Introduction

Air flow is a significant agent of erosion, sediment transport, and deposition over subaerial surfaces that lack a protective cover of vegetation, such as deserts, sandy beaches, floodplains exposed after floods, and outwash plains of glacial meltwater streams. The mechanics of air flow, sediment transport, erosion, and deposition have been studied in the field and in wind tunnels, and are similar to those of unidirectional water flows (e.g., Allen, 1982a; Greeley and Iverson, 1985; Pye and Tsoar, 1990; Nickling, 1994; Lancaster, 1995, 2005). To avoid repetition of material in Chapter 5, emphasis will be placed on the differences between air flows and water flows and the associated sedimentary processes.

Mechanics of air flow

Air has much lower density than water (1.3 kg m− 3 versus 1,000 kg m− 3), and much lower viscosity than water (1.78 × 10− 5 N s m− 3 versus 1.00 × 10− 3 N s m− 3). The density of air increases as temperature decreases and increases as pressure increases because air is compressible. Air speeds regularly exceed water speeds by an order of magnitude. For example, a river in flood may have a mean flow velocity of 1 m s− 1, where as a strong wind may be tens of m s− 1. This means that air flows are always turbulent, and resistance to motion is due to turbulent shear stress entirely.