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.
Fig. 28.
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.