Blowing sand and surface winds in the Pisco to Chala Area, Southern Peru

Abstract

The coastal belt of southern Peru is noted for its extensive and varied sand dunes. A major part of the sand occurs in a 300 km long strip starting 200 km south of Lima between the towns of Pisco on the north and Chala on the south. The sand is picked up from the Pacific Ocean beaches by the nearly invariant south–southeast winds and moved inland to its resting place in huge sand masses (“sand seas” or “ergs,” depending on the author), generally at higher elevations. The writer mapped the moving sands and accumulated sand masses in this interesting region in 1959–1961 with airphotos, work which is partly unpublished and largely unknown (Plate 1). Eight sand masses were mapped; a ninth (Cerro Blanco) occurs just outside the area of airphoto coverage. If an average sand depth of only 10 m is assumed for the largest sand mass at Ica in the northern part of the area, that mass contains nearly 12 billion tons of sand and covers 900 km 2. Probably the average sand depth here is greater. One of the unique features of the present study was the mapping of the direction of sand movement (small arrows in Plate 1) which indicates the direction of prevailing surface winds. Offshore, these winds have a nearly constant south–southeast direction. Onshore, the winds blow in great clockwise-sweeping arcs due to frictional drag and other forces as the wind moves diagonally from the ocean over the uneven ground surface. However, there is some modification of the wind direction by topography, one good example being the obvious partitioning of the wind around both sides of Cerro Huaricangana (elev.=1725 m) in the center of the area (Plate 1b). But the most interesting wind pattern occurs in an area of subdued relief and low elevation near Ica (elev. approx. 400 m), where the sands converge on the Ica sand mass from nearly opposite directions, that is, from the northwest, west, southwest, and southeast. The wide angle sweep (140°) of incoming sands here is believed to be unique in the world. A Landsat image was used to determine how this unusual wind pattern could develop (see Fig. 9), but the extreme local rotation of the winds is not fully understood. One idea proposed here by the author is that the surface/near-surface winds move as horizontal vortices, not in streamlines as is generally assumed, and it is the interaction of the vortices and the land surface that causes the pervasive clockwise swing in wind direction.