In making a contour survey, a base line, as centrally situated and as long as possible, should be laid down, with side lines parallel to it near the boundaries of the tract. The cross lines at half-mile or other intervals should then be laid down. Some of them may run out beyond the side lines. Circuits of levels should be run along the base line, the side lines and the two extreme cross lines and be carefully checked. The remaining cross lines should then be levelled. All the levels having been shown on the map the contours should be sketched in. The scale of the map for a large project may be two inches to a mile. If it is likely that the survey will have to be extended, it will be easier to do this by prolonging the base line and running more cross lines, than by prolonging each of the cross lines already surveyed. This can be borne in mind when selecting the base line.
3. Alignments and Discharges.
—On the contour map the proposed alignments of the canal, branches, distributaries, and escapes, determined after careful consideration of all matters affecting them, are shown. The tracts to be irrigated by each branch and each distributary are now marked off, the “irrigation boundaries” following approximately the valleys and lines of drainage. Any large tracts of land which cannot be irrigated are of course shown and are excluded. Forests or other lands which are not to be irrigated should be similarly dealt with, otherwise confusion is likely to arise later. The commanded area dependent on each distributary is now ascertained from the map. A certain percentage being deducted for scattered unculturable areas the culturable commanded areas are obtained. The proportion to be irrigated (in India in the kharif) having previously been decided, the number of acres to be actually irrigated by each distributary is arrived at.
The next step is to ascertain the discharges.[9] A general duty for the whole canal having been estimated by considering the actual figures for other canals the full supply of the canal at its head is arrived at. ([Chapter I, Art. 6]). In Northern India it will be the kharif duty and kharif full supply. Since some water is lost by absorption in the channels, the duty of the water on a branch is higher than that of the whole canal based on its head discharge, and the duty on a distributary is higher still. In designing a canal, an attempt has to be made to estimate the losses of water in the main canal and branches, so that the duties of the branches and distributaries may be estimated and the channels designed accordingly. On the Western Jumna Canal the figures were estimated to be as follows:—
| Kharif. | Rabi. | |
|---|---|---|
| Average discharge at canal head (c. ft. per sec.) | 3536 | 2755 |
| Duty based on the discharge (acres) | 98 | 138 |
| Estimated loss of water in canal and branches (c. ft. per sec.) | 400 | 300 |
| Average discharge at distributary head (c. ft. per sec.) | 3136 | 2455 |
| Duty based on the discharge (acres) | 111 | 154 |
[9] In this Article and in the rest of this Chapter it is assumed that the canal is a Northern Indian one. Any modifications necessary to suit canals in other countries will readily suggest themselves.
The question of duty is one which if not carefully considered, may cause some confusion. A canal and branches, having been designed with certain assumed duties and with discharges based on certain values of N in Kutter’s co-efficient, have, let it be supposed, been constructed to a greater or less extent. When the time comes for constructing the distributaries, the engineers concerned may have different ideas, based on later experience, as regards the probable duty and the most suitable value of N. If they design the distributaries with a higher duty and a lower value of N, it is obvious that they can provide more distributaries than at first designed, or can increase their lengths. In either case they would provide for an increased commanded area. If they do not do this, they ought to adhere to the values at first proposed, thus making the channels larger than, according to their ideas, would be necessary. These larger channels will be able to do more irrigation, by an increase, not in the commanded area, but in the proportion of it which is irrigated. Any other course would result in the canal carrying more water than could presumably be used by the distributaries. Again, the question how the assumed duty was arrived at may need consideration. It may have been arrived at by taking the duty figures of some existing canal, based on discharge figures which were the result, not of observed but of calculated discharges, and if the calculations were based on a value of N which experience has proved to be wrong, a correction is obviously needed. Many mistakes of the kinds indicated above have been made, not perhaps in the case of a project which has been recently got up and is then quickly carried out in its entirety, but in one which is carried out slowly or after a long period has elapsed or in one which consists of extensions of an existing system. So great, however, is the elasticity of a channel—by which is meant its capacity for adapting itself to varied discharges, a small change in the depth of water causing a great change in the discharge—and so considerable has been the uncertainty as to the real duty to be expected, that any mistakes made have not usually resulted in any serious trouble.
BIFURCATION AT TAIL OF CANAL.