Ian Livingstone

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.

Interactions between aeolian and fluvial systems in dryland environments

Historically, fluvial and aeolian processes in dryland environments have been viewed as mutually exclusive. However, recent research indicates that in many regions dryland aeolian and fluvial systems do not operate independently. There are interactions between the two systems that have important implications for the geomorphology of the landscape. This paper reviews the factors controlling the transfer of sediments between aeolian and fluvial systems, focusing on moisture availability, sediment supply and the magnitude/frequency characteristics of fluvial and aeolian events. We conclude by highlighting areas of future research that will contribute greatly to our understanding of aeolian-fluvial relationships in dryland areas.

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. n nThe 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.

Learning in small groups in university geography courses: designing a core module around group projects

Abstract As class sizes have increased and staff‐student ratios have worsened, group work has been seen as one way in which the quality of the learning experience for students may be maintained or improved. This paper focuses on the use of learning in small groups to undertake geography projects. We explore how the advantages for students and staff of learning through group project work may be achieved, and how the potential disadvantages can be reduced or overcome. This is done in the context of a case study of a course for final‐year undergraduate geography students which the authors taught for two years at Coventry University, UK, which was designed around a field course and two eight‐week group projects.

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.

Role-playing Planning Public Inquiries

In the UK and elsewhere, planning public inquiries are held to allow public debate when there is concern over a land development decision. Using the format of the inquiry provides geography students, usually working in teams, with the opportunity to investigate an environmental issue from a particular point of view and then to present their case as an oral presentation and a written report. Students have to take responsibility for managing the project, collecting supporting information and preparing their presentation. The format therefore allows the delivery of part of a geography curriculum as well as the development of key skills in a non-didactic setting. Experience shows that students greatly enjoy working within this non-standard format, and are highly motivated to produce good performances at the inquiry.

Using the internet

Using the Internet 2 What is the Internet? 2 How are all the computers on the Internet linked together? 2 What do I need in order to access the Internet at home? 3 What will it cost? 3 How do I install this equipment? 3 Getting started 4 What can I do on the Internet? 5 How to go directly to a known website 5 Hyperlinks 6 Searching within a website 7 Searching the Internet for information 9 ‘Navigation’ – or finding your way around a website 12 Searching within a page 13 Bookmarks and favourites 15 Using ‘tabs’ 16 Setting the browser’s ‘homepage’ 18 Interacting with features on a website 19 Notes on online security 21 Appendix – Downloading and Installing Google Chrome 22