Aeolian Ripple Migration and Associated Creep Transport Rates

Sherman Sherman,Martin Martin,Li Li,Ellis Ellis,Kok Kok,Farrell Farrell,Zhang Zhang

doi:10.3390/geosciences9090389

Abstract

Wind-formed ripples are distinctive features of many sandy aeolian environments, and their development and migration are basic responses to sand transport via saltation. Using data from the literature and from original field experiments, we presented empirical models linking dimensionless migration rates, u r / g d ( u r is the ripple migration speed, g is the gravity acceleration, and d is the grain diameter) with dimensionless shear velocity, u*/u*t (u* is shear velocity and u*t is fluid threshold shear velocity). Data from previous studies provided 34 usable cases from four wind tunnel experiments and 93 cases from two field experiments. Original data comprising 68 cases were obtained from sites in Ceará, Brazil (26) and California, USA (42), using combinations of sonic anemometry, sand traps, photogrammetry, and laser distance sensors and particle counters. The results supported earlier findings of distinctively different relationships between u r / g d and u*/u*t for wind tunnel and field data. With our data, we could also estimate the contribution of creep transport associated with ripple migration to total transport rates. We calculated ripple-creep transport for 1 ≤ u*/u*t ≤ 2.5 and found that this accounted for about 3.6% (standard deviation = 2.3%) of total transport.

Highlights

Ripples are near-ubiquitous features of sandy aeolian environments on Earth and elsewhere
We found that wind tunnel data indicated ripple migration rates that were about two to three times faster than field equivalents
There is a linear relationship between shear velocity and ripple migration rate

Summary

Introduction

Ripples are near-ubiquitous features of sandy aeolian environments on Earth and elsewhere. The regularity of their morphology and the complex patterns that they form make them objects of visual and scientific interest. The movement of a ripple form represents a mode of sand transport that occurs at a rate proportional to the speed and height of the bed form, and contributes to the portion of sand moving as creep/reptation rather than saltation. Creep is normally referred to as the movement of grains in continuous or near-continuous contact with a sand bed [1] and reptation is the movement of low-energy hopping particles with no ability to rebound or to eject other grains [2]. We defined creep to be synonymous with reptation, as the terms are often used interchangeably [2,3,4,5,6]

Results

Discussion

Conclusion