Interpretation of the complex dune morphology on Mars: dune activity,modelling and a terrestrial analogue

In this work we analyze a dark erg on Mars that could be considered a mega‐dune (draa) where secondary dunes of different morphology are superposed over a main crescent‐shaped bedform (primary dune). The presence of a complex, multi‐directional wind regime is indicated as one of the main causes for the accumulation of a tall draa, presenting an analogy to the Great Sand Dunes in Colorado. In both cases, main regional winds from the SW blow in opposition to winds from the NE which are enhanced by the topography. Such a complex wind regime leads to the development of star and reversing dunes and is accurately predicted by atmospheric models on a regional and local scale. Signs of activity in the form of grainflow scars are also noted over the slip faces of many dunes, suggesting that easterly winds are actively shaping the study draa in the present‐day climatic setting. The presence of this draa on Mars suggests a complex interaction between regional and local topographically controlled flows and a consistent availability of sand. The future study of an analogue terrestrial site such as the Great Sand Dunes could be fundamental for understanding the evolution of similar Martian dune fields. Copyright © 2012 John Wiley & Sons, Ltd.     

On the shape and migration speed of a proto‐dune

The shape and migration speed of a proto‐dune are mathematically discussed. The migration speed of a low dune is shown to be inversely proportional to its wind‐directional length. Proto‐dunes, whose wind‐directional lengths are about 10 m, are expected to migrate at finite speeds. Copyright © 2002 John Wiley & Sons, Ltd.     

A twenty‐one‐year record of surface change on a Namib linear dune

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.     

Residual dune ridges: Sedimentary architecture,genesis, and implications for palaeo-climate reconstructions

Sedimentary architecture and genesis of residual dune ridges in a temperate climate are presented and implications for their use as archive of changes in long-term precipitation and wind climate are discussed. Residual dunes are common features of wet aeolian systems, where they form sets of shallow ridges, oriented perpendicular to the prevailing wind direction. Residual dune ridges of the study area are vegetated and typically elevate 0.6 to 2.5 m above the surrounding interdune flats. They develop on the lower stoss side of active transgressive dunes, triggered by periods of elevated groundwater table and hence colonization of the foot of the dune by rapid growing pioneer vegetation. Stabilized by plants, the growing ridge detaches from the active transgressive dune and gets abandoned within years in the course of the downwind-migration of the transgressive dune. Grain-size data suggest a main sediment supply from the transgressive dune and only minor input from other sources. Ground-penetrating radar reveals that the residual dune ridges are composed of windward-dipping as well as leeward-dipping sedimentary beds. Leeward-dipping strata reflect sediment supply from the parental dune, whereas windward-dipping beds are seen to result from sediment redistribution along the ridge and sediment supply from the adjacent swales during the ridge growth period. Multi-annual to multi-decadal variability in precipitation leads to the development of sequences composed of tens of ridges, spanning time periods of several centuries. Spacing of individual ridges in these sequences is controlled not by long-term variability in precipitation alone, but probably also reflects variable wind intensity which affects the migration rate of the parental dune. The important role of vegetation in ridge construction makes these landforms a demonstrative example of landscape development by geo-biosphere interacting processes.     

Bridging the gap between field data and global models: current strategies in aeolian research

Modern global models of earth‐atmosphere‐ocean processes are becoming increasingly sophisticated but still require validation against empirical data and observations. This commentary reports on international initiatives amongst aeolian researchers that seek to combine field‐based data sets and geomorphological frameworks for improving the quality of data available to constrain and validate global models. These include a second iteration of the Dust Indicators and Records from Terrestrial Marine Palaeoenvironments (DIRTMAP2) database, the Digital Atlas of Sand Seas and Dunefields of the World and a new geomorphology‐based land surface map produced by the QUEST (Quantifying Uncertainties in the Earth System) Working Group on Dust. Copyright © 2010 John Wiley & Sons, Ltd.     

Growth by extension,and reworking,of a south‐western Kalahari linear dune

Optically stimulated luminescence (OSL) dating studies of linear (longitudinal) dunes have been used extensively to elucidate late Quaternary environments and climates in arid or formerly arid regions, yet understanding of the development of such dunes is incomplete. In particular, conflicting opinions have been presented regarding the propensity of linear dunes to migrate laterally, the degree to which they rework their own sediment during accumulation and whether they form primarily by extension, as opposed to lateral sand movement from adjacent interdunes. This study focuses on this last point, although the importance of the other controversies is discussed in context. A simple linear dune in the south‐western Kalahari, which has a prominent termination on a pan (playa) surface, provides an opportunity to directly test hypotheses of dune extension. Chronostratigraphy along a ~600 m transect along the crest of the dune, constrained by 42 OSL ages, reveals that the dune grew by extension on occasions in the late Pleistocene and early Holocene, but has also been subject to reworking along its length, which has continued until recent times. Dune development by extensional growth is suggested to operate under environmental conditions more conducive to net accumulation, whereas reworking is largely independent of conditions throughout the last ~18 ka, and may represent seasonal fluctuations in the position of the dune crest. The relative significance of these two modes of development is suggested to be a key control on the efficacy of linear dunes as archives of environmental and climatic change. Copyright © 2011 John Wiley & Sons, Ltd.     

Early Holocene dune field development in Dalarna,central Sweden: A geomorphological and geophysical case study

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.     

基于卫星遥感资料的海洋表层流场反演与估算

为解决实际海流资料探测困难,难以大范围有效获取的问题,利用2000～2008年卫星高度计资料和QuikSCAT卫星遥感风场资料,开展了全球海表地转流和Ekman流的反演估算;针对不同纬度地区(赤道和赤道以外地区)地转效应和海洋运动差异,引入不同的运动学模型和反演估算方案,反演得到0.5°×0.5°的逐周全球表层流产品.对比分析表明,卫星资料反演估算的海洋表层流场能较为客观、准确地表现实际海表流场的基本特征,进而为海洋热量和海洋物质输送等研究提供了可用的海流数据信息.     

Suspended sediment transport and deposition over a dune: Río Paraná, Argentina

Field data from the Rio Paraná, Argentina, are used to examine patterns of suspended sediment transport over a sand dune. Measurements of three‐dimensional velocity are made with an acoustic Doppler current profiler whilst suspended sediment concentration and particle size have been quantified using a laser in situ sediment scattering transmissometer. Suspended sediment concentration and streamwise and vertical sediment flux are highest close to the bed, with an upward vertical flux over the stoss side of the dune and downward flux over the lee side. Suspended sediment concentrations are higher over the crest compared with the trough and suspended sediment is coarsest near the bed. About 17% of the suspended‐load transported over the crest is deposited in the lee side before it reaches the trough. Most of this deposited sand is coarser sediment that originates close to the bed over the crest, a result consistent with simulations based on the model of Mohrig and Smith (Water Resources Research 1996; 32: 3207–3217) for the excursion lengths of sediment dispersed in the lee side of a dune. Copyright © 2009 John Wiley & Sons, Ltd.     

Estimation of surface roughness (z0) over a stabilizing coastal dune field based on vegetation and topography

Accurate knowledge of the surface roughness and the resultant wind speed are important for many applications, such as climatic models, wind power meteorology, agriculture and erosion hazards, especially on sand dunes in arid and semi‐arid environments, where vegetation cover is scarce. In this study we aimed at quantifying the effects of vegetation cover and topography on surface roughness over a stabilizing dune field on the southern coast of Israel. Forty‐six wind measurements were made at various distances from the coastline, ranging from 10 to 2800 m, and z₀ values were calculated from the wind measurements based on the ratio between the wind gust and the average wind speed. We estimated vegetation cover using the soil adjusted vegetation index (SAVI) from Landsat satellite images for the upwind sector at various lengths, ranging from 15 to 400 m, and based on digital elevation models and differential GPS field measurements we calculated the topographic variable of the relative heights of the stations. z₀ values were positively correlated with the winter SAVI values (r = 0·87 at an upwind length of 200 m) and negatively correlated with the relative height (r = ?0·68 at an upwind length of 200–400 m for the inland dune stations). Using these variables we were able to create a map of estimated z₀ values having an accuracy of over 64%. Such maps provide a better understanding of the spatial variability in both wind speed and sand movement over coastal dune areas. Copyright © 2007 John Wiley & Sons, Ltd.     

Mars Global Digital Dune Database (MGD3): north polar region (MC‐1) distribution,applications, and volume estimates

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.     

Geomorphic activity of rabbits on a coastal sand dune,De Blink dunes,the Netherlands

A study of the erosion rate and the stability of sandy slopes was conducted on an eastern arm of a parabolic coastal sand dune, De Blink, central Netherlands. The contribution of rabbits to these processes was found to depend on two types of activity; the building of caves and sand mounds of up to 1·5 m² in area; and the digging of shallow burrows, whereby amounts of sand up to 1 kg per burrow were excavated. The burrowing activity was found over the whole dune, while cave holes were dug mainly on the northern slope. The total amount of sand actually transported on the dune due to this activity is not clear yet, but their influence on the development of stepped slopes is well established.     

The relative role of constructive and destructive processes in dune evolution on Cape Lookout National Seashore,North Carolina,USA

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.     

Embryo dune development on a large,actively accreting macrotidal beach: Calais,North Sea coast of France

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.     

Settling-velocity distributions and sorting processes on a longitudinal dune: A case study

The processes acting on a longitudinal dune are inferred from the response in the area size-sorting characteristics of the sediment of the active layer. The sediment size distributions are determined by settling and are best described by a log-hyperbolic model which provides the most information on size-sorting processes. Size-sorting characteristics are markedly different across the low round-crested part of the dune from cross-sections of the high more sharp-crested part. This results in changes in textural parameters along the lee-side consistent with changes in the lee-side separation vortex. This is the first time that the effect of wind speed up on the lee-side is observed to be reflected in the sediment; it can only be detected with the very sensitive log-hyperbolic parameters.     

Influence of topography on wind speed over a coastal dune and blowout system at Jockey's Ridge,NC, USA

This study examines the spatial distribution of wind speed across a coastal dune system located at Jockey's Ridge State Park, North Carolina. The study area consists of a trough blowout through a foredune ridge, and the landforms that have developed behind the foredune. Wind speed and direction were measured simultaneously with single sensors placed at a fixed height in 13 locations across the blowout/dune complex. Fractional wind speed‐up is computed for sampling stations using data from a mast located on the beach as the reference. Results show that wind speeds were generally accelerated across the study site. The highest speeds were recorded on the foredune ridges adjacent to the blowout. Wind was accelerated through the center of the blowout throat and along the downwind lateral wall. Further into the blowout, at the base of the ramp to the depositional lobe, higher wind speeds shifted to the upwind lateral wall and continued to accelerate up the ramp as air exited to the rear. Significant variations in the wind speed‐up pattern were associated with different wind approach angles, with greater speed‐up occurring when the winds were aligned normal to the dune system. The speed‐up decreased as the angle of approach became increasingly oblique to the ridge. The patterns of wind speed‐up across the site point to the influence of topography on airflow. To quantify the relationship, measures of several topographic variables were obtained along sample transects running upwind from each sample station along flow lines representing different wind approach angles. Examination of correlation coefficients between wind speed‐up and topographic variables suggests that for groups of stations with similar topographic characteristics, 30–50% of the variations in speed‐up may be explained by the upwind topographic variability. Copyright © 2014 John Wiley & Sons, Ltd.     

The influence of topography and approach angles on local deflections of airflow within a coastal blowout

The spatial variability of air flow through complex topography is an important, but not fully understood, component of dune development and dynamics. This study examines the spatial variability of the wind field in a linear blowout in coastal dunes at Jockey's Ridge State Park, on the Outer Banks of North Carolina. A spatial array of single‐height anemometers and wind vanes were placed within the blowout. Topography exerted a significant steering effect when onshore winds approached from directions within 50° of the blowout axis. Under those conditions wind flow in the blowout aligned to the axis regardless of approach angle, maximizing the potential for erosion and transport in the trough. In other locations aspect variations caused deflection both proportional and disproportional to changes in the approaching wind. When prevailing winds approached from directions more oblique than 50° to the blowout axis, topographic steering through the blowout trough was reduced and secondary flow generated by flow separation over the trough became more prominent. During those approach angles, wind directions and speeds within the upper blowout trough became erratic as vortices and turbulence dominated the flow, minimizing transport potential. The changing characteristics of airflow in the blowout relative to differing approach angles has implications on dune development and variations in transport potential under changing conditions. Copyright © 2013 John Wiley & Sons, Ltd.     

A continuous model for sand dunes: Review,new developments and application to barchan dunes and barchan dune fields

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.     

Monitoring considerations for a dynamic dune restoration project: Pacific Rim National Park Reserve,British Columbia,Canada.

Historically, management of coastal dune systems has often involved artificial stabilization of active sand surfaces in order for coastal areas to be more easily controlled and modified for human benefit. In North America, the introduction of invasive grasses, namely European and American beach (marram) grasses (Ammophila spp.) has been one of the most successful strategies used for stabilizing active coastal dune sands. Recent research has demonstrated, however, that stabilization of coastal dunes often leads to reduced landform complexity and resilience, as well as declines in species diversity. More ‘dynamic’ restoration efforts have emerged over the past 20 years that encourage dune mobility and aeolian activity in order to provide a more resilient biogeomorphic system. In North America, there is generally little research relating restoration methods and outcomes to geomorphic responses despite the fundamental importance of sedimentary processes and dune morphodynamics in broader ecosystem function. This paper aims to better situate dynamic dune restoration within current geomorphic understanding. A brief review of key terms and concepts used in the emerging field of dynamic dune restoration is provided and expanded upon with respect to geomorphologic considerations. A case study of a recent dynamic restoration effort in Pacific Rim National Park Reserve, British Columbia, Canada is provide to demonstrate how these concepts are applied. Introduction of European marram at this site, coupled with a warming climate and increased precipitation in recent decades at this site, is thought to be associated with a rapid decline in aeolian activity, system stabilization and accelerated ecological succession. Preliminary results on the response of the dune system to mechanical removal of Ammophila are presented to provide the foundation for a research framework to guide the broader restoration project. Recommendations for improving treatment methodologies and monitoring protocols are provided to aid future restoration projects of this nature. Copyright © 2013 John Wiley & Sons, Ltd.     

Regional‐scale sedimentation process models from airborne gamma ray remote sensing and digital elevation data

Airborne gamma ray survey data were used to provide information on potassium, thorium and uranium concentrations in surface soil and rock in arid central Australia. Spatial patterns in these radioelements allow tracing of paths of sediment at catchment scale. Survey elevation data are combined with contour data to produce digital elevation models for terrain analysis, tracing of sediment flow paths and modelling of extreme floods. Gamma ray data show consistent variation with slope, a limited range of drainage areas, and erosion/deposition models derived from the conservation of mass equation. Supply‐limited sediment transport models give a reasonable reproduction of observed radioelement distribution but some elements of the distribution pattern reflect the area inundated by 500–1000 year floods rather than the effects of simple downslope movement. Partial area sediment supply models are derived by downstream accumulation of erosion and deposition rates calculated using the conservation of mass equation with transport laws based on slope alone and stream power. Comparison with observed radioelement patterns suggests that both transport laws apply in different parts of the landscape. Regional‐scale sediment transport models will require a range of models depending on location in the landscape and event frequency. This approach may allow estimation of sediment delivery ratios. Copyright © 2001 John Wiley & Sons, Ltd.