Wind Ripples

Abstract

Two types of wind ripples are distinguished; sand ripples composed of normal wind-blown sand with a median diameter roughly between 0.30 and 0.35 mm., and granule ripples composed in part of particles approaching granule size 2-4 mm. The planimetric patterns and facing directions of ripples on dunes indicate antecedent wind conditions and the microcomplexity of wind currents moving over a topographically varied ground surface under a unidirectional wind aloft. Misinterpretation of non-contemporaneous ripples has supported the fallacious concept of a lee-side eddy on sand dunes. Impregnation of sand ripples permits study of internal relationships. Sand ripples are asymmetrical piles of relatively coarse, homogeneous, essentially structureless sand resting on a smooth platform of much finer, thinly bedded sand. Internal foreset layering is rare. The underlying fine beds may represent material sifted out of the ripples as they travel across the surface. It is suggested that most of the grains composing a sand ripple are traveling by surface creep through saltation impact and only a limited number are experiencing exchange with the saltation curtain. Granting this point, it is argued that size of creeping grains and wind velocity can control the wave length, height, and index (wave length/height) of sand ripples. Thus, it is possible to account for ripple dimensions without recourse to Bagnold's concept of characteristic grain path. Ripple index varies inversely with grain size and directly with wind velocity. By contrast, the degree of asymmetry of individual sand ripples varies directly with grain size and inversely with velocity. Simple experiments with liquid smoke suggest that a flat eddy lies on the lee side of a sand ripple under gentle wind velocities of 2 or 3 mph, but under velocities strong enough to move sand no evidence of such an eddy could be seen. Repeated observations of sand-grain movement on active ripples in the field provide no evidence that a lee-side eddy influences grain movements or the shape, location, and development of sand ripples. The rate of movement of individual sand ripples was measured in the field at 0.35-3.2 inches per minute under wind velocities ranging between 16 and 40 mph. Over this velocity range the relationships are approximately linear. Ripples that move 1 wave length in 1 minute can obviously adjust rapidly to changes in wind regime. Granule ripples are much larger than sand ripples, attaining wave lengths up to 10 feet and heights up to 11 inches in the study areas. They form where lag concentrates of larger particles are developed by deflation under strong winds. Surface creep by saltation impact is clearly the transporting mechanism for these larger grains. Owing to a longevity of months or years and heterogeneous internal constitution, including considerable normal wind-blown sand, granule ripples have a prominent internal foreset structure. A mean index of about 15 seems representative for granule ripples compared to roughly 18 for sand ripples, but departures from these means are large. Relationships observed on granule ripples support conclusions drawn from sand ripples to the effect that wind ripples are composed primarily of creeping grains and that grain size exerts great influence on their dimensions and shape.