The sloping surface downstream of the weir crest (glacis) and its
transition into the stilling basin should be designed so that a hydraulic
jump occurs on it over the full range of discharges. Details of stilling basin
design are given in Chapter 5.
Protection works, such as cut-off piles, aprons and an inverted filter,
are provided both upstream and downstream of the impervious floor of
the weir block (see also worked examples 9.1 and 9.2). Chapters 5 and 8
also provide further details of the design of low-head weirs.
(g) Navigation lock
Special provisions must be made at the barrage site if the river is navig-
able. Navigation locks with appropriate approaches, etc. must be provided
(Chapter 11).
9.1.4 Failures of weir foundations on permeable soils and their remedies
(see also Section 2.6)(a) Exit gradient (Ge) and piping
The exit gradient is the hydraulic gradient (Fig. 9.4) of the seepage flow
under the base of the weir floor. The rate of seepage increases with the
increase in exit gradient, and such an increase would cause ‘boiling’ of
surface soil, the soil being washed away by the percolating water. The flow
concentrates into the resulting depression thus removing more soil and
creating progressive scour backwards (i.e. upstream). This phenomenon is
called ‘piping’, and eventually undermines the weir foundations.
The exit gradient (tan; Fig. 9.4) according to the creep flow theory
proposed by Bligh (Khosla, Bose and Taylor, 1954) is
GeHs/L (9.1)WEIRS AND BARRAGES 369
Fig. 9.4 Seepage flow hydraulic gradients