Unless the banks of the river are high, it is necessary to construct embankments to prevent the river water, when headed up by the weir during the floods, from spilling over the country with possible damage to the canal. If the river has side channels they have to be closed. The stream may also have to be trained, by means of guide banks or spurs, so as to remain in one channel and flow past the canal head and not form shoals against it. Where the river is unstable, it may shift its course so as to strike the weir obliquely and this may cause excessive heading up at one side of the weir. In such cases it is usual to divide the weir into bays or sections, each about 500 feet long, by “divide walls” running at right angles to the weir. 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. [Illustration: FIG. 6.] [Illustration: 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