WATER WORKS RESERVOIRS
It is the purpose of this paper to describe briefly the several classes of water works reservoirs and explain their respective uses and limitations; to touch upon the value of the proper location of reservoirs, and especially of distributing reservoirs; to discuss the governing considerations in reservoir design; to describe some of the difficulties encountered in actual construction, and to give some suggestions, born of experience, which may help others to overcome these difficulties.
Classes of Reservoirs.
Broadly defined, a reservoir is a place in which something is kept in store. In water works reservoirs that which is stored is water. An impounding reservoir is usually formed by building a dam across a stream, so that its waters may be conserved, especially at time of flood, for release during periods of drought. Such reservoirs may be used to store water for power purposes,. or for irrigation, or to diminish the danger of dan age from floods; but none of these uses would have any relation to water works. If, however, they store water for fire protection or domestic use, they may properly be classed as water works reservoirs. Other water works reservoirs include what are known as suction reservoirs, storage basins, clear water reservoirs and distributing reservoirs. The first three of these are generally designed to store water which is pumped into them from wells or streams, or which flows into them from filters or settling basins. Distributing reservoirs are connected to the water distributing system, usually in such a manner that the elevation of the water in the reservoir controls the pressure on the distributing mains. Such reservoirs are naturally built on high ground, and when they are made of steel in the form either of standpipes or of elevated tanks, their flow lines are frequently 100 feet or more above the surface of the ground on which they are located. In any water works system there are usually a number of steps which lie between the taking of water from its original sources and the actual delivery of it to the consumer. These steps include some or all of the following: (a) The collection and storage of the water of a stream in an impounding reservoir, (b) The pumping of water from a stream or lake or from wells to a suction reservoir, or to a settling basin, or to filters, or directly into the distribution system. (c) The purification of the water either by sedimentation, or by filtration, or by both, (d) The pumping of the water from the surface reservoir, or from the sedimentation basin, or from the clear water reservoir of the filters, into the distribution system, (e) The actual delivery of the water from the distribution system to the various consumers.
Field of Usefulness.
The usefulness of any reservoir depends upon, and is limited by, the position which it occupies in the order or procession of the steps just enumerated. For example, if it be an impounding reservoir it can do no more than store the waters of the stream above it, and is of no value in conserving or helping out the capacity of the low lift pumps which take their supply from it; or of any suction reservoir, or filter plant, or clear water reservoir, which may follow it; or of the high lift pumps; or of the distribution system. If it be a suction reservoir or clear water well, then it will help out the capacity of everything back of it, which may be a stream, an impounding reservoir, wells, or low lift pumps taking water from any of these sources and delivering it into the reservoir under consideration, or a water purification plarit which may discharge into it. Such a reservoir does not of course, conserve the capacity of the high lift pumps which draw from it, or of the distribution system into which its waters are discharged. A distributing reservoir, located on the discharge side of the pump and connected to the distribution mains, has more of these steps back of it than any of the other reservoirs just enumerated, and its usefulness may include the conservation of the capacity of stream, or of impounding reservoir, or of wells, or of sedimentation basins, or of filter plant, or of low lift pumps, or of high lift pumps, or of all of these together. A distributing reservoir properly located will, in addition to all of the foregoing, help out the capacity of the distribution mains themselves, and this last and very important function has in the past been frequently overlooked. It follows from what has just been said that the nearer a reservoir is to the beginning of the order or procession of the successive steps in water supply, the less will be its value, other things being equal; and that the further along in this procession of steps, the greater will be the value of the reservoir.
Value of Proper Location of Distributing Reservoirs.
In order to derive the greatest possible benefit from a distributing reservoir, it should be properly located; and the intrinsic value of the proper location of such reservoirs has not always been appreciated in the design of water works systems. Where small water works plants have elevated storage, one frequently sees the tank located on the pumping station lot. A tank so located is in most cases a monument to the bad judgment of the man who designed the plant. To illustrate the point, two cases, out of many that could be mentioned, will be cited: In the first case the main pumping station was two miles north of the center of the congested value district in a small city. The elevated storage reservoir, originally built close to the pumping station, had been destroyed, and it was necessary to provide a new one. For the purpose of computing the friction losses it was assumed that a fire broke out during sprinkling hours on a hot day in summer and that the plant would be required to furnish water at the rate of 4,000 gallons per minute, distributed throughout the city for domestis consumption, in addition to a supply of 2,000 gallons per minute for fire service, which latter amount would be drawn at or near the center of the congested value district. It was found that if the tower and tank were located close to the pumps at the northern end of the city the pressure remaining in the mains in the congested value district would be only 25 pounds, whereas with the same elevated tank located near the southern end of the town the pressure remaining in the mains would be more than 53 pounds To save this difference of 28 pounds by laying additional mains from the pumping station to the congested value district would have involved an expenditure of at least $30,000; so that it may be fairly stated that the advantage obtained in this case by locating the elevated storage near the center of the congested value district instead of at the pumping station was worth not less than $30,000. In the second case, which involved a city of larger size than the one just mentioned, a site for an elevated reservoir of a capacity of 7,500,000 gallons had been selected on an eminence opposite the main pumping station. The congested-value district in this city was comparatively small, and its center was more than three miles south of the pumping station A topographic map had been made of the proposed reservoir site, and test pits had been sunk through the top soil to bedrock, when the writer was called into pass upon the suitability of the topography and soil conditions for a reservoir of the capacity contemplated. It was at once apparent that the location of the reservoir with regard to the pumping station, to the distribution system of mains and to the territory to be supplied, was far from being a desirable one. An examination was made of high ground opposite the congested value district, with the result that a much more suitable site was discovered, which was later purchased by the city. For the purpose of comparing the friction losses incident to the reservoir site originally chosen with those incident to the site recommended by the writer, it was assumed that the rate of domestic consumption during sprinkling hours on a hot day would be 3,500 gallons per minute, which quantity of water would be distributed throughout the city, and that in addition to this 4,000 gallons per minute would be required to meet the demands of a fire near the. center of the congested value district. The computations showed that had the reservoir been located opposite the pumping station the total friction losses would have been 96 feet, or about 41½ pounds, whereas with the reservoir located near the center of the congested value district the friction loss under otherwise similar conditions would be only 3 pounds. The computations further showed, that to reduce the friction loss to 3 pounds under the assumed conditions of demand, by putting in additional mains, would have cost more than $200,000. The expenditure of so large a sum of money would, of course, have been out of proportion to the benefits obtained,. and the best economy would have dictated spending less money and putting up with somewhat greater friction losses. On the other hand, it would not have been economical to make the additional main smaller than 20 inches in diameter, had it been necessary to reduce the frictions by this method. The cost of even a 20-inch main would have been more than $80,000. This sum is, therefore, considered to represent the lowest possible estimate of the advantage possessed by the site finally adopted for the reservoir over the one which had originally been chosen.
Governing Conditions in Reservoir Design.
Among the first points to be determined are the location, capacity and elevation desired. These having been at least approximately determined, the work of designing may be begun. The importance of proper location has already been discussed. On the principle that one cannot have too much of a good thing, the capacity of the reservoir should be made as large as the finances of the local water department will permit. In any event, however, the reservoir should be made large enough to tide over the demand of the tour or five hours of maximum consumption, and, if possible, its capacity should at least be equal to a full day’s pumpage. The elevation would be influenced by a number of conditions, the chief among which would be the topography of the city and the height and character of the buildings to be served. If pressure is increased during fires, it may often be found advisable to install an electric driven booster pump taking its water from the reservoir and pumping into the mains, the pump to be started when the fire alarm is turned in. Generally speaking, where the elevation required is high and the capacity small, the reservoir will be an elevated tank supported by a tower, the whole structure usually being of steel. Other things being equal, that type of tank in which the average elevation of the stored water is highest is to be preferred; and when such tanks can be secured, as at present, at relatively low cost and in safe and attractive designs, it would seem that the old fashioned tall standpipe in which three-fourths of the contained water served no useful purpose except to support the remaining upper one-fourth, has no longer any right to exist. Where larger capacities are required, and where the flowline of the reservoir does not have to be far above the surface of the ground, the choice in most cases naturally falls on reinforced concrete as the material for the reservoir. One point which frequently has to be decided is whether or not the reservoir shall be covered. One advantage of covering the reservoir is that the water is more easily protected against pollution, although it is usually possible so to fence and otherwise safeguard an open reservoir that the danger of pollution from the outside is almost negligible. Another minor advantage is that the roof will keep the temperature of the stored water more uniform, and prevent ice in winter and overheating in summer. Perhaps, however, the best service rendered by the roof would be the prevention of algae by the exclusion of the rays of the sun from the water. As it is possible at small expense and with the exercise of intelligent care to stop the growth of algae by the use of sulphate of copper, and as the addition of a roof almost always adds very greatly to the cost of a reservoir, most reservoirs, and especially those of large capacity, are of the open type. The designer having reached this point can now proceed with the details. From now on he will probably work on the cut-and-try plan, making preliminary sketches and estimates of many different types of design, and abandoning one after the other until convinced that he has finally selected the best one, all things considered, for the local conditions. In a recent case more than a score of such trial computations were made on as many different types of reservoir wall before the wall which seemed to meet most satisfactorily all of the requirements was selected.
Some Construction Difficulties and Some Suggestions.
The first thing naturally demanded of a water works reservoir is that it shall hold water, and as a rule the most difficult part of the construction of a reservoir is making it watertight. A small amount of leakage really does no great damage; but so long as any leakage can be detected, it is an eyesore, and it remains as a reproach to all of those in any way connected with the design or construction of the reservoir, whether contractor, engineer or owner. For these reasons, leaks so small that they could do no harm whatever are not as a rule permitted in the finished structure, even though the cost of stopping them is out of all proportion to the value of the water lost. It was the writer’s good fortune that in all of the reservoirs mentioned in this paper, he had to deal with contractors who endeavored honestly and conscientiously to secure good results. In almost every case they succeeded remarkably well, and when they failed, the failure was due to some oversight or bad judgment on their part, and not to any desire to skimp the work. It is not an easy matter to build a reservoir which shall be absolutely watertight from the time the forms are removed. Fortunately, however, very small leaks will usually become less or “take up” in a short time, especially when the stored water contains iron or sediment, and in most cases it is not very difficult to stop large leaks or at least to reduce them to so small an amount that they will ultimately stop of themselves.
As a result of his experience with a number of concrete reservoirs, the writer would draw the following conclusions: 1. It is entirely possible with proper materials, mixture and workmanship to prevent moisture from passing through a concrete wall a foot thick even under fairly heavy pressures. 2. It is not to be expected, however, that the perfection of workmanship required to produce these results will always be obtained at every single point over an area of thousands of square feet of wall. 3. Such leaks as may show in spite of conscientious efforts to do good work can almost invariably be stopped entirely or be reduced to such a point that they will stop themselves in the course of time, especially if the water carries iron or sediment. 4. Leaks are most likely to occur at construction joints. The use of steel dams will reduce the danger of such leaks, but these dams cannot always be relied upon to prevent the leakage, and their presence should not be allowed to diminish in any way the precautions which should always be taken to prevent leakage at the joints. 5. The surface of concrete which has begun to set should be scratched and roughened, and all dust, rubbish and laitance should be carefully removed with a vacuum cleaner before the next batch of concrete is poured. This will not always prevent leakage, but it will go far towards doing so. 6. In reservoir construction the use ot chutes for conveying concrete to its place in the wall should not be permitted unless the concrete is thoroughly remixed just before it reaches its final place in the wall. 7. While good results can be obtained by very careful spading of the concrete adjacent to the forms, so as to keep the stone away from the inner surface of the wall, it is believed that far better results would be secured by the plan devised by the contractor for the 10,000,000-gallon reservoir already described; namely, the use of a portable sheet of thin metal with means for holding it about 3/4 inch away from the inner form, the concrete to be poured back of this sheet of metal, and cement mortar in front of it and between it and the form.