Variations In Current Velocity Research Articles

The response of the upper ocean to solar heating II: The wind‐driven current

AbstractThe current profile generated by a steady wind stress is disturbed by the diurnal variation of mixed layer depth forced by solar heating. Momentum diffused deep at night is abandoned to rotate inertially during the day when the mixed layer is shallow and then re‐entrained next night when it deepens. The resulting variation of current profile has been calculated with a one‐dimensional model in which power supply to turbulence determines the profile of eddy viscosity. The resulting variations of current velocity at fixed depths are so complicated that it is not surprising that current meter measurements have seldom yielded the classical Ekman solution. However, the progressive vector diagrams do exhibit an Ekman‐like response (albeit with superimposed inertial disturbances) suggesting that the model might be tested by tracking drifters designed to follow the flow at fixed depths. The inertial rotation of the current in the diurnal thermocline leads to a diurnal jet, the dynamical equivalent of the nocturnal jet in the atmospheric boundary layer over land. The role of inertial currents in deepening the mixed layer is clarified, leading to proposals for improving the turbulence parametrizations used in models of the upper ocean. The model predicts that the diurnal thermocline contains two layers of persistent vigorous turbulence separated by a thicker band of patchy turbulence in otherwise laminar flow.

Estimating Water Volume Discharge through Salt-Marsh Tidal Channels: An Aspect of Material Exchange

Water volume discharge estimates were made at a sample cross-section of the Canary Creek salt marsh (Lewes, Delaware). To account for cross-sectional velocity variations, a dense spatial array of current meters was used. Simultaneous measurements were taken at hourly intervals over three complete tidal cycles. A practical method is presented whereby instantaneous average cross-sectional velocity, instantaneous cross-sectional discharge and tidal cycle discharge can be estimated from a dense current meter array, orideal data set. Based on theseideal estimates, it is shown that a simplified spatial array can be used to estimate water discharges within acceptable error limits. Instantaneous cross-sectional discharges and total tidal cycle discharges were estimated with uncertainties of ±11% and ±7%, respectively. To minimize errors when estimating material exchange between salt marshes and adjacent waters, it is suggested that a comprehensive assessment of cross-sectional lateral current velocity variations be conducted.

Influence of Physical Factors on Density of Stocked Brown Trout (Salmo trutta fario L.) in a Danish Lowland Stream

Physical factors and brown trout densities were studied in a small Danish lowland stream. The densities of brown trout larger than 15 cm were significantly correlated with gradient, mean depth, coefficient of variation in current velocity 7 cm above the bottom, the ratio between wetted perimeter and width, amount of overhanging banks and degree of macrophyte cover. Coefficient of variation in current velocity 7 cm above the bottom was the most important factor for brown trout density (rs= 0.8364, 24 df, (P < 0.001)), which supports the idea of this value as a measure of stream complexity. A rather small relation between trout density and amount of overhanging bank cover (rs= 0.4179, 24 df, (P < 0.050)), contrary to the closer relationships found in previous studies, is discussed as an effect of the self-shading capacity of this rather narrow and deep stream.

3‐D Static analysis of cable in nonuniform current

Abstract The analysis of a cable in a current involves geometric, material, and loading nonlinearity. This paper derives the 3‐D static equations for cable analysis and discusses their boundary conditions and the numerical technique. After the simplification to a 2‐D case, the equations are identical to those of Goodman and Breslin. Traditionally, the buoyancy force on a cable element has been taken in the gravitational direction; however, this paper cautiously avoids this conceptual error and verifies that the buoyancy force is in the normal direction of the cable. The result shows that, in the three‐dimensional analysis, the variation of current velocity has a significant effect on the cable geometry, and the neglect of tangential drag force of the assumption of v=0.5 for the material of low modulus of elasticity may cause significant error.

Velocity variation of the Kuroshio during formation of the small meander south of Kyûshû

Historical GEK data provided by JODC is analyzed to investigate the characteristic variation in velocity of the Kuroshio, with special reference to the formation of small meanders south of Kyushu. It is found that, during or prior to the period of small meander formation, there is a tendency for an abrupt increase in the current velocity west of Yaku-Shima (Yaku-Island), representing an increase in the main current intensity upstream. Also, there are apparent time lags in the variation in current velocity along the path of the Kuroshio between the upstream and the downstream regions of the small meander area. Namely, it is apparent that the increase in Kuroshio velocity in the Satsunan Strait procedes that offshore of Shikoku during the period of the small meander formation, by the order of one month. These results indicate that a nonlinear effect due to the increase in current velocity is a possible cause of the generation of small meanders.

Hydrology of Okarito Lagoon and the inferred effects of selective logging in Okarito Forest

Okarito Lagoon (43° 11′S, 170° 14′E) is a small (20 km2) shore‐parallel, predominantly subtidal estuary, deepest near the landward end, and linked to the sea by two subtidal channels incised through shallow subtidal and intertidal flats which occupy the southern third of the lagoon. Tides at sea vary from 2.1m (spring) to 1.2 m (neap), but in the lagoon the tidal range is constant through the lunar cycle and varies from 0.80 m at the entrance to 0.17 m in the upper lagoon. Tidal water level and flow asymmetries in the subtidal channels are separated by a 1.7 h phase difference. Variations in the net discharge through the inlet result from changing flow cross‐sections rather than from variations in current velocities. Both the tidal‐averaged volume and the tidal compartment of the lagoon vary through the lunar cycle, from maxima at spring tides to minima at neap tides. Freshwater inflows vary from less than 11 m3.s‐1 to more than 750 m3.s‐1. During storms, water level in the lagoon rises rapidly by 2–3 m, then declines to normal over several days. Three water masses, two with salinity and turbidity largely controlled by antecedent rainfall, normally occur in the lagoon. Suspended sediment concentrations in both freshwater inflows and lagoon waters are normally low but increase during floods. Most sediment is supplied by the Waitangi‐taona River or by erosion of tidal channel margins. The lagoon is floored with organic‐rich mud and sandy mud, deposited predominantly from suspension. Surface sediment is consistently muddier than subsurface sediment, probably reflecting an increase in the mud supply since diversion of the Waitangi‐taona River in 1967. Comparisons of the estimated sediment yield and water inflow effects of the 1967 river diversion with short‐term observations during selective logging suggest that the effects of logging on sediment yield, water balance of the lagoon, and dissolved solids inputs will be small compared with changes caused by diversion of the Waitangi‐taona River.

Abyssal hydrography, nephelometry, currents, and benthic boundary layer structure in the Vema Channel

New data from closely spaced hydrocasts, thermograd profiles, vertical nephelometer profiles, and direct bottom current observations within the Vema Channel (near 30°13′S) allow an interpretation of the flow regime and the structure of the benthic boundary layer. A sharp gradient in potential temperature, light scattering, concentration of suspended particulates, and excess radon is present in the transition zone between northward flowing antarctic bottom water (AABW) and the overlying north Atlantic deep water. This transition zone, however, exhibits pronounced eastwest asymmetry across the channel axis. To the east the benthic thermocline is relatively deep (∼4250 m) and sharp (0.8°/100 m to 1.7°/100 m). In the channel axis adjacent to the western wall the thermocline is several hundred meters shallower and more gentle (∼0.3°/100 m). Gradients in light scattering exhibit a corresponding asymmetry. Coldest bottom water temperatures (θ < −0.18°C) and highest values of near-bottom turbidity are present on the eastern side of the channel, even though the bathymetric and structural asymmetry of the channel suggests preferential erosion and perhaps strongest current velocities on the western side, where the channel is deepest. Near-bottom current velocities measured in both the main branch and the western branch of the channel exhibit a significant cross-contour component in the mean flow. This deviation is approximately 40° in the main branch and 10°–20° in the western branch. The direction of deviation from contour-following flow is toward the east or in the same direction as would be predicted for Ekman layer transport in a bottom boundary layer. The temperature and light scattering data are consistent with a model of an asymmetrical flow regime such that (1) strongest northward current velocities are adjacent to the western wall of the channel, (2) fractional effects due to the presence of the wall induce strong turbulence in the flow, (3) turbulent mixing within the AABW results in a ‘blurring’ of the benthic thermocline and slight elevation of near-bottom temperatures in the channel axis, and (4) upslope (eastward) advection in a bottom boundary Ekman layer results in a veering of the mean velocity vector and the transport of coldest bottom water (θ = −0.18°C) to the eastern margin of the channel. Uncertainties in estimating AABW transport through the channel are introduced by difficulties in identifying an appropriate level of no motion and by the unknown extent to which frictional effects in the benthic boundary layer invalidate the geostrophic approximation. If a level of no motion corresponding to σ4 = 46.00 is assumed, the geostrophic transport of AABW through the main branch is of the order of 1.4×106 m3/s. Intervals of abrupt increase in bottom water turbidity within the channel axis show no obvious correlation with variations in current velocity observed simultaneously. Consequently, the time variability in the nepheloid layer may reflect varying rates of sediment entrainment upstream in the Argentine Basin rather than resuspension of sediment within the channel itself. The total weight of suspended particulates per unit area within the channel appears to be comparable to that in the basin to the south. The principal effect of the channel on the bottom nepheloid layer is to increase its thickness from a few tens of meters to several hundred meters.

The hydrography of the Murderkill estuary. Delaware

The hydrography of the lower Murderkill River, a small, shallow estuary in southeastern Delaware, was studied from July, 1967, through December, 1970. Variations of current velocity, tide stage, salinity, water temperature, and dissolved oxygen were measured at one station in the estuary during periods of mean and high freshwater runoff. The flushing time and mixing characteristics of the estuary during mean runoff were determined using a water-tracer dye. Seasonal and tidal variations of salinity, water temperature, dissolved oxygen, pH and turbidity were measured at seven stations along the estuary from September, 1969 through December, 1970.

A description of current, wind, and sea level variations during coastal upwelling off the Oregon coast, July-August 1972

The low-frequency (ƒ < 0.6 cpd) variations in current velocity measured at several depths beneath the surface layer in 100 m of water and in nearby wind and sea level measurements are discussed. The measurements were made on the Oregon continental shelf during July and August 1972 as part of the Coastal Upwelling Experiment. The mean alongshore current at 20 m was 1.8 cm/sec southward but at 80 m was 6 cm/sec northward. The shear persisted and was relatively constant (variance of shear <⅓ variance of current) despite major variations in the current. The thermal wind relation held for the alongshore components of current. The major low-frequency fluctuations in the current (at 20 m and deeper) are barotropic, experienced equally at all depths, and highly correlated with sea level variations. Spectral analysis shows very high mutual coherence between wind, sea level, and clockwise currents only in a narrow band at 0.15 cpd.

Determination of Mooring System Design Data - Offshore British Columbia

Abstract An outline is presented of basic data required in the design of a mooring system for a floating drilling vessel. For offshore British Columbia, field measurements to determine the range of soil conditions, anchor performance in the various types of soil and current velocities were required to supplement the available data. Planning techniques, equipment used and procedures followed in accumulating this information are described to serve as possible guide lines for others entering new offshore areas where similar information is needed. Within the area of interest, three general classifications of soils were present:coarse gravels with clay fill,fine sands, andvery hard silts and clays with shear strengths up to 47,000 lb/sq ft immediately below the mud line. A mooring problem was not expected in the gravels and fine sands; however, applicable data on the performance of anchors in soils with shear strengths greater than 200 lb/sq ft was not available. It was necessary to establish the performance of anchors in these soils; otherwise, anchor piles would have to be used. Anchor tests proved conclusively that lightweight-type anchors in the range of weights tested could develop satisfactory holding power in all types of soil encountered. A brief summary of the meterological and oceanographic conditions off the coast of British Columbia and examples of tabular and graphic methods of presenting maximum and average conditions are included in the Appendix. Introduction Prior to the actual design of a mooring system for a floating drilling vessel, considerable data pertaining to the local environmental conditions must be established. The most significant factors may be listed as follows.Meteorological: magnitude, direction, frequency of occurrence and duration of storm winds.Oceanographic:range of water depths,normal seasonal tidal fluctuations and magnitude and frequency of occurrence of maximum storm tides,magnitude, direction, period, frequency of occurrence and duration of storm waves, andnormal and maximum storm fluctuations of current velocity and direction (for deep draft semi-submersible vessels, information on the current velocity variation with water depth should also be obtained).Miscellaneous:variations in the ocean floor soil conditions and related anchor performance must be considered; andother factors such as the proximity of dangerous rocks or reefs, floating hazards such as icebergs and the expected advance warning of severe storms should be considered in the determination of the design safety factor and the speed in which the system can be set and retrieved. In addition to these environmental factors, information on vessel reactions to wind and current at various approach angles is necessary to determine the strength of the required system. To determine flexibility of the mooring lines, vessel heave, surge, pitch, roll and yaw responses to waves of different periods, heights and approach angles must be known. These vessel characteristics are normally obtained by model tests. Regarding the specific area of interest off the coast of British Columbia, a preliminary investigation into the availability of mooring system design data showed that sufficient information was available on winds, tides and waves (Appendix). Water depths had been recorded and some soil samples had been obtained during previous seismic work. Vessel characteristic data for the Sedco 135 vessel were available from model tests.1 A review of the available information showed that individual anchor holding capacities in the order of 300,000 lb would be required to maintain the vessel on location during the maximum anticipated storm. Although a 30,000- or 40,000-lb U.S. Navy Light Weight Type (LWT) anchor is capable of developing the required holding capacity, the shear strength of the soil must be low enough to permit penetration of the anchor flukes, yet high enough to provide sufficient resistance to anchor drag.

EFFECT OF WATER CURRENTS UPON THE ATTACHMENT AND GROWTH OF BARNACLES

1. The work of previous authors, dealing with the effect of water currents upon barnacle attachment, growth and distribution is briefly reviewed.2. Experiments were conducted to determine the effects of water currents upon the attachment and growth of barnacles, and particularly of Balanus amphitrite. Submerged rotating discs and glass tubes of graded cross-sectional diameter were employed to provide variations in current velocity.3. The velocity of water current limiting attachment appears to lie between 0.5 and 0.9 knot for Balanus amphitrite, between 0.4 and 0.7 knot for B. eburneus, and above 1.1 knots for B. improvisus.4. Following attachment the growth rate of barnacles was found to be increased by water currents of velocity less than 1.5 knots and to be decreased by currents with velocities in excess of this. The adverse effects of water currents were found to decrease with increasing age of the barnacles subsequent to attachment. Six hours after attachment, growth rate was reduced to one-third of normal by a 1.5 knot current and completely stopped by a 3 knot current. Five days after attachment, growth was prevented by currents ranging between 3.3 and 8 knots. Loss of attachment appeared to some extent among all barnacles in which growth rate was reduced. This loss was greatest at velocities bringing about complete cessation of growth.5. It is suggested that loss of attachment is due to interference with the feeding process, followed by reduction of growth rate, death, and diminished adherence. Possibly due to an orientation to the current which facilitates feeding, barnacles attached for one day or less show less retardation of growth rate and loss of adherence than barnacles two days old.6. Since tidal currents are almost invariably intermittent it appears from the data presented that they are not sufficient to prevent the colonization of suitable surfaces by barnacles, except where the velocities are unusually high.It also follows from the numerical results obtained that on vessels making short stays in port and relatively long voyages, little permanent barnacle fouling will occur, since those organisms which attach will be killed and at least a portion of them dislodged. The evidence does not preclude, however, the continued growth of barnacles upon slow vessels making longer stays in port, and their geographical distribution by this means.