11 11 dune morphology and dynamics this chapter discusses the key types of sand dunes and how they can be classified and assesses the major controls on their development the processes of dune dynamics are reviewed leading to a discussion of the concept of equilibrium in solitary dune form recent advances in modeling are reviewed that show how interactions between dunes may result in equilibrium at the dune field scale rather than at the single dune scale

flow structure and resistance over subaquaeous high and low angle dunes a prominent control on the flow over subaqueous dunes is the slope of the downstream leeside while previous work has focused on steep 30 asymmetric dunes with permanent flow separation little is known about dunes with lower lee slope angles for which flow separation is absent or intermittent here we present a laboratory investigation where we systematically varied the dune lee slope holding other geometric parameters and flow hydraulics constant to explore effects on the turbulent flow field and flow resistance three sets of fixed dunes lee slopes of 10 20 and 30 were separately installed in a 15 m long and 1 m wide flume and subjected to 0 20 m deep flow measurements consisted of high frequency vertical profiles collected with a laser doppler velocimeter we show that the temporal and spatial occurrence of flow separation decreases with dune lee slope velocity gradients in the dune leeside depict a free shear layer downstream of the 30 dunes and a weaker shear layer closer to the bed for the 20 and 10 dunes the decrease in velocity gradients leads to lower magnitude of turbulence production for gentle lee slopes aperiodic strong ejection events dominate the shear layer but decrease in strength and frequency for low angle dunes flow resistance of dunes decreases with lee slope the transition being nonlinear over the 10 20 and 30 dunes shear stress is 8 33 and 90 greater than a flat bed respectively our results demonstrate that dune lee slope plays an important but often ignored role in flow resistance

application of a boundary layer model to flow over an eolian dune as part of a larger project to model sand transport over isolated dunes and the temporal evolution of such dunes the present paper describes a wind flow simulation that results from an application of the ms3djh 1 5 model walmsley et al 1982 to the dune studied by howard et al 1978 a comparison is made between the calculations and observations of surface wind direction and wind speed at 80 cm above terrain on the upwind side of the dune the mean error and rms deviation from the mean error respectively are about 0 2 and 7 for surface wind direction and 9 09 and 0 08 for normalized wind speed

field measurements of mean and turbulent airflow over a barchan sand dune advances in our knowledge of the aeolian processes governing sand dune dynamics have been restricted by a reliance on measures of time averaged airflow such as shear velocity u it has become clear that such measures are incapable of explaining the complete dynamics of sediment transport across dune surfaces past evidence from wind tunnel and modelling studies has suggested that in some regions on a dune s surface the sediment transport might be better explained through investigations of the turbulent nature of the airflow however to date there have been no field studies providing data on the turbulent characteristics of the airflow around dunes with which to support or refute such hypotheses the field investigation presented here provides mean and turbulent airflow measurements across the centre line of a barchan sand dune in namibia data were collected using arrays of sonic anemometers and were compared with sand flux data measured using wedge shaped traps results support previously published data derived from wind tunnels and numerical models the decline in mean wind velocity at the upwind toe of the dune is shown to coincide with a rise in turbulence whilst mean velocity acceleration on the upper slope corresponds with a general decline in measured turbulence analysis of the components of reynold shear stress u w and normal stresses u 2 and w 2 supports the notion that the development of flow turbulence along the dune centre line is likely to be associated with the interplay between streamline curvature and mean flow deceleration acceleration it is suggested that due to the nature of its calculation turbulence intensity is a measure of less practical use than direct assessments of the individual components of reynolds stress particularly the instantaneous horizontal streamwise component u 2 and shear stress u w whilst increases in reynolds shear stress and the horizontal streamwise component of stress in the toe region of the dune may effectively explain the maintenance of sand flux in a region of declining mean velocity they have much less explanatory power for sand flux on the upper windward slope and in the crestal region of the dune here it is suggested that mean flow acceleration is likely to provide the most significant driving force on sand flux possibly augmented by a rise in the horizontal streamwise component of reynolds stress u 2 in the crest brink region therefore although wind turbulence is considered to be of fundamental importance in explaining the sediment transport dynamics across the dune s surface it is recognised that the interaction between mean flow deceleration acceleration streamline curvature and individual components of reynolds stress is complex and the identification of a single element of flow that offers a panacea for accounting for sand flux and dune dynamics is difficult to find

a numerical model of flow over sand waves in water of finite depth summary a model of turbulent flow above sand waves in water of finite depth is described closure assumptions are based on an eddy viscosity proportional to the square root of the local value of turbulent kinetic energy and mixing length dependent upon distance from the lower boundary results are presented for some idealized cases to investigate the effects of wave slope water depth froude number and wave shape the implications of the model for the transport of sediment are discussed and the development of the wave investigated it is found that the crest of the wave will become sharper for lower flow rates as has been observed in the sea comparisons are made with recent measurements made over sand waves in the columbia river

the role of streamline curvature in sand dune dynamics evidence from field and wind tunnel measurements abstract field measurements on an unvegetated 10 m high barchan dune in oman are compared with measurements over a 1 200 scale fixed model in a wind tunnel both the field and wind tunnel data demonstrate similar patterns of wind and shear velocity over the dune confirming significant flow deceleration upwind of and at the toe of the dune acceleration of flow up the windward slope and deceleration between the crest and brink this pattern including the widely reported upwind reduction in shear velocity reflects observations of previous studies such a reduction in shear velocity upwind of the dune should result in a reduction in sand transport and subsequent sand deposition this is not observed in the field wind tunnel modelling using a near surface pulse wire probe suggests that the field method of shear velocity derivation is inadequate the wind tunnel results exhibit no reduction in shear velocity upwind of or at the toe of the dune evidence provided by reynolds stress profiles and turbulence intensities measured in the wind tunnel suggest that this maintenance of upwind shear stress may be a result of concave unstable streamline curvature these additional surface stresses are not recorded by the techniques used in the field measurements using the occurrence of streamline curvature as a starting point a new 2 d model of dune dynamics is deduced this model relies on the establishment of an equilibrium between windward slope morphology surface stresses induced by streamline curvature and streamwise acceleration adopting the criteria that concave streamline curvature and streamwise acceleration both increase surface shear stress whereas convex streamline curvature and deceleration have the opposite effect the relationships between form and process are investigated in each of three morphologically distinct zones the upwind interdune and concave toe region of the dune the convex portion of the windward slope and the crest brink region the applicability of the model is supported by measurements of the rate of sand transport and the change of the dune surface in the field

a wind tunnel simulation and field verification of desert dust deposition avdat experimental station negev desert the deposition of natural dust in an area of 53 ha situated in the northern negev desert is investigated in detail both in the wind tunnel dust storm simulations over a topographic scale model and in the field the wind tunnel results and the field results show a high degree of agreement indicating that scale model simulation may be considered an important technique for future loess and desert research more dust settles on windward slopes than on leeward slopes which is in contradistinction with the widespread wind shadow concept air flow separation zones immediately downwind of steep windward slopes have an important impact on dust deposition too in the case of dust deposition on topographic scale models a restricted height distortion of the model will not necessarily lead to serious problems in addition wind tunnel blockage percentages up to 13 may be allowed in order to obtain acceptable dust deposition patterns for the scale model a mean gross dust deposition of about 200 250 g m 2 year 1 is calculated for the northern negev desert for 1987 thus if the settled dust can be protected against erosion in the cultivated areas in the negev the dust content of the top soil will markedly increase with time however it has to be borne in mind that cultivation activities themselves may also contribute to a higher soil erosion and hence to a higher dust content in the atmosphere at any rate a higher dust content in the top soil will unquestionably have an important positive effect on agricultural yields from the air dust concentration data and the dust deposition data a deposition velocity of 4 7cms 1 can be calculated for avdat dust

observations theory and modeling of the differential accumulation of antarctic megadunes antarctic megadunes are characterized by significant spatial differences in accumulation rate with higher accumulation on the windward side and near zero accumulation on the lee side this leads to spatial differences in physical properties of snow and surface roughness as well as to the upwind migration of the megadunes while previous studies agree that megadunes are a result of complex interactions between wind topography and snow it is not clear how they form or why they accumulate on the windward side here we use icesat observations dimensional analysis and atmospheric flow modeling to investigate what conditions are responsible for the accumulation patterns and upwind migration of the megadunes first we use icesat data to quantify the pattern of differential surface elevation change across the megadunes we then use dimensional analysis based on supercritical flow theory and atmospheric flow modeling to show that the megadunes topography and a stable atmosphere will always lead to upwind dune migration we show that a combination of persistent katabatic winds strong stability and spatial variability in surface roughness is responsible for the accumulation on the upwind slope and hence the upwind migration of the megadunes we further show that spatial differences in surface roughness are the primary control on accumulation magnitudes and hence dune migration velocity the dune migration velocity in turn influences the degree of snow metamorphism and the physical properties of snow that are relevant for paleoclimate records our findings pertain to the ongoing evolution of the megadunes but their genesis remains an open question

measurements of the flow structure around ailsa craig a steep three dimensional isolated hill observations of neutral flow around an isolated island are presented the island has a fairly smooth shape with slopes of about 30 and a horizontal section which is roughly elliptical with axis ratio 1 5 the flow field 4 m above the surface shows a speed up around the sides and over the summit with reverse flow on the lee slope measurements with an instrumented aircraft of the mean flow in the wake of the island reveal a single very powerful trailing vortex with axis close to the upstream wind direction there are indications that the single vortex was a consequence of the asymmetry caused by the island s elliptical shape measurements of fluctuating quantities with a tethered balloon system show marked changes in the components of the turbulence energy in the flow around the side of the hill

separation of upslope flow over a uniform slope motivated by the importance of understanding mountain weather during periods of thermal convection a laboratory study was conducted to investigate the separation of an upslope anabatic flow on a two dimensional heated mountainous slope in the absence of a background mean flow three flow regimes were identified in the first at slope angles larger than a critical value the separated flow generated a rising plume completely fed by the anterior upslope flow for this case a simple model based on a balance between the opposing vorticities of baroclinicity and shear was proposed to predict the location of the separation point relative to the mountain base the model also predicts the velocity and length scales at separation as well as those of the rising plume after separation in the second regime the volume flow of the separated plume was not fully supplied by the upslope flow requiring entrainment of additional ambient fluid at the base of the plume source the third regime occurred when wherein the plume almost completely engulfed the slope similar to a buoyant plume emanating from a source of finite dimensions thus overshadowing the upslope flow measurements of the separation point conducted during the materhorn field research program were consistent with the results of the laboratory experiments and modelling

coriolis effects in mesoscale flows with sharp changes in surface conditions a general linearized shallow layer perturbation model where the approximately neutral lower layer of thickness h0 is situated below a stable upper layer i e an inversion with temperature change t is developed for steady mesoscale atmospheric flows over low lying topography whose height is less than h0 with the coriolis parameter f sharp changes in surface conditions surface roughness terrain elevation heat flux are modelled as a distributed body force through the lower layer the froude number of this layer is small typical cases of mesoscale discontinuities are examined the results are compared with those of a continuously stratified model and observations and with numerical mesoscale model results for a meteorological case study over the dover straits region of the english channel the main results are i if the wind direction is parallel to the edge line separating the change in surface roughness there are marked increases and decreases in these coastal winds whose maxima can occur over the sea within a distance of order h0 1 km of a coast the strength of these wind jets which do not occur in the absence of coriolis force decrease away from the edge line gradually over transverse length scales of the order of the rossby deformation radius changes to surface roughness lead to an increase in the wind speed perturbation in the downwind direction until limited by non linear effects when the wind is at an angle to a roughness change or coast the maxima occur at the coastline ii where there are sharp changes in the orientation of contours of constant roughness length e g at capes or bays on the coastline or wakes of high drag areas detached jets are formed in the downwind direction iii changes in surface elevation at a coast produce effects different from those of roughness a positive wind jet forms parallel to the coast in the direction of the wind when the coast is on the right looking downwind and a negative jet when the coast is on the left these jets do not increase in strength along the flow and do not persist downwind iv coriolis effects also determine how the inversion height varies near coastlines and surface roughness changes for example increasing decreasing inland over a distance lr when stable airflow approaches from the sea and the coast is on the right left of an observer looking downwind opposite in the southern hemisphere this mechanism is consistent with observed increasing decreasing cloudiness inland from a coast v other effects occur where the surface elevation changes gradually over a distance of order lr e g a wide shallow valley frictional effects are comparable with buoyancy and coriolis forces and flows perpendicular to the elevation change are deflected to the left in the northern hemisphere as observed in the rhine valley vi the shallow layer model simulates the major features of the low level flow field computed using the numerical mesoscale model with a horizontal resolution of 2 km i e of order h0 broad features were captured using a coarser resolution of 12 km vii the analysis provides a method of estimating errors associated with finite grid size in numerical mesoscale models copyright 2004 royal meteorological society

spectra of velocity components over complex terrain spectra have been measured over a variety of types of complex terrain on tops of hills and escarpments over land downstream of a water surface and over rolling terrain differences between spectra over many types of complex terrain and over uniform terrain can be explained by these hypotheses when air moves over terrain with changed characteristics then 1 for wavelengths very short relative to the fetch over the new terrain the spectral densities are in equilibrium with the new terrain 1 for wavelengths long compared to this fetch spectral densities remain unchanged if the ground is horizontal and decrease when the flow is uphill for the longitudinal velocity component only since vertical velocity spectra contain relatively less low wavenumber energy than horizontal velocity spectra energetic vertical velocity fluctuations tend to be in equilibrium with local terrain

reconstructing the geometry of central anatolia during the late cretaceous large scale cenozoic rotations and deformation between the pontides and taurides the central anatolian crystalline complex cacc exposes metamorphic ophiolitic and igneous rocks that were formed and deformed during closure of the neotethyan ocean the cacc is located in central turkey between the pontides in the north and the taurides in the south separated by major fault zones composite plutons intruded the meta sedimentary and ophiolitic units between 95 and 75 ma and form linear magmatic belts 100 km long along the western and northern margins of the cacc exhumation of the metamorphic and igneous complex was finalized by the paleocene time in this study we paleomagnetically study fifteen plutons spanning the entire non deformed upper cretaceous granitoid belt to test whether the initial configuration of the cacc was modified by vertical axis rotations after its exhumation our results show three internally coherent domains with significantly different vertical axis rotations 1 in the north east the akda block ayb records 151

air flow over a two dimensional hill studies of velocity speed up roughness effects and turbulence wind tunnel measurements have been made of the streamwise mean and turbulent velocities over a rough bell shaped two dimensional hill with height h and maximum slope 0 26 placed in a neutrally stable boundary layer of thickness 10 h and roughness length zo 0 02 h close agreement is found between the mean velocity and predictions obtained from taylor s 1977 computational model and jackson and hunt s 1975 analytical linearized model for locations at or upwind of the hill top but not in the wake examination of the models shows that the shear stresses are only important in an inner region close to the hill surface so that as suggested by jackson and hunt 1975 the perturbation to the mean flow outside this region is essentially inviscid the theory shows that over very rough surfaces such as wooded or urban terrain the height of this inner region can be of the same order as the height of the roughness elements so that in our experiments no measurements could be made in this region in a second experiment flow over a smooth hill on a rough surface was studied the additional increase of wind speed over the hill top can be estimated by assuming a linear superposition of the velocity changes produced by the changes in elevation and in surface roughness in this case rough to smooth in the lee of a hill however the change in roughness significantly alters the flow with flow separation being suppressed and here a linear superposition is not appropriate finally we consider why observed changes in turbulence structure close to the surface differ from those well above the surface calculations of these changes based on the simple theoretical arguments of equilibrium shear layers and rapidly distorted turbulent flows agree well with turbulence measurements in wind tunnels and in the field

the effective roughness length for scalar transfer in neutral conditions over hilly terrain numerical weather prediction models are required to represent the area averaged effects of hills on temperature and water vapour in the atmospheric boundary layer the transfer of these scalars within the boundary layer is affected by bouyancy however we have simplified the problem by simulating the behaviour of a passive scalar in a neutrally stratified turbulent boundary layer over hills the numerical model used for these simulations was fully nonlinear and flows were simulated over a range of slopes including ones where separation occurred the area averaged effects of hills are small over low slopes but significant at steeper slopes due to the impact of the separated region the area averaged scalar concentration varies logarithmically with height but in a region that is shallower than the logarithmic wind region we have found that using an effective roughness length for scalar transfer zoseff the effects of hills on area averaged scalars may be parametrized zoseff is smaller than the local value of roughness length for scalar transfer and for hills of certain wavelengths and roughness it is three orders of magnitude smaller a simplified interpretation of the physical processes simulated by the numerical model has been used to derive an expression for zoseff in terms of hill characteristics only this parametrization should be useful for representing the influence of hills on scalar transfer in numerical weather prediction and climate models

the effect of surface curvature on the deposition of loess a physical model summary a physical model for the deposition of fine dusty material loess on curved surfaces is presented the model is tested in a sedimentation experiment with a symmetrical hill with length axis perpendicular to the flow direction in a wind tunnel it appears both from the model and the experiment that loess deposition will occur preferably on windward slopes and not on leeward ones in contrast to the widespread wind shadow concept below the deflation threshold loess deposition increases as wind speed increases above the deflation threshold an effective accumulation of the loess layer on the surface will occur within a well defined wind speed interval the loess layer on the surface will erode away only if wind speed has exceeded this interval for flat and dry surfaces free of roughness elements the upper limit of the interval is 7 m s free stream velocity for wet surfaces however it may be considerably higher 15 m s and more

atmospheric flow over a succession of nearly two dimensional ridges and valleys observations of neutral static stability flow in a nearly two dimensional valley are presented upstream and downstream of the valley are a succession of similar ridges and valleys the valley sides have a slope of about 32 to the horizontal and when the wind is nearly perpendicular to the valley there is a marked flow separation the observations of mean flow and turbulence are compared with numerical simulations of flow over periodic two dimensional ridges and valleys

bedforms in a turbulent stream formation of ripples by primary linear instability and of dunes by nonlinear pattern coarsening it is widely accepted that both ripples and dunes form in rivers by primary linear instability the wavelength of the former scaling on the grain size and that of the latter being controlled by the water depth we revisit here this problem in a theoretical framework that allows to give a clear picture of the instability in terms of dynamical mechanisms a multi scale description of the problem is proposed in which the details of the different mechanisms controlling sediment transport are encoded into three quantities the saturated flux the saturation length and the threshold shear stress hydrodynamics is linearized with respect to the bedform aspect ratio we show that the phase shift of the basal shear stress with respect to the topography responsible for the formation of bedforms appears in an inner boundary layer where shear stress and pressure gradients balance this phase shift is sensitive to the presence of the free surface and the related effects can be interpreted in terms of standing gravity waves excited by topography the basal shear stress is dominated by this finite depth effect in two ranges of wavelength when the wavelength is large compared to the flow depth so that the inner layer extends throughout the flow and in the resonant conditions when the downstream material velocity balances the upstream wave propagation performing the linear stability analysis of a flat sand bed the relation between the wavelength at which ripples form and the flux saturation length is quantitatively derived it explains the discrepancy between measured initial wavelengths and predictions that do not take this lag between flow velocity and sediment transport into account experimental data are used to determine the saturation length as a function of grain size and shear velocity taking the free surface into account we show that the excitation of standing waves has a stabilizing effect independent of the details of the flow and sediment transport models consequently the shape of the dispersion relation obtained from the linear stability analysis of a flat sand bed is such that dunes cannot result from a primary linear instability we present the results of field experiments performed in the natural sandy leyre river which show the formation of ripples by a linear instability and the formation of dunes by a nonlinear pattern coarsening limited by the free surface finally we show that mega dunes form when the sand bed presents heterogeneities such as a wide distribution of grain sizes