The Company's obligations, as far as drainage is concerned, were limited
to the removal and disposal of sewage, no provision being required for
storm-water, which is allowed to find its way to the natural
watercourses. Apart from that fact, however, the best system for a city
like Monterrey, where rainfall for many months at a time is very scarce,
is the strictly "separate system." In the design advantage was taken of
the natural topography of the drainage district, which is almost an
ideal one for a gravitation system of sewers, the general fall in all
directions being northeast; it was also in this direction that the best
available land could be obtained for disposal purposes.
[Illustration: PLATE XX.--DIAGRAM OF THE MAIN SEWERS OF MONTERREY.]
Plate XX shows in skeleton form the general lay-out of the sewers. Two
drainage districts are arranged, divided by Calle de Washington, which may
be regarded as practically the center of the city, and each of these
districts has an independent main collector connecting to the outfall
sewer at the northeast extremity of the city.
The system has been designed so that extensions may be made and may cover
any part within the city limits; the main collectors are large enough for
the whole area when fully built up.
The sewers are designed on a very liberal basis, namely, on the assumption
that when flowing half full the quantity to be dealt with will be 380
liters per capita per day, with a maximum rate of flow of 200 per cent. It
was assumed that each house would be occupied by 7 persons and have a
frontage of 12-1/2 m. The minimum velocities in the sewers, when running
full, vary between 0.91 and 1.5 m. per sec., with the exception of a few
blocks.
The minimum size adopted was 24.3 cm. (8 in.) in internal diameter. The
sewers of diameters between 24.3 and 50 cm., are 0.91 m. (36 in.) long,
and are of salt-glazed vitrified clay, imported from San Antonio, Tex.
Table 12 gives the details of the length of the various sewers laid.