ABSTRACT
This chapter presents one concept pertaining to the notions of the bed shear stress and bed shear velocity which are a result of physical principles, namely the momentum conservation law. The shear stress may be described in simple terms as a force acting on a surface tangential to the force and expresses flow resistance, that is, friction forces. Shear stresses in water are a consequence of water viscosity and flow turbulence and are generated by the transfer of momentum within the water volume. Shear stresses occur within the whole volume of flowing water; however, a particular interest is given to the bed shear stress that represents the friction at the water-bed interface. The explanation of the formation of shear stresses in the water flow in open channels is provided by the boundary layer theory. According to theory, a region could be identified near the channel boundary in which the flow is influenced by the friction exerted by the water-bottom interface.
