Large bottom openings serve as submerged spillways (Section 4.7.7)
and their capacity can be used for flushing sediment from the reservoir
(Section 4.5) and during the dam construction (if only one opening is
available its blockage must be prevented). A typical arrangement for a
large bottom spillway is shown in Fig. 4.21, and for a bottom outlet of a
smaller capacity in Fig. 4.22. Note the inlet construction and shape
designed to reduce the head loss, the flared outflow section (to aid energy
dissipation downstream of the outlet) and the air vent to protect the junc-
tion of the spillway and outlet against possible cavitation damage (in case
of their joint operation). The outflow area has in this case been reduced to
85% of the bottom outlet area to provide cavitation protection (at the cost
of a 10% reduction in outlet capacity). Should the outlet terminate in a
regulating valve, then a similar area reduction should be provided (for the
full valve opening see Section 6.3).
The head loss at an inlet is expressed as hV^2 /2g; for an inlet with
grooves and a screen, 0.150.34. Although the lower values of the
coefficientare usually associated with inlets with curved walls (giving a
gradually decreasing cross-section), sometimes the same (or an even
better) result can be achieved by a transition formed by several plane sur-
232 DAM OUTLET WORKS
Fig. 4.20 Kariba dam spillway, Zimbabwe (after ICOLD, 1987)
