Occasionally there is a wide berm, say 20 ft. or even 50 ft., between a channel and its bank. In such a case the outlet should be built to suit the bank. The long open cut is however objectionable because the people clear it and heap the spoil in Government land. Sometimes the bank, especially if it is crooked, can be shifted so as to come close to the channel at the outlet site. Sometimes the outlets on inundation canals are large. For outlets of more than 2·5 square feet in area, grooves should be provided so that the cultivators can use a gate if necessary.
12. =Masonry Works.=--The positions and descriptions of all the masonry works of a proposed canal or distributary are of course shown on the longitudinal section of the channel and from this the discharges and water levels are obtained. The principles of design to be followed[19] for bridges, weirs, falls, regulators and syphons, are discussed in _River and Canal Engineering_. It is mentioned that there is no special reason for making the waterway of a regulator exactly the same as that of the stream, and that the waterway may be such as to give the maximum velocity considered desirable, and that the foundations of a bridge should be made so deep that it will be possible to add a floor, at a lower level than the bed of the stream--with the upstream and downstream pitching sloping up to the bed--so as to increase the waterway and so save pulling down the bridge in case the discharge of the channel is increased. It remains to consider certain points affecting Irrigation Canals.
[19] So far as concerns their capacity for dealing with flowing water.
The span of a bridge, where there are no piers, is generally made as shown by the dotted lines in Figure 14, so that the mean width of waterway is the same as that of the channel. The arches, in Northern India, used at one time to be 60° as shown by the upper curved line, but in recent years arches of 90° as shown by the lower curved line, have frequently been adopted, the springing of the arch being below the F.S. level, so that the stream is somewhat contracted. The 90° arch gives a reduced thickness and height of abutment. It causes increased disturbance of the water, and this may necessitate more downstream protection. An advantage of having the springing not lower than the F.S. level is that this admits of a raising of the F.S. level in case the channel is remodelled, and this arrangement is still common on distributaries.