PLUNGE POOLS 259
5.4 Plunge pools
Downstream of free-falling jets (jet, ski-jump spillways, flip buckets)
energy dissipation takes place in stilling basins, or more frequently, in
plunge pools, usually excavated fully or partially in the stream bed during
dam construction, but sometimes only scoured by the action of the jet
itself. The scour and protection works have been reviewed by, for
example, Hartung and Häusler (1973), and the equations of the maximum
scourys(m) by Locher and Hsu (1984), Breusers and Raudkivi (1991),
Annandale (2006) and Liu (2005) (who considered both the energy dissi-
pation mechanism and submerged jet diffusion properties in the scour
development).
Bollaert and Schleiss (2005) using investigations of transient water
pressures in rock fissures developed a physically based model evaluating
the time evolution of scour in jointed rocks. Pagliara et al. (2006) exam-
ined the effect of the densimetric Froude number, jet impact angle and air
content, tailwater elevation, sediment nonuniformity and upstream flow
Froude number on the plane plunge pool scour with the result that
the densimetric Froude number (involving average jet velocity and the
particle determining size) had the main influence on the scour.
The general form of the simplified equation for the maximum scour
depth,ys, measured from the tailwater surface (ysysy 0 ) is
ysCqxHy*w/dz (5.16a)whereCis a coefficient, is the angle of the flip bucket with the horizontal,
dis the particle size (mm) and qandH*are the same as in equation (5.15);
the range of the coefficient Cand exponents x,y,zis
0.65C4.7, 0.5x0.67, 0.1y0.5,0 z0.3, 0w0.1.Thus the range of ysis wide, as is only to be expected because equation
(5.16a) covers a wide range of structures with different designs, degrees of
air entrainment, and geological conditions.
Accepting the possibility of substantial ‘errors’, a simplified equation
by Martins (1975) with x0.6,y0.1,wz0 and C1.5 can be used:
ys1.5q0.6H*0.1(m) (5.16b)(in equation (5.16b) H*is the drop from the reservoir level to the flip
bucket lip).
To reduce the variations in the powers in equation (5.16a) Mason
(1989) investigated the effect of air content in the plunge pool and