ENERGY DISSIPATION ON SPILLWAYS 247
economies where geological and morphological conditions are favourable,
and particularly where the spillway can be placed over the power station
or at least over the bottom outlet works (Novak and Cˇábelka, 1981).
The head loss in the jet itself, whether solid or (more frequently) dis-
integrated, is not very substantial – only about up to 12% (Horˇení, 1956).
The energy loss on a ski-jump spillway can be substantially enlarged by
splitting the overfall jet into several streams (see Fig. 5.2) or by using two
spillways with colliding jets.
A substantial benefit for energy dissipation from jet spillways is in
the third phase at impact into the downstream pool. Here most of the
energy losses in the first three phases occur through the collision of masses
of water, and through the compression of air bubbles, both those con-
tained in the overfall jet as well as those drawn into the downstream pool
at the point of impact. The decrease in energy in this phase may be, there-
fore, enhanced by having a dispersed and intensively aerated jet before
impact.
Fig. 5.2 Comparison of (a) normal spillway, (b) ski-jump spillway and (c)
ski-jump spillway with jet splitters (Novak and Cˇábelka, 1981)
Fig. 5.3 Flip bucket