Garret's diagram is based on 

Explanation:

Garret's diagram is a graphical representation of the design of canal dimensions based on Kennedy's theory. The diagram is discharge plotted on the abscissa (X-axis), and slope of the channel on the left side of the ordinate (Primary Y-axis), and the depth of the channel and critical velocity (v∘) on the right side of the ordinate (secondary Y-axis). Garret diagram is applicable only for the side slope of the channel is 0.5 : 1 (H : V).

Kennedy's Theory:

Kennedy proposed his theory based on the charge and grade of silt present in the Upper Doab Canal (Punjab) and its distributaries. He observed that there is always one velocity for a given condition, called "critical velocity", v∘, which characterizes a non-silting and non-scouring channel in a steady regime. This velocity depends on the depth of water in the channel.

v∘ = 0.546 × y0.64

Where y = depth of water, m.

This relation was applicable to the grade of silt present only in the Upper Doab Canal. He introduced a factor called the "critical velocity ratio, (CVR), m" in his equation to account for the varying grades of silt in other regions. Thus, he propounded :

v∘ = 0.546 m y0.64

Where m = v/v∘ = critical velocity ratio.