Ian McEwan

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.

Turbulence structure of open channel flows over permeable and impermeable beds: a comparative study

The behavior of turbulent open channel flows over permeable surfaces is not well understood. In particular, it is not clear how the surface and the subsurface flow within the permeable bed interact and influence each other. In order to clarify this issue we carried out two sets of experiments, one involving velocity measurements in open channel flows over an impermeable bed composed of a single layer of spheres, and another one where velocities were measured over and within a permeable bed made of five such layers. Comparison of surface flow velocity statistics between the two sets of experiments confirmed that bed permeability can significantly affect flow resistance. It was also confirmed that even in the hydraulically rough regime, the friction factors for the permeable bed increase with increasing Reynolds number. Such an increase in flow resistance implies a different distribution of normal form-induced stress between the permeable and impermeable bed cases. Subsurface flow measurements performed within the permeable bed revealed that there is an intense transport of turbulent kinetic energy (TKE) occurring from the surface to the subsurface flow. We provide evidence that the transport of TKE toward the lower bed levels is driven mainly by pressure fluctuations, whereas TKE transport due to turbulent velocity fluctuations is limited to a thinner layer placed in the upper part of the bed. It was also confirmed that the turbulence imposed by the surface flow gradually dissipates while penetrating within the porous medium. Dissipation occurs faster for the small scales than for the large ones, which instead are persistent, although weak, even at the lowest bed levels

Equilibration of saltation

A two-dimensional numerical model of the saltation process was developed on a parallel computer in order to investigate the temporal behaviour of transport rate as well as its downwind distribution. Results show that the effects of unsteady flow on the transportation of particulates (sediment) have to be considered in two spatial dimensions (x, y). Transport rate Q(x, t) appears in the transport equation for mass M(x, t): where A = ΔxW denotes unit area composed of unit streamwise length Δx and width W. S(x, t) (units kg m−2 s−1) stands for the balance over the splash process. A transport equation for transport rate itself is suggested with Uc (x, t) a mean particle velocity at location x as the characteristic velocity of the grain cloud. For a steadily blowing wind over a 50 m long sediment bed it was found that downwind changes in Q cease after roughly 10–40 m, depending on the strength of the wind. The onset of stationarity (∂/∂t=0) was found to be a function of the friction velocity and location. The local equilibrium between transport rate and wind was obtained at different times for different downstream locations. Two time scales were found. One fast response (in the order of 1) to incipient wind and a longer time for equilibrium to be reached throughout the simulation length. Transport rate also has different equilibrium values at different locations. A series of numerical experiments was conducted to determine a propagation speed of the grain cloud. It was found that this velocity relates linearly to friction velocity. Copyright

Sediment transport over a flat bed in a unidirectional flow: simulations and validation

A discrete particle model is described which simulates bedload transport over a flat bed of a unimodal mixed–sized distribution of particles. Simple physical rules are applied to large numbers of discrete sediment grains moving within a unidirectional flow. The modelling assumptions and main algorithms of the bedload transport model are presented and discussed. Sediment particles are represented by smooth spheres, which move under the drag forces of a simulated fluid flow. Bedload mass–transport rates calculated by the model exhibit a low sensitivity to chosen model parameters. Comparisons of the calculated mass–transport rates with well–established empirical relationships are good, strongly suggesting that the discrete particle model has captured the essential elements of the system physics. This performance provides strong justification for future interrogation of the model to investigate details of the small–scale constituent processes which have hitherto been outside the reach of previous experimental and modelling investigations.

Selective bedload transport during the degradation of a well sorted graded sediment bed

The paper presents an analysis of the composition of bedload transport and changes to bed structure and topography during three graded sediment degradation experiments. The analysis suggests that variations in channel hydraulics and active layer composition alone may not explain the observed reductions in sediment transport. Further, the experiments appear to cover a crucial range of mean bed shear stresses for armouring studies, ranging between a condition of passive winnowing, to one of more active armour development in which the coarse grains play a role in determining bed structure. This indicates that the active layer concept, commonly applied in computer models of graded sediment transport, may be limited in its application.

Diffusion of saltating particles in unidirectional water flow over a rough granular bed

We show that the motion of saltating particles on a flat rough bed in unidirectional water flow is diffusive and comprises three ranges (local, intermediate, and global) of spatial and temporal scales with different scaling behaviour and diffusion properties. Our computer simulations suggest that the ratio of the travelling particle diameter to the prevailing diameter of static bed particles (or the height of bed roughness) is one of the key parameters controlling particle diffusion.

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.

THE PREDICTION OF DUST EROSION BY WIND: AN INTERACTIVE MODEL

We have devised a partial differential equation for the prediction of dust concentration in a thin layer near the ground. In this equation, erosion (detachment), transport, deposition and source are parameterised in terms of known quantities. The interaction between a wind prediction model in the boundary layer and this equation affects the evolution of the dust concentration at the top of the surface layer. Numerical integrations are carried out for various values of source strength, ambient wind and particle size. Comparison with available data shows that the results appear very reasonable and that the model should be subjected to further development and testing.