Best Methods of Obtaining a Good Water Supply
Filtration System and Pumping Station—Uses of Auxiliary Reservoir—Causes of Insufficient Pressure—Preparations for Bond Issue—Surveys, Plans and Specifications—Personnel of Engineers
(Continued from page 1301, Vol. LXVlll)
Construction of Filter Plant
If it is necessary to filter the water coining from the reservoir, then it probably will be best to build a filtration plant directly in front of the dam, though, under some conditions, the filtration plant may be located on high ground near the city. In two instances where the writer has designed and supervised the construction of filter plants in connection with gravity systems, the filter plant in each case has been located at the dam. From the dam or from the filtration plant, if such is necessary, one or more conduits will lead to the city. The conduit must be large enough so that when the water flows through it the amount of head or fall lost in friction will not he so great but that there will be sufficient pressure remaining for the needs of the city. There are several types of conduits in use, pressure pipes, tunnels and open channels. All three types of construction, the pressure pipe, the tunnel or the open channel, might have to be used in different sections of the conduit. In the majority of cases a pressure pipe alone will be used.
Pipe lines are built of four different materials, cast iron, steel, wood or reinforced concrete. It is not possible to say which of these materials is the most suitable. It depends entirely upon the conditions which exist in each particular problem.
Uses of Auxiliary Reservoir
If there is a convenient hill located in the city or on its outskirts, an auxiliary reservoir should be constructed to hold enough water so that if the pipe line leading from the storage reservoir to the city should break there would be an adequate supply of water in the city while the break was being repaired. Such a reservoir would serve not only the above purpose, but it would act as an equalizer upon the flow of water through the conduit from the storage reservoir to the city. If there were no reservoir at the city, there would be a great variation in the amount of water flowing through the conduit, for the amount of water that is used during the twenty-four hours in any city varies greatly from hour to hour. During the day when the factories are operating and when everybody is using water there is about four of five times as much water used as during the earlier morning hours when most people are asleep. This being the case, if there were no reservoir in the city there might at certain times during the day be twice as much water flowing through the pipe as the average rate of flow for the entire day. If such were the case there would be approximately four times as much friction in the pipe, and in order to have sufficient pressure in the city at all times it would be necessary to have a pipe much larger than would be the case with the equalizing reservoir located in or near the city. For in the latter case the flows through the conduit would be uniform. During the night the water would flow into the equalizing reservoir and during the day it would flow out again to supply the excess required. If there is no high ground in or adjacent to the city, then it is essential that an elevated steel tank or standpipe be constructed. This will permit of a limited amount of water being stored in the city. However, the cost of such a structure in proportion to the amount of water stored is considerable as compared with a reservoir located on high ground.
Location of Pumping Station
In case there is no chance of getting water by gravity and it is necessary to pump the water, then the essential points to be sure of are, first that the pumping station be so located on the stream from which the water is to be pumped that it will be above any points where the stream is contaminated or may be contaminated in the future. This, of course, is not always possible, but an effort should be made to accomplish it. The writer has recently been studying a water supply situated in a large city where the present pumping station is located a short distance down the river from the most logical outlet of the city sewer system. Had the pumping station been located a short distance farther up the stream this could have been avoided. In pumping water from a stream one of the most essential considerations is the selection of the kind of power to be used in driving the pumps. Ordinarily, with any large installation, there are two methods of doing this: either by steam or electric power. In the first case it is necessary for the city to build its own power house, have its own boilers, and furnish the necessary steam to the pumps. In the second case the electricity may be purchased and the first cost of installation will be much less than in the case of the steam-driven pumps. One of the most important questions to be considered in making a choice of supply is the question of the reliability of service. If the plant is to be operated by electricity, is it certain that the electricity can be furnished at all times; or, if not, will the interruptions in the service be such as to cause an interruption in the supply of water?
As in the case of the gravity system, the construction of a reservoir in or near the city at an elevation high enough to insure sufficient pressure from the water in the reservoir will materially benefit pumping conditions. If such a reservoir has been constructed, then there is a reserve supply and if the pumps fail for any reason the reservoir can furnish what water is needed during the time the pumps are being repaired. Further than that, in the case of electrically driven pumps, it will probably be possible to buy electric power very cheaply for use during certain times of the day, when very little power is being used by other customers of the electric company.
Filtration Plant Near Pumping Station
If the water from the pumping station is to be filtered, the filtration plant should ordinarily be located adjacent to the pumping station. In fact, in many cases the pumping station and the filtration plant are in one building. The water is first pumped to the sedimentation basin passing from there on to the filters, then into the clear water basin, and from there is pumped directly into the city mains. This arrangement requires two sets of pumps and this is in most cases necessary wherever a pumping station and filtration plant are installed. The first set of pumps are usually known as low-lift pumps as they only have to raise the water to the level of the sedimentation basin. The other pumps are spoken of as high-lift pumps as they pump the water under high pressure directly into the city mains. In some cases the reverse is true; that is, the filtration plant is located at an elevation high enough so that the water after it is pumped to the filtration plant need only to be pumped under low pressure into the city system. From the writer’s observations, however, the filtration plant located at the pumping station is a more economical arrangement than to have the two widely separated, though this depends upon the local conditions.
In studying the water supply situation of a city it often develops that water can be obtained in more than one way and it is a question which way is the best. It may be possible to pump and filter water from some nearby river or lake, or it may be possible to get water by gravity by building a storage reservoir on a stream distant from the city. In the latter case the water may or may not have to be filtered. In the case of the pumping proposition the first cost is low and depreciation and operating cost is high. The more water that is pumped the greater is the annual cost, so that the cost increases each year. In the latter case the first cost is ordinarily much higher, but the annual cost decreases as the bonds are paid off, and eventually there will be a very small annual expense. One advantage of the gravity plan over that of pumping, which must not be overlooked, is that there is no danger of the water supply being cut off by inability to operate the pumps, as might be the case with a prolonged coal strike. The subject of getting water by gravity from storage reservoirs is one that has not been given sufficient attention by a great many city officials. It is almost always true that a city located in a hilly or mountainous country can get a much more satisfactory water supply by gravity from a storage reservoir than by pumping. The writer knows this to be a fact by actual experience in designing and building such gravity supplies to replace pumping plants.
Causes of Insufficient Pressure
Even though the supply of water is pure and there is an adequate supply of water, it is not always the case that there is sufficient pressure for ordinary usage, so that in the case of a large fire the pressure may be inadequate. Insufficient pressure results from two causes. The first is that the pressure of the water as it comes from the source of supply, whether it be from pumping or from gravity, is not high enough; and the second is that even though the initial pressure is high enough, the mains through the city are not large enough to carry the water where it is needed without excessive loss in friction. The first condition can be remedied by the installation of what is known as the Booster Pump, which takes the water at a certain pressure and repumps it, adding pressure to the water. If this is not possible for fire protection, the pressure will have to be increased to the required amount by the use of portable pumping engines. In the case where the mains are not large enough the best remedy is to put in larger mains where they are necessary. Until this is done portable pumping engines will have to be used, but if the mains are very small even these may not furnish the required pressure. As soon as it is certain that the funds will be available for the proposed undertaking, surveys, plans and specifications should be made under the direct charge of a competent hydraulic engineer for the carrying out of the proposed work. When this work has been completed contracts should be let and the work carried out to completion. In the letting of contracts for public works there are four principal methods of proceeding with the work: 1st, by day labor, in which case the city pays directly for all labor and materials; 2nd, by the cost-plus percentage contract, in which a certain contractor is engaged to do the work, he buying the materials and paying the labor and receiving a certain percentage of the cost as his compensation for doing the work; 3rd, the costplus fee basis, where the contractor carries on the work, buying the materials and paying the labor himself, and is paid a fixed amount for his services instead of a percentage; 4th, where the contract is advertised and the bids are competitive, the contract being awarded to a contractor who agrees to do the work for a lump sum or at certain unit prices. The unit price is in most cases more satisfactory than the lump sum. It is fairer toboth parties and fewer disputes arise. With the unit price contract the total amount paid the contractor varies in proportion to the amount of work done. That is, he may agree to put in 10,000 cu. yds. of concrete at $15.00 a yard, or a total of $150,000, and if there are 11,000 cu. yds. he would be paid $15,000 additional to the amount bid; or if there were 9,000 cu. yds. he would be paid $15,000 less than the amount bid. If the bid were a lump sum the price would be the same whether there was more or less work done than was originally expected.
Enough has been said about the actual construction work that needs to be done, in improving a city’s water supply system. Once the city official is satisfied that the water supply system needs improvement, the first step to take is to have a competent hydraulic engineer make a thorough investigation of the needs of the city. After such an investigation is made the engineer should make a report to the city officials. This report should in a general way cover the following subjects: A brief description of the existing water supply system, with a study of the conditions of operation, an estimate of the future needs of the city in regard to its water supply, as based upon population statistics, and the general industrial future of the city; a study of the available sources by which the water supply of the city may be increased, estimates of cost of different possible improvements and also estimates of annual cost. These estimates of cost should be on the safe side; that is, if an engineer is satisfied that an improvement will cost a certain sum of money, he should make his estimates large enough so that there will be no possible chance of the work finally costing more than his estimates. Several other important points should be touched upon in the report, depending upon the particular conditions. The entire report should be so worded and arranged and sufficiently free from technical phrases so that the entire water situation as stated by the engineer with his recommendations can be clearly understood by the average citizen. After such a report has been made, there should be no question as to what are the proper steps for the city to take in the improvement of its water supply.
Once a definite improvement has been decided upon by the city officials, the next step is to raise the money to carry out the undertakings. The manner of doing this will depend on the city’s charter and the legal power of the city officials. In some cases money can be appropriated by the city officials without a vote of the taxpayers, and in others a bond issue has to be voted upon by the taxpayers.
Preparation for Bond Issue Election
The greatest care should be exercised by the municipal officials in drawing up resolutions and in proceeding correctly in the preparations for an election where a bond issue is to be voted upon. A great many times a technical error has made the election illegal, and as a result a new election has had to be held before the bonds could be issued and sold. There are attorneys who make a special business of such work, and it oftentimes will result in a considerable saving of time and money if they are consulted. It has been the writer’s own experience that it is much more satisfactory in the successful carrying out of any large improvement that the public be fully informed through the city newspapers as to just what it is proposed to do and the reasons for doing it.
Surveys, Plans and Specifications
(Continued on page 72)
Obtaining a Good Water Supply
(Continued from page 60)
Methods of Carrying Out Work
Of the four methods mentioned above the first method of doing the work—by day labor—especially where any large quantity of work is involved, is rarely satisfactory, and can only be so where one man is put in absolute charge of the work who is a competent executive and who is not hampered in any wav by requests to buy his materials of certain favored people, or to place certain men on the work, regardless of their ability. The second method—that of letting a contract on the cost-plus percentage basis, is also ordinarily unsatisfactory. However honest the contractor might be in his desire to keep the cost of the work down, it will be known throughout the entire job that the work is on a cost-plus percentage basis and that the greater the cost of the work the more will be the contractor’s profit. This being the case, the foremen and men on the work who are working for the contractor are inclined to take their time about doing work, and often deliberately increase the cost, thinking that by so doing they are benefiting their employer. The third method, that of doing work on a cost-plus fixed fee basis, gives no incentive to increase the cost either on the part of the contractor or on the part of the men working for him. If the cost is increased his fee remains the same, and his natural impulse will be to get the work done as soon as possible, so that his plant and his organization may be busy on some other work. The fourth and most common method of letting contracts is to advertise for bids and then award the contract to the lowest bidder or to one of the low bidders. There are some features of doing the work in this way which are not satisfactory. When the bids are received they may vary greatly in their magnitude. One contractor may be willing to do a certain piece of work for $300,000 and there may be several others willing to do it from prices varying from this amount up to $500,000. Even though the contract is let to the lowest bidder whose bid we will say is $300,000, this does not signify that the work can be done for this amount of money. After the contractor has started the work and partially completed it, labor conditions and the prices of material may increase to such an extent or other conditions may arise such that he will be unable to complete the work and he may have to abandon the contract or the city officials may decide to take the contract away from him. Ordinarily in the case of such contracts it has been necessary for the contractor to furnish a bond with the contract. To those unfamiliar with the carrying out of public works this bond may appear to be the greatest safeguard; however, in many cases bonds have been of no value. In case the contractor falls down on his contract the bonding company may immediately begin to look for places whereby it can be released from its responsibility. Almost all public work is carried through with some changes in the original plans. The bonding company may contend that these changes have released them from their responsibility, or there may be several other reasons why they should not fulfill the requirements of the bond and in very many cases it means a long drawn out legal battle which in the end is more costly than if there had been no bonding company whatever. As a safeguard against this last trouble it is essential that the city officials let their contracts to responsible contractors, even though they are not the lowest bidders, but whom they know, even though the work may cost them more than they anticipate, will be able and willing to carry through the work to completion. Of the four methods of letting contracts, the third method is probably the fairest to all concerned and the most satisfactory if the city officials have the authority to let contracts in such a manner. The essential precaution is that a contractor be selected who is absolutely certain to do the work efficiently and economically. If authority is lacking to let a contract on a cost plus fixed fee basis, then the best method is to advertise for bids and award the contract to a contractor who has done work of a similar nature and who without question is financially able to carry out the work.
Personnel of Engineers.
A word as to the personnel of the engineer in charge of making the plans, specifications and carrying out the work. It is necessary first of all that he be a practical man of some business knowledge and not a theorist. It is necessary that he should have had special training in the work which he has to do. It is also essential that he should have had some practical experience in dealing with men. Many contractors have been forced into bankruptcy because the engineers on the work loved to split hairs and insisted on refinements in construction absolutely beyond all reason. Too often the attitude toward contractors is that they are dishonest and will attempt to beat the city whenever they have a chance. The writer’s own experiences with contractors have been that they are just as honest as men in any other business and if treated fairly will do their utmost to give the best kind of work for the money which they receive.