Chapter 4
Dam outlet works
4.1 Introduction
Dam outlet works consist generally of spillways and bottom (high-head)
outlets. Spillways are basically dam appurtenances ensuring a safe passage
of floods from the reservoir into the downstream river reach. The spillway
design depends primarily on the design flood, dam type and location, and
reservoir size and operation. The design of bottom outlet works depends
primarily on the purpose of the reservoir and the sediment inflow and
deposition in the reservoir.
Spillways may be classified in several ways: according to function as
(main) service, emergency and auxiliary spillways; according to mode of
control as free (uncontrolled) or gated (controlled) spillways; according to
hydraulic criteria, i.e. type, as overfall, side channel, chute, shaft, siphon
and tunnel spillways. In the following text this last type classification will
be used.
Apart from economics, the main factors governing the choice of
spillway for a given project are the reliability and accuracy of flood predic-
tion, the duration and amount of spillage, seismicity of project site, topo-
graphy and geology, and the dam type.
In the case of gated spillways the gates may be operated manually,
by remote control, or automatically, depending on the level of water in the
reservoir. Rigid control regulations are required for non-automatic opera-
tion in order to prevent an artificial major flood downstream and/or not to
lose valuable water from the reservoir. When controlling the reservoir
outflow by spillway gates prior to or during a flood period, reliable flood
forecasting methods have to be developed. Gates are of decreasing useful-
ness in lowering the reservoir level as the dam height increases unless they
are submerged as, for example, in tunnel spillways. If the local conditions
(e.g. seismic activity, lack of confidence in maintenance and operating
skills, or difficulties of access) mean that there are doubts about the