However, the spillway may terminate in a vertical face with a free-falling jet
(e.g. in arch dam spillways) or in a ski jump shape followed by a more or less
dispersed jet. As these arrangements (apart from the case where necessit-
ated by the dam shape as, for example, in arch dams) are primarily used to
enhance energy dissipation, they will be discussed further in Chapter 5.
An important design feature is the point at which self-aeration of the
overfall nappe (in contact with the spillway) starts. The mechanics of
the phenomenon of self-aeration will be dealt with in Section 4.7.3; the
distanceLi(m) of the inception point from the crest can be estimated
using Hickox’s equation:
Li14.7q0.53 15 q1/2 (4.23)
whereqis the specific discharge (discharge for unit length, m^2 s^1 ).
In 1980 ICOLD (International Commission on Large Dams) con-
ducted a survey of spillways (of all types) covering 123 structures, with 71
operating over 100 days (Cassidy and Elder, 1984). Most of those where
erosion of the surface was reported were operating with maximum velocities
of flow over 30 m s^1 and specific discharges over 50 m^2 s^1. The survey
covered cases with 20 m s^1 Vmax40 m s^1 and 5 m^2 s^1 q200 m^2 s^1. Not
all damage was necessarily due to cavitation, though most of it was. As
shown in Section 4.5 even ‘smooth’ concrete surfaces require great caution
with velocities over 30–35 m s^1 , corresponding to a head of over 50 m, if no
losses are taken into account; actually, substantially higher dams may have
velocities well below these values if energy losses are included (Chapter 5).
The role of large values of qis apparent in conjunction with equation (4.23),
as the onset of self-aeration, and thus even more so the beginning of air
contact with the spillway surface (affording cavitation protection), is delayed
in proportion to q1/2. For further discussion of the importance of qin energy
dissipation and downstream erosion see Chapter 5.
4.7.2 Side-channel spillways
Side-channel spillways are mainly used when it is not possible or advisable
to use a direct overfall spillway as, e.g., at earth and rockfill dams. They
are placed on the side of the dam and have a spillway proper, the flume
(channel) downstream of the spillway, followed by a chute or tunnel.
Sometimes a spillway that is curved in plan is used, but most frequently it
is straight and more or less perpendicular to the dam axis (Fig. 4.8); the
latter is certainly the case in a gated spillway.
The spillway proper is usually designed as a normal overfall spillway
(Section 4.7.1). The depth, width, and bed slope of the flume must be
designed in such a way that even the maximum flood discharge passes with
210 DAM OUTLET WORKS
