- 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 = 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
According to CWPC the free board should be as follows:
- 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.