Contents preface (VII) introduction 1—37

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436 IRRIGATION AND WATER RESOURCES ENGINEERING
economical alternative. The following features of the two stages should be considered while selecting the site for headworks.
(i) The initial cost of headworks in the boulder stage is generally smaller than that in the alluvial stage because of: (a) local availability of stones, (b) smaller width of river (requiring smaller length of weir), (c) smaller scour depths which reduce the require-ments of cutoffs and other protection works, and (d) close proximity of higher banks which requires less extensive training works.
(ii) An irrigation canal offtaking from a river in the boulder region will have a number of falls which may be utilised for generation of electricity. There is almost no scope for the generation of electricity in this manner in the alluvial reach of a river.
(iii) If the existing irrigation demand is less but is likely to develop with the provision of irrigation facilities, it is desirable to divert the river water into an irrigation channel by constructing a temporary boulder bund across the river. This bund will be washed away every year during the floods and will be reconstructed every year. This will, no doubt, delay the Rabi crop irrigation, but it is worthwhile to use temporary bunds for a certain period; when the irrigation demand grows, permanent headworks may be constructed. In this manner, it would be possible to get returns proportional to ex-penditures incurred on the headworks. Construction of temporary bunds is gener-ally not possible in the alluvial stage of the river.
(iv) An irrigation channel offtaking in the boulder stage of a river will normally require a large number of cross-drainage structures.
(v) Because of the nature of the boulder region, there is always a strong subsoil flow in the river bed. This causes considerable loss of water and is of concern during the periods of short supply. Similarly, there will be considerable loss of water from the head reach of the offtaking channel. In alluvial reach of the river this loss of water is much less.
(vi) The regions close to the hills usually have a wet climate and grow good crops. The irrigation demand in the head reach of the channel offtaking in the boulder stage is, therefore, generally small. However, this demand would increase with the provision of irrigation facilities. In alluvial regions, the demand for irrigation is high right from the beginning.
Once the stage of a river has been chosen for locating the headworks, the site of the headworks is selected based on consideration of its suitability for the barrage (or weir), the undersluices, and the canal head regulator.
For irrigation purposes, the site for headworks should result in a suitable canal alignment capable of serving its command area without much excavation. For siting the headworks, the river reach should, as far as possible, be straight and narrow and have well-defined and non-erodible high banks. In the case of a meandering river, the headworks should be located at the nodal point.
From sediment considerations, the offtaking channel should be located at the downstream end of the outside of a river bend so that it has the advantage of drawing less sediment. However, a curved reach would need costly protection works against the adverse effect of cross currents. Moreover, if canals take off from both the banks, the canal taking off from the inner bank draws relatively more sediment.

CANAL HEADWORKS

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In order to ensure adequate supply to the offtaking canal at all times, the undersluice should be sited in the deep channel. A river reach with deep channels on both banks and shallow channel at the centre is more suitable when canals take off from both sides.

Besides, the site must be accessible and suitable for making the river diversion and other related arrangements at a reasonable cost.