sediment size effects on rip channel dynamics abstract field observations indicate that both wave conditions and sediment characteristics exert the controlling effects on the beach states and determine the development of rip channel system although the role of wave conditions on the rip channel dynamics has been extensively studied little attention has been paid to the effects of sediment characteristics in this paper we use a nonlinear morphodynamic model xbeach to investigate the impact of grain size on the formation and nonlinear evolution of rip channels under the condition of uniform sediments we find that the grain size can affect a number of aspects of rip channel dynamics with increasing grain size both longshore migration speed and growth rate decrease significantly while the response and saturation times increase on the other hand the influence of grain size on the geometrical properties of the rip channel seems to be much less pronounced as the wavelength of the channels hardly changes and the three dimensionality features only vary slightly the global analysis method is applied to investigate the mechanism underlying the variations of growth rate and migration speed for varying sediment size the calculations reveal that these variations are mainly caused by the amount of sea bed sediments being stirred up which is clearly grain size dependent quantitative relationships between predicted rip channel characteristics sediment grain size and the hydrodynamic conditions are tentatively established which is amendable to improvement by extending the analysis to other or more general situations

mean currents and sediment transport in a rip channel measurements of hydrodynamics and suspended sediment transport were carried out along a cross shore transect in a rip channel a tidally induced threshold for rip flow initiation was found this threshold depended upon the degree of wave energy dissipation and could be identified through a critical value of the ratio of significant wave height to water depth s in the rip neck with normally incident waves rip current velocities were well correlated with velocities predicted from a simple model involving the onshore mass transport in asymmetric breaking waves crossing the bar hydrodynamics and sediment transport were tidally modulated with strong offshore directed rip currents and associated offshore sediment transport due to the mean flow at low tide at high tide when the rip was inactive the tendency was for weak onshore directed mean flows and or oscillatory incident waves transporting smaller amounts of sediment landward the net result was a gradual lowering of the topographic relief along the rip channel with net deposition seaward of the rip outside the surf zone transport was mainly landward due to oscillatory incident waves

a physical explanation of the relation between flank uplifts and the breakup unconformity at rifted continental margins rift flank uplifts and the breakup or postrift unconformity are characteristic features of many rifted passive or atlantic type continental margins but are not predicted as primary features of simple lithospheric stretching models the explanations that have been proposed for their origin remain controversial either because they call upon special circumstances or because they are difficult to test plane strain finite element models are used in this paper to explore the dynamics of lithospheric necking during rifting and rupture the results agree with the conceptual interpretation that uplift of the rift boundaries to form flank mountains and uplift of the basin responsible for the breakup unconformity are related consequences of regional isostatic compensation of mass that is redistributed during the necking and rupture phases although the amplitudes of these uplifts depend on the model parameter values the relations between and relative signs of these two effects appear to be fundamental the explanation we propose may have been missed in recent studies because there has been a tendency to concentrate on kinematic stretching models which assume local isostatic equilibrium through out the rifting process such an approach is predicated on the assumption that the lithosphere has an insignificant strength or flexural rigidity during extension which is not true if our explanation is correct

observations of rip spacing persistence and mobility at a long straight coastline three years of daily video observations at a long straight beach were analysed to determine the temporal trends and variability of the location spacing persistence and mobility of rips rips were identified at this site on 684 days of the total 947 days 72 occurrence when suitable images were available a median number of 7 rips were observed within the total 2 km study area on days when rips were present no tendency was identified for rips to reoccur in preferred locations alongshore following storm reset events the average alongshore distance between all rips observed was 209 m but with a high standard deviation of 98 m or approximately 47 of the mean rip spacing there was no evidence at this site that rips tended to be regularly spaced alongshore no clear relationship was identified between the number of rips and the prevailing offshore wave conditions including significant wave height peak wave period and incident wave power the majority of rips persisted for 5 or fewer days with an average of 8 days and standard deviation of 9 days rips were stationary 33 of days with migration rates of less than 5 m day observed on 47 of days when rips were present maximum migrations rates of up to 50 m day were observed and generally though not always occurred in the direction consistent with the prevailing offshore swell direction and resulting alongshore current the occurrence of inshore sea breezes may have accounted for the less frequent observations of rip migration against the opposing regional swell direction the results presented at this site complement a recent four year study at a contrasting short and embayed beach detailed in holman et al holman r a symonds g thornton e b and ransinghe r 2006 rip spacing on an embayed beach journal of geophysical research 111 c01006 doi 10 1029 2005 jc002965 17p the specific mechanism s of rip channel infilling at the long straight and littoral drift dominated coastline may have contributed to the observation of shorter rip persistence relative to the embayed site mean rip spacing was observed at both sites to be highly variable irregular and exhibited no identifiable trend associated with the offshore wave conditions this latter observation supports the notion that rips may become rapidly topographically controlled soon after a storm reset event and their location is then primarily determined by the evolving nearshore morphology rather than hydrodynamic forcing this new dataset of daily multi year observations at an open coast site cannot be reconciled with the majority of existing template and instability models for rip formation that predict a relationship between incident wave conditions and regular spacing of rips alongshore rather these observations tend to support emerging theories of rip genesis that predict the irregular and random spacing of rip channels alongshore

the stratigraphic record of prebreakup geodynamics evidence from the barrow delta offshore northwest australia the structural and stratigraphic evolution of rift basins and passive margins has been widely studied with many analyses demonstrating that delta systems can provide important records of postrift geodynamic processes however the apparent lack of ancient synbreakup delta systems and the paucity of seismic imaging across continent ocean boundaries mean that the transition from continental rifting to oceanic spreading remains poorly understood the early cretaceous barrow group of the north carnarvon basin offshore nw australia was a major deltaic system that formed during the latter stages of continental rifting and represents a rich sedimentary archive documenting uplift subsidence and erosion of the margin we use a regional database of 2 d and 3 d seismic and well data to constrain the internal architecture of the barrow group our results highlight three major depocenters the exmouth and barrow subbasins and southern exmouth plateau overcompaction of pre cretaceous sedimentary rocks in the south carnarvon basin and pervasive reworking of permian and triassic palynomorphs in the offshore barrow group suggests that the onshore south carnarvon basin originally contained a thicker sedimentary succession which was uplifted and eroded prior to breakup backstripping of sedimentary successions encountered in wells in the exmouth plateau depocenter indicates that anomalously rapid tectonic subsidence 0 24 mm yr 1 accommodated barrow group deposition despite evidence for minimal contemporaneous upper crustal extension our results suggest that classic models of uniform extension cannot account for the observations of uplift and subsidence in the north carnarvon basin and may indicate a period of depth dependent extension or dynamic topography preceding breakup

effect of hydrodynamics and bathymetry on video estimates of nearshore sandbar position remote sensing of wave breaker patterns clearly visible as high intensity bands in time exposure video images has become a powerful tool to obtain large scale kilometers and long term years time series of nearshore sandbar position however intensity based bar crest positions xi differ from directly measured positions xb by a time varying distance x which is of o 10 m and depends on the offshore wave height h0 the water level 0 and the bathymetry itself the effect of these parameters on x was investigated from simultaneous video observations and bathymetric surveys obtained in the double barred system at egmond aan zee netherlands and from wave model predictions assuming that the roller energy represents image intensity when the wave field over a bar was predicted to be nonsaturated xi was observed and predicted to move offshore as either 0 decreased or h0 increased under saturated conditions x only responded to changes in 0 additional model investigations showed that an increase in outer bar crest depth similar to that observed during interannual bar behavior significantly reduced the x variability at the outer bar and increased the x variability at the inner bar implications of our observational and model findings for studying sandbar position from video imagery are outlined

precambrian tectonic evolution of part of gondwana southwestern australia southwestern australia has a long dynamic precambrian history of crustal fragmentation and aggregation by continental collision and accretion this interpretation contrasts with the widely held belief that most of australia was part of a proterozoic supercontinent in which all tectonic magmatic and metamorphic activity was intracontinental the pilbara and yilgarn cratons are two quite different fragments of archean continental crust that collided to form the capricorn orogen between 2000 and 1600 ma the albany fraser orogen formed along the southern margin of the yilgarn craton during another major continental collision at 1300 to 1100 ma rifting removed the northeastern part of these combined cratons and orogens between 1100 and 700 ma subsequent continental collision along this rifted margin formed the paterson orogen at 700 to 600 ma therefore the assembly of this considerable part of gondwana may not have been completed until about 600 ma

the geodynamics of lithospheric extension lithospheric extension is a fundamental plate tectonic process controlling the collapse of mountain belts the break up of continents and the formation of new oceanic basins this review summarises major advances on understanding a the nature of the ocean continent transition b the origin and evolution of detachment faults c the dynamic strength of continental lithosphere during extension and d the role of magmatism during extension major steps have been made in mapping out the complexities of the ocean continent transition zone particularly in magma poor rifted margins where extensional structures are not masked by voluminous magmatism the role of detachment faulting during rifting seems to be crucial and explains observations on depth dependent stretching of the lithosphere despite new insights into the dynamic behaviour of the lithosphere its rheological response remains a major uncertainty in modelling the evolution of extensional systems likewise the multiple roles that basaltic magmatism may have in modifying extension remain a key question future insights into the global behaviour of extensional systems will be gained by constraining lithospheric rheology through integrating structural evolution and magmatism in the field geodetic measurements of active extension thermochronology seismic surveys and drilling programs 2008 elsevier b v all rights reserved

rip spacing and persistence on an embayed beach 1 four years of daily time exposure images from an embayed beach were examined to study the spacing persistence and location preferences of rips in a natural rip channel system a total of 5271 rip channels was observed on 782 days occurrence statistics showed no evidence of the preferred location pattern associated with standing edge waves trapped in an embayed beach the histogram of rip spacing the primary diagnostic observable for most models was well modeled by a lognormal distribution mean spacing of 178 m however spacings were highly longshore variable time mean of the standard deviation longshore mean of rip spacing was 39 so they are of questionable merit as a diagnostic variable storm driven resets to the longshore uniform condition required by most models occurred only four times per year on average making rip generation models relevant to only a small fraction of the system behavior rip spacings after the 15 observed reset events were uncorrelated with bar crest distance the lifetime of the 324 individual rip channel trajectories averaged 45 6 days rips were equally mobile in both longshore directions but the coefficient of variation of rip migration rates was large even for high migration rates the mean migration rate was well correlated to a proxy for the longshore current r2 of 0 78 thus there is no significant evidence that the formation spacing and migration of rip channels on this beach can be explained by currently existing simple models moreover the alongshore uniform initial conditions assumed by these models are rare on palm beach making the models generally inapplicable

frequency wavenumber velocity spectra taylor s hypothesis and length scales in a natural gravel bed river 1 macroscale turbulent coherent flow structures in a natural fast flowing river were examined with a combination of a novel 2 mhz acoustic doppler beam adb and a maximum likelihood estimator mle to characterize the streamwise horizontal length scales and persistence of coherent flow structures by measuring the frequency f streamwise wavenumber ks energy density velocity spectrum e f ks for the first time in natural rivers the adb was deployed under a range of froude numbers 0 1 0 6 at high reynolds numbers 106 based on depth and velocity conditions within a gravel bed reach of the kootenai river idaho the mle employed on the adb data increased our ability to describe river motions with relatively long 10 m length scales in 1 m water depths the e f ks spectra fell along a ridge described by v f ks where vis the mean velocity over depth consistent with taylor s hypothesis new consistent length scale measures are defined based on averaged wavelengths of the low frequencye f ks and coherence spectra energetic 50 of the total spectral energy low frequency f 1 m s lmwere found to be significantly longer than their corresponding coherence lengths suggesting that the turbulent structures evolve rapidly under these conditions this is attributed to the stretching and concomitant deformation of preexisting macroturbulent motions by the ubiquitous bathymetry induced spatial flow accelerations present in a natural gravel bed river

the many surface expressions of mantle dynamics plate tectonic theory suggests that present day topography can be explained by the repeated interactions between the tectonic plates moving along earth s surface however mounting evidence indicates that a significant proportion of earth s topography results from the viscous stresses created by flow within the underlying mantle rather than by the moving plates this dynamic topography is transient varying as mantle flow changes and is characterized by small amplitudes and long wavelengths it is therefore often hidden by or confused with the more obvious topographic anomalies resulting from horizontal tectonic movements however dynamic topography can influence surface processes and thus enter the geological record it has for example played a role in the establishment of amazon drainage patterns in turn surface processes such as the erosion of topographical anomalies could affect mantle flow this emerging view of dynamic topography suggests that the concept of plate tectonics as the driver of surface deformation needs to be extended to include the vertical coupling between the mantle and the surface unravelling this coupling back in time with the help of models and the geological record can potentially provide unprecedented insights into past mantle dynamics

magmatism and rift margin evolution evidence from northwest australia deep crustal seismic observations from northwest australia demonstrate that the cuvier margin is a volcanic passive margin that formed as greater india rifted away from australia in the early cretaceous formation of the cuvier basin and initial sea floor spreading resulted in the emplacement of exceptionally thick oceanic crust while contemporaneous spreading off the adjacent exmouth plateau formed normal thickness oceanic crust crust of normal thickness was generated in the cuvier basin within 3 4 m y after spreading began there is no evidence for hot spot activity with the appropriate timing or in the appropriate spatial pattern to account for the difference in volcanism along the margin we believe that rapid rifting of the cuvier margin created strong lateral temperature gradients in the subjacent upper mantle at the time of breakup this induced small scale convection in the upper mantle that increased the rate of upwelling and thus the volume of melt generated by decompression partial melting in turn this resulted in a larger volume of magma at the initiation of sea floor spreading and thicker oceanic crust was initially emplaced rifting on the exmouth plateau happened over a much longer period of time and no such convective partial melting occurred the volume of melt associated with rifting was therefore entirely the result of passive upwelling and a normal thickness of oceanic crust was emplaced immediately adjacent to the exmouth plateau

a refined gravity model from lageos gem l2 a refined gravity field model goddard earth model gem l2 has been derived using the lageos orbital data yielding better baseline measurements for the analysis of tectonic plate motion this field also contributes to an improved understanding of long wavelength features such as the sea slope across broad ocean basins through its significant improvement of the long wavelength geoid through degree and order 4 the geoid for these terms has an accuracy estimated at 8 cm in gem l2 gem l2 as in all recent goddard earth models relies heavily on the precise near earth satellite laser ranging data in this case provided by nasa s crustal dynamics program two and a half years of lageos laser data acquired from over 20 well distributed stations were combined with the existing data from the best satellite derived model gem 9 to develop the new lageos model testing shows that the lageos gravity field error at long wavelengths is less than half that for gem 9 independent tests using well determined longitude accelerations of 24 hour satellites have verified the improved accuracy of the new model a comparison of global laser base stations from independent data sets of alternating 15 day data segments over the 2 1 2 years of lageos show total inter station positioning to 1 8 cm when using this new field the same comparison using the 1979 versus the 1980 lageos data yields 5 2 cm this difference in agreement reflects the change in data distribution and other systematic errors along with the tectonic motion which has occurred between these chronologically distinguishable data sets five day average polar motion values with a precision of 10 cm and change in length of day values accurate to better than 5 msec have been derived in the solution the adjustment of these parameters are necessary to achieve the accurate stations and geopotential results in gem l2

extreme erosion on high energy embayed beaches influence of megarips and storm grouping abstract megarips have long been recognised as an important yet poorly documented mechanism of beach erosion on high energy embayed beaches the persistence and cumulative effect of megarips during storm groups are described from three embayed beaches arrifana mt clerigo and amoreira exposed to high energy wave conditions in the mesotidal bedrock dominated southwestern coast of portugal morphological changes determined by topographic monitoring and supplemented by interpretation of digital imagery over two years revealed the development of storm induced megarips which exerted a major influence on beach erosion differences in megarip influence within and between beaches are related to embayment geometry and orientation which significantly influence the nearshore wave field rip location is topographically controlled being determined by alongshore variations in breaking wave height and obliquity along with interaction of wave driven circulation patterns and embayment nearshore topography moderate beach sand loss during individual storms was linked to the development of megarips with associated rip neck and feeder channels extreme erosion however occurred when megarips and feeder channels persisted during successive storms promoting continued erosion and seaward sediment export observations show that once initiated megarip channels persist for several months and continue to act as conduits for offshore sediment transport under non storm conditions the maintenance of such rip circulation systems driven by morphodynamic feedback reduces beach recovery ability until the rip neck and feeder channels are infilled

shoreline variability at a low energy beach contributions of storms megacusps and sea breeze cycles abstract the temporal and spatial variations in shoreline position were analysed at a low energy beach in southwestern australia fronted by an offshore reef to examine the relative contributions of storm events megacusps and diurnal sea breeze cycles to the beach morphologic response the shoreline position did not exhibit a seasonal sinusoidal erosion accretion pattern as often observed at open coast sandy beaches but instead a dynamic morphology dominated by storm and recovery cycles that generated fluctuations of 10 m in the shoreline position during low swell recovery periods the beach rapidly developed megacusps wavelength 100 m amplitude 5 m which despite generating local erosive changes within the megacusps embayments the beach still tended to accrete during these low swell conditions in most cases the embayments coincided with locations of rip currents suggesting that rip currents might contribute to the formation of the megacusps additionally high frequency measurements collected over a series of diurnal sea breeze cycles typical of the site during summer months demonstrated that the erosive impact due to sea breezes was much smaller than that observed during storms despite wave heights often being similar during these events while the beach eroded during intensification of the diurnal sea breeze the beach mostly exhibited net daily accretion due to the low energy swell that dominated the wave spectrum during the overnight and morning hours the formation and alongshore migration of megacusps during sea breeze cycles also contributed to beach accretion overwhelming the erosion due to the consecutive sea breezes events after a few days

tidal asymmetry in sediment resuspension on a macrotidal beach in northwestern australia abstract a field experiment was conducted on a macrotidal beach in northwestern australia to investigate the relation between sediment resuspension and sea bed morphology continuous measurements of waves currents and suspended sediment concentrations were carried out over a period of several hours around high tide concurrent with half hourly visual observations of the sea bed morphology the incident waves had a significant wave height of 0 35 m and period 11 s most of the data were collected outside the surf zone where sediment resuspension was mainly by wave groups and suspended sediment transport was primarily accomplished by mean longshore and cross shore currents the sediment resuspension process was further characterised by a pronounced tidal asymmetry with suspended sediment concentrations and transport rates during the falling tide significantly larger than during the rising tide the asymmetry is attributed to the combined effects of a larger bed roughness and stronger mean nearshore flows during the falling tide the increase in bed roughness occurred during the rising tide under the influence of decreasing bed shear stresses when post vortex ripple morphology developed on a previously plane bed around high tide the ripples were fully developed and had attained a ripple height of approximately 0 005 m and a ripple length of 0 06 0 08 m application of the van rijn van rijn l c 1989 handbook sediment transport by currents and waves delft hydraulics report h461 sediment transport model further confirmed that enhanced sediment resuspension during the falling tide was primarily the result of the development of ripple morphology with increased mean current strengths playing a secondary role the results have major implications for modelling suspended sediment transport on macrotidal beaches

rip currents mega cusps and eroding dunes abstract dune erosion is shown to occur at the embayment of beach mega cusps o 200 m alongshore that are associated with rip currents the beach is the narrowest at the embayment of the mega cusps allowing the swash of large storm waves coincident with high tides to reach the toe of the dune to undercut the dune and to cause dune erosion field measurements of dune beach and rip current morphology are acquired along an 18 km shoreline in southern monterey bay california this section of the bay consists of a sandy shoreline backed by extensive dunes rising to heights exceeding 40 m there is a large increase in wave height going from small wave heights in the shadow of a headland to the center of the bay where convergence of waves owing to refraction over the monterey bay submarine canyon results in larger wave heights the large alongshore gradient in wave height results in a concomitant alongshore gradient in morphodynamic scale the strongly refracted waves and narrow bay aperture result in near normal wave incidence resulting in well developed persistent rip currents along the entire shoreline the alongshore variations of the cuspate shoreline are found significantly correlated with the alongshore variations in rip spacing at 95 confidence the alongshore variations of the volume of dune erosion are found significantly correlated with alongshore variations of the cuspate shoreline at 95 confidence therefore it is concluded the mega cusps are associated with rip currents and that the location of dune erosion is associated with the embayment of the mega cusp

depth dependent stretching a different approach uplift of the crust during rifting is a major problem that has been left unresolved by uniform stretching models various depth dependent stretching models have been proposed to account for this but these require decoupling of the crust and mantle and result in space problems within the mantle a significant modification of these discontinuous nonuniform or depth dependent stretching models is proposed here that accounts for synrift uplift and doming but does not result in these problems the model involves continuous nonuniform stretching within a polygonal region bounded by symmetrical outward sloping boundaries within the mantle this geometry results in thinning of the lithosphere below crustal regions that are less stretched or unstretched crustal uplift resulting from asthenospheric upwelling below these regions this in turn results in stretching factors for the mantle that are less than those for the crust a relationship opposite to relationships derived by discontinuous nonuniform stretching models corollary predictions for mantle derived heat flow within and outside regions of stretching of these two different depth dependent models may allow them to be distinguished from each other and from regions where uniform stretching predominates

rip current type spacing and persistence narrabeen beach australia abstract long term daily rip observations on narrabeen beach australia were made together with daily measurements of wave height period direction surf zone width and beach state a total of 3513 rips were observed on 270 days over a 19 month period results show that rips accompany moderate to high waves and are associated with intermediate beach types rip spacing follows wave conditions increasing in spacing size and intensity as waves rise and conversely as they fall rips are a function of both the prevailing and antecedent wave conditions and the rate and direction of change in wave conditions rips are classified into three types erosion mega and accretion erosion rips are initiated by rising seas in the transverse bar and rip and higher beach states they accompany general beach erosion they are widely spaced narrabeen ys 300 500 m 100 200 m increasing in size and intensity with the waves until the fully dissipative ripless state is reached they are both temporally and spatially highly variable persisting in one location for only hours to a day or so except where topographically controlled they are a major mechanism for the seaward transport of sediment and under extreme conditions may extend over a kilometer seaward of the breaker zone mega rips are large scale 1 km topographically controlled erosion rips that persist when nearshore and or embayment topography prevents the development of the fully dissipative state by inducing wave refraction and persist longshore gradients in surf zone dynamics that in turn drive the rip circulation accretion rips prevail during stable or falling wave conditions usually following erosion rip formation they are associated with general beach accretion are more closely spaced narrabeen ys 170 250 m 100 m less intense and become increasingly topographically arrested by crescentic bars and rhythmic bar beach morphology of the longshore bar trough rhythmic bar and beach and transverse bar and rip beach states they are relatively stable both spatially and temporally and may persist in one location for days to weeks if wave conditions remain favorable they are fair weather rips which may disappear by infilling in the low tide terrace beach state or be destroyed by rising seas rips generally are absent from the reflective low tide terrace and fully dissipative beach states

thermal uplift and erosion across the continent ocean transform boundary of the southern exmouth plateau thermal evolution of the continental lithosphere at a continent ocean transform margin is examined using a two dimensional heat conduction model all heating is assumed to result from the emplacement of new oceanic ridge against the continent the assumed initial continental temperature gradient is probably best applied to regions having experienced finite duration periods of rifting lasting tens of millions of years a deep seismic reflection profile across the southern paleo transform margin of the exmouth plateau northwest australia tests the predictions of the model using this reflection profile it is estimated that up to 3 5 km of sediments have been eroded from the continental rim diminishing to almost no erosion at 60 km from the continent ocean transform boundary this trend and these values for erosion can be matched approximately using the model under the condition of local isostasy a finite difference scheme is employed where the surface elevation is the result of the competing 1 thermal uplift up 2 surficial erosion down and 3 local isostatic rebound up in response to the erosion the model predicts that most of the erosion ceases by 40 ma after ridge emplacement and that of the order of 1000 km3 eroded sediments are shed for every 10 km of transform length

sedimentation and shale tectonics of the northwestern niger delta front abstract the modern day slope and outer shelf of the niger delta are characterized by fault bounded sedimentary depocentres and intervening shale structures some of which are diapiric using two dimensional seismic reflection data and stratigraphic information from five wells the neogene history of a 70 30 km portion of the northwestern niger delta shelf and slope has been examined the shale structures were established by late miocene time probably in response to lateral shale withdrawal from beneath the advancing deltaic load combined with compressional uplift and folding of pro delta strata during pliocene and pleistocene time these structures were buried by the prograding delta and extensional growth faulting commenced stacked onlap relationships and stacked erosional channels indicate that the shale structures continued to influence sedimentation patterns throughout this period throughout the pliocene extension and subsidence were concentrated in a graben like depocentre bounded by both regional and counter regional growth faults during the early pleistocene subsidence on the regional growth fault ceased and growth was transferred to the counter regional growth fault set the geometry and history of the depocentre is analogous to salt related depocentres typical of the northern gulf of mexico and atlantic margins of brazil and angola to explain the history of the niger delta study area a model is proposed in which the miocene pleistocene sedimentary package has slid basinward breaking up during extension thereby creating seaward stepping depocentres examples from analogue modelling studies are used to suggest possible origins for the large counter regional growth faults

genetic stratigraphic sequences in basin analysis ii application to northwest gulf of mexico cenozoic basin the northwest gulf of mexico cenozoic sedimentary wedge illustrates the application of genetic stratigraphic sequence analysis and documents several general conclusions 1 sequences defined by regional marine flooding are the principal genetic stratigraphic units of the basin fill continental margins are characterized by repetitive episodes of basin margin offlap punctuated by periods of transgression and marine flooding of the depositional platform 2 continental margin outbuilding is concentrated at one or more shelf edge deltaic depocenters separated by interdeltaic bights depocenters remain fixed during a depositional episode but commonly relocate during transgression and flooding 3 a distinct syndepositional structural style in prograding continental margins results in sporadic uplift of a basin fringing peripheral bulge and accentuates preservation of shelf margin facies along zones of extensional normal faulting and enhanced subsidence 4 genetic stratigraphic sequences commonly reflect an evolving interplay among two or even three variables for example early cenozoic gulf sequences are most closely related to tectonic events of the intraplate source terrane which in turn affect rate and location of sediment supply and basin margin response to loading late cenozoic sequences more closely reflect proposed eustatic cycles

volcanic margin formation and mesozoic rift propagators in the cuvier abyssal plain off western australia the western margin of australia is characterized by synrift and postrift magmatism which is not well understood a joint interpretation of magnetic anomaly satellite gravity anomaly and seismic data from the cuvier abyssal plain and margin shows that the breakup between india and australia started circa 136 ma m14 and was followed by two rift propagation events which transferred portions of the indian plate to the australian plate post breakup magmatism continued with the emplacement of the wallaby and zenith plateaus 17 18 km thick at their centers along a transform margin two narrow magmatic edifices adjacent to the wallaby plateau sonne and sonja ridges represent an extinct ridge and a pseudofault respectively they formed by excess volcanism probably by lateral migration of buoyant melt along upside down crustal drainage channels from the melt source underneath the wallaby plateau in a mantle plume scenario a small plume 400 km diameter located underneath the rift could have locally uplifted the bernier platform and exmouth sub basin in the early cretaceous and left a track consistent with the azimuth of the wallaby and zenith plateaus in this case ridge plume interaction would have caused two consecutive ridge propagation events towards the plume while the ridge moved away from the hotspot the abrupt end of the hotspot track west of the zenith plateau would be a consequence of the accelerating south eastward motion of the spreading ridge relative to the mantle after 120 ma leaving the mantle plume underneath the indian plate an alternative nonmantle plume scenario is based on the observation that between breakup and chron m0 120 ma the ocean crust in the southern cuvier abyssal plain was formed while the spreading ridge abutted indian continental crust small scale convection may have been initiated during rifting in the early cretaceous and maintained until the wallaby zenith ridge transform intersection passed by the eastern edge of indian continental crust at chron m0

eustatic curve for the middle jurassic cretaceous based on russian platform and siberian stratigraphy zonal resolution we have used the stratigraphy of the central part of the russian platform and surrounding regions to construct a calibrated eustatic curve for the bajocian through the santonian the study area is centrally located in the large eurasian continental craton and was covered by shallow seas during much of the jurassic and cretaceous the geographic setting was a very low gradient ramp that was repeatedly flooded and exposed analysis of stratal geometry of the region suggests tectonic stability throughout most of mesozoic marine deposition the paleogeography of the region led to extremely low rates of sediment influx as a result accommodation potential was limited and is interpreted to have been determined primarily by eustatic variations the central part of the russian latform thus provides a useful frame of reference for the quantification of eustatic variations throughout the jurassic and cretaceous the biostratigraphy of the russian platform provides the basis for reliably determining age and eustatic events the mesozoic section of the central part of the russian platform is characterized by numerous hiatuses in this study we filled the sediment gaps left by unconformities in the central part of the russian platform with data from stratigraphic information from the more continuous stratigraphy of the neighboring subsiding regions such as northern siberia although these sections reflect subsidence the time scale of variations in subsidence rate is probably long relative to the duration of the stratigraphic gaps to be filled so the subsidence rate can be calculated and filtered from the stratigraphic data we thus have compiled a more complete eustatic curve than would be p ssible on the basis of russian platform stratigraphy alone relative sea level curves were generated by backstripping stratigraphic data from well and outcrop sections distributed throughout the central part of the russian platform for determining paleowater depth we developed a model specifically designed for this region based on paleoecology sedimentology geochemistry and paleogeography the curve describes a series of high frequency eustatic events superimposed on longer term trends many of the events identified from our study can be correlated to those found by haq et al 1988 and other sea level studies from other parts of the world but there are significant differences in the relative magnitudes of events because the eustatic curve resulting from this study is based on a stable reference frame the curve can be used in sedimentary basin modeling and as a tool for quantifying subsidence history from the stratigraphy of passive margins basins and other active regions

analysis of observed two and three dimensional nearshore bar behaviour abstract shore parallel sandbar systems often exhibit considerable quasi regular alongshore variations such as crescentic plan shapes therefore morphological change in such systems commonly consists of two and three dimensional variability the former is related to overall on offshore bar migration whereas the latter is induced by the horizontal amplitude growth migration or length scale change of the quasi regular topography in this paper we used a six week data set of bathymetric surveys and video images of breaking induced foam to quantify short term days weeks two and three dimensional variability in the bar crest position of the double barred beach at egmond aan zee netherlands the alongshore uniform response was computed as the mean of each bar crest line whereas the alongshore non uniform behaviour was characterised by the amplitude around each mean a more sophisticated description of the inner bar data set was provided by complex empirical orthogonal function ceof analysis the first complex mode containing about 85 of the variance in the data set corresponded to the amplitude growth and longshore migration of an approximately 600 m long alongshore non uniformity the longshore migration rate varied between 0 and 150 m day and was found to be well related to the longshore component of the offshore wave energy flux the second complex mode explained about 10 of the variance and largely described the alongshore averaged cross shore bar migration the ceof results suggest that short term variability in bar crest position is largely due to changes in the quasi regular topography and not to alongshore uniform on offshore oriented behaviour

quantification of sand bar morphology a video technique based on wave dissipation a technique is presented to remotely measure the scales and morphology of natural sand bars based on the preferential dissipation of wind waves and swell over the crests of the bar photographic or video images are recorded and statistical uncertainties associated with incident wave height modulations removed by averaging time exposures ground truth testing of the technique was carried out as part of the superduck experiment in october 1986 the time exposures generally provided a good mapping of underlying morphology allowing detection of the bar and determination of cross shore and longshore length scales however during high waves persistent surface foam obscures the relationship of image intensity to local dissipation modeled theoretically by dissipation of a random wave field and an enhancement technique of image differencing must be done to remove the bias errors in the estimate of bar crest distance from the shoreline are generally less than 35 but this value depends on the geometry of the particular bar logistic simplicity and quantitative capabilities make this technique very attractive

oceanographic and geophysical applications of satellite altimetry since the last quadrennial report new satellite altimeter data have not been generally available nevertheless important progress has been made from analyses of geos 3 and seasat data the marine gravity field has been determined to an extraordinary resolution and accuracy and important solid earth geophysical interpretations made in the area of oceanography statistical estimates of the variability of dynamic topography have been published methods for incorporating altimeter data into numerical models have been developed and applied and tentative but interesting determinations of the mean circulation have been made in addition altimeter waveform analysis has made it possible to use altimeter data to determine elevations of major ice fields finally important new insights into the nature of satellite ephemeris error and its influence on altimetrically derived sea surface topography have been achieved

lower crustal extension across the northern carnarvon basin australia evidence for an eastward dipping detachment asymmetry and strain partitioning along conjugate margins are often explained in terms of detachment faults nevertheless cogent evidence for their existence remains limited furthermore even when inferred detachment faults are imaged seismically it is difficult to demonstrate that differential displacement has occurred across these surfaces we present tectonic and stratigraphic evidence from the northern carnarvon basin northwest australia that documents the existence of an intracrustal eastward dipping detachment by integrating sequence stratigraphy with kinematic and isostatic models of basin development we conclude that the northern carnarvon basin was formed as a consequence of four extension events 1 a broadly distributed late permian event 2 a predominantly localized rhaetian event responsible for the inception of the barrow and dampier subbasins 3 a localized callovian fault reactivation within the barrow dampier subbasins and 4 a tithonian valanginian event that generated large post valanginian regional subsidence across the northern carnarvon basin with only minor accompanying brittle deformation and erosional truncation the regional distribution and amplitude of the post valanginian subsidence are not consistent with the minor amounts of tithonian valanginian brittle upper crustal extension observed on the margin large portions of the platform were emergent or at very shallow water depths prior to the tithonian valanginian extension to match the distribution and magnitude of the post valanginian thermal type subsidence requires significant lower crustal and mantle extension across the northern carnarvon basin such a distribution of extension implies the existence of an eastward dipping intracrustal detachment having a ramp flat ramp geometry that effectively thinned the lower crust and lithospheric mantle the detachment breached the surface close to the position of the continent ocean boundary west of the exmouth plateau the flat component of the detachment occurred at midcrustal depths 15 km across the plateau and ramped beneath the australian continent lower crustal ductile extension provides a viable mechanism to generate large regional subsidence with little attendant brittle deformation which may explain the paradox that both sides of many conjugate margins appear to be the upper plate

a comparison of two spectral wave models in the southern california bight abstract two models a spectral refraction model longuet higgins and a parabolic equation method pem refraction diffraction model kirby are used to simulate the propagation of surface gravity waves across the southern california bight the bight contains numerous offshore islands and shoals and is significantly larger 300 km by 300 km than regions typically studied with these models the effects of complex bathymetry on the transformation of incident wave directional spectra s0 f 0 which are very narrow in both frequency and direction are difficult to model accurately as s0 f 0 becomes broader in both dimensions agreement between the models improves and the spectra predicted at coastal sites become less sensitive to errors in the bathymetry grid to tidal changes in the mean water depth and to uncertainty in s0 f 0 itself the smoothing associated with even relatively narrow 0 01 hz 5 bandwidth s0 f 0 is usually sufficient to bring the model predictions of shallow water energy into at least qualitative agreement however neither model is accurate at highly sheltered sites the importance of diffraction degrades the predictions of the refraction model and a positive bias o 10 of the deep ocean energy in the refraction diffraction model estimates believed to stem from numerical noise kirby may be comparable to the low wave energy the best agreement between the predicted spectra generally occurs at moderately exposed locations in deeper waters within the bight away from shallow water diffractive effects and in the far field of the islands in these cases the differences between the models are small comparable to the errors caused by tidal fluctuations in water depth as waves propagate across the bight the accuracy of predicted energies at these sites is likely to be limited by the uncertainty in specifying s0 f 0

interactions between continental breakup dynamics and large scale delta system evolution insights from the cretaceous ceduna delta system bight basin southern australian margin 1 the interpretation of two regional seismic reflection profiles and the construction of a balanced cross section through the southern australian margin bight basin are designed to analyze the influence of the australia antarctica continental breakup process on the kinematic evolution of the cretaceous ceduna delta system the data show that the structural architecture of this delta system consists of two stacked delta systems the lower white pointer delta system late albian santonian is an unstable tectonic wedge regionally detached seaward above late albian ductile shales sequential restoration suggests that the overall gravitational sliding behavior of the white pointer delta wedge 45 km of seaward extension i e 27 is partially balanced by the tectonic denudation of the subcontinental mantle we are able to estimate the horizontal stretching rate of the mantle exhumation between 2 and 5 km ma 1 the associated uplift of the distal part of the margin and associated flexural subsidence in the proximal part of the basin are partially responsible for the decrease of the gravitational sliding of the white pointer delta system lithospheric failure occurs at 84 ma through the rapid exhumation of the mantle the upper hammerhead delta system late santonian maastrichtian forms a stable tectonic wedge developed during initial slow seafloor spreading and sag basin evolution of the australian side margin lateral variation of basin slope related to the geometry of the underlying white pointer delta wedge is associated with distal raft tectonic structures sustained by high sedimentation rates finally we propose a conceptual low angle detachment fault model for the evolution of the australian antarctic conjugate margins in which the antarctic margin corresponds to the upper plate and the australian margin to the lower plate

lagrangian observations of circulation on an embayed beach with headland rip currents abstract this study describes the first comprehensive measurements of nearshore current patterns across the entire extent of an embayed beach bounded by headland rip currents a field experiment at whale beach nsw australia provides valuable insights into i embayment wide spatial and temporal flow behaviour ii rates of cross and alongshore water exchange and iii the influence of geological control by headlands on morphology and circulation lagrangian flow data was obtained using 34 gps drifters with 293 individual deployments over a single ebbing tidal cycle during moderate low energy h s 1 m oblique wave forcing in situ wave and current data and bathymetric data were also collected beach morphology was dominated by a large mid beach rip channel with lesser headland rip channels mean flow rates were 0 6 ms 1 in the mid beach channel and 0 4 ms 1 in the headland channels with the majority of cross shore water volume flux 60 through the central channel a weak alongshore current o 0 1 ms 1 was forced by the oblique offshore wave angle rip current velocities flow variability and rate of surfzone exits by lagrangian drifters increased as water level decreased transient currents on a planar bar along the northern half of the beach with velocity standard deviation up to 0 2 ms 1 were not tidally modulated lagrangian time series were used to differentiate four current regimes rip cell rip head planar bar and offshore low energy zone based on mean velocity velocity variability and degree of tidal modulation an increase in surfzone exit rates by drifters was observed from south upwave to north downwave with exit rates per drifter deployment of 22 at the south headland rip 65 at the mid beach open rip and 80 at the north headland rip the high rate of drifter exits contrasts previous observations on open coast beaches observed flow behaviours are attributed to wave shadowing at the upwave protected end of the beach and longshore currents forced by oblique waves deflected offshore at the downwave headland these field observations are in good agreement with recent numerical modelling a relationship between bathymetric variability and current intensity was determined with cross shore average mean velocity correlating with a parameterisation of bathymetric alongshore non uniformity this study demonstrates that flow behaviour and exchange rates can vary along the length of an embayed beach due to geological control this research has implications for transport of organisms nutrients and pollutants is relevant to beach safety practitioners and can be used in calibration of numerical models

effect of orbit error on determining sea surface variability using satellite altimetry satellite altimetry data provided by geos 3 seasat and currently geosat have been successfully used to measure sea surface variability the determination of the ocean variability has been accomplished through either examination of differences along collinear tracks or differences at points where tracks cross in either case it is first necessary to remove the relatively large long wavelength error that exists in the sea surface heights due to inaccuracies in the estimation of the satellite altitude from ground based tracking data this long wavelength altitude error has generally been an order of magnitude larger than the short wavelength variability that is being sought a number of techniques have been applied to eliminate the altitude error with the most common being a simple modeling of the orbit error as a linear trend over the region of interest the intent of this study is to quantify the effectiveness of using a linear model of altitude error when attempting to estimate sea surface variability although there are many sources of altitude error only the influence of gravity modeling errors will be considered here this is justified since this is the dominant error source in the estimation of the satellite altitude from ground based data additionally gravity model errors are the primary source of altitude errors with wavelengths shorter than once per revolution analyses are presented for the case of geosat and topex altitude errors due to the expected modeling uncertainties of the gem l2 and gem t1 gravity fields results are given in terms of the rms of the crossovers after removing linear trends along track and minimizing the crossovers it is found that for scales below 10 000 km both gravity models are equally effective also the higher altitude of topex provides for significantly better performance of the linear orbit error correction model the results indicate that the current gem t1 model induces crossover discrepancies of less than 5 cm rms at scales up to 10 000 km

climate changes control offshore crustal structure at south china sea continental margin a b s t r a c t rifted continental lithosphere subsides as a consequence of combined crustal thinning and mantle lithosphere cooling yet basins on some continental margins experience anomalous subsidence events that postdate active extension deep basins on the northern margin of the south china sea notably the baiyun sag show basement subsidence accelerating after 21 ma postdating extension by several million years we combine geophysical observations and numerical forward modeling to show that loading of the offshore basins by increased sediment flux caused by faster onshore erosion following early miocene monsoon intensification is a viable trigger for ductile flow after the cessation of active extension this illustrates that offshore basin dynamics at continental margins with weak crust can be controlled by onshore surface processes in a newly recognized form of climate tectonic coupling

how preexisting weaknesses control the style of continental breakup continental breakup is a highly variable process differences occur in the relative timing and extent of volcanism uplift and graben formation as well as in the mode and amount of continental extension before breakup we propose a model which reconciles this variability with the previously recognized tendency for breakup to occur along preexisting weak trends continental lithosphere is viewed as a composite material composed of two strong layers one in the upper mantle and one in the middle crust finite element simulation indicates that extensional failure at weaknesses in the mantle causes concentrated extension in the mantle and diffuse extension in the crust this leads to early melt segregation and volcanism margin uplift during the late stages of the extension process and relatively narrow symmetrical extended margins in contrast failure at weaknesses in the crustal strong zone causes focused extension in the crust and diffuse extension in the mantle this produces initial graben formation cooling in the lower crust and upper mantle and broad asymmetrical extended margins volcanism only occurs late in the process failure at laterally offset weaknesses within both strong layers leads to a deformation pattern dominated simple shear thus differences in the prerift configuration of the continental lithosphere can control the overall style of continental breakup

longshore current and wave height modulation at tidal frequency inside the surf zone data were acquired continuously during the 19 day delilah nearshore experiment with a specific objective of examining variability of the longshore current at tidal frequencies it is hypothesized that breaking wave heights inside the surf zone are strong functions of the depth which are modulated by the tidal variations and since radiation stress is a function of the wave height longshore currents are forced at the tidal frequency inside the surf zone the measured longshore current variations at tidal frequency are the same order of magnitude as the mean longshore current variations for moderate wave height conditions indicating that the tide is a dominant mechanism associated with longshore current variability simulations of the magnitude and phase of the longshore current variability with tide elevation using the model by thornton and guza 1986 are used to explain observations the measured tidal elevation and longshore current are in phase in the inner surf zone and out of phase in the outer surf zone as predicted by the model verifying the hypothesis

ripex observations of a rip current system rip current kinematics and beach morphodynamics were measured for 44 days at sand city monterey bay ca using 15 instruments composed of co located velocity and pressure sensors acoustic doppler current profilers and kinematic gps surveys the morphology consisted of a low tide terrace with incised quasi periodic rip channels representative of transverse bars offshore 17 m depth significant wave height and peak period ranged 0 20 3 0 m and 5 20 s the mean wave direction was consistently near 08 resulting in rip channel morphology which evolved in response to the changing wave characteristics an inverse relationship between sediment accreting on the transverse bar and eroding in the rip channel was found the spatial distribution of sediment is reflected in the background rip current flow field the mean velocity magnitudes within the rip channel transverse bars increased offshore onshore with decreasing tidal elevations and increased with increasing sea swell energy eulerian averaged flows were predominantly shoreward on the transverse bars and seaward within the rip channel throughout the experiment resulting in a persistent cellular circulation except during low wave energy the rip current spacing to the rip channel width was less than or equal to two which suggests that the rip currents are influenced by each other and that no two dimensional bar return flow should be present the vertical velocity profile on the bar indicated that the flow was predominantly shoreward the flow field within the surf zone was depth uniform except for significant shear occurring near the surface owing to stokes drift the wave induced transport hypothesis is evaluated d 2005 elsevier b v all rights reserved

the morphosedimentary behaviour of a headland beach system quantifying sediment transport using fluorescent tracers abstract embayed beaches occurring along rocky coasts with headlands controlling hydrosedimentary processes are considered to be constrained sedimentary systems with little longshore sediment transfer between them such beaches are likely to narrow under rising sea level and a deficit of fluvial sediment and because beaches act to dissipate wave energy cliff erosion is expected to increase knowledge of hydrosedimentary processes in fetch limited environments such as embayed beaches is therefore a fundamental management tool in erosion prone coastal areas such as the algarve region southern portugal the main objective of this work is to quantify longshore sediment transport lst using a sedimentary tracer and compare the results with the ones derived via empirical formulae from several authors to achieve this we establish the spatial and temporal patterns of stained sediment and relate these to marine climate data over six tidal cycles for three adjacent embayed beaches the values of displaced sand estimated using tracer and scaled to mixed layer depth were compared with the results of five well known lst formulae to assess the applicability of each model to mesotidal environments of the lst formulae used the cerc 2013 shows to be the most accurate in our study area this study demonstrates the efficacy of using tracers for tracking and quantifying the sediment transport and as a basis for assessing the viability of different lst formulae for representing transport in these types of environments the work also shows the importance of the morphological control of hydrosedimentary processes including the disruption of longshore drift and the sediment transfer promoted by shore platforms

storm driven changes in rip channel patterns on an embayed beach this paper introduces a semi automatic computer algorithm designed to detect rip current channels in video imagery as a case study this method is applied to 3 3 years of video data from an embayed beach to demonstrate the link between antecedent surf zone morphology wave energy and up down transitions in beach state an objective measure of rip channel change was developed to define six significant rip reconfiguration events and relate these events to wave energy over the period of study no complete resets of the nearshore morphology occurred the analysis indicates that direct correlation of rip patterns with the instantaneous wave conditions is not a useful way to demonstrate how rips and waves interact the average wave energy over a period of ten days combined with storm duration were good indicators of rip channel change demonstrating that in general beach morphology responds with a time lag to changes in forcing rip channels with a short cross shore length and narrow alongshore spacing responded faster to changes in wave conditions than rips with a long cross shore length and wider alongshore spacing to force changes in the rip morphology longer rip channels required wave events of higher energy and or a longer duration offshore islands protect the beach under certain wave approach angles sometimes resulting in a dual width surf zone which was narrow at the sheltered end and wide at the exposed end of the beach the wider surf zone end was characterised by three dominant and persistent rip channels whereas the narrow surf zone section contained a number of smaller rips which evolved rapidly under wave forcing our observations demonstrate the importance of rip channel size in controlling the response time of nearshore morphology

estimating palaeo water depth from the physical rock record abstract palaeo water depth and its change with time is a key factor in environmental analysis and in sedimentology in general the literature published on this topic and on related subjects however is substantial albeit scattered within and between different communities geologists oceanographers engineers physicists etc here i focus on proxies for palaeo bathymetry as obtained from the physical rock record floral faunal and chemical evidence for palaeo water depth being equally important must be dealt with elsewhere an assessment of the main depth related parameters in actualistic settings is presented and their applicability to the fossil sedimentary record discussed the outcome reveals the full complexity of this topic there is for example no such thing as an average wave base depth applicable to a wide range of fossil case settings this because wave and current climates in general and particularly so in neritic and coastal settings are intriguingly complex furthermore observations from modern oceans are not a priori applicable to fossil ones acknowledging these problems and limitations critically evaluated quantitative proposals for depth indicative facies and features in the sedimentary record are presented and error bars discussed

holocene evolution of the western orinoco delta venezuela the pristine nature of the orinoco delta of eastern venezuela provides unique opportunities to study the geologic processes and environments of a major tropical delta remote sensing images shallow cores and radiocarbon dating of organic remains form the basis for describing deltaic environments and interpreting the holocene history of the delta the orinoco delta can be subdivided into two major sectors the southeast sector is dominated by the rio grande the principal distributary and complex networks of anastomosing fluvial and tidal channels the abundance of siliciclastic deposits suggests that fluvial processes such as overbank flooding strongly influence this part of the delta in contrast the northwest sector is represented by few major distributaries and overbank sedimentation is less widespread relative to the southeast sector peat is abundant and occurs in herbaceous and forested swamps that are individually up to 200 km 2 in area northwest directed littoral currents transport large volumes of suspended sediment and produce prominent mudcapes along the northwest coast mapping of surface sediments vegetation and major landforms identified four principal geomorphic systems within the western delta plain 1 distributary channels 2 interdistributary flood basins 3 fluvial marine transitional environments and 4 marine influenced coastal environments coring and radiocarbon dating of deltaic deposits show that the northern delta shoreline has prograded 20 30 km during the late holocene sea level highstand progradation has been accomplished by a combination of distributary avulsion and mudcape progradation this style of deltaic progradation differs markedly from other deltas such as the mississippi where distributary avulsion leads to coastal land loss rather than shoreline progradation the key difference is that the orinoco delta coastal zone receives prodigious amounts of sediment from northwest moving littoral currents that transport sediment from as far away as the amazon system 1600 km late holocene progradation of the delta has decreased delta plain gradients increased water levels and minimized overbank flooding and siliciclastic sedimentation in the northwest sector these conditions coupled with large amounts of direct precipitation have led to widespread peat accumulation in interdistributary basins because peat forming environments cover up to 5000 km 2 of the delta plain the orinoco may be an excellent analogue for interpreting ancient deltaic peat deposits

rip channel development on nonbarred beaches the importance of a lag in suspended sediment transport 1 on approximately planar beaches rip channel development is often preceded by a period in which jet like rip currents develop in apparently random locations and dissipate after minutes to tens of minutes the subsequent development of sharp edged trough like channels extending across the surf zone suggests that rip currents on planar beaches cause local erosion conversely channels are known to cause localized offshore directed flow and once channels have formed on approximately planar beaches rip currents no longer occur in apparently random locations but are restricted to the locations of the channels apparently the excavation of channels by rip currents on planar beaches triggers a positive feedback between the morphological development and the flow however theoretical analysis indicates that when depth increases with distance from shore and sediment transport is treated as a function only of local flow conditions channel development in the vicinity of a rip current may not occur in numerical simulations using a cellular model of nonbathymetrically driven rip currents in which sediment flux on a planar beach is driven by approximately realistic rip current velocity patterns deposition occurs under parts of the rip currents especially in the seaward half of the surf zone and these rip ridges cause a negative feedback in these simulations as in most models treating surf zone sediment transport sediment flux is assumed to be strictly a function of local hydrodynamic conditions however observations of sediment laden rip current plumes extending well beyond the surf zone suggest that suspended sediment transport is not always in equilibrium with local conditions other simulations employ a treatment of suspended sediment transport that allows for a lag in the adjustment of fluxes to the changes in local hydrodynamic conditions that the sediment is advected through with this nonlocal sediment transport the flux of sediment out of the surf zone in a rip current is increased ridges do not develop under any part of the rip currents and the morphodynamic feedback becomes positive

morphodynamics of intermediate beaches a video imaging and numerical modelling study four years of time exposure images of palm beach sydney australia are used to a develop a phenomenological scheme based on surf zone properties for the classification of the four intermediate beach states low tide terrace ltt transverse bar rip tbr rhythmic bar beach rbb and longshore bar trough lbt identified by wright l d short a d 1984 morphodynamic variability of surf zones and beaches marine geology 56 93 118 and b determine environmental conditions associated with the different beach states the overall agreement between beach state classifications obtained by visual classifications of images and those predicted by the phenomenological scheme exceeds 90 analysis of time series of intermediate beach states and wave data indicates that the nearshore morphology continuously cycles through the four beach states in response to variations in the incident wave energy the results also indicate that lbt morphology is produced by high energy wave conditions erosive while the progression towards ltt occurs under lower waves accretive a depth averaged morphodynamic model is employed to determine the morphodynamic processes governing the rbb to tbr and tbr to ltt transitions model results indicate that the rbb to tbr transition is governed by the erosion of the original longshore bars in the initial rbb morphology and the formation and subsequent welding of shoals in the lee of the original longshore bars model results also indicate that rbb to tbr transitions can occur under shore normal and oblique wave conditions the tbr to ltt model simulations indicate that under obliquely incident waves progressive erosion of the transverse shoals and deposition of the eroded sediment in rip channels gradually diminishes the longshore variability in morphology resulting in an ltt morphology

rip current review rip currents are shore normal narrow seaward flowing currents that originate within surf zone extend seaward of the breaking region rip head and can obtain relatively high velocities within the last decade there have been a significant number of laboratory and field observations within rip current systems an overview of rip current kinematics based on these observations and the scientific advances obtained from these efforts are synthesized rip current flows are partitioned into mean infragravity very low frequency vorticity and tidal contributions and it is found that each contributes significantly to the total data from the laboratory and the field suggest that the rip current strength increases with increasing wave energy and decreasing water depths the maximum mean current occurs inside the surf zone where the maximum forcing is present owing to the dissipation of waves

alongshore rip channel migration and sediment transport abstract the relationship between alongshore rip channel migration and sediment transport is investigated using time averaged video images to identify the positions of rip channels at three sites along southern monterey bay california over 1 3 year periods daily migration rates are calculated for each site the basic coastal engineering research center cerc formula for bulk alongshore sediment transport is used to estimate alongshore transport rates at each site based on hourly input of directional wave spectra measured at an offshore buoy and shoaled and refracted to the nearshore the wave transformation model is validated by comparison with measured nearshore directional wave spectra in 13 m water depth an expanded cerc formulation that accounts for the additional forcing of alongshore wave height gradients is also tested but rejected correlation coefficients between daily rates of calculated alongshore transport and rip channel migration are low 0 01 0 44 and 0 38 at the sand city stilwell and marina sites respectively the problem of digitization noise associated with video based rip channel identification is discussed and several filtering methods are used to increase the signal to noise ratio and obtain improved correlations results indicate that higher frequency migration events on time scales shorter than 8 days will likely be obscured below the digitization noise floor but that longer period oscillations such as tidal and seasonal cycles show up clearly in both rip migration and sediment transport datasets daily rates are cumulatively summed to generate time series of representative overall rip migration distance and net alongshore transport and the cumulative summing procedure is shown to be similar to applying a low pass filter mean migration is demonstrated to lag behind net transport in the summertime suggesting that a minimum level of alongshore forcing is required to generate detectable rip migration rip channel migration and net alongshore transport have correlation coefficients that range from 0 76 to 0 94 at the three sites

temporal and spatial variations in sand budgets with application to southern monterey bay california abstract often in calculating a sediment budget there is an unknown contribution which is ascribed to the residual of the balance and the budget cannot be fulfilled the temporal and spatial variations and sediment size within a littoral cell provide additional information to define sediment budgets the approach here is to consider only medium to coarse sand grain size 0 25 mm over a long time period during significant changes of natural and anthropogenic inputs and losses that allow calculating several budgets to isolate the unknown inputs and outputs to close the system considering only medium to coarse sand eliminates including the cross shore transport of fine sand by waves which is poorly quantified to demonstrate these concepts shoreline recession rates measured over a 101 year time period are used to calculate sediment budgets for southern monterey bay california littoral cells the coarse sandy shoreline is backed by extensive bluffs and dunes that reach 44 m in elevation intensive sand mining of coarse sand derived directly from the beach and surfzone started in 1927 and continues today a contribution of about 100 k m 3 year of medium to coarse sand from the salinas river is calculated from measured shoreline accretion for the period 1910 1945 starting when the river first flowed into the littoral cell prior to damming of the river and significant losses owing to sand mining sediment budgets are calculated for 1940 1989 and 1989 2011 to spatially identify the loss of about 200 k m 3 year attributed to different mining operations that captured the littoral transport the primary contributions of medium to coarse sand to the littoral system is approximately 180 k m 3 year from the eroding dunes and beaches only a quarter of the dune sand is found to be compatible with the coarser beach sand with the finer fraction carried offshore a conclusion is that sand mining is the cause of the observed high recession rates

quartz cementation history of sandstones revealed by high resolution sims oxygen isotope analysis abstract evaluating the timing and origin of quartz cement is central to understanding how porosity is lost in sandstones during burial kinetic models of quartz cementation have been calibrated using large scale datasets but have never been tested at the microscopic level at which cement forms here we use high precision in situ oxygen isotope analyses on sandstone from the jurassic ness formation from the north sea to reveal the growth history of single quartz overgrowths to a resolution of 2 m measured 18o cement range from 28 to 20 v smow in early to late cement and are consistent with quartz cementation models that propose the bulk of quartz precipitates as a continuous process beginning at 60 70 c quantitative x ray diffraction analyses and clay mineralogical analysis of interbedded shales are inconsistent with a silica source from shale implying that silica for the cement is sourced internally to the sand these isotope data are broadly consistent with predictive conceptual quartz cementation models and provide a critical link from micron scale measurements to basin scale predictions and observations

coupled sandbar patterns and obliquely incident waves 1 in double sandbar systems the alongshore variability in the inner bar often resembles that of the outer bar suggesting that the outer bar acts as a morphological template for the inner bar earlier observations have indicated that this resemblance also termed coupling may take several forms here we apply a nonlinear 2dh morphodynamic model with time invariant forcing to show that the angle of wave incidence is crucial for the alongshore variable morphodynamic evolution of the inner bar for a given crescentic outer bar in contrast to previous modeling efforts of double barred systems which mostly used highly idealized boundary conditions we force our model with realistic hydrodynamics and bathymetrical data derived from video observations at the double barred gold coast australia the results show that for small angles of wave incidence 10 over a crescentic outer bar cell circulation patterns govern the flow at the inner bar giving rise to rip channels that incise the inner bar at the locations of the landward perturbations in the outer bar horns on the other hand for obliquely incident waves 10 20 over a crescentic outer bar the circulatory nature of the flow disappears and gives way to a meandering alongshore current the offshore directed sections of this meandering alongshore current erode the inner bar downdrift of the outer bar horns leading to landward perturbations of the inner bar that are coupled to the outer bar horns an observed coupling type that had not been reproduced as yet oblique wave incidence thus proves to be crucial to the development of this type of sandbar coupling as previously hypothesized from sandbar coupling observations at the gold coast additional simulations including tidal water level variations and bar depth variations demonstrate the robustness of our findings

uniform sense normal simple shear of the continental lithosphere geophysical studies suggest that the thin crust characteristic of the basin and range province extends eastward beneath the west margin of the colorado plateau and rocky mountain regions in arizona and utah zones perhaps over 100 km wide may be defined bounded on the west by the east limit of upper crustal normal faults that account for more that 10 extension and on the east by the east limit of thinning beneath the colorado plateau a discrepancy exists within these zones between the negligible extension measurable in the upper crust and the substantial extension apparent from crustal thinning assuming the discrepant zone crust was as thick as or thicker than the colorado plateau rocky mountain crust prior to extensional tectonism if various theories appealing to crustal erosion are dismissed mass balance problems evident in the discrepant zones are most easily resolved by down to the east normal simple shear of the crust moving lower and middle crustal rocks that initially were within the zones up and to the west to where they now are locally exposed in the basin and range province west of the discrepant zones in both arizona and utah east directed extensional allochthons with large displacement are exposed these geophysical and geological observations complement one another if it is accepted that the entire crust in both arizona and utah failed during extension on gently east dipping east directed low angle normal faults and shear zones over a region several hundred kilometres wide large scale uniform sense normal simple shear of the crust suggests the entire lithosphere may do the same such a hypothesis predicts major lithospheric thinning without crustal thinning will occur in plateau areas in the direction of crustal shear in the case of the arizona utah and red sea extensional systems and possibly the death valley extensional terrain a broad topographic arch typically 1500 2000 m higher than the extended terrain is present suggesting lithospheric thinning in areas predicted by the hypothesis

analysing decadal scale crescentic bar dynamics using satellite imagery a case study at anmok beach south korea abstract understanding long term sandbar dynamics can be crucial for informed coastal zone management but is often hampered by data availability to increase the number of sandbar observations available from bathymetric surveys this study proposes and evaluates a method to manually extract the sandbar location using freely available satellite imagery for the case study of anmok beach in south korea validation of the satellite extracted sandbar locations against 9 in situ measurements shows good agreement with errors well within the pixel resolution of the satellite imagery i e 30 m for landsat missions the applicability of the method is constrained to locations where 1 the cross shore crescentic length scales are larger than the image resolution 2 frequent wave breaking and clouds are absent and 3 the water clarity is sufficient to enable the manual extraction of the sandbar crest line using the additional sandbar observations from the satellite imagery significantly increases the temporal extent and resolution of the dataset for anmok beach this allows the study of sandbar characteristics dynamics and impacts of human interventions to an extent that would not have been possible without the satellite imagery within the study period 1990 2017 it is found that the sandbar maintains a persistent crescentic pattern that is only altered during prolonged and very intense storm conditions the cumulative alongshore migration of the sandbars is investigated and found to be in the order of hundreds of meters over the 27 years study period comparing the sandbar characteristics prior and after the construction of gangneung port shows that both the amplitudes and wavelengths of the sandbar crescents near the port have decreased after its construction

application of the gem t2 gravity field to altimetric satellite orbit computation as part of a continuing effort to provide improved orbits for use with existing altimeter data we have recomputed ephemerides for both the seasat and geosat exact repeat altimeter missions the orbits were computed in a consistent fashion using the goddard earth model t2 gem t2 gravity field along with available ground based tracking data such an approach allows direct comparisons of sea level between the two altimeter systems evaluation of the resulting ephemerides indicates that root mean square accuracies of 30 50 cm have been achieved for the radial component of the orbits for both satellites an exception occurs for the last year of the geosat exact repeat mission when the rms radial orbit accuracy degrades to the 1 m level at times owing to the inability to adequately model the drag force arising from the increased solar activity

depositional wedge cycles on evolving plate margin western and northwestern australia the barrow province lies at the transition between the western and northwestern rift domains of the australian continental margin as these domains evolved during the mesozoic five depositional wedge sequences developed in the eastern half of the province each sequence possesses distinct stratigraphic and sedimentological characteristics which differs from the others as a function of proximity direction and style of rift related diastrophism along the evolving margin the first depositional sequence developed during the triassic as part of a regional tethyan facing passive margin clastic wedge this event was followed during the early and middle jurassic by a second more provincial sequence in response to early syn rift fault related subsidence by the callovian a deep anastomosing rift complex had formed and deposition of the third late jurassic cycle was initially very localized confined by the new graben topography however stratigraphic continuity returned during post breakup subsidence of the northwestern margin as subregional depositional systems began to bury the fragmented borderland this cycle was terminated at the close of the jurassic in response to renewed rifting between western australia and greater india sediment stripped from t e uplifted rift shoulders fed the northerly prograding barrow delta system to form a fourth depositional sequence during the late jurassic early neocomian the final cycle commenced as greater india uncoupled from australia during the middle neocomian post breakup subsidence is reflected by a major regional transgression and the development of a second margin wide depositional wedge during the later cretaceous the close temporal and spatial relationship that exists between these cycles and the evolution of the western and northwestern australian continental margin emphasizes the importance of tectonic control upon sedimentation

the california coastal wave monitoring and prediction system abstract a decade long effort to estimate nearshore 20 m depth wave conditions based on offshore buoy observations along the california coast is described offshore deep water directional wave buoys are used to initialize a non stationary linear spectral refraction wave model model hindcasts of spectral parameters commonly used in nearshore process studies and engineering design are validated against nearshore buoy observations seaward of the surfzone the buoy driven wave model shows significant skill at most validation sites but prediction errors for individual swell or sea events can be large model skill is high in north san diego county and low in the santa barbara channel and along the southern monterey bay coast overall the buoy driven model hindcasts have relatively low bias and therefore are best suited for quantifying mean e g monthly or annual nearshore wave climate conditions rather than extreme or individual wave events model error correlation with the incident offshore wave energy and between neighboring validation sites may be useful in identifying sources of regional modeling errors

observations and modeling of steep beach grain size variability novel observations of surface grain size distributions are used in combination with intra wave modeling to examine the processes responsible for the sorting of sediment grains on a relatively steep beach slope 1 7 5 the field observations of the mean grain size collected with a digital camera system at consecutive low and high tides for a 2 week period show significant temporal and spatial variation this variation is reproduced by the modeling approach when the surf zone flow circulation is relatively weak showing coarse grain sizes at the location of the shore break and finer sediment onshore and offshore of the shore break the model results suggest that grain size sorting is dominated by the wave breaking related suspended sediment transport which removes finer sediment from the shore break and transports it both on shore and offshore the transport capacity of wave breaking related suspended sediment is controlled by the sediment response time scale in the advection diffusion equation where small large values promote onshore offshore transport comparisons with the observed beach profile evolution suggest a relatively short time scale for the suspended sediment response which could be explained by the vigorous breaking of the waves at the shore break