The free-board or height of the masonry walls and tops of embankments above H.F. Level is about 5 feet.
The span of each opening in the under-sluices is generally 20 to 35 feet. The piers may be 5 feet thick. It is usual to make each alternate pier project upstream further than the others so that long logs coming down the river during floods, broadside on, may be swung round and not be caught and held against the piers.
Fig. 6.
Fig. 6A.
[Figs. 6] and [6A] show the headworks of the Upper Chenab Canal now under construction ([Chapter IV.]) The site is in a low flat plain, but no better site could be found. The weir consists of 8 bays of 500 feet each. The crest is 10 feet above the river bed and the falling shutters 6 feet high. The slopes are 1 in 6 and 1 in 15. The bulk of the work is rubble masonry in lime. The lower layer upstream of the crest is of puddle; upstream of the second line of wells it is rubble masonry in half sand and half lime; upstream of the lower line of wells it is of dry stone and there is an intermediate layer of rubble masonry in lime with the stones laid flat. Below the crest there is a wall of masonry 9 feet thick and on the crest there are two strips of ashlar between which the shutters lie when down. The extreme upstream and downstream portions of the bed protection are of dry stone and 4 feet thick while next to the weir are concrete blocks 2 feet thick resting on dry stone. The width of the crest is 14 feet, of the weir 140 feet, of the protection 70 feet upstream and 110 feet downstream. The guide banks have tops 40 feet wide and 18 and 14 feet above the crest of the weir in the upstream and downstream lengths respectively, the side slopes being 2 to 1 and the water slope being covered, up to H.W. level, by dry stone pitching 4 feet thick. The left guide bank runs upstream for 3,250 feet from the centre line of the canal and the right 2000 feet from the line of crest shutters. The under-sluices have 8 bays of 35 feet each and the canal head regulator 36 openings of 6·5 feet each, the large openings shown in the figure being sub-divided. The crest of the weir is no less than 10 feet above the river bed and the shutters add 6 feet to this. The floor of the under-sluices is 4 feet higher than the river bed. There is thus ample allowance for a possible rise in the river bed.
2. The Contour Map.
—The contour map, besides showing the contours of the country to be irrigated and of a strip of country, even if not to be irrigated, which will be traversed by the main line, should show all its main features, namely:—streams, drainages, railways, roads, embankments, reservoirs, towns, villages, habitations, and the boundaries of woods and cultivated lands. It should also show the highest water levels in all streams or existing canals. A map showing as many as possible of the above features should be obtained and lines of levels run for the contours. In doing this, the points where the lines of levels cut or pass near to any of the above features or boundary lines, should be noted. It may be necessary to correct inaccuracies in the plan or to supply defects in it. The greater the trouble taken to do this the less will be the trouble experienced later on.
The heights of the contour lines will, in very flat country, have eventually to be only 1 foot apart. This will necessitate running lines of levels half-a-mile apart at the most, and preferably 2000 feet apart, the pegs in each line being about 500 feet apart. In less flat country the heights of the contour lines can be further apart than 1 foot. Whatever distance apart is decided on for them, the survey should be done once for all. On one of the Indian canals in flat country, the lines of levels were at first taken 5 miles apart, the branches roughly aligned and then further surveys made. This led to great expense and delay and the procedure has not been repeated.