Flow in Open Channels

In an open channel flow, only part of the current is at contact with walls and a free surface is present, usually at atmospheric pressure. In most cases, the curvature of the trajectories is negligible and a hydrostatic pressure distribution is achieved. Gradient pressure in the direction of flow is rather limited and the flow is driven by differences in the elevation head. The source of energy per unit of weight and per unit of length is the slope of the bed, that exactly balances the energy losses in a uniform regime. In other situations, the source of energy is the slope of the free surface, which can drive flow also in the presence of horizontal or counter-sloping bed. The classical terminology defines the area of the cross-section of the current; the wetted perimeter, i.e. the length of the contact between the liquid and the walls of the channel; the hydraulic radius, equal to the ratio between the two. The kinematics of the currents is described in terms of Froude number, with distinction of the subcritical and supercritical flows also referred to as tranquil and shooting flow, respectively. If the uniform flow is tranquil, the bed slope is defined mild, otherwise it is steep. In critical conditions the current requires the minimum energy to flow and the critical depth corresponds to a stationary point of energy referred to the local bottom of the channel.