Irrigation works

by E. S. Bellasis

A large canal is provided, so far as is practicable, with “escapes” by means of which surplus water may be let out. Surplus water occurs chiefly after rain. At such times the demand for water may suddenly be reduced and if there were no escapes there would probably be serious breaches of the banks before there was time for the reduction of water, effected at the head of the canal, to take effect lower down. There is usually an escape at some point in the main line, preferably at a point where it divides into branches, and this escape runs back to the river. There may also be escapes near the tails of the longest branches. These escapes may discharge into drainages or into reservoirs formed by running a low embankment round a large area of waste land.

The drainage of the whole tract irrigated by a canal must be carefully seen to. The subsoil water level of a tract of country is nearly always raised by an irrigation canal. The rise near to a canal or distributary is due to percolation from the channel and is inevitable.[3] The rise at places further away, if it occurs, is due to over-watering or to neglect of drainage. Immense damage has been done by “water-logging” of the soil when irrigation water has been supplied to a tract of flat country and the clearance and improvement of the natural drainages has not been attended to. Any drainages crossed by the banks of the irrigation channels should be provided with syphons or aqueducts or else the drainage diverted into another channel. Very frequently the main line of a canal—whether great or small—in the upper reaches near the hills, has to cross heavy drainage channels or torrents and large and expensive works are required for this.

[3] But see [Chap. V.] as to reduction of percolation.

Near the head of a canal and of every branch and distributary, there is an ordinary gauge which shows the depth of water and is read daily. The gauge near the head of a main canal is generally self-registering.

The principles sketched out in this article are those generally followed in the designs of modern canals. They have by no means been followed in all cases. In some of the older Indian canals both the canal and the distributaries ran in low ground. Water-courses took off direct from the canals, and the irrigation did not generally extend far from the canal. In fact long distributaries were impracticable because they would have run into high ground. The banks of the channels obstructed drainages and caused pestilential swamps. Most canals of this type have been abolished since the advent of British rule and replaced by others properly designed. Some badly designed canals however, mostly of the inundation class, still exist but in very dry tracts where drainages are of little consequence.

3. Information Concerning Canals.

—Nearly all canal irrigation is done by “flow,” the water running from the water-courses onto the fields, but a small proportion is done by “lift.” This is done in the case of high pieces of land, the lifting being usually done by pumps or, in the east, by bullocks or by manual labour.

Irrigation generally consists in giving the land a succession of waterings, one previous to ploughing and others after the crop is sown, each watering being of quite moderate depth. On inundation canals in India the waterings for the summer crop are thus effected but for the winter crop the land is deeply soaked during the flood season and is afterwards ploughed and sown. In Upper Egypt this system is emphasised, the water flowing into vast basins, formed by dykes, where it stands for some time and, after depositing its silt, is drained off.

Until recent times the whole of the irrigation of Egypt was basin irrigation. In Lower Egypt the construction of the Nile barrage led to the introduction of canals which take off at a proper level and their working is not restricted to the period when the river is in flood. In Upper Egypt most of the irrigation is still basin irrigation but the canals taking off above the Assiut barrage form a notable exception. By means of the Assouan dam which crosses the Nile, the water during the latter part of the flood season and after the floods are over, i.e. from November to March, is ponded up and a vast reservoir formed and the impounded water is let down the river in May and June.

In some of the older irrigation canals of India the velocity was too high and the channels have since had to be remodelled and the crests of weirs raised or new weirs built. The more recent canals are free from grave defects of this kind but every canal undergoes changes of some kind and finality has never yet been quite attained.