Fig. 9.
The scale is 6 feet to an inch. The depth of water, in this particular case, is 7 feet, and the bank, excluding the small raised bank, 2 feet above the water. The inside edge of the bank, where the small raised bank is shown, is kept parallel to the canal for a considerable distance. Its position is got by drawing a line, shown dotted, at, generally, 1¹⁄₂ to 1. The embanked part of the slope is actually made at 1¹⁄₂ to 1, but the excavation is at 1 to 1, so that a berm is left. The width of this berm of course varies as the depth of digging varies. If there is likely to be much falling in of the sides the berm can be made wider, the dotted line starting, not from the edge of the bed, but from a point further in. On an inundation canal in sandy soil the berm may be 20 feet wide. In figure 8, the inside slope above the berm is supposed to have silted up to a slope of 1 to 1. In cases where it is expected the whole inside slope will silt to ¹⁄₂ to 1 the dotted line, to give the edge of the bank, can be shifted towards the channel so that the berm at the ground level when the channel is excavated will be very small for the minimum depth of digging. There is no need for the inner edge of the bank to run parallel to the canal for great distances. Its position can be shifted whenever suitable and the width of the berm at ground level varied. This prevents the occupation of a needlessly great width of land. It used at one time to be not unusual to make a bank with a berm on the land side, similar to that formed by the spoil in [Fig. 8], but at about the level of full supply in the canal. The principle is not a good one. Salient angles are liable to be worn away. If earth has to be added to a bank to strengthen it, the whole can be widened or the rear slope flattened. The roadway is shown 18 feet wide, which is nearly the maximum. For the drainage of rain water it has a transverse slope, away from the canal, of about 1 in 50. The small raised bank on the canal side is to give safety to wheeled vehicles. It is provided on the patrol bank[13] on main lines and places where there is much traffic or where there is plenty of width of bank to spare. When the ground level is, for a considerable distance, above the proper bank level—which is at a fixed height above the F.S. Level—so that the road and its side-drain have to be cut out, much earthwork can be saved by allowing them to be at a higher level and, in the case at least of the non-patrol road, giving the road a reduced width.
[13] A canal has an unmetalled driving road—called the “patrol road” or “inspection road” on one bank. This road is reserved for the use of officials. Otherwise, it would soon be cut up and worn away, and the cost of repairs would be excessive. The patrol road should be on that bank which is, in the morning (the time when inspections are usually made) in the shade of trees planted on the landward side. Trees are not usually planted near the water edge as they are sometimes blown down. In Northern India the canals generally flow in a southerly direction, so that the left bank is best for the patrol bank. On the other bank there is a bridle road which is open to the public. Near a rest house—unless there is a bridge actually at the place—the patrol road should be on the same bank as the rest house. It can if necessary cross at the first bridge. Frequently there is also on one or both sides of the canal a “boundary road,” which is open to the public, along the toe of the outer slope. Along a distributary there may be a boundary road on one side. It is generally the only road which can take wheeled traffic, and in this case it should be reserved for officials unless money is provided to keep it always in repair. Officials have to be on tour for weeks or months at a time, and in all weathers. Their baggage carts also have to precede and follow them. Anything which facilitates their touring about and seeing things for themselves is, in India, most desirable. At a watercourse crossing the boundary road along a distributary should be taken by a curved incline up on to the bank and down again. Thus not only is the cost of a culvert saved, but any touring official who is driving obtains a view of the channel which he cannot get from the boundary road.
Fig. 10.
In shallow digging, the plan of setting back the banks ([Fig. 10]) and letting silt deposit as shown by the dotted lines, is one which should be followed much oftener than it is. It not only gives eventually a very strong bank, but it enables the borrow pits, from which the earth for the banks is got, to be dug inside the banks. Outside borrow pits, besides being a source of expense, owing to compensation having to be paid to those in whose land they are dug, cause great areas of hollows which are not only unsightly, but are often full of stagnant water and are thus a fruitful source of mosquitoes and malaria. Insufficient attention has hitherto been paid to this matter.
In designing each reach of a canal or branch, type cross sections should be drawn out for several different depths of digging, e.g., one for very shallow digging, i.e., where the bed is little, if at all, above the ground level, one for deep digging where the ground is higher than the water level, and one for the “balancing depth,” where the area of the channel excavation is equal to the earth required for the banks. In calculating the earthwork the sectional area of the digging or of the embankment is taken, whichever is the greater.
The proper width and height of bank for any channel depends partly on the maximum depth of water in the channel, and partly on the discharge. Given a depth of water of say 8 feet, a breach will obviously be more disastrous with a great volume of water than with a small volume. The following statement gives some figures suitable to the rather light and friable soils of Northern India, but the question is largely one of judgment. Generally a low and rather wide bank is preferable to a higher and narrower one. If a road, with or without the small raised bank next the canal, is required, special widths can, of course, be arranged for. A 14-foot bank is required for a driving road.
| Top Width of Bank. | Height of Bank above F. S. | Greatest Admissible Discharge. | Greatest Admissible 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.