conceptual framework for assessing the response of delta channel networks to holocene sea level rise abstract recent research has identified two fundamental unit processes that build delta distributary channels the first is mouth bar deposition at the shoreline and subsequent channel bifurcation which is driven by progradation of the shoreline the second is avulsion to a new channel a result of aggradation of the delta topset the former creates relatively small branching networks such as wax lake delta the latter generates relatively few long distributaries such as the mississippi and atchafalaya channels on the mississippi delta the relative rate of progradation to aggradation and hence the creation of accommodation space emerges as a controlling parameter on channel network form field and experimental research has identified sea level as the dominant control on holocene delta growth worldwide and has empirically linked channel network changes to changes in the rate of sea level rise here i outline a simple modeling framework for distributary network evolution and use this to explore large scale changes in holocene channel pattern that have been observed in deltas such as the rhine meuse and mississippi rapid early to mid holocene sea level rise forced many deltas into an aggradational mode where i hypothesize that avulsion and the generation of large scale branches should dominate slowing of sea level rise in the last 6000 yr allowed partitioning of sediment into progradation facilitating the growth of smaller scale distributary trees at the shorelines of some deltas and a reduction in the number of large scale branches significant antecedent topography modulates delta response the filling of large incised valleys for example caused many deltas to bypass the aggradational phase human effects on deltas can be cast in terms of geologic controls affecting accommodation constriction of channels forces rapid local progradation and mouth bar bifurcation while accelerated sea level rise increases aggradation and induces more frequent channel avulsion

hydraulic control of a highly stratified estuarine front abstract observations at the mouth of the fraser river british columbia canada indicate an abrupt frontal transition between unstratified river outflow and a highly stratified river plume with differences in salinity greater than 25 psu across a few meters in the vertical direction and several hundred meters in the horizontal direction the front roughly follows a natural break in the bathymetry crossing the channel at an angle of approximately 45 and is essentially stationary for a period of approximately 3 5 h centered on the low tide following the larger of two daily ebbs the location of the front is coincident with observations of significantly supercritical internal froude numbers at the front based on velocities in the along flow direction this observation contradicts the one dimensional theory which indicates that the froude number should be 1 however because the front is oriented obliquely to the outflow a coordinate system can be selected that is normal to the front and for which a c

advection of the salt wedge and evolution of the internal flow structure in the rotterdam waterway an analysis of field measurements recorded over a tidal cycle in the rotterdam waterway is presented these measurements are the first to elucidate the processes influencing the along channel current structure and the excursion of the salt wedge in this estuary the salt wedge structure remained stable throughout the measuring period the velocity measurements indicate decoupling effects between the layers and that bed generated turbulence is confined below the pycnocline the barotropic m4 overtide structure is imposed at the mouth of the estuary and the generation of m4 overtides within the estuary is found to be relatively small internal tidal asymmetry does not make a significant contribution to the m4 velocity frequency band instead the combination of barotropic and baroclinic forcing in conjunction with the suppression of turbulence at the interface provides the main explanation for the time dependence and mean structure of the flow in the rotterdam waterway this gives rise to the observed differences in the length of the flood and ebb in the magnitudes of the flood and ebb velocities in the length of the slack water periods and in the timing of the onset of slack water at the surface and near the bed it results in the formation of distinct exchange flow profiles at the head of the salt wedge around slack water and the creation of maximal velocities at the pycnocline during flood advection governs the displacement and structure of the salt wedge since turbulent mixing is suppressed the tidal displacement of the salt wedge controls the height of the pycnocline above the bed at a particular site hence it controls the height to which bed generated turbulence can protrude into the water column consequently the authors find asymmetries in the structure of the internal flow turbulent mixing and bed stresses that are not related to classical internal tidal asymmetry

characteristics of a surface buoyant jet tidal outflow from leschenault estuary in winter forms an unsteady surface buoyant jet in koombana bay on each outgoing tide as the water moves away from the exit channel and spreads radially over the bay it assumes a plumelike nature the mean features of this buoyant outflow such as the plume shape frontal celerity internal circulation and the bulk entrainment were measured in an intensive field program these features are shown to compare favorably with results from previous laboratory investigations and predictions from a simple inertia buoyancy force balance which includes entrainment at the front the stability of the plume frontal region was examined and it is shown that subfronts were present but their behavior cannot be explained by reference to previous work

radial spreading of buoyant surface plumes in coastal waters shallow buoyant surface plumes in coastal waters sometimes develop by nearly radial spreading from a continuous source such as at a river mouth such plumes often display a ring structure where buoyant water is concentrated between fronts at both the leading and trailing edges of the expanding ring these rings often occur in multiple concentric forms a model of the radial time dependent spreading of a shallow buoyant layer over a motionless deep ambient layer is developed it includes fronts as discontinuities and treats the remainder of the plume flow as inviscid gravitational spreading controlled by nonlinear internal waves calculations by the method of characteristics are presented that show the evolution of a clear ring structure the combination of rapid spreading in the radial geometry together with reflection of internal waves off the frontal boundary at the plume leading edge is shown to be necessary for ring formation a mechanism is suggested for the formation of multiple rings through the shear flow instability inherent in radial spreading

two layer hydraulics at the river ocean interface liftoff is the hydraulically forced detachment of buoyant freshwater from the channel bottom or seabed that occurs as river water discharges into the coastal ocean it is a key feature of strongly stratified systems occurring well upstream in the channel or seaward of the river mouth under sufficiently strong forcing we present a two layer hydraulic solution for the river ocean interface that considers the river estuary and near field river plume as a single interlinked system extending previous work that considered them separately this unified approach provides a prediction of the liftoff location and free surface profile for a wide range of forcing conditions which are characterized in terms of the freshwater froude number f f equiv q b 0 sqrt g 0 prime h 0 3 here q is the river discharge b 0 is the channel width g 0 prime equiv unicode stix x0394 unicode stix x1d70c 0 unicode stix x1d70c 2 g is the reduced gravitational acceleration unicode stix x0394 unicode stix x1d70c 0 is the density contrast between fresh and ocean water and h 0 is the total water depth at the river mouth the solution is validated with laboratory experiments using an experimental apparatus consisting of a long sloping river channel that discharges into a deep wide saltwater basin the experiments simulate the full range of hydraulic behaviours predicted by the model from saltwater intrusion to offshore liftoff for f f 1 liftoff occurs in the estuary channel and our results show that the relationship between intrusion length and f f depends on the channel slope for f f 1 corresponding to flood conditions in many natural systems liftoff is forced outside the river mouth and the hydraulic coupling between the channel and shelf becomes more important for these conditions and for intermediate to steeply sloped shelves the offshore liftoff distance varies linearly with f f 1 a particularly simple scaling given the nonlinearity and relative complexity of the governing equations the model and experimental results support a conceptual description of the river ocean interface that relates the liftoff location free surface elevation and the spreading rate of the buoyant river plume

oblique internal hydraulic jumps at a stratified estuary mouth abstractobservations and analyses of two tidally recurring oblique internal hydraulic jumps at a stratified estuary mouth columbia river oregon washington are presented these hydraulic features have not previously been studied due to the challenges of both horizontally resolving the sharp gradients and temporally resolving their evolution in numerical models and traditional observation platforms the jumps both of which recurred during ebb formed adjacent to two engineered lateral channel constrictions and were identified in marine radar image time series jump occurrence was corroborated by i a collocated sharp gradient in the surface currents measured via airborne along track interferometric synthetic aperture radar and ii the transition from supercritical to subcritical flow in the cross jump direction via shipborne velocity and density measurements using a two layer approximation observed jump angles at both lateral constrictions are shown to lie within the theoretical bounds given by th

ebb tide dynamics and spreading of a large river plume abstract momentum balances in the near field region of a large tidally pulsed river plume are examined the authors concentrate on a single ebb tide of the columbia river plume using the regional ocean modeling system roms configured to hindcast flow conditions on the washington and oregon shelves and in the columbia river estuary during ebb plume interior streamwise balances are largely between advection pressure gradient and frictional forces stream normal balances in this region reduce to centrifugal cross stream pressure gradient and coriolis terms i e the gradient wind balance commonly assumed in river plume bulge investigations temporal derivatives are most important at the plume front and as the ebb progresses winds were light and contributed little to the force balance midebb stress and vertical salt flux were largest at a midplume depth where stratification and vertical shear were also high consistent with shear induced mixing internal stress slows the spreading plume consi

initial expansion of the columbia river tidal plume theory and remote sensing observations 1 analysis of the columbia river tidal plume using lagrangian frontal equations provides a concise description of the evolution of frontal depth h velocity u reduced gravity g and frontal internal froude number fr because the estuary mouth is narrow the initial radial plume motion is supercritical fr 1 for up to 12 hours understanding this supercritical phase is vital because plume properties change rapidly with strong ecosystem impacts to analyze this expansion analytical and numerical models the latter with three mixing formulations were tested model results are compared to synthetic aperture radar images to verify that the predicted frontal properties are realistic lagrangian theory provides especially simple constraints independent of the mixing model on spatial variations in fr and ug h for parameters representative of the columbia river plume the plume spreads 10 35 km and thins to 25 60 of its initial depth before becoming subcritical after liftoff fr increases as the front accelerates and thins it then decreases to unity g decreases but more slowly than h and u h controlled by a balance between spreading and mixing first decreases then increases the strength of vertical mixing and the mixing efficiency ef ratio of buoyancy flux to dissipation both play a significant role in determining plume properties and it is important to include both the horizontal gradient in g and surface slope component of the pressure gradient a model with an ellison turner type entrainment scheme predicts the plume trajectory better than one that assumes a constant interfacial gradient richardson number

laboratory investigation of the impact of lateral spreading on buoyancy flux in a river plume abstractthe relationship between lateral spreading and mixing in stratified gravity currents is investigated by comparing laterally confined and unconfined currents in a series of laboratory experiments the vertical turbulent buoyancy flux is determined using a control volume approach with velocity and density fields derived from combined particle image velocimetry piv and planar laser induced fluorescence plif lateral spreading is determined in the unconfined experiments based on plan view imaging using the optical thickness method otm the authors find that lateral spreading dramatically modifies the plume structure the spreading plume layer consists of approximately linear density and velocity profiles that extend to the surface whereas the channelized plume profiles are uniform near the surface lateral spreading decreases the average plume density relative to laterally confined currents with similar inflow conditions however the local turbulent buoyancy flux in the spreading experiments i

source conveyance and fate of suspended sediments following hurricane irene new england usa hurricane irene passed directly over the connecticut river valley in late august 2011 intense precipitation and high antecedent soil moisture resulted in record flooding mass wasting and fluvial erosion allowing for observations of how these rare but significant extreme events affect a landscape still responding to pleistocene glaciation and associated sediment emplacement clays and silts from upland glacial deposits once suspended in the stream network were routed directly to the mouth of the connecticut river resulting in record breaking sediment loads fifteen times greater than predicted from the pre existing rating curve denudation was particularly extensive in mountainous areas we calculate that sediment yield during the event from the deerfield river a steep tributary comprising 5 of the entire connecticut river watershed exceeded at minimum 10 40 years of routine sediment discharge and accounted for approximately 40 of the total event sediment discharge from the connecticut river a seriesof surface sediment corestaken in floodplainponds adjacent to the tidalsec

nonlinear evolution of ocean internal solitons propagating along an inhomogeneous thermocline measurements of the upper ocean thermal structure show that on the continental shelves the thermocline depth may shoal or deepen generally depending on the bottom topography thermocline shoaling and deepening cause changes in the phase speeds of internal waves as described by linear wave theories on the other hand the ocean area where internal waves have variable phase speeds may be treated as a dynamically inhomogeneous medium in this case theories of nonlinear dispersive wave propagation in inhomogeneous media developed by tappert and zabusky 1971 may stand we used these theories to analyze the evolution of ocean internal solitary waves passing over a seamount in the gulf of aden the results indicate that a surprisingly sharp recess of an internal solitary wave packet imaged by the space shuttle discovery is a signature of spatial phase delay caused by thermocline shoaling over the seamount soliton fission due to thermocline shoaling was also observed in the imagery the observed number of transmitted solitons over the seamount agrees with theoretical predictions relative soliton amplitudes measured from the imagery also agree qualitatively with predictions

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 shear stresses in a small highly stratified estuary 1 this paper examines near bottom shear stresses at two locations in a small highly stratified estuary the navesink river new jersey usa the shear stresses obtained from acoustic doppler current profilers adcps operating in the recently developed mode 11 at a high spatial resolution are examined in detail measurement noise of the adcp is estimated using autocorrelation analyses and is found to be small but correlated with velocity measurement noise in estimates of reynolds stress is found to be somewhat smaller than that obtained in earlier studies for the adcp operating in mode 4 internal waves may have produced observed correlated oscillatory motions the effect of these motions on reynolds stress estimates is quantified and shown to be at least an order of magnitude less than the total measured stress and probably much less the high spatial resolution profiles from the adcp operating in mode 11 allow excellent measurements of near bed vertical velocity gradients log profile estimates of stress are poorly related to reynolds estimates even when stability measures based on near bed density gradients are taken into account we show that upper layer pycnoclines likely inhibit turbulence and bias log profile estimates even at a half meter above the bed where the water column is well mixed

river plume liftoff dynamics and surface expressions the water surface expression of liftoff and its dependence on discharge are examined using numerical simulations with the regional ocean modeling system roms liftoff is the process by which buoyant river water separates from the bed and flows over denser saltwater during low discharge conditions liftoff occurs in the river and is accompanied by a change in the surface slope during high discharge conditions liftoff occurs outside the mouth and generates a ridge on the water surface the location and height of the ridge can be described by analytical equations in terms of discharge shelf slope and river mouth aspect ratio the offshore distance and height of the ridge are proportional to the river discharge and vary inversely with river mouth aspect ratio for steep shelf slopes liftoff occurs close to the river mouth and generates a large ridge the ridge is modified but not eliminated by the presence of tides the water surface slope change at the ridge peak is large enough to be detected by the upcoming surface water and ocean topography swot altimeter and can be used to identify the liftoff location during high discharge however during low discharge the water surface slope change at the liftoff location is too small to be detected by swot these results indicate that remote measurements of the presence or absence of the ridge may be useful to distinguish between low and high flows and remote measurements of the ridge location or height could be used to estimate freshwater discharge

the sensitivity of salt wedge estuaries to channel geometry abstractthe authors develop a two layer hydraulic model to determine the saline intrusion length in sloped and converging salt wedge estuaries they find that the nondimensional intrusion length cil hs depends significantly on the channel bottom slope and the rate and magnitude of landward width convergence in addition to the freshwater froude number in the definition of ci is a quadratic interfacial drag coefficient l is the salt wedge intrusion length and hs is the depth at the mouth of the estuary bottom slope is found to limit the saline intrusion length and this limitation accounts for the deviation of the observed exponent n in a scaling relationship with the river discharge of the form l q n from the canonical value of 2 to 2 5 predicted by the theory of schijf and schonfeld for a flat prismatic estuary the authors find that estuary convergence is important only when the ratio of the slope limited intrusion length to the convergence length is greater than one and that the effects of

phytoplankton bloom during the northeast monsoon in the luzon strait bordering the kuroshio article were discernible for both situations a twin core structure of the bloom was identified the major core lay far offshore in the deep basin to the northwest of luzon with a maximum horizontal scale of 250 km and the other core was nearshore in the babuyan channel with a maximum scale of 100 km variations in cli matological monthly mean aph indicate that the lzs bloom as a whole persists from november to february peaking in december further investigations with satellite altimetry and sea surface temperature suggest that the bloom is primarily associated with the kuroshio and its intrusion which generate various meso scale pro cesses and thus induce enhanced nutrient pumping entrainment from depth when interacting with the topog raphy of the luzon strait and the circulation of the northeastern south china sea i e the luzon gyre meanwhile upwelling at the center of the luzon gyre contributes to the offshore bloom the nearshore bloom is substantially enhanced by nutrient input from the cagayan river

lateral spreading of a near field river plume observations and numerical simulations 1 data collected from the near field region of the merrimack river massachusetts plume were analyzed to examine plume spreading estimates of the plume spreading rate were derived from a direct assessment of the spreading of clustered surface drifters and from density and velocity data along a cross plume arc additionally plume spreading rates were also derived from highly resolved numerical model output roms these three distinct observational and numerical approaches have reasonable agreement and are compared favorably with observations using a control volume approach in a previous study it is demonstrated that these three methods are valid to estimate the plume spreading rate the observations and numerical simulations also allowed estimation of terms in the lateral momentum balance along streamline normals perpendicular to surface layer streamlines one principal finding of this study is that the dominant terms in the lateral momentum balance are the centrifugal force term associated with streamline curvature the buoyancy term the coriolis term and the interfacial stress term furthermore the lateral momentum balance shows that the lateral spreading process is significantly affected by interfacial stress within the first 1 to 2 km but thereafter is locally inviscid however turbulent mixing continues to play an important role in modifying the spreading rate by adjusting the internal wave speed

diverse volumetric faulting patterns in the san jacinto fault zone we examine locations magnitudes and faulting types of post 2000 earthquakes in the trifurcation area of san jacinto fault zone to clarify basic aspects of failure processes in the area most m 3 5 events have strike slip mechanisms occur within 1 km of the main faults clark buck ridge and coyote creek and have hypocenter depths of 10 13 km in contrast many smaller events have normal source mechanisms and hypocenters in intrafault areas deeper than 13 km additional small events with hypocenter depth 13 km occur in off fault regions and have complex geometries including lineations normal to the main faults five moderate earthquakes with m 4 7 5 4 have high aftershock rates 150 m 1 5 events within 1 day from the mainshock to obtain more details on aftershock sequences of these earthquakes we detect and locate additional events with the matched filter method there are almost no aftershocks within 1 km from the mainshocks consistent with large mainshock stress drops and low residual stress the five aftershock sequences have almost no spatial overlap while the mainshocks are on the main faults most aftershocks are located in intrafault and off fault regions their locations and spatial distribution reflect the mainshock rupture directions and many also follow structures normal to the main faults the significant diversity of observed features highlights the essential volumetric character of failure patterns in the area the increasing rate of moderate events productive aftershock sequences and large inferred stress drops may reflect processes near the end of a large earthquake cycle

island mass effect in the marquesas islands time variation 1 chlorophyll a in the marquesas islands has been analyzed for the period 1997 2002 as well as simultaneous sea surface height anomaly sea surface temperature wind and rainfall for the first time the seasonality of phytoplankton blooms is shown around the marquesas islands clearly the data show three types of blooms in this area seasonal blooms episodic ones and la nina related bloom initiation of blooms is due to the island mass effect and has been explained by the dynamic interaction of the circulation and the topography the island mass effect blooms are strongly correlated with the total ekman plus geostrophic surface current and episodically correlated with the geostrophic current strong correlation is also observed with sea surface temperature analysis of rainfall data suggests that land drainage can enhance a bloom if the current dispersing the nutrients is sufficiently strong

backwater controls of avulsion location on deltas river delta complexes are built in part through repeated river channel avulsions which often occur about a persistent spatial node creating delta lobes that form a fan like morphology predicting the location of avulsions is poorly understood but it is essential for wetland restoration hazard mitigation reservoir characterization and delta morphodynamics following previous work we show that the upstream distance from the river mouth where avulsions occur is coincident with the backwater length i e the upstream extent of river flow that is affected by hydrodynamic processes in the receiving basin to explain this observation we formulate a fluvial morphodynamic model that is coupled to an offshore spreading river plume and subject it to a range of river discharges results show that avulsion is less likely in the downstream portion of the backwater zone because during high flow events the water surface is drawn down near the river mouth to match that of the offshore plume resulting in river bed scour and a reduced likelihood of overbank flow furthermore during low discharge events flow deceleration near the upstream extent of backwater causes enhanced deposition locally and a reduced channel fill timescale there both mechanisms favor preferential avulsion in the upstream part of the backwater zone these dynamics are fundamentally due to variable river discharges and a coupled offshore river plume with implications for predicting delta response to climate and sea level change and fluvio deltaic stratigraphy

maximal two layer exchange over a sill and through the combination of a sill and contraction with barotropic flow the analysis of two layer exchange flow through contractions with a barotropic component treated by armi farmer 1986 is extended to include exchange flows over sills and through a combination of a sill and contraction it is shown that exchange over a sill is fundamentally different from exchange through a contraction control at the sill crest acts primarily through the deeper layer into which the sill projects and only indirectly controls the surface layer this asymmetry in the control results in asymmetrical flows the interface depth above the crest is not one half the total depth as assumed in other studies by analogy with flow through contractions but is somewhat deeper the maximal exchange rate is less than for flow through a contraction of equal depth when both a sill and a contraction are present the contraction influences control at the sill crest only if it lies between the sill and the source of denser water the response to barotropic flow is also asymmetrical the transition to single layer flow occurs at much lower speeds for a barotropic component in one direction than the other results of the analysis are applied to exchange flow through the strait of gibraltar which includes both a sill and a contraction it is shown that maximal exchange conditions apply throughout part of the tidal cycle and observations illustrate several of the analytical predictions for barotropic flows including the formation of fronts single layer flow submaximal exchange and reverse flow

backwater and river plume controls on scour upstream of river mouths implications for fluvio deltaic morphodynamics sediment flux from rivers to oceans is the fundamental driver of fluvio deltaic morphodynamics and continental margin sedimentation yet sediment transport across the river to marine boundary is poorly understood coastal rivers typically are affected by backwater a zone of spatially decelerating flow that is transitional between normal flow upstream and the offshore river plume flow deceleration in the backwater zone as well as spreading of the offshore plume should render rivers highly depositional near their mouths leading to sedimentation and eventual elimination of the backwater zone at steady state this reasoning is counter to observations of riverbed scour erosional bed forms and long lived backwater zones near the mouths of some coastal rivers e g mississippi river united states to explain these observations we present a quasi 2 d model of a coupled fluvial backwater and offshore river plume system and apply it to the mississippi river results show that during high discharge events the normal flow depth can become larger than the water depth at the river mouth resulting in drawdown of the water surface spatial acceleration of flow and erosion of the riverbed as proposed by lane 1957 the transition to drawdown and erosion is ultimately forced by spreading of the offshore river plume this points to the need to model coupled river and river plume systems with a dynamic backwater zone under a suite of discharges to accurately capture fluvio deltaic morphodynamics and connectivity between fluvial sediment sources and marine depositional sinks

penetration of nonlinear rossby eddies into south china sea evidenced by cruise data national basic research program of china 2009cb421208 2007cb411803 natural science foundation of china 40976013 41121091 united states of america national oceanic atmospheric administration national environmental satellite data and information service ocean remote sensing 3000 11 03241

mechanism of internal waves in the luzon strait 1 this study suggests a western boundary current instability generation mechanism for the ocean internal waves a linear model describing this instability is developed and an analytical solution is derived from a zero order complex frequency wavenumber relation the real part of the relation represents a dispersion relation of the generated internal waves and the imaginary part is an exponential growth rate the dispersion relation describes the wave propagation characteristics while the growth rate represents the instability properties the theoretical results are used for the case of the kuroshio east of the luzon strait based on the analysis it is found that for the westward propagating disturbance the kuroshio west wing is unstable and the east wing is stable while the reverse is true for the eastern propagating disturbance the results are used to interpret satellite sar images of the ocean internal waves which were generated in the luzon strait and propagated westward reasonable estimates for the growth rate and propagation velocity are derived from a scale analysis of the kuroshio east of the luzon strait

tide induced variation of the dynamics of a salt wedge estuary abstract the time variations of the salt wedge intrusion in the fraser estuary british columbia were monitored during a variety of tidal and run off conditions using instruments and sampling methods that provided high resolution of the velocity and water properties in space and time the salt wedge was found to vary considerably in position and vertical structure through the tidal cycle due to the interaction of the tidal flow with the density driven motion of the salt wedge during the flood the salt wedge progressed up the estuary as a gravity current while during the ebb the salinity structure was eroded by shear instability the difference in character of the flow between flood and ebb is attributed to the transition between a subcritical and supercritical internal froude number during flood tide the internal state is subcritical and the estuarine shear flow is maintained with a small amount of vertical mixing while in the supercritical ebb flow vertical shears become so large that the pycnoc

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

a numerical technique to incorporate frontal boundaries in two dimensional layer models of ocean dynamics abstract this paper describes a novel numerical technique that exploits the method of bicharacteristics to allow small fronts to he included in numerical layer models of the ocean the approach treats a front as a free boundary and solves the interior problem using a standard finite difference scheme it employs a method based on the bicharacteristics of the equations to locate the boundary and enforce the boundary conditions the bicharacteristics for the nonlinear shallow water wave equations are derived and their application to the problem is outlined the method is demonstrated and evaluated by compared calculated approximate solutions first to the exact solution of a simple problem and second to a numerical solution of the more complicated problem of the radially symmetric discharge of a buoyant fluid into a stationary deep ambient fluid obtained using a simple alternative method the scheme is then successfully applied to investigate the effect of a uniform crossflow in the ambient fluid on the

structure and dynamics of the columbia river tidal plume front 1 time dependent buoyant plumes form at the outflow of tidally dominated estuaries when estuary discharge velocity exceeds plume internal wave speed c a sharp front forms at the plume s leading edge that expands from the time dependent source using observations of the columbia river tidal plume from multiple tidal cycles we characterize time evolving plume structure and quantify front speed uf plume internal wave speed c front curvature and ultimate extent we identify three distinct stages of propagation 1 initially the plume is strongly influenced by shallow bathymetry near the river mouth 2 as the front advances offshore the plume detaches from the bottom and expands as a freely propagating gravity current with relatively constant uf c and frontal froude number f uf c ambient currents explain intracycle variability in uf and winds alter front shape variability in ambient stratification associated with previous cycles plume remnants leads to complex fronts and internal waves 3 finally the plume decelerates adjusts toward geostrophy and may radiate additional internal waves using a simple kinematic model we suggest that constant frontal propagation speed uf 0 9 0 1 m s during stage 2 is primarily controlled by linearly increasing volume flux from the columbia river mouth as this discharge rate subsides the plume expands as a fixed volume with decreasing front speed stage 3 the plume s final extent is controlled by the rossby radius which scales with a length based on the total volume discharged this provides an integral description of plume front evolution based on the time dependent estuary discharge