Part 10
The diagram, Fig. 21, is a record of the rainfall during the latter flood, and was plotted from intermittent readings of standard gauges. It demonstrates that the intensity increased toward the mountains on the south, which form the tributary water-shed of the Santa Catarina River, showing a difference of 10.54 in. between the city and the Estanzuela Dam, which is not quite 12 miles to the southeast.
An estimate of the volume of discharge of the river at the time of maximum flood is only a reasonable conjecture which (without special reference to accuracy) aims to impress those who have not witnessed such occurrences with the tremendous volume coming from barren steep surfaces previously saturated.
The original computation, referred to by the author, was obtained from the average of two different methods which gave results close to each other. In one method the extent and nature of the water-shed were considered, together with the maximum period of precipitation that occurred, sufficient to gather a maximum volume of water in the river. In the other method the volume was derived from a cross-section of the wetted perimeter of the river at the time of maximum flow, in combination with velocity approximations obtained by using rough floats. This gave 271,500 cu. ft. per sec. The figure submitted by the author, 235,000 cu, ft. per sec., is in accord with the proposed formula[9] for impervious surfaces by C. E. Gregory, M. Am. Soc. C. E. In the first and last methods, the intensity, a governing factor, is more or less of an assumption, and the cross-sectional method is also unreliable, as the river-bed was greatly disturbed, due to the high velocity of the water, which deepens the channel to a considerable extent at times of maximum flood, the gravels being redeposited during the period of subsidence. Such was the case during the flood of September, 1910, when the depth of gravel above the roof of the San Geronimo Infiltration Gallery was diminished to such an extent that it was so inefficient as a filter for the flood as to permit the percolation of turbid water into the underground supply.
[9] _Transactions_. Am. Soc. C. E., Vol. LVIII. p. 458.
During the floods of August, 1909, Shafts Nos. 2 and 3 were damaged beyond repair, and sand and gravel, entering through them, blocked up the gallery to within about 150 ft. of Shaft No. 1. The interior timbering probably collapsed, due to cavings and disturbance in the river-bed during the period of maximum flood, but no explorations have been possible on account of the great quantity of water still coming through (at present more than 650 liters per sec.). For this reason the work of driving the gallery, as well as lining Shaft No. 1, has been suspended.
On reaching the city, the flood of August, 1909, swept away two streets adjoining the river. These streets had been built on made ground, in what was originally the river-bed. The sewers and water mains laid in them were destroyed entirely, and some 460 ft. of the 24-in. cast-iron pipe, buried under the river-bed at a depth of 8 ft., were carried away. In relaying this portion of the main, and for protecting the remainder of it across the river, it is now proposed to encase it in a solid rubble concrete block, 8 ft. square, which will impart weight and stability against the scouring effect of floods.
The South Reservoir is circular in shape, with an interior diameter of 165.68 ft. at the top, and is partly excavated in the ground and partly completed by an embankment of vast proportions (Fig. 10). Right after the flood of August, 1909, a wet spot appeared on the northeastern toe of the embankment, and it was supposed for some time that it was the effect of the saturation produced by the preceding rains, but, as it persisted for several months, it was obvious that its origin was in the interior of the reservoir, which was emptied when the writer took charge of the work. The first inspection revealed a horizontal crack in the concrete lining, about 310 ft. long and extending about 153° around the circumference on the north side. Throughout its length it coincided with the line of cut and fill. Vertical cracks, coinciding with the panel points in the lining, had also developed, and extended from the main horizontal crack to the roof. The circumstances originating this development can be conjectured by considering the position of the main crack, its characteristic features, and the conditions that preceded its formation. The coincidence of the crack with the joint of cut and fill, points to this line as a source of danger. An examination showed, besides, that the fracture was clean and sharp, ranging in thickness from a hair line at the ends to 3/16 in. at the center, and that its upper border projected over the lower one perceptibly, a proof that horizontal motion had taken place. The vertical cracks were a secondary effect, the consequence of the displacement immediately after it was scoured. A fracture was discovered in the floor of the reservoir. It started at the center and branched out into two diverging lines in a radial direction.
The circumstance of two abnormal rainfalls, giving 35 in. in 9 days, the precipitation being concentrated in two periods, not far apart, of 42 hours and 98 hours, respectively (Fig. 4), together with lack of provision for shedding the water from the roof of the reservoir and from the surrounding embankment, lead to the inference that the latter became saturated, increasing thereby in weight and decreasing in stability, especially in its steep inner face. A settlement and the consequent horizontal displacement, under these conditions, was natural. The concrete lining, only 16 in. thick at that height, was not sufficient to sustain the resulting strain, and the main fracture developed, permitting the stored-up water to leak into the bank. In time this seepage found its way under the bottom of the reservoir, softening the ground and producing a slight settlement which caused the crack in the floor. Had under-drainage been provided, as at the Obispado Reservoir, the actual conditions would have been noticed earlier. However, as the embankment is of vast proportions, stable in itself to sustain with a large margin of safety the weight of the stored-up water, there was no actual danger of failure, except for the fact that the material forming the structure, on account of its calcareous nature, is dissolved by water. Long exposure to this condition would, in time, open passages in the embankment, and it is certain that there would be cavings in its interior.
The necessary grouting has been done, and provision is being made for water-proofing the interior of the reservoir and shedding the water from the roof and from the embankment, thus relieving the structure of the consequent strain.
Another place in the works where floods have had a damaging effect is the Estanzuela intake basin, which, when the dam was completed, was filled to the overflow level in order to test its water-tightness. As this basin, when cleaned, was found to be slightly fissured on the north side, it was decided to line it with concrete. As shown in Fig. 8, the lining does not cover its entire area, but only the central portion, leaving a strip on either side without protection. The flood of September, 1910, coming in greater volume than the previous ones of August, 1909, in passing through the narrow gorge at the entrance, undermined the lining in those places where it was not founded on solid rock. Figs. 1, 2, and 3, Plate XXXIII, show some of the damage caused by this flood. The buoyant effect of the water and the impact of large rolling boulders caused fractures all over the surface, and lifted the concrete lining bodily; but the dam proper, being founded on rock bottom, did not suffer any injury. In the future, in order to avoid the seepage of the ordinary supply, alluded to by the author, the water will be carried to the valve-house in an open rubble concrete channel, lined with cement mortar and built high up against the western hillside. The remainder of the basin will be paved with large boulders.
In conclusion, the writer wishes to emphasize the point that, notwithstanding the severity of the test, relatively small damage was inflicted on the extensive works carried out under the author's design and direction. A test so severe that it caused serious damage and immense losses in the entire region, washing away kilometers of railroad track and destroying practically all the bridges within reach of the flood, is an occurrence of paramount importance, and should be remembered as a leading factor in the design of engineering works.
GEORGE T. HAMMOND, M. AM. SOC. C. E. (by letter).--In a country, such as that described in this paper, where water is valuable, and a shortage is at times possible, where the majority of the population is very poor, and water and sewage discharge are both to be paid for on a basis of volume, the question of the expected quantity of daily water supply and sewage flow per capita is of primary importance. This question, notwithstanding its difficulty, should be given a first place in the studies for water-works and sewerage projects, and should never be lost sight of in the design, which should be such that, while proper for the expected future flow for a reasonable time, should also be proper and economical for conditions which at present obtain and may change but slowly.
It is desirable, of course, to get as much capacity in works as one can for the outlay, but there are instances where one can get too much for the money, as where a larger pipe than is necessary is used for a sewer, merely because it costs about the same as a smaller one, and as a result the cost of maintenance is permanently increased.
The water-works were designed to supply 40,000,000 liters (10,582,000 gal.) daily, which it was assumed would be sufficient for all future developments in Monterrey for a population of 200,000 at a per capita consumption of 200 liters (about 53 gal.) per day. The present population of the city is given as less than 90,000, there having been an increase of 22,000 in ten years (1891-1901), but it is evident that in the last ten years (1901-1911) this rate of increase has not continued. Taking into account all the data known to the writer, it does not seem that the city will attain a population of 200,000 in a great many years, if it ever does; but this is a matter of personal opinion, and is only stated as such.
The present requirements of the city's population, assuming that each person uses 200 liters (53 gal.) per day, would be, at that rate, which is a very liberal one, only 18,000,000 liters (4,762,000 gal.) per day, or less than half the amount which may be provided.
If the water were not to be metered and the sewage discharge paid for by measure, it is possible that the free use of water might lead to the usual waste with which all are fairly familiar; but the use of meters, and the rates charged, will reduce the water consumption to a minimum. This end will especially result from Section 5 of the Tariffs which provides that:
"Groups can be formed of two or more small houses so as to obtain a joint service under the proportion shown in the tariff."
This provision will keep down the per capita supply, among the majority of the people, to about 37-1/2 liters (10 gal.) per day. A similar provision led to abuse in Santiago de Cuba, as well as in other Cuban cities, where one householder, taking water, frequently delivers it to adjoining houses and tenements through rubber hose. As many as ten or twelve families are sometimes found to be supplied from one tap in this manner. Indeed, it may be stated as a rule, having but few exceptions, that where water is paid for by meter its use is always restricted.
The water mains and distribution system, however, are so well laid out, and the whole design is so good, that the writer would not anticipate much difficulty because it is on rather too liberal lines for the present or probable future. It may, perhaps, be argued that it may cost more to keep the mains in such a system clean; but this extra cost will scarcely be of much moment, and will be offset by the greater lasting quality of the larger pipes. There is another feature of the problem, however, which is not affected favorably by a too liberal forecast of the per capita water supply, namely, the sewerage system.
If it is assumed that, using 200 liters per capita per day, the total water supply of the city for the present population will be 18,000,000 liters, and that this may double in fifty years, or even amount to 40,000,000 liters in that time, it would seem that a rather liberal provision has been made for the water supply, and that this will scarcely be exceeded by the sewage, for the latter must come from the water supply, there being little or no ground-water and no storm-water taken into the sewers. Designing the sewers to flow half full for all diameters less than 18 in., and seven-tenths full for all larger sizes, it would seem that this would give ample capacity for all time to come in such a city, and that good practice would not exceed these figures, it being more desirable that the sewers should not be too large to work well, than that they should be large enough in all places to meet every possible contingency. If all the sewers of a system are too large, the condition is incurably bad; while, if a few miles prove to be too small, on account of growth and prosperity not anticipated by the designer, it will be easy enough to relay such parts when this becomes necessary.
Mr. Conway states that:
"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 [100 gal.] per capita per day, with a maximum rate of flow of 200 per cent."
If the writer understands this statement correctly, it means that the sewers, flowing half full, will carry 380 liters per capita in 12 hours, or are designed with 200% of the capacity required to take the assumed flow in 24 hours.
It was assumed that each house would be occupied by 7 persons and have a frontage of 12-1/2 m. (about 41 ft.), that is, about 700 gal. per day per house, the maximum flow rate being 200%, or at the rate of 700 gal. per house in 12 hours.
It is to be remembered that nearly all the houses are of one story, and that, as a rule in tropical and sub-tropical countries, the per capita use of water diminishes with some function of the increasing number of inhabitants in one house. Most of the water is used in the kitchen, and where there are 7 persons instead of 5, the quantity used by the smaller number will generally serve the larger.
The writer is unable to understand how this quantity of sewage will be produced, especially as the author states that, as far as the company is concerned, it is limited to the removal and disposal of the sewage, and is not required to provide for storm-water. He also states that:
"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'."
The minimum velocities in the sewers, when running full, vary between 0.91 and 1.5 m. (from 3 to 5 ft.) per sec., and will be the same flowing half full.
From the foregoing data it will be observed that:
(1) The water supply is the only source from which sewage flow is anticipated;
(2) The water supply is very liberally estimated at 200 liters (53 gal.) per capita daily;
(3) For purposes of sewer design, the daily flow of sewage expected (all of which is derived from the water supply of 200 liters per capita) is estimated at 380 liters per capita, with a maximum rate of flow of 200% (or at the rate of 760 liters per capita), and with this quantity the sewers are designed to flow only half full;
(4) The gradients are such that a velocity of from 3 to 5 ft. (0.91 to 1.5 m.) per sec. will be secured in the sewers flowing half full with the above quantity of flow per capita.
The writer does not agree with this method of computation, as he feels sure that it will give sewers which are too large, with grades too steep for the best obtainable results. His experience, extending over more than twenty years in sewer design and hydraulic work, convinces him that the method pursued is wrong in principle.
The principles involved in sewer design are first of all hydraulic. The quantity of flow, in the nature of things, cannot be forecasted accurately; success depends on getting the nearest possible approximation to average conditions. If 200 liters per capita per day is a liberal allowance, and 40,000,000 liters per day is a liberal expectation at this rate for double the present population, and the sewers are designed to flow half full only, why should this again be doubled?
The design of a sewer system for a city such as Monterrey is, in fact, a very difficult problem, especially as the quantity of sewage will be very limited, flush-water will have to be used in considerable quantities, and water in that part of the world is precious at all times and often scarce. Under these circumstances, the size or shape of the pipes selected for the lateral sewers, should have been such as would more nearly agree with the requirements than does the 8-in. circular.
A. P. Folwell, M. Am. Soc. C. E., writing of the 8-in. circular size, states:[10]
[10] "Sewerage," by A. P. Folwell, M. Am, Soc. C. E.
"To secure a flow in this pipe having an average depth of 4 inches would require the sewage from a population of 6,500. In general it may be said that the ordinary depth of flow in any sewer should not be less than 2 inches, nor should it be less than 1/2 the radius of the invert, since if it is so there is much more danger of deposits forming along the edges and even in the center of the stream. It will sometimes be impossible to meet this requirement fully, but it should be kept in mind as extremely desirable."
Sewers of small size should be proportioned throughout the system so that the depth of the minimum daily flow in the invert, and the velocity of flow, will be the best possible to prevent deposits. The transporting power of water is dependent mainly on the depth of flow, a minimum velocity being selected rather than a minimum depth of flow. To those who have had charge of the maintenance of sewers, as well as of their design and construction, this principle seems so obvious that it is always a surprise to see it disregarded by designers, who in these days seem inclined to consider sewerage as a system of grades and sizes of pipes installed for ideal, rather than for actual, conditions. Messrs. Staley and Pierson have well stated the principle involved as follows:
"A stream having a depth of flow sufficient to immerse solid matter held in suspension, to a certain extent lifts it and carries it forward. The entire surface is also exposed to the action of the current. A stream having an equal velocity but a less depth in proportion to the diameter of the solid matters to be transported, evidently has less transporting power. * * * An amount of sewage which can be properly transported by a circular sewer of a given size, cannot be as efficiently transported by one of larger diameter."
From some strange idea, which is apparently without foundation in logic or based on any actual justification from experience, it has of late years become the practice of designing engineers to make the 8-in. circular pipe the smallest size for sewers; and it is not improbable that the designer of the Monterrey system has merely followed this example. It has also become the frequent practice of designers to give every length of sewer all the grade possible, regardless of the fact, taught both by hydraulics and experience, that the best grade is that which will give as much depth of flow as is consistent with a scouring velocity.
Some years ago it was the standard practice, in the "strictly separate system" of sewers, to use the 6-in. pipe as the minimum size, and, as far as the writer has been able to discover, after giving the matter a rather extensive investigation, the 6-in. size has given excellent results wherever its use was proper. In places where it has not succeeded there were excellent reasons why it should not have been selected, and these could easily have been observed at the time the designs were made. The best sizes for the sewers in a given system is always a matter to be determined by local conditions; but there seems to be no reason why the 6-in. size should not be used where the flow is so slight that the 8-in. will not work well; or where the velocity must of necessity be so great that a flotation depth of flow cannot be maintained in the larger size. As to likelihood of clogging and stoppage, the writer's opinion, based on the maintenance of three rather extensive systems in different parts of the United States, in each of which the 6-in. size comprises more than 75% of the whole length of pipe, and of three other systems, one having 12-in. and two having 8-in. as the minimum sizes, is that the 6-in. size, where properly used, is less likely to become clogged than either of the others used improperly. The cost of maintaining the 6-in. pipe lateral, under these circumstances, is much less than that of maintaining the 8-in. lateral.
The 6-in. pipe is not being used now as much as the 8-in., and in most cases this is probably because the capacity of the latter is nearly double that of the 6-in., and costs only a few cents more per foot. If there is a sufficient population per acre, or if, within 30 or 40 years, such a population is anticipated as will fill the 8-in. pipe half full, its use, of course, is justified and necessary; but where it is quite evident that this will never occur, its use is counter-indicated.
In Monterrey, where the building lots have a frontage of 41 ft., where the houses, as a rule, are only one story high, where the water service is metered and paid for, and the sewage flow is also paid for, there seems to be no reason to justify the use of 8-in. pipe for the upper reaches of the smallest sewers. The sewage flow to be anticipated will probably never be sufficient to keep an 8-in. pipe sewer in a good clean condition at the upper ends of the lines of sewers without excessive flushing; and the sharper or steeper the grade on which it is placed, the worse will be the result, because the sharper the grade, the thinner the flowing thread of sewage will be drawn out in the invert; on the other hand, if the grades are flat, the slight quantity of sewage flow will be spread out in a sluggish stream, without sufficient depth, on the bottom of the 8-in. pipe.