a significant proportion of the water column, and influences the profile of wind-induced currents. The analysis of current profiles using both harmonic methods, and fitting EOFs in the vertical shows that to first order the motion in the centre of the channel can be described by linear dynamics. However, non-linear coupling between the time-varying eddy viscosity and wind shear in the surface layer modifies the M 2 tidal current in this region. In shallower water the quadratic bottom friction term is important as are the non-linear advection of momentum terms. With open boundary forcing, the flow is to first order aligned with the orientation of the channel, and dominated by currents at the period of the boundary forcing. In the near bed region, higher frequency harmonics are generated, although the M 2 harmonic found in the wind-driven flow case does not appear at any depth. The highly sheared surface layer found with wind forcing is not present for currents induced by open boundary forcing, where shear is confined to the bottom boundary layer. The application of harmonic analysis methods, and the use of EOFs in the vertical are shown to be powerful techniques in understanding the physical nature of time-dependent flow in the North Channel.

Surface Flow Measurement using VHF Ocean Radar in a Stratified Coastal Water and Data Assimilation into a 3-D Baroclinic Flow Model

Doppler ocean radar is being increasingly used in estuarine research. The advantage of using this instrument is that it can measure sea surface flow velocity field in all weather conditions due to its remote sensing capability. In the present study, the accuracy of the surface current velocities in Osaka Bay measured by very high-frequency (VHF) Doppler radar is investigated. The radar system uses a frequency of 41.9 MHz and allows a radial range resolution of 500 m at ranges up to 20 km from a coastline. Data from VHF ocean radar are compared with that using Acoustic Doppler Current Profilers (ADCP) in order to quantitatively verify the surface shear and wind-induced current. They are also used to enhance an assimilation technique in order to assimilate the observed surface flow velocities into a three-dimensional baroclinic flow model. The correlation coefficient and rms difference between the velocities measured by the VHF ocean radar and the in situ current profiler axe in the order of 0.69 and 6.4 cm/s, respectively.Assimilation of the surface velocity is carried out using ocean radar observations. Two assimilation schemes, blending and nudging, are examined. The blending scheme including the vertical mixing induced by wind stress and with a logarithmic profile for the decrease in surface velocity downward performs better than the nudging scheme. Application of the assimilation model shows good agreement with the in situ current and salinity data.

Wind-induced Currents in Seas

Analysis of currents in the Caspian and Black seas is made to show that the role of wind-induced currents in the dynamics of seas is largely overestimated and the conventional methods used to evaluate current velocities based on wind speed yield values higher than those observed in the field.

Courants et transports sédimentaires dans la zone littorale : le rôle des courants orbitaux et de downwelling / Currents and sediment transport in the coastal zone: the role of orbital and downwelling currents

Extended abstract Several types of currents occur along sandy shorelines. They are responsible for significant sediment transport and represent major processes causing coastal changes. Coastal currents are generated by various forcing mechanisms of atmospheric and/or marine origin which have distinct effects on the morphology and sedimentology of the littoral zone. The aim of this paper is to examine the role of coastal currents on the sediment dynamics of sandy shorelines. Tidal currents generally have limited effects on the morphodynamics of sandy coasts where wave- and wind-induced currents appear to be the main factors contributing to sediment transport in the coastal zone. The morphological and sedimentological effects that may be caused by these different currents are exemplified by a series of wave and current measurements carried out in the nearshore zone of the Mediterranean Sea and the Beaufort Sea. Examples of measurements of hydrodynamic parameters during fair-weather and storm conditions are shown in order to illustrate typical patterns of water circulation and sediment transport generated by different forcing mechanisms. Wave-generated currents result in both longshore and offshore sediment transport through shore-parallel littoral drift and seaward-directed rip currents and undertow. Wind-generated currents may result in more significant offshore sediment dispersal, particularly during storms. Some of the results presented in this paper show that the seaward-flowing downwelling near-bottom currents that develop in the nearshore zone during storm surges may be responsible for high magnitude offshore sediment transport. These currents differ from undertow and rip currents because they are not related to wave action but are generated by onshore wind stress causing a sea -level set-up at the coast. Because the coastal zone is not solely affected by surface gravity waves and associated currents, but also by other mechanisms such as wind-driven storm currents, existing models of beach/nearshore changes that are based only on wave-energy dissipation over an equilibrium profile may be seriously inadequate.