to control excess water, while volume of flow during longer periods of time is of
interest in designing projects for use of water. The answers to this question are
found through the application of hydrology, the study of the occurrence and
distribution of the natural waters of the earth. Since the* future ca nnot be accurately
TABLE 1.2
Water balance of the coterminous United States* Water balance of the coterminous United States*
Component 109 bgd 106 AF/yr *n./yr lO’ irrVyr
Precipitation 4200 4704 29.7 5786 Evapotranspiration 2800 3136 19.8 3857 Diversions for Irrigation ___
_
_ _ 152 170 1.1 209 Public use 42 47 0.3 58 Industry! 256 287 1.8 353 Total diversionsf 450 504 3.2 620 Consumption Irrigation 84 94 0.60 116 Public use 10 11 0.07 13 Industry 6 7 0.04 9 Total consumption 100 112 0.71 138 Outflow to ocean 1300 1456 9.2 1791• Adapted from W. B. Solley, E. B. Chase, and W. B. Mann, IV, Estimated Use of Water In the United States 1980, U.S. Geol. Surv. Circ. 1001, 1983.
+ Approximately 87% of the water withdrawn by industry in the United States is used for the cooling of thermoelectric power plants.
Í Twenty percent of the diverted water comes from groundwater. The remaining $0% is from surface water. Reclaimed water accounts for less than 0.2%.
4
4 WATER-RESOURCES ENGINEERING
forecast, hydrology involves assessment of probability. The principles of hydrology are outlined in Chaps. 2 to 5.
The water flowing in a stream is not necessarily available for use by every person or group desiring it. The right to use water has considerable value, especially in regions where water is scarce. Like other things of value, water rights are protected by law, and a legal answer to the question Who may use this water? may be required before the quantities of available water can be evaluated. Diversion of natural streamflow may cause property damage and alterations in natural flow conditions are governed by legal restrictions that should be investi gated before completion of the project plan.
1.
1.3 3 Water Water QuaQua litylity
In addition to being adequate in quantity, water must often withstand certain tests of quality. Problems of water quality are encountered in planning water-supply and irrigation projects and in the disposal of wastewater. Polluted streams create problems for fish and wildlife, are unsuited for recreation, and are often unsightly
and sometimes odorous. Chemical and bacteriologic tests are employed to de termine the amount and character of impurities in water. Plant and human physiologists must evaluate the effect of these impurities on crops or hum an
consumers and set standards of acceptable quality. The engineer must then provide the necessary facilities for removing impurities from the water by physical, chemical, or biologic methods. Hydrologic studies are necessary to evaluate the effectiveness of the wastewater management plan. Governmental agencies having the authority to regulate the disposal of wastes are required to safeguard our waters against pollution.
1.4
1.4 HydraHydra ulic ulic StructuresStructures
Structural design^oLfacilities for water-resources projects utilizes the techniques of civil engineering. The shape and dimensions of the structure are often dictated by the hydraulic characteristics it must possess and hence are determined by
application of the principles of fluid mechanics. Many hydraulic structures are relatively massive as compared with buildings and bridges, and the structural design involves much less fine detail. However, hydraulic structures frequently involve complex curved and warped surfaces and sometimes intricate detail for gates, valves, control systems, etc. Almost all the conventional engineering mater ials are employed in hydraulic structures. Earth, mass and reinforced concrete, timber, clay tile, asphaltic compounds, and most of the common metals are found in such structures.
Largely because of topographic controls, it is not always possible to select the most satisfactory location for a hydraulic structure from the structural viewpoint. Hence, geologic investigations are an important part of the preliminary planning. These investigations should be aimed at selecting the best of the
INTRODUCTION 55
the particular conditions at the site, and locating sources of native material suitable for use in the proposed structure.
1.
1.5 5 EconoEcono mics mics in in Water-ResourWater-Resour ces ces EnginEngineeringeering
Little skill is required to design a structure for some purpose if unlimited funds are available. The special ability of the engineer is reflected in the planning of projects that serve their intended purpose at a cost com mensurate with the benefits (value engineering). An economic analysis to determine the best of several
alternatives is required in planning most projects. It must usually be demonstrated that the project cost is sufficiently less than the expected benefits to warrant the required investment. In many cases the estimated benefits serve also as a basis for determining a schedule of payments by the beneficiaries who will repay the project cost to the construction agency.
Precipitation and streamflow vary widely from year to year. It is usually uneconomic to design a project to provide protection against the worst possible flood or to assure an adequate water supply during the most severe drought that could conceivably occur. Instead the project design is gaged against a scale of probability so that the probability of-the project failing to serve its purpose is
small but still positive. Economic analysis (Chap. 13) is dependent on hydrologic analysis of the pr obability of occurrence of extreme floods or d rou ghts (Chap. 5).
1.
1.6 6 Social Social Aspects Aspects oof f Water-ResouWater-Resou rces rces EnginEngineeringeering
Most water projects are planned for and financed by some governmental unit—a municipal water-supply or sewerage system, a state highway department, or a federal irrigation or flood-mitigation project—or by a public utility. Many such projects become controversial political issues and are debated at length by people
whose understanding of the basic engineering aspects of the problem is limited. It is a clear responsibility of an engineer who has the necessary facts concerning such a project to take a firm position in the public interest if the final decision is not to be made on political and emotional grounds. It is particularly important that the engineer carefully analyze the facts and present a sound case in simple terms and avoid championing a “pet” project that is of limited benefit to the public. Throughout any negotiations concerning a publicly financed project, the engineer should adhere carefully to the code of ethics of the professional society that represents the civil engineering profession in his or her country. Failure to do so prejudices the case and the entire profession in the eyes of the public.
1.
1.7 7 Planning Planning oof f Water-ReWater-Re sources sources ProjectsProjects
Planning is an important step in the development of a water-resources project. The planning of a project (Fig. 1.1) generally involves a political incentive or recognition of the need for a project. This is followed by the conception of
6
6 WATER-RESOURCES ENGINEERING
FIGURE 1.1
Steps in planning a water-resources project.
alternative technically feasible solutions that would satisfy the need. The alterna tive proposals are subjected to an economy study that analyzes their benefits and costs and thus determines their economic feasibility. Evaluation of social and environmental impacts is also an important step in planning. Finally, financial feasibility (can the project be paid for?) and political practicality (is the project
acceptable to the public?) play an important role in the choice of alternatives. A detailed discussion of planning for water-resources development is presented in Chap. 21.
1.8
1.8 History History oof f WaterWater -Resources -Resources EngineeEngineeringring
The importance of water to human life justifies the supposition that some ancient man conceived the idea of diverting streamflow from a natural channel to an artificial one in order to convey water to some point where it was needed for crops or humans. The Old World contains numerous evidences of water projects of considerable magnitude. The earliest large-scale drainage and irrigation works are attributed to Menes, founder of the first Egyptian dynasty, about 3200 b.c. These
works were followed by many varied projects in the Mediterranean and Near East area, including dams, canals, aqueducts, and sewer systems. Some 381 mi of aqueducts were constructed to bring water to the city of Rome. An irrigation project in Szechwan Province of China dating from about 250 b.c. is still in use.
Even in the New World, projects of considerable scope antedate the coming of Europeans. Ruins of elaborate and extensive irrigation projects constructed about
a.d . 1100 by Hohokam Indians in what is now Arizona and similar Aztec works
in Mexico indicate flourishing irrigation economies.
These early works were not designed and built by engineers in the modern sense of the word. The ancient builders were master craftsmen and technicians (the Greek architekton , or archtechnician) who employed amazing intuitive judg ment in planning and executing their works. Rules of thumb developed through experience guided the leading builders, but these trade secrets were not necessarily conveyed to other men. The great thinkers of the Greek era contributed much to science, but since manual labor was considered demeaning, the application of their knowledge in practical pursuits was retarded. Many erroneous concepts and gaps in understanding delayed the development of engineering as it is known today. It was not until the time of Leonardo da Vinci (about a.d . 1500) that the idea that
precipitation was the source of streamflow received any real support and many years later before it was def initely proved. The limita tions of available construc tion materials also influenced early engineering works. Since no materials suitable for
INTRODUCTION 77