STILLING BASINS 253
5.1). For k5 there is practically no cavitation danger; for k3.5 there is a
2% probability of the occurrence of cavitation during the time of basin
operation (for well constructed basins). In reality, this probability of cavita-
tion could be appreciably higher because the inception of cavitation will
occur at pressures higher than pv, irregularities in the basin floor will cause
local pressure reduction, and the incoming flow upstream of the jump may
already have a substantial degree of turbulence (Narayanan, 1980).
Physical models provide, with certain safeguards, a suitable tool for
evaluating the amplitude and frequency characteristics of macroturbulent
pressure fluctuations and for assessing the tendency towards cavitation
with intermittent cavitation in prototype (Lopardo, 1988) (Chapter 16).
Potential cavitational damage is not the only danger in hydraulic
jump stilling basins (as well as in other types of basins). Probably the most
serious structural problem is the effect of uplift pressures due to the dam
drainage system or the tailwater level or the water table in the basin bank.
This pressure excess over the hydrostatic pressure in the basin is aggra-
vated by the macroturbulent pressure fluctuations underneath (and on the
side of )the jump. Although the pressure fluctuations have an unequal spatial
distribution (which alleviates this part of the problem) it is only sensible to
design the floor slab for the more severe of the two situations: either the full
downstream uplift pressure applied over the whole area of the floor with the
basin empty or the uplift pressure head equal to the root mean square (r.m.s.)
value of pressure fluctuations of the order of 0.12V 12 /2g(V 1 is the inlet super-
critical velocity) applied under the whole full basin (this is half of the
maximum point pressure fluctuation resulting from index max0.05 and
kmax5). Furthermore, all contraction joints should be sealed, no drain open-
ings should be provided, and floor slabs should be as large as possible and
connected by dowels and reinforcement (ICOLD, 1987).
Fig. 5.5 Comparison of streamwise variation of pressure fluctuations in
free and submerged jumps for Fr5 (after Locher and Hsu, 1984)