lower than the upstream water level, inside the dam, with flow passing
through the main body of the rockfill (Lawson, 1987).
4.8 Bottom outlets
Bottom outlets are openings in the dam used to draw down the reservoir
level. According to the type of control gates (valves) (see Section 6.3) and
the position of the outflow in relation to the tailwater, they operate either
under pressure or free flowing over part of their length. The flow from the
bottom outlets can be used as compensating flow for a river stretch down-
stream of the dam where the flow would otherwise fall below acceptable
limits: outlets can also serve to pass density (sediment-laden) currents
through the reservoir (Section 4.5).
BOTTOM OUTLETS 231
Fig. 4.19 Oued Sarno dam, Algeria; general arrangement of spillway
and outlets (after ICOLD, 1987)