The preceding rules and principles being taken into consideration, the channels are designed. The bed levels, gradients and depths are so arranged as to give the velocities suited to the soil and to maintain the proper relation of depth to velocity. The bed width is arranged so as to give the proper discharge. The full supply level of the canal and branches has also to be so arranged that it shall be higher, at each distributary off-take, than the full supply level of the distributary. It is desirable to be able to give a distributary its full supply even when the canal is low. Generally the slope of the country along any line is greater than would be suitable for the bed, and “falls” are introduced. The off-take of a distributary is generally just above a fall and there is generally an ample margin between its F.S. level and that of the canal. The discharge of the canal during the greater part of the rabi may be only about half the full supply. This discharge should be estimated and the water level corresponding to it calculated and shown on the longitudinal section. If possible the levels should be so arranged that even with its least supply the water level in the canal will enable full supply to be given to a distributary. If this cannot otherwise be managed it may be necessary to construct a regulator in the canal below the head of the distributary so that, during low supplies, the water can be headed up. It has been stated in _River and Canal Engineering_, Chapter IV., that such heading up, if temporary, is not at all likely to cause silt deposit in the canal. The designing of the distributaries is not proceeded with at this stage. Since no irrigation is usually done directly from the canal and branches, they are designed without any particular connection between the level of the water and that of the country traversed. Dangerously high embankments are of course avoided as far as possible. The bed is designed at such a level that the excavation and embankment at any place will be, as nearly as possible, equal. Land in India is cheap. When the excavation exceeds the embankment the balance is made into a spoil bank. When the excavation is less than the embankment the balance is got from borrow-pits.