Irrigation works : $b the principles on which their design and working should be based, with special details relating to Indian canals and some proposed improvementsBellasis, E. S. (Edward Skelton)
Science
Irrigation works : $b the principles on which their design and working should be based, with special details relating to Indian canals and some proposed improvements
Bellasis, E. S. (Edward Skelton)
Canals; Irrigation
It has been stated (CHAPTER I, Art. 2) that it is not desirable to let
one channel tail into another. In old canals a distributary used
sometimes, after running parallel to a canal, to be brought back towards
it and tail into it. The advantage of this was that the distributary had
not to be made very small towards the tail and that, if the demand
abruptly ceased, the distributary was not likely to breach. The
principle was, however, essentially bad. The lower part of the
distributary was obviously too near the canal and not centrally
situated as regards the irrigated strip. The portion at the extreme tail
was superfluous. Again, whatever volume of water was carried through the
distributary and back into the canal, was needlessly detached instead of
being kept in bulk. Moreover the duty of water on such a distributary
cannot be ascertained without a tail gauge and the observation of
discharges at the tail. There are similar objections to one distributary
tailing into another. Each should be separate and distinct.
A major distributary is one whose discharge is more than 40 c. ft. per
second. It may be as much as 250 c. ft. per second. A branch, as soon as
it reaches a point where its discharge becomes only 250 c. ft. per
second should be considered as a major distributary. A minor
distributary is one whose discharge is from 8 to 40 c. ft. per second. A
minor distributary is nearly always a branch of a major distributary.
There are instances of “direct minors,” i.e., minors taking off from
canals or branches. Such a minor, unless its discharge is a large
fraction of that of the canal which supplies it--and this can seldom be
the case--is objectionable because the petty native official who has to
see to the regulation of supplies can manipulate the supply easily and
without detection, and the number of persons irrigating from it being
small, he can make private arrangements with them. On the Sidhnai Canal
there are some half-dozen distributaries each of which had one or two
minors which took off close to the head of the distributary. The people
who irrigated from the minors managed to get the heads shifted and taken
off direct from the canal, on the ground that, the water level in the
canal being higher than in the distributary, there would be better
command and less silt deposit. The irrigation on all these minors ran up
to a figure far in excess of what had been intended, to the detriment of
lands further down the canal. The minor heads have all been
retransferred to the distributaries, the difficulty as to command being
got over, as it should have been at first, by constructing weirs in the
distributaries. The fall in the water surface at the distributary head,
i.e., the difference between the water level in the canal and that in
the distributary downstream of its head but upstream of the weir, is
quite trifling or even inappreciable.
Public-domain text, read in full here on John Shaqi.
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