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.
