Non-uniform Aeolian transport over a patch of large, virtually immobile grains

An array of fixed spheres buried in a sand bed in a wind tunnel was observed to be gradually exposed as the sand particles became mobile due to the action of wind of sufficient strength. The rate of exposure was found to be a function of the distance from the leading edge of the sand bed. The eroded sand bed stabilised as the exposed spheres provided progressively more protection to the sand. The time required to attain stabilisation was also found to be a function of the distance from the leading edge. A numerical model of the process was developed and showed that the boundary-layer shear stress was gradually transferred from the saltating grains to the bluff-body resistance of the emergent spheres, which in turn shielded the sand surface from the abrasion caused by inter-saltation collisions. The model successfully portrays many of the features of the process that have been observed, in particular the exposure of spheres at rates dependent on distance from the leading edge and the progressive stabilisation of the sand bed advancing downwind from the leading edge. However, the model was observed to attain stability too rapidly.The characteristics of this non-uniform behaviour are of practical importance because gravel or pebble layers are an economical means of controlling wind erosion in some circumstances