formation and stability of transverse and longitudinal sand dunes the shape of dunes depends on the history of wind regimes and sand availability in deserts exposed to winds from two different directions but with comparable magnitude dunes are found to be linear ridges which are either perpendicular or parallel to the mean wind direction depending on the angle between the two wind directions these dunes respectively observed for small and large angles between winds are called transverse and longitudinal dunes in both cases their large width hundreds of meters and evolution time scale years strongly limit the investigation of their dynamics and thus our understanding of such structures here we show that under water similar structures can be obtained but at much smaller space and time scales performing controlled experiments together with numerical simulations we highlight the physical mechanisms at play in the formation and long term evolution of these structures we show in particular that while longitudinal dunes are stable and extend in time transverse dunes are unstable they evolve into wavy ridges and eventually break into barchans if the sand supply is too low this fundamental difference is understood through the study of single sand piles and bars exposed to two winds in the case of a large angle between winds a sand pile grows a finger pointing in the average wind direction and transforms into a longitudinal dune such an elongation does not occur for a small angle where a sand pile evolves into a barchan these results explain the morphological differences between straight and long longitudinal dunes and sinuous transverse dunes while giving keys to infer the wind history or pattern state of development from the observation of dune shapes in the field

incipient bedforms in a bidirectional wind regime most terrestrial sand seas form at horse latitudes where the wind direction exhibits seasonal variation here we extend the two dimensional linear stability analysis of a flat sand bed associated with a unidirectional wind to the three dimensional case in order to account for multidirectional wind regimes focusing on the simplest case of bidirectional flow regimes we show that the transition from transverse to oblique or longitudinal patterns is controlled by the transport ratio and the divergence angle between the two flows our predictions agree with previous results for dune orientation and also provide a wider range of possible alignments depending on flow strength especially when the two winds are perpendicular at which the transition occurs this analysis also predicts the selected pattern wavelength which either decreases close to the transition angle for strong winds due to a geometric effect or increases at low winds when the bed slope affects the transport this theoretical analysis is complemented by analogous subaqueous experiments where bedforms are submitted to alternate water flows for transverse bedforms the experimental data validate the model at strong flows providing evidence for the predicted geometric effect but also for the increase of the wavelength close to the transport threshold for longitudinal bedforms a discrepancy is observed which we interpret as the sign of enhanced nonlinearities induced by the development of slip faces when the flow alternately blows on both sides of the dune

factors determining desert dune type while most observers recognize four elemental types of desert dunes longitudinal transverse barchan and star1 3 there is little agreement about which factors determine these types the angular relationships between the resultant of sand shifting winds and both the crest and principal slipfaces of the elemental types have been discussed qualitatively for many decades these relationships have been quantified but the wide range of dune types that combine the elemental forms and the juxtaposition of different dune types show that wind regime is not the only determinant for example sand availability has been considered important we have now quantified the importance of both wind regime and sand availability as determinants of dune type we show here that the four elemental types occur in areas uniquely defined by both the equivalent sand thickness of sediments held in the dunes and a measure of directional variability of sand moving winds wind strength does not appear to be important vegetation has an ambiguous role and the particle size of dune sediments is unimportant

early stage aeolian protodunes bedform development and sand transport dynamics early stage aeolian bedforms or protodunes are elemental in the continuum of dune development and act as essential precursors to mature dunes despite this we know very little about the processes and feedback mechanisms that shape these nascent bedforms whilst theory and conceptual models have offered some explanation for protodune existence and development until now we have lacked the technical capability to measure such small bedforms in aeolian settings here we employ terrestrial laser scanning to measure morphological change at the high frequency and spatial resolution required to gain new insights into protodune behaviour on a 0 06 m high protodune we observe vertical growth of the crest by 0 005 m in two hours our direct measurements of sand transport on the protodune account for such growth with a reduction in time averaged sediment flux of 18 observed over the crestal region detailed measurements of form also establish key points of morphological change on the protodune the position on the stoss slope where erosion switches to deposition is found at a point 0 07 m upwind of the crest this finding supports recent models that explain vertical dune growth through an upwind shift of this switching point observations also show characteristic changes in the asymmetric cross section of the protodune flow form feedbacks result in a steepening of the lee slope and a decline in lower stoss slope steepness by 3 constituting a reshaping of protodune form towards more mature dune morphology the approaches and findings applied here a demonstrate an ability to quantify processes at requisite spatial and temporal scales for monitoring early stage dune evolution b highlight the crucial role of form flow feedbacks in enabling early stage bedform growth alluding to a fluctuation in feedbacks that require better representation in dune models and c provide a new stimulus for advancing understanding of aeolian bedforms

experimental study on progressive failure process and permeability characteristics of red sandstone under seepage pressure abstract rock masses in underground spaces often experience in situ stress field and seepage field coupling which gives them complex mechanical behavioural and permeability characteristics in this study a high pressure triaxial automated system for rock hptas is employed in triaxial compression testing of red sandstone under different seepage pressures and the variation laws of strength deformation axial strain stiffness and permeability during rock failure are analysed the results show that with changes in the seepage pressure there are also changes in the rock s strength and ability to resist deformation and the stress threshold decreases with an increase in the seepage pressure in addition seepage pressure does not affect the strain stiffness trend but it affects the value throughout the process of stress and strain the permeability curve firstly decreases and then increases rapidly before finally stabilizing and the peak value of the permeability curve lags behind that of the stress strain curve according to the law of permeability evolution during the pre peak progressive failure process a piecewise functional relationship model between permeability and stress is established in this study and theoretical values agree well with experimental results these experimental results can be used as a reference for monitoring and controlling rock stability during geological engineering when a seepage stress coupling action is applied

morphology and dynamics of star dunes from numerical modelling star dunes are common in sand seas but the mechanisms driving their formation are unclear numerical modelling indicates that the morphology of the dunes is controlled by the frequency of changes in the wind regime

influence of water content on the strength of rock water content is one of the most important factors influencing rock strength considerable research has been carried out to investigate rock strength under both dry and water saturated conditions according to these results the petrophysical properties of rocks decrease with increasing moisture and this can result in an increase in the mechanical compliance in some cases in several cases the strength decrease is remarkable after only 1 water saturation for rock mechanics and rock engineering projects it is strongly recommended that the dry uniaxial compressive strength is used for the purposes of strength classification while for the actual engineering design it is essential to establish the wet strength and ideally the water sensitivity of the rock in order to asses their potential change in strength and deformability the goal of this paper is to show a method for estimating the sensitivity of sandstone rocks to water content using the published data

investigations on fluid transport properties in the north german rotliegend tight gas sandstones and applications tight gas reservoirs are an important part of the world gas resources such reservoirs have very low permeability usually below 0 1 md and show a strong stress sensitivity to fluid transport properties and a considerable productivity decline during the production process due to increasing effective stress in an experimental study several measurement series were performed on plugs from the north german rotliegend tight gas reservoirs to improve knowledge and understanding of the effects of changing stress and pore pressure conditions on reservoir rocks during gas production in addition to the experimental study an interactive rock data catalog irdc has been developed which contains a database coupled with a correlation module the database contains metadata of the fields and wellbores included in the irdc the corresponding log data and petrophysical data the correlation module contains correlations derived both from the study measurements and from literature the results of this study will enable reservoir engineers to select specific data from the database and process it in the correlation module to generate secondary sets of data which can then be used for modeling and simulation of tight gas reservoirs

effects of water on rock strength in a brittle regime abstract failure strengths of granite and andesite have been measured under various conditions of strain rate and confining pressure both in the dry and wet states constant stress creep tests for granitic samples have been also conducted all specimens show that the failure strength decreased linearly as the logarithm of the strain rate decreased the strain rate dependence of the failure strength is increased at higher confining pressures the strain rate effect is more apparent on the failure strengths of wet samples than dry samples in lower confining pressure ranges in the constant stress creep experiments the creep failure strength decreased as the logarithm of the time to failure increased time dependent failure strength of all rocks observed in the regime of the present study may be interpreted by subcritical crack growth assisted by the stress corrosion mechanism

measurements of the aeolian sand transport saturation length the wavelength at which a dune pattern emerges from a flat sand bed is controlled by the sediment transport saturation length which is the length needed for the sand flux to adapt to a change in wind strength the influence of the wind shear velocity on this saturation length and on the subsequent dune initial wavelength has remained controversial in this paper we present direct measurements of the saturation length performed in a wind tunnel experiment complementary to this initial dune wavelengths are measured under different wind conditions in particular after storms using the linear stability analysis of dune formation it is then possible to deduce the saturation length from field data both direct and indirect measurements agree that the saturation length is almost independent of the wind strength this result supports the idea that grain inertia is the dominant dynamical mechanism limiting sediment transport saturation on dunes

barchan dune corridors field characterization and investigation of control parameters 1 the structure of the barchan field located between tarfaya and laayoune atlantic sahara morocco is quantitatively investigated and compared to that in la pampa de la joya arequipa peru on the basis of field measurements we show how the volume the velocity and the output sand flux of a dune can be computed from the value of its body and horn widths the dune size distribution is obtained from the analysis of aerial photographs it shows that these fields are in a statistically homogeneous state along the wind direction and present a corridor structure in the transverse direction in which the dunes have a rather well selected size investigating the possible external parameters controlling these corridors we demonstrate that none among topography granulometry wind and sand flux is relevant we finally discuss the dynamical processes at work in these fields collisions and wind fluctuations and investigate the way they could regulate the size of the dunes furthermore we show that the overall sand flux transported by a dune field is smaller than the maximum transport that could be reached in the absence of dunes i e in saltation over the solid ground

an agent based model of dune interactions produces the emergence of patterns in deserts 1 crescent shaped barchan dunes are highly mobile dunes which are ubiquitous on earth and other solar system bodies although they are unstable when considered separately they form large assemblies in deserts and spatially organize in narrow corridors that extend in the wind direction collision of barchans has been proposed as a mechanism to redistribute sand between dunes and prevent the formation of very large dunes here we use an agent based model with elementary rules of sand redistribution during collisions to access the full dynamics of very large barchan fields we tune the dune field density by changing the sand load lost ratio and follow the transition between dilute fields where barchans barely interact and dense fields where dune collisions control and stabilize the dune field in this dense regime barchans have a small well selected size and form flocks the dune field self organizes in narrow corridors of dunes as it is observed in real dense barchan deserts

mechanical and permeability characteristics of rock under hydro mechanical coupling conditions a study on the mechanical characteristics and permeability properties of sandstone and limestone from a coal mine under 3d stresses has been carried out with the mts815 rock mechanics test system and the characteristics of mechanics and permeability before and after the rock failure under hydro mechanical coupling conditions have been analyzed and discussed the research revealed the difference of mechanics and permeability before and after the rock failure and obtained the relationship among the strength deformation and differential water pressure it also obtained the correlation among the permeability and stress strain and revealed the features of fractures which influenced the correlation the results show that there are obvious weakening effects of differential water pressures on the mechanical properties after the rock damages the permeability of rock mass in practical engineering is mainly controlled by the features of fractures the permeability has an negative index correlation with the axial strain e 1 and has a power function correlation with the mean stress m m 1 2 3 3 for fractured limestone and coarse sandstone the permeabiity will tend to be stable gradually when the mean stress reaches or exceeds a certain value ml

shear wave splitting observations in the chinese tianshan orogenic belt 1 a linear array of 26 3 component broadband digital stations across the chinese tianshan orogenic belt ctob was deployed for 13 months to measure the shear wave splitting fast direction of teleseismic shear waves beneath the southern ctob is nww see and gradually turns into nee sww in the north delay times are 0 65 1 48s with average value of 0 97 0 15s and the maximum delay time is observed in the middle mountain and decrease in the south and the north fast directions are generally parallel to the strike of the orogenic belt indicating that more than one hundred kilometers of upper mantle is involved in collisional deformation compared to other stations change in fast direction for stations in the south and north ends of the array is probably resulted from subduction of the tarim block and the junggar block into the tianshan range respectively

development and steady states of transverse dunes a numerical analysis of dune pattern coarsening and giant dunes we investigate the development and steady states of transverse dunes for ranges of flow depths and velocities using a cellular automaton dune model subsequent to the initial bed instability dune pattern coarsening is driven by bed form interactions collisions lead to two types of coalescence associated with upstream or downstream dominant dunes in addition a single collision ejection mechanism enhances the exchange of mass between two adjacent bed forms throughpassing dunes the power law increases in wavelength and amplitude exhibit the same exponents which are independent of flow properties contrary to the wavelength dune height is limited not only by flow depth but also by the strength of the flow superimposed bed forms may propagate and continuously destabilize the largest dunes we identify three classes of steady state transverse dune fields according to the periodicity in crest to crest spacing and the mechanism of size limitation in all cases the steady state is reached and maintained through the dynamic equilibrium between flow strength and dune aspect ratio in the limit of low flow strength where it becomes the primary factor of size limitation the bed shear stress in the dune trough regions is close to its critical value for motion inception comparisons with natural dune fields suggest that many of them may have reached a steady state finally we infer that the sedimentary patterns in the model may be used to bring new constraints on the development of modern and ancient dune fields

effect of confining presurre pore pressure and specimen dimension on permeability of yinzhuang sandstone abstract tests on yinzhuang sandstone for measuring permeability under different confining pressure pore pressure and specimen dimensions were carried out with an orthogonal experimental design table l 16 4 3 having 3 experimental factors and 4 levels a total of 16 tests had been performed the permeability of yinzhuang sandstone was found as a function of the axial stress and strain strongly but the effect of confining pressure pore pressure and specimen dimensions on permeability was not obvious for most regions in a complete stress strain path only in some individual regions some individual factors discovered its own significant effect for example the confining pressure gave most significant influence on permeability in the strain softening region whereas the ratio of pore pressure to confining pressure discovered an obvious effect only on the value of highest permeability

setting the length and time scales of a cellular automaton dune model from the analysis of superimposed bed forms 1 we present a new 3 d cellular automaton model for bed form dynamics in which individual physical processes such as erosion deposition and transport are implemented by nearest neighbor interactions and a time dependent stochastic process simultaneously a lattice gas cellular automaton model is used to compute the flow and quantify the bed shear stress on the topography local erosion rates are assumed to be proportional to the shear stress in such a way that there is a complete feedback mechanism between flow and bed form dynamics in the numerical simulations of dune fields we observe the formation and the evolution of superimposed bed forms on barchan and transverse dunes using the same model under different initial conditions we perform the linear stability analysis of a flat sand bed disturbed by a small sinusoidal perturbation comparing the most unstable wavelength in the model with the characteristic size of secondary bed forms in nature we determine the length and time scales of our cellular automaton model thus we establish a link between discrete and continuous approaches and open new perspectives for modeling and quantification of complex patterns in dune fields

global map of eolian features on mars ten basic categories of eolian features on mars were identified from a survey of mariner 9 and viking orbiter images the ten features mapped are 1 light streaks including frost streaks 2 dark streaks 3 sand sheets or splotches 4 barchari dunes 5 transverse dunes 6 crescentic dunes 7 anomalous dunes 8 yardangs 9 wind grooves and 10 deflation pits the features were mapped in groups not as individual landforms and recorded according to their geographic positions and orientations on maps of 1 12 5 million or 1 25 million scale in the north polar region light and dark streaks indicate winds from the west and northwest frost streaks show northeast and northwest winds barchan dunes show southwest and west winds transverse dunes show east and west winds local erosional features show winds from the northwest or southeast and northeast or southwest in the middle and low northern latitudes streaks show northeast winds yardangs and deflation pits show mostly easterly and northeasterly winds in the low southern latitudes light streaks and dunes record northwest and northeast winds dark streaks record southeast winds in high southern latitudes most streaks and dunes record southeast and east winds in the south polar region light and frost streaks record southwest and northwest winds whereas dark streaks and transverse dunes show southeast winds the patterns recorded for ephemeral features conform to global conditions of strong southern spring and summer northern fall and winter wind circulation erosional features in bedrock indicate long term and perhaps ancient wind trends whereas depositional features may record relatively more recent winds wind directions indicated by yardangs pits and other erosional features seldom correspond to those shown by streaks these erosional features indicating winds of different directions than those that formed the streaks may have been carved when the effective regional winds on mars were different due to relative hemisphere insolation differences which are related to obliquity variations alternatively many yardangs and pits may be largely controlled by structure and carved along surface structural patterns or features that channeled both ancient and present day winds deflation pits in the mantled terrain may contain the best record of ancient wind trends these pits seem to have stratigraphically related orientations different stratigraphic units composing the mantle contain wind erosion pits that possibly record large scale wind direction changes because of long term changes in the obliquity of the mars

modeling emergent large scale structures of barchan dune fields in nature barchan dunes typically exist as members of larger fi elds that display striking enigmatic structures that cannot be readily explained by examining the dynamics at the scale of single dunes or by appealing to patterns in external forcing to explore the possibility that observed structures emerge spontaneously as a collective result of many dunes interacting with each other we built a numerical model that treats barchans as discrete entities that interact with one another according to simplifi ed rules derived from theoretical and numerical work and from fi eld observations 1 dunes exchange sand through the fl uxes that leak from the downwind side of each dune and are captured on their upstream sides 2 when dunes become suffi ciently large small dunes are born on their downwind sides calving and 3 when dunes collide directly enough they merge results show that these relatively simple interactions provide potential explanations for a range of fi eld scale phenomena including isolated patches of dunes and heterogeneous arrangements of similarly sized dunes in denser fi elds the results also suggest that 1 dune fi eld characteristics depend on the sand fl ux fed into the upwind boundary although 2 moving downwind the system approaches a common attracting state in which the memory of the upwind conditions vanishes this work supports the hypothesis that calving exerts a fi rst order control on fi eld scale phenomena it prevents individual dunes from growing without bound as single dune analyses suggest and allows the formation of roughly realistic persistent dune fi eld patterns

how does the water rock interaction of marly rocks affect its mechanical properties in the three gorges reservoir area china marly rock is a sensitive rock group of landslides in the three gorges reservoir area china it is composed predominantly of carbonate and clay minerals water rock interaction wri of which could activate landslides in the reservoir area to study the mechanism by which wri affects the mechanical properties of marly rock samples were collected from two boreholes depth 301 78 and 307 14 m and slope surface then laboratory tests were designed to study the quantitative relationship between mineral contents and mechanical properties of intact rock to analyze both change process of mineral composition and microstructure under short term and long term wri finally the change in mechanical properties and its effect on slope stability are suggested this study indicates that the uniaxial compressive strength and poisson ratio can be estimated by linear regression equations 1 c 8 959 c q 0 744 cm 58 516 2 0 014 c q 0 001 cm 0 234 the chemical reactions of wri mainly included dissolution and ion exchange on the slope surface dissolution mainly acts on calcite illite dolomite feldspar and other minerals dissolved in water underground both chemical reactions of dissolution and ion exchange easily approach equilibrium with long term seepage small size minerals and micropores damage the stable microstructure of marly rock these changes of minerals and microstructure can trigger shallow slope failure and develop deep creep deformation along some crash zones in the reservoir shoreline

the sand seas of titan cassini radar observations of longitudinal dunes the most recent cassini radar images of titan show widespread regions up to 1500 kilometers by 200 kilometers of near parallel radar dark linear features that appear to be seas of longitudinal dunes similar to those seen in the namib desert on earth the ku band 2 17 centimeter wavelength images show 100 meter ridges consistent with duneforms and reveal flow interactions with underlying hills the distribution and orientation of the dunes support a model of fluctuating surface winds of 0 5 meter per second resulting from the combination of an eastward flow with a variable tidal wind the existence of dunes also requires geological processes that create sand sized 100 to 300 micrometer particulates and a lack of persistent equatorial surface liquids to act as sand traps

bedform alignment in directionally varying flows many kinds of sediment bedforms are presumed to trend either normal or parallel to the direction of sediment transport for this reason the trend of bedforms observed by remote sensing or by field observations is commonly used as an indicator of the direction of sediment transport such presumptions regarding bedform trend were tested experimentally in bidirectional flows by rotating a sand covered board in steady winds transverse oblique and longitudinal bedforms were created by changing only two parameters the angle between the two winds and the proportions of sand transported in the two directions regardless of whether the experimental bedforms were transverse oblique or longitudinal as defined by the bedform trend relative to the resultant transport direction they all had trends that yielded the maximum gross transport across the bedforms the fact that many of the experimental bedforms were neither transverse nor parallel to the resultant transport direction suggests that transport directions cannot be accurately determined by presuming such alignment

giant aeolian dune size determined by the average depth of the atmospheric boundary layer sand dunes common on earth and other planetary bodies still hold some mysteries the ways that small dunes form those with lengths of tens of metres are well understood but much less clear is how giant kilometre scale dunes come into existence what dictates their shapes and what limits their size andreotti et al use a combination of field measurements and aerodynamic calculations to show that the growth of giant dunes is limited by the confinement of the sand transport flow over the average depth of the atmospheric boundary layer the new findings can explain the mean spacing of wind generated giant dunes up to 3 5 km in length this paper shows that the growth of aeolian giant dunes ascribed to the non linear interaction between small scale superimposed dunes is limited by the confinement of the flow over the average depth of the atmospheric boundary layer the findings explain the mean spacing of aeolian giant dunes ranging from 300 metres in coastal deserts to 3 5 kilometres depending on the wind regime1 2 sand dunes exhibit linear3 4 crescent shaped or star like forms5 resulting from the interaction between dune morphology and sand transport6 7 8 small scale dunes form by destabilization of the sand bed9 10 11 with a wavelength a few tens of metres determined by the sand transport saturation length11 12 13 the mechanisms controlling the formation of giant dunes and in particular accounting for their typical time and length scales have remained unknown using a combination of field measurements and aerodynamic calculations we show here that the growth of aeolian giant dunes ascribed to the nonlinear interaction between small scale superimposed dunes4 10 14 15 is limited by the confinement of the flow within the atmospheric boundary layer16 17 aeolian giant dunes and river dunes form by similar processes with the thermal inversion layer that caps the convective boundary layer in the atmosphere18 acting analogously to the water surface in rivers in both cases the bed topography excites surface waves on the interface that in turn modify the near bed flow velocity this mechanism is a stabilizing process that prevents the scale of the pattern from coarsening beyond the resonant condition our results can explain the mean spacing of aeolian giant dunes ranging from 300 m in coastal terrestrial deserts to 3 5 km we propose that our findings could serve as a starting point for the modelling of long term evolution of desert landscapes under specific wind regimes

two modes for dune orientation earth s sand seas dune fields experience winds that blow with different strengths and from different directions in line with the seasons in response dune fields show a rich variety of shapes from crescentic barchans to star and linear dunes these dunes commonly exhibit complex and compound patterns with a range of length scales and various orientations which up to now have remained difficult to relate to wind cycles here we develop a model for dune orientation that explains the coexistence of bedforms with different alignments in multidirectional wind regimes this model derives from subaqueous experiments which show that a single bidirectional flow regime can lead to two different dune orientations depending on sediment availability i e the erodibility of the bed sediment availability selects the overriding mechanism for the formation of dunes increasing in height from the destabilization of a sand bed with no restriction in sediment availability or elongating in a finger on a non erodible surface from a localized sand source these mechanisms drive the dune orientation therefore dune alignment maximizes dune orthogonality to sand fluxes in the bed instability mode while dunes are aligned with the mean sand transport direction in the fingering mode applied to earth s deserts the model quantitatively predicts the orientation of rectilinear dunes and their superimposed patterns this field study suggests that many linear dunes on earth elongate from sources and are simply aligned with the mean sand transport direction

extraterrestrial dunes an introduction to the special issue on planetary dune systems abstract aeolian dune fields have been described on earth mars venus and titan the plethora of data returned from recent planetary missions has enabled a new era in planetary geomorphic studies much of our understanding of planetary dune systems comes from the application of earth analogs wind tunnel experiments and modeling studies despite the range of atmospheric pressures composition and gravity many of the dune forms on extraterrestrial surfaces are similar to those on earth although some have notable differences in bedform scale and composition as an introduction to the special issue on planetary dune systems this paper summarizes the current state of knowledge of planetary dune studies and highlights outstanding questions that require further investigation

sediment flux from the morphodynamics of elongating linear dunes although dunes are very common bedforms in terrestrial sand seas the description of linear dune growth either by extension or lateral accretion is still hindered by our limited understanding of the underlying mechanisms therefore sand flux estimates from remote imagery rely essentially on the migration speed of barchan dunes but not on the dynamics of linear dunes here we use 50 yr of high resolution aerial and satellite imagery of the tenere desert niger the world s largest source of mineral aerosols to demonstrate that linear dunes can elongate in the direction of the resultant sand flux with no lateral migration as they elongate from topographic obstacles in a zone of low sediment availability with multimodal winds these elongating lee dunes are ideal to isolate and quantify linear dune growth only by extension using similar conditions in a numerical model we show how deposition downstream of low hills may result in nucleation and development of bedforms from elongation we derive the local sand flux parallel to the linear dune crests this study shows that the morphodynamics of linear dunes under complex wind regimes can also be used for assessing sediment flux and wind conditions comparably to the more established method of using sand flux estimates perpendicular to the barchan dune crests in zones of unidirectional wind

fluid permeability of sedimentary rocks in a complete stress strain process abstract the permeability of sedimentary rocks during triaxial compression tests was investigated to relate it to the complete strain stress process it was found that the permeability was not constant but varied with the stress and strain states in the rocks prior to the peak strength the permeability decreases with increasing load a dramatic increase in permeability occurs during the strain softening period in the present study in situ measurements of fluid flow and pressure in floor strata was carried out in a double longwall mining face in the yangzhuang colliery these measurements show that both the strata pressure and the position with respect to the mining face influence the hydrogeologic properties the permeability increased in the floor strata behind the mining face because those mining induced fractures opened as the strata pressure decreased to better understand this change in hydraulic behavior around the mining faces 3 d numerical modeling was carried out the model provides the general picture of the stress distribution and failure zone both in the floor and roof strata the field and model results demonstrate the importance of changes in the stress and strain states on the hydrogeology of a site

aeolian dune field pattern boundary conditions aeolian dune field patterns reflect the complex external environment within which the pattern evolves these external environmental controls are the boundary conditions on aeolian dune field pattern formation the influences of boundary conditions such as wind regime and sediment supply are well known however boundary conditions represent a relatively unexplored area of dune field pattern formation source area geometry and areal limits are two newly recognized boundary conditions measurements of crest spacing and crest length from satellite images of dune field patterns with point and line source area geometries show an increase in spacing and crest length over distance whereas spacing and crest length in plane sourced patterns emerge equally across the dune field the impact of the size and shape of a dune field on crest spacing is tested using a previously established analytical model model results indicate that the area of a dune field limits the maximum spacing that can occur within a given area and that dune fields that are five times longer in the direction of dune migration than in the crest parallel direction can achieve the greatest spacing within a given area empirical measurements of spacing defect density and dune field area from ten different dune fields ranging over four orders of magnitude in size show that spacing increases and defect density decreases as dune field area increases because the formation of all dune fields involves a wind regime and sediment supply there must be interplay of boundary conditions within the same dune field at different stages of pattern development one boundary condition may dominate over others and with changing climatic eustatic or tectonic parameters boundary conditions may change to modify an existing pattern recognizing boundary condition controls on aeolian dune field pattern formation provides a framework for recognizing the signature of the external environment in which a pattern developed and can be used for reconstructing past dune constructional events and climatic change

growth mechanisms and dune orientation on titan dune fields on titan cover more than 17 of the moon s surface constituting the largest known surface reservoir of organics their confinement to the equatorial belt shape and eastward direction of propagation offer crucial information regarding both the wind regime and sediment supply herein we present a comprehensive analysis of titan s dune orientations using automated detection techniques on nonlocal denoised radar images by coupling a new dune growth mechanism with wind fields generated by climate modeling we find that titan s dunes grow by sediment transport on a nonmobile substratum to be fully consistent with both the local crestline orientations and the eastward propagation of titan s dunes the sediment should be predominantly transported by strong eastward winds most likely generated by equinoctial storms or occasional fast westerly gusts additionally convergence of the meridional transport predicted in models can explain why titan s dunes are confined within 30 latitudes where sediment fluxes converge