Chapter XVII: Appendix: F

ESCAPES.

(See page 9.)

There are no definite rules regarding the capacity of the escapes to be provided on a canal. On some canals in dry tracts of country the discharging power of the escapes is a mere fraction of that of the canal. In other cases it is about half that of the canal. In a district liable to heavy rain an escape, say at a point where a canal divides into branches, should be able to discharge about half of the main canal supply. On branches, escapes, if provided at all, usually discharge into reservoirs, and their period of working is very limited: it may be only twenty-four hours.

On distributaries, escapes are seldom provided. It has been suggested, in connection with modules, that the people irrigating from each watercourse should be responsible for disposing, by means of it, of a certain quantity of surplus water. This would be too rigid a rule. On some watercourses there is much waste land or land under rice cultivation; in such cases surplus water can be passed off without damage. The canal subordinates are fully cognisant of such cases, and they arrange accordingly. In other cases surplus water would do some damage; but on nearly every distributary the full supply, even when there is no demand for water, can be got rid of for a few hours, or even more, without a breach occurring.

Escapes at outlets, in connection with modules, can be arranged by means of waste weirs or by means of Gregotti’s syphons (_sifoni autolivelatori_). The following is an abridged translation of part of a pamphlet by Gregotti:--

The figure represents one of the syphons installed in the “Centrali
Milani.”

A is the supply basin of the “Centrali,” which ends in the syphon B.
The latter is constructed with mouthpiece of rectangular section _a_,
which is submerged in the basin A. A weir divides the mouthpiece of
the syphon from the descending branch, _c_, of the same, also
rectangular in section. The weir crest is at level _dd_, from 2 to 7
cm. below the maximum level of water surface which it is desired not
to exceed in the supply basin.

The descending branch, _c_, has at its base a small tank _e_, which
forms a water seal. The syphon is completed by a tube _f_, which is
attached to the intake branch of the syphon and which ends at a level
of 2 to 7 cm. above the previously mentioned surface _dd_.

As soon as the water surface in the supply basin tends to rise above
the plane _dd_, a filament of water, in falling over the weir _b_,
pours down the descending branch _c_, and when the water has risen
from 2 to 7 cm. above the crest of the weir, the thickness of the
falling stream has become such that it is able, by lapping, with a
wave-like course, the wall _gg_, to extract the air that has become
enclosed in the syphon, and which cannot be replaced because the space
in which the stream acts is closed at its base by the water in the
tank _e_; and at the top also the aeration tube is closed by the rise
in the water surface of the supply basin. From this point the syphon
action quickly becomes fully established and begins to give its full
discharge.

The discharge that is given is equal to that of an orifice in a thin
partition if certain limitations are allowed for between the fall used
in the syphon and the height of the arch, that is, the distance from
the crest of the weir to the inside roof of the syphon.

The discharge is given by the formula

Q = μA√(2_g h_).

Q = discharge of syphon in cubic metres per sec.

μ = a coefficient of reduction of discharge which varies between wide
limits.

A = the minimum cross-sectional area of the syphon in square metres.

_g_ = value of acceleration due to gravity.

_h_ = the fall, or the difference of level in metres between the water
surfaces in the supply basin A and in the small tank _e_.

As soon as the supply basin surface falls, the opening of the aeration
tube becomes uncovered and air is drawn into the syphon. But until the
surface has fallen some centimetres the supply of air is not
sufficient to cause the syphon action to stop completely, and thus the
escape varies gradually from the maximum discharge to zero as the
water surface falls a few centimetres till it reaches its original
level.

In certain cases it is possible to do without the aeration tube,
especially when the fall used in the syphon is not great and when it
is possible to arrange matters so that the velocity of the water
flowing past in front of the syphon is small.

The syphon with a width of 3 metres escapes 8 cubic metres per sec. of
water.