Spillways are normally uncontrolled, i.e. they function automatically
as the water level rises above NWL, but they may be controlled by gates.
In some instances additional emergency spillway capacity is provided by a
fuse plug (see Section 4.7.7), i.e. an erodible subsidiary bank designed to
wash out if a predetermined extreme flood level is attained. Alternative
emergency provision can be made by reinforced concrete flap-gates
designed to tip over by hydrostatic pressure under extreme flood con-
ditions or by the use of crest-mounted fusegates (see Section 6.2.8). Con-
crete dams normally incorporate an overfall or crest spillway, but
embankments generally require a separate side-channel or shaft spillway
structure located adjacent to the dam.
1.5.2 Outlet worksControlled outlet facilities are required to permit water to be drawn off as
is operationally necessary. Provision must be made to accommodate the
required penstocks and pipework with their associated control gates or
valves. Such features are readily accommodated within a concrete dam, as
noted in Section 1.4. For embankment dams it is normal practice to
provide an external control structure or valve tower, which may be quite
separate from the dam, controlling entry to an outlet tunnel or culvert.
A bottom discharge facility is provided in most dams to provide an
additional measure of drawdown control and, where reasonable, to allow
emptying of the reservoir. The bottom outlet must be of as high a capacity
as economically feasible and consistent with the reservoir management
plan. In most cases it is necessary to use special outlet valves (Section 6.3)
and/or structures to avoid scouring and damage to the stream bed and
banks downstream of the dam.
1.5.3 River diversionThis provision is necessary to permit construction to proceed in dry con-
ditions. An outlet tunnel or culvert may be temporarily adapted to this
purpose during construction, and subsequently employed as a discharge
facility for the completed dam. In the absence of such a tunnel of adequate
capacity alternative steps will be necessary, involving the construction of
temporary upstream and downstream cofferdams or, in the case of con-
crete dams, by programming construction of one monolith or block to
leave a temporary gap or formed tunnel through the structure.
The hydraulic aspects of river diversion are dealt with in detail in
Vischer and Hager (1998).