CHAPTER V.
PROPOSED IMPROVEMENTS IN IRRIGATION CANALS.
1. =Preliminary Remarks.=--The chief improvements which have been under consideration during recent years are three in number. The first is increased economy of water in its actual use in the fields; the second is reduction of the losses by absorption in the channels; and the third is distribution by means of modules.
Regarding the first, it has long been known that the ordinary methods of laying on the water are more or less wasteful. In California, when the water instead of being applied to the surface of the ground, is brought in a pipe and delivered below the ground level, the duty is increased from 250 to 500 acres. In India a field is divided, by means of small ridges of earth, into large compartments. The water is let into a compartment and gradually covers it. By the time the further side is soaked the nearer side has received far too much water. Frequently the water for a compartment, instead of being carried up to it by a small watercourse, is passed through another compartment and this adds to the waste. Also the number of waterings given to a crop is often 5 or 6, when 4 would suffice. Experiments made on the Upper Bari Doab Canal, by Kennedy, showed that the water used in the fields was nearly double what it might have been. The 53 c. ft. shown in Chapter 1, Art. 4, as reaching the fields, were used up when 28 c. ft. would have sufficed. It is not certain that the waste is generally quite as much as the above. It is possible that the restricted supplies might have given smaller yields of crops. More recent experiments made by Kanthack on the same canal give the needless waste as about 25 per cent. The field compartments ought, according to Kennedy, to be 70ft. square, the small branch watercourses being 140ft. apart. It would be better to have still smaller compartments, but this would be rather hard on the people.
At one time Government issued orders, in Northern India, that compartments of 1296 square feet were to be used, and that, otherwise, increased water rates would be charged, but the orders were never enforced. They were thought to press too hardly on the people. Extreme measures for enforcing economy in the use of water in any country are likely to be introduced only when they become absolutely necessary owing to the supplies of water being otherwise insufficient.
2. =Reduction of Losses in the Channels.=--For several years experiments have been going on in the Punjab as to the effect of lining watercourses with various materials. The following conclusions have been arrived at[51]:--
[51] _Punjab Irrigation Paper_ No. 11 C. “Lining of Watercourses to reduce absorption losses. Experiments of 1908-1911.”
I. ORDINARY UNLINED TRENCHES.
(_a_) The rate of absorption varies greatly, and this is due probably to unequal fissuring of the upper layers of the soil.
(_b_) The rate of absorption in the three hottest months averaged ·0571 feet per hour, or more than double the rate (·026) in the three coldest months. The difference is ascribed to the greater viscosity of the water when cold.
(_c_) The average losses with canal water were ·0315 feet per hour, or 8·75 c. feet per second per million sq. feet.[52] With well water the figures were ·1096 and 30·5. The conclusion is that the silt in canal water reduces the losses by more than two-thirds.
[52] This loss of 8·75 c. ft. per second was in water only about a foot deep. This confirms the conclusion arrived at in Chapter I, Art. 4, that the depth of water is not a factor of much importance.
(_d_) With canal water the average loss decreased by 40 per cent. (from ·0491 to ·0293) in about four years. This was no doubt due to the effect of the silt. With well water the loss at the end of four years (·2293) was nearly four times as great as at first (·0591). This may have been due to removal of the finer particles of soil by the water, but the experiments were made at only one place, and were not conclusive.
II. LINED TRENCHES.
(_e_) With trenches lined with crude oil ¹⁄₁₆ inch thick, or with Portland cement ¹⁄₁₆ inch thick, or with clay puddle 6 inches thick, the “efficiency ratios,” as compared with unlined trenches, are respectively about 4·0, 5·7 and 5·7, the age of the lining being four years. The efficiency ratio is the inverse of the loss. Thus with an efficiency ratio of 3 the loss in the lined trench is 33 per cent. of that in the unlined trench.
(_f_) The efficiency ratio in the case of oil may diminish at the rate of 10 per cent. per annum, but in the case of cement and clay puddle it tends to increase rather than to decrease.
Assuming that the efficiency ratios are only 3·0, 4·5 and 4·5, and that the loss in an unlined channel is 8 c. feet per second per million sq. feet, the saving in water by using channels lined with oil, cement and puddle respectively would be 5·33, 6·25 and 6·25 c. feet per second. The average duty of the water at the canal head is about 242 acres, and the average revenue per acre is Rs 3·93. The revenue from 1 c. ft. of water at the canal head is thus Rs 950. Only about half the water reaches the fields (Chapter I., Art. 4), and the revenue from 1 c. ft. of water which reaches the fields is about Rs 1900. The mean of the above two sums is Rs 1425. If 6 c. ft. of water per second could be saved the revenue would be increased by Rs 8,550 per annum.
The cost of lining a million square feet of channel with oil, cement and puddle is estimated at Rs 30,000, Rs 27,500 and Rs 35,000 respectively. Allowance has to be made for the fact that watercourses flow intermittently, and that a lined channel gives no saving when it is not in flow, also that extensions of canals might have to be undertaken in order to utilise the water saved. After making these allowances it is estimated, in the paper above quoted, that the saving effected by lining a million square feet with oil, cement or puddle represents the interest on a capital sum of Rs 69,300, Rs 81,250 and Rs 81,250 respectively, or 2 or 3 times the sums sunk in constructing the linings.
Hitherto the experiments have been carried out on a moderate scale, but extensive operations are now being undertaken on the Lower Chenab Canal, and possibly on others.
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.
On nearly every distributary there are some watercourses whose command is bad, and it has been stated (Chapter II, Art. 9) that in an ordinary unmoduled distributary the sizes of the outlets in such cases should be extremely liberal. To module any such outlet would cause a lowering of the water level in the watercourse and would interfere with the irrigation. Such outlets should not be moduled. Again, there are some few outlets which are not submerged, _i.e._, there is a free fall into the watercourse. The discharge does not depend on the water level in the watercourse, and it is not affected by any enlargement or clearance of it. It depends only on the water level in the distributary. This water level, if most of the outlets are moduled, will be fairly constant. Such outlets need not be moduled, and they should not be moduled unless the other unmoduled outlets in the reach concerned are sufficiently numerous, and perhaps not even then, because moduling involves some expense.
A distributary generally has some falls which divide it into reaches. Immediately upstream of a fall the water level for a given discharge is not affected by the silting or scouring of the channel. Any outlets near to and upstream of the fall are less subject than others to variation in discharge, and are suitable for non-moduling in case a sufficient number of unmoduled outlets is not otherwise obtainable.
Regarding the watercourses at the extreme tail of a distributary it has been pointed out (Chapter III., Art. 7) that in an ordinary case they should not be left without masonry outlets, because they may then lower the water level and so unfairly reduce the supply of any watercourse, even though upstream of them, which has such an outlet. But any outlets near the tail of a distributary can suitably be left unmoduled because of the difficulty of ensuring that the supply at the tail shall be exactly what is needed.
Gibb’s modules have been tried on various distributaries in the Punjab and found to give good results. It is believed however that in only one case has a whole distributary been moduled. The distributary is a large one, its length being 35 miles. It appears that the discharge reaching the tail of the distributary is not constant but varies, as was to be expected, when the head discharge varies for any length of time. The command on the distributary is good. There is nothing to show that matters would not have been improved, and money saved, by leaving some of the outlets without modules.
It has been remarked above, that at the downstream end of a reach ending in a fall, the F.S. level of a distributary is not affected by silt. At the upstream end of the reach it is affected. There are thus two gradients, one flat, and one steep. It appears to have been decided in one case in the Punjab, that the minimum limit of supply for the module should be about half an inch below the flat line and the maximum limit ·3 feet above the steep line. In many cases a greater range would be required,[55] say a foot.
[55] It is understood that a range of a foot can easily be arranged for, and that ranges of 3 or 4 feet can be introduced at slightly increased cost.
In Chapter III. Art. 7, the case of a distributary without modules but with the outlets carefully adjusted, was considered. The question to be decided in each case is whether such an arrangement is preferable to moduling some of the outlets. This turns largely on the amount of attention which would be bestowed on the case. In view of the difficulty of securing such attention and of the trouble of constantly making alterations in a certain number of outlets, it is probable that moduling will in many cases be considered preferable.
The question of moduling the heads of distributaries has also been considered in the Punjab. For minor or small distributaries modules are feasible. For a large distributary a module would be expensive and it appears that the present system of regulating is preferable.
Kennedy’s “Gauge Outlet,” which is a kind of semi-module is described in Appendix K. It is being tried in the Punjab.
APPENDICES.
APPENDIX A.
DIVIDE WALL ON LOWER CHENAB CANAL.
(See page 50, first footnote.)
The Gagera branch of the Lower Chenab Canal--the left-hand branch in fig. 27--was found to silt. It was proposed to make a divide wall (fig. 27) extending up to full supply level. The idea is unintelligible. The silt does not travel by itself but is carried or rolled by the water. As long as water entered the Gagera branch, silt would go with it. The authorities, who had apparently accepted the proposal, altered the estimate when they received it, and ordered the wall to be made as shown dotted and of only half the height. This was done. The idea seems to have been that the wall would act as a sill and stop rolling silt. This is intelligible, but such sills do not always have much effect on rolling silt. Moreover, there was a large gap, A B, in the wall. The work is said to have proved useless, and proposals have been made to continue the wall from A to B. In this form it is conceivable that it may be of use.
APPENDIX B.
SPECIFICATION FOR MAINTENANCE OF CHANNELS.
(See page 138.)
I. ROADS AND BANKS.
1. =Filling Holes.=--Holes to be all dug out and thoroughly opened and inspected, then to be filled in with rammed earth. Never to be filled in a hurry or without digging out.
2. =Dressing.=--Heavy soil to be dressed even. Light sandy soil to be disturbed as little as possible, and grass in such soil not to be removed except when in large tufts. When dressing is done, the road to be given (as far as possible) a transverse slope from the canal side of about 1 in 50.
3. =Trees.=--Branches to be lopped so as not to obstruct riders. Great care is needed to see that the men do not lop needlessly high. Roots, if projecting on road, to be covered up or cut out.
4. =Petty Repairs.=--Settlement or wearing down, if slight, should be made good on maintenance estimates, otherwise on special estimates. Cracks should be dug out and filled in and rammed. Old “dead men” or walls of earth should be utilised or at least levelled down.
5. =Sand or “Reh” Soil.=--Can be dug out to a depth of 9 inches and removed to a distance, and (the places having been inspected by the Subdivisional Officer) replaced by good soil got from pits or berms, the places being selected with care. If the lead is slightly askew, the stuff removed can be put in the same pits from which earth is got.
6. =Laying long coarse Grass on Road.=--This can be done in cases where the removal of sand or “reh” is not practicable or has proved ineffective. The grass is laid crosswise to prevent wheels sinking in.
II. JUNGLE AND TREES.
1. =Jungle.=--To be cut close to the ground or to be dug out by the roots when ordered. To be burned as soon as dry. Dead branches, twigs, etc., to be burned or removed to rest-houses, and not left about on canal land. Precautions to be taken against damage by fire to forests, etc. Clearance to include the channel[56] and both roads, and any jungle on the slopes of the spoil which obstructs the roads.[57]
[56] Jungle on inside slopes not to be cleared where banks fall in or where channel is too wide.
[57] When an embankment runs parallel to an inundation canal, a chain or so distant, the intervening space need not be cleared, nor need the top of a bank be cleared if it is so uneven that it is not a road.
2. =Trees.=--Trees which fall into a channel or across a road to have their branches cut away at once. The trunk to be removed so far as is possible. Trees which are dead or broken off should be felled, also those which have been blown into inclined positions, unless bad gaps will be caused. Trees (unless required for stock) to be sold as they lie and removed, including the parts below ground, by purchasers, within a fixed time. Logs, etc., not to be left lying about on canal land. Stumps, etc., to be made into charcoal and the holes filled up.
_Note._--The above works (Parts I. and II.) to be done immediately after the rains (repairs to roads and removal of trees, branches, etc., being also done during the rains or whenever necessary) and finished at latest by 31st October.
III. CATTLE CROSSINGS OR GHÁTS.
1. =Repairs.=--Gháts to be dressed, strengthened, and kept neat, the bank being thrown back and curved so as to give a long inner slope, and lumps, etc., levelled off.
2. =Closures.=--To be closed (by order of Subdivisional Officer and no one of lower rank) only when very near to a bridge or near to another ghát.[58] If closed, to be staked up and bushing to be added. Not to be closed by loose thorny branches. Not to be allowed close to any milestone, outlet, etc.
[58] Regarding gháts at bridges, see Chap. II., Art. 12.
3. =Small Gháts.=--Gháts where only foot-passengers cross, can run diagonally up the slopes or as may be convenient. They should be dressed and kept in order.
4. =Canal Road at Gháts.=--At all gháts care must be taken that the canal road, especially if used for driving, is not cut up and is kept in proper order.
IV. MISCELLANEOUS ITEMS.
1. =Rubbish or Obstructions in Bed of Channel.=--To be removed from the channel when it is laid dry, and not left till it is about to be reopened.[59] Old stakes, etc., to be sawn off when crooked or too high.
[59] Where the bed is too low, no rubbish clearance should be done except in the case of very large snags, etc.
2. =Temporary Aqueducts or Damaged Wooden Bridges.=[60]--To be removed before water is expected (but not sooner than is necessary) and the banks repaired and made good.
[60] This applies to inundation canals.
APPENDIX C.
SPECIFICATION FOR MAINTENANCE OF MASONRY WORKS.
(See page 138.)
1. =General Repairs.=--Masonry, plaster, pitching, etc., to be kept in repair. Pitching, where defective or out of line, to be made right. Bumping posts to be fixed in proper positions. Earth to be added to ramps, etc., where needed. Metalling to be regularly seen to. Needles, planks, hooks, railings, winches, lamp-posts, lamps, etc., to be kept in order and complete. Bricks, bats, etc., to be properly stacked. Needles, etc., to be neatly stacked on rests or with bricks under them. All surplus and useless needles, etc., to be removed. Huts to be kept in repair. Extra mud walls or screens not to be allowed when unsightly. All verandah openings to be edged with a 6-inch band of whitewash.
2. =Jungle.=--All masonry to be kept free from jungle growth, and all piers free from caught jungle. For this purpose long bamboo weed-hooks to be supplied.
3. =Dressing, etc.=--Rubbish, lumps of earth, logs, etc., to be cleared away, pits and holes filled up. Banks, slopes, etc., of main and branch channels in the neighbourhood of the work to be specially levelled and dressed.
_Note._--All works should be specially seen to in October, and everything be in order by 31st October.
APPENDIX D.
WATCHING AND PROTECTING BANKS AND EMBANKMENTS.[61]
(See page 138.)
[61] This is reprinted from _Punjab Rivers and Works_. It was drawn up for inundation canals and flood embankments.
1. =Watching.=--Every watchman employed to have a fixed headquarters and a fixed beat. If there is no permanent hut on or near the bank, grass huts should be erected by the men at the places fixed. The presence or absence of the men to be frequently tested by the mate and suboverseer. The suboverseers tests to be recorded in a book and to form the subject of frequent inquiry by the Subdivisional Officer, who will also record his remarks and take proper action in case the suboverseer is in fault.
2. =Gauge Readers, Regulating Establishment, Bungalow Watchmen, etc.=--To be made to assist whenever possible. The allotment of a beat to each such man has been separately ordered.
3. =Employment of Men on Repairs.=--The men, when not otherwise occupied, to do petty repairs, etc., within their beats, but not to be put on miscellaneous duties and sent about as messengers, nor to act as orderlies or khalassies.
4. =Strength of Establishment.=--Should generally be greater for one and a half months in July and August than at other times. Care to be taken as to this and as to dismissing men when no longer needed.
5. =Stakes and Mallets.=--To be collected beforehand, if necessary, at suitable places, to be accounted for at end of flow season and balance taken care of.
6. =Breaches.=--The Establishment to be trained by the Subdivisional Officer to report every breach to all officials with the greatest possible speed. The mate, daroga, and suboverseer to remain there till the breach is closed and to promptly send a report on the prescribed form to the Subdivisional Officer.
7. =Serious Breaches.=--In case of serious breaches of main channels the Subdivisional Officer to himself reach the spot as soon as possible.
8. =Breach Reports.=--See printed form M[62] attached. To be promptly submitted for each breach to the Executive Engineer. The report contains a column for cost of closure. This means the stoppage of the flow and not the complete making up of the banks. The column for remarks of the Executive Engineer should be filled in and the report promptly returned to the Subdivisional Officer, who will, in the meantime, be making up the banks and preparing a requisition or estimate.
[62] Not printed.
9. =Progress Report.=--With the Executive Engineer’s monthly progress report a list of breaches will be submitted, canal by canal, with columns showing date of occurrence and cost of closure. The return should be on the attached form G.[63] The Subdivisional Officer should also submit this form to the Executive Engineer.
[63] Not printed. The form differs slightly from a form prescribed by the Chief Engineer for general use in the Province.
10. =Estimates.=--The cost of breaches is not to be charged to maintenance estimates. At the close of each month the Executive Engineer should submit or sanction an estimate, accompanied by the breach reports, for closing any breaches which have occurred and making up the banks.
11. =Breaches in the Flooded Area near Canal Heads.=--These may be of special importance. It may be impossible to do any good and money may be uselessly spent. In any such cases the Subdivisional Officer should at once proceed to the spot and the case should be reported by wire to the Executive Engineer and, if necessary, to the Superintending Engineer.
12. =Breaches in Flood Embankments.=--The Subdivisional Officer must at once proceed to the spot and the case be reported by wire to the Executive Engineer and Superintending Engineer. The Breach Report forms can be submitted partially filled in at the earliest possible moment and a complete form afterwards.
APPENDIX E.
SPECIFICATION FOR BUSHING.
(See page 139.)
1. The object of bushing is to form a silt berm and thus prevent or stop the falling in of the banks.
2. The branches must be thickly packed in order that the water among them may become still, and also in order that they may not be shifted by the stream. If thickly packed, the pegs required will also be fewer. Most of the branches should be leafy and freshly cut, but, mixed with these, there may be a proportion of kikar or other leafless branches. Frequently it is possible to utilise jungle trees of small value, bushes, scrub jungle, or even long grass.
3. Except when the bushes are to be very small or the length to be bushed very short, the proposed line for the edge of the berm should be marked out by long stakes driven in the water at fairly close intervals. Otherwise the work may be badly done and the berm formed imperfect and out of line.
4. As the berm formed is not likely in any case to be perfectly straight, and as subsequent additions to it will be difficult, while trimming it will be easy, the bushes should extend slightly beyond the line of the proposed berm. Care should be taken that the lower branches, which cannot be seen when once submerged, are long enough.
5. The branches should be piled up to above water-level, so that, as they settle, they will assume the position desired, but to lay them high above full-supply level on the slopes is useless and wasteful. If the pegs have to be driven at a high level, the branches should be attached to them by thin ropes or twine. Long pegs standing up high above the ground are also wasteful. The pegs should as far as possible be kept in line and their heads at one level.
6. If bushing is begun during low supply, it need not, at first, extend up to full-supply level. More branches, freshly cut, can be added as the supply rises. In any case it is generally necessary to make some additions to bushing from time to time, and this should be explained to contractors and others when fixing the rates.
7. If the trees from which branches are cut are in desirable places, the branches should be cut with judgment; but where trees are in places where they should not be (_e.g._, on the inside slopes of the channels), all the branches may be cut off. The trunk may be left temporarily in order to supply more branches.
APPENDIX F.
ESCAPES.
(See page 9.)
There are no definite rules regarding the capacity of the escapes to be provided on a canal. On some canals in dry tracts of country the discharging power of the escapes is a mere fraction of that of the canal. In other cases it is about half that of the canal. In a district liable to heavy rain an escape, say at a point where a canal divides into branches, should be able to discharge about half of the main canal supply. On branches, escapes, if provided at all, usually discharge into reservoirs, and their period of working is very limited: it may be only twenty-four hours.
On distributaries, escapes are seldom provided. It has been suggested, in connection with modules, that the people irrigating from each watercourse should be responsible for disposing, by means of it, of a certain quantity of surplus water. This would be too rigid a rule. On some watercourses there is much waste land or land under rice cultivation; in such cases surplus water can be passed off without damage. The canal subordinates are fully cognisant of such cases, and they arrange accordingly. In other cases surplus water would do some damage; but on nearly every distributary the full supply, even when there is no demand for water, can be got rid of for a few hours, or even more, without a breach occurring.
Escapes at outlets, in connection with modules, can be arranged by means of waste weirs or by means of Gregotti’s syphons (_sifoni autolivelatori_). The following is an abridged translation of part of a pamphlet by Gregotti:--
The figure represents one of the syphons installed in the “Centrali Milani.”
A is the supply basin of the “Centrali,” which ends in the syphon B. The latter is constructed with mouthpiece of rectangular section _a_, which is submerged in the basin A. A weir divides the mouthpiece of the syphon from the descending branch, _c_, of the same, also rectangular in section. The weir crest is at level _dd_, from 2 to 7 cm. below the maximum level of water surface which it is desired not to exceed in the supply basin.
The descending branch, _c_, has at its base a small tank _e_, which forms a water seal. The syphon is completed by a tube _f_, which is attached to the intake branch of the syphon and which ends at a level of 2 to 7 cm. above the previously mentioned surface _dd_.
As soon as the water surface in the supply basin tends to rise above the plane _dd_, a filament of water, in falling over the weir _b_, pours down the descending branch _c_, and when the water has risen from 2 to 7 cm. above the crest of the weir, the thickness of the falling stream has become such that it is able, by lapping, with a wave-like course, the wall _gg_, to extract the air that has become enclosed in the syphon, and which cannot be replaced because the space in which the stream acts is closed at its base by the water in the tank _e_; and at the top also the aeration tube is closed by the rise in the water surface of the supply basin. From this point the syphon action quickly becomes fully established and begins to give its full discharge.
The discharge that is given is equal to that of an orifice in a thin partition if certain limitations are allowed for between the fall used in the syphon and the height of the arch, that is, the distance from the crest of the weir to the inside roof of the syphon.
The discharge is given by the formula
Q = μA√(2_g h_).
Q = discharge of syphon in cubic metres per sec.
μ = a coefficient of reduction of discharge which varies between wide limits.
A = the minimum cross-sectional area of the syphon in square metres.
_g_ = value of acceleration due to gravity.
_h_ = the fall, or the difference of level in metres between the water surfaces in the supply basin A and in the small tank _e_.
As soon as the supply basin surface falls, the opening of the aeration tube becomes uncovered and air is drawn into the syphon. But until the surface has fallen some centimetres the supply of air is not sufficient to cause the syphon action to stop completely, and thus the escape varies gradually from the maximum discharge to zero as the water surface falls a few centimetres till it reaches its original level.
In certain cases it is possible to do without the aeration tube, especially when the fall used in the syphon is not great and when it is possible to arrange matters so that the velocity of the water flowing past in front of the syphon is small.
The syphon with a width of 3 metres escapes 8 cubic metres per sec. of water.
APPENDIX G.
GAUGES.
(See Chap. III., Arts. 2 and 3; also see _Hydraulics_, Chap. VIII., Art. 5, and Appendix H.)
1. The gauge should be placed on that bank and facing in that direction which enables it to be most conveniently read by the gauge reader and by officials passing the place.
2. The gauge should be of enamelled iron secured by copper screws to a post of squared and seasoned wood which is either driven beforehand[64] into the channel or spiked to a masonry work. Even in the deepest channel a long enough post can be arranged for. A masonry pillar is not necessary. The post may be rectangular in cross-section, with upstream and downstream edges cut sharp. This prevents, or greatly reduces, the heaping up of water at the upstream side and the formation of a hollow downstream. If the “Ward” gauge of two vertical planks is used, the planks should meet at an acute angle, not a right angle, and not be wider than 7 inches each.
[64] Driving after the gauge is attached may loosen or break the screws.
3. The top of the gauge should be slightly above the highest probable water-level. The post should extend up to the top of the gauge.
4. If ever the graded bed of the channel is altered the zero of the gauge should be altered. There may be some risk of confusion at first, but it can be avoided by exercising due care and making notes. The levels of the old and new zeros should be recorded.
5. A gauge at a distance from the bank is objectionable. It collects jungle, cannot be properly read, and is liable to be damaged by floating logs or boats. A gauge should be as near as possible to one bank or the other. If the bank is vertical, the gauge should be quite close to it. If, owing to silt deposit, the gauge is dry at low supply, the deposit can be removed by the gauge reader.
6. Every regulator should be given a name, generally that of a neighbouring village and not that of a channel, and the gauge book headings should be drawn up in an intelligent and systematic manner. Each main channel should be entered in order, and each regulator on the channel--together with the head gauges of all channels which take off there--should be entered, commencing from upstream. A specimen is given on page 109. Thus the head gauge of any branch appears in the register of the main channel from which it takes off, other gauges on the branch appearing in the register for the branch. And similarly as regards a distributary which has gauges other than the head gauge.
7. Each gauge reader should be supplied with a register, each page having, besides the counterfoil, as many detachable slips--marked off by perforations--as there are officials--usually the Subdivisional Officer, zilladar and suboverseer--to whom daily gauge reports are to be sent. The titles and addresses of these officials are printed on the backs of the respective slips. The slips and counterfoil have printed on them a form--similar to part of the specimen shown on page 109--showing the names of all the gauges read by that particular gauge reader, so that he has merely to fill in the date and readings, tear off the slips and despatch them. The posting of the register in the subdivision is facilitated if each gauge has a number and if the corresponding numbers are printed--besides the names--on the gauge slips. If the gauge reader does not know English, the headings of the slips are printed in the vernacular. If the gauge readings are telegraphed, there may be only one slip--besides the counterfoil--which is sent to the telegraph signaller.
APPENDIX H.
GIBB’S MODULE.[65]
(See p. 164.)
[65] This description has been supplied by Glenfield & Kennedy, Kilmarnock. The modules can, it is understood, be obtained from them.
The attributes of a perfect module are many and varied, but in Gibb’s module they have all been successfully embodied in what is probably the simplest piece of apparatus of its kind ever devised. The following summary of the characteristics of Gibb’s module is, therefore, equivalent to an enumeration of the attributes of a perfect module:--
Gibb’s module
Cannot be tampered with, } Cannot get out of order, } since it has no moving Silt or other solid matter in the water } parts, and because of its cannot affect its action, } extreme simplicity. Requires no attention, }
It is accurate, } being designed on scientific Works with very small loss of head, } hydraulic principles.
It is portable, and can be erected at any desired site very simply and easily.
It is strong and durable.
The range of variation of both up- and downstream water-levels through which the discharge remains constant is more than sufficient to meet all the requirements of irrigation canals.
The sufficiency of the delivery can be ascertained at a glance.
The water can be drawn from any desired depth in the parent channel.
When desired, means are provided whereby the supply can be closed or opened at will.
Means are provided, if desired, for a sudden increase of discharge when the upstream water-level exceeds a certain limit, so that surplus water, which might endanger the safety of the canal, is allowed to escape into the branch whenever the danger limit is reached. The upstream water-level at which escapement begins can be fixed in accordance with the requirements of each site, and the action of the escape notch is independent of the opening and closing of the module.
No designing or calculations are required. These have already been worked out. Known the discharge required, the module is supplied complete and ready for setting in position in the canal bank.
HYDRAULIC PRINCIPLE.
The entire absence of moving parts is the chief feature of Gibb’s module; the water simply regulates itself by using up all the excess of energy over and above that required to discharge the correct supply of water. The way in which this takes place will be understood from the following analogy:--
We all know that when we stir tea in a cup so as to make it spin, the liquid rises at the rim of the cup and curves down into a depression in the middle, and the greater the spin the more marked this effect is. It is, we know, the centrifugal force produced by the spin that makes the tea remain high at the rim of the cup. If, while the tea is thus spinning, a teaspoon is held so that it dips slightly below the surface of the liquid near the rim, it will obstruct the flow of the outer portion of the liquid, which will fall in towards the depression in the middle. The reason for this, of course, is that the centrifugal force is absorbed when we interrupt any part of the spin with the teaspoon; hence the liquid must fall, and we know that when liquid falls it uses up “head” or energy.
In Gibb’s module a similar action is made to take place in a steel chamber, semicircular or spiral in plan, through which the water flows in a semicircular path instead of circulating round and round as in the teacup. The surface of the stream, however, assumes the same form as it does in a cup, because it flows under the same conditions. Across the chamber are fixed a number of vertical steel diaphragm plates which take the place of the teaspoon in the above analogy. The lower edges of these plates are of such a shape, and they are fixed at such a height from the bottom of the chamber, as to allow a stream of just the correct required discharge of water to flow under them without interference. But if, owing to an increase of head caused by a rise in the upstream water-level, the water tends to rise higher at the circumference of the chamber, then the water at the surface of the stream strikes against the diaphragm plates, and its centrifugal force being absorbed, it will fall in towards the centre just as happened in the teacup when the spoon was used in place of these plates. In this way the excess head that caused the additional rise of water at the circumference is used up by the fall back towards the centre. The full capacity of the semicircle or spiral for using up excess head or energy in this way is made available by the use of a sufficient number of diaphragm plates fixed at suitable intervals. When the range of head to be dealt with is not large, then a semicircular chamber is sufficient; but for large ranges of head the chamber is made of spiral form so as to lead the water round a complete revolution or more, as may be necessary.
STRUCTURAL DETAILS.
Fig. 29 shows the general form and structure of the type of module suitable for irrigation. Fig. 30 is from a photograph.
The working chamber or shell A is constructed of mild steel plating securely riveted to a framework of angle steel, and the semicircular form of the shell with the rigid diaphragm plates B B riveted to the walls makes a very strong structure, and ensures durability.
The “leading-in” bend C is of cast iron strongly bolted to the steel shell, and is so designed as to deliver the water into the module chamber in a completely established vortex condition.
The socket D on this “leading-in” bend is made so as to allow of considerable latitude in the vertical alignment of the straight leading-in pipe, so that the water can be drawn from any desired depth in the parent channel, and the proportion of silt drawn off is thus brought under control.
Grooves E E and a shutter F, as illustrated, for closing off the flow through the module, are provided, if required, but all modules are not fitted in this way, because many irrigation authorities consider it undesirable to provide the consumers with unrestricted facilities for closing off their supplies without previously giving notice of such an action.
An escape notch H is provided in the position indicated when desired. It may, however, be found difficult to determine beforehand the upstream water-level at which it is necessary to allow this escape of surplus supply, so that it is generally more satisfactory to cut the escape notch after the modules have been installed and actual experience has indicated a suitable level for the notch crest.
In the standard type of module for irrigation purposes the top of the module chamber is completely open, as shown, and this is the type generally recommended, as it is found that consumers have greater confidence in an apparatus which hides nothing from them. To meet the needs of special cases, however, a second type is also made in which the chamber is completely closed and considerably reduced in height, being thus specially suitable for sites where space is confined.
Pipes I, of diameter suitable for all sizes of modules, are also supplied. These may either be welded steel or cast iron, as desired. An 18-feet length of pipe is usually found sufficient to bring the supply through the canal bank to the module.
All modules supplied are treated with anti-corrosive paint, which ensures the protection of the metal.
APPENDIX K.
KENNEDY’S GAUGE OUTLET.[66]
(See p. 168.)
[66] See _Punjab Irrigation Branch Paper No. 12_, “Results of Tests of Kennedy’s Gauge Outlet.”
FIG. 31 shows a bell-mouthed orifice discharging into an air-space. The jet springs across the air-space and traverses a gradually diverging tube. Let _a_, A be the sectional areas of the stream at the air-space and the downstream end of the tube respectively, and let V, _v_, and P_ₐ_, P₁ be the corresponding velocities and pressures. Let resistances be neglected. Since the pressure in the air-space is P_ₐ_,
V = √(2_g h_₀)
or the discharge through the tube depends only on _h_₀ and not on _h_₁.
By Bernouilli’s theorem,
V² P_ₐ_ _v_² P₁ ---- + ---- = ----- + -- 2_g_ W 2_g_ W
or
P₁ - P_ₐ_ V² - _v_² --------- = ---------. W 2_g_
This quantity (since _v_ is small) is not much less than _h_₀ or V²/2_g_. In other words, the water levels of two cisterns with an air-space between them differ only a little, or _h_₁ is small.
The above case (two cisterns and air-space) is mentioned in _Hydraulics_, Chap. V. The principle is simply that the velocity head at the air-space is reconverted into pressure head by passing the stream through a gradually diverging tube. In the absence of such a tube the velocity head would be wasted by causing eddies in the downstream cistern.
If the downstream cistern is a watercourse whose water-level is considerably lower than that of the upstream cistern or distributary, V is obviously unaffected. Also P₁ is obviously reduced. Therefore, by Bernouilli, _v_ is increased, or the stream does not fill the expanded tube and there are eddies in the tube. The water-level in the watercourse may even be lower than the end of the tube. The discharge is unaffected.
In practice there are, of course, resistances, but this fact does not affect the general conclusions stated above. The minimum working head (difference between the two water-levels) which gives a constant discharge is greater than would be the case in the absence of resistances. This “minimum working head for modularity” has been found to be ·21 foot, ·42 foot, and ·61 foot, the corresponding values of the “depression,” _h_₀, being respectively 1 foot, 2 feet, and 3 feet. When the working head is less than the above, the discharge is less and it depends on the working head. The depression should, according to Kennedy, be about 1·75 feet, but it may be more.
The chief difficulty in using the gauge outlet as a module is that the air vent can be stopped up. This converts the apparatus into a compound diverging tube (_Hydraulics_, Chap. III., Art. 17). The discharge is, of course, increased, and it becomes dependent at all times on the working head. Another difficulty is that any rise or fall in the water-level of the distributary (and such rises and falls may occur owing to silting or scour, however carefully the discharge may be regulated) alters the discharge somewhat, though not to the same degree as in an ordinary outlet with a working head of, say, ·5 foot. In short, Kennedy’s gauge outlet, or “semi-module” as it is sometimes called, can modify but not do away with the variations of the discharges of outlets.
INDEX.
Absorption, 16, 159. Alignment, principles of, 4. Alignment, centrality in, 5. Alteration in line, 59. Assiut Barrage, 11. Assouan Dam, 11.
Banks, construction of, 138. -- protection of, 139, 175, 178. -- width and height of, 56. Banks and Roads, 53. Basin Irrigation, 11. Berms, 53. Bifurcations, 47. Bifurcation, head needed at, 45. Borrow pits, 55. Branches of Canals, 3. Breaches in Banks, 176. Bricks used for canal work in India, 88. Bridges, 8, 80, 87, 130. -- Skew, 29, 42. Bushing of banks, 178.
Canal and branches, 20, 47. Canal, bed width of, 51. -- supplied from reservoirs, 3, 13. -- inundation, 1, 45, 79, 127, 156. -- perennial, 1. Capillarity, 16. Cattle Gháts, 81, 173. Cement for lining channels, 160. Chainage, 93, 118. Channels, alterations in, 97. -- enlargement of, 97, 139. -- gradients of, 50. -- side slopes of, 52. Colonization Schemes, 64. Command, 2. Commanded area, 4. Contour lines, 37. -- plan, 26, 36, 63. -- survey, 37. Crops, failure of, 103, 142. -- kinds of, 101. Culturable commanded area, 26, 113. Curves and bends in channels, 8, 139.
“Delta,” 22, 110. Deputy Collector, 96. Designs and Estimates, 60. Design of canals, 2, 26, 30, 47, 147, 156. Discharge of canal during rabi, 52, 118. Discharge observations, 107. Discharges of Punjab rivers, 145, 157. Discharge tables, 106. -- through an outlet, 61. Distance marks, 93. Distribution of water, 14, 118, 125. Distributaries, 3, 20, 44, 46. -- best system of, 71. Distributary, bed width of, 68. -- design of, 60. -- height and width of banks, 68. -- kharif, 156. -- longitudinal section of, 69. -- major and minor, 41. -- off-take of, 51. -- remodelling of, 128. -- side slopes of, 69. -- strip of land for, 69. -- with three fourths full supply, 45, 68, 166. Divide Walls, 33, 169. Divisions, canal, 96. Drainage, 10. Drainage Crossings, 8, 156. Duty of water, 21, 25, 39, 148, 153, 154. Duty, improvement of, 24, 102, 158.
Eastern Jumna Canal, 31. Efflorescence called “Reh,” 15. Egypt, irrigation in, 11. Embankments, 9, 33, 156. Escapes, 9, 100, 180. Estimates for work, 59. Evaporation, 16. Executive Engineer, 96. Extensions of canals, 127. Extra land, 58.
Falling Shutters, 32. Falls, 8, 81, 87. -- incomplete, 87, 130. -- notch, 86. Field book, 101. -- map, 101. -- register, 101. Final line, 59. Flow and lift, 11. Full supply duty, 64. Full supply factor, 64.
Ganges Canal, 31. Gauges, 10, 103, 104, 183. Gauge reader, 96, 98, 105, 184. Gauge reading, 105, 121. -- register, 105, 106, 108, 111. Gibb’s module, 164, 186. Guide banks, 35.
Head for distributary, 82, 83. Headworks, 2, 30, 98, 99, 137.
Indents for water, 98, 108, 110. Inundation canals, 1, 45, 79, 127, 156. Irrigation boundaries, 33, 129. -- in various countries, 1. -- registers, 113. -- unauthorised, 101.
Kennedy’s gauge outlet, 168, 193. -- Rules for channel design, 48. Kharif or Summer Crop, 23. Kutter’s co-efficients, 51.
Lift irrigation, 11, 142. Lime for making channels watertight, 162. Longitudinal section, 69, 130. Losses of water in channels, 16, 38, 159. Low supplies, 118, 123. Lower Chenab Canal, 25, 144. Lower Egypt, 11. Lower Jhelum Canal, 144.
Maintenance work, 138, 171, 174. Marginal Embankments, 9. Masonry works, 29, 80, 89. -- -- large scale site plan, 89. -- -- type designs, 89. Mills, 8. Minors, question of desirability of, 75. Modules, 162, 186.
Navigation, 12. Needles and horizontal planks, 85.
Oil for lining channels, 160. Older Indian canals, 10, 68, 98. Outlet, discharge of, 61. -- registers, 113. Outlets, 15, 61, 66, 67. -- applications regarding, 140. -- design of, 76. -- on inundation canals, 79. -- on older canals, 68. -- positions of, 65. -- remodelling of, 131. -- register of, 113. -- size of, 114, 134. -- temporary, 78. -- variability of duty on, 66.
Parapets, width between, 77. Patwari, 96, 101. Percolation, 16. Perennial Canals, 1. Pitching, 90. Plan, large scale, 59. Postal system, 97. Profile walls, 91, 94. Project, sketch of, 26. Proportion of land to be irrigated, 27, 156. Puddle for lining channels, 160. Punjab, projects for canals in, 144. Punjab rivers, 145, 147.
Quarters for regulating staff, 88.
Rabi or winter crop, 23. Railings, 88. Rain, 9, 22, 100. Ramps, 88. Ratio of bed width to depth, 50. Reduction in size of channel, 128. Registers, irrigation, 113. Regulation of supply, 103, 121, 127. Regulators, 7, 80, 84, 184. -- permissible heading up, 85. Remodellings of channels, 127. -- of outlets and watercourses, 131, 134. Rest Houses, 95. Reservoirs, 13. Rules for designing canals, Kennedy’s, 48.
Scheme, cost of, 28, 59. Sides, falling in of, 139. Sidhnai canal minors, 41. Silt, clearance of, 97, 138, 139. -- deposit, 15, 97. -- trapping at Headworks, 47. Silting and scouring, 15, 48, 98, 139. Sirhind Canal, 19, 144. -- -- silting in the head reach of, 98. Soil, water-logging of, 10, 24, 102. Spoil Banks, 52, 57. Subdivisions, canal, 95. Subdivisional officer, 96. Suboverseer, 96. Superintending Engineer, 96. Supply carried, 100. -- distribution of, 14, 118. -- mean and full, 27, 46, 122. -- regulation of, 103, 104, 106, 127. Syphons, 7, 71, 87, 181.
Tailing of one channel into another, 40. Telegraph, line of, 97. Training of rivers, 33. Trial lines, 58. Trial pits, 58. Triple canal project, 144. Tunnels, 13. Turns, or rotational periods of flow, 14, 118. Type cross sections, 56.
Under-sluices, 32. Upper Bari Doab Canal, 19, 144. -- Chenab Canal, 33, 144. -- Egypt, 11. Upper Jhelum Canal, 50, 144.
Velocity, 12, 50. Village lands, 62.
Watching banks, 175. Water, payment for, 12, 100, 102, 142, 143. Water level, fluctuation in, 100, 124. Watercourse, limit of size of, 74. Watercourses, 4, 20, 65. -- applications regarding, 140. -- for trees, 58. -- remodelling of, 131. -- with poor command, 67, 133, 166. Waterings, 11, 24. Water-logging of the soil, 10, 24, 102. Wave, travel of, down a channel, 124. Western America, canals in, 12. -- Jumna Canal, 31, 39. Wing Walls, 89. Works, arrangement of, 89. -- two or more close together, 89. -- urgent repairs of, 137.
Zilladar, 96.
Harvey & Healing, Printers, Manchester Street, Cheltenham.
Transcriber’s Notes
The inconsistent use of periods after Roman numerals has been retained; other inconsistencies (spelling, hyphenation, formatting and lay-out) have been retained as well, except when mentioned below.
Depending on the hard- and software used and their settings, not all elements may display as intended. Some of the tables are best viewed in a wide window.
Page 21: The equation does not agree with the calculations given.
Page 66, Fig. 11: There are two illustrations labelled Fig. 11, the hyperlinks point to the appropriate illustration.
Changes made
Obvious typographical errors have been corrected silently. Footnotes and illustrations have been moved out of text paragraphs. Some tables have been re-arranged or split; in several tables, the data alignment has been standardised.
Page 18, table, Total of second column: 8·93 changed to 8·01 Page 39: Kutters changed to Kutter’s Page 93: marked out changed to marked at Page 69: 3 Depth of digging changed to 13. Depth of digging Page 109, first average ·1 changed to 4·1 Page 117: Net Areas Irrigated in Areas changed to Net Areas Irrigated in Acres Page 150: Cusecs. added as in similar tables Index: Cattle Ghats changed to Cattle Gháts; Line for making ... changed to Lime for making ...; Lower Chenal Canal changed to Lower Chenab Canal.