In the general arrangement of a headworks a great deal depends on local conditions. Sometimes the river runs in a fairly straight and defined channel and the weir can then be run straight across it. Sometimes, as in the case of the Ganges Canal, there is a succession of islands and various short weirs are required in the different channels. At the heads of the Eastern and Western Jumna Canals, the river, on issuing from the hills, widens out (Fig. 5.) and the weir is built obliquely and not in a straight line. Its crest is higher at the east than at the west side. There are under-sluices at both sides. The upstream end and west side of the island are revetted. The old head of the Western Jumna Canal, as shown in the figure, existed long before the advent of the British, and a temporary weir, made of gabions filled with stones, was constructed across the river every year during the low water period and swept away during the floods. To have carried the weir along the line shown dotted, the head of the Western Jumna Canal being of course brought up to it, would apparently have been feasible and cheaper, but the off-take would have been in shallow water because of the curve in the river, and there would have been no current along the face of the head regulator of the canal. The level of the floor of the under-sluices is generally about the same as that of the bed of the canal. The sill--made to exclude shingle and sand as far as possible--of the canal head regulator may be 3 feet higher and the crest of the weir 6 to 9 feet higher. The top of the weir shutters is 1 to 2·5 feet above the F.S. level of the canal which may be 5 feet or more above the bed of the canal. If the weir is provided with falling shutters the width of the waterway of the under-sluices may be about ¹⁄₁₂th of the width of the waterway of the weir alone, otherwise about ¹⁄₈th. In nearly all cases the weir has a flat top and flat slopes both upstream and downstream. In a case where the river bed is of sand, the depth of water on the crest of the weir in floods may be 15 feet and the velocity 14 or 15 feet per second. The downstream slope of the weir may be about 1 in 15, and the upstream slope 1 in 6. Where the river bed is of boulders the velocity may be still higher. The faces of the weir are usually of hammer-dressed stone. A lock for the passage of rafts is added if necessary.