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 bank | 14. | Bed level |
| 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.
