energy flux of nonlinear internal waves in northern south china sea 1 we analyze three sets of adcp measurements taken on the dongsha plateau on the shallow continental shelf and on the steep continental slope in the northern south china sea scs the data show strong divergences of energy and energy flux of nonlinear internal waves nliw along and across waves prevailing westward propagation path the nliw energy flux is 8 5 kw m 1 on the plateau only 0 25 kw m 1 on the continental shelf 220 km westward along the propagation path and only 1 kw m 1 on the continental slope 120 km northward across the propagation path along the wave path on the plateau the average energy flux divergence of nliw is 0 04 w m 2 which corresponds to a dissipation rate of o 10 7 10 6 w kg 1 combining the present with previous observations and model results a scenario of nliw energy flux in the scs emerges nliws are generated east of the plateau propagate predominantly westward across the plateau along a beam of 100 km width that is centered at 21 n and dissipate nearly all their energy before reaching the continental shelf

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

interpreting avulsion process from ancient alluvial sequences guadalope matarranya system northern spain and wasatch formation western colorado alluvial deposits of the guadalope matarranya system oligocene ebro basin spain and the wasatch formation eocene western colorado provide time integrated records of the process of river channel avulsion these sequences consist of isolated channel belt sandstones incised into and abruptly overlain by flood plain siltstones indicating deposition by avulsive river systems the geometry and distribution of channel incisions suggest that avulsion was not controlled by tectonics climate or base level changes but formed by autocyclic processes measurements from 221 channel fills in the guadalope matarranya system and 38 from the wasatch formation allow us to statistically characterize channel geometries we infer to be associated with establishment and abandonment of individual river avulsions paleoflow depths in both systems average 1 4 to 1 6 m aggradation height superelevation of channel margin levees are on average 0 6 and 1 1 times paleoflow depth in the guadalope matarranya and wasatch systems respectively these results are consistent with values from recently avulsed modern rivers and suggest that 1 flow depth is the appropriate parameter against which to scale the critical superelevation necessary for channel avulsion and 2 the increase in potential energy due to channel perching drives the lateral instability that is needed for avulsion to be successful numerous stacked channel fills indicate repeated reoccupation of the same site by avulsing channels these reoccupation channels indicate that inherited flood plain topography here abandoned channel forms was an important control on the arrival site of newly avulsed channels comparison of our results to others suggests two end member types of avulsion can take place incisional avulsion seen here is characterized by an early incision phase followed by infilling by migrating bar forms aggradational avulsion begins with aggradation followed in time by stream integration into a single downcutting channel we suggest that the type of avulsion is strongly influenced by whether or not the adjacent flood plain is well or poorly drained in both cases subsequent aggradation and channel perching increase the chances that some triggering event will lead to avulsion

tidal mixing events on the deep flanks of kaena ridge hawaii abstract a 3 month mooring deployment august november 2002 was made in 2425 m depth on the south flank of kaena ridge hawaii to examine tidal variations within 200 m of the steeply sloping bottom horizontal currents and vertical displacements inferred from temperature fluctuations are dominated by the semidiurnal internal tide with amplitudes of 0 1 m s 1 and 100 m respectively a series of temperature sensors detected tidally driven overturns with vertical scales of 100 m a thorpe scale analysis of the overturns yields a time averaged dissipation near the bottom of 1 2 10 8 w kg 1 10 100 times that at similar depths in the ocean interior 50 km from the ridge dissipation events much larger than the overall mean up to 10 6 w kg 1 occur predominantly during two phases of the semidiurnal tide 1 at peak downslope flows when the tidal stratification is minimum n 5 10 4 s 1 and 2 at the flow reversal from downslope to upslope flow when the tidal stratification is ordinarily increasi

formation of coastline features by large scale instabilities induced by high angle waves along shore sediment transport that is driven by waves is generally assumed to smooth a coastline this assumption is valid for small angles between the wave crest lines and the shore as has been demonstrated in shoreline models but when the angle between the waves and the shoreline is sufficiently large small perturbations to a straight shoreline will grow here we use a numerical model to investigate the implications of this instability mechanism for large scale morphology over long timescales our simulations show growth of coastline perturbations that interact with each other to produce large scale features that resemble various kinds of natural landforms including the capes and cuspate forelands observed along the carolina coast of southeastern north america wind and wave data from this area support our hypothesis that such an instability mechanism could be responsible for the formation of shoreline features at spatial scales up to hundreds of kilometres and temporal scales up to millennia

tidally generated turbulence over the knight inlet sill very high turbulent dissipation rates above 5 1024 wk g 21 were observed in the nonlinear internal lee waves that form each tide over a sill in knight inlet british columbia this turbulence was due to both shear instabilities and the jumplike adjustment of the wave to background flow conditions away from the sill turbulent dissipation was significantly lower 5 1027 to 5 1028 wk g 21 energy removed from the barotropic tide was estimated using a pair of tide gauges a peak of 20 mw occurred during spring tide approximately twothirds of the barotropic energy loss radiated away as internal waves while the remaining one third was lost to processes near the sill observed dissipation in the water column does not account for the near sill losses but energy lost to vortex shedding and near bottom turbulence though not measured could be large enough to close the energy budget

internal tide scattering at seamounts ridges and islands 1 the scattering of mode 1 internal tides from idealized gaussian topography in a nonrotating ocean with constant and realistic stratifications is examined with a primitive equation numerical model incident mode 1 energy fluxes of 20 and 2000 w m 1 are used to examine the linear regime and a more realistic situation simulations using two dimensional or infinite ridges compare well with ray tracing methods and illustrate how the size and shape of the topography influence wave scattering the height affects energy transmission and reflection while the slope and width determine the conversion of low mode internal tides into beams or higher modes three dimensional topographic scattering is considered for seamounts finite width ridges and islands scattering from finite ridges focuses wave energy directly downstream while scattering from seamounts produces azimuthal energy dispersion scattering to higher wave modes occurs in the lee of near critical and supercritical seamounts and ridges nonlinear interactions transfer energy into the mode 1 m4 internal tide the mellor yamada level 2 5 submodel parameterizes turbulent mixing for the near critical and supercritical ridges with realistic stratification elevated mixing is found over the leading edge of the topography and along a tidal beam up to the first surface bounce a transition from a beam structure near the topography to a low mode structure farther away occurs due to an increased contribution from the mode 1 internal tide as it refracts around the topography and not due to turbulent dissipation internal tide scattering at topography leads to a loss of energy to mixing and to a redistribution of energy flux in space frequency and mode number

simplified process modeling of river avulsion and alluvial architecture connecting models and field data abstract modeling is an invaluable tool for studying sedimentary basin filling and for understanding depositional processes with long recurrence intervals including channel avulsion simplified modeling approaches such as cellular models and process analogue experiments are particularly useful for efficiently exploring alternative hypotheses and evaluating first order controls on river avulsion and alluvial architecture here we review the history and current state of the art in simplified avulsion and alluvial architecture models with a particular focus on how results and insights from these models can be incorporated into field and subsurface studies and vice versa simplified avulsion and alluvial architecture models have proliferated in the past decade providing a wide variety of models to serve as a basis for future coupled field modeling studies we compare features of leading models and discuss avenues for effectively pairing model capabilities with hypotheses and field data outstanding questions highlighted by recent modeling efforts include 1 what thresholds control avulsion initiation in different systems 2 how do floodplain processes and topography influence avulsion dynamics and alluvial architecture 3 what factors determine where avulsion channels stabilize answering these questions will require targeted modeling efforts coupled to data from ancient systems hence our model comparison emphasizes features that can be used to choose or design fit for purpose models and we outline how quantitative data useful for model selection and validation can be obtained from modern systems and ancient deposits matching model goals with targeted questions and model parameters and predictions with quantitative field data will help tighten communication between field and model oriented sedimentary geologists facilitating advances in our understanding of river avulsion and alluvial architecture

observations of overturning in the thermocline the context of ocean mixing abstract the time sequence of events that lead to internal wave breaking and ocean turbulence is investigated data are obtained from depths 100 400 m with a repeat profiling ctd and a coded pulse doppler sonar the instruments were deployed from r p flip during february march 1995 while stationed 30 km west of point arguello california as an aspect of the marine boundary layer experiment although the water depth at the site is 1500 m both rms shear and diapycnal diffusivity as inferred from the average rate and size of overturning events increase with depth below 250 m a deep source of wave energy is implied depth time series of 6 4 m shear s 2 m strain n2 n2 where n is the buoyancy frequency 6 4 m gradient richardson number ri n2 s2 and 2 m effective strain rate the depth derivative of ctd inferred vertical velocity are obtained at 4 minute intervals over a 9 day 100 400 m domain the occurrence of overturns static instabilities of vertical scale 2 m in the observed density

energetics of m2 barotropic to baroclinic tidal conversion at the hawaiian islands abstract a high resolution primitive equation model simulation is used to form an energy budget for the principal semidiurnal tide m2 over a region of the hawaiian ridge from niihau to maui this region includes the kaena ridge one of the three main internal tide generation sites along the hawaiian ridge and the main study site of the hawaii ocean mixing experiment the 0 01 horizontal resolution simulation has a high level of skill when compared to satellite and in situ sea level observations moored adcp currents and notably reasonable agreement with microstructure data barotropic and baroclinic energy equations are derived from the model s sigma coordinate governing equations and are evaluated from the model simulation to form an energy budget the m2 barotropic tide loses 2 7 gw of energy over the study region of this 163 mw 6 is dissipated by bottom friction and 2 3 gw 85 is converted into internal tides internal tide generation primarily occurs along the flanks of the kaena ridge and

internal tide energy over topography 1 the method used to separate surface and internal tides ultimately defines properties such as internal tide generation and the depth structure of internal tide energy flux here we provide a detailed analysis of several surface internal tide decompositions over arbitrary topography in all decompositions surface tide velocity is expressed as the depth average of total velocity analysis indicates that surface tide pressure is best expressed as the depth average of total pressure plus a new depth dependent profile of pressure which is due to isopycnal heaving by movement of the free surface internal tide velocity and pressure are defined as total variables minus the surface tide components corresponding surface and internal tide energy equations are derived that contain energy conversion solely through topographic internal tide generation the depth structure of internal tide energy flux produced by the new decomposition is unambiguous and differs from that of past decompositions numerical simulations over steep topography reveal that the decomposition is self consistent and physically relevant analysis of observations over kaena ridge hawaii and the oregon continental slope indicate o 50 w m 1 error in depth integrated energy fluxes when internal tide pressure is computed as the residual of pressure from its depth average while these errors are small at major internal tide generation sites they may be significant where surface tides are larger and depth integrated fluxes are weaker e g over continental shelves

high resolution sequence stratigraphy of fluvio deltaic systems prospects of system wide chronostratigraphic correlation a basin scale numerical model with a sub grid parameterization of fluvio deltaic processes and stratigraphy was used to study the relation between alluvial sedimentation and marine deltaic deposition under conditions of time invariant forcing the experiments show that delta evolution is governed by a robust morphodynamic feedback loop which provides a link between major avulsions delta lobe switches and sequestration of sediments on the delta plain major avulsions driven by local superelevation result in abandonment of delta lobes and initiation of new lobes progradation of the delta front lengthens the fluvial profile and reduces its gradient which induces aggradation upstream the aggradation in turn causes local superelevation of the channel belt each major avulsion causes a wave of incision to migrate upstream whereas downstream of the avulsion point the rate of aggradation temporarily increases until a new equilibrium situation has been established the feedback loop explains storage and release of fluvial sediments without the need to invoke changes in upstream or downstream controls and provides a plausible mechanism for the generation of high frequency incision aggradation cycles as the sole result of compensational stacking the stratigraphic expression of a depocentre shift is an essentially isochronous surface hence the stratigraphic record of fluvio deltaic systems may be subdivided into a series of units representing intervals during which a channel belt and delta lobe were forming at a fixed location in the basin so called chronosomes fluvio deltaic chronosomes are bounded by abandonment surfaces which are clearly expressed in the marine as well as the fluvial domain the surface marking the abandonment of a particular channel belt and delta lobe correlates with the surface at the base of a new delta lobe landward this surface forms the base of an aggradational package of fluvial sediments downstream of an avulsion site associated with a lobe switch upstream of this site the conformable surface passes into a surface of fluvial incision and terrace formation onlapped by aggradational channel belt deposits identification of the isochronous bounding surfaces of chronosomes in the field offers the attractive prospect of high resolution chronostratigraphic correlation throughout fluvio deltaic systems

multistage exposure history of the 74261 soil constituents gray soil 74261 was collected on the rim of shorty crater in the taurus littrow valley this soil is of particular interest because it may represent old lunar regolith material the cosmic ray exposure history and the isotopic composition of the trapped noble gases of clinopyroxene plagioclase basalt ropy glass and orange glass separates from the 74261 soil were investigated grain size separates from bulk soil were studied in order to determine the isotopic composition of the trapped noble gases all constituents of this soil experienced an exposure to cosmic rays prior to the excavation by the shorty crater formation 17 m y ago except for the orange glasses all soil constituents were preexposed for about 370 m y at an average shielding depth of 75 g cm2 the orange glasses experienced a preexposure for 90 m y at about 60 g cm2 depth the time when the preexposure occurred was riot determined because the fission xe component could not be derived with sufficient precision for 235u 136xe dating the relatively high 40ar 36ar 11 ratio of 5 8 to 8 3 indicates however that the preexposure to volatiles in the lunar atmosphere and to solar and cosmic radiations occurred early in the history cf the moon perhaps about 3 aeons ago in such a scenario the 74261 soil was buried after the preexposure period beneath some shielding material until it was reexcavated by the shorty crater impact

boundary layer intrusions from a sloping bottom a mechanism for generating intermediate nepheloid layers 1 laboratory experiments were used to investigate the growth of intrusions due to internal wave reflection from a sloping boundary when normalized by the incident energy density flux the average intrusion spreading velocity was found to be a linear function of the frequency ratio c where is the frequency of the incident wave and c is the critical frequency at which the wave characteristic has the same angle as the bottom slope evenly spaced layers indicating thin perturbations in the background density gradient developed within the mixing region and spread into the tank interior the vertical spacing of these layers also bore a linear relationship to c a linear model of internal wave reflection suggests that these layers may be related to an isopycnal displacement or overturn scale intrusion growth occurred at a range around the critical frequency and was strongest at slightly supercritical conditions a balance relating the spreading rate of intrusions to the divergence of energy density flux across the boundary layer is derived fitting the laboratory results to this theoretical prediction suggested a weak net buoyancy flux this balance might be of use in predicting spreading rates of intermediate nepheloid layers generated by internal wave mixing at oceanic margins

conditions for branching in depositional rivers it is often taken for granted that rivers organize transport into a single active channel in some net depositional environments however flow of water and sediment is distributed in several stable channels such branching rivers may be confined in valleys anabranching or anastomosed or unconfined on deltas distributaries and their existence confronts us with the very basic question of what governs the spatial organization of channel patterns in sedimentary landscapes current models for equilibrium channel morphology cannot predict the occurrence of branching rivers because they do not consider dynamical processes such as avulsion i e the rapid abandonment of a channel in favor of a new path at lower elevation the requisite conditions for avulsion have been the subject of ongoing debate here we resolve the conditions leading to channel avulsion and show that branching rivers occur when avulsion is the dominant mechanism of lateral channel motion a compilation of field and laboratory data demonstrates that avulsion frequency scales with the time required for sedimentation on channel beds to produce a deposit equal to one channel depth from the relative rates of bank erosion and channel sedimentation we derive a dimensionless mobility number that accurately predicts the conditions under which anabranching and distributary channels occur results may be directly applied to modeling landscape evolution over human and geologic time scales and for inverting formative environmental conditions from channel deposits on earth and other planetary surfaces

generation of diurnal k1 internal tide in the luzon strait and its influence on surface tide in the south china sea 1 the predominance of diurnal surface tides over semidiurnal surface tides in the south china sea scs has been attributed to the near resonance response of the former in the scs recent observations further revealed vigorous internal tides in the northern scs conceivably internal tides generated in the luzon strait could modify the surface tide in the scs we use a three dimensional tide model of the east asian seas to address this issue with a typical summertime stratification of the scs as the initial condition energy budget indicates that one third of the incident k1 surface tide energy is converted to the baroclinic energy over topographic ridges in the luzon strait in comparison with a global tidal model matsumoto et al 2000 our numerical experiments that annihilated or reduced the k1 internal tide in the luzon strait led to up to 50 amplification of the simulated k1 surface tide in the scs this suggests that the baroclinic energy conversion substantially reduces the amplitude of k1 surface tide in the scs the simulated phases in the scs differ little from those calculated from matsumoto s tide model suggesting that the modification is primarily on the amplitude two dimensional surface tidal models lack baroclinic energy conversion in the luzon strait the consequent overestimation of surface k1 tide can be reduced only through precise prescription of sea levels in the luzon strait or assimilation of sea level data

internal solitary wave reflection near dongsha atoll the south china sea internal solitary wave isw reflection is rarely observed in satellite images even in the south china sea scs where the strongest and most energetic isws in the world have been observed compared to the large number of satellite images showing shoaling isw in the scs fewer than ten satellite images have been reported showing isw reflection in this study we collect recent satellite images and implement a numerical model to analyze isw reflection near dongsha atoll in the scs satellite observations show that the reflection appears to be associated with the large amplitude isws generated by strong tidal currents in luzon strait numerical simulations show that isws break when reaching the sloping bottom part of isw energy is reflected by mode 1 waves and their trailing mode 2 waves the mode 1 waves have two types long inertial gravity waves and breaking isw induced short waves they propagate quickly but induce weak vertical velocity and surface imprints mode 2 waves induce strong vertical velocity showing visible signature in satellites horizontal distribution of the energy indicates that a maximum energy of about 2 of the incident energy is contained in a single reflected wave this could explain why the reflected waves are rarely observed because reflected waves must be sufficiently large to be detected in satellite images although individual wave s energy is small in total up to 20 of the incident wave is reflected by the groups of mode 1 and mode 2 waves this suggests that isw reflection has a significant impact on energy distribution over the continental slopes

predicting delta avulsions implications for coastal wetland restoration river deltas create new wetlands through a continuous cycle of delta lobe extension avulsion and abandonment but the mechanics and timing of this cycle are poorly understood here we use physical experiments to quantitatively define one type of cycle for river dominated deltas the cycle begins as a distributary channel and its river mouth bar prograde basinward eventually the mouth bar reaches a critical size and stops prograding the stagnated mouth bar triggers a wave of bed aggradation that moves upstream and increases overbank flows and bed shear stresses on the levees an avulsion occurs as a time dependent failure of the levee where the largest average bed shear stress has been applied for the longest time r 2 0 93 these results provide a guide for predicting the growth of intradelta lobes which can be used to engineer the creation of new wetlands within the delta channel network and improve stratigraphic models of deltas

bifurcation dynamics and avulsion duration in meandering rivers by one dimensional and three dimensional models at river bifurcations water and sediment are divided over two branches the dynamics of the bifurcation determine the long term evolution centuries of the downstream branches potentially leading to avulsion but the dynamics are poorly understood the long term evolution can only be studied by one dimensional models because of computational costs for such models a relation describing the sediment division is necessary but only few relations are available and these remain poorly tested so far we study the division of sediment and the morphodynamics on a timescale of decades to centuries by idealized three dimensional modeling of bifurcations with upstream meanders and dominantly bed load transport an upstream meander favors one bifurcate with more sediment and the other with more water leading to destabilization the bifurcations commonly attain a highly asymmetrical division of discharge and sediment after a few decades to a few centuries depending on combinations of the relevant parameters although past work on avulsions focused on slope advantage we found that bifurcations can be quasibalanced by opposing factors such as a bifurcate connected to the inner bend with a downstream slope advantage nearly balanced bifurcations develop much slower than unbalanced bifurcations which explains the observed variation in avulsion duration in natural systems which branch becomes dominant and the timescale to attain model equilibrium are determined by the length of the downstream bifurcates the radius of the upstream bend a possible gradient advantage for one bifurcate and notably the width depth ratio the latter determines the character of the bars which may result in overdeepening and unstable bars the distance between the beginning of the upstream bend and the bifurcation determines the location of such bars and pools which may switch the dominant bifurcate in fact when the bifurcation is quasibalanced by opposing factors any minor disturbance or a different choice of roughness or sediment transport predictor may switch the dominant bifurcate the division of sediment is nearly the same as the division of flow discharge in most runs until the discharge division becomes very asymmetrical so that a bifurcate does not close off entirely this partly explains the sustained existence of residual channels and existence of anastomosing rivers and the potential for reoccupation of old channel courses we develop a new relation for sediment division at bifurcations in one dimensional models incorporating the effect of meandering the flow and sediment divisions predicted by two existing relations and the new relation for one dimensional models are in qualitative agreement with the three dimensional model these one dimensional relations are however of limited value for wider rivers because they lack the highly three dimensional bar dynamics that may switch the direction of bifurcation evolution the potential effects of bed sediment sorting bank erosion and levee formation on bifurcation stability and avulsion duration are discussed

rates of delta progradation during highstands consequences for timing of deposition in deep marine systems estimated times required for 24 modern river systems to form a shelf edge delta range from 8 5 ka to 116 5 ka depending on fluvial sediment supply delta width shelf volume and shelf transport rates these values indicate that transport of sand into deep marine systems is likely to be significant during third order highstands of relative sea level factors such as shelf transport dynamics may slow delta progradation while submarine canyons cutting the shelf may reduce the time before deep marine deposition occurs interpreting ancient sand rich deep marine strata as lowstand deposits without sufficient palaeogeographic information may not therefore always be appropriate

tidally forced internal waves and overturns observed on a slope results from home abstract tidal mixing over a slope was explored using moored time series observations on kaena ridge extending northwest from oahu hawaii during the survey component of the hawaii ocean mixing experiment home a mooring was instrumented to sample the velocity and density field of the lower 500 m of the water column to look for indirect evidence of tidally induced mixing and was deployed on a slope in 1453 m water depth for 2 months beginning in november 2000 the semidiurnal barotropic tidal currents at this site have a significant cross ridge component favorable for exciting an internal tidal response a large amplitude response is expected given that the slope of the topography 4 5 is nearly the same as the slope of the internal wave group velocity at semidiurnal frequency density overturns were inferred from temperature profiles measured every 2 min the number and strength of the overturns are greater in the 200 m nearest the bottom with overturns exceeding 24 m present at any depth nearly

thermohaline circulation energetics and variability in a single hemisphere basin model a series of numerical experiments have been carried out to explore the variability of the thermohaline circulation and the energetics of halocline catastrophe it is found that when the amplitude of surface freshwater flux is smaller than a critical value the thermohaline circulation is in a thermal mode with deep water formed in the north when the freshwater flux amplitude is supercritical the thermohaline circulation does not reach a single steady state instead the model ocean is in a continuous transition between a slow quasi steady saline mode and an energetic unsteady thermal mode for cases with no wind stress the saline mode is characterized by sinking along the equator for cases with wind stress the saline mode is characterized by intermediate water formation at midlatitude during the saline mode phase the deep water gradually becomes warm and salty thus at the end of the saline mode phase within the northern basin there is cold and relatively fresh water lying on top of warm and salty water such a vertical structure is potentially very unstable because small perturbations can grow supported by the release of potential energy during strong cooling a quantity called the diabatic available potential energy index is introduced as an indicator for such convective instability thus a final equilibrium in the saline mode cannot be reached instead the model ocean flips to a very energetic thermal mode in which violent overturning destroys the vertical stratification after the energy is released the saline cell supported by precipitation in the subpolar basin advances southward and the model ocean returns to the saline mode the whole cycle will be repeated

quantitative metrics that describe river deltas and their channel networks 1 densely populated river deltas are losing land at an alarming rate and to successfully restore these environments we must understand the details of their morphology toward this end we present a set of five metrics that describe delta morphology 1 the fractal dimension 2 the distribution of island sizes 3 the nearest edge distance 4 a synthetic distribution of sediment fluxes at the shoreline and 5 the nourishment area the nearest edge distance is the shortest distance to channelized or unchannelized water from a given location on the delta and is analogous to the inverse of drainage density in tributary networks the nourishment area is the downstream delta area supplied by the sediment coming through a given channel cross section and is analogous to catchment area in tributary networks as a first step we apply these metrics to four relatively simple fluvially dominated delta networks for all these deltas the average nearest edge distances are remarkably constant moving down delta suggesting that the network organizes itself to maintain a consistent distance to the nearest channel nourishment area distributions can be predicted from a river mouth bar model of delta growth and also scale with the width of the channel and with the length of the longest channel analogous to hack s law for drainage basins the four delta channel networks are fractal but power laws and scale invariance appear to be less pervasive than in tributary networks thus deltas may occupy an advantageous middle ground between complete similarity and complete dissimilarity where morphologic differences indicate different behavior

the shaping of continental slopes by internal tides the angles of energy propagation of semidiurnal internal tides may determine the average gradient of continental slopes in ocean basins 2 to 4 degrees intensification of near bottom water velocities and bottom shear stresses caused by reflection of semi diurnal internal tides affects sedimentation patterns and bottom gradients as indicated by recent studies of continental slopes off northern california and new jersey estimates of bottom shear velocities caused by semi diurnal internal tides are high enough to inhibit deposition of fine grained sediment onto the slopes

longshore drift trapped in an expected universe abstract on the basis of a review of current practice in coastal science and engineering with regard to quantitative determination of longshore drift we conclude that coastal scientists and engineers have been trapped in an expected universe of longshore transport sand volumes without critical assessment of assumptions made in pioneering studies as a result workers in this field have come to accept a range of sand volumes and a range of techniques to measure or predict these volumes stabilized by opinion of the leading experts the shortcomings of previous studies and subsequent practice however indicate that these transport volumes and techniques may well be wrong certainly at present we have no dependable verifiable and consistent field measure of net or gross instantaneous or annual longshore transport volumes against which predictions can be compared both field measurements and mathematical model results especially as used for applied purposes and expressed in annual terms are suspect for a number of reasons these include questionable assumptions that lag far behind our current understanding of shoreface processes fudge factor coefficients for equations the lack of storm transport measurement and a cascade of uncertainties that moderate current practice once determined annual longshore transport sand volumes tend to be long lived and in all cases are unverifiable by field measurement more fundamentally the question is raised whether an earth surface system as complex and variable as longshore transport on beaches can ever be quantitatively modeled or measured successfully qualitative mathematical modeling remains a valid and useful approach to understanding the nature of sedimentary processes

remote sensing evidence for baroclinic tide origin of internal solitary waves in the northeastern south china sea 1 evidence for baroclinic tide origin of internal solitary waves isws in the northeastern south china sea is presented based on 116 internal wave packets observed in satellite images from 1995 to 2001 these wave packets can be divided into two types a single wave isw packet containing only one isw with without an oscillating tail and a multiple wave isw packet composed of a group of rank ordered isws all of the 22 single wave isw packets occur in the deep water zone it is suggested that the isws instead of being generated by the lee wave mechanism are developed by nonlinear steepening of the baroclinic tides which are produced by the strong tidal currents flowing over a ridge in luzon strait this suggestion is verified by an ers 2 sar image which records such an evolution process from a baroclinic tide to a single isw in its spatial domain

a simple parameterization of turbulent tidal mixing near supercritical topography abstract a simple parameterization for tidal dissipation near supercritical topography designed to be applied at deep midocean ridges is presented in this parameterization radiation of internal tides is quantified using a linear knife edge model vertical internal wave modes that have nonrotating phase speeds slower than the tidal advection speed are assumed to dissipate locally primarily because of hydraulic effects near the ridge crest evidence for high modes being dissipated is given in idealized numerical models of tidal flow over a gaussian ridge these idealized models also give guidance for where in the water column the predicted dissipation should be placed the dissipation recipe holds if the coriolis frequency f is varied as long as hn w f where n is the stratification h is the topographic height and w is a width scale this parameterization is not applicable to shallower topography which has significantly more dissipation because near critical processes dominate the observed turbul

from tides to mixing along the hawaiian ridge the cascade from tides to turbulence has been hypothesized to serve as a major energy pathway for ocean mixing we investigated this cascade along the hawaiian ridge using observations and numerical models a divergence of internal tidal energy flux observed at the ridge agrees with the predictions of internal tide models large internal tidal waves with peak to peak amplitudes of up to 300 meters occur on the ridge internal wave energy is enhanced and turbulent dissipation in the region near the ridge is 10 times larger than open ocean values given these major elements in the tides to turbulence cascade an energy budget approaches closure

temporal variability of internal tides in the northeast south china sea 1 the northeast south china sea is perhaps the largest internal tide energy source in the world the temporal variability of internal tides was investigated on the basis of 8 month moored acoustic doppler current profiler observations on the continental slope at the dongsha plateau the daily amplitude and phase of diurnal and semidiurnal internal tides were determined from complex demodulation and the dominant spatial and temporal patterns were extracted with empirical orthogonal function eof analysis the eof modal structures showed good agreement with the normal mode solution although vertical phase propagations were apparent the first semidiurnal eof mode corresponded to the first normal mode and the first two diurnal eof modes corresponded to the second and third normal modes respectively the modal structure and energy flux also were consistent with previous observations near the shelf break from the asian seas international acoustic experiment on the other hand the amplitudes of diurnal and semidiurnal eof modes both indicated large irregular fortnightly variations that were not phase locked incoherent with astronomical forcing the study highlighted the importance of incoherent internal tidal motion which accounted for about three fourths of the observed tidal energy

the role of internal tides in mixing the deep ocean abstract internal wave theory is used to examine the generation radiation and energy dissipation of internal tides in the deep ocean estimates of vertical energy flux based on a previously developed model are adjusted to account for the influence of finite depth varying stratification and two dimensional topography specific estimates of energy flux are made for midocean ridge topography weakly nonlinear theory is applied to the wave generation at idealized topography to examine finite amplitude corrections to the linear theory most internal tide energy is generated at low modes associated with spatial scales from roughly 20 to 100 km the richardson number of the radiated internal tide typically exceeds unity for these motions and so direct shear instability of the generated waves is not the dominant energy transfer mechanism it also seems that wave wave interactions are ineffective at transferring energy from the large wavelengths that dominate the energy flux instead it appears that most of

three dimensional numerical simulation of m2 internal tides in the east china sea the east china sea and adjacent seas are one of the most significant generation regions of the m 2 internal tide in the world s oceans in the present study we investigate the distribution and energetics of the m 2 internal tide around the continental shelf edge in the east china sea using a three dimensional numerical model the numerical experiment shows that m 2 internal tides are effectively generated over prominent topographic features such as the subsurface ridges in the bashi luzon and tokara straits the ridges along the ryukyu island chain and the continental shelf slope in the east china sea the former particularly so all of these topographic features are characterized by steep slopes at the depth of the thermocline onto which the m 2 barotropic tide is almost normally incident the m 2 internal tides propagating away from these multiple source regions interfere with each other to create a complicated wave pattern it is found that the calculated pattern of the m 2 internal tide agrees well with topex poseidon altimeter observations the conversion rate from m 2 barotropic to baroclinic energy over the whole analyzed model domain is estimated to be 35 gw roughly 10 of the energy in the m 2 surface tide incident on the prominent topographic features is converted to the m 2 internal tide although about half of the m 2 internal tidal energy is subject to local dissipation in close proximity to the generation sites

piping coarse grained sediment to a deep water fan through a shelf edge delta bypass channel tank experiments 1 it is now generally accepted that deltas that prograde to the shelf edge are able to transport coarse sediment to deep water either with or without sea level changes however it is still unclear how feeder rivers behave differently in the shelf edge delta case to rivers found in a delta that progrades over the shelf a series of nine shelf edge delta experiments are presented to investigate the lateral mobility of the feeder channel at the shelf edge and the associated deep water depositional system under a range of sediment supply rates and shelf front depths in the experiments constant sediment supply from an upstream point source under static sea level led the fluviodeltaic system to prograde over the shallow shelf surface and advance beyond the shelf edge into deep water the feeder river of the fluviodeltaic system became a bypass system once the toe of the delta front reached the shelf edge after the delta front was perched at the shelf edge a submarine fan developed in deep water although remaining disconnected from the delta in this bypass stage no regional avulsion or lateral migration of the feeder river occurred and all sediment from the upstream source bypassed the river delta front and shelf front slope the duration of the bypass stage is proportional to shelf front depth and inversely proportional to sediment discharge the combined duration of the shelf transit phase of the fluviodeltaic system and the bypass phase is the characteristic time scale for the continental margin to anneal transgression inducing perturbation due to high frequency and or high amplitude relative sea level rise the sequential evolution in the experiment compares favorably to the eocene sobrarbe formation a shelf edge delta in spain although natural variations are noted this comparison justifies the application of concepts proposed herein to natural systems and provides insight into interpreting processes from ancient shelf edge delta systems

role of mesoscale eddies in modulating the semidiurnal internal tide observation results in the northern south china sea abstractthe role of mesoscale eddies in modulating the semidiurnal internal tide sit in the northern south china sea scs is examined using the data from a cross shaped mooring array from novem

wave angle control of delta evolution 1 wave influenced deltas with large scale arcuate shapes and demarcated beach ridge complexes often display an asymmetrical form about their river channel here we use a numerical model to demonstrate that the angles from which waves approach a delta can have a first order influence upon its plan view morphologic evolution and sedimentary architecture the directional spread of incoming waves plays a dominant role over fluvial sediment discharge in controlling the width of an active delta lobe which in turn affects the characteristic rates of delta progradation oblique wave approach and a consequent net alongshore sediment transport can lead to the development of morphologic asymmetry about the river in a delta s plan view form this plan form asymmetry can include the development of discrete breaks in shoreline orientation and the appearance of self organized features arising from shoreline instability along the downdrift delta flank such as spits and migrating shoreline sand waves features observed on natural deltas somewhat surprisingly waves approaching preferentially from one direction tend to increase sediment deposition updrift of the river this morphodynamic groin effect occurs when the delta s plan form aspect ratio is sufficiently large such that the orientation of the shoreline on the downdrift flank is rotated past the angle of maximum alongshore sediment transport resulting in preferential redirection of fluvial sediment updrift of the river mouth

baroclinic energy flux at the hawaiian ridge observations from the r p flip abstract estimates of baroclinic energy flux are made in the immediate nearfield september october 2002 and 450 km offshore farfield october november 2001 of the kaena ridge an active barotropic to baroclinic conversion site the flux estimates are based on repeated profiles of velocity and density obtained from the research platform floating instrument platform flip as an aspect of the hawaii ocean mixing experiment energetic beams associated with both semidiurnal and diurnal internal waves are observed in the kauai channel beam depths and orientations are consistent with generation along the upper flanks of the ridge at the far field site the baroclinic energy flux is borne primarily by first mode semidiurnal waves the energy flux associated with the entire spectrum of internal waves is computed by cross spectral analysis significant energy fluxes are found in the inertial diurnal semidiurnal and twice semidiurnal frequency bands the semidiurnal energy flux strongly dominates the s

the generation of large amplitude unsteady lee waves by subinertial k1 tidal flow a possible vertical mixing mechanism in the kuril straits numerical experiments with a two dimensional nonhydrostatic model are performed to investigate tidally generated internal waves in the kuril straits and their effect on vertical mixing the results show that sill scale internal waves at the k1 tidal frequency are confined to the sill slopes because the k1 tide is subinertial in the kuril straits in contrast to previous theories the authors show that intense short internal waves generated at the sill breaks by the subinertial k1 tidal current can propagate upstream as the tidal current slackens theoretical considerations identify these short waves as unsteady lee waves which tend to be trapped at the generation region and grow into large amplitude waves eventually inducing vigorous mixing along their ray paths in particular superposition of a propagating unsteady lee wave and a newly generated lee wave over a sill causes significant wave breaking leading to a maximum vertical diffusivity of 103 cm2 s21 this quite intense mixing reaches down to the density layer of the north pacific intermediate water npiw in contrast the m2 tidal current does not cause such strong vertical mixing because most of generated internal waves propagate away as first mode internal tides and because the barotropic flow amplitude is small the authors therefore suggest the possibility that generation of lee waves through interactions between the k1 current and the bottom topography of the kuril straits contributes to the observed modification of the okhotsk sea water required in the formation of the npiw

quantitative models of sedimentary basin filling summary quantitative modelling of the filling of sedimentary basins was begun in earnest in the 1960s dozens of themes and variations have been proposed since then and have yielded an abundance of idealized stratigraphic patterns as functions of both imposed changes and basin properties post plate tectonic modelling began with rigid lid models which show the stratigraphic signature of subsidence variation this work introduced the connection between stratigraphy and the rheology of the lithosphere rigid lid models are the simplest type of geometric model in which the sediment surface is assigned prescribed geometries usually corresponding to different depositional environments these can reproduce many aspects of overall stratal geometry but are formally restricted to relatively long timescales for which quasi steady surface topography can be assumed so called dynamic models attempt to represent the morphodynamics of the sediment surface by abstracting and averaging short term transport processes most of the dynamic models proposed to date can be seen as special cases of a single general morpho dynamic equation the most important result of the first wave of quantitative basin filling models is that even relatively simple models can produce reasonable stratal patterns we now have a wide array of tools for exploring scenarios searching for general behaviours and effects and making initial quantitative predictions we have also learned that basin response to external forcing as recorded in stratigraphy can be as sensitive to the characteristics of the basin as to the forcing the main brake on the development of basin modelling is not computing power but lack of methods and data for testing the models we have already developed physical experiments which are only just beginning are one means of doing this experimental stratigraphy is a bridge to quantitative field tests which will require collaboration among academic researchers from a wide range of areas and between academia and industry on projects of greater scale and degree of integration than we have seen so far the advancement of quantitative sedimentary geology will also require significant changes in the way the subject is taught at all levels

geomorphic origin of merritt island cape canaveral florida usa a paleodelta of the reversed st johns river abstract the merritt island cape canaveral miccsc sedimentary complex consists of a series of adjacent non conformable beach ridge sets that suggest a multi phase constructional history but the feature s geomorphic and sedimentary origins are not well understood in spite of its notable sedimentary volume surface area 1200 km2 the miccsc lacks a clear sediment source or supply mechanism to explain its presence today previously published u th radiocarbon and osl dates indicate that beach ridge deposition was active during mis 5 130 80 ka on merritt island but has occurred over a shorter younger time interval on cape canaveral proper 6 ka to present in this paper it is proposed that the miccsc is an abandoned paleodelta whose fluvial source provided a sediment supply sufficient for coastal progradation although the miccsc today does not receive an appreciable sediment supply the nearly 23 000 km2 drainage basin of the st johns river may well have provided such a sediment supply during mis 5 times this low gradient fluvial system currently empties to the atlantic ocean some 200 km north of the miccsc near jacksonville florida but may have flowed southward during the time of miccsc sedimentary construction then experienced flow reversal since mis 5 times three possible uplift mechanisms are proposed to explain the northward down tilting that may have reversed the flow direction of the st johns abandoning deltaic construction of the miccsc 1 karst driven flexural isostatic uplift in response to carbonate rock dissolution in central florida 2 glacio hydro isostatic tilting back tilting cycles during loading and unloading of the laurentide ice sheet during the pleistocene and 3 mantle convection driven dynamic topography operating within southeastern north america since the pliocene this example testifies to the sensitivity of low gradient low relief landscapes to various sources of uplift be they isostatic or otherwise

diagnosing a partly standing internal wave in mamala bay oahu 1 an internal partly standing wave in mamala bay hawaii is studied using new observations and the princeton ocean model pom previous work suggested a convergence in the bay of east and westbound waves emanating from kaena ridge and makapuu point respectively new energy flux measurements with shipboard adcp ctd confirm that makapuu point is the eastern source after validating the pom results against observations the model output is modally decomposed and compared with the expected theoretical patterns of kinetic and available potential energy energy flux and group velocity for a partly standing wave agreement is seen for the first baroclinic mode which also contains most of the energy the results confirm previous suggestions of standing wave dynamics in mamala bay

three dimensional model of alluvial stratigraphy theory and applications abstract a three dimensional model of alluvial stratigraphy has been developed to simulate the spatial distribution proportion and connectedness of coarse grained channel belt deposits in alluvial strata as a function of channel belt width floodplain width bankfull channel depth channel belt and overbank sedimentation rates avulsion location and period compaction and tectonism tilting and faulting in this model a floodplain surface of variable width and length is occupied by a single channel belt changes in floodplain topography are produced by spatial and temporal variation of channel belt and floodplain deposition rates and by compaction and local or regional tectonism the location and timing of avulsions are determined by local changes in floodplain slope relative to channel b lt slope and by flood magnitude and frequency the diverted channel belt follows the locus of maximum floodplain slope at the end of each simulation architectural parameters are calculated including channel deposit proportion and connectedness and the dimensions of channel belt sandstone bodies three dimensional perspective diagrams mesh surfaces and two dimensional stratigraphic sections can be plotted to illustrate depositional surfaces time planes and the location and geometry of coarse grained channel belt deposits within finer grained overbank deposits the model predicts that channel belt proportion and connectedness and dimensions of sandstone bodies vary as a function of distance from avulsion points and cross section orientation upstream from avulsion points sandstone bodies have low width thickness ratios because of aggradation in a fixed channel belt immediately downstream from avulsion points channel belts tend to be connected resulting in sandstone bodies with high width thickness ratios avulsion sequences develop where points of avulsion shift up valley with a progressive decrease in avulsion period such sequences may produce successions in which channel belt proportion and connectedness vary vertically with a cyclic period of 103 to 105 years down valley increases in aggradation rate or down valley decreases in floodplain slope for example associated with a rise in base level may result in an increase in channel belt proportion and connectedness because of high avulsion frequencies in down valley regions of the floodplain down valley decreases in aggradation rate as in alluvial fans foreland basins and during base level fall may result in high avulsion frequencies in up valley parts of the floodplain tectonic tilting and faulting locally increase avulsion probabilities and channel belts generally shift toward areas of maximum subsidence under certain conditions however depositional topography may cause channels to shift away from areas of maximum subsidence channel deposit proportion and connectedness are gen rally high near downthrown areas of the floodplain but distribution clustering of channel belts may not be a reliable indicator of fault geometry or displacement models of alluvial architecture that consider only sediment accumulation rate as the main controlling factor are oversimplified the three dimensional model presented here predicts many of the features of channel behavior observed in modern rivers but there is a pressing need for better models and adequate natural data to test them

middle and late holocene avulsion history of the river rhine rhine meuse delta netherlands at present little evidence is available on avulsion rates avulsion frequencies and interavulsion periods of aggrading fluvial systems over time scales of millenia the large number of high quality 14c ages from the holocene rhine meuse delta central netherlands enables a detailed assessment of such avulsion parameters and an evaluation of the factors controlling avulsion frequency 14c histogram analysis indicates that instantaneous avulsion coexistence of successive channel belts for less than a few centuries has dominated in this area interavulsion periods were typically 1000 yr although values of 500 or 5000 yr have also been found avulsion frequency was especially large prior to 4300 cal yr b p indicating a strong control by aggradation rate in turn controlled by sea level rise in addition periods with high avulsion frequency appear to correlate with time intervals characterized by large scale transgressive tendencies the time interval with the largest number of coexisting distributaries corresponds to the period with dominance of anastomosing fluvial style

model estimates of m2 internal tide energetics at the hawaiian ridge 1 a primitive equation model is used to examine the structure and energetics of m 2 internal tides generated at the hawaiian ridge recent estimates based on altimeter data suggest that 20 gw of barotropic tidal dissipation occurs at the ridge with conversion to internal tides believed to be the dominant dissipation mechanism the model simulates an internal tide that accounts for 9 7 gw of radiated energy away from the ridge in the northeast and southwest directions the strongest generation occurs at three sites where enhanced barotropic currents flow across elongated topographic features the depth integrated baroclinic energy flux and energy densities at these sites are on the order of 10 4 w m 1 and 10 4 j m 2 respectively a modal decomposition indicates that 62 of the outgoing energy flux is accounted for by the first internal mode 15 is accounted for by the second mode and less than 5 is accounted for by each subsequent higher mode the tidal dissipation due to bottom friction along the ridge is estimated to be 0 1 gw the level of turbulent dissipation near the ridge owing to tidal energy conversion remains to be determined to assess fully the barotropic baroclinic energy budget

fluviodeltaic response to sea level perturbations amplitude and timing of shoreline translation and coastal onlap 1 alluvial river systems respond to changing sea level by adjusting channel length through shoreline translation and coastal onlap offlap at the alluvial basement transition the magnitude and timing of shoreline translation and coastal onlap offlap relative to the driving sea level signal depend on the dynamics of the fluvial system and have important implications for interpreting ancient fluviodeltaic deposits i obtain analytical solutions to a diffusive geometric model of fluviodeltaic response to sea level perturbations that treats the shoreline and alluvial basement transition as moving boundaries fluviodeltaic behavior is most sensitive to the ratio of the period of eustatic cycling to the fluvial response time which varies with the length and water discharge of the fluvial system the square root of this ratio provides a first order estimate of the fraction of the fluvial long profile affected by eustatic forcing the fluviodeltaic system behaves as a low pass filter with respect to eustatic cycling the amplitude of both shoreline translation and coastal onlap offlap increase with increasing cycle period within a relatively narrow bandwidth in which the fluvial response time is comparable to the cycle period only small time lags develop between shoreline response and eustatic forcing the time lag between maximum coastal onlap and maximum sea level increases without bound as the forcing period decreases relative to the fluvial response time model predictions agree with experimental data and differ considerably from the model predictions of pitman 1978 and angevine 1989 which suggest that fluviodeltaic systems are strong high pass filters to sea level cycling

coastal progradation and sediment partitioning in the holocene waipaoa sedimentary system new zealand abstract over the late holocene highstand the shoreline at poverty bay nz migrated 12 km seaward fed by sediment from the waipaoa river paleo shorelines indicate steadily decelerating progradation possibly signaling changes in forcing on the waipaoa sedimentary system to isolate the cause of this progradation slowdown we reconstruct late holocene tectonics and stratigraphy over the waipaoa coastal plain and nearshore from 7 ka present we find that decreasing rates of sediment storage by coastal progradation were driven by increasing tectonic storage in the steadily subsiding but rapidly growing coastal plain such that net terrestrial storage remained constant at 0 8 mt yr hence changes in shoreline migration were due to autogenic increases in accommodation rather than allogenic changes in forcing furthermore while the waipaoa sediment load is primarily mud reconstructions suggest that progradation was largely controlled by the supply of coarse grained sediment our results suggest that in coastal systems such as the waipaoa where progradation is confined and wave energy is high net accumulation of muds occurs only behind the prograding sandy shoreface which shelters them from wave attack accounting for mud storage in the waipaoa coastal plain and poverty bay suggests that export of muddy sediment to the waipaoa shelf remained roughly constant at 2 0 mt yr from 7 ka until the onset of anthropogenic deforestation in the 19th century

turbulent dissipation on the margins of the south china sea 1 measurements of turbulence along the continental slope shelfbreak and shelf of the northern south china sea are presented from a 10 day measurement program during april 2005 the shelfbreak region was characterized by a deep stratification that appears to be typical of the late winter monsoon this appears to shift the wave field at the shelfbreak from the soliton like depression anomalies observed in previous years to a more complex wave field supporting high frequency waves in both upper and lower depth intervals of the water column on average 30 of the depth integrated turbulent dissipation occurs within 10 m of the bottom where baroclinic energy is preferentially dissipated dissipation levels reaching 50 mw m2 are an order of magnitude larger than those attainable through frictional dissipation of the local barotropic tide and an order of magnitude larger than levels typical in the open ocean

direct breaking of the internal tide near topography kaena ridge hawaii abstract barotropic to baroclinic conversion and attendant phenomena were recently examined at the kaena ridge as an aspect of the hawaii ocean mixing experiment two distinct mixing processes appear to be at work in the waters above the 1100 m deep ridge crest at middepths above 400 m mixing events resemble their open ocean counterparts there is no apparent modulation of mixing rates with the fortnightly cycle and they are well modeled by standard open ocean parameterizations nearer to the topography there is quasi deterministic breaking associated with each baroclinic crest passage large amplitude small scale internal waves are triggered by tidal forcing consistent with lee wave formation at the ridge break these waves have vertical wavelengths on the order of 400 m during spring tides the waves are nonlinear and exhibit convective instabilities on their leading edge dissipation rates exceed those predicted by the open ocean parameterizations by up to a factor of 100 with the disparity in

the breaking and scattering of the internal tide on a continental slope abstracta strong internal tide is generated in the luzon strait that radiates westward to impact the continental shelf of the south china sea mooring data in 1500 m depth on the continental slope show a fortnightly averaged incoming tidal flux of 12 kw m 1 and a mooring on a broad plateau on the slope finds a similar flux as an upper bound of this 5 5 kw m 1 is in the diurnal tide and 3 5 kw m 1 is in the semidiurnal tide with the remainder in higher frequency motions turbulence dissipation may be as high as 3 kw m 1 local generation is estimated from a linear model to be less than 1 kw m 1 the continental slope is supercritical with respect to the diurnal tide implying that there may be significant back reflection into the basin comparing the low mode energy of a horizontal standing wave at the mooring to the energy flux indicates that perhaps one third of the incoming diurnal tidal energy is reflected conversely the slope is subcritical with respect to the semidiurnal tide and the observed

scaling relationships and evolution of distributary networks on wave influenced deltas 1 no genetic model can explain the variability in distributary network pattern on modern deltas here we derive scaling relationships for two processes known to create distributary channels and with these laws construct a simple model for distributary network evolution the first process is mouth bar deposition at the shoreline and subsequent channel bifurcation the second is avulsion the wholesale abandonment of a channel in favor of a new path the former creates relatively small networks with power law distributions of channel length the latter generates relatively few backwater scale distributaries frequency magnitude plots of channel length on natural deltas agree with theoretical predictions and show a clear separation in scale that reflects these processes mouth bar distributary lengths scale with the width of the parent channel and avulsive distributary lengths scale with the backwater length intermediate channel lengths are relatively rare wave energy controls network topology by suppressing mouth bar development thereby preferentially eliminating smaller scale distributaries

energy of nonlinear internal waves in the south china sea 1 four sets of adcp measurements were taken in the south china sea scs these results were combined with previous satellite observations and internal tide numerical model results analysis suggests that strong internal tides are generated in luzon strait propagate as a narrow tidal beam into the scs are amplified by the shoaling continental slope near tungsha island become nonlinear and evolve into high frequency nonlinear internal waves niw internal waves in the scs have geographically distinct characteristics 1 west of luzon strait the total internal wave energy eiw is 10 that predicted by garrett munk spectra egm levine 2002 there is no sign of niw 2 near tungsha island eiw 13 egm strong nonlinear and high harmonic tides are present repetitive trains of large amplitude niw appear primarily at a semidiurnal periodicity with their amplitudes modulated at a fortnightly tidal cycle the rms vertical velocity of niw shows a clear spring neap tidal cycle and is linearly proportional to the barotropic tidal height in luzon strait with a 1 85 day time lag consistent with the travel time of internal tides from luzon strait to tungsha island 3 at the northern scs shelfbreak eiw 4 egm single depression waves are found but no multiple waves packets are evident 4 on the continental shelf eiw 2 egm both depression and elevation niw exist

on the generation and evolution of nonlinear internal waves in the south china sea the nonhydrostatic regional ocean modeling system is applied to the nonlinear internal waves or solitons that are generated at the luzon ridge in the south china sea the luzon ridge near the batan islands is represented by an idealized ridge with a height of 2 6 km on a flat bottom model runs are performed for various ridge shapes and a symmetric tidal forcings the model is in the mixed tidal lee wave regime the barotropic tide over the ridge generates first mode waves through the internal tide release mechanism westward traveling solitons emerge from these first mode waves through nonlinear steepening in the internal tide release mechanism asymmetric tides with strong eastward currents can generate strong westward solitons the eastward current creates an elevation wave with a higher energy density west of the ridge and as soon as the current slackens the wave is released westward on its backslope strong solitons develop the energy density is further enhanced by nonlinearities such as differences in phase speeds and energy fluxes related to lee waves a modal and harmonic decomposition shows the generation of vertical modes and higher temporal harmonics and indicates significant wave wave interaction e g triads in the mixed tidal lee wave regime more energy is contained in the first mode compared to the higher modes hence linear internal tide beams are less well defined and strong solitons develop

energy flux and dissipation in luzon strait two tales of two ridges abstractinternal tide generation propagation and dissipation are investigated in luzon strait a system of two quasi parallel ridges situated between taiwan and the philippines two profiling moorings deployed for about 20 days and a set of nineteen 36 h lowered adcp ctd time series stations allowed separate measurement of diurnal and semidiurnal internal tide signals measurements were concentrated on a northern line where the ridge spacing was approximately equal to the mode 1 wavelength for semidiurnal motions and a southern line where the spacing was approximately two thirds that the authors contrast the two sites to emphasize the potential importance of resonance between generation sites throughout luzon strait baroclinic energy energy fluxes and turbulent dissipation were some of the strongest ever measured peak to peak baroclinic velocity and vertical displacements often exceeded 2 m s 1 and 300 m respectively energy fluxes exceeding 60 kw m 1 were measured at spring tide at the wester

relative importance of fluvial input and wave energy in controlling the timescale for distributary channel avulsion 1 existing avulsion models are decoupled from nearshore processes here i explore quantitatively how the interplay of wave energy with fluvial input of sediment and water controls the aggradation rate and avulsion timescale of a single distributary channel my approach rigorously couples a diffusive moving boundary theory of fluvial morphodynamics with a diffusive treatment of shoreface morphodynamics i use this deterministic model to quantify the time required for channel belt superelevation normalized with channel depth to attain a threshold value for nodal avulsion at a specified channel location increasing the long term wave energy relative to fluvial input by an order of magnitude increases longshore sediment dispersal thereby reducing the rate of channel belt aggradation and associated seaward extension and increasing the avulsion timescale by a factor of approximately 50 far field processes eventually limit the ability of wave energy to suppress avulsion

internal hydraulic jumps and overturning generated by tidal flow over a tall steep ridge abstract recent observations from the hawaiian ridge indicate episodes of overturning and strong dissipation coupled with the tidal cycle near the top of the ridge simulations with realistic topography and stratification suggest that this overturning has its origins in transient internal hydraulic jumps that occur below the shelf break at maximum ebb tide and then propagate up the slope as internal bores when the flow reverses a series of numerical simulations explores the parameter space of topographic slope barotropic velocity stratification and forcing frequency to identify the parameter regime in which these internal jumps are possible theoretical analysis predicts that the tidally driven jumps may occur when the vertical tidal excursion is large which is shown to imply steep topographic slopes such that dh dxn 1 the vertical length scale of the jumps is predicted to depend on the flow speed such that the jump froude number is of order unity the numerical results agree with the theoreti

on the geological significance of clastic parasequences abstract parasequences recognized in clastic sedimentary successions of shallow marine origin are considered by some geologists to be the fundamental building blocks of depositional sequences even though problems in their definition and application have been identified by others who instead advocate their abandonment as formal sequence stratigraphic units to elucidate the geological significance of clastic parasequences and inform the debate on their use in stratigraphy a quantitative characterization of the geometry facies characteristics and timescale of deposition of 1163 parasequences has been undertaken based on a synthesis of data from outcrop and subsurface studies that are available in the scientific literature through a database compilation the attributes of the studied parasequences are analysed with respect to the interpreted geological origin of the units and with consideration of sources of bias and uncertainty particular emphasis is placed on assessing the following i the importance of heuristics and of data types and coverage in the recognition of parasequences ii differences in parasequence characteristics observed across deltaic and shoreface depositional systems and between the quaternary and the ancient rock record iii possible explanations for the range in timescales of deposition of parasequences and iv the role of autogenic dynamics on the development of deltaic parasequences partly based on a comparison with the recent evolution of modern deltas the results demonstrate that parasequence definition and physical correlation suffer from subjectivity and that significant variability exists in the spatio temporal and architectural attributes of clastic parasequences this gives rise to uncertainty that affects the use of parasequences as a framework for comparison of the architecture of packages of strata originating via shoreline regression this uncertainty must be considered when using analogue data for subsurface predictions or when attempting comparative studies of clastic successions

a unifying framework for shoreline migration 1 multiscale shoreline evolution on sedimentary coasts 1 a unifying framework for shoreline migration is presented which synthesizes problems from coastal engineering geomorphology and stratigraphy into a common quantitative basis consideration of characteristic problems from these fields reveals that multiscale shoreline evolution problems naturally break into three classes on the basis of the morphologic unit of evolution shoreface bed versus shoreline planform versus stratigraphic shoreline trajectory shoreline evolution models correspondingly break into morphodynamic morphokinematic and hybrid approaches a generalized shoreline continuity equation is derived the shoreline exner equation which subsumes equilibrium profile models commonly used in geomorphology and engineering and provides a natural framework for interpreting shoreline trajectories preserved in the sedimentary record the shoreline exner equation provides a rigorous basis for geometric and moving boundary models which have significant untapped potential for shoreline migration problems recent advances in computational methods for evolving interface problems make hybrid models particularly attractive for problems of large scale planform shoreline dynamics such as delta evolution

hotspots of deep ocean mixing on the oregon continental slope 1 two deep ocean hotspots of turbulent mixing were found over the oregon continental slope thorpe scale analyses indicate time averaged turbulent energy dissipation rates of e 10 7 w kg and eddy diffusivities of k 10 2 m2 s at both hotspots however the structure of turbulence and its generation mechanism at each site appear to be different at the 2200 m isobath sustained 100 m high turbulent overturns occur in stratified fluid several hundred meters above the bottom turbulence shows a clear 12 4 h periodicity proposed to be driven by flow over a nearby 100 m tall ridge at the 1300 m isobath tidally modulated turbulence of similar intensity is confined within a stratified bottom boundary layer along slope topographic roughness at scales not resolved in global bathymetric data sets appears to be responsible for the bulk of the turbulence observed such topography is common to most continental slopes providing a mechanism for turbulence generation in regions where barotropic tidal currents are nominally along isobath

controls on river delta formation insights from numerical modelling abstract primary hydraulic and sedimentary controls on river deltas were previously postulated in classification schemes in the field of sedimentary geology based on many field observations however detailed mechanistic models were restricted to the river dominated delta class analogous to the concept of morphological conditioning here we show that antecedent stratigraphy controls morphometrics of prograding river dominated delta distributary networks under steady sea level further we use coupled hydrodynamic morphodynamic stratigraphic detailed numerical modelling to assess the influence of wind generated waves and tides on clastic river delta formation our synthetic simulations show that deltas forming under mere riverine forcing prograde via sequences of mouth bar induced flow bifurcation and upstream channel shifting windwave action suppresses sequestration of fine sediments on the developing delta plain entailing relatively smooth shorelines perturbed by a limited number of distributary channels in contrast tide influenced river deltas are found to prograde mainly via lengthening of initially formed relatively stable distributaries as well as being characterized by cyclicity in deposits interbedding of sands and silts these results provide a framework for physics based river delta modelling under various environmental conditions particularly our findings suggest that relatively low energetic basin conditions can already significantly impact morphological and stratal patterns of prograding river deltas

a coupled model for laplace s tidal equations in a fluid with one horizontal dimension and variable depth abstracttide topography interactions dominate the transfer of tidal energy from large to small scales at present it is poorly understood how low mode internal tides reflect and scatter along the continental margins here the coupling equations for linear tides model celt are derived to determine the independent modal solutions to laplace s tidal equations lte over stepwise topography in one horizontal dimension celt is i applicable to arbitrary one dimensional topography and realistic stratification without requiring numerically expensive simulations and ii formulated to quantify scattering because it implicitly separates incident and reflected waves energy fluxes and horizontal velocities obtained using celt are shown to converge to analytical solutions indicating that flat bottom modes which evolve according to lte are also relevant in describing tides over sloping topography the theoretical framework presented can then be used to quantify simultaneous incident and reflected energy fl

influence of steady base level rise on channel mobility shoreline migration and scaling properties of a cohesive experimental delta 1 here we describe results from an alluvial delta physical experiment with and without steady base level rise using a unique cohesive sediment mixture that promotes the development of persistent channels and a rugose shoreline we quantitatively characterize channel network properties as a function of base level rise in a distributary system that reproduces many aspects of the geometry of natural deltas analysis of the experimental data shows clear dynamical differences between the predominantly progadational and aggradational phases of the experiment the experiment was conducted in two phases a first in which the delta prograded into standing water of constant depth in the absence of base level rise and a second during which steady base level rise was imposed on the system forcing a twofold increase in topset aggradation due to greater sediment retention in the fluvial reach the shift in sediment mass balance to enhanced fluvial deposition in the second phase caused channel network mobility to increase reducing the autogenic channel time scale from 23 5 to 12 5 h and supporting a positive correlation between deposition and channel avulsion frequency an independent shoreline time scale that characterizes the dominant time over which shoreline regression is persistent closely correlates with measurements of the channel network 28 3 h during fan progradation and 9 5 h during fan aggradation these metrics suggest a strong coupling between channel network and shoreline kinematics and a more active fluvial surface during fan aggradation by a factor of 2 to 3 similar to the increase in aggradation rate spatial scaling of shoreline roughness reveals that maximum shoreline variability is set by the scale of distributary lobes strong coupling exists between delta growth and shoreline geometry during progradation during aggradation however shoreline variability is not solely due to distributary lobe growth but is also set by shoreline transgression over inactive portions of the delta illustrating a decoupling between fan kinematics and fan geometry this decoupling together with the matched increase in channel network mobility and fluvial aggradation suggest the stratigraphic architecture may not be a strong geometric signal of different fluvial surface conditions either

internal tides in monterey submarine canyon abstract the m2 internal tide in monterey submarine canyon is simulated using a modified version of the princeton ocean model most of the internal tide energy entering the canyon is generated to the south on sur slope and at the head of carmel canyon the internal tide is topographically steered around the large canyon meanders depth integrated baroclinic energy fluxes are up canyon and largest near the canyon axis up to 1 5 kw m 1 at the mouth of the upper canyon and increasing to over 4 kw m 1 around monterey and san gregorio meanders the up canyon energy flux is bottom intensified suggesting that topographic focusing occurs net along canyon energy flux decreases almost monotonically from 9 mw at the canyon mouth to 1 mw at gooseneck meander implying that high levels of internal tide dissipation occur the depth integrated energy flux across the 200 m isobath is order 10 w m 1 along the majority of the canyon rim but increases by over an order of magnitude near the canyon head where internal ti

prototypical solitons in the south china sea 1 surface signatures associated with non linear internal waves are often seen in satellite images of the western south china sea scs slope and shelf observation in the deep sea to the east are rare here we report on the evolution of an energetic packet as it propagated through the deep central basin of the scs toward the western slope and shelf the waves have amplitudes estimated at 170 m half widths of 3 km and phase speeds of 2 9 0 1 m s faster than the mode 1 linear phase speed of 2 6 m s the shape and observed phase speed were consistent with the korteweg devries kdv model over the 65 km path that they were tracked the intrinsic velocity shear of the waves is small compared to pre existing shears and the waves exhibit weak turbulence the kdv fit and a satellite derived estimate of horizontal wave extent imply a westward energy flux of 4 5 gw for each crest

long range propagation and associated variability of internal tides in the south china sea the variability of internal tides during their generation and long range propagation in the south china sea scs is investigated by driving a high resolution numerical model the present study clarifies the notably different processes of generation propagation and dissipation between diurnal and semidiurnal internal tides internal tides in the scs originate from multiple source sites among which the luzon strait is dominant and contributes approximately 90 and 74 of the baroclinic energy for m2 and k1 respectively to the west of the luzon strait local generation of k1 internal tides inside the scs is more energetic than the m2 tides diurnal and semidiurnal internal tides from the luzon strait radiate into the scs in a north south asymmetry but with different patterns because of the complex two ridge system the tidal beams can travel across the deep basin and finally arrive at the vietnam coast and nansha island more than 1000 1500 km away during propagation m2 internal tides maintain a southwestward direction whereas k1 exhibit complicated wave fields because of the superposition of waves from local sources and island scattering effects after significant dissipation within the luzon strait the remaining energy travels into the scs and reduces by more than 90 over a distance of 1000 km inside the scs the k1 internal tides with long crests and flat beam angles are more influenced by seafloor topographical features and thus undergo apparent dissipation along the entire path whereas the prominent dissipation of m2 internal tides only occurs after their arrival at zhongsha island this article is protected by copyright all rights reserved

speed and evolution of nonlinear internal waves transiting the south china sea abstract in the south china sea scs 14 nonlinear internal waves are detected as they transit a synchronous array of 10 moorings spanning the waves generation site at luzon strait through the deep basin and onto the upper continental slope 560 km to the west their arrival time speed width energy amplitude and number of trailing waves are monitored waves occur twice daily in a particular pattern where larger narrower a waves alternate with wider smaller b waves waves begin as broad internal tides close to luzon strait s two ridges steepening to o 3 10 km wide in the deep basin and o 200 300 m on the upper slope nearly all waves eventually develop wave trains with larger steeper waves developing them earlier and in greater numbers the b waves in the deep basin begin at a mean speed of 5 greater than the linear mode 1 phase speed for semidiurnal internal waves computed using climatological and in situ stratification the a waves travel 5 10 faster than b waves until they reach

internal tide reflection and turbulent mixing on the continental slope abstract observations of turbulence internal waves and subinertial flow were made over a steep corrugated continental slope off virginia during may june 1998 at semidiurnal frequencies a convergence of low mode onshore energy flux is approximately balanced by a divergence of high wavenumber offshore energy flux this conversion occurs in a region where the continental slope is nearly critical with respect to the semidiurnal tide it is suggested that elevated near bottom mixing k 10 3 m2 s 1 observed offshore of the supercritical continental slope arises from the reflection of a remotely generated low mode m2 internal tide based on the observed turbulent kinetic energy dissipation rate the high wavenumber internal tide decays on time scales o 1 day no evidence for internal lee wave generation by flow over the slope s corrugations or internal tide generation at the shelf break was found at this site

spatial variation of overbank aggradation rate and its influence on avulsion frequency river avulsions are commonly considered to be driven by the aggradation and growth of alluvial ridges and the associated increase in cross valley slope relative to either the down channel slope or the down valley slope the latter is termed the slope ratio in the present paper therefore spatial patterns of overbank aggradation rate over stratigraphically relevant time scales are critical in avulsion dominated models of alluvial architecture detailed evidence on centennial to millennial scale floodplain deposition has to date been largely unavailable new data on such long term overbank aggradation rates from the rhine meuse and mississippi deltas demonstrate that the rate of decrease of overbank deposition away from the channel belt is much larger than has been supposed hitherto and can be similar to observations for single overbank floods this leads to more rapid growth of alluvial ridges and more rapid increase in slope ratios potentially resulting in increased avulsion frequencies a revised input parameter for overbank aggradation rate was used in a three dimensional model of alluvial architecture to study its effect on avulsion frequency realistic patterns of avulsion and interavulsion periods 1000 years were simulated with input data from the holocene rhine river with avulsions occurring when the slope ratio is in the range 3 5 however caution should be practised with respect to uncritical use of these numbers in different settings evidence from the two study areas suggests that the avulsion threshold cannot be represented by one single value irrespective of whether critical slope ratios are used as in the present study or superelevation as has been proposed by other investigators

asymmetry in holocene river deltas patterns controls and stratigraphic effects abstract understanding controls on sediment distribution in river deltas is paramount to predicting facies relationships and stratal architecture most classifications of deltas have focused on subaerial plan morphology as a simplistic function of river tide and wave energy new work however is revealing the importance of subaqueous deposits and in particular shore parallel sediment redistribution in the shaping of delta planform delta asymmetry has emerged as an important characteristic reflective of patterns of alongshore sediment dispersal asymmetry has been described from onshore and offshore environments from several different types of modern deltas but aspects of asymmetry have not been fully documented and the degree to which these patterns are recorded in deltaic strata is not yet known this study is a comprehensive literature review of sediment distributions in modern deltas focusing on studies with high resolution geomorphic geophysical and geochronological datasets we reviewed 27 deltas using over 100 papers primarily from the past 15 20 years morphological facies and stratigraphic aspects were analyzed across the entire spectrum of deposits from the delta apex to the most distal muds of the prodelta we define quantifiable indices of asymmetry describing upcurrent vs downcurrent distribution of sediment volume sediment area sediment caliber and distributary channels most deltas are asymmetrical to some degree with respect to one or more of these parameters many deltas are increasingly skewed toward the downcurrent side from proximal to distal sub environments i e delta plain delta front prodelta some deltas are skewed toward the upcurrent side in one sub environment and toward the downcurrent side in another sub environment sand is preferentially deposited on the downcurrent side of most deltas but distributary channels tend to develop toward the upcurrent side the highest sand mud ratios are often on the upcurrent side in the lower delta plain but in the delta front these ratios are highest on the downcurrent side these complex patterns of asymmetry reflect different combinations of controls including discharge partitioning lobe abandonment and localized transgression plume deflection by coastal currents dominance of longshore current direction variable subsidence and other factors these processes may result in upcurrent downcurrent variations in clinoform geometry rates of progradation and stratal lapping relationships asymmetry has multiple aspects manifestations and controls even within a single delta but the long term preservation potential of these patterns likely varies depending on depositional setting and shoreline trajectory studies of ancient deltas will be better informed by recognizing the wide variety of controls on sediment distribution and avoiding the tendency toward a single model of delta asymmetry

experimental study of avulsion frequency and rate of deposition in existing models of alluvial architecture it is typically assumed that mean avulsion frequency is independent of sedimentation rate however if avulsion is driven by superelevation of a river bed above its surrounding flood plain one might expect avulsion rate to increase with sedimentation rate we have carried out a series of experiments with laboratory scale fluvial fans in which we measured the frequency of apical avulsions as a function of mean sedimentation rate on the fan avulsion frequency increased strongly with increasing sedimentation rate and then stabilized as mass flows began to influence deposition in the regime of increasing avulsion frequency the added volume of sediment needed to trigger an avulsion decreased with increasing sedimentation rate although our experimental results cannot simply be scaled up to natural rivers they suggest the possibility of coupling between avulsion frequency and sedimentation rate that would be strong enough to change qualitatively the results of existing models of alluvial architecture these models should be applied with caution until avulsion mechanics are better understood

wave controls on deltaic shoreline channel morphodynamics insights from a coupled model it is widely recognized that waves inhibit river mouth progradation and reduce the avulsion timescale of deltaic channels nevertheless those effects may not apply to downdrift deflected channels in this study we developed a coupled model to explore the effects of wave climate asymmetry and alongshore sediment bypassing on shoreline channel morphodynamics the shoreline position and channel trajectory are simulated using a shoreline module which drives the evolution of the river profile in a channel module by updating the position of river mouth boundary whereas the channel module provides the sediment load to river mouth for the shoreline module the numerical results show that regional alongshore sediment transport driven by an asymmetric wave climate can enhance the progradation of deltaic channels if sediment bypassing of the river mouth is limited which is different from the common assumption that waves inhibit delta progradation as such waves can have a trade off effect on river mouth progradation that can further influence riverbed aggradation and channel avulsion this trade off effect of waves is dictated by the net alongshore sediment transport sediment bypassing at the river mouth and wave diffusivity based on the numerical results we further propose a dimensionless parameter that includes fluvial and alongshore sediment supply relative to wave diffusivity to predict the progradation and aggradation rates and avulsion timescale of deltaic channels the improved understanding of progradation aggradation and avulsion timescale of deltaic channels has important implications for engineering and predicting deltaic wetland creation particularly under changing water and sediment input to deltaic systems

relationship between sediment supply and avulsion frequency in braided rivers the interplay between sediment supply s s sedimentation rate s r and the frequency of channel avulsion a f exerts a primary control on alluvial architecture in order to investigate the effect of sediment supply on avulsion frequency four froude scale model experiments of an aggrading braided river were undertaken in which the magnitude of s s was progressively increased over an eightfold range the value of a f increases at a rate slower than the increase in s s contrary to the trend previously reported by bryant falk and paola in their experimental study on alluvial fan dynamics these results suggest that the relationship between a f and s s is dependent upon bed slope and that the response of a f to an increase in s s in unconfined braided rivers may be different than that on steep alluvial fans

necessary conditions for a meandering river avulsion there is as yet no rational basis for predicting the conditions leading to avulsion of a meandering river here we present a conceptual model and quantify it under simplifying assumptions as a first step toward the construction of a stability diagram for avulsion initiation it is assumed initially that a rectangular crevasse channel of arbitrary depth is cut into the levee of a meandering river because the water entering the crevasse channel is derived from relatively high in the main flow it contains low concentrations of suspended solids consequently the crevasse flow is under capacity and the entrance is eroded deepening of the entrance increases the crevasse channel discharge and the concentration of suspended solids supplied because more sediment laden waters are tapped from the deeper flow in the main channel the crevasse entrance is predicted to deepen until its sediment carrying capacity is satisfied by the suspended solids entering from the main channel whether an avulsion will occur for a particular combination of initial conditions depends upon whether there exists steady state hydraulic and sediment transport conditions for crevasse channel depths that are equal to or less than the main channel depth this conceptual model is quantified by writing the unsteady gradually varied one dimensional conservation of mass and momentum equations for water and sediment transport through a main and crevasse channel sediment transport is computed as the integral of a cross sectional mean velocity and a rouse concentration profile solutions show that stable crevasse channels exist only for particular combinations of the initial height of the crevasse bed relative to the water depth in the main channel and the ratio of initial crevasse bed slope to main channel slope

peruvian sediments as recorders of an evolving hiatus for the last 22 thousand years highlights review of sediment archives from the peruvian margin since the lgm focus on the evolutionary feature of the hiatus found in archives modern analogue for current dominated environments for paleo reconstructions new results for erosional potential of the non linear internal waves nliws abstract the peruvian continental margin is characterized by the presence of one of the strongest and most distinct oxygen minimum zones omzs in today s oceans therefore it has long been in the focus of oceanographic and geological investigations observations indicate that omzs are expanding in relation with currently changing climate to advance understanding of the temporal evolution of omzs and climate change complete paleoceanographic and palaeoclimatological reconstructions are needed however the development of paleoenvironmental scenarios for the period since the last glacial maximum at this region was hampered by a ubiquitous hiatus and short term interruptions of the stratigraphical record in the present study we combined the stratigraphical information from 31 sediment cores from the peruvian margin located between 3 and 18 s and water depths of 90 to 1300 m within and below today s omz in order to determine the extent of the hiatus and assess the responsible mechanisms a widespread unconformity and related erosional features omission surfaces and phosphorites were observed in sediment cores from the area south of 7 s depicting a prograding feature on the continental slope from south to north during the deglaciation combining recent oceanographic and sedimentological observations it is inferred that tide topography interaction and resulting non linear internal waves nliws shape the slope by erosion carry sediments upslope or downslope and leave widespread phosphoritic lag sediments while the peru chile undercurrent pcuc transports the resuspended sediments southward causing non deposition this exceptional sedimentary regime makes the peruvian margin a modern analogue for such environments overall our compilation of downcore records showed that enhanced bottom currents due to tide topography interaction were progressively evolving and affected a wider area with the onset of the last deglaciation elevated tidal amplitudes and variability of mid depth water masses i e density changes and hydrodynamics in relation with changing climate were potential reasons of this evolving feature of erosion and reworking additionally erosion and non deposition was observed widest and even was encountered on the continental shelf during the early holocene potentially indicating a strong phase of the pcuc mirroring today s el nino like conditions

an estimate of tidal energy lost to turbulence at the hawaiian ridge abstract an integrated analysis of turbulence observations from four unique instrument platforms obtained over the hawaiian ridge leads to an assessment of the vertical cross ridge and along ridge structure of turbulence dissipation rate and diffusivity the diffusivity near the seafloor was on average 15 times that in the midwater column at 1000 m depth the diffusivity atop the ridge was 30 times that 10 km off the ridge decreasing to background oceanic values by 60 km a weak factor of 2 spring neap variation in dissipation was observed the observations also suggest a kinematic relationship between the energy in the semidiurnal internal tide e and the depth integrated dissipation d such that d e1 0 5 at sites along the ridge this kinematic relationship is supported by combining a simple knife edge model to estimate internal tide generation with wave wave interaction time scales to estimate dissipation the along ridge kinematic relationship and the observed vertical and cross ridge st

sediment transport model for the eocene escanilla sediment routing system implications for the uniqueness of sequence stratigraphic architectures abstract stratigraphic architectures are fundamentally controlled by the interplay at different temporal and spatial scales of accommodation and sediment supply modulated by autogenic responses of the sediment routing system and its constituent segments the flux and caliber of sediment supply is a function of climate catchment area and tectonics in the source regions and unraveling these forcing mechanisms from the observed stratigraphic architecture remains a key research challenge the mid to late eocene escanilla sediment routing system had its source regions in the south central pyrenean orogen northern spain and transported sediment from wedge top basins along tectonic strike to marine depocenters by constructing a volumetric budget of the sedimentary system it has been demonstrated that there were marked changes in the grain size distribution released from the sediment sources and also in the position of the gravel front across three 2 6 myr time intervals from 41 6 to 33 9 ma classical sequence stratigraphic interpretations would relate the movement of depositional boundaries such as the gravel front to changes of base level either in isolation or in combination with sediment supply herein we explore the possibility that the position of the gravel front was primarily driven by variability of grain size distributions released from the source regions as a result of changes in catchment uplift rate and or surface run off using a simple model of sediment transport that captures first order processes we simulate the lateral movement of gravel deposition in the proximal part of the escanilla sediment routing system movement of the gravel front is a function of both accommodation generation and the transport capacity of the sediment routing system we assume that the transport capacity is a linear function of the local slope and the water flux by assuming that the observed thickness of deposits is equivalent to the accommodation available during deposition we then use the stratigraphic architecture to constrain the change in catchment size and water flux over the three time intervals of the escanilla paleo sediment routing system multiple scenarios are investigated in order to find the most plausible tectonic and climatic history model results indicate that during the mid eocene there was an increase in catchment length and sediment flux most likely driven by tectonic uplift in the pyrenean orogen subsequent marked progradation of the gravel front during the late eocene was the consequence of reduced transport capacity due to a reduction in surface run off the latter model result is in agreement with records of pollen taxa that indicate increased climatic aridity in the late eocene the combination of a sediment transport model with a full sediment budget makes it possible to test the non uniqueness of these results

numerical and analytical estimates of m2 tidal conversion at steep oceanic ridges abstract numerical calculations of the rate at which energy is converted from the external to internal tides at steep oceanic ridges are compared with estimates from analytic theories the numerical calculations are performed using a hydrostatic primitive equation ocean model that uses a generalized s coordinate system as the vertical coordinate the model regional ocean modeling system roms estimates of conversion compare well with inviscid and nondiffusive theory in the sub and supercritical regimes and are insensitive to the strength of viscosity and diffusivity in the supercritical regime the nondissipative analytic solution is singular all along the internal tide beams because of dissipation the roms solutions are nonsingular although the density gradients are strongly enhanced along the beams the agreement between model and theory indicates that the prominent singularities in the inviscid solution do not compromise the estimates of tidal conversion and that the linearization used in derivi