The distributaries of the older canals were constructed before Kennedy’s laws regarding silting were known, and it has been necessary to remodel many of them. In some cases the gradient was wrong, in others the cross-section.[33] In some cases a distributary ran in rather low ground, and it was proposed to abandon it and construct a new one on high ground. It was however pointed out by Kennedy (_Punjab Irrigation Paper No. 10_, “Remodelling of Distributaries on old Canals,”) that irrigation had become established along the course of the distributary, that most of it would remain there and that a new alignment would result in increased length of watercourses. Such distributaries have therefore been allowed to remain very much as they were.
[33] The difficulty of reducing the size of a channel which is too large is well known and has been discussed in _River and Canal Engineering_, Chapter VIII. It is there explained that a moderate reduction of width can be effected by “bushing,” but that for great reductions, groynes or training walls are necessary. When the bed of a distributary is too low it has been suggested that it could be raised by filling in earth in each alternate length of 500 feet, and leaving the rest to silt, but this would be expensive.
Remodelling should not be considered piecemeal, but regard should be had to the whole channel. When a distributary is remodelled the outlets should of course be dealt with as well as the channel. The chief thing to consider is not whether the channel as it exists is exactly as it was originally designed to be, but how it is doing its work and what kind of alteration it needs. Even when a simple silt clearance or berm cutting of a channel has to be undertaken, the work need not always consist in blindly restoring the channel to its original condition. It may be both feasible and desirable to remodel it to a slight extent, lowering the water for instance in reaches where the outlets draw off very good supplies and thus benefiting less fortunate reaches lower down.
The irrigation boundaries of the extended or remodelled channel should as far as possible follow drainages, but these are not always important or pronounced. The actual irrigation boundaries should be shown and also those of any neighbouring channels of other canals, and any suitable adjustments should be made.