Introduction

Unidirectional, turbulent water flow is probably the most important agent of erosion, sediment transport, and deposition on land. Together with sediment gravity flows, unidirectional water flows are the prime sculptors of the land surface, with air and ice flows of secondary importance. Water may flow in channels or as sheet-like flows over floodplains and hillslopes. However, unidirectional water flows also occur in the sea and lakes. This chapter deals with principles of unidirectional water flow, sediment transport and bed forms, erosion, and deposition. These principles are also relevant to unidirectional air flows and sediment gravity flows, and even periodically reversing water flows, as discussed in subsequent chapters. The extensive published literature on this subject is spread over the disciplines of Earth science, engineering, and physics. Useful reviews of various aspects of this subject are given in Allen (1982a, 1985), Best (1993, 1996), Bridge (2003), Leeder (1999), Middleton and Wilcock (1994), Nezu and Nakagawa (1993), Van Rijn (1990, 1993), and Yalin (1992).

Fundamentals of unidirectional water flow

Definition of physical properties of water and its motion

The physical properties of water that are of most concern here are density and viscosity, both of which depend on temperature. The flow of water is normally described in terms of depth, width, cross-sectional area, velocity, discharge, bed shear stress, Reynolds number, and Froude number. Methods of measurement of fluid flows are discussed in the appendix.