Spatial and Temporal Development of Saltation in Air.

Abstract

A resurgence of interest in the concept of equilibrium in the aeolian saltation system has been witnessed in the 1990's. Throughout the aeolian field of research, i. e. wind tunnel, field and numerical models, many highly successful individual investigations have been conducted. Despite these data, however, the timing and location of the mass flux equilibrium have not been quantified. This research investigates the simultaneous downwind spatial and temporal developments of the aeolian saltation system. Experiments were conducted in the laboratory and the field. By unification of the spatial and temporal dimensions in both environments one of the major limitations of contemporary aeolian science, the inability to relate data from different experimental environments, is addressed. In the wind tunnel the development of the saltation system was measured over a streamwise length of 8m. Sediment transport was measured at 1m intervals by the downwind deployment of seven Aarhus sand traps. In the field the development of the saltation system was monitored over distances of 10m and 20m. Mass flux was measured by the downwind deployment of five 'total load' sand traps. In both environments temporal wind velocity and mass flux data were collected simultaneously at a single site. Spatial profile velocity data were later obtained by a streamwise traverse along the experimental area. The downwind spatial development of the saltation system, from a point of initiation, in the laboratory and the field is manifest by an overshoot in mass flux and shear velocity. It is shown that in both environments mass flux increases with distance to a maximum at 4m downwind. This result is in remarkable agreement with existing data of a comparable scale. In the wind tunnel and the field experiments it V' is found that shear velocity overshoots between 2-4m downwind of the overshoot in mass flux. The distance between the overshoot in mass flux and the overshoot in shear velocity is termed the 'separation distance'. The existence of a 'separation distance' between the overshoots of mass flux and shear velocity questions the appropriateness of traditional mass flux formulae. It is found that conventional mass flux relationships with shear velocity, generated from data collected simultaneously at the same site, have the lowest predictive capability. The greatest confidence in the ability of shear velocity to predict the rate of mass flux is shown to occur when shear velocity data are collected downwind of mass flux data. The critical distance between the data collection points is demonstrated to be defined by the 'separation distance'. The downwind spatial development of the saltation system without a point of initiation in the laboratory and the field is influenced by sand entering from upwind. The existence of high energy bombardment by saltation processes throughout the experimental area is shown to produce an accelerated development of the saltation system. It is found that the precise downwind development of mass flux and shear velocity are dependent on the exact rate of sand entering from upwind. The temporal development of the saltation system is controlled essentially by the availability of transportable grains from the sand bed. In both the wind tunnel and the field experiments it is demonstrated that the saltation system develops through time from a transport-limited to a supply-limited system. The depletion of the sand bed through time limits the existence of the state of equilibrium. The equilibrium concept is thus shown to be inappropriate for the universal prediction of mass flux.