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

density driven exchange flow in terms of the kelvin and ekman numbers 1 the pattern of density induced flow influenced by basin s width friction and earth s rotation is investigated as a function of the ekman ek and kelvin ke numbers a semianalytical solution is used to determine the conditions under which the density induced exchange flow is vertically sheared or horizontally sheared solutions are obtained over diverse laterally varying bathymetries it is found that the exchange flow is horizontally sheared under high frictional conditions ek 1 independently of the width of the basin ke the horizontally sheared pattern describes inflow in the channel and outflow over shoals with the inflow occupying the entire water column the exchange flow pattern is also horizontally sheared under weak friction ek 0 and in wide ke 2 basins in that case however the outflow is concentrated on the left looking into the basin in the northern hemisphere portion of the cross section and inflow appears on the right also under weak friction the exchange pattern becomes more vertically sheared with outflow at surface and inflow underneath as the width of the basin becomes small ke 1 bathymetry is not very influential in the weak friction exchange patterns finally under moderate friction 0 01 ek 0 1 the exchange pattern is both horizontally and vertically sheared for all widths the horizontally sheared pattern is best defined in wide basins high ke whereas the vertically sheared pattern practically dominates in narrow basins low ke these findings allow classification of various estuaries in the ek ke parameter space

particle trapping in estuarine tidal flows particle trapping in estuarine turbidity maxima etm is caused primarily by convergent mean and or tidal fluxes of sediment the result is an approximately bell shaped along channel distribution of vertically integrated tidal cycle mean suspended sediment concentration observations from the columbia river estuary suggest that 1 strong two layer or internal along channel residual and overtide flows are generated by time varying stratification and 2 correlations between the near bed velocity and the suspended sediment fields at these frequencies are important in landward transport of sediment a new spatially and temporally integrated form of the sediment conservation equation has been derived to analyze this trapping process time changes in tidally averaged sediment concentration between two estuarine cross sections can be shown to be related to the divergence of the seaward river flow transport the divergence of velocity shear sediment stratification correlations for the mean flow and each tidal constituent and net erosion or deposition at the bed vertically integrated variables other than seaward river transport are absent from this integrated balance analysis of sediment fluxes using this balance supports the idea that internal residual and overtide circulations are primarily responsible for the landward sediment transport on the seaward side of etm found near the upstream limits of salinity intrusion the balance also shows that attempts to represent fluxes causing trapping of sediment in an etm as a product of a time mean vertically integrated along channel gradient and a diffusivity inevitably lead to the appearance of countergradient transport and thus a negative diffusivity on the seaward side of the etm this result occurs because the trapping process is inherently nonlinear and at least two dimensional and because a one dimensional representation is physically unrealistic

a dynamic equation for the potential energy anomaly for analysing mixing and stratification in estuaries and coastal seas abstract in this paper a time dependent dynamic equation for the potential energy anomaly is rigorously derived from dynamic equations for potential temperature and salinity the continuity equation and the equation of state for sea water the terms locally changing are a the advection b the depth mean straining c the non mean straining d the vertical advection e the vertical mixing f surface and bottom density fluxes g inner sources of density e g due to absorption of solar radiation and the non linearity of the equation of state and h horizontal divergence of horizontal turbulent density fluxes in order to derive the equation in concise form a vertical velocity linearly varying with depth with respect to depth proportional vertical coordinates had to be defined the evaluation of the terms in the equation is then carried out for a one dimensional tidal straining study and a two dimensional estuarine circulation study comparisons to empirical estimates for these terms are made for the one dimensional study it is concluded that the equation provides a general reference for empirical bulk parameterisations of stratification and mixing processes in estuaries and coastal seas and that it is a tool for complete analysis of the relevant terms from numerical models

the formation of estuarine turbidity maxima due to density effects in the salt wedge a hydrodynamic process study abstract by means of a numerical model of an idealized flat bottom estuary the paper studies the hydrodynamic control of the turbidity zone by the combined effect of the salt wedge and tidal movements the model is of two dimensional x z finite difference type with high resolution in time and space it computes momentum surface elevation salinity suspended particulate matter spm turbulent kinetic energy and dissipation rate as prognostic state variables at the seaward boundary a tidal forcing is applied at the landward boundary a weir is situated where a constant freshwater discharge is prescribed the initial spm concentration is horizontally homogeneous after simulating a few tidal periods the model results exhibit the evolution of a stable spm peak the estuarine turbidity maximum or etm at the tip of the salt wedge an inspection of the tidal mean velocity profiles around the etm shows that this trapping of spm is due to a residual near bottom upstream current in the region of the salt

evidence of tidal straining in well mixed channel flow from micro structure observations 1 this study presents for the first time micro structure observations in tidally energetic weakly stratified regimes obtained in the wadden sea a tidal shallow coastal area in the south eastern north sea characterised by barrier islands separated by tidal gulleys the tidal currents are typically overlaid by a weak horizontal density gradient due to freshwater run off from land the observations in an energetic tidal channel clearly show the expected effects of tidal straining destratification during flood and increased stratification during ebb microstructure observations are consistent with the tidal straining dynamics during the flood relatively high values of viscous dissipation are observed whilst during ebb the values are substantially smaller it is also shown that the tidal cycle of stratification and destratification depends on the position in the tidal channel in parts of the channel increased stratification occurs during full flood a phenomenon which can only be explained by advection of stratified water masses formed outside the tidal channel the observations presented here show the general significance of the tidal straining process for tidally energetic weakly stratified regimes

creation of residual flows in a partially stratified estuary the creation of residual flows in estuaries is examined using acoustic doppler current profiler data sets from northern san francisco bay the data sets are analyzed using principal component analysis to examine the temporal variability of the flows which create the residual circulation it is seen that in this periodically and partially stratified estuary the residual flows are created through a series of pulses with strong variability at the 24 hour timescale through the interaction of shear stratification and mixing this interaction is captured through the use of a dimensionless number the horizontal richardson number rix which is developed to examine the local balance between the stratifying and destratifying forces at the tidal timescale it is seen that rix is a valuable parameter in predicting the onset of the residual creating events with a threshold value of 3 on ebb tides this critical value is argued to be a threshold above which the stratification and shear flow create a feedback effect each further intensifying the other this feedback results in a highly variable exchange flow which creates the estuarine residual in intermittent pulses rather than as a steady flow although typically attributed to baroclinic forcing an argument is made that these pulses of residual creating exchange flow could be created by barotropic forcing in the presence of variable stratification which is asymmetric between flood and ebb tides this result poses a great challenge for turbulence modeling as the timing and magnitude of stratification and shear must be correctly simulated on the tidal timescale in order to reproduce the effects seen in the data sets presented

combined effects of wind tide and horizontal density gradients on stratification in estuaries and coastal seas abstract the aim of this one dimensional water column study is to combine modified versions of three characteristic parameters for periodic tidal flow under the influence of a longitudinal buoyancy gradient the horizontal richardson number the inverse strouhal number and the inverse ekman number into a parameter space study including constant wind forcing from various directions it is shown how the underlying dynamical equations can be cast into nondimensional form depending mainly on these three nondimensional parameters plus the relative wind speed and the wind direction idealized model simulations are carried out for the whole realistic range of horizontal richardson and inverse strouhal numbers for various latitudes showing the amplitude of the tidally induced stratification for a wide range of scenarios it is found that classical threshold values for the horizontal richardson number indicating the switch from periodic stratification to permanent stratification are valid only for special cas

effects of tidal straining on the semidiurnal cycle of dissipation in the rhine region of freshwater influence comparison of model and measurements 1 the rhine region of freshwater influence in the north sea is a region of the coastal ocean that exhibits strong cross shore density gradients interacting with the progressive kelvin tidal wave which generates mixing when mixing is low enough to allow stratification to develop the stratification modifies the tidal currents and generates a strong tidal shear that will strain the density gradients generating a semidiurnal cycle of stratification observations of turbulent kinetic energy dissipation during calm conditions show that the bottom half of the water column has a predominantly m 4 cycle higher values of occurring during flood flow are associated with the residual flow in that direction following the strong peak in dissipation near the bed enhanced dissipation is apparent extending up the water column in the upper part of the water column shows a predominantly m 2 variation with the maximum occurring at a time when vertical stability is negligible and the effect of tidal straining is to create instability in the water column as the cross shore shear forces higher salinity water over fresher water the inference is that potential energy released by straining is responsible for convective motions and hence the enhanced turbulent activity observed the cycle of stratification and dissipation has been simulated using a one dimensional mellor yamada levels 2 and 2 2 turbulence closure model the model correctly predicts the amplitude and phase of the stratification while the model is in qualitative agreement in the description of the dissipation and its variability the absolute values of appear to be too high

saline intrusion in partially mixed estuaries abstract restricting interest to partially mixed estuaries earlier studies of tidally averaged linearised theories relating to the vertical structure of salinity and velocities accompanying saline intrusion are extended to take account of tidal straining and associated convective overturning the applicability of these theories is evaluated by reference to a single point numerical model in which the time varying cycle of depth averaged tidal current amplitude u and a temporally and vertically constant saline gradient sx are specified this model highlights the importance of convective overturning in counteracting unstable density structures introduced by tidal straining by omitting overturning in the model results agree closely with linearised theoretical derivations however incorporating overturning substantially increases tidally averaged surface to bed differences for both residual currents u and salinity s the vertical structure of tidal currents is a maximum and hence the effect of tidal straining in shallow macro tidal estuaries the propagation of tidal elevations and currents remains insensitive to saline intrusion in partially mixed estuaries the applicability of the model was evaluated by simulation of recent measurements by rippeth et al j phys oceanogr 31 2001 2458 to explore the generality of estuarine responses the model was run for a wide range of values of saline intrusion lengths l and water depths d additional sensitivity analyses were made for changes in u and bed stress coefficient k response frameworks are shown for u s potential energy anomaly work done by bed friction and internal shear rates and efficiency of saline mixing and ratios of relative mixing by diffusion to overturning by equating the rate of mixing associated with vertical diffusion with river flow q an expression for saline intrusion length l d 2 k u u o uo river flow velocity was derived this formulation agrees with an earlier derivation based on flume tests and showed reasonable agreement with observed values in six estuaries eight cases however in funnel shaped estuaries axial migration of the intrusion introduces major variations in d u and uo thereby complicating the application of the above expression for l moreover the time lag involved in the adjustment of l to changes in u and q may explain much of the complexity encountered in the observations

role of tidally periodic density stratification in the creation of estuarine subtidal circulation 1 tidally periodic stratification is explored as a mechanism for estuarine subtidal flow creation through the use of a numerical water column model using a two equation turbulence closure a tidally periodic steady state is established and the subtidal circulation is calculated the results demonstrate that the subtidal circulation caused by this forcing is of the same vertical structure as the expected estuarine exchange flow further the subtidal exchange is of similar magnitude to that given by typical scalings of the baroclinic circulation driven by the longitudinal salinity gradient but is strongly dependent upon the timing of the stratification event within the tidal cycle even reversing sign for certain phases the dependence of this subtidal flow mechanism on the local depth the tidal forcing and the magnitude of the stratification is explored through the use of a bulk richardson number for which the subtidal circulation is maximal at intermediate values and is reduced for both large and small bulk richardson numbers this dependence of the subtidal circulation on the bulk richardson number is demonstrated to be consistent with an analysis of the energetics of mixing and buoyancy on the tidal timescale

semidiurnal switching of stratification in the region of freshwater influence of the rhine observations in the rhine region of freshwater influence rofi system in the north sea show evidence of large semidiurnal oscillations in stability superimposed on a mean stratification occurring throughout the stratified region at times of reduced mixing the amplitude of this semidiurnal variation is of the same order as the mean stability and frequently results in conditions being mixed or nearly mixed once per tide it is deduced that this semidiurnal variation results primarily from cross shore tidal straining which interacts with the density gradient to induce stratification this conceptual picture of the contributing processes has been tested in a one dimensional point model forced by the observed slopes and the local density gradients the model exhibits the same qualitative behavior as the observations produces oscillations in stratification of the amplitude observed and confirms the critical role of cross shore tidal straining the large cross shore shear nder stratified conditions is identified with the changes in ellipse configuration which are observed between mixed and stratified conditions the occurrence of semidiurnal variations in stability in the rhine rofi is thus inferred to be a consequence of the development of mean stability whenever the horizontal density gradients relax in conditions of low stirring

semi diurnal tidal ellipse variability in a region of freshwater influence 1 new insight into a key process instrumental in the onset of stratification in tidally driven systems is presented for the first time using long term field observations from the liverpool bay region of freshwater influence rofi tidal ellipses respond to changes in stratification becoming more circular with greater vertical structure and reveal variability in response to tidal straining an inflection point that occurs before the maximum vertical density difference at the time of maximum positive bed surface cross shore shear is identified as the crucial point where the water column abruptly decouples the surface and bed layers then rotate in clockwise and anti clockwise directions respectively the maximum vertical stratification is observed when the maximum negative bed surface along shore shear is reached decoupling to a two layer system is therefore not the smooth progression predicted using turbulence closure models but rather a very sudden switch highlighting the importance of the role of reynolds stresses

quantifying the contributions of tidal straining and gravitational circulation to residual circulation in periodically stratified tidal estuaries abstract this numerical modeling study quantifies for the first time the contribution of various processes to estuarine circulation in periodically stratified tidal flow under the impact of a constant horizontal buoyancy gradient the one dimensional water column equations with periodic forcing are first cast into nondimensional form resulting in a multidimensional parameter space spanned by the modified inverse strouhal number and the modified horizontal richardson number as well as relative wind speed and wind direction and the residual runoff the along tide momentum equation is then solved for the tidal mean velocity profile in such a way that it is equated to the sum of the contributions of tidal straining resulting from the temporal correlation between eddy viscosity and vertical shear gravitational circulation resulting from the depth varying forcing by a constant horizontal buoyancy gradient wind straining and depth mean residual flow resulting from net freshwater runoff this definitio

interferometric radar measurement of ocean surface currents we describe a new method of measuring surface currents using an interferometric synthetic aperture radar an airborne implementation has been tested over the san francisco bay near the time of maximum tidal flow resulting in a map of the east west component of the current only the line of sight component of velocity is measured by this technique where the signal to noise ratio was strongest statistical fluctuations of less than 4 cm s 1 were observed for ocean patches of 60 60 m

the cycle of turbulent dissipation in the presence of tidal straining in regions of large horizontal density gradient tidal straining acts to produce a periodic component of stratification that interacts with turbulent mixing to control water column structure and flow a 25 h series of measurements of the rate of dissipation of turbulent kinetic energy e in the liverpool bay region of freshwater influence rofi have revealed the form of this interaction and indicate substantial differences from regions where horizontal gradients are weak in the rofi system there is a pronounced difference between flood and ebb regimes during the ebb the water column stratifies and strong dissipation is confined to the lower half of the water column by contrast during the flood stratification is eroded with complete vertical mixing occurring at high water and high values of dissipation 3 mw m23 extending throughout the water column the cycle of dissipation is therefore predominantly semidiurnal in the upper layers whereas near the bottom boundary the principal variation is at the m4 frequency as observed in regions of horizontal uniformity toward the end of the flood phase of the cycle tidal straining produces instabilities in the water column that release additional energy for convective mixing confirmation of increased vertical motions throughout the water column during the late flood and at high water is provided by measurements of vertical velocity and the error velocity from a bottom mounted acoustic doppler current profiler

thickness weighted averaging in tidal estuaries and the vertical distribution of the eulerian residual transport abstractthis paper presents thickness weighted averaging twa in generalized vertical coordinates as a unified framework for a variety of existing tidal averaging concepts in seas and estuaries v

the importance of tidal and lateral asymmetries in stratification to residual circulation in partially mixed estuaries measurements collected in the york river estuary virginia demonstrate the important impact that tidal and lateral asymmetries in turbulent mixing have on the tidally averaged residual circulation a reduction in turbulent mixing during the ebb phase of the tide caused by tidal straining of the axial density gradient results in increased vertical velocity shear throughout the water column during the ebb tide in the absence of significant lateral differences in turbulent mixing the enhanced ebb directed transport caused by tidal straining is balanced by a reduction in the net seaward directed barotropic pressure gradient resulting in laterally uniform two layer residual flow however the channel shoal morphology of many drowned river valley estuaries often leads to lateral gradients in turbulent mixing tidal straining may then lead to tidal asymmetries in turbulent mixing near the deeper channel while the neighboring shoals remain relatively well mixed as a result the largest lateral asymmetries in turbulent mixing occur at the end of the ebb tide when the channel is significantly more stratified than the shoals the reduced friction at the end of ebb delays the onset of the flood tide increasing the duration of ebb in the channel conversely over the shoal regions where stratification is more inhibited by tidal mixing there is greater friction and the transition from ebb to flood occurs more rapidly the resulting residual circulation is seaward over the channel and landward over the shoal the shoal channel segregation of this barotropically induced estuarine residual flow is opposite to that typically associated with baroclinic estuarine circulation over channel shoal bathymetry

residual circulation in shallow estuaries 2 weakly stratified and partially mixed narrow estuaries long wave theory is used herein to analyze circulation in weakly stratified and partially mixed estuaries unlike the highly stratified systems considered in part 1 the flows considered here have only a minimal tidal frequency internal wave component these estuaries may therefore be modeled as the sum of weakly interacting barotropic and baroclinic modes the dominant factors driving the residual flow are finite amplitude barotropic effects in weakly stratified estuaries and a combination of barotropic effects and steady horizontal density gradient forcing in partially mixed estuaries the dominant vertical exchange mechanism in the weakly stratified case is bottom boundary induced turbulence and that in partially mixed estuaries is believed to be random internal wave interactions a model of the columbia river estuary under weakly stratified conditions accurately predicts the observed residual velocity and salinity fields and the tidal amplitude at which a transition to a highly stratified state takes place the partially mixed state is unstable in the columbia river estuary because the tides and buoyancy input are both too strong

mapping small elevation changes over large areas differential radar interferometry a technique that uses synthetic aperture radar sar images to measure very small 1 cm or less surface motions with good resolution 10 m over large swaths 50 km is presented along with experimental results the method could be used for accurate measurements of many geophysical phenomena including swelling and buckling in fault zones residual displacements from seismic events and prevolcanic swelling the method is based on sar interferometry where two images are made of a scene by simultaneously flying two physically separated antennas then the phases of corresponding pixels are differenced and altitude formation is deduced from some simple computation and image rectification it is also possible to use one antenna flown twice over the same scene then if the second flight exactly duplicates the track of the first an interesting possibility occurs there would be no phase changes between the images at all unless there was a physical change in the scene such as ground swelling that would alter the distance from some resolution element to the antenna since the phase changes all occur at the short carrier wavelength the basic limitation on sensitivity is only the phase noise in the system when the two imaging passes are made from flight tracks that are separated which is the case with the seasat images used here it is no longer possible to distinguish surface changes from the parallax caused by topography however with some additional computation a third image made at some other baseline may be used to remove the topography and leave only the surface changes this method was applied using seasat data to an imaging site in imperial valley california where motion effects were observed that were ascribed to the expansion of water absorbing clays phase change images of this area are shown along with associated ground truth about the presence of water problems with the technique are explored along with a discussion of future experimental possibilities on upcoming sar missions like earth observing system eos earth resources satellite ers 1 sir c and the venus imaging radar magellan

gravity current dynamics and entrainment a process study based on observations in the arkona basin abstract a 19 h time series of dissipation stratification and horizontal velocities has been obtained for a dense gravity current flowing into the arkona basin in the western baltic sea the observations are compared with one dimensional quasi steady theory in which the gravity component in the flow direction is balanced by bottom friction while that in the cross flow direction is balanced by the coriolis force the observations deviate from the theory in that the bottom shear stress is more than 2 times as large as that required to balance the gravity several reasons for this discrepancy are discussed a 1d turbulence model is also compared with the observations profiles of velocity stratification and dissipation rates generally show similar variations with depth as the observations although the observed dissipation rates are somewhat larger than the modeled and the modeled transverse velocities are much larger than the observed subsequently the model is used to investigate the variation of t

on the derivation of coseismic displacement fields using differential radar interferometry the landers earthquake we present a map of the coseismic displacement field resulting from the landers california june 28 1992 earthquake derived using data acquired from an orbiting high resolution radar system we achieve results more accurate than previous space studies and similar in accuracy to those obtained by conventional field survey techniques data from the ers 1 synthetic aperture radar instrument acquired in april july and august 1992 are used to generate a high resolution wide area map of the displacements the data represent the motion in the direction of the radar line of sight to centimeter level precision of each 30 m resolution element in a 113 km by 90 km image our coseismic displacement contour map gives a lobed pattern consistent with theoretical models of the displacement field from the earthquake fine structure observed as displacement tiling in regions several kilometers from the fault appears to be the result of local surface fracturing comparison of these data with global positioning system and electronic distance measurement survey data yield a correlation of 0 96 thus the radar measurements are a means to extend the point measurements acquired by traditional techniques to an area map format the technique we use is 1 more automatic 2 more precise and 3 better validated than previous similar applications of differential radar interferometry since we require only remotely sensed satellite data with no additional requirements for ancillary information the technique is well suited for global seismic monitoring and analysis

primary or secondary distal volcaniclastic turbidites how to make the distinction an example from the miocene of new zealand mahia peninsula north island abstract miocene marine volcaniclastic deposits occur in mahia peninsula north island new zealand and were sedimented in a forearc setting related to the hikurangi trench subduction system these deposits are interbedded with hemipelagic marls and correspond to simple or amalgamated centimetric to metric thick turbiditic sequences volcaniclastic material is mainly composed of vitric particles with crystals quartz plagioclase with minor biotite amphibole pyroxene and oxides which are well represented in the coarse grained fraction the glass shards are mainly rhyolitic in composition three types of volcaniclastic turbidites were distinguished with geochemical data because distinction is impossible on sedimentary characteristics 1 primary monomagmatic turbidites contain both magmatic bubble wall pumice and shards and phreatomagmatic blocky shards with few vesicles and hydroclastically fragmented pyroclasts vitric particles the chemical compositions of the vitric particles and the crystals are very homogeneous suggesting a cogenetic origin these turbidites directly result from unique eruptive events and are probably related to the entrance of hot subaerial pyroclastic flows into the sea which also led to their transformation into subaqueous gravity flows 2 secondary monomagmatic turbidites never contain phreatomagmatic pyroclasts and the glass compositions display a trend from andesites to rhyolites there is a strong linear correlation in the compositions that suggest that the glass particles are derived from the same magma crystals also show a compositional homogeneity these deposits reflect the succession of several eruptions related to a unique magmatic system and result of the reworking of volcaniclastic material after relatively short storage on the shelf 3 secondary multimagmatic turbidites do not display compositional homogeneity of their vitric and mineral components this implies that the volcaniclastic material has been stored during a relative long period on the shelf before remobilization and that this sedimentation records the volcanic activity of multiple magmatic sources consequently it has been possible to distinguish primary volcaniclastic turbidites that are directly related to the volcanic activity from secondary turbidites that result from reworking of previously deposited material on the shelf monomagmatic turbidites can be used as stratigraphic and magmatic markers whereas multimagmatic cannot multimagmatic turbidites however record the evolution of the volcanic arc during longer periods this leads to the conclusion that the mahia peninsula volcaniclastic turbidites distally record the evolution of the source volcanic arc

topographic mapping from interferometric synthetic aperture radar observations interferometric synthetic aperture radar observations provide a means for obtaining high resolution topographic terrain maps from data acquired simultaneously at two slightly displaced antennas calculation of the three dimensional coordinates of all the points in a radar image can be made from the combination of along track slant range and interferometer fringe measurements thus the result compensates for the layover exhibited by conventional radar maps and removes the false targets generated by multiple signal paths to a given object in the scene we have derived a topographic map of a portion of the san francisco bay area utilizing data that were recorded by a radar system mounted on a nasa cv990 aircraft and processed by a general purpose digital computer this map displays the height above sea level of a region approximately 11 km by 10 km in size sampled on an 11 m pixel grid uncertainties in the estimated height result from imprecise knowledge of the observational geometry radar bandwidth limitations and finite signal to noise ratios for our system each factor contributed approximately 3 4 m to a relative rms error of about 6 m additional global errors of the order of 10 m can result from inaccurate monitoring of aircraft attitude the statistical variation of the height measurements from the ocean portion of the map which we can presume to be quite flat is 2 10 m rms which is consistent with the theoretical estimate comparison of our results with u s geological survey contour maps indicates a high degree of correlation between the two sets of altitude data

tidal riverine and wind influences on the circulation of a macrotidal estuary abstractthe effect of tides river wind and earth s rotation on the three dimensional circulation in the dee a macrotidal estuary are investigated using a fine resolution model the interactions of the large tidal amplitude currents river and wind generated circulation require baroclinic and unsteady studies to properly understand the estuarine dynamics assessment of the model skill has been carried out by model observation comparisons for salinity which is the main control for density surface elevation current and turbulence stationary nondimensional numbers were only partially able to characterize the dynamics in this real complex macrotidal estuary at low water tidal straining and constrained river flow cause stratification large spatial variability occurs in the current and residual patterns with flood dominated maximum values occurring within the tidal channels the tides control residual circulation by modulating stratification through tidal straining and bathymetric constraint on

remote sensing of ocean currents a method of remotely measuring near surface ocean currents with a synthetic aperture radar sar is described the apparatus consists of a single sar transmitter and two receiving antennas the phase difference between sar image scenes obtained from the antennas forms an interferogram that is directly proportional to the surface current the first field test of this technique against conventional measurements gives estimates of mean currents accurate to order 20 percent that is root mean square errors of 5 to 10 centimeters per second in mean flows of 27 to 56 centimeters per second if the full potential of the method could be realized with spacecraft then it might be possible to routinely monitor the surface currents of the world s oceans

lateral entrapment of sediment in tidal estuaries an idealized model study two physical mechanisms leading to lateral accumulation of sediment in tidally dominated estuaries are investigated involving coriolis forcing and lateral density gradients an idealized model is used that consists of the three dimensional shallow water equations and sediment mass balance conditions are assumed to be uniform in the along estuary direction a semidiurnal tidal discharge and tidally averaged density gradients are prescribed the erosional sediment flux at the bed depends both on the bed shear stress and on the amount of sediment available in mud reaches for resuspension the distribution of mud reaches over the bed is selected such that sediment transport is in morphodynamic equilibrium that is tidally averaged erosion and deposition of sediment at the bed balance analytical solutions are obtained by using perturbation analysis results suggest that in most estuaries lateral density gradients induce more sediment transport than coriolis forcing when frictional forces are small ekman number e 0 02 the lateral density gradient mechanism dominates and entraps sediment in areas with fresher water results also show that the lateral sediment transport induced by the semidiurnal tidal flow is significant when frictional forces are small e 0 02 model predictions are in good agreement with observations from the james river estuary

drivers of residual estuarine circulation in tidally energetic estuaries straight and irrotational channels with parabolic cross section the generation of residual circulation in a tidally energetic estuary with constant longitudinal salinity gradient and parabolic cross section is examined by means of a two dimensional cross sectional numerical model neglecting river runoff and stokes drift it is shown how the longitudinal and lateral residual circulation can be decomposed into contributions from various processes such as tidal straining circulation gravitational circulation advectively driven circulation and horizontal mixing circulation the sensitivity of the residual circulation and its components from various processes to changes in forcing is investigated by varying the simpson number nondimensional longitudinal buoyancy gradient and the unsteadiness parameter nondimensional tidal frequency as well as the bed roughness and the width of the estuary for relatively weak salinity gradient forcing the tidal straining circulation dominates the residual exchange circulation in support of classical estuarine circulation up estuary flow near the bed and down estuary flow near the surface the strength of the longitudinal estuarine circulation clearly increases with increased salinity gradient forcing however when the simpson number exceeds 0 15 the relative contributions of both gravitational circulation and advectively driven circulation to estuarine circulation increase substantially lateral residual circulation is relatively weak for small simpson numbers and becomes flood oriented divergent flow near the bed and convergent flow near the surface for larger simpson numbers because of increasing contributions from gravitational and advectively driven circulation increasing the unsteadiness number leads to decreased longitudinal and lateral residual circulation although changes in bed roughness result in relatively small changes in residual circulation results are sensitive to the width of the estuary mainly because of changes in residual exchange circulation driven by tidal straining

modeling the lateral circulation in straight stratified estuaries the dynamics of lateral circulation in an idealized straight estuary under varying stratification conditions is investigated using a three dimensional hydrostatic primitive equation model in order to determine the importance of lateral circulation to the momentum budget within the estuary for all model runs lateral circulation is about 4 times as strong during flood tides as during ebbs this flood ebb asymmetry is due to a feedback between the lateral circulation and the along channel tidal currents as well as to time varying stratification over a tidal cycle as the stratification is increased the lateral circulation is significantly reduced because of the adverse pressure gradient set up by isopycnals being tilted by the lateral flow itself when rotation is included a timedependent cross channel ekman circulation is driven and the tidally averaged bottom lateral circulation is enhanced toward the right bank when looking toward the ocean in the northern hemisphere this asymmetry in the tidally averaged bottom lateral circulation may lead to asymmetric sediment transport leading to asymmetric channel profiles in straight estuaries for the weakly stratified model run advection due to lateral currents is a dominant term in both the along channel and cross channel momentum equations over a tidal cycle and for the tidally averaged momentum equations in the tidally averaged along channel momentum equation lateral advection acts as a driving term for the estuarine exchange flow and can be larger than the along channel pressure gradient therefore it should not be ignored when estimating momentum budgets in estuaries