late quaternary geological history of the dead sea area israel abstract a 34 5 m borehole which was drilled near the dead sea coast altitude 394 m in the southern part of the fan delta of wadi zeelim reveals the geological history of that area from the latest pleistocene to present the depositional time frame is based on six 14c dates and two u th dates an erosional or nondepositional period is implied by the hiatus between 21 100 yr b p u th age depth 33 m and 11 315 yr b p 14c age depth 32 m a subsequent arid phase is recorded by a 6 5 m thick layer of halite based on 14c dates this phase relates to the abrupt younger dryas cold period reported in temperate to polar regions the fragility of the environment in this region is indicated by the fact that the region experienced such a severe short aridification phase less than 1000 yr evidence of which is found widely in the desert fringes of the middle east and north africa the aragonite found in most of the holocene section indicates that the well site was covered by the lake for most of the holocene exceptions are the intervals at 0 3 and 10 14 m depths which represent low stands of the lake

a unifying framework for particle entrainment 1 a new framework is presented that describes the hydrodynamic entrainment of sediments the framework is derived from the general equations of particle stability and fluid motion and covers a wide range of scales from that of the individual particle to en masse entrainment at the scale of the stream reach particle entrainment is explicitly shown to be driven by the combination of instantaneous fluid stress and pressure gradients particle surface contact forces particle submerged weight and the weight and motion of particle associated fluid of these factors bed shear stresses across particle differences in pressures and fluxes of momentum and sediment bed characteristics are found to be key factors in particle entrainment the conventional shields entrainment parameter is shown to be applicable to averaged en masse entrainment by steady uniform 2 d flows this nondimensionalized bed shear stress at threshold is further demonstrated to be a function of interparticle forces and bed normal hydrodynamic forces and thereby sediment size and other fluid sediment and bed characteristics observed shields parameter dependencies are discussed the presented framework which includes all relevant particle and fluid characteristics and flow dynamics may aid understanding of entrainment mechanics the design and analysis of further studies of entrainment the design of parameterizations that lead to the solution of entrainment problems and numerical modeling of combined fluid and sediment dynamics

will the dead sea die the level of the dead sea the lowest surface on earth is currently declining at a rate of 0 8 m yr and has dropped about 20 m since the beginning of the twentieth century it reached 410 m in 1997 we address the question of whether the level of the dead sea will continue to decline a numerical model developed in this study to determine the water balance accounts for the increase in salinity and the concomitant decrease in the rate of evaporation that accompanies reduction in the activity of the water simulations based on ranges of water withdrawal scenarios suggest that the dead sea will not die rather a new equilibrium is likely to be reached in about 400 yr after a water level decrease of 100 to 150 m

the origins of s forms form similarity process analogy and links to high energy subglacial meltwater flows abstract this paper first presents the history of ideas on s form formation with an emphasis on meltwater models the main approach is based on form similarity between modern erosional marks and subglacial s forms form similarity is then extended to process similarity which promotes understanding of s forms three classes of erosional marks are identified obstacle scours basin shaped erosional marks and landforms related to escarpment flow identification is based on form with recognition of elements though it mainly depends on recognition of the whole form in much the same way as we recognize family members or familiar flowers flow conditions related to the various flow features are determined by flow visualization and computational fluid dynamics some s forms are produced in flume models though most are natural features observed in the field examples from beneath past ice sheets are seen in the field and illustrated by air and ground photographs satellite images and dem digital elevation model the results of this work show remarkable similarity between modern erosional marks and ancient subglacial s forms consequently established processes are attributed to ancient forms in turn s forms may be used abductively to interpret subglacial hydrological conditions the lack of waning stage bedforms scoured into primary forms suggests that erosional marks form under abruptly terminating catastrophic flow

3d numerical simulation of turbulence and sediment transport within a tidal inlet abstract turbulence and sediment transport models are incorporated into a three dimensional hydrodynamics model to investigate the mechanisms of morphologic evolution of scour holes within the indian river inlet delaware usa the inlet bed had eroded slightly since stabilizing the channel walls in late 1930s through 1976 the mean rate of bed erosion roughly doubled as a response to anthropogenic activities within the inlet such as the removal of 80 piles remaining from an old bridge severe erosion near the in water bridge supports and cofferdams for the replacement bridge necessitated channel bed stabilization that along with the remained debris from the removal of old bridge piles enhanced the growth of two deep scour holes on the bayside and oceanside of the bridge cofferdams scour hole and channel bed evolution has decreased drastically since 1994 the present investigation suggests that initially flow concentration through the cofferdams of the replacement bridge was the main reason for scour hole development bed shear stress over the forward facing slope of the scour hole entrains sediment from the bed and extends the scour hole along the inlet and in the vertical direction flow separation within the developed scour holes after channel bed stabilization enhances turbulence and appears to be the dominant mechanism for further scour hole development

flow structure caused by a local cross sectional area increase and curvature in a sharp river bend horizontal flow recirculation is often observed in sharp river bends causing a complex three dimensional flow structure with large implications for the morphological and planimetric development of meanders several field observations in small scale systems show that sharp bends are often found in association with a strong increase in cross sectional area the deposition of outer bank benches and reattachment bars near the inner bank recent studies show that these bends can also occur in large scale systems in this study we present field measurements of a sharp bend in the mahakam river east kalimantan indonesia the cross sectional area increases by a factor of 3 compared with the reach averaged cross sectional area along a river reach of about 150 km cross sectional area correlates strongly with curvature the field measurements are analyzed together with the results from numerical simulation with a three dimensional finite element model which yields a comprehensive view of the intricate flow structure in turn the model is used to validate a new equation that captures the water surface topography dependence on cross sectional area variation and curvature the results show the importance of the increase in cross sectional area in the development of horizontal recirculation vertical acceleration of the flow into the scour causes the pressure to deviate from a hydrostatic pressure distribution strong downflow up to 12 cm s 1 advects longitudinal momentum toward the bed causing the flow to concentrate in the lower part of the cross section this increases the velocity magnitude throughout the cross section which is expected to maintain the large scour depth found in several bends along the mahakam river

plutonism in three orogenic pulses eastern blue ridge province southern appalachians department of geological sciences university of north carolina mitchell hall chapel hill nc 27599 3315

numerical simulation of sand waves in a turbulent open channel flow we develop a coupled hydro morphodynamic numerical model for carrying out large eddy simulation of stratified turbulent flow over a mobile sand bed the method is based on the curvilinear immersed boundary approach of khosronejad et al adv water resour vol 34 2011 pp 829 843 we apply this method to simulate sand wave initiation growth and evolution in a mobile bed laboratory open channel which was studied experimentally by venditti church j geophys res vol 110 2005 f01009 we show that all the major characteristics of the computed sand waves from the early cross hatch and chevron patterns to fully grown three dimensional bedforms are in good agreement with the experimental data both qualitatively and quantitatively our simulations capture the measured temporal evolution of sand wave amplitude wavelength and celerity with good accuracy and also yield three dimensional topologies that are strikingly similar to what was observed in the laboratory we show that near bed sweeps are responsible for initiating the instability of the initially flat sand bed stratification effects which arise due to increased concentration of suspended sediment in the flow also become important at later stages of the bed evolution and need to be taken into account for accurate simulations as bedforms grow in amplitude and wavelength they give rise to energetic coherent structures in the form of horseshoe vortices which transport low momentum near bed fluid and suspended sediment away from the bed giving rise to characteristic boil events at the water surface flow separation off the bedform crestlines is shown to trap sediment in the lee side of the crestlines which coupled with sediment erosion from the accelerating flow over the stoss side provides the mechanism for continuous bedform migration and crestline rearrangement the statistical and spectral properties of the computed sand waves are calculated and shown to be similar to what has been observed in nature and previous numerical simulations furthermore and in agreement with recent experimental findings singh et al water resour res vol 46 2010 pp 1 10 the spectra of the resolved velocity fluctuations above the bed exhibit a distinct spectral gap whose width increases with distance from the bed the spectral gap delineates the spectrum of turbulence from the low frequency range associated with very slowly evolving albeit energetic coherent structures induced by the migrating sand waves overall the numerical simulations reproduce the laboratory observations with good accuracy and elucidate the physical phenomena governing the interaction between the turbulent flow and the developing mobile bed

a unified model for subaqueous bed form dynamics 1 bed form evolution remains dynamic even in the special case of steady uniform flow data from the sandy braided north loup river nebraska show that roughness features on the channel bottom display a statistical steady state and robust scaling that are maintained through the collective interactions of transient short lived bed forms motivated by such field data and laboratory observations of bed form growth we develop a nonlinear stochastic surface evolution model for the topography of bed load dominated sandy rivers in which instantaneous sediment flux explicitly depends on local elevation and slope this model quantitatively reproduces laboratory observations of initial growth and saturation of bed forms from a flat surface and also generates long term dynamical behavior characteristic of natural systems we argue that the variability in geometry and kinematics of bed forms in steady flow and the existence of roughness at all wavelengths up to the largest dunes are a consequence of the nonlinear relationship between sediment flux and topography subject to noise

observations and analysis of the horizontal structure of a tidal jet at deep scour holes scour in the vicinity of a hydraulic structure may compromise its geotechnical stability or that of nearby structures much fundamental research has been dedicated to the understanding and prediction of these processes in order to enable the efficient design of mitigation measures while most of these efforts consider laterally uniform flows nonuniformity is the rule rather than the exception this study presents field observations near the storm surge barrier in the eastern scheldt estuary the netherlands where significant scour holes have developed the tidal flow through this semi open barrier exhibits characteristics of a shallow jet a pronounced contraction of this jet toward the nearby scour hole during particular stages of the tidal cycle is revealed attributed to potential vorticity conservation the depth averaged vertical vorticity increases proportionally to the depth increase measurements show a vertically uniform velocity profile in the scour hole that is attributed to a reduction of the adverse pressure gradient due to the contraction that suppresses boundary layer separation from the upstream slope of the scour hole a positive feedback mechanism is thus revealed lateral velocity gradients lead to relatively high near bed velocities in the scour hole which enhances erosion causing an even stronger horizontal contraction maintaining the scouring potential if the upstream flow field is laterally uniform observed around slack tide horizontal contraction does not occur and the boundary layer separates from the upstream slope the scour depth will then tend to an equilibrium the results infer the importance of the nonuniformity of the horizontal flow field

variation in bed level shear stress on surfaces sheltered by nonerodible roughness elements 1 direct bed level observations of surface shear stress pressure gradient variability turbulence intensity and fluid flow patterns were carried out in the vicinity of cylindrical roughness elements mounted in a boundary layer wind tunnel paired corkscrew vortices shed from each of the elements result in elevated shear stress and increased potential for the initiation of particle transport within the far wake while the size and shape of these trailing vortices change with the element spacing they persist even for large roughness densities wake interference coincides with the impingement of the upwind horseshoe vortices upon one another at a point when their diameter approaches half the distance between the roughness elements while the erosive capability of the horseshoe vortex has been suggested for a variety of settings the present study shows that the fluid stress immediately beneath this coherent structure is actually small in comparison to that caused by compression of the incident flow as it is deflected around the element and attached vortex observations such as these are required for further refinement of models of stress partitioning on rough surfaces

deep seismic reflection data from the bay of fundy and gulf of maine tectonic implications for the northern appalachians three deep penetration seismic reflection profiles were collected off southwest nova scotia to determine the crustal structure and geometry beneath the avalon and meguma zones of the appalachian orogen in canada onshore geological features have been traced seawards using new gravity and magnetic anomaly maps the seismic data can also be correlated with the previous united states geological survey profile in the central gulf of maine two seismically distinct lower crustal blocks are identified the avalon and sable lower crustal blocks separated by a major north dipping reflection zone that cuts the entire crust the recognition of the sable block adds a fourth block to the three already identified in the canadian appalachians the sable block is overlain by the meguma zone the avalon zone overlies at least the northern part of the avalon lower crustal block although offshore extension of geological features is not unequivocal it appears that a north dipping reflection zone southwest of nova scotia m

three dimensional numerical model for flow and bed deformation around river hydraulic structures this paper describes a numerical model developed to simulate flow and bed deformation around river hydraulic structures the model solved the fully three dimensional reynolds averaged navier stokes equation expressed in a moving boundary fitted coordinate system to calculate the flow field with water and bed surfaces varying in time a nonlinear k e turbulence model was employed in order to predict flow near the structure where three dimensional flow is dominant the temporal change in bed topography was calculated by coupling a stochastic model for sediment pickup and deposition using a momentum equation of sediment particles in order to account for the effect of nonequilibrium sediment transport in validating the numerical model a spur dike and a bridge pier which are considered to be typical river engineering structures were selected by comparing the numerical results with observed laboratory experimental data the model was found to reproduce flow and scour geometry around these structures with

nature of flow and turbulence structure around an in stream vertical plate in a shallow channel and the implications for sediment erosion 1 detailed knowledge of the dynamics of large scale turbulence structures is needed to understand the geomorphodynamic processes around in stream obstacles present in rivers detached eddy simulation is used to study the flow past a high aspect ratio rectangular cylinder plate mounted on a flat bed relatively shallow channel at a channel reynolds number of 2 4 105 similar to other flows past surface mounted bluff bodies the large amplification of the turbulence inside the horseshoe vortex system is because the core of the main necklace vortex is subject to large scale bimodal oscillations the presence of a sharp edge at the flanks of the obstruction fixes the position of the flow separation at all depths and induces the formation and shedding of very strong wake rollers over the whole channel depth compared with the case of a circular cylinder where the intensity of the rollers decays significantly in the near bed region because the incoming flow velocity is not sufficient to force the wake to transition from subcritical to supercritical regime in the case of a high aspect ratio rectangular cylinder the passage of the rollers was found to induce high bed shear stresses at large distances 6 8 d behind the obstruction also the nondimensional values of the pressure root mean square fluctuations at the bed were found to be about 1 order of magnitude higher than the ones predicted for circular cylinders overall this shows that the shape of the in stream obstruction can greatly modify the dynamics of the large scale coherent structures the nature of their interactions and ultimately their capability to entrain and transport sediment particles and the speed at which the scour process evolves during its initial stages

an investigation of the flow and scour mechanisms around isolated spur dikes in a shallow open channel 1 conditions corresponding to the initiation of the erosion and deposition process 1 the present study investigates the flow physics and the role played by the main coherent structures in the scouring processes around a vertical spur dike in a straight channel at conditions corresponding to the start flat bed of the scouring process large eddy simulation les is performed at a relatively low channel reynolds number re 18 000 in the range where most flume studies with clear water scour conditions are conducted similar to these studies the incoming flow is fully turbulent and contains realistic turbulence fluctuations visualization experiments are conducted to better understand the nature of the interactions between the dominant coherent structures playing a role in the erosion process it is found that the structure of the horseshoe vortex hv system at the base of the spur dike changes considerably in time and in vertical sections perpendicular to the trajectory defined by the axis of the main necklace vortex however its intensity is the largest at vertical sections situated around the tip of the spur dike it is in this region that the core of the main necklace vortex oscillates aperiodically between two preferred modes in one of them zero flow mode the necklace vortex is closer to the spur dike and more compact and the near bed jet flow beneath it is weak in the other one back flow mode a strong near bed jet flow convects the primary necklace vortex away from the spur dike and its core is more elongated and less compact this explains the large amplification by about 1 order of magnitude compared to the surrounding turbulent flow of the turbulent kinetic energy and pressure fluctuations inside the hv system in the region situated around the tip of the spur dike and the double peak distribution of the turbulent kinetic energy past the spur dike in the legs of the necklace vortex the intensity of the bimodal oscillations decreases such that they are not observed in spanwise sections situated at more than one channel depth behind the spur dike it is found that the legs of the horseshoe vortices can interact at times with the vortex tubes shed in the detached shear layer dsl and with the tip of the spur dike these events typically result in a significant change in the coherence of the hv system the largest bed shear stress values in the mean flow are present in the strong acceleration region near the tip of the spur dike but high bed shear stress values are also observed beneath the upstream part of the dsl the bed shear stress fluctuations around the local mean values can be very high especially in the region situated beneath the upstream part of the dsl at random times some of the vortices shed in the dsl merge or interact with eddies from the recirculation region this leads to an increase in their strength and to a large increase of the bed shear stress along their path

suppression of vertical flow separation over steep slopes in open channels by horizontal flow contraction flow separation and its control have been the subject of intensive research for decades flow separation occurs when the boundary layer loses contact with the associated confining wall which is usually caused by a pressure gradient acting against the local flow direction numerous strategies exist to control flow separation and in this study we demonstrate experimentally that vertical flow separation over steep slopes in shallow free surface flows may be suppressed by contracting the flow horizontally upstream of the slope we found that unexpectedly introducing lateral non uniformity in the upstream flow field could suppress vertical flow separation for steep slopes up to 1 in 2 this study reveals the possibility of two different flow states over steep slopes i a vertically attached flow combined with horizontal convergence and ii a vertically detached flow combined with horizontal divergence a detailed analysis of the dynamics of the two different flow states is presented although a predictive relation determining the transition point between the two flow states was not found in the current study the observed phenomena were shown to be strongly related to the magnitude of the lateral gradient at the upstream edge of the slope the results demonstrate a significant influence of the vertical flow state separated or attached on the shear stress at the confining boundaries of the flow

numerical computation of sand dune migration with free surface flow 1 this paper presents a morphodynamic model for free surface flows over bed forms the model reproduces bed form evolution and migration in a physically based manner the flow model component of the coupled morphodynamic model which is two dimensional in the streamwise and vertical directions explicitly treats unsteadiness and nonhydrostatic effects comparison with measurements shows that the flow model accurately simulates both mean flow and turbulence structure induced by bed forms under free surface flow the flow model performance is enhanced by the use of a high order godunov scheme known as the cubic interpolated pseudoparticle cip technique to compute the advection phase of the navier stokes equation in addition an enhanced k turbulence closure with nonlinear terms is incorporated in the present model in order to allow better predictions of reynolds stress anisotropy in regions with flow separation performance of the nonlinear k model is compared with standard k closure in the context of the flow as well as morphodynamic simulation nonequilibrium bed load sediment transport is treated using a eulerian stochastic formulation of the sediment exchange process in terms of pickup and deposition functions the proposed approach for sediment transport explicitly considers the local flow variability during morphodynamic computation model results show that the vertical grid distribution pattern affects the simulation result particularly for bed form morphodymics the model successfully simulates some important physical features of bed form evolution such as amalgamation and asymmetric geometry the simulation results are in agreement with laboratory experiments as well as some commonly used prediction methods for geometric characteristics of bed forms

shoreline processes and the age of the lake lahontan highstand in the jessup embayment nevada the well developed shoreline record of pluvial lake lahontan in the jessup embayment nevada is used to refine the history of late pleistocene lake level fluctuations and to assess controls on shoreline development and distribution controls on the strength and type of shorelines developed include local slope the amount and characteristics of sediment available for transport the availability of accommodation space and length of time the lake level resides at a particular shoreline elevation at the sehoo highstand and during the early part of the regression strong storm winds and waves from the south southeast set up a clockwise net shore drift pattern near the head of the embayment although significant differences in local slope geometry of the shoreline and wave energy existed in the embayment crestal heights of constructional shoreline features formed at the highstand vary 2 6 m in elevation and hence provide a relatively precise marker of the highstand elevation radiocarbon dating of a camel bone preserved in high shoreline deposits indicates that the lake reached its highest elevation of 1338 5 m in the embayment and receded from that elevation immediately prior to 13 070 60 yr b p similar and slightly older radiocarbon ages on gastropod shells preserved in barrier deposits at 1327 m 13 280 110 yr b p and 1331 m 13 110 110 yr b p suggest that the final rise to the highstand was very rapid and that the lake maintained its highest stand for a very brief period of time perhaps only for years or decades the brevity of the highstand is reasonable in light of the recent formation of similar barrier features in modern pyramid lake which formed in less than seven months due to a rapid increase in lake level

abrupt increase in greenland snow accumulation at the end of the younger dryas event the warming at the end of the last glaciation was characterized by a series of abrupt returns to glacial climate the best known of which is the younger dryas event1 despite much study of the causes of this event and the mechanisms by which it ended many questions remain unresolved1 oxygen isotope data from greenland ice cores2 4 suggest that the younger dryas ended abruptly over a period of about 50 years dust concentrations2 4 in these cores show an even more rapid transition 20 years this extremely short timescale places severe constraints on the mechanisms underlying the transition but dust concentrations can reflect subtle changes in atmospheric circulation which need not be associated with a large change in climate here we present results from a new greenland ice core gisp2 showing that snow accumulation doubled rapidly from the younger dryas event to the subsequent preboreal interval possibly in one to three years we also find that the accumulation rate change from the oldest dryas to the bo11ing allerod warm period was large and abrupt the extreme rapidity of these changes in a variable that directly represents regional climate implies that the events at the end of the last glaciation may have been responses to some kind of threshold or trigger in the north atlantic climate system

detailed simulation of morphodynamics 3 ripples and dunes we present a 3 d physics based high resolution modeling approach to the dynamics of underwater ripples and dunes the flow is modeled by large eddy simulation on a cartesian grid with local refinements the sediment transport is modeled by computing pickup transport over the bed transport in the water column and deposition of rigid spherical particles in a lagrangian framework the morphological development of the bed is modeled by a sediment balance equation in which the pickup and deposition from the sediment motion submodels appear as source and sink terms the model realistically replicated the formation and migration of dunes model results showed a good agreement with data from five flume experiments we subsequently applied the model to investigate the effect of sediment grain size on ripples finer sediments were found to yield more superimposed ripples than coarser sediments moreover under the same hydrodynamic conditions the finer sediments yielded two dimensional bed forms whereas for coarser sediment irregularities increased we extended the tests to pronounced 3 d morphologies by simulating the development of local scour at a pier the results agreed well with experimental data the model contributes to unraveling the complex problem of small scale morphodynamics and may be used in a wide range of applications for instance to develop more reliable parameterizations of small scale processes for application in large scale morphodynamic models

the influence of topography on the stability of jets in this article the effect shelflike topography has on the stability of a jet that flows along the smooth shelf is addressed the linear stability problem is solved to determine for which nondimensional parameters a shelf can either destabilize or stabilize a jet these calculations reveal an intricate dependence of growth rate on topography in particular the authors determine that retrograde topography with the shallow water on the left always stabilizes the jet in relation to the flat bottom equivalent whereas prograde topography with the shallow water on the right can either stabilize or destabilize the jet depending on the particular values of the rossby number and topographic parameters for rossby numbers of order 1 and larger prograde topography is strictly stabilizing for small rossby numbers small amplitude topography destabilizes whereas large topography stabilizes the nonlinear evolution of these instabilities is explored to confirm the predictions from the linear theory and also to illustrate how stabilization is directly related to fluid transport across the shelf

tidal jets nutrient upwelling and their influence on the productivity of the alga halimeda in the ribbon reefs great barrier reef abstract a field experiment was carried out to study water circulation and benthic biological productivity near a passage through the ribbon reefs in the northern great barrier reef of australia the currents through the passage were phase locked with the tide during rising tides strong currents through the passage generated localized upwelling on the upper continental slope enriching the depleted surface waters in nutrients particularly nitrate and phosphate simultaneously on the shelf side of the passage a tidal jet vortex pair system developed which separated from the ribbon reefs so that the coral reefs themselves received little of the upwelled water this was propagated as a bottom trapped layer towards the meadows of the calcareous alga halimeda situated several kilometres inshore of the reefs halimeda can accumulate nutrients particularly nitrate from the relatively low concentrations available from the upwelling events the quantity of nitrogen upwelled was more than sufficient to supply the total nitrogen requirements of the halimeda vegetation a tidal jet also formed offshore from the reef passage during falling tides and the coral covered offshore side of the ribbon reefs may be sustained by the subsequent vertical turbulent entrainment into the jet of deep nutrient rich water immediately offshore from the reef passages these processes require a continuous barrier reef with only narrow passages several kilometers apart numerical models successfully reproduce the observations of jet driven upwelling and of the dynamics of the tidal jet vortex pair system the model predictions are very sensitive to the details of the bathymetry of reef passages as such data are presently unavailable it is not yet possible to use these models to calculate the jet driven nutrient upwelling for the whole great barrier reef

structure and evolution of tidal starting jet vortices at idealized barotropic inlets 1 we present laboratory experiments for tidal starting jet vortices forming at idealized barotropic inlets using dye visualization and particle image velocimetry piv of the surface velocity field vortices are identified in the piv data using the local swirl strength and metrics are calculated for each identified vortex including position equivalent circular diameter maximum vorticity circulation and upwelling potential these quantitative metrics are presented for four different inlet layouts including narrow and wide barrier islands and short and long jetties in each case starting jet eddies initially form attached to the inlet mouth with a rapid increase in vorticity circulation and size as water exiting the inlet flows directly into the starting jet dipole once the vortices reach a critical size they are entrained into the tidal jet and detach from the inlet as they advect away from the inlet their size remains steady while the maximum vorticity and circulation gradually decrease because of the effect of bottom friction and the reduction of input to the vortices from the tidal flow as a result of their advection within the tidal jet secondary vortices shed from the inlet during the quasi steady tidal jet are also entrained into the starting jet vortices decreasing the overall rate at which their vorticity and circulation decay downstream of the inlet the quantitative results indicate that the starting jet eddies grow detach from the inlet mouth and decay at predictable non dimensional rates and times

three dimensional calculation of scour around cylinders the flow of water around a cylinder was calculated by a numerical model the model used a finite volume method to solve the nontransient navier stokes equations for three dimensions on a general nonorthogonal grid the k e turbulence model was used to solve the reynolds stress term the flow field gave the shear stress on the boundaries which was used to calculate sediment concentration for the bed elements the diffusion convection equation for the sediment concentration was solved and continuity for the sediment concentration gave changes in the bed elevation using the modified bed a new flow field was calculated and also a new sediment concentration this sequence was repeated until the depth of the calculated scour hole was identical to the depth of the scour hole from a physical model study the rest of the geometry of the resulting scour hole compared well with the measured scour hole from the experimental study also other flow characteristics given by the numerical model were found to be in ag

flow structure and resistance over subaquaeous high and low angle dunes a prominent control on the flow over subaqueous dunes is the slope of the downstream leeside while previous work has focused on steep 30 asymmetric dunes with permanent flow separation little is known about dunes with lower lee slope angles for which flow separation is absent or intermittent here we present a laboratory investigation where we systematically varied the dune lee slope holding other geometric parameters and flow hydraulics constant to explore effects on the turbulent flow field and flow resistance three sets of fixed dunes lee slopes of 10 20 and 30 were separately installed in a 15 m long and 1 m wide flume and subjected to 0 20 m deep flow measurements consisted of high frequency vertical profiles collected with a laser doppler velocimeter we show that the temporal and spatial occurrence of flow separation decreases with dune lee slope velocity gradients in the dune leeside depict a free shear layer downstream of the 30 dunes and a weaker shear layer closer to the bed for the 20 and 10 dunes the decrease in velocity gradients leads to lower magnitude of turbulence production for gentle lee slopes aperiodic strong ejection events dominate the shear layer but decrease in strength and frequency for low angle dunes flow resistance of dunes decreases with lee slope the transition being nonlinear over the 10 20 and 30 dunes shear stress is 8 33 and 90 greater than a flat bed respectively our results demonstrate that dune lee slope plays an important but often ignored role in flow resistance

assessment of the record of the 1982 el chichon eruption as preserved in greenland snow variability in the so42 and cl time series for the 1980s from 12 shallow snow pits across the greenland ice sheet is used to evaluate the record of the 1982 el chichon eruption and the potential for recording a moderate northern equatorial eruption in a single greenland ice core composition of volcanic glass found in spring 1983 snow in one of the pits in the summit region matches that from el chichon glass thereby verifying the deposition of material from the eruption high na and cl concentrations in this same layer probably represent deposition of the reaction products of halite and h2so4 as observed in the stratosphere following the eruption these findings and the presence of a cl signal in five of the other pits indicate that the cl aerosol component of some eruptions has the potential to remain aloft for at least 1 year after the eruption some of these aerosols may be adsorbed onto tephra particles distinct so42 peaks that can be confidently linked to el chichon were found in only 50 of the records developed through subseasonal sampling however in several other pits an elevated baseline thought to represent more lengthy periods of el chichon aerosol deposition as opposed to deposition from a single snowfall event were observed smoothing of the original data by the calculation of annual so42 flux resulted in the presence of high flux values between 1982 and 1984 years thought to be affected by el chichon aerosol deposition in 9 of the 12 pits these results suggest that a single ice core from anywhere in greenland may record a signal from a northern equatorial eruption of magnitude similar to that of el chichon about 75 of the time this is despite the overall high levels of so2 4 deposition from anthropogenic sources that now make identification and quantification of the volcanic so2 portion of the record more difficult than obtaining the same data for preindustrial volcanism nevertheless composite records from all the pits sampled yielded stratospheric loading 20 mt and optical depth 0 13 estimates similar to stratospheric and satellite based measurements following the eruption equally high so42 concentration and flux values in snow from 1980 to 1982 reflect deposition from the series of middle to high northern latitude volcanic eruptions 1979 1981

the kingston terrane southern new brunswick canada evidence for an early silurian volcanic arc the fault bounded kingston terrane of southern new brunswick consists of felsic and less abundant mafic volcaniclastic rocks and flows and minor interbedded sedimentary rocks of the kingston group intruded by mainly granitic plutons and abundant younger mafic sheets the granitic plutons form about half of the terrane and are characterized by fine grain size and typically granophyric and porphyritic textures indicative of high level emplacement mafic sheets were emplaced in the granitic plutons and parallel to primary layering in the kingston group which is typically steeply dipping and trends northeast to north northeast hence in contrast to some earlier interpretations the kingston terrane does not consist of a bimodal dike complex the kingston group granitic rocks and mafic sheets contain mineral assemblages consistent with metamorphism to upper greenschist lower amphibolite facies new u pb zircon ages of 442 6 ma from a dacitic tuff unit and 437 10 ma from a granitic pluton corroborate previous u pb dates that had suggested that the volcanic and granitic rocks in the kingston terrane are early silurian rather than precambrian as had been earlier assumed the felsic volcanic and granitic rocks are interpreted to be comagmatic on the basis of their field relationships and similar u pb ages and chemical character compositions of both the felsic rocks and less abundant mafic and intermediate and mafic rocks are consistent with calc alkalic affinity and emplacement in a continental margin volcanic arc herein termed the kingston arc a u pb date of 435 5 ma was also obtained from compositionally similar granite that forms a dike in an adjacent fault bounded amphibolite facies metasedimentary unit the similarity in age and composition of the granite dike suggests a link between these metasedimentary rocks and the kingston terrane possibly as part of an accretionary complex if so its location southeast of the kingston terrane indicates that subduction was to the present northwest mafic sheets in the kingston terrane are tholeiitic but have some arc like characteristics consistent with their origin in a former arc setting but the sheets were likely emplaced in a subsequent extensional setting the compositional similarity of the mafic sheets to late silurian mafic volcanic rocks and plutons of the coastal maine magmatic province suggests that they may be related a possible model for evolution of the kingston terrane is proposed wherein the kingston group and related granite formed in an early silurian arc on the margin of ganderia as represented by the new river terrane of late precambrian and early paleozoic age northwestward subduction culminated in collision between the kingston arc and a continental block now represented by the brookville terrane to the southeast subsequent transpressive mo tions in the late silurian may have been responsible for the crustal scale extensional environment in which the coastal maine magmatic province was formed including emplacement of mafic dikes into the deformed former kingston arc transpressive motions related to juxtaposition of the avalon and meguma terranes with the previously accreted more inboard terranes continued through the devonian and into the carboniferous

a numerical investigation of coherent structures and mass exchange processes in channel flow with two lateral submerged groynes 1 large eddy simulation is used to investigate the dynamics of the main coherent structures present in the flow around two vertical submerged groynes situated in a long flatbed open channel the mass exchange processes between the embayment region and the main channel are investigated by studying the ejection of a contaminant introduced instantaneously inside the embayment the instantaneous and mean structure of the horseshoe vortex system forming at the base of the upstream groyne and the bed shear distributions that determine the evolution of the scour in the groyne region are investigated it is found that the amplification of the bed shear stress in the accelerating region around the tip of the upstream groyne is around one order of magnitude larger relative to the mean bed shear stress in the incoming flow analysis of the instantaneous flow fields shows that the eddies that are shed inside the horizontal and vertical detached shear layers play an important role in controlling the mass exchange at the lateral and roof interfaces it is found that most of the pollutant leaves the embayment through the roof and bottom lateral sections the overall mass exchange process is qualitatively different and substantially accelerated compared to the case when the groynes are emerged however it is shown that similar to the emerged case the decay of the mass of contaminant within the embayment cannot be characterized by a unique value of the exchange coefficient used in simple dead zone theory models

catastrophic arid episodes in the eastern mediterranean linked with the north atlantic heinrich events the response of continental climate to the well documented climate oscillations during the last glacial period has been a subject of intense interest yet much less is known about the influence on regional continental climates than in the marine or polar realms of earth the detailed lake level history of the closed lake lisan paleo dead sea in the middle east has been reconstructed from shoreline indications and high resolution u th and 1 4 c chronologies thus providing data on the response of the lake s catchment area to the climate changes during the corresponding period we present a correlation between the newly developed lake lisan level curve for the past 55 k y and the north atlantic heinrich events the correlation indicates a closely connected climate response between these north atlantic events and the hydrologic conditions that prevailed in the eastern mediterranean our findings show that although the generally cooler conditions that prevailed during the last glaciation favored high levels of the lake catastrophic events in the north atlantic which are associated with maximum cooling have been responsible for droughts in the eastern mediterranean we infer that cold water input to the mediterranean originating in the collapse of the north atlantic deep water circulation caused the reduction of evaporation and less precipitation in the eastern mediterranean

complex exposure histories of chert clasts in the late pleistocene shorelines of lake lisan southern israel activities of 26 al and 10 be in five chert clasts sampled from two beach ridges of late pleistocene lake lisan precursor of the dead sea in southern israel indicate low rates of chert bedrock erosion and complex exposure burial and by inference transport histories the chert clasts were derived from the senonian mishash formation a chert bearing chalk which is widely exposed in the nahal zin drainage basin the drainage system that supplied most of the material to the beach ridges simple exposure ages assuming only exposure at the beach ridge sampling sites range from 35 to 354 ky using the ratio 26 al 10 be total clast histories range from 0 46 to 4 3 my unrelated to the clasts current position and exposure period on the late pleistocene beach ridges 160 177 m below sea level optically stimulated luminescence dating of fine sediments from the same and nearby beach ridges yielded ages of 20 0 s 1 4 ka and 36 1 s 3 3 ka these ages are supported by the degree of soil development on the beach ridges and correspond well with previously determined ages of lake lisan which suggest that the lake reached its highest stand around 27 000 cal years bp if the clasts were exposed only once and than buried beyond the range of significant cosmogenic nuclide production then the minimum initial exposure and the total burial times before delivery to the beach ridge are in the ranges 50 1300 ky and 390 3130 ky respectively alternatively the initial cosmogenic dosing could have occurred during steady erosion of the source bedrock back calculating such rates of rock erosion suggests values between 0 4 and 12 m my 1 t he relatively long burial periods indicate extended sediment storage as colluvium on slopes and or as alluvial deposits in river terraces some clasts may have been stored for long periods in abandoned pliocene and early pleistocene routes of nahal zin to the mediterranean before being transported again back into the nahal zin drainage system and washed on to the shores of lake lisan during the late pleistocene copyright 2003 john wiley sons ltd

the statistical connection between tropospheric and stratospheric circulation of the northern hemisphere in winter abstract the associated anomaly patterns of the stratospheric geopotential height field and the tropospheric geopetential and temperature height fields of the northern hemisphere are determined applying the canonical correlation analysis with this linear multivariate technique the coupled modes of variability of lime series of two fields are isolated in the space of empirical orthogonal functions the one dataset is the 50 hpa geopotential height field the other set consists of different height fields of the tropospheric pressure levels 200 500 700 and 850 hpa and the temperature of the 850 hpa pressure level for the winter months december january february two natural coupled modes a barotropic and a baroclinic one of linear relationship between stratospheric and tropospheric circulation are found the baroclinic mode describes a connection between the strength of the stratospheric cyclonic winter vortex and the tropospheric circulation over the north atlantic the corresponding temperature

impacts of abrupt climate changes in the levant from last glacial dead sea levels abstract a new detailed lake level curve for lake lisan the last glacial dead sea reveals a high frequency of abrupt fluctuations during marine isotope stage 3 mis3 compared to the relatively high stand characterizing mis2 and the significantly lower holocene lake the lake level fluctuations reflect the hydrological conditions in the large watershed of the lake which in turn reflects the hydro climatic conditions in the central levant region the new curve shows that the fluctuations coincide on millennial timescales with temperature variations recorded in greenland four patterns of correlation are observed through the last ice age 1 maximum lake elevations were reached during mis2 the coldest interval 2 abrupt lake level drops to the lowest elevations coincided with the occurrence of heinrich h events 3 the lake returned to higher stand conditions along with warming in greenland that followed h events 4 significant lake level fluctuations coincided with virtually every greenland stadial interstadial cycle over glacial interglacial time scales northern hemisphere glacial cooling induces extreme wetness in the levant with high lake levels reaching 160 m below mean sea level mbmsl approximately 240 m above typical holocene levels of 400 mbmsl these orbital time scale shifts are driven by expansions of the european ice sheet which deflect westerly storm tracks southward to the eastern mediterranean resulting in increased sea air temperature gradients that invoke increased cyclogenesis and enhanced moisture delivery to the levant the millennial scale lake level drops associated with greenland stadials are most extreme during heinrich stadials and reflect abrupt cooling of the eastern mediterranean atmosphere and sea surface which weaken the cyclogenic rain engine and cause extreme levant droughts during the recovery from the effect of heinrich stadials the regional climate configuration resumed typical glacial conditions with enhanced levant precipitation and a rise in lake lisan levels similar cyclicity in the transfer of moisture to the levant affected lake levels during all of the non heinrich stadial interstadial cycles

hairpin erosional marks horseshoe vortices and subglacial erosion abstract crescentic erosional marks wrapped around obstacles and extending into straight grooves are termed hairpin erosional marks they are u shaped to semi circular in cross section are deepest around the upstream shoulders of the obstacles and range over seven orders of magnitude in scale some are asymmetrical with one groove better developed than the other the straight grooves may broaden and shallow rapidly in the downflow direction upstream grooves merge with rather than cross downstream crescentic marks grooves are distinct from glacial abrasion forms and many are not striated hairpin erosional marks are interpreted as being products of scour by horseshoe vortices in turbulent fluids this interpretation is supported by the close similarity of these marks to scours produced experimentally and in nature by horseshoe vortices estimated reynolds numbers show that such vortices are probable around obstacles in wind and flowing water but are extremely unlikely in ice aeolian hairpin erosional marks related to yardangs are illustrated to support this conclusion the surface texture cross sectional shapes and spatial patterns of hairpin erosional marks also discount formation by ice a model for the evolution of isolated erosional drumlins from an initial field of hairpin erosional marks extends earlier research on a meltwater hypothesis for drumlins

late holocene lake levels of the dead sea this work presents a high resolution lake level record of the late holocene dead sea a hypersaline terminal lake whose drainage basin encompasses both mediterranean and hyperarid climatic zones the lake level curve reflects the regional hydrologic variations in the drainage basin which in turn represent the levant paleoclimates the curve is based on 46 radiocarbon ages of organic remains from well exposed sedimentary sequences along the dead sea shores these sequences record fluvial and lacustrine depositional environments the paleolakeshores are marked by shore ridges coarse sand units and aragonite crusts in the modern dead sea such features indicate the exact elevation of the shore the late holocene dead sea level fluctuated within the range of 390 to 415 m below sea level mbsl for most of the time the lake was below the topographic sill 402 mbsl separating the northern and southern basins of the dead sea and was confined to the deep northern basin nevertheless short term rises in the late holocene dead sea level caused the flooding of the shallow and flat southern basin highstands occurred in the second and first centuries b c and the fourth century a d during the roman and early byzan tine periods respectively in the eleventh and twelfth centuries a d during the crusader period and at the end of the nineteenth century a d the rises mark a significant change in the annual rainfall in the region which likely exceeded the instrumentally measured modern average the curve also indicates drastic drops that exposed the sedimentary sequences to erosion the oldest and probably deepest drop in the lake level culminated during the fifteenth and fourteenth centuries b c after a retreat from a higher lake stand the longest lowstand occurred after the byzantine period and continued at least until the ninth century a d this arid period coincided with the invasion of moslem arab tribes into the area during the seventh century a d the dramatic fall of the dead sea level during the twentieth century is primarily artificial and has been caused by the diversion of runoff water for the drainage basin but the magnitude is not considered exceptional for the late holocene although the past drops in the lake never exceeded the modern artificial drop rates they do represent extreme arid conditions that occurred frequently over the past several thousand years

neoproterozoic and cambrian u pb zircon ages from grand manan island new brunswick implications for stratigraphy and northern appalachian terrane correlations pre mesozoic rocks are exposed on the southeastern third of grand manan island and adjacent offshore islands in the bay of fundy u pb zircon ages reported here demonstrate that most of these rocks are late neoproterozoic to early cambrian the oldest dated unit is the ingalls head formation from which two felsic tuff samples yielded ages of 617 6 3 2 and 618 3 2 8 ma the three islands granite exposed only on offshore islands is younger than the ingalls head formation at 611 1 2 4 ma it provides a minimum age for marble of the kent island formation which occurs as large xenoliths in the granite the high duck island granite yielded an age of 547 3 1 1 ma providing a minimum late neoproterozoic age for the host long island bay formation an age of 539 0 3 3 ma from a dacitic lithic crystal tuff demonstrates that the priest cove formation is earliest cambrian this supersedes a silurian age previously suggested on the basis of poorly documented fossil evidence the long pond bay formation

hydromorphological processes at submerged solitary boulder obstacles in streams abstract a series of 51 experiments was conducted in a laboratory flume to investigate hydromorphological processes at submerged solitary boulder like obstacles as part of a programme to validate the experimental results five sites with obstacle marks at large boulders were surveyed in two gravel bed ephemeral streams located in eastern spain three hydromorphological system states i iii were observed each exhibiting characteristic dynamic interactions between hydraulic and sedimentary processes i wake vortex domination and absent scouring ii wake vortex domination and downstream scouring iii horseshoe vortex domination frontal scouring and ridge like sediment accumulation at the lee side the latter system state iii comprises typical fluvial obstacle marks in mobile alluvial beds obstacles may tilt upstream into the scour hole when frontal scour depths exceed 0 6 times obstacle length favouring the rapid achievement of steady state conditions state iii field and experimentally modelled obstacle marks are related to each other when frontal scour depth width and length as well as ridge width are used as morphometric variables an analytical model was adapted and successfully applied for determination of these variables at the steady state condition the model was also used to assess the critical threshold for the formation of obstacle marks it integrates mean flow velocity water depth obstacle height and width obstacle shape median grain size relative to obstacle width and grain size distribution dynamic interactions between hydraulic processes and sediment exchange around obstacles together with well defined boundaries between evolutionary states critical thresholds suggest a self organised scale invariant mechanism driving the generation of obstacle marks

transient eddy formation around headlands eddies with length scales of 1 10 km are commonly observed in coastal waters and play an important role in the dispersion of water borne materials the generation and evolution of these eddies by oscillatory tidal flow around coastal headlands is investigated with analytical and numerical models using shallow water depth averaged vorticity dynamics eddies are shown to form when flow separation occurs near the tip of the headland causing intense vorticity generated along the headland to be injected into the interior an analytic boundary layer model demonstrates that flow separation occurs when the pressure gradient along the boundary switches from favoring accelerating to adverse decelerating and its occurrence depends principally on three parameters the aspect ratio b a where b and a are characteristic width and length scales of the headland h cda where h is the water depth cd is the depth averaged drag coefficient and uo a where uo and are the magnitude and frequency of the far field tidal flow simulations with a depth averaged numerical model show a wide range of responses to changes in these parameters including cases where no separation occurs cases where only one eddy exists at a given time and cases where bottom friction is weak enough that eddies produced during successive tidal cycles coexist interacting strongly with each other these simulations also demonstrate that in unsteady flow a strong start up vortex forms after the flow separates leading to a much more intense patch of vorticity and stronger recirculation than found in steady flow

digital topography and bathymetry of the area of the dead sea depression abstract the dead sea depression occupies the central part of that segment of the syrian african rift extending from the mountains of southern lebanon to the arava valley north of the gulf of aqaba on the red sea the lowest continental feature in the world it encloses the dead sea whose surface now lies at about 409 m this shrunken sea consists of a shallow southern basin occupied by evaporation pans and a northern basin 50 km in length by 13 17 km in width with a flat floor some 731 m below msl the depression is asymmetric with the mountains to the east being up to 500 m higher the closed depression has two outlets one at 60 5 m altitude into the mediterranean via the jezreel valley near afula and a second into the red sea at about 200 m altitude in the arava valley a digital terrain model dtm based upon the local 1 50 000 scale topographic maps and modern bathymetric surveys gives elevations to decimeter resolution on a grid with 25 m spacing when visualized as shaded relief or in coloured hypsometric format the dtm graphically shows the overall morphology and its many features these include the local tectonics from the major boundary faults to the associated subsidiary faults and lineaments and their possible relationships with the incised canyons offshore also clearly visible are the volcanic cones on the golan heights the coast paralleling kurkar ridges and the halokinetic disruptions to the deep offshore topography the dtm was used to compute a hypsometric curve for the closed depression from 731 m to 60 m the maximum area and volume of a lake extending to the afula spillway would be 5985 km 2 and 1602 km 3 respectively inflection points were observed at 715 m where the dead sea s flat bottom meets its steep flanks at 385 m where the northern basin meets the flat floor of the southern basin and surrounding plain at 228 with the step up to the basin of the sea of galilee and at mediterranean sea level

north atlantic influence on 19th 20th century rainfall in the dead sea watershed teleconnections with the sahel and implication for holocene climate fluctuations abstract the importance of understanding processes that govern the hydroclimate of the mediterranean basin is highlighted by the projected significant drying of the region in response to the increase in greenhouse gas concentrations here we study the long term hydroclimatic variability of the central levant region situated in the eastern boundary of the basin as reveled by instrumental observations and the holocene record of dead sea level variations observations of 19th and 20th century precipitation in the dead sea watershed region display a multidecadal anti phase relationship to north atlantic natl sea surface temperature sst variability such that when the natl is relatively cold jerusalem experiences higher than normal precipitation and vice versa this association is underlined by a negative correlation to precipitation in the sub saharan sahel and a positive correlation to precipitation in western north america areas that are also affected by multidecadal natl sst variability these observations are consistent with a broad range of holocene hydroclimatic fluctuations from the epochal to the millennial and centennial time scales as displayed by the dead sea lake level by lake levels in the sahel and by direct and indirect proxy indicators of natl ssts on the epochal time scale the gradual cooling of natl ssts throughout the holocene in response to precession driven reduction of summer insolation is associated with previously well studied wet to dry transition in the sahel and with a general increase in dead sea lake levels from low stands after the younger dryas to higher stands in the mid to late holocene on the millennial and centennial time scales there is also evidence for an anti phase relationship between holocene variations in the dead sea and sahelian lake levels and with proxy indicators of natl ssts however the records are punctuated by abrupt lake level drops which appear to be in phase and which occur during previously documented abrupt major cooling events in the northern hemisphere we propose that the mechanisms by which natl ssts affect precipitation in the central levant is related to the tendency for high low pressure anomalies to persist over the eastern north atlantic western mediterranean region when the basin is cold warm this in turn affects the likelihood of cold air outbreaks and cyclogenesis in the eastern mediterranean and consequently rainfall in the central levant region depending on its phase this natural mechanism can alleviate or exacerbate the anthropogenic impact on the regions hydroclimatic future

volcanism and the greenland ice cores the tephra record abstract in recent years increased awareness of the considerable potential offered by tephrochronology in palaeoclimatic studies has fuelled a renewed interest in the tephra record preserved within the greenland ice cores this renewal has occurred in tandem with the development of continuous flow analysis techniques which provide high resolution volcanic aerosol records these chemical records are of considerable value for identifying tephra horizons composed of glass shards and also provide the basis for a new high precision ice core timescale thus allowing ages to be assigned to volcanic events tephra horizons of glass shards in the ice cores play a critical role for 1 testing and building chronological frameworks for the ice cores and other sequences 2 synchronising disparate palaeoclimatic sequences and testing phase relationships 3 atmospheric pathway reconstruction and 4 verifying the source of volcanic aerosol peaks here we review the results of over 25 years of research into tephra horizons in the grip gisp2 and ngrip cores we provide high precision ages for horizons based on recently developed timescales and draw together geochemical characterisations of the products of 45 volcanic events deposited on the greenland ice sheet and identified in the cores there is a clear disparity between the number of volcanic aerosol peaks and the number of tephra horizons of glass shards identified thus far however this synthesis highlights the critical role of the greenland ice cores firstly in the reconstruction of the history of icelandic volcanic eruptions and secondly the considerable value of some tephra horizons in facilitating the correlation of these key climatic archives to north atlantic marine sequences other noteworthy issues include the potential of these cores to preserve tephra horizons from more distal volcanic sources such as north america to resolve the debate surrounding the thera eruption and the observation that some tephra horizons are preserved within the ice without coeval volcanic aerosol signals

a wind tunnel study of flow structure adjustment on deformable sand beds containing a surface mounted obstacle roughness elements of varied scale and geometry commonly appear on the surfaces of sedimentary deposits in a wide range of planetary environments they perturb the local fluid flow so that the entrainment transport and deposition of particles surrounding each element is fundamentally altered fluid dynamists have expended much effort in examining the flow structures surrounding idealized elements mounted on fixed planar walls however self regulation occurs in sedimentary systems as a result of the bed surface undergoing rapid topographic modification with sediment transport until it reaches a stable form that enhances the net physical roughness the present wind tunnel study examines how the flow pattern surrounding an isolated cylinder a problem extensively studied in classical fluid mechanics is altered through morphodynamic development of a deep well that envelopes the windward face and side walls of the roughness element spatial patterns in the fluid velocity turbulence intensity and reynolds stress obtained from laser doppler anemometer lda measurements suggest that the flow structures surrounding such a cylinder are fundamentally altered through self regulation of the bed topography as it reaches steady state for example flow stagnation and the turbulent dissipation of momentum are substantially increased at selected points surrounding the upwind face and side walls of the cylinder respectively along the center line of the wake flow to the rear of the cylinder several structures arising from flow separation are annihilated by strong upwelling of the airflow exhausted from the terminus of the well feedback plays a complex time dependent role in this system

controlling mechanism of local scouring experimental study of clear water scouring around a circular cylinder shows that the scour mechanism is coupled to the three dimensional separation of the upstream boundary layer and the periodic vortex shedding in the wake of the cylinder the first scour appears in the wake of the cylinder the main scouring agent in the upstream region is a system of horseshoe vortices the vortices have a periodical character that causes a triple scour profile to develop in the upstream region during scouring the number and periods of horseshoe vortex shedding undergo no appreciable change despite the clear water stage the transport phenomenon is periodical transport of sediment takes place through turbulent scales of comparable size to macro length scales the size of horseshoe vortices are representative for the macroscale wake scouring is caused by the primary wake vortices and the accelerated side flow the process is characterized by a strong periodical transport and the formation of ripples the periodicity is controlled by the shedding frequency of the wake vortices collars attached to the cylinder cannot prevent the formation of the vortices

bed load at low shields stress on arbitrarily sloping beds alternative entrainment formulation 1 according to the bagnold hypothesis for equilibrium bed load transport a necessary constraint for the maintenance of equilibrium bed load transport is that the fluid shear stress at the bed must be reduced to the critical or threshold value associated with incipient motion of grains it was shown in a companion paper seminara et al 2002 however that the bagnold hypothesis breaks down when applied to equilibrium bed load transport on beds with transverse slopes above a relatively modest value that is well below the angle of repose an investigation of this failure resulted in a demonstration of its lack of validity even for nearly horizontal beds the constraint is here replaced with an entrainment formulation according to which a dynamic equilibrium is maintained by a balance between entrainment of bed grains into the bed load layer and deposition of bed load grains onto the bed the entrainment function is formulated so that the entrainment rate is an increasing function of the excess of the fluid shear stress at the bed over the threshold value the formulation is implemented with the aid of a unique set of laboratory data that characterizes equilibrium bed load transport at relatively low shear stresses for streamwise angles of bed inclination varying from nearly 0 to 22 the formulation is shown to provide a description of bed load transport on nearly horizontal beds that fits the data as well as that resulting from the bagnold constraint the entrainment formulation has the added advantage of not requiring the unrealistically high dynamic coefficient of coulomb friction resulting from the bagnold constraint finally the entrainment formulation provides reasonable and consistent results on finite streamwise and transverse bed slopes even those at which the bagnold formulation breaks down completely

a 45 kyr laminae record from the dead sea implications for basin erosion and floods recurrence abstract recording and analyzing how climate change impacts flood recurrence basin erosion and sedimentation can improve our understanding of these systems the aragonite detritus laminae couplets comprising the lacustrine formations that were deposited in the dead sea are considered as faithful monitors of the freshwater supply to the lakes we count a total of 5600 laminae couplets deposited in the last 45 kyr mis3 mis1 at the dead sea center which encompass the upper 142 m of the icdp core 5017 1 the present study shows that aragonite and detritus laminae are thinner and occur at high frequency during mis 3 2 while they are much thicker and less frequent during mis 1 by analyzing multiple climate connected factors we propose that significant lake level drops enhanced dust input and low vegetative cover in the drainage basin during the last deglaciation 22 11 6 ka have considerably increased erodible materials in the dead sea watershed we find a decoupling existed between the significant lake level drop lake size reduction and lamina thickness change during the last deglaciation we argue that during the last glacial and the holocene the variation of lamina thickness at the multiple millennium scale was not controlled directly by the lake level size change we interpret this decoupling implying the transport capacity of flash floods is low and might be saturated by the oversupply of erodible materials and indicating a transport limited regime during the time period we suggest the observed thickness and frequency distribution of aragonite detritus laminae points to the high frequency of small magnitude floods during the last glacial in contrast to low frequency but large magnitude floods during the holocene

anomalous sulfur isotope compositions of volcanic sulfate over the last millennium in antarctic ice cores 1 the reconstruction of past volcanism from glaciological archives is based on the measurement of sulfate concentrations in ice this method does not allow a proper evaluation of the climatic impact of an eruption owing to the uncertainty in classifying an event between stratospheric or tropospheric this work develops a new method using anomalous sulfur isotope composition of volcanic sulfate in order to identify stratospheric eruptions over the last millennium the advantages and limits of this new method are established with the examination of the 10 largest volcanic signals in ice cores from dome c and south pole antarctica of the 10 seven are identified as stratospheric eruptions among them three have been known to be stratospheric tambora kuwae the 1259 unknown event and they exhibit anomalous sulfur isotope compositions three unknown events circa 1277 1230 1170 a d and the serua eruption have been identified as stratospheric eruptions which suggests for the first time that they could have had significant climatic impact however the kuwae and the 1259 unknown event stratospheric eruptions exhibit different anomalous sulfur isotope compositions between south pole and dome c samples differences in sulfate deposition and preservation patterns between the two sites can help explain these discrepancies this study shows that the presence of an anomalous sulfur isotope composition of volcanic sulfate in ice core indicates a stratospheric eruption but the absence of such composition does not necessarily lead to the conclusion of a tropospheric process because of differences in the sulfate deposition on the ice sheet

on interfacial instability as a cause of transverse subcritical bed forms 1 recent experimental results suggest there are at least two distinct bed form initiation processes bed forms may be generated from local bed defects that are propagated down and across stream by flow separation processes when sediment transport is patchy and sporadic alternatively bed form development may occur over the whole bed at once when sediment transport is general and widespread herein we critically test a simple model for this latter bed form initiation mode that was originally presented by h k liu in 1957 but not tested because of the complex nature of the measurements required the theory is based on the idea that a moving sand bed might be likened to a dense fluid and that a hydrodynamic instability develops at the interface between the sediment transport layer and the near bed fluid leading to the organization of the transport system into laterally extended discrete bed forms our predicted initial bed form lengths are within 10 of the lengths observed in a series of flume experiments suggesting that transverse waveforms on sand beds sheared by unidirectional currents can be initiated as an interfacial hydrodynamic instability of kelvin helmholtz type when the current is sufficiently vigorous to produce general sediment movement that is to create a sediment transport layer that acts as a pseudofluid with density composed of solid and fluid components

a new greenland ice core chronology for the last glacial termination 1 we present a new common stratigraphic timescale for the north greenland ice core project ngrip and grip ice cores the timescale covers the period 7 9 14 8 kyr before present and includes the bolling allerod younger dryas and early holocene periods we use a combination of new and previously published data the most prominent being new high resolution continuous flow analysis cfa impurity records from the ngrip ice core several investigators have identified and counted annual layers using a multiparameter approach and the maximum counting error is estimated to be up to 2 in the holocene part and about 3 for the older parts these counting error estimates reflect the number of annual layers that were hard to interpret but not a possible bias in the set of rules used for annual layer identification as the grip and ngrip ice cores are not optimal for annual layer counting in the middle and late holocene the timescale is tied to a prominent volcanic event inside the 8 2 kyr cold event recently dated in the dye 3 ice core to 8236 years before a d 2000 b2k with a maximum counting error of 47 years the new timescale dates the younger dryas preboreal transition to 11 703 b2k which is 100 150 years older than according to the present grip and ngrip timescales the age of the transition matches the gisp2 timescale within a few years but viewed over the entire 7 9 14 8 kyr section there are significant differences between the new timescale and the gisp2 timescale the transition from the glacial into the bolling interstadial is dated to 14 692 b2k the presented timescale is a part of a new greenland ice core chronology common to the dye 3 grip and ngrip ice cores named the greenland ice core chronology 2005 gicc05 the annual layer thicknesses are observed to be log normally distributed with good approximation and compared to the early holocene the mean accumulation rates in the younger dryas and bolling periods are found to be 47 2 and 88 2 respectively

sediment patterns near a model patch of reedy emergent vegetation this laboratory study describes the sediment patterns formed in a sand bed around circular patches of rigid vertical cylinders representing a patch of reedy emergent vegetation the patch diameter was much smaller than the channel width two patch densities solid volume fraction 3 and 10 and two patch diameters 22 and 10 cm were considered for flows above the threshold of sediment motion patterns of sediment erosion and deposition were observed around and within the patch scouring within the patch was positively correlated with turbulent kinetic energy in the patch for sparse patches sediment scoured from within the patch was mostly deposited within one patch diameter downstream of the patch for dense patches which experience greater flow diversion sediment scoured from the patch was carried farther downstream before deposition along the patch centerline differences between the sparse and dense patch patterns of deposition are explained in the context of flow diversion and wake structure which are related to a nondimensional flow blockage parameter while sediment was redistributed near the patch observations suggest that net deposition was not recorded at the reach scale

characterization of land degradation along the receding dead sea coastal zone using airborne laser scanning abstract the dead sea the lowest place on the earth s continents was at its highest level in 1896 reaching an elevation of 388 4 m below mean sea level m b m s l and 390 m in the early 1920s since then it has almost constantly been dropping reaching the level of 426 m b m s l in 2013 since the late 1990s its level has been decreasing by approximately 1 my 1 the rapid lake retreat accelerates large scale environmental deterioration including soil erosion land degradation rapid headcut migration and widespread development of collapse sinkhole fields these geomorphic elements threaten the natural environment and anthropogenic infrastructure we provide an overview of the geomorphic processes in the form of soil erosion channel incision land degradation and the development of collapse sinkholes we take advantage of the high resolution airborne laser scanning technology for three dimensional detection of surficial changes quantification of their volumes and documentation of the present state of the terrain with utmost accuracy and precision this type of information and the identification of future trends are vital for proper planning of any rapidly changing environment

initial stages of erosion and bed form development in a turbulent flow around a cylindrical pier 1 bed load transport and erosion in fine sediment beds are mainly driven by the dynamics of the near bed turbulent flow in situations when the shear stress is not sufficiently high to produce significant transport the presence of an obstacle can initiate erosion and trigger the development of bed forms which are produced by the emergence of the turbulent horseshoe vortex thv system we develop a numerical model to investigate the initial stages of erosion and the development of ripples produced by the thv system in the vicinity of a surface mounted cylindrical pier the flow is simulated using the detached eddy simulation approach which has been shown to accurately resolve most of the turbulent stresses produced by the thv to compute the erosion the exner equation is coupled to a new bed load transport model that directly incorporates the effect of the instantaneous flow field on sediment transport the morphodynamic model is integrated simultaneously with the flow equations using an arbitrary lagrangian eulerian method for moving boundaries even though the time rate of scour is slower compared to the observations the computed results exhibit essentially all the dynamics of erosion including the emergence of ripples reported in the experiments of dargahi 1990 the bed forms show similar velocities as reported in the experiments and are shown to be statistically similar to ripples measured in laboratory experiments and in nature to our knowledge this is the first three dimensional simulation to capture the ripple dynamics that evolve naturally from the nonlinear interactions between the flow and the bed

erosion by planar turbulent wall jets new scaling laws are presented for the spatial variation of the mean velocity and lateral extent of a two dimensional turbulent wall jet flowing over a fixed rough boundary these scalings are analogous to those derived by wygnanski et al 1992 for the flow of a wall jet over a smooth boundary they reveal that the characteristics of the jet depend weakly upon the roughness length associated with the boundary as confirmed by experimental studies rajaratnam 1967 these laws are used in the development of an analytical framework to model the progressive erosion of an initially flat bed of grains by a turbulent jet the grains are eroded if the shear stress exerted on the grains at the surface of the bed exceeds a critical value which is a function of the physical characteristics of the grains after the wall jet has been flowing for a sufficiently long period the boundary attains a steady state in which the mobilizing forces associated with the jet are insufficient to further erode the boundary the steady state profile is calculated separately by applying critical conditions along the bed surface for the incipient motion of particles these conditions invoke a relationship between the mobilizing force exerted by the jet the weight of the particles and the local gradient of the bed use of the new scaling laws for the downstream variation of the boundary shear stress then permits the calculation of the shape of the steady state scour pit the predicted profiles are in good agreement with the experimental studies on the erosive action of submerged water and air jets on beds of sand and polystyrene particles rajaratnam 1981 the shape of the eroded boundary at intermediate times before the steady state is attained is elucidated by the application of a sediment volume conservation equation this relationship balances the rate of change of the bed elevation with the divergence of the flux of particles in motion the flux of particles in motion is given by a semi empirical function of the amount by which the boundary shear stress exceeds that required for incipient motion hence the conservation equation may be integrated to reveal the transient profiles of the eroded bed there is good agreement between these calculated profiles and experimental observations rajaratnam 1981

quantifying the pinatubo volcanic signal in south polar snow recent snow and firn core samples from south pole contain increased sulfate so42 concentrations during 1992 1994 as a result of the june 1991 pinatubo eruption and the august 1991 cerro hudson eruption in chile traces of pinatubo tephra volcanic ash were identified in the 1993 and 1994 snow layers supporting the conclusion that increased so42 in 1993 1994 is from the pinatubo eruption although the pinatubo eruption preceded hudson its so42 signal in south polar snow follows and is resolved from that of hudson the deposition of the pinatubo so42 aerosol was delayed due to the long transport to the high southern latitudes and its initial existence at high altitudes in the antarctic atmosphere multi year multi site sampling demonstrates that the volcanic signals are well preserved and spatially consistent measurements on 2 firn cores show that the south pole so so42 flux from pinatubo is 10 9 1 2 kg km 2 over 2 2 years while the hudson flux is 3 2 0 6 kg km 2 in 1 1 years these results when combined with satellite determined pinatubo sulfur dioxide so2 emission make it possible to link quantitatively the atmospheric aerosol mass loading from a low latitude volcanic eruption to its signal in polar ice cores

numerical simulation of dune flat bed transition and stage discharge relationship with hysteresis effect 1 this work presents recent advances on morphodynamic modeling of bed forms under unsteady discharge this paper includes further development of a morphodynamic model proposed earlier by giri and shimizu 2006a this model reproduces the temporal development of river dunes and accurately replicates the physical properties associated with bed form evolution model results appear to provide accurate predictions of bed form geometry and form drag over bed forms for arbitrary steady flows however accurate predictions of temporal changes of form drag are key to the prediction of stage discharge relation during flood events herein the model capability is extended to replicate the dune flat bed transition and in turn the variation of form drag produced by the temporal growth or decay of bed forms under unsteady flow conditions some numerical experiments are performed to analyze hysteresis of the stage discharge relationship caused by the transition between dune and flat bed regimes during rising and falling stages of varying flows the numerical model successfully simulates dune flat bed transition and the associated hysteresis of the stage discharge relationship this is in good agreement with physical observations but has been treated in the past only using empirical methods a hypothetical relationship for a sediment parameter the mean step length is proposed to a first level of approximation that enables reproduction of the dune flat bed transition the proposed numerical model demonstrates its ability to address an important practical problem associated with bed form evolution and flow resistance in varying flows

ice cores as an index of global volcanism from 1850 to the present to evaluate an important cause of past climate change climate researchers need reliable estimates of volcanic aerosol loading in the atmosphere previous indices the dust veil index volcanic explosivity index and those by mitchell 1970 sato et al 1993 and khmelevtsov et al unpublished manuscript 1993 all have drawbacks ice core acidity and sulfate records because they contain physical evidence of atmospheric loading are a promising source of information on past volcanic aerosols but these records contain large nonvolcanic signals as well we have compared northern hemisphere nh and southern hemisphere sh annual average versions of five indices with eight nh and six sh ice core records for the period 1850 to the present in an attempt to identify the volcanic signal common to all records the indices are all highly correlated with each other for the nh although the individual ice core records are in general not well correlated with each other or with any of the indices a composite derived from averaging the cores the ice core volcano index ivi shows promise as a new index of volcanic aerosol loading this new index correlates well with the existing non ice core volcanic indices and with high frequency temperature records for the sh the individual ice cores and indices are better correlated the shivi is again highly correlated with all indices and individual ice cores but not with high frequency temperature records for both hemispheres the southern oscillation index shows no significant correlation with the volcanic indices ice cores or the ivi thus providing no evidence for the impact of volcanic eruptions on el nino southern oscillation events

geochemistry of silurian devonian volcanic rocks in the coastal volcanic belt machias eastport area maine evidence for a pre acadian arc the coastal volcanic belt in easternmost maine consists of silurian devonian volcanic rocks related to the convergence of avalonia with the leading edge of composite laurentia just before and during early stages of the acadian orogeny new major and trace element analyses from the machias eastport area indicate that the early to late silurian volcanic rocks in the dennys edmunds and leighton formations formed in a continental arc environment in contrast to some earlier interpretations these rocks have a continuous compositional range from basalt through andesite to rhyolite the mafic rocks in these formations are calc alkaline and show subduction related geochemical signatures on various discrimination diagrams whereas the felsic rocks have geochemical characteristics of volcanic arc granites in contrast the thick volcanic deposits of the younger late silurian eastport formation are bimodal and tholeiitic and have transitional to within plate geochemical signatures thus the transition over time as the acadian seaway closed from subduction and arc settings to continental extension can been seen in this rock sequence and it constrains the time of avalonia s docking regionally volcanic rocks in the kingston terrane of southern new brunswick are likely correlatives of the earlier arc volcanic rocks dennys edmunds and leighton formations while eastport formation analogues are found in the coastal maine magmatic province cranberry island series and probably the passamaquoddy bay volcanic series which similarly appear to have formed after the transition to an extensional environment occurred although other extensional settings cannot be entirely ruled out basalts in the post acadian perry formation indicate continental extension was also active in the late devonian

scour on alluvial bed downstream of grade control structures this paper describes an approach for predicting local scour downstream of grade control structures the developed analysis applies the incomplete self similarity iss theory for deducing some physically based dimensionless groups controlling the geometrical pattern of the scour profile the scour measurements available in the literature in conjunction with numerous unpublished data allow a multiregressive calibration of the iss relationships the experimental sample includes different bed grain size distributions and scales of the erosive phenomenon the results prove that the ratio between the upstream water head and the weir height is able to explain the measurements of scour depth carried out in both small and large scale installations this estimation of maximum scour depth was improved introducing variables representative of both the jet contraction and the bed particle grain size distribution variables related to the longitudinal sizes of the scour profile tend to be predicted with more accuracy than those related to the scour depth and appear more influenced by the coarsest component of the alluvial bed

coherent structures in the flow field around a circular cylinder with scour hole large eddy simulation les and laboratory flume visualizations were used to investigate coherent structures present in the flow field around a circular cylinder located in a scour hole the bathymetry corresponds to equilibrium scour conditions and is fixed in les the flow parameters in the simulation correspond to the experimental conditions in which the approach flow is fully turbulent detailed consideration is given to the interaction of the horseshoe vortex hv system within the scour hole with the detached shear layers formed from the cylinder and the near bed turbulence it is found that the overall structure of the hv system varies considerably in space and time though a large relatively stable primary necklace vortex is present at practically all times inside the scour hole the simulation captures the presence of bimodal chaotic oscillations inside the hv system as well as the sharp increase in the resolved turbulent kinetic energy levels and pressure fluctuations reported in prior experimental investigations high levels of the mean bed shear stress are observed beneath the primary necklace vortex especially over the region where the bimodal oscillations are strong as well as beneath the small junction vortex at the base of the cylinder it is also found that the detachment and advection of patches of vorticity from the downstream part of the legs of the necklace vortices can induce large instantaneous bed shear stress values when the critical bed shear stress value for sediment entrainment on a flat surface is adjusted for bed slope effects the les simulation correctly predicts that the distribution of the mean bed shear stress is consistent with equilibrium scour conditions

quantitative assessment of glacial fluctuations in the level of lake lisan dead sea rift abstract a quantitative understanding of climatic variations in the levant during the last glacial cycle is needed to support archaeologists in assessing the drivers behind hominin migrations and cultural developments in this key region at the intersection between africa and europe it will also foster a better understanding of the region s natural variability as context to projections of modern climate change detailed documentation of variations in the level of lake lisan the lake that occupied the dead sea rift during the last glacial cycle provides crucial climatic information for this region existing reconstructions suggest that lake lisan highstands during cold intervals of the last glacial cycle represent relatively humid conditions in the region but these interpretations have remained predominantly qualitative here i evaluate realistic ranges of the key climatological parameters that controlled lake level based on the observed timing and amplitudes of lake level variability i infer that a mean precipitation rate over the wider catchment area of about 500 mm y 1 as proposed in the literature would be consistent with observed lake levels if there was a concomitant 15 50 increase in wind speed during cold glacial stadials this lends quantitative support to previous inferences of a notable increase in the intensity of mediterranean winter storms during glacial periods which tracked eastward into the levant in contrast to highstands during regular stadials lake level dropped during heinrich events i demonstrate that this likely indicates a further intensification of the winds during those times

late holocene climates of the near east deduced from dead sea level variations and modern regional winter rainfall the dead sea is a terminal lake of one of the largest hydrological systems in the levant and may thus be viewed as a large rain gauge for the region variations of its level are indicative of the climate variations in the region here we present the decadal to centennialresolution holocene lake level curve of the dead sea then we determine the regional hydroclimatology that affected level variations to achieve this goal we compare modern natural lake level variations and instrumental rainfall records and quantify the hydrology relative to lake level rise fall or stability to quantify that relationship under natural conditions rainfall data pre dating the artificial dead sea level drop since the 1960s are used in this respect jerusalem station offers the longest uninterrupted pre 1960s rainfall record and jerusalem rains serve as an adequate proxy for the dead sea headwaters rainfall principal component analysis indicates that temporal variations of annual precipitation in all stations in israel north of the current 200 mm yr 1 average isohyet during 1940 1990 are largely synchronous and in phase 70 of the total variance explained by pc1 this station also represents well northern jordan and the area all the way to beirut lebanon especially during extreme drought and wet spells we a determine the modern and propose the past regional hydrology and eastern mediterranean em climatology that affected the severity and length of droughts wet spells associated with multiyear episodes of dead sea level falls rises and b determine that em cyclone tracks were different in average number and latitude in wet and dry years in jerusalem the mean composite sea level pressure and 500 mb height anomalies indicate that the potential causes for wet and dry episodes span the entire em and are rooted in the larger scale northern hemisphere atmospheric circulation we also identified remarkably close association within radiocarbon resolution between climatic changes in the levant reflected by level changes and culture shifts in this region

use of paleo records in determining variability within the volcanism climate system abstract volcanic eruptions that inject large quantities of sulfur rich gases into the stratosphere have the capability of cooling global climate by 0 2 0 3 c for several years after the eruption equatorial eruptions will impact global climate whereas mid latitude eruptions can cool climate in the hemisphere of origin magnitude of cooling varies by latitude and it is possible for warming to occur in certain regions primarily during the winter although instrumental records have been used to quantify the volcanic impact on climate they are limited in their temporal and spatial coverage and the style and magnitude of eruptions occurring over the two centuries of instrumental records is limited a thorough evaluation of the range of variability in the volcanism climate system requires a multidisciplinary approach that includes the analysis of ice core records geological data atmospheric measurements and visual phenomena tree ring records and other proxy data evaluation of these longer time series indicates that multiple volcanic eruptions have the potential to force climate over decadal to multi decadal time frames especially when these eruptions enhance or extend pre existing cool conditions on the other hand a lack of climatically significant eruptions may result in warmer average temperatures over decadal time frames because the volcanic cooling component within the climate system is absent mega eruptions like the toba eruption of 71 000 73 000 yr ago may impact climate on centennial time frames through positive feedback mechanisms evidence exists which indicates that environmental changes associated with rapid climatic fluctuations such as crustal loading unloading with glaciation deglaciation and variability in glacial meltwater loading on ocean basins may cause an increase in volcanic activity

natural versus human control on subsurface salt dissolution and development of thousands of sinkholes along the dead sea coast one of the most hazardous results of the human induced dead sea ds shrinkage is the formation of more than 6000 sinkholes over the last 25 years the ds shrinkage caused eastward retreat of underground brine replaced by fresh groundwater which in turn dissolved a subsurface salt layer to generate cavities and collapse sinkholes the areal growth rate of sinkhole clusters is considered the most pertinent proxy for sinkholes development analysis of light detection and ranging lidar digital elevation models dems and interferometric synthetic aperture radar insar allows translation of the areal growth rate to a salt dissolution rate of the salt layer revealing two peaks in the history of the salt dissolution rate these peaks cannot be attributed to the decline of the ds level instead we show that they are related to long term variations of precipitation in the groundwater source region the judea mountains and the delayed response of the aquifer system between the mountains and the ds rift this response is documented by groundwater levels and salinity variations we thus conclude that while the ds level decline is the major trigger for sinkholes formation the rainfall variations more than 30 km to the west dominate their evolution rate the influence of increasing rainfall in the judea mountains reaches the ds at a typical time lag of four years and the resulting increase in the salt dissolution rate lags by a total time of 5 6 years

coherent structure dynamics and sediment erosion mechanisms around an in stream rectangular cylinder at low and moderate angles of attack 1 local scour around elongated in stream structures e g high aspect ratio rectangular bridge piers is mainly driven by the interactions between the erodible bed and the large scale coherent structures generated by the presence of the flow obstruction the present investigation uses eddy resolving numerical simulations to study the mean flow and turbulence structure around a high aspect ratio rectangular cylinder placed in a flat bed channel simulations are conducted for three angles of attack 0 15 and 30 at a channel reynolds number of 2 4 105 this paper focuses on the dynamics of the large scale coherent structures forming around the rectangular cylinder and their role in controlling sediment entrainment at conditions corresponding to the start of the scour process simulation results show that most of the sediment is entrained from the bed by the eddies shed inside the separated shear layers ssls by the legs of the necklace vortices and by the strongly accelerated flow on the outer side of the ssls for 0 and 15 the horseshoe vortex hv system plays a relatively minor role in the entrainment of sediment in front of the cylinder and the passage of the wake vortices rollers results in a small amplification of the bed friction velocity in contrast for 30 the unsteady dynamics of the main necklace vortices part of the hv system and of the roller vortices results in a significant amplification of the instantaneous bed friction velocity the mean flux of sediment entrained from the bed calculated on the basis of the mean flow field is found to underestimate by 2 3 times the same quantity when estimated more correctly on the basis of the instantaneous flow fields the primary reason for this underestimation is sediment entrainment in regions where the mean flow bed friction velocity is smaller than the critical value for entrainment quantitative information on the extent of the regions of high values of the bed friction velocity and its standard deviation and on the sediment entrainment flux as a function of the angle of attack is essential to guide scour protection measures around the in stream noncircular obstructions

abrupt aridities and salt deposition in the post glacial dead sea and their north atlantic connection abstract abrupt arid events in the post glacial 17 4 10 kyr bp dead sea basin dsb were recorded by significant lake level declines in lake lisan and massive deposition of gypsum and salt between 17 4 and 16 kyr cal bp the lake level dropped from its late mis2 stand of 260 m below mean sea level m bmsl to 330 m bmsl depositing a thick sequence of gypsum between 16 and 15 kyr cal bp the lake level recovered but dropped abruptly again at 14 kyr cal bp to below 465 m bmsl probably the lowest late pleistocene stand then between 13 and 11 kyr cal bp the younger dryas time interval the lake rose above 400 m bmsl and declined at 11 10 kyr cal bp depositing a thick sequence of salt the abrupt lake level drops and salt deposition coincided with times of ice and meltwater discharges into the north atlantic na heinrich event h1 and meltwater pulse mwp1a similar coincidence between ice and meltwater discharges in the na e g h events and arid episodes at the levant was recorded during the colder last glacial period demonstrating a persistent effect of the north atlantic hydrology and sea ice advances on the levant climate the climatically turmoil post glacial period was accompanied by significant developments in human culture the collapse of the natufian culture during the bolling allerod and the rise of the pre pottery neolithic cultures ppn a and b around the 11 10 kyr cal bp salt deposition interval marking the initiation of the neolithic revolution in the region

a meltwater origin for antarctic shelf bedforms with special attention to megalineations the geomorphology of troughs crossing the antarctic shelf is described and interpreted in terms of ice stream hydrology the scale of tunnel channels on the inner shelf and the absence of sediment at their mouths are taken to infer catastrophic drainage drumlins on the inner and outer shelves with pronounced crescentic and hairpin scours are also interpreted as products of catastrophic flow gullies and channels on the continental slope and turbidites on the rise and abyssal plain point to abundant meltwater discharge across the shelf attempts to explain this morphology and sedimentology in terms of release or discharge of meltwater by pressure melting strain heating darcian flow or advection in deforming till are shown to be unrealistic we suggest that meltwater flow across the middle and outer shelves might have been in broad turbulent floods which raises the possibility that megascale glacial lineations msgl on the shelf might originate by erosion in turbulent flow this possibility is explored by use of analogs for msgl from flood and eolian landscapes and marine environments an extended discussion reflects on objections that stand in the way of the flood hypothesis

holocene climate variability and cultural evolution in the near east from the dead sea sedimentary record abstract a comprehensive record of lake level changes in the dead sea has been reconstructed using multiple well dated sediment cores recovered from the dead sea shore interpreting the lake level changes as monitors of precipitation in the dead sea drainage area and the regional eastern mediterranean palaeoclimate we document the presence of two major wet phases 10 8 6 and 5 6 3 5 cal kyr bp and multiple abrupt arid events during the holocene the arid events in the holocene dead sea appear to coincide with major breaks in the near east cultural evolution at 8 6 8 2 4 2 3 5 cal kyr bp wetter periods are marked by the enlargement of smaller settlements and growth of farming communities in desert regions suggesting a parallelism between climate and near east cultural development

u th dating of lake lisan late pleistocene dead sea aragonite and implications for glacial east mediterranean climate change u series dating can be an effective means to obtain accurate and precise ages on quaternary carbonates however most samples require a correction for u and th in admixed detritus this complication is often addressed through generation of u th isochrons requiring analyses of several coeval samples in addition presence of water derived hydrogenous th in the carbonate can cause inaccuracies in isochron ages this study reports a high resolution u series chronology of sediments deposited by lake lisan the last glacial precursor of the dead sea the strategy employed combines multiple measurements from a few stratigraphic heights and fewer analyses from many heights in a single described and measured section the resulting chronology is based on ages at 22 heights in a 40 m thick section covering the interval of 70 14 calendar ka bp the effects of admixed detritus are evaluated using trace elements nearly pure aragonite samples indicated by very low abundances of insoluble elements such as nb and zr were found to contain hydrogenous th which causes the uncorrected u 230th age of a modern sample to be 2 5 ka nevertheless accurate ages have been obtained by correcting for the detrital and aqueous interferences the resulting ages are in stratigraphic order and their accuracy is evidenced by consistency of lisan formation u series and 14c ages with the coral based calendar radiocarbon age calibration the u th ages provide a context to unravel the limnological history of lake lisan boundaries between the lower middle and upper stratigraphic units correspond to the mis 4 3 and 3 2 transitions respectively during mis 2 and 4 the lake generally showed a stable two layer configuration and a positive fresh water balance reflected by deposition of laminated aragonite detritus dry intervals during mis 2 and 4 are indicated by thick gypsum layers and an inferred depositional hiatus which are temporally associated with heinrich events h1 at 17 ka and h6 at 65 ka respectively during mis 3 the lake level was unstable with intermittent dry periods indicated by abundant clastic layers and a significant hiatus between 43 49 ka clastic layers are associated with dansgaard oeschger events during mis 3 and indicate lake level declines during abrupt northern hemisphere warmings overall the climate of the eastern mediterranean region shows a strong linkage to the northern hemisphere climate with increasing lake size and stability during cold periods and fluctuations and dessication during warmings and heinrich events

possible connection between large volcanic eruptions and level rise episodes in the dead sea basin abstract the june 1991 pinatubo volcanic eruption perturbed the atmosphere triggering short term worldwide changes in surface and lower troposphere temperatures precipitation and runoff the following winter was anomalously wet in the levant with a 2 m increase in the dead sea level that created a distinct morphological terrace along the lake s shore given the global radiative and chemical effects of volcanogenic aerosols on climatic systems we tested the hypothesis that the 1991 92 winter shore terrace is a modern analogue to the linkage between past volcanic eruptions and a sequence of shore terraces on the cliffs around the dead sea basin dsb sixteen shore terraces detected using airborne laser scanning data were interpreted as indicating short term level rises due to episodes of enhanced precipitation and runoff during the dramatic drop in lake lisan s palaeo dead sea level at the end of the last glacial maximum the terraces were compared with a dated time series of volcanogenic sulfate from the gisp2 ice core and similar numbers of sulfate concentration peaks and shore terraces were found furthermore a significant correlation was found between so 4 concentration peaks and the heights of the terraces this correlation may indicate a link between the explosivity of past eruptions the magnitude of stratospheric injection and their impact on the northern hemisphere water balance the record of such short term climato hydrological effects is made possible by the dramatic desiccation of lake lisan detailed records of such events albeit rare because of their vulnerability and short longevity provide an important demonstration of global climatic teleconnections

a potential vorticity theory for the formation of elongate channels in river deltas and lakes 1 rivers empty into oceans and lakes as turbulent sediment laden jets which can be characterized by a gaussian horizontal velocity profile that spreads and decays downstream because of shearing and lateral mixing at the jet margins recent experiments demonstrate that this velocity field controls river mouth sedimentation patterns in nature diffuse jets are associated with mouth bar deposition forming bifurcating distributary networks while focused jets are associated with levee deposition and the growth of elongate channels that do not bifurcate river outflows from elongate channels are similar in structure to cold filaments observed in ocean currents where high potential vorticity helps to preserve coherent structure over large distances motivated by these observations we propose a hydrodynamic theory that seeks to predict the conditions under which elongate channels form our approach models jet velocity patterns using the flow vorticity both shearing and lateral spreading are directly related to the vertical component of vorticity we introduce a new kind of potential vorticity that incorporates sediment concentration and thus allows study of jet sedimentation patterns the potential vorticity equation reduces the number of fluid momentum equations to one without losing generality this results in a compact analytical solution capable of describing the streamwise evolution of the potential vorticity of a sediment laden jet from initial conditions at the river mouth our theory predicts that high potential vorticity is a necessary condition for focused levee deposition and the creation of elongate channels comparison to numerical laboratory and field studies indicates that potential vorticity is a primary control on channel morphology our results may be useful for designing river delta restoration schemes such as the proposed mississippi delta diversion

turbulent open channel flows with variable depth across the channel the flow of water in straight open channels with prismatic complex cross sections is considered lateral distributions of depth mean velocity and boundary shear stress are derived theoretically for channels of any shape provided that the boundary geometry can be discretized into linear elements the analytical model includes the effects of bed generated turbulence lateral shear turbulence and secondary flows experimental data from the science and engineering research council serc flood channel facility are used to illustrate the relative importance of these three effects on internal shear stresses new experimental evidence concerning the spatial distribution of reynolds stresses yx and zx is presented for the particular case of compound or two stage channels in such channels the vertical distributions of zx are shown to be highly nonlinear in the regions of strongest lateral shear and the depth averaged values of yx are shown to be significantly different from the depth mean apparent shear stresses the importance of secondary flows in the lateral shear layer region is therefore established the influence of both reynolds stresses and secondary flows on eddy viscosity values is quantified a numerical study is undertaken of the lateral distributions of local friction factor and dimensionless eddy viscosity the results of this study are then used in the analytical model to reproduce lateral distributions of depth mean velocity and boundary shear stress in a two stage channel the work will be of interest to engineers engaged in flood channel hydraulics and overbank flow in particular

experimental investigation of shallow recirculating flows experiments were conducted to study the bed friction influence in the recirculating zone of a shallow open channel flow velocity measurements were made using a hot film anemometer the recirculating flow rate and the length of the recirculating zone were determined from the measurements and they are related to a horizontal length scale and a bed friction length scale for deep flows the results are independent of the bed friction length scale for shallow flows the results are independent of the horizontal length scale turbulence in the recirculating flows is generated on one hand by the bed friction and on the other hand by the transverse shear the bed generated component is proportional to the bed friction velocity the transverse component is confined to a transverse mixing region in the limit of shallow water depth when the flow is under strong bed friction influence both the transverse mixing region and the recirculating zone have the same length of about one friction length scale

subglacial water sheet floods drumlins and ice sheet lobes the effect of subglacial lakes upon ice sheet topography and the velocity patterns of subglacial water sheet floods is investigated a subglacial lake in the combined michigan green bay basin great lakes north america leads to 1 an ice sheet lobe in the lee of lake michigan 2 a change in orientations offlood velocities across the site of a supraglacial trough aligned closely with green bay in agreement with drumlin orientations 3 low water velocities in the lee of lake michigan where drumlins are absent and 4 drumlinization occurring in regions of predicted high water velocities the extraordinary divergence of drumlin orientations near lake ontario is explained by the presence of subglacial lakes in the ontario and erie basins along with ice sheet displacements of up to 30 km in eastern lake ontario the megagrooves on the islands in western lake erie are likely to be the product of the late stage of a water sheet flood when outflow from eastern lake ontario was dammed by displaced ice and instead flowed westward along the erie basin the finger lakes of northern new york state northeastern u s a occur in a region of likely ice sheet grounding where water sheets became channelized green bay and grand traverse bay are probably the products of erosion along paths of strongly convergent water sheet flow

formation of riffle pool sequences field evidence for an autogenetic process abstract riffle pool sequences are the characteristic reach scale bedforms of mixed and gravel bedded rivers while most research has been directed to the quantitative description of these sequences and to their maintenance comparatively little attention has been given to the processes responsible for their formation in this paper field results from quasi continuous velocity records obtained with an array of electro magnetic current meters are analysed with respect to a model of macro scale structure in the turbulent velocity field of natural channels this model has been suggested as the most plausible basis for riffle pool formation the results demonstrate the occurrence of a variety of coherent flow structures some of which scale on channel dimensions as predicted by the model but which also suggest that some revision of the association between flow structure and riffle pool formation is necessary it is suggested that riffle pool units are initiated with the generation of roller eddies upstream and downstream from a major flow obstacle corresponding to engineering criteria for scour around bridge piers the obstacle persists long enough to fix the flow pattern and hence account for significant modification of channel form but is ultimately removed as part of the process of bed modification via the extension of scour three distinct stages in the process are involved local scour of a single pool creates deposition downstream which then generates the next downstream flow irregularity a riffle pool sequence is thus created autogenetically as the summation of a sequence of irregularities each unit of which is formed and maintained only by flow dynamics operating at the local scale the process is essentially both deterministic and statistical in character it is further suggested that spatial differences in the near bed turbulence field arising from incipient riffle pool topography themselves create differences in surface sediment entrainment which enhance and maintain the sequence in a form process feedback mechanism similar reasoning might possibly be extended to the entire range of bedform sizes

u s geological survey deep seismic reflection profile across the gulf of maine deep seismic reflection and magnetic data suggest that the gulf of maine is underlain by four crustal blocks of differing reflection and magnetic character two of these blocks the gulf of maine fault zone and adjacent central plutonic zone can be correlated with avalonian rocks in southern new england and new brunswick the boundary between them the fundy fault projects onshore near the ponkapoag fault in southeastern massachusetts in a region where no major crustal boundary has yet been identified a third block called the southern plutonic zone is interpreted as avalonian although the reflection and magnetic data are ambiguous the fourth block along the rifted continental margin is correlated with meguma rocks of nova scotia the late paleozoic variscan front crosses the gulf of maine within the gulf of maine fault zone and indicates significant compressional movement rather than strike slip the moho surface throughout the region is essentially flat and may have been produced by mesozoic crustal extension associated modification of the lower crust was minimal in the northern gulf and may have been moderate in the central and southern gulf the franklin rift basin formed by reactivation of the inferred avalon meguma boundary beneath georges bank as a low angle detachment

numerical modeling of flow and scour at pipelines a numerical model for the description of fluid flow and suspended and bed load sediment transport is presented density effects are included in the momentum reynolds equations and in the turbulence kappa and epsilon equations changes in bed levels are calculated from sediment continuity and the finite element grid is adapted to the geometry the reynolds equations and the transport equation for suspended sediment are solved numerically using a taylor galerkin finite element method the flow at a surface mounted cylinder in a steady flow is predicted in good agreement with experiments periodic vortex shedding from a cylinder placed above a rigid bed is predicted in good agreement with laboratory experiments provided that sufficiently detailed grids approximately 5 000 nodes are used scour calculations are performed for a cylinder in a steady flow with its underside placed at the level of the original flat bed predicted scour at a pipeline in steady flow is in good agreement with laboratory measurements reported in the literature