13.4.2. Rockfill Weirs with Sloping Aprons Figure 13.4 shows the longitudinal section of a typical rockfill weir whose main body consists of dry boulders packed in the form of glacis with few intervening walls. This type of weir is the simplest one, but requires a large quantity of stones for construction as well as maintenance. As such, this type of weir is suitable in areas where a large quantity of stones is available in the vicinity of the site and where labour is cheap.
H.F.L. 203.10
H.F.L. 202.5
Pond level 201.67
1:20
12 m
9 m
12 m
42.5 m
Fig. 13.4The Okhla weir (old) on the Yamuna river near Delhi
13.4.3. Concrete Weirs with Glacis Figure 13.5 shows the longitudinal section of a typical concrete weir in which the excess energy of overflowing water is dissipated by means of a hydraulic jump which forms near the downstream end of the glacis. Barrages are also constructed like concrete weirs. Design of such weirs is mainly based on the method proposed by Khosla et al. discussed in Chapter 9. On pervious foundations, only concrete weirs are constructed these days. Their detailed design requires the knowledge of: (i) the maximum flood discharge and corresponding level of the river at and near the selected site for weir, (ii) the stage-discharge curve of the river at the weir site, and (iii) the cross-section of the river at the weir site.
440 Pond level
1:4
IRRIGATION AND WATER RESOURCES ENGINEERING
Weir crest
1:5
Downstream glacis
Intermediate cutoff
Upstream cutoff
Downstream cutoff
Fig. 13.5 Concrete weir
Based on the site conditions, general and economic considerations, and other data, the designer decides (i) the afflux, (ii) the pond level, (iii) the minimum waterway (or the maximum discharge per metre length of weir), and (v) the weir crest level.
