Читать книгу ASCE 1193: The Water-Works and Sewerage of Monterrey, N. L., Mexico - G. R. G. Conway - Страница 8

Table of Contents

The question of the best sources from which Monterrey should be supplied with potable water was one that had been long under discussion, and was the subject of many investigations prior to the granting of the present concession. Several of the original schemes called for an impounding reservoir in the Cañon of Santa Catarina and it was on the assumption that a dam would be built that a clause was inserted in the concession for the purpose of making its construction obligatory. The general character of the physical and geological conditions surrounding Monterrey has already been referred to. A thorough study of these conditions proved that no suitable site for impounding the Santa Catarina River could be found, apart from the fact that periodically this river is subject to enormous floods which tear through the steep cañon with tremendous velocity.

At the site originally proposed for the dam, a considerable underflow was found, and later investigations, carried out under the present concession, proved that, although borings were carried to a depth of 54 m., bed-rock could not be found, the strata being composed of gravels, conglomerate and sand. Assuming that such a dam could have been built, the quality of the water draining from a comparatively barren water-shed, on which many thousands of goats are pastured, would have made its filtration an absolute necessity before it could be delivered to the consumers.

The various available sources from which water could be delivered to the city by gravity were investigated by Mr. F. S. Hyde, in the autumn of 1905, and also by J. D. Schuyler, M. Am. Soc. C. E., who was afterward retained as Consulting Engineer for the Company. The various investigations made from time to time showed that the question of a satisfactory supply was one of extreme difficulty, requiring prolonged observation and study, more particularly into the character of the underground sources of supply.

One of the chief characteristics of many of the streams in the State of Nuevo León, is their disappearance and reappearance at different points along their routes, and the Santa Catarina River, under normal conditions, as already remarked, is a very notable example of a river which is very dry at the surface for many kilometers of its length. In the writer's opinion, the waters of this and similar rivers in the State pass through many open caverns underground, so that experience gained in the investigation of underflow waters in other places would be insufficient to determine the quantity passing at any point along the river if ascertained by merely computing it from the velocity of the underflow and the area of the water-bearing gravels. The rainfall on the water-shed of the Santa Catarina River is probably 25% greater than at Monterrey, and all ordinary rains sink rapidly into the limestone soils and quickly disappear. In another water-shed of a very similar character, namely, that of the Rio Blanco, in the southern part of the State, the underflow waters appear at the surface at a place called Mezquital, where a metamorphosed sandstone barrier prevents them from disappearing underground. At this point the normal quantity of water is about 5,660 liters (200 cu. ft.) per sec., but it gradually disappears, and a few kilometers below it has sunk to an insignificant stream, finally disappearing altogether for about 20 km. In the neighborhood of Monterrey similar conditions exist with regard to the surface-water supplies, and investigations, therefore, were directed toward obtaining unpolluted supplies from springs and underground sources.

Santa Catarina Sources.—The chief points from which it was thought desirable to obtain underflow supplies were (1) at the barrier of San Geronimo, and (2) at the Cañon of Santa Catarina, both shown on Plate II.

Conditions at San Geronimo, which is only 6¹⁄₂ km. west of Monterrey, were investigated by the State Government in 1892, to determine the depth of bed-rock, the rock on either side of the valley being shale, with its original bedding planes standing almost vertical. To determine this depth, borings were made by driving 2-in. tubes until it was assumed that bed-rock had been reached, a method which, in strata containing so many boulders, was obviously unreliable. These borings indicated that bed-rock was from 12 to 15 m. below the surface. If these had proved to be correct, there is no doubt that a development of the underground water at this point, by constructing a submerged dam combined with an infiltration gallery, would have yielded a large supply.

In March, 1906, the Company commenced operations at San Geronimo by sinking a well a few meters north of the then dry bed of the river. Water was found in considerable quantities a few meters below the surface, practically at the level of the river, that is, 570 m. above datum. This supply was used for provisional purposes, and will be referred to later in describing the San Geronimo gravity supply works.

Between August, 1906, and January, 1907, 4-in. bore-holes were sunk in the river bed and on the high ground to the north with a "Keystone" driller outfit. These borings showed bed-rock immediately under the river bed, at a depth of from 15 to 45 m., but dipping gradually as the borings were carried northward.

Boring operations were also carried on at Santa Catarina, during November and December, 1906, and in January, 1907, to determine the geological conditions, and the results are shown on Fig. 7. From the area of water-bearing gravels found, it was proposed to tap the underflow water at the 630-m. level by an infiltration gallery. This would have necessitated a gravitation tunnel 3,000 m. long, and an aqueduct of 14 km., which it was proposed to carry to one of two distributing reservoirs at Guadalupe, on the south side of the river, opposite Monterrey. In May, 1907, the writer, after making a study of all the available data which had been accumulated, had additional borings sunk farther across the valley to the north, and these revealed a considerable area of water-bearing gravels, and proved that, in former geological times, the Santa Catarina flowed about 500 m. north of its present position, and to the back of Obispado Mountain, instead of through the city. This aspect of the subject was discussed with Mr. Schuyler, who agreed with the writer that, in the interest of economy, it was better to tap this supply by an infiltration gallery at the 560-m. level, and bring the water thus obtained to a reservoir to be placed at the western limits of the city, dividing the city, for distribution purposes, into two interchangeable systems, a high- and a low-pressure, the high-pressure system being supplied from Estanzuela, 18 km. south of the city. One advantage to be gained from this change was that the scheme was capable of considerable extension, and any future developments at Santa Catarina Cañon would form part of the works to be constructed for both high- and low-pressure districts.

Fig. 7.—Cross-section Of Santa Catarina River At Santa Catarina.

The future extension of the Santa Catarina sources, the writer believes, can be developed best by driving an infiltration gallery 10 m. below the surface of the Santa Catarina River, a little west of the village of the same name, and then conveying the water through a comparatively short gravitation tunnel and pressure conduit to a main reservoir near San Geronimo having a top water level at an elevation of about 590 m. above datum.

Southern Sources of Supply.—The available sources of supply southward from Monterrey include a number of springs at various points in a distance of 40 km. Many of these springs are of uncertain quantity, and some are quite dry during periods of drought. The chief perennial springs near Monterrey are those which contribute to form the Estanzuela and Diente Rivers, both tributaries of the Silla, while farther south, at the Potrero Cerna, near El Porvenir, there are excellent springs, at a considerable elevation, with a minimum flow of from 170 to 200 liters (from 6 to 7 cu. ft.) per sec. The total quantity of water available from all these springs during the driest season would probably not be less than about 560 or 700 liters (from 20 to 25 cu. ft.) per sec.

The Estanzuela springs issue at the foot of the Sierra Madre Mountains, and have a normal flow of from 56 to 85 liters (2 to 3 cu. ft.) per sec. in an ordinary dry year; they probably derive their water, through the limestone formation, from the neighboring water-shed of Santa Catarina, as the catchment area of the stream is only 910 hectares, and the stream has never been known to fail, even in the driest periods of prolonged drought. The rainfall on the area is about 30 in. per annum, and the catchment area is well wooded and covered with abundant vegetation. The El Diente springs have an ordinary dry-weather flow of about 28¹⁄₂ liters (1 cu. ft.) per sec.; but part of the water is carried underground, and the real quantity is much greater and could be developed by a small submerged dam carried down to bed-rock.

The elevation and the extreme purity of the water of the Estanzuela River made its acquisition very desirable, and the Company, therefore, purchased the Federal water rights owned by various members of the Estanzuela community, amounting to 91 liters per sec., and has since acquired a Federal concession to all the flood-waters of that river. It was decided, therefore, to adopt the Estanzuela River as the first step toward developing the water to the south of Monterrey for a high-pressure supply, the advantage of the scheme being that from time to time extensions could be made to tap other sources by gravity, as the demands of the city required. The Estanzuela scheme, therefore, is a preliminary step toward future extensions which will be necessary in this direction as the city grows. The springs near El Porvenir, and others which contribute to the San Juan River, can be tapped at a sufficiently high level to convey them by a gravity pressure line to the Estanzuela Aqueduct near Mederos.

The two sources definitely decided on in July, 1907, were those from Estanzuela and San Geronimo. The works were designed to supply 40,000,000 liters daily, which it was assumed would be sufficient for all future developments for a population of 200,000 at a per capita consumption of 200 liters per day. The present requirements of the city's population, assuming that all the water was supplied by the Company, would be, at that rate, which is a very liberal one, only 18,000,000 liters daily. This, it was thought, would be easily met by the San Geronimo source alone, as it was estimated that it would provide not less than 20,000,000 liters, if the infiltration gallery was driven far enough into the water-bearing gravels.

The question of a high-pressure water supply for domestic use in a city like Monterrey is not a serious one, as practically nine-tenths of the houses are of one story. The increase in the number of large commercial buildings, however, will make the demand greater in the future, and this point has been kept in mind in arranging the division of the distribution systems.