avoid pressures below about 7 m vacuum (3 m absolute) in hydraulic
engineering design. It has to be stressed, however, that in turbulent flows
the mean pressure may be well above the danger limit but cavitation can
still occur owing to fluctuating instantaneous pressures that fall below the
limit. To assess the danger of cavitation in this case it is necessary to
analyse the turbulent pressure fluctuations, e.g. under the hydraulic jump
in a stilling basin (Chapter 5).
To ascertain the danger of cavitation in any particular situation,
model (Chapter 16) or prototype measurements are necessary, or the
design has to follow well-established principles. Should there be a clear
danger of cavitation damage then either the design or the mode of opera-
tion of a particular structure has to be changed, or some other safeguard
has to be applied. The most frequent of these is the introduction of air at
the endangered parts, i.e. artificial aeration, preventing the occurrence of
extremely low pressures. The use of special epoxy mortars can also sub-
stantially postpone the onset of cavitation damage on concrete surfaces,
and is a useful measure in cases where the cavitation is not frequent or
prolonged.
4.7 Spillways
4.7.1 Overfall spillways
The basic shape of the overfall (ogee) spillway is derived from the lower
envelope of the overall nappe flowing over a high vertical rectangular
notch with an approach velocity V 0 0 and a fully aerated space beneath
the nappe (pp 0 ), as shown in Fig. 4.5 (e.g. Creager, Justin and Hinds,
1945; US Bureau of Reclamation, 1987).
206 DAM OUTLET WORKS
Fig. 4.5 Overfall spillway shape derivation
