120.96 120.360.60 m. Adopting a proportional open flume intake
(outlet), H/D0.9. Therefore the head, H, over the crest
0.9D0.90.960.864 m. The crest level120.96 0.864120.096 m
AOD. As the crest level is greater than the bed level of the offtake,
this may be adopted. From the flume formula, the throat width,
bQ/CH3/20.5/1.60.8643/20.39 m.
Note that suggested values of Care as follows:
Q (m^3 s^1 ) Offtake angle, C values
60° 45°
0.55 1.60 1.61
0.55–1.5 1.61 1.63
The width of the offtake canal2.0 m. Therefore the flume ratio, fr
0.39/20.195. The crest length 2 H1.728 m; say 1.75 m.
For the glacis and stilling basin (cistern), the discharge intensity
(assuming 67% for splay), q0.670.50/0.390.86 m^3 m^1. The work-
ing head, h120.96 120.360.60 m. The critical depth, hc(q^2 /g)1/3
0.42 m, and therefore h/hc0.60/0.421.43. The non-dimensional energy
curve,h/hcversusE/hc, gives the relationship
E/hch/hc
1
2
(hc/h)^2
from which E0.706 m. Therefore the depth of the cistern below the bed
level of offtake
E water depth in offtake
0.206 m (minimum depth'0.075 m).
Therefore the floor level of the cistern119.86 0.206119.654 m AOD,
and hence the depth of the cistern floor below the crest, d
120.096 119.6540.442 m. The length of the downstream glacis
2.5d1.10 m. The length of the cisternofftake flow depthworking
head1.10 m.
DOWNSTREAM EXPANSION LENGTH
- 1.5(offtake bed width b)2.4 m, or
- length of glacislength of cistern2.20 m, or
- with a minimum splay of 1 in 10, length requiredfr/splay1.95 m.
Therefore adopt the largest of the three, i.e. 2.4 m. The downstream
curtain wall depth below the cistern floor level is one-half of the offtake
406 DIVERSION WORKS
