Profile walls ([Fig. 21], [page 92]) used occasionally to be built at frequent intervals along a distributary. They will not prevent scour occurring, if the stream is tending to scour, unless very close together. Such walls are of some use as showing whether the channel is altering, but they are expensive and have to be altered if, as often happens, the channel is remodelled. It is a much better plan to lay down blocks—about 1¹⁄₄ foot cubes—of masonry or concrete, along the centre line at every 500 feet, with their upper faces level with the bed. If the bed scours they may be displaced but otherwise they are useful not only for showing what silt, if any, has deposited, but for showing the centre line of the channel. Without them the centre line is liable to be altered in silt clearances or berm cuttings. To enable a block to be readily found and to be replaced in proper position if displaced, there should be two small concrete pillars exactly opposite to it and equidistant from it, one on either bank of the channel. Such blocks and pillars may with advantage be placed at quite short intervals on curves.
The rest houses for the use of officials on tour are generally at intervals of about 8 to 14 miles. There is generally a rest house near to a large regulator and frequently there is one near to a small regulator. This facilitates inspection work and discharge observations and it saves money, because the house can be looked after by one of the regulating staff. Not infrequently the house is placed just too far away from the regulator. Similarly if a rest house is near a railway station it should be within a quarter of a mile of it—always provided that this does not bring it too near to villages or huts—and not a mile or more away as is sometimes the case. It is also a mistake to place a rest house off the line of channel unless perhaps when it is on a district road which crosses the channel.
CHAPTER III.
The Working of a Canal.
1. Preliminary Remarks.
A large canal is under a Superintending Engineer and it often constitutes his sole charge. It consists generally of three to five “divisions,” each under an Executive Engineer. A division has two to four subdivisions, each under a Subdivisional Officer. A subdivision is divided, for purpose of engineering work and maintenance, into several, generally three or four, sections, each consisting of some 20 miles of canal and some 40 miles of distributary, and being in charge of a native overseer or suboverseer, and for purposes of water distribution and revenue, into a few sections each having, perhaps, some 30,000 acres of irrigation and being in charge of a native zilladar. As far as possible the boundaries of divisions and subdivisions are co-terminous with those of the branches of the canal. A distributary is always wholly within a subdivision. At every regulator there is a gauge reader, who, supplied when necessary with permanent assistants, sees to the regulation of the supply. If there is a telegraph office at the regulator the telegraph “signaller” may have charge of the regulation. The zilladar has a staff of some ten or twelve patwaris, who record in books the fields watered and who are in touch with the people and know when the demand for water is great, moderate or small, and for what kind of crops it is needed. In each division there is generally a Deputy Collector who is a native official, ranking as a Subdivisional Officer. His duty is to specially supervise the revenue staff in the whole division. Both he and the Subdivisional Officer have magisterial powers which are exercised in trying petty cases connected with the canal.
Along a main canal and its branches there is nearly always a “canal dak” or system of conveyance of bags containing correspondence for the officials stationed on the canal or touring along it. Along the main line, and most of the way down the branches, there is a line of telegraph for the special use of the canal officials. The telegraph offices are at the chief regulators, with tapping stations, for the use of officials on tour, at the rest houses near to which the line runs.
However carefully a canal has been designed, alterations in the channels from silting and scour soon take place and they go on more or less without cessation. In a distributary, especially if the gradient has of necessity been made somewhat flat, there is quite likely to be a deposit in the upper reach. The deposit is generally greatest at the head and decreases, in going downstream, at a fairly uniform rate. It may extend for half-a-mile or less or more. Or a deposit may occur on the sides, which grow out and contract the channel. This often occurs over a great length of a distributary or even over the whole of it. Sometimes a distributary scours its bed, or the sides may fall in somewhat. Clearances of the silt and cutting of the berms are effected at intervals. Falling in of the sides may be stopped by means of bushing, and scour of the bed may be stopped by raising the crest of a fall or by introducing a weir, but in the meantime the changes cause the discharge tables for the distributary to become more or less erroneous. In many cases silt deposits in the upper part of the distributary during the summer months when the river water is heavily silted and scours away again in the winter, the régime of the channel being, on the whole, permanent. The changes which occur in the branches and main canal are similar to the above and the remedies adopted are similar. On some of the older canals the scour was so serious that many intermediate weirs had to be constructed. The remarkable silting in the head reach of the Sirhind Canal has been described in River and Canal Engineering, Chapter V. The remedy consisted in keeping the gates of the under-sluices properly closed so that a pond was formed in which the river silt deposited. When necessary the canal is closed, the sluices opened, and the silt scoured away. For a plan of the headworks see [fig. 24.]
In working a canal, it is necessary to arrange so that the water sent down any channel is as nearly as possible in accordance with the demand. The zilladar supplies the Subdivisional Officer, every week or ten days, with an “indent” showing how much water is required in each distributary and the Subdivisional Officer makes indents on the subdivision next above. The officer in charge of the headworks thus knows what the demand is. When it is more than the supply available, the water is dealt out to the various divisions according to rules approved of by the Superintending Engineer of the canal.
