Contents preface (VII) introduction 1—37

Fig. 13.2 Longitudinal section of Sarda barrage
The procedure of design of a barrage is similar to that of a weir. Weirs are of the following three types:
(i) Masonry weirs with vertical downstream face, (ii) Rockfill weirs with sloping apron, and

(iii) Concrete weirs with glacis.

13.4.1. Masonry Weirs with Vertical Downstream Face
Figure 13.3 shows the typical sketch of a masonry weir which consists of a horizontal masonry floor and a masonry crest with vertical (or nearly vertical) downstream face. Shutters provided at the top of the crest raise the water level further. During floods, these shutters are dropped down to pass the floods effectively and reduce the afflux upstream. The stability of the crest should be examined for the following conditions:
(i) The water level on the upstream side is up to the top of the shutters with no flow on the downstream side and all the water is diverted into the offtaking canal. The overturning moment caused by the water pressure on the upstream side must be resisted by the weight of the crest without any tension at its upstream end. The stability of the crest against sliding due to water pressure should also be examined.

CANAL HEADWORKS

439

Pond level 290.00

Crest level 289.18

			286.03
			1.5 m
			282.98
7.5 m	4.5 m	15 m	22.5 m
			Block protection

Fig. 13.3 The Bhimgoda weir (old) on the Ganga river at Hardwar
(ii) When the shutters are dropped down, water flows over the crest and the overturning moment is reduced due to the lowered water level on the upstream and presence of water on both sides of the crest. However, there will be some loss of weight (and, hence, the resisting moment) of the crest due to floatation because of the crest not being completely impervious. It is impossible to determine the amount of this loss of weight accurately. The reduced resisting moment is calculated on the basis of full weight of the masonry above the downstream level and submerged weight below the downstream level. The safety of the crest is examined for different stages of dis-charge up to the maximum flood discharge. At all such stages, the resisting moment must be more than the overturning moment and there should be no tension at the upstream end of the crest.