Introduction

In this chapter we present an introduction to dense granular flows and their stability by discussing two classes of phenomena: avalanches on an inclined plane, and particle transport on an erodible bed sheared by a fluid flow. These granular flows lead to the appearance of surface waves, called ripples or dunes depending on whether their wavelength is of a few centimeters or a few meters (the relevance of this common distinction will be discussed later on). Owing to the difficulty – both experimental and theoretical – of studying granular media, the mechanisms responsible for these waves remain poorly understood, and so the results presented in this chapter are definitely less well established than those in the preceding chapters.

Avalanches, ripples, and dunes present serious problems for human activities. Among natural phenomena, snow and mud avalanches are well known for their destructive nature; the displacement of a sand dune by the wind – the so-called aeolian dunes – while less dramatic, can cut communication links and threaten habitation and industrial installations. Subaqueous dunes perturb navigation in rivers and shallow seas such as the North Sea, while on river bottoms such dunes increase friction and raise the water level, thereby contributing to flooding. Granular flows are also omnipresent in industry: flow and transport of coal, construction materials (cement, sand, gravel), agricultural foodstuffs, pharmaceutical materials, and sand from oilfields are some examples. Instabilities occur in the conduits used to transport these materials, giving rise to dunes which perturb the flow and may form obstructions, causing serious damage to operating equipment.