conventional methods, particularly a suction dredger with a bucket wheel,
or special techniques (e.g. pneumatic or jet pumps) may be used. The
environmental impact of all techniques for maximizing sediment through-
flow or/and the recovery of storage require very careful consideration.
For further discussion of methods of dealing with sedimentation in
reservoirs see ICOLD (1999) and Batuca and Jordaan (2000).
4.6 Cavitation
Cavitation occurs whenever the pressure in the flow of water drops to the
value of the pressure of the saturated water vapour, pv(at the prevailing
temperature); cavities filled by vapour, and partly by gases excluded from
the water as a result of the low pressure, are formed. When these ‘bubbles’
are carried by the flow into regions of higher pressure, the vapour quickly
condenses and the bubbles implode, the cavities being filled suddenly by
the surrounding water. Not only is this process noisy, with disruption in
the flow pattern, but – more importantly – if the cavity implodes against a
surface, the violent impact of the water particles acting in quick succession
at very high pressures (of the order of 1000 atm), if sustained over a period
of time, causes substantial damage to the (concrete or steel) surface, which
can lead to a complete failure of the structure. Thus cavitation corrosion
(pitting) and the often accompanying vibration is a phenomenon that has
to be taken into account in the design of hydraulic structures, and pre-
vented whenever possible (Knapp, Daily and Hammit, 1970; Galperin et
al., 1977; Arndt, 1981).
Low pressures – well below atmospheric pressure – will occur at
points of separation of water flowing alongside fixed boundaries, particu-
larly if the flow velocity is high. Thus there are two factors, pressure pand
velocityu, which influence the onset of cavitation. They are combined with
density in the cavitation number, , which is a form of the Euler number:
2(p pv)/ u^2. (4.14)
Cavitation occurs if the cavitation number falls below a critical value c
which is a function of the geometry and can vary widely. As an example,
the incipient cavitation number for sloping offsets and triangular protru-
sions, as determined from data by Wang and Chou (Cassidy and Elder,
1984), is shown in Fig. 4.4.
According to Ball and Johnson (Cassidy and Elder, 1984), a 3 mm
perpendicular offset into the flow can cause cavitation at velocities as low
as 11 m s^1 ; for an equally high recess from the flow the critical velocity is
about 32 m s^1. In spillway design we certainly should be very wary of cavi-
tation problems at velocities exceeding 35 m s^1 , even if the spillway
204 DAM OUTLET WORKS
