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

settling and compaction of olivine in basaltic magmas an experimental study on the time scales of cumulate formation a series of centrifuge assisted settling experiments of 30 vol olivine in 70 vol basaltic melt was conducted to elucidate the formation mechanisms and time scales of gravitational cumulates the settling experiments were performed in a centrifuging piston cylinder at 200 1 500g 1 270 1 280 c and 0 8 1 1 gpa on previously annealed and texturally equilibrated samples the mechanical settling of the dense olivine suspension occurs at about 1 6 the speed of simple stokes settling resulting in a sedimentation exponent n 4 1 6 in agreement with predictions from analogue systems the porosity m of the orthocumulate resulting from gravitational settling of crystals is about 54 and formation times of olivine orthocumulates result to 0 1 10 m day 1 for an initial crystal content of the melt of 1 5 and grain sizes of 2 10 mm after mechanical settling olivine grains rest on each other and further compaction occurs through pressure dissolution at grain contacts olivine reprecipitation where olivine is in contact with melt and concomitant expulsion of excess liquid from the cumulate layer with centrifugation at 400g for 50 h porosities as low as 30 3 vol were achieved the olivine content at the bottom of the gravitational cumulate is 1 m log h a t where is the density difference between crystals and melt h the crystal layer thickness a the acceleration and t the time of centrifuging compaction is hence proportional to effective stress integrated over time indicating that pressure dissolution is the dominant mechanism for chemical compaction the compaction limit that is the lowermost porosity to be reached by this mechanism is calculated by equating the lithostatic and hydraulic pressure gradients in the cumulate and results to 3 5 porosity for the experiments crystal size distribution curves and a growth exponent n of 3 1 3 indicate that diffusion controlled ostwald ripening is the dominant crystal growth mechanism the above relationship combined with a linear scaling for grain size as appropriate for reaction controlled pressure solution creep allows calculation of formation times of adcumulates if chemical compaction is dissolution reprecipitation limited then single layers of natural olivine adcumulates of m thickness with 70 75 vol olivine at the base as observed in the rhum layered intrusion would have typical formation times of 0 4 3 years for grain sizes of 2 10 mm this time scale compares favourably with characteristic cooling times of sills if a greater than 20 m thick series of cumulate layers pressurizes a base layer with the porosity still filled by a melt then compaction proceeds to the compaction limit within a few years it can thus be expected that in layered mafic intrusions where cumulates are continuously deposited from a large magma chamber and which characteristic cooling times of more than decades a compaction zone of several tens of metres forms with adcumulates only maintaining porosities in the order of 5 in conclusion gravitational settling and gravitation driven chemical compaction are feasible cumulate forming processes for dense mafic minerals in basaltic magmas and in particular in large layered intrusions

quantifying provenance of reservoir sediment using multiple composite fingerprints in an arid region experiencing both wind and water erosion abstract quantitative information on sediment sources is useful for developing targeted protection measures that effectively control soil erosion in a watershed in this study a multiple composite fingerprints method was tested and used for quantifying the sediment provenance of the danghe river and for providing effective sediment control strategies for the watershed in an arid region experiencing both wind and water erosion surface samples were taken from three geomorphic source areas of dunes gobi and upstream mountains and sediment samples from the watercourses near the reservoir to accommodate the wide range of sediment size distributions resulting from the fluvial and aeolian processes sediment contributions from each source were estimated for each of six particle size groups the results showed that the proportional contributions from each source to the reservoir sediments were 56 73 from upstream mountains 26 36 from gobi and 16 91 from dunes with the relative contributions in each size group following a similar trend the largest fraction of the sediment was from the 0 063 0 1 mm particle size group accounting for about 37 09 of the total in which 4 7 59 and 25 50 were from dunes gobi and upstream mountains our results also indicated that selective erosion existed in both erosion processes especially in wind erosion process and multiple particle size tracking should be used in the cases of severe selective erosion to improve the accuracy of estimation also the sediment contribution rate per unit area was the largest from dunes thus controlling wind blown sand to the river by stabilizing the dunes is recommended to reduce the reservoir siltation

effect of embedded rock fragments on slope soil erosion during rainfall events under simulated laboratory conditions abstract quantifying the role of embedded rock fragments in soil water processes on a slope is crucial for the application of rock fragments in soil and water conservation practice in this study a laboratory runoff and erosion experiment was conducted in the soils embedded with rock fragments including five rock fragment contents rc of 0 0 1 0 2 0 3 and 0 4 kg kg 1 in a metal flume under three slopes 5 10 and 15 results showed that the mean steady infiltration rate in the soil without rock fragment null was lower than that in the soils containing rock fragments low rock fragment contents of 0 1 0 2 kg kg 1 and high rock fragment contents of 0 3 0 4 kg kg 1 labelled as low and high respectively as slope gradient 5 but was higher than the latter two soils as slope gradients 10 and 15 initial infiltration rate significantly increased with rc p

relations between interrill erosion processes and sediment particle size distribution in a semiarid mediterranean area of se of spain abstract erosion and sediment characteristics were measured using simulated rainfall on two cultivated soils of contrasting lithology quaternary calcareous colluvium and tertiary marls in a semiarid mediterranean area of se spain two rainfall intensities high 56 0 2 4 mm h 1 and medium 31 4 1 4 mm h 1 were used in order to know the mechanisms involved in each selected condition for each simulated event runoff and sediment were sampled at 1 min intervals on a 1 m wide by 2 m long erosion plot the erosion rate was calculated as the total amount of soil lost divided by the time period of the test the duration of the test was that needed to reach steady state runoff an average time of 24 5 min for quaternary calcareous colluvium and 17 7 min for tertiary marls the size distribution of the transported sediment in the field effective size distribution was compared with equivalent measurements of the same samples after chemical and mechanical dispersion ultimate size distribution to investigate the detachment and transport mechanisms involved in sediment mobilisation the results showed that the soil type determined the hydrological response regardless of the rainfall intensity the erosional response was however determined by the rainfall and soil surface characteristics in quaternary calcareous colluvial soils the predominant erosion process depended on the rainfall intensity with a prevailing detachment limited condition in high intensity events and prevailing transport limited conditions in those events of medium intensity these differences in the main erosion processes were reflected in the size of the transported material and in the change in sediment size within the storm thus a time dependent size distribution of the eroded material decreasing coarse fractions and increasing fine fractions with runoff time was observed for high intensity events in medium intensity events on the other hand the time independent size distribution of the eroded material indicated transport limited erosion due to the rapid surface crusting on the tertiary marl soil no differences in the main erosion processes or in the sediment size distribution occurred for the different rainfall intensities tested the erosion of marl soils was determined mainly by the limited quantity of available sediment the effective size of material was a more sensitive parameter than the ultimate size of the sediment to study the way in which the sediment was transported

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

major ion chemistry of groundwater in a river basin a study from india groundwater is the prime source in the area of varaha river basin andhra pradesh india groundwater samples collected during pre and post monsoon were analyzed for major ion chemistry to understand the operating mechanism of geochemical processes for variation of groundwater quality low ionic concentration is observed in the groundwater occurring at topographic highs and towards the river compared to the rest of the area this is caused by the influence of recharge water geochemistry of groundwater is observed to be mainly controlled by the rock weathering mineral dissolution leaching ion exchange and evaporation and is subsequently modified by the anthropogenic and marine activities contribution of residual soluble salts below the depth of 5 m from the ground surface is high while it is low beyond the depth of 5 m anthropogenic activities cause an accumulation of excess residual soluble salts in the former depth and the content of this soluble matter is reduced by the clay products beyond the latter depth seasonal ionic concentration of groundwater is greatly influenced by the recharge process with reference to topographical features lithological characteristics and anthropogenic activities groundwater quality is classified as mixed and non carbonate alkali groups due to a combined action of geogenic anthropogenic and marine sources respectively the groundwater quality is above the adequacy level for both drinking and irrigation and appropriate management measures are recommended for sustainable development

gis based simulation of erosion and deposition patterns in an agricultural landscape a comparison of model results with soil map information abstract this paper presents a relatively simple two dimensional hillslope erosion deposition model the model differs from most geomorphological models in that it does not assume that erosion by surface wash is always transport limited as a first test the model is applied to an agricultural catchment in central belgium here patterns of past erosion and deposition can be deduced from information contained in the belgian soil map so that a qualitative validation of the model results is possible the results indicate that a simple model formulation allows to reproduce the basic pattern of erosion and deposition as it is observed in the landscape this confirms the major influence of topography on erosion and deposition patterns in such landscapes and indicates that in some cases a simple model formulation may well be sufficient to allow adequate modelling of landscape evolution

settling and compaction of chromite cumulates employing a centrifuging piston cylinder and application to layered mafic intrusions the time scales and mechanics of gravitationally driven crystal settling and compaction is investigated through high temperature 1 280 1 500 c centrifuge assisted experiments on a chromite basalt melt system at 100 1 500g 0 5 gpa subsequently the feasibility of this process for the formation of dense chromite cumulate layers in large layered mafic intrusions lmis is assessed centrifugation leads to a single cumulate layer formed at the gravitational bottom of the capsule the experimentally observed mechanical settling velocity of a suspension of 24 vol chromite is calculated to be about half 0 53 of the stokes settling velocity with a sedimentation exponent n of 2 35 3 gravitational settling leads to an orthocumulate layer with a porosity of 0 52 all porosities as fraction formation times for such a layer from a magma with initial chromite contents of 0 1 1 vol are 140 3 5 days equal to a growth rate of 0 007 0 3 m day for grain sizes of 1 2 mm more compacted chromite layers form with increasing centrifugation time and acceleration through chemical compaction an increase of grain contact areas and grain sizes together with a decrease in porosity is best explained by pressure dissolution at grain contacts reprecipitation and grain growth into the intergranular space and a concomitant expulsion of intergranular melt the relation between the porosity in the cumulate pile and effective pressure integrated over time h a t is best fit with a logarithmic function in fact confirming that a pressure dissolution reprecipitation process is the dominant mechanism of compaction the experimentally derived equation allows calculating compaction times 70 80 chromite at the bottom of a 1 m thick chromite layer are reached after 9 250 years whereas equivalent compaction times are 0 2 0 9 years for olivine both for 2 mm grain size the experiments allow to determine the bulk viscosities of chromite and olivine cumulates to be of magnitude 109 pa s much lower than previously reported as long as melt escape from the compacting cumulate remains homogeneous fluidization does not play any role however channelized melt flow may lead to suspension and upward movement of cumulate crystals in lmis chromitite layers are typically part of a sequence with layers of mafic minerals compaction occurs under the additional weight of the overlying layers and can be achieved in a few years to decades

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

vertical fluxes of sediment in oscillatory sheet flow time series of vertical sediment fluxes are derived from concentration time series in sheet flow under waves while the concentrations c z t vary very little with time for z 10d50 the measured vertical sediment fluxes qzs z t vary strongly with time in this vertical band and their time variation follows to some extent the variation of the grain roughness shields parameter 2 5 t thus sediment distribution models based on the pickup function boundary condition are in some qualitative agreement with the measurements however the pickup function models are only able to model the upward bursts of sediment during the accelerating phases of the flow they are so far unable to model the following strong downward sediment fluxes which are observed during the periods of flow deceleration classical pickup functions which essentially depend on the shields parameter are also incapable of modelling the secondary entrainment fluxes which sometimes occur at free stream velocity reversal the measured vertical fluxes indicate that the effective sediment settling velocity in the high 0 3 c z t 0 4 concentration area is typically only a few percent of the clear water settling velocity while the measurements of richardson and jeronimo chem eng sci 34 1979 1419 from a different physical setting lead to estimates of the order 20 the data does not support gradient diffusion as a model for sediment entrainment from the bed that is detailed modelling of the observed near bed fluxes would require diffusivities that go negative during periods of flow deceleration an observed general trend for concentration variability to increase with elevation close to the bed is also irreconcilable with diffusion models driven by a bottom boundary condition

effects of soil surface roughness on interrill erosion processes and sediment particle size distribution abstract soil surface roughness significantly impacts runoff and erosion under rainfall few previous studies on runoff generation focused on the effects of soil surface roughness on the sediment particle size distribution psd which greatly affects interrill erosion and sedimentation processes to address this issue a rainfall simulation experiment was conducted with treatments that included two different initial soil surface roughnesses and two rainfall intensities soil surface roughness was determined by using photogrammetric method for each simulated event runoff and sediment samples were collected at different experimental times the effective undispersed psd of each sediment sample and the ultimate after dispersion psd were used to investigate the detachment and transport mechanisms involved in sediment movement the results show that soil surface roughness significantly delayed runoff initiation but had no significant effect on the steady runoff rate however a significant difference in the soil loss rate was observed between the smooth and rough soil surfaces sediments from smooth soil surfaces were more depleted in clay size particles but more enriched in sand size particles than those from rough soil surfaces suggesting that erosion was less selective on smooth than on rough soil surfaces the ratio of different sizes of transported sediment to the soil matrix indicates that most of the clay was eroded in the form of aggregates silt size particles were transported mainly as primary particles and sand size particles were predominantly aggregates of finer particles soil surface roughness has a crucial effect on the sediment size distribution and erosion processes significant differences of the enrichment ratios for the effective psd and the ultimate psd were observed under the two soil surface roughness treatments these findings demonstrate that we should consider each particle size separately rather than use only the total sediment discharge in assessing and modeling interrill erosion processes

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

effect of particle density and inflow concentration of suspended sediment on bedload transport in rill flow laboratory flume experiments were carried out to evaluate the effect of particle density on bedload transport of sand sized particles and the effect of a suspended load of clay particles kaolinite on bedload transport of sand sized particles in rill flow conditions three materials in the range 400 600 m were selected to simulate bedload transport of primary particles and aggregates sand 2650 kg m3 crushed brick 2450 kg m3 and anthracite 1300 1700 kg m3 in the two first experiments two different methods were applied to determine bedload transport capacity of coarse particles for various conditions of flow discharge from 2 to 15 l min and slope 2 2 3 and 4 in the third experiment clear water was replaced with kaolinite water mixture and bedload transport capacity of crushed brick particles was determined for a 4 slope and different concentrations of kaolinite 0 7 41 and 84 g l the results showed that bedload transport increased significantly with the decrease in particle density but the effect of particle density on transport rates was much less important than flow discharge velocity measurements of clear flow flow mixed with coarse particles and coarse particles confirmed the existence of a differentiation between suspended load and bedload in these experimental conditions suspended load of kaolinite did not affect bedload rates of crushed brick particles three transport capacity formulae were tested against observed bedload rates a calibration of the foster formula revealed that the shear stress exponent should be greater than 1 5 the low and the govers unit stream power usp equations were then evaluated the low equation was preferred for the prediction of bedload rates of primary particles but it was not recommended in the case of aggregates of low density because of the limited experimental conditions applied to derive this equation copyright 2008 john wiley sons ltd

hindered settling of silt abstractsediment particles smaller than 64 m with a noncohesive base mineral are referred to as silt the settling velocity of suspended sediment decreases with increasing concentration due to hindered settling effects compared with clay and sand hindered settling of silt is poorly documented and understood a formulation is proposed for the hindered settling of silt water mixtures derived from physical reasoning the effect of silt on the mixture s viscosity is addressed in particular the formulation compares favorably with laboratory experiments on the settling of silt sized sediment this study shows that effective settling velocities can be derived from a single concentration time series measured within the settling mixture provided that the sediment is initially homogeneously distributed

flow regime associated with partially penetrating large diameter wells in hard rocks abstract in case of a large diameter well which penetrates an aquifer only partially significant vertical flows may set in as a result of withdrawal from the well provided of course the vertical permeability below the well bottom is appreciable a finite difference model of groundwater flow regime associated with a partially penetrating well in a hard rock terrain has been presented use of this model in the estimation of hydrogeological parameters in such a situation has been illustrated by a field example the analysis of pump test data shows that parameter estimation will be in error if the vertical flows are neglected the same model has also been used for prognosing the efficacy of a borewell in a large diameter partially penetrating well to increase the yield

size characteristics of sediment in interrill overland flow on a semiarid hillslope southern arizona this study examines the size characteristics of sediment removed from a semiarid hillslope by interrill overland flow rainfall simulation experiments were conducted on a runoff plot 18 m wide and 35 m long established on a piedmont hillslope in southern arizona the top of the plot coincided with the hillslope divide and its outlet was located within a shallow rill samples of runoff were obtained from two cross sections located in the interrill portion of the plot upslope of the rill and from a calibrated flume through which was directed interrill overland flow reaching the bottom of the plot analyses of sediment contained in these samples showed that sediment in interrill flow is finer than the matrix soil the fineness of the interrill sediment compared to the matrix soil appears to be due to the inability of interrill overland flow to transport the coarser fraction of the sediment supplied to it by raindrop detachment this finding implies that the rate of soil erosion in interrill areas is not as is commonly supposed limited by the rate at which raindrops can detach sediment but by the rate at which they detach sediment of a size that the overland flow is competent to transport the relative fineness of sediment eroded from this hillslope is consistent with other evidence for the recent evolution of shrub covered hillslopes in southern arizona

major ion chemistry of groundwaters in the continental terminal water table of southwestern niger africa numerous chemical analyses undertaken during the last 4 a in the niamey area of the continental terminal groundwaters sw niger africa reveal large seasonal variations and high no3 content up to 2 87 meq l the positive correlation between conductivity and depth to the water table indicates that the chemical change is associated with the recharge in the shallow wells this highly mineralised recharge reflects the rain composition variably concentrated by evaporation processes

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

rainwash experiments on the erodibility of loose sediments laboratory experiments using simulated rainfall splash boards and trays were conducted in order to obtain information on the erodibility of loose sediments as a time dependent phenomenon in all ten different sediments ranging from silt d50 24 m to coarse sand d50 593 m were studied the effect of three different initial states of the sediments namely the air dry state t1 the field capacity state t2 and the desiccated state t3 on the erodibility was also assessed variations in detachability during a rain erosion experiment can be explained by change in water content including liquefaction and the development of a water layer on the surface cohesion and granulometric composition of the top layer the coarsest sandy sediments almost always yielded the highest detachability when starting from an initial field capacity state t2 for the finer sediments the highest detachability was found for the homogeneous and not yet compacted initial air dry state t1 the effect of the initial desiccated state t3 on the detachability of the finer sediments was also a function of the rainfall duration the relationship between detachability and median grain size of the sediments was very similar to the relation between grain size and the susceptibility to runoff and wind erosion established by other investigators the highest detachability was found for a fine very well sorted sand with a median grain size of 96 m

hydrology of the hapex sahel central super site surface water drainage and aquifer recharge through the pool systems abstract the hydrology of the sahel is characterized by the degradation of the drainage network resulting in the lack of large watersheds over which the spatial integration of the hydrological processes could be studied the main hydrological units are small endoreic areas measuring a few hectares to a few square kilometres and the surface runoff is collected into pools a detailed investigation of the role of these pools in the hydrology of the hapex sahel central super site was carried out from 1991 to 1993 the first results of this investigation are presented a typology in three classes of the endoreic systems valley bottoms sinks plateaux is proposed the behaviour of one representative pool in each class is analysed showing that the partition between evaporation and deep infiltration depends on the level of filling of the pools the bottom of the pool is clogged by clay deposits which prevent infiltration above a threshold varying between 1 and 2 m most of the water stored in the pool after runoff infiltrates contributing to the recharge of the aquifers on a seasonal basis deep infiltration accounts for less than 50 of the water collected by the plateau pool and more than 80 for the valley bottom pools almost all the water running off to the sink pools infiltrates rapidly and deeply into the ground the valley pools both valley bottoms and sinks appear to be the major contributors to the recharge of the upper aquifer the proportion of the hapex sahel central super site water balance that is taken by the deep infiltration from the pools varies greatly depending on the temporal distribution of rainfall whereas similar seasonal rainfalls were recorded in 1991 and 1992 it is estimated that 5 of the water precipitated over the valley pool watershed infiltrated towards the aquifer in 1991 and 20 in 1992 this difference is explained by a very irregular time distribution of precipitation in 1992 most of the major rainfall events being observed over a short period during the intensive observation period in conclusion some preliminary figures are given regarding the importance of recharge from the pools as compared with in situ recharge

interpretation of pumping test data from large diameter wells on an oceanic island estimation of aquifer parameters is vital for the assessment of groundwater potential and groundwater flow regime on an oceanic island where fresh water lens is fragile and sensitive to various stresses it is even more essential that in order to assess the potential of fresh water lens and the effect of various stresses on the fresh groundwater regime the parameters should be representative to the field hydrogeological set up pumping tests conducted on existing large diameter wells on an oceanic island have been analyzed a finite difference method has been used to take into account the well storage partial penetration and upconing effect into the aquifer forward modeling has been carried out to estimate aquifer parameters from the pumping test data field examples are described

evaluation of the simple settling theory for predicting sediment deposition by overland flow experiments were conducted in the laboratory to evaluate the influence of sediment concentration sediment grain size distribution bed slope and flow discharge on sediment deposition rates and patterns associated with a reduction in bed slope the experimental data clearly indicate that sediment deposition by overland flow is a very selective process fine particles remain almost entirely in suspension and coarse particles are deposited quickly analysis of the data shows that up to a critical unit discharge a simple settling equation without a transport term assuming continuous mixing of the sediment and water gives a good prediction of the overall sediment delivery ratio and the grain size distribution of the deposited and the exported sediment however there are some discrepancies for the clay the coarse silt and the sand fractions the assumption of continuous mixing is tested by investigating the sedimentation patterns of very narrow size classes the observed decrease of sediment concentration versus distance from the inflow point for these individual sediment size classes closely agrees with the prediction assuming continuous mixing when the critical unit discharge is exceeded hydraulic properties of the overland flow do influence the sediment delivery outcomes at discharges exceeding the threshold value the simple settling theory underpredicts the sediment delivery ratio in these hydraulic conditions a transport term needs to be incorporated into the simple settling theory it is shown that the transport capacity and the re entrainment model yield similar expressions for the description of sediment transport by overland flow over an area of net deposition the experimental data indicate that the re entrainment of previously deposited sediment is non selective copyright 1999 john wiley sons ltd

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

deep infiltration through a sandy alluvial fan in semiarid niger inferred from electrical conductivity survey vadose zone chemistry and hydrological modelling abstract in semiarid southwestern niger most of the groundwater recharge is indirect and occurs through endoreic ponds elsewhere in the landscape there is no evidence of deep infiltration with a possible exception for gullies and alluvial fans on sandy slopes in order to verify this hypothesis a detailed geophysical and geochemical survey was conducted on a large representative mid slope fan 6 ha at this site distributed hydrological modelling conducted over the encompassing endoreic catchment 190 ha showed high losses of runoff water by infiltration electromagnetic mapping and 2 d electrical imaging survey were used to investigate the 35 m deep vadose zone in addition 8 boreholes were drilled following the geophysical survey to constrain the interpretation variations in apparent electrical conductivity measured in boreholes appear to be mainly linked with changes in the soil solution mineralization an extrapolation throughout the area shows that apparent electrical conductivity of the ground is systematically lower below channels this suggests localised leaching through the unsaturated zone a physically based 2 d distributed hydrologic model was used to estimate the amount of surface water loss by infiltration for the 1992 2002 period depending on year infiltrated volumes range from 1000 to 24 000 m3 this represents between 5 and 16 of the runoff that reaches the final outlet of the basin an endoreic valley bottom pond where recharge to the aquifer has been shown to occur because leaching of the vadose zone is observed down to a depth of 10 m below channels episodic groundwater recharge through sandy mid slope fans is highly probable during rainy years

assessment of dispersive pressure as a beach placer mechanism the concept of sediment sorting extending from the bagnold constitutive relationship for sediment under shear is examined in the context of beach placer formation the traditional interpretation of shear sorting is found to be inconsistent with data from new granular flow experiments a binary mixture satisfying the shear equivalence relationship displayed pronounced segregation during shear this observation is contrary to shear sorting as applied in the literature where no such segregation for this pairing of mineral species is expected further experiments minimizing the potential of grain percolation of smaller grains or kinetic sieving also demonstrated sorting patterns inconsistent with the existing shear sorting interpretation consequently the shear sorting process as currently formulated may not be an appropriate means to describe beach placer formation kinetic sieving appears to be a more likely mechanism in this context enhanced by the shearing and dilation of the granular fluid flow 2009 the authors journal compilation 2009 international association of sedimentologists

comment on improving noble gas based paleoclimate reconstruction and groundwater dating using 20 ne 22 ne ratios by f peeters et al 2003 geochim cosmochim acta 67 587 600 1 1associate editor b marty

paleopedology of middle wisconsin weichselian paleosols in the merida andes venezuela abstract buried spodosols podzols within a thick lithostratigraphic section in the northern venezuelan andes are interbedded with tills and a succession of multicyclic lacustrine clays silts and sands from the early and middle stades of the wisconsinan weichselian glaciation the peats dated by ams radiocarbon allow calculations of the time required for pedogenesis these spodosols were studied to determine compositional changes from initial to full soil development during lake low water stands peats of variable thickness developed along with soils belonging to the spodosol order on a relative time scale the initial stage of soil genesis produced peat with a weathered mineral zone bs initial spodic horizon of a few centimeters thickness with a longer time for soil genesis of a few centuries a thin spodosol developed with an e bs horizon sequence with still longer time spans of a few thousand years between 55 and 60 ka more mature spodosols with e bs cox cu profiles formed beneath very thick peat in conjunction with groundwater fluctuations the lowermost peat vii in the section varies from 15 to 26 5 cm in thickness the paleosols are sandy with little evidence of either clay transformations or silt accretion from airfall influx apparent from the particle size analysis xrd analysis of the clay fraction shows minor weathering of illite to vermiculite and chlorite in the bs horizons chemically extractable forms of fe and al show increases of both soluble and insoluble minerals from the e to the bs horizons in the buried spodosols a considerably different trend compared with the surface soils entisols geochemical trace element analysis shows a moderate degree of translocation of soluble chemical elements from the e to the bs horizons

retraction note development prospects of groundwater resources in an elliptical shape island union territory of lakshadweep india a major problem of the islanders is the availability of fresh water for drinking purpose groundwater is the only source of fresh water for the islanders the demand for groundwater is increasing very year due to growing population and urbanization a proper understanding of the groundwater condition is important in order to meet this increasing demand and to formulate future development and management strategies it is in this context principal hydrogeologic units water table fluctuation pattern general groundwater potential existing groundwater withdrawal structures and draft water quality etc have been studied in an elliptical shape andrott island of union territory of lakshadweep india through field investigation and secondary data collection groundwater occurs under phreatic condition and seawater is in hydraulic continuity with the groundwater as evidenced by the tidal influence in almost all the wells groundwater level fluctuation due to seasonal variation varies from 0 to 0 542 m depending on the distance of the well from the coast depth to groundwater level varies from less than 1 234 to 3 520 m depending on the topography groundwater level fluctuation is due to the combination of factors like rainfall tidal activities sub surface runoff and draft large diameter dug wells are the main groundwater extraction structures in this island there are 2 143 dug wells with almost each family having its own well and the density of the dug wells is about 437 km2 the stage of groundwater development is estimated as 37 and hence safe for further groundwater development in this island however considering the very limited fresh water resources and also the growing demand for groundwater various management strategies such as rainwater harvesting artificial recharge of groundwater public participation in water conservation and wise use of groundwater etc have been suggested

hindered bedload settling as a model of sand bed planation by water waves the interrelationships between fluid flows and the surface forms of underlying movable beds are crucial in interpreting sedimentary structures and in predicting hydraulic drag energetic flows can erode all features from a sediment bed and transition to such a planar bed is important to the processes of sediment transport by waves1 the following analysis connects bed planation to a threshold effect in hindered settling with increasing concentration of noncohesive sediment moving near the bed calculated fluid velocity from the resulting quantitative criterion for this bed transition agrees with extensive laboratory data2 3 in oscillatory flows

flow pattern and residence time of groundwater within the south eastern taoudeni sedimentary basin burkina faso mali the knowledge about groundwater flow conditions within the southeastern taoudeni basin aquifer shared by burkina faso and mali is relatively limited with very little information on potentiometric heads recharge processes residence time and water quality a better evaluation of groundwater resources in this area is a strategic point for water resources management in the entire soudano sahelian region which endures since the beginning of the twentieth century a continuous decrease in precipitation amount this paper provides a transboundary synthesis using water 18o 2h and 3h and carbon isotopes 13c and 14c in conjunction with hydrogeological and hydrochemical data the objectives are to improve the conceptual model of groundwater recharge and flow within this sandstone reservoir and to assess the changes in the aquifer due to water abstraction and recent climate changes including an insight into sahelian aquifers palaeorecharge processes the local meteoric water line for the bobo dioulasso station is proposed 2h 8 0 0 5 18o 10 2 2 1 two main tendencies can be derived from groundwater chemistry first a slight evolution from the ca mg hco3 type towards a na k hco3 type that indicates developed interactions between groundwater and clay minerals related to the residence time of groundwater a second tendency towards cl no3 so4 hco3 water types indicates the anthropogenic influence on groundwater related to the poor sanitary conditions observed around wells the carbon 14 activity measured on the tdic varies between 0 3 and 122 pmc so our record contains samples covering a wide period from actual to pleistocene suggesting a continuous recharge of the system through time even if the sahel region has endured many different climate phases which have influenced the infiltration and recharge processes all groundwater samples have stable isotope compositions in the range of the present day regional and global meteoric water line which suggests that the sampled groundwater was not significantly affected by evaporation during recharge evolved waters are depleted relative to unevolved samples by 1 5 2 in 18o and 10 15 in 2h the whole dataset support the hypothesis of a largely unified homogeneous aquifer system with a multilayered structure but it also points out the very low renewability of the resource and a strong anthropogenic contamination of the shallowest horizons

determination of the distance of the fresh water salt water interface extending into coastal confined aquifers in a coastal zone an understanding of the distance of the fresh water salt water interface and its extension inland is important for prevention of sea water intrusion in this article estimating methods are described for calculating the distance of a fresh water salt water interface in a coastal confined aquifer based on the submarine fresh groundwater discharge this groundwater discharge is controlled not only by the aquifer properties and hydraulic head difference but also by the position of the fresh water salt water interface in the coastal zone a homogeneous and isotropic coastal confined aquifer is considered and fresh groundwater flow in the confined aquifer is thought to be at a steady state two observation wells at different distances in a profile perpendicular to the coastline are required in calculation of the distance of the interface toe in the coastal zone four coastal confined aquifers with horizontal and sloping confining beds and with varying thickness are also considered reasonable results are obtained when examples are used to illustrate the application of the methods the methods require hydraulic head data at the two wells and thickness of the confined aquifers but the hydraulic conductivity and fresh groundwater flow rate of the confined aquifers are not needed

roughness coefficients and velocity estimation in well inundated sheet and rilled overland flow without strongly eroding bed forms even with the flow of water over a soil surface in which roughness elements are well inundated and in less erosive situations where erosional bed forms are not pronounced the magnitude of resistance coefficients in equations such as those of darcy weisbach chezy or manning vary with flow velocity at least using both original laboratory and field data and data from the literature the paper examines this question of the apparent variation of resistance coefficients in relation to flow velocity even in the absence of interaction between hydraulics and resulting erosional bed forms resistance equations are first assessed as to their ability to describe overland flow velocity when tested against these data sources the result is that manning s equation received stronger support than the darcy weisbach or chezy equations though all equations were useful the second question addressed is how best to estimate velocity of overland flow from measurements of slope and unit discharge recognizing that the apparent flow velocity variation in resistance coefficients is probably a result of shortcomings in all of the listed resistance equations a new methodology is illustrated which gives good agreement between estimated and measured flow velocity for both well inundated sheet and rill flow comments are given on the predictive use of this methodology copyright 1999 john wiley sons ltd

correction factors in the determination of mean velocity of overland flow the velocity of overland flow has been conventionally measured using tracers but it is difficult to measure the mean flow velocity directly because the centroid of the tracer plume is not easily identified consequently previous investigators have measured the velocity of the leading edge of the plume and multiplied it by a correction factor to obtain an estimate of mean velocity an alternative method is to measure the velocity of the peak concentration in the tracer plume and multiply this velocity by another correction factor to estimate mean velocity to investigate the controls of and and develop predictive models for these correction factors 40 experiments were performed in a flume with a mobile sand bed multiple regression analyses reveal that both and vary inversely with slope and directly with reynolds number the derived regression equations may be used to calculate the mean velocity of other shallow overland flows at least within the range of slope and reynolds number for which the equations were developed in the experiments slope ranged from 2 7 to 10 and reynolds number from 1900 to 12 600

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

psem_2d a physically based model of erosion processes at the plot scale 1 this paper presents the development and first applications of the plot soil erosion model 2d psem 2d infiltration is computed using a green and ampt model overland flow is computed using the depth averaged two dimensional unsteady flow equations saint venant equations and soil erosion is computed by combining the equation of mass conservation of sediment and a detachment transport coupling model for erosion by runoff a shear stress approach is used to determine the transport capacity the formation of a covering cohesionless layer as a result of depositing sediment and action of rainfall impact before runoff is considered the erosion processes involved are rainfall and runoff detachment of original soil rainfall redetachment and overland flow entrainment of sediment from the deposited layer and deposition the model uses a single representative particle size complex rainfall events on natural slopes can be simulated the accuracy of the predictions for erosion of planar surfaces is tested by comparison with observed data obtained from experiments and with an analytical solution good agreement between the calculated results and measured data was found a sensitivity analysis was also performed limitations related to the description of overland flow on plane soil surface are pointed out finally an application to a nonplanar natural surface of 75 m2 illustrated the distribution of erosion and sedimentation over the plot

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

the dynamic response of splash erosion to aggregate mechanical breakdown through rainfall simulation events in ultisols subtropical china rainfall splash erosion is a temporal dynamic process that is determined by the interaction of soil surface and rainfall conditions the properties of aggregate such as aggregate stability and size distribution are recognized as factors that influence the variation in soil surface structure and sediment selectivity during splash erosion event this study based on aggregate stability to investigate the effect of aggregates breakdown dynamics on splash erosion and sediment size selectivity processes of ultisols in subtropical china during rainfall events laboratory experiments of rainfall simulation involving rainstorm simulations at 58 1 2 8 mm h 1 were conducted on four soils derived from quarternary red clay and shale we compared the size distributions of aggregates produced by raindrop impact with the sediment size distributions resulted from transportation fractal dimension d and enrichment ratio er were used to characterize aggregates size distribution and sediment size selectivity aggregates were prone to breakdown at the beginning of rainfall and a multiple regression equation d 1 02rmi 1 77 t0 06 r2 0 95 p 0 01 n 60 was obtained to describe aggregate breakdown dynamics considering a relative aggregate stability index rmi and rainfall time t total masses of erosion sediment decreased as a power function with an increasing size fraction the behaviors of sediment size fractions 0 25 mm 0 5 0 25 mm 1 0 5 mm 2 1 mm 5 2 mm showed that the transport selectivity was decided by the interaction of aggregates size distributions and the characteristics of soil surface structure sediment size selectivity led to the enrichment of 1 0 25 mm fraction compared with the breakdown products this research contributes to knowledge about the dynamics of the splash mechanism and the results could be valuable for the development of rainfall erosion models

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

effects of rain intensity slope gradient and particle size distribution on the relative contributions of splash and wash loads to rain induced erosion abstract soil erosion during rainfall is a complex phenomenon resulting from detachment by raindrop impact and overland flow the objective of this study is to investigate the transportation rate of splash load sl and that of wash load wl and their relative contributions to the rain induced erosion rate affected by rain intensity slope gradient and particle size distribution a total of 60 simulation runs were carried out using a detachment tray under simulated rainfall with no inflow the experiments were done on two soil samples aggregate sizes finer than 2 and 4 75 mm from an agricultural land use at different rain intensities 57 and 80 mm h 1 and varying slope gradients 0 5 2 5 5 10 and 20 under unsteady state conditions wl showed relatively high dependency on slope gradient whereas approaching steady state conditions nearly similar values of wl occurred at different slopes sl and wl increased with increasing rain intensity and slope gradient implying the importance of rain induced erosion on bare agriculture lands especially at steeper slopes the ratio of wl sl decreased when slope gradient increased however wl increased more significantly than sl as soil aggregates became finer the result indicates that at all the slope gradients wl was much larger than sl indicating that wash load significantly contributes to the rain induced erosion in the agricultural soil furthermore with increasing slope gradient the contribution of wash load to rain induced erosion decreased while it was greater in the soil containing finer aggregates also with increasing rain intensity at lower slope gradients 10 the finding of this study revealed that the rain induced erosion was transport limited at the gentler slopes whereas at the steeper slopes it shifted to detachment limited conditions

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

splash detachment runoff depth and soil cohesion summary a set of laboratory experiments has been carried out in order to investigate the importance of soil cohesion and the interference due to the depth of the surface runoff on the detachment rate of soil particles and aggregates experimental data show that the soil detachment rate decreases as the runoff depth increases this indicates that the detachment power of the raindrops is partially dispersed by the water layer an inverse relationship between soil detachment and soil cohesion has been confirmed nevertheless the grain size distribution must also be used in order to define soil detachability

interrill erosion of carbon and phosphorus from conventionally and organically farmed devon silt soils abstract globally between 0 57 and 1 33 pg of soil organic carbon soc may be affected by interrill processes also a significant amount of phosphorus p is contained in the surface soil layer transformed by raindrop impact runoff and crust formation in the eu the p content of a crusted 2 mm surface layer corresponds to 4 to 40 kg ha 1 of p on arable land 1 094 mil km2 therefore the role of interrill processes for nutrient cycling and the global carbon cycle requires close attention interrill erosion is a complex phenomenon involving the detachment transport and deposition of soil particles by raindrop impacted flow resistance to interrill erosion varies between soils depending on their physical chemical and mineralogical properties in addition significant changes in soil resistance to interrill erosion occur during storms as a result of changes in surface roughness cohesion and particle size as a consequence erosion on interrill areas is selective moving the most easily detached small and or light soil particles this leads to the enrichment of clay phosphorous p and carbon c such enrichment in interrill sediment is well documented however the role of interrill erosion processes on the enrichment remains unclear enrichment of p and c in interrill sediment is attributed to the preferential erosion of the smaller lighter soil particles in this study the p and organic c content of sediment generated from two devon silts under conventional cs and organic os soil management were examined artificial rainfall was applied to the soils using two rainfall scenarios of differing intensity and kinetic energy to determine the effects on the p and c enrichment in interrill sediment interrill soil erodibility was lower on the os irrespective of rainfall intensity sediment from both soils showed a significant enrichment in p and c compared to the bulk soil however sediment from the os displayed a much greater degree of p enrichment this shows that the net p export from organically farmed soils is not reduced by a similar degree than soil erosion compared to conventional soil management the enrichment of p and c in the interrill sediment was not directly related to soc p content of the soil and soil interrill erodibility a comparison of soil and sediment properties indicates that crusting p and c content as well as density and size of eroded aggregate fragments control p and c enrichment due to complex and dynamic interactions between p soc and interrill erosional processes the nutrient and c status of sediments cannot be predicted based on soil p content soc or interrill erodibility alone clearly further research on crust formation and the composition of fragments generated by aggregate breakdown and their transport in raindrop impacted flow under different rainfall conditions is required attaining this critical missing knowledge would enable a comprehensive assessment of the benefits of organic farming on nutrient budgets off site effects of interrill erosion and its role in the global c cycle

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

occurrence and formation of water laid placers a placer is a deposit of residual or detrital mineral grains in which a valuable mineral has been concentrated by a mechanical agent the agent is usually running water and the valuable mineral is usually denser than quartz for example gold diamond cassiterite ilmenite or chromite it is difficult to overemphasize the importance of placers as sources of mineral wealth the witwatersrand south africa paleoplacers discovered in 1886 alone have provided over half of all the gold ever mined in the world pretorius 1976 and alluvial placer deposits of cassiterite in southeast asia presently are the world s major source of tin toh 1978 the earliest evidence of placer mining comes from stone carvings in egypt dated at 2500 b c macdonald 1983 and as early as 7 b c strabo pointed out that in turdetania spain more gold was procured from washing sand in the rivers than by digging in the mines given the economic importance of water laid placers and the long history oftheir exploitation it is remarkable that they have received so little attention in sedimentary research mining and exploration practices for the most part have been guided by empirical rules of thumb that are locally useful but lack any firm physical basis of understanding see bateman

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

geoelectrical method in the investigation of groundwater resource and related issues in ophiolite and flysch formations of port blair andaman island india geoelectrical resistivity investigation using vertical electrical sounding ves technique was conducted at port blair south andaman island to locate the fractures in different formations and to decipher its groundwater potential a total of 40 ves were carried out covering the entire study area using schlumberger electrode configuration out of which 34 ves fall in andaman flysch formations and the remaining ves in ophiolite formations the interpreted resistivity results were integrated with nine borehole lithologs for the subsurface analysis the combination of ves with borehole litholog data has provided a close correspondence on subsurface hydrogeological conditions the interpreted ves data of various formations showed drastic variations in the resistivity ranging from higher in ophiolite moderate in andaman flysch and very low in valleys of andaman flysch formations the study further revealed that the weathered and fractured volcanics of ophiolite groups of rocks and sandstone that occur in the andaman flysch formations constitute the productive water bearing zones categorized as good groundwater potential zone based on the geoelectrical parameters viz thickness of layers and the layer resistivity values a groundwater potential map was prepared in which good moderate and poor groundwater zones were demarcated further numerical spatial and litho geoelectric models of resistivity were analyzed in terms of groundwater potential and these models have thus enabled to prepare a comprehensive groundwater development and management plans proving its efficacy in this art of exploratory investigations

layered intrusions and traffic jams a wide range of explanations has been proposed for the origin of repetitive layering in mafic ultramafic and in per alkaline intrusions here we propose that the interaction of mineral grains that sink and float in the crystallizing magma is an alternative mechanism that can explain many of the features of layered intrusions without the need to invoke extrinsic factors similar to traffic jams on a motorway small perturbations in crystal density develop that impede further ascent or descent of buoyant or heavy minerals respectively these traffic jams separate layers of magma from the rest of the magma chamber the magma in the individual layers further evolves as a largely independent subsystem with gravitational sorting organizing the mineral distribution within each layer layering can develop in the intermediate range between full mineral separation in low viscosity or slowly cooling magma chambers and homogeneous crystallization in high viscosity or fast cooling chambers this self organization mechanism provides a novel explanation for the formation of rhythmic layering in low viscosity magmas for example in the ilimaussaq igneous complex in southwest greenland

the concept of transport capacity in geomorphology the notion of sediment transport capacity has been engrained in geomorphological and related literature for over 50 years although its earliest roots date back explicitly to gilbert in fluvial geomorphology in the 1870s and implicitly to eighteenth to nineteenth century developments in engineering despite cross fertilization between different process domains there seem to have been independent inventions of the idea in aeolian geomorphology by bagnold in the 1930s and in hillslope studies by ellison in the 1940s here we review the invention and development of the idea of transport capacity in the fluvial aeolian coastal hillslope debris flow and glacial process domains as these various developments have occurred different definitions have been used which makes it both a difficult concept to test and one that may lead to poor communications between those working in different domains of geomorphology we argue that the original relation between the power of a flow and its ability to transport sediment can be challenged for three reasons first as sediment becomes entrained in a flow the nature of the flow changes and so it is unreasonable to link the capacity of the water or wind only to the ability of the fluid to move sediment secondly environmental sediment transport is complicated and the range of processes involved in most movements means that simple relationships are unlikely to hold not least because the movement of sediment often changes the substrate which in turn affects the flow conditions thirdly the inherently stochastic nature of sediment transport means that any capacity relationships do not scale either in time or in space consequently new theories of sediment transport are needed to improve understanding and prediction and to guide measurement and management of all geomorphic systems

concentrated flow erosion as a main source of sediments in galicia spain knowledge of soil loss rates by water erosion under given climate soil topography and management conditions is important for establishing soil conservation schemes in galicia a region with atlantic climatic conditions in spain field observations over the last decade indicate that interrill rill and ephemeral gully erosion may be an important sediment source the aim of this work was to assess concentrated erosion rates describe types of rills and ephemeral gullies and determine their origin evolution and importance as sediment sources soil surface state and concentrated flow erosion were surveyed on medium textured soils developed over basic schists of the ordenes complex series coruna province spain from 1997 to 2006 soil surface state was characterized by crust development tillage features and roughness degree soil erosion rate was directly measured in the field concentrated flow erosion took place mainly on seedbeds and recently tilled surfaces in late spring and by autumn or early winter during the study period erosion rates were highly variable and the following situations could be distinguished a no incision or limited rill incision i e below 2 mg ha 1 year 1 b generalized rill and ephemeral gully incision in the class of mean values between 2 5 and 6 25 mg ha 1 year 1 this was the most common erosion pattern and c heavy erosion as observed during an extremely wet winter period between october 2000 and february 2001 with erosion figures that may be about ten orders of magnitude higher up to 55 60 mg ha 1 year 1 therefore low values of soil losses are dominant but also large values of rill and ephemeral gully erosion occurred during the study period copyright 2009 john wiley sons ltd

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

rain infiltration into loess soils from different geographic regions abstract loess soils are among the most erodible soils therefore evaluating and enhancing infiltration is paramount in controlling soil loss a laboratory study was conducted to evaluate the relative difference in infiltration among selected loess surface and subsurface soils from belgium canada china and the united states representing the major loess belts of the northern hemisphere soils were subjected to simulated rainstorms of constant intensity i 41 1mm h 1 duration 2 hours and energy rate 27 0 j m 2 per mm of rain infiltration runoff and soil water pressure were continuously monitored infiltration was described by a linear relationship for the pre ponding period and by a power series for the post ponding period differences in infiltration response were attributed to differences in soil properties such as differences in organic matter particle size swelling clay content fe oxyhydroxides and carbonates despite the high organic carbon content and coarser texture of the canadian loess surface soil the presence of highly expansive smectitic clay caused a rapid reduction in infiltration rates indicating the importance of soil mineralogical constituents in surface seal development

stochastic sediment transport in soil erosion a stochastic model governing the downslope transport of a sediment particle during an erosion event is described particles alternate between resting on the soil bed and being transported by the overland flow probability densities and moments are constructed for the distribution of a particle s position at a given time and also for time of passage to a given location we show that a suitable averaging of the stochastic particle motions in our model gives rise to the deterministic erosion differential of hairsine and rose hairsine p rose c 1991 soil sci soc am j 55 2 320 324 the model generalized h einstein s stochastic model der geschiebebetrieb als wahrscheinlichkeitsproblem verlag rascher zurich 1937 for bedload transport in streams

modelling soil erosion in a clayey subsurface drained agricultural field with a three dimensional flush model summary soil erosion is an important environmental issue in agricultural areas of northern europe where clayey soils are prevalent clayey soils are routinely subsurface drained to accelerate drainage which creates an additional discharge route for suspended sediment previously assessment of the sediment load from clayey fields has been difficult because process based models were only able to simulate sediment loads via surface runoff a new distributed process based erosion model was developed and incorporated into the flush modelling system to fulfil this void the model facilitates simulation of spatially distributed soil erosion on the field surface and sediment loads via surface runoff and subsurface drainflow soil erosion on the field surface is simulated with the two dimensional sediment continuity equation coupled with hydraulic and rain drop splash erosion sediment settling and transport capacity processes subsurface sediment transport in macropores is described with the three dimensional advection dispersion equation the model was applied to a clayey subdrained field section 3 6 ha in southern finland the results demonstrated the capability of the model to simulate soil erosion and sediment transport in terms of the match between the measured 2669 kg ha 1 and modelled 2196 kg ha 1 sediment loads via surface runoff and the measured 2937 kg ha 1 and modelled 2245 kg ha 1 loads via drainflow during the validation period of 7 months the model sensitivity analysis pointed out the importance of the flow model parameters in simulation of soil erosion through their control on the division of total runoff into surface runoff and drainflow components the key parameters in the erosion model were those that affected hydraulic and splash erosion rates the model application in the experimental field suggested that both hydraulic and splash erosion were the factors behind the sediment losses during the growing season and early autumn whereas high sediment loads in late autumn were caused by hydraulic erosion due to overland flow in tilled soil

water table fluctuation and recharge in semi arid climate some results of the hapex sahel hydrodynamic survey niger groundwater level measurements taken over a 4 year period from an extensive network of wells and boreholes within the hapex sahel hydrologic atmospheric pilot experiment in the sahel degree square south niger together with existing data have provided an insight into infiltration and recharge processes taking place in the porous phreatic aquifer of the continental terminal formation despite high spatial and temporal variability of aquifer response to rainfall rises of between 0 and 9 m are recorded a pattern of recharge can be recognised aquifer responses vary from site to site but the type of response at any single point tends to be consistent from year to year recharge is dominated by infiltration from temporary drainage networks pools and streams and aquifer response depends to a large extent on aquifer hydraulic characteristics and distance from the nearest infiltrating zone in many wells for which data extending back to 1987 is available water levels show a consistent year by year rise this is interpreted as a process of aquifer recovery following the severe drought of the 1970s and early 1980s though part may also be attributable to changing patterns of land management e g woodland clearance initial estimates of regional recharge are from 50 60 mm year 1 or in other words about 10 of annual rainfall the figure is supported by other methods of investigation hydrochemical analyses water budgets of pools

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

detachment and transportation of loose sediments by raindrop splash summary laboratory experiments on sediments ejected by splash consisted of two parts quantities ejected and mean projected distances were measured since the splash tray was 100 cm long and 20 cm wide and since both measurements were made on a receiving screen of one square metre it was obvious that only part of the translocated sediment was caught it was subject to serious doubt that the measured distances were representative for all sediment translocated if it is stated that the ejected sediment is distributed according to an exponential function around any point of impact located on the tray and if by definition tray and rainfall are perfectly homogeneous the ratio of the true to the measured distance and the ratio of the caught to the translocated sediment are only a function of the exponent of the real function the apparent distance being also a function of this exponent is thus the only factor needed to solve the two questions hence all empirical data were transformed into true data

micromorphological and mineralogical components of surface sealing in loess soils from different geographic regions mineralogy of clay sized particles and rainstorm characteristics are among the major factors that determine the nature of soil sealing this research was designed to further our knowledge about the role of clay mineralogy and rainstorm characteristics on crust formation four loess soils from different geographic locations canada usa china and belgium were used in this study preliminary studies were carried out on grenada bt usa lishi china and bierbeek bt belgium horizons detailed infiltration and microscopic studies were performed only on the bm1 horizon of the saskatchewan soil canada and the ap horizon of the grenada soil mississippi usa two levels of rainstorm were applied 40 and 80 mm h 1 while smectite was the major component in the saskatchewan bm 1 horizon vermiculite and illite were found to be the dominant clay minerals in the bt horizon of the grenada soil formation of the washed layer together with thin lamellar crust coincided with the preferential movement of clay sized particles through runoff in both the saskatchewan bm1 and grenada ap horizons the higher initial infiltration rate with higher rainfall intensity in both soils were likely due to higher matric suction but rapid reduction in infiltration in this treatment was due to rapid aggregate destruction and a dense packing of the fundamental soil particles no washed in zone could be observed in the bm1 horizon of the saskatchewan soil in both rainstorm intensities however drastic reduction in infiltration especially with high rainstorm intensity after 7 8 min was attributed mainly to the swelling of smectite in this soil material washed in materials were visible in the ap horizon of grenada under high rainstorm intensity due to the presence of iron oxides and clay coatings aggregates were stable and no sealing could be observed in the grenada bt and bierbeek bt materials the degree of seal development in the lishi soils was low this was due to low clay and high carbonate contents this study shows clearly that higher and continuous rainstorm intensity causes rapid seal development and as a result more erosion would occur especially in the soils with high smectite content