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Buildings affect aeolian sediment transport and bedform development in sandy environments. Cellular automaton (CA) models have, however, only been used to simulate natural bedform dynamics. This study extends a well-known aeolian CA model to include sediment dynamics around buildings, and uses this model to explore the interaction of building-induced deposition and erosion with natural bedform dynamics. New CA rules are introduced to represent acceleration, deceleration and sideward transport of sediment around obstacles. The simulated deposition and erosion patterns show good agreement with field experiments. The model reproduces the shape and location of the morphological pattern around a single building, and effects of building spacing on this pattern for building groups. Model results further demonstrate that building-induced effects interact with local bedform dynamics and can alter the shape, growth and migration of sand dunes.

Linear dunes are the most common type of dune found on Earth and exist on several extra-terrestrial bodies, but despite this abundance their internal stratigraphy has not been commonly agreed. A cellular automaton is deployed to simulate the development of linear dunes, starting from a flat bed, under bi-modal oblique wind regimes of varying degrees of asymmetry. The internal stratigraphy of the linear dunes is monitored by keeping track of (buried) erosion surfaces, avalanche deposits and vertical accumulation, as well as the age of last subaerial exposure of the sediments. The simulations show the initial pattern-coarsening of a network of small dunes into fewer larger longitudinal ridges via bedform interactions and Y-junction dynamics. Three newly recognized types of bedform interaction are identified that relate to initial Y-junction dynamics: longitudinal crest-splitting, which creates free dune tips that can interact with adjacent dunes, and laterally oscillating interactions that lead to ephemeral Y-junctions (normal or reverse). The results show that these three bedform interactions leave no persistent signatures in the stratigraphic record. However, a further three bedform interactions involving the superposition of one dune onto another – merging, cannibalizing and repulsion (known from transverse dune field dynamics) – do leave specific evidence in the internal stratigraphy of the remaining dune, a buried interaction surface at a specific inclination. The preservation potential of this interaction surface varies between the three types. After the initial pattern-coarsening phase, the linear dunes become larger and more independent and their crest orientation follows the net resultant transport direction. The stratigraphies of mature dunes under wind regimes of differing asymmetry show that under (nearly) symmetrical winds the dune accumulates mainly vertically, with strata dipping parallel to the flanks, while under more asymmetrical wind regimes the internal stratigraphy resembles that of transverse dunes.

We apply a mean-field model of interactions between migrating barchan dunes, the CAFE model, which includes spontaneous calving, aggregation, fragmentation, and mass-exchange, yielding a steady-state size distribution that can be resolved for different choices of interaction parameters. The CAFE model is applied to empirically measured distributions of dune sizes in two barchan swarms in the north circumpolar region of Mars, three swarms in Morocco, and one in Mauritania, each containing more than 1000 bedforms. When the sizes of bedforms are rescaled by the mean size in each zone two attractor states appear, with the Tarfaya zones all displaying a common distribution and the Martian and Mauritanian zones sharing a different distribution. Comparison of these attractor states with the outputs of the CAFE model reveals that the Tarfaya-type distribution results from a preference for aggregation and fragmentation interactions whereas the Mars-Mauritania distribution is more likely a result of exchange-dominated interactions. We observe that there appears to be a greater number of collisions happening in Tarfaya than in the other areas which is consistent with a greater rate of aggregation-fragmentation processes as suggested by our model. Our comparison with the CAFE model also predicts a universal rule for the outputs of the different types of interactions with exchange interactions favouring the production of two dunes roughly equal in size. Fragmentation interactions often result in three bedforms with volumes approximately in the ratio 1:1:2. Finally, we find that spontaneous calving of dunes does not play an important role in shaping the size distributions in barchan swarms., 31 Pages, 11 figures, Published in Physica A: Statistical Mechanics and its Applications (2022)

It is well known that barchan dunes are not isolated bedforms but are able to interact with one another both directly, through collisions and the emission/absorption of flux, and indirectly, due to the effects of turbulence in the wake of a dune. In recent years, wave-tank experiments, continuum simulations, and cellular automata models have enabled researchers to model barchan-barchan interactions. The findings from these studies have been fed into object-based models of entire fields of barchans and used to predict the size distributions. Although there has been some success with these techniques, each model has failed to reproduce certain known properties on the field-scale; for instance, that the mean width is constant with downwind distance. Furthermore, previous attempts have not been based on a theoretical understanding of the role of interactions in determining the dune size distribution, thus limiting their potential as universal models of barchan swarms.Mean-field models are relatively simple in terms of the mathematics, but have shown some degree of success in the modelling of barchan fields, although previous work has focused only on specific cases of interaction rules. We have developed a more general mean-field model which can include many different forms of interaction, making it applicable to a variety of problems, including socio-economic systems as well as fields of interacting barchans. Despite the generality of our model, we have been able to derive expressions for the dependence of the steady-state size distribution, and its moments, on the choice of interaction rules. This means that, by making a measurement of the size distribution of a barchan field, we are able to infer properties of the interactions at play. To demonstrate the power of such a model we have measured size distributions of several barchan fields in the area of Tarfaya, Morocco. Measurements were made by recording locations of seven distinct points on each barchan to yield morphometric parameters of each dune and compile the size-distribution. By comparing the distribution and its moments to those predicted by the model, we can infer certain properties of the interaction rules, such as the relative probabilities of the different forms of collision. The results show an example of how our model provides a more comprehensive understanding of the way in which dune-dune interactions determine properties on the scale of the field.

Conventional aeolian sand transport models relate mass transport rate to wind speed or shear velocity, usually expressed and empirically tested on a 1-s time scale. Projections of total sand delivery over long time scales based on these models are highly sensitive to any small bias arising from statistical fitting on empirical data. We analysed time series of wind speed and sand transport rate collected at 14 independent measurement stations on a beach during a prior field experiment. The results show that relating total sand drift to cumulative above-threshold wind run yields models which are more statistically robust when fitted on empirical data, generating smaller prediction errors when projected to longer time scales. Testing of different power exponents indicates that a linear relationship between sand drift and above-threshold wind run yields the best results. These findings inspire a speculative novel phenomenological model relating the mass flow of air in the boundary layer to the mass transport of sand over the surface.

This paper presents results of laboratory testing and proof-of-concept field deployment of large-scale particle image velocimetry (LSPIV) using a consumer-level digital video camera to measure advection velocities of streamers and sand transport patterns over a beach surface. The LSPIV algorithm developed in this study employs interrogation windows of ten different sizes to resolve the displacement vector field at a range of scales. Laboratory testing shows that the vertical downward velocities and accelerations of falling batches of sand measured with the LSPIV method agree very closely with the theoretically predicted fall velocities and accelerations. The field experiment was successful in recording the advection of streamers at a frequency of 25 Hz over a field of view of 1.3 × 1.6 m2 and over a period of 144 s. The vector field of streamer propagation could be resolved to a scale of 108 mm, exhibiting a mean advection speed of 4.01 m/s. Individual time-series segments however reveal that speeds fluctuate continuously between 3 and 7 m/s and with directional variability of 15–20°. Because of the experimental design these advection results may be representative of horizontal flight velocities of the saltating grains. 2D variograms and a time-space collage indicate length and time scale characteristics of streamers that are largely in line with previous studies. The high spatio-temporal variability of the measured advection and the lack of any link between local propagation speeds and grain cloud densities inside streamers confirm the importance of measuring boundary layer turbulence concurrently with the small-scale sand transport dynamics.

Parabolic dunes are widely distributed on coasts and margins of deserts and steppes where ecosystems are vulnerable and sensitive to environmental changes and human disturbances. Some studies have indicated that vegetated parabolic dunes can be activated into highly mobile barchan dunes and the catastrophic shift of eco-geomorphic systems is detrimental to land management and social-economic development; however, no detailed study has clarified the physical processes and eco-geomorphic interactions that control the stability of a parabolic dune and its resistance to unfavorable environmental changes. This study utilizes the Extended-DECAL (Discrete Eco-geomorphic Aeolian Landscapes) model, parameterized by field measurements of dune topography and vegetation characteristics combined with remote sensing, to explore how increases in drought stress, wind strength, and grazing stress may lead to the activation of stabilizing parabolic dunes into highly mobile barchans. The modeling results suggest that the mobility of an initial parabolic dune at the onset of a perturbation determines the capacity of a system to absorb environmental change, and a slight increase in vegetation cover of an initial parabolic dune can increase the activation threshold significantly. The characteristics of four eco-geomorphic interaction zones control the processes and resulting morphologies of the transformations. A higher deposition tolerance of vegetation increases the activation threshold of the dune transformation under both a negative climatic impact and an increased sand transport rate, whereas the erosion tolerance of vegetation influences the patterns of resulting barchans (a single barchan versus multiple barchans). The change in the characteristics of eco-geomorphic interaction zones may indirectly reflect the dune stability and predict an ongoing transformation, whilst the activation angle may be potentially used as a proxy of environmental stresses. In contrast to the natural environmental changes that tend to affect relatively weak and young plants, grazing stress can exert a broader impact on any plant indistinctively. A small increase in grazing stress just above the activation threshold can accelerate dune activation significantly.

Repeat observations underpin our understanding of environmental processes, but financial constraints often limit scientists’ ability to deploy dense networks of conventional commercial instrumentation. Rapid growth in the Internet-Of-Things (IoT) and the maker movement is paving the way for low-cost electronic sensors to transform global environmental monitoring. Accessible and inexpensive sensor construction is also fostering exciting opportunities for citizen science and participatory research. Drawing on 6 years of developmental work with Arduino-based open-source hardware and software, extensive laboratory and field testing, and incorporation of such technology into active research programmes, we outline a series of successes, failures and lessons learned in designing and deploying environmental sensors. Six case studies are presented: a water table depth probe, air and water quality sensors, multi-parameter weather stations, a time-sequencing lake sediment trap, and a sonic anemometer for monitoring sand transport. Schematics, code and purchasing guidance to reproduce our sensors are described in the paper, with detailed build instructions hosted on our King’s College London Geography Environmental Sensors Github repository and the FreeStation project website. We show in each case study that manual design and construction can produce research-grade scientific instrumentation (mean bias error for calibrated sensors –0.04 to 23%) for a fraction of the conventional cost, provided rigorous, sensor-specific calibration and field testing is conducted. In sharing our collective experiences with build-it-yourself environmental monitoring, we intend for this paper to act as a catalyst for physical geographers and the wider environmental science community to begin incorporating low-cost sensor development into their research activities. The capacity to deploy denser sensor networks should ultimately lead to superior environmental monitoring at the local to global scales.

The transformation of barchans into parabolic dunes has been observed in various dune systems around the world. Precise details of how environmental controls influence the dune transformation and stabilisation mechanism, however, remain poorly understood. A ‘horns-anchoring’ mechanism and a ‘nebkhas-initiation’ mechanism have previously been proposed and selected environmental controls on the transformation have been explored by some modelling efforts, but the morphodynamic processes and eco-geomorphic interactions involved are unclear and comparison between different dune systems is challenging. This study extends a cellular automaton model, informed by empirical data from fieldwork and remote sensing, to fully explore how vegetation characteristics, boundary conditions, and wind regime influence the transformation process and the resulting dune morphologies. A ‘dynamic growth function’ is introduced for clump-like perennials to differentiate between growing and non-growing seasons and to simulate the development of young plants into mature plants over multiple years. Modelling results show that environmental parameters interact with each other in a complex manner to impact the transformation process. The study finds a fundamental power-law relation between a non-dimensional parameter group, so-called the ‘dune stabilising index’ (S⁎), and the normalised migration distance of the transforming dune, which can be used to reconstruct paleo-environmental conditions and monitor the impacts of changes in climate or land-use on a dune system. Four basic eco-geomorphic interaction zones are identified which bear different functionality in the barchan to parabolic dune transformation. The roles of different environmental controls in changing the eco-geomorphic interaction zones, transforming processes, and resulting dune morphologies are also clarified.

This paper presents a comparison between two methods for estimating shear stress in an atmospheric internal boundary layer over a beach surface under optimum conditions, using wind velocities measured synchronously at 13 heights over a 1.7 m vertical array using ultrasonic anemometry. The Reynolds decomposition technique determines at-a-point shear stresses at each measurement height, while the Law-of-the-Wall yields a single boundary layer estimate based on fitting a logarithmic velocity profile through the array data. Analysis reveals significant inconsistencies between estimates derived from the two methods, on both a whole-event basis and as time-series. Despite a near-perfect fit of the Law-of-the-Wall, the point estimates of Reynolds shear stress vary greatly between heights, calling into question the assumed presence of a constant stress layer. A comparison with simultaneously measured sediment transport finds no relationship between transport activity and the discrepancies in shear stress estimates. Results do show, however, that Reynolds shear stress measured nearer the bed exhibits slightly better correlation with sand transport rate. The findings serve as a major cautionary message to the interpretation and application of single-height measurements of Reynolds shear stress and their equivalence to Law-of-the-Wall derived estimates, and these concerns apply widely to boundary layer flows in general. © 2015 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.

AIM Within fluvial and coastal ecosystems world-wide, flows of water, wind and sediment generate a shifting landscape mosaic composed of bare substrate and pioneer and mature vegetation successional stages. Pioneer plant species that colonize these ecosystems at the land–water interface have developed specific traits in response to environmental constraints (response traits) and are able to modify habitat conditions by modulating geomorphic processes (effect traits). Changes in the geomorphic environment under the control of engineer plants often feed back to organism traits (feedback traits), and thereby ecosystem functioning, leading to eco-evolutionary dynamics. Here we explain the joint foundations of fluvial and coastal ecosystems according to feedback between plants and the geomorphic environment. LOCATION Dynamic fluvial and coastal ecosystems world-wide. METHOD Drawing from a pre-existing model of ‘fluvial biogeomorphic succession’, we propose a conceptual framework showing that fluvial and coastal ‘biogeomorphic ecosystems’ are functionally similar due to eco-evolutionary feedbacks between plants and geomorphology. RESULTS The relationships between plant traits and their geomorphic environments within different fluvial and coastal biogeomorphic ecosystems are identified and classified within a framework of biogeomorphic functional similarity according to three criteria: (1) pioneer plants develop specific responses to the geomorphic environment; (2) engineer plants modulate the geomorphic environment; (3) geomorphic changes under biotic control within biogeomorphic ecosystems feed back to organisms. MAIN CONCLUSIONS The conceptual framework of functional similarity proposed here will improve our capacity to analyse, compare, manage and restore fluvial and coastal biogeomorphic ecosystems world-wide by using the same protocols based on the three criteria and four phases of the biogeomorphic succession model.

The Namib Sand Sea in southern Africa offers an ideal location in which to consider general questions about the evolution of sand seas, about the fluxes of sand through contemporary dune fields and about the patterns of dune form that are created. This paper aims to provide a concise account of the approaches and techniques that are currently being used and will be used in the future to address these questions. The paper considers the techniques employed to investigate wind climate, the morphometry of the dunes, the internal structure of dune sediments, the age of the dunes and the potential to model dune development.

Aeolian streamers are common in prototype saltation systems. Streamers are elongate, flow-aligned features within which the concentrations of saltating grains are large relative to a spanwise average concentration. The occurrence of streamers introduces substantial spatial and temporal variability in local sand transport rates. There have been few studies to attempt to characterize the scales of streamers, and the results of those studies have been constrained because they use Eulerian approaches to measure an inherently Lagrangian process. We describe the results from a field experiment designed to address this methodological problem. Field experiments were conducted at Jericoacoara, Ceará, Brazil, in October, 2011. The wind field was measured with ultrasonic anemometers and ruggedized thermal probes. Transport rates were measured using Miniphones, Wenglor Particle Counters, and hose traps. A set of three video cameras, deployed in a triangular array, was used to capture images of streamers. The field of vision for the central, upwind-facing camera was a minimum of about 15 m, expanding to more than 100 m in the middle distance. Video images were time-averaged over a number of intervals, ranging from 1-64 seconds, to establish characteristic path lengths and spatial and temporal scales. The results of these analyses show that this methodological approach is technically sound. Streamer characteristics are center-to-center spacings of about 1 m, length-scales exceeding 50 m and time scales of individual streamers exceeding 64 s. © Coastal Education & Research Foundation 2013.

Improvements in the design and affordability of ultrasonic anemometers have provided significant contributions to aeolian research, by facilitating high frequency monitoring of three dimensional wind velocities. From these data it is possible to calculate quasi-instantaneous Reynolds stresses to evaluate boundary layer turbulence, moving beyond time-averaged measures, such as shear velocity ( U * ). As ultrasonic anemometry is used more frequently in aeolian geomorphology it is important to question accepted conventions concerning data processing and analysis. This paper examines data processing questions associated with the application of ultrasonic anemometry to field studies in aeolian geomorphology, through an investigation of three streamline correction routines, the two-step, three-step and planar-fit methods, on data recorded on a gently sloping beach at Magilligan Strand, Northern Ireland in May 2010. The planar-fit technique has not previously been used in aeolian geomorphology. Results are compared with data that have been corrected only for wind direction (yaw). The effects that these different methods have on quadrant analysis and Reynolds stress calculation are discussed. Streamline correction is applied as a time-variable procedure using a characteristic timescale of 8 s following analysis of the resultant wind speed energy spectrum. It is found that Reynolds shear stress is dependent on streamline correction method, with run mean estimates of resultant horizontal shear stress ranging from 0.05 to 0.11 N m − 2 depending on the technique. The two-step method consistently maximises the shear stress and when the resultant horizontal shear is calculated, it produces the most robust estimate for application to aeolian research. In contrast, the different methods have little effect on the identification or sequencing of turbulent structures using quadrant analysis. Streamline correction is an essential processing step when using Reynolds decomposition, however, the method used has an effect on data quality as each correction re-distributes stress across the whole tensor, altering both normal and shear stresses. The total stress tensor magnitude itself is a conserved value and may provide a suitable parameter to quantify boundary layer turbulence.

A second-generation, source-to-sink cellular automaton-based model presented here captures and quantifies many of the factors controlling the evolution of aeolian dune-field patterns by varying only a small number of parameters. The role of sediment supply, sediment availability and transport capacity (together defined as sediment state) in the development and evolution of an aeolian dune-field pattern over long time scales is quantified from model simulations. Seven dune-field patterns can be classified from simulation results varying the sediment supply and transport capacity that control the type and frequency of dune interactions, the sediment availability of the system and, ultimately, the development of dune-field patterns. This model allows predictions to be made about the range of sediment supply and wind strengths required to produce the dune-field patterns seen in the real world. A new clustered dune-field pattern is identified from model results and used to propose an alternative mechanism for the formation of superimposed dunes. Bedforms are hypothesized to cluster together, simultaneously forming two spatial scales of bedforms without first developing a large basal dune with small superimposed dunes. Manipulation of boundary conditions produces evolving dune fields with different spatial configurations of sediment supply. Trends of spacing and crest length increase with decreasing variability as the dune field matures. This simple model is a valuable tool which can be used to elucidate the dominant control of aeolian sediment state on the construction and evolution of aeolian dune-field patterns.

A new digital atlas of the geomorphology of the Namib Sand Sea in southern Africa has been developed. This atlas incorporates a number of databases including a digital elevation model (ASTER and SRTM) and other remote sensing databases that cover climate (ERA-40) and vegetation (PAL and GIMMS). A map of dune types in the Namib Sand Sea has been derived from Landsat and CNES/SPOT imagery. The atlas also includes a collation of geochronometric dates, largely derived from luminescence techniques, and a bibliographic survey of the research literature on the geomorphology of the Namib dune system. Together these databases provide valuable information that can be used as a starting point for tackling important questions about the development of the Namib and other sand seas in the past, present and future.

The following is a white paper (adapted here for print) for the U.S. National Research Council's committee on Challenges and Opportunities in Earth Surface Processes, drafted at a National Science Foundation sponsored workshop associated with the 38th Binghamton Geomorphology Symposium, “Complexity in Geomorphology,” held at Duke University in October 2007.

Aeolian dune field development in coastal and semi-arid environments is a function of complex ecogeomorphic interactions which are sensitive to fluctuations in climatic and environmental conditions. We explore the relationships between ecological and geomorphic processes in the development of these landscape patterns and speculate on their response to variations in vegetation vitality and sediment transport capacity, indicating possible consequences of climate and land use change, using the Discrete ECogeomorphic Aeolian Landscape (DECAL) cellular automaton algorithm. This algorithm models dune field behaviour that reflects long-term trends prevalent in palaeo-records, but also elucidates possible evolutionary progressions, relaxation period sequences and threshold sensitivities. The landscape response is sensitive both to the perturbation itself and the state of the system when the disturbance occurs. Response amplitude decreases in simulated systems with reduced mobility unless an external disturbance mimicking fire or land clearance is applied concurrently with a reduction in growth vigour triggering a threshold type response when sufficient vegetation is removed. The model demonstrates that the relative response characteristics of the multiple vegetation types and their mutual feedback with geomorphic processes impart a significant influence on landscape equilibrium or attractor states. Fast growing vegetation enables the formation of hairpin (long-walled) parabolic dune systems, which eventually become sediment starved and stabilise, whereas inhospitable conditions inhibiting vegetation growth contribute to the development of active transgressive transverse dune fields. This simple vegetated dune model illustrates the power and versatility of a cellular automaton approach for exploring thresholds, sensitivities and possible evolutionary trajectories associated with the interactions between ecology, geomorphology and climatic conditions in complex earth surface systems.

Vegetation plays an important role in shaping the morphology of aeolian dune landscapes in coastal and semi-arid environments, where ecogeomorphic interactions are complex and not well quantified. We present a Discrete ECogeomorphic Aeolian Landscape model (DECAL) capable of simulating realistic looking vegetated dune forms, permitting exploration of relationships between ecological and morphological processes at different temporal and spatial scales. The cellular automaton algorithm applies three simple rules that lead to self-organization of complex dune environments, including nebkhas with distinctive deposition tails that form in association with mesquite-type shrubs, and hairpin (long-walled) parabolic dunes with trailing ridges that evolve from blowouts in association with vegetation succession. Changing the conditions of simulations produces differing landscapes that conform qualitatively to observations of real-world dunes. The model mimics the response of the morphology to changes in sediment supply, vegetation distribution, density and growth characteristics, as well as initial disturbances. The introduction of vegetation into the model links spatial and temporal scales, previously dimensionless in bare-sand cellular automata. Grid resolutions coarser than the representative size of the modelled vegetation elements yield similar morphologies, but when cell size is reduced to much smaller dimensions, the resultant landscape evolution is dramatically different. The model furthermore demonstrates that the relative response characteristics of the multiple vegetation types and their mutual feedback with geomorphological processes impart a significant influence on landscape equilibria, suggesting that vegetation induces a characteristic length scale in aeolian environments. This simple vegetated dune model illustrates the power and versatility of a cellular automaton approach for exploring the effects of interactions between ecology and geomorphology in complex earth surface systems.

This paper reviews the challenges involved with instrumentation, data acquisition, analysis, and the application of theory when conducting field experiments in natural aeolian environments. This is presented in the context of a field investigation into the formation and behaviour of aeolian sand streamers. Also known as sand snakes, streamers are a familiar manifestation of pronounced spatial and temporal variability in wind-blown sand over beaches, dunes, and other sedimentary surfaces. Streamers represent complex and dynamic transport patterns that change over temporal scales on the order of 0.1 s and over a range of spatial scales from 0.1 to 10 m. These transport features are likely governed by near-surface turbulent structures in the wind, necessitating detailed measurements of turbulence dynamics in association with sediment transport patterns. This paper reviews all stages of the investigation, from field site selection to the evaluation of theoretical models. The discussion covers a range of topics, including: research strategy, site selection, instrumentation requirements and limitations, experimental design and measurement techniques, scale and precision of data acquisition, data processing and visualization, description and parameterisation of results, challenges of spatio-temporal data analysis, turbulence measurements in the saltation layer, scaling issues and non-dimensionalisations in fluid dynamics theory, and the application of theory and conceptual models to the complexities of real natural environments. The challenges discussed here apply to many geomorphological field investigations in general, and three principal issues are identified: 1) the need for inexpensive, compact, and robust instruments with high spatio-temporal resolution, 2) the lack of extended quantitative methods for the computational analysis of shapes, forms and patterns, and 3) the conceptual discrepancies and ambiguities of applying scaling parameters from experimental engineering and idealized theoretical models to complex natural geomorphic systems.

Aeolian geomorphology provides a rich ground for investigating Earth surface processes and landforms as complex systems. Sand transport by wind is a classic dissipative process withnon-linear dynamics, while dunefield evolution is a prototypical self-organisation phenomenon.Bothofthesebroadareasofaeoliangeomorphologyarediscussedandanalysedinthecontextofcomplexityandasystems approach.Afeedbackloopanalysisoftheaeolianboundary-layer-flow/sediment-transport/bedforminteractions,basedoncontemporary physicalmodels,revealsthatthesystemisfundamentallyunstable(oratmostmeta-stable)andlikelytoexhibitchaoticbehaviour.Recent field-experimental research on aeolian streamers and spatio-temporal transport patterns, however, indicates that sand transport by wind may be wholly controlled by a self-similar turbulence cascade in the boundary layer flow, and that key aspects of transport event timeseriescanbefullyreproducedfromacombinationof(self-organised)1/fforcing,motionthreshold,andsaltationinertia.Theevolutionof various types of bare-sand dunes and dune field patterns have been simulated successfully with self-organising cellular automata that incorporateonlysimplifiedphysically-basedinteractions(rules).Becauseoftheirundefinedphysicalscale,however,itnotclearwhether they in fact simulate ripples (bedforms) or dunes (landforms), raising fundamental cross-cutting questions regarding the difference between aeolian dunes, impact ripples, and subaqueous (current) ripples and dunes. An extended cellular automaton (CA) model, currently under development, incorporates the effects of vegetation in the aeolian environment and is capable of simulating the development of nebkhas, blow-outs, and parabolic coastal dunes. Preliminary results indicate the potential for establishing phase diagrams and attractor trajectories for vegetated aeolian dunescapes. Progress is limited, however, by a serious lack of appropriate concepts for quantifying meaningful state variables at the landscape scale. State variables currently used in the bare-sand models are far from capturing the rich 3D topography and patterns and are not sufficiently discriminative to distinguish different attractors. The vegetationcomponentintheextendedmodel,andindeedecogeomorphicsystemsingeneral,poseevengraverchallengestoestablishing

Aeolian sand transport on beaches and in dune environments shows a great spatial and temporal variability that has important implications for modeling and monitoring of coastal systems. Yet there have been few quantifications or statistical characteristics of transport variability in natural environments. Transport variability can result from bed surface control in the form of differentiation in grain size, surface moisture, and microtopography, or can be induced by fluid forcing in the form of gusts, burst-sweep events, and streamwise vortices. A field experiment was conducted on a coastal dune near Guadalupe, California, to quantify transport variability over spatial scales of 0.1–4.0 m and temporal scales of 1–120 seconds. Results show that spanwise (lateral) variability increases with spatial scale and decreases with temporal scale. Minimum transport variability over the smallest distances and longest time scales is on the order of 30%, providing error margins for transportrate measurements a...

The measurement of aeolian sand transport rates on small scales is of interest to the development and testing of detailed models of sand movement by wind. This paper reports on laboratory evaluations and preliminary field tests of a new design of a piezo-electric impact responder, called a 'Safire', capable of measuring saltation impacts at a frequency of 20 Hz. The advantages of the Safire are: (1) that it provides high-frequency measurements, (2) that it presents a minimal obstruction to the wind flow (no scour observed in the field), and (3) that it is of a (relatively) low-cost. Laboratory calibrations were performed with a vertical gravity flume generating known sand grain fluxes using both mixed sand and specific size fractions. Initial tests investigated three fundamental characteristics: correspondence between digital and analogue signals generated by the instrument, directional response of the probe, and linearity of instrument response to mass flux. Instrument calibration included determination of the momentum threshold required for the sensor to register a grain impact. Based on this lower limit and the known distribution of grain size and speed at different fall heights, a prediction is made as to the sand grain flux the Safire ought to measure, which is then compared with the signal response. The result of this comparison is an assessment of the instrument's efficiency in counting saltating grains. These Safires were also deployed in the field as part of a larger investigationof spatio-temporal transport variability.Thisexperiment providedthe opportunity to compare the instrument's performancewithtraditional sandtraps,andthispaperdevelopsmethodsandassumptionsrequiredtoconvertmeasurements from impact responders to traditional mass transport rates. The evaluations indicate that improvements to the instrument production process are required to ensure a standard momentum threshold among individual instruments. Furthermore, the sensor design needs to be reconsidered in order to eliminate the variation in response depending on azimuth direction, so that the sensor is uniformly omni-directional. D 2003 Elsevier B.V. All rights reserved.

The formation and evolution of parabolic aeolian dunes depend on vegetation, and as such are particularly sensitive to changes in environmental controls (e.g., temperature, precipitation, and wind regime) as well as to human disturbances (e.g., grazing, agriculture, and recreation). Parabolic dunes can develop from the stabilisation of highly mobile barchan dunes and transverse dunes as well as from blowouts, as a consequence of colonisation and establishment of vegetation when aeolian sand transport is reduced and/or when water stress is relieved (by increasing precipitation, for instance). Conversely, existing parabolic dunes can be activated and may be transformed into barchan dunes and/or transverse dunes when vegetation suffers environmental or anthropogenic stresses. Predicted increases in temperature and drought severity in various regions raise concerns that dune activation and transformations may intensify, and this intensification would have far-reaching implications for environmental, social, and economic sustainability. To date, a broad examination of the development of parabolic dunes and their related transformations across a variety of climate gradients has been absent. This paper reviews existing literature, compares data on the morphology and development of parabolic dunes in a comprehensive global inventory, and scrutinises the mechanisms of different dune transformations and the eco-geomorphic interactions involved. This knowledge is then integrated into a conceptual framework to facilitate understanding and prediction of potential aeolian dune transformations induced by changes in environmental controls and human activities. This conceptual framework can aid judicious land management policies for better adaptations to climatic changes.

Coastal dunes are the primary defence protecting the coastline from the destructive forces of the sea in The Netherlands. Aeolian processes are important in this context as they contribute to dune accretion and thus the safety of the coastal hinterland. In this study, we analyze horizontal and vertical variability of event scale aeolian sand transport on a wide beach on the island of Ameland, The Netherlands. Data were obtained from a meteorological station, groundwater monitoring wells and a camera installed on the beach. Fifteen aeolian transport events (two involving onshore winds, seven longshore and six offshore) were measured using a comprehensive grid of 37 customized MWAC traps. The highest sand transport rates and largest variability was found for alongshore events. Surface moisture, governed by groundwater, was found to be an important controlling parameter for aeolian transport rates and vertical flux profiles. Groundwater levels were largely dominated by beach inundation, influencing the groundwater table for a two week period. Variations in vertical flux profiles between traps were larger for wet sand transport events than dry ones. In general, sand transport rates were highest at the foreshore and lowest at the dune toe. Sand transport dynamics are dependent on local conditions such as beach dimensions, beach orientation and also meteorological and surface characteristics. Moderate (high frequency, low magnitude) events are also capable of transporting large amounts of sand. Future studies should include spatially explicit measurements of elevation and surface moisture to obtain a more complete understanding of the complex sand transport dynamics.

Complex nonlinear dynamic systems are ubiquitous in the landscapes and phenomena studied by earth sciences in general and by geomorphology in particular. Concepts of chaos, fractals and self-organization, originating from research in nonlinear dynamics, have proven to be powerful approaches to understanding and modeling the evolution and characteristics of a wide variety of landscapes and bedforms. This paper presents a brief survey of the fundamental ideas and terminology underlying these types of investigations, covering such concepts as strange attractors, fractal dimensions and self-organized criticality. Their application in many areas of geomorphological research is subsequently reviewed, in river network modeling and in surface analysis amongst others, followed by more detailed descriptions of the use of chaos theory, fractals and self-organization in coastal geomorphology in particular. These include self-organized behavior of beach morphology, the fractal nature of ocean surface gravity waves, the self-organization of beach cusps and simulation models of ripples and dune patterns. This paper further presents a substantial extension of existing dune landscape simulation models by incorporating vegetation in the algorithm, enabling more realistic investigations into the self-organization of coastal dune systems. Interactions between vegetation and the sand transport process in the model—such as the modification of erosion and deposition rules and the growth response of vegetation to burial and erosion—introduce additional nonlinear feedback mechanisms that affect the course of selforganization of the simulated landscape. Exploratory modeling efforts show tantalizing results of how vegetation dynamics have a decisive impact on the emerging morphology and—conversely—how the developing landscape affects vegetation patterns. Extended interpretation of the modeling results in terms of attractors is hampered, however, by want of suitable state variables for characterizing vegetated landscapes, with respect to both morphology and vegetation patterns. D 2002 Elsevier Science B.V. All rights reserved.

Equilibrium concepts are widespread in aeolian geomorphology. The main objective of this chapter is to reflect critically on whether a new paradigm based on coherent flow structures (CFS) is beginning to emerge as an alternative to our current understanding of aeolian processes, which is predicated on time-averaged flow and transport assessments. A survey of recent research addressing the existence and nature of CFS in aeolian systems indicates that: (1) there is as yet no convincing evidence in support of the classic bursting process or of any discrete flow structure with a characteristic topology (e.g., hairpin vortex) for the case of near-surface boundary layer flow over flat sandy surfaces (with or without saltation); (2) there are instances when near-surface vortices of varying geometry are found in association with small obstacles (e.g., logs, pebbles, vegetation clumps) on sandy surfaces, but it is not always apparent what the pattern of erosion or deposition in the vicinity of the obstacle will be given that there are additional controls that involve surface hardness and sediment delivery from upwind; (3) flow over aeolian dunes is generally well understood and very similar to flow over fluvial dunes with characteristic zonation, including flow acceleration and streamline compression on the stoss slope and a recirculation eddy and wake region in the lee; and (4) the importance of these dune-related flow zones to sediment transport response is more reliably understood through examination of a range of turbulence parameters (e.g., Reynolds stress, turbulent kinetic energy) rather than mean flow quantities alone. In order to advance our understanding of CFS in aeolian geomorphology, it will be essential to develop better technologies for high-frequency monitoring of wind and sediment flux, to focus attention on sediment transport events with specific topologies (e.g., streamers) rather than attempt to capture flow events and hope that there is an accompanying transport signal, to move beyond simple quadrant analysis and adopt analytical methods that elucidate the time-history of event signatures, and to reformulate existing time-averaged parameterizations of sediment flux to incorporate event-based information in ways that are not immediately apparent.

Drivers of land degradation often co-occur and their effects are often non-additive because of internal system feedbacks. Therefore, to understand how drivers of land degradation alter ecogeomorphic patterns and processes, novel tools are required. In this chapter we explore different modelling approaches that have been developed to simulate pattern formation, and ecological and geomorphic processes. These modelling approaches reflect some of the best available tools at present, but notably, they tend to simulate only one or at best two components of the ecogeomorphic system. The chapter culminates with a discussion of these different modelling approaches and how they provide a foundation upon which to develop much needed ecogeomorphic modelling tools.

On all sandy coastlines fringed by dunes, understanding localised air flow allows us to examine the potential sand transfer between the beach and dunes by wind-blown (Aeolian) action. Traditional thinking into this phenomenon had previously included only onshore winds as effective drivers of this transfer. Recent research by the authors, however, has shown that offshore air-flow too can contribute significantly, through lee-side back eddies, to the overall windblown sediment budget to coastal dunes. Under rising sea levels and increased erosion scenarios, this is an important process in any post-storm recovery of sandy beaches. Until now though, full visualisation in 3D of this newly recognised mechanism in offshore flows has not been achieved. Here we show for the first time this return flow eddy system, using 3D Computational Fluid Dynamics modelling, and reveal the presence of complex corkscrew vortices and other phenomena. The work highlights the importance of relatively small surface undulations in the dune crest which act to induce the spatial patterns of airflow (and transport) found on the adjacent beach.

Understanding the morphodynamics of beach-dune systems requires knowledge of the spatio-temporal variability of the sediment transport system. It is common in aeolian studies to employ a single transect instrument set up, oriented parallel to the wind direction. This experimental design assumes that there is no significant variation in sediment transport lateral to this direction. A limited number of recent studies into this lateral (or spanwise) variability have revealed substantial differences in transport rates over very short spanwise distances (

In this chapter, three broad categories of geomorphological models are considered: (1) traditional physically based computer models; (2) cellular-automata models; and (3) statistical models of observations or simulated data. Nine considerations for constructing and running geomorphological models within these categories are then explored: (1) suitability of the model for the question and observational data at hand; (2) model parsimony; (3) dimensional analysis; (4) benchmarks; (5) sensitivity analysis; (6) calibration; (7) observation and model data exploration; (8) uncertainty assessment; and (9) alternative models, data, and questions. For each consideration, good practices within the context of the literature are highlighted.

This introductory chapter of the volume on ‘Quantitative Modeling of Geomorphology’ considers different types of models in geomorphology as recognized in past research and review literature and the various ways of classifying and characterizing different kinds of modeling approaches and philosophies. This is followed by reflections on one particular aspect that is not generally considered in the context of geomorphic models: questions of beauty and aesthetics, what these might be, and whether they may be fruitful to consider in this realm. The remainder of the introduction explains the general development approach and objectives of the volume, describes the volume structure and logic of the sequence of chapters, and concludes with important acknowledgements.

The processes that shape the Earth's surface have a variation or rhythm that can be measured in the field using a number of instruments and techniques. This chapter describes electronic field equipment that has increased our capacity to investigate sediment transport and fluid flows in coastal, fluvial, and aeolian environments. This equipment includes: (1) anemometers, velocimeters, profilers, and hydrophones for Eularian flow measurement; (2) traps, impact sensors, and backscatterance sensors for sediment transport measurement; (3) drogues, particle image velocimeters, and electronic tracers for measuring Lagrangian flow and transport; and (4) erosion pins, distance sensors, and sonar altimeters for measuring bed elevation change. Modern deployment of these instruments and past applications are described with respect to the spatial and temporal scale of the processes being examined, and the methodological and interpretative limitations inherent to field deployments.

International audience; This review article presents recent advances in the field of biogeomorphology related to the reciprocal coupling between Earth surface processes and landforms, and ecological and evolutionary processes. The aim is to present to the Earth Science community ecological and evolutionary concepts and associated recent conceptual developments for linking geomorphology and biota. The novelty of the proposed perspective is that (1) in the presence of geomorphologic-engineer species, which modify sediment and landform dynamics, natural selection operating at the scale of organisms may have consequences for the physical components of ecosystems, and particularly Earth surface processes and landforms; and (2) in return, these modifications of geomorphologic processes and landforms often feed back to the ecological characteristics of the ecosystem (structure and function) and thus to biological characteristics of engineer species and/or other species (adaptation and speciation). The main foundation concepts from ecology and evolutionary biology which have led only recently to an improved conception of landform dynamics in geomorphology are reviewed and discussed. The biogeomorphologic macroevolutionary insights proposed explicitly integrate geomorphologic niche-dimensions and processes within an ecosystem framework and reflect current theories of eco- evolutionary and ecological processes. Collectively, these lead to the definition of an integrated model describing the overall functioning of biogeomorphologic systems over ecological and evolutionary timescales.

Cellular automaton modelling for the simulation of dune field formation and evolution has developed progressively in aeolian geomorphology in the last decade or so. A model that incorporates the effects of vegetation and its interactions with geomorphic landscape development - the Discrete Ecogeomorphic Aeolian Landscapes (DECAL) model - can replicate a number of important visual and qualitative aspects of the complex evolution of aeolian dune landscapes under the influence of vegetation dynamics in coastal environments. A key challenge in this research area is the analysis and comparison of both simulated and real-world vegetated dune landscapes using objective and quantifiable principles. This study presents a methodological framework or protocol for numerically quantifying various ecogeomorphic attributes, using a suite of mathematically defined landscape metrics, to provide a rigorous and statistical evaluation of vegetated dune field evolution. Within this framework the model parameter space can be systematically explored and simulation outcomes can be methodically compared against real-world landscapes. Based on a simplified scenario of parabolic dunes developing out of blow-outs the resulting dune field realizations are investigated as a function of variable growth vigour of two simulated vegetation types (pioneer grass and successional woody shrub) by establishing a typological phase-diagram of different landscape classes. The set of simulation outcomes furthermore defines a higher-dimensional phase-space, whose axes or dimensions can be interpreted by analysing how individual ecogeomorphic landscape metrics, or state variables, contribute to the data distribution. Principal component analysis can reduce this to a visual three-dimensional (3D) phase-space where landscape evolution can be plotted as time-trajectories and where we can investigate the effects of changing environmental conditions partway through a simulation scenario. The use of landscape state variables and the construction of a 3D phase-space presented here may provide a general template for quantifying many other eco-geomorphic systems on the Earth's surface.

This letter presents a self-organising cellular automaton model capable of simulating the evolution of vegetated dunes with multiple types of plant response in the environment. It can successfully replicate hairpin, or long-walled, parabolic dunes with trailing ridges as well as nebkha dunes with distinctive deposition tails. Quantification of simulated landscapes with eco-geomorphic state variables and subsequent cluster analysis and PCA yields a phase diagram of different types of coastal dunes developing from blow-outs as a function of vegetation vitality. This diagram indicates the potential sensitivity of dormant dune fields to reactivation under declining vegetation vitality, e.g. due to climatic changes. Nebkha simulations with different grid resolutions demonstrate that the interaction between the (abiotic) geomorphic processes and the biological vegetation component (life) introduces a characteristic length scale on the resultant landforms that breaks the typical self-similar scaling of (un-vegetated) bare-sand dunes.

[1] This letter reports on wavelet spectral analysis of collocated time-series of streamwise wind speed and aeolian sand transport collected at 0.04 m above the bed in a field experiment on top of a large sand mound in a desert environment. Results suggest three distinct spatio-temporal scales of interaction between boundary layer turbulence and sediment transport: a long-term scale associated with the regional mixing layer, an integral time-scale associated with the internal boundary layer, and a streamer scale associated with the inertial subrange (eddy cascade) and saltation inertia.

Received 5 December 2004; revised 31 March 2005; accepted 9 June 2005; published 7 September 2005. (1) Field experiments were conducted to determine the characteristic spatial and temporal dimensions and behavior of aeolian streamers and to identify the processes responsible for their formation. An instrument array that included anemometer towers, piezo-electric transport sensors (Safires), and hot-film probes was deployed to measure streamers and airflow dynamics on both a coastal dune and a desert sand mound in California. Aeolian transport occurs predominantly in the form of families of intertwining and bifurcating streamers, while under intense wind forcing, more complex patterns of nested streamers and clouds with embedded streamers develop. The streamers display a characteristic width of approximately 0.2 m and an average lateral spacing of about 1 m. These dimensions appear to be independent of mean airflow characteristics. The observations and measurements of streamers under different environmental conditions suggest that bed surface control in the form of differentiation in moisture content, grain size, or microtopography is not a necessary condition for the formation of streamers. The results show little correlation between possible streamwise vortices or burst sweep events and streamers. It is proposed that streamers are generated by near-surface gusts that originate from large eddies propagating to the ground from higher regions of the boundary layer. These elongated and stretched eddies scrape across the surface and initiate saltation along their path. This concept accounts for the characteristic size and spacing of streamers, their rapid propagation, and the fast response of saltation to wind speed fluctuations.

The Earth’s pulse is evident in a variety of geomorphic processes that shape its surface. This chapter describes how electronic instrumentation has dramatically increased our capacity to investigate sediment transport by wind and water and to relate processes to morphological change. A number of instruments and techniques for measuring fluid flow and sediment transport in fluvial, coastal, and aeolian environments are discussed, including current meters, current profilers, pressure transducers, optical backscatter sensors, anemometers, hot-film probes, photoelectric erosion pins, sediment traps, and saltation impact responders. The deployment of such instruments is placed in the context of scale, methodology, limitations, and interpretation of spatiotemporal records of measured processes.

Vegetation density on foredunes exerts an important control on aeolian sediment transport and deposition, and therefore on profile development. In a long-term monitoring field experiment, three plots were planted with regular grids of reed bundles in three different densities: 4, 2 and 1 bundles per m2. This study reports on the differences in profile development under the range of vegetation densities. Topographic profiles were measured between May 1996 and April 1997. Results indicate important differences in profile development for the three reed bundle densities: in the highest density plot a distinct, steep dune developed, while in the lowest density a more gradual and smooth sand ramp was deposited. When the stems had been completely buried, differences in profile evolution vanished. After a second planting of reed stems in January 1997 the process was repeated. In May 1997, all plots had gained a sand volume ranging from 11·5 to 12·3 m3 m−1, indicating that the sediment budget is relatively constant, regardless of the particular profile evolution. The field evidence is compared with simulations of profile development, generated by the foredune development model SAFE. The model successfully reproduces the overall profile development, but in general, the equations used for vegetation–transport interaction overestimate the effect of vegetation. This causes some deviations between field and model results. Several reasons for this are discussed. Based on the experiments reported here, recommendations are given for further research. Copyright © 2001 John Wiley & Sons, Ltd.