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1.
The purpose of this study was to quantify relationships between season, sediment availability, sediment transport pathways, and beach/foredune morphology at Greenwich Dunes, PEI. This was done for periods ranging from a few days to multiple decades using erosion pins, bedframe measurements, annual surveys, and digital photogrammetry using historical aerial photographs. The relative significance of seasonal/annual processes versus response of the foredune system to broader geomorphic controls (e.g. relative sea level rise, storms, etc.) was also assessed. The data show that there are clear seasonal differences in the patterns of sand supply from the beach to the foredune at Greenwich and that there are differences in sediment supply to the foredune between the east and west reaches of the study area, resulting in ongoing differences in foredune morphology. They also demonstrate that models that incorporate wind climate alone, or even models that include other factors like beach moisture, would not be able to predict the amount of sediment movement from the beach to the foredune in this environment unless there were some way to parameterize system morphology, especially the presence or absence of a dune ramp. Finally, the data suggest that the foredune can migrate landward while maintaining its form via transfers of sediment from the stoss slope, over the crest, and onto the lee slope. Although the rate of foredune development or recovery after disturbance changes over time due to morphological feedback, the overall decadal evolution of the foredune system at Greenwich is consistent with, and supports, the Davidson‐Arnott (2005) conceptual model of dune transgression under rising sea level. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

2.
Sediment transport and short‐term morphologic change were evaluated at a site where sand fences are deployed and the beach is raked (Managed Site) and a site where these human adjustments are not practiced (Unmanaged Site). Data were gathered across the seaward portion of a low foredune when winds blew nearly shore‐normal at mean speeds 8.9 to 9.3 m s‐1. Data from traps revealed sediment transport rates at unvegetated portions of the foredune crest (40.2 to 43.5 kg m‐1 h‐1) were greater than on the backshore (4.9 to 11.2 kg m‐1 h‐1) due to onshore decreases in surface moisture and speed‐up of the wind passing over the foredune. Data from erosion pins indicate sediment input to the dune was 1.48 m3 m‐1 alongshore at the Managed Site and 1.25 m3 m‐1 at the Unmanaged Site. The Unmanaged Site had deposition at the dune toe, erosion at mid‐slope, and deposition at the crest. Deposition occurred at mid‐slope on the Managed Site near a partially buried (0.58 m high) fence with a porosity of about 65%. Deposition at partially buried wrack on the upper backshore and dune toe at the Unmanaged Site was about twice as great as deposition in this zone at the Managed Site. Results indicate that: (1) the seaward slope of the foredune can be a more important source of sand to the lee of the crest than the beach; (2) wrack near the toe can decrease transport into the foredune; (3) a scour zone can occur on the foredune slope above the wrack line; (4) a fence placed in this location can promote deposition and offset scour, but fences can restrict delivery of sediment farther inland. Evaluation of alternative configurations of fences and strategies for managing wrack is required to better determine the ways that humans modify foredunes. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

3.
This paper examines patterns of foredune vegetation along an embayment in southern Brazil and the relationships between variations in percent cover and diversity, and environmental factors such as beach/surfzone type and exposure to wind and wave energy. The study was conducted on Moçambique Beach, Brazil, which displays significant alongshore variations in exposure to the prevailing winds and waves, beach/surfzone morphodynamic type, type and dimensions of the dune systems and foredune vegetation cover and diversity. Two vegetation surveys were carried out in contiguous 1 m2 quadrats across the foredune. The presence/absence, percent cover of the species, diversity and similarity between the vegetation of the profiles surveyed and their relation with environmental conditions are examined. The results show that the vegetation cover decreases from south to north, possibly reflecting the increasing exposure to wind and wave energy. Distinct patterns of species distributions occur along Moçambique beach, such that different plant species are dominant on the southern, middle and northern ends of the beach. A cluster analysis demonstrated two associations: the first one is represented by the profiles located in the lower energy zone of the beach, and the second association is represented by the profiles more exposed to wind and wave energy, sediment deposition and salt spray. The diversity of species decreases northwards, possibly influenced by the surfzone type, number of breaking waves and degree of aeolian transport. The presence/absence of the species and the vegetation cover on the foredune reflect the varying levels of exposure of the beach and foredune to the winds and waves and also reflect the volume of sediment deposition on the foredune and the beach mobility determined by the morphodynamic beach/surfzone type. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

4.
Near‐surface airflow over a morphologically simple, vegetated, 8 m high foredune with a small wave‐cut scarp was measured for onshore to oblique‐onshore conditions during a low‐moderate (5–6 m s‐1 ) wind event and a high velocity (11–18 m s‐1) sand‐transporting gale event. Flow across the foredune was characterized by significant flow compression and acceleration up and across the foredune during both events. During the gale, a pronounced jet (speed bulge) developed at the foredune crest, which increased in magnitude with increasing wind speed. The vertical (W) velocity component of the 3D flow field was positive (upwards) across the stoss slope under low wind conditions but negative (downwards) during gale wind conditions, with upslope acceleration. During the low velocity event, there was speed‐down within the vegetation canopy, as would be expected for a porous roughness cover. During the strong wind event there was speed‐up in the lower portion of the vegetation canopy, and this was found up the entire stoss slope. Sediment transport during the gale force event was substantial across the beach and foredune despite the moderate vegetation cover and minimum fetch. Aeolian suspension was evident in the lee of the dune crest. The observations presented herein show that strong storm winds are an effective mechanism for translating sediment landwards across a high vegetated foredune, contributing sediment to the stoss slope, crest and leeward slopes of the foredune and backing dunes. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

5.
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   

6.
At Pedro Beach on the southeastern coast of Australia a series of foredune ridges provides an opportunity to explore the morphodynamic paradigm as it applies to coastal barrier systems using optically stimulated luminescence (OSL) dating, ground penetrating radar (GPR) and airborne LiDAR topography. A series of sandy dune-capped ridges, increasing in height seawards, formed from c. 7000 years ago to c. 3900 years ago. During this time the shoreline straightened as the embayment filled and accommodation space for Holocene sediments diminished. Calculation of Holocene sediment accumulation above mean sea level utilising airborne LiDAR topography shows a decline in average sediment supply over this time period coupled with a decrease in shoreline progradation rate from 1.2 m/yr to 0.38 m/yr. The average ridge ‘exposure lifetime’ during this period increases resulting in higher ridges as dune-forming processes have longer to operate. Increasing exposure to wave and wind energy also appears to have resulted in higher ridges as the sheltering effect of marginal headlands was diminished. An inherited disequilibrium shoreface profile will drive onshore accumulation of sandy sediments forming a prograded barrier; however, if there is no longer ‘accommodation space’ for sediment, this will be an overriding factor causing the cessation of progradation, as occurred c. 3900 years ago at Pedro Beach. Excess sediment in the nearshore zone after 3900 years ago may have been moved northward to nourish downdrift beaches in the compartment. A high outer foredune has formed through vertical accretion after 500 years ago, evidenced by GPR subsurface structures and OSL ages, with a distinct period of vertical and lee slope accretion and dated to the period 1890–1930 AD. The increased dune sediment transport resulting in foredune building is attributed to recent human disturbance. © 2018 John Wiley & Sons, Ltd.  相似文献   

7.
Vegetation surveys were conducted on a variety of coastal foredunes in a largely natural region along the Gulf County region of the Florida panhandle. Species presence, absence and percentage cover were surveyed on 12 foredune profiles during different seasons. The vegetation data were analyzed using the Shannon–Wiener Diversity Index and Sørensen Index. Uniola sp. and Andropgon sp. were the dominant species on foredunes. Uniola sp. was found predominantly on the gulfward facing or stoss slopes, and Andropgon sp. was found to be dominant on the inland or lee slopes of foredunes. While they are present on all foredunes, their presence and percentage cover are dominant on rapidly prograding coasts. Prograding/accretional beaches had higher Sørensen Index values (i.e. higher similarities) than did the foredune‐vegetation profiles on eroding beaches. Diversity as indicated by the Shannon–Wiener analysis (H’) is greatest on the highest, and generally eroding dunes. Foredune diversity increased with foredune height, and the tallest foredunes were found on shorelines with relatively low erosion rates, where dunes were slowly translating landwards, cannibalizing older dunes, and moving into areas colonized by late successional species, such as Quercus sp. These observations of foredune species richness, diversity, profile similarities, and the use of ecological indices can provide excellent proxy evidence of shoreline dynamics, and in particular the degree of beach erosion and accretion, in the absence of historical erosion/accretion data. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

8.
Wind flow and sand transport intensity were measured on the seaward slope of a vegetated foredune during a 16 h storm using an array of sonic anemometers and Wenglor laser particle counters. The foredune had a compound seaward slope with a wave‐cut scarp about 0.5 m high separating the upper vegetated portion from the lower dune ramp, which was bare of vegetation. Wind direction veered from obliquely offshore at the start of the event to obliquely onshore during the storm peak and finally to directly onshore during the final 2 h as wind speed dropped to below threshold. Sand transport was initially inhibited by a brief period of rain at the start of the event but as the surface dried and wind speed increased sand transport was initiated over the entire seaward slope. Transport intensity was quite variable both temporally and spatially on the upper slope as a result of fluctuating wind speed and direction, but overall magnitudes were similar over the whole length. Ten‐minute average transport intensity correlates strongly with mean wind speed measured at the dune crest, and there is also strong correlation between instantaneous wind speed and transport intensity measured at the same locations when the data are smoothed with a 10 s running mean. Transport on the beach for onshore winds is decoupled from that on the seaward slope above the small scarp when the wind angle is highly oblique, but for wind angles <45° from shore perpendicular some sand is transported onto the lower slope. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

9.
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10.
The Oceano Dunes near Pismo Beach, California is part of a large transgressive dune system that extends up to 5 km inland and hosts a state park that has been managed for off-highway vehicle recreation since 1982, although vehicle activity has existed in the dunes for almost 90 years. As a result, foredunes have been largely obliterated and sand surfaces in vehicle use areas are highly emissive of dust-sized particles, causing frequent exceedances of state air quality standards. To reduce dust emissions from the dunes, a nature-based foredune restoration strategy using five different treatments over a 20-ha site was implemented in February 2020. The research hypothesis is that treatments will differ in their ability to promote deposition and dune development and that more intensive planting-based treatments will outperform simpler treatments. We test this using biannual high-resolution uncrewed aerial system (UAS) surveys to quantify sediment budgets, sand exchanges between beach, foredune, and backdune components, changes in plant cover, and related dune development over a 2-year period (October 2019–2021). After two full wind and plant-growth seasons, results show that all treatments are maintaining a positive sediment budget (net accumulation), most are developing sizable nebkha dunes (an important stage in foredune development in this region), and some are increasing plant cover and species richness. There is no clear winner, yet two treatments (broadcast native plant and sterile grass seeds, and a high-density straw planting node with native seedlings) are performing well toward developing an incipient foredune. These results will inform an adaptive management process that could entail further modifications to enhance foredune development. Based on this experience, and with reference to other types of restoration projects, we refine existing criteria used to assess the performance of “dynamic” dune restoration efforts to include settings that involve revegetation (vs. devegetation) as a means for foredune re-establishment.  相似文献   

11.
Erosion of hard‐rock coastal cliffs is understood to be caused by a combination of both marine and sub‐aerial processes. Beach morphology, tidal elevation and significant wave heights, especially under extreme storm conditions, can lead to variability in wave energy flux to the cliff‐toe. Wave and water level measurements in the nearshore under energetic conditions are difficult to obtain and in situ observations are rare. Here we use monthly cliff‐face volume changes detected using terrestrial laser scanning alongside beach morphological changes and modelled nearshore hydrodynamics to examine how exposed cliffs respond to changes in extreme wave conditions and beach morphology. The measurements cover the North Atlantic storms of 2013 to 2014 and consider two exposed stretches of coastline (Porthleven and Godrevy, UK) with contrasting beach morphology fronting the cliffs; a flat dissipative sandy beach at Godrevy and a steep reflective gravel beach at Porthleven. Beach slope and the elevation of the beach–cliff junction were found to influence the frequency of cliff inundation and the power of wave–cliff impacts. Numerical modelling (XBeach‐G) showed that under highly energetic wave conditions, i.e. those that occurred in the North Atlantic during winter 2013–2014, with Hs = 5.5 m (dissipative site) and 8 m (reflective site), the combination of greater wave height and steeper beach at the reflective site led to amplified wave run‐up, subjecting these cliffs to waves over four times as powerful as those impacting the cliffs at the dissipative site (39 kWm‐1 compared with 9 kWm‐1). This study highlighted the sensitivity of cliff erosion to extreme wave conditions, where the majority (over 90% of the annual value) of cliff‐face erosion ensued during the winter. The significance of these short‐term erosion rates in the context of long‐term retreat illustrates the importance of incorporating short‐term beach and wave dynamics into geomorphological studies of coastal cliff change. © 2017 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.  相似文献   

12.
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   

13.
Jet flow over foredunes   总被引：1，自引：0，他引：1       下载免费PDF全文
Jet flows, which are localized flows exhibiting a high speed maxima, are relatively common in nature, and in many devices. They have only been occasionally observed on dunes, and their dynamics are poorly known. This paper examines computational fluid dynamic (CFD) two‐dimensional (2D) modelling of jet flow over a foredune topography. Flow was simulated in 10° increments from onshore (0°) to highly oblique alongshore (70°) incident wind approach angles. CFD modelling reveals that the formation of a jet is not dependent on a critical wind speed, and an increase in incident wind velocity does not affect the magnitude of jet flow. A jet is first formed at ~1.0 m seawards of the foredune crest on the Prince Edward Island foredune morphology example examined here. A jet is not developed when the incident wind is from an oblique approach angle greater than ~50° because there is significantly less flow acceleration across a much lower slope at this incident angle. The presence of a scarp does influence the structure of the crest jet, in that the jet is more pronounced where a scarp is present. Surface roughness affects the magnitude of jet expansion and jets are better developed on bare surfaces compared to vegetated ones. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

14.
Changes in wind speed and sediment transport are evaluated at a gap and adjacent crest of a 2 to 3 m high, 40 m wide foredune built by sand fences and vegetation plantings on a wide, nourished fine sand beach at Ocean City, New Jersey. Anemometer masts, cylindrical sand traps and erosion pins were placed on the beach and dune during two obliquely onshore wind events in February and March 2003. Results reveal that: (1) changes in the alongshore continuity of the beach and dune system can act as boundaries to aeolian transport when winds blow at an angle to the shoreline; (2) oblique winds blowing across poorly vegetated patches in the dune increase the potential for creating an irregular crest elevation; (3) transport rates and deflation rates can be greater within the foredune than on the beach, if the dune surface is poorly vegetated and the beach has not had time to dry following tidal inundation; (4) frozen ground does not prevent surface deflation; and (5) remnant sand fences and fresh storm wrack have great local but temporary effect on transport rates. Temporal and spatial differences due to sand fences and wrack, changes in sediment availability due to time‐dependent differences in surface moisture and frozen ground, combined with complex topography and patchy vegetation make it difficult to specify cause–effect relationships. Effects of individual roughness elements on the beach and dune on wind flow and sediment transport can be quantified at specific locations at the event scale, but extrapolation of each event to longer temporal and spatial scales remains qualitative. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

15.
A remote sensing technique for assessing beach surface moisture was used to provide insight into beach‐surface evolution during an aeolian event. An experiment was carried out on 21 October 2007 at Greenwich Dunes, Prince Edward Island National Park, Canada, during which cameras were mounted on a mast on the foredune crest at a height of about 14 m above the beach. Maps of beach surface moisture were created based on a calibrated relationship between surface brightness from the photographs and surface moisture content measured in situ at points spaced every 2.5 m along a transect using a Delta‐T moisture probe. A time sequence of maps of surface moisture content captured beach surface evolution through the transport event at a spatial and temporal resolution that would be difficult to achieve with other sampling techniques such as impedance probes. Erosion of the foreshore and berm crest resulted in an increase in surface moisture content in these areas as the wetter underlying sediments were exposed. Flow expansion downwind of the berm crest led to sand deposition and a consequent decrease in surface moisture content. Remote sensing systems such as the one presented here allow observations of the combined evolution of beach surface moisture, shoreline position, and fetch distances during short‐term experiments and hence provide a comprehensive rendering of sediment erosion and transport processes. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

16.
This paper addresses a series of geomorphic questions relating to large‐scale (> 1 km), long‐term (100 – 1,000 years) coastal planshape evolution. Previous research on soft‐cliff coasts has recognised the role of protective fronting beach volumes on reducing rates of cliff toe retreat. However, it is the maintenance of this critical threshold that ultimately determines two contrasting modes of shoreline behaviour: Mode A, in which there is little beach sediment and shoreline evolution is controlled by material strength; and, Mode B, when ample beach sediment means that shoreline evolution is controlled by longshore sediment transport. Here we use a numerical model (SCAPE) to investigate temporal and spatial changes in beach volume on a broader range of feedbacks than considered in previous models. The transition between Mode A and Mode B coasts is defined by relative sediment inputs to outputs and used to explore how these contrasting modes control the evolution of an initial linear frontage exhibiting longshore changes in cliff lithology (material resistance and the proportion of beach grade material in the eroded bedrock). Under Mode A, relative changes in material resistance result in long term heterogeneous rates of retreat, which result in the development of persistent headland and embayment features. However, under Mode B, feedbacks between coastal planshape, longshore sediment transport, beach volume and wave energy result in steady state retreat rates regardless of longshore variations in resistance. Results are compared and contrasted to previous simulations and site specific examples and a conceptual model of Mode A and Mode B interactions presented. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

17.
Evidence from a field study on wind flow and sediment transport across a beach–dune system under onshore and offshore conditions (including oblique approach angles) indicates that sediment transport response on the back‐beach and stoss slope of the foredune can be exceedingly complex. The upper‐air flow – measured by a sonic anemometer at the top of a 3·5 m tower located on the dune crest – is similar to regional wind records obtained from a nearby meteorological station, but quite different from the near‐surface flow field measured locally across the beach–dune profile by sonic anemometers positioned 20 cm above the sand surface. Flow–form interaction at macro and micro scales leads to strong modulation of the near‐surface wind vectors, including wind speed reductions (due to surface roughness drag and adverse pressure effects induced by the dune) and wind speed increases (due to flow compression toward the top of the dune) as well as pronounced topographic steering during oblique wind approach angles. A conceptual model is proposed, building on the ideas of Sweet and Kocurek (Sedimentology 37 : 1023–1038, 1990), Walker and Nickling (Earth Surface Processes and Landforms 28 : 111–1124, 2002), and Lynch et al. (Earth Surface Processes and Landforms 33 : 991–1005, 2008, Geomorphology 105 : 139–146, 2010), which shows how near‐surface wind vectors are altered for four regional wind conditions: (a) onshore, detached; (b) onshore‐oblique, attached and deflected; (c) offshore, detached; and (d) offshore‐oblique, attached and deflected. High‐frequency measurements of sediment transport intensity during these different events demonstrate that predictions of sediment flux using standard equations driven by regional wind statistics would by unreliable and misleading. It is recommended that field studies routinely implement experimental designs that treat the near‐surface wind field as comprising true vector quantities (with speed and direction) in order that a more robust linkage between the regional (upper air) wind field and the sediment transport response across the beach–dune profile be established. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

18.
The formation of an inner nearshore bar was observed during a high‐energy event at the sandy beach of Vejers, Denmark. The bar accreted in situ during surf zone conditions and the growth of the bar was associated with the development of a trough landward of the bar. Measurements of hydrodynamics and sediment fluxes were obtained from electromagnetic current meters and optical backscatter sensors. These process measurements showed that a divergence in sediment transport occurred at the location of the developing trough, and observed gradients in cross‐shore net sediment flux were consistent with the morphological development. The main cause for the flux gradients were cross‐shore gradients in offshore‐directed mean current (undertow) speed which depended upon local relative wave height and local bed slope. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

19.
More than 4000 hourly wind profiles measured on three topographically different foredunes are analysed and discussed. Wind flow over the foredunes is studied by means of the relative wind speed: the ratio between wind speed at a certain location and the reference wind speed at the same height. Relative wind speeds appear to be independent of general wind speed but dependent on wind direction. For perpendicular onshore winds the flow over the foredune is accelerated due to topographic changes and decelerated due to changes in surface roughness. Accelerations dominate over decelerations on the seaward slope. The pattern of acceleration and deceleration in relation to wind direction is more or less comparable for different foredunes, but the magnitudes differ. An increase in foredune height from 6 to 10m leads to an increase in speed-up near the top of the seaward slope from 110 to 150 per cent during onshore wind, but further increase of foredune height from 10 to 23m appears to have little effect, due to increased roughness and deflection of flow. Topography also influences the direction of the flow. Between beach and top, the flow deflects in the direction of the normal during onshore winds. During offshore winds the flow is deflected to the parallel. Near the dunefoot, deflection is always in the direction of the parallel, and increases with steeper topography. The maximum deflection near the dunefoot was 90°, over a 23 m high dune, observed during offshore winds. Patterns of erosion and sedimentation resulting from winds from different directions can be explained by the observed accelerations and decelerations. Owing to speed-up on the seaward front of the foredune, sand transport capacity of the wind increases, which results in erosion if vegetation is absent. During strong onshore wind, sand is lifted near the dunefoot and moves over the foredune in suspension. During weaker winds, vertical wind velocities do not exceed fall velocities of the sand grains, and most of the sand is deposited near the dunefoot.  相似文献   

20.
The nature of wind flow over a small, 0.6 m high foredune scarp is investigated on the Sir Richard Peninsula, South Australia during a variety of incident wind directions and speeds. The study provides additional supporting evidence that the presence of the scarp and the dune exerts a strong influence on a landwards trending reduction in wind velocity and an increase in turbulence, with the greatest area of turbulence occurring near and at the foredune scarp base. For an incident oblique wind, an alongshore helicoidal flow is formed within a separation region along the scarp basal region. In this region, the coefficient of variation (CV) of wind speed is high and displays significant fluctuations. The flow at the scarp crest is compressed, streamlined and accelerated, turbulence is suppressed, and local jets may occur depending on the incident wind approach angle. Jets are more likely where the incident flow is perpendicular or nearly so. A flow separation region does not develop downwind of the scarp crest where the morphology of the foredune stoss slope downwind of the scarp is more convex (as in this case) rather than relatively flat, and possibly due to the presence of vegetation at the scarp crest. A tentative model of the flow regions developed across a backshore–scarp–foredune region during oblique incident flow is provided. © 2018 John Wiley & Sons, Ltd.  相似文献   

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