- Email: [email protected]
Contemporary research in aeolian geomorphology
Geomorphology 105 (2009) 1–5
Contents lists available at ScienceDirect
Geomorphology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / g e o m o r p h
Review
Contemporary research in aeolian geomorphology B.O. Bauer Earth & Environmental Sciences and Geography, University of British Columbia — Okanagan, Kelowna, BC, Canada V1V 1V7
A R T I C L E
I N F O
Article history: Accepted 2 June 2008 Available online 13 June 2008 Keywords: Aeolian processes and landforms Dust emissions Transport modeling Desert and coastal dunes Aeolian measurement and instrumentation
A B S T R A C T The first International Conference on Aeolian Geomorphology (ICAR) was held in 1986, and every four years since then, aeolian geomorphologists from around the world have assembled to discuss their research and to showcase recent advancements in understanding and modeling of aeolian processes. A content analysis of the “Bibliography of Aeolian Research” [Stout, J.E., Warren, A., Gill, T.E., 2009. Publication trends in aeolian research: An analysis of the Bibliography of Aeolian Research. Geomorphology 105, 6–17 (this volume)] shows that the number of publications on aeolian topics has increased exponentially from the mid-20th Century with approximately 50 publications per year to about 500 publications per year when the first ICAR was held, to almost 1000 publications per year currently. Areas of focus have shifted historically from initial concerns with aeolian erosion and dust events as isolated phenomenon of localized curiosity or only regional importance, to comprehensive physically-based investigations and modeling of the mechanics of aeolian transport. Recently, more applied studies have been motivated by the recognition of the importance of aeolian processes to dust emissions into the atmosphere (with relevance for human health and for meteorological conditions and climate change) and within regional management contexts (especially on coasts where impending sea-level rise is of great concern and in arid and semi-arid environments given the dependence of sediment surface stability and remobilization on meteorological and ecological conditions). Aeolian geomorphology is a rapidly growing sub-discipline of Geomorphology that offers rich opportunities for interdisciplinary collaborations with colleagues from the Atmospheric Sciences, Climatology, Sedimentology, Quaternary Geology, Fluid Mechanics, Physics, Mathematics, Computer Science, Physical Geography, Ecology, and Agricultural Sciences, as well as our counterparts in fluvial, coastal, and arid-lands geomorphology who are similarly concerned with fluid-sediment interactions and the consequent genesis of landforms. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The 6th International Conference on Aeolian Geomorphology (ICAR VI) was held from July 23–28, 2006 at the University of Guelph (Ontario, Canada) with more than 175 participants representing 25 countries. This was the latest in a series of international conferences on aeolian processes and landforms that began with a seminal meeting in Aarhus, Denmark (1986) followed by increasingly wellattended meetings in Sandberg, Denmark (1990), Zzyzx, California (1994), Oxford, England (1998), and Lubbock, Texas (2002). The next meeting (ICAR VII) will be held in Santa Rosa, Argentina from July 5–9, 2010 (http://www.icarvii.com.ar/index.htm). Each of these ICAR meetings has facilitated the dissemination of recent research results through the publication of important conference-related proceedings, monographs, or special issues of journals that provide a unique, contemporary snapshot and synopsis of the state of the science. These volumes also provide insight into the
E-mail address: [email protected]. 0169-555X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2008.02.014
breadth of aeolian research, which often leads to cross-fertilization of ideas within the aeolian community and related fields (e.g., fluid mechanics, atmospheric processes, dynamical modeling, sedimentology, agriculture, dryland ecology, etc.). More importantly, these volumes serve as a theoretical, conceptual, and empirical foundation upon which aeolian researchers are able to situate their own research agendas and advance their efforts toward the creation of new knowledge of relevance to societal concerns. The ICAR series of conferences, therefore, serves as a ‘touch-stone’ against which progress can be gauged and evaluated. This special volume of Geomorphology contains 16 manuscripts based on presentations made at ICAR VI dedicated specifically to geomorphic processes — measurement and assessment, and interaction with landforms and landscapes. A special companion volume of the Journal of Geophysical Research — Earth Surface contains 15 manuscripts derived from the conference on topics honouring the career contributions of Dr. Gillette who maintains research interests in dust emissions, sediment transport mechanics, and shear stress partitioning. Another special volume of Earth Surface Processes and Landforms contains additional manuscripts that round out the many contributions from the conference.
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B.O. Bauer / Geomorphology 105 (2009) 1–5
2. The state of aeolian science The increasing popularity of aeolian geomorphology as a legitimate, thriving, and autonomous branch of Earth science is assessed by Stout et al. (2009-this volume) who estimate that the rate of publication of aeolian-related manuscripts has increased from fewer than three per year in the 17th Century to approximately three per day in the 21st Century. The Bibliography of Aeolian Research, which attempts to list every known scientific manuscript published in the field of aeolian research, currently contains over 29,000 bibliographic citations. This list does not include the huge literatures that are peripheral to, but inform, aeolian research on such issues as boundary layer mechanics, atmospheric circulation and global climate modeling, sedimentology, mineralogy, dating techniques, remote sensing methods, land use/land cover change, etc., except as they pertain specifically to or are used explicitly within aeolian studies. The breadth of aeolian research is reflected in the organization of the sessions at ICAR VI, which included oral and poster presentations under the following categories: Mechanics of Aeolian Processes 1 (Non-erodible elements and drag portioning) Mechanics of Aeolian Processes 2 (Saltation controls and features) Mechanics of Aeolian Processes and Dune Systems Dunes and Dune Systems Modelling Sand Transport Systems Paleo-Aeolian Systems Modeling Sand Transport Systems, Paleo-Aeolian Systems, and Coastal Systems Coastal Aeolian Systems Wind Tunnel Facilities Dust Entrainment, Transport, and Deposition (1 + 2) Modelling Dust Transport Systems Anthropogenic Interactions with Aeolian Systems (1 + 2). The ICAR series of conferences enables contemporary aeolian researchers to assemble once every four years to discuss challenges and problems, to share ideas and innovative solutions, and to explore commonalities within the various sub-disciplines as well as interconnections with a large number of more prominent disciplines (e.g., Geology, Geography, Atmospheric Sciences, Fluid Mechanics, Ecology, Sedimentology). Thirty years ago, a process geomorphologist studying desert dunes would likely have been limited to presenting her research at a geologically-oriented conference targeted exclusively at sedimentary systems and Quaternary environments. Since the advent of the ICAR series, however, such an individual is now able to interact with all manner of colleagues with similar aeolian interests including, for example, coastal geomorphologists who study air-flow and sediment transport pathways over and through coastal dune systems. Similarly, someone interested in understanding and modeling the importance of dust in the atmosphere will find opportunities at ICAR to interact with field-based researchers who are investigating the processes by which dust is emitted from various natural and humanmodified surfaces, how these emission processes are coupled to saltation dynamics or vegetation patterns, and how surface variability leads to complex spatial and temporal patterns of dust emissions into the atmosphere. The recent explosion of interest in aeolian studies, which warranted the creation of a specific conference venue such as ICAR (dedicated specifically to exchange of information and ideas on aeolian phenomena), has facilitated new opportunities for aeolian researchers that are stimulated explicitly by (1) who one comes into contact with, (2) what problems are deemed worthy of study and discussion, and (3) how one practices aeolian geomorphology. In short, aeolian science is no longer studied at the margins of other disciplines as an esoteric nicety, but rather, it has evolved into a
mature science in its own right with legitimate problems, methodologies, theories, paradigms, and organizational structures. 3. Contemporary research themes 3.1. Aeolian transport mechanics A great deal of effort in the aeolian community continues to be directed at understanding how a moving fluid (air, in this case) is able to entrain and transport sediment under the complex conditions found in nature. A large number of models (e.g., Bagnold, Horikawa and Shen, Hsu, Kadib, Kawamura, Lettau and Lettau, White, Zingg) are available to predict the rate of sediment transport (see Sherman and Hotta, 1990) based on attributes of the flow field – typically mean velocity, shear velocity, or some measure of shear stress – as well as on the character of the sediment (e.g., grain size, density). Unfortunately, these models have proven relatively unreliable except under the most ideal conditions, such as those found in wind tunnels or in the middle of vast, uniform sand sheets. This reality has stimulated intensive research into various aspects of the models, such as the assumption that an equilibrium relationship exists that adequately represents the dynamical dependence of sediment flux on a uniform and steady wind field. Aeolian researchers have invested a great deal of effort into investigating the character of small-scale boundary layer flows, especially the shape of near-surface velocity profiles within saltation layers, the nature of turbulence within sediment-laden flows, the scales and sources of flow unsteadiness, the existence and character of semi-coherent structures such as sand streamers and hairpin vortices, and the effect of changing surface features (e.g., bedform roughness, vegetation, topography) on boundary layer dynamics. Carefully planned and executed experiments in wind tunnels have revealed a great deal about the mechanics of aeolian transport, but the nature of the experimental apparatus necessarily constrains the scales of turbulent interactions that can be simulated, and, thus, a need continues to exist for field-based studies that employ sensitive instrumentation and clever experimental designs. Although apparent similarities exist between the mechanics of aeolian and fluvial transport, especially because many of the fluid mechanical laws and principles are thought to be universal, it is increasingly apparent that the dominance of saltation as the primary mode of transport in air leads to complexities that are not found in water. These are rooted in the extreme difference in density between the fluid (air) and the solid (sand), which leads to: (1) ballistic trajectories of grains that have the potential to impact the surface and cause cratering and the ejection of other surface particles (the “splash” process); and (2) mutually efficient exchange of momentum between the fluid and the grains. The splash process has been investigated intensively using high-speed cinematography and theoretical modeling (e.g., McEwan et al., 1992). Although the importance of impact cratering in maintaining the character of saltation is accepted widely, some uncertainty remains regarding the assumptions central to the models, such as whether grain liftoff is constant and proportional to shear velocity (e.g., Namikas, 2006). After a single grain is entrained from the surface, whether by fluid forces or impact cratering, it is accelerated by the fluid and thereby gains momentum from the fluid. In consequence, the air is decelerated and loses momentum to the grain. This is much more noticeable in air than in water because of density differences, and, therefore, this momentum exchange needs to be taken into account when modeling the mechanics of aeolian transport. As the grain returns back down to the surface, it passes through near-surface fluid that moves more slowly than the grain, and momentum is again exchanged — the fluid is accelerated and gains momentum from the grain whereas the grain is slowed down. Only recently has it proven possible to secure highspeed measurements of the velocity profile and grain trajectories (e.g.,
B.O. Bauer / Geomorphology 105 (2009) 1–5
Zhang et al., 2007) but as yet, no unambiguous treatment of momentum exchange in saltation layers has been forthcoming. 3.2. Surface controls, spatial variability, and feedback mechanisms Much of our understanding of the mechanics of aeolian transport is predicated on various idealistic assumptions such as infinitely flat surfaces, uniform grain sizes, limitless supplies of dry sediment, and steady, uniform wind systems. These can be assumed in theoretical models, and to a large extent, can be simulated in wind tunnels. Natural systems are not typically ideal, however, and a host of complications need to be taken into account. At small scale, aeolian researchers have investigated the influence of a range of surface roughness elements (e.g., patches of coarse sediments, individual boulders or shrubs, vegetation canopies, trees, fences, buildings) to determine how near-surface flows respond to these physical impediments and also how transport pathways are altered (e.g., horseshoe vortices around pieces of driftwood or coppice dunes, funneling through foredune blowouts or between buildings). In most arid and semi-arid landscapes, patches of scrub-like vegetation and isolated shrubs are the norm, and considerable attention is directed at understanding how the existence of these fixed (but often flexible), surface roughness elements influence the aeolian transport system. The Raupach model (Raupach et al., 1993) offers one commonly adopted approach, and it is based on a shear-partitioning scheme by which a portion of the total shear stress in the boundary layer is ascribed to the roughness elements themselves. The remaining stress is ascribed to the intervening sediment surfaces and is, therefore, available to drive sediment transport. Much of the recent research effort within this paradigm has been directed at understanding the influence of roughness element size, density, spacing, and porosity (e.g., Gillies et al., 2007; Brown et al., 2008). Even though a great deal is being learned about how the configuration of surface roughness elements can influence boundary layer flows (consistent with what meteorologists have discovered for forests, agricultural fields, and urban environments) and, therefore, shear stress distributions, the manner in which this knowledge can be implemented in models that are intended to aid in the prediction of the potential of sediment transport or dust emission at landscape scales remains elusive. At the landscape scale, the focus begins to move away from the micro-scale mechanics of aeolian processes to more complicated assemblages of natural interactions within aeolian systems as a whole. For example, aeolian researchers have been investigating the influence of topography on the flow field, and hence, the spatial distribution of erosion and deposition within an otherwise uniform atmospheric boundary layer. Sophisticated computational fluid dynamics (CFD) models have been employed (e.g., Parsons et al., 2004) to determine how flow is compressed and expanded depending on the curvature and geometrical scales of the underlying surface (e.g., sloping beaches, isolated dunes). Such topographic modulation has ramifications for flow acceleration and deceleration, and, therefore, for the distribution of shear stress and for pathways of sediment transport. The recent availability of reasonably inexpensive 3-D sonic anemometry has facilitated the collection of detailed data sets in the field (in coastal and desert dune systems), and these are being used to test the reliability of the CFD schemes and to provide insight into how dunes migrate and how dune systems are formed and maintained (e.g., Livingstone et al., 2005). The use of remote sensing technologies, as well as field-based measurements of the spatial distribution of surface moisture, surface temperature, and vegetation density across large sections of beach or desert, is providing new insights into how aeolian transport can be modulated by these factors in ways that cannot be parameterized easily within the non-spatial paradigm implicit to the one-dimensional (equilibrium) transport models. For example, the idea of ‘fetch distance,’ which was originally proposed for agricultural fields (Chepil,
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1957) and extended by Gillette et al. (1996) to arid lands in general, was recently used as a central concept in the modeling framework proposed by Bauer and Davidson-Arnott (2002) for beaches. This model indicates clearly that the foreshore zone of beaches will typically be an eroding aeolian environment with small transport flux despite experiencing large shear stresses that would otherwise suggest a zone of intense sediment transport. Whether maximum transport flux (as predicted by the equilibrium models) is ever achieved higher on the back beach depends on the complex interactions between boundary layer development, grain size distribution, surface moisture content, and wind-approach angle favouring a situation in which the maximum fetch distance on the beach exceeds the critical fetch distance needed to achieve the maximum potential. Such complex feedback mechanisms on the beach are critical to understanding the magnitude and timing of sediment delivery into the foredunes. Moreover, event-based and seasonallybased elements must be considered, along with vegetative controls. Process-based field experiments that deploy sophisticated electronic instrumentation serve as nice complements to more traditional studies of dune systems, which have employed detailed (repetitive) topographic mapping, stratigraphic reconstruction, paleo-environmental and isotopic dating, historical archives, and remote sensing approaches (e.g., LIDAR). Investigations into the origin and evolution of large dune complexes and sand seas – once the domain of the Quaternary geologist – are increasingly of interest to process geomorphologists motivated by the need to understand the impacts of global climate change, changing land use and land cover dynamics, and the inexorable imprint of human activities on the landscape. All these regional and global scale influences will have ramifications for altered vegetation assemblages and, therefore, the stabilization or reactivation of dune systems, sand sheets, and sediment surfaces. An increasingly large range of conceptual models are being proposed to account for these interactions (e.g., Breshears et al., 2009-this volume). 3.3. Dust emission and transport Interest in dust-related process has increased significantly in the past two decades, in part because of recognized adverse health impacts related to very small airborne particles (PM10, PM2.5), but also because dust plays an important role in the overall dynamics of the atmosphere. Hygroscopic nuclei serve as possible nucleation sites for moisture condensation. The presence of dust in significant concentrations can influence the radiative components of the energy balance of the atmosphere. The motivation for studying dust emission and dust transport, therefore, arises from practical concerns outside of aeolian geomorphology, but as it happens, the processes by which dust is emitted from the surface are influenced to large extent by the presence of mixed grain sizes in the transport population, especially large grain sizes experiencing saltation. Thus, the traditional “sand” and “dust” communities have interacted more frequently recently and to great mutual benefit. As in the past, much of the interest derives from the need for erosion control on agricultural fields, especially the effectiveness of certain tillage and planting techniques. Other humandisturbed systems are also being studied with increasing intensity, including logging trails, military bases that are used for maneuvers, off-road vehicle recreation areas, and the shorelines of large water reservoirs that experience seasonal drawdown, to name but a few. These studies indicate that human and animal disturbance to soil surfaces in arid and semi-arid environments is a critical factor because undisturbed surfaces typically have biological or chemical crusts that preclude dust emission except under the most severe meteorological events. Disturbing these naturally occurring crusts and binding elements enhances the susceptibility to dust events. Spatial variability and complexity is the norm, severely affecting the prospects for any kind of universal approach that incorporates standard parameters and generally impeding progress toward the modeling of global dust
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B.O. Bauer / Geomorphology 105 (2009) 1–5
emission. Isolated sources of extreme dust emission (“hot spots”) have been studied using satellite imagery acquired during extreme events, and these are then ground-truthed and assessed for factors that may explain the susceptibility to emission (Lee et al., 2009-this volume). Comprehensive, regional-scale dust models that integrate atmospheric processes and land-surface characteristics (including soil type, vegetation, moisture, etc.) have been developed, but the results are dependent on the accuracy with which localized dust emissions can be predicted. This is an area of considerable importance and research opportunity (see papers in companion volumes in JGR and ESP&L). 3.4. Advanced instrumentation and methods A major factor in the advancement of any science is the degree to which in situ measurements of critical system parameters can be made accurately, rapidly, and with great precision. Aeolian geomorphology has benefited immensely in the past two decades from significant technological advancements. Hot-wire and sonic anemometry have facilitated high frequency (sub 1 Hz) measurements of velocity and turbulence in three dimensions, simultaneously. Irwin sensors have been deployed in wind tunnels and in the field to measure surface shear stress within and around vegetation in an attempt to quantify various shear-partitioning schemes. The methods by which data for transport rate are acquired have advanced from manual integrating traps (that provide information on the scale of minutes to tens of minutes) to mechanisms that incorporate continuously measuring load cells and high precision balances that provide information on the scale of seconds or less. Devices, such as the Saltiphone, Sensit, and Safire, provide data on saltation intensity at frequencies of around 20 Hz (Baas, 2004) whereas a recent microphone-based device (Ellis et al., 2009-this volume) can be used at kiloHz frequencies. Laser-based systems, originally designed for windtunnel applications, are now finding their way into field deployments. These instruments provide fast response measurements, and because of the small size and cheap cost, deploying them in tight configurations to examine spatial trends is possible (e.g., the vertical profile of mass transport). A broad suite of instrumentation from meteorology and agronomy is facilitating the measurement of associated parameters, such as relative humidity, surface temperature (infra red thermometry), and near-surface moisture content (resistance methods). Although field measurements will always be critical to understanding the mechanics of aeolian transport and in providing ground-truth data, the advent of sophisticated but easily accessible remotely sensed data promises major advances in understanding large aeolian systems, the emission of dust, and the importance of surface cover and changing land-use patterns on aeolian transport. Some of these techniques are being used to understand the lunar and Martian surfaces, for which aeolian processes dominate. Similarly, new developments in subsurface stratigraphic rendering (e.g., ground penetrating radar) as well as advances in dating techniques (e.g., optically stimulated luminescence) are yielding phenomenal insights into the historical development of existing dune fields and loess environments as well as the relationship of various depositional or erosional events to known climatological or oceanographic phases. Such information is critical in calibrating and constraining our predictions of the future based on the limited knowledge of process mechanics derived from contemporary windtunnel and field studies. 4. Conclusions Aeolian geomorphology is a thriving sub-discipline of Earth Science that is witnessing rapid growth and increasing relevance. Like many other branches of geomorphology, the imperative of global change (including trends toward climate warming, intensified
meteorological extremes, and enhanced human footprints) provides unprecedented opportunities to apply knowledge to problems of great societal concerns in ways that reduce future uncertainty. A collective scientific response is mandated, and new and innovative ways of measuring, monitoring, and modeling landscapes are essential. Old paradigms predicated on equilibrium relationships, one-dimensional (localized) interactions, and one-way forcings from fluid to sediment are in need of revision because they are simply not serving us well. Acknowledgements Sincerest thanks are extended, on behalf of the aeolian community, to the organizers and many volunteers who contributed to the success of ICAR VI, especially Dr. William Nickling, Dr. Jack Gillies, and Dr. Nick Lancaster. Sponsorship was provided by the Wind Erosion Laboratory (Department of Geography, University of Guelph), the Office of Research at the University of Guelph, the College of Social and Applied Human Sciences (University of Guelph), Rowan Williams Davies & Irwin, Inc. (Guelph, Ontario), the Division of Atmospheric Sciences and Earth & Ecosystem Sciences and the Center for Arid Lands Environmental Management (Desert Research Institute, Nevada), and IGCP 500 (Westerlies and Monsoons: Impacts of Climate Change and Variability on Dryland Environments, Hydrogeology and People). A large number of individuals provided expert review of the manuscripts published in this special volume, including: Alan Arbogast, Bas Arens, Andreas Baas, Grady Blount, Brenda Buck, Mary-Louise Byrne, Robin Davidson-Arnott, Jeff Dech, Patrick DeDeckker, Deanna van Dijk, Jean Ellis, Eugene Farrell, Suskia Faye-Visser, Paul Gares, Dale Gillette, Dirk Goossens, Andrew Goudie, Lawrence, Hagen, Patrick Hesp, Paul Hesse, Chris Houser, Joseph Hupy, Werner Illenberger, Derek Jackson, James King, Gary Kocurek, Aart Kroon, Jeff Lee, Natalie Mahowald, Cheryl McKenna Neuman, Grant McTainsh, Steve Namikas, Karl Nordstrom, Jeff Ollerhead, Patrick Pease, Norb Psuty, Keld Rasmussen, David Scott, Douglas Sherman, Geert Sterk, Ina Tegan, Haim Tsoar, Richard Washington, Jeff Whicker, and Giles Wiggs. A great debt of gratitude is owed to Jack Vitek for his editorial oversight. References Baas, A.C.W., 2004. Evaluation of saltation flux impact responders (Safires) for measuring instantaneous aeolian sand transport rates. Geomorphology 59, 99–118. Bauer, B.O., Davidson-Arnott, R.G.D., 2002. A general framework for modeling sediment supply to coastal dunes including wind angle, beach geometry, and fetch effects. Geomorphology 49, 89–108. Breshears, D.D., Whicker, J.J., Zou, C.B., Field, J.P., Allen, C.D., 2009. A conceptual framework for dryland aeolian sediment transport along the grassland-forest continuum: effects of woody plant canopy cover and disturbance. Geomorphology 105, 28–38 (this volume). Brown, S., Nickling, W.G., Gillies, J.A., 2008. A wind tunnel examination of shear stress partitioning for an assortment of surface roughness distributions. Journal of Geophysical Research, Earth Surface 113. doi:10.1029/2007JF000790. Chepil, W.S., 1957. Width of field strips to control wind erosion. Kansas Agricultural Experimental Station Bulletin 92. Ellis, J.T., Morrison, R.F., Priest, B.H., 2009. Detecting impacts of sand grains with microphone system in the field conditions. Geomorphology 105, 87–94 (this volume). Gillette, D.A., Herbert, G., Stockton, P.H., Owen, P.R., 1996. Causes of the fetch effect in wind erosion. Earth Surface Processes and Landforms 21, 641–659. Gillies, J.A., Nickling, W.G., King, J., 2007. Shear stress partitioning in large patches of roughness in the atmospheric inertial sublayer. Boundary-Layer Meteorology 122 (2), 367–396. doi:10.1007/s10546-006-9101-5. Lee, J.A., Gill, T.E., Mulligan, K.R., Acosta, M.D., Perez, A.E., 2009. Land use/land cover and point sources of the 15 December 2003 dust storm in Southwestern North America. Geomorphology 105, 18–27 (this volume). Livingstone, I., Wiggs, G.F.S., Baddock, M., 2005. Barchan dunes: why they cannot be treated as ‘solitons’ or ‘solitary waves’. Earth Surface Processes and Landforms, 30 (2), 255–257. McEwan, I.K., Willetts, B.B., Rice, M.A., 1992. The grain-bed collision in sand transport by wind. Sedimentology 39, 971–981. Namikas, S.L., 2006. A conceptual model of energy partitioning in the collision of saltating grains with an unconsolidated sediment bed. Journal of Coastal Research 22 (5), 1250–1259. Parsons, D.R., Walker, I.J., Wiggs, G.F.S., 2004. Numerical modelling of flow structures over idealised transverse aeolian dunes of varying geometry. Geomorphology 59, 149–164.
B.O. Bauer / Geomorphology 105 (2009) 1–5 Raupach, M.R., Gillette, D.A., Leys, J.F., 1993. The effect of roughness elements on wind erosion threshold. Journal of Geophysical Research 98, 3023–3029. Sherman, D.J., Hotta, S., 1990. Aeolian sediment transport: theory and measurement. In: Nordstrom, K.F., Psuty, N.P., Carter, W.G. (Eds.), Coastal Dunes: Form and Process. John Wiley & Sons Ltd., Chichester, UK, pp. 17–37.
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Stout, J.E., Warren, A., Gill, T.E., 2009. Publication trends in aeolian research: an analysis of the Bibliography of Aeolian Research. Geomorphology 105, 6–17 (this volume). Zhang, W., Kang, J.-H., Lee, S.-J., 2007. Tracking of saltating sand trajectories over a flat surface embedded in an atmospheric boundary layer. Geomorphology 86, 320–331.
Contents lists available at ScienceDirect
Geomorphology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / g e o m o r p h
Review
Contemporary research in aeolian geomorphology B.O. Bauer Earth & Environmental Sciences and Geography, University of British Columbia — Okanagan, Kelowna, BC, Canada V1V 1V7
A R T I C L E
I N F O
Article history: Accepted 2 June 2008 Available online 13 June 2008 Keywords: Aeolian processes and landforms Dust emissions Transport modeling Desert and coastal dunes Aeolian measurement and instrumentation
A B S T R A C T The first International Conference on Aeolian Geomorphology (ICAR) was held in 1986, and every four years since then, aeolian geomorphologists from around the world have assembled to discuss their research and to showcase recent advancements in understanding and modeling of aeolian processes. A content analysis of the “Bibliography of Aeolian Research” [Stout, J.E., Warren, A., Gill, T.E., 2009. Publication trends in aeolian research: An analysis of the Bibliography of Aeolian Research. Geomorphology 105, 6–17 (this volume)] shows that the number of publications on aeolian topics has increased exponentially from the mid-20th Century with approximately 50 publications per year to about 500 publications per year when the first ICAR was held, to almost 1000 publications per year currently. Areas of focus have shifted historically from initial concerns with aeolian erosion and dust events as isolated phenomenon of localized curiosity or only regional importance, to comprehensive physically-based investigations and modeling of the mechanics of aeolian transport. Recently, more applied studies have been motivated by the recognition of the importance of aeolian processes to dust emissions into the atmosphere (with relevance for human health and for meteorological conditions and climate change) and within regional management contexts (especially on coasts where impending sea-level rise is of great concern and in arid and semi-arid environments given the dependence of sediment surface stability and remobilization on meteorological and ecological conditions). Aeolian geomorphology is a rapidly growing sub-discipline of Geomorphology that offers rich opportunities for interdisciplinary collaborations with colleagues from the Atmospheric Sciences, Climatology, Sedimentology, Quaternary Geology, Fluid Mechanics, Physics, Mathematics, Computer Science, Physical Geography, Ecology, and Agricultural Sciences, as well as our counterparts in fluvial, coastal, and arid-lands geomorphology who are similarly concerned with fluid-sediment interactions and the consequent genesis of landforms. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The 6th International Conference on Aeolian Geomorphology (ICAR VI) was held from July 23–28, 2006 at the University of Guelph (Ontario, Canada) with more than 175 participants representing 25 countries. This was the latest in a series of international conferences on aeolian processes and landforms that began with a seminal meeting in Aarhus, Denmark (1986) followed by increasingly wellattended meetings in Sandberg, Denmark (1990), Zzyzx, California (1994), Oxford, England (1998), and Lubbock, Texas (2002). The next meeting (ICAR VII) will be held in Santa Rosa, Argentina from July 5–9, 2010 (http://www.icarvii.com.ar/index.htm). Each of these ICAR meetings has facilitated the dissemination of recent research results through the publication of important conference-related proceedings, monographs, or special issues of journals that provide a unique, contemporary snapshot and synopsis of the state of the science. These volumes also provide insight into the
E-mail address: [email protected]. 0169-555X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2008.02.014
breadth of aeolian research, which often leads to cross-fertilization of ideas within the aeolian community and related fields (e.g., fluid mechanics, atmospheric processes, dynamical modeling, sedimentology, agriculture, dryland ecology, etc.). More importantly, these volumes serve as a theoretical, conceptual, and empirical foundation upon which aeolian researchers are able to situate their own research agendas and advance their efforts toward the creation of new knowledge of relevance to societal concerns. The ICAR series of conferences, therefore, serves as a ‘touch-stone’ against which progress can be gauged and evaluated. This special volume of Geomorphology contains 16 manuscripts based on presentations made at ICAR VI dedicated specifically to geomorphic processes — measurement and assessment, and interaction with landforms and landscapes. A special companion volume of the Journal of Geophysical Research — Earth Surface contains 15 manuscripts derived from the conference on topics honouring the career contributions of Dr. Gillette who maintains research interests in dust emissions, sediment transport mechanics, and shear stress partitioning. Another special volume of Earth Surface Processes and Landforms contains additional manuscripts that round out the many contributions from the conference.
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2. The state of aeolian science The increasing popularity of aeolian geomorphology as a legitimate, thriving, and autonomous branch of Earth science is assessed by Stout et al. (2009-this volume) who estimate that the rate of publication of aeolian-related manuscripts has increased from fewer than three per year in the 17th Century to approximately three per day in the 21st Century. The Bibliography of Aeolian Research, which attempts to list every known scientific manuscript published in the field of aeolian research, currently contains over 29,000 bibliographic citations. This list does not include the huge literatures that are peripheral to, but inform, aeolian research on such issues as boundary layer mechanics, atmospheric circulation and global climate modeling, sedimentology, mineralogy, dating techniques, remote sensing methods, land use/land cover change, etc., except as they pertain specifically to or are used explicitly within aeolian studies. The breadth of aeolian research is reflected in the organization of the sessions at ICAR VI, which included oral and poster presentations under the following categories: Mechanics of Aeolian Processes 1 (Non-erodible elements and drag portioning) Mechanics of Aeolian Processes 2 (Saltation controls and features) Mechanics of Aeolian Processes and Dune Systems Dunes and Dune Systems Modelling Sand Transport Systems Paleo-Aeolian Systems Modeling Sand Transport Systems, Paleo-Aeolian Systems, and Coastal Systems Coastal Aeolian Systems Wind Tunnel Facilities Dust Entrainment, Transport, and Deposition (1 + 2) Modelling Dust Transport Systems Anthropogenic Interactions with Aeolian Systems (1 + 2). The ICAR series of conferences enables contemporary aeolian researchers to assemble once every four years to discuss challenges and problems, to share ideas and innovative solutions, and to explore commonalities within the various sub-disciplines as well as interconnections with a large number of more prominent disciplines (e.g., Geology, Geography, Atmospheric Sciences, Fluid Mechanics, Ecology, Sedimentology). Thirty years ago, a process geomorphologist studying desert dunes would likely have been limited to presenting her research at a geologically-oriented conference targeted exclusively at sedimentary systems and Quaternary environments. Since the advent of the ICAR series, however, such an individual is now able to interact with all manner of colleagues with similar aeolian interests including, for example, coastal geomorphologists who study air-flow and sediment transport pathways over and through coastal dune systems. Similarly, someone interested in understanding and modeling the importance of dust in the atmosphere will find opportunities at ICAR to interact with field-based researchers who are investigating the processes by which dust is emitted from various natural and humanmodified surfaces, how these emission processes are coupled to saltation dynamics or vegetation patterns, and how surface variability leads to complex spatial and temporal patterns of dust emissions into the atmosphere. The recent explosion of interest in aeolian studies, which warranted the creation of a specific conference venue such as ICAR (dedicated specifically to exchange of information and ideas on aeolian phenomena), has facilitated new opportunities for aeolian researchers that are stimulated explicitly by (1) who one comes into contact with, (2) what problems are deemed worthy of study and discussion, and (3) how one practices aeolian geomorphology. In short, aeolian science is no longer studied at the margins of other disciplines as an esoteric nicety, but rather, it has evolved into a
mature science in its own right with legitimate problems, methodologies, theories, paradigms, and organizational structures. 3. Contemporary research themes 3.1. Aeolian transport mechanics A great deal of effort in the aeolian community continues to be directed at understanding how a moving fluid (air, in this case) is able to entrain and transport sediment under the complex conditions found in nature. A large number of models (e.g., Bagnold, Horikawa and Shen, Hsu, Kadib, Kawamura, Lettau and Lettau, White, Zingg) are available to predict the rate of sediment transport (see Sherman and Hotta, 1990) based on attributes of the flow field – typically mean velocity, shear velocity, or some measure of shear stress – as well as on the character of the sediment (e.g., grain size, density). Unfortunately, these models have proven relatively unreliable except under the most ideal conditions, such as those found in wind tunnels or in the middle of vast, uniform sand sheets. This reality has stimulated intensive research into various aspects of the models, such as the assumption that an equilibrium relationship exists that adequately represents the dynamical dependence of sediment flux on a uniform and steady wind field. Aeolian researchers have invested a great deal of effort into investigating the character of small-scale boundary layer flows, especially the shape of near-surface velocity profiles within saltation layers, the nature of turbulence within sediment-laden flows, the scales and sources of flow unsteadiness, the existence and character of semi-coherent structures such as sand streamers and hairpin vortices, and the effect of changing surface features (e.g., bedform roughness, vegetation, topography) on boundary layer dynamics. Carefully planned and executed experiments in wind tunnels have revealed a great deal about the mechanics of aeolian transport, but the nature of the experimental apparatus necessarily constrains the scales of turbulent interactions that can be simulated, and, thus, a need continues to exist for field-based studies that employ sensitive instrumentation and clever experimental designs. Although apparent similarities exist between the mechanics of aeolian and fluvial transport, especially because many of the fluid mechanical laws and principles are thought to be universal, it is increasingly apparent that the dominance of saltation as the primary mode of transport in air leads to complexities that are not found in water. These are rooted in the extreme difference in density between the fluid (air) and the solid (sand), which leads to: (1) ballistic trajectories of grains that have the potential to impact the surface and cause cratering and the ejection of other surface particles (the “splash” process); and (2) mutually efficient exchange of momentum between the fluid and the grains. The splash process has been investigated intensively using high-speed cinematography and theoretical modeling (e.g., McEwan et al., 1992). Although the importance of impact cratering in maintaining the character of saltation is accepted widely, some uncertainty remains regarding the assumptions central to the models, such as whether grain liftoff is constant and proportional to shear velocity (e.g., Namikas, 2006). After a single grain is entrained from the surface, whether by fluid forces or impact cratering, it is accelerated by the fluid and thereby gains momentum from the fluid. In consequence, the air is decelerated and loses momentum to the grain. This is much more noticeable in air than in water because of density differences, and, therefore, this momentum exchange needs to be taken into account when modeling the mechanics of aeolian transport. As the grain returns back down to the surface, it passes through near-surface fluid that moves more slowly than the grain, and momentum is again exchanged — the fluid is accelerated and gains momentum from the grain whereas the grain is slowed down. Only recently has it proven possible to secure highspeed measurements of the velocity profile and grain trajectories (e.g.,
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Zhang et al., 2007) but as yet, no unambiguous treatment of momentum exchange in saltation layers has been forthcoming. 3.2. Surface controls, spatial variability, and feedback mechanisms Much of our understanding of the mechanics of aeolian transport is predicated on various idealistic assumptions such as infinitely flat surfaces, uniform grain sizes, limitless supplies of dry sediment, and steady, uniform wind systems. These can be assumed in theoretical models, and to a large extent, can be simulated in wind tunnels. Natural systems are not typically ideal, however, and a host of complications need to be taken into account. At small scale, aeolian researchers have investigated the influence of a range of surface roughness elements (e.g., patches of coarse sediments, individual boulders or shrubs, vegetation canopies, trees, fences, buildings) to determine how near-surface flows respond to these physical impediments and also how transport pathways are altered (e.g., horseshoe vortices around pieces of driftwood or coppice dunes, funneling through foredune blowouts or between buildings). In most arid and semi-arid landscapes, patches of scrub-like vegetation and isolated shrubs are the norm, and considerable attention is directed at understanding how the existence of these fixed (but often flexible), surface roughness elements influence the aeolian transport system. The Raupach model (Raupach et al., 1993) offers one commonly adopted approach, and it is based on a shear-partitioning scheme by which a portion of the total shear stress in the boundary layer is ascribed to the roughness elements themselves. The remaining stress is ascribed to the intervening sediment surfaces and is, therefore, available to drive sediment transport. Much of the recent research effort within this paradigm has been directed at understanding the influence of roughness element size, density, spacing, and porosity (e.g., Gillies et al., 2007; Brown et al., 2008). Even though a great deal is being learned about how the configuration of surface roughness elements can influence boundary layer flows (consistent with what meteorologists have discovered for forests, agricultural fields, and urban environments) and, therefore, shear stress distributions, the manner in which this knowledge can be implemented in models that are intended to aid in the prediction of the potential of sediment transport or dust emission at landscape scales remains elusive. At the landscape scale, the focus begins to move away from the micro-scale mechanics of aeolian processes to more complicated assemblages of natural interactions within aeolian systems as a whole. For example, aeolian researchers have been investigating the influence of topography on the flow field, and hence, the spatial distribution of erosion and deposition within an otherwise uniform atmospheric boundary layer. Sophisticated computational fluid dynamics (CFD) models have been employed (e.g., Parsons et al., 2004) to determine how flow is compressed and expanded depending on the curvature and geometrical scales of the underlying surface (e.g., sloping beaches, isolated dunes). Such topographic modulation has ramifications for flow acceleration and deceleration, and, therefore, for the distribution of shear stress and for pathways of sediment transport. The recent availability of reasonably inexpensive 3-D sonic anemometry has facilitated the collection of detailed data sets in the field (in coastal and desert dune systems), and these are being used to test the reliability of the CFD schemes and to provide insight into how dunes migrate and how dune systems are formed and maintained (e.g., Livingstone et al., 2005). The use of remote sensing technologies, as well as field-based measurements of the spatial distribution of surface moisture, surface temperature, and vegetation density across large sections of beach or desert, is providing new insights into how aeolian transport can be modulated by these factors in ways that cannot be parameterized easily within the non-spatial paradigm implicit to the one-dimensional (equilibrium) transport models. For example, the idea of ‘fetch distance,’ which was originally proposed for agricultural fields (Chepil,
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1957) and extended by Gillette et al. (1996) to arid lands in general, was recently used as a central concept in the modeling framework proposed by Bauer and Davidson-Arnott (2002) for beaches. This model indicates clearly that the foreshore zone of beaches will typically be an eroding aeolian environment with small transport flux despite experiencing large shear stresses that would otherwise suggest a zone of intense sediment transport. Whether maximum transport flux (as predicted by the equilibrium models) is ever achieved higher on the back beach depends on the complex interactions between boundary layer development, grain size distribution, surface moisture content, and wind-approach angle favouring a situation in which the maximum fetch distance on the beach exceeds the critical fetch distance needed to achieve the maximum potential. Such complex feedback mechanisms on the beach are critical to understanding the magnitude and timing of sediment delivery into the foredunes. Moreover, event-based and seasonallybased elements must be considered, along with vegetative controls. Process-based field experiments that deploy sophisticated electronic instrumentation serve as nice complements to more traditional studies of dune systems, which have employed detailed (repetitive) topographic mapping, stratigraphic reconstruction, paleo-environmental and isotopic dating, historical archives, and remote sensing approaches (e.g., LIDAR). Investigations into the origin and evolution of large dune complexes and sand seas – once the domain of the Quaternary geologist – are increasingly of interest to process geomorphologists motivated by the need to understand the impacts of global climate change, changing land use and land cover dynamics, and the inexorable imprint of human activities on the landscape. All these regional and global scale influences will have ramifications for altered vegetation assemblages and, therefore, the stabilization or reactivation of dune systems, sand sheets, and sediment surfaces. An increasingly large range of conceptual models are being proposed to account for these interactions (e.g., Breshears et al., 2009-this volume). 3.3. Dust emission and transport Interest in dust-related process has increased significantly in the past two decades, in part because of recognized adverse health impacts related to very small airborne particles (PM10, PM2.5), but also because dust plays an important role in the overall dynamics of the atmosphere. Hygroscopic nuclei serve as possible nucleation sites for moisture condensation. The presence of dust in significant concentrations can influence the radiative components of the energy balance of the atmosphere. The motivation for studying dust emission and dust transport, therefore, arises from practical concerns outside of aeolian geomorphology, but as it happens, the processes by which dust is emitted from the surface are influenced to large extent by the presence of mixed grain sizes in the transport population, especially large grain sizes experiencing saltation. Thus, the traditional “sand” and “dust” communities have interacted more frequently recently and to great mutual benefit. As in the past, much of the interest derives from the need for erosion control on agricultural fields, especially the effectiveness of certain tillage and planting techniques. Other humandisturbed systems are also being studied with increasing intensity, including logging trails, military bases that are used for maneuvers, off-road vehicle recreation areas, and the shorelines of large water reservoirs that experience seasonal drawdown, to name but a few. These studies indicate that human and animal disturbance to soil surfaces in arid and semi-arid environments is a critical factor because undisturbed surfaces typically have biological or chemical crusts that preclude dust emission except under the most severe meteorological events. Disturbing these naturally occurring crusts and binding elements enhances the susceptibility to dust events. Spatial variability and complexity is the norm, severely affecting the prospects for any kind of universal approach that incorporates standard parameters and generally impeding progress toward the modeling of global dust
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emission. Isolated sources of extreme dust emission (“hot spots”) have been studied using satellite imagery acquired during extreme events, and these are then ground-truthed and assessed for factors that may explain the susceptibility to emission (Lee et al., 2009-this volume). Comprehensive, regional-scale dust models that integrate atmospheric processes and land-surface characteristics (including soil type, vegetation, moisture, etc.) have been developed, but the results are dependent on the accuracy with which localized dust emissions can be predicted. This is an area of considerable importance and research opportunity (see papers in companion volumes in JGR and ESP&L). 3.4. Advanced instrumentation and methods A major factor in the advancement of any science is the degree to which in situ measurements of critical system parameters can be made accurately, rapidly, and with great precision. Aeolian geomorphology has benefited immensely in the past two decades from significant technological advancements. Hot-wire and sonic anemometry have facilitated high frequency (sub 1 Hz) measurements of velocity and turbulence in three dimensions, simultaneously. Irwin sensors have been deployed in wind tunnels and in the field to measure surface shear stress within and around vegetation in an attempt to quantify various shear-partitioning schemes. The methods by which data for transport rate are acquired have advanced from manual integrating traps (that provide information on the scale of minutes to tens of minutes) to mechanisms that incorporate continuously measuring load cells and high precision balances that provide information on the scale of seconds or less. Devices, such as the Saltiphone, Sensit, and Safire, provide data on saltation intensity at frequencies of around 20 Hz (Baas, 2004) whereas a recent microphone-based device (Ellis et al., 2009-this volume) can be used at kiloHz frequencies. Laser-based systems, originally designed for windtunnel applications, are now finding their way into field deployments. These instruments provide fast response measurements, and because of the small size and cheap cost, deploying them in tight configurations to examine spatial trends is possible (e.g., the vertical profile of mass transport). A broad suite of instrumentation from meteorology and agronomy is facilitating the measurement of associated parameters, such as relative humidity, surface temperature (infra red thermometry), and near-surface moisture content (resistance methods). Although field measurements will always be critical to understanding the mechanics of aeolian transport and in providing ground-truth data, the advent of sophisticated but easily accessible remotely sensed data promises major advances in understanding large aeolian systems, the emission of dust, and the importance of surface cover and changing land-use patterns on aeolian transport. Some of these techniques are being used to understand the lunar and Martian surfaces, for which aeolian processes dominate. Similarly, new developments in subsurface stratigraphic rendering (e.g., ground penetrating radar) as well as advances in dating techniques (e.g., optically stimulated luminescence) are yielding phenomenal insights into the historical development of existing dune fields and loess environments as well as the relationship of various depositional or erosional events to known climatological or oceanographic phases. Such information is critical in calibrating and constraining our predictions of the future based on the limited knowledge of process mechanics derived from contemporary windtunnel and field studies. 4. Conclusions Aeolian geomorphology is a thriving sub-discipline of Earth Science that is witnessing rapid growth and increasing relevance. Like many other branches of geomorphology, the imperative of global change (including trends toward climate warming, intensified
meteorological extremes, and enhanced human footprints) provides unprecedented opportunities to apply knowledge to problems of great societal concerns in ways that reduce future uncertainty. A collective scientific response is mandated, and new and innovative ways of measuring, monitoring, and modeling landscapes are essential. Old paradigms predicated on equilibrium relationships, one-dimensional (localized) interactions, and one-way forcings from fluid to sediment are in need of revision because they are simply not serving us well. Acknowledgements Sincerest thanks are extended, on behalf of the aeolian community, to the organizers and many volunteers who contributed to the success of ICAR VI, especially Dr. William Nickling, Dr. Jack Gillies, and Dr. Nick Lancaster. Sponsorship was provided by the Wind Erosion Laboratory (Department of Geography, University of Guelph), the Office of Research at the University of Guelph, the College of Social and Applied Human Sciences (University of Guelph), Rowan Williams Davies & Irwin, Inc. (Guelph, Ontario), the Division of Atmospheric Sciences and Earth & Ecosystem Sciences and the Center for Arid Lands Environmental Management (Desert Research Institute, Nevada), and IGCP 500 (Westerlies and Monsoons: Impacts of Climate Change and Variability on Dryland Environments, Hydrogeology and People). A large number of individuals provided expert review of the manuscripts published in this special volume, including: Alan Arbogast, Bas Arens, Andreas Baas, Grady Blount, Brenda Buck, Mary-Louise Byrne, Robin Davidson-Arnott, Jeff Dech, Patrick DeDeckker, Deanna van Dijk, Jean Ellis, Eugene Farrell, Suskia Faye-Visser, Paul Gares, Dale Gillette, Dirk Goossens, Andrew Goudie, Lawrence, Hagen, Patrick Hesp, Paul Hesse, Chris Houser, Joseph Hupy, Werner Illenberger, Derek Jackson, James King, Gary Kocurek, Aart Kroon, Jeff Lee, Natalie Mahowald, Cheryl McKenna Neuman, Grant McTainsh, Steve Namikas, Karl Nordstrom, Jeff Ollerhead, Patrick Pease, Norb Psuty, Keld Rasmussen, David Scott, Douglas Sherman, Geert Sterk, Ina Tegan, Haim Tsoar, Richard Washington, Jeff Whicker, and Giles Wiggs. A great debt of gratitude is owed to Jack Vitek for his editorial oversight. References Baas, A.C.W., 2004. Evaluation of saltation flux impact responders (Safires) for measuring instantaneous aeolian sand transport rates. Geomorphology 59, 99–118. Bauer, B.O., Davidson-Arnott, R.G.D., 2002. A general framework for modeling sediment supply to coastal dunes including wind angle, beach geometry, and fetch effects. Geomorphology 49, 89–108. Breshears, D.D., Whicker, J.J., Zou, C.B., Field, J.P., Allen, C.D., 2009. A conceptual framework for dryland aeolian sediment transport along the grassland-forest continuum: effects of woody plant canopy cover and disturbance. Geomorphology 105, 28–38 (this volume). Brown, S., Nickling, W.G., Gillies, J.A., 2008. A wind tunnel examination of shear stress partitioning for an assortment of surface roughness distributions. Journal of Geophysical Research, Earth Surface 113. doi:10.1029/2007JF000790. Chepil, W.S., 1957. Width of field strips to control wind erosion. Kansas Agricultural Experimental Station Bulletin 92. Ellis, J.T., Morrison, R.F., Priest, B.H., 2009. Detecting impacts of sand grains with microphone system in the field conditions. Geomorphology 105, 87–94 (this volume). Gillette, D.A., Herbert, G., Stockton, P.H., Owen, P.R., 1996. Causes of the fetch effect in wind erosion. Earth Surface Processes and Landforms 21, 641–659. Gillies, J.A., Nickling, W.G., King, J., 2007. Shear stress partitioning in large patches of roughness in the atmospheric inertial sublayer. Boundary-Layer Meteorology 122 (2), 367–396. doi:10.1007/s10546-006-9101-5. Lee, J.A., Gill, T.E., Mulligan, K.R., Acosta, M.D., Perez, A.E., 2009. Land use/land cover and point sources of the 15 December 2003 dust storm in Southwestern North America. Geomorphology 105, 18–27 (this volume). Livingstone, I., Wiggs, G.F.S., Baddock, M., 2005. Barchan dunes: why they cannot be treated as ‘solitons’ or ‘solitary waves’. Earth Surface Processes and Landforms, 30 (2), 255–257. McEwan, I.K., Willetts, B.B., Rice, M.A., 1992. The grain-bed collision in sand transport by wind. Sedimentology 39, 971–981. Namikas, S.L., 2006. A conceptual model of energy partitioning in the collision of saltating grains with an unconsolidated sediment bed. Journal of Coastal Research 22 (5), 1250–1259. Parsons, D.R., Walker, I.J., Wiggs, G.F.S., 2004. Numerical modelling of flow structures over idealised transverse aeolian dunes of varying geometry. Geomorphology 59, 149–164.
B.O. Bauer / Geomorphology 105 (2009) 1–5 Raupach, M.R., Gillette, D.A., Leys, J.F., 1993. The effect of roughness elements on wind erosion threshold. Journal of Geophysical Research 98, 3023–3029. Sherman, D.J., Hotta, S., 1990. Aeolian sediment transport: theory and measurement. In: Nordstrom, K.F., Psuty, N.P., Carter, W.G. (Eds.), Coastal Dunes: Form and Process. John Wiley & Sons Ltd., Chichester, UK, pp. 17–37.
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Stout, J.E., Warren, A., Gill, T.E., 2009. Publication trends in aeolian research: an analysis of the Bibliography of Aeolian Research. Geomorphology 105, 6–17 (this volume). Zhang, W., Kang, J.-H., Lee, S.-J., 2007. Tracking of saltating sand trajectories over a flat surface embedded in an atmospheric boundary layer. Geomorphology 86, 320–331.