whereVais the approach velocity.
IfLeis the effective length of the crest, the head causing flow is given
by the weir formula:
H(Q/CdLe)2/3 (10.41)
whereQis the discharge and Cdis the discharge coefficient of the crest.
Therefore, the RL of the crest is E H.
Two types of crest are used (Fig. 10.18); the rectangular one for dis-
charges up to 10 m^3 s^1 and the trapezoidal one for larger discharges (see
Punmia and Lal, 1977).
The following are the design criteria established by extensive model
studies at the Irrigation Research Institute in India.
- For a rectangular crest,
top width, B0.55d1/2(m), (10.42)
base width, B 1 (Hd)/Ss, (10.43)
whereSsis the relative density of the crest material (for masonry,
Ss2). The discharge is given by the following formula:
Q1.835LH3/2(H/B)1/6. (10.44)
- For a trapezoidal crest,
top width, B0.55(Hd)1/2(m). (10.45)
For the base width, B 1 , upstream and downstream slopes of around 1
452 CROSS-DRAINAGE AND DROP STRUCTURES
Fig. 10.18 Sarda fall crests
