shear instability in a highly stratified estuary abstract shear instability is found to be the principal mechanism of vertical exchange within the pycnocline of a salt wedge estuary a field program involving high resolution velocity and density measurements as well as high frequency acoustic imagery allowed direct comparison of instantaneous richardson number distributions to the occurrence of shear instability the theoretical stability threshold of 0 25 is consistent with the measurements based on estimates of gradients that contain the mean as well as fluctuations due to internal waves an effective stability threshold based on mean gradients is found to be approximately one third reflecting a significant contribution of internal wave shear the integral effect of the mixing process is to homogenize the gradients of velocity and density producing linear profiles of these quantities across the pycnocline a turbulent prandtl number of unity is suggested by the vertical distributions of velocity and density during periods of active vertical mixin

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

mixing at camarinal sill in the strait of gibraltar during the gibraltar experiment in 1985 1986 observations of shear stratification microstructure and acoustic backscatter revealed a complex mixing regime at camarinal sill in the strait of gibraltar the mixing is forced by strong semidiurnal tidal currents and weaker but more steady baroclinic flows flows are characterized as outflow or inflow nearly equivalent to westward or eastward rising water at gibraltar coincides with outflow at all depths we observed two modes of outflow at the sill in may 1986 the transition layer separating atlantic and mediterranean water was between 0 75 mpa and 1 mpa when we sampled east of the sill near neap tide the transition turned down just before reaching the sill crest where the flow became critical increasing shear soon destabilized the transition producing billows which grew to 30 75 m on the west flank of the sill the overturning generated intense turbulence with peak dissipation rates e exceeding 10 2 w kg 1 this turbulence rapidly thickened the transition from z 30 50 m east of the sill to z 130 150 m within 1 2 km west of the sill from the rate of thickening we estimate the rate of formation of new transitional water during neap tide as 0 8 1 2 105 m3 s 1 this outflow mode did not release eastward propagating internal bores when outflow turned to inflow but the inflow did appear to arrest the deep outflow in may the largest average dissipation rate over and just west of the sill had the same magnitude as estimates of the net energy lost by adjustment of velocity and mass across the sill during outflow other averages are smaller than the energy loss presumably because the limited sampling severely underestimates the average dissipation rate during outflow strong shears were observed to rise to 100 times those in the background internal wave field however probably owing to spatial offsets between the ship s acoustic doppler current profiler and our tethered free fall profiler the only statistically significant correlations were found between overturning scales measured directly and those estimated from e in the same profile we observed the other outflow mode in october 1985 when we profiled over the sill near spring tide and the transition was east of the sill centered near 1 5 mpa after encountering the sill the transition rose sharply and remained within 10 20 m of the surface across the sill plunging steeply downward just past the west edge of the sill this was accompanied by intense turbulence and at least one rebound of the transition we did not observe release of eastward propagating bores when inflow turned to outflow but observations by armi and farmer 1988 in april 1986 found bores released at spring tide in regimes similar to the one we observed in october 1985

the vertical structure of turbulent dissipation in shelf seas abstract the free fall fly profiler has been used to determine the variation in energy dissipation in the water column over a tidal cycle at mixed and stratified sites in the irish sea it was found that exhibits a strong m4 variation with a pronounced phase lag that increases with height above the bed in mixed conditions this m4 signal which extends throughout the water column is reasonably well reproduced by turbulent closure models of the vertical exchange in the summer stratified situation the m4 signal in is confined to about 40 m above the seabed with phase delays of more than 4 h relative to the seabed the lowest levels of dissipation 10 5 w m 3 measured in the pycnocline are significantly above the system noise level and much higher than predicted by a model using the mellor yamada level 2 closure scheme my2 0 however when allowance is made for the diffusion of tke the model my2 2 simulates the depth time distribution of dissipation in the stratified case satisfactorily if

coastal evolution changes in sea level and molluscan fauna in northeastern argentina during the late quaternary abstract the northern argentine shoreline underwent a remarkable progradation during the last 6890 years equivalent to about 4000 km 2 the shoreline moved approximately 30 km seaward on the average two types of accretionary shorelines were formed high energy environments produced barrier islands and barrier spits which began to form approximately synchronously low energy environments located between those of high energy consisted of tidal flats and salt marshes four groups of molluscan species represent the high energy palaeoenvironments of the barrier islands and barrier spits the other fossils represent a low energy environment of tidal flats the formation of the barrier islands began 6000 years b p it ended 3500 years ago due to a slight fall in relative sea level of 1 5 m above m s l and subsequent formation of barrier spits seaward as a consequence a very rapid faunal change occurred in less than 5000 6000 years the dominant high energy fossil fauna was replaced by a modern low energy fauna over most of the area palaeoenvironmental changes were probably greater and more rapid during the high energy episode when the fossils were more varied than those living now in the same area at samborombon bay whilst along the oceanic coastal area the molluscan fauna increased in diversity to the more varied extant one

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

microstructure observations of turbulent mixing in a partially mixed estuary part ii salt flux and stress abstract turbulent mixing in the water column was observed with a microstructure profiler in the hudson river estuary during two cruises in summer and fall 1995 the focus is on the estimation of turbulent salt flux and turbulent stress from measured viscous dissipation rates e and on the tidal and fortnightly variability of these fluxes in estimating eddy viscosity km and eddy diffusivity k the authors follow measurement modeling techniques of busch and osborn while prescribing a variable flux richardson number rf dependent upon the gradient richardson number ri it is argued that a steady state production dissipation balance holds in the turbulent kinetic energy budget all turbulence characteristics varied strongly over semidiurnal tidal cycles and over the fortnightly cycle subject to complications arising from nontidal flows the strongest mixing occurred during flood on neap tides and during ebb on spring tides in the lower part of the water column during floods and spring ebb km

marine molluscs and late quaternary environmental studies with particular reference to the late glacial period in northwest europe a review abstract although it is now commonly thought that temperature is a minor factor in limiting the distribution of marine invertebrates particularly on a local scale there is evidence to suggest that some boreal molluscs particularly those which occur in similar environments on both sides of the north atlantic are valuable as oceanic temperature indicators this applies particularly to planktotrophic species whose northern limits of distribution are apparently controlled by summer surface temperature the oceanography of the eastern north atlantic is briefly considered in relation to zoogeographic subdivisions based on temperature and it is concluded that the latter are fairly crude and apply only to shelf seas most glaciomarine molluscan faunas are not specific to that environment an exception being that dominated by the bivalve portlandia arctica the role of marine molluscs in palaeoenvironmental research at the local level for the 13 11 ka bp period is illustrated by a site at inchinnan scotland it is concluded that a shell bed at this locality consists partly of a little modified life assemblage and partly of shells which have been gently reworked under conditions of generally rapid deposition during the same time interval the changing composition of molluscan faunas on the nw european seaboard suggests that a weak drift of north atlantic water replaced polar water at or a little later than 13 ka bp off scotland and a few hundred years later off northern norway evidence for relatively warm intervals early and late in the 13 11 ka bp period has been found particulary on the scottish coast and to a lesser extent more generally the later warm interval may be related to the southward movement of the polar front at the beginning of the younger dryas the value of the bivalve shell itself in the determination of temperature seasonality and calendar years is briefly discussed

legacy impacts of acid sulfate soil runoff on mangrove sediments reactive iron accumulation altered sulfur cycling and trace metal enrichment abstract the off site impacts of iron rich acidic runoff from drained acid sulfate soils ass on the geochemistry of estuarine sediments are relatively poorly constrained in this study we examine the geochemistry of intertidal zone mangrove sediments and porewaters from an estuarine tributary that received 15 20 years of acute ass drainage compared to adjacent control sites mixing of acidic iron rich ass drainage waters with circumneutral ph estuarine waters drove precipitation of fe iii floc and has led to significant localized accumulation of reactive fe species fer 1 m hcl and citrate buffered dithionite extracts up to 4200 mol g 1 as well as significantly elevated porewater fe2 1000 m in ass impacted sediments x ray diffraction indicates that goethite lepidocrocite and ferrihydrite are important fe iii minerals in surficial sediments at ass impacted sites the abundant fer is shifting the equilibrium of reduced inorganic sulfur ris mineral formation pathways towards accumulation of s 0 and acid volatile sulfide avs species and is buffering porewater s ii to low concentrations in contrast ris speciation at the control sites is dominated by pyrite scr and porewater s ii is present some porewater metals i e al mn cu cr are significantly elevated at sites proximal to the source of ass drainage compared to control sites abundant fer also provides a substrate for sorption and accumulation of metals and correspondingly there is significant relative enrichment of reactive metal fractions 1 m hcl extractable in sediments at the ass impacted sites with al mn u cu cr ni this study demonstrates that ass drainage which causes accumulation of reactive fe in mangrove environments can substantially influence sediment biogeochemistry by altering reduced inorganic sulfur cycling and enhancing the availability of some trace metals

detailed observations of a naturally occurring shear instability simultaneous profiles of microstructure horizontal velocity and acoustic backscatter allow one of the most complete descriptions of a naturally occurring shear instability to date shear increased rapidly after passing through a lateral constriction which formed a hydraulic control a kilometer long set of 20 m tall billows grew on a middepth density interface where the richardson number fell below 0 25 the velocity interface thickened steadily after the billows formed consistent with rapid momentum mixing across a shear layer with a reynolds number of 3 106 the billows generated large density overturns and dissipation rates greater than 10 5 w kg 1 even within the first large overturn indicating that these structures were fully turbulent early in their development as the billows grew a well mixed layer developed at the interface and survived as an actively turbulent layer for up to 6 buoyancy periods 3 times longer than in laboratory studies at low reynolds number variations in the mean density of the billows lead us to infer that the vertical offset of the velocity and density interfaces varied with time where the billows first formed with data from the large overturns within the shear layer we find e n2 3 104 an average root mean square overturn scale lrms of 2 6 m and a buoyancy scale lb of 2 7 m despite having sampled the billows at varying stages of their evolution we find no indication that the ratio lrms lb is ever significantly different than 1 for this shear instability

hydraulic control of sill flow with bottom friction abstract the hydraulics of strait and sill flow with friction is examined using a reduced gravity model it is shown that friction moves the critical or control point from the sill to a location downstream if the strait has constant width the control point lies where the bottom slope is the negative of the drag coefficient cd if cd exceeds the bottom slope everywhere the flow cannot be controlled in the classical sense that energy and flow force are minimized friction also decreases the minimum obstacle height required to establish hydraulically controlled flow in the classical laboratory towing experiment also friction greatly encourages the establishment of stationary hydraulic jumps in the lee of the sill and under certain conditions gives rise to stationary jumps on the upstream face of the obstacle some consequences of these results for deep ocean overflows are given using the iceland faroe overflow as an example

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

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

observations of turbulence in a partially stratified estuary the authors present a field study of estuarine turbulence in which profiles of reynolds stresses were directly measured using an adcp throughout a 25 h tidal day the dataset that is discussed quantifies turbulent mixing for a water column in northern san francisco bay that experiences a sequence of states that includes a weak ebb and flood that are stratified followed by a strong and eventually unstratified ebb and flood these measurements show that energetic turbulence is confined to a bottom mixed layer by the overlying stratification examination of individual reynolds stress profiles along with profiles of richardson number and turbulent froude number shows that the water column can be divided into regions based on the relative importance of buoyancy effects using the measured turbulence production rate p the dissipation rate e is estimated the observed turbulence had values of e nn 2 20 all of the time and e nn 2 200 most of the time suggesting that the observed motions were buoyancy affected turbulence rather than internal waves however at times turbulent froude numbers in much of the upper water column were less than one indicating important stratification effects taken as a whole the data show that stratification affects the turbulent velocity variance q2 most severely that is observed reductions in u9w9 are largely associated with small values of q2 rather than with a dramatic reduction in the efficiency with which turbulent motions produce momentum fluxes finally the dataset is compared to predictions made using the popular mellor yamada level 2 5 closure these comparisons show that the model tends to underestimate the turbulent kinetic energy in regions of strong stratification where the turbulence is strongly inhomogeneous and to overestimate the turbulent kinetic energy in weakly stratified regions the length scale does not appear to compensate for these errors and as a result similar errors are seen in the eddy viscosity predictions it is hypothesized that the underestimation of q2 is due to an inaccurate parameterization of turbulence self transport from the near bed region to the overlying stratification

turbulent mixing in a strongly forced salt wedge estuary up to about 400 m 3 s 1 above which a majority of the mixing occurs offshore observed buoyancy fluxes were more consistent with the k turbulence closure than the mellor yamada closure and more total mixing occurred in the estuary with k calculated buoyancy fluxes were sensitive to horizontal grid resolution as a lower resolution grid yielded less integrated buoyancy flux in the estuary and exported lower salinity water but likely had greater numerical mixing

late pleistocene and holocene palaeoenvironments in golfo san jorge patagonia molluscan evidence palaeoenvironments suggested by molluscan assemblages preserved in late pleistocene and holocene raised marine terraces along the golfo san jorge central patagonia coastline argentina are reviewed in this work a better precision of the stratigraphical and geographical ranges of the molluscs through their systematic review shows that all taxa have living representatives mostly within the argentine and or magellanean zoogeographical provinces in general no significant taxonomic appearances or disappearances are evident along this coastal area since the last pleistocene highstand reinforcing that the modern palaeoceanographical pattern dominated by the cool malvinas falkland current has not been substantially modified and that the magellanean malacological province has lasted at least since the pleistocene although with varying boundaries the so called marine terrace v recently assigned to the last interglacial highstand oxygen isotope substage 5e showed in this area a molluscan assemblage characterised by very low diversity dominantly large shells of mulinia edulis and by cold water taxa in disagreement with higher sea surface temperatures within oxygen isotope substage 5e last interglacial maximum and suggesting softer substrates than in the modern adjacent littoral zone

hydraulics and mixing in the hudson river estuary a numerical model study of tidal variations during neap tide conditions three dimensional numerical modeling is performed to study intratidal and along channel variability in stratification and mixing in the hudson river estuary the modeled fields show good agreement

broadly distributed and locally enhanced turbulent mixing in a tidal estuary abstract spatial variations of vertical turbulent mixing along a stretch of the hudson river estuary are examined with focus on the vicinity of a hydraulic control point at the george washington bridge at which the cross section narrows and the thalweg takes a bend richardson numbers are lowered and mixing is enhanced downstream of this modest morphological feature qualitatively following predictions based on hydraulic theory by chant and wilson the enhancement in the viscous dissipation rate e is only a modest factor of 2 3 however extending over 2 km along the river upon averaging over tidal and fortnightly cycles and multiple cruises streamwise variations of e along the probed 15 km stretch of the estuary are surprisingly small given that individual depth averages of e across the halocline vary by over three orders of magnitude the principal result is that the observed part of the hudson river showed strong broadly distributed mixing everywhere with little local concentration

internal hydraulics and mixing in a highly stratified estuary shipboard acoustic doppler current profiler and conductivity temperature depth data obtained during highly stratified conditions in the hudson river estuary along a section of variable width and breadth are presented the observations emphasize tidal period asymmetries in the vertical structure of current and salinity however these asymmetries exhibit significant along channel structure which is determined by channel morphology during the ebb the flow is linearly sheared and steep halocline slopes in the vicinity of channel contractions are maintained by momentum advection a minimum in vertical shear across the pycnocline occurs in channel contractions during food the pycnocline sharpens and flattens with a middepth velocity maximum embedded in the pycnocline which separates a stratified surface layer from a bottom mixed layer the along channel structure in vertical shear is consistent with a lateral vorticity equation estimates of richardson numbers suggest that vertical mixing across the pycnocline is enhanced downstream of channel contractions

hydraulics and mixing in a laterally divergent channel of a highly stratified estuary author posting american geophysical union 2017 this article is posted here by permission of american geophysical union for personal use not for redistribution the definitive version was published in journal of geophysical research oceans 122 2017 4743 4760 doi 10 1002 2016jc012455

microstructure observations of turbulent mixing in a partially mixed estuary part i dissipation rate variations of turbulent mixing in the water column and in the benthic boundary layer were observed with a microstructure profiler in the hudson river estuary during two cruises in summer and fall of 1995 variability patterns of stratification shear richardson number ri and turbulent dissipation rates e were similar to those of earlier observations with strong turbulence in the weakly stratified bottom layer weak turbulence in the halocline during neap tides and low ri and high e spanning the water column during spring ebbs depthintegrated turbulent dissipation rates re dz approximately equaled the work done by the tidal pressure gradient force adjusted for the change in tidal kinetic energy alternatively a suitable scaling for re dz is also provided by the product of bottom shear stress and depth average velocity t by a relationship that fails at slack tide however at heights above bottom of z 0 3 m and again excepting slack tides observed e were highly correlated with a law of the wall dissipation rate eb 5 kz ratios e eb were close to 1 here the friction 3 u