(ramps), offsets, or grooves, or a combination of two or all three of them
(Fig. 4.12). The most frequent shape used on spillways is the combination
of a deflector with an offset. Air is supplied to the spillway surface auto-
matically through air ducts as the flow separation causes the pressure
downstream of the aerator to drop below atmospheric; the air duct(s) must
be designed to control this pressure drop within acceptable limits. As air
bubbles will not stay in contact with the spillway but rise to the water
surface, a series of aerators is sometimes considered necessary to achieve
the minimum required air concentration at the spillway surface. The
higher the flow velocity, the larger can be the distance between the aera-
tors; roughly, the distance in metres should be equal to about 2 times the
mean flow velocity (in m s^1 ). However, recent research (Kramer 2004)
seems to indicate that for certain flow conditions the average minimum air
concentration never falls below a critical value, suggesting that one aerator
may be sufficient to protect a chute from cavitation damage.
For the computation of the air discharge through an aerator Pinto
(1991) recommends (from prototype measurements)
220 DAM OUTLET WORKS
Fig. 4.12 Types of aeration facilities (after Cassidy and Elder, 1984)
