Tidal Current Speed Research Articles

Seasonal changes in oceanographic processes at an equatorial macrotidal beach in northern Brazil

Coastal waters influenced by the Amazon River are highly dynamic and productive environments, although few studies have focused on the seasonal changes in oceanographic processes, except for the Amazon plume. The aim of this paper is to evaluate the effects of short- (daily) and medium-term (seasonal) variations in meteorological and hydrodynamic conditions on hydrological variables in a sub-tidal zone located to the southeast of the Amazon estuary, in an area influenced by other rivers. The study was undertaken between November 2008 and September 2009. The month with the lowest fluvial discharge (November, dry season) was characterized by strong tidal currents and high significant wave heights, the highest salinity and pH, and the lowest dissolved nutrient concentrations and turbidity. During the equinoctial spring tide (March, rainy season), when discharge was increasing, similar significant wave height and tidal current speeds were recorded, with high tide elevations, and the highest concentrations of chlorophyll a and nitrate. The maximum fluvial discharge (June, rainy season) was characterized by the lowest hydrodynamic energy, salinity and pH, but also the highest silicate concentrations. During the other equinoctial spring tide (September, dry season), when discharge was decreasing, tides and tidal currents were similar to those recorded in March, whereas phosphate and nitrite concentrations were the highest recorded in the study. Overall, coastal circulation and hydrological variables shifted seasonally, responding primarily to atmospheric, hydrographic, and astronomical factors, including winds, waves, fluvial discharge, tides, and tidal currents. The study also confirmed that coastal areas to the southeast of the Amazon plume are highly dynamic and productive.

100 kW급 조류발전용 터빈의 성능에 관한 연구

As the problems of global warming are brought up recently, many skillful solutions for developing new renewable energy are suggested. One of the most remarkable things is ocean energy. Korea has abundant ocean energy resources owing to geographical characteristics surrounded by sea on three sides, thus the technology of commercialization about tidal current power, wave power is demanded. Especially, Tidal energy conversion system is a means of maintaining environment naturally. Tidal current generation is a form to produce electricity by installing rotors, generators to convert a horizontal flow generated by tidal current into rotating movement. According to rotor direction, a tidal current turbine is largely distinguished between horizontal and vertical axis shape. Power capacity depends on the section size crossing a rotor and tidal current speed. We therefore investigated three dimensional flow analysis and performance evaluation using commercial ANSYS-CFX code for an 100 kW class horizontal axis turbine for low water level. Then We also studied three dimensional flow characteristics of a rotating rotor and blade surface streamlines around a rotor. As a result, We found that torque increased with TSR, the maximum torque occurred at TSR 3.77 and torque decreased even though TSR increased. Moreover we could get power coefficient 0.38 at designed flow velocity.

Tidal transport through the Tsugaru Strait — part I: Characteristics of the major tidal flow and its residual current

The potential role of the tide-induced time-mean flow (the tidal residual current) in determining transport through the Tsugaru Strait (located between the East/Japan Sea and the North Pacific) is investigated using a high-resolution numerical barotropic model. The calculated K1, O1, M2, and S2 tidal fields agree well with available observational records derived from both tide gauge and current meter measurements in the strait and the adjacent seas. The tidal residual current speed reaches 0.3 ms−1 in two narrow “neck” areas where topographic sills are located. This result suggests that tides should be taken into account in estimating the long-term water mass and nutrient transport through narrow regions between the East/Japan Sea and the North Pacific. An interesting aspect of the tidal residual current field is the prediction of several active eddy zones in which sequences of eddy triplets develop in the vicinity of capes. Our vorticity analysis reveals that the interplay of topographic effects arising from both the headland and the sill around capes plays a critical role in the formation of these triple eddy patterns.

Performance of horizontal axis tidal current turbine by blade configuration

The west and south coastal region of Korea has very strong tidal current speeds and therefore accommodates many suitable sites for the application of TCP (Tidal Current Power). The maximum current speed recorded in the south is up to 6.5 m/s. Unlike other renewable energy sources, TCP is an extremely reliable, predictable and continuous energy source as the current pattern and speed can be predicted throughout the year. One of the essential components in a TCP device is the rotor converting the inflow current into the rotational energy. The design optimization of the rotor is very important to maximize the power production. The performance of the rotor can be determined by various parameters including the number of blades, shape, sectional size, hub, diameters etc. The blade of the rotor is one of the essential components which can convert tidal current energy into rotational energy to generate electricity. The variable blade properties determine the performance, efficiency and stability of the turbine system. This paper presents the design procedure for a 300 kW tidal current turbine blade. The HAT turbine model was designed based on the wind mill turbine design principles together with known turbine theories. To verify the compatibility of the turbine design method and to analyze the properties of design factors, the 3D CFD model was applied with the ANSYS CFX program. The characteristics and performances of the blades can be applied in the design of 300 KW and larger capacity TCP rotors.

Phase speciation of mercury (Hg) in coastal water of the Yellow Sea, China

The objective of this study is to investigate the phase distribution of mercury (Hg) in water and the relative importance of processes affecting its distribution and behavior at a coastal site of the Yellow Sea with high tidal current speeds in the eastern continental margin of China. Total Hg (THg) and reactive Hg (RHg) were measured in unfiltered and filtered (0.45 μm) water in four different seasons at 1–3 h intervals using ultra-trace Hg techniques. The Hg in this coastal water showed typical Hg-unpolluted coastal region levels. Mean (± SD) concentrations of THg in unfiltered water were 13.4 ± 3.9 pM with a range of 6.7 to 27.5 pM and a considerable RHg fraction (%RHg/THg = 36.9%). Particulate Hg (PHg) was the dominant phase in unfiltered water samples (%PHg/THg = 70 ± 11%), and the distribution coefficient (Log K d ) of Hg in particulate and dissolved (0.45 μm) phases averaged 4.96 ± 0.20 with a range of 4.26 to 5.38. Enhanced suspended particulate matter (SPM) in water induced by wind and tidal mixing probably explained most of the variation in the unfiltered THg. In the dissolved phase, RHg accounted for a major fraction of THg (%RHg/THg = 86 ± 16%). The weak correlation between dissolved organic carbon (DOC) and THg in the dissolved phase indicated that Hg in this study region might primarily originate from atmospheric deposition and also reflected the importance of other competing ligands (e.g., Cl −) in the water column. Interestingly, measurements covering four different seasons showed no seasonality in these Hg species. Although there was some uncertainty in the measurements, using the difference of RHg between unfiltered and dissolved samples revealed that a considerable fraction (28 ± 19%) of RHg could bind to SPM.

Habitat-forming cold-water corals show affinity for seamounts in the New Zealand region

Determining the distribution of habitat- forming scleractinian corals in the New Zealand region is necessary in order to understand the ecological significance of these taxa and the likely impact of an- thropogenic activities on their persistence. Historical records from early publications, research trawl surveys, commercial fishing bycatch and recent biodiversity sur- veys were compiled for the habitat-forming coral spe- cies Madrepora oculata, Solenosmilia variabilis, Gonio- corella dumosa, Enallopsammia rostrata and Oculina virgosa. These data were used to describe the observed depth, geographic distribution and geomorphic habitat associations of the study corals in the region. A boosted regression trees analysis was also used to identify which of 11 environmental variables best describe the distribu- tion of the 5 species across the New Zealand region, and to predict their spatial distribution. The contribution of the environmental variables differed greatly between species, but consistently identified depth and seamount occurrence as important factors de scribing observed coral species distribution. The models identified that M. oculata, S. variabilis and E. rostrata occurred in deep waters (>1000 m) where seabed slopes were steep, tidal current and orbital velocities were slow, sea surface pri- mary productivity was low, and where seamounts were generally present. By contrast, G. dumosa and O. vir- gosa were found in relatively shallower waters, where sea surface primary productivity was high and tidal current speeds were generally fast. Spatial predictions were consistent with the recorded observations and identified that all species, apart from O. virgosa, were distributed throughout the region a nd were found primarily between ~200 and 2000 m.

A wave‐driven jet over a rocky shoal

Field observations and model simulations are presented of flow generated by waves breaking over a shoal at the entrance to a shallow bay. The shoal is composed of a series of steep and narrow bedrock ridges with depths of 2–8 m at the ridge crests. Observations from instruments on the shoal indicated peak significant wave heights during a storm event were 2.4–4.0 m across the observation sites; this spatial variability is due to wave breaking over the ridges. The 2‐D depth‐averaged hydrodynamic model Delft3D, coupled to the wave model SWAN, was used to simulate the waves and wave‐driven flow over the shoal and throughout the entire bay with a nested fine grid (5 m resolution) to resolve the shoal bathymetry. The model predicts a well‐defined jet behind the shoal, with mean axial speeds of 0.4–0.7 m/s. The observations indicate maximum speeds behind the shoal of 0.3–0.4 m/s, exceeding the maximum tidal current speed by more than a factor of 4, that are consistent in timing and direction with the model predictions. The model overpredicts wave breaking over the steep slopes and as a result the current speeds are overestimated.

Retention-favorable timing of propagule release in barnacles and periwinkles

Recent studies on the ecology of marine larvae suggest that retention near coastal areas and self-recruitment are probably much more common than previously thought. In light of this, the advective potential of pelagic stages can be partially determined by the timing of propagule release. We sampled the upper-shore levels of a subtropical coastline in southeastern Brazil to examine the temporal patterns of propagule release for the common barnacle Chthamalus bisinuatus and the peri- winkle Nodilittorina lineolata. The release timing in both species was very consistent between sites a few kilometers apart. Barnacles released nauplii in a rhythmic pattern, mostly coinciding with neap tides, when the speed of tidal currents was lowest. There was no variation in propagule release in relation to diel or flood-ebb tidal cycles. Periwinkles released propagules in a very irregular pattern, which remarkably matched a time series of wave heights. Egg capsules were released during periods of rough seas, when onshore surface currents were expected to prevail. Eggs were released mostly during ebb tides, and there was no diel variation. Propagule release rhythms are usually viewed as a means to ensure fast offshore advection, thus avoiding presumed hazardous conditions for larvae. However, propagule release strategies in these upper-shore species would more likely contribute to the retention of offspring close to parental stocks.

Residency and migratory behaviour by adult Pomatomus saltatrix in a South African coastal embayment

Acoustic telemetry was used to study patterns of habitat use and movements of Pomatomus saltatrix L. (common name elf/shad/bluefish/tailor) within the Saldanha Bay with Langebaan Lagoon coastal embayment on the west coast of South Africa. Thirty six mature P. saltatrix were tagged with acoustic transmitters and released within the lagoon in May 2006 and November–December 2007, and their positions were monitored until late-November 2008 using 28 hydrophones positioned throughout the embayment. The detection pattern of P. saltatrix suggested a tendency to residence within the embayment throughout the thirty month long study period, with nearly 60% of released individuals only being detected within the lagoon in the inner part of the embayment. However, there was a long-term trend of movement from the lagoon into the bay. One individual was recaptured off the east coast of South Africa 21 months after being tagged, 1760 km away, suggesting that P. saltatrix are capable of undertaking long along-shore migrations. Over finer scales within the inner lagoon, P. saltatrix ground speed increased (1) with an increase in tidal current speed, (2) with an increase in photoperiod, and (3) during day. Pomatomus saltatrix tended to move seaward during ebb tides, and to occupy greater depths during day.

Modelling and control of a marine current turbine-driven doubly fed induction generator

This paper deals with the modelling and control of a variable speed doubly fed induction generator-based marine current turbine with and without tidal current speed sensor. The proposed maximum power point tracking control strategy relies on the resource and the marine turbine models that were validated by experimental data. The sensitivity of the proposed control strategy is analysed regarding the swell effect because it is considered as the most disturbing one for the resource model. Tidal current data from the Raz de Sein (Brittany, France) are used to run simulations of a 7.5-kW prototype over various flow regimes. Simulation results are presented and fully analysed.

Tidal movements of East Pacific green turtle Chelonia mydas at a foraging area in Baja California Sur, México

We tracked East Pacific green turtles Chelonia mydas using GPS-VHF telemetry in Estero Banderitas, a tidally-influenced foraging area in Bahia Magdalena, Baja California Sur, Mexico. Tidal currents were measured with a bottom-mounted Acoustic Doppler Profiler (ADP) and the data used to predict tidal current speed and direction at the location and time during which turtles were tracked. Twenty-nine turtles were tracked in the summers of 2000 to 2003. Vagility (mean ± SD; 18.6 ± 11.4 km d -1 ) and speed (0.83 ± 0.47 km h -1 ) of turtles was the greatest so far reported for green turtles at foraging areas. Turtles displayed highly linear movements, and move- ment patterns were circatidal. Vector correlation was used to compare turtle speed and direction with tidal speed and direction. Correlation coefficients were significant for 11 out of 13 tracks, indicating significant linear interdependence between turtles and tides. Speed and direction contributed equally to the correlation. Results indicated a new paradigm for green turtles in feeding areas, where turtles are transported on continual tides that allow them to exploit a patchy and seasonal distribution of algae, their main diet component. This tidal transport is markedly different than Selective Tidal Stream Transport, in which animals use either the ebb or flood tide for transport. Tidal currents may be an accurate indicator of turtle movement in tidal areas, and this transport system has implications for foraging ecology, energetics, and growth.

Sediment dynamic responses of coastal salt marsh to typhoon “KAEMI” in Quanzhou Bay, Fujian Province, China

In order to understand the mechanisms of coastal protection by salt marshes during typhoon events, in situ measurements of water level, tidal current speed and direction, and suspended sediment concentration (SSC) were carried out using Electromagnetic Current Meter (EMCM, AEM HR), miniature pressure sensor (MkV/D) and Seapoint Turbidity Meter (STM) sensor on a tidal flat in Quanzhou Bay, during the period when the typhoon “KAEMI” was passing through the region. The analysis of the data obtained shows that the near-bed current speed within the Spartina alterniflora marsh was generally below 5 cm s−1, which was apparently smaller than on the adjacent bare flat (i.e. 5–30 cm s−1). The change in the near-bed current speed in response to the typhoon event was not significant within the S. alterniflora marsh, but the current direction was influenced by the typhoon. The effect of the typhoon on the SSC was highly significant, with the SSC reaching 13 to 19 times the values on the bare flat or within the marsh under fair weather conditions; the near-bed SSC within the marsh was higher than on the bare flat, after the typhoon landed. The near-bed suspended sediment fluxes within the marsh and on the bare flat during the typhoon event were both enhanced, i.e., 4 times larger than under fair weather conditions. During the ebb, the bottom shear stress on the bare flat exceeded the critical shear stress for sediment motion for most of the ebb duration of the tide and, therefore, the bed sediment was eroding, with the erosion flux after the typhoon landed being around 2 to 3 times the value associated with fair weather conditions. In contrast, within the S. alterniflora marsh, the bottom shear stress was mostly lower than the critical shear stress for sediment motion, or lower than the critical shear stress for the maintenance of suspension; hence, the marsh surface was dominated by settling processes, with a settling flux during the typhoon being 3 to 6 times compared with the fair weather situations. The settling flux during the ebb was up to twice the magnitude for the flood. The comparison of sediment dynamic processes between the S. alterniflora marsh and the bare flat demonstrates that during the typhoon event the bare flat was dominated by erosion, whilst the presence of S. alterniflora favored the accretion of suspended sediment.

Tidal modification and its effect on sluice-gate outflow after completion of the Saemangeum dike, South Korea

This study examined tidal modification and change in tidal currents caused by the construction of the Saemangeum dike, based on field observations and a numerical model. The Saemangeum dike was completed in April 2006, enclosing an estuarine area along the mid-western coast of South Korea. After closure of the dike, the tidal range outside the dike decreased slightly but significantly, while the inside tidal range decreased drastically. The numerical model results show that the dike construction has influenced tidal energy propagation and the tidal system in the Yellow Sea. The tidal current speed near the dike decreased abruptly following closure of the dike, except in front of the sluice gates. Since completion of the dike, outflow water discharged from the sluice gates has longer residence times due to the weakened tidal current; the change in the tidal current field has also caused greater northward expansion of outflow water. The sluice gates release fresher water, which spreads over the sea surface mainly by inertial momentum near the gate; this water is then gradually mixed with sea water farther from the gate. The less saline, possibly more contaminated outflow impacts the marine environment near the Saemangeum dike. Controlling the discharge and gate-opening timing can partially mitigate these impacts on the marine environment.

Boils and Turbulence in a Weakly Stratified Shallow Tidal Sea

Abstract Measurements of turbulence are made in a weakly but variably stratified region of tidal straining in the eastern Irish Sea using turbulence sensors profiling vertically through the water column on the Fast Light Yo-yo (FLY) profiler and horizontally on the Autonomous Underwater Vehicle (AUV) Autosub. The tidal currents exceed 1 m s−1 at the location of the measurements in water of a depth of about 43.5 m, and result in turbulence extending from the seabed to the surface with a cycle period that is half that of the tides, as previously observed. The time of onset of enhanced turbulence that is measured by the sensors on FLY and Autosub as the speed of tidal currents increases are in good agreement, as are their mean levels. Boils on the sea surface are identified using the Autonomously Recording Inverted Echo Sounder, version 2 (ARIES II), a two-beam upward-pointing side-scan sonar mounted on a rig resting on the seabed. The boils have mean horizontal dimensions of about 25 m. They are continually present when turbulence within the water column near the surface is large, typically when the rate of dissipation of turbulent kinetic energy per unit mass ɛ exceeds about 3 × 10−6 W kg−1, compared to the rate of about 1 × 10−7 W kg−1 near times of slack tide when no boils are observed. The top of a region with a relatively high ɛ gradually extends upward from the seabed as the tidal flow increases and, in stably stratified conditions resulting from tidal straining during ebb tides, can be detected in the ARIES II sonographs as it approaches the sea surface, although only after sound reflection from the sea surface. Upward-moving bursts of enhanced turbulence below the surface with horizontal dimensions of about 5–9 m in the direction of the tidal flow are identified in the AUV record in periods of high tidal flow using conditional sampling. The bursts result in slight upward displacements of the AUV from its set operational depth. Boils first appear at times before the region of generally enhanced turbulence reaches the surface: large localized upward-moving bursts appear to precede the main advancing region of small-scale turbulence.

A sedimentological approach to P-A relationships for tidal inlet systems: an example from Yuehu Inlet, Shandong Peninsula, China

Power-law relationship between tidal prism (P) and the cross-sectional area of the entrance channel (A) is applicable to assess the equilibrium conditions of a tidal inlet system. The classic method of determining P-A relationships proposed by O’Brien depends on datasets from multi-tidal inlet systems, which has shown some limitations and is unable to assess equilibrium of a single tidal inlet. This paper focuses on establishing a new P-A relationship for a single tidal inlet. Our experimental result shows that in order to maintain the status, power n should be > 1, implying that the inlet width will narrow and current speed within the entrance will increase as tidal prism becomes smaller. A possible explanation for power n <,1.0, as many researchers argued before, is that the influence of tidal prism has been exaggerated. Meanwhile, the magnitude of coefficient C is dependent on many factors such as longshore drift, freshwater discharge, etc, resulting in a wide range of variation of C. It should be pointed out that P-A relationship given by the sediment dynamical approach is still a representative of average status for tidal inlets in equilibrium. As tide, wave, freshwater discharge and tidal inlet morphology change with time, actual P-A relationships will fluctuate also. The problems that need to be solved when applying sediment dynamic methods to P-A relationships include the cross-sectional distribution pattern of tidal current speeds in the entrance channel, the relationship between the tidal current and the tidal water level at the entrance, and the calculation of the ratio of width to depth. This paper will establish a sediment dynamical approach of P-A relationship for a single tidal inlet. The results are tested for P-A relationships of Yuehu Inlet, a small inlet-lagoon system located in Shandong Peninsula, China.

Form drag is a major component of bed shear stress associated with tidal flow in the vicinity of an isolated sand bank, Torres Strait, northern Australia

Tidal current and elevation data were collected from five oceanographic moorings during October 2004 in Torres Strait, northern Australia, to assess the effects of large bedforms (i.e., sand banks) on the drag coefficient ( C D ) used for estimating bed shear stress in complex shallow shelf environments. Ten minute averages of tidal current speed and elevation data were collected for 18 days at an on-bank site (<7 m water depth) and an off-bank site (<10 m). These data were compared to data collected simultaneously from two shelf locations (<11 m) occupied to measure regional tidal behaviour. Overall C D estimates at the on- and off-bank sites attained 7.0±0.1×10 −3 and 6.6±0.1×10 −3, respectively. On-bank C D estimates also differed between the predominant east–west tidal streams, with easterly directed flows experiencing C D =7.8±0.18×10 −3 and westerly directed flows C D =6.4±0.12×10 −3. Statistically significant differences between the off-bank and on-bank sites are attributed to the large form drag exerted by the sand banks on the regional tidal currents, and statistically significant differences between the westward and eastward flows is ascribed to bedform asymmetry. Form drag from the large bedforms in Torres Strait comprises up to 65% of the total drag coefficient. When constructing sediment transport models, different C D estimates must therefore be applied to shelf regions containing steep bedforms compared to regions that do not. Our results extend the limited inventory of seabed drag coefficients for shallow shelf environments, and can be used to improve existing regional seabed mobilisation models, which have direct application to environmental management in Torres Strait.

Modelling and Testing the Vertical Axis, Impulse Rotor Tidal Power Pontoon

The specification of a new tidal power pontoon is described. The device consists of a 500kW asynchronous generator connected through gearbox and drive shaft to a twelve bladed, vertical axis rotor 4.00m × 4.00m mounted at the impact point of a jet in a symmetrical Bernoulli duct beneath a 16.00m × 10.00m pontoon. Analytical models are developed for (i) the flow through the duct, (ii) the torque and speed of the rotor and (iii) the efficiency as functions of the device’s dimensions and the tidal current speed. Results from computational fluid dynamic simulations confirm the principles of the theory and emphasise the importance of the duct as both accelerator and diffuser. The overall efficiency is shown to depend upon the tip speed ratio of the blade to the jet velocities. Results from laboratory experiments with a 1/40th scale model in a unidirectional flume support the theoretical considerations, provide empirical relationships for the device’s efficiency as a function of the jet Reynolds Number and confirm that the optimum TSR lies in the range 0.45–0.50.

Boils and turbulence in a weakly stratified shallow tidal sea

Measurements of turbulence are made in a weakly but variably stratified region of tidal straining in the eastern Irish Sea using turbulence sensors profiling vertically through the water column on the Fast Light Yo-yo (FLY) profiler and horizontally on the Autonomous Underwater Vehicle (AUV) Autosub. The tidal currents exceed 1 m s−1 at the location of the measurements in water of a depth of about 43.5 m, and result in turbulence extending from the seabed to the surface with a cycle period that is half that of the tides, as previously observed. The time of onset of enhanced turbulence that is measured by the sensors on FLY and Autosub as the speed of tidal currents increases are in good agreement, as are their mean levels. Boils on the sea surface are identified using the Autonomously Recording Inverted Echo Sounder, version 2 (ARIES II), a two-beam upward-pointing side-scan sonar mounted on a rig resting on the seabed. The boils have mean horizontal dimensions of about 25 m. They are continually present when turbulence within the water column near the surface is large, typically when the rate of dissipation of turbulent kinetic energy per unit mass ε exceeds about 3 × 10−6 W kg−1, compared to the rate of about 1 × 10−7 W kg−1 near times of slack tide when no boils are observed. The top of a region with a relatively high ε gradually extends upward from the seabed as the tidal flow increases and, in stably stratified conditions resulting from tidal straining during ebb tides, can be detected in the ARIES II sonographs as it approaches the sea surface, although only after sound reflection from the sea surface. Upward-moving bursts of enhanced turbulence below the surface with horizontal dimensions of about 5–9 m in the direction of the tidal flow are identified in the AUV record in periods of high tidal flow using conditional sampling. The bursts result in slight upward displacements of the AUV from its set operational depth. Boils first appear at times before the region of generally enhanced turbulence reaches the surface: large localized upward-moving bursts appear to precede the main advancing region of small-scale turbulence.

Sediment transport under wave and current combined flows on a tide-dominated shoreface, northern coast of France

Hydrodynamic circulation and sediment transport patterns were investigated on a macrotidal shoreface off the southern North Sea coast of France from in situ directional wave and current measurements and through numerical modeling of sediment flux. Current meters equipped with wave gauges were deployed at two sites on the mid-upper shoreface, in approximately 5–6 m water depth, during 6 and 15 days. Sediment flux was estimated using the SEDTRANS96 numerical model (Li, M.Z., Amos, C.L, 2001. SEDTRANS96: the upgraded and better calibrated sediment transport model for continental shelves. Comp. Geosc. 27, 619–645) based on Grant and Madsen's (Grant, W.D., Madsen, O.S., 1986. The continental shelf bottom boundary layer. Ann. Rev. Fluid Mech. 18, 265–305) combined flow boundary layer theory. The hydrodynamic measurements and modeling results show that sediment on the shoreface is essentially transported alongshore in response to shore-parallel tidal currents. The direction of the net sediment transport is largely determined by the asymmetry of the tidal flows, which can be flood- or ebb-dominated, but during important wind speed events, winds blowing in the same direction as tidal flow can significantly reinforce tidal currents, or conversely, can limit tidal current speeds when acting in the opposite direction. Although sediment transport on this macrotidal shoreface is strongly controlled by tidal processes, waves can significantly enhance bed shear stresses, resulting in increased sediment remobilization, and hence higher sediment flux. Wave-induced onshore sediment transport may occasionally occur, but is several orders of magnitude lower than longshore transport. This macrotidal shoreface is thus characterized by a hydraulic regime strongly dominated by shore-parallel tidal flows that favour longshore sediment dispersal rather than shore-normal transport. Our results imply that the water depths from which sediment may be transported shoreward to beaches is certainly much more restricted on a macrotidal shoreface, such as the one bordering the northern coast of France, than on micro- and mesotidal shoreface environments where weaker tidal currents do not significantly limit cross-shore sediment movements.

Spatial and temporal variability in juvenile bivalve dispersal: effects of sediment transport and flow regime

Many species of benthic marine invertebrates, including bivalves, continue to disperse as juveniles (post-settlement). This dispersal has the potential to alter patterns set up at the time of settlement. Great spatial and temporal variability in rates of dispersal of juvenile bivalves has been observed in the field. We made synoptic measurements of current speeds and rates of bedload trans- port of sediment and dispersal of juvenile bivalves in the Navesink estuary, New Jersey, USA to examine the contribution of spatial and temporal variation in current speed to dispersal patterns. Daily rates of juvenile bivalve dispersal were high and varied strongly across sites. Bivalve dispersal was positively related to rates of sediment transport and current velocities, but not significantly to ambient density of bivalves. Variability in bivalve dispersal across dates was considerably less than that across sites, likely because tidal current speeds varied less by date than by site. The strong rela- tionship between bivalve dispersal and sediment transport and velocity suggests that dispersal of the bivalve species in this estuary is initiated by sediment transport.