ABSTRACT
Rivers often experience changes in bed elevation, which can be caused by the presence of hydraulic structures, other forms of human interference within a stream and/or its watershed, or occur as part of natural geomorphological processes. Furthermore, human-induced changes tend to occur at an accelerated pace compared to natural processes. Whatever the cause, there are three generally accepted forms of riverbed elevation change: (1) aggradation or degradation, (2) general scour, and (3) local scour. Aggradation and degradation are long-term processes that take place over long stream reaches. The bed topography evolution associated with these processes is often related to changes in sediment load (e.g., when a braided river changes its planform geometry into a meandering river or downstream of a dam, where the outlet flow contains no sediment load). General or contraction scour is often caused by alteration of the flow patterns and consequently the bed shear stress within a short reach. For example, this type of scour can occur during the passage of a flood when the bed scours on the rising limb and fills during the falling limb. Local scour, on the other hand, is much more complicated than the other two processes leading to bed elevation change. In this case, the removal and transport of bed material occurs in the vicinity of hydraulic structures due to the action of unsteady energetic coherent vortices, which are induced as the approach flow in the reach encounters the hydraulic structure and wraps around it. The classical example of this type of
scour is the scour hole that develops around a bridge pier in a river reach due to the so-called horseshoe vortex (see Figure 6.1). Often the effects of general and local scour cannot be readily separated as, e.g., in the case of a rapid flooding event passing through a river reach with hydraulic structures. The process that leads to changes in bed elevation in such cases is the result of the combined action of both scouring processes and will be collectively referred to as scour.
