An existing canal or distributary may need remodelling for various reasons, and in various degrees. If the velocity is too high and the bed has scoured, or the sides have fallen in, it may be necessary to raise the crests of falls, or to construct intermediate weirs, or to widen the channel and reduce the depth. If the command is not good it may be necessary to regrade the channel. If silt deposit occurs, the cross-section of the channel may have to be altered, to give a better relation between D and V. If there is surplus water, extensions or enlargements of channels may be desirable and these can sometimes be undertaken to a moderate extent merely by restricting a somewhat too lavish supply to existing distributaries. If the water level is dangerously high it may have to be lowered, or the banks raised and strengthened. Sometimes it is desirable to cut off bends either to shorten the channel and gain command or because the bends are sharp and cause falling in of the banks or, if numerous, silting. In all cases the general principles are the same as for entirely new projects, but certain details require consideration.
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.
Regarding the percentage of area to be irrigated, it has already been stated that one canal or distributary irrigates a far higher percentage than another. Generally when there is a high percentage in any tract, it is undesirable to cut it down unless it has very recently sprung up to the detriment of other tracts. In some remodelling projects a uniform percentage is taken on the whole area including both new and old irrigation. This plan is suitable when the percentage of old irrigation is not very high. In other cases the old irrigation to be provided for may be taken as the maximum area actually irrigated, a little being perhaps added for extensions. If the irrigation of considerable areas of jungle tracts is contemplated and if these consist of numerous small patches, a further percentage can be added for them. If there are large jungle tracts they can of course be dealt with separately and any suitable percentage adopted for them. The percentage for each portion of a remodelling project is not necessarily the same.
If the discharge of a channel is increased, the waterways of bridges may need increasing. This can often be done ([Chapter II., Art. 12]) by making a floor at a low level. Or the waterway may be allowed to remain small, the floor being added at the bed level and the bridge then becoming an incomplete fall, ([page 87]). The fall in the water surface, though small, can be recognised and shown on the longitudinal section.
In remodelling schemes, the longitudinal section should give all possible information. It should show not only the levels of bed and banks, but the F.S. levels (in blue figures) above and below all falls or regulators, and the levels of floors and waterways of bridges. The plan should show all watercourses and the “chaks” or areas assigned to them.[34] On each chak the actual average irrigation can be shown in blue figures and the proposed irrigation in red. The “draw-off” for each proposed outlet can then be shown on the longitudinal section. The area actually irrigated, as shown on the map, should in each case be the mean of at least three years, and if possible of five years. The number of years should be mentioned in a note on the map. Cross-sections of channels should always be drawn to natural scale, and not with the horizontal scale differing from the vertical.
[34] The field maps mentioned on [page 101] are prepared to a very large scale and show all watercourses. The maps should always be corrected up to date by the patwaris. The chak maps which are on a smaller scale—say 4 inches to the mile—can thus be kept correct.