the effect of cutoff walls on saltwater intrusion and groundwater extraction in coastal aquifers summary we use numerical simulations to study the effect of cutoff walls on saltwater intrusion and assess their protective effect on groundwater extractions near the coast results are presented in terms of dimensionless variables with ranges suitable for field applications we find that in the absence of groundwater extractions the effectiveness of cutoff walls is determined by the wall depth its distance from the coast the velocity ratio the intensity of mixing the conductivity anisotropy and the relative conductivity of the wall to assess the effectiveness of cutoff walls in practical applications we present graphs and empirical equations also we assess the protective effect of cutoff walls on groundwater extractions by means of drains and single wells we do so by calculating the maximum safe extraction rate before and after the construction of a cutoff wall i e the maximum rate of groundwater extraction for which the maximum salt concentration entering the drain or the well is below a certain threshold we find that the protective effect of cutoff walls is stronger for single wells rather than drains and for cases when the extractions are located at relative small distances from the coast relative large depths and in aquifers with small velocity ratio weak mixing and high anisotropy to facilitate the design of safe extractions we use numerical results to develop analytical approximations for the influence of cutoff walls

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

aeolian dune migration along the ceara coast north eastern brazil aeolian dune dimensions and migration rates are analysed along the ceara coast north east brazil dunes that are currently mobile along the ceara coast are composed of barchans and sand sheets the results show that barchans maintain an equilibrium form which can be characterized by values of dimensionless shape parameters h w and w l where h is the dune height w is the wing to wing width and l is the dune length dunes are highly mobile with average migration rates of 17 5 m year 1 for barchans and 10 m year 1 for sand sheets the calculated migration rates were found to depend strongly on dune dimensions for both barchans and sand sheets i e the larger the dune is the lower the migration rate will be this size dependence was associated with the existence of a representative common transport rate along the dune fields which induces a different dune migration rate dependent on dune size finally from the observed dune evolution an aggregated scale aeolian sediment transport was inferred this bulk transport rate of the order of 90 100 m3 m 1 year 1 is only valid for a timescale of years to decades which is the timescale used in dune evolution analysis

conceptual models of the evolution of transgressive dune field systems abstract this paper examines the evolutionary paths of some transgressive dune fields that have formed on different coasts of the world and presents some initial conceptual models of system dynamics for transgressive dune sheets and dune fields various evolutionary pathways are conceptualized based on a visual examination of dune fields from around the world on coasts with high sediment supply dune sheets and dune fields tend to accumulate as large scale barrier systems with little colonization of vegetation in arid hyper to arid climate regimes and as multiple active discrete phases of dune field and deflation plain couplets in temperate to tropical environments active dune fields tend to be singular entities on coasts with low to moderate sediment supply landscape complexity and vegetation richness and diversity increases as dune fields evolve from simple active sheets and dunes to single and multiple deflation plains and basins precipitation ridges nebkha fields and a host of other dune types associated with vegetation e g trailing ridges slacks remnant knobs gegenwalle ridges and dune track ridges tree islands and bush pockets three principal scenarios of transgressive dune sheet and dune field development are discussed including dune sheets or dune fields evolving directly from the backshore development following foredune and or dune field erosion and development from the breakdown or merging of parabolic dunes various stages of evolution are outlined for each scenario knowledge of evolutionary patterns and stages in coastal dune fields is very limited and caution is urged in attempts to reverse change and or modify dune fields to restore some perceived loss of ecosystem or dune functioning

subsurface seawater intrusion barrier analysis seawater intrusion barriers are currently being constructed in the coastal aquifers of japan based on the results of a computer model employing finite difference numerical techniques this model provides a practical design and management tool for predicting the likelihood of seawater intrusion through a semipervious subsurface barrier in an unconfined coastal aquifer the analysis assumes an abrupt interface the dupuit approximations and various simplifying boundary conditions the movements of both the water table and the freshwater saltwater interface are numerically simulated in the unsteady state the computed results are validated with observations obtained from a vertical two dimensional sand box model the application of the computer model is illustrated by a site specific analysis for a proposed subsurface barrier location in a limestone coastal aquifer on okinawa jima island in the western pacific ocean from this analysis it is found that the subsurface barrier is able to delay seawater intrusion for about 2 mo under critical conditions of continual pumping of the aquifer without recharge the subsurface barrier is technically feasible and is a viable solution to the problem of seawater intrusion in coastal aquifers

a simple model for the formation of vegetated dunes a simple model for the dynamics of dunes associated with vegetation is proposed using the model the formation processes of transverse dunes parabolic dunes and elongated parabolic dunes are simulated according to two environmental factors i the amount of sand at the source ii the wind force the results have qualitative correspondence to the real counterpart and the simplicity of the algorithm and the consequent ease of handling this model provide us with wide applicability for the investigation of the complex interplay between vegetation and dunes copyright 2001 john wiley sons ltd

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

late highstand patterns of shifting and stepping coastal barriers and washover fans late messinian sorbas basin se spain abstract the late messinian sorbas member up to 75 m thick consists in its type area of a parasequence set of three prograding coastal barriers sequences i iii associated with lagoon and washover sediments around the town of sorbas these strata can be studied exceptionally well due to absence of burrowing by raised messinian salinities and exposure along a network of up to 30 m deep canyons fifteen vertical sections were logged and carefully correlated this permits to reconstruct and discuss patterns of relative sea level movements between decimetres up to 15 m within a parasequence excellent examples of non tidal transgressive facies are characterized by lagoon and washover sediments instead of the usual combination of washover and tidal deposits channel and flood tidal delta implications for the sandstone connectivity are given the lower two sequences are deposited in a relatively large tectonically enhanced wedge shaped accommodation space they show both fining up deepening sequences followed by prograding coarsening up shoaling sequences and can be compared to the classical parasequences of the western interior basin usa progradation of sequence ii was interrupted by a major slide event most likely triggered by an earthquake which caused more than 400 m seaward slumping of a stretch of 1 km of coastal sands the architecture of sequence iii is more complex due to limited accommodation space characteristic for the late highstand so that this setting was very sensitive to sea level fluctuations this resulted in an intricate pattern of juxtaposed and superposed lagoonal muds washover fans and swash zones so called stranded coastal barriers occur which were left behind after seaward jump of the coastline over more than 1 5 km during forced regression the pattern of reconstructed sea level positions is well comparable to the sequential pattern shown by the correlated equivalents along the northern basin margin which belong to the so called terminal carbonate complex the problem of ranking the complex sequence iii as one or more parasequences and its consequences for cyclostratigraphy are shortly discussed two models of washover formation are given respectively during more rapid and more slow sea level rise the influence of synsedimentary folding on the location of barriers is discussed and also the source area of extra and intraclasts supplied to barriers and washovers after deposition of the sorbas member the sea withdrew from the sorbas basin probably as a result of the major downdrop in the mediterranean at the maximum isolation during the salinity crisis it is suggested that the semi enclosed setting of the basin resulted only in limited localized erosion in contrast to the deeper adjacent vera basin which was more open to the mediterranean

high resolution holocene environmental changes in the thar desert northwestern india sediments from lunkaransar dry lake in northwestern india reveal regional water table and lake level fluctuations over decades to centuries during the holocene that are attributed to changes in the southwestern indian monsoon rains the lake levels were very shallow and fluctuated often in the early holocene and then rose abruptly around 6300 carbon 14 years before the present 14c yr b p the lake completely desiccated around 4800 14c yr b p the end of this 1500 year wet period coincided with a period of intense dune destabilization the major harrapan indus civilization began and flourished in this region 1000 years after desiccation of the lake during arid climate and was not synchronous with the lacustral phase

a numerical solution for the movement of an interface in a layered coastal aquifer abstract this paper deals with seawater intrusion in a coastal aquifer where close to the coast the aquifer is divided into a number of subaquifers by impervious relatively thin layers as a result of changing the total fresh water discharge to the sea seawater intrusion occurs as a separate advancing wedge in each of the subaquifers assuming everywhere flow normal to the coast line and essentially horizontal in both the fresh water and the salt water zones the complete mathematical model is first stated in terms of depth to the interface in each subaquifer a numerical finite difference scheme is then presented for solving the model for the case of a two layered aquifer principles of a method for extending the model to a larger number of subaquifers is described

modelling vegetated dune landscapes this letter presents a self organising cellular automaton model capable of simulating the evolution of vegetated dunes with multiple types of plant response in the environment it can successfully replicate hairpin or long walled parabolic dunes with trailing ridges as well as nebkha dunes with distinctive deposition tails quantification of simulated landscapes with eco geomorphic state variables and subsequent cluster analysis and pca yields a phase diagram of different types of coastal dunes developing from blow outs as a function of vegetation vitality this diagram indicates the potential sensitivity of dormant dune fields to reactivation under declining vegetation vitality e g due to climatic changes nebkha simulations with different grid resolutions demonstrate that the interaction between the abiotic geomorphic processes and the biological vegetation component life introduces a characteristic length scale on the resultant landforms that breaks the typical self similar scaling of un vegetated bare sand dunes

influence of high water levels on aeolian sand transport upper beach dune evolution on a macrotidal coast wissant bay northern france abstract aeolian sand transport measurements and detailed topographic surveys were carried out during 1 year along a macrotidal upper beach dune system experiencing rapid coastal retreat aeolian transport measurements coupled with wind records showed extreme spatial and temporal variability along a spatially limited upper beach sector analysis of wind velocity rainfall and water level showed that aeolian sand transport may occur all year long on the upper beach with the most energetic conditions occurring during winter and spring analysis of volume change revealed that aeolian sedimentation on the upper beach occurred only during the summer period while the rest of the year was characterised by upper beach and dune scarp erosion this erosion due to storm events combined with high water levels completely eliminated summer aeolian sand accumulation our results show that aeolian transport and upper beach dune evolution on this macrotidal beach is strongly controlled by the magnitude and frequency of occurrence of high water levels this study illustrates the fact that upper beach dune evolution cannot be completely understood and satisfactorily modelled if only potential aeolian sand transport is considered

model for the genesis of coastal dune fields with vegetation vegetation greatly affects the formation and dynamics of dune fields in coastal areas in the present work we use dune modeling in order to investigate the genesis and early development stages of coastal dune fields in the presence of vegetation the model which consists of a set of coupled equations for the turbulent wind field over the landscape the saltation flux and the growth of vegetation cover on the surface is applied to calculate the evolution of a sand patch placed upwind of a vegetated terrain and submitted to unidirectional wind and constant sand influx different dune morphologies are obtained depending on the characteristic rate of vegetation growth relative to wind strength barchans transverse dunes with trailing ridges parabolic dunes and vegetated alongshore sand barriers or foredunes the existence of a vegetation free backshore is found to be important for the nucleation timescale of coastal dune generations the role of the sand influx and of the maximum vegetation cover density for the dune shape is also discussed

model for a dune field with an exposed water table abstract aeolian transport in coastal areas can be significantly affected by the presence of an exposed water table in some dune fields such as in lencois maranhenses northeastern brazil the water table oscillates in response to seasonal changes of rainfall and rates of evapotranspiration rising above the ground during the wet season and sinking below in the dry period understanding dune mobility in an environment with a varying groundwater level is essential for the study of long term evolution of many dune fields here we apply a model for aeolian dunes to study the genesis of coastal dune fields in the presence of an oscillating water table we find that the morphology of the field depends on the time cycle tw of the water table and the maximum height hw of its oscillation our calculations show that long chains of barchanoids alternating with interdune ponds such as those found at lencois maranhenses arise when tw is of the order of the dune turnover time whereas hw dictates the growth rate of dune height with distance downwind

formation of parabolic dunes from barchan and transverse dunes along israel s mediterranean coast the mediterranean coastal dunes of israel underwent a land use change during the second half of the 20th century due to intense agricultural and pastoral activity the coastal dunes were stripped of natural vegetation until the end of the first half of the 20th century the barchan and transverse dunes were shaped by strong southwesterly winter winds a decrease in human activity during the second half of the 20th century brought about a renewal of natural vegetation on the dune crest the only area with neither erosion nor deposition the establishment of vegetation on the crest changes the dynamics of these barchan and transverse dunes so that not all of the sand eroded from the windward side is carried to the lee slip face some is trapped by plants consequently there is a change in the shape of the windward slope from convex to concave and the dune gradually becomes parabolic in this study we trace the morphodynamics of the dunes by analysing 12 sets of aerial photographs which were taken from 1944 to 1995 the average rate of advance of 15 dunes has decreased from 3 4 m a 1 to 1 9 m a 1 while the vegetation cover has increased from 4 3 to 17 per cent during this period copyright 2002 john wiley sons ltd

stratigraphic evolution and preservation of aeolian dune and damp wet interdune strata an example from the triassic helsby sandstone formation cheshire basin uk new and previously published models of wet aeolian system evolution form a spectrum of types that may be explained in terms of aeolian dune dynamics rate of water table rise and or periodicity of interdune flooding this is illustrated with an example from the mid triassic anisian helsby sandstone formation cheshire uk lenses of damp and wet interdune strata exhibit an intertonguing transitional relationship with the toe sets of overlying aeolian dune units this signifies dune migration that was contemporaneous with water table controlled accumulation in adjacent interdunes downwind changes in the geometry and facies of the interdune units indicate periodic expansion and contraction of the interdunes in response to changes in the elevation of the groundwater table and episodic flooding during which accumulation of dune strata continued relatively uninterrupted this contrasts with other models for accumulation in wet aeolian systems where interdune flooding is associated with a cessation in aeolian bedform climbing and the formation of a bypass or erosional supersurface architectural panels document the detailed stratigraphy in orientations both parallel and perpendicular to aeolian transport direction enabling a quantitative three dimensional reconstruction of genetically related aeolian dune and interdune elements sets of aeolian dune strata are composed of grainflow and translatent wind ripple strata and are divided by a hierarchy of bounding surfaces originating from oblique migration of superimposed dunes over slipfaceless sinuous crested parent bedforms together with lee slope reactivation under non equilibrium flow conditions silty mudstone and sandstone interdune units are characterized by wind ripple wavy and subaqueous wave ripple laminae desiccation cracks mud flakes raindrop imprints load casts flutes intraformational rip up clasts and vertebrate and invertebrate footprint impressions and trackways these units result from accumulation on a substrate that varied from dry through damp to wet surface conditions interdune ponds were flooded by either fluvial incursions or rises in groundwater table and were periodically subject to gradual desiccation and reflooding red silty mudstone beds of subaqueous origin pass laterally into horizontally laminated wind ripple beds indicating a progressive transition from wet through damp to dry surface conditions within a single interdune

how does layered heterogeneity affect the ability of subsurface dams to clean up coastal aquifers contaminated with seawater intrusion abstract the main purpose of this work was to examine how aquifer layering impacts the ability of subsurface dams to retain seawater intrusion swi and to clean up contaminated coastal aquifers using both experimental and numerical techniques four different layering configurations were investigated including a homogeneous case case h and three different layered cases where a low permeability layer was set at the top of the aquifer case lh at the middle part of the aquifer as interlayer case hlh and at the lower part of the aquifer case hl the subsurface dam was able to retain the saltwater wedge associated with a drop of the hydraulic gradient from 0 0158 down to 0 0095 in all the cases thereby achieving up to 78 reduction in the saltwater toe length in cases lh and hlh the start of the saltwater spillage was delayed compared to the homogeneous case and the time taken for the freshwater zone to be fully contaminated post spillage was twice and three times longer respectively by contrast the existence of a low k layer at the bottom of the aquifer case hl considerably weakened the ability of dams to retain the intrusion allowing for quicker saltwater spillage past the wall the natural cleanup of swi contaminated coastal aquifers was for the first time evidenced in heterogeneous settings depending on the stratification pattern the presence of stratified layers however prolonged the cleanup time to various degrees compared to the homogeneous scenario particularly in case hl where the cleanup time was nearly 50 longer

foredunes and blowouts initiation geomorphology and dynamics this paper reviews the initiation dynamics geomorphology and evolution of incipient foredunes established foredunes and blowouts incipient foredunes may be initiated in a variety of ways leading to the formation of ramps terraces or ridges depending on progradation rates vegetation type and cover sediment transport rates and scale of erosional processes vegetation cover morphology growth rates and types combine with aerodynamic processes to determine dune and swale morphology established foredunes are classified into five morpho ecological types and the flow behaviour over two types is detailed a new model of foredune evolutionary paths at various possible time scales is presented blowout initiation dynamics and sediment transport and evolution is examined throughout the review the gaps in present understanding of dune processes and a range of ideas for new research possibilities are provided

wind velocity and sand transport on a barchan dune we present measurements of wind velocity and sand flux performed on the windward side of a large barchan dune in jericoacoara northeastern brazil from the measured profile we calculate the air shear stress using an analytical approximation and treat the problem of flow separation by an heuristic model we find that the results from this approach agree well with our field data moreover using the calculated shear velocity we predict the sand flux according to well known equilibrium relations and with a phenomenological continuum saltation model that includes saturation transients and thus allows for nonequilibrium conditions based on the field data and theoretical predicted results we indicate the principal differences between saturated and nonsaturated sand flux models finally we show that the measured dune moves with invariant shape and predict its velocity from our data and calculations

dune and dune field development on padre island texas with implications for interdune deposition and water table controlled accumulation abstract portions of the back island dune fields on padre island texas undergo seasonal destructive and constructive phases in which they are reduced to a nearly planar surface during the winter and then reform during the spring and summer initial depositional sites for dry sand that we observed were associated with roughness elements that caused a lowering of the transport capacity of the flow bedforms develop through a series of morphologic and dynamic stages 1 irregular patches of dry sand 2 wind ripple protodunes with leeward flow expansion 3 grainfall protodunes with a range from leeward flow expansion to flow separation 4 barchan dunes with grainflow and 5 crescentic ridges that characterize the mature field the survival and further development of protodunes and dunes is f vored by increased size a more advanced stage of development unidirectional and moderate wind and a plentiful sand supply these factors can be viewed as the result of the interrelationship of aspects of eolian transport the nature of leeward secondary airflow patterns and sand supply dune field ordering occurs by repeated merging splitting cannibalization and lateral linking of dunes on padre island and other areas of dunes that we studied flow conditions across portions of interdune flats favor the transport of dry loose sand from interdune flats onto the dunes so that dunes grow at the expense of interdune flats dry surface deposits are therefore unlikely to accumulate on interdune flats but rather to characterize interdune depressions that may represent the minimum d stance to which an interdune flat can be closed otherwise accumulations on interdune flats most commonly should be formed on wet or damp surfaces or occur associated with some mechanism that protects the deposits from deflation padre island dune fields represent a type of eolian system where accumulations of dunes and interdune flats occur during periods of a water table rise with bypassing and deflation occurring as the water table becomes static or falls respectively

early diagenesis of eolian dune and interdune sands at white sands new mexico abstract the degree of early diagenesis in eolian dune and interdune sands at white sands new mexico is largely a function of the relationship between sand location and the water table most active and vegetation stabilized dune sands are in the vadose zone whereas interdune sands are in the capillary fringe and phreatic zones crystallographically controlled dissolution of the framework gypsum grains results in elongate prismatic etch pits on sand grains from the capillary fringe and phreatic zones whereas dissolution of sand grains in the vadose zone is slight causing minute irregularities on grain surfaces vadose water percolating through the sand is manifest as meniscus layers consequently dune sands in the vadose zone are cemented mainly by meniscus shaped gypsum at grain contacts pendant cements formed on the lower margins of some sand grains cementation in the capillary fringe and the phreatic zone is more extensive than the vadose regardless of strata type typically well developed gypsum overgrowths form along the entire edge of a grain or may encompass the entire grain complex diagenetic histories are suggested by multiple overgrowths and several episodes of dissolution on single grains attesting to changing saturation levels with respect to gypsum in the shallow ground water these changes in saturation are possibly due to periods of dilution by meteoric recharge alternating with periods of concentration of ions and the formation of cement due to evaporation through the capillary fringe

morphologic and sedimentologic patterns of active aeolian dune fields on the east coast of maranhao northeast brazil the eastern portion of the maranhao coast is characterized by large active dune fields located in a tropical setting this article combines the analysis of dune field morphologic patterns with the classical analysis of grain size and heavy minerals to study the sedimentary dynamics of the active aeolian system in this region based on the heavy mineral analysis the main sedimentary suppliers feeding the system are the westward alongshore drift bringing sand from the coast east of the parnaiba river mouth and the river itself the absence of well defined variation patterns of the two morphometric parameters studied dune spacing and crest length reflects complex sedimentary dynamics and transport pathways inside the aeolian system despite the unidirectional wind during the wet season the interdune plains are flooded and the system is intensely reworked by intermittent drainages during the dry season the deposits formed by the drainages and interdune lakes become temporary internal sedimentary sources for the system due to this combined aeolian fluvial transport the sediment source area has a planar geometry with contributions from the beach and interdune plains and not linear as expected in a typical case of source only in the beach the areal limits of the dune fields is the main boundary condition controlling the dune field patterns as dune spacing and crest length by changing the sedimentary inflow outflow balance and interactions between migration dunes copyright 2015 john wiley sons ltd

modelling the formation of residual dune ridges behind barchan dunes in north east brazil residual dune ridges are often formed by vegetation growing along a line some distance upwind of the lower stoss slope of migrating dunes this process is common in areas where vegetation germinates along the edge of the water during the rainy period when the water level is higher and interdune areas are flooded the phenomenon occurs on a large scale in north east brazil because of the rise and fall in groundwater level at the end of the rainy season each residual dune ridge corresponds to the position of the dune during the wet period in each year therefore variations in the distance between these residual dune ridges could be used potentially to monitor climatic fluctuations in rainfall and wind to examine the potential use of these residual dune ridges for the reconstruction of past climatic fluctuations a model that simulates them under varying conditions of wind rainfall and evaporation rates was formulated the model was tested for sensitivity to climatic variability in north east brazil and validated against residual dune ridge displacements as measured in the field and from high spatial resolution satellite images based on the results it is concluded that residual dune ridges may not form in north east brazil in years which are exceptionally

a new physical barrier system for seawater intrusion control abstract the construction of subsurface physical barriers is one of various methods used to control seawater intrusion swi in coastal aquifers this study proposes the mixed physical barrier mpb as a new barrier system for seawater intrusion control which combines an impermeable cutoff wall and a semi permeable subsurface dam the effect of the traditionally used physical barriers on transient saltwater wedge dynamics was first explored for various hydraulic gradients and the workability of the mpb was thereafter thoroughly analysed a newly developed automated image analysis based on light concentration conversion was used in the experiments which were completed in a porous media tank the numerical code seawat was used to assess the consistency of the experimental data and examine the sensitivity of the performance of the barriers to various key parameters the results show that the mpb induced a visible lifting of the dense saline flux upward towards the outlet by the light freshwater this saltwater lifting mechanism observed for the first time induced significant reduction to the saline water intrusion length the use of the mpb yielded up to 62 and 42 more reduction of the saltwater intrusion length than the semi permeable dam and the cutoff wall respectively the performance achieved by the mpb with a wall depth of 40 of the aquifer thickness was greater than that of a single cutoff wall with a penetration depth of 90 of the aquifer thickness about 13 extra reduction this means that the mpb could produce better seawater intrusion reduction than the traditionally used barriers at even lower cost

dune formation under bimodal winds the study of dune morphology represents a valuable tool in the investigation of planetary wind systems the primary factor controlling the dune shape is the wind directionality however our understanding of dune formation is still limited to the simplest situation of unidirectional winds there is no model that solves the equations of sand transport under the most common situation of seasonally varying wind directions here we present the calculation of sand transport under bimodal winds using a dune model that is extended to account for more than one wind direction our calculations show that dunes align longitudinally to the resultant wind trend if the angle w between the wind directions is larger than 90 under high sand availability linear seif dunes are obtained the intriguing meandering shape of which is found to be controlled by the dune height and by the time the wind lasts at each one of the two wind directions unusual dune shapes including the wedge dunes observed on mars appear within a wide spectrum of bimodal dune morphologies under low sand availability

the effect of climate change on the mobility and stability of coastal sand dunes in ceara state ne brazil the coast of ceara state in ne brazil is covered by vast fields of active and stabilized coastal sand dunes its tropical climate is characterized by two seasons wet and dry with wind intensity determined by the meridional shift of the intertropical convergence zone the wind power is negatively correlated with precipitation and precipitation is negatively correlated with the difference between sea surface temperatures of the tropical atlantic north and south of the equator we present a model suggesting that during the late pleistocene wind power determined the mobility and stability of the dunes sand dunes accumulated during periods of high wind power as it is today and stabilized when wind power was low once the dunes were stabilized by vegetation they could not be activated even by increased wind power samples that were taken for luminescence dating from 25 stabilized dunes along the coasts of ceara gave ages ranging from135 ka to 100 yr we postulate that these luminescence ages fall at the beginning of wet periods in ne brazil characterized by low wind power these paleoclimatic wet periods correlate well with the cold periods of stades in greenland ice core records

chaos fractals and self organization in coastal geomorphology simulating dune landscapes in vegetated environments complex nonlinear dynamic systems are ubiquitous in the landscapes and phenomena studied by earth sciences in general and by geomorphology in particular concepts of chaos fractals and self organization originating from research in nonlinear dynamics have proven to be powerful approaches to understanding and modeling the evolution and characteristics of a wide variety of landscapes and bedforms this paper presents a brief survey of the fundamental ideas and terminology underlying these types of investigations covering such concepts as strange attractors fractal dimensions and self organized criticality their application in many areas of geomorphological research is subsequently reviewed in river network modeling and in surface analysis amongst others followed by more detailed descriptions of the use of chaos theory fractals and self organization in coastal geomorphology in particular these include self organized behavior of beach morphology the fractal nature of ocean surface gravity waves the self organization of beach cusps and simulation models of ripples and dune patterns this paper further presents a substantial extension of existing dune landscape simulation models by incorporating vegetation in the algorithm enabling more realistic investigations into the self organization of coastal dune systems interactions between vegetation and the sand transport process in the model such as the modification of erosion and deposition rules and the growth response of vegetation to burial and erosion introduce additional nonlinear feedback mechanisms that affect the course of selforganization of the simulated landscape exploratory modeling efforts show tantalizing results of how vegetation dynamics have a decisive impact on the emerging morphology and conversely how the developing landscape affects vegetation patterns extended interpretation of the modeling results in terms of attractors is hampered however by want of suitable state variables for characterizing vegetated landscapes with respect to both morphology and vegetation patterns d 2002 elsevier science b v all rights reserved

aeolian dune field development in a water table controlled system skeiđararsandur southern iceland aeolian dune fields characterized by partly vegetated bedforms undergoing active construction and with interdune depressions that lie at or close to the water table are widespread on skei ararsandur southern iceland the largest aeolian dune complex on the sandur covers an area of 80 km2 and is characterized by four distinct landform types i spatially isolated aeolian dunes ii extensive areas of damp and wet flooded interdune flat with small fluvial channels iii small aeolian dune fields composed of assemblages of bedforms with simple morphologies and small predominantly damp interdune corridors and iv larger aeolian dune fields composed of assemblages of complex bedforms floored by older aeolian dune deposits that are themselves raised above the level of the surrounding wet sandur plain the morphology of each of these landform areas reflects a range of styles of interaction between aeolian dune interdune and fluvial processes that operate coevally on the sandur surface the geometry scale orientation and facies composition of sets of strata in the cores of the aeolian dunes and their relationship to adjoining interdune strata have been analysed to explain the temporal behaviour of the dunes in terms of their mode of initiation construction pattern of migration style of accumulation and nature of preservation seasonal and longer term flooding induced changes in water table level have caused episodic expansion and contraction of the wet interdune ponds most of the dunes are currently undergoing active construction and migration and although sediment availability is limited because of the high water table substantial aeolian transport must occur especially during winter months when the surface of the wet interdune ponds is frozen and sand can be blown across the sandur without being trapped by surface moisture bedforms within the larger dune fields have grown to a size whereby formerly damp interdune flats have been reduced to dry enclosed depressions and dry aeolian system accumulation via bedform climb is ongoing despite regional uplift of the proximal sandur surface in response to glacial retreat and unloading over the past century sediment compaction induced subsidence of the distal sandur is progressively placing aeolian deposits below the water table and is enabling the accumulation of wet aeolian systems and increasing the likelihood of their long term preservation wet dry and stabilizing aeolian system types all co exist on skei ararsandur and the dunes are variously undergoing coeval construction accumulation bypass stabilization and destruction as a result of interactions between localized factors

replenishing an unconfined coastal aquifer to control seawater intrusion injection or infiltration in this study we compare the performances of well injection and pond infiltration in controlling seawater intrusion in an unconfined coastal aquifer through two scenario groups 1 a single injection well vs an elliptic infiltration pond and 2 an injection extraction well pair system vs an elliptic infiltration pond extraction well system comparison is based on quantitative indicators that include the interface toe location saltwater volume and maximum net extraction rate for scenario 2 we introduce a method to determine the maximum net extraction rate for cases where the locations of stagnation points cannot be easily derived analytical analysis shows that the performances of injection and infiltration are the same provided that the pond shape is circular the examination of scenario group 1 suggests that the shape of the infiltration pond has a minor effect on the interface toe location as well as the reduction in the saltwater volume given the same total recharge rate the investigation of scenario group 2 indicates by contrast that the maximum net extraction rate increases significantly with the increasing ratio of b to a where a and b are semi axes of the ellipse parallel and perpendicular to the coastline respectively specifically for a typical aquifer assumed an increase of 40 is obtained for the maximum net extraction when b a increases from 1 200 to 200 despite that the study is based on a simplified model the results provide initial guidance for practitioners when planning to use an aquifer recharge strategy to restore a salinized unconfined coastal aquifer

a new late pleistocene fauna from arid coastal india implications for inundated coastal refugia and human dispersals abstract a diverse late pleistocene fossil assemblage was recovered from a sea cliff locality near gopnath in gujarat northwestern india these remains are the first large sample of pleistocene faunal materials from arid northwestern india several taxa known primarily from coarse alluvial deposits of central india are documented for the first time from an undisturbed open air site adjoining the great indian desert the sample includes a new species of antelope from a lineage considered extinct outside of africa since the early pleistocene the paleoenvironmental context faunal composition and type of fossil preservation reported here is unique the gopnath fauna accumulated in a pond within a carbonate dune field that formed part of a larger coastal oasis ecosystem this paleoscape occupied the cambay gulf during hyper arid glacial low stands the gopnath fossils are correlated to late acheulean lithics from a coastal cliff locality

investigating parabolic and nebkha dune formation using a cellular automaton modelling approach vegetation plays an important role in shaping the morphology of aeolian dune landscapes in coastal and semi arid environments where ecogeomorphic interactions are complex and not well quantified we present a discrete ecogeomorphic aeolian landscape model decal capable of simulating realistic looking vegetated dune forms permitting exploration of relationships between ecological and morphological processes at different temporal and spatial scales the cellular automaton algorithm applies three simple rules that lead to self organization of complex dune environments including nebkhas with distinctive deposition tails that form in association with mesquite type shrubs and hairpin long walled parabolic dunes with trailing ridges that evolve from blowouts in association with vegetation succession changing the conditions of simulations produces differing landscapes that conform qualitatively to observations of real world dunes the model mimics the response of the morphology to changes in sediment supply vegetation distribution density and growth characteristics as well as initial disturbances the introduction of vegetation into the model links spatial and temporal scales previously dimensionless in bare sand cellular automata grid resolutions coarser than the representative size of the modelled vegetation elements yield similar morphologies but when cell size is reduced to much smaller dimensions the resultant landscape evolution is dramatically different the model furthermore demonstrates that the relative response characteristics of the multiple vegetation types and their mutual feedback with geomorphological processes impart a significant influence on landscape equilibria suggesting that vegetation induces a characteristic length scale in aeolian environments this simple vegetated dune model illustrates the power and versatility of a cellular automaton approach for exploring the effects of interactions between ecology and geomorphology in complex earth surface systems

holocene stratigraphic evolution of saline lakes in nhecolandia southern pantanal wetlands brazil nhecol ndia is a fossil lobe of the taquari river megafan and a prominent geomorphic subunit of the pantanal wetlands because of the presence of 10 000 small lakes we investigated the stratigraphic records of three saline lakes from nhecol ndia to explore their potential as quaternary hydroclimate archives radiocarbon data indicate that accumulation at two lakes was approximately continuous in the late holocene and chemostratigraphic variability suggests sensitivity to environmental change with multicentennial resolution a basal sandy unit and an upper muddy unit comprise the shallow stratigraphy of each lake a pronounced change in depositional environment from freshwater wetlands to saline lakes at 3300 3200 cal yr bp best explains the lithofacies transition ephemeral freshwater wetlands formed on the abandoned megafan lobe which was molded by deflation in the arid early holocene wind scouring of the megafan lobe generated topographically closed depressions with complex marginal sand ridges which allowed permanent lakes to evolve when rainfall increased in the late holocene the lakes became highly saline and alkaline after 910 cal yr bp which influences biogeochemistry and aquatic ecology the results hold implications for understanding the response of the southern pantanal to climate change as well as the development of pans in tropical megafan settings

white sands dune field new mexico age dune dynamics and recent accumulations abstract the white sands dune field situated within the tularosa basin in southern new mexico is thought to have been largely derived by a stepwise progressive deflation of pleistocene lake otero strata with the onset of regional aridity optically stimulated luminescence osl dating of samples from a core that penetrated the gypsum accumulation of the dune field confirm a time of origin at 7000 yr dune sediment is characterized as lagged influx from previously stored lake otero sediment and contemporaneous influx derived from subsequent playas sediment became available for aeolian transport and dune field construction because of a falling water table driven by the regional aridity the current dune field is primarily a wet aeolian system in which the behavior of the accumulation surface over time is a function of the water table although surface cementation by gypsum also imparts aspects of a stabilizing system dune crests are oriented borderline transverse to the annual transport resultant and a monitored dune and its cross strata show that transverse winds from the sw during the late winter and spring account for most of the crest normal migration but a significant along crest component of migration of dune sinuosity occurs with fall and winter winds from the nw and n that strike the crests obliquely trenches across three interdune areas show dune sets and interdune strata climbing at about 0 1 depth of interdune scour increases with interdune streamwise length which acts to enhance the probability of dune cross strata deflation during arid years but also increases the probability of interdune accumulation because the water table shows a net rise over time within the trenches dune sets show a bundling of foresets between reactivation surfaces interpreted as annual cycles that reveal lee face reworking by the oblique nw and n winds and slipface progradation fostered by transverse winds from the sw a progression from grainflow dominated to wind ripple dominated cross strata within a single set revealed in the longest trench is consistent with along crest migration of dune sinuosity interdune laminations show light dark couplets interpreted as annual varves reflecting dry wet portions of the year the number of laminations preserved in vertical sections across an interdune area shows that the record is incomplete and endorses the interpretation of periods of interdune deflation a calculation of the current accumulation rate as based upon the climbing strata within the trenches is significantly greater than the long term accumulation rate determined by osl dates taken from the core in addition to compaction and possible dissolution of gypsum as partial explanations the long term accumulation rate may reflect significant variation in the accumulation rate over time short term deflation of accumulations on a regular basis and or the presence of unconformities within the gypsum body

evidence for indurated sand dunes in the martian north polar region 1 we investigate the relevance of induration as an explanation for the occurrence of dunes of unusual morphology in the martian north polar region the evidence for induration of aeolian deposits on mars is reviewed an explanation for rounded elongated barchans in chasma boreale is presented the role played by induration in the formation of linear dunes is discussed our arguments are supported by the results of a computer simulation model

a continuous model for sand dunes review new developments and application to barchan dunes and barchan dune fields basically sand dunes are patterns resulting from the coupling of hydrodynamic and sediment transport once grains move they modify the surface topography which in turns modifies the flow this important feedback mechanism lies at the core of continuous dune modelling here we present an updated review of such a model for aeolian dunes including important modifications to improve its predicting power for instance we add a more realistic wind model and provide a self consistent set of parameters independently validated as an example we are able to simulate realistic barchan dunes which are the basic solution of the model in the condition of unidirectional flow and scarce sediments from the simulation we extract new relations describing the morphology and dynamics of barchans that compare very well with existing field data next we revisit the problem of the stability of barchan dunes and argue that they are intrinsically unstable bed forms finally we perform more complex simulations first a barchan dune under variable wind strength and second barchan dune fields under different boundary conditions the latter has important implications for the problem of the genesis of barchan dunes copyright 2010 john wiley sons ltd

an assessment of the importance of some parameters for seawater intrusion in aquifers and a comparison of dispersive and sharp interface modelling approaches abstract the problem considered is that of determining the location and shape of the steady interface in a coastal aquifer discharging fresh water to the sea for homogeneous isotropic aquifers the location of the interface is obtained by the boundary integral method for an abrupt interface model these results are compared with the distribution of salt in the dispersion zone where the two fluids meet as determined by a finite difference solution of the flow and convective dispersion equations considerations are restricted to a rectangular section of isotropic homogeneous aquifer and examples are limited to those showing the effects of different convection to dispersion ratios including anisotropic dispersion different fresh water flow gradients and different relative densities of seawater these examples give an indication of the relative accuracy of the abrupt interface model for different ratios of convection to dispersion and enable the influence of various parameters on the shape and extent of the dispersion zone to be studied

dune whitening and inter dune freshwater ponds in ne brazil the changes in color of sand dunes from white to yellow or red is often interpreted to signify their age or their source materials in this study we demonstrate the effect seasonal inter dune freshwater ponds have on the bleaching of the color of sand dunes by iron reduction in the anaerobic conditions they create by combining spectral measurements of field samples of dune sand together with the analysis of satellite images covering three dune fields in ne brazil lencois maranhenses jericoacoara and canoa quebrada we demonstrate its existence in the field in areas where ponds cover 41 of the dune field as in lencois maranhenses an almost total bleaching in the color of the sand occurs after which the dunes remain white with no relation to distance from the coastline in areas with less ponds e g jericoacoara or canoa quebrada dune whitening is less active and there are areas where dune rubification occurs as this process may have occurred in other dune fields during past climate conditions that may have been different from current ones interpretations of dunes age based on their color should be handled with care

the enigmatic messinian pliocene section of cuevas del almanzora vera basin se spain revisited erosional features and strontium isotope ages abstract the cuevas del almanzora section was in the late seventies the focus of a discussion because of alleged continuous marine messinian to pliocene sedimentation however a discontinuity has been shown to exist in the shape of laminated strata including the late messinian brackish lacustrine lago mare biofacies more recently benson and rakic el bied 1991 concluded that the section is still one of the best biostratigraphic successions for the western mediterranean in which to document terminal miocene events but that it entirely has an early messinian age i e it antedates deposition of the main evaporites this paper presents strontium isotope ages indicating that the classic threefold division in an earlier marine messinian a lago mare and a pliocene interval sensu geerlings et al 1980 cita et al 1980 should be maintained moreover the sr isotopic composition of the euryhaline cyprideis ostracodes from the lago mare laminites is similar to those from central mediterranean basins this stresses the importance of late messinian water exchange between the vera basin and the then enclosed mediterranean it thus refutes the opinion of benson and rakic el bied 1991 that this interval is a local facies of no particular stratigraphic importance a time gap of up to 0 8 ma between the youngest marine strata of the messinian and the overlying pliocene provides a maximum timing for the duration of the messinian salinity crisis during which very little net sedimentation occurred compared to coeval deposits in for example the nearby nijar and sorbas basins in the cuevas section an inconspicuous and hitherto overlooked erosional gap has been observed this separates the lago mare marls from the pliocene marls more field observations have been made and are discussed in the light of existing interpretations in order to demonstrate the importance of more widespread erosion in the vera basin gypsum containing mass flow deposits filling up a late messinian palaeorelief in the garrucha area are shown to be derived from the basin these probably continue offshore as feeder channels related to a late messinian sea level fall a holistic approach of a key section thereby not overlooking the regional geology is a necessary step to be made before far reaching claims can be made about its interregional significance

multiple origins of linear dunes on earth and titan the formation of dunes is controlled by the direction of the prevailing winds and the characteristics of the sediments linear dunes in the qaidam basin china are shown to form from cohesive sediments a model that could be applicable to dunes on titan

profile measurement and simulation of a transverse dune field in the lencois maranhenses in this work we report measurements of the height profile of transverse dunes in the coastal dune field known as lencois maranhenses northeastern brazil our measurements show that transverse dunes with approximately the same height present a variable brink position relative to the crest in contrast to the case of barchan dunes based on our field data we present a relation for the dune spacing as a function of the crest brink distances of transverse dunes furthermore we compare the measurements with simulations of transverse dunes obtained from a two dimensional dune model where a phenomenological definition is introduced for the length of the separation streamlines on the lee side of closely spaced transverse dunes we find that our model reproduces transverse dune fields with similar interdune distances and dune aspect ratios as measured in the field

the response of the water table in coastal aeolian systems to changes in sea level abstract the water table in some aeolian systems is a control on the type and rate of accumulation and defines the depth to which deflation will occur in the formation of regional bounding surfaces at the basinal scale over time the elevation of the water table may be a function of climate tectonism and or sea level the latter invokes the hypothesis that the water table underlying an arid coastal sand sea without significant recharge adjusts to changes in sea level for tens to hundreds of kilometers inland the simplest physical and mathematical model for this hypothesis states that the water table re equilibrates as an inland directed diffusion wave to each increment of change in sea level model simulations incorporating a range of hydraulic parameters and periods of sea level cycles demonstrate scenarios ranging from one in which the continental water table is essentially unaffected by sea level to one in which the water table exactly mimics the sea level curve using a realistic set of mid range parameters the model produces a suite of water table curves that evolve from the coast to inland because of a diminishing amplitude for all cycles a loss of more minor cycles and a time lag the resulting stratigraphic record from the coast to more inland positions shows a diminishing number of recorded events a thinning of the section removal of previously preserved strata during lowstands and the appearance of new stratigraphic units far inland most of these characteristics are compatible with those of the jurassic page sandstone in regional cross sections in northern arizona and southern utah but are difficult to distinguish from the effects of tectonism

field evidence for surface wave induced instability of sand dunes barchans are crescent shaped dunes that move faster over desert surfaces than most other dune types a three year field study shows how they do it collisions and changes in wind direction destabilize the dunes generating surface waves that form new smaller barchans this prevents the formation of a single giant dune so plays a fundamental role in the development of dune patterns field studies of barchans crescent shaped dunes that propagate over solid ground under conditions of unidirectional wind1 have long focused on the investigation of an equilibrium between sand transport by wind and the control of air flow by dune topography2 3 4 which are thought to control dune morphology and kinematics5 6 7 because of the long timescale involved however the underlying dynamic processes responsible for the evolution of dune fields remain poorly understood8 here we combine data from a three year field study in the moroccan sahara with a model study to show that barchans are fundamentally unstable and do not necessarily behave like stable solitary waves as suggested previously9 10 11 12 we find that dune collisions and changes in wind direction destabilize the dunes and generate surface waves on the barchans because the resulting surface waves propagate at a higher speed than the dunes themselves they can produce a series of new barchans of elementary size by breaking the horns of large dunes the creation of these new dunes provides a mechanism for sand loss that prevents dune fields from merging into a single giant dune and therefore plays a fundamental role in the control of size selection and the development of dune patterns

coastal dune fields at the sao francisco river strandplain northeastern brazil morphology and environmental controls extensive coastal dune elds occur on the quaternary strandplain associated with the sao francisco river mouth two different generations of dunes are identi ed one is inactive already xed by vegetation comprising parabolic dunes the other generation is active bordering the present day shoreline and transgressing over the inactive dune eld three morphological provinces in the active coastal dune elds are recognized on the updrift side of the sao francisco river mouth they are a sand sheet with shrub coppice and shadow dunes b isolated dunes of the barchan transversal type up to 5 m high and interdune areas and c a 23 m high compound dune with superimposed small dunes the same provinces are recognized on the downdrift side of the river mouth with two important exceptions the barchan transversal and compound dunes are replaced respectively by i zibar type dunes up to 5 m high and ii a 19 m high precipitation dune which is associated with numerous blowouts the prevailing eastern winds from august to january favour the development of the aeolian bedforms and the migration of dunes the shoreline orientation almost transversal to the winds and the great supply of ne grained sediments contribute to the formation of barchan transversal types and compound dunes in the updrift side on the other hand in the downdrift side the shoreline orientation is almost parallel to the prevailing winds this fact in association with a coarser grain size in the beachface favours the formation of zibar type and precipitation dunes with numerous blowouts the rate of migration of individual dunes is about 20 to 24 m per year this study suggests that the aeolian sedimentation is a relatively recent phenomenon at the quaternary strandplain of the sao francisco river the rst generation of dune elds initiated some time after 3000 years bp and the second generation originated some centuries ago copyright 2004 john wiley sons ltd

measurements and numerical simulations of the degree of activity and vegetation cover on parabolic dunes in north eastern brazil in this work we present measurements of vegetation cover over parabolic dunes with different degrees of activation along the north eastern brazilian coast we extend the local values of the vegetation cover density to the whole dune by correlating measurements with the relative brightness index c of high resolution quickbird panchromatic satellite images of the dune field we then introduce the vegetation data into a continuous model for vegetated dunes coupling sand erosion and vegetation growth and perform simulations of the evolution of the morphology and vegetation cover of parabolic dunes finally from the comparison of both the measurements and the simulation results we show that the model is able to predict the dune shape and the vegetation distribution of real parabolic dunes as result of the evolution of a blow out