0 Can transverse dunes move sideways ? Secondary flow deflection in the lee of transverse 1 dunes with implications for dune alignment and migration 2 3 4 5 6 7 8 9 10 11 12 (original) (raw)
Related papers
Earth Surface Processes and Landforms, 2013
Measurements of lee-side airflow response from an extensive array of meteorological instruments combined with smoke and flow streamer visualization is used to examine the development and morphodynamic significance of the lee-side separation vortex over closely spaced transverse dune ridges. A differential deflection mechanism is presented that explains the three-dimensional pattern of lee-side airflow structure for a variety of incident flow angles. These flow patterns produce reversed, along-dune, and deflected surface flow vectors in the lee that are inferred to result in net 'lateral diversion' of sand transport over one dune wavelength for incident angles as small as 10 from crest-transverse (i.e. 80 from the crest line). This lateral displacement increases markedly with incident flow angle when expressed as the absolute value of the total deflection in degrees. Reversed and multi-directional flow occurs for incident angles between 90 and 50 . These results document the three-dimensional nature of flow and sand transport over transverse dunes and provide empirical evidence for an oblique migration model. Figure 8. Time-averaged flow vectors for events 6 (a) and 7 (b). Upper values indicate direction AE SD and lower values are speed and [coefficient of variation]. Vectors show directions at three levels: surface, crest height (AE 30 cm), and at half dune height. White arrows at crest locations are at outer flow (5Á2 h on east and 4Á6 h on west profiles). Dashed vertical line shows lee-slope base and interdune.
Dynamics of secondary airflow and sediment transport over and in the lee of transverse dunes
Progress in Physical Geography, 2002
Recent research literature on secondary airflow and sediment transport patterns over flow-transverse dunes is reviewed. Various issues surrounding the behaviour, modelling and sedimentological implications of near-surface airflow dynamics over dunes are discussed, including: the Law of the Wall; the Jackson and Hunt airflow model; the effects of streamline compression, acceleration and curvature on stoss slope shear stress; and, in particular, recent efforts to characterize secondary lee-side airflow patterns. A revised conceptual model of leeside airflow is presented and areas for further research are identified regarding the implications of such patterns for dune sedimentary dynamics, morphology, and migration.
Secondary airflow and sediment transport in the lee of a reversing dune
Earth Surface Processes and Landforms, 1999
Lee-side windspeed and sediment transport were measured over a small (1Á2 m) transverse ridge in the Silver Peak dunefield, west-central Nevada, USA, using an intensive array of 25 cup anemometers and seven total flux traps. During crest-transverse and transporting flow conditions (u 0Á3crest % 8Á4 m s À1 ), windspeed near the surface of the lee slope averaged half (48 per cent) that of crest speeds. Dimensionless speeds in the separation zone ranged from 0Á2 to 0Á8 that of the outer flow (u 12 ). Along the boundary of the separation cell, windspeed increased by 10 per cent of the crest speed before separation. Equilibrium of upper and lower wake regions was not observed by the documented eight dune heights, suggesting that wake recovery may not occur over closely spaced dunes. Sediment transport measured directly on both the lee slope and interdune surfaces averaged approximately 15 per cent of crest inputs. This suggests that a significant amount (c. 70±95 per cent) of sediment transported over the crest moved as fallout. For this data set, flux was approximately proportional to the cube of the near-surface windspeed (u 0Á3 ) and in general there was an order of magnitude difference between flux measured at the crest and that measured within the separation zone. Transport direction in the separation zone was acutely oblique to the incident direction owing to secondary flow deflection. Beyond the interdune, transport direction progressed from oblique to crest-transverse. This indicates that an appreciable amount of sediment may move laterally along the lee slope and interdune corridor under crest-transverse flows. Regarding the grain size and sorting properties of transported sediment, there was no significant difference in mean grain size over the dune, although in general particles were finer and more poorly sorted in the lee.
The geomorphological significance of airflow patterns in transverse dune interdunes
Geomorphology, 2007
The interdunes between aeolian dunes have been relatively ignored when compared with the research attention on the morphodynamics of the dune bodies themselves. This neglect is in spite of the possible significance of interdune dynamics for the geomorphology of the sand dune system as a whole, especially with regard to dune spacing. This paper considers the mean airflow within four relatively simple transverse dune interdunes. The study locations were chosen in order to sample interdunes with different size and surface characteristics, the dynamics of which were investigated for when incident flow was normal to the upwind crest. The findings confirm existing models of flow reattachment length and recovery for aeolian dune lee-side flow, and show a consistent pattern of increasing near-surface velocity downwind of reattachment that supports a mechanism for interdunes as sand-free features. Flow dynamics are characterised for the different types of interdune observed, where two groups are recognised. The flow patterns in relatively short interdunes (where dunes are closely-spaced) with a sandy surface were accordant with those of the flow response model. In 'extended' interdunes, where bounding dunes were spaced with a length well over that for flow separation, evidence at the downwind edge of the interdunes suggested that flow reacted to the subsequent dune. For the case of these 'extended' interdunes, a new descriptive model is presented to characterise their dynamics. In this model, the variation in near-surface flow allowed process zones to be identified through the interdune. The geomorphological significance of the processes dominating each zone is discussed, and comparisons are made between the flow response case and the new interdune model from this study. In a discussion on the controls of spacing between dunes, where reattachment length exerts a fundamental control, the role of sediment availability is also highlighted as a significant factor. The presence of a sandy bed can, in some circumstances, determine whether dune development, and therefore spacing, is controlled primarily by elements of flow response.
Field Evidence for the Upwind Velocity Shift at the Crest of Low Dunes
Boundary-Layer Meteorology, 2013
Wind topographically forced by hills and sand dunes accelerates on the upwind (stoss) slopes and reduces on the downwind (lee) slopes. This secondary wind regime, however, possesses a subtle effect, reported here for the first time from field measurements of nearsurface wind velocity over a low dune: the wind velocity close to the surface reaches its maximum upwind of the crest. Our field-measured data show that this upwind phase shift of velocity with respect to topography is found to be in quantitative agreement with the prediction of hydrodynamical linear analysis for turbulent flows with first order closures. This effect, together with sand transport spatial relaxation, is at the origin of the mechanisms of dune initiation, instability and growth.
Geomorphology, 2009
This study reports the responses of three-dimensional near-surface airflow over a vegetated foredune to variations in the conditions of incident flow during an 8-h experiment. Two parallel measurement transects were established on morphologically different dune profiles: i) a taller, concave-convex West foredune transect with 0.5-m high, densely vegetated (45%), seaward incipient foredune, and ii) a shorter, concavestraight East foredune transect with lower, sparsely vegetated (14%) seaward incipient foredune. Five stations on each transect from the incipient dune to the crest were equipped with ultrasonic anemometers at 0.6 and 1.65 m height and logged at 1 Hz. Incident conditions were recorded from a 4-m tower over a flat beach. Winds increased from 6 m s − 1 to N 20 m s − 1 and were generally obliquely onshore (ENE, 73°). Three subevents and the population of 10-minute averages of key properties of flow (U, W, S, CV U ) from all sample locations on the East transect (n = 235) are examined to identify location-and profile-specific responses over 52°of the incident direction of flow (from 11 to 63°onshore). Topographic steering and forcing cause major deviations in the properties and vectors of near-surface flow from the regional wind. Topographic forcing on the concave-straight dune profile increases wind speed and steadiness toward the crest, with speed-up values to 65% in the backshore. Wind speed and steadiness of flow are least responsive to changes in incident angle in the backshore because of stagnation of flow and are most responsive at the lower stoss under pronounced streamline compression. On the steeper concaveconvex profile, speed and steadiness decrease toward the crest because of stagnation of flow at the toe and flow expansion at the slope inflection point on the lower stoss. Net downward vertical velocity occurs over both profiles, increases toward the crest, and reflects enhanced turbulent momentum conveyance toward the surface. All of these flow responses are enhanced with faster speeds of incident flow and/or more onshore winds. Significant onshore steering of near-surface vectors of flow (to 37°) occurs and is greatest closer to the surface and during highly oblique winds (~15°onshore). Therefore, even subtle effects of streamline compression and amplification of flow under alongshore conditions effectively steer flow and sand transport toward the dune. As topographic forcing and steering cause significant, three-dimensional deviations in near-surface properties of flow, most regional-scale and/or two-dimensional models of dune process-response dynamics are insufficient for characterizing coastal and desert dune sediment budgets and morphodynamics. In particular, deflection of sand transport vectors with greater fetch distances than those derived from regional winds may occur. Coincident flow, transport and morphological response data are required to better quantitatively model these processes.
Lee Angle Effects in Near Bed Turbulence: An Experimental Study on Low and Sharp Angle Dunes
International Journal of Hydraulic Engineering, 2012
This research presents recent advances on morphodynamic modeling over gravel dunes. Boundary-layer separation over gravel fixed dunes is investigated by Acoustic Doppler Velocimetry (ADV). Using the measurements of flow over dunes at laboratory scale, examined the influence of dune lee sides on the separation of flow. Experiments were conducted in a horizontal flu me. A train of 2D fixed dunes were installed on the bottom of the flu me starting just downstream of the entrance on the flu me. Experiments were carried out using 2 types of dunes with different lee slopes, (38 and 8 degree). The results of quadrant analyses base on the stress fraction SHf versus the hole size at t wo elevations near the bed and near water surface for two angles of lee side, ,at the central axis of the flu me also investigated. Both experimental observations and quadrant results were in agreement about the influence of dune characteristics on the flow separation in different dune lee angles.
Numerical modelling of flow structures over idealized transverse aeolian dunes of varying geometry
Geomorphology, 2004
A Computational Fluid Dynamics (CFD) model (PHOENICSk 3.5) previously validated for wind tunnel measurements is used to simulate the streamwise and vertical velocity flow fields over idealized transverse dunes of varying height (h) and stoss slope basal length (L). The model accurately reproduced patterns of: flow deceleration at the dune toe; stoss flow acceleration; vertical lift in the crest region; lee-side flow separation, re-attachment and reversal; and flow recovery distance. Results indicate that the flow field over transverse dunes is particularly sensitive to changes in dune height, with an increase in height resulting in flow deceleration at the toe, streamwise acceleration and vertical lift at the crest, and an increase in the extent of, and strength of reversed flows within, the lee-side separation cell. In general, the length of the separation zone varied from 3 to 15 h from the crest and increased over taller, steeper dunes. Similarly, the flow recovery distance ranged from 45 to >75 h and was more sensitive to changes in dune height. For the range of dune shapes investigated in this study, the differing effects of height and stoss slope length raise questions regarding the applicability of dune aspect ratio as a parameter for explaining airflow over transverse dunes. Evidence is also provided to support existing research on: streamline curvature and the maintenance of sand transport in the toe region; vertical lift in the crest region and its effect on grainfall delivery; relations between the turbulent shear layer and downward forcing of flow re-attachment; and extended flow recovery distances beyond the separation cell. Field validation is required to test these findings in natural settings. Future applications of the model will characterize turbulence and shear stress fields, examine the effects of more complex isolated dune forms and investigate flow over multiple dunes. D
Variations in wind velocity and sand transport on the windward flanks of desert sand dunes
Sedimentology, 1985
The magnitudes of increases in wind velocity, or speed-up factors, have been measured on the windward flanks of transverse and linear dunes of varying height. On transverse dunes, velocity speed-up varied with dune shape and height. For linear dunes, speed-up factors varied principally with wind direction relative to the dune, with dune shape and dune height. The main effect of velocity speed-up on the windward flanks of dunes is to increase potential sand transport rates considerably in crestal areas. This is greatest for large dunes, with winds of moderate velocity blowing at a large angle to the dune. Changing ratios of base to crest sand-transport rates on transverse dunes tend to reduce dune steepness as overall wind velocities increase. On linear dunes, the tendency for crestal lowering is counteracted by deposition in this area when winds reverse in a bi-directional wind regime.
Emergence of oblique dunes in a landscape-scale experiment
Nature Geoscience, 2014
Aeolian dunes in many arid environments on Earth are shaped by seasonally varying bimodal wind regimes. However, the dynamics of dune evolution under such wind regimes are difficult to investigate at the time and length scales of laboratory experiments 1 . These bedforms, in their natural environments 2-4 , may also be influenced by unknown initial conditions and a variety of factors such as sediment availability 5 , vegetation 6 and cohesion 7 . Here we report results from a landscape-scale experiment in which we examine the evolution of bedforms under asymmetric bimodal winds. After flattening an experimental dunefield across 16 hectares of the Tengger Desert in Inner Mongolia, we measured winds and topography from March 2008 to October 2011 to reveal the development of regular dune patterns with a constant wavelength and increasing amplitude. On a seasonal timescale, we show that individual dunes propagate in different directions according to the prevailing wind. We find that the orientation of dune crests is controlled by the combination of the normal contributions of the two dominant winds, with respect to their relative strengths and directions, such that crests form an oblique angle of 50 • with the resultant sand flux. Our landscape-scale experiment suggests that the alignment of aeolian dunes can be used to determine wind forcing patterns on the Earth and other planetary bodies.