Chapter 5
Energy dissipation
5.1 General
Energy dissipation at dams and weirs is closely associated with spillway
design, particularly with the chosen specific discharge q, the difference
between the upstream and downstream water levels (H*) and the down-
stream conditions. Chapter 4 dealt mainly with the actual spillway inlet
works and certain standard types of conduits conveying the flow from the
spillway inlet, i.e. chutes, tunnels, etc. In this chapter the main concern is
the concept of energy dissipation during the whole passage of the flow
from the reservoir to the tailwater and, in particular, the stilling basin
(energy dissipator) design.
The magnitude of energy that must be dissipated at high dams with
large spillway discharges is enormous. For example, the maximum energy
to be dissipated at the Tarbela dam service and auxiliary spillways could
be 40 000 MW, which is about 20 times the planned generating capacity at
the site (Locher and Hsu, 1984).
In the design of energy dissipation, environmental factors have to be
considered; some of the most important ones are the effect of dissolved
gases supersaturation on fish in deep plunge pools, and of spray from flip
bucket jets which can result in landslides and freezing fog.
The passage of water from a reservoir into the downstream reach
involves a whole number of hydraulic phenomena such as the transition
into supercritical flow, supercritical non-aerated and aerated flow on the
spillway, possibly flow through a free-falling jet, entry into the stilling
basin with a transition from supercritical to subcritical flow, and echoes of
macroturbulence after the transition into the stream beyond the basin or
plunge pool. It is, therefore, best to consider the energy dissipation process
in five separate stages, some of which may be combined or absent (Novak
and Cˇábelka, 1981) (Fig. 5.1):