Hydraulics
Flow
An open channel is a duct which a liquid flows with a free surface. At all points along its length the pressure on this surface will be the same, usually atmospheric pressure. A channel may be covered, provided that it is not running full; a partly filled pipe is treated as an open channel. Since the pressure at the surface of the liquid is constant, flow is not due to the pressure differences along the channel but is caused by the differences in the potential energy head due to the slope of the channel.
Open channel flow, a branch of hydraulics, is a type of liquid flow with in a conduit with a free surface, known as a channel. The other type of flow with in a conduit is pipe flow. These two types of flow are similar in many ways but differ in one important respect, the open channel has a free surface whereas pipe flow does not.
Steady flow- the depth of flow does not change over time or if it can be assumed to be constant during the time interval under consideration.
Un steady flow-the depth of flow does change with time.
Uniform flow: the depth of flow is the same at every section of the channel. Uniform flow can be steady or unsteady, depending on whether or not the depth changes with time (although the unsteady uniform flow is rare).
Varied flow – the depth of flow changes along the length of the channel.
Rapidly varied flow- the depth changes abruptly over comparative short distance. examples include the hydraulic jump and the hydraulic drop.
NB: a hydraulic jump is a phenomenon in the science of hydraulics which is frequently observed in open channel flow such as rivers or spillways. When a liquid at high velocity discharges into a zone of lower velocity a rather abrupt rise occurs in the liquid surface. The rapid flowing liquid is abruptly slowed and increases in height converting some of the flow’s initial kinetic energy into an increase in potential energy.
Properties of channels.
Artificial channels are made by man; they include irrigation canals, navigation canals, and spillways. Sewers, culverts and drainage ditches. They are constructed in a regular crossectional shape throughout. They are made from concrete, steel, or earth and they have the surface roughness reasonably well defined.
Natural channels are not regular and their surface roughness will often change with time distance and even elevation.
The wetted perimeter.
The length of the wetted surface measured normal to the direction of flow.
Area – the crossectional area of flow normal to the direction of flow.
In open channels the wetted perimeter can be defined as the surface of the channel bottom and sides in direct contact with the aqueous body. Friction losses typically increase with an increasing wetted perimeter.
For a rectangular channel of width B in which the depth of liquid is D.
Then Perimeter P= B+2D
Hydraulic gradient: the slope of the energy grade line of slope of the line representing the sum of the kinetic and potential energy along the channel length. It is equal to the slope of water surface in steady uniform flow.
Hydraulic radius (hydraulic mean depth): a measure of water depth in a channel defined as crossectional area of flowing water divided by the length of wetted perimeter.

The Chezy formula.

In fluid dynamics the Chezy formula describes the mean velocity of steady, or turbulent open channel flow. This was named after Antoine de Chezy the French hydraulics engineer in 1775.

This formula can be used to calculate mean flow velocity in pipes and channels and

worked example

worked example

prepared by Gilbert Niwamanya.

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