Irrigation works

by E. S. Bellasis

The channels of a large irrigation system should run on high ground. In the case of a distributary, this is necessary in order that the water-courses may run downhill, and since the water in the canal and branches has to flow into the distributaries, the canal and branches must also be in high ground. Another reason for adopting high ground is that all the channels should, as far as possible, keep away from the natural drainage lines of the country and not obstruct them. Also a channel in high ground is cheapest and safest. When a channel is in low ground it must have high banks which are expensive to make and liable to breach. Every tract of country possesses more or less defined ridges and valleys. When the ridges are well defined, the irrigation channels, especially the distributaries, follow them approximately, deviating slightly on one side or the other from the very top of the ridge in order to secure a more direct course. If any part of a ridge is so high as to necessitate deep digging the channel does not necessarily go through it. It may skirt it and return to the crest of the ridge further on, especially if this arrangement shortens the channel or at least does not lengthen it much. A channel also goes off the ridge sometimes when adherence to it would give a crooked line. Of course all the channels—canals, branches and distributaries—have to flow more or less in the direction of the general slope of the tract being dealt with.

Fig. 2.

The alignments of the channels do not, however, depend exclusively on the physical features of the country. Centrality in the alignment is desirable. It will be shown ([Chap. II. Art. 10]) that a distributary works most economically when it runs down the centre of the tract which it has to irrigate. It is better to have short watercourses running off from both sides of a distributary than long watercourses from only one side. The same is true of a branch; it should run down the centre of its tract of country. Again the angles at which the channels branch off have to be considered. If branches were taken off very high up the canal and ran parallel to and not far from it, there would be an excessive length of channel. But neither should the branches be so arranged as to form a series of right angles. In the case shown in [Fig. 2] the size of the main or central canal would of course be reduced at the point A. By altering the branches to the positions shown in dotted lines their length is not appreciably increased while the length A B is made of the reduced instead of the full size. Moreover the course B C is more direct than BAC and this may be of the greatest importance as regards gaining the necessary command. When a channel bifurcates, the total wet border always increases and there is then a greater loss from absorption. The water is always kept in bulk as long as possible. If the alignment of a branch is somewhat crooked it does not follow that straightening it—supposing the features of the country admit of this—will be desirable. It may increase the length of distributaries taken off near the bends. It will be shown ([Chap. II. Art. 10]) that a distributary ought, when matters can be so arranged, to irrigate the country for two miles on either side of it, and watercourses should be two or three miles long. A distributary need not therefore extend right up to the boundary of the commanded area but stop two or three miles from it. Generally it is not desirable to prolong a distributary and make it “tail” into another channel ([Chap. II. Art. 3]). A distributary, like a canal, may give off branches.

None of the rules mentioned in the preceding paragraph are intended to be other than general guides, to be followed as far as the physical features of the country permit, or to assist in deciding between alternative schemes. It may for instance be a question whether to construct one distributary or two, between two nearly parallel branches. The two-mile rule may enable the matter to be decided or it may influence the decision arrived at as to the exact alignments of the branches. The flatter the country and the less marked the ridges the more the alignment can be based on the above rules. Sometimes, as in the low land adjoining a river, the ridges are ill defined or non-existent and the alignment is based entirely on the above rules. The rule as to following high ground need not be adhered to at the tail of any distributary if all the land to be irrigated at the tail is low and if there is a deep drainage line or other feature of the country such as to preclude the possibility of an extension of the distributary. Possible extensions should always be considered. In hilly districts an irrigation canal may have to run in sidelong ground along the side of a valley.

In flat valleys, owing to the land nearest the river having received successive deposits of silt in floods, the ground generally slopes away from the river and a canal can irrigate the low land even if taken off at right angles to the river. But to irrigate the high land near the river and the land where it rises again towards the hills or watershed, a canal taking off higher up the river is necessary. Of course much depends on whether the canal is to irrigate when the river is low or only when it is high, and whether or not there is to be a weir in the river. In Upper Egypt, it is common for a high level canal taking off far upstream, to divide into two branches, one for the land near the river and one for the land towards the watershed, and for both branches to be crossed—by means of syphons—by a low-level canal which irrigates the low ground. Similar arrangements sometimes occur on Indian inundation canals.

Regulators are usually provided at all off-takes of branches. In the case of a channel taking off from another channel many times its own size there is generally only the head regulator of the smaller channel but in other cases there is a regulator in each channel below the bifurcation. Thus, when the number of bifurcating channels is two it is called a double regulator. Regulators, with the “falls”—introduced to flatten the gradients when the slope of the country is too steep—and drainage crossings and the bridges, provided at the principal roads, constitute the chief masonry works on a canal. At a fall, mills are often constructed or the fall may be used for electric power.

Regarding curves and bends in channels, it is explained in River and Canal Engineering that, as regards increased resistance to flow and consequent tendency to silt deposit, curves of fair radius have very little effect, that a curve of a given angle may perhaps have the same effect whether the radius is great or small but that if the radius is large a succession of curves cannot be got into a short length, that a succession of sharp curves in a short length may have great effect, amounting to an increase of N in Kutter’s co-efficient, that a single sharp curve has not much effect, that the chief objection to such a curve is the tendency to erosion of the bank, that at a place where the channel has, in any case, to be protected, as for instance just below a weir or fall, there is no objection to the introduction of a sharp bend and that such bends, in fact right-angled elbows, exist without any evil effects at many regulators when the whole supply is being turned into a branch. It is remarkable that on perennial canals no advantage is ever taken of the last mentioned fact. Cases undoubtedly occur, though somewhat infrequently, in which the most suitable and cheapest arrangement would be to give a canal an abrupt bend at a fall. In order to reduce eddying, the bend need not be an absolute elbow but can be made within the length of the pitching which would be curved instead of straight. This is frequently done on inundation canals, without the slightest drawback, even when there is no fall, the pitching at bridges being utilised. A pitched bend can be made anywhere.

When a river floods the country along its banks as in parts of Egypt and of the Punjab, it is generally necessary to construct marginal embankments before irrigation can be introduced. The canal may take off at a point where flooding does not occur or it may pass through the embankment.[2] If it passes through at a point where flooding occurs, a masonry regulator is constructed to prevent the floods from enlarging the gap and breaking into the country.

[2] For detailed accounts of such embankments and canals see Punjab Rivers and Works (Spon) 1912.