Wind-Tilted Pebbles in Sand - Some Field Observations and Simple Experiments

Pebbles on a wind-exposed sand surface have a tendency to tilt forward in the direction against the wind. This paper describes field observations and basic experiments aimed at increasing the knowledge of wind-tilting processes. The tilting velocity was related among other factors to wind velocity, mass balance conditions, and the geometry and weight of the obstacles. In addition basic studies on the flow pattern and micro-morphology around the objects were performed. Two main types of tilting could be distinguished: cavity tilting and deflational tilting. The former type is characteristic of obstacles with a high front edge; the latter is typical of flat obstacles. By the cavity tilting a hollow is excavated in front of the obstacle. In due course the obstacle tilts down into this depression. By the deflational tilting the surrounding sand surface is subsequently lowered and the obstacle will rest on a »sand pillar«, which is successively weakened on the windward side. Consequently the object is turned over. A basic model of the wind-tilting processes in dry sand is found at the end of the paper.