Aquaculture Engineering

Unit 11 - Water distribution and control structures

Design of Canals

Channel of most economical cross section

A channel which gives maximum discharge for a given cross–sectional area and bed slope is called a channel of most economical cross–section. In other words, it is a channel, which involves lesser excavation for a designed amount of discharge. It is some time, also defined as a channel which has minimum wetted perimeter so that there is a minimum resistance to flow and thus resulting in a maximum discharge. The conveyance of a channel section increases with increase in the hydraulic radius or with decrease in wetted perimeter.

Therefore, the channel section having the least wetted perimeter or maximum hydraulic radius for a given area has the maximum conveyance and is known as the best section. The most efficient section is usually also the most economical section but not necessarily. The semicircle has the least perimeter among all sections with the same area. Hence it is the most hydraulically efficient of all sections. For a most efficient trapezoidal section or best hydraulic section of trapezoidal shape, the most economical side slopes are

However, because of constructional difficulties it may not be possible to adopt this most economical side slope. In general, a channel section should be designed for the best hydraulic efficiency but should be modified for practicability. The principle of best hydraulic section, however applies only to the design of non-erodible channel.