Open Channel Flow
An open channel flow is a fluid flow that is not entirely enclosed in a medium such as that of a pipe
and has its surface open to atmospheric pressure. It is the liquid flow within a channel with a free
surface. Examples of an open channel flow are streams, floods, drainage ditches, and the like. It is
classified into flows: (1) Uniform flow, and (2) Non-uniform flow.
A uniform flow is characterized by a constant velocity, flow depth, area, and slope. From its name
“uniform”, it entails that the stream surface or fluid surface is parallel to the channel bed which is the
indication of a constant slope.
A non-uniform flow is just the opposite of the uniform flow. It is described by the changing depth and
velocity of the flow. It is classified in the forms of Gradually Varied Flow (GVF) and Rapidly Varied Flow
(RVF). The GVF and RVF are results of obstructions in the channel flow. In short distances near the
obstruction, RVF occurs. For obstructions that varies the flow over larger distances, GVF occurs.
Hydraulic jumps are created with the RVF occurrence. It is the sudden rise of the fluid level and it also
dissipates energy. Surface profiles of GVF has not been understood.
Uniform and non-uniform flows may coexist in a single channel (e.g. a river with rocks half-submerged
in the water). Obstructions can affect the flow depth and velocity of the flow.
Froude number determines the criticality of the flow. A subcritical flow is Fr<1 which denotes that the
water is deep and flowing slowly. A supercritical flow defines a shallow and fast flowing water of which
Fr>1.
Specific energy is the total mechanical energy of the fluid flow in the form of heads. It affects the
flow level.
The flow in a channel can be controlled or regulated with the presence of sluice gates, weirs and etc.
Obstructions can vary the flow of a section in a given channel resulting to RVF and GVF under steady
flow conditions. Geometries of weirs and other obstructions can give varying effects to the flow of the
fluid throughout the channel.
