Farhoudi and Narayanan (1991) investigated the forces acting on
slabs of different lengths and widths beneath a hydraulic jump giving
further details of effects of slab size, position and width–length ratio. Pin-
heiro, Quintela and Ramos (1994) give a summary and comparative analy-
sis of methodologies for computing the hydrodynamic forces acting on
hydraulic jump stilling basin slabs.
The prevention of vibration of basin elements (due to the turbulence
of the flow) also requires massive slabs, pinned to the foundation when
possible (see also Fiorotto and Salandin, 2000).
Abrasion of concrete in the basin could take place if this is also used
for bottom outlets carrying abrasive sediment (although this is unlikely to
happen for velocities below 10 m s^1 , or from sediment drawn into the
basin from downstream either by bad design or operation. The basin
should be self-cleaning to flush out any trapped sediment.
Uplift, abrasion, and cavitation are, of course, closely connected, and
provision for maintenance and repairs should be considered in the basin
design.
The discharge used in stilling basin design is in most cases the spill-
way design (maximum) discharge (in cases 1, 2 and 4 above). This,
however, need not always be the case. Sometimes it may be more econom-
ical to take a calculated risk and design the basin for a smaller and more
frequently occurring discharge (say Q 1000 or smaller instead of PMF –
Chapter 4) and carry out repairs should this chosen Qbe exceeded. Great
care and experience is necessary when opting for this alternative.
5.3.2 Other types of stilling basins
Although the stilling basin based purely on a simple hydraulic jump works
well and relatively efficiently, in certain conditions other types of basins
may produce savings in construction costs. Standard basins were
developed with baffles, chute blocks and special end sillsby the USBR
(Bradley and Peterka, 1957; Peterka, 1963; US Bureau of Reclamation,
1987). An example of a basin with chute and baffle blocks – USBR Type
III – which can be used for velocities V18.2 m s^1 andq18.6 m^2 s^1 is
shown in Fig. 5.6. As this basin is shorter than others, the temptation is to
use it outside these limits; however, the danger of cavitation damage in
these cases is substantial and great care must be exercised in the design
and positioning of the blocks. Basco (1969) and Nothaft (2003) carried out
a detailed investigation of the trend in design of baffled basins and of drag
forces, pressure fluctuations, and optimum geometry; the whole area of
baffled basins is also reviewed by Locher and Hsu (1984).
Theplain and slotted roller bucketdissipators developed mainly in
the USA (Peterka, 1963) (Fig. 5.7) require substantially higher tailwater
254 ENERGY DISSIPATION
