settling velocity characteristics of sediment detached from a soil surface by raindrop impact summary the aim of the study was to ascertain whether the detachment of sediment from a soil surface by raindrop impact is a selective or aselective process relationships between particle size and settling velocity were established two contrasting soil types with a mean surface water depth of 5 mm were subjected to simulated rainfall at a rate of 56 mm h 1 in modified splash cups mean drop diameter was 2 24 mm the two soil types were a cracking clay black earth or vertisol and a slightly dispersive sandy clay loam solonchak or aridisol for both soil types sediment that was ejected into the air from drop impact sites had a selective preference for removing sediment with settling velocities corresponding to fine sand sized primary particles and fine to coarse sand sized aggregates investigations of the settling velocity distributions of detached sediments sampled from the surface water before ejection or deposition could occur suggested that within experimental uncertainties the impact detachment process is aselective with respect to sediment size or settling velocity for the drop diameter and water depth investigated

identification of multiple reflected phases from migration receiver function profile an example for the indepth iii passive teleseismic p waveform data 1 the crustal multiple reflected phases usually interfere with those converted phases from the topmost mantle discontinuities which make them difficult to be identified and should be carefully interpreted in a migration receiver functions profile we propose a simple scheme to identify the crustal multiples in a migration receiver functions profile the scheme is based on the differences in both geometric and dynamic features resolved using avp amplitude variation verse ray parameter and dvp discontinuity depth variation verse ray parameter synthetic data shows that the crustal multiples can be distinguished from those converted phases in the receiver function migration profile especially when the receiver functions are migrated using velocities lower than the actual velocities as an example we apply our procedure to the indepth iii passive teleseismic p waveform data our results indicate that a possible converted phase from the 220 km discontinuity is a crustal multiple reflected phase

structural analysis of a sampling of tesserae implications for venus geodynamics abstract understanding the nature and timing of tessera formation is fundamental to venus tectonic and geodynamic models tesserae are commonly considered to exhibit complex deformation histories to represent the oldest global stratigraphic unit to have formed during a global phase of tesserization and to require weak lithosphere for their formation although these characterizations of tesserae are gaining widespread mention in the literature they are essentially unsupported by an assemblage of data yet they figure prominently in the way we frame and address questions about venus tectonics thus they are quickly becoming accepted paradigms despite a lack of scientific foundation open discussion of these hypotheses or assumptions is crucial because of their implications concerning geodynamic models of the surface evolution and planetary dynamics of venus we examined tessera terrain in ishtar terra crustal plateaus and as inliers within the plains using high resolution magellan radar imagery we describe several types of tessera terrain that record a wide range of structural histories fold and s c terrains are found only in ishtar terra ribbon lava flow and basin and dome terrains reside within the interior of crustal plateaus whereas folded ribbon terrain and extended folded terrain comprise margins of crustal plateaus inliers are divisible into fracture dominated and graben dominated tesserae although some inliers host early contractional fabrics the range of deformation histories recorded by the various tessera types indicates that tesserae should not be considered a single map unit tessera deformation records local to regional surface strain patterns and reflects near surface rheology at the time of deformation progressive deformation fabrics in some tessera terrain record changes in shallow crustal rheology through time thus structural analysis of individual tessera types and tessera provinces will allow us to better understand the tectonic processes responsible for tessera formation tesserae likely formed in several tectonic environments including 1 as a result of subsurface flow in ishtar terra 2 as sequences of surface layer extension and contraction in crustal plateaus 3 as flooded crustal plateaus and 4 as densely fractured surface layers fractured as a result of corona and chasma formation

hindered settling of sand grains the sedimentation rate of sand grains in the hindered settling regime has been considered to assess particle shape effects the behaviour of various particulate systems involving sand has been compared with the widely used richardson zaki expression the general form of the expression is found to hold in as much as remaining as a suitable means to describe the hindered settling of irregular particles the sedimentation exponent n in the richardson zaki expression is found to be significantly larger for natural sand grains than for regular particles the hindered settling effect is therefore greater leading to lower concentration gradients than expected the effect becomes more pronounced with increasing particle irregularity at concentrations around 0 4 the hindered settling velocity of fine and medium natural sands reduces to about 70 of the value predicted using existing empirical expressions for n using appropriate expressions for the fluidization velocity and the clear water settling velocity a simple method is discussed to evaluate the sedimentation exponent and to determine the hindered settling effect for sands of various shapes

impact craters and venus resurfacing history venusian impact crater size frequency distributions locations and preservation states were analyzed to reconstruct the history of resurfacing by tectonism and volcanism an atmospheric transit model for meteoroids demonstrates that for craters larger than about 30 km the size frequency distribution is close to the atmosphere free case with this result and assuming that the surface records a crater production population a catastrophic resurfacing model crm an age of cessation of rapid resurfacing of 500 ma is obtained crater locations are widely dispersed across venus and the hypothesis that they are completely spatially random csr cannot be rejected however craters that show embayment by plains materials or modification by throughgoing faults i e tectonized are preferentially found in areas with relatively few craters overall the primary region where these modified craters are found is the aphrodite volcanotectonic zone extending from ovda regio on the west to the region east of atla regio these results together with the appearance of plains material on most crater floors and evidence for complex volcanic stratigraphy imply that a range of surface ages are recorded by the impact crater population e g the aphrodite zone is relatively young an end member model equilibrium resurfacing model erm was developed to quantify resurfacing scenarios in the erm venus has been resurfacing at an average rate of approximately 1 km2 yr 1 however the crm and erm are idealized end member representations of possible resurfacing histories for both models the resurfacing rate can be expressed as the product of resurfacing patch area a normalized by planetary surface area and the frequency of resurfacing events numerical simulations of resurfacing showed that there are two solution branches that satisfy the csr constraint a 0 1 74 diameter circle the former range corresponds to resurfacing diameters smaller than the average intercrater distance whereas the latter is associated with large infrequent events resurfacing 10 of the planet every 50 ma to 100 every 500 ma the observed fraction of embayed and tectonized craters further constrains values of a and only values near 0 0003 are admissible the resurfacing model that best fits all of the statistical and geological constraints has resurfacing with small patches that occurs in any given geological episode in only a limited number of regions on the planet

measurements and analysis of setting velocities of natural quartz sand grains abstract settling velocities in water were measured for natural quartz sand grains in the sieve size range 0 75 to 1 50 0 35 1 68 mm obtained from beach sand samples the grains were selected so as to provide a wide range of roundness from very angular to very well rounded r 0 6 5 1 in a first set of experiments only two axial diameters were measured under a microscope the longest axial diameter d1 and the intermediate diameter di in a second data set the smallest axial diameter ds was measured as well settling velocities were measured in a 6 meter long settling tube it is found that the intermediate grain diameter is on average equal to the nominal diameter computed as dn dsdid1 1 3 this permits a simpler analysis of grain settling through microscopic measurements of di alone but does not allow for corrections of grain shape effects the measured settling velocity wm is then compared with the settling velocity of a sphere ws calculated using di as the sphere diameter and the relationship wm 0 977w0 913s is found to be consistent with the data an alternative approach is provided where ws is calculated directly from the sieve diameter and then corrected to the actual settling velocity wm for that mean grain size the results thus provide graphs and empirical relationships which permit the evaluation of actual grain settling rates rather than settling velocities of spheres from the sieve diameter or from microscopic measurements of di for individual grains these procedures provide a mean settling velocity that takes into consideration the average degree of grain irregularities found in natural sands a more detailed analysis is then conducted to examine the effects of roundness and sphericity on the settling of individual grains it is found that grain roundness has no measurable effect on the settling rate the settling velocity is shown to be a function of grain sphericity either expressed as the corey shape factor or e shape factor grain asymmetries ust also be important accounting for much of the scatter in the results

modeling overland flow and soil erosion on nonuniform hillslopes a finite volume scheme 1 this paper presents a finite volume scheme for coupling the saint venant equations with the multiparticle size class hairsine rose soil erosion model a well balanced monotone upstream centered schemes for conservation laws hancock muscl hancock method is proposed to minimize spurious waves in the solution arising from an imbalance between the flux gradient and the source terms in the momentum equation additional criteria for numerical stability when dealing with very shallow flows and wet dry fronts are highlighted numerical tests show that the scheme performs well in terms of accuracy and robustness for both the water and sediment transport equations the proposed scheme facilitates the application of the hairsine rose model to complex scenarios of soil erosion with concurrent interacting erosion processes over a nonuniform topography

effect of raindrop splash and transversal width on soil erosion laboratory flume experiments and analysis with the hairsine rose model the parameter consistency of the one dimensional hairsine rose h r erosion model under conditions of significant rainfall splash was examined to account for the splash characteristic length scale and its interaction with the transverse erosion width experiments were carried out using erosion flumes of the same length 6 m but different widths with sediment concentrations measured at the flume exits total sediment concentration and the concentration of seven size fractions 1000 m were measured at high rainfall intensity 60 mm h and with a gentle slope 2 2 the conditions employed ensured that erosion was predominantly precipitation driven the experimental results showed that raindrop splash affected particularly the sediment breakthrough from the wider flumes flumes 1 and 2 1 and 0 5 m wide respectively however the raindrop splash effect was less significant in observed sediment concentrations from the narrower flumes flumes 3 and 4 both 0 25 m wide for these flumes the detached sediment was affected by the transversal width of the flume in that an amount of detached sediment adhered to the barriers instead of being removed in the overland flow the one dimensional h r model was fitted to the experimental results and good agreement was found in particular for the finer size classes the data for the coarser grain sizes were more scattered suggesting sediment motion by mechanisms other than as a suspension in the overland flow e g rolling along the soil surface the optimized parameters indicated that the shield layers where the shield consists of redeposited eroded sediment of the wider flumes 1 and 2 developed within 5 10 min from the start of the experiment whereas in the narrower flumes 3 and 4 they never fully developed the optimized detachment rates were consistent with previous findings but the estimated thickness of the deposited layer was too small to provide complete protection of the original soil against raindrop detachment indicating that the shield was not uniform the experimental design allowed us to investigate directly the effect of flow non uniformity on soil erosion by inclusion of an offset drainage point in flume 4 the observations taken during and after the experiment as well as surface elevation data confirmed the noticeable impact of non uniform flow on the erosion process

splash and wash dynamics an experimental investigation using an oxisol abstract limited data are available in the literature on the partitioning of interrill erosion into splash and wash components laboratory rainfall simulation experiments were conducted on an oxisol at 5 10 and 20 slopes at a constant rainfall intensity rainwash was separated from front splash and lateral splash using a novel experimental design results indicate that output of sediment from the 0 18 m 2 erosion plots was dominated by splash with a geometric mean aggregate diameter gmad similar to that of the original soil the most easily detached aggregate size by splash was 500 to 1000 m i e coarse sand sized aggregates wash was dominated by aggregates with a gmad significantly finer than the soil matrix and this reflected the limited energy available to transport coarse aggregates detached by splash wash and splash sediment flux and the runoff coefficient increased significantly with slope angle peak front splash was associated with a different dimensionless water depth dwd for each slope angle as slope angle increased the dwd at peak output increased from 0 to 0 4 at 5 and from 1 4 to 1 7 at 20 time trends of front splash output for 5 and 10 slopes were similar to those presented in the literature however data for the 20 slopes were more complicated with a series of pulses related to periodic migrating waves of wash sediment superimposed on the general pattern these migrating bedload waves are interesting in that they seem analogous to those documented in the fluvial environment at steady or even decreasing discharge data indicate that front splash flux exceeded wash transport on all slopes tested these data reflect the boundary conditions of the experiment but may be applicable under certain field situations such as on short steep furrow side slopes

dating volcanism and rifting on venus using impact crater densities although the distribution of impact craters on venus is indistinguishable from a completely spatially random population on the basis of crater data alone the addition of geologic information indicates that the craters are not random with respect to geology areas of low crater density correlate with concentrations of tectonic and volcanic features as well as high proportions of faulted and embayed craters high resolution mapping of large volcanoes flood type lava flow fields rifts and coronae shows that these features have low crater densities that are unlikely to have occurred merely by chance because these features also appear young based on stratigraphic data difference in age is the most likely reason for differences in crater density between geologically defined terrains crater densities may be used to determine reliable relative ages for sufficiently large geologic terrains containing at least eight to nine craters as long as the mapping criteria are independent of impact crater locations this result provides a tool to begin identifying and dating major geologic provinces and to begin developing a geologic history of venus using a simple data model and calculating crater ages relative to an assumed global mean age of 300 ma we show that the ages of large volcanoes 72 45 ma flow fields 128 91 ma rifts 130 145 ma and coronae 120 115 ma are all substantially younger than the mean plains age and probably represent ongoing volcanic and tectonic activity rather than the end of a brief global resurfacing event with a mean age of 300 ma

a new view of the stratigraphic history of venus abstract studies of venus using magellan data have so far generated two views about the way venus has evolved on the one hand venus has been suggested to have had a history in which there was a series of epochs each represented by a different volcanic or tectonic process on a global scale basilevsky and head 1995 j w head et al 1996 lunar planet sci 27th 525 526 p t basilevsky et al 1997 in venus ii univ of arizona press tucson this we define as a directional history on the other hand there is evidence to suggest that coronae rifts wrinkle ridges small and large edifices and large flow fields have each formed throughout the portion of venus history revealed by presently exposed rock units we propose that the plains have been built up by lavas erupted in a number of different styles each occurring throughout the history represented by the exposed stratigraphy of the planet dates derived from crater counts accumulated from the combined area of specific types of feature such as coronae e g m h price et al 1996 j geophys res 101 4657 4672 must be interpreted with care as the method is based upon the assumption that features of like morphology have the same age detailed studies from full resolution magellan data indicate that venus has had a complex history in which most geologic processes have operated in a nondirectional fashion to a greater or lesser extent throughout the planet s history

size fractions resulting from an aggregate stability test interrill detachment and transport aggregate stability is a soil characteristic often linked with soil erodibility but this relationship is generally empirical the aggregate size distribution is a recognized factor in uencing crust characteristics and transport during interrill erosion events this parameter was used in this work in order to improve the understanding of the relationship between aggregate stability and erodibility a second aspect of this study was to investigate the size selectivity of interrill erosion processes i e breakdown detachment by splash and transport by overland ow this experimental work was based on rainfall simulations and aggregate stability tests which were performed using two different soils a clay loam slightly sensitive to erosion and a more susceptible silt loam size distributions of soil fragments either produced by aggregate stability tests or detached by splash or transported by runoff were measured by both wet sieving and laser diffraction sizer analysis the clay loam is particularly stable and has coarse breakdown products with mean weight diameters larger than 1400 m aggregate breakdown is more ef cient for the silt loam and leads to ner soil fragments with mean weight diameters between 229 and 380 mm for the more stable soil the clay loam splash rate and sediment concentration are respectively two and six times lower and less time variable than for the silt loam for the two soils detachment by splash and short distance transport are size selective interrill erosion processes this selectivity leads to the enrichment of silt and clay fractions of the eroded aggregates in comparison with the breakdown products the low erodibility of the more stable soil is in accordance with results of aggregate stability tests copyright 2004 john wiley sons ltd

geological evolution of venus rises plains plumes and plateaus crustal plateaus and volcanic rises major physiographic features on venus both formed over mantle plumes crustal plateaus were produced by large degrees of plume melting beneath thin lithosphere the oldest tectonic features in crustal plateaus are ribbon like troughs indicating early uplift and tensile stretching their shallow depths suggest that surface temperature there was higher in the past widespread volcanic plains derived from the broad upwellings of internally heated mantle convection were continually erupted during the time of crustal plateau formation discrete volcanic rises younger than crustal plateaus formed over a thicker lithosphere with less plume partial melting the presence of only one transitional form indicates that the lithosphere thickened rapidly thermal and magmatic models show that the formation of these major features can be tied to an expected change in mantle convective style about 1 billion years ago

surface formation rates and impact crater densities on venus impact crater density has been used to estimate relative ages of major venus geologic units typically based on the assumption of short globally synchronous formation periods the total number density of craters is the only parameter which may be quantified for most venus subregions in contrast to the size frequency analysis often used for other planetary studies this paper examines the statistical limits on such crater counts and the predicted crater density for various possible time varying surface formation rate functions these results show that 1 the true mean crater density is poorly constrained due to the limited sample size provided by aggregate geologic units and 2 any given crater density can be accommodated by a very short pulse of unit formation at some age t gaussian distributed formation functions of mean age t exponential functions with initiation ages between t and 2t or a host of other more complicated rate functions these results when coupled with the unknown rate of crater removal by tectonic or volcanic activity indicate that the crater counts for specific landforms on venus offer little constraint on the relative timing or duration of their development the hypothesis of globally synchronous formation of landforms such as volcanoes or coronae cannot be validated with the available crater data

comment on impact craters of venus a continuation of the analysis of data from the venera 15 and 16 spacecraft by a t basilevsky et al

impact craters on regional plains on venus age relations with wrinkle ridges and implications for the geological evolution of venus 1 a global assessment of impact craters on venus 30 km in diameter shows that 146 craters are completely or partially superposed on the regional plains that dominate the surface and typically are deformed by wrinkle ridges in this study we found that 10 of these craters are deformed by wrinkle ridges pre wr 93 craters are superposed on wrinkle ridges post wr and 43 craters were found to be ambiguous cases we compared our findings with the results of a similar study by mcgill 2004 done for craters of all sizes but for 1 3 of the surface of venus we found that the percentage of pre wr craters found by mcgill 2004 among the unambiguous cases 15 is close to our percentage 10 but we are in disagreement on the classification of a significant number of specific craters and in this analysis we describe the major reason for the disagreements we then summarize published data on the crater densities for different plains on venus we assess monte carlo modeling of volcanic resurfacing which competed with the ongoing formation of impact craters on the basis of these data and analyses we conclude that the mean age of the regional plains is close to the mean global surface age of venus and that the potential time span of regional plains emplacement was relatively short we interpret our results as evidence that wrinkle ridging on venus was a distinct tectonic phase that occurred rather early in post regional plains time probably within the first 10 although the exact duration remains uncertain regional and global stratigraphic relationships strongly support the hypothesis that wrinkle ridging on venus predominantly occurred during a specific phase of history the geologic record of wrinkle ridging and post plains tectonic evolution of volcanic plains on the moon and mars show interesting similarities to those of venus

radar properties as clues to relative ages of ridge belts and plains on venus 1 an ongoing issue regarding the crustal evolution of venus is whether or not certain features and processes occurred in the same sequence and at the same time globally this paper seeks to test one aspect of directional models for the geologic history the inference that all ridge belts are older than regional plains this test involves traditional geologic analysis but also involves the use of radar properties in a unique way we use a simplified model for the ridge shape and an estimate of the diffuse scattering behavior of the venus surface to determine the true backscatter coefficient of the slope of the ridge tilted away from the radar and ratio this value to the echoes from nearby plains units in general ridge forming terrain in these belts appears to have greater inferred backscatter coefficients and thus decimeter scale roughness than the regional plains plains units closely adjacent to many ridge belts have higher backscatter than plains at some distance which we infer is generally due to aeolian deposition of debris characterized by centimeter scale microdunes or ripples where the belt topography interferes with wind patterns the rougher apparent surface texture within the belt forming terrain is not consistent with the gentle folding that forms the ridges and so is likely a result of weathering or tectonic processes operating prior to belt deformation we thus conclude that the material incorporated in the ridge belts we have studied predates the surrounding regional plains

equilibrium resurfacing of venus results from new monte carlo modeling and implications for venus surface histories abstract venus impact crater population imposes two observational constraints that must be met by possible model surface histories 1 near random spatial distribution of 975 craters and 2 few obviously modified impact craters catastrophic resurfacing obviously meets these constraints but equilibrium resurfacing histories require a balance between crater distribution and modification to be viable equilibrium resurfacing scenarios with small incremental resurfacing areas meet constraint 1 but not 2 whereas those with large incremental resurfacing areas meet constraint 2 but not 1 results of monte carlo modeling of equilibrium resurfacing strom et al 1994 is widely cited as support for catastrophic resurfacing hypotheses and as evidence against hypotheses of equilibrium resurfacing however the monte carlo models did not consider intermediate size incremental resurfacing areas nor did they consider histories in which the era of impact crater formation outlasts an era of equilibrium resurfacing we construct three suites of monte carlo experiments that examine incremental resurfacing areas not previously considered 5 1 0 7 and 0 1 and that vary the duration of resurfacing relative to impact crater formation time 1 1 suite a 5 6 suite b and 2 3 suite c we test the model results against the two impact crater constraints several experiments met both constraints the shorter the time period of equilibrium resurfacing or the longer the time of crater formation following the cessation of equilibrium resurfacing the larger the possible areas of incremental resurfacing that satisfy both constraints equilibrium resurfacing is statistically viable for suite a at 0 1 suite b at 0 1 and suite c for 1 0 7 and 0 1 areas of incremental resurfacing

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

new crater calibrations for the lunar crater age chronology abstract measuring the spatial density of craters emplaced on geologic units is the primary method used for remotely estimating ages of solar system surfaces the calibration for this method which enables conversion of crater density to absolute age comes from apollo and luna lunar samples for which absolute radiometric ages have been determined researchers throughout the 1970s worked to establish the lunar chronology system based on that calibration correlating crater densities with absolute ages however no uniform crater study has been conducted on all calibration terrains a limitation that was previously unaddressed until this study the latest lunar images from the lunar reconnaissance orbiter were used here to re map the eleven main sampling sites and new crater counts of those surfaces were conducted these show significant differences for many sites crater counts in many cases having more craters than previously identified these results calibrated to the radiometric ages show a revised lunar crater chronology that changes previously established crater based ages by up about 1 billion years surfaces younger than 3 6 gyr and older than 3 9 gyr under the classic chronology are younger in this system and those in between are older in this new system

erosion induced exposure of soc to mineralization in aggregated sediment during slope scale erosion events re distribution of eroded soil and the associated soil organic carbon soc is not always uniform but very often affected by preferential transport and deposition under given flow conditions the site of soc deposition depends on the transport distances of sediment particles containing the soc very often soil and soc erosion risk are assessed by applying mineral particle specific soc distributions to erosion models however soil is not always eroded as individual mineral particles but mostly in a form of aggregates aggregates are likely to increase settling velocities of individual mineral particles which may considerably reduce the transport distance of sediment fractions and the associated soc skewing soc redistribution and its subsequent fate yet little is known about the potential effects of aggregation on the movement and fate of eroded soc to assess the effect of preferential deposition a simulated rainfall was applied to two soils in this study with the movelier silty clay having greater soc content and aggregate stability than the mohlin silty loam the eroded sediments of the two soils were fractionated by a settling tube apparatus according to their potential transport distances the co2 emissions of the fractionated and incubated sediments were then measured for 50 days enabling the assessment of the bioactivity of eroded soc for weeks after deposition our results show that 1 the re deposition of eroded soc into terrestrial systems increased by 64 if considering the actual aggregate specific rather than the mineral particle specific soc distribution 2 the co2 emission rates differed across settling fractions with the most pronounced rates in the finest fractions from the mohlin silty loam sediment and in the medium size fractions from the movelier silty clay sediment 3 over 50 day incubation the co2 emissions from the mohlin silty loam sediment was 114 greater than that from the non eroded mohlin soil whereas co2 emissions were roughly equivalent for the movelier sediment and non eroded movelier silty clay these data demonstrate that erosion and preferential deposition of soc enriched aggregates can enhance terrestrial soc deposition this can further result in greater co2 efflux than commonly applied mineral particle size specific soc distribution would suggest the different performances of deposition and co2 emissions between the two soils also suggest that these effects can vary with soil type our observations certainly illustrate that sediment aggregation requires further investigation to assess the redistribution and subsequent fate of eroded soc appropriately

evaluating a multiclass net deposition equation in overland flow conditions 1 the movement of sediment and related pollutants in overland flow from the point of erosion to a stream normally occurs through a series of deposition zones here we present the evaluation of a new multiclass model of sediment transport through these zones this model simulates size selective net deposition as a result of simultaneous deposition by settling and nonselective reentrainment of previously deposited sediment hairsine et al 2002 sander et al 2002 multiclass sediment deposition models provide a useful basis for simulating off site effects of soil erosion but information on the grain size composition of deposited and exported sediment is crucial for calibrating and evaluating such models therefore the multiclass sediment deposition model was tested using detailed experimental data information on the amount of sediment deposition on the grain size distribution of the deposited and exported sediment and on the spatial pattern of the deposits was collected during sediment deposition experiments calibration of the multiclass model using one part of these data was performed by adjusting the fraction of stream power available for sediment reentrainment by minimizing errors between both simulated and observed total outflow sediment concentration and grain size distribution of the exported sediment evaluation was conducted using the other part of the data over a wide range of conditions the evaluation shows that the deposition equation is capable of reasonably simulating the observed size selectivity of the deposition process however the model is not capable of simulating correctly the spatial pattern of the deposited sediment because of simplifications used in the hydraulic model

settling velocity of sediments at high concentrations new data on the settling velocity of artificial sediments and natural sands at high concentrations are presented the data are compared with a widely used semiempirical richardson and zaki equation trans inst chem eng 32 1954 35 which gives an accurate measure of the reduction in velocity as a function of concentration and an experimentally determined empirical power n here a simple method of determining n is presented using standard equations for the clear water settling velocity and the seepage flow within fixed sediment beds the resulting values for n are compared against values derived from new and existing laboratory data for beach and filter sands for sands the appropriate values of n are found to differ significantly from those suggested by richardson and zaki for spheres and are typically larger corresponding to a greater reduction in settling velocity at high concentrations for fine and medium sands at concentrations of order 0 4 the hindered settling velocity reduces to about 70 of that expected using values of n derived for spheres at concentrations of order 0 15 the hindered settling velocity reduces to less than half of the settling velocity in clear water these reduced settling velocities have important implications for sediment transport modelling close to and within sheet flow layers and in the swash zone

influence of rock fragment coverage on soil erosion and hydrological response laboratory flume experiments and modeling two laboratory flume experiments on the effect of surface rock fragments on precipitation driven soil erosion yields were carried out the total sediment concentration the concentration of seven individual size classes and the flow discharge were measured digital terrain models dtms were generated before and after one of the experiments the results revealed that the rock fragments protected the soils from raindrop detachment and retarded the overland flow therefore decreasing its sediment transport capacity rock fragments were found to affect selectively the different size classes in a manner that changed according to the time scale for short times the rock fragment coverage reduced erosion of the finer particles 100 m was unaffected at long times the rock fragment cover decreased the concentration of the individual size classes in proportion to effective rainfall intensity and the area exposed to raindrops an area based modification of the hairsine and rose h r soil erosion model was employed to analyze the experimental data the h r model predictions agreed well with the measured sediment concentrations when high rainfall intensity and low rock fragment cover were used predictions were instead less accurate with low rainfall intensity and high rock fragment cover the dtm results showed that the presence of rock fragments on the soil surface led to increased soil compaction perhaps due to higher soil moisture content from greater infiltration within the rock fragment covered flumes

influence of sediment settling velocity on mechanistic soil erosion modeling we report on a series of soil erosion experiments performed on the 2 m x 6 m epfl erosion flume total sediment concentrations and the concentrations of seven size fractions 1000 mm were measured during 14 high intensity 47 5 52 5 mm h rainfall experiments on three different slopes 2 2 12 4 the short time and long time analytical solutions for the hairsine rose erosion model were rewritten to account for infiltration the newly collected data were used to test the model under conditions that had not been explored before steeper slopes infiltration more realistic soil composition the analytical solutions could predict the observed total sediment concentration well however the observed sediment concentrations for the individual size classes could be predicted only when adjusted settling velocities were used the adjusted settling velocities were estimated through manual optimization the optimized settling velocities for the smallest and midsize particles 315 mm were smaller than measured the effective settling velocities could also be calculated from the calculated amount of material in the shield of each size class at the end of the experiments this calculated settling velocity distribution agreed very well with the optimized settling velocities this study moves the hairsine rose model another step closer to an operational soil erosion model for field applications

exact solutions of the hairsine rose precipitation driven erosion model for a uniform grain sized soil hairsine and rose developed a mechanistic one dimensional precipitation driven erosion model that since its appearance has been validated by several sets of experimental results the model allows any sediment particle to be present in one of three zones viz the flow zone the deposited layer or the original soil the model has the general form of a two region model in which advection is the only transport process for the special case of a soil composed of a single particle size and for overland flow that occurs at a steady rate and with a uniform depth it is possible to derive fully explicit analytical solutions to the model details of the solutions for a slightly generalized mathematical form of the model are provided the goldstein j function which appears commonly in two region model solutions was modified to accommodate some of the solutions presented the form of the model analyzed indicated that based only on sediment concentrations in runoff water it is not possible to distinguish one mechanistic feature of the hairsine rose model i e that raindrop induced detachment of the undisturbed soil moves directly into the flowing water from the point of view of the model it is equally plausible for raindrop impact to move sediment directly into the deposited layer

soil loss by splash and wash during rainfall from two loess soils abstract physical processes occurring during surface seal formation through a rainstorm are well understood but limited information is available regarding the quantity and particle size distribution of splash and runoff at certain time intervals in this study we evaluated the quantity and particle size distribution of suspensions of both splash and interrill runoff in two loess soils with different mineralogy and aggregate stability and somewhat different particle size distribution but similar organic matter content the soils were subjected to simulated rainstorms of 40 turn h 1 and 100 mm h 1 intensities the amount of splash was about four times higher for the saskatchewan soil typic haploboroll with high smectite than for the grenada soil typic fragiudalf which is rich in fe oxyhydroxides the amount of splashed material and sediment load increased with increased rainfall intensity for both soils splash was decreased after wetting of the soil surface the decrease in splash rate was more rapid with high rainfall intensity the amount of clay size particles of the splash was similar to the original soil material micromorphological observations confirmed the fluctuations in clay content with time at the very surface soil materials splashed were much higher 10 to 20 times than the interrill runoff losses the latter was controlled by the rainfall intensity the soil material from saskatchewan produced more than 11 mg ha 1 of interrill runoff with low rainfall intensity high rainfall intensity produced 10 times more soil loss than low rainfall intensity high amount of soil loss clearly shows that the saskatchewan soil would benefit from erosion control measures both rainfall intensities removed preferentially more clay from the saskatchewan b horizon material this has important agronomic and environmental implications for this soil interrill soil losses from the granada a horizon material were much less with no clear evidence of preferential removal of clay size particles

venus crater distribution and plains resurfacing models detailed analysis of the distribution of craters on venus using mth nearest neighbor analysis coupled with models based upon surface morphology constraints indicates that the hypothesis of complete spatial randomness csr cannot be rejected but is not a unique model of the observed crater distribution based on morphologic mapping the extensive volcanic plains can be divided into four units that have a spread in age of the order of 0 5t the mean surface age of the planet this four unit plains model along with its derivatives produce test statistics that indicate such models also cannot be rejected further the probability of obtaining a result at least as extreme as the observed test statistic given that the null hypothesis model corresponds to venus is true is lowest for the csr model there is no particular reason to pick a csr model along with its implications for catastrophic resurfacing as a constraint on the evolution of venus and there are geological reasons to choose the multiage models we find that we cannot distinguish statistically among models that have two three or four distinct production ages within the plains however the hypothesis that the variation in crater density within all of the plains is due to a single random process can be rejected for two reasons first the binomial probability that such a process could exist within each of the plains units is 0 05 except the smallest and youngest unit pl1 second using a chi squared statistic to test the hypothesis that four plains units have the same age gives a p value of 10 4 indicating confident rejection of the hypothesis thus csr cannot be used as a constraint on models of resurfacing or planetary evolution of venus because of the non uniqueness in matching such a model to the observed crater distribution and the strong indication of distinct ages within the plains with a significant spread in age geological and geophysical constraints provide our best clues for understanding venus

impact crater densities on volcanoes and coronae on venus implications for volcanic resurfacing the density of impact craters on large volcanoes on venus is half the average crater density for the planet the crater density on some classes of coronae is not significantly different from the global average density but coronae with extensive associated volcanic deposits have lower crater densities these results are inconsistent with both single age and steady state models for global resurfacing and suggest that volcanoes and coronae with associated volcanism have been active on venus over the last 500 million years

a model for the formation of wrinkle ridges in volcanic plains on venus abstract ridged plains the most abundant geologic terrain on venus are volcanic plains deformed by wrinkle ridges after their emplacement it has been suggested that the ridges are the product of thermal stresses induced in the lithosphere by the increase of surface temperature due to the greenhouse effect of gases released during the emplacement of the volcanic plains a model for the formation of wrinkle ridges is proposed where the ridges are assumed to be the surface effect of dislocations produced by compressive stress in the venusian lithosphere the lithosphere is modeled as a thermoelastic half space the surface of which is subject to a linear temperature increase during 100 ma the state of stress is calculated and the maximum distance is obtained at which dislocations can be placed in order that the compressive stress at the venusian surface is everywhere lower than the rock strength the model predictions in terms of dislocation density and crustal shortening appear to be consistent with observations

the geologic history of venus a stratigraphic view on the basis of regional and global stratigraphic analyses we outline the major events in the geologic history of venus determined by photogeological study of surface features because the morphological signatures of terrain emplaced prior to the time of tessera formation are not preserved the stratigraphic record presented comprises only the last 10 20 of the total history of venus the estimated range of the mean crater retention age of the surface from 200 to 1600 million years leads us to describe the timing and duration of different events in terms of fractions of the mean surface age t the beginning of the observed history of venus was characterized by intensive tectonic deformation of global or semi global scale which formed the tessera terrain termination of the compressional stage is estimated to have occurred at about 1 4t while the tensional stage lasted for another 0 1 0 2t after tessera formation several stages of extensive volcanism occurred burying vast areas of tessera and forming what are now observed as regional plains the combined duration of the emplacement of these plains is estimated to be about 0 2 0 3t with an implied average global rate of volcanism of a few cubic kilometers per year regional plains forming materials can be subdivided and are separated from each other and from underlying and overlying units by unconformities these unconformities are formed from oldest to youngest by tessera forming deformation dense fracturing broad ridging and finally wrinkle ridging these tectonic episodes are interpreted to be generally globally synchronous and to represent successive episodes characterized by the dominance of compression then tension then again compression and finally tension the last global scale tectonic episode extensive wrinkle ridging happened at about time t which was very close in time to the emplacement of the most areally abundant plains unit this marked the transition to the present stage of the history of venus which is characterized by a predominance of regional rifting and related volcanism this stage appears to have lasted from about time t to the present making it the longest time duration among the stratigraphic units considered although the resulting tectonic and volcanic features and deposits cover only 10 20 of the surface of venus these observations mean that the general intensity of tectonics and the flux of volcanism a few tenths of a cubic kilometer per year in this latest period were much lower than those in earlier times in summary the morphologically observable part of the history of venus was characterized by two key characteristics that stand in contrast to the comparable period of earth history approximately the phanerozoic when global geodynamic processes were dominated by plate tectonics 1 venus shows no signature of plate tectonics instead its global tectonic environment passed from an initial dominance of compression through tension then again compression and finally tension with the density of deformational structures and the strain rate declining with time 2 in the beginning of this period of time on venus plains forming volcanism occurred at a rate comparable to volcanism at mid ocean ridges but was emplaced in an entirely different style the predominant component of volcanism on earth during this time was the extrusive volcanism at mid oceanic ridges for the last few hundred million years venus has been dominated primarily by rift associated volcanism emplaced at a production rate comparable to or even lower than present intraplate volcanism production rates on earth

geology and distribution of impact craters on venus what are they telling us magellan has revealed an ensemble of impact craters on venus that is unique in many important ways we have compiled a data base describing the 842 craters on 89 of venus surface mapped through orbit 2578 the craters range in diameter from 1 5 to 280 km we have studied the distribution size density morphology geology and associated surface properties of these craters both in the aggregate and for some craters in greater detail we find that 1 the spatial distribution of craters is highly uniform 2 the size density distribution of large craters diameters 35 km is similar to the young crater populations on other terrestrial planets but at a much lower density that indicates an average age of only about 0 5 ga based on the estimated population of venus crossing asteroids 3 unlike the case on other planets the density of small craters diameters 35 km declines rapidly with decreasing diameters because of atmospheric filtering 4 the spectrum of crater modification differs greatly from that on other planets 62 of all craters are pristine only 4 are embayed by lavas and the remainder are affected by tectonism but none are severely and progressively depleted as extrapolated from the size density distribution of larger craters 5 large craters have a progression of morphologies generally similar to those on other planets but small craters are typically irregular or multiple rather than bowl shaped 6 diffuse radar bright or dark features surround some craters and 367 similar diffuse splotches with no central crater are observed and 7 other crater features unique to venus include radar bright or dark parabolic arcs opening westward and extensive outflows originating in crater ejecta the first three of these observations are entirely unexpected we interpret them as indicating that the planet s cratering record was erased by a global resurfacing event or events the latest ending about 0 5 ga after which volcanic activity declined but did not cease entirely since the last resurfacing event a maximum of 10 of the planet has been resurfaced and only about 4 of the craters have been obliterated convective thermal evolution models support this interpretation arkani hamed and toksoz 1984 observations 3 7 confirm quantitatively the expectation that the dense atmosphere of venus has strongly affected the production of craters large impactors have been relatively unaffected intermediate sized ones have been fragmented and have produced overlapping or multiple craters a narrow size range has produced shock induced splotches but no craters and the smallest bodies have had no observable effect on the surface the number of craters eliminated by the atmospheric filter is enormous about 98 of the craters between 2 and 35 km in diameter that magellan might have observed on a hypothetical airless venus unique crater related features such as parabolas and outflow deposits demonstrate the roles of venus high atmospheric density and temperature in modifying the crater formation process finally heavily fractured craters and lava embayed craters are found to have higher than average densities along the major fracture belts and rifted uplands connecting aphrodite terra and atla beta themis and phoebe regiones these craters thus provide physical evidence for recent volcanic and tectonic activity at a low level

a conceptual model for the relationship between coronae and large scale mantle dynamics on venus 1 among the most enigmatic features on venus are the more than 500 quasi circular structures collectively referred to as coronae associated volcanism and tectonism suggest that coronae result from small transient mantle plumes although only about 20 have clearly associated positive gravity anomalies indicative of current activity coronae are concentrated within a region defined by the dynamically active highland areas of beta atla and themis regiones bat coronae avoid the lowest or highest topography and geoid subsets of the corona population indicate that the spatial distribution of coronae has evolved from a previously less concentrated global distribution to the current bat concentration recent laboratory experiments on thermal convection at high rayleigh number using fluids with temperature dependent viscosity suggest a straightforward interpretation of these relationships small scale upwellings from a hot lower thermal boundary layer tend to be excluded by longer wavelength downwelling structures from the cold upper boundary layer the relative paucity of coronae in the broad lowland plains regions is consistent with the inference of mantle downwellings beneath the plains coronae are largely excluded from compensated highlands perhaps because the thickened crust is not easily disrupted in these regions laboratory experiments indicate that small transient boundary layer instabilities can coexist with and be focused toward large scale upwellings possibly explaining the spatial concentration of coronae in the bat region thus coronae provide important insights into interacting scales of convective flow in venus s mantle and into the evolution of venus s mantle dynamics direct tests of our model are possible through geological mapping

themis regio venus evidence for recent volcanism from virtis data abstract themis regio is interpreted to be a hotspot rise underlain by one or more mantle plumes many volcanic features in the themis region have high emissivity anomalies in venus express visible and infrared thermal imaging spectrometer virtis data other high emissivity anomalies have been found to correspond to volcanic flows interpreted to be relatively recent smrekar et al 2010a science 328 605 608 similarly at themis the majority of the high emissivity anomalies closely correspond to flows associated with coronae and volcanic edifices consistent with the interpretation that they represent recent volcanism these volcanic features also have negative bouguer gravity anomalies indicating low density at depth consistent with hot mantle upwellings leading to pressure release melting in addition several volcanic features at themis have low emissivity anomalies suggestive of more evolved compositions combining geologic mapping data with gravity and emissivity data provide support that themis is an active hotspot swell with associated long lived volcanoes and coronae that are likely associated with small scale upwellings

effect of antecedent conditions and fixed rock fragment coverage on soil erosion dynamics through multiple rainfall events the effect of antecedent conditions and specific rock fragment coverage on precipitation driven soil erosion dynamics through multiple rainfall events was investigated using a pair of 6 m 1 m flumes with 2 2 slope four sequential experiments denoted e1 e2 e3 and e4 involved 2 h precipitation rates of 28 74 74 and 28 mm h 1 respectively and 22 h without rainfall were conducted in each experiment one flume was bare while the other had 40 rock fragment coverage the soil was hand cultivated and smoothed before the first event e1 only and left untouched subsequently sediment yields at the flume exit reached steady state conditions over time scales that increased with sediment size experiments were designed such that both steady and non steady effluent sediment yields were reached at the conclusion of e1 results from subsequent experiments showed that short time soil erosion was dependent on whether steady state erosion was achieved during the preceding event although consistent steady state effluent sediment yields were reached for each sediment size class steady state erosion rates were however dependent on the rainfall intensity and its duration if steady state sediment yields were reached for a particular size class that class s effluent sediment yield peaked rapidly in the next rainfall event the early peak was followed by a gradual decline to the steady state condition on the other hand for size classes in which steady state was not reached at the end of the rainfall event i e e1 in the following event e2 the sediment yields for those classes increased gradually to steady state i e the sharp peak was not observed the effect of rock fragment cover 40 on the soil surface was also found to be significant in terms of the time to reach steady state i e their presence reduced the time for steady conditions to be attained effluent sediment yields for the bare and rock fragment covered flumes e1 showed steady conditions were reached for the latter in contrast to the former we used the hairsine rose h r model to simulate the experimental data as it explicitly models soil particle size classes experiments e1 and e2 involved soil compaction by raindrops and in this case the model predictions were found to be unsatisfactory however compaction was effectively completed by the end of experiment e2 and the model provided reasonable predictions for experiments e3 and e4

raindrop interaction in interrill erosion for steady rainfalls a probabilistic approach the main processes involved in interrill erosion are soil particles detachment and transport detachment is caused by shear stresses created by the impacts of raindrops after sediments are lifted in the water layer they are transported over a distance that depends on their settling velocities and the water flow velocity this study calculates the probabilities of interactions between raindrops during soil detachment and between raindrops and particles during their sedimentation raindrops are assumed to be consistent with the poisson process and their densities are described by raindrop size distribution functions marshall palmer gamma and lognormal laws interaction probabilities are calculated based on characteristic time and length scales of the shear stresses and the perturbation created by the raindrop impact inside the water layer under the hypothesis of a constant rainfall intensity the results show that during soil detachment raindrops are almost independent thus the total amount of soil detached by a rainfall is practically the sum of soil detached by its individual raindrops whereas during sediment transport the probability of interaction between raindrops and settling particles is very high whatever the rainfall intensity and particle size emphasizing the need to study further the interaction between raindrops and settling particles

constraining the relative importance of raindrop and flow driven sediment transport mechanisms in postwildfire environments and implications for recovery time scales national geographic society proposal002929 2009 0046 1 national science foundation 02 39749 09 34131 u s geological survey usgs landslide hazards program

duration of tessera deformation on venus the density and distribution of impact craters superposed on the highly deformed tessera terrain on venus permit analysis of the amount and duration of deformation prior to the emplacement of the stratigraphically younger global volcanic plains eighty percent of tesserae craters are undeformed no existing craters exhibit evidence of contractional deformation suggesting that the early compressional stage of tessera deformation ended abruptly the small number of craters fractured by late stage tessera extension constrains the duration of this phase to less than 20 of the average crater retention age of the tesserae or approximately 30 60 ma these results suggest a geologically rapid decline in the magnitude of surface strain rates associated with the transition from the terminal stages of tessera compressional deformation to the eruption of the global volcanic plains

venus s shield terrain plains planitiae or lowlands expanses of gentle long wavelength 1000 km basins cover 80 of venus s surface these regions are widely accepted as covered by volcanic fl ows although the mechanism s responsible for resurfacing remains elusive in addition a volcanic origin for the lowland surface may be open to question lowland resurfacing is typically attributed to catastrophic emplacement 10 100 m y of globally extensive thick 1 3 km fl ood type lava this model of resurfacing has been postulated on the basis of impact crater distribution taken together with a lack of obvious volcanic fl ows or edifi ces and a lack of viable alternative models ongoing geologic mapping of 15 000 000 km2 0 25n 90 150e using 225 m pixel and 75 m pixel nasa magellan sar synthetic aperture radar data indicates that small edifi ces called shields 1 15 km diameter edifi ces 1 km high play a major role in lowland resurfacing individual shields are radar smooth or rough quasi circular to circular features with or without a central pit shield shapes which have been previously documented range from shield dome or cone to fl at topped or fl at shield deposits typically coalesce forming a thin regionally extensive but discontinuous mechanically strong layer herein called shield paint shield paint conforms to delicate local topography providing evidence of its thin character and indicating generally low viscosity during emplacement shield terrain shields and shield paint covers more than 10 000 000 km2 within the study area detailed mapping of fi ve 2 2 regions using coregistered normal and inverted right and left illumination sar imagery indicates shield densities of 3500 33 500 shields 106 km 2 thus the map area hosts more than 35 000 335 000 shields shield terrain generally postdates but is also locally deformed by fractures and wrinkle ridges indicating time transgressive formation relative to local deformation and or reactivation the regional scale crust was strong throughout shield terrain formation shield terrain may extend across much of venus s surface

particle size characteristics of suspended sediment in hillslope runoff and stream flow this study examines the particle size characteristics of hillslope soils and fluvial suspended sediments in an agricultural catchment samples of surface runoff and stream flow were collected periodically and analysed for the size distributions of the effective undispersed sediment this sediment was subsequently dispersed and the ultimate size distributions determined the median effective particle size of stream suspended sediment was considerably coarser than the median ultimate particle size indicating that most of the load included a substantial proportion of aggregates moreover the proportion of fine material i e silt and clay increased and the proportion of sand sized material decreased with increasing discharge this decrease in sediment size with increased flow which is contrary to the traditional assumption of a positive discharge particle size relationship is thought to reflect i the influx of silt and clay predominantly the former originating on the catchment slopes and brought to the stream by overland flow along vehicle wheelings roads and tracks and ii erosion of fine material from the channel bed and banks during large storms however the proportion of sand sized sediment increased during the rising limb of the hydrograph as a result of the entrainment of coarser source material from the valley floor during overbank flooding the stream suspended sediment was finer than the catchment soils and considerably finer than material eroding from the catchment slopes during storms the degree of clay and silt enrichment in the suspended sediments was largely the result of preferential deposition of the coarser fraction during the transport and delivery of sediment from its source to basin outlet the data from this study confirm that a significant mode of sediment transport in fluvial systems is in the form of aggregates and that the dispersed sediment size distribution is inappropriate for determining the transportability of sediment by flow 1997 by john wiley sons ltd