Design of Canals

If silting occurs, the capacity of the canal reduces as its cross-sectional area reduces, and hence it will irrigate lesser areas. Thus, alluvial channels must be designed for a velocity which is neither scouring nor silting and results in a stable (regime) channel. Such a velocity is known as non-silting, and non-scouring velocity and a channel in which no silting and no scouring occurs is called Regime or Stable Channel (canal).

Kennedy introduced a term called critical velocity which is defined as the mean velocity which will just keep the channel free from silting and scouring.

It is the first stage of stability attained by a channel after it has been put into service. 

It is the ultimate state of regime in which all the geometrical elements are suitably adjusted. This stability approaches the true regime and the equations developed by Lacey are applicable to a channel in either true or final regime.

Since Kennedy’s studies were based on a given canal system only, to validate his expression of critical velocity throughout India for various types of soils, he introduced a term called critical velocity ratio denoted by ‘m’. The value of ‘m’ accounts for silt grade (type and size of silt).