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IRRIGATION WORKS

By the Same Author

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Frontispiece.

IRRIGATION WORKS

THE PRINCIPLES ON WHICH THEIR DESIGN AND WORKING SHOULD BE BASED, WITH SPECIAL DETAILS RELATING TO
INDIAN CANALS
AND SOME PROPOSED IMPROVEMENTS

BY

E. S. BELLASIS, M.Inst.C.E.

RECENTLY SUPERINTENDING ENGINEER IN THE IRRIGATION BRANCH OF THE PUBLIC WORKS DEPARTMENT OF INDIA

37 Illustrations

London
E. & F. N. SPON, Ltd., 57 HAYMARKET, S.W.
New York
SPON & CHAMBERLAIN, 123 LIBERTY STREET
1913

CONTENTS

PREFACE

When River and Canal Engineering was written it was decided to omit Irrigation works and to deal with them separately because the subject interests chiefly specialists.

The present book deals with the principles which govern the design and management of Irrigation works, and it discusses the Canals of Northern India—the largest and best in the world—in detail.

Some years ago a number of rules for designing distributaries were framed, at the request of the Punjab Government, by the late Colonel S. L. Jacob, C.I.E., R.E., and comments on these rules were obtained from many experienced engineers and recorded. The author has had the advantage of reading all these opinions. Generally the weight of opinion on any point agrees with what most experienced engineers would suggest, and direct conflicts of opinion scarcely occur. Important papers have been printed by the Punjab Irrigation Branch on Losses of Water and the Design of Distributaries, on the great Triple Canal Project, on Gibb’s Module, on Kennedy’s Gauge Outlet, and on the Lining of Watercourses. These papers are not always accessible to engineers, and the chief points of interest in them are not, in most cases, discernible at a glance. Such points have been extracted and are given in this book.

E. S. B.

Cheltenham, May 20th, 1913.

IRRIGATION WORKS.

CHAPTER I.
INTRODUCTION

1. Preliminary Remarks.

—The largest irrigation canals are fed from perennial rivers. When the canal flows throughout the year it is called a “Perennial Canal.” Chief among these are the canals of India and particularly those of Northern India, some of which have bed widths ranging up to 300 feet, depths of water up to 11 feet and discharges up to 10,000 c. ft. per second. Other large canals as for instance many of those in Scinde, Egypt and the Punjab, though fed from perennial rivers, flow only when the rivers are high. These are called “Inundation Canals.” Many canals, generally of moderate or small size, in other countries and notably in the Western States of America, in Italy, Spain, France and South Africa, are fed from rivers and great numbers of small canals from reservoirs in which streams or rain-water have been impounded. Sometimes water for irrigation is pumped from wells and conveyed in small canals. In Australia a good deal of irrigation is effected from artesian wells. Irrigation works on a considerable scale are being undertaken in Mexico and the Argentine. In this book, irrigation works of various countries are referred to and to some extent described, but the perennial canal of Northern India, with its distributaries, is the type taken as a basis for the description of the principles and methods which should be adopted in the design, working and improvement of irrigation channels and it is to be understood that such a canal is being referred to where the context does not indicate the contrary. Any reader who is concerned with irrigation in some other part of the world will be able to judge for himself how far these principles and methods require modification. The branches and distributaries—all of which are dealt with—of a large perennial canal cover all possible sizes.

[Chapter II.] of this book deals with the design of canals and [Chapter III.] with the working of canals but as the two subjects are to some extent interdependent, they will both be dealt with in a preliminary manner in the remaining articles of the present Chapter. [Chapter IV.] describes the Punjab Triple Canal Project.[1] [Chapter V.] deals with certain proposed improvements in the working of canals.

[1] The latest example of canal design.

2. General Principles of Canal Design.

—The head of a canal has to be so high up the river that, when the canal is suitably graded, the water level will come out high enough to irrigate the tract of land concerned. If a river has a general slope of a foot per mile and if the adjoining country has the same slope and is a foot higher than the water level of the river, and if a canal is made at a very acute angle with the river, with a slope of half a foot per mile, the water level about two miles from the canal head will be level with the ground.

The headworks of the canal consist of a weir—which may be provided with sluices—across the river, and a head “regulator,” provided with gates, for the canal. There are however many canals, those for instance of the inundation canal class, which have no works in the river and these may go dry when the river is low. They usually have a regulator to prevent too much water from going down the canal during floods. If a canal is fed from a reservoir the headworks consist simply of a sluice or sluices.

A canal must be so designed as to bring the water to within reasonable distance of every part of the area to be irrigated. Unless the area is small or narrow the canal must have “branches” and “distributaries.” A general sketch of a large canal is given in [Fig. 1]. On a large canal, irrigation is not usually done directly from the canal and branches. It is all done from the distributaries.

Fig. 1.

From each distributary “watercourses” take off at intervals and convey the water to the fields. A small canal, say one whose length is not more than 15 miles or whose discharge is not more than 100 c. feet per second, may be regarded as a distributary and the word distributary will be used with this extended meaning.

It is not always the case that the whole tract covered by a system of canal channels is irrigated. In the case of a canal fed from a river, the land near the river is often high or broken and the main canal runs for some distance before it reaches the tract to be irrigated. Again, within this tract there are usually portions of land too high to be irrigated. Those portions of the tract which can be irrigated are called the “commanded area.”

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.

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.

On some Indian irrigation canals made about 30 years ago, great sums of money were wasted in making the canals navigable. There is nothing like enough navigation to pay for the extra cost. The idea has now been quite given up except as regards timber rafting from upstream. This requires no curtailment of the velocity in the channels. The requirements of the irrigation and navigation were always in conflict. The mere fact that branches have to be worked in turns is enough to prevent navigation succeeding.

In India the water used for irrigation is paid for, not according to the volume used but according to the area irrigated. The volume used in any particular watercourse is not known. The areas sown are measured. Certain kinds of crops use up more water than others and the charges are fixed accordingly.

In the canals which have their headworks among the mountains of Western America there are frequent tunnels and syphons and the canals often run in steep sidelong ground. There are great lengths of tunnel and syphon in the Marseilles and Verdun Canals and there are long tunnels in the Periyar Canal in Madras and in the Upper Swat Canal in the North West Frontier Province of India.

The Tieton Canal, Washington, U.S.A., traverses steep sidelong ground which would be liable to slip if a large cutting were made. The cross-section of the channel is a circle, 8-ft. 3¹⁄₂-ins. in diameter, with the upper part removed, so that the depth is 6 feet. It is made of reinforced concrete 4 inches thick and the sides are tied together by iron bars which run across the channel above the water. In the Santa Ana Canal the channel consists for 2¹⁄₂ miles of a flume made of wooden “staves.”

A canal constructed in Wyoming, U.S.A., after taking off from a river, passes through a tunnel into another valley and is turned into another stream which thus becomes the canal. This is said to save loss of water by percolation. The stream is winding while a canal could have been made straighter. There may, owing to the ground near the stream being saturated, have been less loss of water at first than there would have been in the artificial channel but, owing to the smaller wetted area, there would probably have been an eventual saving in adopting the latter. The real advantage of adopting the natural stream was probably a saving in the cost of construction. (Min. Proc. Inst. C.E. Vol. CLXII.)

Irrigation from canals which are supplied from reservoirs differs in no respect from that from other canals. The principles on which reservoir capacities should be calculated and earthen and masonry dams constructed are given in River and Canal Engineering. Sometimes, as for instance when a reservoir becomes seriously reduced in size owing to silt deposit, the water is run off after the bed of the reservoir has been soaked, and crops are grown on the soaked soil.

The distribution of the water of a canal as between the main channel and the branches, is effected by means of the regulators at the bifurcations. When the supply is ample and the demand great, the channels may all be running nearly full. When the demand exceeds the supply, the water may be reduced proportionately in each branch but this may result in the water of a branch being too low to give proper supplies to the distributaries or some of them, and in the water of a distributary not commanding the higher ground. Moreover it violates the principle of keeping the water in bulk as far as possible. It is more usual to give each branch full supply, or a certain large fraction of the full supply, in turn, and similarly with the distributaries.

The method of distribution from a distributary to the watercourses varies. In many modern canals there is, at each watercourse head, a sluice which is adjusted at frequent intervals according to the supply and the demand. One method, which is excellent because it fulfils in the highest degree the principle of keeping the water in bulk, is to have very large watercourses and, by means of regulators which are built at frequent intervals, to turn the whole of the water of the distributary into a few watercourses at a time, beginning with those nearest the head of the distributary and working downstream. But a system which seems eminently suitable may be impracticable because of local circumstances. In India, any such arrangement would need an army of officials and would lead to unbounded corruption.

In India the water from a distributary enters the watercourses through “outlets” which are small masonry tubes passing through the banks of the distributary. There is no easy way of closing these outlets or at least of keeping them closed if the cultivators choose to open them, but it is easy to close a whole distributary and so regulate the supply. This is the chief reason why watercourses in India do not usually take off direct from the canals.

The presence of silt in the water of a river from which a canal is drawn is often spoken of as being a great evil. If it is an evil at all it is a very mixed evil. The deposits of silt in the channels have been enormously reduced by the application of scientific principles of design. The clayey silt which remains in the water and reaches the fields, brings to them greatly increased fertility.

In India the fertility of the soil is often reduced or destroyed by the formation on the surface of the ground of an efflorescence called “reh.” It consists of various salts or compounds of sodium and occurs chiefly where there is an impervious layer of subsoil. The salts exist as an ingredient of the upper soil. This becomes saturated with rain or canal water and as the water evaporates the salts are left on the surface. Remedies are drainage, or flooding the soil and running the water off, or deep tilling, or chemical treatment with lime or gypsum. (Indian Engineering, 8th Jan., 1910).

The inundation canals of the Punjab have been described in Punjab Rivers and Works. All descriptions and remarks in the present book regarding Indian canals must be assumed to refer to perennial canals unless the contrary is stated or implied.

4. Losses of Water.

—When water flows or stands in an earthen channel or tank, or is spread over a field, losses occur from evaporation, percolation and absorption. Of these, absorption is by far the most important and, unless the contrary is stated or implied, it will be taken to include the others. The losses by evaporation are very small. The loss by evaporation from the surface of the water, even in the hot season in India when a hot wind often blows, does not exceed half an inch in 24 hours and on the average in India is only about a tenth of an inch in 24 hours.

Fig. 3.

Percolation and absorption are described as follows by Beresford in Punjab Irrigation Paper, No. 10, “The Irrigation Duty of Water.” Percolation consists in flow through the interstices of boulders, shingle, gravel or coarse sand. The flow is similar to that in pipes. The water percolating into the soil from a channel, extends downwards and spreads outwards as it descends. None of it goes upwards. In fine sand and ordinary soil the interstices act like capillary tubes. The water is absorbed as by a sponge and it remains in the soil by virtue of capillarity. Owing to the combined action of capillarity and gravity the water spreads in the manner shown by the dotted lines in [Fig. 3]. The amount of absorption from a channel will be greater the greater the area of the wetted surface. In a high embankment with narrow banks, the absorption ceases when the water reaches the outer slopes, except in so far as it is evaporated from the slopes. Moreover high embankments are generally in clayey soil. If banks of sand are constructed on a layer of clay ([Fig. 4.]) and well rammed, the absorption ceases as soon as the banks are saturated and the channel then holds water as well as any other except for evaporation from the outer slopes, but if the bed and subsoil are also of sand the absorption of the water will be far greater. Absorption ceases when the water extends nearly down to the level of the subsoil water, i.e., to a point where the effect of absorption from above plus gravitation is equal to the effect of absorption from below minus gravitation. If a bottle is filled with water and a small sponge jammed into the neck and the bottle turned upside down, the sponge becomes saturated but no water will be given out. But if a dry sponge is placed in contact with the wet one it will absorb moisture until saturated.

Fig. 4.

It is known that the loss of water is greatly influenced by the nature of the soil. When water is turned into a dry channel or onto a field, the loss is at first great. It decreases hourly and daily and eventually becomes nearly constant, tending to reach a fixed amount when the water extends down to nearly the level of the subsoil water. Observations made by Kennedy on loamy fields near the Bari Doab Canal in India showed that on a field previously dry the rate of absorption is given by the equation

y = ·0891 x ^·86.

Where y is the depth of water absorbed in feet and x is the time in hours. The observations extended over eight days. Denoting by c the depth of water in feet absorbed in one hour, it was found that on a field on which no rain had fallen for two months, c was ·04 to ·05 but on the second watering of the crop about a month later c was ·02 to ·03 and about the same on a third watering. It was found that at the first commencement the rate of absorption was much affected by the state of the surface of the ground but that the effect was only temporary. The losses were found to be as follows:

In the eight days the total loss was almost exactly eight feet.

The losses by absorption in the various channels of certain canals has been estimated to be as follows:—

[4] When the channel was in continuous flow.

[5] Maximum value when flow was intermittent.

Some information as to losses of water is also given in [Chapter IV. Art. 2].

The relative losses of water in the channels of the Upper Bari Doab Canal were as follows:—

The reasons for the great variation in the value of c are not properly known. The depth of water is not likely to have much influence on it. It is well known that the fine silt carried by the water tends to render the channels watertight when it deposits. The canals and branches receive either no deposits or deposits consisting chiefly of sand. The distributaries, especially in their lower reaches, receive deposits of fine silt which is only occasionally cleared away. The watercourses receive similar deposits but they are very frequently cleared out by the cultivators. This is perhaps the reason why the rate of loss of water in the watercourses is nearly three times as great as the rate of loss in the distributaries of the same canal. On the Sirhind canal the distributaries have more branches than on the Bari Doab canal and the watercourses are smaller. This accounts for the different relative losses in the two cases. The sandy nature of the soil on the Sirhind canal accounts for the general higher value of c on that canal.

The following formula has been deduced as giving the loss by absorption on a Punjab Canal.

P = 3·5 √d WL 1,000,000

Where P is the loss by absorption in c. ft. per second in a reach whose length is L, width (at water level) W and depth d. According to the formula the loss per million square feet is 10·5 c. ft. per second when d is 4 ft. and 7 c. ft. per second when d is 2 feet, These figures do not agree with those in the preceding table and it is clear that there are not yet sufficient data from which to construct a formula.

The first steps taken on the Bari Doab Canal, and subsequently on other canals, to reduce the losses of water, consisted in the reduction in the number of watercourses. This will be referred to again ([Chapter II. Art. 9]). Further steps will be considered in [Chapter V.]

5. Duty of Water.

—The number of acres irrigated annually by a constant discharge of 1 c. ft. of water per second is called the “duty” of water. In India on perennial canals the duty may be as much as 250 or even 300 acres. On inundation canals which flow for only five months in the year and are situated in tracts of scanty rainfall and light or sandy soil, the duty may be only 70 acres. The duties of most existing canals whether in India or elsewhere, are known only approximately. The duty is calculated on the average discharge entering the canal at its head less the water which is passed out at escapes. It thus includes all losses of water. The duty varies not only as between one canal and another but on the same canal from year to year. It depends on the character of the soil, a sandy soil requiring more water than a clayey soil. It also depends on the rainfall. A moderate amount of rain causes the canal water to go further, but heavy rain may enable some crops to do without canal water or may permit of the concealment of canal irrigation. The duty also depends on the kind of crops grown, on the losses in the channels by absorption and on the quantity of water available. A liberal supply of water leads to carelessness in the use, but a very restricted supply is largely wasted owing to the shortness of the “turns” or rotational periods of flow in the different channels.

There is an obvious connection between the duty of water and the total depth of the water, known in India as “delta,” given to the fields. Calculations are much simplified, while still being accurate enough for all practical purposes, by assuming that the number of seconds, (86,400) in a day is twice the number of square feet, (43,560) in an acre. Assuming this to be the case a discharge of 1 c. ft. per second for a day gives 2 acre-feet, i.e., it will cover an acre of ground to a depth of 2 feet in a day; and in six months it will cover 100 acres to depth of 3·65 feet. In Northern India the year is divided into two halves in each of which a crop is grown and the duty is calculated for each crop. In this case, if the flow of a canal has been continuous, a duty of 100 acres per cubic foot of its mean discharge per second, corresponds to a total depth of 3·65 feet over the area irrigated. Generally the flow in the half-year has not been continuous. In other countries, and in India on canals other than the perennial canals, the periods of flow vary a great deal. The duty cannot be calculated from delta or vice versa until the period of flow is stated.

The daily gauge-readings and daily discharges corresponding to them, having been booked, the discharges are added up. The total, divided by the number of days on which the canal has been running, gives the average daily discharge. Suppose that during the “kharif” or summer crop which is considered to last from 1st April to 30th September or 183 days, the canal was closed for 13 days and that the total of the daily discharges on the remaining 170 days comes to 850,000 c. ft. per second. The average daily discharge is 5,000 c. ft. per second. Suppose the kharif area irrigated to be 500,000 acres, the kharif duty is 100 acres. To find delta the total of the daily discharges has to be multiplied by the number of seconds in a day and divided by the number of square feet in an acre (these figures are, as already stated, very nearly in the ratio of two to one) and divided again by the number of acres irrigated. Thus, in the above case, delta is very nearly 850,000 × 2500,000 or 3·4 feet. For comparing the results of one canal or one year with another, delta is the more convenient figure to take. As soon as the areas irrigated by the canals are known for any crop, the Chief Engineer of the province issues a statement of the value of delta for each perennial canal and compares them with those for previous years. The value of delta for the Punjab canals ranges from 3 to 4 feet for the kharif and from 1·8 to 2·1 feet for the “rabi” or winter crop. Individual canals vary greatly, the worst having nearly twice as high a figure as the best. The differences are due to the causes already mentioned.

Although the figures of duty take no account of the number of days a canal was closed, they are the most convenient standard for judging generally of the work likely to be done by a projected canal. It will readily be seen that figures of duty are not exact and are only an approximate guide. The delta figures are, on the perennial canals of the Punjab, also worked out for each month of the crop, the volume of water used from the beginning of the crop up to the end of the month being divided by the area irrigated up to the end of the month. But when irrigation is in full swing, some little delay occurs in booking the fields. Moreover the same field is watered a number of times during the crop and much depends on whether waterings have just been given or are just about to be given. The figures are useful to some extent for comparison. The figures for the rabi crop of 1908-09 were as follows, the figure for March being the final figure for the crop.

One great principle to be followed in order to obtain a high duty is to restrict the supply of water. A cultivator whose watercourse is always running full may waste great quantities of water, but if he knows that it is only to run for a few days out of a fortnight he will use the water carefully. It is not, of course, meant that the water kept back is run into escapes and wasted. It goes to irrigate other lands. The available supply of water should be spread over as large an area of land as just, and only just, to suffice.[6] Other methods of improving the duty are the reduction in the number of watercourses, the apportionment of the sizes of outlets, watercourses and distributaries to the work that they have to do, careful attention to the distribution of the water and the prevention of wastage due to carelessness.

[6] A system of lavish supply is in most cases likely to lead to harm by water-logging of the soil or its exhaustion by over-cropping or to raising of the spring level and injury to the public health.

The following information concerning duties is taken from Buckley’s Irrigation Pocket Book:—

[7] Occasionally as low as 98 or even 62.

[8] The most recent canal.

The period of flow in each case would be six months or less.

The average rabi duties on the Lower Chenab and Upper Bari Doab Canals, in the Punjab, calculated on the discharges at the distributary heads, for periods of 3 and 5 years respectively, ending March, 1904, were 208 and 263 acres respectively, but in the latter case 11 per cent. of the area received only “first waterings.” For the kharif the figures are 100 and 98 respectively.

In Italy the duty is 55 to 70 acres, in Spain from 45 to 205 acres, in the Western States of America generally 60 to 150 acres. In South California the duty is 150 to 300 acres, when, as is usual, surface irrigation is employed, but 300 to 500 acres with subsoil irrigation, the water being delivered in a pipe below ground level ([Chapter V.])

In basin irrigation in Egypt the duty is 20 to 25 acres, but the period of flow is only 40 days. The basins are flooded to about 3 feet in depth.

6. Sketch of a Project.

—The tract of country to be dealt with in an irrigation project may be limited either by the natural features of the country, by its levels, by the quantity of water available or by financial considerations. If the tract is small or narrow, and particularly if it is not very flat, it may be obvious that there is only one line on which the irrigation channel can conveniently be constructed but in any considerable scheme a contour plan of the whole tract is absolutely necessary. The surveys for such a plan are expensive and take time and it is desirable, as far as possible, to settle beforehand the area over which they are to extend. This may be done to some extent by the examination of any existing levels and of the tract itself. Very high, sandy or swampy ground, whether occurring at the edge of the tract or in the middle of it, may have to be left out. The remainder, as already mentioned, is called the commanded area. When land occupied by houses or roads or which is very much broken, or which for any reason cannot be irrigated, has also been deducted, the balance is the “culturable commanded area.”

Either before or after the culturable commanded area has been approximately ascertained, the proportion of it which is to be irrigated must be settled. This depends on local circumstances. In India the supply of water is calculated on the supposition that a fraction, generally from ¹⁄₃ to ³⁄₄, of the culturable commanded area will be irrigated each year. The rest will be lying fallow or be temporarily out of use or be used for crops which do not require canal irrigation. The restriction of the area is necessary either because the supply of water is limited or in the interests of the people. Too liberal a supply of water tends, as already stated, to over cultivation, and exhaustion and water-logging of the soil.

The next step is to estimate the duty and the discharge of the canal and then to fix its main dimensions. In Northern India the duty in the rabi is higher than in the kharif. It may be 200 acres in the rabi and 100 acres in the kharif. Local circumstances determine which crop has the greater area. Suppose that it is estimated that both will be equal. Then the total annual area for which water is to be provided must be divided by two and this gives the kharif area. During the kharif there is usually an ample supply of water and the kharif mean supply of the canal is based on the kharif area and the kharif duty. The full supply is not run all through the crop because the demand fluctuates, the demand being greatest when all the crops have been sown and when there is no rain, but from experience of other canals the ratio of the kharif full supply to the kharif mean supply can be estimated. The ratio is generally about 1·25. On the kharif full supply depends the size of the channel, every channel being constructed so as to carry a certain “full supply” or maximum discharge and the top of the bank being made at such a height that there shall be a sufficient margin or “free-board” above the “full supply level.” The canal runs full provided that there is a sufficient supply in the river or that the water level of the river is high enough—this last condition referring to canals which have no weir in the river—and provided also that there is a sufficient “demand” for the water. At other times a canal runs with less than full supply. This generally occurs throughout most of the rabi, the supply of water in the river being then restricted. The distributaries are generally run full or ³⁄₄ths full, some being closed, in turn, to give water to the others. In the case of a country where there is only one crop in the year, the average discharge of the canal can be found by dividing the area by the estimated duty. The F.S. discharge can be assumed to bear such a relation to the average discharge as may be found by experience to be suitable. On some Indian inundation canals the F.S. discharge is taken as twice the average discharge.

The F.S. discharge of the canal having been arrived at, the alignments of the canal and branches are next sketched out on the contour plan and certain tracts and discharges are assigned to each branch. The gradients can be ascertained from the levels of the country and the cross-section of the channel can then be sketched out. If the velocity is too great for the soil “falls” can be introduced. The above procedure will enable a rough idea to be formed of the cost of the earthwork of the scheme. The cost of the headworks and masonry works and distributaries can be best estimated by obtaining actual figures for existing works of similar character, the distributaries being reckoned at so much per mile. The probable revenue which the canal will bring in will depend upon the rate charged for the water and the cost and maintenance, matters which can only be determined by local considerations based on the figures for existing canals.

The masonry works on a canal consist of the headworks and of bridges, regulators and drainage crossings. The principles of design for such works have been dealt with in River and Canal Engineering. It is of course economical to make a bridge and fall in one. If the off-take of a distributary is anywhere in the neighbourhood the fall should of course be downstream of it. The positions of the falls should be fixed in accordance with these considerations. If the longitudinal section is such that the position of the fall cannot be much altered, it may be feasible to divert a road so that the bridge may be at the best site for the fall. In the case of a railway crossing, a skew bridge is often necessary. In the case of a road crossing it may be feasible to introduce curves in the road but here also a skew bridge is often necessary.

CHAPTER II.
The Designing of a Canal.

1. Headworks.

—In the design of head works no very precise rules can be laid down. Some general ideas can however be given as to the chief points to be attended to and some general and approximate rules stated. In every case a large scale plan of the river is of course required and also a close examination of it and study of its character. An attempt to forecast its action is then possible. Gauge readings for several years and calculations of discharges are of course necessary. If the bed of the river, in course of time, rises upstream of the weir or scours downstream of it, a large amount of protection to the bed and banks will become necessary. Some description of headworks and weirs, with a plan of the headworks of the Sirhind Canal, India, has been given in River and Canal Engineering, Chapters IV. and X. Remarks regarding the collection of information for such works are given in Chapter II. of the same work. It is also explained how, by keeping the gates of the under-sluices closed, a “pond” is formed between the divide wall and the canal head so that heavy sand deposits in the pond and does not enter the canal. By closing the canal and opening the under-sluices the deposit is scoured away.

The best site for the headworks of a canal depends on the stability and general character of the bed of the river but in deciding between any two proposed sites, the question of the additional cost of the canal, if the upper site is adopted, has to be taken into account. Such cost may, in rugged country, be considerable.

In the case of Indian perennial canals, the head is often close to the hills where the river bed is of boulders and shingle and fairly stable, but it is often at a distance from the hills and in such cases a gradual rise in the bed of the river, even in the absence of a weir, is more probable than scour. Such a rise may necessitate a raising of the crest of the weir and of the bed of the canal.

Fig. 5.

In the general arrangement of a headworks a great deal depends on local conditions. Sometimes the river runs in a fairly straight and defined channel and the weir can then be run straight across it. Sometimes, as in the case of the Ganges Canal, there is a succession of islands and various short weirs are required in the different channels. At the heads of the Eastern and Western Jumna Canals, the river, on issuing from the hills, widens out ([Fig. 5.]) and the weir is built obliquely and not in a straight line. Its crest is higher at the east than at the west side. There are under-sluices at both sides. The upstream end and west side of the island are revetted. The old head of the Western Jumna Canal, as shown in the figure, existed long before the advent of the British, and a temporary weir, made of gabions filled with stones, was constructed across the river every year during the low water period and swept away during the floods. To have carried the weir along the line shown dotted, the head of the Western Jumna Canal being of course brought up to it, would apparently have been feasible and cheaper, but the off-take would have been in shallow water because of the curve in the river, and there would have been no current along the face of the head regulator of the canal.

The level of the floor of the under-sluices is generally about the same as that of the bed of the canal. The sill—made to exclude shingle and sand as far as possible—of the canal head regulator may be 3 feet higher and the crest of the weir 6 to 9 feet higher. The top of the weir shutters is 1 to 2·5 feet above the F.S. level of the canal which may be 5 feet or more above the bed of the canal. If the weir is provided with falling shutters the width of the waterway of the under-sluices may be about ¹⁄₁₂th of the width of the waterway of the weir alone, otherwise about ¹⁄₈th.

In nearly all cases the weir has a flat top and flat slopes both upstream and downstream. In a case where the river bed is of sand, the depth of water on the crest of the weir in floods may be 15 feet and the velocity 14 or 15 feet per second. The downstream slope of the weir may be about 1 in 15, and the upstream slope 1 in 6. Where the river bed is of boulders the velocity may be still higher. The faces of the weir are usually of hammer-dressed stone. A lock for the passage of rafts is added if necessary.

Unless the banks of the river are high, it is necessary to construct embankments to prevent the river water, when headed up by the weir during the floods, from spilling over the country with possible damage to the canal. If the river has side channels they have to be closed. The stream may also have to be trained, by means of guide banks or spurs, so as to remain in one channel and flow past the canal head and not form shoals against it. Where the river is unstable, it may shift its course so as to strike the weir obliquely and this may cause excessive heading up at one side of the weir. In such cases it is usual to divide the weir into bays or sections, each about 500 feet long, by “divide walls” running at right angles to the weir.

The free-board or height of the masonry walls and tops of embankments above H.F. Level is about 5 feet.

The span of each opening in the under-sluices is generally 20 to 35 feet. The piers may be 5 feet thick. It is usual to make each alternate pier project upstream further than the others so that long logs coming down the river during floods, broadside on, may be swung round and not be caught and held against the piers.

Fig. 6.

Fig. 6A.

[Figs. 6] and [6A] show the headworks of the Upper Chenab Canal now under construction ([Chapter IV.]) The site is in a low flat plain, but no better site could be found. The weir consists of 8 bays of 500 feet each. The crest is 10 feet above the river bed and the falling shutters 6 feet high. The slopes are 1 in 6 and 1 in 15. The bulk of the work is rubble masonry in lime. The lower layer upstream of the crest is of puddle; upstream of the second line of wells it is rubble masonry in half sand and half lime; upstream of the lower line of wells it is of dry stone and there is an intermediate layer of rubble masonry in lime with the stones laid flat. Below the crest there is a wall of masonry 9 feet thick and on the crest there are two strips of ashlar between which the shutters lie when down. The extreme upstream and downstream portions of the bed protection are of dry stone and 4 feet thick while next to the weir are concrete blocks 2 feet thick resting on dry stone. The width of the crest is 14 feet, of the weir 140 feet, of the protection 70 feet upstream and 110 feet downstream. The guide banks have tops 40 feet wide and 18 and 14 feet above the crest of the weir in the upstream and downstream lengths respectively, the side slopes being 2 to 1 and the water slope being covered, up to H.W. level, by dry stone pitching 4 feet thick. The left guide bank runs upstream for 3,250 feet from the centre line of the canal and the right 2000 feet from the line of crest shutters. The under-sluices have 8 bays of 35 feet each and the canal head regulator 36 openings of 6·5 feet each, the large openings shown in the figure being sub-divided. The crest of the weir is no less than 10 feet above the river bed and the shutters add 6 feet to this. The floor of the under-sluices is 4 feet higher than the river bed. There is thus ample allowance for a possible rise in the river bed.

2. The Contour Map.

—The contour map, besides showing the contours of the country to be irrigated and of a strip of country, even if not to be irrigated, which will be traversed by the main line, should show all its main features, namely:—streams, drainages, railways, roads, embankments, reservoirs, towns, villages, habitations, and the boundaries of woods and cultivated lands. It should also show the highest water levels in all streams or existing canals. A map showing as many as possible of the above features should be obtained and lines of levels run for the contours. In doing this, the points where the lines of levels cut or pass near to any of the above features or boundary lines, should be noted. It may be necessary to correct inaccuracies in the plan or to supply defects in it. The greater the trouble taken to do this the less will be the trouble experienced later on.

The heights of the contour lines will, in very flat country, have eventually to be only 1 foot apart. This will necessitate running lines of levels half-a-mile apart at the most, and preferably 2000 feet apart, the pegs in each line being about 500 feet apart. In less flat country the heights of the contour lines can be further apart than 1 foot. Whatever distance apart is decided on for them, the survey should be done once for all. On one of the Indian canals in flat country, the lines of levels were at first taken 5 miles apart, the branches roughly aligned and then further surveys made. This led to great expense and delay and the procedure has not been repeated.

In making a contour survey, a base line, as centrally situated and as long as possible, should be laid down, with side lines parallel to it near the boundaries of the tract. The cross lines at half-mile or other intervals should then be laid down. Some of them may run out beyond the side lines. Circuits of levels should be run along the base line, the side lines and the two extreme cross lines and be carefully checked. The remaining cross lines should then be levelled. All the levels having been shown on the map the contours should be sketched in. The scale of the map for a large project may be two inches to a mile. If it is likely that the survey will have to be extended, it will be easier to do this by prolonging the base line and running more cross lines, than by prolonging each of the cross lines already surveyed. This can be borne in mind when selecting the base line.

3. Alignments and Discharges.

—On the contour map the proposed alignments of the canal, branches, distributaries, and escapes, determined after careful consideration of all matters affecting them, are shown. The tracts to be irrigated by each branch and each distributary are now marked off, the “irrigation boundaries” following approximately the valleys and lines of drainage. Any large tracts of land which cannot be irrigated are of course shown and are excluded. Forests or other lands which are not to be irrigated should be similarly dealt with, otherwise confusion is likely to arise later. The commanded area dependent on each distributary is now ascertained from the map. A certain percentage being deducted for scattered unculturable areas the culturable commanded areas are obtained. The proportion to be irrigated (in India in the kharif) having previously been decided, the number of acres to be actually irrigated by each distributary is arrived at.

The next step is to ascertain the discharges.[9] A general duty for the whole canal having been estimated by considering the actual figures for other canals the full supply of the canal at its head is arrived at. ([Chapter I, Art. 6]). In Northern India it will be the kharif duty and kharif full supply. Since some water is lost by absorption in the channels, the duty of the water on a branch is higher than that of the whole canal based on its head discharge, and the duty on a distributary is higher still. In designing a canal, an attempt has to be made to estimate the losses of water in the main canal and branches, so that the duties of the branches and distributaries may be estimated and the channels designed accordingly. On the Western Jumna Canal the figures were estimated to be as follows:—

[9] In this Article and in the rest of this Chapter it is assumed that the canal is a Northern Indian one. Any modifications necessary to suit canals in other countries will readily suggest themselves.

The question of duty is one which if not carefully considered, may cause some confusion. A canal and branches, having been designed with certain assumed duties and with discharges based on certain values of N in Kutter’s co-efficient, have, let it be supposed, been constructed to a greater or less extent. When the time comes for constructing the distributaries, the engineers concerned may have different ideas, based on later experience, as regards the probable duty and the most suitable value of N. If they design the distributaries with a higher duty and a lower value of N, it is obvious that they can provide more distributaries than at first designed, or can increase their lengths. In either case they would provide for an increased commanded area. If they do not do this, they ought to adhere to the values at first proposed, thus making the channels larger than, according to their ideas, would be necessary. These larger channels will be able to do more irrigation, by an increase, not in the commanded area, but in the proportion of it which is irrigated. Any other course would result in the canal carrying more water than could presumably be used by the distributaries. Again, the question how the assumed duty was arrived at may need consideration. It may have been arrived at by taking the duty figures of some existing canal, based on discharge figures which were the result, not of observed but of calculated discharges, and if the calculations were based on a value of N which experience has proved to be wrong, a correction is obviously needed. Many mistakes of the kinds indicated above have been made, not perhaps in the case of a project which has been recently got up and is then quickly carried out in its entirety, but in one which is carried out slowly or after a long period has elapsed or in one which consists of extensions of an existing system. So great, however, is the elasticity of a channel—by which is meant its capacity for adapting itself to varied discharges, a small change in the depth of water causing a great change in the discharge—and so considerable has been the uncertainty as to the real duty to be expected, that any mistakes made have not usually resulted in any serious trouble.

BIFURCATION AT TAIL OF CANAL.

The Distributaries have Gates and Winches.

To face p. 41.

It has been stated ([Chapter I, Art. 2]) that it is not desirable to let one channel tail into another. In old canals a distributary used sometimes, after running parallel to a canal, to be brought back towards it and tail into it. The advantage of this was that the distributary had not to be made very small towards the tail and that, if the demand abruptly ceased, the distributary was not likely to breach. The principle was, however, essentially bad. The lower part of the distributary was obviously too near the canal and not centrally situated as regards the irrigated strip. The portion at the extreme tail was superfluous. Again, whatever volume of water was carried through the distributary and back into the canal, was needlessly detached instead of being kept in bulk. Moreover the duty of water on such a distributary cannot be ascertained without a tail gauge and the observation of discharges at the tail. There are similar objections to one distributary tailing into another. Each should be separate and distinct.

A major distributary is one whose discharge is more than 40 c. ft. per second. It may be as much as 250 c. ft. per second. A branch, as soon as it reaches a point where its discharge becomes only 250 c. ft. per second should be considered as a major distributary. A minor distributary is one whose discharge is from 8 to 40 c. ft. per second. A minor distributary is nearly always a branch of a major distributary. There are instances of “direct minors,” i.e., minors taking off from canals or branches. Such a minor, unless its discharge is a large fraction of that of the canal which supplies it—and this can seldom be the case—is objectionable because the petty native official who has to see to the regulation of supplies can manipulate the supply easily and without detection, and the number of persons irrigating from it being small, he can make private arrangements with them. On the Sidhnai Canal there are some half-dozen distributaries each of which had one or two minors which took off close to the head of the distributary. The people who irrigated from the minors managed to get the heads shifted and taken off direct from the canal, on the ground that, the water level in the canal being higher than in the distributary, there would be better command and less silt deposit. The irrigation on all these minors ran up to a figure far in excess of what had been intended, to the detriment of lands further down the canal. The minor heads have all been retransferred to the distributaries, the difficulty as to command being got over, as it should have been at first, by constructing weirs in the distributaries. The fall in the water surface at the distributary head, i.e., the difference between the water level in the canal and that in the distributary downstream of its head but upstream of the weir, is quite trifling or even inappreciable.

In some of the older Indian canals it was the custom to place the heads of distributaries, not just above a fall but several hundred feet above it, the idea being that the distributary then received less silt. This practice has now been discontinued. There is no valid reason for following it.

The question whether, when a channel crosses a road on the skew, a skew bridge should be constructed or curves introduced into the road or channel, is one which requires some consideration. As far as possible the lines of channels should be fixed so as to cross important[10] roads on the square or with a small angle of skew. In the case of main canals or branches, the introduction of special curves is generally out of the question, but if the road is not straight something can be done by shifting the line one way or the other. In the case of “major” distributaries, curves can to some extent be introduced. In the case of “minor” distributaries it is often possible to curve the channel, with a radius of say 500 feet, so that it will cross the road at right angles. There is very little objection to a skew bridge if the angle of skew is not great. The angle of crossing having been made as near to 90° as possible, the bridge can be made skew though not necessarily so much askew as the road. Slight curves can be introduced into the road. When the road is made askew, a bridge on the square involves at least three considerable curves ([Fig. 7]) and the taking up of extra land. It also causes, in perpetuity most likely, a more or less inconvenient and unsightly arrangement and one which, in most countries, would not be tolerated. When the angle of skew is not great, it is best to introduce no curve at all into the road. In the case of a “village” road, which may be more or less undefined and liable to be shifted, the difficulty about land may not be great, but even in this case the angle of crossing should, if possible, be kept near to 90°, especially in the case of minors, and where curves have to be introduced into the road they should be suitable ones. Abrupt angles are not only unsightly but are unfair to the cart drivers. The crossings of village roads by the minors of a certain great modern canal have been stigmatised as “hideous.” Indian canals can afford to do work properly.

[10] In India “district” and “provincial” roads.

Fig. 7.

4. Remarks on Distributaries.

—Before a canal system can be properly designed, it is necessary to determine certain points in connection with the working of the distributaries. A distributary is intended to irrigate a certain kharif area. Its average kharif supply is determined from the assumed kharif duty. It generally runs full in the kharif but not always. In a very dry tract such as the Montgomery district of the Punjab, the demand is so great and so steady that a distributary practically runs full through the greater part of the kharif. In such a case the canal or branch must be so designed that it can keep all distributaries full at the same time. Its F.S. discharge will be the sum of all the F.S. discharges of the distributaries plus the losses of water by absorption.

But in other cases, especially if the rainfall is considerable, a distributary does not require its full supply, either all through the kharif or for long at a time. An estimate must then be made of what it will require. It may be estimated that its requirements will be met if, during the period of greatest demand, it is closed for two days out of a fortnight and receives full supply for the remaining twelve days. In this case, since the various distributaries need not all be closed on the same days, the canal or branch can be so designed that it will carry a full supply equal (after deducting losses) to ⁶⁄₇ths of the aggregate full supplies of the distributaries. In other cases the fraction may be ³⁄₄ths. It is likely to be lower the greater the rainfall of the district. Even in the case when the distributaries run full through nearly the whole of the kharif, there will be periods when they only run with about ³⁄₄ths full supply. If full supply were run at such times, many of the outlets would discharge more water than was required, the cultivators would partly close them, and breaches in the banks of the distributary might result. Thus the water level of a distributary must always be so arranged that it will have a good “command” when it is running with about three-fourths of the full supply discharge. The water level with ³⁄₄ths full supply is generally ·5 to ·75 feet below the full supply level but it should be calculated in each case. Generally it will be correct to make the water level, when ³⁄₄ full supply is run, about 1 foot above the high ground traversed by the distributary, excluding any exceptionally high portions of small area. A more exact method is given in [Art. 9]. The greater the proportion of the culturable area which is to be irrigated, the less should be the area of any high land which is excluded. The F.S. levels of the distributaries at their off-takes must be settled in accordance with the foregoing remarks, and these F.S. levels must be entered on the plan. Neglect to thus fix the F.S. levels of distributaries before designing the canals has frequently led to trouble.

The head needed at a bifurcation in order to get the supply into a branch or distributary is always small unless the velocity is high. For a velocity of 3 feet per second the head required is only about ·16 ft., for 2 ft. per second ·1 ft.

On an Inundation Canal which has no weir across the river, the mean supply downstream of the regulator (which is built a few miles down the canal lest it should be damaged by the shifting of the river) is, as has been mentioned, about half the full supply. The command in such canals is not generally very good. A distributary can often obtain only mean supply and it should be designed so as to command the country when it is carrying mean supply. A detailed description of Inundation Canals in Northern India, is given in Punjab Rivers and Works.

Let M, F, m, f, be the mean and full supply discharges at the heads of a canal and of an average distributary on it and let the number of distributaries be n. It has been seen ([Chap. I. Art. 6.]) that M = ·8F about. Let k be the proportion of the supply lost by absorption in canal and branches. Then n m = (1 - k) M = ·8 (1 - k) F. If the distributaries all run with full supplies—at the time of greatest demand—for 4 days out of 5, then,

n f = 1·25 (1 - k) F

fm = 1·25·8 = 1·56

Since k depends on the wetted area, it is not likely to be so great for F as for M, but the above gives a general idea of the ratio of the full kharif discharge to the mean kharif discharge. On a large canal the circumstances of the distributaries will not all be similar. Some will run full for a greater proportion of their time than others. They can be divided into groups and the ratio of the full to the mean supply calculated for each group. The mean supply is, as above stated, obtained from the area to be irrigated, and the duty as estimated at the distributary head.

At one time a system was introduced of making distributaries of large size with the idea of running them for short periods. One reason given for abandoning this arrangement, was that there was a tendency to run such a distributary for too long. This reason is not very intelligible. It would be applicable to any distributary which was not intended to be run without cessation. The result would be that some other distributary would be kept short of water and this would imply extremely bad management. The chief reason against such a distributary is the greater cost of its construction. It would effect a saving of water. The ratio of the discharge to the wetted area would be high, though this would be to some extent neutralized by the greater frequency of closures, since, when water is admitted to a dry channel, the absorption is at first great. There would also be some difficulty in the distribution of the water because of the short period for which it would remain open. It will be seen ([Chapter III. Art. 5]), that it is desirable to open and close always at the same hour of the day. An ordinary distributary might run for 11 days out of 14. One of double the size could not conveniently be run for 5¹⁄₂ days. A distributary can always be enlarged if necessary, but if made too large it is extremely difficult to make it smaller.

It was also, at one time, usual to make minors, when there were several on a distributary, of large capacities so that they ran in turns. The preceding remarks apply to this case. The system has been abandoned.

5. Design of Canal and Branches.

—The apportioning of discharges to the various channels having been effected as described in [Art. 2], the designing of the canal and branches is proceeded with. Rough longitudinal sections of all the lines are prepared by means of the contour map, the ground levels being shown at intervals of one foot—or whatever the vertical distance between the contours may be—and the horizontal distances obtained from the map by scaling.

On these longitudinal sections the lines proposed for the bed and F.S. levels are shown reach by reach and also the mean velocities and discharges.

The laws of silting and scouring and the principles on which channels should be designed are fully gone into in River and Canal Engineering. It is there explained that, for a channel of depth D, there is a certain critical velocity, V₀, which just prevents the deposit of the silt, consisting of heavy clay and fine sand, found in Indian rivers—this silt enters the canal in such immense quantities that the canal silt clearances would be impossible if much of it was deposited in the channels—that sand of grades heavier than ·1 may deposit in the head of a canal and well nigh threaten its existence, that the clear water entering the canal in winter may pick up and carry on some of the sand but that proper steps for preventing the deposit in the canal can be taken at the headworks. This last question has been referred to in [Art. 1]. The following additional rules for designing canals in Northern India are chiefly taken from those given by Kennedy in the explanatory notes to his Hydraulic Diagrams, which are in use in the Irrigation Branch in Northern India.

• (1) Near the hills where the bed is of shingle the velocity may exceed V₀. A few other soils will stand 1·1 V₀.
• (2) In ordinary channels any excess over V₀ will give much trouble lower down.
• (3) In the first four or five miles of a distributary, V₀ should be allowed and gradually be reduced to ·85 V₀ at the tail, the gradient being reduced if convenient, while a minor or branch distributary should have less than V₀ at its off-take and still less at the tail. The sand is drawn off by the outlets and in the lower part of a distributary it is often non-existent.
• (4) If there is efficient silt trapping at the head of the canal any figures arrived at by the preceding rules should be multiplied by ·9.
• (5) In the case of a canal having its head far from the hills, the sand is finer and any figures arrived at as above may be multiplied by, perhaps, about ·75, but further experience is needed to decide this.
• (6) If the soil is very poor, especially if the depth of water is more than 6 or 7 feet, the velocity should be less than V₀—say ·9 V₀—so as not to cause falling in of the banks. Depths of more than 9 or 9·5 feet should, as far as possible, be avoided for the same reason.
• (7) At a bifurcation, one branch channel may have no raised sill, and, owing to its smaller depth, it may draw off no surface water and get an undue share of rolling sand. Its velocity should be greater than V₀ and that of the other branch be less than V₀.
• (8) At such a bifurcation it may be necessary, during times of low supply, to head up the water in the main channel and some silt may temporarily be deposited in it. When the heading up ceases, the silt is scoured away but it mostly goes into the branch whose bed level is the lower. It is best to design such bifurcations so that the sill levels of the two branches are equal and, if possible, so that their bed levels are equal.[11] Otherwise the channel which is likely to get most silt should have the steeper gradient.
• (9) Any existing well established régime should not be tampered with.

[11] Appendix A in River and Canal Engineering deals with some instances of fallacies in questions concerning flow in open streams. An extract from it describing a remarkable divide wall recently constructed at the head of the Gagera branch, Lower Chenab Canal, is given in [Appendix A] of this book.

Experience shows that in designing Irrigation Channels in the plains of India in accordance with Kennedy’s figures, the maximum ratio of bed width to depth of water is as follows:—

The actual gradients of the canals generally range from about 1 in 8,000 for a main canal to 1 in 2,000 for the tail of a distributary, but near the head of a canal where the bed is of boulders and shingle, the gradient may be as steep as 1 in 1,000.[12] The velocity in this last case may be 5 feet per second but generally it is not more than 3 or 4 feet per second in canals and branches, and 1 to 2 feet per second in distributaries.

[12] On the Upper Jhelum Canal, 1 in 970.

In designing the channels, N, in Kutter’s co-efficient, may be taken as ·0225 or ·020, according to judgment. For new and smooth channels ·020 is generally correct. A channel generally becomes rougher by use but sometimes it becomes smoother. Cases have occurred in which N has been found to be ·016. This question is discussed in Hydraulics, Chap. VI.

The bed width of a canal is reduced, where a distributary takes off, in such a way that when the canal and distributary are both running full, the depth of water in the canal continues to be uniform and the flow to be uniform. When the distributary is closed there is heading up in the canal upstream of the off-take, but not enough to make any appreciable difference unless the capacity of the distributary is a large fraction of that of the canal and even then no harm is likely to result.

The preceding rules and principles being taken into consideration, the channels are designed. The bed levels, gradients and depths are so arranged as to give the velocities suited to the soil and to maintain the proper relation of depth to velocity. The bed width is arranged so as to give the proper discharge. The full supply level of the canal and branches has also to be so arranged that it shall be higher, at each distributary off-take, than the full supply level of the distributary. It is desirable to be able to give a distributary its full supply even when the canal is low. Generally the slope of the country along any line is greater than would be suitable for the bed, and “falls” are introduced. The off-take of a distributary is generally just above a fall and there is generally an ample margin between its F.S. level and that of the canal. The discharge of the canal during the greater part of the rabi may be only about half the full supply. This discharge should be estimated and the water level corresponding to it calculated and shown on the longitudinal section. If possible the levels should be so arranged that even with its least supply the water level in the canal will enable full supply to be given to a distributary. If this cannot otherwise be managed it may be necessary to construct a regulator in the canal below the head of the distributary so that, during low supplies, the water can be headed up. It has been stated in River and Canal Engineering, Chapter IV., that such heading up, if temporary, is not at all likely to cause silt deposit in the canal. The designing of the distributaries is not proceeded with at this stage.

Since no irrigation is usually done directly from the canal and branches, they are designed without any particular connection between the level of the water and that of the country traversed. Dangerously high embankments are of course avoided as far as possible. The bed is designed at such a level that the excavation and embankment at any place will be, as nearly as possible, equal. Land in India is cheap. When the excavation exceeds the embankment the balance is made into a spoil bank. When the excavation is less than the embankment the balance is got from borrow-pits.

The side slopes of channels in excavation are generally 1 to 1, in embankment 1¹⁄₂ to 1. The sides of channels of small or moderate size usually become about ¹⁄₂ to 1, or even vertical, by the deposit of silt on the slopes. This reduction of area is allowed for in the design i.e. the bed width is so designed that the channel will carry the required discharge, not with the side slopes as executed, but when they have become ¹⁄₂ to 1. In large canals however the sides do not always silt up but rather tend to fall in. When this is expected to occur the allowance above described is not made. Berms are left so that if any part of the sides fall in, the bank will not also fall in. The berms also allow of the channel being widened if that ever becomes necessary. Type sections are given in [Figs. 8] and [9].

6. Banks and Roads.

—[Figs. 8] and [9] show the banks and spoil.

Fig. 8.

Fig. 9.

The scale is 6 feet to an inch. The depth of water, in this particular case, is 7 feet, and the bank, excluding the small raised bank, 2 feet above the water. The inside edge of the bank, where the small raised bank is shown, is kept parallel to the canal for a considerable distance. Its position is got by drawing a line, shown dotted, at, generally, 1¹⁄₂ to 1. The embanked part of the slope is actually made at 1¹⁄₂ to 1, but the excavation is at 1 to 1, so that a berm is left. The width of this berm of course varies as the depth of digging varies. If there is likely to be much falling in of the sides the berm can be made wider, the dotted line starting, not from the edge of the bed, but from a point further in. On an inundation canal in sandy soil the berm may be 20 feet wide. In figure 8, the inside slope above the berm is supposed to have silted up to a slope of 1 to 1. In cases where it is expected the whole inside slope will silt to ¹⁄₂ to 1 the dotted line, to give the edge of the bank, can be shifted towards the channel so that the berm at the ground level when the channel is excavated will be very small for the minimum depth of digging. There is no need for the inner edge of the bank to run parallel to the canal for great distances. Its position can be shifted whenever suitable and the width of the berm at ground level varied. This prevents the occupation of a needlessly great width of land. It used at one time to be not unusual to make a bank with a berm on the land side, similar to that formed by the spoil in [Fig. 8], but at about the level of full supply in the canal. The principle is not a good one. Salient angles are liable to be worn away. If earth has to be added to a bank to strengthen it, the whole can be widened or the rear slope flattened. The roadway is shown 18 feet wide, which is nearly the maximum. For the drainage of rain water it has a transverse slope, away from the canal, of about 1 in 50. The small raised bank on the canal side is to give safety to wheeled vehicles. It is provided on the patrol bank[13] on main lines and places where there is much traffic or where there is plenty of width of bank to spare. When the ground level is, for a considerable distance, above the proper bank level—which is at a fixed height above the F.S. Level—so that the road and its side-drain have to be cut out, much earthwork can be saved by allowing them to be at a higher level and, in the case at least of the non-patrol road, giving the road a reduced width.

[13] A canal has an unmetalled driving road—called the “patrol road” or “inspection road” on one bank. This road is reserved for the use of officials. Otherwise, it would soon be cut up and worn away, and the cost of repairs would be excessive. The patrol road should be on that bank which is, in the morning (the time when inspections are usually made) in the shade of trees planted on the landward side. Trees are not usually planted near the water edge as they are sometimes blown down. In Northern India the canals generally flow in a southerly direction, so that the left bank is best for the patrol bank. On the other bank there is a bridle road which is open to the public. Near a rest house—unless there is a bridge actually at the place—the patrol road should be on the same bank as the rest house. It can if necessary cross at the first bridge. Frequently there is also on one or both sides of the canal a “boundary road,” which is open to the public, along the toe of the outer slope. Along a distributary there may be a boundary road on one side. It is generally the only road which can take wheeled traffic, and in this case it should be reserved for officials unless money is provided to keep it always in repair. Officials have to be on tour for weeks or months at a time, and in all weathers. Their baggage carts also have to precede and follow them. Anything which facilitates their touring about and seeing things for themselves is, in India, most desirable. At a watercourse crossing the boundary road along a distributary should be taken by a curved incline up on to the bank and down again. Thus not only is the cost of a culvert saved, but any touring official who is driving obtains a view of the channel which he cannot get from the boundary road.

Fig. 10.

In shallow digging, the plan of setting back the banks ([Fig. 10]) and letting silt deposit as shown by the dotted lines, is one which should be followed much oftener than it is. It not only gives eventually a very strong bank, but it enables the borrow pits, from which the earth for the banks is got, to be dug inside the banks. Outside borrow pits, besides being a source of expense, owing to compensation having to be paid to those in whose land they are dug, cause great areas of hollows which are not only unsightly, but are often full of stagnant water and are thus a fruitful source of mosquitoes and malaria. Insufficient attention has hitherto been paid to this matter.

In designing each reach of a canal or branch, type cross sections should be drawn out for several different depths of digging, e.g., one for very shallow digging, i.e., where the bed is little, if at all, above the ground level, one for deep digging where the ground is higher than the water level, and one for the “balancing depth,” where the area of the channel excavation is equal to the earth required for the banks. In calculating the earthwork the sectional area of the digging or of the embankment is taken, whichever is the greater.

The proper width and height of bank for any channel depends partly on the maximum depth of water in the channel, and partly on the discharge. Given a depth of water of say 8 feet, a breach will obviously be more disastrous with a great volume of water than with a small volume. The following statement gives some figures suitable to the rather light and friable soils of Northern India, but the question is largely one of judgment. Generally a low and rather wide bank is preferable to a higher and narrower one. If a road, with or without the small raised bank next the canal, is required, special widths can, of course, be arranged for. A 14-foot bank is required for a driving road.

The spoil in [Fig. 8] is shown at a different level from the bank proper, as it should be to give a neat straight edge to the bank. The width of the spoil may vary every chain. In [Fig. 9] the spoil is raised to avoid taking up too much land. The spoil presents the best appearance when its height is kept uniform for as long a length as possible, the width varying according to necessity, When the height has to be altered, the change should be made by means of a short ramp. When the spoil is higher than the road, gaps in it are left at intervals so that rain water can pass away. When the spoil is heavy for a very short length it can, in order to avoid a short and unsightly heap, which would result from the adoption of the section shown in [Fig. 9], be placed as in [Fig. 8], some of it being led askew.

The small channel shown outside the bank in [Fig. 8] is a watercourse for enabling trees to be grown. It has, of course, to be graded, and it may be in cutting or in embankment. If any silt clearances of the canal are likely to be necessary, the watercourse must be set back to allow room for the spoil. Such spoil, if sandy, is to a large extent washed down or blown away and does not accumulate to anything like the extent that would be expected.[14] Moreover the spoil can extend onto the watercourse when the trees have grown big, and no longer need watering. Outside the watercourse is shown the boundary road and the land boundary pillar. The small channel in [Fig. 9] is a drain for rain water. It can be used as a plantation watercourse if the water is lifted.

[14] This fact has been quoted (The Pioneer Mail, “Silt,” 8th March, 1913) as showing that the silt supposed to be cleared is not really cleared. This may be the case to some extent, but shortage of spoil is little proof of it.

Where there is no spoil, some extra land, perhaps 20 feet on either bank, is usually taken up for getting earth from for repairs.

7. Trial Lines.

—The proposed lines of channel, determined as explained in Art. 5 should next be laid down on the ground. A line should consist of a number of straight portions. The curves should not be put in. Trial pits should be dug at intervals. Some defects in the line may at once become apparent because the contour map, owing chiefly to the lines of levels having been taken a considerable distance apart, is not perfect. A line may pass through a patch of very high or very low ground or too near to some building or other object with which it is desirable not to interfere. Alteration may be desirable at a drainage crossing or at the off-take of a branch. The lines should be corrected where necessary. Sometimes the corrections may be very considerable. Allowance can be made for the alterations which will occur when the curves are laid out. Where there is doubt as to which line is the best, trial pits may be dug to obtain further information regarding the soil.

The line should now be levelled, careful checks being made, a longitudinal section of it prepared and the proposed bed, bank and F.S. level shown. The ground levels ascertained by levelling the line, are certain to disagree, to some extent, with the contour lines. The latter were got only by inference from the levels of points in the survey lines, and they should be corrected in accordance with the fresh levels now available. If the line does not seem to be the best that can be got, a fresh line can be marked on the plan and the above procedure repeated.

8. Final Line and Estimate.

—As soon as the best line seems to have been found, a large scale plan of the country along its course should be made by taking bearings or off-sets from points in it to the various objects and noting where the line cuts them. On this plan will be shown the exact alignment, the curves being put in and the straight portions slightly shifted where necessary so that the line may pass at a proper distance from any buildings or other objects. But before this procedure is carried out, or while it is being carried out, the estimate for the work can be prepared from the longitudinal section already taken. Such a section is of course amply sufficient for a “project estimate,” in which only approximate figures are given, and it is quite near enough for any estimate. In the case of small works which have often to be executed with great promptitude, lamentable delays have occurred owing to the engineer deferring the preparation of his estimate till he had got the line exactly fixed. Moreover there is a chance of the labour being thrown away in case the sanctioning authority directs any change in the alignment to be made.

In the case of a large scheme, a project estimate is prepared. In this the earthwork and the area of land to be occupied are calculated pretty accurately. Designs and estimates are also prepared for the headworks and for the chief regulators. For works of which there are to be many of one type—bridges, falls, distributary heads and small drainage syphons—the cost is arrived at from lump sum figures, one drawing of each kind being submitted as a type. The distributaries are approximately estimated at mileage rates. In the case of a small scheme everything is estimated in detail except perhaps the distributaries or some of them.

CANAL WITH BRIDGE AND DISTRIBUTARY HEAD.

The Head has Gates and Winches.

To face p. 61.

9. Design of a Distributary.

—A distributary is a canal in miniature and, like a canal, it may have branches. It has masonry bridges, falls and drainage syphons. It has, as already mentioned, a masonry regulator at its head. At the off-take of any branch or distributary there is a regulator in the head of the branch. If the branch takes off a large proportion of the water there is a double regulator. A distributary gives off watercourses as a canal gives off distributaries. The watercourses belong to the people and not to Government and they are cleared and maintained by the people. Each watercourse has a masonry head known as an “outlet” ([Fig. 11]). The outlet is the point where the water passes from the hands of Government officials to those of the cultivators. The outlet is of masonry and its opening is not adjustable but is fixed in such a way that its discharge, when the distributary is full, bears, as nearly as can be arranged, the same ratio to the F.S. discharge of the distributary as the area intended to be irrigated by the watercourse bears to that intended to be irrigated by the distributary.

Fig. 11.

The floor of the outlet is level with the bed of the distributary. It thus draws off rolling sand which might otherwise accumulate in the distributary. Small outlets are made of earthenware pipes, about ·4 feet in diameter, laid in concrete. Two pipes, or three, may be laid side by side. If more than three would be required, a masonry opening is adopted. The discharge through an outlet, is generally 2 to 5 c. feet per second per square foot of outlet area, and the head ·1 to ·5 feet.

For the tract of country allotted to any distributary, a contour map is prepared on a fairly large scale, say 4 inches to a mile. On the map the line is laid down and a rough longitudinal section, showing the ground level, is prepared as in the case of a canal.

It has already been stated ([Art. 4]) that a distributary is so designed that its water level, when three-fourths of the full supply is run, shall be well above the level of most of the ground along its course. In other words it should have a good command. A good rule is to allow a fall of ·5 feet from the level of the water in the distributary to that in the watercourse, a slope of 1 in 4,000 for the water flowing along the watercourse, and a fall of ·3 feet for the water at the tail of the watercourse to the level of the ground. This last level is, like the other ground levels, taken from the contour map. This procedure, in short, consists in making the water level of the watercourse at its head govern that of the distributary, just as the water level in the distributary at its head was made to govern that in the canal.

The enlarged contour map of the distributary area shows, among other things, the boundaries of the lands belonging to each village. Generally a watercourse supplies water to only one village. When, however, a village is far from the distributary, its watercourse has to pass for a long distance through other villages and it would be wasteful of water to have two separate watercourses. In such cases one watercourse may serve two villages or more. When a village is near to the distributary and its land extends for a long distance parallel to the distributary, it may have several watercourses for itself alone. A watercourse can generally be most conveniently dug along the boundary line of two villages, or there may be some other line which the people particularly desire.[15] Subject to, or modified by, these considerations a watercourse is designed to run on high ground like a distributary.

[15] They also frequently wish the “chak”—the area irrigated by a watercourse—so arranged that two men who are “enemies” shall not be included in the same “chak.” This condition can be complied with only up to a certain point. Arrangements may be modified but not in such a way as to upset the proper rules.

Contour Map (part) and Line of Distributary.

The scale is 1 inch to 2 miles. The contour lines at 1 foot intervals are shown dotted, the roads by double lines. The line of the distributary, in order to follow the ridge of the country, would have gone more to the left of the plan near the village. The shifting of the line to the right brings it nearer to the centre of the irrigated tract—supposed to be the whole area shown—and enables a single bridge to be built at the bifurcation of the two roads. Suitable lines for main watercourses are shown in thin firm lines. It is assumed that the command is sufficient to enable the watercourses to run off at the considerable angles shown.

To face page 63

The great object is to reduce the total length of channels, i.e., minors and watercourses. No watercourse can be allowed to run alongside of or near to another. It may run alongside a canal or distributary when really necessary to gain command but not otherwise. The longer the watercourse the larger the chak. The discharge of an outlet may be anything up to 4 or 5 c. feet per second. This limits the size of a chak. If a chak is too big it can be split up or a minor can be designed. Very small chaks are to be avoided, but it is difficult to fix a minimum size. The irrigation boundary of the distributary, as fixed in the project, is shown on the map but in practice it will not be exactly followed. For various reasons the boundaries of a chak may run somewhat outside it or stop short of it.

Where a distributary gives off a minor and there is a double regulator, watercourses should, as far as possible, be taken off from one or other of the branch channels and not from upstream of the double regulator. Otherwise, irregularities are likely to occur, both of the regulators being partially closed at the same time—a thing which is never necessary in legitimate distribution of the supply—in order to head up the water and increase the discharges of the outlets.

A watercourse nearly always gives off branches and generally a system of turns is arranged by the farmers among themselves, each branch in turn taking the whole discharge of the watercourse for a day or part of a day, the other branches being closed by small dams of earth. To irrigate a field alongside the watercourse a gap is cut in its bank. For fields further away, smaller channels run off from the watercourses at numerous points. Several gaps and several field channels may be in flow at one time, and there is a dam in the watercourse below the lowest one.

Occasionally, on an old canal, one watercourse crosses another, the lands irrigated being at different levels, but such crossings do not often occur in systems of watercourses laid out according to modern methods. They are, however, quite legitimate.

The lines of the main watercourses are sketched on the map, their irrigation boundaries shown on it, and F.S. discharges allotted to them according to the areas which are to be dependent on them. In order that this may conveniently be done the “full supply duty” or “full supply factor” for the distributary is obtained. It bears the same ratio to the ordinary duty that the mean supply bears to the full supply. The total of the F.S. discharges of all the watercourses should, with an allowance for loss by absorption in the distributary, be the same as the F.S. discharge of the distributary. If the results are very discrepant it shows that the sizes of the outlets need revision. Possibly they may all be too large.

In “colonization” schemes where a canal is constructed to irrigate waste lands—which are the property of Government and which are divided into square blocks and given out to colonists—Government has complete control of the watercourse system, and can arrange it exactly as desired, but in other cases landowners often strenuously oppose the passage of watercourses through their lands. Compulsory procedure according to legal methods is tedious, but the practical rule is not to let anyone have water until any watercourses which are to pass through his land have been not only agreed to but constructed.

In ordinary cases Government possesses no power as to the precise line on which a watercourse is dug. It fixes the site of the outlet and assigns certain land to it, and sketches out the line of the watercourse. If the people choose to alter the line they can do so, but great alterations in the main watercourses are not generally feasible.

The positions of the outlets[16] having been settled after discussion with the cultivators, a table is prepared showing the chainage of the outlets, the probable head or difference between the F.S. level of the distributary and of the watercourse, and the F.S. discharge. From this the sizes of the outlets are calculated and shown in another column. If the length of the outlet barrel is not more than 5 or 6 times the diameter—in the case of a barrel whose cross section is not round or square, the mean diameter—the discharge can be calculated as for a “short tube,” but if longer the formula for flow in pipes should be used, allowance being, of course, made for the head lost at the entrance. The outlets generally consist at first of wooden “shoots” or long tubes, rectangular in cross section. This is because, after they have been tested by a year or two years’ working, the sizes nearly always require adjustment and the cultivators often wish to have the site shifted.

[16] The positions can be slightly altered by the Engineers for any sufficient reason.

Fig. 11.

The uncertainty as to the proper size of an outlet is due to several causes. If the command is very good there may be a clear fall from the outlet into the watercourse. In this case the discharge depends only on the depth of water in the distributary, and is known pretty accurately. But ordinarily the outlet is submerged, and its discharge depends on the difference between the water levels in the distributary and in the watercourse. The latter level is not fixed. The cultivators can lower it, to an extent which depends chiefly on the distance of the fields from the distributary, by deepening or widening the watercourse. In this way the discharge of the watercourse is increased except when a dam is temporarily made in it for the purpose of irrigating any comparatively high land. This uncertainty as to the discharge can in some cases be got over by building a cistern ([Fig. 11]). This has the same effect as raising the level of the barrel, the real outlet being no longer submerged, and the discharge depending on the depth of the crest of the overfall below the water in the distributary. But such cisterns add greatly to the cost of an outlet, and they can only be adopted when there is good command. A great cause of uncertainty as to the proper size of an outlet is the variability of the duty of the water on the watercourse. The soil may be clayey or sandy, the watercourse may be short or long, the crops grown may be ordinary ones or may be chiefly rice, which requires three or four times as much water as most other crops, and the cultivators may be careful or the opposite. Again, the people may, if the outlet gives a plentiful supply, often keep it closed, but there is no record of such closures nor would the people admit that they occur. These causes may all operate in one direction—on a whole distributary this cannot happen to the same extent—and thus enormous differences in duty may occur. There is no way of arriving at the proper size for an outlet except trial. Observations of the discharges of the outlets are of very limited use. The discharge may vary according to the particular fields being irrigated. Observations of discharges will be useful in cases where the people complain, or when the discharge is obviously much greater or much less than intended and will in such cases enable temporary adjustments to be made, but by placing a dam in a watercourse and turning the water on to a high field near its head the people can make it appear that the discharge is only a fraction of what it should be.

On any distributary there are generally some watercourses which have a poor command, the head at the outlet being, say, ·1 ft. or even less. Probably the irrigation is a good deal less than it should be. In such cases the rules may be set aside and a liberal size of outlet given. The size may be 2 or 3 times the calculated size. There is no harm in this. The irrigation cannot increase much. Similar cases frequently occur on inundation canals especially near the heads of canals or distributaries.

The construction of masonry outlets on a distributary is not usually a final settlement of the matter. Further adjustments become necessary. This matter will be dealt with in [Chapter III].

On the older canals little or insufficient attention was given to the question of the sizes of outlets. The sizes were far too great and, as long as all the outlets in a distributary remained open, water could not reach the tail. The distributary used to be divided into two or three reaches and the outlets in the upstream reaches used to be closed periodically. The closures had to be effected through the agency of native subordinates and the system gave rise to corruption on a colossal scale. The tail villages never obtained anything like their proper share of water. The upper villages were over-watered and the soil was often water-logged and damaged. Moreover, even if all concerned had the best intentions, it was impossible to stop all leakage in the closed outlets, except by making earthen dams in the watercourses, and great waste of water resulted from this.

The water level of the distributary with ³⁄₄ full supply, designed so as to be at least ·5 ft. above the water level in the watercourse heads—or to be 1 foot above high ground if this simpler plan is adopted—is drawn on the rough longitudinal section and also the line of F.S., falls being introduced where desirable and the gradients, F.S. depths of water and widths of channels being arranged, just as in the case of a canal, so as to give the required discharges, velocities suited to the soil and a suitable ratio of depth to velocity. The bed width of a distributary decreases in whole numbers of feet. The decrease occurs at outlets but not at every outlet. As the channel becomes smaller its velocity becomes less and this necessitates, according to the laws of silting and scour, a reduced depth of water. The height and width of the banks in the tail portion of a distributary should be made rather greater than elsewhere—regard being had to the depth and volume of the water—so that breaches may not occur when the demand abruptly slackens. The longitudinal section of a distributary should have horizontal lines for showing the following:

[17] Called “Natural Surface” in India.

[18] Called “Reduced Distance” in India.

A specimen of a longitudinal section is shown in [Fig. 12]. It shows only a few of the above items. In practice all would be shown, large sheets of paper being used with all the lines and titles printed on them.

When a distributary is constructed the side slopes are made 1 to 1 in excavation and 1¹⁄₂ to 1 in embankment. The sides usually silt up till they are ¹⁄₂ to 1 or even vertical. The silting up to ¹⁄₂ to 1 is, as in the case of a canal, allowed for in the designing. The berms are left so that, if any part of the side falls in, the bank will not also fall in. They also allow of widening of the channel. The remarks made in Art. 6 regarding the design of banks, apply to distributaries, especially large ones.

On a distributary there is seldom much spoil. Where there is no spoil, a strip of land, outside the bank and 10 feet wide, can be taken up on either bank from which to obtain earth for repairs. On a minor the width of the strip is sometimes only 5 feet.

Fig. 12.

[Larger illustration] (82 kB)

When a distributary passes through land which is irrigated from wells, it frequently cuts through the small watercourses which run from the well to the fields. In such cases, either a syphon or a supplementary well is provided at Government cost. If several watercourses, all from the same well, are cut through, it is generally possible to combine them for the purpose of the crossing. The wishes of the cultivators in this matter are met as far as possible.

The procedure as regards laying out the line on the ground, digging trial pits, correcting the line and preparing the estimate are the same as for the case of a canal.

Fig. 13.

10. Best System of Distributaries.

—Let AB ([Fig. 13]) represent a portion of a distributary, the irrigation boundary CD being two miles from AB. In order to irrigate a rectangular plot ACDB, the main and branch watercourses would be arranged somewhat as shown by the full and dotted lines respectively. Generally, the whole supply of the main watercourse would be sent in turn down each branch, the other branches being then dry. The average length open is AGE. The ends of the branches lie on a line drawn say 200 feet from the lines BD and DC, since it is not necessary for the watercourses to extend to the outside edges of the fields. Within the field there are small field watercourses which extend to every part of it. By describing three rectangles on AC, making AB greater than, equal to and less than AC, it can be seen that the average length of watercourse open is least—relatively to the area of the block—when AB is equal to AC, i.e., when the block served by the watercourse is square as in the figure. If AB is 4 times AC, the average length of watercourse open is increased—relatively to the area of the block—in about the ratio of 3 to 2. Moderate deviations from a square are of little consequence.

Suppose two parallel distributaries to be 4 miles apart, each of them being an average Indian one, say sixteen miles long with a gradient of one in 4,000, and side slopes of ¹⁄₂ to 1, the bed width and depth of water at the head being respectively 13·5 feet and 2·9 feet, and at the tail 3 feet and 1 foot. The discharge of the distributary, with N = ·0225, will be 72 c. ft. per second. The discharge available for the 2 mile strip along one bank will be 36 c. ft. per second. If the duty is 300 acres per c. ft. the area irrigated in this strip will be 10,800 acres, or 1,350 acres for each of the eight squares like ACDB. Each main watercourse would then have to discharge 4·5 c. ft. per second. Supposing its gradient to be 1 in 4,000 and its side slopes ¹⁄₂ to 1 and N to be ·0225, its bed width would be 3 feet and depth of water 1·45 feet. Its wet border would be 6·3 feet, and its average length 5280√2 + 5280 - 200 or 12,546 feet. Its wetted area would be 79,040 square feet, and the total wetted area of the 16 watercourses—on the two sides of the distributary—would be 1,264,640 square feet. The wetted border of the distributary itself is 19·5 feet at the head and 5 feet at the tail, average 12·25 feet, and its wetted area is 5,280 × 16 × 12·25 or 1,034,880 square feet.

If the distributaries were two miles apart, there would be twice the number of distributaries, and each square would be one square mile instead of four. Each watercourse would have to discharge 1·125 c. ft. per second. It would have a bed width of 2 ft., depth of water ·8 ft., wet border 3·8 feet, length 6,173 feet, and wetted area 23,457 feet. The total wetted area of the 64 water courses would be 1,501,248 square feet, or 18 per cent. more than before. Each distributary would discharge 36 c. ft. per second, the bed width and depth at the head being 10 feet and 2·24 feet, and at the tail 2 feet and ·75 feet. The wet border at the head and tail would be 14·5 and 3·5 feet, mean 9 feet, and the wetted area of the two distributaries would be 1,520,640 square feet or 50 per cent. more than before. Supposing that, in the case of the larger distributary considered above, the 2-mile square was considered too large, and that rectangles 1 mile wide were adopted, so that the watercourses were a mile apart, their number would be doubled and their length and size reduced. Their total wetted area would not be greatly affected, but the difference in the wetted areas of the two small distributaries as compared with the one large one, would be the same as before. In practice, of course, distributaries are not always parallel, nor are the blocks of irrigation all squares, and frequently, owing to peculiarities in the levels of the ground or the features of the country, or the boundaries of villages, it is necessary to align the watercourses in a particular manner, or to construct more than one watercourse where one would otherwise have sufficed, but the above calculations show in a general way the advantages of large watercourses and of not placing the distributaries too near together.

It is commonly said that a watercourse discharging more than 4 or 5 c. ft. per second is objectionable because the cultivators, if there are too many of them on one watercourse, cannot organize themselves in order to work it and keep it in order. This matter is much exaggerated. On the inundation canals of the Punjab a watercourse often discharges 10 c. ft. per second, and is several miles long and requires heavy clearances, but the people have no particular difficulty in managing it. Kennedy, a great authority on questions of irrigation, states that the length of a watercourse may be three miles. This, if the angle made by a watercourse with the distributary is 45°, gives rather more than two miles as the width of the strip to be irrigated.

Suppose that a distributary instead of being two miles from each side of the irrigated strip, ran along one side of it, and was four miles from the other side. If the block were square, as before, the side of a square would be 4 miles, and each watercourse would have to discharge 18 c. ft. per second, which is far too much. The blocks would have to be rectangles, each being only one mile wide measured parallel to the distributary. It has been already seen that the length of watercourse in this case is greater than when the block is square and each side is two miles. Thus centrality in the alignment of the distributary is an advantage.

A minor distributary has been defined ([Chapter II., Art. 3]) as being one discharging not more than 40 c. ft. per second, but the term has come to be used to designate a branch of a major distributary, and in that sense it will be used in this article. When the shape of the area commanded by a distributary is such that watercourses exceeding 2 miles in length would otherwise be required, one or more minors are often added. Frequently it is a question whether to let some of the watercourses be more than two miles long, or to construct a minor and thus shorten the watercourses to perhaps only one mile. Which method is best has not been definitely settled. It is known that the loss of water in watercourses is heavy, but if a minor is added the loss in it has to be considered. The loss must be high in any channel in which the ratio of wet border to sectional area is small. The minor also costs money in construction and in maintenance. On the whole the matter, as far as concerns cost and loss of water, is, perhaps, almost evenly balanced, but as regards distribution of the supply a system without minors is preferable. The off-take of a minor is generally far from the canal, i.e., in a more or less out-of-the-way place, and it is impossible to see that the regulation is properly carried out. Irregularities and corruption are sure to arise. Even if the supply is fairly distributed as between the minor and the distributary it is almost certain that the regulator, if a double one, will be manipulated for the illegal benefit of outlets in the distributary upstream of the bifurcation. There are sure to be some such outlets not very far distant. In any case each minor adds one, if not two, to the already very large number of gauges which have to be entered daily in the sub-divisional officer’s register ([Chapter III., Art. 3]), and adds also to the mileage of channel to be inspected and maintained. These considerations should, in many cases, though of course not in all, turn the scale against the construction of a minor. At one time it became usual to construct minors even when watercourses more than two miles long would not otherwise have resulted. This custom was condemned some years ago, and is not likely to be re-established. Most of the difficulties just mentioned can, in the case of a minor which is not too large, either absolutely or relatively to the main distributary downstream of the off-take, be got over by making the minor head like a watercourse outlet, building it up to the proper size, removing the regulating apparatus and abolishing the reading of the gauge, but in this case the minor is not likely to be bigger than a large watercourse. Such minors should not be constructed, and any existing ones should, after the head has been treated as above, be made over to the people and considered as watercourses.

11. Outlets.

—The top of the head and tail walls of an outlet are level with the F.S. levels in the distributary and watercourse respectively. The steps in the head wall enable the cultivators to go down either to stop up the outlet or to remove any obstruction. The stepping is arranged so as to fall inside the side slope ultimately proposed. It is usual, in some places, to have the entrance to the “barrel” of the outlet made of cast iron. The cast iron pieces are made of various standard sizes. This to some extent prevents the “barrel” being built to a wrong size. A discrepancy between the size of the masonry barrel and that of the iron would be noticed, but if the masonry barrel is built too large the iron head does not always restrict the discharge. The action is the same as in a “diverging tube” well known in hydraulics.

For sizes up to about 50 or 60 square inches the barrel should be nearly square. For larger sizes the height should exceed the width. Up to about 100 or 120 square inches the width can be kept down to 7 or 8 inches so that an ordinary brick can be laid across to form the roof. For larger outlets the height can be from 1·5 to 3 times the width, and the roof can be made of large bricks, concrete blocks or slabs of stone or of a flat arch of brickwork or by corbelling, but in this last case there should be two complete courses above the top of the outlet. The less the width the cheaper the roof, the easier the adjustment of size and the less the tendency to silt deposit during low supplies. If pipes are used they should be laid in concrete. If cast iron head pieces are to be used there should be several sizes of one width and the widths of the masonry outlets should be made to suit these widths.

A masonry outlet is not generally built till the watercourse has been sometime in use. The exact position of the outlet should then be so fixed that the watercourse shall run out straight or with a curve and should not be crooked.

The width between parapets should be, for a driving road or one to be made into such, 10 ft. (if the bank is wider, it should be narrowed just at the outlet site) and for a non-driving road, 8 feet to 3 feet according to the ultimate width of the bank. Earth backing should be most carefully put in and rammed, otherwise a breach may occur and the outlet be destroyed.

Various attempts have been made to provide gates or shutters for outlets. The chief result has been trouble and increased cost. If grooves are made and shutters provided, the shutters are soon broken or lost by the people. Hinged flap shutters are objectionable because they are often closed by boys or by malicious persons or by neighbours who wish to increase the supply in their own outlet. The cultivator, when he wishes to reduce the supply or to close the outlet, can easily do this by obstructing the orifice with a piece of wood or an earthenware vessel or a bundle of brushwood or grass.

As regards temporary outlets, wooden outlets if large (unless made of seasoned wood and therefore costly) are liable to give great trouble. Water escapes round the outside or through the joints. Pipes may do well if laid in puddle but are brittle and costly if of large size. The irrigators may interfere both with wooden outlets and pipes and they are liable to be displaced or broken. A temporary outlet, if small, can be made of bricks laid in mud. The joints can be pointed with lime mortar. When the outlet is made permanent the same bricks are used again. But all kinds of temporary outlets are liable to give trouble especially in light or sandy soil. There is much to be said in favour of building masonry outlets at the first, making a barrel only, i.e., omitting the head and tail walls and taking the chance of having to alter the size. The alteration is not very expensive. The head and tail walls are built when the size has been finally settled. The adjustment can be made by raising or lowering the roof. This should be done over the whole length of the outlet but lowering can be done temporarily over a length of 3 feet at the tail end of the outlet. This can be done even when the distributary is in flow. A reduction over a short length at the upstream end of a barrel does not, as already remarked, necessarily reduce the discharge much.

On inundation canals the rules regarding outlets have to be modified. Great numbers of watercourses take off directly from the canals. In such cases, especially near the head of a canal, the ground to be watered is often 5 to 8 feet above the canal bed and it is wholly unsuitable to place the outlet at bed level. The cost of the tail wall would be excessive. The floor level in such cases must be at about the lowest probable cleared bed level of the watercourse, say, in order to be safe, a foot or half a foot below the usual cleared bed of the watercourse, so that water need never be prevented from entering the watercourse. The irrigators should be consulted as to the floor level and their wishes be attended to as far as possible. For lift outlets the floor should be at the bed level of the canal or distributary. If this bed is to be raised in the course of remodelling, the floor should be at the old bed level until the bed has actually been raised, unless there is a weir which raises the water. It is necessary that lift outlets should work however small the canal supply may be. In a distributary or small canal, the head wall should be built up to F.S. level but in a canal with deep water the head wall should reach up to just above the roof of the outlet and be submerged in high supplies. The stepping of the head wall should be set back if the channel is to be widened and should project into the channel if the channel is to be narrowed. The centre line of the channel near the outlet site must always be laid down and the outlet built at right angles to it and also at the correct distance from it.

Occasionally there is a wide berm, say 20 ft. or even 50 ft., between a channel and its bank. In such a case the outlet should be built to suit the bank. The long open cut is however objectionable because the people clear it and heap the spoil in Government land. Sometimes the bank, especially if it is crooked, can be shifted so as to come close to the channel at the outlet site. Sometimes the outlets on inundation canals are large. For outlets of more than 2·5 square feet in area, grooves should be provided so that the cultivators can use a gate if necessary.

12. Masonry Works.

—The positions and descriptions of all the masonry works of a proposed canal or distributary are of course shown on the longitudinal section of the channel and from this the discharges and water levels are obtained. The principles of design to be followed[19] for bridges, weirs, falls, regulators and syphons, are discussed in River and Canal Engineering. It is mentioned that there is no special reason for making the waterway of a regulator exactly the same as that of the stream, and that the waterway may be such as to give the maximum velocity considered desirable, and that the foundations of a bridge should be made so deep that it will be possible to add a floor, at a lower level than the bed of the stream—with the upstream and downstream pitching sloping up to the bed—so as to increase the waterway and so save pulling down the bridge in case the discharge of the channel is increased. It remains to consider certain points affecting Irrigation Canals.

[19] So far as concerns their capacity for dealing with flowing water.

The span of a bridge, where there are no piers, is generally made as shown by the dotted lines in [Figure 14], so that the mean width of waterway is the same as that of the channel. The arches, in Northern India, used at one time to be 60° as shown by the upper curved line, but in recent years arches of 90° as shown by the lower curved line, have frequently been adopted, the springing of the arch being below the F.S. level, so that the stream is somewhat contracted. The 90° arch gives a reduced thickness and height of abutment. It causes increased disturbance of the water, and this may necessitate more downstream protection. An advantage of having the springing not lower than the F.S. level is that this admits of a raising of the F.S. level in case the channel is remodelled, and this arrangement is still common on distributaries.

Fig. 14.

When a fall and bridge are combined, the bridge is placed below the fall as this gives a lower level for the roadway. The side walls of the fall are produced downstream to form those of the bridge.

The roads in India are generally unfenced and the banks of canals close to bridges, on both sides of the canal and both above and below the bridge, are generally more or less worn down by cattle, which, when being driven home in the evening and out to graze in the morning, go down to the stream to drink. In order to prevent this damage the banks are sometimes pitched, above the bridge as well as below it, but the cattle generally make a fresh “ghát” further away. The best plan is to allow a “ghát” on one bank either above or below the bridge and to protect the other three places.