what determines the downstream evolution of turbidity currents abstract seabed sediment flows called turbidity currents form some of the largest sediment accumulations deepest canyons and longest channel systems on earth only rivers transport comparable sediment volumes over such large areas but there are far fewer measurements from turbidity currents ensuring they are much more poorly understood turbidity currents differ fundamentally from rivers as turbidity currents are driven by the sediment that they suspend fast turbidity currents can pick up sediment and self accelerate ignite whilst slow flows deposit sediment and dissipate self acceleration cannot continue indefinitely and flows might reach a near uniform state autosuspension here we show how turbidity currents evolve using the first detailed measurements from multiple locations along their pathway which come from monterey canyon offshore california all flows initially ignite typically initially faster flows then achieve near uniform velocities autosuspension whilst slower flows dissipate fractional increases in initial velocity favour much longer runout and a new model explains this bifurcating behaviour however the only flow during less stormy summer months is anomalous as it self accelerated which is perhaps due to erosion of surficial mud layer mid canyon turbidity current evolution is therefore highly sensitive to both initial velocities and seabed character

palaeohurricane reconstructions from sedimentary archives along the gulf of mexico caribbean sea and western north atlantic ocean margins hurricanes annually threaten the atlantic ocean margins historical hurricane records are relatively short and palaeohurricane sedimentary archives provide a geological and climatic context that sheds light on future hurricane activity here we review palaeo trends in hurricane activity elucidated from sedimentary archives we discuss dating methods site selection and stat istics associated with previously published records these archives have been useful for under standing the long term evolution of coastal systems and the response of intense hurricane activity to climatic changes regional shifts in hurricane overwash on centennial to millennial timescales have been linked to various climatic modes of variability including el nino southern oscillation and the north atlantic oscillation but could also reflect regional scale controls on hurricane activity

eruption dynamics at pahvant butte volcano utah western usa insights from ash sheet dispersal grain size and geochemical data shallow to emergent subaqueous basaltic explosive eruptions here referred to as surtseyan are a source of pyroclastic material expelled from subaqueous vents and transferred to the water column and atmosphere they can threaten coastal communities in different ways and one of these is through the generation of pyroclastic density currents travelling over water once the eruption becomes subaerial these may be preceded or accompanied by eruption fed currents moving along the sea lake floor one approach for studying these eruptions is through analysis of their deposits but these are commonly either inaccessible under water or poorly preserved by the time they are exposed subaerially the site selected for this work pahvant butte ut usa avoids these limitations the volcano was formed at 18 600 cal year b p in the giant pleistocene lake bonneville by an eruption initiated at a relatively shallow water depth 85 m the water is now drained and deposits of the edifice and of the ash sheet at medial distances on the old lake floor medial lake are accessible and well preserved the ash sheet deposits allow us to investigate dispersal of ash from a surtseyan eruption including the subaqueous phase of edifice building by comparing edifice deposits with ash sheet deposits correlated using geochemistry and textural analysis results suggest that medial deposited ash from pahvant butte travelled at least 25 km from the vent in eruption fed pyroclastic density currents moving along the lake floor this transport began early in the eruption apparently prior to the volcano s subaerial emergence

the physics of wind blown loess implications for grain size proxy interpretations in quaternary paleoclimate studies abstract loess deposits are recorders of aeolian activity during past glaciations since the size distribution of loess deposits depends on distance to the dust source and environmental conditions at the source during transport and at deposition loess particle size distributions and derived statistical measures are widely used proxies in quaternary paleoenvironmental studies however the interpretation of these proxies often only considers dust transport processes to move beyond such overly simplistic proxy interpretations and toward proxy interpretations that consider the range of environmental processes that determine loess particle size distribution variations we provide a comprehensive review on the physics of dust particle mobilization and deposition furthermore using high resolution bulk loess and quartz grain size datasets from a last glacial interglacial sequence we show that because grain size distributions are affected by multiple often stochastic processes changes in these distributions over time allow multiple interpretations for the driving processes consequently simplistic interpretations of proxy variations in terms of only one factor e g wind speed are likely to be inaccurate nonetheless using loess proxies to understand temporal changes in the dust cycle and environmental parameters requires i a careful site selection to minimize the effects of topography and source distance and ii the joint use of bulk and quartz grain size proxies together with high resolution mass accumulation rate calculations if possible

a simple universal equation for grain settling velocity abstract a new equation is presented for sediment fall velocity as a function of grain diameter for given values of fluid viscosity and fluid and solid density sediment fall velocity is a fundamental parameter in the modeling and interpretation of fluviatile and coastal deposition the equation applies to the entire range of viscous to turbulent conditions and its simple explicit form makes it easy to use in computer models and other applications in sedimentology geomorphology and engineering the equation is derived from dimensional analysis and converges on stokes law for small grains and a constant drag coefficient for large grains its two physically interpretable parameters are easily adjusted for shape effects or for the use of sieve diameter rather than nominal grain diameter it gives a close fit to published and new experimental data for both quartz sand and low density materials with no more error than previous equations of more complicated form

fluvial erosion transport equation of landscape evolution models revisited we present a mesoscale erosion deposition model which differs from previous landscape evolution models equations by taking explicitly into account a mass balance equation for the streamflow the geological and hydrological complexity is lumped into two basic fluxes erosion and deposition and two averaged parameters unit width discharge q and stream slope s the model couples the dynamics of streamflow and topography through a sediment transport length function x q which is the average travel distance of a particle in the flow before being trapped on topography this property reflects a time lag between erosion and deposition which allows the streamflow not to be instantaneously at capacity the so called x q model may reduce either to transport limited or to detachment limited erosion modes depending on x but it also may not we show in particular how it does or does not for steady state topographies long term evolution and high frequency base level perturbations apart from the unit width discharge and the settling velocity the x q function depends on a dimensionless number encompassing the way sediment is transported within the streamflow using models of concentration profile through the water column we show the dependency of this dimensionless coefficient on the rouse number we discuss how consistent the x q model framework is with bed load scaling expressions and einstein s conception of sediment motion

experimental alluvial fan evolution channel dynamics slope controls and shoreline growth 1 river deltas and alluvial fans have channelization and deposition dynamics that are not entirely understood but which dictate the evolution of landscapes of great social economic and ecologic value our lack of a process based understanding of fan dynamics hampers our ability to construct accurate prediction and hazard models leaving these regions vulnerable here we describe the growth of a series of experimental alluvial fans composed of a noncohesive grain mixture bimodal in size and density we impose conditions that simulate a gravel sand fan prograding into a static basin with constant water and sediment influx and the resulting fans display realistic channelization and avulsion dynamics we find that we can describe the dynamics of our fans in terms of a few processes 1 an avulsion sequence with a timescale dictated by mass conservation between incoming flux and deposit volume 2 a tendency for flow to reoccupy former channel paths and 3 bistable slopes corresponding to separate entrainment and deposition conditions for grains several important observations related to these processes are an avulsion timescale that increases with time and decreases with sediment feed rate fan lobes that grow in a self similar quasi radial pattern and channel geometry that is adjusted to the threshold entrainment stress we propose that the formation of well defined channels in noncohesive fans is a transient phenomenon resulting from incision following avulsion and can be directly described with dual transport thresholds we present a fairly complete process based description of the mechanics of avulsion and its resulting timescale on our fans because the relevant dynamics depend only on threshold transport conditions and conservation of mass we show how results may be directly applied to field scale systems

magnetic mineral transport and sorting in the swash zone northern lake erie canada a combined field and laboratory study in northern lake erie has provided new insights into the origin and dynamics of heavy mineral placer deposits on beaches consisting primarily of non magnetic sediment work was conducted on the cross shore and longshore transport of heavy magnetic minerals using magnetic susceptibility and fluorescent paints to trace the movement in the field of samples of magnetic magnetite and non magnetic quartz and calcite grains respectively laboratory experiments examined how the burial of small dense magnetic minerals is affected by the grain size of the non magnetic host material and how grain burial affects magnetic susceptibility measurements at the surface the field experiments demonstrated that the magnetic mineral tracers were buried rapidly beneath coarser non magnetic grains under low to moderate wave conditions and subsequently were unable to move in the longshore or cross shore directions the laboratory experiments showed that the magnetic susceptibility rapidly decreased with the rate and depth of burial of the magnetic minerals and that magnetic grain burial was most effective beneath coarser rather than finer non magnetic sand and for the latter sediments under less rather than more energetic conditions the results imply that magnetic mineral concentrations develop in this area through magnetic grain burial under fairly mild conditions and subsequent settling exposure and concentration in the upper swash zone during more energetic periods when the non magnetic grains are eroded it is probably during these erosional periods when the magnetic minerals are exposed in fairly homogeneous deposits that longshore and cross shore transport takes place

grain size sediment transport regime and channel slope in alluvial rivers the general relationship between channel morphology and the grain size of sediment in the channel bed is an important but poorly known aspect of alluvial rivers an analysis of an equation for total sediment flux in the limits of suspension bedload and mixed modes of transport indicates distinct steady state regimes of channel morphology such regimes are readily seen in published data for modern alluvial rivers by way of a conventional shields plot or a plot of channel slope as a function of relative grain size d h and the ratio ws u where d and ws are respectively mean diameter and fall speed of bed sediments and h and u are respectively mean depth and friction velocity of the flow with slope and mode of transport in an alluvial river constrained by grain size and channel depth alone estimates of discharge and sediment flux follow directly introduction of the sediment flux relationship into conventional diffusion models for the evolution of an alluvial system provides nominal estimates of

fall velocity of multi shaped clasts abstract accurate settling velocity predictions of differently shaped micro or macroclasts are required in many branches of science and engineering here a single dimensionally correct equation is presented that yields a significant improvement on previous settling formulas for a wide range of clast shapes for smooth or irregular clasts with known axial dimensions a partially polynomial equation based on the logarithmic values of dimensionless sizes and settling velocities is presented in which the values of only one coefficient and one exponent need to be adapted for different shapes irrespective of the reynolds number for irregular natural clasts with unknown axial dimensions a polynomial equation of the same form is applied but with different coefficients comparison of the predicted and measured settling velocities of 8 different shape classes as well as natural grains with unknown axial dimensions in liquids representing a total of 390 experimental data points shows a mean percentage error of 0 83 and a combined r2 value of 0 998 the settling data of 169 differently shaped particles of pumice glass and feldspar falling in air were also analyzed which demonstrates that the proposed equation is also valid for these conditions two additional shape classes were identified in the latter data set although the resultant equations are less accurate than for liquids an excel spreadsheet is provided to facilitate the calculation of fall velocities for grains settling individually and in groups or alternatively to determine the equivalent sieve size from the settling velocity which can be used to calibrate settling tubes

effects of riparian vegetation on topographic change during a large flood event rio puerco new mexico usa 1 the spatial distribution of riparian vegetation can strongly influence the geomorphic evolution of dryland rivers during large floods we present the results of an airborne lidar differencing study that quantifies the topographic change that occurred along a 12 km reach of the lower rio puerco new mexico during an extreme event in 2006 extensive erosion of the channel banks took place immediately upstream of the study area where tamarisk and sandbar willow had been removed within the densely vegetated study reach we measure a net volumetric change of 578 050 490 000 m3 with 88 3 of the total aggradation occurring along the floodplain and channel and 76 7 of the erosion focusing on the vertical valley walls the sediment derived from the devegetated reach deposited within the first 3 6 km of the study area with depth decaying exponentially with distance downstream elsewhere floodplain sediments were primarily sourced from the erosion of valley walls superimposed on this pattern are the effects of vegetation and valley morphology on sediment transport sediment thickness is seen to be uniform among sandbar willows and highly variable within tamarisk groves these reach scale patterns of sedimentation observed in the lidar differencing likely reflect complex interactions of vegetation flow and sediment at the scale of patches to individual plants

the gravel sand transition along river channels abstract this paper discusses a hitherto neglected aspect of fluvial downstream fining the transition from a gravel bed to a sand bed from the few known examples and by additional study of new sites some general features of such transitions are given these transitions are generally rapid and involve a change in bed texture from unimodal gravels through bimodal sediments to exclusively sand material and are often associated with a break in slope we propose three possible causes local base level control an excess of sand supply and abrasion breakdown of fine gravel we further argue that such transitions occur in a variety of fluvial environments and are geographically more widespread than previously supposed and on this basis we suggest they may also aid in interpreting ancient sediment showing bimodality and rapid proximal to distal changes in grain size

controls on the geometry of potholes in bedrock channels potholes circular depressions carved into bedrock are the dominant roughness elements in many bedrock channels here we show using data from previous studies and new data from the smith river oregon that pothole depths increase in proportion to both the mean pothole radius such that the most common pothole depth to radius ratio is 2 and the diameter of the largest clasts episodically stored in potholes we present a theory for these observations based on computational fluid dynamics and sediment transport modeling of vortices in cylindrical cavities of different shapes and sizes we show that the shear stress at the bottom of a pothole which controls the rate of pothole growth is maximized for potholes with a depth to radius ratio of approximately 1 and decreases nonlinearly with increasing depth to radius ratio such that potholes with depth to radius ratios larger than 3 are uncommon our model provides a mechanistic explanation for pothole shapes and sizes

new estimation of nile delta subsidence rates from insar and gps analysis increase of water level in most of the world s oceans and seas including the mediterranean due to global warming may have a serious impact on the nile delta the nile delta is considered as one of the most important regions in egypt as more than 50 of the egyptians are living there to study the thread of a potentially sinking and flooding of the nile delta here we estimate present vertical motion by use of the persistent scatterer insar psi analysis in addition to the available gps stations in the nile delta the subsidence rate of the nile delta is estimated by applying the psi processing for all archived envisat sar data from the descending tracks 207 and 436 from these tracks 36 and 37 sar scenes were collected respectively moreover the results of a time series analysis for six gps permanent stations in the nile delta were used to validate the estimated subsidence rates from psi analysis the psi and gps solutions show that the estimated ground deformation is strongly localized at the big cities from time series for selected ps pixels from cairo tanta mahala mansoura damietta and port said the estimated subsidence rates from envisat scenes are around 6 4 0 4 mm year 4 0 0 6 mm year 4 8 1 0 mm year 10 0 1 2 mm year 10 3 1 6 mm year and 4 9 1 6 mm year respectively the localization of subsidence rates at big cities implies that the detected signals may be due to human activities such as ground water pumping and that the delta as a whole presently is not subsiding

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

paleoslope reconstruction in sandy suspended load dominant rivers constraining the depositional gradient of ancient alluvial river systems can aid in reconstructing landscapes and estimating paleodischarge by establishing boundaries on the climatic and tectonic history of continental sequences we present three methods for estimating ancient depositional gradients based on the interpreted mode of sediment transport for the range of particle sizes found on the beds of modern rivers or preserved in channel fills for sandy rivers with suspension as the dominant mode of sediment transport these methods take advantage of observations that can be directly obtained from preserved strata including measurements of paleo flow depth and grain size the first method relates river slope to shields number at bankfull flow the second method is similar to the first but allows for variation in shields number at bankfull flow with grain diameter the final method relies on criteria required for both suspended and bed material load sediment transport in the same system and is the most comprehensive and physically justified method we present each method and test them with modern river datasets to verify accuracy and help constrain uncertainty the first step to adapting them to ancient systems results indicate that all methods estimate slope within a factor of two these methods are potentially very powerful for interpreting sandy fluvial deposits because they can provide reasonably accurate quantitative estimates of paleoslope an elusive yet important environmental variable

terminal settling velocity of commonly occurring sand grains published measurements of terminal settling velocity for commonly occurring sands are used to develop three equations which join into a single segmented curve of dimensionless form results are noticeably different from those for spheres of similar diameter and permit calculation of the settling velocity for usual sand grains without specification of exact grain shape for quartz in water the three equations of different settling regimes correspond approximately to very fine sand fine to coarse sand and very coarse sand