Question

Calculate the depth of cut-off for scour holes for an irrigation channel of F.S.Q equal to 354 cumecs and H.F.Q of natural drainage equal to 600 cumecs. Assume the value of friction factor as 1.

(a) 7 m

(b) 7.5 m

(c) 10.5 m

(d) 8 m

This question was addressed to me in a dream while sleeping

The origin of the question is Cross Drainage Work Design in section Cross Drainage Work of Irrigation Engineering

Answer 1

Right option is (c) 10.5 m

To explain I would say: Lacey’s normal depth of scour = R = 0.47 (Q/f)^1/3 where Q = 354 cumecs and f = 1.

R = 0.47 (354/1)^1/3 = 7 m

The depth of cut-off provided for scour holes = 1.5 R (on both sides)

= 1.5 x 7 = 10.5 m.