Aeolian Geomorphology

Binghamton Geomorphology Symposium 17

William G. Nickling

• Publisher: Routledge
• Year: 2020
• Print ISBN: 9780367210540
• eBook ISBN: 9780429560170
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Geography
• Index: Geography

This book, first published in 1986, stems from the 1986 Binghamton Geomorphology Symposium. The topic was chosen because of the advances in the study of aeolian processes and landforms, particularly in the area of desertification, and the papers collected here clearly indicate that their study is not constrained by discipline boundaries but are of interest to geologists, physical geographers, soil scientists, meteorologists and engineers.

1

Collision in aeolian transport: the saltation/creep link

Brian B. Willetts and M. Ann Rice

Abstract

Collision, it is contended, is the mechanism which is necessary to investigate in order to better understand the launch conditions for saltation, creep transport and ripple formation, and surface re-sorting.

Observations of landing and take-off velocities at individual collisions are incorporated in approximate calculations of momentum change for three size fractions of each of two sands. One sand is a quartz sand with compact grain shape, and the other, a shell sand with more platy grains, particularly in larger fractions. Calculations are made for horizontal bed and for collision with a bed having a local adverse slope.

Grain shape is found to have a marked effect on collision. The shell sand grains lose rather less forward momentum on average than do quartz grains, and undergo a much smaller change of vertical momentum. It is inferred that collision is much less effective in sustaining a saltating load in the case of shell sand than in that of quartz sand.

Adverse bed slope reduces the loss of forward momentum at collision, but increases more dramatically the change of vertical momentum. Therefore, as one moves up a ripple back towards the crest, creep activity increases, but the vigour of launch of new saltations diminishes. These saltations will land downwind with uneven spatial distribution. The high rate of creep activity near the crest will produce a coarser than average grain population there.

Introduction

It is well known that grains of wind-blown sand move by three processes called, respectively, suspension, saltation, and creep. Saltation is believed to be particularly important. Grains in saltation receive momentum from the wind and deliver it to the bed at a rapid rate. This keeps the bed surface in a disturbed state, prompting further saltation and stimulating both creep and the release of fine material into suspension. Measurement of the transport rate of suspended material is virtually impossible. Of the remaining mobile material, roughly three-quarters of the transport rate is by saltation and roughly one quarter by creep (Bagnold 1941, Willetts & Rice in press a).

Its acknowledged importance has led to several studies of saltation. Films and multiple exposure still photographs have been taken which enabled detailed measurements to be made on the higher portion of the trajectory (White & Schultz 1977, Hunt & Nalpanis in press), and produced data from which velocity and acceleration have been computed. Several theoretical models of the saltation trajectory have been published (e.g. White & Schultz 1977, Hunt & Nalpanis in press, Jensen & Sorensen 1983). There are two significant uncertainties in using such models. One concerns the initial conditions for the trajectory (velocity and angle of launch), and the other concerns the rate and effect of particle spin. Both of these features are closely related to the outcome of the collision which usually precedes the saltation.

Creep has been studied less than saltation. It takes place at the base of a layer of moving sand which becomes deep and populous when transport is active. In these circumstances it is difficult to make detailed observations. Creep transport rates can be obtained by using traps designed to exclude most saltating grains, and grain velocities estimated by using such traps in conjunction with dyed grains originating at a known distance upwind (Willetts & Rice in press a). However, the amount of quantitative information available about creep is very small. Yet a large proportion of the moving grains in unit bed area are moving in creep, and the process must, therefore, be influential in ripple formation, in the re-sorting of the surface population, and in the release of dust. It is generally accepted that creep motions are instigated by collision, as saltating grains return to the bed.

Collision then, is at the root both of the unknown features of saltation, and of the important, but obscure process of creep. Nevertheless, there have been a limited number of studies of collision. Tsuchiya and Kawata (1972) incorporated intelligent estimates of the outcome of collision in a calculation of repeated saltation. Rumpel (1985) performed a simplified analysis of the mechanics of idealized collisions in order to generate data by means of which he could demonstrate the construction and use of probability distribution matrices for the outcome of saltation sequences. Willetts & Rice (in press b) made direct observations of individual collisions and produced sets (of restricted size) of measured data which can be used in the way in which Rumpel used his artificial data.

It is the purpose of this paper to explore the links between saltation, collision, and creep. Data derived from direct observation of collision will be used, ...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface
• Acknowledgements
• 1 Collision in aeolian transport: the saltation/creep link
• 2 Small scale wind tunnel modelling of particle transport - Froude number effect
• 3 Mass momentum and kinetic energy fluxes of saltating particles
• 4 Particle transport by atmospheric winds on Venus: an experimental wind tunnel study
• 5 The role of vegetation in the formation of linear sand dunes
• 6 Geomorphological significance of wind flow patterns over a Namib linear dune
• 7 The Toshka-Canal dune: analysis of development and dynamics
• 8 Aeolian processes and dune characteristics of a developed shoreline: Westhampton Beach, New York
• 9 Quaternary dunes of the Pacific Coast of the Californias
• 10 Origions of low-angle stratification in aeolian deposits
• 11 Aeolian landforms: laboratory simulations and field studies
• 12 Aeolian processes and landforms in glaciofluvial environments of southeastern Baffin Island, N.W.T., Canada
• 13 The frequency and source areas of dust storms
• 14 Effect of antecedent moisture conditions on dust storm generation in Arizona
• 15 Maneuver-caused wind erosion impacts, South Central New Mexico
• 16 Rock varnish as an indicator of aeolian environmental change
• Index