262 ENERGY DISSIPATION
Sudden expansion energy dissipators(Locher and Hsu, 1984) which
utilize the principle of energy loss at a sudden enlargement are a fairly
recent development. Although almost inevitably associated with cavita-
tion, this occurs away from the boundaries without undue danger to the
structure. As an example of this type of structure, the section of one of the
three circular cross-section expansion chambers of the New Don Pedro
dam, designed to pass a total of 200 m^3 s^1 at a gross head of 170 m, is
shown in Fig. 5.10. The chambers dissipate about 45 m of head and the
remainder is dissipated in pipe resistance and the 9.14 m diameter tunnel
downstream of the gates.Fig. 5.10 Sudden expansion dissipator, New Don Pedro dam; dimen-
sions in metres (Locher and Hsu, 1984)Worked example
Design a hydraulic jump stilling basin for the maximum discharge of
25 m^3 s^1 m^1 flowing from an overfall spillway, with the spillway crest 50 m
above the downstream gravel river bed with a slope S 0 0.001 and
n0.028. Check the possibility of cavitation in the basin floor and esti-
mate the scour downstream of the basin; compare this with the depth of a
plunge pool downstream of a flip bucket.Solution
For the design head Cd0.75 (Section 4.7.1).Fromq2/3Cd 2
g1/2H3/2,H
2/3
5.032 m.25 3
2 0.75(19.62)1/2