Kennedy's Canal Design Theory | Design Steps

R G Kennedy (an executive engineer of Punjab PWD) in 1895 carried out extensive investigations on some of the canals reaches in the Upper Bari Doab canal system and came up with this theory.

In this place no silting and no scouring occurred in the past 30 years so the friction between the flowing water and bed creates vertical eddies, which help the silt to keep in suspension.

This theory can be used for unlined canals design in India. Scouring and sedimentation are two important considerations in this theory.

Excessing silting and scouring leads to some issues in the canal system. They are listed below.

• Reduction in the capacity of the canal

• Canal bed erosion

• An increase in canal cross-sectional area

• A decrease in the canal flow depth

• A decrease in the command area of crops

As per Kennedy’s theory, erosion and sedimentation balance each other over the depth. In this theory, Kennedy gave some definitions for stable channels and critical velocity.

Stable/Regime channel

This is a very idle channel which is not practically possible as no erosion and no sedimentation occur in this type of channel. The silt which has entered the channel should be carried by the suspension.

One of the important considerations of this channel is that the velocity of water in the canal should be so as not to erode the canal bed nor allow silting.

Critical velocity

The critical velocity of a channel is the velocity that is just enough to produce eddies.

The possible values of m are listed below:

• m= 1 for Bari Doab Canal

• m<1 If the silt is finer than the Bari Doab canal

• m>1 if the silt is coarser than the Bari Doab canal.

Now let's see how to design a canal using Kennedy's theory.

Design

The design steps are-

• Step 1: We have to assume a trial depth, first. For example, take trial depth d = 1.5 m.

• Step 2: Find critical velocity using the formula Q = A x Vo, where Q will be given or has to be assumed and the critical velocity is known from the above formula.

• Step 3: We need to find canal area A using the above formula.

• Step 4: Use Kutter's equation to measure the actual mean velocity

Here, n - Rugosity coefficient, S - slope, R = A/P, P - wetted perimeter

• Step 5: Check whether the mean velocity and critical velocity are equal, if not repeat with the different depth.

Kennedy's theory can also be applied in a graphical solution using the GARRET plots.

Graphical solution using GARRET plots

With known Q, S, m, n find the optimum depth from the GARRET plots.

Even though Kenndy proposed this theory after proper observations, there are many drawbacks to this theory.

Drawbacks of Kennedy's theory

• The empirical formulae are difficult to remember and involve tedious calculations.

• This theory cannot be applied where any channel is just forming and has not attained a stable slope.

• A single factor m cannot explain the silt properties of the canal

• The effect of eddies from the sides of the channel is ignored.

Try this question out!

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