FREE BOARD

• 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.

$F= \sqrt{CD}$

where

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

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

CANAL BANKS

• 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.