secondary circulation in a region of flow curvature relationship with tidal forcing and river discharge 1 on the basis of a 301 day acoustic doppler current profiler mooring in an estuarine tidal strait the strength and structure of secondary circulation in a region of flow curvature is related to variations in tidal forcing and river discharge during low flow conditions the structure of secondary flow is consistent with a centrifugally forced helical flow with bottom flow toward the inside of the bend and surface flow toward the outside of the bend the strength of secondary flow increases linearly with tidal range and is consistent with a vertical eddy viscosity that is linearly dependent on tidal current speed during times of high river discharge the strength of secondary flow is significantly reduced and its vertical structure undergoes a fundamental change over the spring neap cycle during spring tides the classic helical flow pattern is evident albeit weaker than during low flow conditions however during neap tides a more complex two cell structure is evident the change between these two states occurs with a spring neap transition in the subtidal flow indicating that it is also accompanied by changing stratification simple scaling analysis suggests that during weakly stratified conditions secondary circulation will influence stream wise dynamics and dispersion for channels with widths on order or less than 0 1 h cd where h is the water column depth and cd is a quadratic bottom drag coefficient in contrast during highly stratified conditions lateral excursions due to secondary flows are limited to approximately one tenth of the channel s width and are an ineffective lateral mixing agent

observations of stratified turbulent mixing in an estuary neap to spring variations during high river flow abstract observations of turbulent mixing stratification and currents were made with a free falling microstructure profiler and a 600 khz adcp in the hudson river off manhattan in may 1994 the ship was anchored in a relatively straight and uniform channel about 15 m deep over 4 days of intense observations from 19 to 24 may 1994 the progression from neap tides to spring tides caused a severe reduction in stratification from an initial top to bottom salinity difference of 18 to 3 6 while low gradient richardson numbers ri occurred only in the weakly stratified bottom layer on flood during neap tides low ri spanned the water column on the second half of ebb during spring tides turbulent dissipation rates varied roughly inversely to ri during neap conditions turbulent mixing was intense only in the bottom layer on flood where eddy diffusivities k reached more than 10 2 m 2 s 1 while mixing was weak in the centre of the halocline near 5 m during spring tides strong mixing occurred throughout the water column during the latter part of ebb with typical k 5 10 3 m 2 s 1 and typical vertical salt flux of 0 0015 kg m 2 s 1 if it existed in isolation this flux would eliminate the haline stratification on a time scale of 4 h ebb time bands of low ri and intense mixing showed increasing phase lag with increasing distance from the bottom the spring tide ebb time internal turbulent stress was relatively uniform with depth with a typical magnitude of 0 4 pa 4 dyne cm 2 turbulent overturning thorpe scales ranged from below 1 cm in the neap tide halocline to a few metres in the weakly stratified bottom layer on flood the strong spring tide ebb time mixing had thorpe scales of typically 0 5 m throughout the water column ozmidov scales tracked thorpe scales except in the weakly stratified layer above the bottom

on the nature of transverse variability in a coastal plain estuary even though estuarine processes are inherently three dimensional most of the previous studies have focused primarily on the axial longitudinal and vertical variabilities recent surveys in the lower delaware bay indicate the presence of significant transverse variability in the salinity distribution and residual circulation pattern conceptual models are developed to examine the mechanisms which might explain the observed transverse structure results indicate that both the gravitational effect and wind forcing are capable of generating substantial transverse structure in drowned river valley estuaries with triangular cross sections

the effects of channels and shoals on exchange between the chesapeake bay and the adjacent ocean the role of bathymetric changes in determining the transport of water and salt in the lower chesapeake bay lcb was investigated using high resolution acoustic doppler current profiler adcp and conductivity temperature depth profiles a cross channel transect was repeated eight times during neap tides on october 6 7 1993 which illustrated the lateral structure of the longitudinal and transverse flow fields and the intratidal variations in the flow structure across the lcb amplitude and phase of the m2 tidal component as well as the mean flow velocity were calculated using least squares fitting at every point of a uniform grid obtained from the adcp data the results differ from the classical two layer pattern of estuarine circulation modified by coriolis effects but are consistent with recent hydrographic observations semidiurnal flow was highest over the navigational channels and lateral gradients were strongest in regions of sharp bathymetric changes the phase lag of the semidiurnal flow also showed lateral and vertical gradients that represented advances at the bottom with respect to the surface and over the shoals in relation to the channels the section of the water column measured indicated a mean outflow of 0 7 104 m3 s and a mean inflow of 1 3 104 m3 s the apparent gain of water by the estuary during the period of observation can be explained by meteorologically forced net barotropic inflow the depth averaged mean longitudinal flow consisted of inflow in the navigational channels and outflow over the shoals the mean transverse flow showed near surface convergence over the channels we propose a possible explanation for the observed flow and density structure as follows the barotropic and baroclinic forcing interact with the bathymetry to extend the inflow from the bottom to the surface thereby inducing a transverse circulation that yields near surface convergence over the channels

modeling influence of stratification on lateral circulation in a stratified estuary abstract the dynamics of lateral circulation in the passaic river estuary is examined in this modeling study the pattern of lateral circulation varies significantly over a tidal cycle as a result of the temporal variation of stratification induced by tidal straining during highly stratified ebb tides the lateral circulation exhibits a vertical two cell structure strong stratification suppresses vertical mixing in the deep channel whereas the shoal above the halocline remains relatively well mixed as a result in the upper layer the lateral asymmetry of vertical mixing produces denser water on the shoal and fresher water over the thalweg this density gradient drives a circulation with surface currents directed toward the shoal and the currents at the base of the pycnocline are directed toward the thalweg in the lower layer the lateral circulation tends to reduce the tilting of isopycnals and gradually diminishes at the end of the ebb tide a lateral baroclinic pressure gradient is a dominant dri

secondary circulation in a highly stratified estuary observations from the hudson river estuary are presented to discuss the interaction between secondary flows and stratification in the vicinity of a headland during highly stratified conditions analysis of the cross stream momentum balance suggests that centrifugal accelerations are in near balance with the cross stream baroclincity results emphasize that the interaction between stratification and secondary flows can set up a cross stream internal seiche this results in a reduction and subsequent reversal of the secondary flows downstream of a headland simple scaling arguments suggest that the spin down time for tidally driven eddies is related to stratification

the dynamics of a partially mixed estuary measurements of velocity density and pressure gradient in the lower hudson river estuary were used to quantify the dominant terms in the momentum equation and to characterize their variations at tidal and spring neap timescales the vertical momentum flux assumed to be due mainly to turbulent shear stress was estimated indirectly based on the residual from the acceleration and pressure gradient terms the indirect estimates of stress compared favorably to bottom stress estimates using a quadratic drag law supporting the hypothesis that the tidal momentum equation involves a local balance between tidal acceleration pressure gradient and stress divergence estimates of eddy viscosity indicated that there was significant tidal asymmetry with flood tide values exceeding ebb values by a factor of 2 as a consequence of the asymmetry the vertical structure of the tidally averaged stress bore no resemblance to the tidally averaged shear in spite of the asymmetry of vertical mixing the tidally averaged estuarine circulation was found to depend simply on the intensity of bottom turbulence which could be parameterized by a rayleigh drag formulation based on the tidal velocity magnitude and the tidally averaged near bottom flow this seemingly paradoxical result indicates that the estuarine circulation can be modeled without detailed knowledge of the effective eddy viscosity only requiring an estimate of the bottom drag coefficient the tidal forcing conditions and the baroclinic pressure gradient a notable characteristic of this solution is an inverse dependence of the estuarine circulation on the amplitude of the tides

axial convergence in a well mixed estuary abstract transverse secondary circulations involving surface convergence observed in a well mixed estuary in north wales are made visible by the collection of surface material along an axial line which extends continuously for many kilometres through the estuary the circulation and axial convergence however are seen only during the flood phase of the tide and no similar behaviour has been observed during the ebb phase convergent circulations in the estuary are associated with small but steady transverse density gradients in the cross section produced by non uniform advection of the longitudinal gradient through the channel a diagnostic model using measured mean distributions of cross sectional density indicates surface transverse velocities 0 1 ms 1 similar to those observed in the estuary the model further predicts appreciable transverse divergent currents at a fractional depth of 0 75 a prediction which has been tested in the estuary using a vertical array of accurately resolving current direction indicators

lateral circulation generates flood tide stratification and estuarine exchange flow in a curved tidal inlet abstractcross channel transect measurements of microstructure and velocity in a well mixed and curved tidal inlet in the german wadden sea show the occurrence of significant late flood stratification this stratification is found to be a result of lateral straining this study observes a strong single cell lateral circulation which is strongly pronounced at late flood and absent during most of ebb this tidal asymmetry is caused by a systematic interplay between centrifugal forcing and the lateral baroclinic pressure gradient during flood a positive feedback between the terms generates strong lateral circulation whereas during ebb a negative feedback leads to a suppression of the cross channel exchange a theoretical framework based on vorticity is developed which allows lateral and longitudinal circulation to be studied in a consistent way with this framework it is possible to show that the tidal asymmetry of the lateral flow is a major driver of residual longitudinal estuarine circulation here ide

lateral circulation and suspended sediment transport in a curved estuarine channel winyah bay sc usa 1 shipborne vertical profiles of flow and suspended sediment concentration collected on a transect across a curved nonsymmetrical estuarine channel are presented analysis of the transient cross channel momentum balance equation shows that the lateral circulation pattern is controlled by the interaction between centrifugal and lateral baroclinic forcings although those two might not be necessarily in balance as suggested earlier by seim and gregg 1997 instead differential along channel advection and local acceleration appear to influence greatly lateral circulation dynamics during ebb when the water column is highly stratified the interaction between centrifugal acceleration and opposite directed lateral baroclinic forcing results in weak lateral flows during flood lateral flows are dominated by centrifugal acceleration which is directed toward the outside of the curvature at the mid depth because of the nonlogarithmic current profile and reinforced by lateral baroclinic forcing this results in strong two layer clockwise circulation looking up estuary during flood the eastward directed bottom currents during ebb deliver only a small amount of suspended sediment from the relatively narrow western shoal to the channel bed during flood the west directed near bed currents deliver a significant amount of sediments from the gentle broad eastern shoal which in conjunction with the locally resuspended sediment load promotes the development of the estuarine turbidity maximum increased lateral advection of sediments during flood reinforces a tidal asymmetry in the development of turbidity maximum decomposition analysis of lateral sediment fluxes averaged over a tidal cycle suggests convergence of sediment toward the center of the channel is driven mainly by the oscillatory tidal component

numerical modelling of the sydney harbour estuary new south wales lateral circulation and asymmetric vertical mixing abstract a fully calibrated three dimensional hydrodynamic model of the sydney harbour estuary was used to determine the dominant forcing of regulating estuarine circulation in a sinuous channel under conditions of low river discharge during dry weather the system characterized by stratified flood and mixed ebb during spring tides and stratified flood and ebb during neap tides was analysed using model results of current profiles density field eddy viscosity and gradient richardson number at a mooring station and cross channel transect the magnitude and direction of lateral circulation was modulated in the flood ebb and spring neap cycles due to changes in the interactions between the tides and the complex geometry during spring tides a differential advection induced lateral baroclinic pressure gradient tends to drive the lateral circulation and stratifies the water column when the along estuary circulation is strong enough to break the stratification induced by the lateral baroclinic pressure gradient centrifugal force works in concert with nonlinear advection establishing a classical two layer helical flow across the channel channel bends also induce vertical mixing as a result of the overturning of the density fields the lateral circulation redistributes the differential advection induced lateral shear of the along estuary momentum and is transferred to either reinforce or cancel the along estuary circulation the lateral bathymetry variability in a channel is therefore the key factor causing the intratidal asymmetries in the along estuary circulations stratification suppresses lateral circulation during neap tides and generates a three layer lateral circulation structure in the water column during stratified ebb

water and suspended sediment division at a stratified tidal junction 1 tidal junctions play a crucial role in the transport of water salt and sediment through a delta distributary network water salt and sediment are exchanged at tidal junctions thereby influencing the transports in the connecting branches and the overall dynamics of the system this paper presents observations of water salt and sediment transports in three channels that connect at a stratified tidal junction flow variation in one channel was found to lag behind flow variation in a connected channel by more than 1 h which is largely attributed to channel length differences from the junction to the sea the water columns in the three channels were periodically stratified during spring tide whereas the salinity structure represented a salt wedge during neap tide salinity differences between the three channels were substantial the channels contain water bodies of different salinity and act largely independently flow velocities in the upper and lower layers differed substantially flow in the lower layer was generally in the direction of acceleration produced by the baroclinic pressure gradient interestingly baroclinic pressure gradients were sometimes directed landward indicating the presence of saltier water at the land side of the estuary in sharp channel bends close to the junction secondary flow was strongest at the highest axial flow velocity during spring tide in one channel bend these circulations steered the suspended sediment toward the inner bend which affected the suspended sediment division

near bottom turbulence measurements in a partially mixed estuary turbulent energy balance velocity structure and along channel momentum balance abstract a set of moored bottom mounted and shipboard measurements obtained in a straight section of the lower hudson estuary during late summer and early fall of 1995 determine velocity density and along channel pressure gradient throughout the 15 m water column as well as providing direct eddy correlation estimates of reynolds stress and indirect inertial range estimates of dissipation within 3 m of the bottom the analysis focuses on testing 1 a simplified turbulent kinetic energy equation in which production balances dissipation 2 the prandtl karman law of the wall which is a relationship between bottom stress and near bottom velocity gradient and 3 a simplified depth integrated along channel momentum balance involving local acceleration pressure gradient and bottom stress estimates of production and dissipation agree well throughout the entire record the relationship between bottom stress and velocity gradient is consistent with the law of the wall within approximately 1 m of the sea

turbulent stresses and secondary currents in a tidal forced channel with significant curvature and asymmetric bed forms acoustic doppler current profilers are deployed to measure both the mean flow and turbulent properties in a channel with significant curvature direct measurements of the reynolds stress show a significant asymmetry over the tidal cycle where stresses are enhanced during the flood tide and less prominent over the ebb tide this asymmetry is corroborated by logarithmic fits using 10 min averaged velocity data a smaller yet similar tendency asymmetry in drag coefficient is inferred by fitting the velocity and estimated large scale pressure gradient to a one dimensional along channel momentum balance this smaller asymmetry is consistent with recent modeling work simulating regional flows in the vicinity of the study site the asymmetry in drag suggests the importance of previously reported bed forms for this channel and demonstrates spatial and temporarily variations in bed stress secondary circulation patterns observed in a relatively straight section of channel appear driven by local curvature rather th

three dimensional tidal flow around headlands field measurements of tidal flow around a headland indicate secondary circulation induced by flow curvature the secondary flow defined to be the flow in the plane normal to the direction of the vertically averaged current is directed toward the headland near the bottom and seaward near the surface consistent with theoretical predictions the strength of the secondary flow varies from 5 to 15 of the streamwise flow it is strongest when the water column is stratified because of both the enhanced shear of the streamwise flow and the reduced frictional damping of the secondary flow the transverse exchange accomplished by the secondary flow significantly influences the structure of the streamwise flow causing a broadening of the transverse shear and changing the horizontal and vertical distributions of momentum it may also have a significant influence on horizontal dispersion in the vicinity of headlands

the importance of aspiration and channel curvature in producing strong vertical mixing over a sill on the basis of observations of the time dependent tidally forced flow over a long sill we find that aspiration and channel curvature set the flow structure and condition the flow to allow intense vertical mixing aspiration reduces the potential energy of the water column by thinning it while maintaining its density contrast channel curvature induces a cross channel circulation that can rapidly overturn a stratified flow eighteen along channel sections of density velocity and dissipation rate of turbulent kinetic energy were collected in and around the tacoma narrows of puget sound a site suspected of driving a strong vertical circulation in adjoining main basin rapid inflow to the narrows on flood from one channel of a triple junction reduces dynamic pressure allowing dense water from below sill depth to be uplifted or aspirated into the narrows we estimate water from below 150 m 3 times the sill depth is drawn into the narrows on a 3 m flood tide once in the narrows the flow remains stratified until it passes a 50 bend where a strong secondary circulation overturns the 50 m deep water column and generates intense turbulent mixing cross channel velocities of up to 0 4 m s 1 are observed and maximum values of exceed 10 3 w kg 1 upon leaving the sill stratification is reestablished and turbulence decays a similar set of sequences occurs on ebb except that the outflow bypasses the flood inflow channel and instead discharges into colvos passage the third branch of the triple junction colvos passage ultimately discharges the ebb effluent back into main basin enhancing the impact of mixing at the narrows by discharging the mixed product far from the source scaling of the cross channel momentum equation suggests that below a threshold value of along channel velocity stratification should suppress secondary circulation for a given vertical shear radius of curvature and channel width above the threshold velocity the magnitude of the cross channel velocity is roughly consistent with predictions for unstratified flow we estimate the maximum effective eddy diffusivity that aspiration and mixing in the narrows can produce in main basin to be 10 3 m2 s 1

frictional estuarine flow in juan de fuca strait with implications for secondary circulation estuarine flow in juan de fuca strait is highly seasonal in nature and an appreciable along channel surface pressure gradient is coincident with the peak fraser river discharge an upper layer momentum balance requires the vertical eddy viscosity av to be of the order of 0 02 m2 s 1 slightly larger than implied by conventional empirical formulae and used in current models mixing in an estuarine channel and a large vertical eddy viscosity should lead to significant cross channel flows this secondary circulation which may resemble internal ekman layers and intrusions of water mixed at the sloping sides of the strait is much less understood than the basic along channel estuarine exchange unfortunately historical current meter data in juan de fuca strait are of insufficient spatial resolution to show the predicted patterns

the synoptic regulation of dryline intensity abstract to investigate the role of synoptic scale processes in regulating the strength of the dryline a dataset is constructed of all drylines occurring within the west texas mesonet wtm during april may and june of 2004 and 2005 in addition dewpoint and wind data were collected from stations on the western morton mort and eastern paducah padu periphery of the wtm domain 230 km across generally oriented east west across the typical location of the dryline in west texas drylines were characterized by two variables the difference in dewpoint between mort and padu hereafter dryline intensity and the difference in the eastward component of the wind between mort and padu hereafter dryline confluence a high degree of correlation existed between the two variables consistent with a strong role for dryline confluence in determining dryline intensity some cases departing from the strong correlation between these variables represent synoptically quiescent drylines whose strength is likely

cycle of vertical and horizontal mixing in a shallow tidal creek 1 an experimental study of vertical mixing and along channel dispersion parameterized in terms of horizontal mixing near the mouth of the duplin river a tidal creek bordered by extensive intertidal salt marshes on sapelo island georgia was carried out over several spring neap cycles in the fall of 2005 vertical mixing is modulated on both m4 and fortnightly frequencies with maximum turbulent stresses being generated near the bed on periods of maximum flood and ebb and propagating into the water column showing a linear dependence with depth values are significantly greater on spring tide than on neap horizontal mixing evaluated by salt fluxes is driven and dominated by tidal dispersion which is also modulated by the fortnightly spring neap cycle net export of salt from the lower duplin is shown to be due to residual advection modified by upstream tidal pumping the tidal dispersion coefficient exhibits a pulsating character with greater values on spring tide followed by smaller values on neap tide