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1.
Embryo dunes are often ephemeral, but can develop to become established coastal foredunes. In 2001 a patch of embryo dunes 13.11 m2 appeared on a beach in north Lincolnshire, UK and had expanded to over 3600 m2 by 2011. The rate of expansion is linked to storm occurrence, where expansion is slowed during years with a higher incidence of storm surges. From July 2009–October 2010 seasonal changes in dune field topography were determined using terrestrial laser scanning (TLS) data. Vegetation is important in the development of embryo dunes, but can cause errors in TLS data. Tests evaluating the impact of vegetation on the TLS data suggest the minimum elevation value from the TLS point cloud within a 0.05 m grid cell gives a good approximation of the ground surface. Digital elevation models (DEMs) of the dunes constructed using filtered data showed the embryo dunes underwent a classic seasonal cycle of erosion during the winter and accretion during the summer. For example from October 2009 to April 2010 over 375 m3 of sediment was eroded from the dunes whereas during spring and summer 2010 the dune field gained over 600 m3 of sand. The overall magnitude of change in dune height and volume from season to season exceeded the errors associated with the construction of the DEM from the TLS data and the vegetation filtering process, which suggests TLS can be useful for documenting topographic change in vegetated dunes. After 10 years, the patch of embryo dunes is still expanding but has not yet merged with more established foredunes to landward. Aeolian process measurements indicate that, at present, the embryo dunes do not prevent sand from reaching the foredunes, however the rate of foredune progradation has slowed concurrently with the expansion of the embryo dune field. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

2.
Longitudinal dunes can move sideways   总被引：3，自引：0，他引：3  
Longitudinal dunes occur in all major sand areas in the world. Their dominant mode of migration or extension is considered to be either prevailing- or resultant wind-parallel, the dunes extending downwind via accretion of the terminal nose. In the Qaidam Pendi in Northwest China a series of active longitudinal dunes extend downwind at 5–10 m yr?1. Internal sedimentary structures examined in the dunes, however, display beds dipping in one direction rather than two opposed directions as is expected. Analysis of aerial photographs confirms that these dunes migrate laterally up to three metres per year, whilst maintaining a symmetrical longitudinal dune morphology.  相似文献   

3.
Simple, and locally compound, transverse and barchanoid dunes dominate the 2000 km2 Skeleton Coast dunefield in northwestern Namibia/South West Africa. Dune height and spacing are closely correlated (r = 0-89) and decrease across the dunefield from southwest to northeast, with an accompanying change from transverse to barchanoid ridges and ultimately barchans. The dunes are aligned transverse to the dominant strong south and south southwest onshore winds. Alignment patterns indicate that surface roughness changes between coastal plain and dunes cause dune-forming winds to swing to the right over the dunes, but resume their original direction beyond. Grain size and sorting vary at three scales: the dune, the dune landscape and through the dunefield. Overall the sands, derived from three localities by deflation from beaches supplied by vigorous longshore drift, become progressively finer and better sorted across the dunefield paralleling changes in dune height and spacing. A statistically significant relationship (r = ?0?65) was established between dune spacing and the phi grain size of the coarser fraction of the dune sands, demonstrating the importance of the protective effects of coarse grains, and suggesting that the morphometry of simple transverse dunes may be controlled by the scale of turbulence associated with the threshold wind speed required to move the coarsest fraction of the dune sand.  相似文献   

4.
Barchan dunes are common on Earth, Mars and Titan. Previous studies have shown that their formation, migration and evolution are influenced by the wind regime and other factors, but details vary among regions. Understanding barchan morphology and migration will both improve our understanding of dune geomorphology and provide a basis for describing the environmental conditions that affect the formation and development of these dunes on Earth and other planets. Here, we provide detailed measurements of barchan dune migration in China's Quruq Desert, in the lower reaches of the Tarim River. We monitored their migration direction and rate, and their morphological changes during migration, by comparing Google Earth images acquired in 2003 and 2014. The dunes migrated west-southwest, close to the local resultant drift direction. The migration rate averaged 8.9 to 32.1 m year−1, with obvious spatial variation. In addition to the wind regime, the migration rate depended on dune morphology, density and vegetation cover; the rate was negatively related to dune height, density and vegetation cover, but positively linearly related to the length/width ratio (LU/W) and to the decrease in this ratio from 2003 to 2014. We found correlations among the dune morphometric parameters, but the relationships were weaker than in previous research. Due to the complexity of the factors that affect the processes that underlie sand dune development and migration, the morphological changes during dune migration were also complex. Our measurements suggest that the aeolian environment played a dominant role in dune migration and its spatial variation in the Quruq Desert. These results will support efforts to control dune migration in the western Quruq Desert and improve our understanding of dune morphodynamics. © 2019 John Wiley & Sons, Ltd.  相似文献   

5.
Although extensive data exist on runoff erosion and rates for non‐sandy hillslopes, data for arid dune slopes are scarce, owing to the widespread perception that the high infiltrability of sand will reduce runoff. However, runoff is generated on sandy dunes in the Hallamish dune field, western Negev Desert, Israel (P ≈ 95 mm) due to the presence of a thin (usually 1–3 mm) microbiotic crust. The runoff in turn produces erosion. Sediment yield was measured on ten plots (140–1640 m2) on the north‐ and south‐facing slopes of longitudinal dunes. Two plots facing north and two facing south were subdivided into three subplots. The subplots represented the crest of the active dune devoid of crust, the extensively crusted footslope of the dune, and the midslope section characterized by a patchy crust. The remaining plots extended the full length of the dune slope. No runoff and consequently no water‐eroded sediments were obtained from the crest subplots devoid of crust. However, runoff and sediment were obtained from the mid‐ and footslope crusted subplots. Sediment yield from the footslope subplots was much higher than from the midslopes, despite the higher sediment concentration that characterized the midslope subplots. The mean annual sediment yield at the Hallamish dune field was 432 g per metre width and was associated with high average annual concentrations of 32 g l?1. The data indicate that owing to the presence of a thin microbiotic crust, runoff and water erosion may occur even within arid sandy dune fields. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   

6.
Long-term and seasonal geomorphological changes at Padre Island, Texas are identified and linked with potential external drivers. Aerial and satellite images from 1950 to 2018, monthly images from 2019 to 2020, and a 2018 LiDAR data set are used to assess long-term and seasonal geomorphological changes within a 50 km2 area of Padre Island near Port Mansfield, Texas. Trends in landcover are evaluated by mapping and comparing the relative areal coverage of each facies. Vegetated dunes, absent initially, emerged in the fore-island and expanded into the back-barrier to cover 14% of the study area. The active vegetation-free back-barrier dune field steadily decreased in areal extent from 12% to 6% as vegetation spread. Nebkha dune coverage fluctuated between 4% and 7%. Expansive microbial mats colonized the wind tidal and deflation flats surrounding the vegetated dunes and back-barrier dune field giving rise to a remarkably different landscape over the 50-year period studied. An assessment of external forcing factors identifies increased rates of relative sea level rise and decreased sediment influx as the most likely primary factors driving the geomorphological changes. These changes have induced a widespread shift toward stabilization of island sediments by vegetation and microbial mats, which in turn has starved the back-barrier of sediments resulting in low rates of accretion and increased flooding. These findings highlight the sensitivity of the back-barrier and, in particular, the dune facies to changes in sea level and sediment supply, and show that microbial mats are effective at stabilizing island sediments and may be harbingers to barrier island response to rising sea level. As shown in this study, long-term monitoring of geomorphic facies changes and topography can detect important shifts in the island state that can be used to inform decision making for these sensitive coastal landscapes.  相似文献   

7.
Sunset Crater in north‐central Arizona (USA) is a 900‐year‐old scoria‐cone volcano. Wind action has redistributed its widespread tephra deposit into a variety of aeolian dune forms that serve as a terrestrial analog for similar landforms and aeolian processes on Mars. Fieldwork was conducted to collect essential geomorphological and sedimentological data, and to establish a baseline for the type and morphometry of dunes, physical properties, interactions with topography, and saltation pathways. Our analyses focused primarily on coppice dunes, falling dunes, wind ripples, and sand streaks. For all collected volcaniclastic aeolian sediment samples, the sand‐size fraction dominated, ranging from almost 100% sand to 74.6% sand. No sample contained more than 1.6% silt. The composition is overwhelmingly basaltic with non‐basaltic particles composing 2 to 6% of the total. Coppice (nebkha) dunes form where clumps of vegetation trap saltating particles and create small mounds or hummocks. Mean grain size for coppice dune samples is coarse sand. Measured dune height for 15 coppice dunes ranged from 0.3 to 3.3 m with a mean of 1 m. Mean length was 6.7 m and mean width was 4.8 m. Falling dunes identified in this study are poorly developed and thin, lacking a prominent ramp‐like structure. Mean wavelength for three sets of measured ripples ranged from 22 to 36 cm. Sand streaks extend downwind for more than a kilometer and are up to 200 m in width. They commonly occur on the lee side of mesas and similar landforms and are typically the downwind continuation of falling dunes. Falling dunes, wind ripples, and sand streaks have been identified on Mars, while coppice dunes are similar to Martian shadow or lee dunes in which sand accumulates in the lee of obstacles. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

8.
Extensive coastal dune ?elds occur on the Quaternary strandplain associated with the São 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 São 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 São 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.  相似文献   

9.
This paper documents application of an established geostatistical methodology to detect significant changes in a foredune–transgressive dune complex where Parks Canada Agency (PCA) implemented a dynamic restoration program to remove invasive marram grasses (Ammophila spp.) and enhance dynamic dune habitat for an endangered species. Detailed topographic surveys of a 10 320 m2 site in the Wickaninnish Dunes in Pacific Rim National Park, British Columbia, Canada for the first year post‐treatment are compared to a pre‐restoration LiDAR baseline survey. The method incorporates inherent spatial structure in measured elevation datasets at the sub‐landscape scale and models statistically significant change surfaces within distinct, linked geomorphic units (beach, foredune, transgressive dune complex). Seasonal and annual responses within the complex are discussed and interpreted. All geomorphic units experienced positive sediment budgets following restoration treatment. The beach experienced the highest differential volumetric change (+1656 m3) and net sediment influx (+834 m3, 0 · 19 m3 m–2) mostly from supply to the supratidal beach and incipient dune. This sediment influx occurred independent of the restoration effort and was available as a buffer against wave erosion and as supply to the landward dunes. The foredune received +200 m3 (0 · 13 m3 m‐2) and its seaward profile returned to a similar pre‐restoration form following erosion at the crest from vegetation removal and scarping by high water events. Sediment bypassing and minimal change was evident at the mid‐stoss slope with appreciable extension of depositional lobes in the lee. The transgressive dune complex experienced high accretion following restoration activity (+201 m3) and over the year (+284 m3, 0 · 07 m3 m–2) mostly from depositional lobes from the foredune, precipitation ridge growth along the downwind boundary, and growth of existing lobes within the complex. Further integration of this methodology to detect significant geomorphic changes is recommended, particularly for applications where sampling densities are limited or logistically defined. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

10.
Submarine dune dynamics are controlled by tidal currents and wind forces. According to the relative influence of these forces and the nature of dune sediment, different bedform behaviors can be observed. The footprint of the different hydrodynamic agents is recorded into the internal architecture of dunes. This paper is concerned with bedforms that compose the thick sediment wedge located in the eastern English Channel, off the Bay of Somme. This sedimentary archive constitutes an interesting feature to achieve a better understanding of seabed sediment dynamics and its timeline building stages. The dynamics of large submarine dunes, which are organized in fields, are studied thanks to bathymetric and seismic data over the periods 1937–1993 and 1993–2007. Dune morphology presents low lee and stoss side slopes (on average 8° and 3°, respectively) and dune migration rate is not very high. Dune movements are in the direction of residual tidal currents, i.e. toward the east, with mean migration rates around 0·8 to 5 ± 0·25 m yr?1 and up to 6·6 ± 0·7 m yr?1, respectively, at multi‐decennial and decennial time scales. The dune internal architecture is complex with superimposed eastward prograding units, displaying locally opposite progradation. Second‐order discontinuities (dip of 0·5°–4° perpendicular to dune crests) constitute dune master bedding. By counting the number of second‐order reflectors between 1937–1993 and 1993–2007, the formation periodicity of these bounding surfaces is estimated to range from 4 to 18 years. These time intervals coincide with the long‐term tidal cyclicities and also with the inter‐annual to decennial variability of storm activity in northern Europe. Two theories were made to interpret the dune internal structures: the second‐order surfaces are interpreted either as the depositional surfaces corresponding to the marks of weak energy periods (weak tidal and storm action), or as erosive surfaces due to an opposite direction of dune migration provoked temporarily by exceptional storms from the northeast. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

11.
The Mars Global Digital Dune Database (MGD3) now extends from 90°N to 65°S. The recently released north polar portion (MC‐1) of MGD3 adds ~844 000 km2 of moderate‐ to large‐size dark dunes to the previously released equatorial portion (MC‐2 to MC‐29) of the database. The database, available in GIS‐ and tabular‐format in USGS Open‐File Reports, makes it possible to examine global dune distribution patterns and to compare dunes with other global data sets (e.g. atmospheric models). MGD3 can also be used by researchers to identify areas suitable for more focused studies. The utility of MGD3 is demonstrated through three example applications. First, the uneven geographic distribution of the dunes is discussed and described. Second, dune‐derived wind direction and its role as ground truth for atmospheric models is reviewed. Comparisons between dune‐derived winds and global and mesoscale atmospheric models suggest that local topography may have an important influence on dune‐forming winds. Third, the methods used here to estimate north polar dune volume are presented and these methods and estimates (1130 km3 to 3250 km3) are compared with those of previous researchers (1158 km3 to 15 000 km3). In the near future, MGD3 will be extended to include the south polar region. Published in 2011. This article is a US Government work and is in the public domain in the USA.  相似文献   

12.
A test of granulometric control of desert dune geometry   总被引：1，自引：0，他引：1  
I. G. Wilson's hypothesis of coarse-tail grain-size control of dune spacing was derived from three dunefields in the Sahara and, although it was supplemented by measurements of spacing from other parts of the world, the grain-size data come only from North Africa. In this paper the hypothesis is tested in the Australian dunefields. Australian dunes do not form separate categories on a P20/s (twentieth percentile/spacing) plot and, when placed on Wilson's diagram, the Australian data form a continuum between dunes and draas. Ripples maintain their identity, suggesting that the average saltation length of sand controls ripple wavelength while dunes and Wilson's draas are formed by secondary flow of some kind. In Australia the spread of data on the P20/s diagram indicates that grain-size is not the prime control on s. Data from Australia and the Sahara indicate that direct linear relationships between s and h (dune spacing and height means respectively for blocks of dunes) occur but they have different slopes in different areas. These differences possibly reflect variations in vegetation and substrate as well as differences in wind regime. The separation of dunes from draas in the Sahara reflects the greater role of grain-size in an area where coarse grains frequently occur in dune crests. The relative paucity of coarse grains in Australian dune crests may reflect the fine-grained alluvium from which the dunes are derived. These differences may be ascribed to differing topography in Australia and the Sahara.  相似文献   

13.
Coastal dunes are dynamic features that are continuously evolving due to constructive (e.g., wind- and wave-driven sediment transport) and destructive (e.g., elevated total water levels during storm events) processes. However, the relative importance of these processes in determining dune evolution is often poorly understood. In this study, ten lidar datasets from 1997 to 2016 are used to determine the relative role of erosion and accretion processes driving foredune change on the coast of Cape Lookout National Seashore, North Carolina, USA. Beach and dune morphometrics reveal that dune toe locations have generally retreated since 1997, while dune crest heights accreted by 0.01–0.02 m/year. We develop three univariate metrics that represent (1) the potential for erosion, i.e., total water level impact hours per year, (2) accretion, i.e., dune building hours per year, and (3) the relative net effect of foredune accretion and erosion processes, i.e., constructive–destructive dune forcing (CDDF) ratio, and test the correlative power of these metrics in explaining changes in foredune morphology. The total water level impact hours per year metric explained as much as 66% and 67% of the variance in dune crest and toe elevations, respectively, across the nearly two decades of dune evolution. The greatest number of dune building hours per year and largest dunes within the study site co-occurred at locations exposed to the dominant cross-shore wind direction as a result of varying shoreline orientation. The CDDF ratio was positively correlated to changes in the dune toe elevation in approximately 70% of dunes within the study site, outperforming the impact and dune building hours per year metrics. Our results show that these three metrics can provide first-order estimates of dune morphometric change across multiple spatial and temporal scales, which may be particularly useful at sites where lidar acquisition is intermittent.  相似文献   

14.
There is little understanding of the flow-field surrounding semi-vegetated linear dunes, and predictions of dune mobility are hampered by a lack of empirical data concerning windflow. In an attempt to characterize the near-surface airflow upwind of and over partially vegetated linear dunes in the southwest Kalahari Desert, this study presents measurements of vertical and horizontal wind velocity profiles across cross-sectional transects of seven partially vegetated linear dunes. Vegetation surveys combined with velocity measurements from vertical arrays of cup-anemometers, placed up to 2·3 m above the ground surface, were used to gain information concerning the modification of airflow structure caused by the intrusion of the dunes into the atmospheric boundary layer and to predict the variability of aerodynamic roughness (z0) from interdune to crest. The results suggest an acceleration of flow up the windward slopes of the dunes and, as such, the data correspond to classical theory concerning flow over low hills (essentially Jackson and Hunt (1975) principles). Where the theory is incapable of explaining the airflow structure and acceleration characteristics, this is explained, in part, by the presence of a spatially variable vegetation cover over the dunes. The vegetation is important both in terms of the varying aerodynamic roughness (z0) and problems concerning the definition of a zero-plane displacement (d). It is considered that any attempts to characterize surface shear stress over the Kalahari linear dunes, in order to predict sand transport and dune mobility, will be hampered by two problems. These are the progressively non-log-linear nature of the velocity profiles over the dunes caused by flow acceleration, and the production of thin near-surface boundary layers caused by areally variable aerodynamic roughness as a result of the partially vegetated nature of the dunes.  相似文献   

15.
Sand dunes as potential sources of dust in northern China   总被引：1，自引：0，他引：1  
While saltation bombardment of sand grains on a fine substrate can produce considerable dust, the well-sorted nature of sand dunes tends to preclude them from consideration as major dust sources. Recent research, however, has revealed that sand dunes can, in some cases, be large sources of dust. We used the PI-SWERL(Portable In-Situ Wind Erosion Laboratory) to measure in the field the potential of sand dunes and other desert landforms to emit particulate matter 10 μm(PM-10) dust in the Tengger, Ulan Buh, and Mu Us deserts of northern China. Combined with high resolution particle size measurements of the dune sand, an assessment of sand dunes as a dust source can be made. Large active transverse dunes tend to contain little to no stored PM-10, yet they produce a low dust flux. Coppice dunes stabilized by vegetation contain appreciable PM-10 and have very high dust emission potential. There is a positive correlation between the amount of PM-10 stored in a dune and its potential dust flux. Saltation liberates loose fines stored in dunes, making them very efficient dust emitters compared to landforms such as dry lake beds and washes where dust particles are unavailable for aeolian transport due to protective crusts or sediment cohesion. In cases where large dunes do not store PM-10 yet emit dust when active, two hypotheses can be considered:(1) iron-oxide grain coatings are removed during saltation, creating dust, and(2) sand grains collide during saltation, abrading grains to create dust. Observations reveal that iron oxide coatings are present on some dune sands. PI-SWERL data suggests that low dust fluxes from dunes containing no stored dust may represent an estimate for the amount of PM-10 dust produced by removal of iron oxide coatings. These results are similar to results from dunes in the United States. In addition, PI-SWERL results suggest that dust-bearing coppice dunes, which cover vast areas of China's sandy deserts, may become major sources of dust in the future if overgrazing, depletion of groundwater, or drought destabilizes the vegetation that now partially covers these dunes.  相似文献   

16.
Within the greater Ar Rub' al Khali (Empty Quarter) sand sea lies an internal depocentre, the Al Liwa Basin, which comprises a variety of mega‐scale dune types. Crescentic dunes dominant the north of the basin while megadunes of stellate or star form are a major landform of the south‐eastern reaches. Their development into dune fields is determined by the style and rate of dune–dune interactions, the boundary conditions imposed by a multi‐modal wind regime, fluctuating groundwater levels, and sediment availability under an assortment of climatic conditions throughout the Quaternary. As a result, dune field patterns are a collective response to these perturbations in space, time and environment. The R‐statistic is a collective measure of these responses, and is a metric capable of identifying the degree of pattern maturity or self‐organization of the aeolian system, and the pathways from which patterns evolve. The spatial signature of the southerly located star dunes is characterized by two definitive patterns of organization: the first, one of complete spatial randomness, the second, a low degree of spatial uniformity. In isolation, these results appear to be unrelated to those for crescentic dunes of the region in which a significantly higher degree of pattern dispersion is the norm. However, when spatial statistical measures are integrated with the theoretical understanding of dune–dune interactions and the involvement of environmental agents, the complex morphodynamic pathways and linkages between regional dune fields is better understood. In this case, both constructive (e.g. merging, lateral linking) and regenerative activity (e.g. calving) have played important roles in the development of dune size, and associated adjustments in spacing, and dune numbers, and subsequently dune field patterns. Synergetic patterns are emblematic of this vast dunescape, whereby transitional geographic, morphologic, dimensional and environmental modifications exist between the mega‐crescentic and mega‐stellate dunes of the Empty Quarter. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

17.
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   

18.
The Pacific margin of Canada has been subjected to tectonism, dramatic sea level change and vigorous storm and tidal energy since glacial times resulting in a complex seafloor. Extensive multibeam mapping of this shelf has provided an opportunity to understand how these processes have impacted sedimentology and morphology. Bathymetric restriction of the tidally dominated flow between the inland seas and the open Pacific has resulted in the development of very large subaqueous dune fields and terrace moats. For example, in the southern Strait of Georgia nearly symmetrical dunes with wavelengths between 100 and 300 m, dune heights up to 28 m, cover the seafloor in 170–210 m water depth. In northern Hecate Strait a 72 km2 area of large 2D dunes occurs at the transition with Dixon Entrance which opens to the Pacific Ocean and steep (>10°) wave-cut terraces and drowned spits, a result of sea level changes during the Holocene, are now being undercut to generate moats 7 m deep, in a narrowing shelf trough. Currents, with velocities ranging between 0.2 and 2.2 m s?1, are dominated by semi-diurnal tidal streams that are continually modified by wind and estuarine circulation. There appears to be a clear association of grain size, water depth and flow velocity controlling the size of the subaqueous dunes.  相似文献   

19.
Changes in vegetation cover within dune fields can play a major role in how dune fields evolve. To better understand the linkage between dune field evolution and interdune vegetation changes, we modified Werner's (Geology, 23, 1995: 1107–1110) dune field evolution model to account for the stabilizing effects of vegetation. Model results indicate that changes in the density of interdune vegetation strongly influence subsequent trends in the height and area of eolian dunes. We applied the model to interpreting the recent evolution of Jockey's Ridge, North Carolina, where repeat LiDAR surveys and historical aerial photographs and maps provide an unusually detailed record of recent dune field evolution. In the absence of interdune vegetation, the model predicts that dunes at Jockey's Ridge evolve towards taller, more closely‐spaced, barchanoid dunes, with smaller dunes generally migrating faster than larger dunes. Conversely, the establishment of interdune vegetation causes dunes to evolve towards shorter, more widely‐spaced, parabolic forms. These results provide a basis for understanding the increase in dune height at Jockey's Ridge during the early part of the twentieth century, when interdune vegetation was sparse, followed by the decrease in dune height and establishment of parabolic forms from 1953‐present when interdune vegetation density increased. These results provide a conceptual model that may be applicable at other sites with increasing interdune vegetation cover, and they illustrate the power of using numerical modeling to model decadal variations in eolian dune field evolution. We also describe model results designed to test the relative efficacy of alternative strategies for mitigating dune migration and deflation. Installing sand‐trapping fences and/or promoting vegetation growth on the stoss sides of dunes are found to be the most effective strategies for limiting dune advance, but these strategies must be weighed against the desire of many park visitors to maintain the natural state of the dunes. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

20.
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   

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