Hydraulic Structures: Fourth Edition

SPILLWAYS 223

Fig. 4.14 Shapes of shaft spillways (after Wagner, 1956)

approximated by Cd0.773 1 0.4
D

H

c

; for 

D

H

c

0.1 the coefficient has

practically the same value as for a straight-crested overfall (0.745).
Cd 1 in equation (4.61) has to take into account losses in the shaft and
bend (and possibly also in the tunnel for ‘pipe control’, i.e. when the
tunnel is flowing full – Fig. 4.17(a)).
In the case of a drowned spillway (or a spillway in the transition
region) a vortex, which reduces the spillway capacity, is formed; to prevent
it, piers or other anti-vortex devices are sometimes used, as is the case for
free-flowing spillways if these are placed close to the reservoir bank or
even in a cut in the bank; in this situation the anti-vortex device actually
plays a role in erosion prevention (Fig. 4.16). The coefficient m2/3Cd
is, in this case, reduced to mm 1 s, where  1 f(l/Dc, H/Dc) and
s(S/Dc, H/Dc) with 0.5 1 1 and 0.8s1.0. For optimum
performancea1.75Dc,l 6 Dc, and 0n0.5 (see Fig. 4.16).