Giles F.S. Wiggs

Remobilization of southern African desert dune systems by twenty-first century global warming

Although desert dunes cover 5 per cent of the global land surface and 30 per cent of Africa, the potential impacts of twenty-first century global warming on desert dune systems are not well understood. The inactive Sahel and southern African dune systems, which developed in multiple arid phases since the last interglacial period, are used today by pastoral and agricultural systems that could be disrupted if climate change alters twenty-first century dune dynamics. Empirical data and model simulations have established that the interplay between dune surface erodibility (determined by vegetation cover and moisture availability) and atmospheric erosivity (determined by wind energy) is critical for dunefield dynamics. This relationship between erodibility and erosivity is susceptible to climate-change impacts. Here we use simulations with three global climate models and a range of emission scenarios to assess the potential future activity of three Kalahari dunefields. We determine monthly values of dune activity by modifying and improving an established dune mobility index so that it can account for global climate model data outputs. We find that, regardless of the emission scenario used, significantly enhanced dune activity is simulated in the southern dunefield by 2039, and in the eastern and northern dunefields by 2069. By 2099 all dunefields are highly dynamic, from northern South Africa to Angola and Zambia. Our results suggest that dunefields are likely to be reactivated (the sand will become significantly exposed and move) as a consequence of twenty-first century climate warming.

The role of streamline curvature in sand dune dynamics: evidence from field and wind tunnel measurements

Abstract Field measurements on an unvegetated, 10 m high barchan dune in Oman are compared with measurements over a 1:200 scale fixed model in a wind tunnel. Both the field and wind tunnel data demonstrate similar patterns of wind and shear velocity over the dune, confirming significant flow deceleration upwind of and at the toe of the dune, acceleration of flow up the windward slope, and deceleration between the crest and brink. This pattern, including the widely reported upwind reduction in shear velocity, reflects observations of previous studies. Such a reduction in shear velocity upwind of the dune should result in a reduction in sand transport and subsequent sand deposition. This is not observed in the field. Wind tunnel modelling using a near-surface pulse-wire probe suggests that the field method of shear velocity derivation is inadequate. The wind tunnel results exhibit no reduction in shear velocity upwind of or at the toe of the dune. Evidence provided by Reynolds stress profiles and turbulence intensities measured in the wind tunnel suggest that this maintenance of upwind shear stress may be a result of concave (unstable) streamline curvature. These additional surface stresses are not recorded by the techniques used in the field measurements. Using the occurrence of streamline curvature as a starting point, a new 2-D model of dune dynamics is deduced. This model relies on the establishment of an equilibrium between windward slope morphology, surface stresses induced by streamline curvature, and streamwise acceleration. Adopting the criteria that concave streamline curvature and streamwise acceleration both increase surface shear stress, whereas convex streamline curvature and deceleration have the opposite effect, the relationships between form and process are investigated in each of three morphologically distinct zones: the upwind interdune and concave toe region of the dune, the convex portion of the windward slope, and the crest-brink region. The applicability of the model is supported by measurements of the rate of sand transport and the change of the dune surface in the field.

Exposure to airborne dust contaminated with pesticide in the Aral Sea region.

The Aral Sea region is one of the worlds foremost ecological disaster zones and there is increasing local concern for the health of millions of people living in this region. We have found that dust deposition rates across eastern Turkmenistan are among the highest in the world and that the dust is contaminated with pesticide.

Desert dune processes and dynamics

This article reviews the advances made and problems encountered in the measurement, modelling and understanding of desert dune dynamics and processes in the last two decades. The main findings of three methods of investigation are reviewed: field studies, wind tunnel studies and mathematical modelling. Whilst major advances in field techniques have allowed an appreciation of the aerodynamic nature of sand dunes, particular problems with field research are evident in the measurement of aeolian processes on dune surfaces. Specifically, it is shown that attempts to ascertain shear stresses on dune windward slopes in the field and relate changes in stress to sand transport rate and erosion/deposition measurements have generally failed. These difficulties have arisen because the non-log-linear nature of wind velocity profiles on dune surfaces as a result of windflow acceleration has made the calculation of surface shear stresses unviable. Significant advances have been achieved in wind tunnel modelling where high-frequency hot-wire anemometer measurements have enabled shear stress and turbulence characteristics to be determined, although problems have been encountered in choosing appropriate scaling parameters. Empirical field and wind tunnel data have allowed the calibration of mathematical models which are now at a stage where the flow field around dunes can be calculated. It is considered, however, that the emerging technique of modelling using complex systems theory may hold the key to constructing a reliable framework for future investigations. New complex systems models have emphasized the need to return to a larger-scale perspective where dunes are not considered as individual elements, but as an integral part of a dunefield where aeolian processes at the dune scale are not thought to be significant.

Air flow and sand transport over sand-dunes

Developments in the modelling of turbulent wind over hills and sand dunes of different shapes by Hunt et al. [1], Carruthers et al. [2] are briefly described, and compared with earlier studies of Jackson and Hunt [3] and Walmsley et al. [4]. A new model (FLOWSTAR) is described; it has a more accurate description of airflow close to the surface, which is not in general logarithmic at typical measurement heights. Comparisons are made between the new model and the results of non-linear models using higher-order turbulence schemes, especially for surface shear stress.

Airflow and roughness characteristics over partially vegetated linear dunes in the southwest Kalahari Desert

There is little understanding of the flow-field surrounding semi-vegetated linear dunes, and predictions of dune mobility are hampered by a lack of empirical data concerning windflow. In an attempt to characterize the near-surface airflow upwind of and over partially vegetated linear dunes in the southwest Kalahari Desert, this study presents measurements of vertical and horizontal wind velocity profiles across cross-sectional transects of seven partially vegetated linear dunes. Vegetation surveys combined with velocity measurements from vertical arrays of cup-anemometers, placed up to 2·3 m above the ground surface, were used to gain information concerning the modification of airflow structure caused by the intrusion of the dunes into the atmospheric boundary layer and to predict the variability of aerodynamic roughness (z0) from interdune to crest. The results suggest an acceleration of flow up the windward slopes of the dunes and, as such, the data correspond to classical theory concerning flow over low hills (essentially Jackson and Hunt (1975) principles). Where the theory is incapable of explaining the airflow structure and acceleration characteristics, this is explained, in part, by the presence of a spatially variable vegetation cover over the dunes. The vegetation is important both in terms of the varying aerodynamic roughness (z0) and problems concerning the definition of a zero-plane displacement (d). It is considered that any attempts to characterize surface shear stress over the Kalahari linear dunes, in order to predict sand transport and dune mobility, will be hampered by two problems. These are the progressively non-log-linear nature of the velocity profiles over the dunes caused by flow acceleration, and the production of thin near-surface boundary layers caused by areally variable aerodynamic roughness as a result of the partially vegetated nature of the dunes.

Numerical modelling of airflow over an idealised transverse dune

Abstract The general flow structure over transverse aeolian dunes is now well documented through both field studies and wind tunnel experiments. Research on windward (stoss) slopes of dunes is extensive and has recently been complemented by research on the lee-side flow structure. However, a number of technical deficiencies in wind tunnel instrumentation and a lack of detailed resolution in and appropriate turbulence instrumentation for field research have resulted in an incomplete quantified characterisation of the flow structure over aeolian dunes. This study applies a two-dimensional numerical model with an RNG-modified κ-ϵ turbulence model to simulate the time-averaged flow field over an idealized aeolian dune. The model is successfully validated with wind tunnel experimental data. Results indicate that the model accurately predicts the flow patterns over the dune, producing regions of flow stagnation at the toe, acceleration up the stoss slope and a region of flow separation and reversal in the lee. Further development and application of this model will allow examination of flow-form interaction, the testing of more complex isolated dune morphologies, and characterisation of flow over multiple dunes.

Analysis of linear sand dune morphological variability, southwestern Kalahari desert

Abstract Linear dunes are the most common desert dune form, usually occurring in extensive dunefields rather than as isolated individuals. As part of a wider project investigating the dynamics and environmental significance of linear dunes, the extensive linear dunefield of the southwestern Kalahari Desert, southern Africa, was investigated for planimetric pattern variability. Considerable intradunefield variability was identified through aerial photograph analysis of a 4000 km 2 area, leading to the development of a five-class classification scheme. This scheme was validated statistically utilising data for key planimetric pattern variables: Y-junctions, termini, orientation range, and wavelength. The application of the classification scheme thoughout the dunefield permits the identification of trends in planimetric patterns. This provides a basis for first attempts to explain aspects of planimetric variability in terms of the behaviour of linear dunes and their responses to key environmental variables.

WIND ENERGY VARIATIONS IN THE SOUTHWESTERN KALAHARI DESERT AND IMPLICATIONS FOR LINEAR DUNEFIELD ACTIVITY

The southwestern Kalahari linear dunefield, which displays marked morphological variability, possesses a partial but temporally and spatially variable vegetation cover and has frequently been described as a palaeodunefield. Palaeo status has been ascribed on the basis of several criteria including the presence of vegetation, but also because dunes are thought to be out of alignment with modern resultant potential sand-moving wind directions and because present-day wind energy is regarded as low. For the period 1960–1992, wind data from eight dunefield meteorological stations are analysed in detail to examine these assertions. Potential sand transport directions, including spatial and temporal variations, and potential drift directions for the windiest three month periods, are calculated and explained. It is concluded that the present-day potential sand transport environment is markedly variable from year to year and from place to place. While periods of low sand transport energy do occur, it is also noted that the 1980s possessed considerable potential for sand transport in the dunefield. Directional variability is also relatively high, perhaps exceeding that under which linear dunes can be expected to form. Because linear dune aeolian activity has a number of states, however, the present-day wind environment may allow dune surface aeolian activity to occur which does not alter the overall pattern of the dunes.

Estimating aerodynamic roughness over complex surface terrain

[1] Surface roughness plays a key role in determining aerodynamic roughness length (zo) and shear velocity, both of which are fundamental for determining wind erosion threshold and potential. While zo can be quantified from wind measurements, large proportions of wind erosion prone surfaces remain too remote for this to be a viable approach. Alternative approaches therefore seek to relate zo to morphological roughness metrics. However, dust-emitting landscapes typically consist of complex small-scale surface roughness patterns and few metrics exist for these surfaces which can be used to predictzofor modeling wind erosion potential. In this study terrestrial laser scanning was used to characterize the roughness of typical dust-emitting surfaces (playa and sandar) where element protrusion heights ranged from 1 to 199mm, over which vertical wind velocity profiles were collected to enable estimation of zo. Our data suggest that, although a reasonable relationship (R 2 >0.79) is apparent between 3-D roughness density and zo, the spacing of morphological elements is far less powerful in explaining variations in zo than metrics based on surface roughness height (R 2 >0.92). This finding is in juxtaposition to wind erosion models that assume the spacing of larger-scale isolated roughness elements is most important in determining zo. Rather, our data show that any metric based on element protrusion height has a higher likelihood of successfully predicting zo. This finding has important implications for the development of wind erosion and dust emission models that seek to predict the efficiency of aeolian processes in remote terrestrial and planetary environments.