Water-Resources Engineering - Parte 1

WATER-RESOURCES

ENGINEERING

RAY K. LINSLEY

JOSEPH B.

FRANZINI

FRANZINI

DAVID L.

DAVID L. FREYBERG

GEORGE

GEORGE TCHOBANOGLOUS

FOURTH EDITION FOURTH EDITION Me Me Graw Graw Hill Hill Education

WATER-RESOURCES ENGINEERING WATER-RESOURCES ENGINEERING

McGraw-Hill Series in Water Resources and Environmental

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ABOUT THE AUTHORS

ABOUT THE AUTHORS

R n K.

Linsley,Linsley,

senior author of this book, passed away on November 6, 1990

He graduated from Worchester Polytechnic Institute in 1937 and served as an

engineer for the Tennessee Valley Authority and head hydrologic engineer for the

U S. Weather Bureau before joining the faculty at Stanford University, where he

remained for 25 years. He took early retirement from Stanford in 1975 to devote

his efforts to consulting. Linsley and his graduate students contributed greatly to

the understanding of hydrologic processes. He was a pioneer in the development

of procedures for hydrologic simulation employing continuous deterministic

models. Linsley was also senior author of several textbooks, including

Applied Hydrology

and

Hydrology for Engineers

, and he authored numerous technical

papers and reports. He received many honors including an Honorary D.Sc. from

| the University of Pacific and an Honorary D.Eng. from his alma mater. At the

time of his death, Linsley was Chairman of Linsley, Kraeger, and Associates, Ltd.,

a consulting firm in Santa Cruz, California, that does hydrologic modeling.

j

j JosJoseph Beph B. . FrFrananzinzinii

received B.S. and M.S. degrees from the California Institute of

i

Technology and a Ph.D. fro m Stanford Univ ersity. All his degrees were in civil

engineering. Franzini served on the faculty at Stanford University from 1950 to

1986. At Stanford he taught courses in fluid mechanics, hydrology, sedimentation,

and water resources and also did research on a number of topics in those fields.

Franzini is coauthor of the widely used text

Fluid Mechanics with Engineering Applications

and has authored numerous technical papers. He was also coauthor

with Linsley of

Elements of Hydraulic Engineering

, the predecessor to this book.

Through the years Franzini has been active as a consultant to various private

VIII ABOUT THE AUTHORS

organizations and governmental agencies in both the United States and abroad. He has been associated with Nolte and Associates, a consulting civil engineering firm in San Jose, California, for over 30 years and is a registered civil engineer in California.

David L. Freyberg is an associate professor of civil engineering at Stanford University in the Water Resources Program. He is also associate dean of the School of Engineering for Undergraduate Education. After completing A.B. and B.E. degrees at Dartmouth College in 1972, he served for several years as an engineer and project engineer with Anderson-Nichols & Company in Boston. His graduate eduction was at Stanford, where he completed both the M.S. and Ph.D. After receiving the Ph.D. in 1981 he joined the faculty of Stanford’s department of civil engineering. At Stanford he teaches or has taught courses in water resources, subsurface flow and transport, watershed hydrology, stochastic hydro logy, and fluid mechanics. The au thor of a nu mber of technical papers, Frey berg ’s current research focuses on the prediction of contaminant transport in ground water, with emphasis on the interpretation of field experiments, and on the relationship between prediction uncertainty and geologic variability. In 1985 he was named a Presidential Young Inves tigator by the Na tional Science Fou ndation . George Tchobanoglous is a professor of civil engineering at the University of California at Davis. He received a B.S. degree in civil engineering from the University of the Pacific, an M.S. degree in sanitary engineering from the University of California at Berkeley, and a Ph.D. in environmental engineering from Stanford University. His principal research interests are in the areas of wastewater treatment, wastewater filtration, aquatic wastewater management systems, individual onsite treatment systems, and solid waste management. He has authore d or co autho red over 200 technical publications and 6 textbooks. Professor Tchobanoglous serv es nationally and inter nationally as consultant to b oth gove rn mental agencies and private concerns. An active member of numerous professional societies, he is past president of the Association of Environmental Engineering Professors. He is a registered civil engineer in California.

CONTENTS

CONTENTS

Preface

Preface

xin

Comments on Units

Comments on Units

xv

1

1

Introduction

Introduction

i

i

*

*

2

2

De

Desc

scrriipt

ptiive

ve Hy

Hydr

drol

olog

ogy

y

9

9

The hydrologic cycle, precipitation, streamflow, evaporation and transpiration, collecting hydrologic data

*

3

3

Quantitative Hydrology

Quantitative Hydrology

43

Hydrograph analysis, estimating volume of runott, runoff from snow, hydrographs of basin outflow, storage routing, computer simulation

4

4

Groundwater

Groundwater

89

Occurence, groundwater hydraulics, wélls, yield, artificial recharge, groundwater quality

5

5

Probability Concepts in Planning

Probability Concepts in Planning

135 Flood frequency, flood formulas, rainfall frequency, drought,

stochastic hydrology

X CONTENTS

6

Water

Water Law

Law

169

Common law, state water codes, groundwater law, federal law, interstate problems, drainage law

7 Reservoirs

7 Reservoirs

185

Physical characteristics, yield, capacity, reliability, sedimentation, waves, reservoir clearance

8 Dams

8 Dams

219

Forces on dams, gravity dams, arch dams, buttress dams, earth dams, miscellaneous types, failures, safety and rehabilitation

9

9 Spillways,

Spillways, Ga

Ga tes,

tes, and Outlet Works

and Outlet Works

269 Spillways, crest gates, outlet works, protection against scour

10

10 Open

Open Channe

Channels

ls

312

Hydraulics of open-channel flow, measurement of flow, types of channels, appurtenances

1

11

1

Pre

Pressur

ssure

e Con

Condui

duits

ts

,,

346

Hydraulics of pressure conduits, measurement of flow, forces on pipes, pipe materials, appurtenances for pressure conduits, inverted siphons

1

12

2 Hydraulic

Hydraulic Machin

Machin ery

ery

397

Turbines, centrifugal and axial-flow pumps, cavitation, displacement pumps, miscellaneous pumps

1

13

3 Engineering

Engineering Econ

Econ om

omy

y in

in Wa

Water-R

ter-R esou

esou rces

rces

Planning

Planning

438

Social importance, annual-cost comparisons, interest and taxes, frequency and economy, economy studies for public works, cost allocation

14 Irrigation

14 Irrigation

__

^ 461

Water requirements, soil-water relationships, water quality, irrigation methods, irrigation structures, legal aspects of irrigation

15

15 Water-Supply

Water-Supply Sys

Sys tem

tem ss

497

Water uses and quantities, water characteristics and quality, treatment, distribution systems

1

16

6 Hydroelectric

Hydroelectric Power

Power

568

Thermal versus water power, systems and load, project arrangement, electrical equipment, operation

CONTENTS XI

17

17 Rive

River

r Nav

Nav iga

iga tion

tion

592

Requirements of a navigable waterway, navigation dams, navigation locks

18 Drainage

18 Drainage

615

Estimates of flow, municipal storm drainage, land drainage, highway drainage, culverts and bridge waterways

1

19

9 Sewerage

Sewerage an

and

d Waste

Waste water

water Trea

Trea tmen

tmen tt

660

Quantity of wastewater, characteristics of wastewater, collection and pum pin g , w ast ew ate r tr eatm e n t, w ast ew ate r m a nag e m en t

20

20 Flood-Da

Flood-Da mage

mage Mitigati

Mitigati on

on

743

The design flood, flood-mitigation reservoirs, levees and flood walls, floodways, channel improvement, evacuation and floodproofing, land management and flood mitigation, flood-plain management, economics of flood mitigation

2

21

1

Planning

Planning for

for Water-Resources

Water-Resources Develo

Develo pmen

pmen tt

i n

Levels of planning, phases, objectives, data requirements, project formulation and evaluation, environmental considerations, systems anal ysis , m ultiple-purpose project s

Appe

Appendix

ndix A

A Usefu

Usefu l

l Tables

Tables

803

Append

Appendix

ix B

B Metric

Metric Versions

Versions o

of

f Figures

Figures 7.14 and 10.2

7.14 and 10.2

813

Append

Appendix

ix C

C Drinking-W

Drinking-W ater

ater Stan

Standards

dards

816

Indexes

Indexes

Name Index

Name Index

819

Subject Index

Subject Index

\ 826

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PREFACE

PREFACE

This is the fourth edition of this book. During the preparation of this edition two new coauthors were brought aboard: Dr. David L. Freyberg and Dr. George Tchobanoglous. Shortly before the manuscript for this edition was completed the senior author, Dr. Ray K. Linsley, passed away after a lengthy illness. Dr. Linsley was a leader in the field of hydrology and was an authority on water-resources planning. He is the senior au thor of the widely used textbook Hydrology for Engineers. He will be missed greatly by his many friends and colleagues.

This edition has been updated to conform with changing technology. Its goal is the same as that of the first edition—to give the student an up-to-date background for the planning and design of systems to manage water resources.

World population continues to grow, placing greater pressure on available water supplies for human use, industrial production, and sanitation and for the growing of food and fiber. Floods result in property damage and loss of life and curtail the production of industrial and agricultural products. Pollution of both surface and groundwater reduces the available supply of potable water for many uses.

Efficient water management today is necessary to ensure the availability of adequate water supplies in the future. Management in this sense includes more than engineering activities. Economic, social, political, and environmental con siderations are an important part of the decision-making process. Planning in the true sense of the word is a complex process in which competing uses for water must be considered in the light of physical, economic, and environmental con straints. Water-resources engineering draws on the student’s background in science, the humanities, social studies, and design. A course in water-resources engineering should present relevant material in a unified framework, emphasizing

XIV PREFACE

why things are done along with how they are done. That is what this book was designed to do.

This edition of the book is set up in the same format as previous editions. The first five chapters deal with the subjects of hydrology, the determination of where water can be found, and how the available amounts can be estimated. Legal aspects, often critical constraints on water management, are discussed in Chapter 6. Physical works—dams, canals, pipelines, hydraulic machines, and so on, which are utilized in water management—are considered in Chapters 7 through 12. Cost effectiveness, an important consideration in planning water projects, is reviewed in Chapter 13 along with relevant principles of engineering economy applicable to water-resources planning. Specific purposes of water management with special attention to ways in which planning differs among the various purposes are presented in Chapters 14 through 20. The planning procedure for single and

multi-purpose projects is summarized in Chapter 21.

We feel that students learn best by working problems. There are many new problems in this edition, and nearly all of the problems retained from the previous edition have been revised with new data. About 40 percent of the problems are in SI metric units.

Dr. Frey berg’s expertise is in water-resources engineering, particularly in the fields of grou ndw ater an d surface-water hydrology. He was responsible for Cha pter 4 and the solutions manual, and prepared the prdblems and solutions for all chapters except Chapters 15 and 19, which were written by Dr. Tchobanoglous, who also prepared those chapters for the third edition of this book. Dr. Tchoba-noglous’s expertise is in the fields of water quality and water and wastewater treatment.

The authors wish to express their special appreciation to Professor Eugene L. Grant, who prepared Chapter 13 for the first edition. The list of persons who contributed to previous editions is long and we thank them all, including the reviewers who provided us and our publisher with many useful suggestions.

Joseph B. Franzini David L. Freyberg George Tchobanoglous

COMMENTS ON UNITS

COMMENTS ON UNITS

Those working in the field of water-resources engineering must be versed in the English system of units as well as the International System of Units (SI). Though conversion to the SI metric system is gradually taking place in the United States, the English system of units is still widely used. In contrast, the use of the SI metric system is almost universal throughout the rest of the world. In this edition most units are expressed in the English system with corresponding SI units given in parentheses.

Many abbreviations are used in the English system. The student should become familiar with them. Some of the abbreviations a re more widely used than

others. A list of abbreviations for English units is as follows:

Correct Form of Correct Form of Abbreviation

Abbreviation RepresentationRepresentation English UnitsEnglish Units

cfs

\

cubic feet per second \ ft3/sec

cfs/ft cubic feet per second per foot1 (ft3/sec)/ft

cfs/sq mi cubic feet per second per square mile (ft3/sec)/mi2

cu ft cubic feet ft3

cu yd cubic yards yd3

fps feet per second ft/sec

sped gallons per capita per day gal/capitaday

gpd/acre gallons per day per acre gal/dayac

spd gallons per day gal/day

gpd/ft2 gallons per day per square foot gal/dayft2

gpm gallons per minute gal/min

mgd million gallons per day Mgal/day

mph miles per hour mi/hr

pcf pounds per cubic feet lb/ft3

psf pounds per square foot lb/ft2

psi pounds per square inch lb/in2

sfd second-foot-day, equals one cfs

flowing for one day

ft3 day/sec

second-foot cubic foot per second ft3/sec

sq ft square foot ft2

x xvv

xvi

xvi COMMENTS ON UNITS

In this edition of the book a few of the English Units are expressed in terms In this edition of the book a few of the English Units are expressed in terms of the abbreviations cfs, gpm, and psi, for example. Often, however, the correct of the abbreviations cfs, gpm, and psi, for example. Often, however, the correct dimensional form shown in the right-hand column of the preceding list is used in dimensional form shown in the right-hand column of the preceding list is used in the literature.

CHAPTER

CHAPTER

1

INTRODUCTION

INTRODUCTION

The management and control of our water resources requires the conception, planning, and execution of designs to make use of the water or avoid damage

from too much water. For most of the twentieth century this has been viewed as the work of civil engineers. It is becoming apparent that engineering structures are not always the preferred solution. In some cases a nonstructural solution is superior. This means that more alternatives must be considered in the planning phase and may require the service of other disciplines—economies, social and political science, biology, and geology. Each problem involves a unique set of physical conditions and constraints, which can be resolved by the careful c oordina

tion of the various disciplines. 1

1.1 .1 Fields Fields of of WaterWater -Resourc-Resourc es es EngineeEngineeringring

Water is controlled and regulated to serve a wide variety of purposes. Flood mitigation, storm drainage, sewerage, and highway culvert design are applications of water-resources engineering to the control of water so that it will not cause excessive damage to property, inconvenience to the public, or loss of life. Municipal water supply, irrigation, hydroelectric-power development, and navigation im provements are examples of the utilization of water for beneficial purposes.

Pollution threatens the utility of water for municipal and irrigation uses and seriously despoils the aesthetic value of rivers—hence pollution control or water- quality management has become an important phase of water-resources engineer ing. Finally, the potential of nonstructural measures such as zoning to avoid flood

1 1

*T'

damage and the preservation of natural beauty are factors the water-resources engineer must consider. There has been a tendency toward specialization within these applications in the water-resources field, but actually the problems en countered and the solutions to these problems have much in common. Table 1.1 summarizes the problems that may be encountered within the nine main functional fields of water-resources engineering.

2

2 WATER-RESOURCES ENGINEERING

TABLE 1.1

Problems of water-resources engineering Problems of water-resources engineering

Conser Conser vation vation

Studies and facilities Studies and facilities required

required

C

Coonnttrorol l of of exex cece ss ss wwaatteer r CCoonnseserrvvaattiioon n ((qquuaannttitityy)) (quality)(quality)

Flood Flood miti miti gation gation Storm Storm drain drain age age Bridges, Bridges, culverts culverts Sewer Sewer age age Water Water supply supply Irriga Irriga tion tion Hydro Hydro power power Navi Navi gation gation Pollution Pollution control control

How much water is

needed? _— -- - - - XX XX X X XX XX How much water* can be

expected? Minimum flow* — — —— X X X X XX XX X X XX Annual yield* - - -- X X X X X X X XX X XX Flood peaks X X X X XX —— X X XX X X XX Flood volume XX XX -- - - —— —— XX Groundwater* —— XX — X X XX XX —— — XX Who may use the water? - - — - X X X X XX X X XX What kind of water is it?

Chemical —— XX —— X X X X XX — — —— XX Bacteriological — XX — X X X X XX — — XX Sediment X X XX X X X X X X X XX X X X XX What structural problems

exist? I Geology / XX XX X X X X X X X X X X XX XX Dams XX —— — — -- X X X X X X XX XX Spillways XX —— —— — X X X X X X X XX X Gates X X XX —— XX XX XX X X X X XX Sluiceways XX — — — — —— X X X X XX X X Intakes — — — — —— —— X X X X XX Channel works X X X X X X XX — — —— - r - r -- XX Levees X X XX XX Pipelines —— XX —— XX X X X X XX —— XX Canals X X XX —— X X X X X X XX Locks — — — — — — — — —— XX Pumps X X XX —— XX X X X X X X X X XX Turbines —— —— —— —— —— —— XX Purification -- XX —— X X X X XX —— XX Does project affect wild

life or nat ural beauty? XX X X X X X X X X XX XX XX XX Is the project economic? X X XX X X XX X X X X X X XX . . XX

INTRODUCTION 33 1.

1.2 2 Quality Quality oof f WaterWater

V. jome risk of oversimplification, the job of the water-resources engineer may be

-graced to a number of basic questions. Since the water-resources project is for

l&e control or use of water, the first questions naturally deal with the quantities

iSÍ

’* ater. Where utilization is proposed, the first question is usually How much

«war¿T l s needed?

This is probably the most difficult of all the design problems to

accurately because it involves social and economic aspects as well as

sacineering. On the basis of an economic analysis, a decision must also be made

OTaeerning the span of years for which the proposed project will serve.

Table 1.2 summarizes 1980 water use in the United States in relation to gross

mater supply-precipitation. In discussing water use it is important to distinguish

between diversion (withdrawal), or water taken into a system, and consumption,

mater that is evaporated or combined in a product and is no longer available for

use.

Almost all project designs depend on the answer to the question How much

+axer can be expected?

Peak rates of flow are usually the basis of design of projects

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

large pressure pipes were available to the Romans, their aqueducts were designed

as massive structures to carry water under atmospheric pressure at all times.

The first effort at organized engineering knowledge was the founding in 1760

>f ihe École des Ponts et Chaussées in Paris. As late as 1850, however, engineering

designs were based mainly on rules of thumb developed through experience and

tempered with liberal factors of safety. Since that date, utilization of theory has

increased rapidly until today a vast amount of careful computation is an integral

part of most project designs. A considerable lag seems to exist between research

and application. The answers to many professional problems are available in

laboratory records and even published papers, but they have not yet been

extensively employed by practicing engineers.

1.

1.9 9 The The Future Future oof f Water-ResouWater-Resou rces rces EngineerEngineer inging

Laymen, unfamiliar with engineering problems, often view the enormous activity

in flood mitigation, irrigation, and other phases of water-resources engineering

with the thought that opportunities for further work must be negligible. Actually

modern civilization is far more dependent on water than were the civilizations of

the past. Modern medical science together with modern sanitary engineering has

reduced death rates and increased life expectancy. Modern standards of personal

cleanliness require vastly more water than was used a century ago. The increasing

population requires expanded acreage for agriculture, much of which must come

through land drainage or irrigation. Increasing urban populations require more

attention to storm drainage, water supply, and sewerage. Industrial progress finds

increasing uses for water in process industries and for electric-power production.

The emphasis of water-resources engineering shifts more or less continuously. The

major work in this field during the early years of the United States was the

construction of canals for transport. Other modes of transportation have made

the canal bpat obsolete, but these new means of transport have introduced new

problems^ of drainage for highways, railroads, and airports.

The development of civilization has increased the importance of

water-resources engineering, and there is no prospect of a decline of activity in this field

in the foreseeable future. In fact, the increasing pressure for water is forcing the

development of marginal projects that might not have been considered only a few

years ago. If a project of marginal value is to be successful, it must be planned

with more care and thought than was required for the more obvious projects of

the past. More accurate hydrologic methods must be employed in estimating

available water. More efficient methods and better construction material must be

utilized to reduce costs so that difficult projects may become economically feasible.

The water-resources engineers of the future will find themselves deeply

involved with new technology and new concepts. Reclamation of wastewater,

weather modification, land management to improve water yield, and new water

saving techniques in all areas of water use are topics of increasing interest and

research. An expanding world population is changing ecologic patterns in many

ways, and water planning must include evaluation of ways to minimize undesirable

8

8 WATER-RESOURCES ENGINEERING

ecologic conseqences. Concern for the preservation of the natural environment ecologic conseqences. Concern for the preservation of the natural environment will be increasingly important in water planning of the future.

will be increasingly important in water planning of the future.

The conflict between preserving our ecosystem and meeting the “needs” of The conflict between preserving our ecosystem and meeting the “needs” of people for water management must certainly lead to new approaches in water people for water management must certainly lead to new approaches in water management and quite possibly to new definitions of

management and quite possibly to new definitions ofneed. _{It will not be sufficient It will not be sufficient}

to attack water problems of the future by simply copying methods of the past. to attack water problems of the future by simply copying methods of the past.

BIBLIOGRAPHY

BIBLIOGRAPHY

Biswas, Asit K.: “A History of Hydrology,” North Holland Publishing Company, Amsterdam, 1970. Chow, Ven Te (Ed.): “Handbook of Applied Hydrology,” McGraw-Hill, New York, 1964.

Kelly, D.: Estimated Use of Water in the United States, U.S. Geol. Surv. Circ. _{876, 1983.}

Langbein, W. B., and W. G. Hoy t: “ Wate r Facts for the N atio n’s Futu re, ” Ronald, New York, 195 9. Maass, Arthur, M. M. Hufschmidt, Robert Dorfman, H. A. Thomas, S. A. Marglin, and G. M. Fair:

“Design of Water-Resource Systems,” Harvard, Cambridge, Mass., 1962.

Merdinger, Charles J.: Civil Engineering through the Ages, Trans. ASCE , Vol. CT, pp. 1-27, 1953. “The Nation’s Water Resources,” U.S. Water Resources Council, Washington D.C., 1968.

Rouse, Hunter, and S. Ince: “History of Hydraulics,” Institute of Hydraulic Research, University of Iowa, Iowa City, Iowa, 1957.

van der Leeden, Frits Fred L. Troise, and David K. Todd: “The Water Encyclopedia,” 2d ed., Lewis Publishers, Boca Raton, Fla. 1989.

“Water Policies for the Future,” Report of the U.S. National Water Commission, Washington D.C.,

1973. '

White, Gilbert F.: “Strategies of American Water Management,” University of Michigan Press, Ann Arbor, Mich., 1969.

CHAPTER

CHAPTER

2

2

DESCRIPTIVE

DESCRIPTIVE

HYDROLOGY

HYDROLOGY

1

2.1

2.1 The

The Hydrolo

Hydrologic

gic Cycle

Cycle

The wor ld’s supply of fresh wate r is quite small comp ared to the enormous volumes of salt water in the oceans. Fortunately the freshwater supply is renewed by the hydrologic cycle, which is an immense solar distillation system. Water evaporated from the oceans is transported over the continents by moving air masses. When this moisture-bearing air is cooled to its dewpoint temperature, the vapor con denses into water droplets forming fog or cloud. The cooling occurs when the moist air is lifted to higher elevations. Since air pressure decreases with elevation (Table A-3), the air expands as it is lifted and cooled in accordance with the Ideal Gas Law

p V /T =

const (2.1)

Lifting occurs in three ways. Orographic lifting occurs when the air is forced up over the underlying terrane. Frontal lifting occurs when the air mass is pushed up by a cooler air mass. The boundary between the two air masses is called a frontal

surface. Finally, the moist air may be heated from below as it passes over a warmer

1 “Hy drology is the science tha t treats of the waters of the Earth, their occurrence, circulati on, and distribution, their chemical and physical properties, and their reaction with their environment, including their relation to living things.” (From “Scientific Hydrology,” U.S. Federal Council for Science and Technology, June 1962.)

9 9

10 WATER-RESOURCES ENGINEERING

FIGURE 2.1

Schematic diagram of the hydrologic cycle.

surface, causing convective lifting, which may result in a convective thunderstorm. Often two or more of these mechanisms may take place together.

About two-thirds of the precipitation that reaches the land surface is returned to the atmosphere by evaporation from water surfaces, soil, and vegetation and through plant transpiration. The remaining third of the precipitation returns ultimately to the ocean through surface or underground channels. The large percentage of precipitation that is evaporated has often led to the belief that

increasing this evaporation by construction of reservoirs or planting of trees will increase the moisture available in the atmosphere for precipitation. Actually only a small portion of the moisture (usually much less than 10 percent) that passes over any given poin t on the ea rth ’s surface is pre cipitate d.1 Hence, moisture evaporated from the land surfaces is a minor part of the total atmospheric moisture.12

The hydrologic cycle is depicted diagramfnatically in Fig. 2.1. No simple figure can do justice to the complexities of the cycle ass it occurs in nature. The science of hydrolog y is devoted to a study of the rate of exchange of water between phases of the cycle and in particular to the variations in this rate with time and

1G. S. Benton, R. T. Blackburn, and V. O. Snead, The Role of the Atmosphere in the Hydrologic Cycle, Trans. Am. Geophys. Union, Vol. 31, pp. 61-73, February 1950.

2 F. A. Huff and G. E. Stout, A Preliminary Study of Atmospheric-moistur e-precipitation Relationships over Illinois, Bull. Am. Meteorol. Soc., Vol. 32, pp. 295-297, 1951.

DESCRIPTIVE HYDROLOGY 11

place. This information provides the da ta necessary for the hydraulic design of physical works to control and utilize natural water.

2.2

2.2 The The River River BasBasinin

A river basin (catchm ent)1 is the area tri butary to a given point on a stream and is separated from adjacent basins by a divide, or ridge, that can be traced on topographic maps. All surface water srcinating in the area enclosed by the divide is discharged through the lowest point in the divide through which the main stream of the catchment passes, it is commonly assumed that the movement of ground-water conforms to the surface divides, but this assumption is not always correct, and large quantities of water may be transported from one catchment to another as groundwater.

PRECIPITATION PRECIPITATION 2.

2.3 3 Types Types of of PrecipitationPrecipitation

Precipitation includes all water that falls from the atmosphere to the earth’s surface. Precipitation occurs in a variety of forms that are of interest to the meteorologist, but the hydrologist is interested in distinguishing only between liquid precipitation (rainfall) and frozen precipitation (snow, hail, sleet, and freezing rain). Rainfall runs off to the streams soon after it reaches the ground and is the cause of most floods. Froz en precipita tion may r emain where it falls for a long time before it melts. Melting snow is rarely the cause of major floods although, in

combination with rainfall, it may contribute to major floods such as that on the upper Mississippi River in 1969. Mountain snowpacks are often important sources of water for irrigation and other purposes. The snowfields serve as vast reservoirs that store water precipitation until spring thaws release it near the time it is required for irrigation. ^

2.4

2.4 FoFo g g Drip Drip anand d DewDew

•

Fog consists of water droplets so small that their fall velocities are negligible. Fog particles that contact vegetation may adhere, coalesce with other droplets, and eventually form a drop large enough to fall to the ground. Fog drip is an important source of water for native vegetation during the rainless summers of the Pacific Coast of North America.

On clear nights the loss of heat by radiation from the soil causes cooling of the ground surface and of the air immediately above it. Condensation of the water vapor present in the air results in a deposit of dew. The small quantities of dew

1The words river basin, drainage basin, watershed , and catchment are used interchangeably. A subbasin is a tributary basin of a larger drainage basin.

1

122 WATER-RESOURCES ENGINEERING

FIGURE 2.2

Standard 8-in. nonrecording precipitation gage. ( U.S . National Weather Service)

and fog drip deposited in any day do not contribute to streamflow or groundwater. They do, however, offer a source of water that may be exploited locally. Research in Israel1 has shown that br oad-lea ved crops such as cabbage may be efficient dew collectors that can be grown in an arid region with little or no irrigation.

2.5

2.5 Precipitation Precipitation MeaMea suremsurem ent—ent—

Amount of precipitation is expressed as the depth in inches or millimeters that falls on a level surface. This may be measured as the depth of water deposited in an open, straight-sided container. The standard gage1 2 used in the United States (Fig. 2.2) consists of a funnel 8 in. (20.32 cm) in diameter discharging into a tube 2.53 in. (6.43 cm) in diameter. The area of the inner tube is 0.1 that of the funnel, and a stick graduated in inches and tenths can be used to measure precipitation to the nearest 0.01 in. (0.25 mm). Precipitation in excess of 2 in. (50 mm) overtops the inner tube and collects in the overflow can. By removing the funnel and inner

1D. Ashbel, Frequency and Distribution of Dew in Palestine, Geogr. Rev., Vol. 39, pp. 291-297, April 1949.

2 Worldwide, a variety of different types of gages are used. Practically, there is little difference in accuracy in measuring rain, but smaller gages are not suitable for snowfall.

DESCRIPTIVE HYDROLOGY 13

tube from the gage, the 8-in.-diameter overflow can may be used to collect snowfall, which is melted and measured in the inner tube. Large storage gages are used in remote areas to catch and store precipitation for periods of 30 days or more. If snowfall is expected, an initial charge of calcium chloride brine is placed in the gage to melt the snow and to prevent the freezing of the liquid in the gage. A thin film of oil is used to prevent evaporation from the gage between observations.

Wind sets up air currents around precipitation gages that usually cause the gages to catch less prec ipita tion th an they sh ould.1 The low fall velocity of snowflakes makes this effect even more marked for snowfall than for rain. The deficiency in catch may vary from 0 to 50 percent or more depending on the type of gage, wind velocity, and local terrane. The U.S. National Weather Service1 2 uses an Alter shield consisting of a series of metal slats pivoted about a circular ring near the top of the gage and joined by a chain at the bottom. The tops of the slats are about 2 in. (5 cm) above the top of the gage. The flexible construction is intended to permit wind to move the slats and minimize the accumulation of snow on the shield.

In order to determine rates of rainfall over short periods of time, recording rain gages are used. The weighing rain gage has a bucket supported by a spring or lever balance. Movement of the bucket is transmitted to a pen that traces a record of the increasing weight of the bucket and its contents on a clock-driven chart or punched paper tape. The tipping-bucket gage consists of a pair of buckets pivoted under a funnel in such a way that when one bucket receives 0.01 in.

(0.25 mm) of precipitation, it tips, discharging its contents into a reservoir and bringing the other bucket under the funnel. A recording mechanism indicates the

time of occurrence of each tip. The tipping-bucket gage is well adapted to the measurement of rainfall intensity for short periods, but the more rugged construc tion of the weighing-type gage and its ability to record snowfall as well as rain make it preferable for many purposes._{Subsequent to the development of radar in World War II it was found that} microwave ra da r (1 to 20 bm wavelength) would indicate the presence of rain3 within its scanning area. The amount of reflected energy is dependent on the raindrop size and the distance from the transmitter. Drop size is roughly correlated with rain intensity, and the image on the radar screen (isoecho map) can be interpreted as an approximate indication of rainfall intensity. A calibration may also be determined from actual rain-gage measurements in the area scanned by the radar. Radar offers a means of obtaining information on a real rainfall distribution, which would be only roughly defined by the usual network of rain gages.

1C. C. Warnick, Experiments with Windshields for Precipitation Gages, Trans. Am. Geophys. Union, Vol. 34, pp. 379-388, June 1953.

2 The U. S. Weather Bureau was changed to the National Weather Service in 1970.

3 L. J. Battan, “ Rada r O bserva tion of the A tmosp here,” University of Chicago Press, Chicago, 1973.

1

144 WATER-RESOLJRCF.S ENGINEERING

2.6

2.6 Computation Computation oof f Average Average PrecipitationPrecipitation

Large differences in precipitation are observed within short distances in mountain ous terrane or during showery precipitation in level country. The average density of rain gages in the United States is about one per 250 mi2 (700 km2), and the data so obtained represent only a scattered sample of precipitation over large areas. It is sometimes necessary to estimate the average precipitation over a given area. The simplest method of doing this is to compute the arithmetic average of the recorded precipitation values at stations in or near the area. If the precipitation is nonuniform and the stations unevenly distributed within the area, the arithmetic average may be incorrect. To overcome this error, the precipitation at each station may be weighted in proportion to the area the station is assumed to represent.

A common method of determining weighting factors is the Thiessen network (Fig. 2.3). A Thiessen network is constructed by connecting adjacent Stations on a map by straight lines and erecting perpendicular bisectors to each connecting line. The polygon formed by the perpendicular bisectors around a station encloses an area that is everywhere closer to that station than to any other station. This area is assumed to be best represented by the precipitation at the enclosed station. This is often a reasonable assumption but may not always be correct. To compute the average rainfall, the area represented by each station is expressed as a percentage of the total area. The average rainfall is the sum of the individual station amounts, each multiplied by its percentage of area. An alternative method is shown in Fig. 2.3. If the stations are uniformly distributed in the area, the Thiessen areas will be equal and the computed average rainfall will equal the arithmetic average.

The basis for the Thiessen method is the assumption that a station best represents the area that is closest to it. If precipitation is controlled by topography or results from intense convection, this assumption may not be valid. An isohyetal map (Fig. 2.4) showing contours of equal precipitation may be drawn to conform to other pertinent information in , addition to the precipitation data and thus present a more accurate picture of the rainfall distribution. Since precipitation

FIGURE 2.3 Thiessen network.

DESCRIPTIVE HYDROLOGY 1155 Isohyets Isohyets Area between Area between isohyets. isohyets. m m ii2 Average Average precipitation, precipitation, in. in. Product Product m m ii2 in .in . 3 3 .0.0 3 3 .5.5 1 1 99 3.453.45 66 66 4 4 .0.0 1 1 00 66 3.753.75 33 99 88 4 4 .5.5 1 1 00 22 4.254.25 44 33 44 5 5 .0.0 66 0 0 44 .. 77 55 22 88 55 5 5 .5.5 1 1 55 00 5.255.25 77 88 88 6 6 .0.0 8 8 44 55 .. 77 5 5 44 88 33 6 6 .5.5 4 4 77 6.206.20 22 99 11 Total Total 55 66 88 —— 27452745 FIGURE 2.4 \\ ee isohyetal map.

usually increases with elevation, the isohyets may be made to conform approx-imately with the contours of elevation.

To compute average precipitation from an isohyetal map, the areas enclosed between successive isohyets are measured and multiplied by the average

precipita-tion between the isohyets. The sum of these products divided by the total area is the average precipitation. If the isohyets are interpolated linearly between stations, the computed average precipitation will not differ appreciably from that computed with a Thiessen network.

2.7 Snow 2.7 Snow

The measurement of snowfall has been discussed in Sec. 2.5. Snow on the ground is measured in terms of its depth (in inches or centimeters). Shallow depths are measured with any convenient scale, while large depths are measured on a snow stake, a graduated post permanently installed at the desired site. Because of variations in snow density, a depth measurement is not sufficient to tell how much water is contained in the snow pack. The water equivalent , or depth of water that would result from melting a column of snow, is measured by forcing a small tube into the snow, withdrawing it, and weighing the tube to determine the weight of the snow core removed. There are a number of types of snow samplers, but the most common type is the Mt. Rose pattern with an internal diameter of 1.485 in. (3.772 cm) so th at each ounce of snow in the core represents 1 in. (25 mm) of water equivalent. The specific gravity of freshly fallen snow is usually about 0.1. Thus, its water equivalent is 0.1 in. for each inch of snow depth. The specific gravity increases with time as the snow remains on the ground and may reach a maximum of abou t 0.5 in heavy mountain snowpacks.1 The term density of snow is often