Irrigation works

by E. S. Bellasis

In cases where it is not desired to incur much expenditure, it may be a good plan to construct watercourses to a cross section somewhat larger than that ultimately desired. The silt deposited on the bed and sides forms, in most cases, a more impervious lining than the original soil. The same plan can be adopted in the tail portion of a distributary. In a larger channel there would be less certainty that any deposit would take place unless short lengths, at frequent intervals, were excavated to the true or ultimate section, so as to form weirs and spurs; and even these might not stand.

In Italy, in cases where the water naturally contains lime in suspension, the beds of canals have become gradually watertight by the deposit of lime in the channel.[53] In some cases lime has been artificially added. It appears that a considerable period of time is necessary for the process.

[53] Min. Proc. Inst. C. E. Vol. CXVI.

3. Modules.

—A module is an appliance which automatically gives a constant discharge through an aperture, however the water level on either the upstream or downstream side of the aperture may fluctuate. In an old and simple form of module there is a horizontal orifice in which works loosely a tapering rod attached to a float. The water passes through the annular space surrounding the rod. If the water level rises, the rise of the float brings a thicker part of the rod to the orifice and reduces the annular space. In another kind of module the water is discharged through a syphon. If the water level alters, the syphon moves in such a way that the head, or difference between the levels of its two ends, remains the same. The great objections to modules are that they are liable to get out of order or to be tampered with. A module recently invented and patented by Gibb[54] has no movable parts, and is not liable to these objections.

[54] For description see [Appendix H].

A few years ago the question of the desirability of using modules for the outlets of distributaries in India was raised. The opinions of a large number of the senior canal engineers were called for and considered, and since then the subject has been thoroughly discussed. There are certain inherent difficulties in the way of moduling the outlets of a distributary. Owing, for instance, to rain further up the canal, or to the closure of a distributary owing to a breach in it, the canal supply may increase, and it may be necessary to let more water into the distributary under consideration. Under the present system any excesses of water are automatically taken by the outlets. If all outlets were rigidly moduled they would discharge no more than before the excess supply came in, and the excess supply would all go to the tail of the distributary, and, most likely, breach the banks. To get over this difficulty, the module has to be so arranged that when the water level in the distributary rises to a certain “maximum limit” the module ceases to act as such, and the discharge drawn off from the distributary increases as the water level rises. Again, the discharge of the distributary may at times be considerably less than its full supply. In order that, in such a case, the outlets towards the tail of the distributary may not be wholly deprived of water, it has to be arranged so that when the water level in the distributary falls below a certain “minimum limit” the modules cease to act as such, and draw off supplies which are less the lower the water level. Such supplies are not in proportion to the full supplies of the outlets. It will, however, be shown presently that low supplies need seldom be run. When a distributary, say the upper reach, contains silt, the water level corresponding to a given discharge is higher than before, and care has to be taken that the maximum limit is high enough. At the same time the minimum limit must be so low that it will not be passed when the silt scours out. The difference between the maximum and minimum limits is called the “range” of the module.

In Gibb’s module the above conditions can be complied with. The module is placed outside the bank of the distributary. The water is drawn off from the distributary by a pipe, whose lower edge is at the bed level of the distributary, and delivered from the module into the watercourse through a rectangular aperture at a higher level than that of the pipe. It is possible that, owing to the high level of the aperture, some rolling silt which would otherwise have passed out of the distributary may remain in it. The height of the aperture also prevents the watercourse from drawing off any water at all when the water level of the distributary falls below a certain level, but this objection is not important. An escape weir or notch is provided so that when the water level in the distributary rises to the maximum limit some water overflows into the watercourse. On the whole it appears that all difficulties can be got over, though a good deal of care and precision is necessary in fixing the exact height of the maximum and minimum limits.

The difficulties under consideration will all be reduced if some of the outlets on a distributary are left unmoduled, and this is desirable on other grounds. When the supply is normal, i.e. between the maximum and minimum limits, and all modules are working, the supply entering the distributary must be regulated with great precision. The outlets draw off a certain supply. If less than this enters the distributary the tail outlets must go short. If more enters there will be a surplus at the tail, though it can probably be disposed of, because the tail water will rise above the maximum limit. For short periods, say an hour or two, no trouble arises because the distributary acts as a reservoir, the water level rising to take in any excess supply, and falling to allow for a deficiency. At the tail the rise and fall may be hardly perceptible. But if the supply were deficient for a whole night the tail outlets would certainly go short. This could theoretically be remedied to some extent by letting in an excess supply for a short time and causing the water level at the tail to rise above the maximum limit, but in practice no such system of compensation could be worked. The very fact of the tail outlets having gone short for a night would not be known. The proper method of preventing any such troubles as those under consideration is to leave some of the outlets on the distributary un-moduled.

It has been more than once mentioned that there are periods when a distributary is run, not full, but about three-fourths full. If that were done in the case of a distributary whose outlets were mostly moduled, the water level would probably be below the minimum limit, and the modules would not be acting as such. The outlets would not, under these circumstances, obtain their proper proportionate supplies. This difficulty can, no doubt, be got over by running the distributary full for short periods at a time instead of three-fourths full for longer periods. The people, when once they understood the case, could arrange to use the water in greater volume for two days instead of in smaller volume for three. If this arrangement comes into force it will not be necessary to design distributaries—see [Chapter III, Art. 4]—so as to have a good command when three-fourths full supply is run.