A considered evaluation of the generalized characteristics in con-
junction with Figs 1.3 and 1.4 will suggest further conclusions as to the cor-
responding advantages of embankment and concrete dams. However, the
limitations of generalizations on the merits of either type must be appreci-
ated. An open mind must be maintained when considering possible dam
types in relation to a specific site, and evaluation must attach proper
weight to local circumstances. Economic comparisons apart, other non-
engineering factors may be of importance: this is referred to further in
Section 1.6.
The variants of the concrete dam illustrated and their merits are
further compared with those for the embankment dam in Thomas (1976),
Golzé (1977) and USBR (1987).
1.5 Spillways, outlets and ancillary works
Dams require certain ancillary structures and facilities to enable them to
discharge their operational function safely and effectively. In particular,
adequate provision must be made for the safe passage of extreme floods
and for the controlled draw-off and discharge of water in fulfilment of the
purpose of the reservoir. Spillways and outlet works are therefore essen-
tial features. Other ancillary facilities are incorporated as necessary for the
purpose of the dam and appropriate to its type. Provision for permanent
flood discharge and outlet works and for river diversion during construc-
tion can prove to be technically difficult and therefore costly.
In this section, the more important structures and ancillary works
associated with impounding dams are identified and briefly described. As
such, it is intended as an introduction to subsequent chapters dealing with
the design of dams (Chapters 2 and 3), spillways and outlets (Chapter 4),
energy dissipators (Chapter 5) and gates and valves (Chapter 6). For
a review of hydraulics of spillways and energy dissipators see also
Khatsuria (2005)
1.5.1 Spillways
The purpose of the spillway is to pass flood water, and in particular the
design flood, safely downstream when the reservoir is overflowing. It has
two principal components: the controlling spillweir and the spillway
channel, the purpose of the latter being to conduct flood flows safely down-
stream of the dam. The latter may incorporate a stilling basin or other
energy-dissipating devices. The spillway capacity must safely accommodate
the maximum design flood, the spillweir level dictating the maximum reten-
tion level of the dam, i.e. the normal maximum water level (NWL).
20 ELEMENTS OF DAM ENGINEERING
