climate instability during the last interglacial period recorded in the grip ice core isotope and chemical analyses of the grip ice core from summit central greenland reveal that climate in greenland during the last interglacial period was characterized by a series of severe cold periods which began extremely rapidly and lasted from decades to centuries as the last interglacial seems to have been slightly warmer than the present one its unstable climate raises questions about the effects of future global warming

formation of a jump by the dam break wave over a granular bed a previously unreported shock feature associated with the scouring of a horizontal granular bed by a dam break wave is discussed near the wave centre the present study shows the free surface breaks backward and a hydraulic jump forms this behaviour is described from the standpoint of shallow water theory suitably extended to deal with non equilibrium sediment transport the shock formation involves a particularly strong coupling between flow free surface evolution and bed morphodynamics support for our conclusions is sought through experimental and numerical approaches in order to magnify the observed phenomena measurements were performed for the case of light bed particles moving in sheet and debris flow modes a detailed picture of the transient two phase flow is presented based on whole field acquisition of the grain motions by particle tracking techniques corresponding shallow water solutions are constructed numerically using a shock capturing scheme finally an interpretation of the jump formation is proposed based on the theory of characteristics

origin and consequences of cyclic ice rafting in the northeast atlantic ocean during the past 130 000 years deep sea sediment cores recovered from the northeast atlantic ocean were examined in order to elucidate the influence of the earth s orbital parameters on major ice rafting analyses of coarse grained ice rafted debris and planktonic foraminifers revealed a strong reaction to the precession signal since 130 000 yr b p dropstone layers have been deposited each half period of a precessional cycle 11 000 1000 yr ice rafting occurs during times of winter minimum summer maximum insolation and summer minimum winter maximum insolation in the first case high summer insolation forces meltwater discharge from the ice sheets into the polar seas which subsequently enhances formation of sea ice during the winter in the second case growth of continental ice enhances iceberg production which also leads to a salinity reduction of surface seawater both situations result in a southward penetration of polar water thus the marine record of dropstones documents ice rafting not only during weichselian stades but also during cold events within interstades the regularity of ice rafting yields a useful framework to calibrate and elucidate climatic changes not only in the region of the north atlantic ocean but also in remote areas such as the pacific ocean and the antarctic

nonhydrostatic dam break flows ii one dimensional depth averaged modeling for movable bed flows abstractthe dam break flow over a movable bed is an important problem in fluvial flow processes these flows are usually predicted by a one dimensional 1d approach based on a hydrostatic pressure distribution recent three dimensional 3d nonhydrostatic simulations of dam break waves over movable beds based on the reynolds averaged navier stokes rans equations revealed that the saint venant theory is not accurate in predicting the flow dynamics within the scour hole developed in this work a generalized 1d nonhydrostatic model for flow over movable beds is proposed assuming a linear nonhydrostatic pressure distribution the new set of 1d equations accounts for the vertical acceleration which is important in dam break waves over movable beds given the instantaneous curved beds generated over the erodible terrain these equations account for both the bed and suspended load transport modes a high resolution finite volume numerical scheme with a semi implicit treatment of nonhydrostatic terms is dev

computational dam break hydraulics over erodible sediment bed this paper presents one of the first dedicated studies on mobile bed hydraulics of dam break flow and the induced sediment transport and morphological evolution a theoretical model is built upon the conservative laws of shallow water hydrodynamics and a high resolution numerical solution of the hyperbolic system is achieved using the total variation diminishing version of the second order weighted average flux method in conjunction with the hllc approximate riemann solver and superbee limiter it is found that a heavily concentrated and eroding wavefront first develops and then depresses gradually as it propagates downstream in the early stage of the dam break a hydraulic jump is formed around the dam site due to rapid bed erosion which attenuates progressively as it propagates upstream and eventually disappears while the backward wave appears to migrate at the same speed as over a fixed bed the propagation of the forward wavefront shows a complex picture compared to its fixed bed counterpart as a result of the domination of rapid bed erosion initially the density difference between the wavefront and the downstream ambient water in the intermediate period and the pattern of the deformed bed profile in the long term it is also found that the free surface profiles and hydrographs are greatly modified by bed mobility which has considerable implications for flood prediction the computed wave structure in the intermediate period exhibits great resemblance to available experiments qualitatively and yet the existence of a shear wave is found in lieu of a secondary rarefaction postulated in an existing analysis finally the use of the complete rather than simplified conservation equations is shown to be essential for correct resolution of the wave and bed structures which suggests that previous models need reformulating

pleistocene climates in china dated by magnetic susceptibility the magnetic susceptibility of loess and interbedded soils in central china varies with the degree of pedogenesis and serves as a proxy measure of climate the concentration of magnetic minerals in the sediment is inversely proportional to sedimentation rates throughout the brunhes epoch susceptibility measurements combined with the reversal stratigraphy provide a time scale that is independent of astronomic chronology on this scale the susceptibility record closely parallels the oxygen isotope fluctuations in deep sea sediments suggesting a close interdependence of the chinese dust falls the volume of land based ice and global climate

two phase flow modeling of sediment motion in sheet flows above plane beds most models of sediment transport are based on the hypothesis of a weak interaction between the fluid and the sediment phase where the main flow is subjected to the mass and momentum conservation of the fluid phase with small corrections due to the presence of the sediments these models usually give a correct answer for conditions where the concentration of sediment is really low in the case of high sediment transport some models analyze the two subdomains separately the high and the low concentration and employ different constitutive equations in the present model there is a two phase description of the whole domain the closure of the turbulence and the interaction between the sediments and the fluid introduce approximations but results are consistent with experiments and previous models

two dimensional two phase granular sediment transport model with applications to scouring downstream of an apron we present a two dimensional two phase model for non cohesive sediment transport this model solves concentration weighted averaged equations of motion for both fluid and sediment phases the model accounts for the interphase momentum transfer by considering drag forces a collisional theory is used to compute the sediment stresses while a two equation k e fluid turbulence closure is implemented a benchmark sediment transport problem concerning the scouring downstream of an apron is carried out as an example and numerical results agree with existing experimental data

grain scale nonequilibrium sediment transport model for unsteady flow abstracta one dimensional 1d finite volume model is developed for simulating nonequilibrium sediment transport in unsteady flow the governing equations are the 1d mass and momentum conservation equations for sediment laden flow and the sediment continuity equation for both bed load and suspended load transport the rouse profile is modified to consider the nonequilibrium transport of suspended sediment the spatial lag between the instantaneous flow properties e g velocity bed shear stress and the rate of bed load transport in unsteady flow is quantified by using an adaptation length which is derived theoretically by applying the momentum principle in the bed load layer this new method for calculating the adaptation length is verified using data from several experiments and yields better results than other empirical formulas for a wide range of shear stress the nonequilibrium model is applied to simulate a series of laboratory dam break flows over erodible beds and the simulated results agree

riemann wave description of erosional dam break flows this work examines the sudden erosional flow initiated by the release of a dam break wave over a loose sediment bed extended shallow water equations are formulated to describe the development of the surge accounting for bed material inertia a transport layer of finite thickness is introduced and a sharp interface view of the morphodynamic boundary is adopted approximations are sought for an intermediate range of wave evolution in which equilibration of the sediment load can be assumed instantaneous but momentum loss due to bed friction has not yet been felt the resulting homogeneous hyperbolic equations are mathematically tractable using the riemann techniques of gas dynamics dam break initial conditions give rise to self similar flow profiles the wave structure features piecewise constant states two smoothly varied simple waves and a special type of shock an erosional bore forming at the forefront of the wave profiles are constructed through a semi analytical procedure yielding a geomorphic generalization of the stoker solution for dam break waves over rigid bed for most flow properties the predictions of the theoretical treatment compare favourably with experimental tests visualized using particle imaging techniques

three dimensional numerical modeling of dam break flows with sediment transport over movable beds abstracta three dimensional 3d numerical model has been developed to simulate dam break flows with sediment transport over movable beds the hydrodynamic model solves the 3d reynolds averaged navier stokes equations using a finite volume method on collocated hexahedral meshes the volume of fluid vof technique with the compressive interface capturing scheme for arbitrary meshes is used to track the water surface boundary the sediment transport model solves the nonequilibrium transport equations of suspended load and bed load separately and in turn calculates the resulting bed change a moving mesh technique is adopted to track the time evolution of bed topography the grid moving velocity and computational cell volume change due to the moving mesh are taken into consideration when the hydrodynamic and sediment transport equations are discretized the developed model has been tested using several experimental dam break flows over movable beds the calculated temporal and spatial variations of water a

simulation of complex seepage field of a gravity dam foundation using a cfd based approach abstractdam foundation seepage is a major concern for the stability of a hydropower project in southwest china referred to here as project x the complicated geological structure multiple grouti

sensitivity of simulated flow fields and bathymetries in meandering channels to the choice of a morphodynamic model morphodynamic models are used by river practitioners and scientists to simulate geomorphic change in natural and artificial river channels it has long been recognized that these models are sensitive to the choice of parameter values and proper calibration is now common practice this paper investigates the less recognized impact of the choice of the model itself all morphodynamic models purport to simulate the same flow and sediment dynamics often relying on the same governing equations yet in solving these equations the models have different underlying assumptions for example regarding spatial discretization turbulence sediment inflow lateral friction and bed load transport these differences are not always considered by the average model user who might expect similar predictions from calibrated models here a series of numerical simulations in meandering channels was undertaken to test whether six morphodynamic codes basement cche 2d nays ssiim 1 telemac 2d and telemac 3d would yield significantly different equilibrium bathymetries if subjected to identical initial flow conditions we found that despite producing moderately similar velocity patterns on a fixed flat bed regression coefficient r of 0 77 0 20 the codes disagree substantially with respect to simulated bathymetries r 0 49 0 31 we relate these discrepancies to differences in the codes assumptions results were configuration specific i e codes that perform well for a given channel configuration do not necessarily perform well with higher or lower sinuosity configurations finally limited correlation is found between accuracy and code complexity the inclusion of algorithms that explicitly account for the effects of local bed slope and channel curvature effects on transport magnitude and direction does not guarantee accuracy the range of solutions obtained from the evaluated codes emphasizes the need for carefully considering the choice of code we recommend the creation of a central repository providing universal validation cases and documentation of recognized fluvial codes in commonly studied fluvial settings copyright 2015 john wiley sons ltd

one dimensional modeling of dam break flow over movable beds a one dimensional model has been established to simulate the fluvial processes under dam break flow over movable beds the hydrodynamic model adopts the generalized shallow water equations which consider the effects of sediment transport and bed change on the flow the sediment model computes the nonequilibrium transport of bed load and suspended load the effects of sediment concentration on sediment settling and entrainment are considered in determining the sediment settling velocity and transport capacity in particular a correction factor is proposed to modify the van rijn formulas of equilibrium bed load transport rate and near bed suspended load concentration for the simulation of sediment transport under high shear flow conditions the governing equations are solved by an explicit finite volume method with the first order upwind scheme for intercell fluxes the model has been tested in two experimental cases with fairly good agreement between simulations and measurements the sensitivities of the model results to parameters such as the sediment nonequilibrium adaptation length manning s roughness coefficient and the proposed correction factor have been verified the proposed model has also been compared to an existing model and the results indicate the new model is more reliable

experimental study and numerical verification of silted up dam break abstract in this study multiphase flood waves caused by failure of silted up dams have been scrutinized experimentally and efficacy of the public domain cfd software openfoam numerical models to represent such phenomenon is evaluated experimental observations are sparse on dam break for reservoirs with high sediment depths called silted up reservoirs sediment layer in this case may behave as a viscous fluid and produce a complex multi layer flow of air and water overtopping a sediment layer massive sediment quantities can have a substantial impact on the wave propagation pattern caused by dam break this study contributes to an experiment based understanding of dam break phenomenon with silted up reservoir and elaborates the propagation of multi layer waves herein different hydraulic conditions were experimentally observed including 8 different upstream sediment depths in the range of 0 0 24 m and 4 downstream initial water levels of 0 0 02 0 04 and 0 05 m which collectively created 32 scenarios the resulted waves were filmed by high speed cameras and data were extracted through image processing an open source cfd software namely openfoam was employed to simulate the experiments using two distinct numerical approaches volume of fluid vof and eulerian results of both numerical approaches were in good agreement with the experimental measurements with mae and rmse error values varying from 0 006 m to 0 031 m and 0 008 m to 0 043 m respectively 2 to 10 and 2 6 to 14 with respect to the reservoir height of 0 3 m the eulerian approach showed marginally superior performance to vof especially in cases with high sediment depths which is likely due to accommodating phase mixing a wide collection of high quality data is made available online in the appendix data and could be utilized in future studies

two dimensional two phase depth integrated model for transients over mobile bed abstractfast geomorphic transients may involve complex scenarios of sediment transport occurring near the bottom as bed load i e saltating sliding and rolling or as suspended load in the upper portion of the flow the two sediment transport modalities may even coexist or alternate each other during the same event especially when the shear stress varies considerably modeling these processes is therefore a challenging task for which the usual representation of the flow as a mixture may result in being unsatisfactory in the present paper a new two phase depth averaged model is presented that accounts for variable sediment concentration in both bed and suspended loads distinct phase velocities are considered for bed load whereas the slip velocity between the two phases is neglected in the suspended load it is shown that the resulting two phase model is hyperbolic and the analytical expression of the eigenvalues is provided the entrainment deposition of sediment between the bottom and the bed

a depth averaged non cohesive sediment transport model with improved discretization of flux and source terms author s zhao j ozgen xian i liang d wang t hinkelmann r abstract 2019 elsevier b v this paper presents novel flux and source term treatments within a godunov type finite volume framework for predicting the depth averaged shallow water flow and sediment transport with enhanced the accuracy and stability the suspended load ratio is introduced to differentiate between the advection of the suspended load and the advection of water a modified harten lax and van leer riemann solver with the contact wave restored hllc is derived for the flux calculation based on the new wave pattern involving the suspended load ratio the source term calculation is enhanced by means of a novel splitting point implicit discretization the slope effect is introduced by modifying the critical shear stress with two treatments being discussed the numerical scheme is tested in five examples that comprise both fixed and movable beds the model predictions show good agreement with measurement except for cases where local three dimensional effects dominate

the south to north progression of rainfall anomalies across india during the summer monsoon season the low frequency spatio temporal intraseasonal evolution of monsoon rainfall over india is studied by considering the pentad rainfall averages taken over 2 5 wide and 5 0 long blocks lying across the central longitudinal and latitudinal transacts the time latitude cross sections show south to north progressions of rainfall anomalies with a recurrence period of about 40 days the progressions show considerable inter annual and intra seasonal variation in frequency amplitude and rate of progression the extended empirical orthogonal function analysis shows that the first two most important functions correspond to this northward progression of rain anomalies recurring after an interval of about 40 days the rate of progression of anomalies is about 0 5 lat day the time longitude cross sections show the movement of rainfall anomalies towards both directions east and the west with slight preference towards the east the importance of these signals in rainfall forecasting is foreseen

evidence for massive discharges of icebergs into the north atlantic ocean during the last glacial period sediments in the north atlantic ocean contain a series of layers that are rich in ice rafted debris and unusually poor in foraminifera1 here we present evidence that the most recent six of these heinrich layers deposited between 14 000 and 70 000 years ago record marked decreases in sea surface temperature and salinity decreases in the flux of planktonic foraminifera to the sediments and short lived massive discharges of icebergs originating in eastern canada the path of the icebergs clearly marked by the presence of ice rafted detrital carbonate can be traced for more than 3 000 km a remarkable distance attesting to extreme cooling of surface waters and enormous amounts of drifting ice the cause of these extreme events is puzzling they may reflect repeated rapid advances of the laurentide ice sheet perhaps associated with reductions in air temperatures yet temperature records from greenland ice cores appear to exhibit only a weak corresponding signal moreover the 5 10 000 yr intervals between the events are inconsistent with milankovitch orbital periodicities raising the question of what the ultimate cause of the postulated cooling may have been

a simplified physically based model for coastal dike and barrier breaching by overtopping flow and waves abstract the simplified physically based breach model dlbreach has been developed to simulate the overtopping breaching of coastal dikes and barriers which can occur either from the sea side or the bay side the breaching process is divided into two stages intensive breaching and general inlet evolution in which the flows are calculated using the weir flow equation and the keulegan equation respectively the keulegan equation is a simplified energy equation for steady nonuniform flow with local head loss due to channel contraction and expansion revised herein by adding the wind driving force empirical formulas are adopted to calculate phase averaged wave overtopping discharge wave setup and wind setup setdown the wave overtopping discharge is combined with the surge overflow discharge and the wave setup and wind setup setdown are added to the sea and bay water levels for the hydrodynamic and sediment routing alongshore sediment is considered as a source boundary condition for the non equilibrium sediment transport model at the breach the model has been tested using the 94 field experiment of sea dike breaching by overflow in the zwin channel estuary a laboratory experiment of sea dike breaching initiated by wave overtopping and a field observation of the eight day breaching and closure event of the mecox inlet at eastern long island of new york during sept 10 18 1985 the model results agree generally well with the measurements

variations in 87sr 86sr ratios of calcites in chinese loess a proxy for chemical weathering associated with the east asian summer monsoon abstract 87sr 86sr ratios of loess paleosol calcites of luochuan in china over the past 130 ka display a regular change with increasing trend of 87sr 86sr ratios in the following order loess calcite weakly pedogenetic paleosol calcite paleosol calcite pure secondary calcite the observed variations in the 87sr 86sr record of loess paleosol calcites correlate closely with the records of the magnetic susceptibility coarse grain fraction content in the same profile an et al chin sci bull 39 1994 182 184 and the specmap stacked 18o record of global ice volume imbrie et al in berger et al eds milankovitch and climate part i reidel boston 1984 pp 269 305 strata from the last glacial consist mostly of loesses and weakly developed paleosols and have lower 87sr 86sr ratios averaging 0 71079 paleosols which developed during the last interglacial have higher 87sr 86sr ratios averaging 0 71131 the four periods ca 125 110 98 90 80 75 and 9 5 ka b p of increased summer monsoonal rainfall coincide with the four peaks in 87sr 86sr values a series of 87sr 86sr high frequency high amplitude fluctuations is superimposed on the last glacial interglacial cycle over the past 130 ka occurring on millennium timescales with an average time span of about 3 ka during the last glacial the fluctuations of the 87sr 86sr ratios of the luochuan loess paleosol calcites lie in the range 0 71036 0 71176 for the past 130 ka the study shows that the sr isotopic system in loess paleosol calcites may therefore be an excellent proxy of chemical weathering on continents associated with the east asian summer monsoon

formulas for sediment porosity and settling velocity several existing formulas for the initial porosity and settling velocity of sediment have been tested by using extensive data collected from different countries and regions and modified to achieve better reliability or convenience in use