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.