9.1.3 Barrage components
(a) Guide banks
Guide banks direct the main river flow as centrally as possible to the diver-
sion structure. They also safeguard the barrage from erosion and may be
designed so that a desirable curvature is induced to the flow for silt exclu-
sion from the canals. The side slopes of the guide banks must be protected
by stone pitching, with a sufficient ‘self-launching’ stone apron at the
lowest feasible level (Fig. 9.2). The top levels of the guide banks will
depend on the maximum increase in the flood level upstream of the
barrage. The afflux (level difference between the headwater and tailwater
during the passage of maximum flood flow) results in a backwater curve
upstream of the barrage, and flood banks have to be provided along the
upstream reach of the river to contain the flood flow.
(b) Wing walls
Wing walls flanking the barrage and supporting the abutting earth bunds
are designed as retaining walls. Cut-off walls (taken below the scour
levels) below the wings and abutment walls at both sides, in addition to the
upstream and downstream sheetpile cut-offs across the river, form an
enclosed compartment providing good weir foundation conditions.
(c) Gates
Gates used on barrages are of the same type as those used on spillway
crests (Chapter 6). Vertical lift and Tainter gates are most frequently used
to control the flow rate over the crest (sill) of a barrage. The discharge
capacity of a gated crest depends on the conditions of free (Chapter 4) or
366 DIVERSION WORKS
Fig. 9.2 Design criteria for launching aprons on weir and guide banks
