The Barchans of Southern Peru

Abstract

The crescentic sand-dune formation known as the "barchan" occurs extensively in the desert coast of southern Peru. Quantitative measurements were made on 75 of these to determine relationships between height, width, and length of horns. A consistently longer west horn was measured in most of the barchans, but this distortion cannot be readily explained. Mechanical analyses of the dune sand were performed on samples taken from the Pampa de Clemesi and the principal barchan deposits in the Pampa de la Joya, as well as from deposits further downwind from La Joya. There is a consistent increase in the proportion of the fine material in the samples taken farther from the Pampa de Clemesi. The bulk specific gravity also decreases with distance from Clemesi. Bulk specific gravity varied with dune height. Mineralogical analyses of these same samples indicate a decrease in the heavier minerals with increasing distance from the Pampa de Clemesi, and an increase in the volcanic, fibrous glass. This evidence indicated that the La Joya barchans originated from the Pampa de Clemesi and extend over a strip 100 km. downwind, unobstructed by the transversal river channels, which are from 600 to 800 meters deep. The forward movement of the barchans was measured by superimposing two aerial photographs made in 1955 and 1958, adjusted to equal scale by means of ground control. For 45 barchans whose heights were measured in the field, the horizontal displacement in three years was found to be as follows: $$1/D = 0.00726+ 0.0037H.$$ Further measurements were made on 75 other barchans, whose heights were derived from measurements of the aerial photographs, and a similar relationship was discovered. The relationship of dune height to annual forward motion is summarized as follows: Average Crest height (meters) 1.0 2.0 3.0 4.0 5.0 6.0 7.0 3.67 Displacement (meters).... 32.3 22.0 16.8 14.5 12.2 11.5 9.2 15.4 These data are strikingly in agreement with the predicted motion based upon Bagnold's theory and daily wind data recorded at the nearby Vitor Air Base. The mass distribution of over 1,000 dunes was studied statistically, and certain significant mathematical relationships were derived for both the longitudinal and transverse spread of the barchans across a flat plain.