Wind-Driven Sand in Coachella Valley, California

Abstract

An 11-year study provides data on saltating sand driven across a barren, bouldery alluvial plain by strong unidirectional wind. By weight, 50 per cent of the grains travel within 5 inches, and 90 per cent, within 25 inches of the ground. Maximum height attained exceeds 10 feet, even for grains of 3–4 mm. Weight of each grain-size fraction decreases with height, except for particles smaller than 0.062 mm which are presumably in suspension. The weight percentage of grains smaller than 0.125 mm increases with height, that of grains between 0.25 and 0.125 mm decreases with height, and that of still larger grains peaks out at heights increasing with size. This suggests that grains smaller than 0.125 mm are affected by turbulence, and that the rebound height of particles larger than 0.25 mm varies directly with grain size. Grains between 0.125 and 0.25 mm are too large to be significantly affected by turbulence and too small for high rebound. Arithmetic mean grain size within the saltating curtain attains its maximum at heights between 10 and 40 inches, the higher values occurring with stronger winds. Maximum wear of 0.038 inch in 10 years on a vertical lucite rod occurred 9 inches above the ground. This represents the height at which grain size, number, and velocity combine to give maximum impact energy. Common red bricks were worn 1.6 inches in 6 years and 2.25 inches in 11 years. Cubes of commercial gypsum cement (hydrocal) showed maximum wear of 1.9 inches in 10 years. Sound gneissic boulders displayed no measurable wear in 10.5 years. Cubic hydrocal blocks became tilted upwind by ground scour, and one 6-inch cube turned over in 6 years. Smaller cubes similarly turned over and in addition fell sidewise several times in 11 years. Rotation up to 90° on a near-vertical axis accompanied some overturns. Movement of blocks was principally southward, indicating that overturns were due to unusually strong winds from the north. Wind scour is a major mechanism for the shifting of ventifacts in this environment.