field measurements of the flux and speed of wind blown sand a field experiment was conducted to measure the flux and speed of wind blown sand under known conditions in a natural setting the experiment run at pismo beach california involved a tract 100 m long parallel with the wind by 20 m wide the site was instrumented with four arrays of anemometers to obtain wind velocity profiles through the lower atmospheric boundary layer temperature probes to determine atmospheric stability and wind vanes to determine wind direction from these measurements wind friction speeds were derived for each experimental run in order to measure sand saltation flux a trench 3 m long by 10 m wide transverse to the wind direction by 0 5 m deep was placed at the downwind end of the tract and lined with 168 collector bins forming an egg box pattern the mass of particles collected in each bin was determined for four experimental runs in order to assess various sand trap systems used in previous experiments 12 leatherman traps one fryberger trap and one array of ames traps were deployed to collect particles concurrently with the trench collection particle velocities were determined from analysis of high speed 3000 and 5000 frames per second motion pictures and from a particle velocimeter sand samples were collected from the trench bins and the various sand traps and grain size distributions were determined fluxes for each run were calculated using various previously published expressions and then compared with the flux derived from the trench collection results show that bagnold s 1941 model and white s 1979 equation most closely agree with values derived from the trench comparison of the various collector systems shows that the leatherman and ames traps most closely agree with the flux derived from the trench although these systems tended to under collect particles particle speeds were measured from analysis of motion pictures for saltating particles in ascending and descending parts of their trajectories results show that particle velocities from the velocimeter are in the range 0 5 7 0 m s 1 compared to a wind friction velocity of 0 32 0 43 m s 1 and a wind velocity of 2 7 3 9 m s 1 at the height of the particle measurements descending particles tended to exceed the speeds of ascending particles by 0 5 m s 1

sudden stratospheric warmings and tropospheric blockings in a multi century simulation of the ipsl cm5a coupled climate model the relation between sudden stratospheric warmings ssws and blocking events is analyzed in a multi centennial pre industrial simulation of the institut pierre simon laplace coupled model ipsl cm5a prepared for the fifth phase of the coupled model intercomparison project the ipsl model captures a fairly realistic distribution of both ssws and tropospheric blocking events albeit with a tendency to overestimate the frequency of blocking in the western pacific and underestimate it in the euro atlantic sector the 1000 year long simulation reveals statistically significant differences in blocking frequency and duration over the 40 day periods preceding and following the onset of ssws more specifically there is an enhanced blocking frequency over eurasia before ssws followed by an westward displacement of blocking anomalies over the atlantic region as ssws evolve and then decline the frequency of blocking is reduced over the western pacific sector during the life cycle of ssws while the model simulates no significant relationship with eastern pacific blocks finally these changes in blocking frequency tend to be associated with a shift in the distribution of blocking lifetime toward longer lasting blocking events before the onset of ssws and shorter lived blocks after the warmings this study systematically verifies that the results are consistent with the two pictures that 1 blockings produce planetary scale anomalies that can force vertically propagating rossby waves and then ssws when the waves break and 2 ssws affect blockings in return for instance via the effect they have on the north atlantic oscillation

successive aeolian saltation studies of idealized collisions as observed by bagnold and experimentally reconfirmed by other workers the impact angles of saltating grains are remarkably constant over a wide range of conditions lying between 10 and 16 it can be shown that successive saltation contains a mechanism which very effectively confines impact angles to that range this control mechanism is most effective at windspeeds less than about 15 30 m s 1 depending on grain diameter and mass the control mechanism is evaluated from model calculations of grain populations saltating over a level bed consisting of a layer of loose grains the grains are assumed to be spherical and uniform in size and mass also rigid and perfectly elastic the model also describes distributions of maximum height of grain paths and of lift off angles compared to other processes involved in aeolian saltation successive saltation is the only process with a high probability of transferring energy from horizontal into vertical grain movement this fact together with the calculations presented strongly suggests that successive saltation plays a major role in saltation in air successive saltation of uniform grains is theoretically impossible if the ground over which saltation occurs is tilted by about 15 against wind direction values of tilt angles in this range are observed in nature as stoss side angles of dunes and ripples leading to the concept that stoss sides are tilted up by deposition until successive saltation is subdued

palaeotsunamis along canada s pacific coast abstract to fully understand tsunami hazards along canada s pacific coast we must consider all potential sources including local and region wide earthquakes volcanic activity slope failure events and bolide impacts furthermore as climate changes future extreme meteorological conditions will exacerbate slope failures on steep and over steepened slopes and potentially augment the magnitude and frequency of earthquake generated tsunamis there have been numerous reviews of the geological records of palaeotsunamis along the cascadia margin but as yet there has been no attempt made to pull together all data related to palaeotsunamis along the pacific coast of canada we summarise all geological data related to palaeotsunamis reported in the region all relevant non geological data and other information concerning potential tsunamigenic sources evidence for between 10 and 22 palaeotsunamis prior to european arrival have been documented with the cascadia subduction zone y event ad1700 being the most widely recognised geological archaeological and anthropological data all indicate a complex multi sourced prehistory of seismic tsunami activity that is poorly understood a clustering of potential activity off the se coast of vancouver island suggests that we should also attempt to better understand locally generated events along canada s pacific coast

a blocking view of the stratosphere troposphere coupling dynamical influence from the stratosphere is known to play a role in shaping the wintertime tropospheric circulation patterns observations suggest that this influence is strongest following weak and strong polar vortex events termed sudden stratospheric warmings ssws and vortex intensification vi events respectively in this work stratosphere troposphere coupling is studied through the modulation by extreme vortex events of the northern hemispheric tropospheric blocking frequency and eddy driven jet displacements this is done using three reanalysis data sets and the centro euro mediterraneo sui cambiamenti climatici climate model with a resolved stratosphere cmcc cms coupled model control run reanalysis results suggest the existence of distinct patterns of blocking activity following extreme vortex events over the atlantic basin ssws are shown to lead by about 20 50 days the occurrence of increased reduced blocking frequency on the poleward equatorward side of the atlantic jet stream anomalies of the opposite sense with poleward reduction and equatorward enhancement of the blocking frequency occur following vi events the response over the pacific sector is less clear compared to reanalyses cmcc cms shows a similar but weaker response especially over the atlantic a possible explanation is identified in the different structure of the polar vortex and weaker wind shear anomalies with respect to reanalysis we finally highlight that patterns identified following vortex extremes show similarities with the northern annular mode over the atlantic but not over the pacific this suggests that the stratosphere troposphere coupling is more a regional than annular feature

sediment characteristics of an extreme flood 1993 upper mississippi river valley the 1993 mississippi river flood was notable for its high magnitude long duration summer occurrence and low sediment discharge a field survey of a 70 km long reach in the vicinity of quincy illinois revealed that the event was characterized by 100 yr flood had remarkably little sedimentological or geomorphological impact on the flood plain within the study reach because the transport effectiveness of floods in large drainage basins is influenced by event sequencing in the same manner as floods in small watersheds and the cohesive flood plain soils were not susceptible to erosion

morphological and sedimentological responses of streams to human impact in the southern blue ridge mountains usa abstract morphological and sedimentological responses of streams to basin scale impact have been well documented for intensively agricultural or urban areas sensitivity thresholds of streams to modest levels of disturbance however are not well understood this study addresses the influence of forest conversion on streams of the southern blue ridge mountains a region that has received little attention with respect to human impact on stream channels basins were chosen for this study to represent the end members of the range of human impact in the area with the forest cover of the basin used as a proxy for level of impact ranging from about 70 100 regionally two pairs of lightly impacted 90 forest and moderately impacted 70 80 forest sub basins of the upper little tennessee river were identified for comparison reach characteristics e g slope drainage area and riparian cover were aligned in each pair to isolate contrasting forest cover as the primary driver of any detected differences in morphology and sedimentology a suite of standard cross sectional and longitudinal data was collected for each reach for characterization of the sedimentology and morphology of the streams difference of means tests were conducted to identify parameters significantly differing between the lightly and moderately impacted streams in both pairs consistent and significant differences within both pairs were demonstrated in bankfull width depth ratios baseflow wetted width and particle size on the stream bed both in the thalweg and throughout the channel bed the moderately impacted streams are narrower than the lightly impacted streams and the bed texture of the moderately impacted streams is finer than that of the lightly impacted streams the moderately impacted streams contain a higher percentage of

the life cycle of the northern hemisphere sudden stratospheric warmings motivated by recent evidence of strong stratospheric tropospheric coupling during the northern hemisphere winter this study examines the evolution of the atmospheric flow and wave fluxes at levels throughout the stratosphere and troposphere during the composite life cycle of a sudden stratospheric warming the composite comprises 39 major and minor warming events using 44 years of ncep ncar reanalysis data the incipient stage of the life cycle is characterized by preconditioning of the stratospheric zonal flow and anomalous quasistationary wavenumber 1 forcing in both the stratosphere and troposphere as the life cycle intensifies planetary wave driving gives rise to weakening of the stratospheric polar vortex and downward propagation of the attendant easterly wind and positive temperature anomalies when these anomalies reach the tropopause the life cycle is marked by momentum flux and mean meridional circulation anomalies at tropospheric levels that are consistent with the negative phase of the northern hemisphere annular mode the anomalous momentum fluxes are largest over the atlantic half of the hemisphere and are associated primarily with waves of wavenumber 3 and higher

3d ejection behavior of different sized particles in the grain bed collision process abstract the impact ejection process on a mixed grain size bed with granular packing was simulated with the discrete element method in order to understand the interaction between different sized grains in natural aeolian sand transport in this model the granular bed was formed by settling the randomly generated two sized particles under gravity and then a foreign particle was shot onto the granular bed at different speeds and angles the recorded speed direction and number of the ejected particles were then analyzed it was found that the probability distributions of the ejection speed and angle for different sized particles are all identical to those for the single size grain bed collision process the mean ejection speeds of different sized particles are nearly equal and the mean ejection angles of different sized particles are all equal to a constant of 60 the average number of each size of ejected particles grows linearly with the increasing impact speed but remains invariant for various impact angles moreover the smaller particles are preferentially ejected and the ratio between the mean numbers of different sized particles is independent of both the impact speed and angle additionally the ejected particles were found to move in a 3d space they become distributed symmetrically around the incident plane and jump not only forward but also backward these results are critical to understanding the grain size induced inhomogeneity in aeolian sand transport

the pattern of grainfall deposition in the lee of aeolian dunes a simple model for the deposition pattern in the lee of aeolian dunes is presented that relies heavily upon a recently developed understanding of aeolian saltation grainfall deposition at any position on the lee face is the result of all saltation trajectories that leave any point on the surface of the dune upwind of the brink with sufficient initial velocity to travel the intervening distance the deposition rate at any position on the lee slope is obtained by integrating over all combinations of initial position and required velocity the velocity being weighted by its probability density the resulting calculated total deposition rate patterns show distinct maxima on the order of one to a few decimetres from the brink beyond which deposition rates fall off roughly exponentially an important length scale emerges that characterizes this decay with distance from the brink the length increasing with wind velocity and decreasing with grain diameter it is shown that this length scale is on the order of one metre for typical grain size and wind conditions that this is typically smaller than the length of the lee slope is what gives rise to the oversteepening and eventual avalanching of the lee sides of aeolian dunes the position of a pivot point on the lee slope may be predicted separating source regions from accumulation regions for grainflow avalanche deposits the calculated patterns provide not only a means for quantitative interpretation of active and fossil dune grainfall deposits but they provide the initial geometry for grainflow avalanches the initial failures should coincide with the steepest gradient in grainfall deposition slightly downslope from the grainfall maximum

tree root mounds and their role in transporting soil on forested landscapes the growth and decay of tree roots can stir and transport soil this is one process that contributes to the mass movement of soil on hillslope to explore the efficiency of this process we document the mounding of soil beside ponderosa and lodgepole pine trees in the forests that dominate the mid elevations of colorado s boulder creek watershed mounds are best expressed around ponderosa pines reaching vertical displacements above the far field slopes of order 10 20 cm fading into the slope by roughly 100 cm distance from the trunks with common diameters of 30 cm positive mounding occurs on all sides of trees on slopes indicating that the mounding is not attributable to deflection of a creeping flow of soil around the tree but rather to the insertion of root volume on all sides in the subsurface mounding is commonly asymmetric even on cross slope profiles significant variation in the mound sizes results in no clear relationship between tree diameter and root volume displaced these observations motivated the development of a discrete element model of tree root growth using the liggghts model in which grains we specified to be root cells were allowed to enlarge within the simulated granular matrix mounding could be reproduced with the majority of the vertical displacement of the surface being attributable to reduction of the bulk density due to dilation of the granular matrix during root enlargement finally we develop a previous analysis of the role of roots in transporting soil during growth and decay cycles we find that even in shallow soils the root cycle can drive significant soil transport down forested montane slopes copyright 2013 john wiley sons ltd

the impact process in aeolian saltation two dimensional simulations a critical event in the trajectory of a sand grain saltating in air is its interaction with the surface we examine the phenomenon of grain bed impacts in two dimensions using a combination of dynamical computer simulations analytical models and physical reasoning the results indicate that the grain bed collisions can be treated as two body collisions with the bed particle assuming an effective mass greater than its true mass also the presence of geometrical surface irregularities has a strong bearing on the interaction between saltating and surface grain populations as well as on the formation of small scale bedforms

geographical dependence observed in blocking high influence on the stratospheric variability through enhancement and suppression of upward planetary wave propagation abstractprevious studies have suggested the importance of blocking high bh development for the occurrence of stratospheric sudden warming ssw while there is a recent study that failed to identify their statistical linkage through composite analysis applied to high amplitude anticyclonic anomaly events observed around every grid point over the extratropical northern hemisphere the present study reveals a distinct geographical dependence of bh influence on the upward propagation of planetary waves pws into the stratosphere tropospheric bhs that develop over the euro atlantic sector tend to enhance upward pw propagation leading to the warming in the polar stratosphere and in some cases to major ssw events in contrast the upward pw propagation tends to be suppressed by bhs developing over the western pacific and the far east resulting in the polar stratospheric cooling this dependence is found to arise mainly from the sensitivity of the interference between the climatological pws and upward p

a study of the number of sand grains lifting off per unit time and per unit sand bed area aeolian sand flow is an especial two phase flow of gas and solid resulted from sands carried by wind to cause the spatial displacement to simplify this complex process researchers have divided it into four interacting subprocesses jumping of sand grains movement of sand grains in the airflow changes of the airflow due to the moving sand grains and impacts between falling sand grains and sand grains in the underlying sand bed although it simplifies aeolian sand flow enough to permit the development of models of these subprocesses no theory of aeolian sand flow will be complete without a consideration of coupling among the subprocesses in addition two key physical variables must be defined the probability density distribution for the initial liftoff velocities of saltating sand grains and the number of sand grains lifting off per unit time and per unit sand bed area also called liftoff rates little data exist on liftoff rates because of the difficulty of direct measurements and theoretical analysis this paper combines wind tunnel experimental data with a theoretical analysis to provide a preliminary solution based on the probability density distribution for the initial liftoff velocities of sand grains the result shows that liftoff rates increase rapidly with increasing of friction wind velocities the relationship between friction wind velocities and liftoff rates can be expressed as n a 2 1 2a 1 b 1 u b 2 1 u 2 a 1 9858 87 b 1 14 830 07 r 2 0 97 although the results of this study require additional experimental validation they provide a strong basis for future research

eolian ripples as examples of self organization in geomorphological systems abstract ripples arise spontaneoulsy from sandy beds subjected to winds sufficient to cause the saltation of surficial grains while the initially fastest growing wavelengths are 4 6 times the mean hop length of grains impelled downwind by long trajectory high energy saltating grains the ripple wavelengths developed over long periods can be several times this numerical simulations based upon techniques pioneered by p k haff and coworkers reveal that a complex process of ripple coalescence is responsible for this evolution of the ripple field small fast moving ripples catch up with and are absorbed by larger slower forms each such merger resulting in a growth of the mean wavelength and a decline in the relative dispersion of the wavelengths an heuristic argument is presented which accounts for the fast initial growth and subsequent slowing of growth of the mean wavelength as the ripple field evolves ripple cross sectional shape appears to be controlled in large part by a distrianment function which represents the declining likelihood of a grain stopping on a surface as it slopes more steeply downwind that the resulting asymptotic wavelength in the simulations depends strongly upon impact angle supports sharp s 1963 suggestion that the length of the shadow zone plays an important role in controlling the final ripple wavelength

effects of the magnus and saffman forces on the saltation trajectories of sand grain abstract saltating sand grains are the primary component of airborne sand and account for 75 of all sand transport flux the saltation height and horizontal distance traveled by sand grains are key factors in sand control engineering in addition to gravity and aerodynamic drag the magnus and saffman forces also play important roles in saltation to quantify the magnitudes of these forces in saltation we used high speed multi flash photography to observe the movement of saltating sand grains in a wind tunnel this proved to be an efficient technique for determining the movement and rotational velocities of grains of natural sand with grain sizes ranging from 0 2 to 0 3 mm and shear velocities u of 0 67 0 83 and 0 87 m s 1 the rotational speed of saltating sand grains varied between 200 and 800 rev s 1 mean clockwise and anticlockwise rotational speeds were nearly identical and both increased with increasing saltation height with saltation heights divided into 1 cm intervals the rotational speeds followed a lorentzian distribution calculations based on a saltation model showed that the maximum increases in saltation height and in horizontal distance due to the magnus force were 10 2 and 24 9 respectively the rate of increase of both parameters increased with increasing lift off angle the maximum increases in saltation height and horizontal distance of sand grains caused by the saffman force were only 4 6 and 3 7 respectively

discrete particle model for sheet flow sediment transport in the nearshore fully three dimensional discrete particle computer simulations of high concentration sheet flow transport in oscillatory flows quantify the effect of fluid acceleration on bed load transport in highly unsteady flows typical of nearshore marine environments a simple impulse momentum approach explains simulation results and forms the basis for adding an acceleration related term to widely used energetics sediment transport formulae transport predicted by the acceleration term becomes increasingly significant as wave shape approaches the sawtooth profile characteristic of surf zone bores simulations integrate f ma and a corresponding set of equations for the torques for each sphere normal and tangential forces between contacting particles are linear functions of the distance between sphere centers and the relative tangential displacement at the contact point respectively particle interactions are both inelastic and frictional pressure gradient forces generated by the passage of surface gravity waves drive fluid and particle motion in a stack of thin horizontal fluid layers that exchange momentum and exert fluid drag added mass and buoyancy forces on particles transport properties of the simulated granular fluid assemblage are robust to large variations in material properties of the particles simulated transport rates agree with available experimental data for unsteady transport of coarse sands the mode of bed load motion dispersion of bed load particles and particle segregation by size and density are qualitatively consistent with available particle scale observations of bed load transport of natural particles

paleoflood hydrology origin progress prospects from an origin in diverse studies of flood geomorphology and quaternary geology paleoflood hydrology emerged as a geophysical and an applied hydrological science during the 1970s and 1980s since acquiring its formal name in 1982 the most productive approach in paleoflood hydrology has become energy based inverse hydraulic modeling of discrete paleoflood events recorded in appropriate settings as slackwater deposits and other paleostage indicators swd psi or as various threshold indicators of non exceedence technological advances particularly in hydraulic modeling and geochronology were instrumental in moving the discipline to its present status the most recent advances include 1 new techniques for the accurate geochronology of flood sediments notably tams radiocarbon analyses and osl dating and 2 the phenomenal increase in computer power that allows complex hydraulic calculations to become feasible for routine studies from its initial demonstration in the southwestern united states swd psi paleoflood hydrology proved its widespread applicability to various landscape environments particularly important studies have been accomplished in australia china india israel south africa spain and thailand paleoflood hydrology has also generated its share of controversy in part because of the differing viewpoints and attitudes of the two scientific traditions from which it emerged quaternary geology geomorphology versus applied hydrologic hydraulic engineering nevertheless the future growth of the discipline is assured given the rapid pace of discoveries that it engenders indeed so many international studies exist that it is appropriate to pursue global syntheses to address interesting and timely questions of extreme flood phenomena in relation to global climatic change

an experimental study of multiple grain size ejecta produced by collisions of saltating grains with a flat bed collision data are presented from coloured high speed films of three size fractions of sand grains saltating over a bed of the total grain population each fraction was colour tagged and the proportion of each size ejected by grains colliding with the surface was recorded on a number of films taken as the bed was progressively eroded the results confirm earlier findings that v3 v1 0 5 0 6 vn v1 08 and the rebound angle increases with decreasing grain size ejected grains are examined in relation to their size the impactor size ejection speed and angle and the number of ejecta per collision in addition changes in grain parameters are observed with time for fine impactors ejection speeds generally increase with a decrease in ejecta size but the fine fraction does not follow this trend for the coarse and medium impactors ejection angles are typically between 40 and 60 with coarse grains having shallower mean angles than fine ejecta the number of ejections per collision increases with a decrease in particle size for each impactor size the general tendency for coarse particles to be ejected at lower speeds and shallower angles than fine particles will lead to sorting of the grain sizes there is poor correlation between the forward momentum loss of the saltating grams at collision and both the forward momentum of the ejected grains and the number of ejected grains much of the forward momentum of the saltating grains is transfered to creeping grains the composition and geometry of the bed are considered to be important factors in the evolution of the saltation cloud

mechanisms determining the winter atmospheric response to the atlantic overturning circulation abstractin climate models an intensification of the atlantic meridional overturning circulation amoc precedes a warming in the north atlantic subpolar basin by a few years in the ipsl cm5a lr model this warming may explain the atmospheric response to the amoc observed in winter which resembles a negative phase of the north atlantic oscillation nao to firmly establish the causality links between the ocean and the atmosphere and illustrate the underlying mechanisms in this model ensembles of atmosphere only simulations are conducted prescribing the sst and sea ice anomalies that follow an amoc intensification in late winter the north atlantic sst and sea ice anomalies drive atmospheric circulation anomalies similar to those found in the coupled model simulations only driven by the sst anomalies related to the amoc show that the largest oceanic influence is due to the warm subpolar sst anomaly which enhances the oceanic heat release and decreases the lower tropospheric baroclinicity in the reg

annular modes in the extratropical circulation part i month to month variability the leading modes of variability of the extratropical circulation in both hemispheres are characterized by deep zonally symmetric or annular structures with geopotential height perturbations of opposing signs in the polar cap region and in the surrounding zonal ring centered near 458 latitude the structure and dynamics of the southern hemisphere sh annular mode have been extensively documented whereas the existence of a northern hemisphere nh mode herein referred to as the arctic oscillation ao has only recently been recognized like the sh mode the ao can be defined as the leading empirical orthogonal function of the sea level pressure field or of the zonally symmetric geopotential height or zonal wind fields in this paper the structure and seasonality of the nh and sh modes are compared based on data from the national centers for environmental prediction national center for atmospheric research reanalysis and supplementary datasets the structures of the nh and sh annular modes are shown to be remarkably similar not only in the zonally averaged geopotential height and zonal wind fields but in the mean meridional circulations as well both exist year round in the troposphere but they amplify with height upward into the stratosphere during those seasons in which the strength of the zonal flow is conducive to strong planetary wave mean flow interaction midwinter in the nh and late spring in the sh during these active seasons the annular modes modulate the strength of the lagrangian mean circulation in the lower stratosphere total column ozone and tropopause height over mid and high latitudes and the strength of the trade winds of their respective hemispheres the nh mode also contains an embedded planetary wave signature with expressions in surface air temperature precipitation total column ozone and tropopause height it is argued that the horizontal temperature advection by the perturbed zonal mean zonal wind field in the lower troposphere is instrumental in forcing this pattern a companion paper documents the striking resemblance between the structure of the annular modes and observed climate trends over the past few decades

steady state saltation in air coupled equations of motion for steady state saltation over an infinite plane are derived and solved for a simplified model of the grain surface impact process experimentally observed features of the wind velocity profile in saltation are qualitatively reproduced including a diminution of the sub saltation layer mean wind speed as the friction speed increases in this model the surface impact velocity of the saltating grains remains relatively constant over a wide range of free stream shear stresses and the grain mass flux increases with friction speed uf less rapidly than uf3

palaeoflood record of the tagus river central spain during the late pleistocene and holocene abstract palaeoflood hydrology of the tagus river central spain was reconstructed from slackwater deposits and palaeostage indicators in two canyon reaches located at el puente del arzobispo in the central part of the catchment 35 000 km 2 in drainage area and in alcantara at the lower part of the catchment 52 000 km 2 in drainage area near the portuguese border the palaeoflood record with more than 80 flood events shows clusters of floods at specific periods from 9440 to 9210 14 c yr bp 8540 8110 bc 8500 to 8000 14 c yr bp 7500 7000 bc 6750 14 c yr bp 5000 bc 1200 to 950 14 c yr bp ad 785 1205 410 14 c yr bp ad 1450 1500 170 to 80 14 c yr bp ad 1670 1950 the largest flood s occurred during the periods 9440 to 9210 14 c yr bp 6750 14 c yr bp and 1200 to 950 14 c yr bp reaching minimum discharge estimates of between 4000 and 4100 m 3 s 1 in the el puente del arzobispo reach and 13 700 15 000 m 3 s 1 in the alcantara reach these periods with increased flood magnitude and or frequency in the tagus river are strongly related to increased moisture influx and winter precipitation in the iberian peninsula especially in the western zone proxy records sensitive to winter precipitation such as lake levels and vegetation changes indicated by pollen records are in good agreement with the clusters of floods found in the tagus river this flood variability seems to correspond to changes in the prevailing atmospheric circulation pattern affecting the iberian peninsula

paleochannels indicating wet climate and lack of response to lower sea level southeast georgia large meandering paleochannels on the flood plain of the middle ogeechee river in southeast georgia represent wet paleoclimate during late pleistocene and early holocene time and show no indication of downcutting in response to eustatically lower sea level radiocarbon dates indicate that the paleomeanders were active at 31 28 ka and 8 5 4 5 ka bivariate regression models that correlate modern channel dimensions to the discharge of low magnitude floods 1 5 yr recurrence interval indicate that paleodischarge of floods during those times was at least double that of modern floods thus suggesting a wetter paleoclimate these data corroborate independent studies of pollen and paleoclimate simulations that indicate wet early to middle holocene 9 3 ka conditions that were characterized by intensified monsoonal circulation paleoclimate conditions at 31 28 ka are less well known our analysis of the middle ogeechee river flood plain indicates the absence of base level response downcutting to eustatic sea level lowering because the beds of the paleochannels are at approximately the same elevation as the bed of the modern channel this supports recent arguments that the geomorphic response of coastal plain streams to sea level lowering is most apparent in deltaic and shelf environments and may not be recognized very far upstream from the coast

particle dislodgement from a flat sand bed by wind the rate with respect to area and time at which grains are dislodged from a sand bed for given wind conditions is an important factor in determining the grain transport rate and the intensity of grain activity in each of the transport modes the literature of the subject contains little direct information about particle dislodgement the paper describes a series of experiments in which dyed sand grains spread on the surface of quartz dune sand in a wind tunnel were photographed at five second intervals while the sand was exposed to wind the data on rate of loss of coloured grains was used for two of three chosen size fractions to deduce the dislodgement rate for each size fraction the variation of this dislodgement rate with shear velocity is shown graphically for values of u between 24 cm s 1 and 50 cm s 1 because of the artificial method of distribution of the coloured sand grains the results should be applied with caution to natural conditions the interpretation of the observations of dyed grain loss involved the numerical simulation of the process which comprises removal of coloured grains slightly offset by replenishment as upwind coloured grains settle briefly in the observed zone an estimation of grain excursion length has to be incorporated in the simulation this estimation was made by trial but general corroboration was found from earlier work comparisons are made between dislodgement rates obtained thus and rates estimated by anderson 1986 and by jensen and sorensen 1986 reasonable agreement with the latter is found in the u range 30 cm s 1

experimental verification of aeolian saltation and lee side deposition models we report results of experiments intended to test the validity of a model for aeolian saltation and the resulting pattern of deposition on the lee side of aeolian dunes in steady sea breeze conditions on a 3 m tall dune at point ano nuevo california we measured simultaneously the near brink wind speed and the deposition on both horizontal and lee face collector platforms we then used the details of the deposition patterns to constrain approximate values of parameters in a numerical model of the deposition rate that incorporates the essence of the saltation process best fits to the data constrain a parameter that controls the probability distribution of liftoff speeds in addition the total vertical number flux of grains is constrained to roughly 107 108 grains m 2 s 1 at shear velocities of 0 33 0 40 m s 1 the lee side deposition pattern which shows the expected maximum in deposition rate at a distance of several decimetres from the brink is also well fit by the model in addition simultaneous collection of horizontal and lee deposition patterns along with the numerical simulation of these patterns strongly implies that the windfield in the lee of this particular dune is best described as a non recirculating wake grainflows on the lee face are caused by failure of grainfall depositional bumps our results suggest that the principal effect of increased wind speed is to increase the frequency of grainflows rather than to increase their size implying that very large thick grainflows require a different mechanism

climatology of planetary waves in the northern stratosphere a 30 year record of subjectively analyzed daily temperature and geopotential height data from the lower and middle stratosphere 50 30 and 10 hpa is used to derive a climatology of planetary waves with zonal wavenumbers s 1 2 and 3 of temperature t and geopotential height z this should be useful as 1 a reference data set for intercomparison with shorter records and validation of general circulation models and 2 for fundamental understanding of the nature of stratospheric wave disturbances first seasonal december january and february statistics are presented the steady standing waves are strongest at 60 70 n and generally lead to northward eddy heat and momentum fluxes consistent with vertically propagating dissipated disturbances in broad agreement with previous studies based on shorter data sets at 30 hpa the stationary eddies provide about 50 60 of the net northward heat flux but the transients contribute a similar proportion to the net northward momentum flux statistical analysis of the transient wave amplitudes reveals unimodal distributions which are skew for higher wavenumbers transient variations in the eddy fluxes are shown to be mainly due to amplitude variations rather than changes in the barotropic or baroclinic structure of the eddies it is shown that changes in the geographical location of the waves contribute a considerable amount to the transience so that a composite mean based on the wave amplitude maximum contributes a much larger fraction of the net eddy fluxes than the stationary waves the importance of the long data record for these statistical distributions is emphasized by examining a 10 year subsample finally the occurrence of preferred flow regimes through the 30 year data record is examined neither trends nor definite links with possible external forcing mechanisms are found despite considerable low frequency biennial to decadal variability

a comprehensive numerical model of steady state saltation comsalt 1 the blowing of sand by wind known as saltation ejects dust aerosols into the atmosphere creates sand dunes and erodes geological features we present a comprehensive numerical model of steady state saltation comsalt that in contrast to most previous studies can reproduce a wide range of measurements and can simulate saltation over mixed soils comsalt calculates the motion of saltating particles due to gravity fluid drag particle spin fluid shear and turbulence and explicitly accounts for the retardation of the wind due to drag from saltating particles furthermore we included a novel physically based parameterization of the ejection of surface particles by impacting saltating particles which matches experimental results comsalt is the first numerical saltation model to reproduce measurements of the wind shear velocity at the impact threshold i e the lowest shear velocity for which saltation is possible and of the aerodynamic roughness length in saltation it also reproduces a range of other saltation processes including profiles of the wind speed and particle mass flux and the size distribution of saltating particles as such comsalt is the first physically based numerical model to reproduce such a wide range of experimental data since we use a minimum of empirical relations comsalt can be easily adapted to study saltation under a variety of physical conditions such as saltation on other planets saltation under water and saltating snow

mechanics of wind ripple stratigraphy stratigraphic patterns preserved under translating surface undulations or ripples in a depositional eolian environment are computed on a grain by grain basis using physically based cellular automata models the spontaneous appearance growth and motion of the simulated ripples correspond in many respects to the behavior of natural ripples the simulations show that climbing strata can be produced by impact alone direct action of fluid shear is unnecessary the model provides a means for evaluating the connection between mechanical processes occurring in the paleoenvironment during deposition and the resulting stratigraphy preserved in the geologic column vertical compression of small laminae above a planar surface indicates nascent ripple growth supercritical laminae are associated with unusually intense deposition episodes and a plane erosion surface separating sets of well developed laminae is consistent with continued migration of mature ripples during a hiatus in deposition

a study of particle splash on developing ripple forms for two bed materials a custom designed particle tracking velocimetry ptv system was developed to measure particle splash dynamics in two materials mixed quartz sand and ground acrylic particles approximating the density of snow the strengths and limitations of the ptv system are described in the context of five experiments carried out at friction velocities slightly greater than the threshold friction velocity for particle entrainment and saltation analysis of particle collisions on the bed surface splash demonstrates that the simulated snow particles impact the surface at a smaller angle than quartz sand they also eject a greater number of particles though at lower velocities and angles than observed for the sand particles these relations appear to be highly sensitive to the porosity of the bed surface while particle shape and elasticity play a secondary role with regard to ptv measurements carried out along the wind aligned axis of migrating aeolian ripples particles ejected from the lee slope tend to be slightly larger than those on the stoss slope and in general are ejected with a lower speed and greater angle to the surface irrespective of the type of granular material when the average impact angle is close to or greater than the negative slope of the evolving bedform the particle ejection rate from the stoss slope was measured to be approximately twice that for the lee slope this empirical observation confirms the assumption made in models of ripple development that the ejection flux governs the bedform migration rate however systematic under prediction of this rate by an early model suggests that additional processes inclusive of surface creep must also be accounted for

grain size dependence of eolian saltation lengths during snow drifting eolian saltation a primary process in the transport of fine granular material by wind produces a variety of geophysical effects on earth and other planetary surfaces wind tunnel experiments were carried out to investigate the dependence of saltation on grain size the saltation length of snow particles was estimated at size intervals of 0 05 mm in diameter by measuring local vertical mass fluxes in 17 snow collectors arrayed at the lee end of the snow surface the measured mean saltation length of snow particles of 0 01 1 mm in diameter ranged from 0 1 to 1 0 m at wind velocities of 5 10 m s mean saltation length decreased with increasing diameter and decreasing wind speed we suggest that the probability of the saltation length of a particle at each diameter is described by a monotonically decreasing distribution function that is the shorter the saltation length the higher the frequency of its occurrence one ramification of this distribution is that the mean saltation length does not imply the dominance of saltating particles of this length

sediment transport by wind toward a general model a general model of eolian sediment transport by both saltation and suspension is developed saltation is analyzed through calculations of single trajectories given initial conditions of liftoff speed angle and spin profiles of particle concentration and mass flux generated by assuming that all trajectories are identical display distinct maxima at the top of the trajectory by incorporating the realistic distribution of trajectories that arises from the stochastic nature of grain impacts with a granular bed the model yields monotonic decreases in particle concentration and mass flux with height in accordance with published empirical data the analysis of suspension as the balance between downward advective flux as a result of the settling of grains and their upward diffusive flux as a result of turbulence results in concentration profiles that fall off as power laws with height also in accordance with empirical data a relation for reference level concentrations is presented for both blowing snow and dust profiles the solutions for saltation and suspension are combined to yield mass flux profiles for the entire range of grain sizes in the bed the saltation layer emerges naturally as that region where the total flux of saltating grains exceeds that of suspended grains the model points clearly to the need for further research on the stochastic grain bed interactions which control both the probability distribution of liftoff velocities in saltation and the production of fine particles in suspension

the seasonality of east central north american precipitation based on three coeval holocene speleothems from southern west virginia abstract we present a replicated record of holocene climate change from a cave in west virginia usa based on analysis of stable isotopes in precipitation 18ow from the closest global network of isotopes in precipitation gnip station in coshocton oh we interpret enriched oxygen isotopes in calcite 18oc as representing an increase in the relative contribution of summer precipitation to annual totals significant coherence between local summer july and august precipitation and the summer north atlantic oscillation snao index on multi decadal timescales suggests that summer precipitation in our study area is strongly influenced by changes in the bermuda high a strengthened bermuda high circulation would increase summer precipitation amounts in the region and lead to enriched 18oc of speleothem calcite our 18oc record achieves maximum values during the mid holocene consistent with previous studies indicating an intensified bermuda high circulation at that time at 4 2 ka 18oc transitions to lower values at a time when numerous records around the globe document significant changes in the hydrologic budget over the last 2000 years there are intervals of substantially decreased 18oc coincident with aridity in the yucatan alpine glacial advances and a weakened asian summer monsoon

late quaternary climates and river channels of the atlantic coastal plain southeastern usa abstract late quaternary river patterns and sedimentation styles reflect past climate changes in atlantic drainages of eastern georgia and the carolinas past climate episodes for the region are summarized from pollen and eolian sedimentary records that exhibit correlation to major changes in river channel patterns records of past river responses are derived from luminescence and radiocarbon dated landforms and strata slow tectonic uplift does not explain temporal changes in channel patterns drainage networks have not received meltwater from continental or mountain glaciers eustatic influence would have been most pronounced on the now submerged continental shelf prior to 6 ka and currently manifests only within 60 80 km of the present highstand shoreline human impact overshadows climatic changes during historic time but was negligible during prehistoric time the late quaternary fluvial system primarily responded to climatic fluctuations that drove changes in vegetation precipitation and runoff patterns prehistoric meandering paleochannels of the holocene stand as important references to guide natural channel rehabilitation efforts during the coldest climate phases of the late wisconsin stage ca 30 16 ka when forest vegetation was sparse and interspersed with grasses and herbs many rivers and streams were braided with eolian dunes on the floodplains the terminal wisconsin stage ca 16 11 ka exhibited rapid warming and forest expansion characterized by pronounced change from braided to meandering patterns with intermediate forms of sandy scroll bar patterns the younger dryas cold excursion is not readily apparent in the regional fluvial record unusually large meandering channels of the terminal wisconsin stage continue into the first half of the holocene and indicate significantly larger bankfull flood discharges during those times than during the last half of the holocene bankfull channels during the last half of the holocene retained meandering patterns but decreased to modern like dimensions suggesting more modern like flood regimes no clear fluvial response occurs to the presumed middle holocene climatic optimum hypsithermal the medieval warm period and little ice age or to recent instrumentally observed fluctuations in climate but little research has been done in these realms

dating tsunami deposits present knowledge and challenges abstract dating approaches for fine sediment tsunami deposits can be divided into two categories 1 dating directly using materials from within a tsunami deposit and 2 dating indirectly using materials from above and below it for the former optically stimulated luminescence osl dating has a possible advantage in that is can directly date the burial age of sediments laid down by a tsunami however because of methodological limitations osl dating cannot always provide a reliable age although 14c and u th dating of reworked material from within a tsunami deposit are possible means of determining the age of the event the results merely represent a maximum age of deposition for these reasons dating is commonly achieved indirectly from 14c dates taken from samples in sections immediately above and below a tsunami deposit however indirectly dated results do not provide a precise timing of the event therefore there are distinct limitations in the ability to obtain high precision dating of past tsunamis recent methodological developments including the application of bayesian statistical analysis to radiocarbon dating results and the selection of appropriate dating materials has significantly improved the accuracy of age determination by reducing errors this review presents several methodological improvements that have enhanced tsunami deposit age estimation the most important points for a more precise tsunami age estimation are the numbers of 14c samples dated coupled with the use of bayesian statistical analysis these points are applicable not only to tsunami deposits but also to many other past geohazard events

the motion of sediment water mixtures during intense bedload transport computer simulations a new model which couples fluid and particle dynamics has been developed to study the motion of the sediment water mixture during intense bedload transport including the velocity profiles of both sediment and water the roughness length of an upper plane bed and the thickness of moving sediment layers standard mixing length theory is used to model the motion of water above the boundary between the overlying water and the sediment water mixture the turbulent flow within the moving sediment layers is described by a shear stress model in which the effective viscosity of the flowing water is proportional to the velocity difference between the fluid and the sediment the particle dynamics method in which the equations of motion of each of many particles are solved directly is applied to model the movement of sediment particles the particle fluid interaction is expressed by a velocity squared fluid drag force exerted on each sediment particle both computer simulation results and theoretical analysis have shown that the velocities of both sediment and fluid during intense sediment transport decrease exponentially with depth in the top layers of a fast moving sediment water mixture the thickness of the moving sediment layers obtained from the computer simulation results is proportional to the shear stress which agrees with previous experimental observations

late quaternary records of vegetation and fire in southeastern north carolina from jones lake and singletary lake abstract we conducted fossil pollen charcoal and geochemical analyses of sediment cores from jones lake and singletary lake spanning the last 50 000 cal yr b p to examine the linkages between climate vegetation and fire activity on the atlantic coastal plain and particularly emphasize changes since the last glacial maximum application of the modern analog technique mat to fossil pollen data allowed for quantitative estimates of holocene climate but pleistocene assemblages had no modern analogues preempting their use for quantitative reconstructions the mat data indicate markedly lower mean annual precipitation and temperatures during the late pleistocene relative to the holocene increased charcoal accumulation during interstadials indicates increased fire activity during these warm intervals geochemical data 13c 15n c n and pollen concentrations indicate a sparsely vegetated pleistocene landscape that produced few fires followed by an increase in biomass and fire activity around the lakes during the holocene transition a quercus spp maximum in the early holocene is associated with low charcoal abundances while increased dominance of pinus spp during the middle holocene is associated with dramatic increases in charcoal it is unclear if the quercus pinus transition was the result of changing fire regimes or if the fire regime changed in response to vegetation the regional asynchronicity of the quercus pinus transition may indicate another forcing mechanism besides climate change such as prehistoric human activity is responsible for the ecological change macroscopic charcoal and c n ratios reach unprecedented values during the late holocene possibly as a result of deforestation by both prehistoric native americans and later immigrant populations

collisions of quartz grains with a sand bed the influence of incident angle the velocities with which grains were observed to emerge from a sand bed after an intersaltation collision at u 40 cm s 1 are reported for four bed attitudes from horizontal bed to adverse bed slope 15 the principal effect of bed angle is to alter the magnitude and direction of the ricochet velocity however emergent velocities of dislodged grains are consistent with reptation path lengths comparable to the length of the upwind face of ripples in the corresponding wind calculations of the loss of forward momentum at collision using the data for the range of bed attitudes studied suggest that creep is most vigorous on the sloping upwind face of the ripple and diminishes at the crest as a result the crest would be expected to accumulate the coarse material which moves predominantly by creep the saltations originating in ricochet from the sloping back of the ripple are more vigorous and more concentrated in plan than are those originating at the crest however the saltation path length is at least an order of magnitude greater than the ripple wavelength and the probability distribution of path lengths is quite dispersed consequently it is very unlikely that these spatial patterns of ricochet are preserved sufficiently distinctly in the saltation cloud and subsequent collision distribution to be the agent of ripple development this study therefore supports a view of moving grain interaction with the bed in which saltation provides the power to mobilize grains but ripple growth is associated with reptation and particularly with a pattern of impact which develops with the bed relief creep is more active on upwind facing slopes than at the crest which therefore is a zone of net creep grain deposition

the connection between extreme stratospheric polar vortex events and tropospheric blockings the variations of tropospheric blockings and their connections with the stratospheric polar vortex during different stages of the life cycle of extreme stratospheric polar vortex events i e strong vortex sv events and weak vortex wv events are investigated the blocking frequency decreases over the euro atlantic sector and increases over the western north pacific during the onset and maturation stages of sv events there are more blocking days over the western north pacific and weakened upward planetary wave fluxes in the stratosphere during the maturation stage of shorter time scale sv events than during longer time scale sv events and the weakening of the upward planetary wave fluxes mainly results from the linear wave interference effect the blocking frequency is increased over the euro atlantic sector during the decline stage of sv events this increase is found to be related to the descending stratospheric zonal wind anomalies and the poleward displacement of the eddy driven jet stream which lasts for about 10 days and is associated with the descending positive stratospheric northern annular mode nam anomalies the increased blocking frequency leads to an enhancement of the planetary wave fluxes in the stratosphere via both the linear wave interference for shorter time scale sv events and nonlinear wave interference for longer time scale sv events effect the changes of the blocking frequency during the growth stage of wv events are almost opposite to those observed during the onset and maturation stages of sv events the blocking frequency decreases over the euro atlantic sector after the central dates of wv events which are related to the descending stratospheric zonal wind anomalies and the equatorward displacement of the eddy driven jet stream preceded by the descending negative stratospheric nam anomalies about 10 days we found that the nonlinear wave interference begins to make a considerable contribution to the weakening of the upward wave fluxes in the stratosphere after the central date of wv events and this is not emphasized in previous literature

use of multi proxy flood records to improve estimates of flood risk lower river tay scotland abstract proxy flood records from sediment stacks in floodplain palaeochannels provide an opportunity to extend short instrumental records and thus improve current estimates of flood risk the bloody inches a meander cutoff on the lower river tay scotland has been infilling with flood deposits since c 1761 agricultural flood embankments locally breach with flows 850 m3 s 1 introducing silts into the palaeochannel and extensively fail with flows 1200 m3 s 1 which deposit sand repeated cores at the site up to 1 4 m in depth consistently reveal sand rich flood units in the upper section of the core 137cs dating enables these units to be correlated with clusters of floods in the post 1950 discharge record sand units in the lower part of the profile are correlated with major floods from 1814 onwards using a 210pb based chronology these dates are independently corroborated by flood marks inscribed on smeaton s bridge in perth 15 km downstream estimates of historic high flows back to 1814 can also be recovered from these flood stage levels at smeaton s bridge when combined with the 48 year long flow record these historic floods yield an augmented pot series extending back to 1847 using a relatively high threshold of 1361 m3 s 1 and the generalised pareto distribution to model this pot series estimates of the 50 100 and 200 year floods are 1875 2250 and 2050 m3 s 1 respectively these results provide independent and robust confirmation of the flood risk at perth using the standard statistical procedures advocated in the flood estimation handbook if successfully replicated at other sites radiometric dating of sediment stacks in palaeochannels provides a new technique to further advance palaeohydrology and the recovery of historic floods

blocking precursors to stratospheric sudden warming events 1 the primary causes for the onset of major midwinter stratospheric sudden warming events remain unclear in this paper we report that 25 of the 27 events objectively identified in the era 40 dataset for the period 1957 2001 are preceded by blocking patterns in the troposphere the spatial characteristics of tropospheric blocks prior to sudden warming events are strongly correlated with the type of sudden warming event that follows vortex displacement events are nearly always preceded by blocking over the atlantic basin only whereas vortex splitting events are preceded by blocking events occurring in the pacific basin or in both basins contemporaneously the differences in the geographical blocking distribution prior to sudden warming events are mirrored in the patterns of planetary waves that are responsible for producing events of either type the evidence presented here suggests that tropospheric blocking plays an important role in determining the onset and the type of warmings

terminal pleistocene braided to meandering transition in rivers of the southeastern usa abstract thirteen paleomeanders on the oldest parts of meander belts on floodplains in the coastal plain of georgia and the carolinas southeastern usa were selected for radiocarbon dating to determine the onset of meandering following braiding during the late pleistocene during oxygen isotope stage 2 the radiocarbon ages were compared to previously reported late pleistocene ages for braid bars and eolian dunes results indicate that meandering commenced at circa 15 000 to 16 000 cal years bp and continued throughout the terminal pleistocene and holocene correlation with other paleoenvironmental records indicates that this shift to meandering was associated with global warming and moister conditions in the southeastern united states that led to a denser vegetation cover and a reduction in sediment yield the shift to meandering was also associated with some incision and terracing of the late pleistocene braided fluvial surfaces paleodischarge of the bankfull condition of early holocene meandering channels was apparently greater than under modern conditions suggesting wetter conditions at that time than at present this braided to meandering transition in the southeastern united states provides an example of river response to global climate change in a relatively low latitude region of the world that was not influenced by glacial or periglacial landscape conditions

textural analysis and modelling of a flood deposit river severn u k samples of sediment collected from the severn floodplain between worcester and gloucester following the severe flooding in january and february 1990 were analysed for their grain size distribution the results show that most sand was deposited within 20 m of the channel bank but that fine sand may contribute to flood sediment across the width of the floodplain james 1985 numerical model of overbank sedimentation attempts to predict the transfer of sediment to the floodplain during flooding geometrical and hydraulic data relating to the severn flood are used as input for a computer program of james 1985 model the pattern of sediment concentrations predicted by the model was compared with that obtained from statistical analysis of the flood sediment the patterns were found to be similar so james 1985 model was considered to predict in a relative sense the distribution of flood sediment

the early major warming in december 2001 exceptional the early major warming in december 2001 is described and compared to the two other december major warmings in 1998 and 1987 showing a strong tropospheric stratospheric coupling in all three cases we argue that the occurrence of free westward propagating rossby waves interacting with a forced quasi stationary wave number 1 led to these three early events the possible excitation of these waves is discussed with respect to the tropospheric circulation which showed strong blockings over the northern atlantic prior to the early major warmings

an analysis of soil crust strength in relation to potential abrasion by saltating particles wind erosion depends on the ease with which particles can be detached from the soil surface but suitable tests to characterize this property are not available two possible methods to determine surface soil strength in the field were therefore compared on a range of artificially crusted surfaces these were made by spraying or tension wetting aggregates 10 2 2 0 5 and 0 5 mm from a structurally unstable sandy loam followed by drying each test involved measuring the force exerted on a probe driven at a steady rate into the surface using either a flat tipped 0 6 mm diameter penetrometer or a flat ended cylindrical punch with inner and outer diameters of 5 and 6 mm respectively both probes showed that crusts could be produced reproducibly depending on the probe and aggregate size penetration mainly occurred either as a result of aggregates being deflected out of the pathway of the probe or by genuine rupture of aggregates or of the crusted surface the penetrometer because it was comparable to the size of sand grains gave results that can be used to characterize surface erodibility to saltating particles the punch gave results that would be unsuitable for this purpose as would other strength tests that are on too large a scale penetrometer results were analyzed to calculate the energy required for penetration it was thus possible to demonstrate that only the spray wetted fine aggregates had a surface that could undergo large scale rupture by saltating sand grains for all other surfaces saltating particles would be unable to supply sufficient energy to rupture aggregates or the crusted surface erosion could only occur by a slower process of abrasion in which smaller particles or aggregates are chipped away from the surface however it is shown that saltating particles could rupture the interaggregate bonding in the 2 0 5 mm aggregate surfaces thus permitting creep an alternative and potentially simpler way of characterizing surface erodibility by using a surface modulus of elasticity is also discussed our results demonstrate that the small diameter penetrometer is a promising technique for characterizing erodibility of aggregated and crusted surfaces 1997 by john wiley sons ltd

spatial variations in major u s hurricane activity statistics and a physical mechanism abstract the authors provide a statistical and physical basis for understanding regional variations in major hurricane activity along the u s coastline on long timescales current statistical models of hurricane activity are focused on the frequency of events over the entire north atlantic basin the exception is the lead author s previous work which models the occurrence of hurricanes over the caribbean sea gulf of mexico and the southeast u s coast separately here the authors use statistics to analyze data from historical and paleoclimatic records to expand this work in particular an inverse correlation in major hurricane activity across latitudes at various timescales is articulated when activity is above normal at high latitudes it tends to be below normal at low latitudes and vice versa past research paleoclimatic records and historical data hint at the potential of using the north atlantic oscillation nao as an indicator of where storms will likely track over long timescales an excite

late holocene paleofloods in the upper little tennessee river valley southern blue ridge mountains usa we derive a paleoflood chronology for the past 2000 years from three stratigraphic sections of overbank sediments with dates from radiocarbon luminescence 137cs techniques and historical records particle sizes were measured in 6 15 year intervals in post 1870 sediments and in 45 170 year intervals in pre 1870 sediments using an automatic laser analyzer the sedimentological characteristics of ad 1948 2009 deposits were compared with gaging records demonstrating that fine sand content and sorting discern time intervals of large floods but flood magnitudes are not well resolved this modern analog was applied to pre 1870 sediments and revealed two periods in the last 2000 years with large floods during ad 650 850 and ad 1100 1350 which are times when the regional tree ring record showed extreme wetness and no severe or extreme droughts our findings indicate flood prone phases of transitional climate at the beginning and end of the medieval warm period mwp and relatively subdued flooding during

field measurement and numerical modelling of aeolian mass flux distributions on a sandy beach the vertical and horizontal distributions of aeolian mass flux were measured at oceano dunes california and these data were used to evaluate a numerical model of saltation grain size analyses showed that the distributions of the modal sediment size class corresponded closely to those of the total sediment population and modelling thus focused on replicating the distributions of the mean grain size although much previous work has assumed that the mean launch speed of saltating particles varies in proportion to shear velocity simulations using a constant mean launch speed were found to yield the closest approximations to the mass flux distributions observed in the field both exponential and gamma distributions of launch velocity produced realistic simulations although the latter approach required the inclusion of an additional reptation component to achieve good results a range of mean launch angles and an equivalent sphere correction were also found to generate comparable results providing the other input parameters could be varied freely all the modelling approaches overestimated the proportion of mass flux occurring at the bottom of the vertical distributions and underestimated the proportion occurring at the upwind end of the horizontal distributions no theoretical shortcoming that would account for these small but systematic discrepancies could be identified and experimental error thus represents a more plausible explanation the conclusion that mean grain launch speeds are essentially constant and independent of shear velocity suggests that the additional kinetic energy extracted by grains under more energetic wind conditions is largely transferred to the bed and that increases in the transport rate are therefore driven primarily by the ejection of additional grains it is suggested that the kinetic energy of rebounding grains is constrained by the ability of the bed to resist deformation equivalent to a plastic limit hence grains of larger mass diameter rebound from the bed at lower speeds and follow shorter lower trajectories as has been widely observed previously

atmospheric blocking space time links to the nao and pna in the northern hemisphere regions characterized by an enhanced frequency of atmospheric blocking overlap significantly with those associated with the major extra tropical patterns of large scale climate variability namely the north atlantic oscillation nao and the pacific north american pna pattern there is likewise an overlap in the temporal band width of blocks and these climate patterns here the nature of the linkage between blocks and the climate patterns is explored by using the era 40 re analysis data set to examine 1 their temporal and spatial correlation and 2 the interrelationship between blocks and the nao pna it is shown that a strong anti correlation exists between blocking occurrence and the phase of the nao pna in the north atlantic western north pacific and that there are distinctive inter basin differences with a clear geographical over north atlantic and quantitative over north pacific separation of typical blocking genesis lysis regions during the opposing phases of the climate patterns an empirical orthogonal function eof analysis points to a significant influence of blocking upon the nao pattern identifiable as the leading eof in the euro atlantic and a temporal analysis indicates that long lasting blocks are associated with the development of negative nao pna index values throughout their life time in addition an indication of a cause and effect relationship is set out for the north atlantic linkage

wet early to middle holocene conditions on the upper coastal plain of north carolina usa abstract a peat core from a cutoff paleochannel of little river on the upper coastal plain of north carolina provides a continuous pollen record of environmental change for the past 10 500 years and includes a sedimentary record of overbank floods palynological and sedimentary data indicate that the early to middle holocene was wetter than previously suggested from lake sites in the southeastern united states the period from 9000 to 6100 cal yr b p is characterized by high pollen percentages of nyssa and quercus but low percentages of pinus fifteen large overbank flood events are present within this period 5 floods 1000 yr in contrast only 6 large overbank flood events occurred since 6100 cal yr b p 1 flood 1000 yr the increases in moisture and flood events probably were controlled by changes in atmospheric circulation related to shifts in the position of the bermuda high sea surface temperatures and el nino activity that together may have affected the frequency of large floods generated from tropical storms in the region

floodplain sedimentation in the upper mississippi valley natural versus human accelerated abstract understanding the time scales and pathways for response and recovery of rivers and floodplains to episodic changes in erosion and sedimentation has been a long standing issue in fluvial geomorphology floodplains are an important component of watershed systems because they affect downstream storage and delivery of overbank flood waters and they also serve as sources and temporary sinks for sediments and toxic substances delivered by river systems here 14c and 137cs isotopic dating methods are used along with ages of culturally related phenomena associated with mining and agriculture to determine rates of sedimentation and morphologic change for a reach of the upper mississippi river and adjacent tributaries in southwestern wisconsin and northwestern illinois the most important environmental change that influenced fluvial activity in this region during last 10 000 years involved the conversion of a late holocene mosaic of prairie and forest to a landscape dominated by cropland and pastureland associated with euro american settlement results presented herein for the upper mississippi valley umv show that the shift from pre agriculture natural land cover to landscape dominance by agricultural land use of the last 175 200 years typically increased rates and magnitudes of floodplain sedimentation by at least an order of magnitude accelerated overbank flooding led to increased bank heights on tributary streams and in turn contributed to more frequent deep flows of high energy these high energy flows subsequently promoted bank erosion and lateral channel migration and the formation of a historical meander belt whose alluvial surface constitutes a new historical floodplain inset against the earlier historical floodplain the new historical floodplain serves as a flume like channel that provides efficient downstream transport of water and sediment associated with moderate and large magnitude floods floodplains on lower tributaries however continue to experience rates of overbank sedimentation that are of anomalously high magnitude given improved land cover and land conservation since about 1950 this lower valley anomaly is explained by minimal development of historical agriculture period meander belts because of relatively low stream power in these channel and floodplain reaches of relatively low gradient in general long term pre agriculture rates of vertical accretion between about 10 000 and 200 years ago averaged about 0 2 mm yr 1 in tributary watersheds smaller than about 700 km2 and about 0 9 mm yr 1 on the floodplain of the upper mississippi river where the contributing watershed area increases to about 170 000 km2 on the other hand rates of historical vertical accretion during the period of agricultural dominance of the last 200 years average between 2 and 20 mm yr 1 with short episodes of even higher rates during times of particularly poor land conservation practices significant hydrologic effects of mining and agricultural started by the 1820s and became widespread in the study region by the mid 19th century the hydrologic and geomorphic influences of mining were relatively minor compared to those related to agriculture high resolution dating of floodplain vertical accretion deposits shows that large floods have frequently provided major increments of sedimentation on floodplains of tributaries and the main valley upper mississippi river the relative importance of large floods as contributors to floodplain vertical accretion is noteworthy because global atmospheric circulation models indicate that the main channel upper mississippi river should experience increased frequencies of extreme hydrologic events including large floods with anticipated continued global warming instrumental and stratigraphic records show that coincident with global warming a shift to more frequent large floods occurred since 1950 on the upper mississippi river and these floods generally contributed high magnitudes of floodplain sedimentation

characteristics of particle size for creeping and saltating sand grains in aeolian transport creep and saltation are the primary modes of surface transport involved in the fluid like movement of aeolian sands although numerous studies have focused on saltation few studies have focused on creep primarily because of the experimental difficulty and the limited amount of theoretical information available on this process grain size and its distribution characteristics are key controls on the modes of sand movement and their transport masses based on a series of wind tunnel experiments this paper presents new data regarding the saltation flux obtained using a flat sampler and on the creeping mass obtained using a specifically designed bed trap associated with four friction velocities 0 41 0 47 0 55 and 0 61 m sec 1 these data yielded information regarding creeping and saltating sand grains and their particle size characteristics at various heights which led to the following conclusions i the creeping masses increased as a power function q 1 02 14 19u 3 of friction wind velocities with a correlation r2 of 0 95 ii the flux of aeolian sand flow decreases exponentially with increasing height q a exp z b and increases as a power function q 26 30 428 40 u 3 of the friction wind velocity iii the particle size of creeping sand grains is ca 1 15 times of the mean diameter of salting sand grains at a height of 0 to 2 cm which is 1 14 times of the mean diameter of sand grains in a bed and iv the mean diameter of saltating sand grains decreases rapidly with increasing height whereas while at a given height the mean diameter of saltating sand grains is positively correlated with the friction wind velocity although these results require additional experimental validation they provide new information for modelling of aeolian sand transport processes

the distribution of velocity and energy of saltating sand grains in a wind tunnel abstract sand transport by wind is a special case of two phase flow of gas and solids with saltating grains accounting for about 75 of the transport rate this form of flow is not only the main external agent moulding aeolian landforms but also the motive force responsible for transport sorting and deposition of aeolian sediments high speed multiflash photography is an effective method of studying the distribution of velocity and energy of saltating grains within the boundary layer of wind tunnel the experimental wind shear velocities were set at 0 63 0 64 0 74 and 0 81 ms 1 the statistical study of the results showed that there is a power function relation between mean velocity and height of saltating grains as the height is divided into 0 5 cm intervals the sand grain velocities at various levels are consistent with the pearson vii distribution pattern the variations in kinetic energy and total energy of sand grains with height accord with the pulse peak modified with power term pulsepow law the maximum values occur at heights of 6 cm or so and tend to shift upward with increasing wind velocity

bedload sheets in heterogeneous sediment field observations in streams with beds of coarse sand and fine gravel have revealed that bedload moves primarily as thin migrating accumulations of sediment and coarse grains cluster at their leading edge these accumulations are one or two coarse grains high and are much longer 0 2 0 6 m long in sand 0 5 2 0 m in fine gravel than their height the authors propose the term bedload sheet for these features and the authors argue that they result from an instability inherent to bedload movement of moderately and poorly sorted sediment in essence coarse particles in the bedload slow or stop each other trap finer particles in their interstices and thus cause the coarse particles to become mobile again bedload sheets develop on the stoss side of dunes causing the dune to advance incrementally with the arrival of each sheet successive deposition of coarse sediment from the leading edge followed by fine sediment may generate the grain size sorting that distinguishes cross bedding available flume experiments and field observations indicate that bedload sheets are a common but generally unrecognized feature of heterogeneous sediment transport

observed and simulated precursors of stratospheric polar vortex anomalies in the northern hemisphere the northern hemisphere stratospheric polar vortex is linked to surface weather after stratospheric sudden warmings in winter the tropospheric circulation is often nudged towards the negative phase of the northern annular mode nam and the north atlantic oscillation nao a strong stratospheric vortex is often associated with subsequent positive nam nao conditions for stratosphere troposphere associations to be useful for forecasting purposes it is crucial that changes to the stratospheric vortex can be understood and predicted recent studies have proposed that there exist tropospheric precursors to anomalous vortex events in the stratosphere and that these precursors may be understood by considering the relationship between stationary wave patterns and regional variability another important factor is the extent to which the inherent variability of the stratosphere in an atmospheric model influences its ability to simulate stratosphere troposphere links here we examine the lower stratosphere variability in 300 year pre industrial control integrations from 13 coupled climate models we show that robust precursors to stratospheric polar vortex anomalies are evident across the multi model ensemble the most significant tropospheric component of these precursors consists of a height anomaly dipole across northern eurasia and large anomalies in upward stationary wave fluxes in the lower stratosphere over the continent the strength of the stratospheric variability in the models was found to depend on the variability of the upward stationary wave fluxes and the amplitude of the stationary waves

modelling flood hydraulics and overbank deposition on river floodplains this paper outlines a numerical model for the prediction of floodplain inundation sequences overbank deposition rates and deposit grain size distributions the model has two main components first a simplified hydraulic scheme which predicts floodwater flow depths and velocities and second a sediment transport element which employs a mass balance relation describing suspended sediment dispersion by convective and diffusive processes and sediment deposition as a function of particle settling rates these relationships are solved numerically on a finite difference grid that accurately replicates the complex topographic features typical of natural river floodplains the model is applied to a 600 m reach of the river culm devon u k using data derived from a range of field and laboratory techniques continuous records of river stage and suspended sediment concentration provide the model s upstream boundary input requirements these are supplemented by measurements of the in situ settling characteristics of the suspended sediment load the model s sediment transport component is calibrated with the aid of a dataset of measured overbank deposition amounts derived from flood events over a 16 month period the model is shown to predict complicated floodwater inundation sequences and patterns of suspended sediment dispersion and deposition which are largely a product of the complex topography of the floodplain these results compare favourably with observations of overbank processes and are an improvement over those of previous models which have employed relatively simple representations of floodplain geometry 1997 by john wiley sons ltd

a steady state model of wind blown sand transport a model for steady state eolian saltation is presented in which computation of sand grain trajectories in the wind and of the wind velocity profile modified by saltating grain drag forces is combined with experimental data on grain bed collisions in an iterative simulation scheme which resembles the time development of natural saltation qualitative features of eolian saltation are reproduced in the simulations including a height in the saltating layer at which the wind velocity remains roughly constant with changing free stream friction velocity and an approximate separation of the saltating population into successively hopping grains and reptating or creeping grains the numerical solutions give rise to more general arguments in support of the conclusions that fluid entrainment of grains is unimportant in steady state saltation that the mass flux profile obeys roughly an inverse power law dependence on height and that the vertically integrated mass flux dependence on the free stream friction velocity

holocene ethnobotanical and paleoecological record of human impact on vegetation in the little tennessee river valley tennessee abstract human occupation and utilization of plant resources have affected vegetation in the lower little tennessee river valley of east tennessee for 10 000 yr changes in indian cultures and land use are documented by radiocarbon chronologies lithic artifacts ceramics settlement patterns and ethnobotanical remains from 25 stratified archaeological sites within the holocene alluvial terrace the ethnobotanical record consists of 31 500 fragments 13 7 kg of wood charcoal identified to species and 7 7 kg of carbonized fruits seeds nutshells and cultigens from 956 features pollen and plant macrofossils from small ponds both in the uplands and on lower stream terraces record local vegetational changes through the last 1500 to 3000 yr human impact increased after cultigens including squash and gourd were introduced ca 4000 yr b p during the archaic cultural period forest clearance and cultivation disturbed vegetation on both the floodplain and lower terraces after 2800 yr b p during the woodland period permanent indian settlements and maize and bean agriculture extended to higher terraces 1 5 km from the floodplain by the mississippian period 1000 to 300 yr b p after 300 yr b p extensive land clearance and cultivation by historic overhill cherokee and euro americans spread into the uplands beyond the river valley

a kinematic model for the structure of lee side deposits a kinematic model for the structure of the lee side deposit of a dune like bedform gilbert type delta or similar step is developed based on the assumptions that initial deposition is entirely by grainfall that the rate of deposition decreases as a power function of distance downflow from the brink of the slipface and that the resulting steepening of the slipface is periodically interrupted by avalanching the parameters used in the model are 1 the deposition rate at a given distance from the brink 2 the exponent in the equation relating the deposition rate to distance from the brink 3 the bedform migration rate 4 the bedform height 5 the avalanche speed 6 the angle of initial yield and 7 the residual angle after avalanching from these parameters can be calculated structural characteristics such as the proportions of bottomset and foreset deposits the proportions of avalanche and grainfall deposits in the foreset deposit and the spacing of avalanche grainfall couplets the model correctly predicts the trends of changing avalanche activity and changing structural character with changes in flow character grain size and bedform height in both air and water moreover the model correctly predicts certain consistent structural differences between aeolian and subaqueous lee side deposits quantitative evaluation of the model requires more accurate data on the values of the input parameters than are presently available

response of extreme floods in the southwestern united states to climatic variations in the late holocene a regional synthesis of paleoflood chronologies on rivers in arizona and southern utah reveals that the largest floods over the last 5000 years cluster into distinct time periods that are related to regional and global climatic fluctuations the flood chronologies were constructed using fine grained slackwater deposits that accumulate in protected areas along the margins of bedrock canyons and selectively preserve evidence of the largest events high magnitude floods were frequent on rivers throughout the region from 5000 to 360014c yrs bp dendrocalibrated age 3800 2200 bc and increased again after 2200 bp 400 bc with particularly prominent peaks in magnitude and frequency around 1100 900 bp ad 900 1100 and after 500 yrs bp ad 1400 in contrast the periods 3600 2200 bp 2200 400 bc and 800 600 yrs bp 1200 1400 ad are marked by sharp decreases in the occurrence of large floods on these rivers in the modern record storms that generate large floods 10 year in the region fall into three categories 1 winter north pacific frontal storms 2 late summer and fall storms that draw in moisture from recurved pacific tropical cyclones and 3 summer storms mainly convective thunderstorms winter storms and tropical cyclones are associated with the most severe floods on the rivers in this study and are the most probable causes of the paleofloods over the last 5000 years floods from both winter storms and tropical cyclones occur when deep mid latitude troughs steer storm systems into the region composite anomaly maps of daily 700 mbar heights indicate that these floods are associated with a low pressure anomaly off the california coast and a high pressure anomaly over the aleutians or gulf of alaska a strong connection exists between the negative phase of the southern oscillation index often associated with el nin o conditions and the large floods associated with winter storms and tropical cyclones the paleoflood records confirm the existence of centennial scale variations in the conditions conducive to the occurrence of extreme floods and flood generating storms in this region the episodes with an increased frequency of high magnitude floods coincide with periods of cool wet climate in the western u s whereas warm intervals such as the medieval warm period are times of dramatic decreases in the number of large floods a positive relationship between the paleofloods and long term variations in the frequency of el nin o events is evident over the last 1000 years this relationship continues over at least the last 3000 years with warm coastal sea surface temperatures indicative of el nin o like conditions

trouble with diffusion reassessing hillslope erosion laws with a particle based model 1 many geomorphic systems involve a broad distribution of grain motion length scales ranging from a few particle diameters to the length of an entire hillslope or stream studies of analogous physical systems have revealed that such broad motion distributions can have a significant impact on macroscale dynamics and can violate the assumptions behind standard local gradient flux laws here a simple particle based model of sediment transport on a hillslope is used to study the relationship between grain motion statistics and macroscopic landform evolution surface grains are dislodged by random disturbance events with probabilities and distances that depend on local microtopography despite its simplicity the particle model reproduces a surprisingly broad range of slope forms including asymmetric degrading scarps and cinder cone profiles at low slope angles the dynamics are diffusion like with a short range thin tailed hop length distribution a parabolic convex upward equilibrium slope form and a linear relationship between transport rate and gradient as slope angle steepens the characteristic grain motion length scale begins to approach the length of the slope leading to planar equilibrium forms that show a strongly nonlinear correlation between transport rate and gradient these high probability long distance motions violate the locality assumption embedded in many common gradient based geomorphic transport laws the example of a degrading scarp illustrates the potential for grain motion dynamics to vary in space and time as topography evolves this characteristic renders models based on independent stationary statistics inapplicable an accompanying analytical framework based on treating grain motion as a survival process is briefly outlined

vertical accretion sand proxies of gaged floods along the upper little tennessee river blue ridge mountains usa abstract understanding environmental hazards presented by river flooding has been enhanced by paleoflood analysis which uses sedimentary records to document floods beyond historical records bottomland overbank deposits e g natural levees floodbasins meander scars low terraces have the potential as continuous paleoflood archives of flood frequency and magnitude but they have been under utilized because of uncertainty about their ability to derive flood magnitude estimates the purpose of this paper is to provide a case study that illuminates tremendous potential of bottomland overbank sediments as reliable proxies of both flood frequency and magnitude methods involve correlation of particle size measurements of the coarse tail of overbank deposits 0 25 mm sand from three separate sites with historical flood discharge records for the upper little tennessee river in the blue ridge mountains of the southeastern united states results show that essentially all floods larger than a 20 probability event can be detected by the coarse tail of particle size distributions especially if the temporal resolution of sampling is annual or sub annual coarser temporal resolution 1 0 to 2 5 year sample intervals provides an adequate record of large floods but is unable to discriminate individual floods separated by only one to three years measurements of 0 25 mm sand that are normalized against a smoothed trend line through the down column data produce highly significant correlations r 2 values of 0 50 to 0 60 with p values of 0 004 to

transport mass of creeping sand grains and their movement velocities 1 aeolian sand transport is an important component of material circulation above terrestrial surfaces and can include processes of creep saltation and suspension the complex movement of material and energy during aeolian transport has meant that these processes have previously been examined in isolation although a significant amount of research has been conducted on aeolian sand transport this focused primarily on saltation as a result there are few data available on sand grain creep primarily due to a lack of theoretical models and the difficulty of direct measurements in this study we present novel methods and instrumentations to accurately measure the transport mass and the velocity of creeping sand grains using data collected with above instrumentations and a ladder sampler at four friction velocities u 0 26 0 35 0 47 and 0 56 m s 1 in a wind tunnel we studied the transport mass of creeping sand grains and their movement velocities as well as other key parameters of aeolian sand transport four major conclusions can be drawn from this study 1 the transport mass q0 of creeping sand grains increases with increasing frictional wind velocity u the relationship between these variables is represented by the power function q0 0 053 9 195u 2 800 2 the contribution of creep to total aeolian sand transport decreases with increasing frictional wind velocity creep contributed 57 of total aeolian transport at the lowest frictional wind velocity but only 19 at the highest velocity 3 the threshold frictional wind velocity for entrainment was 0 158 m s 1 4 the movement velocity of creeping sand grains ranged from 0 to 0 14 m s 1 but more than 70 of recorded velocities were less than 0 02 m s 1 although the results of this study require further validation they provide a strong basis for future research and help deepen our understanding of aeolian sand transport

a theoretical model for aeolian impact ripples new insights into the grain bed impact process arising from both numerical and physical experiments involving single grain impacts lead to a more complete conceptual model of the aeolian saltation process that in turn allows a simple model of aeolian impact ripples to be developed the saltating population may be idealized as consisting of 1 long trajectory high impact energy constant impact angle successive saltations and 2 short trajectory low impact energy reptations it is argued that the spatial variations in mass flux due to the reptating population lead to the growth and translation of impact ripples using the sediment continuity equation an expression for the spatial variation in the ejection rate of reptating grains from a sinusoidally perturbed bed and a probability distribution for the reptation lengths a simple stability analysis demonstrates that the flat bed is unstable to small amplitude perturbations a fastest growing wavelength emerges that is roughly six times the mean reptation length and is only weakly dependent upon the detailed shape of the probability distribution of reptation lengths the results match well with the observed initial wavelengths in wind tunnel experiments

a physically based bedload transport model developed for 3 d reach scale cellular modelling abstract a bedload transport model is outlined which aims to provide a more physically based description of bed surface and bedload grain size distributions gsds than existing empirical bedload models but is simple enough to be incorporated in a reduced complexity modelling framework one motivation for this is the increasing need for 3 d models of decadal reach scale channel and floodplain evolution for use in river management the computational demands of simulation at these scales mean that the appropriate models are necessarily of the reduced complexity type the bedload model is derived through analysis of sediment beds created using a discrete element model these beds are eroded using a probabilistic entrainment algorithm which predicts the bedload gsd there are two parts to the bedload model the first identifies a functional relationship between the applied shear stress and the fraction of the bed undergoing active transport while the second uses this fraction in conjunction with the surface gsd to produce a bedload gsd the model produces bedload coarsening as shear stress increases from a value of one to a value of five times a reference shear stress this model is tested in a series of numerical experiments applied using a simple cellular model of a straight flume which is run in both recirculation and feed modes in both modes the equilibrium gsds of the bedload and surface are consistent with expected flume behaviour suggesting that the model provides a reasonable reproduction of sediment entrainment transport and sorting

a numerical study of particle motion and two phase interaction in aeolian sand transport using a coupled large eddy simulation discrete element method wind blown sand movement considered as a particle laden two phase flow was simulated by a new numerical code developed in the present study the discrete element method was employed to model the contact force between sand particles large eddy simulation was used to solve the turbulent atmospheric boundary layer motions of sand particles were traced in the lagrangian frame within the near surface region of the atmospheric boundary layer interparticle collisions will significantly alter the velocity of sand the sand phase is quite dense in this region and its feedback force on fluid motion cannot be ignored by considering the interparticle collision and two phase interaction four way coupling was achieved in the numerical code profiles of sand velocity from the simulations were in good agreement with experimental measurements the mass flux shows an exponential decay and is comparable to reported experimental and field measurements the turbulence intensities and shear stress of sand particles were estimated from particle root mean square velocities distributions of slip velocity and feedback force were analysed to reveal the interactions between sand particles and the continuous fluid phase

rain splash of dry sand revealed by high speed imaging and sticky paper splash targets 1 rain splash transport of sediment on a sloping surface arises from a downslope drift of grains displaced ballistically by raindrop impacts we use high speed imaging of drop impacts on dry sand to describe the drop to grain momentum transfer as this varies with drop size and grain size and to clarify ingredients of downslope grain drift the splash of many grains involves ejection of surface grains accelerated by grain to grain collisions ahead of the radially spreading front of a drop as it deforms into a saucer shape during impact for a given sand size splash distances are similar for different drop sizes but the number of displaced grains increases with drop size in proportion to the momentum of the drop not infiltrated within the first millisecond of impact we present a theoretical formulation for grain ejection which assumes that the proportion of ejected grains within any small azimuthal angular interval dq about the center of impact is proportional to the momentum density of the spreading drop within dq and that the momentum of ejected grains at angle q is on average proportional to the momentum of the spreading drop at q this formulation consistent with observed splash distances suggests that downslope grain transport involves an asymmetry in both quantity and distance more grains move downslope than upslope with increasing surface slope and on average grains move farther downslope this latter effect is primarily due to the radial variation in the surface parallel momentum of the spreading drop surface parallel transport increases approximately linearly with slope

grain scale simulations of loose sedimentary beds the example of grain bed impacts in aeolian saltation sediment transport by wind is one of many processes of interest to the geomorphologist in which grain to grain contacts play an important role in order to illustrate the modelling of collections of frictional inelastic sedimentary grains with the particle dynamics method pdm we use the grain impact process in aeolian saltation as a specific example in pdm all the forces on each particle are evaluated at a sequence of small time steps and the newtonian equations of motion are integrated forward in time interparticle forces at grain contacts are treated as springs with prescribed stiffness normal force and by a coulomb friction law tangential force particle inelasticity is represented by spring damping the granular splash resulting from saltation impacts is assessed for sensitivity to the choice of grain properties and the integration time step we find that for the range of impact speeds and impactor masses relevant to aeolian settings grain splashes are relatively insensitive to grain stiffness grain inelasticity and grain friction and that the pattern of ejection from the bed is largely controlled by bed microtopography a large set of impact realizations involving a variety of impact points on a small set of target beds is used to collect the appropriate statistics for describing the stochastic splash process the splash function representing these statistics is then available for use in calculations over longer time scales such as the evolution of the saltation curtain the details given here will enable the interested reader to adapt pdm modelling to other types of clastic sedimentary systems