Aquaculture Engineering

Canals - Formula to calculate the cross-sectional area

Unit 11 - Water distribution and control structures

Water supply systems
Canals

Formula used to calculate the cross- sectional area of the canal
For earthen channels trapezoidal cross sections are more suitable. The area of the channel should be kept sufficient to carry the required volume of water for filling the pond to the optimum level. The cross-sectional area may be determined by using the following formula.

A = Q/V
Where,
A = Cross sectional area of the channel (m²)
Q = Discharge or rate of flow in the channel m³/sec
V = Velocity of flow in the channel (m/sec)

The velocity of flow in the channel should be non-covering and non-silting. The average velocity of water in the channel of loose earth or silt is 30 cm to 45 cm/sec. For channel in ordinary earth or alluvial soil is 75 cm to 90 cm/sec. The bed slope of the channel should be gentle, so that flow in all the cases is uniform. For channels in alluvial soil, a bed slope of one in thousand (1 in 1000) may be found safe.

Area of trapezoidal section
A = BY+KY²
Where,
A = Cross sectional area of the section (m²)
Y = Depth of water (m)
K = Side slope of the channel, is the ratio of change in horizontal distance to 1 meter vertical distance
f = Free board of the channel