Chapter II: The Designing of a Canal (2)

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.

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.

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:

1. Datum |5. Draw-off | 9. Bank width |13. Depth of
| | | digging
2. Bed gradient |6. F.S. discharge|10. Height of |14. Bed level
| | bank |
3. Village |7. Velocity |11. F.S. depth |15. Ground
| | | level[17]
4. Land width |8. V₀ |12. Bed width |16. Chainage[18]
-----------------+-----------------+-----------------+----------------

[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.

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.

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.

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.

In the Punjab the widths of roadways between the kerbs and parapets of bridges respectively have been fixed as follows:--

--------------+----------------+-------------------
KIND OF ROAD.| NEAR TOWNS.[20]|IN THE COUNTRY.[21]
--------------+------+---------+--------+----------
|Kerbs.|Parapets.| Kerbs. | Parapets.
--------------+------+---------+--------+----------
Provincial | 22 | 23·5 | 16 | 17·5
District | 18 | 19·5 | 14 | 15·5
Village | 14 | 15·5 | 8·5 | 10
--------------+------+---------+--------+----------

[20] The figures show the maximum. The general width should be the
same as for neighbouring bridges on the same road.

[21] The parapets should be whitewashed so as to be visible at night.

Fig. 15 shows a head regulator for a distributary. The scale is 10 feet to an inch. It has a double set of grooves for the insertion of the planks with which the regulation is effected. Only one set of grooves is ordinarily used, but when the distributary has to be closed for silt clearance and all leakage stopped, both sets of grooves can be used and earth rammed in between the two sets of planks. The floor is shown a foot lower than the bed of the distributary. This reduces the action of the water on the floor, and enables the bed of the distributary to be lowered if ever the occasion for this should arise. This is a good rule--in spite of the fact that in re-modellings the tendency is for the beds to be raised--for all regulators or bridges, a raised sill being added (in regulators) to reduce the length of the needles or the number of the planks. Such sill should, where needles are to be used, be fairly wide, especially if regulation is to be done while the masonry is somewhat new. The distributary shown has a bed width of 10 ft. The span of the two openings in the head might have been four feet each, but are actually five feet, and this enables the distributary to be increased in size at any time. The pitched portion of the channel tapers. Unless needles are used, instead of horizontal planks, spans are not usually greater than 5 or 6 feet. Longer spans would give rise to difficulties in manipulating the planks. Sometimes distributary heads are built skew, but there is seldom or never any good reason for this. A curve can always be introduced below the head to give the alignment the desired direction.[22] The small circles shown on the plan are “bumping posts.” On the left is shown a portion of the small raised bank at the edge of the road.

[22] The curve can be quite sharp (see CHAP. I., Art. 2), and can be
made, if necessary, within the length of the pitching.

Figure 16 is a double regulator with needles. The scale is 30 feet to an inch. The spans are 15 feet. The roadway is on arches, but the regulating platform on steel beams. The needles are seen at the upstream sides of the regulators. They are worked from the platforms to which access is obtained through the gaps in the upstream parapets. The regulating platform should generally be only just clear of the F.S. level, and therefore lower than the roadway.

Needles lying on Bank.

_To face p. 85._]

Frequently the roadway of a bridge or small regulator is carried, not on arches, but on steel beams. The railings may be of wood or of gas pipe with the ends plugged, running through angle iron posts. In the case of such a regulator the roadway is sometimes so light that camels are not allowed to cross over. This causes unnecessary hardship. Bridges are not too numerous. If the regulation is done by gates, both road and platform are carried on arches.

The regulators on inundation canals, and some on perennial canals, are not strong enough to admit of the flow of water being entirely stopped, so that the depth of water would be perhaps 10 feet upstream and nil downstream. This might cause the overturning of the piers, or the formation of streams under the floor. In such cases a maximum permissible heading up is decided on. Such orders are, in India, liable to be lost sight of in course of time, and they are, at least on inundation canals, where sudden emergencies often occur, hardly reasonable. An engine driver is not told that he must never entirely close his throttle valve. Regulators should be so designed that the water can be completely shut off.

The following remarks show the chief points in favour of needles and horizontal planks respectively.

_Advantages of Needles._ Needles can be placed or removed by one man.

Needles do not require hooks, etc., which are liable to be broken or
lost.

A needle regulator requires few piers, and is therefore cheap.

Water falling over planks throws a strain on the floor.

Regulation with needles is easy and rapid. A jammed plank, especially
if low down and not horizontal, may give great trouble.

_Advantages of Planks._ Floating rubbish is not liable to collect
above the Regulator because the water flows over the planks.

By means of double grooves and earth filling, leakage can be quite
stopped.

For large works the advantages are generally with needles, but for small works, _e.g._ distributary heads and shallow water, with planks. Needles 14 feet long are not too long for trained men. Planks are more likely than needles to arrest rolling sand, and this can be taken into consideration in designing double regulators. See number 8 of Kennedy’s rules, Article 5. When planks are used there should be two sets of grooves. Planks are very suitable for escape heads which have only occasionally to be opened, earth being filled in between the two sets of planks.

Regarding notched falls, in the case of small distributaries the notches are so narrow that they are extremely liable to be obstructed either accidentally by floating rubbish or wilfully by persons whose outlets are upstream of them. Weirs are not open to this objection, and are frequently adopted. There is not the least chance of their causing any silting worth mentioning. A simple weir if made of the proper height for the F.S. discharge, will cause a slight heading up with ³⁄₄ths of the F.S. discharge, and this unfairly benefits any outlets for a considerable distance upstream of the weir. This difficulty can be got over by making the weir as in Fig. 16A.

In this case the usual practice of placing the bridge downstream of the fall has not been followed.

The gauge well is seen on the left bank.

_To face p. 87._]

For cisterns below falls the usual rule for the depth is

K = H + ∛H √D

where H is the depth of water in the upstream reach, and D is the difference between the upstream and downstream water levels. Another rule for distributaries is

H + D
K = -----
3

the length of the cistern being 3 H and its width the bed width of the channel.

At “incomplete” falls, i.e., where the tail water level is above the crest, it is not unusual to construct a low-level arch, which forms a syphon. The object is to allay the surging of the surface water.

The question of skew bridges has been dealt with in Art. 3. Another question is that of the heights of bridges. Irrigation channels, especially the smaller ones, are very frequently at a high level, and bridges have ramps which are expensive to make and to maintain, and are inconvenient. The lowering of distributary bridges in such cases, so that they become syphons, or nearly so, has often been advocated and is frequently desirable. The bed should slope down to the floor and up again. The heading up can be reduced by giving ample waterway, but it will not be necessary to do this if there is head to spare. The fall in the water surface can be recognised and shown on the longitudinal section. The structure becomes one of the incomplete falls above described. The crown of the arch can, if desirable, be kept above F.S. level, so that floating rubbish will not accumulate.

The width between the parapets of a regulator can be 10 feet in the case of a driving road. It may be less, according to the width of the bank, in other cases.

The upper layer of the floor of a bridge or regulator is of brick on edge. Below this there is a layer of brick laid flat, and below this, concrete of a thickness ranging from ·5 feet to 3 feet. The thicknesses of piers range from 1·5 to 3 feet.

The bricks used for canal work in Northern India are 10 inches long, 4⁷⁄₈ inches wide, and 2³⁄₄ inches thick. The thicknesses of walls are about ·83, 1·25, 1·7, 2·1, 2·5 feet, and so on.

The slopes of ramps should be about 3 in 100 for district roads, and 5 in 100 for village roads.

Railings should be provided along the tops of high walls and top of pitching near to public roads or canal patrol roads. Bumping posts should be provided for all parapets, and should not be so placed as to seriously obstruct the roadway.

The quarters for the regulating staff should, when convenient, be in the fork between the two principal branches. They may be on the bank--with foundations on pillars carried down to ground level--but not in such a position as to obstruct the road or any road likely to be made. Rests consisting of two parallel timbers bolted to blocks of masonry reaching up a foot from the ground, should be provided for the needles or planks. The bolt head should be countersunk so as not to damage the needles and planks when they are hurriedly laid down.

When two or more works are close together they should be made to conform, and the whole site should be considered with reference to a neat and suitable arrangement of works, ramps and roadways. If an outlet is near to a minor or distributary head the parapets of the two should be in line. If two masonry works of any kind are near together it is often suitable to pitch the intervening space. If there are outlets or distributaries on opposite banks they should be exactly opposite each other. Where a road crosses a bridge or regulator, the bank should be at the same level as the road, the bank being gradually ramped back to its original level. The space in front of any quarters should have a slight slope for drainage, but otherwise be at one level and be connected with the road or bank by proper ramps. The berm or bank should be made at the exact level of the top of any pitching or side wall which adjoins it. Wing walls are frequently made too short, so that the earth at their ends forms a steep slope and is worn away, and the bank or roadway is cut into. The walls should extend to such a point that the earth at their ends cannot assume a slope steeper than the slope of the bank.

It is obvious that for every masonry work there should be a large scale site plan[23] showing all roads, ramps, and adjoining works, both existing and proposed roads being shown for some little distance from the work.

[23] It is, or was until recently, in some parts of India, the custom
to omit the preparation of site plans, and to leave the fixing of the
exact site of a work and the arrangement of ramps and other details to
the judgment of the assistant engineer who was building it. Much
unsightly work resulted. A chief engineer in the Punjab recently
issued some orders on the subject.

For each kind of masonry work there is usually a type design. A few of its dimensions, which are fixed, are marked on it. The other dimensions are variable. It would be a great advantage to add to the design a tabular statement to show how these dimensions should vary under different circumstances.

13. =Pitching.= The object of pitching upstream of bridges or regulators or downstream of bridges where there may be little or no scouring action, may be partly to protect the bank from damage by cattle or wear, or to prevent sandy sides from falling in. In such cases there may be pitching of the sides only, and it may be of brick on edge laid dry and under this one brick flat resting on rammed ballast (Fig. 17). Downstream of regulators or weirs and downstream of bridges if contracted or having piers which cause a rush of water, especially if the soil is soft, the side pitching may be as above, but with the bricks over one-sixth of the area placed on end and projecting for half their length. This “roughened pitching” tends somewhat to reduce the eddying. The bed protection should be solid concrete or blocks of concrete or masonry. Immediately downstream of regulators or weirs where there is great disturbance, both side and bed pitching may consist of solid concrete or of concrete or masonry blocks (Fig. 18).

Three kinds of toe walls are shown in Figures 17, 19 and 20. The kind shown in Fig. 19 contains, for a given depth below the bed, far more masonry than the one shown in Fig. 17. It is also liable to be displaced and broken if scour occurs.

The earth should in all cases be carefully cut to the proper slope, so that no made earth has to be added. If the slope has already fallen in too much, well rammed earth should be added. The flat brick and rammed ballast can be varied as the work proceeds, more being used in soft places and less in hard.

In some parts of the Punjab, large bricks, the length, breadth, and thickness being about twice the corresponding dimensions of an ordinary brick, are made, and are extremely useful and cheap for pitching. Where the soil is sandy such bricks can be burned without cracking.

Sometimes the curtain wall which runs across the bed at the downstream end of the pitching is carried into the banks and built up so as to form a profile wall (Fig. 21). This is not very suitable, because the pitching of the sides is apt to settle and leave the profile wall standing out. It is better to lay a row of blocks on the slope. If a hole tends to form in the bed downstream of the curtain wall, blocks of masonry or concrete can be laid and left to take up their own positions (Fig. 22).

When scour of the bed or sides occurs downstream of pitching, it is sometimes said that any extension of the pitching downstream is followed by extension of the scour. This may happen if the cross section of the stream downstream of the pitched section has become greater than the pitched section. In this case there is eddying, due to abrupt enlargement of the stream where the pitching ends. The increased width and lowered bed level (not counting mere local hollows) of the stream should be adhered to in the pitching. Where the masonry of the regulator ends and the pitching begins, there will be an abrupt or tapered enlargement, but the eddies--at very low supplies there may be a fall--cannot do harm.

This principle of enlarging the pitched cross section can be followed, even in a new channel, if the soil is light and scour is feared, and for this reason the matter is mentioned in the present Chapter instead of in Chapter III. It was once the custom to splay out the sides of a channel, downstream of a regulator or weir, so as to form a sort of pool in which the eddies exhausted themselves, but this gives curved banks and requires extra land and is not a very convenient or neat arrangement. Where scour of the sides is likely to occur, or has occurred, immediately downstream of the pitching the latter may be turned in as shown in Fig. 23.

Pitching has constantly to be replaced or extended owing, generally, to failure to pitch a sufficient length or to ram well the earth under the pitching, or to use properly rammed ballast or flat brick, or to give proper bed protection, or to the use of dry brick pitching when a stronger kind is needed.

The side slopes of pitching should be 1 to 1. They can be ¹⁄₂ to 1 in rare cases, _e.g._, when there is no room for 1 to 1, or in continuation of existing ¹⁄₂ to 1 pitching. No absolute rule can be laid down as to the length to be pitched, but in a Punjab distributary it is often about 5 times the bed width.

14. =Miscellaneous Items.= On Indian canals the chainage[24] is marked at every thousand feet. Five thousand feet is called a “canal mile.” The distance marks are often cast iron slabs, fixed in a cylindrical block of brickwork about 2·1 feet in diameter and 1·5 feet high, the upper edge being rounded to a radius of ·4 feet. The wedge-shaped bricks for these blocks are specially moulded. The iron slab should project about eight inches and have about a foot embedded in the brickwork.

[24] In India, instead of the simple word “chainage” the term “reduced
distance” is used. It is the distance reduced to a common starting
point as levels are reduced to mean sea level. The expression is
puzzling to non-professionals and new comers.

On a canal having a wide bank the distance mark is put at the outer edge of the patrol bank, earth being added, if necessary, to increase the width. On a distributary with a narrow bank the mark should be on the opposite bank not the patrol bank. To enable the miles to be easily distinguished the masonry block can be sunk only ·5 foot in the ground, the others being sunk a foot. In all cases the masonry block rests on a pillar, 1·7 feet square, of bricks laid in mud, carried down to the ground level.