Aquaculture Engineering

Design of Canals - Design of side slope and bottom slope of the channel

Unit 11 - Water distribution and control structures

Design of Canals

Design of side slope and bottom slope of the channel
The bottom slope of a channel mainly depends upon the topography and energy head required for the flow of water. The natural slope of the land is usually the deciding factor in determining the channel bed slope. The steeper the channel the more will be the discharge. However, excess gradients produce very high velocities which cause erosion. Normally a channel slope about 0.1%, silting may take place in the channel if the slope is less than 0.5%. For determining the bed slope of the channels, the velocities should be checked and the maximum grade designed so that the velocities do not exceed the maximum permissible limit.

The side slope of the channel depends mainly on kind of material the channel section is made of. A general idea of the slopes suitable for use with various kinds of materials is given below.

The free board of a channel is the vertical distance from the top of the channel to the water surface at the design condition. The free board should be sufficient to prevent waves or fluctuation in the water surface from overflowing the sides. Under ordinary condition, the free board requirement can be approximately computed by using the equation.

f= (CD) ½
Where,
f= freeboard in feet
D = Depth of water in the channel (feet)
C = co-efficient
Co-efficient varying from 1.5 for a channel capacity of 20 cubic feet per second to 2.5 for a channel capacity of 3,000 cubic feet per second or more.