wave induced advective transport below a rippled water sediment interface the exchange between the water column and the sediment bed and the transport inside the permeable sediment layer are important processes in the cycles of chemical elements in this paper we examine quantitatively the effects of modulations in the profile of the water sediment interface on this exchange and on the advective transport inside the sediment bed the flow field inside a sediment layer bounded between spatially periodic ripples on top and an impermeable bottom surface is modeled using darcy s law the forcing is due to progressive gravity waves in the water above the results of two different models for the pressure variation imposed on the upper boundary are compared the two pressure profiles are derived from potential flow theory and from a numerical solution to the navier stokes equations for the oscillatory flow over a rippled bed from an analytic solution to the two dimensional model the trajectories of pore water particles immediately below the ripple profile are found to be quite different from the simple elliptical pattern found below a flat bed the shapes of these trajectories can be quite complicated and vary considerably both along the length of the ripple and over the depth of the sediment layer close to its surface the total exchange across the water sediment interface averaged over one wave period is significantly higher across a rippled interface than across a flat bed this difference increases with increasing ripple slope and the strength of the wave motion and it decreases with increasing thickness of the sediment layer relative to the length of the gravity wave since rippled bed forms are commonly found in coastal waters the increase in the total exchange across a rippled water sediment boundary can enhance the exchange of solutes due to wave pumping immediately below the water sediment interface circulation cells with net advective transport over a wave period are found such net advection patterns can lead to spatial in the horizontal direction inhomogeneities of the vertical concentration or temperature profiles if the overlying water column and or the sediment bed act s as source or sink this gives a plausible physical mechanism to explain the spatial variations in vertical concentration profiles found in field measurements

sediment dynamics over sand ridges on a tideless mid outer continental shelf abstract a sand ridge field with crests oriented ne sw 52 located between 55 and 85 m water depth on the valencia continental shelf spain was mapped with multibeam swath bathymetry and characterized with seismic profiling and sediment sampling boundary layer hydrodynamic measurements conducted over a sand ridge at 66 m depth show evidence of wave and current sediment resuspension during a major storm event with a three year recurrence period from the measured intrawave sediment concentration profiles it is shown that the suspended sediment time evolution is related not only to mean currents but also to the wave field the near bottom shear stress generated during storm conditions reached around 1 n m2 and was able to resuspend and transport sandy and muddy sediments for more than 32 h the total suspended sediment transport in a layer of 1 m above the bottom was around 4000 kg per meter width and directed 160 it is argued that these sand ridges were developed in a shallow environment and there is no evidence of present day ridge migration or morphological degradation it is hypothesized that sediment dynamics affecting sand ridges during storms favors the maintenance of their morphology

combining seismic noise techniques for landslide characterization a strong topographic relief and the presence of weakly consolidated sediments create favorable conditions for the development of landslides around the eastern rim of the fergana basin central asia in summer 2012 a field experiment employing small aperture seismic arrays was carried out on an unstable slope using ambient vibration recordings the aim of the study was to constrain the seismic response of a potential future landslide and to map lateral and vertical changes in the shear wave velocity of the surficial soil layers strong variations of horizontal to vertical spectral ratios in terms of amplitude and directionality indicated clear differences in local site effects probably reflecting the stability of different sections of the slope results further showed resonant frequencies of both the entire unstable block as well as for smaller individual parts the use of an ad hoc passive seismic tomography approach based on noise correlograms allowed for the mapping of the shear wave velocities of the sliding material even in cases of significant topography relief based on the recording of seismic noise only we clearly identified a low velocity body of weakly consolidated claystone and limestone material which can be interpreted as the landslide body with laterally varying thickness

modified topographic amplification factors for a single faced slope due to kinematic soil structure interaction in this paper we evaluate the additive effects of topography soil nonlinearity and soil structure interaction ssi along the crest of an idealized 40 m high cliff type topographic feature with slope inclination 300 where excessive damage was observed during the athens 1999 earthquake the objective of this paper is to investigate the relative contribution of topographic amplification and kinematic ssi as a function of the incident motion frequency content and geotechnical site conditions for a surface and an embedded structure located at the cliff crest for this purpose we perform elastic parametric and nonlinear site specific two dimensional finite element simulations using three profiles and six input motions we illustrate the role of ssi in altering the response at the location of peak topographic amplification potential behind the crest the effects of incident motion incoherency on the transient structural response and the beneficial contribution of structural embedment we finally suggest that empirical models for base slab averaging of shallow foundations developed as a function of site conditions structural dimensions and center line location could be combined with topographic amplification factors to predict realistic design spectra for structures located on irregular topographic features

velocity measurements close to rippled beds in oscillatory flow the results are reported of velocity measurements in oscillatory flow over rippled beds velocities were measured with a laser doppler anemometer in both an oscillating tray rig and an oscillatory flow water channel both self formed and artificial ripples were examined in addition some measurements were made with an apparently plane bed with intense sediment motion the experimental results were compared with the predictions of kalkanis 1964 kajiura 1968 bakker 1973 1974 and sleath 1974 the closest agreement between theory and experiment was obtained with the method of sleath measurements of the eulerian drift velocities showed drift towards the adjacent ripple crest in the immediate vicinity of the bed and away from the crest further out

sediment transport in oscillatory flow over flat beds the motion of sediment in oscillatory flow over a flat bed is investigated both theoretically and experimentally the theory makes use of results for the relationship between stress and strain in steady flow the experiments were performed in two different oscillatory flow water tunnels with two different sediments they were designed to measure the variation of velocity with height in the moving layer of sediment under conditions of intense sediment transport although certainly not perfect the agreement between theory and experiment is reasonable provided the stress ratio is chosen correctly calculated values of the mean sediment transport rate during a half cycle are compared with the predictions of currently available formulas

loose boundary hydraulics and fluid mechanics selected advances since 1961 s u m m a r y a maj or and acknowledged role in the advance o f sedimentological knowledge is played by the methods of physics as expressed through fluid mechanics and loose boundary hydraulics turbulence is increasingly being seen as involving orderly flow structures and these are significant for the origin of several sedimentary structures and for suspension transport recent advances in sediment transport theory are rooted in the concept of the sediment load as a downward acting force and in the notion of the flow as a transporting machine laboratory experiments are increasing our understanding of bedforms due to rivers and waves but there is a need for well framed field studies to clarify the influence of change in natural environments physical explanations are being proposed for tidal bedforms field studies in both modern environments and the stratigraphic record suggest that tidal patterns and strengths are recognizable from the internal features of cross bedded units formed by sand waves tidal patterns are complex however and a wider range of situations needs to be explored than has hitherto been the case mathematical modelling could help to define the expectable bedding patterns although successful mathematical models have been constructed of flow and sedimentation in channel bends secondary flow in real bends and the evolution of those bends are not yet well understood turbidity currents on a geologically significant scale have not been observed directly but valuable insights into their character and processes have come from laboratory experiments recent laboratory work points to the limitations of lock exchange experiments and emphasizes the various ways in which turbidity currents mix with the ambient medium these studies seem to have implications yet to be worked out for the internal features and some sole marks of turbidites

an exact solution for effects of topography on free rayleigh waves an exact solution for the effects of topography on rayleigh wave amplification is presented the solution is obtained by incorporating conformal mapping into complex variable stress functions developed for free rayleigh wave propagation in an elastic half space with a flat upper surface results are presented for free rayleigh wave propagation across isolated symmetric ridges and valleys it is found for wavelengths that are comparable to ridge widths that horizontal rayleigh wave amplitudes are amplified at ridge crests and that vertical amplitudes are strongly reduced near ridge crests relative to horizontal and vertical amplitudes of free rayleigh waves in the flat case horizontal amplitudes are strongly deamplified at valley bottoms relative to those for the flat case for rayleigh wavelengths comparable to valley widths wave amplitudes in the symmetric ridges and valleys asymptotically approach those for the flat case with increased wavelengths increased ridge and valley widths and with horizontal distance from and depth below the isolated ridges and valleys also prograde particle motion is predicted near crests of narrow ridges and near the bottoms of narrow valleys finally application of the theory at two sites known for topographic wave amplification gives a predicted surface wave amplification ratio of 3 80 at the ridge center for a frequency of 1 0 hz at robinwood ridge in northern california and a predicted surface wave amplification ratio of 1 67 at the ridge center for the same frequency at the cedar hill nursery site at tarzana in southern california

topographical and geological amplifications determined from strong motion and aftershock records of the 3 march 1985 chile earthquake site response experiments were performed 5 months after the ms 7 8 central chile earthquake of 3 march 1985 to identify amplification due to topography and geology topographical amplification at canal beagle a subdivision of vina del mar was hypothesized immediately after the main event when extensive damage was observed on the ridges of canal beagle using frequency dependent spectral ratios of aftershock data obtained from a temporarily established dense array it is shown that there is substantial amplification of motions at the ridges of canal beagle the data set constitutes the first such set depicting topographical amplification at a heavily populated region and correlates well with the damage distribution observed during the main event dense arrays established in vina del mar also yielded extensive data which are quantified to show that in the range of frequencies of engineering interest there was substantial amplification at different sites of different geological formations to substantiate this spectral ratios developed from the strong motion records of the main event are used to show the extensive degree of amplification at an alluvial site as compared to a rock site similarly spectral ratios developed from aftershocks recorded at comparable stations qualitatively confirm that the frequency ranges for which the amplification of motions occur are quite similar to those from strong motion records in case of weak motions the denser arrays established temporarily as described herein can be used to identify the frequency ranges for which amplification occurs to quantify the degree of frequency dependent amplification and used in microzonation of closely spaced localities

measurement and prediction of wave generated suborbital ripples 1 experiments in a large scale wave flume using regular and irregular waves with periods between 4 s and 6 s and heights between 0 2 m and 1 55 m have examined the formation of wave generated ripples using sediment beds composed of four grain sizes d50 0 349 mm 0 329 mm 0 220 mm and 0 162 mm in a water depth of approximately 4 m estimates of wave generated ripple height and wavelength were obtained using zero down crossing analyses of bed profiles measured by acoustic means along a 4 m transect normal to the ripple crests further information pertaining to was obtained from bed images obtained using scanning sonar the analyses reported here focus on the sequence bedforms that evolved in response to stepwise increases and decreases in wave height results show that ripples for the most part are suborbital in nature and do not conform well to empirical equations used frequently to predict and values in the field on the basis of the present data two new equations for prediction of and are obtained and their use in field situations where hydrodynamic and sedimentary conditions favor development of suborbital bedforms is recommended

cyclostratigraphic record of the triassic a critical examination high frequency fourth and fifth order cyclicity is a common feature of sedimentary sequencesinalldepositionalsettings whiletectonismandautocyclicprocessesare clearlyrespon sible for this cyclicity in some instances many cases are interpreted as resulting from orbitally forced variations in solar insolation at the milankovitch frequencies that is the precession and short and long eccentricity cycles at scales of tens of thousands to hundreds of thousands of years this forcing is presumed to have controlled sedimentation through periodic changes in climate or sea level examples of interpreted milankovitch frequency cyclicity occur throughout the triassic record and include much of the german triassic the alpine triassic and the newark supergroup of north america the cyclostratigraphy of these sections has been used as a tool for intrabasinal and interbasinal correlation and for chronostratigraphy these interpretations are not always without controversy however as conceptual arguments and radio isotopic age data have called some of these conclusions into question the stratigraphic record is inherently cyclical but the scale of this cyclicity varies in scale from the outcrop to the continental the largest scale or first order cycles are those that are truly continental in scope as they are thought to record changes in sea level caused by the operation of the wilson cycle at a scale of hundreds of millions of years prothero 1990 jacquin graciansky 1998 second order cycles corresponding to the sedimentary sequences of sloss 1963 last from tens of millions up to one hundred million years and are generally considered the result of long term trends in eustasy potentially in direct proportion to the activity of the mid ocean ridges the cause of the third order sea level cycles of millions of years duration is not well understood these cycles are often apparent at the basin scale and may be related to short term tectonic activity higher frequency fourth and fifth order cycles frequently are evident at the outcrop scale fourth order cycles include the classic cyclothems of the carboniferous e g heckel 1986 and have periods in the range of hundreds of thousands of years whilefifth order cyclesoperate at frequencies of tens of thousands of years these high frequency fourth and fifth order cycles are often referred to as milankovitch cycles due to the match of their interpretedperiodswiththecalculatedmilankovitch orbital frequences it is these high frequency cycles thatarethefocusofmuchmoderncyclostratigraphy and the record and application of these cycles in the triassic system is the focus of this chapter

topographic effects on the seismic response of steep slopes a frequency domain parametric study using generalized consistent transmitting boundaries has been performed to evaluate the significance of topographic effects on the seismic response of steep slopes the results show that the peak amplification of motion at the crest of a slope occurs at a normalized frequency h 0 2 where h is the slope height and is the wavelength of the motion the importance of the natural site frequency is illustrated by the analysis of a stepped layer over a half space it was found that the natural frequency of the region behind the crest can dominate the response relative to the topographic effect for the conditions studied moreover the effect of topography can be handled separately from the amplification due to the natural frequency of the deposit behind the crest of the slope this concept of separating the amplification caused by topography from that caused by the natural frequency is advantageous to the development of a simplified method to estimate topographic effects

geometry prediction for wave generated bedforms published data on wave generated bedform geometry from field and laboratory measurements are re evaluated in terms of relative fluid orbit diameter at the bed and a wave period parameter d 8t2 laboratory bedform data shows a strong dependence on these parameters however for very small values of the period parameter typically field data there is evidence that bedform length depends only on sediment grain size the disappearance of sediment bedforms at high fluid velocities apparently occurs through a decrease in bedform height improved methods for the prediction of bedform geometry under waves are provided in graphical form and computer code

magnetostratigraphy and high resolution lithostratigraphy of the permian triassic boundary interval in central germany abstract a comprehensive lithostratigraphic and magnetostratigraphic study was carried out in order to provide a magnetic polarity scale for the lower buntsandstein of the classic germanic trias in its type area of central germany the 300 m thick lower buntsandstein consists of 20 small scale fining upward cycles they can be correlated over large parts of the central european basin using a combination of high resolution cyclic stratigraphy and gamma ray logging on the basis of this robust lithostratigraphic framework a very precise positioning and correlation of paleomagnetic results has been realized eight outcrops and one completely cored well were collected yielding a total of 900 paleomagnetic samples from about 80 of them a characteristic remanence was obtained the consistent correlation of lithostratigraphic units and magnetozones supports an early acquisition of the characteristic magnetizations in both the magnetite bearing gray lithologies as well as the hematite bearing red brown lithologies the inter section correlation of all investigated profiles allows the creation of a well defined composite magnetic polarity scale the uppermost zechstein comprises one normal znz and two reversed zry zrz magnetozones in the overlying lower buntsandstein are delineated three normal sn1 to sn3 and three reversed sr1 to sr3 well documented magnetozones which can be correlated with magnetic records from the boreal and tethyan realms the combination of magnetostratigraphic and biostratigraphic data suggests the base of the oolite alpha 2 within the beds of the so called graubankbereich lower buntsandstein as the position of the hindeodus parvus calibrated permian triassic boundary within the germanic trias it is situated within the lowermost part of a remarkable thick normal magnetozone sn1 which is a distinctive feature occurring in virtually all magnetic records across the permian triassic boundary

the application of finite element analysis to body wave propagation problems summary the finite element method is shown to be a powerful tool for the numerical modelling of seismic body wave propagation problems applications extend to both problems on a scale of interest to engineers and also to large scale seismological problems solutions are sought in the time domain efficient programs have been written to accomplish this the scope of numerical solutions has been greatly enhanced by the use of a previously reported scheme for exactly cancelling reflections at the boundaries of the model the finite difference results of boore and the analytical results of trifunac for the amplification due to a mountain and an alluvial valley respectively are compared with new finite element results the new results agree well although there are some difficulties with resonance in the alluvial valley problem boore s sh results have been extended to vertical p and sv incidence a deep earthquake zone has been modelled realistically in two dimensions and earthquakes simulated at depth it is suggested that the variation in observed amplitude across the top of the zone due to refraction away from the slab may be used to provide an estimate of the thickness of the slab from long period observations of local earthquakes

underground and ridge site effects a comparison of observation and theory abstract this paper compares experimental observations and theoretical predictions of the effects of underground and ridge sites on the amplitude of seismic waves observations by tucker et al 1984 of narrow band earthquake independent site effects of a factor of 3 at underground sites down to a depth of 40 m can be predicted well using a one dimensional theoretical model and information about seismic velocity as a function of depth the observations can be explained as being due to the interference of upcoming incident and downgoing surface reflected waves within a thin low velocity near surface layer an important effect of this layer is to make the interference pattern at shallow depths i e within the layer insensitive to the incidence angle of the incoming signal and thus stable from earthquake to earthquake our model suggests that at depths of about 100 m site effects as great as a factor of 6 would be observed at about 5 hz while at much greater depths site effects would be small currently available theoretical techniques are sufficient to assess the expected ground motion at underground sites provided that the velocity profile is known this is useful for buried pipes tunnels and critical storage facilities observations by tucker et al 1984 and others of narrow band earthquake independent site effects of about a factor of 8 on ridges cannot be predicted completely using theoretical techniques currently available to us accurate predictions could probably be made with a model that included the effects of ridge topography neighboring topography and near surface velocity gradients use of such a model if it were available would require detailed expensive knowledge of regional topography and velocity structure the observed insensitivity of ridge site effects on ridges to incidence angle can however be explained with existing theoretical models as being due to a near surface low velocity layer the size of site effects on ridges and the stability of these effects from earthquake to earthquake suggests that they should be considered when interpreting seismograms obtained from ridge sites and when designing structures such as communication antennae and power transmission towers that will be located on mountain ridges accurate determination of the site effects of ridges and mountain tops can probably best be done using experimental techniques

sediment transport in oscillatory sheet flow motivated by an interest in high rates of sediment transport an experimental investigation was conducted to examine sediment motion in conditions of intense sediment transport fluid velocity and sediment concentration were measured simultaneously in a sheet flow regime generated in a u shaped oscillatory water tunnel for two types of noncohesive sediments a probe was developed to measure the instantaneous variations in sediment concentration based on the electrical conductivity of the fluid grain mixture unlike previous concentration devices this probe is nonintrusive and is capable of measuring a wide range of sediment concentrations from extremely dense concentrations within the bed to low concentration suspended load well above the moving layer of sediment horizontal fluid velocities were measured simultaneously using laser doppler anemometry in backscatter mode the sediment concentration measurements provide a comprehensive data set for sediment transport in sheet flow and near sheet flow regimes the temporal variation of sediment transport over the half cycle of oscillation was examined most of the sediment was transported within the moving bed layer with the total transport occurring between the stationary bed height and a distance about twice the thickness of the moving bed layer the temporal variation in sediment transport was found to be proportional to cos t the magnitude of the sediment transport increased with increasing bottom shear stress fluid velocity and sediment concentration measurements were used to calculate instantaneous and mean rates of sediment transport which were compared with predictions from existing models

kinematic soil structure interaction from strong motion recordings earthquake strong motion recordings from 29 sites with instrumented structures and free field accelerographs are used to evaluate variations between foundation level and free field ground motions the focus of the paper is on buildings with surface and shallowly embedded foundations the foundation free field ground motion variations are quantified in terms of frequency dependent transmissibility function amplitude uhu procedures are developed to fit to uhu analytical models for base slab averaging for the assumed conditions of a rigid base slab and a vertically propagating incoherent incident wave field characterized by ground motion incoherence parameter k the limiting assumptions of the model are not strictly satisfied for actual structures and the results of the identification are apparent k values denoted k a that reflect not only incoherence effects but also possible foundation flexibility and wave inclination effects nonetheless a good correlation is found between k a values and soil shear wave velocity for sites with stiff foundation systems based on these results recommendations are made for modifying free field ground motions to estimate base slab motions for use in response analyses of buildings

ground motion amplitude across ridges abstract local amplification and wave diffraction on an elongated ridge near sourpi in central greece were studied by the analysis of seismic records of local and regional earthquakes data were obtained during field work especially designed for this purpose these data were analyzed in the frequency and time domains in the frequency domain spectral ratios show amplifications of 1 5 to 3 at the ridge top relative to the base of the ridge the horizontal components of motion are more amplified than the vertical component and the observed spectral ratios seem stable for different earthquake locations theoretical spectral ratios calculated by the indirect boundary element method are dependent on earthquake location but are in general agreement with the observed spectral ratios another dataset from mont st eynard in the french alps showed similar characteristics with spectral amplitudes on the top of the ridge up to four times those on the flank these relative amplifications are within the range predicted by numerical simulations the numerical simulations also show that the topographic effect involves the emission of diffracted waves propagating from the top toward the base of the ridge the use of a seven station array on the ridge at sourpi made it possible to identify such waves the analysis was performed with wave separation methods using singular value decomposition and spectral matrix filtering our results show agreement between experimental data and theoretical results supporting the use of numerical simulations for estimation of purely topography induced amplification on ridge tops our results also show that such amplification is moderate for the ridges under study

comparison of time averaged acoustic backscatter concentration profile measurements with existing predictive models time averaged concentration profiles over bedforms measured using acoustic backscatter abs instrumentation on a macrotidal beach at whitsand bay cornwall uk were dominated by vortex type suspension profiles wave heights were typically in the range 0 25 1 0 m with wave periods of between 5 and 8 s mean currents measured at 0 2 m above the bed were generally weak 0 1 m s 1 near bed concentrations were compared with two reference concentration cref models smith j d mclean s r 1977 boundary layer adjustments to topography and suspended sediment mem soc r sci liege 11 123 151 nielsen p 1986 suspended sediment concentrations under waves coastal eng 10 23 31 the resuspension coefficient 0 showed a strong dependence on the maximum skin friction shields parameter the smith and mclean smith j d mclean s r 1977 boundary layer adjustments to topography and suspended sediment mem soc r sci liege 11 123 151 model implies that cref z0 increases with excess skin friction at the bed excess however field data showed no such correlation and the inclusion of a variable 0 gave poor model predictions that tended to have cref z0 decreasing with increasing excess reference concentrations at the bed c0 were determined from the time averaged concentration profiles using regression analysis the nielsen nielsen p 1986 suspended sediment concentrations under waves coastal eng 10 23 31 model predicts that c0 increases rapidly with the modified skin friction shields parameter r however extrapolated values of c0 showed no dependence on r in addition model predictions were orders of magnitude higher than extrapolated values of c0 the influence of the bedforms on the magnitude of near bed concentration measurements has been examined we conclude that the observed variations in cref over different bedforms vortex ripples lunate ripples lunate mega ripples will need to be taken into account if model predictions of cref as a function of the skin friction shields parameter are to improve the mixing height l predictions of the nielsen nielsen p 1992 combined convection diffusion modelling of sediment entrainment proc 3rd int conf on coastal engineering venice 4 9 oct 1992 pp 3202 3215 and the vincent and osborne vincent c e osborne p d 1995 predicting suspended sand concentration profiles on a macrotidal beach cont shelf res 15 1497 1514 models have been compared with measured data although the predictions of both models were typically within a factor of two of l measurements the vincent and osborne vincent c e osborne p d 1995 predicting suspended sand concentration profiles on a macrotidal beach cont shelf res 15 1497 1514 model is believed to be more appropriate for modelling l over bedforms as field data showed a strong dependence of l on and model predictions do not rely on bedform data

numerical evaluation of the effect of cross coupling of different components of ground motion in site response analyses abstract the effect of cross coupling between the three components of ground motion in the evaluation of site response functions such as standard spectral ratios ssrs and horizontal to vertical spectral ratios hvsrs is analyzed in this article numerical analyses of the seismic response of fully 3d geological structures namely a real topographic irregularity and an ideal stratigraphic inclusion have been carried out to obtain a 3d transfer function in the form of a 3 3 matrix each element of this matrix contains the frequency response in the i th direction due to an input motion in the j th direction a synthetic set of acceleration time histories at the surface of the geological irregularity has been created by convolution with the 3d transfer function using as input motion different real multicomponent strong motion accelerograms recorded at stiff soil or rock sites the ssrs and hvsrs are calculated and compared with the theoretical 3d transfer function in order to highlight the effect of cross coupling terms these are found to generate a rather large dispersion in the site response functions as well as response peaks that could be misleading in the interpretation of both numerical and observed spectral ratios

biostratigraphic and radiometric age data question the milankovitch characteristics of the latemar cycles southern alps italy the combination of age diagnostic fossils in platform carbonates geometrical relationships with coeval basinal successions and single zircon u pb ages from volcaniclastic layers in basinal deposits constrain the age of a cyclic succession in the interior of the latemar platform to a narrow time span in the middle triassic with a duration shorter than 5 m y precession induced milankovitch band sea level oscillations are therefore excluded as controls for most of the 600 platform cycles reported from latemar statistical evaluation and graphic space time analyses of cycle stacking patterns alone may therefore not allow an unambiguous recognition of milankovitch frequencies in ancient platform carbonates

a multi frequency acoustic concentration and velocity profiler acvp for boundary layer measurements of fine scale flow and sediment transport processes the use of acoustics to measure sediment transport boundary layer processes has gained increasing acceptance over the past two decades this has occurred through the development of increasingly sophisticated measuring systems and theoretical developments which have enabled flow and suspended sediment parameters to be obtained from acoustic data with a high degree of accuracy until relatively recently separate acoustic systems were used to measure flow and suspended sediment concentration over the past few years however the technology has become sufficiently advanced so that flow and sediment measurements can be integrated into a single system this integration provides quasi instantaneous non intrusive co located high temporal spatial resolution measurements of benthic flow and sediment processes here the development of such an instrument the acoustic concentration and velocity profiler acvp is described the theory underpinning its application is outlined new approaches to velocity de aliasing and suspended sediment inversion instabilities using multi frequency capabilities are presented and the application of the system to sediment transport processes over a sandy ripple bed is illustrated the observations clearly show the value of such instrumentation for studying the dynamical interaction between the bed the flow and the sediments at and within the bottom boundary layer

the lower triassic cyclic deposition in chaohu anhui province china abstract the lower triassic strata developed in chaohu anhui province of east central china including the yinkeng helongshan and nanlinghu formations is about 268 m thick and consists of carbonate rocks such as micrite limestone and nodular limestone as well as shale and argillaceous mudstone the deficiency of sedimentary structures and bioclastic particles and presence of pyrite and high ratios of v v ni suggest that the lower triassic strata formed in a quiet open marine basin environment bordering carbonate slope and lower slope within the lower triassic the cyclic deposits include two orders of thickening and coarsening upward successions respectively called basic unit bu and basic sequence bs which represent shallowing upward cycles five basic units are recognized and they are mostly composed of shale argillaceous mudstone calcareous mudstone and nodular limestone and overlying nodular limestone and micrite limestone about 0 25 1 4 m thick the shale and mudstone mainly formed in a deep water basin and the nodular limestone and micrite limestone formed on the rise of basin margin or the base of slope thus the basic units are interpreted to reflect cyclic sedimentation from the deep water basin to the rise of basin margin or from the rise to base of slope the basic units are combined with medium to thick bedded micrite limestones and thus form four types of basic sequences average 6 5 14 62 m thick likewise they are interpreted to be shallowing upward cycles with long period for example from the deep water of basin through the rise of basin margin to the base of slope farther reaching to lower slope the various orders of cycle in the lower triassic are laterally very extensive regionally which suggests that the fluctuation of sea level was likely a control on the origin of the cycles these small scale transgressive regressive cycles possibly reflect milankovitch cyclicity the associations of basic sequences constitute two transgressive regressive cycles which comprise strata of yinkeng helongshan and nanlinghu stages respectively which is similar to the carbon isotope cycles of the lower triassic tectonic activities can control the origin of the cycles the collision of south china block and north china block in the lower middle triassic may have resulted in the development of the two cycles and caused the shallowing upward sequence of the lower triassic from deep water basin to slope

suspended sediment concentrations under waves abstract a predictive formula is derived for calculation of the magnitude of time averaged suspended sediment concentrations under waves by taking into account the effect of flow contraction near ripple crests the formula applies equally well for rippled beds and sheet flow conditions the formula is in qualitative agreement with the bed load flux formula of madsen and grant 1976

laboratory investigations into the threshold of movement of natural sand sized sediments under unidirectional oscillatory and combined flows the threshold of movement of sediment obtained from sandbanks within the bristol channel uk is investigated under unidirectional oscillatory and combined flows the experiments were undertaken in a recirculating unidirectional laboratory flume containing an oscillating plate to simulate wave action with movement along the same axis as the unidirectional flows the sand samples consisted of cohesionless quartz grains with median grain sizes between 0 315 and 0 513 mm the experiments were performed under flow velocities measured at 2 cm above the bed ranging between 0 and 24 cm s 1 and oscillatory currents wave periods of 5 12 and 15 s ranging from 0 to 28 cm s 1 the critical conditions for the initiation of sediment movement were assessed by visual observation using the yalin criterion the results show that under unidirectional flow there is a slight overestimation of the threshold of naturally graded sediments derived on the basis of empirically derived threshold curves for artificially prepared sediments under similar flow conditions in the case of oscillatory flows the threshold for the natural sands is found to be higher than that predicted by previously derived empirical curves under combined flows wave period is shown to control threshold conditions with the unidirectional and oscillatory flow components combining in a linear fashion for long period 12 s and 15 s waves in contrast in the presence of short period 5 s waves the unidirectional and oscillatory components of the flow appear to decouple for high orbital velocities in both cases the effect of the wave period on threshold diminishes

boundary layer dynamics and sediment transport under storm and non storm conditions on the scotian shelf abstract near bed measurements of waves currents seabed responses and ripple migration rates were obtained using an instrumented tripod at a water depth of 39 m on the scotian shelf during the winter of 1992 93 these data and the grant and madsen s 1986 grant w d madsen o s 1986 the continental shelf bottom boundary layer annu rev fluid mech 18 265 305 combined flow boundary layer model are used to examine wave current interaction various sediment dynamic thresholds and sediment transport on an exposed high energy continental shelf the seabed was found to be rippled for most time of the deployment and thus the ripple enhanced combined skin friction shear velocity had to be used to determine the initiation of bedload transport under combined flows this indirectly indicates adequate predictions of the skin friction shear velocities inside the wave boundary layer by the grant and madsen model at high transport stages bedload roughness must be added to the grain size roughness to obtain a transportrelated combined shear velocity in order to predict correctly the onsets of saltation suspension and sheet flow transports otherwise the initiation of suspended load transport and the total sediment transport rates will be severely under estimated the comparison between the prediction by the grant and madsen 1986 model and the calculation of a quadratic stress law suggests that the total current shear velocity was enhanced by a factor of 2 3 due to the wave current interaction during storms while the apparent bottom roughness was increased by more than an order of magnitude the non linear coupling between waves and currents also causes a 20 increase of the combined skinfriction shear velocity inside the wave boundary layer this non linear coupling is most important when waves and currents are roughly equal in magnitude and the angles between them are less than 30 four sediment transport formulae were tested while the engelund hansen total load and yalin bedload methods did not perform well the einstein brown and bagnold formulae were found to respectively give reasonable predictions of the bedload and total load sediment transport rates under the observed combined flow conditions the predicted sediment transport direction is also in good agreement with the observed ripple migration direction the gsc sediment transport model sedtrans92 predicts that the net daily transport rates during the storms reached 822 kg m 1 day 1 and were 2 3 orders of magnitude higher than the non storm transport suggesting the dominance of storm processes in sediment transport on the scotian shelf these values are in good agreement with the results from sand tracer experiments conducted in this region we thus conclude that sedtrans92 can properly simulate boundary layer dynamics and sediment transport on continental shelves

suspended sediment reference concentration under waves field observations and critical analysis of two predictive models two commonly adopted but fundamentally different approaches for predicting time averaged suspended sediment reference concentration cref under waves are tested against field measurements and compared with each other the first model relates cref to the cube of the non dimensional skin friction whereas the second model adopts a more complex function of excess skin friction incorporating the empirical constant 0 the dataset is from the zone of wave shoaling seaward of an open coast surfzone and includes measurements of waves currents suspended sediment and bedforms estimates of cref are derived from acoustic backscatter data and the seabed and suspension process are described from video footage when waves were energetic the bed was deformed into large hummocks during less energetic conditions the bed was rippled the time averaged concentration profiles over the ripples were consistent with settling flux balanced by pure gradient diffusion and a sediment diffusivity that is constant with elevation above the bed cref in that case is shown to apply at z 0 where z is the elevation above the bed over the hummocks there was a sheet flow at the base of the suspension and cref is shown to apply at z 1 cm the concentration profiles over the hummocks implied sediment diffusivity that varied linearly with elevation within 10 cm of the bed and constant sediment diffusivity above that level for both rippled and hummocky beds 0 derived from the field data was found to be sensitive to the value assumed for critical stress for initiation of sediment motion which could explain the range of values reported in the literature for 0 0 was also found to vary in a complex way with skin friction which suggests that the reference concentration model based on excess skin friction is not correctly formulated nevertheless two functions for 0 one applying to rippled beds and the other to hummocky beds were contrived to make the model fit the data the model based on non dimensional skin friction was found to be a good predictor of cref when a correction was made for flow contraction over ripples the correction was not required for the hummocky bed where sediment was being entrained in a thin sheet flow layer the model based on non dimensional skin friction correctly portrayed the relationship between flow and sediment response without contrivance and therefore should be the favoured approach in predicting reference concentration

geometry migration and evolution of wave orbital ripples at leo 15 observations of the temporal evolution of the geometric properties and migration of wave formed ripples are analyzed in terms of measured suspended sand profiles and water velocity measurements six weeks of bedform observations were taken at the sandy medium to coarse sized sand leo 15 site located on beach haven ridge during the late summer of 1995 with an autonomous rotary sidescan sonar during this period six tropical storms several of hurricane strength passed to the east of the study site ripples with wavelengths of up to 100 cm and with 15 cm amplitudes were observed the predominant ripples were found to be wave orbital scale ripples with ripple wavelengths equal to 3 4 of the wave orbital diameter although orbital diameters become larger than 130 cm during the maximum wave event it is unclear if a transition to non orbital scaling is occurring ripple migration is found to be directed primarily onshore at rates of up to 80 cm day suspended transport due to wave motions calculated by multiplying acoustic backscatter measurements of suspended sand concentrations by flow velocity measurements are unable to account for a sufficient amount of sand transport to force ripple migration and are in the opposite direction to ripple migration thus it is hypothesized that the onshore ripple migration is due to unobserved bedload transport or near bottom suspended transport bedload model calculations forced with measured wave velocities are able to predict the magnitude and direction of transport consistent with observed ripple migration rates sequences of ripple pattern temporal evolution are examined showing mechanisms for ripple directional change in response to changing wave direction as well as ripple wavelength adjustment and erosion due to changing wave orbital diameter and relative wave to current velocities

geometric characteristics of coarse sand ripples generated by oscillatory flows a full scale experimental study abstract the shape of vortex ripples generated by wave boundary layers determines the local hydrodynamics and sediment motions and therefore is critical for understanding coastal sediment transport in the rippled bed regime in this study coarse sand vortex ripples were generated by periodic oscillatory flows mimicking full scale field conditions in an oscillatory water tunnel a laser based bottom profiler was applied for measuring the movable bed s profile the observed ripple development from a flat bed is in agreement with many previous studies small ripple marks appeared within a couple of flow periods and gradually became 3 dimensional 3 d transient ripples which eventually evolved to equilibrium 2 dimensional 2 d ripples after o 100 1000 flow periods the shape of equilibrium 2 d ripple was studied based on a representative ripple profile obtained by ensemble averaging a periodic ripple train the shape of ripples formed under sinusoidal flows become increasingly sinusoidal as the flow becomes stronger larger shields parameter or wave period becomes longer which is possibly because the influence of coherent vortex becomes more significant wave nonlinearities have different effects on the ripple shape generally speaking wave skewness makes ripples onshore leaning while wave asymmetry makes ripples crests sharper and troughs flatter our measurements also revealed some detailed geometric characteristics i e the roundness of ripple crests and the maximum local slope along ripple flanks

orbitally forced lofer cycles in the dachstein limestone of the julian alps northeastern italy lofer cyclothems of the upper triassic dachstein limestone of the julian alps northeastern italy are the focus of a new sedimentological cyclostratigraphic study a 112 cycle long field section reveals that the meter scale basic cycle shallows upward from subtidal megalodont bearing subfacies to intertidal laminites capped by a paleosol subfacies time series analysis of a correlative grayscale scan of a high resolution field photograph shows that a 166 m thick succession of lofer cycles has time frequency characteristics consistent with earth s precession index suggesting that the lofer cycles were generated by eustatic sea level oscillations that were forced by milankovitch cycles

sedimentary structures sorby and the last decade henry clifton sorby pioneered in the last century the description and especially the hydrodynamic interpretation of sedimentary structures together with their use as palaeocurrent indicators research completed since the last syntheses were published a decade ago shows that work along these lines continues to be necessary and relevant particularly as regards the physical explanation of structures and to present significant challenges and opportunities perhaps the most pressing needs are for a better understanding of 1 bedforms in gravels silts and carbonate sediments 2 tidal and especially sand wave bedding 3 hummocky and swaley cross stratification and 4 soft sediment and dewatering structures in turbidites many sedimentary structures present a little exploited opportunity to quantify process rates and define short time periods from the rock record

the effect of topography on earthquake ground motion a review and new results a brief review of experimental and theoretical results about the effect of topography on seismic motion shows that they are consistent only on a qualitative basis amplification at mountain tops for wavelengths comparable with mountain widths is predicted and observed but the numerical simulations often underestimate the actual observed effects we propose that this disagreement is because current models are not complex enough we therefore computed the response to incident sh waves of a set of different complex configurations that include two dimensional surface topography with or without periodic ridges and subsurface layering the results show that 1 the topographic effect in itself is difficult to isolate from other effects like surface layering and therefore the amplification on top of geomorphologically complex sites cannot be predicted by a priori estimations based solely on topography and 2 the high crest base amplification ratio observed in the field cannot usually be matched even with complex two dimensional structures with incident plane sh waves which suggests that more complex models are needed to incorporate more complex wave fields e g sv surface and three dimensional geologic configurations

effects of local soil conditions on the topographic aggravation of seismic motion parametric investigation and recorded field evidence from the 1999 athens earthquake during the 1999 athens earthquake the town of adames located on the eastern side of the kifissos river canyon experienced unexpectedly heavy damage despite the particular geometry of the slope that caused significant motion amplification topography effects alone cannot explain the uneven damage distribution within a 300 m zone parallel to the canyon s crest which is characterized by a rather uniform structural quality in this article we illustrate the important role of soil stratigraphy and material heterogeneity on the topographic aggravation of surface ground motion for this purpose we first conduct an extensive time domain parametric study using idealized stratified profiles and gaussian stochastic fields to characterize the spatial distribution of soil properties and using ricker wavelets to describe the seismic input motion the results show that both topography and local soil conditions significantly affect the spatial variability of seismic motion we next perform elastic two dimensional wave propagation analyses based on available local geotechnical and seismological data and validate our results by comparison with aftershock recordings

cyclic carbonate sedimentation in the upper triassic dachstein limestone austria the role of patterns of sediment supply and tectonics in a platform reef basin system abstract factors that controlled the deposition of i peritidal lofer cycles on a carbonate platform and ii platform margin reef slope deposits have been deduced at localities in the austrian alps the steinernes meer and hochkonig massif these locations are part of an entire late triassic platform reef basin system that is preserved with most original stratigraphic relationships intact platform lofer cycles shallow upward from a subtidal grainstone through a variety of intertidal dolomitic mudstones to a supratidal weathering horizon soil lofer cycles show random non hierarchical stacking patterns limited lateral continuity varied progradation directions complete shoaling 98 of cycles and very low stratigraphic completeness only 1 20 exponential frequency distributions of cycle thickness suggest random aperiodic cycle deposition rather than regular deposition in response to regular eustatic sea level oscillations sediments in the adjacent reef complex record storms and the lateral migration of sand shoals and stromatolite capped banks not sea level fluctuations and intermittent subaerial exposure on the basis of these observations in contrast to many previous interpretations lofer cycles are interpreted to be mostly autocycles formed within a tidal flat island system by lateral migration of wide low emergent banks separated by shallow subtidal areas preservation potential of individual cycles is thought to have been low reworking was almost certainly very important in this system extensional regional tectonics in the late triassic exerted a long term control over the development of the tidal flat island system on the platform top and appears to have left a tectonic overprint in lofer cycle successions differential subsidence of individual platforms across the region is suggested by substantial regional thickness variations 1200 3000 m in the norian rhaetian platform carbonates of the northern calcareous alps two important tectonic deepening events in the steinernes meer section almost caused platform drowning and correlate with a lowermost rhaetian transgression in the western tethys many other tectonic events may have gone unrecorded on the platform within a 716 m thick measured succession of lofer cycles intervals of enhanced paleokarst development nd stacked intertidal to supratidal beds are present with 20 75 m vertical spacing these platform units are interpreted to represent prolonged periods in which greater areas of the platform were occupied by intertidal to supratidal sediments these units correlate with thick units of debris flows on the reef slope what may be tectonic highstands the result of a vertical tectonic movement are recorded as packstone to grainstone deposition on the reef slope a holistic model driven by aperiodic fault controlled downdropping resulting in switching loci of sediment export patterns from a continuously operative subtidal carbonate factory the reef complex best explains vertical facies patterns in the platform and reef slope successions it is not a static sea level model although thi d fourth and fifth order eustasy is not required the model could explain the sporadic occurrence of shallowing up cycles in the adjacent kossen basin late triassic eustatic sea level fluctuations were ineffective in controlling sedimentation as a result of the processes described above