OPEN CHANNEL FLOW
In this chapter, the fundamentals of rigid boundary open channel flow are described. In open
channel flow, the water surface is not confined; surface configuration, flow pattern and pressure
distribution within the flow depend on gravity. In rigid boundary open channel flow, no
deformation of the bed and banks is considered. Mobile boundary hydraulics refers to flow which
can generate deformation of the boundary through scour and fill. Mobile boundary hydraulics will
be discussed in later chapters. In this chapter, the discussion is restricted to a one-dimensional
analysis of rigid boundary open channel flow where velocity and acceleration are large only in
one direction and are so small as to be negligible in all other directions.
Open channel flow can be classified as: (1) uniform or nonuniform flow; (2) steady or unsteady
flow; (3) laminar or turbulent flow; and (4) tranquil or rapid flow. In uniform flow, the depth and
discharge remain constant with respect to space. Also, the velocity at a given depth does not
change. In steady flow, no change occurs with respect to time at a given point. In laminar flow,
the flow field can be characterized by layers of fluid, one layer not mixing with adjacent ones.
Turbulent flow on the other hand is characterized by random fluid motion. Tranquil flow is
distinguished from rapid flow by a dimensionless number called the Froude number, Fr. If Fr < 1,
the flow is subcritical; if Fr > 1, the flow is supercritical, and if Fr = 1, the flow is called critical.
Open channel flow can be nonuniform, unsteady, turbulent and rapid at the same time. Because
the classifying characteristics are independent, sixteen different types of flow can occur. These
terms, uniform or nonuniform, steady or unsteady, laminar or turbulent, rapid or tranquil, and the
two dimensionless numbers (the Froude number and Reynolds number) are more fully explained
in the following sections.
Velocity: The velocity of a fluid particle is the time rate of displacement of the particle from one
point to another. Velocity is a vector quantity. That is, it has magnitude and direction. The
mathematical representation of the fluid velocity is a function of the increment of length ds during
the infinitesimal time dt; thus,
Streamline: An imaginary line within the flow which is everywhere tangent to the velocity vector is
called a streamline.
Acceleration: Acceleration is the time rate of change in magnitude or direction of the velocity
vector. Mathematically, acceleration a is expressed by the total derivative of the velocity vector or