Development of the saltation system under controlled environmental conditions

摘要

The transport of sand by the wind occurs predominantly by the process of saltation. Following the entrainment of sand by an above threshold wind, the saltation system is regulated by the mutual interaction of the atmospheric boundary-layer, the sand cloud and the sand bed. Despite existing data on the spatial and temporal development of the sand transport system, very little is known about the development of the saltation system towards equilibrium. Results are presented from wind-tunnel experiments that were designed to address the simultaneous spatial and temporal development of the saltation system, with and without artificial sand feed. The development of the saltation system was monitored over a streamwise length of 8 m during a period of 3600 s. Mass flux data were measured simultaneously at 1 m intervals by the downwind deployment of seven Aarhus sand traps. Wind velocity data were collected throughout the experiments. The downwind spatial development of the saltation system is manifested by an overshoot in mass flux and friction velocity prior to declining towards a quasi-equilibrium. Mass flux overshoots at approximately 4 m downwind, in remarkable agreement with existing data of a comparable scale. Friction velocity overshoots at approximately 6 m downwind, a result not previously witness in saltation studies. The overshoot of mass flux prior to the overshoot in friction velocity is a spatial manifestation of the time lag between the entrainment of grains and the deceleration of the wind by the grains in transport. Temporally, the development of the saltation system is controlled by the availability of entrainable grains from the sand bed. Through time the saltation system develops from a transport-limited to a supply-limited system. The depletion of the sand bed through time limits the appropriateness of the assumption of 'equilibrium' for the universal prediction of mass flux.