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1.
David M. Kennedy 《地球表面变化过程与地形》2010,35(15):1751-1762
Farewell Spit is a 25 km long barrier spit that marks the end of a littoral drift system, almost 1000 km in length that runs along South Island, New Zealand. The spit is composed of barchan dunes over 20 m high, sand sheets over 1 km wide and vegetated linear dunes. Analysis of aerial photography indicates a rapid colonization of the spit by vegetation which has expanded in area by 75% since 1950. Vegetation colonization preferentially occurs on the southern side of the spit, with its northern margin characterized by barchan dunes which migrate at rates of up to 64 m/yr. Sand sourced from longshore drift appears to be the primary source of beach sediment, which is then transported into the dune field by the persistent westerly winds of the Roaring 40s. While there has been significant dune roll‐over on the surface of the spit, its overall area has remained much the same for the past 54 years. Occasional cyclone events cause erosion, but this is balanced by aeolian sediment transport. It would appear that extension of the subaerial portion of the spit is related to the development of shells banks at its downdrift end which are periodically welded to the main spit by dune extension. Farewell Spit therefore provides an ideal example of a barrier environment where longshore sediment supply and aeolian transport dominates geomorphic evolution. This differentiates the study site from other barrier environments where overwash or tidal inlet development often characterizes recent landform evolution. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
2.
The formation of the complex linear dunes in the central Taklimakan Sand Sea is discussed based on analyses of wind regimes, sand grain size distributions on the topography of the dunes, and a combination of geomorphic and geophysical investigations into the morphology of the dunes. Complex linear dune formation is shown to have ?ve stages. Analysis clearly shows that under the control of wind regime, sand supply and other factors, the simple linear dunes move sideways while they evolve. This is the main cause for the formation of complex linear dunes in the central Taklimakan Sand Sea. We have not collected enough evidence to show whether the complexity of the complex linear dunes is left over from previous wind regimes or whether the previous wind regimes had different dominant wind directions compared to those of modern winds. The evolutionary processes of complex linear dunes in the region partly support the theory of ‘barchan evolution’ but do not support the ‘roll‐vortex’ and ‘bimodal wind regime’ hypotheses. After the complex linear dunes were developed, the local wind regime and the other controls such as sand supply suggest it is possible for them to maintain their linear shape. The evolutionary process discussed is limited to the region indicated in this paper, and may not be applicable to the whole Taklimakan Sand Sea. There are different evolutionary processes in different dune?elds because of variations in the factors that control complex linear dune formation. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
3.
The introduction of vegetation to bare barchan dunes can result in a morphological transformation to vegetated parabolic dunes. Models can mimic this planform inversion, but little is known about the specific processes and mechanisms responsible. Here we outline a minimalist, quantitative, and process‐based hypothesis to explain the barchan–parabolic transformation. The process is described in terms of variations in the stabilization of wind‐parallel cross‐sectional dune slices. We hypothesize that stabilization of individual ‘dune slices’ is the predictable result of feedbacks initiated from colonization of vegetation on the slipface, which can only occur when slipface deposition rates are less than the deposition tolerance of vegetation. Under a constant vegetation growth regime the transformation of a barchan dune into a parabolic dune is a geometric response to spanwise gradients in deposition rates. Initial vegetation colonization of barchan horns causes shear between the anchored sides and the advancing centre of the dune, which rotates the planform brinkline angle from concave‐ to convex‐downwind. This reduces slipface deposition rate and allows vegetation to expand inward from the arms to the dune centre. The planform inversion of bare barchans dunes into vegetated parabolic dunes ultimately leads to complete stabilization. Our hypothesis raises several important questions for future study: (i) are parabolic dunes transitional landforms between active and vegetation‐stabilized dune states? (ii) should stabilization modelling of parabolic dune fields be treated differently than linear dunes? and (iii) are stabilized parabolic dune fields ‘armoured’ against re‐activation? Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
4.
Although dunes fronted by sandy beaches constitute approximately 80 per cent of South Africa's coastline, few studies have addressed the formation and life cycle of coastal foredunes, the small, ephemeral shore‐parallel dune ridges typically less than 5 m high and 20 m wide, which form seaward of the storm line. This study used regular, detailed topographic surveys of embryo and foredunes at Tugela mouth, an aggrading stretch of shoreline on the subtropical east coast of South Africa, over a 32‐month period, to gain insight into the formation and motion of these highly mobile landforms over the short term. Average wind drift potential at Tugela mouth during the study period, at 2·35 m s?1, was an order of magnitude lower than that typical of most parts of the eastern South African coast. The dominant sand‐moving wind for the region was from the southwest to west‐southwest at 10·7 to 13·8 m s?1, with a secondary vector from north to north‐northeast at 10·8 to 13·8 m s?1. Signi?cant shoreline retreat, a result of the low sediment yield of the Tugela River during the study period, was one of the main results. This provided the context for redistribution of sand from the inland to the seaward side of the study area, a consequence of the dominant wind direction, and for frequent creation and destruction of short‐lived embryo dunes. Those foredunes which survived the whole study period tended to increase in height, but there was no consistent directional trend in foredune crest movement throughout the 32 months. The study results generally supported Psuty's model of foredune development, but could not con?rm his contention of landward retreat of dune forms under conditions of shoreline erosion. This may be due to the relatively short duration of the study, or possibly to low wind drift potential at the site. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
5.
Transverse dunes appear in regions of mainly unidirectional wind and high sand availability. A dune model is extended to two‐dimensional calculation of the shear stress. It is applied to simulate dynamics and morphology of three‐dimensional transverse dunes. In the simulations they seem to reach translational invariance and do not stop growing. Hence, simulations of two‐dimensional dune ?elds have been performed. Characteristic laws were found for the time evolution of transverse dunes. Bagnold's law of the dune velocity is modi?ed and reproduced. The interaction between transverse dunes led to the interesting conclusion that small dunes can travel over bigger ones. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
6.
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. 相似文献
7.
Concepts derived from previous studies of offshore winds on natural dunes are evaluated on a dune maintained for shore protection during three offshore wind events. The potential for offshore winds to form a lee‐side eddy on the backshore or transfer sediment from the dune and berm crest to the water are evaluated, as are differences in wind speed and sediment transport on the dune crest, berm crest and a pedestrian access gap. The dune is 18–20 m wide near the base and has a crest 4.5 m above backshore elevation. Two sand‐trapping fences facilitate accretion. Data were obtained from wind vanes on the crest and lee of the dune and anemometers and sand traps placed across the dune, on the beach berm crest and in the access gap. Mean wind direction above the dune crest varied from 11 to 3 deg from shore normal. No persistent recirculation eddy occurred on the 12 deg seaward slope. Wind speed on the berm crest was 85–89% of speed at the dune crest, but rates of sediment transport were 2.27 times greater during the strongest winds, indicating that a wide beach overcomes the transport limitation of a dune barrier. Limited transport on the seaward dune ramp indicates that losses to the water are mostly from the backshore, not the dune. The seaward slope gains sand from the landward slope and dune crest. Sand fences causing accretion on the dune ramp during onshore winds lower the seaward slope and reduce the likelihood of detached flows during offshore winds. Transport rates are higher in access gaps than on the dune crest despite lower wind speeds because of flatter slopes and absence of vegetation. Transport rates across dunes and through gaps can be reduced using vegetation and raised walkover structures. Copyright © 2017 John Wiley & Sons, Ltd. 相似文献
8.
The protocratonic core of the São Francisco craton assembled during the 2.1–2.0 Ga Transamazonian orogeny. Orosirian Fe‐rich sequences that extend from the northwestern border of the São Francisco protocraton (Colomi Group) to the southeast under the Espinhaço Belt (the < 1.99 Ga Serra da Serpentina Group) record the opening of an intracratonic basin with the episodically developed ferruginous waters prior to the initiation of the Espinhaço rift at 1.8 Ga. Ferruginous conditions developed again during deposition of the Canjica Iron Formation of the < 1.7 Ga Serra de São José Group in the Espinhaço rift (contemporaneously with felsic magmatism; Conceição do Mato Dentro Rhyolite and Borrachudos Granitic Suite) and extensive sandstones of the < (1666 ±32) Ma Itapanhoacanga and < (1683 ±11) Ma São João da Chapada Formations. In the upper São João da Chapada Formation, banded hematitic phyllite also records input of Fe‐rich fluids. The young age of these iron formations with respect to the conventionally accepted 1.88 Ga age for the youngest shallow‐marine Paleoproterozoic iron formations, the apparent absence of granular facies (granular iron formations), and yet shallow‐water (above fair‐weather base) depositional environment indicate that an unusual setting developed in a large basin after the Great Oxidation Event, in the aftermath of the Transamazonian orogeny. We propose that mantle plumes led to the opening of a previously unrecognized rift system, that could have caused the magmatism, supplied hydrothermal Fe and led to the opening of the Espinhaço, Pirapora, and Paramirim rifts, later obliterated by the Araçuaí orogenic belt during the Neoproterozoic to Early Paleozoic Brasiliano orogeny. The rift system did not develop into an open continental margin but probably evolved into a broad sag basin, stretching across the São Francisco and Congo cratons. 相似文献
9.
Ian Livingstone 《地球表面变化过程与地形》2003,28(9):1025-1031
Repeated surveying of two sites on a Namib linear dune between 1980 and 2001 provides a 21‐year record of dune surface change. The surveys con?rm the view that the dunes are not inactive relics but are responding to the present‐day wind regime. They also provide no evidence that the dunes are migrating laterally. Examination of wind data for the survey period provides some evidence that the form of the crest of the dunes is actively responding to the natural year‐by‐year climate variability, such that an increase in the frequency of easterly winds leads to the development of a double‐crested form while fewer easterly winds lead to a single‐crested form. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
10.
11.
Mark A. Bishop 《地球表面变化过程与地形》2004,29(1):125-131
Distribution‐free statistical methods of comparative data analysis have identi?ed subtle granulometric differences attributed to the evolution of barchan form at Gurra‐Gurra waterhole. Geomorphic locations on the barchan dunes display statistically signi?cant grain‐size differences that assist in the interpretation of aeolian processes. In summer, very ?ne sands mantle the dunescape and are the fraction that most affects the parameters of sorting and skewness. The sur?cial sedimentological character is one of subtle contrasts between the processes of grain winnowing and intergranular protection. The second and third moment measures are parameters that best demonstrate the spatial granulometric differences. Dune‐forming processes at Gurra‐Gurra have produced dune sands that have a very narrow range of grain size, which, in turn, re?ects textural and mineralogical maturity, and hence an extensive transport history. The statistical techniques employed in this study can also be used for the comparison of temporal (seasonal) sedimentological change, and for the granulometric analysis and association of process for dunes of different morpho‐types. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
12.
Early Holocene dune field development in Dalarna,central Sweden: A geomorphological and geophysical case study 
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下载免费PDF全文 Bonäsheden, Sweden's largest continuous dune field, situated in the county of Dalarna, central Sweden, has been investigated using LiDAR (light detection and ranging) remote sensing, ground penetrating radar as well as by field observations and luminescence dating. The use of LiDAR in conjunction with geographic information system (GIS) software proved to be efficient in mapping the inactive dune field and classifying the dune morphology, especially when slope raster images were used. The dunes have formed mostly by winds from the northwest (NW) and are of a transverse type. Still other dune types, such as parabolic dunes, and transverse dunes with a deviating orientation are present. Also, there seems to be different generations of dunes, suggesting a complex palaeowind environment with a change from predominantly north‐westerly winds to more westerly winds. Luminescence dating finally allows us to have an absolute chronology of the development of the Bonäsheden dune field, revealing formation of the dune field closely following the de‐glaciation of this part of Sweden (c. 10.5 ka). The well preserved transverse shape of the majority of the dunes suggests rapid stabilization by vegetation, although sand drift still seems to have been active on a noticeable scale for at least 1500 years and also, occasionally and patchy, as coversand deposition during the Late Holocene. A simple model is proposed for the dune field development of Bonäsheden based on our findings. This model is a useful addition since the majority of present day dune field models focus on the formation of parabolic dunes or large unvegetated dune fields. Our results suggest that most models cannot adequately simulate the formation of such small dune fields as that of Bonäsheden, with apparently rapidly fixated transverse dunes in a previously glaciated, now vegetated area. Copyright © 2017 John Wiley & Sons, Ltd. 相似文献
13.
Transformation of parabolic dunes into mobile barchans triggered by environmental change and anthropogenic disturbance 下载免费PDF全文
下载免费PDF全文 Parabolic dunes are widely distributed on coasts and margins of deserts and steppes where ecosystems are vulnerable and sensitive to environmental changes and human disturbances. Some studies have indicated that vegetated parabolic dunes can be activated into highly mobile barchan dunes and the catastrophic shift of eco‐geomorphic systems is detrimental to land management and social‐economic development; however, no detailed study has clarified the physical processes and eco‐geomorphic interactions that control the stability of a parabolic dune and its resistance to unfavorable environmental changes. This study utilizes the Extended‐DECAL (Discrete Eco‐geomorphic Aeolian Landscapes) model, parameterized by field measurements of dune topography and vegetation characteristics combined with remote sensing, to explore how increases in drought stress, wind strength, and grazing stress may lead to the activation of stabilizing parabolic dunes into highly mobile barchans. The modeling results suggest that the mobility of an initial parabolic dune at the onset of a perturbation determines the capacity of a system to absorb environmental change, and a slight increase in vegetation cover of an initial parabolic dune can increase the activation threshold significantly. The characteristics of four eco‐geomorphic interaction zones control the processes and resulting morphologies of the transformations. A higher deposition tolerance of vegetation increases the activation threshold of the dune transformation under both a negative climatic impact and an increased sand transport rate, whereas the erosion tolerance of vegetation influences the patterns of resulting barchans (a single barchan versus multiple barchans). The change in the characteristics of eco‐geomorphic interaction zones may indirectly reflect the dune stability and predict an ongoing transformation, whilst the activation angle may be potentially used as a proxy of environmental stresses. In contrast to the natural environmental changes that tend to affect relatively weak and young plants, grazing stress can exert a broader impact on any plant indistinctively. A small increase in grazing stress just above the activation threshold can accelerate dune activation significantly. Copyright © 2017 John Wiley & Sons, Ltd. 相似文献
14.
Monitoring surface change on a Namib linear dune 总被引：1，自引：0，他引：1
Ian Livingstone 《地球表面变化过程与地形》1989,14(4):317-332
In tackling the apparently intractable problem of linear dune initiation and maintenance there has been a move away from large-scale deductive models to smaller-scale field studies of individual dunes. This paper reports a study of surface change on a large, complex linear dune in the Namib Desert, southern Africa. The dune surface responds to a markedly seasonal wind regime. In summer westerly winds erode sand from the west flank of the dune and deposit it on the easterly lee side of the dune crest. In winter this pattern is reversed. Easterly winds erode sand from the east slope and deposit it on the west slope. The crest therefore moves back and forth some 15 m each year returning at the end of a year's cycle to its position at the beginning. The position of the base of the dune appears to remain fixed, even though sand is moving throughout the dune system. The dune does extend northward along some resultant of the westerly and easterly winds. Despite relatively high levels of activity, especially at the dune crest, there is no evidence of the breakdown of the linear dune form. The conclusion must therefore be that linear dunes can be maintained in bimodal wind regimes and are not necessarily related to unidirectional parallel regimes as others have suggested. 相似文献
15.
K. Pye 《地球表面变化过程与地形》1983,8(4):371-381
Several previous attempts have been made to explain the apparent poor development of coastal dunes in the humid tropics in terms of lack of wind energy, failure of sand supply to the shoreline, excessive climatic wetness, salt crust formation on beaches, and the character of tropical back-beach vegetation. However, recent published reports indicate that coastal dune occurrences are more common in the humid tropics than was formerly thought, throwing suspicion on the idea that environmental conditions militate against dune formation in these areas as a whole. Evidence from the humid tropical sector of the North Queensland coast suggests that the poor development of dunes in this area primarily reflects poor sediment sorting in the beach and nearshore zone and low wind energy at the shoreline due to the nature of the coastal orientation and physiography in relation to the prevailing southeasterly winds. These limiting factors are not unique to humid tropical climates. 相似文献
16.
Sediment budget data from an 18‐month topographic survey were analysed with data from brief experiments on wind parameters, beach moisture contents, bedforms and sand mobilization in order to monitor conditions and patterns of embryo dune development over a flat 150–1000 m wide accreting upper beach. The surface conditions over the upper beach locally affect aeolian transport, but net dune development over time depends on sustained strong winds and their orientation. Incoming marine sand supplied by storms and onshore winds is reorganized by the dominant offshore to longshore winds into elongated embryo dunes over this upper beach, imprinting a regional morphology of long‐term longshore dune ridge development. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
17.
Barchans, isolated crescent‐shaped bedforms, are believed to be formed under almost unidirectional wind or water ?ows and limited sand supply. The formation of barchan morphologies under the action of purely oscillatory wave motion has not yet been fully investigated. The present study attempted to form barchan topography in a wave ?ume and to compare this with barchans in the ?eld. Barchan morphologies of ripple size, called the barchan ripples, were generated from a ?at bed by the action of waves. The horn width, the distance between horn tips, of the barchan ripples increased linearly with an increase in the total length, the overall length projected on the centre line of the barchan, with a coef?cient common to barchan dunes in deserts. The ratio of horn length to horn width of the barchan ripples was smaller than that of barchan dunes, but similar to that of subaqueous barchans in the ?eld. The longer the wave period was, the larger the ratio of the body length to horn width became. Most subaqueous barchans formed under waves (in the laboratory) and unidirectional ?ows (in the ?eld) had blunter horns than subaerial barchans. The shape of the barchan ripples changed with wave period. The outer rim became rounder with increasing wave period. The relationship between the base area and the height of barchan morphologies seems to be linear, with a constant coef?cient for the scale from ripples to dunes. The barchan ripples showed a linear relationship between the height and the horn width, similar to that for barchan dunes. The migration speed of the barchan ripples was proportional to the cube of the ?ow velocity and was inversely proportional to height. The same relation with a different value of the coef?cient was obtained for barchan dunes. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
18.
As with most dune fields, the White Sands Dune Field in New Mexico forms in a wind regime that is not unimodal. In this study, crescentic dune shape change (deformation) with migration at White Sands was explored in a time series of five LiDAR‐derived digital elevation models (DEMs) and compared to a record of wind direction and speed during the same period. For the study period of June 2007 to June 2010, 244 sand‐transporting wind events occurred and define a dominant wind mode from the SW and lesser modes from the NNW and SSE. Based upon difference maps and tracing of dune brinklines, overall dune behavior consists of crest‐normal migration to the NE, but also along‐crest migration of dune sinuosity and stoss superimposed dunes to the SE. The SW winds are transverse to dune orientations and cause most forward migration. The NNW winds cause along‐crest migration of dune sinuosity and stoss bedforms, as well as SE migration of NE‐trending dune terminations. The SSE winds cause ephemeral dune deformation, especially crestal slipface reversals. The dunes deform with migration because of differences in dune‐segment size, and differences in the lee‐face deposition rate as a function of the incidence angle between the wind direction and the local brinkline orientation. Each wind event deforms dune shape, this new shape then serves as the boundary condition for the next wind event. Shared incidence‐angle control on dune deformation and lee‐face stratification types allows for an idealized model for White Sands dunes. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
19.
We herein report the results of a ?eld study that was designed to test the feasibility of using ground‐based LIDAR to map the topography of a sand dune in high spatial resolution. A portable Cyrax 2500 three‐dimensional (3D) laser scanner was used to digitally capture the topography of a barchan, roughly 4 m tall and 50 m long, located in the White Sands National Monument, New Mexico. We performed eleven scans around the barchan and obtained the elevation relative to the inter‐dune ?at at roughly 1/4 million points on the dune surface. The elevation point data were then interpolated to yield a continuous surface model of the dune topography with c. 10 cm spatial resolution and c. 6 mm position accuracy. The results from this ?eld study clearly demonstrate the potential of ground‐based LIDAR as a mapping tool for use in aeolian research and other earth science applications. The 3D surface model of the dune can describe the morphology with hitherto unprecedented detail. Moreover, the surface of the dune is mapped with a minimum of foot traf?c on the dune itself. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
20.
A field study of mean and turbulent flow characteristics upwind,over and downwind of barchan dunes 总被引：1，自引：0，他引：1
Field‐measured patterns of mean velocity and turbulent airflow are reported for isolated barchan dunes. Turbulence was sampled using a high frequency sonic anemometer, deriving near‐surface Reynolds shear and normal stresses. Measurements upwind of and over a crest‐brink separated barchan indicated that shear stress was sustained despite a velocity reduction at the dune toe. The mapped streamline angles and enhanced turbulent intensities suggest the effects of positive streamline curvature are responsible for this maintenance of shear stress. This field evidence supports an existing model for dune morphodynamics based on wind tunnel turbulence measurements. Downwind, the effect of different dune profiles on flow re‐attachment and recovery was apparent. With transverse incident flow, a re‐attachment length between 2·3 and 5·0h (h is dune brink height) existed for a crest‐brink separated dune and 6·5 to 8·6h for a crest‐brink coincident dune. The lee side shear layer produced elevated turbulent stresses immediately downwind of both dunes, and a decrease in turbulence with distance characterized flow recovery. Recovery of mean velocity for the crest‐brink separated dune occurred over a distance 6·5h shorter than the crest‐brink coincident form. As the application of sonic anemometers in aeolian geomorphology is relatively new, there is debate concerning the suitability of processing their data in relation to dune surface and streamline angle. This paper demonstrates the effect on Reynolds stresses of mathematically correcting data to the local streamline over varying dune slope. Where the streamline angle was closely related to the surface (windward slope), time‐averaged shear stress agreed best with previous wind tunnel findings when data were rotated along streamlines. In the close lee, however, the angle of downwardly projected (separated) flow was not aligned with the flat ground surface. Here, shear stress appeared to be underestimated by streamline correction, and corrected shear stress values were less than half of those uncorrected. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献