Water Conveyance Loss and Designing Conveyance System - Practices of Irrigation and On-farm Water Management - page 12

1.2.3.4 Freeboard

The required freeboard above the maximum design water level shall be at least one-
fourth of the design flow depth (0.25 d ) and shall not be less than 0.3 m.

1.2.3.5 Water Surface Elevations

Water surface elevations should be designed to provide enough hydraulic head for
successful operation of all ditches or other water conveyance structures diverting
from the canal or lateral.

1.2.3.6 Side Slopes

Canals, laterals, and field channels should be designed to have stable side slopes.
Local information on side-slope limits for specific soils and/or geologic materials
should be used if available. If such information is not available, the design of side
slopes for the banks of canals or laterals shall not be steeper than those shown below:

Materials Side slope (horizontal to vertical)

Sandy to loam soil 2:1

Silty clay 1.5:1

Heavy clay 1:1

1 / 4 :1

Loose rock to solid rock

1.2.4 Calculation of Velocity of Flow in Open Channel

The irrigation or drainage channel design should be such that it provide adequate
capacity for the design discharge or flow resulting from the design storm. The
velocity of flow in open channels can be determined by using Chezy's equation
or Manning's equation.

1.2.4.1 Chezy's Equation

The earliest formula for open channel was proposed by Chezy (in 1775). The
Chezy's equation can be expressed as

C √ RS V = (1.7)

where

V = velocity of flow (m/s)

R = hydraulic radius of the flowing section (m)

S = slope of water surface (taken as equal to the slope of channel bed (m/m))

C = Chezy's constant, which varies with surface roughness and flow rates

( ∼ 45-55).