Worked Example 9.5
Design a suitable outlet to discharge 50 l s^1 from a canal with a full supply
depth of 1 m. The working head available is 150 mm.
Solution 1: open flume outlet
For non-submergence of the flume, i.e. modular flow conditions, the
minimum working head is approximately 0.2 times the head over the crest.
Therefore the maximum head over the crest, H0.15/0.20.75 m. Hence
the throat width, b(from the weir formula)0.05 m. Therefore, adopt the
minimum value of b0.06 m; this gives the head, H0.65 m, and hence
the minimum working head, h0.20.650.13 m, which is satisfactory,
as the available head is 0.15 m. This sets H/D0.65/1.00.65, and the
flexibility,F0.9/0.651. The design may be acceptable but may not
draw its fair share of silt from the supply canal because of the excessive sill
height of 0.35 m (depth of flow head over sill).
Solution 2: pipe and open flume outlet
An open flume type outlet is expensive, particularly if the supply canal
bank is very wide, and in such cases the pipe semimodule is used. The
outlet is also suitable for drawing its share of silt, with its lead pipe set at
or near the bed level (Fig. 9.32). The pipe delivers the water into a tank on
the downstream side, to which an open flume or an orifice semimodule is
fitted.
Q50 l s^1 ; D1.0 m; h0.15 m.
Assume a bank width of 10 m and adopt a concrete pipe with a kvalue of
0.1 mm. The head loss through the pipe, h 1 (1.5L/d)V^2 /2g. Assuming
a pipe diameter, d300 mm, V0.707 m s^1 andV^2 /2g0.0255 m. There-
fore the Reynolds number, Re 2 105 ;k/d3.3 10 4 , and so 0.0175
from the Moody chart. Hence the head loss0.0531 m, giving the avail-
able working head for the semimodule as 0.15 0.05310.0969 m. There-
fore the maximum head over the crest, H0.0969/0.20.484 m. The
throat width, b(from weir formula)0.093 m. Therefore provide a throat
width of 10 cm, which gives H0.46 m and thus hminimum0.092 m, which is
satisfactory as the available head is 0.0969 m. The layout of the proposed
design of the pipe and open flume outlet is shown in Fig. 9.32(a). Two
other alternative proposals which are in use are shown in Figs 9.32(b)
and (c).