several configurations; Baker (1994) included in his investigations flat blocks
and reinforced grass and reported stability for downward sloping wedge-
shaped blocks for velocities up to about 7 m s^1. In spite of their proven
stability attention must be paid to the detailing of the block edges, the
drainage slots connecting the low pressure zone on top of the blocks to the
drainage layer below them and to the subsoil or underlayer consisting of
granular material and possibly geotextiles (Hewlett et al., 1997).
4.7.7 Other spillways
The previous six sections have dealt with the main and most frequent
types of spillways. Their combinations, as well as other types of spillways,
are also sometimes used. If of unusual shape, their design should be
developed with the aid of model studies (Chapter 16), particularly if they
are intended to convey major discharges.
Unusual spillway shapes (in plan) are often associated with the
desire to increase the effective spillway length (even if the specific dis-
charge,q, may be decreased owing to interference to flow); examples are a
spillway with a ‘duck bill’crest,labyrinthorspecial shape(shaft) spillways
(Fig. 4.19 – note also the aeration vent for the shaft bend). The discharge
coefficients of these spillways are usually (but not always) somewhat
smaller than those of conventional spillways described earlier. The main
objective of model studies is to ascertain these coefficients and the
modular limits of the spillway.
Fuse plugsare used as auxiliary spillways. They are basically broad-
crested weirs with a crest higher than the main spillway crest but below the
maximum water level, and an earth embankment on top of the spillway,
designed to fail at a predetermined reservoir level. The sudden flow after
the fuse plug failure must be taken into account when choosing the site of
the auxiliary spillway, which usually discharges into a (side) valley other
than the main spillway. The downstream face of the weir must be suitably
protected, e.g. by concrete plain or wedge-shape blocks (Section 4.7.6) or
reinforced grass (CIRIA, 1987).
Tunnelledspillways, free flowing or under pressure, usually convey
flow from side channel or shaft spillways (Sections 4.7.2, 4.7.3 and 4.7.4).
Large-capacity outlets placed below the dam crest and controlled by gates
are usually called orificeorsubmergedspillways (Fig. 4.20). Apart from
the gates (Chapter 6) their important design features are the inlet arrange-
ment and associated head losses and the prevention of vortices (Section
4.8), and the control and effect of the outflow jet (Chapter 5). All of these
aspects are again best studied on suitable scale models.
The elimination of a conventional spillway is a unique feature of
some rockfill dams; this is achieved by the inclusion of an impervious wall,
230 DAM OUTLET WORKS
