Strong Tidal Flows Research Articles

Structure and dynamics of fluid muds on the Amazon Continental Shelf

Thick layers of fluid mud occur in strong tidal flows over the inner portion of the Amazon continental shelf in regions of strong salinity fronts associated with the plume discharged from the Amazon River. Detailed shipboard profile measurements obtained in this region during A Multidisciplinary Amazon Shelf Sediment Study (AMASSEDS) provide an unprecedented opportunity to examine the structure and dynamics of fluid muds under natural conditions. The analysis focuses on flows in which the motion is fully turbulent and suspended sediment dominates the stratification. Under these conditions a comparison of measurements and one‐dimensional model calculations indicates that vertical transport is controlled by suppression of turbulent mixing at gradient Richardson numbers near 1/4. This constraint produces a distinctive vertical structure and leads to an upper bound on the total amount of suspended sediment that may be carried in a turbulent suspension by a tidal flow.

Numerical experiments of internal wave generation by strong tidal flow across a finite amplitude bank edge

Results of some idealized numerical experiments of strong tidal flow of a stratified fluid across a finite amplitude bank edge are presented. These experiments were motivated by a need to develop an understanding of some of the complex internal wave phenomena observed on Georges Bank (Loder et al., 1992) and at other locations where tidal forcing is strong. The numerical model solves the fully nonlinear, nonhydrostatic Boussinesq equations on an ƒ plane. The model is two‐dimensional, with spatial variation in the vertical and cross‐bank directions only. Model forcings are based on the Georges Bank observations. A horizontally uniform stratification is used. The model successfully reproduces some observed features including the formation of a large depression and a hydraulic jump over the bank edge during off‐bank flow and two on‐bank propagating depressions every tidal period. An undular bore propagating away from the bank is in agreement with other observations (La Violette et al., 1990). Rotational effects are shown to be responsible for the formation of the second of the on‐bank propagating depressions. Sensitivity of the results to the topographic slope, tidal current strength, stratification, and model initialization is explored.

Energetics of the Internal Tide on Northern Georges Bank

Abstract This paper discusses the energetics of the internal waves generated by the tide on the northern edge of Georges Bank in the outer Gulf of Maine–-a region of strong tidal flow, a density/temperature front, and abrupt topography. A series of bank-edge cross sections in early July 1988, using the towed vertically profiling Batfish, shows that during off-bank flow a large internal hydraulic jump develops over the bank slope. This disturbance propagates on bank during the reverse phase of the tide, evolving into two internal solitonlike features that impinge upon the frontal zone separating stratified bank-edge water from the homogeneous water on bank. Thermistor chain data from the bank edge show that two internal wave packets per tidal cycle are characteristic of the region during this time of year. The available potential energy plus kinetic energy of the internal disturbances, estimated using the Batfish and thermistor chain data, is found to be 35 J m−3 in a plug of fluid 60 m deep and 4 km long ...

A new species ofCerapusfrom Cudjoe Channel, Lower Florida Keys, USA, with notes on male behaviour (Crustacea: Amphipoda: Corophioidea)

Cerapus cudjoe is described from Cudjoe Channel in the Lower Florida Keys, USA, where it lives primarily on the hydroid, Cnidoscyphus marginatus (Allman), in channels subject to strong tidal flow. The species is characterized by the shape of the palm of male gnathopod 2, and the huge peduncular hook on male uropod 1 which distinguishes it from all congeners. Males were observed swimming during slack tide and actively seeking females in presumed precopulatory behaviour.

Degradation of some mechanical current meter measurements by high-frequency mooring or wave motion

Comparison of current measurements from moored (paddle-wheel rotor) Aanderaa current meters and acoustic Doppler current profilers in a strong tidal flow on Georges Bank indicates rate under-reading by the Aanderaa meters at some vertical positions. The under-reading may arise from mooring-line vibrations induced by vortex shedding from spherical buoyancy packages, and shielding of the paddle-wheel rotors due to the meters' inability to remain aligned with the fluctuating relative water velocity. Field tests and a simple model are used to investigate this explanation. The authors briefly review the Georges Bank observations that instigated this investigation. An explanation for the degradation of the Aanderaa measurements is proposed, observational information from field tests is discussed, and the model and its implications are presented. The model results support the hypothesis that high-frequency mooring-line vibration causes the degradation of the Aanderaa measurements.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Nitrogen budget in Tokyo Bay with special reference to the low sedimentation to supply ratio

In order to determine why the sedimentation to supply ratio of nutrients in Tokyo Bay is markedly small, the nitrogen budget was investigated for 1979, when a systematic and continuous observation of flow and salinity was carried out. The data were analyzed by use of a simple advective-diffusive box model and dissolved oxygen balance in the lower layer was also examined. The calculated values of two-layer flow, settling, primary production, mineralization, denitrification, and dissolved oxygen consumption were comparable to those observed. The factors making the sedimentation to supply ratio makedly small were summarized as: 1) a strong and stable two-layer flow generated by a large freshwater supply, 2) further intensification of this two-layer flow by the northern winter monsoon, 3) coincidence of the discharge region with the supply region of nutrients caused by the transverse inclination of the interface, probably due to the earth's rotation. 4) effective discharge of nutrients from the bay due to a strong tidal flow and a possible cyclonic tidal residual circulation in the inner bay mouth, 5) incomplete consumption of nutrient salts by phytoplankton in the upper layer even in the most productive season, and 6) possible denitrification in the anaerobic bottom water in summer and in the bottom sediment itself throughout the year in the inner bay.

Generation mechanism of internal waves by a vertically sheared tidal flow over a sill

The internal response over a sill to strong tidal flow superimposed on a vertically sheared steady flow is examined with a simple two‐layer analytical model. As in the case where the vertical shear is absent, the upstream propagating internal wave is gradually formed through interference of infinitesimal‐amplitude internal waves (elementary waves) emanating from the sill at each instant of time (Hibiya, 1986); with the composite internal Froude number exceeding (falling below) unity in the accelerating (decelerating) stage of the two‐layer flow, the upstream propagating elementary waves come close together over the sill. In this case, however, the amplitude of the elementary wave is increased when the lower layer velocity is larger than the upper layer velocity, whereas it is reduced when the upper layer velocity is larger than the lower layer velocity. As a result, the internal wave field over the sill differs markedly between the ebb and flood tidal phases. It follows from this analytical result that in the Strait of Gibraltar, the internal wave propagating Mediterraneanward is significantly amplified during the ebb (westward) phase of the tidal flow, whereas such amplification does not take place throughout the tidal cycle for the internal wave propagating Atlanticward. This is consistent with the result from satellite observation that the sea surface signatures caused by underlying large‐amplitude internal wave trains appear only on the Mediterranean side and not on the Atlantic side.

Study of internal wave generation by tide-topography interaction

The generation mechanism of internal waves by a relatively strong tidal flow over a sill is clarified analytically. Special attention is directed to the role of the tidal advection effect, which is examined by use of characteristics. An internal wave which propagates upstream is gradually formed through interference of infinitesimal amplitude internal waves (elementary waves) emanating from the sill at each instant of time. In the accelerating (or decelerating) stage of the tidal flow, the effective amplification of the internal wave takes place as the internal Froude number exceeds (or falls below) unity because during this period the internal wave slowly travels downstream (or upstream) while crossing over the sill where elementary waves are efficiently superimposed. In fact, the variability in the internal wave field actually observed in a realistic situation (Stellwagen Bank in Massachusetts Bay) is shown to be satisfactorily interpreted in terms of this mechanism. Furthermore, by using this analytical model, the relation between the strength of the tidal advection effect and the resulting internal waveform is clarified. This theory is easily extended to include a vertically sheared steady flow. In this case, although the fundamental generation mechanism is the same as above, the amplitude of the elementary wave varies with time depending on the relative direction of the tidal flow and steady shear flow, so that the internal wave field over the sill differs markedly between the ebb and flood tidal phases. As an example, the internal wave generation process over the sill in the Strait of Gibraltar is qualitatively discussed on the basis of this analytical model. The effect of vertical mixing caused by breaking of these large-amplitude internal waves on the coastal environment is also pointed out. In particular, a brief discussion is made on the control of water exchange by the fortnightly modulation of tidal mixing processes at the sills and constrictions in channels connecting freshwater sources with the ocean.

Environmental monitoring of caged fish farming in macrotidal environments

Recent years have seen considerable expansion in fish farming using sea cages. These have until recently been located in the sheltered and low current environments of sea lochs and fjords. Significant benthic enrichment is known to occur at these sites, but the effect is highly localized. Fish farm development is now occurring in areas with strong tidal flows. The siting of caged fish farming in macro-tidal environments may reduce the environmental impact of the industry on benthic communities at the farm site. However there exists the potential for accumulation of farm wastes in nearby sedimentary sinks. Additionally the longer residence time of the wastes in the water column has the potential, especially in regions with already high nutrient loadings and/or long flushing times, to lead to stimulation of phytoplankton blooms. Environmental monitoring of such developments therefore needs to consider these addition effects.

Shelf sediment dispersal during the dry season, Princess Charlotte Bay, Great Barrier Reef, Australia

Princess Charlotte Bay, located on the northern Great Barrier Reef, is an environment of terrigenous and carbonate deposition. The dynamics on this shelf are controlled by the Great Barrier Reef at the edge of the shelf, and the mid-shelf, shore-normal reefs. This study examines the dynamics during the dry season, with six time-series records from instrumented tripod deployments and numerous hydrographic stations. The shallow nearshore waters and the estuaries prove to be the sites where most active sediment resuspension and transport takes place. Sediment resuspension is effected primarily by waves in the nearshore, and channeling of tidal currents in the estuaries. Bedload transport did not occur during this study, mainly because current velocities were too low. Suspended particulate matter (SPM) transport in the bay is governed by tides and winds. Strong tidal flow imparts a strong offshore component to the transport, and strong southeast winds impart an alongshore component that transports SPM out of the bay to the northwest. Rattlesnake Channel, east of Princess Charlotte Bay, is another route by which SPM leaves the bay. Flow through this channel is predominantly tidal, with ebb waters (leaving Princess Charlotte Bay) carrying higher SPM concentrations than flood waters. SPM flux in the nearshore was an order of magnitude higher than at offshore stations, with highest fluxes generally occurring at times of sustained southeast winds. Transect data show that SPM drops to average bay values in water 11 m deep, indicating most SPM is transported in nearshore waters.

Generation mechanism of internal waves by tidal flow over a sill

The generation mechanism of internal waves by relatively strong tidal flow over a sill is clarified analytically. Special attention is directed to the role of the tidal advection effect, which is examined by use of characteristics. An internal wave which propagates upstream is gradually formed over the sill through the interference among infinitesimal amplitude internal waves (elementary waves) emanated from the sill at each instant of time. In the accelerating (decelerating) stage of tidal flow, the effective amplification of an internal wave takes place as the Froude number exceeds (falls below) unity, because during this period the internal wave slowly travels downstream (upstream), crossing over the sill, where elementary waves are efficiently superimposed. When the strength of the tidal advection effect is appropriate, the internal wave formed in the accelerating stage (Ac wave) and that formed in the decelerating stage (Dc wave) overlap, so that the resultant wave height becomes very large. Since the relative position of Ac and Dc waves varies depending on the strength of the tidal advection effect, the resultant internal wave form is strongly affected by it. When the tidal advection effect is strong, the Ac wave is carried too far downstream to interfere with the Dc wave, and hence the resultant wave form consists of two crests and two troughs. When the tidal advection effect is moderate, the optimum interference between Ac and Dc waves occurs, and the resultant wave form consists of one crest and one trough, with its horizontal scale approaching that of the sill. When the tidal advection effect is weak, the internal wave height becomes very small, since elementary waves propagate almost freely in the tidal flow without coming close together, and the resultant wave form approaches a sinusoidal wave of tidal frequency.

Temperate skeletal carbonate sediments on Scott shelf, northwestren Vancouver Island, Canada

Scott shelf (∼2000 km 2), centred at Scott Islands, northwestern Vancouver Island, is mainly shallower than 150 m, topographically diversified, and floored by bedrock outcrops and terrigenous lithic gravels and sands left stranded following the post-glacial rise in sea level (13,000 yrs B.P.). It lies in a zone of vigorous wind-wave currents and strong tidal flows, but is largely starved of modern terrigenous sediment. As a consequence it is slowly accumulating a thin, discontinuous blanket of clean, skeletal carbonate sands and gravels, admixed to varying degrees with the underlying terrigenous deposits. Principal skeletal contributors are infaunal bivalves (on coarse sandy and gravelly substrates), barnacles (on low-amplitude gravelly ridges), bryozoans (on bedrock outcrops and boulders) and benthic foraminifera (on fine sands in deep (> 100 m) waters south of the Scott Islands). Living carbonate benthos are scattered and generally sparse, occupying specific ecologic niches, and the shelf-wide rate of carbonate production is low. Skeletons are fragmented, transported and mixed during storms and are concentrated within bedrock hollows and crevices, and shallow depressions between gravel ridges. Where infaunal bivalves are abundant the carbonates are dominated by aragonite, but otherwise the skeletal hashes are predominantly calcitic. Many grains, and especially aragonitic ones, are corroded and weakened by epilithic and endolithic bioerosion, and probably also by marine and dissolution. The most corroded shells have ages of only about 1000 yrs so that their preservation potential is low. The character of Scott shelf skeletal carbonate deposits reflects their temperate latitude, cold-water heritage.

Intercomparison of AMF VACM and Aanderaa current meters moored in fast tidal currents

Records between one and two months long have been obtained from two AMF VACM and four Aanderaa RCM's in a shallow sea wtih strong rectilinear tidal flows and weak residuals. The tidal currents and the residual currents with a daily time scale as recorded by the two meter types were in good agreement. There was poor agreement for the overall mean currents.

The tidal physics of headland flows and offshore tidal bank formation

The general tidal characteristics in the neighbourhood of a promotory are derived using a numerical model developed in plane polar coordinates. These same tidal features are found for a model of the tides off the Start Point promontory using a Cartesian reference frame. It is shown that promontories are characterised by strong tidal flows and it is suggested that vorticity generated near the headland plays a key role in the dynamics of offshore tidal bank formation.

Shoreface-connected sand ridges on American and European shelves: A comparison

Large scale sand ridges, 10 m or more in height and 2–4 km apart have long been accepted as characteristic of shelves experiencing strong tidal flow. However, they also occur on inner continental shelves whose strongest flows are wind-induced. The Middle Atlantic Bight of North America is the best known example, but inner shelf ridge topographies occur extensively on other Atlantic shelves. We wish to call attention to similarities and differences among the inner shelf ridges of North America, South America, and Europe, as a necessary step prior to the framing and testing of hypotheses concerning the hydrodynamics of ridge formation. Inner shelf ridge fields occur primarily on low, unconsolidated coasts whose substrates provide an ample supply of sediment for ridge formation. The constituent materials are commonly coarse to fine sand, but on the Guiana coast of South America, similar ridges appear to be forming in a mud substrate. Here ridges have much greater spacing (tens of km versus km), less relief, and much flatter slopes (1,3000 versus 1,500). Inner shelf ridges are characteristically aligned obliquely to the shoreline, with acute angles opening into the prevailing flow direction. Downcurrent slopes tend to be steeper and finer grained. The ridges tend to migrate downcoast and offshore, extending their crestlines so as to maintain contact with the shoreface. Where ridges are nearly shore-parallel (current parallel), the movement is expressed as downcurrent growth of ridges and downcurrent extension of swales between ridges. The textural and morphologic asymmetry of inner shelf sand ridges can be explained in terms of existing models for sand wave formation. The oblique orientation with respect to the shoreline is also possibly compatible with these models. An alternative, or perhaps complementary explanation for sand ridge genesis requires helical flow structure in the shelf flow field during peak events, but both theory and observation are less well developed for this model. p]Inner shelf sand ridges appear to be responses to periods of intense flow induced by the passage of storms. The extent to which a given shelf sector develops ridge topography may depend on the efficiency with which the local water mass responds to storm passage. Ridge behavior, and especially ridge migration rates, are necessary information for marine environmental management, since ridges are most active in the nearshore zone where the shelf floor is used for sewage outfalls, deep water ports, nuclear reactor sites, and other structures requiring a measure of sea floor stability.

Wave and tidal current sorting of shelf sediments southwest of England

ABSTRACTThe relative roles of waves and tidal currents in transporting bottom sediment on the continental shelf off Lands End, southwest England, are evaluated by study of (a) sediment grain size in relation to boundary layer measurements in tidal currents, (b) regional variation in sediment parameters in relation to peak tidal and wave‐induced currents, and (c) visual observation of bedforms.(a) The sediments are mainly zoogenic sands. The average hydraulic equivalent median diameter is Mdφ=1.40φ (medium grade sand), and two‐thirds of the median grain sizes fall between 0.97φ and 1.83φ. The linear bottom current which will just move this range of sizes is exceeded only slightly by the highest tidal drag velocities Ū* measured in the area. Thus, sediment movement by tidal currents alone is restricted to areas of high bed roughness and strong peak tidal flows. In contrast, wave‐induced oscillatory currents at 100 m depth (typical of the area) attain sufficient speed to disturb the same particle sizes over 3% of the time. This includes storm periods when much greater velocities occur.(b) The average Mdφ of the sediment decreases southwest and northeast from south of the Lizard. This correlates well with the pattern of maximum tidal current speeds, suggesting that tidal currents control the areal distribution of sediment median grain size.Most sediments are well sorted (mean σi=0.48φ). Sorting improves at shallower depths but does not improve in areas of faster tidal currents, suggesting that wave‐induced currents exert the major control on sorting.Silt and clay proportions increase west of the Scilly Isles and are influenced by both wave and tidal currents.(c) Photographs and television pictures show that symmetrical bedforms due to wave action are dominant north and west of the western Channel. Asymmetric bedforms are more common in the western Channel itself, where tidal currents and bed roughness are both high.Results are used to construct a sediment transport model for the study area. Since medium grade well sorted sands occur in depths of over 100 m, many ancient, extensive, well sorted sand sheets may have been deposited at depths greater than previously suspected.

Recent Carbonate Sedimentation and Diagenesis in Walker Cay-Grand Cays Area, Little Bahama Bank: ABSTRACT

The Walker Cay-Grand Cays area is on the northwestern part of the Little Bahama Bank, 130 nautical mi northeast of Miami, Florida. This study was made to provide detailed data on the sediment changes that occur within a small, complex, shelf-edge carbonate area. On a typical traverse seaward across the area, the environments observed are: (1) shallow bank with muddy calcarenite; (2) oolite bars; (3) islands of lithified Pleistocene calcarenite, (4) patch reefs with a Gorgonacea, Montastrea, Agaricia, Diplora assemblage; (5) the fringing reef, largely dead and algal encrusted, except for scattered growth of Acropora and Millepora; (6) a deeper, forereef zone of live Montastrea with patches of coarse coralgal calcarenite; and (7) a rock platform with little sediment cover, lying at a depth of about 65 ft and sloping gradually seaward until it reaches a depth of 85-125 ft where it intersects the very steep continental slope. On traverses across these environments, 135 bottom samples were collected. The sand-size fractions of the samples were impregnated with polyester resin and thin-sectioned. The data resulting from point counts of these thin sections were analyzed by R and Q mode factor analysis. In the Q-mode analysis, 5 factors accounted for 96 percent of the variation in the data. Mapping of these factors led to a relatively simple pattern of sedimentation: a broad area of skeletal sand composed of variable percentages of Halimeda, mollusks, foraminifers, and pellets, separated in protected areas by carbonate mud, around the reefs by coralgal sand, and in areas of strong tidal flow, by sand bars of superficial oolite. Lithification of Recent carbonates occurs as: (1) beachrock; (2) subaerial crusts about 3 in. thick on beach ridges; (3) thin, well-indurated algal mats near the edge of a mangrove swamp; and (4) semilithified blocky submarine crusts on some oolite bars. Another diagenetic change in the sediment is micritization of the grains, particularly on some of the older stabilized oolite bars, where the oolitic coatings are being destroyed. This process, if continued, might lead to the misinterpretation of these deposits as pellet sands. End_of_Article - Last_Page 526------------

The Respiratory Currents in the Tracheal System of the Adult Honey-Bee

ABSTRACT The propodeal spiracles of the honey-bee remain closed in normal air but in air plus CO2 they open as the abdomen contracts, and close as the abdomen expands. The first thoracic spiracles remain open in normal air but in air plus CO2 they nearly close as the abdomen contracts, and open as the abdomen expands. When the bee is in normal air there is a strong tidal flow of air in and out of the first thoracic spiracles, but only a weak and variably directed stream of air through the body. It is deduced that when an active bee is producing metabolic CO2, directed streams of air enter the first thoracic spiracles and the abdominal spiracles and leave via the propodeal spiracles.