Etude du rôle des obstacles topographiques dans la circulation éolienne d'après les images satellites et les photographies aériennes, de l'échelle continentale à celle de la butte-témoin

Abstract

The meteorological satellite imagery NOAA and Météosat reveals synoptic scale wind systems. Three saharian models have been analysed showing the long distance eolian sand transport especialy by Harmattan. This transport is submitted often over more than two million kilometers to the effect of the major relief of Sahara : Eglab, Tassili-N'Ajjer - Hoggar and Tibesti. In the wind direction, in the three analysed systems, the same succession of areas with different dynamical behaviours can be observed : 1° -upwind of the topographical obstacles, positive sand budget sandseas ; 2° - in the area of subdivision of currents by the obstacle, negative sand budget sandseas or areas with dominant corrasion ; ; 3 - leeward of the relief, a shadow area without sand deposit or negative flow ; 4 - farther away from the obstacle, where the wind currents are meeting, positive sans budget sandseas. The three studied models are interesting because of the continental scale of the obstacle effects. At smaller dimensions, regional or local scales, the same schemes are produced with the same successions, the variety of which results from different paramètres : forms, orientations, places of the obstacles in the eolian system, and value of its upwind slopes. This last paramètre is the most relevant.