56 research outputs found
Early-stage aeolian protodunes: bedform development and sand transport dynamics
No full textEarly-stage aeolian bedforms, or protodunes, are elemental in the continuum of dune development and act as essential precursors to mature dunes. Despite this, we know very little about the processes and feedback mechanisms that shape these nascent bedforms. Whilst theory and conceptual models have offered some explanation for protodune existence and development, until now, we have lacked the technical capability to measure such small bedforms in aeolian settings. Here, we employ terrestrial laser scanning to measure morphological change at the high frequency and spatial resolution required to gain new insights into protodune behaviour. On a 0.06 m high protodune, we observe vertical growth of the crest by 0.005 m in two hours. Our direct measurements of sand transport on the protodune account for such growth, with a reduction in time-averaged sediment flux of 18% observed over the crestal region. Detailed measurements of form also establish key points of morphological change on the protodune. The position on the stoss slope where erosion switches to deposition is found at a point 0.07 m upwind of the crest. This finding supports recent models that explain vertical dune growth through an upwind shift of this switching point. Observations also show characteristic changes in the asymmetric cross section of the protodune. Flow-form feedbacks result in a steepening of the lee slope and a decline in lower stoss slope steepness (by 3°), constituting a reshaping of protodune form towards more mature dune morphology. The approaches and findings applied here, a) demonstrate an ability to quantify processes at requisite spatial and temporal scales for monitoring early-stage dune evolution, b) highlight the crucial role of form-flow feedbacks in enabling early-stage bedform growth, alluding to a fluctuation in feedbacks that require better representation in dune models, and c) provide a new stimulus for advancing understanding of aeolian bedforms
Coupling leeside grainfall to avalanche characteristics in aeolian dune dynamics
No full textAvalanche (grainflow) processes are fundamental drivers of dune morphodynamics and are typically initiated by grainfall accumulations. In sedimentary systems, however, the dynamism between grainfall and grainflow remains unspecified because simple measurements are hampered by the inherent instability of lee slopes. Here, for the first time, terrestrial laser scanning is used to quantify key aspects of the grainfall process on the lee (slip face) of a barchan sand dune. We determine grainfall zone extent and flux and show their variability under differing wind speeds. The increase in the downwind distance from the brink of peak grainfall under stronger winds provides a mechanism that explains the competence of large avalanches to descend the entire lee slope. These findings highlight important interactions between wind speed, grainfall, and subsequent grainflow that influence dune migration rates and are important for correct interpretation of dune stratigraphy
Surface and Meteorological Data at Medano Creek, Great Sand Dunes National Park, Colorado, USA on 15th April 2019
No full textWind and surface morphological data collected at Medano Creek on the 15th April 2019 to investigate protodune initiation. Surface morphological data: This is terrestrial laser scanned (TLS) data collected of the creek sand surface using three different co-located Leica TLS (C10, P20 and P50). The data is raw point cloud format in text columns of x, y and z coordinate data. It has been orientation into the same local coordinate system. Each data set uses the same coordinate system. Data can be viewed in any spatial software. Data is labelled using C10, P20 or P50, followed by the scan number. Scan times are indicated in a separate file. Wind data were collected from a fixed point next to the TLS instruments using a Gill 3D sonic anemometer. The data is in csv file format with column titles and can be viewed in any text or database software.</span
Surface and Meteorological Data of Saltation and Ripple Dynamics at Huab Dune Field, Skeleton Coast National Park, Namibia 2014, 2016 and 2018
No full textThis dataset includes raw point cloud data from repeat terrestrial laser scans (TLS) of rippled surfaces on barchan and dome dunes within the Huab Dune Field, Skeleton Coast National Park, Namibia. This raw data can be used to extract saltation height dynamics as well as 3D ripple data including celerity. As well as the TLS data, additional measurements of the wind speed through a CSAT 3D sonic anemometer or cup anemometer and sediment transport using a Sensit and Wenglor gate sensor.</span
Surface and Meteorological Data at Huab River Valley, Skeleton Coast National Park, Namibia in September 2019
No full textWind, sediment transport and surface/saltation data collected at Huab River Valley during a field campaign in September 2019 to investigate saltation on gravel and sand surfaces. Surface/saltation data: This is terrestrial laser scanned (TLS) data collected over sand and gravel surfaces during multiple days when saltation was active, on a surface approximately 8 m from the TLS, perpendicular to the wind direction. The data is raw point cloud format in text columns of x, y and z coordinate data. Files are named *_^_scan&amp; where * is the date that the data was collected in yymmdd format, ^ is surface type (sand or gravel) and &amp; is the scan number. Each data set uses the same coordinate system. Data can be viewed in any spatial software. Wind and sediment data were collected from a fixed point on each surface, directly downwind of the TLS data. The data is in csv file format with column titles and can be viewed in any text or database software. Data include hot wire measurements at different heights, Wenglor counts, sensit counts and 3D sonic measurements on some days. Sonic data is at 10 Hz, hotwire data at 10 second intervals, transport data is given within both datasets.</span
Topographic perturbation of turbulent boundary layers by low-angle, early-stage aeolian dunes
No full textDecimeter-scale early-stage aeolian bedforms represent topographic features that differ notably from their mature dune counterparts, with nascent forms exhibiting more gently sloping lee sides and a reverse asymmetry in their flow-parallel bed profile compared to mature dunes. Flow associated with the development of these “protodunes,” wherein the crest gradually shifts downstream towards its mature state, was investigated by studying the perturbation of the turbulent boundary layer over a succession of representative bedforms. Rigid, three-dimensional models were studied in a refractive-index-matched experimental flume that enabled near-surface quantification of mean velocities and Reynolds stresses using particle-image velocimetry in wall-normal and wall-parallel measurement planes. Data indicate strong, topographically induced flow perturbations over the protodunes, to a similar relative degree to that found over mature dunes, despite their low-angled slopes. The shape of the crest is found to be an important factor in the development of flow perturbations, and only in the case with the flattest crest was maximal speed-up of flow, and reduction in turbulent stresses, found to occur upstream of the crest. Investigation of the log-linearity of the boundary layer profile over the stoss sides showed that, although the profile is strongly perturbed, a log-linear region exists, but is shifted vertically. A streamwise trend in friction velocity is thus present, showing a behavior similar to the trends in mean velocity. Analysis of the growth of the internal boundary layer on the dune stoss sides, beginning at the toe region, reveals a similar development for all dune shapes, despite clear differences in mean velocity and turbulent stress perturbations in their toe regions. The data presented herein provide the first documentation of flow over morphologies broadly characteristic of subtle, low-angle, aeolian protodunes, and indicate key areas where further study is required to yield a more complete quantitative understanding of flow–form–transport couplings that govern their morphodynamics.</p
Aerosol concentration and meteorological data at Etosha Pan, Namibia (July 2015 - June 2016)
No full textAerosol concentration and meteorological data collected at five meteorological stations situated around Etosha Pan in Namibia during the year July 2015 to June 2016. Data are logged every 10 minutes. LiDAR data measured at Okaukuejo on 8/9/10 July 2015. Data support the publication Wiggs et al. 2022, Quantifying mechanisms of aeolian dust emission: field measurements at Etosha Pan, Namibia. Journal of Geophysical Research: Earth Surface.
Met and Dust Data.xlsx = Excel file detailing 10 minute measurements of aerosol concentration and meteorological data (2015-2016).
MSG3-SEVI-MSG15-0100-NA-*-NA_dust_250.tif - SEVIRI false colour composite data related to LiDAR data 8-10 July 2015
Processed_Wind_Profile_81_YYYYMMDD_L2.nc - LiDAR horizontal wind speed 8-10 July 2015
Stare_81_YYYYMMDD_L2.nc - LiDAR aerosol backscatter 8-10 July 201
Modeling the dynamics of aeolian meter-scale bedforms induced by bed heterogeneities
No full textDesert surfaces are typically nonuniform, with individual sand dunes generally surrounded by gravel or nonerodible beds. Similarly, beaches vary in composition and moisture that enhances cohesion between the grains. These bed heterogeneities affect the aeolian transport properties greatly and can then influence the emergence and dynamics of bedforms. Here, we propose a model that describes how, due to transport capacity being greater on consolidated than erodible beds, patches of sand can grow, migrate, and spread to form bedforms with meter-scale length. Our approach has a quantitative agreement with high-resolution spatiotemporal observations, where conventional theory would predict the disappearance of these small bedforms. A crucial component of the model is that the transport capacity does not instantly change from one bed configuration to another. Instead, transport capacity develops over a certain distance, which thereby determines the short-term evolution of the bedform. The model predicts various stages in the development of these meter-scale bedforms, and explains how the evolution of bed elevation profiles observed in the field depends on the duration of the wind event and the intensity of the incoming sand flux. Our study thus sheds light on the initiation and dynamics of early-stage bedforms by establishing links between surface properties, emerging sand patterns, and protodunes, commonly observed in coastal and desert landscapes
Airflow dynamics in transverse dune interdunes
No full textAeolian dune interdunes have been relatively ignored when compared with the research attention on the morphodynamics of the dune bodies themselves. This neglect is in spite of the possible significance of interdune dynamics for the geomorphology of the sand dune system as a whole, especially with regard to dune spacing.;This project involved the collection of geomorphologically relevant airflow data for four relatively simple transverse dune interdunes. The study locations were chosen in order to sample interdunes with different size and surface type characteristics, the dynamics of which were investigated for when incident flow was normal to the upwind crest. The findings confirm existing models of aeolian dune lee-side flow in terms of flow re-attachment length and recovery attributes. A consistent pattern of increasing near-surface velocity downwind of re-attachment provides a mechanism for interdunes as sand-free features. Where studies for comparison from other aeolian examples are limited, the field-measured turbulence shows the importance of the shear layer as a source of turbulence, and agrees with studies from subaqueous bedforms. The importance of shear stress variability and the possible contribution of turbulence structures to the maintenance of sediment transport at re-attachment where velocity and mean stress is low or negative is also emphasised. At the downwind edge of interdunes, the mean and turbulent velocity properties, and therefore morphodynamics, vary according to the interdune size. In this case, interdune length leads to greater recovery, and a balance exists in this region between the recovering flow at the surface, dissipating wake from above and the obstacle effect of the dune.;The flow dynamics are characterised for the different types of interdune observed. Dynamics accordant with the flow response model are seen to characterise the interdune setting with the closest spacing. The occurrence of other "extended" aeolian interdunes with a length well over that for flow separation demanded the development of a new descriptive model to characterise the dynamics therein. In this model, the variation in near-surface flow allowed process zones to be identified through the interdune. The geomorphological significance of the processes dominating each zone are discussed and comparisons are made between the flow response case and the new interdune model from this study
Dune initiation in a bimodal wind regime
No full textEarly-stage bedforms develop into mature dunes through complex interactions between wind, sand transport and surface topography. Depending on varying environmental and wind conditions, the mechanisms driving dune formation and, ultimately, the shape of nascent dunes may differ markedly. In cases where sand availability is plentiful, the emergence and growth of dunes can be studied with a linear stability analysis of coupled transport and hydrodynamic equations. Until now, this analysis has only been applied using field evidence in uni-directional winds. However, in many areas of the world and on other planets, wind regimes are more often bimodal or multimodal. Here, we investigate field evidence of protodune formation under a bimodal wind regime by applying linear stability analysis to a developing protodune field. Employing recent development of the linear stability theory and experimental research, combined with in-situ wind, sediment transport, and topographic measurements during a month-long field campaign at Great Sand Dunes National Park, Colorado, USA, we predict the spatial characteristics (orientation and wavelength) and temporal evolution (growth rate and migration velocity) of a protodune field. We find that the theoretical predictions compare well with measured dune field attributes as characterized by high-resolution Digital Elevation Models measured using repeat terrestrial laser scanning. Our findings suggest that linear stability analysis is a quantitative predictor of protodune development on sandy surfaces with a bimodal wind regime. This result is significant as it offers critical validation of the linear stability analysis for explaining the initiation and development of dunes towards maturity in a complex natural environment
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