Introduction

The ‘benthic’ boundary layer (bbl) is the layer overlying the deep ocean seabed within which the mean speed of the overlying water is reduced from its ambient value distant from the boundary to zero at the seabed (or nearly to zero relative to the solid Earth if the seabed is covered by mobile and moving sediment particles). In shallow water the layer, discussed in Chapter 10, is usually referred to as the bottom boundary layer, but many (not all) of its properties are similar, if not identical, to those of the bbl. Turbulence in the bbl is sustained by shear resulting from the stress exerted on the seabed by the water flowing over it, and by flow separation over bottom roughness. The stress can equally be thought of as that exerted by the seabed on the water in reducing its speed to zero.

Because of its importance in the understanding of flow over aircraft wings and of the variation in wind speed and gustiness near the ground, the nature of turbulence near rigid boundaries has been the subject of intensive investigation since the early years of the twentieth century, through both laboratory experiments and measurements in the atmospheric boundary layer over land, supplemented in more recent years by numerical simulations. Much of what is know about the structure of turbulence in such boundary layers comes from these extensive and comprehensive studies.