Question 9.3: Design an intake structure for a minor canal, using the foll......

The upstream full supply depth, D = 120.96 – 120.00 = 0.96 m. The water depth in the offtake = 120.36 – 119.86 = 0.50 m. The working head = 120.96 – 120.36 = 0.60 m. Adopting a proportional open flume intake (outlet), H/D = 0.9. Therefore the head, H, over the crest = 0.9 D = 0.9 × 0.96 = 0.864 m. The crest level = 120.96 – 0.864 = 120.096 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, b = Q / C H^{3 / 2}=0.5 / 1.6 \times 0.864^{3 / 2}=0.39 \, m .
Note that suggested values of C are as follows:

Table 1

The width of the offtake canal = 2.0 m. Therefore the flume ratio, {f_{\mathrm{r}}} = 0.39/2 = 0.195. The crest length = 2H = 1.728 m; say 1.75 m.
For the glacis and stilling basin (cistern), the discharge intensity (assuming 67% for splay), q = 0.67 × 0.50/0.39 = 0.86 m³ {m}^{-1} . The working head, h = 120.96 – 120.36 = 0.60 m. The critical depth, h_{\mathrm{c}}=(q^{2}/g)^{1/3}=  0.42 m, and therefore  h/h_{\mathrm{c}}=0.60/0.42=1.43. The non-dimensional energy curve, h/h_{\mathrm{c}} versus E/h_{\mathrm{c}} , gives the relationship

from which E = 0.706 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 cistern = 119.86 – 0.206 = 119.654 m AOD, and hence the depth of the cistern floor below the crest, d = 120.096 – 119.654 = 0.442 m. The length of the downstream glacis  = 2.5 d = 1.10 m. The length of the cistern = offtake flow depth + working head = 1.10 m.

DOWNSTREAM EXPANSION LENGTH

1. 1.5 × (offtake bed widthb) \simeq{^{}} 2.4 m, or
2. length of glacis + length of cistern = 2.20 m, or
3. with a minimum splay of 1 in 10, length  required = f_{\mathrm{r}}/\mathrm{splay}= 1.95 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 flow depth = 0.25 m. Adopt a minimum depth of 0.5 m. The upstream curtain wall depth is one-third of the FSL flow depth = 0.32 m. Adopt a minimum depth of 0.5 m.

DOWNSTREAM BED PROTECTION

Length of protection = offtake flow depth + working head = 1.10 m. Provide a gravel apron, about 1 m long and 150 mm thick, downstream of the cistern. The upstream approach length of the crest

= 0.64 m.

The radius joining the crest =2 H \simeq 1.728 m. The details of the layout are shown in plan and section in Fig. 9.31.