Solution
CREST DESIGN
Adopt a rectangular crest (Fig. 10.18). B0.55d1/2; for a trapezoidal crest,
B0.55 (Hd)1/2andHdD 1 99.00 97.502.5 m. Hence B(trape-
zoidal crest)0.87 m. Choose a rectangular crest, width B0.80 m.
Adopting a crest length of L6 m, equation (10.51) gives the head over
the crest,
H0.43 m.
Check for B: the crest level100.00 0.4399.57 m AOD.
Therefored99.57 97.502.07 m and B0.55d0.79 m. Therefore a
crest width of 0.8 m is satisfactory.
The base width, B 1 1.25 m (equation (10.50) with Ss2.0). The
velocity of approach (assuming a 1:1 trapezoidal channel)2.8/(61)1
0.4 m s^1. Therefore the upstream total energy level (TEL)100.000
0.008100.008 m AOD, and E100.008 99.570.438 m.
The depth of the cistern, dc0.19 m (equation (10.55)), and the
length of the cistern, Lc4 m (equation (10.54)). The RL of the cistern
bed97.50 0.1997.31 m AOD.
IMPERVIOUS FLOOR DESIGN (WORKED EXAMPLE 9.1 PROVIDES DETAILED
CALCULATIONS)
The maximum seepage head, Hsd(with no water downstream, and the
upstream water level at crest level). Adopting nominal upstream and
downstream cut-off depths of 1 m and 1.5 m respectively, the base length of
the impervious floor for the exit gradient of 1 in 5 is approximately 13 m.
The length of the impervious floor downstream of the crest is approxi-
mately 6 m (equation 10.56)). The upstream floor thickness (nominal
thickness of 0.3 m) at the toe of the crest is approximately 1.5 m, and at 5 m
from the toe it is approximately 0.14 m; adopt a minimum of 0.3 m.
Appropriate energy-dissipating devices (for large discharges) and
upstream and downstream bed protection works must be provided. The
detailed layout of the design is shown in Fig. 10.17.
References
d’Aubuisson, J.F. (1940) Traité d’Hydraulique, 2nd edn, Pitois, Levraut et Cie, Paris.
Blaisdell, F.W. and Anderson, CL. (1988a) A comprehensive generalized study of
scour at cantilevered pipe outlets. Journal of Hydraulic Research, 26 (4):
357–76.
458 CROSS-DRAINAGE AND DROP STRUCTURES
