Solution
Figure 8.22 shows the details of a Crump weir profile. Figure 8.23 shows a
typical layout of the design flow chart based on which appropriate soft-
ware could be developed to solve the problem. The detailed solution and
design calculations are described below.
Select the width, b, of the weir approximately as the width of the bed
B, i.e. bB10 m (Fig. 8.21). The minimum head over the crest to avoid
surface tension effects is approximately 0.06 m. Therefore, using the
Crump weir equation for discharge (modular),
QCdg1/2bH 1 3/2. (i)WithCd0.626 (Water Resources Board, 1970), H 1 for the minimum flow
of 0.5 m^3 s^1 is 0.0864 m0.06 m, and is satisfactory. A log–log plot of the
stage–discharge curve (with stagehzin cm) is plotted in Fig. 8.24.
From the given cross-sectional details of the stream, the area A
(Fig. 8.21) is calculated for different stages, and the velocity computed.
Hence the energy head H 2 downstream of the weir is calculated from
h+zV^2 /2g.The tailwater rating curve (hence H 2 versusQ) is plotted in Fig. 8.25.
Using equation (i) the upstream rating curve (the weir formula gives H 1
versusQ) is also plotted on Fig. 8.25.
WORKED EXAMPLES 355
Fig. 8.22 Crump weir layout