Chapter 9
Diversion works
9.1 Weirs and barrages; worked examples
9.1.1 General
Weirs and barrages are relatively low-level dams constructed across a river
to raise the river level sufficiently or to divert the flow in full, or in part,
into a supply canal or conduit for the purposes of irrigation, power genera-
tion, navigation, flood control, domestic and industrial uses, etc. These
diversion structures usually provide a small storage capacity. In general,
weirs (with or without gates) are bulkier than barrages, whereas barrages
are always gate controlled. Barrages generally include canal regulators,
low-level sluices to maintain a proper approach flow to the regulators, silt-
excluder tunnels to control silt entry into the canal and fish ladders for
migratory fish movements.
Weirs are also used to divert flash floods to the irrigated areas or for
ground water recharging purposes. They are also sometimes used as flow-
measuring structures. Figure 9.10 gives a detailed description of the
various parts of a typical barrage constructed on rivers flowing over per-
meable beds (see also Baban, 1995).
The site selection of a barrage depends mainly on the location and
elevation of the off-take canal, and a site must be selected where the river
bed is comparatively narrow and relatively stable. The pondage require-
ment and interference with the existing structures such as bridges, urban
development, valuable farmland, etc., must be considered, as well as avail-
able options to divert the flow during construction.