Design Criteria for Irrigation Canals - Sediment Transport in Irrigation Canals: A New Approach - page 67

on the bed or side slopes). For a given design discharge, the canal dimen-
sions and bed slope are determined considering the flow velocity not
exceeding a permissible velocity or boundary shear stress. These values
are related to the critical values for the bed and bank material. Under the
tractive force theory two methods are in use:
- maximum permissible velocity;
- critical shear stress.

A summary of the available equations for the prediction of velocity is
given by Raudkivi (1990) and Simons and Senturk (1992).
The minimum velocity in an irrigation canal should not induce sedi-
mentation and at the same time the velocity should limit unwanted aquatic
weed growth and reduce health risks (for example, schistosomiasis). The
minimum velocity is a function of the shape of the canal. For large
canals, a minimum velocity of 0.30 m/s is recommended (Dahmen, 1999).
Smaller velocities result in uneconomic large cross sections. A veloc-
ity of 0.10 to 0.15 m/s is recommended for minor canals (tertiary and
small secondary canals); smaller velocities result in uneconomic wide
sections.
The maximum permissible velocity should not cause erosion of the
bottom and side slopes and a critical shear stress criterion is applied,
which is extremely dependent on the fact that:
•

the resistance to erosion increases when smaller particles are washed
out;

•

an aged canal has more resistance to erosion than a newly constructed
one;

•

colloidal matter in the water will increase the cohesion of the particles
that form the boundaries, resulting in a larger resistance;

• a higher ground water table than the canal water level will decrease the

resistance; a lower ground water level will increase the resistance.

The tractive force depends on the shear stress at the bottom, which can
be expressed as (Dahmen, 1994):

τ = cρgyS o (4.13)

with:
τ = tractive force per unit wetted area (N/m 2 )

c = correction factor; the correction factor c depends on the B/y ratio:

0 . 77e 0 . 065( B/y )

c = for 1 < B/y < 4

c = 1 for B/y ≥ 4

density of water (1000 kg/m 3 )

ρ = ρ w =

S o = bed slope (m/m)

y = water depth (m)

acceleration due to gravity (m/s 2 )

g =