Contents preface (VII) introduction 1—37

Fig. 13.1 Typical layout of headworks (1)
13.4. WEIR (OR BARRAGE)
A weir is an ungated barrier across a river to raise the water level in the river. It raises the water level in the river and diverts the water into the offtaking canal situated on one or both of the river banks just upstream of the weir. Weirs are usually aligned at right angles to the direction of flow in the river. Such weirs will have minimum length and normal uniform flow through all the weir bays thereby minimising the chances of shoal formation and oblique flow
(1).
To increase the water level, the weir crest is raised above the river bed. Part of the raising of the water level is obtained by shutters provided at the top of the weir crest. These shutters are dropped down during floods so that the afflux is minimum. The afflux is defined
(2) as the difference in water level between the upstream and downstream of a structure under free flow conditions as a result of construction of the structure across a river. Controlling pond levels by means of shutters becomes difficult when the difference between the pond level and the crest level is higher than 2.0 m. In such cases, a gate-controlled weir, better known as

438 IRRIGATION AND WATER RESOURCES ENGINEERING
barrage, is preferred. Barrage is a gate-controlled weir with its crest at a lower level. A barrage and weir are similar structures and differ only in a qualitative sense. The crest of a barrage is usually at a lower level and the ponding up of the river for diversion into the offtaking canal is achieved by means of gates (instead of shutters). Barrages are considered better than weirs due to the following reasons:
(i) Barrages offer better control on the river outflow as well as discharge in the offtaking canal.
(ii) With proper regulation and with the help of undersluices and sediment excluders, the upstream region in the vicinity of the headworks can be kept free of sediment deposition so that sediment-free water enters the offtaking canal.
(iii) Because of the lower crest level of a barrage, the afflux during floods is small.
(iv) It is possible to provide a roadway across the river at a relatively small additional cost.
Because of these advantages, barrages are usually constructed at the site of headworks on all important rivers. At some barrages, the raised crest may not be provided at all and the complete ponding is obtained only by means of gates. Figure 13.2 shows the longitudinal section of Sarda barrage which does not have a raised crest.


H.F.L.226.00
Pond level 222.65

219.60	219.00	1:20
			Block
		protection
9 m	33.5 m	24.5 m