Chapter II: The Designing of a Canal (1)

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.

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.

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:--

Kharif. Rabi.
Average discharge at canal head (c. ft. per sec.) 3536 2755
Duty based on the discharge (acres) 98 138
Estimated loss of water in canal and branches (c. ft. per
sec.) 400 300
Average discharge at distributary head (c. ft. per sec.) 3136 2455
Duty based on the discharge (acres) 111 154

[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.

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.

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,

nf = 1·25 (1 - k)F

f 1·25
- = ---- = 1·56
m ·8

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:--

Discharge, c. ft. per second 10 25 100 200 500 1,000
Ratio 3·5 4 4·5 5 6 6

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.

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.

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.

------------+--------------+---------------+---------------
Top Width of|Height of Bank| Greatest | Greatest
Bank. | above F. S. | Admissible | Admissible
| | Discharge. |Depth of Water.
------------+--------------+---------------+---------------
Feet. | Feet. |C. ft. per sec.| Feet.
20 | 2 | 12,000 | 12
18 | 2 | 8,000 | 12
16 | 2 | 5,000 | 11
14 | 2 | 3,000 | 10
16 | 1·5 | 2,000 | 9
14 | 1·5 | 1,500 | 9
12 | 1·5 | 1,200 | 8
10 | 1·5 | 1,000 | 7
9 | 1·5 | 700 | 6
8 | 1·5 | 500 | 5·5
7 | 1·5 | 400 | 5
6 | 1 | 300 | 4·5
5 | 1 | 200 | 4
4 | 1 | 100 | 3·5
3 | 1 | 50 | 3
------------+--------------+---------------+---------------

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.

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.

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.