Aquaculture Engineering

Design of Canals - Feeder channel

Unit 11 - Water distribution and control structures

Design of Canals

Feeder channel
Main feeder channel starts form the main gate or pumping station and usually traverses the central portion of the farm. Depending upon the area of the farm, the main feeder channel could feed a number of secondary feeder channels before water is conveyed to the ponds. Flow of water into the pond is regulated by using gate or water inlet structure. Depending upon the shape, a channel is prismatic or non-prismatic. A channel is said to be prismatic when the cross-section is uniform and bed slope is constant. In a non-prismatic channel either the cross-section or the bed slope or both change.

Aquacultural farms usually have prismatic channels which can be of any regular shape such as rectangular, parabolic, trapezoidal or circular. However, most of the unlined channels are designed in trapezoid shapes, for it provides side slope for stability. Rectangular shape is commonly used for channels built of stable materials. The channels used for supplying water into the ponds are usually in the farm of the canal or flume. The canals is usually a long and mild–sloped channel built in the ground which may be unlined or lined with stone masonry, concrete, cement, wood or bituminous materials. The flume is a channel of wood, metal, concrete or masonry usually supported on or above the surface of the ground to carry water. Elevated flumes are most efficient water conveyors than earth canals but they require high initial investments. They are usually installed on top of earthen dikes. The cross–section of a flume is normally that of a rectangular channel.

The flow in a channel can be either uniform or non-uniform. The flow is uniform when the rate of flow, velocity of flow, depth of flow, area of flow and slope of bed remains constant over the given length of the channel. Change in any one of the above conditions causes the flow to be non-uniform. An obstruction constructed across a channel of uniform width will also cause the flow to be non-uniform. Aquaculture farms are usually designed for a uniform flow in the channel.

The discharge of uniform flow in the channel may be expressed as the products of the velocity and water area.

Q = Ax V
Where,
Q = Discharge m³/sec
A = Water area/Area of flow m²
V = Mean velocity m/sec
C = Chezzy’s co-efficient

Due to presence of a free surface and to the friction along the channel wall, the velocities in a channel are not uniformly distributed in a channel section. The measured maximum velocities in ordinary channels usually appear to occur below the free surface at a distance of 0.05 to 0.25 of the depth. The mean velocity in any vertical section occurs at the depth of approximately 0.6 of the water depth below the free surface. A more accurate value of the mean velocity is obtained by measuring the velocities at 0.2 and 0.8 of the water depths from the surface and then taking the average of these velocities. The average or mean velocity of flow in an open channel can be determined with the help of a pitot tube or a current meter.
Several equations are available to calculate the mean velocity in an open channel. Among them the Chezy’s equation and Manning equation are commonly used.

Owing to its simplicity and fairly good accuracy the manning equation is the most commonly used formula for the computation of mean velocity and flow in open channels.
Thus discharge of uniform flow in an open channel.

The lowest velocity in a channel that will not start sedimentation and induce the growth of aquatic plant and mass is minimum permissible velocity. Usually a mean velocity of 0.6 to 0.9 m/sec may be used safely when the percentage of silt present in the channel is small and a mean velocity is not less than 0.75m/sec, will prevent a growth of vegetation on it.