Brian B. Willetts

A review of recent progress in our understanding of aeolian sediment transport

We review recent progress in our understanding of aeolian sediment transport, with emphasis on work published since 1985. The current conceptual model of sediment transport is discussed at length, with attention given to problems of definition that have arisen. We discuss in depth the collision (grain impact) and aerodynamic entrainment (initial motion) processes. The effect of the evolving population of moving grains on the wind (the wind feedback mechanism) is treated in the context of recent modelling of the self-regulating saltation process. The link between saltation and suspension is discussed briefly. We conclude by outlining future research directions that must involve a greater symbiosis of experimentalists and theoreticians, working both at the grain and the bedform scales.

Adaptation of the near-surface wind to the development of sand transport

A model of wind-blown sand transport is described with particular emphasis on the feedback between the grain cloud and the near-surface wind. The results from this model are used to develop Owens (1964) hypothesis that ‘the grain layer behaves, so far as the flow outside it is concerned, as increased aerodynamic roughness whose height is proportional to the thickness of the layer’. The hypothesis is developed to show the influence this dynamic roughness has on the turbulent boundary layer above the saltation layer. Two processes are identified which influence the path of the system towards equilibrium. The first is the feedback between the near-surface wind and the grain cloud in which the quantity of sand transported is limited by the carrying capacity of the wind. The second is due to the temporal development of an internal boundary layer in response to the additional roughness imposed on the flow above the grain layer by the grain cloud. A similarity is noted between the temporal response of a turbulent boundary layer to sand transport and the spatial response of a turbulent boundary layer downstream of a step increase in surface roughness. Finally it is noted that the work may have important implications for transport rate prediction in unsteady winds.

WIND EROSION OF CRUSTED SOIL SEDIMENTS

Saltating particles increase the rate of dust release from sediments in arid and semi-arid areas. They also break interparticle bonds in aggregated and crusted soils, thereby increasing the number of particles available for entrainment. This pilot study examines rates of erosion in relation to the flux of saltating grains for three crusted sediments of different strengths. Dislodgement of surface particles decreases with increasing crust strength, as measured by a cylindrical flat-ended penetrometer. In addition, initial dust release from craters formed by single impactors in unaggregated soil is examined in relation to the associated saltator. The volume of material removed depends linearly on the kinetic energy of the abraders.

Sand transport by wind: a review of the current conceptual model

Abstract Rapid progress has been made in the last decade towards a more comprehensive model of wind blown sand transport extending and sharpening Bagnold’s classic model in significant aspects. This paper reviews the current physical model and attempts to indicate future directions of research. The currently accepted physical model of aeolian sand transport reduces the sand transport system to four distinct sub-processes: aerodynamic entrainment; the trajectory of the wind driven sand grains; the grain/bed collision; and the modification of the wind by the driven sand. The isolation and separate treatment of these sub-processes has been an important factor in the recent rapid development of aeolian sand transport mechanics. It is, however, their interaction that produces the rich behaviour of the system. Anderson & Haff (1991) and McEwan & Willetts (1991) have synthesized the four sub-processes and constructed full saltation models which follow the system from incipience to steady-state saltation. These computer simulations provide a stern test for the physical model as the results calculated can be compared to experimental observations from field and wind tunnel. The models compare well with available data; thus we can have some confidence that the physical model is realistic. Moreover, these computer simulations have become a powerful investigative tool and have highlighted areas where our understanding is deficient.

Transient sand transport rates after wind tunnel start‐up

Wind tunnel experiments were conducted with a well mixed, flat sand bed, 5·7 m in length, to study the initial sand flux response at three different shear velocities. In some experiments, the bed was allowed to deplete without replenishment; in others, sand was fed 10·8 m upstream of the monitored cross-section. The results indicated that the transport rate increases rapidly during the first minute, and then adjusts slowly towards a steady rate. The time to reach such an equilibrium was observed to be on the order of 2–4 min in non-fed experiments and on the order of 8–9 min in fed experiments. Many factors may affect such development and bring about non-stationarity in total sand transport rate. Among these factors are differences in the natural composition of the sand bed, changes in both the topographical features of the sand bed (ripples) and its surface texture, and any artificial features that influence the adjustment between the boundary layer profile and the sand load on the wind. A useful key to the influence of each factor is obtained by noting that each has a typical and distinct ‘time constant’. The nature and relative importance of each is discussed by reference to the reported wind tunnel experiments and to the behaviour of saltation cloud numerical models.

USING FENCES TO CREATE AND STABILISE SAND DUNES

The equations describing conservation of mass, momentum and energy in a turbulent free surface flow are derived for a controle volume extending over the whole depth. The effect of the turbulent surface oscillations are discussed but neglected in the following analysis, where the equations are applied to the energy balance in a surf zone wave motion. This leads to results for the wave height variation and the velocity of propagation. The results cannot be reconciled completely with measurements and the concluding discussion is aimed at revealing how the model can be improved.A three-dimensional morphodynamic model of sequential beach changes Is presented. The model Is based on variations in breaker wave power generating a predictable sequence of beach conditions. The spectrum of beach conditions from fully eroded-dissipatlve to fully accreted reflective is characterised by ten beach-stages. Using the breaker wave power to beach-stage relationship the model Is applied to explain temporal, spatial and global variations In beach morphodynamlcs.The agents of initial damage to the dunes are water, which undermines them, and animals (including man) which damage the protective vegetation by grazing or trampling. Of these, man has recently assumed predominant local importance because of the popularity of sea-side holidays and of the land-falls of certain marine engineering works such as oil and gas pipelines and sewage outfalls. The need is therefore increasing for active dune management programmes to ensure that under these accentuated pressures, the coast retain an equilibrium comparable with that delicately balanced equilibrium which obtains naturally at a particular location.