• The vertical distance between F.S.L. and top of the lowest bank of the channel is know free board.
  • It is provided to prevent waves or fluctuations in water surface from overtopping the banks.
  • Freeboard depends upon the canal size, wind action, soil characteristics and location.
  • According to USER, free board may be worked out from following formula, under ordinary conditions.

\[F= \sqrt{CD}\]


F = free board

C = a constant whose value varies from 0.46 to 0.76

D = the depth of water in metres

Lacey gave following formula for the free board

\[F= 0.20+0.15Q^{\frac{1}{3}}\]

According to CWPC the free board should be as follows:

Free board, Dowla, and service road.
Free board, Dowla, and service road.


  • The purpose of banks is to prevent spread of water beyond the specified limit.
  • The width of the banks should be enough so that a minimum cover of 0.5 in soil is available everywhere above the saturation line.
  • The hydraulic gradient in ordinary soils is kept 1 in 4 and for light soils 1 in 6.
  • If banks become very high counterberms may be provided on the outer slopes of the banks.
  • Banks should be properly compact while making.
  • CWPC has given following bank widths depending upon the discharge.