STILLING BASINS 257
to low dams was developed particularly in the USSR (e.g. Skladnev, 1956)
and is reviewed by Novak and Cˇábelka (1981). This type of basin is really
only one example of spatial hydraulic jumpbasins. Others involve a
sudden change in width (Fig. 5.8(a)) or a jump combined with side inflows
form chutes (Fig. 5.8(c)).
Yasuda and Hager (1995) investigated the formation of a hydraulic
jump in a linearly contracting channelwith the conclusion that although
there was substantial agreement with the classical jump (equation (5.8))
there were significant differences in its structure. Bremen and Hager
(1993) investigated jumps in abruptly expanding channelswith the conclu-
sion that jumps with the toe just in the upstream channel are more effi-
cient than classical jumps, but in stilling basin construction this advantage
has to be offset against the fact that they may become asymmetric for
expansion ratios larger than 1.4 and – being longer – require more excava-
tion volume than basins based on the classical jump.
For gated barrages various types of stilling basins other than those
already discussed have been developed; one of the most common is a
sloping apron (glacis) followed by a horizontal sill with or without appur-
tenances. The shape of the basin is usually dependent on the morphology
of the river bed and the amount of excavation needed for its construction
and its function has to be considered together with various modes of gate
operation. For further details see Section 9.1.
All these and other types of stilling basins and energy dissipators are
best developed with the aid of scale model studies (Novak and Cˇábelka,
1981).
Mason (1982) carried out a survey of 370 dissipators constructed
since 1950; the survey included rock basins (unprotected bed with sub-
merged roller bucket), simple jump basins, baffle basins and various types
of dissipators involving free trajectory jets. The results of the survey are
summarized in Fig. 5.9, and confirm that to operate successfully baffle
basins need a certain minimum inflow velocity (head), but their range is
limited by cavitation problems for H30 m. Simple hydraulic jumps have
been used for heads that are larger than indicated on the figure (H50 m)
but great care is needed in design and construction. Various types of jet
dissipators need a minimum head (H10 m) to work properly.
5.3.3 Erosion downstream of stilling basinsIt has been demonstrated in Section 5.3.1 that at the outflow from the
basin there remains a certain proportion of energy to be dissipated.
Because of this and the uneven velocity distribution, there will always be
some local erosion downstream of the basin. To eliminate this is almost
impossible and, above all, uneconomical. The main purpose of the basin is