geological survey water-supply paper 2175 eusgs 467.pdfofthose needs, the geological survey has...

Measurement and Computation

of Streamflow: Volume 1.

Measurement of Stage

and Discharge

By S. E. RANTZ and others

GEOLOGICAL SURVEY WATER-SUPPLY PAPER 2175

EUSGSscience for a changing world

UNITED STATES GOVERNMENT PRINTING OFFlCE, WASHINGTON: 1982

II. Series.81-607309AACR2

UNITED STATES DEPARTMENT OF THE INTERIOR

JAMES G. WATT, Secretary

GEOLOGICAL SURVEY

Dallas L Peck, Director

First printing 1982Second printing 1983

Library of Congress Cataloging in Publieation Data

Measurement and computation of streamflow.

(Geological Survey Water-Supply Paper 2175)Includes bibliographies.Contents: v. L Measurement of stage and discharge.-v. 2. Computation of discharge. L Stream

measurements.L Rantz, S. E. (Saul Edward), 1911-

TC175.M42 551.48'3'0287

For sale by the Superintendent of Documents, U.s. Government Printing OfficeWashington, D.C. 20402

PREFACE

The science and, in part, art of stream gaging has evolved throughthe years, largely from the collective experiences and innovations ofits practitioners. The earliest truly comprehensive manual onstream-gaging procedures and equipment, based on techniques de-veloped up to that time, was published in 1943 by the GeologicalSurvey as Water-Supply Paper 888, "Stream-Gaging Procedures," byD. M. Corbett and others. That report was an instant classic; it wasreceived enthusiastically by the hydraulic-engineering professionthroughout the world and became a major training document for atleast two generations of stream gagers, hydrologists, and hydraulicengineers, both in the United States and abroad.

The need for updating Water-Supply Paper 888 has been obviousfor some time as a result of later developments in stream-gagingtechniques and equipment. Furthermore, it has been felt for sometime that the scope of that report should be expanded to cover not onlythe newer field techniques but also the office procedures required toproduce a published annual record ofstream discharge. In recognitionof those needs, the Geological Survey has sporadically produced sup-plementary reports that update the description of specific techniquesand equipment or that describe specific field or office procedures notcovered by Water-Supply Paper 888. These reports-some adminis-trative, SOme open-file, and others in the published series titled"Techniques of Water-Resources Investigations"-are fragmentaryin the sense that they deal only with selected aspects of the broadsubject of stream gaging. Similar, but less detailed, reports onselected topics have also been produced by international agencies inthe form of Technical Notes by the World Meteorological Organiza-tion and in the form of Recommendations and Standards by the In~ternational Standards Organization.

The present report evolved from that background. Its two volumesprovide a comprehensive compilation of time-tested techniques pre-sented as an up-to-date (1980) standardized manual of stream-gagingprocedures.

Acknowledgments.-The material presented in the manual isbased on the work of many hydraulic engineers and technicians, eachof whom has contributed to the current state-of-the~artin the meas-urement and computation of streamflow. Many of those who contrib-uted directly to the manual are listed in the references appended toeach chapter. However, the authors wish to acknowledge a specialgroup of colleagues who have made major contributions to this man-ual in the form of written (cited) methods and techniques, significantsuggestions for correction and revision of the manuscript and illus~

III

IV PREFACE

trations, and all-out efforts to bring about development and publica-tion of the material. These colleagues include:

Harry H. Barnes, Jr. Thomas J. BuchananRolland W. Carter Frederick A. KilpatrickGeorge F. Smoot Ernest D. CobbHoward F. Matthai Manual A. BensonAlvin F. Pendleton, Jr. Tate DalrympleHarry Hulsing Carl E. KindsvaterG. Lawrence Bodhaine Hubert J. TracyJacob Davidian James F. WilsonThe authors also wish to acknowledge with thanks the suggestions

of their many other colleagues, who are too numerous to mention.

CONTENTS

IArt,de h\!adlngs are listed in the table of contents only In the vulume of the manual In whlch they occur, but aUchapter tltle# for the two volumes are listed 'n both volUlnes A complete lndex c'''''enng both volumes of themanual appears in each volume]

VOLUME 1. MEASUREMENT OF STAGE AND DISCHARGEPage

Preface _______ __ ___ __ __ __ __ __ ____ ________ ___ __ __ __ __ __ _ _ IIIConversion factors XIV

Chapter I.-IntroductionPurpose of the manual 1Scope ofthe manuaL ~ . '"__________ 1Streamflow records __ __ ____ __ _ __ ___ _ _____ 2General stream*gaging procedures 3Selected reference __ _ __ __ __ _ __ ___ ___ __ _____ _ ___ 4

Chapter 2.-Selection of Gaging-Station SitesIntroduction ___ ____ __ ___ _______ ___ _ ____ ____ ___ ___ ___ 4Considerations in specific site selection __ 4Selected references ____ ____ __ __ _______ ___ ___ ____ ___ ___ _ __ __ __ 9

Chapter 3.-Gaging-Station ControlsTypes of control__ __ ________ ______ ___ __ ____ __ __ __ __ ____ _ 10Attributes of a satisfactory control __ 11Artificial controls .__ __ 12

Choice of an artificial control . ..___ __ 17ChOIce between weir and flume 18

Choice between critical-flow flume and supercritical-flow flume________ 20Summary_ __ ___ __ __ __ __ _ __ __ __ ___ ___ ___ ___ __ 20

Design of an artificial control ", __ ." .______________ 21Selected references .__ _____ 22

Chapter 4.-Measurement of StageGeneral __ _ ___ _ __ _____ ___ ___ ____ __ __ ___ __ ____ 22Datum of gage __ __ __ _____ __ ____ __ __ _ __ __ ___ __ __ __ 23Nonrecording stream-gaging stations 24

Staff gage . "" ____ __ ___ __ ___ __ ___ __ __ ____ ____ _ 26Wire-weight gage ..__ .-____________ 26Float-tape gage ~,___ 26Electric-tape gage __ _ __ ____ __ ___ _ ___ __ 28Chain gage ". __ __ __ 31

Recording stream-gaging stations 32

v

VI CONTENTS

Recording stream-gaging stations-continued PageMethods of sensing stage for automatic recording ~~ ~~ ~~~~~~ +~~~~~~~~~~~~~ 32

Float sensor ~~~ + ~ + ~ ~~~~ ~ ~ ~ ~ ~ + ~~ ~~ ~~ ~ ~ ~~ ~ ~ ~~~ ~~~ _~ ~~ ~ ~ ~ ~ ~ ~ ~~_ ~~~ ~ ~ ~ 32Bubble-gage sensor ___ ___ 32

Water+stage recorders ~ ~ ~~ ~~~ ~~ ~~~ ~ ~~ ~ ~ ~ ~ ~ _~~~ ~_~~ ~ ~ ~ ~ ~ ~~ ~~ ~~ ~~ + ~ ~ + ~ ~~ ~~ 34Digital recordeL_ _ 36

Graphic recorder ~ ~ _~ +~ ~~~ ~~ ~ ~~ ~~~ ~~ ~~~~~ ~ ~ ~ _ ~ ~~ ~~ ~~ ~~ ~~ ~ ~~~ ~~ ~ ~ ~ ~ _ 39Stilling wells ~ ~~ ~ ~ ~ ~ .. ~ ~ ~~~_~~_~~~~~ ___ __ ___ ___ ____ __ __ __ ____ ___ __ 41Instrument shelters _ __ ___ _ _____ __ __ ______ _ _______ 51Reference and auxiliary gages at recording gaging stations 53Telemetering s~ystems + _ _ __ _ _ __ __ _ _ _ _ _ _ _ _ _ 54

Position-motor system "' ~~ ~~ ~ ~_ ~ ~ ~ ~ 55

Impulse system ~~ ~ ~ ~ +~ ~~ ~~~~ ~~ ~~ +~ ~ ~_~ ~~ ~ ~ ~ ~ ~~ ~~ ~~~~ ++ ~ ~ ~~~ ~~ ~ +~ ~ ~ ~ 55Telemark system ~ ~~ ~ ~~ ~~ ~~ ~~~ ~~~ ~ ~ ~ ~~ ~~ ~~~ ~~~+ ~ ~~ ~ ~~~ ~~~ ~~ ~ ~ ~ _~~ ~ ~ 55Resistance system____________ . 57

Satellite data+collection system ~~"~~~~~~~~~~_.~~~~~~~~~_~~~~~~~~~_~~ 57Operation ofa recording stream-gaging station ~~~~~~~~~~~~~~~~~.~~~~~~~~~~.~ 59Factors affecting the accuracy of the stage record ~___ 63

Nonrecording gages ~~~ ~~ ~~~~ ~~~ ~ ~ +~ ~ ~~~~ ~~ ~ ~ ~ ~ ~ ~~ ~ ~ ~~ ++~~~ ~~~~~ ~ ~ ~ ~~~ ~ ~ 64Staff gage ~~~~~~ ~ ~ _~ ~~~ ~~ ~ ~ ~~ ~~ ~~~ ~ ~ ~ ~_~ ~~~~ +~~ ~~ ~ ~~ ~ ~~~ ~ +~ ~ +_~ ~ ~ ~ 64Wire~weight gage ~~ _~ ~ ~ ~~ ~ ~ ~~ ~~~ ~ +~ ~_~ ~ ~~M~~ ~ ~~. ~ ~ ~ ~ ~ ~ ~ ~~ ~ ~~~~~~~ 64Float-tape gage _ _____ _____ __ __ ________ __ ____ __ __ __ 65

Electric~tape gage ~ ~ ~ ~ ~~~~ ~ ~ ~ ~ +~~~ ~~~ ~~ ~ ~ ~ ~ _, ~~ ~~ ~ _~ ~ ~ ~~~ ~~~~ _~ ~ ~~~ ~ ~ 66Chain gage ~~~ ~~ ~~~ ~~ ~~ ~ ~ ~~ ~ + ~~ ~ ~ ~~ ~~ ~ ~ ~~~ ~ ~ ~~ ~~ ~_ ~_ ~~ ~ ~ ~~ ~~~ ~~ ~ ~ ~ ~ 67

Accuracy offioat-operated recorders ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 68Accuracy ofbubble+gage recorden;; ~~~~~ .~~~~~~~~~~ ~~~~~~~~~~~~ 71

Special pUrjXlse gages ~~~ ~ ~~~ ~ ~ ~~~_.~~~ ~ ~~ ~ _~ ~ ~ ~~ ~~~ ~~~~~~ ~ ~~~~~~~ ~~ ~ ~ ~ ~ ~ ~ ~ ~~ 74Model T recorder ~'-~_ _ ~~~~~_ ~~~~~~~~~_~~____ 74SR recorder ~~ ~~ ~ ~~~~ ~~ ~ ~ ~~~ ~~ ~~~ ~~ ~ ~ ~~ ~ ~ ~~ ~~ __ ~ ~~~~ ~~~~ ~ ~ ~ ~ '. ~ ~ ~ ~~~ ~ ~ ~ ~ __ ~ 76Cr€st~stage gage ~~~ ~ ~~ ~ ~~~~~~~ ~~~ ~ ~ ~~ ~~ ~ ~~ ~~ ~ ~ ~~ ~ +~ __ ~ ~~~ ~ ~~~ ~ ~~ ~ ~ ~ ~ ~ ~~ 77

Selected references ~~~~~~~~__ ~~~~~~~~~+~~~~~~~~~+~_~~~~~~~~ ._+~~~~~~~~~~~~~~ 78

Chapter 5_-Measurement of Dischargeby Conventional Current-Meter Method

Introduction ~~ ~ ~ ~ ~~ ~ ~ ~~ ~ ~~~~ ~ ~~~ ~~ ~ ~ ~_~ ~~ ~ +~ ~ ~ ~ ~~~ ~~ ~~ ~ ~ ~~~ ~~~ ~~ ~ ~ __~ ~~~ ~ ~ 79General description of a conventional current~metermeasurement of discharge 80Instruments and equipment ~+~~~~~~~~~~~~~~~~~~~~~~~~.~~~~~~~~_~~~~~~__ ~~~~~ 82

Current meters ~ _ _'"~_~~~~~. ~~ ~~~~~~~~ ~~~~~~ 84Verlical·axiscurrentmeters~~~~~~_ ~~~~~~~~_ 85Horir..-ontal+axis current meters _~~ ~~ __ ~_~,_~_~~ _ 88Comparison of performance of vertical-axis and horizontal~axiscurrent·

meters _~~ ~~~ ~~~ ~ ~ ~ ~~~ ~ ~ ~ ~~ ~~~_~~~+~ ~~ ~ ~~ ~~ ~~~ ~~ ~ ~~ ~ ~ ~ ~ ~~~ ~~ ~~~ ~ ~ 89Optical current meter ~~~~~~~~~~~~~~~~~~_~~~~ __ ~~~~~~~~~~~~~~~~~~~~~ 91Care of the current meter ~~~~~~+~~~~~~~~~ ~ ~~~~~~_~~~~~~~~~~~+~~~~~~ 93Rating of current meters +~~~~~~~~~~~~~~~~~~+~~~~~~~~~+~~~~~~~~~~~~ 94

Sounding equipment ~ ~ ~ _~~~~~~~~~ .._~~~~~~ ~~~~~~~~~~~~~~~ ~~ __ ~~~~~~~~ 97Wading rods ~ __ ~_ __ __ ~_~~~~~~~~~~~~~_~~~_ _~~~~~_~~_ 97Sounding weights and accessories ~~~~~~~~~~ ~ '_~~~~~~~._~~~~~~~~~_~~~~ 101Sounding reels ~ _~ ~~~~~~ ~ .__ ~~~~ ~~ ~ __ ~ ~ ~~ ~~~ ~~~ ~ ~ ~~ ~ ~ ~ ~~ ~ _~ _~~ ~ ~ ~ ~ ~ ~ ~ 102Handlines ~ ~~ ~ ~ ~ ~~~~ __ ~~ ~ ~ ~~ ~~~ ~~ ~ ~~~ ~ ~ ~~~_ + ~~ ~~~ ~ ~~ ~~ ~~ ~_~ ~ ~ ~ ~ ~~~~ 104Sonic sounder~ ~ ~ ~ ~ +~ ~ ~~ ~~ ~ ~ ~~ ~~~~ ~ ~~ ~ ~ ~ ~~ ~~ ~~~ ~ ~ ~~~~~~ ~ ~ ~ ~~~ ~ ~ ~ ~ ~ ~ ~ 108

CONTENTS Vl!

Instruments and equipment-Continued PageWidth~measuring equipment ~~~~~~~~~~_~~_~_~~~"~~~~~~~~~~~~_~ ~~~~~~ 110Equipment assemblies~_~ ~ ~~~~ ~ ~~ ~~~ ~~~ ~ ~~ ~ ~ ~ ~_ ~ ~~~ ~~~ ~ ~ ~~~ ~ ~~ ~ _~__ 110

CabJeway equipment "" _" ,.", "__ "_."____ 110Bridge equipment ,__ 117Boat equipmenL ~ __ ~ ~_______ 120Ice equipment ___ ______ __ ___ _____ ___ ____ __ __ ___ __ 124Veloci ty-azimuth-depth assembly __ ____ __ ___ _ ___ ____ _ __ 129

Miscellaneous equipment ~ 130

Measurement of velocity ~ ~~ ~ ~ ~~ ~~ ~ ~ ~ ~ ~ ~ ~ ~ ~_ ~ ~ ~~ ~_ ~ ~ ~ ~ ~ ~~~ ~ ~~~~~ __~ ~~~~ ~ ~~ 131Vertical-velocity curve method ~~~_~_~_~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~_~132Two-point method ~ ~~ ~ ~~ ~ ~~ ~~ ~ ~ ~ ~ ~~ ~ ~ ~~ ~ ~~ ~ ~~ ~ ~~ ~ _~_~__ ~_ ~~ ~ ~ ~~ ~~ ~ ~~ ~ ~~ ~ 134Six~tenths·depth method~ ~ ~ ~ ~ ~~ ~ ~ ~~ ~~~ ~~ ~~~ ~~ ~ ~~ ~ ~ ~~ ~ ~ ~~ ~ ~~~ ~~~ ~~ _~ .~.~ 134Three-point method ~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~_~_~~~~~~~~~~m~~__ 135Two-tenths-depth method __ ~~~~~~~ ~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~_ 135Subsurfac('~velocity method ~ _~_~~~~~~~~~~..~~ ~~~ ~~~~~~~~ ~~~~~~~~~~~_~ 136Surface-velocIty method ~ ~ __~~~~~~~~~~~~~~~~~~~_~ ~_ ~~ ~~~~~~~~~~~ 137Integration method ~~ ~ ~~~~ ~_ ~~ ~ ~ ~~~ ~~ ~ ~~ ~~ ~_~__ ~ ~~ ~~ ~~ ~ .. ~~ ~ ~ ~~~ ~ ~ ~ 138Five-point method~~~~ ~~~ __ ~~~~ ~~ ~~~ ~ ~~~ ~ ~~~ __ ~ _~~~ ~~ ~~~~~ ~ ~~~ ~~ ~ ~_ ~~ __ ~ 138Six-point method ... ~~~~~~~~~~_~~~~~~~_~_~ ~_ .. ~~~~~~ __ ~~~~~~~~~~~~~~~~ 138

Procedure for conventional current~metermeasurement of discharge ~~~~~~~~_~ 139Current-meter measurements by wading ~~ ~~ ~~~~~~~~~~_~_~~~~~~_~~~~ __ ~~~~143Current~meter measurements from cableways~~~~~~~~~~~~~~~~~~~~_~__ ~~~~ 146Current-meter measurements from bridges ~~~~~~~..~~~~~~~~~~~~~ __~~~~~~~ 149Current~meter measurements from ice cover ~~~~~~~~~~~~~~~~~~~~~ ~ 151Current-meter measurements from boats ~~.~~~~~~~~~~~~~~~~~~_~~~~~~~~~~155Networks of current meters ~~~~~ ~~~~~_~~~~~~~__ ~~~_~~~~ ~~~~~ ~~~~~_~_ ~__ 158

Special problems in conventional current~meter measurements_~~~~~~~~~~~_~~~ 159Measurement of deep, swift streams ~ ~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~ 159

Case A. Depth can be sounded~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~ 159Case R Depth cannot be sounded, but standard cross section is available 168Case C. Depth cannot be sounded and no standard cross section is avail~

able __ ~~~~~~~~~~~~~ '" ~_ 169Case D. Meter cannot be submerged ~~~ ~~~~~~~~~~~~~~~__ ~~~~~~~~ ~~~~ 170

Computation of mean gage height of a discharge measuremenL __~~~~~~~~~ 170Measurement procedures during rapidly changing stage ~~~~ _~ ~~~~~~~ 174

Case A. Large streams ~~~~~~ ~~~~_~~~~~~~~~~~~~~~~~~~~~~~~ ~~~ 174Case B. Small streams ~~~~~~_~_~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~175

Correction of discharge for storage during measurement ~~~~~~~~~~~~~~__ ~_ 177Summary of factors affecting the accuracy of a discharge measurement ~~~~ 179Accuracy of a discharge measurement made under average conditions 181Selected references ~ ~~~ ~~~ _~ _~ ~ ~~~~~~~ ~ ~~~~~ ~ ~ ~ ~~ _~ _~ __ ~~ ~~~ ~ ~~ ~~ ~~_~ ~~ ~ 183

Chapter 6.-Measurement of Discharge by theMoving-Boat Method

Introduction ~~ ~~ ~~ ~ ~~~~~~~~ _~ ~ ~~~ ~~~~~~~__ ~~~ ~~~~ ~ ~_~ ~~~~ __ ~~ ~ ~~ ~ ~ ~ ~ ~~~ 183Theory of the movmg-ooat method _ _ 184Equipment . . 187

Vane and angle indicator ~~~~~~~~~_~~__ ~ __ __ ~~~_~ .. ~ ~"'~ ~~_ ~_ 187

VlII CONTENTS

Equipment-Continued PageCurrent meter _~. ~ ~ ~ __ ~~_~_~~_~~ 188Rate indicator and counter ~ ~~ __"__ ~~ 190Battery charger _~ ~ ~ ~ _~ __ ~ ~ __ ~ ~ _~ _~ ~ ~ ___ __ 193Sonic sounder ~~_~__ ~~____ 193Boat ~ __ ~ ~ ~ ~.~ ~ ~ ~ _~ . ~ __ e. ~ ~. _ ~ _ ~ ~ ~ _ _ 195

Measurement procedures ~ ~ __ ~ ~ _~ ~ ~ ~ ~~ ~ __ ~ __ 197Selection and preparation of the measurement site ~ ~~_~__ ~~_~ __ 197Preparation of the equipment ~ ~ ~ __ ~ __ ~ ~ __ 198

Assembly of the equipment ~_~ __ ~ ~ ~_________ 199Selection of the instrument settings ~~_~_~ ~ 200

Function of the crew members _~_~~_~_~~_ 201Boat operator .. ~ ~ __ ~ ~ __ ~ ~ _~ _~ ~ ~ ~ __ 201Angle observer _~ ~ __ ~ ~ ~ __ ~ .. __ . _,_ ~ ~ _~ __ ~~ 202

Notekeeper ~~ ~ ~ ~ ~~ ~ ~ ~~ ~ ~ ~~ ~~ ~ ~ ~ ~~~ ~ ~ ~ ~ ~ ~~ ~~~ ~ ~ ~ ~ ~ ~ ~ ~ ~~ ~ ~ ~ ~ ~~ ~~~~~~ ~ 203Computation of the discharge measuremenL .. ~~ ~~ __ ~ 204

Computation of unadjusted discharge ~ ~. __ ~_~ ~_._~~ 204Adjustment of total width and area __ ~~., ~.,_~~ __ ~~ ~_~~_~ __ ~__ 207Adjustment of mean velocity and total discharge ~ __ ~ ~_ 208

Detemlination of vertical~velodtyadjustment factor" __ ~ ,____ 210Application of velocity adjustment to computed discharge ~ 211

Selected references ~ ~ ~ __ ~ ~ ~ _~_ . ~ ~ ~ __ ~ __ ___ 211

Chapter 7.-Measurement of Discharge by Tracer Dilution

General ~ ~_ ~ ~ _~ ~~ __ ._', ~ ~ _~ __ ~ ., ~ ~ _~ _~_Theory of tracer~dilutionmethods ~ ~ _~~"

Theory of the constant-rate-injection method ~ __ . ~~ ~ _Theory of the sudden~injectionmethod . ~ ~ ~ .,_ ,Factors affecting the accuracy of tracer~dilution methods~~~~ ~~

Turbidi ty ~ ~ ~ ~ ~ . .-. ~ _~ ~ ~ ~ _~Loss of tracer ~ ~ __ ~ _~ ~ ~ __ ~ ~ ~ _ ~ ~ _~ _Criteria for satisfactory mixing ~,,_~~ __ ~ ~.

Calibration of measurement reach ~. ~ __ ~ __ ~ . __ . _Effect of inflow or outflow between injection and sampling SitesMeasurement of discharge by fluorescent-dye dilution ~~ '

Fl Uorescent dyes ' . ~,~ ~ _~ _~ _~ ~ ~ ~ __ .FlUorometer ~ ~ .. _" . _" ~ ~ ~ ~"' _ ,,_ ~ ~ ~ __ ~ __

Description of fluorometer ~~~ __ ~__ ~ ~_~ ~~_Effect of temperature on fluorometry ~ __ ~ __ ~ ~ ~ ~_~~~_~ _Calibration characteristics of the f1urometer ~~~_ ~ ~" ~Preparation of standard dye solutions for flurometer calibration ~ _Operation of the fluorometer ~ __ ~~_~ ~ _'_~ ~~ ~ _

Dye-injection apparatus ~ ~ ~ ~ ~ __ ~ ~" ,, ~ ~Mariotte vessel ~_____ _ ~~_~~~ ~~~ ~~ ~ ~ __Floating siphon "~_ ~_ _ ~ ,~_Pressure tank ~ ~ __ ~_~~_~_~~ ~ ~~_~_~ ~_~ _

Determination of quantities of fluorescent dye for measuring discharge _Quantity of dye needed for measurement by the constant-rate~injection

method ~ _~ __ ~ ~ __ ~ ~ _~ ~ ~

Quantity of dye needed for measurement by the sudden~injectionmethod

211212213214215215216216220222223223223224226226228231232232233234235

235236

CONTENTS IX

Measurement of discharge by fluorescent-dye dilution-Continued PageProcedures for measuring discharge by the dye-dilution method 237

Field procedures ~~ __ ~~_~ __ ~_~~~~~_~~_~~~~~_~~~~~~~~~~~~ .~~~~ 237Analysis and computations ~ 240Sample computation-constant-rate-injection method 241

Simplified procedures for making numerous dye-dilution measurements ofdischarge ~~ ~ ~ .. 246

Measurement of discharge by sodium dichromate dilution .____ 249G-eneral _______ __________ ___ _ m __ _ _ _ _ 249

Principle of colorimetric analysis, __ ."._.""~~~~~~~~~~~~~~~~~~_~~~~~~~ 249Method of analysis by colorimeter _~~~_~~~~~_~~~~.. ~ .. ~~~ ~~~~~~~_~ .. _~~~ 250

Measurement of discharge by salt dilution ~~~~_~__ ~_~ __ ~_~~ _~_~~~~~ _~~_~~~~_ 250General _~~ ~ ~ ~~ _ ___ .- ~ _~~ _~ _~~ ~ ~ ~~ ~ ~~ ~~ _~ _~ _~_~ ~ ~_ ~ ~ ~~~ ~ ~~~_~~ ~_ 250Preparation and injection of the concentrated salt solution ~_~ __ ~~~~~~~_ 251Measurement of relative conductance at the sampling site ~~~_~~~~. ~ ~ 252Computation of discharge ~~~ __ ~ _~ __ ~~~~~~~~~~~~_~~ __ ~~~~_~~ 255

Measurement of discharge by dilution of radioactive tracers _~~~~_.,~~_~~_~~~~~ 256General ~~~ ~ __ ~ ~ _~~ ~ __ ~ ~ ~~ _~ ~ ~ __ ~ ~~ ~ ~ ~~ ~ ~ ~ ~~ _~ ~~~ __ -._ 256Methodology _. ~ __ ~~~ ~ ~~ ~ ~ ~ ~~ ~~ ~ __ ~~ _~ ~_ ~_ ~~ ~~ ~ __~~ ~~ ~ ~_ ~~ ~~_~ ~ ~~ __ ~ ~ 258

Selected references _~ ~ ~~~ _~ ~ ~ _~_ ~ ~ ~~ ~~ ~~ ~ ~~~ _~ ~~~ ~ _~ _~ ~~ ~_ ~~ _~ ~ ~~ ~ __ ~~ ~ ~~~ ~ 259

Chapter 8.-Measurement of Discharge by Miscellaneous Methods

General ~~~~ ~ __~_~~~ __ ~~ ~_~ ~~~_~_~_~~_~~~ __ ~~~~~ 260Floats _, ~~ ~ __ ~ ~ ~ _~ _~ ~ ~ __ ~ ~~~ _~ ~ _~ ~ _~ ~ __ ~~ ~ ~ _~ _~ ~~~~~ ~ , _" _"' _~ ~~ _ 261Volumetric measurement ~~~~~_~~~~__ ~~~~ ~~~_~ ~~~_~~_ ~_".~ .. ~~ __ ~~~~~~ 262Portable wei, plate ........................••........••.......•............ 263Portable Parshall flume __._. __ ~~~_~~~~~~~~_~~~~~~~~~~~_~~_~~~~~ 265Measurement of unstable ftow~-roll waves or slug flow ~~ __ ~~~ ~~~~~~~ __ 268

Charactenstic'i of unstable flow ~ __ ~_~~_~~ . ~~~ ~ ~ __ ~_~ __ 268Determination of discharge ~ _~ _~ ~ _~ _~_~ _~~~ _~_, ~ _~ ~~~ ~~ ~ __ ~~_ ~ __ ~ ~ __ ~ __ ~ 269Examples of discharge determination~_,_~~_~~_~__~~~~~~~~~~~~~~_~ __ ~~~~_~ 270Proposed instrumentation _~ __ ~ ~ ~ _~ ~ ~ _~~~_~ ~ ~~ ~~ ~ _~~ ~~ __ ~ ~~ _~~ ~ ~ 272

Selected references 272

Chapter g.-Indirect Determination of Peak Discharge

Introduction ~ _~ _~ ~~ ~_~ __ ~~ ~ _~_ ~~~~ ~ _~ ~ ~_ ~ _~~ ~ ~ ~~ ~~ ~ ~ ~~ ~~~ _~ _~~ _ 273Collection of field data~ _~ _~ ~~ ~ _~ ~ ~ _~ _~~ _~ ~ __ ~ _~ ~~ ~_ ~ ~_ ~ ~~ ~ ~~ ~ __ ~~_ ~ __ 274

Slope·area metbod ...................................•..............••.... 274Contracted-opening method ~_~ _~~~_~~~~~~~~_~_~~~~_~~~~ ~~~ _~~~_~~~~~~~_~ 277Flow over dams and weirs ~ ~~~~_~_~~~~~~~~~~_~~~_~~~_~~~~__ ~~~~~~ ~~~~ 279Flow through cuiverts _, ~ ~ __ ~ -. ~ ~ .. _~ __ ~ ~~ ~ ~ ~~ ~~ ~ ~~ ~~ ,. __ ~ ~ ~~~~ __ ~ _ 279

General classification of flow __ ~ _~ ~ ~ ~~ ~ ~~ ~~ ~ ~~ ~~~~ _~ ~ ~ __ ~ ~ __ ~ 281Estimating discharge from superelevation in bends ~~_~~ ~_~~~~~~~~_~_~_~_~ 281Selected references ~ _~ ~~ ~~ _~ ~ _~ ~ ~_~ __ ~~~ _~ ~ __ ~ ~ ~ ~~_~ ~~~ __ ~ _~ _~ ~ ~ ~ ~~ ~ ~~ ~ 284

Index

VOLUME 2. COMPUTATION OF DISCHARGEChapter

10 Discharge ratings using simple stage-discharge relations ~~_Page

285

x CONTENTS

11 Discharge ratings using slope as a parameter :~YO12 Discharge ratings using a velocity index as a parameter __ 42913 Discharge ratings for tidal streams n _~ _ 471

14 Discharge ratings for miscellaneous hydraulic faciHties 48615 Computation of discharge records_________________________ 54416 Presentation and publication of stream-gaging data 601

ILLUSTRATIONS

Page

29.30.31.32.33.34.

9.10.11.12.13.14.15.16.17.18.19.

20.21.22.23.24,25.26.27.28.

FIGURE 1. Gage, concrete control, outside gage on bridge, and an engineermaking a wading measurement, Kaskaskia River at Bondville,III _

2, Gage and natural control, Little Spokane River at Elk, Wash3. Rectangular weir on Trifolium Drain 23 of Imperial lITigation Dis-

trict, near Westmorland, Calif _

4. Four-foot Parshall flume discharging 62 ffj/s 0.76 m 3/s) underfree-flow conditions .. .. _

5. Flow through a 3·ft trapezoidal supercritlcal-fiow flume showingtransition from sulx:ritical to supercritical flow . _

6. Concrete artificial control on Mill Creek near Coshocton, Ohio __7. Artificial control on Delaware River near Red Bluff, N. Mex., with

shallow V notch in the broad"crested weir _8. Cover of book and weekly report card for recording manual gage

ohservations _ . _Vertical staff gage . ._Type-A wire-weight gage . .Float.tape gage _" . .. _Electnc-tape gage ,____________________ . _

Chain gageMajor units of the bubble gage ._ . .InstallatIOn of bubble orifice In unstable streambed _Digital recorder . . .Sample digital recorder tape .. _.. ,__ , _Digita I-recorder timer . _.. . _Continuous strip-chart recorder . ,Horizontal--drum recorder _Section of a typical strip chart _Reinforced.concrete well and shelter _Concrete well and wooden shelter with asphalt-shingle sidingCorrugated-galvanized-steel.pipe well and shelterConcrete-pipe well and shelter . . _Concrete-block shelter _ __ _ __

Steel-pipe well and look-i'1 shelter attached to bridge abutmentCorrugated--steel-piPf' well and wooden shelter attached to bridge

pier _ _

Flushing system for intakesStatic tube for intakes _. _

Typical bubhle-gage InstallationTelemark gage _

Resistance system transmitter HOltResistance system indicator unit "___________ _ _

913

14

15

1617

18

2527282930313335363839404142434445464748

49505052565758

i'~IGURE 35.36.

37.38.39.40.41.

42.

43.

44.45.

46.47.48.49.

50.51.52.53.5455.56.57,58.59.60.61.6263.64.

65.66.67.68.69.

70.71.72.73.74.75.76.77.78.

CONTENTS XI

Sample log sheet for bubble gage ~~_ ~~ _~~ __ ~ _~~~~~~~~~~~~_ ~~~~~ 62Diagram for determining allowable length of bubble tubing for

maximum friction error of 0,01 fL~~~~_~~ ~ _~~~~~~~_ ~ _~~~~~ 72Diagram for determining required bubble rate~~~~ ~ __ ~~ ~~~~ 73Model T recorder ~~~~~~~~~~~~_ ~~~~ ~ ~_ ~ .__ .~~~~~~~~_ ~~~~~ 75Model SR recorder ~~~_ ~~_.~~~~~~_~~~~~~~~~~~~~~~~~~~~~~_~ ~~ 76Crest-stage gage~~_~~~~~ ~~ ~ ~ ~~ _~ _~ ~~ ~ ~~ ~ ~ ~ ~~~~ ~~ ~~~ ~ _~ ~~~~ ~ ~~ ~ 78Definition sketch of midsection method of computing cross-section

area for d;scharge measurements... 81Computation notes of a current~meter measurement by the midsec~

tion method __ ~ _~ _~ ~~ ~~ ~ ~~ ~~ ~~ ~ ~~~ ~ ~ ~_~ ~ ~ ~_ ~ _~ ~~~ ~~ ~ ~~~ ~ ~~~ ~ 83Comparison of pulsations for two different mean velocities mea-

sured in a laboratory flume, 12 ft wide __ ~_~_~~_~~~_~~~_~ ~ 85Price type AA meter, top; Price pygmy meter, bottom ~~~~~~~ 86Vane ice meter, top; vane meter with cable-suspension yoke, bot-tom_~~~~~~~__ ~~_____ ___ __~ __ ~ ~_ ~ ~ __ ~~ _~~_ 87

Ott current meter ~~ __ ~ ~ ~_____ 88

Velocity components measured by Ott and Price current meters 88Hoff current meter ~~~~~~~~~~~~~~~~_._~_~~_~~~~~~~~~_~~~~_~~~ 89Comparison of mean velocities measured simultaneously by var·

IOUS current meters during 2-min periods, Stella Niagara section,Panel point 5 _~ ~~_~~_~~~~~~~_ 90

Optical current meter _~ __ ~~~ ~~~~~~~ __ ~_ ~ __ ~_ ~~~_ ~~~~_ 91Current-meter rating table ~~~~~~~~~~~~~~~ _~~~~~_~~ _~~~~~ _~_.~_ ~ 95Top-setting wading rod with meter attached ~~_.~~_,,~_~_ 98Round wading rod w;th meter attached .. .. 99Lower section of ice rod for use with vane ice meter ~~~_~_~~_ 100Lower sectIOn of ice rod for use with Price meteL ~ __ ~ __ ~_ 101Columbus 15-,30-,50·,75-, and 100-lb sounding weights~~~~~~_~ 102Sounding-weight hangers and hanger pins ~~~~~~~~~~~~.~~~~~~~_ 103Canfield reel ~ ~ ~~~ ~ ~~~ ~~ ~ ~ ~~ ~ ~ ~~ ~ ~~ ~~ ~~ ~ ~ _~ ~ ~_ ~ __ ~ ~~ ~ 104B--56 reel ~~~~~~~_~_~ ~~~~~ ~_~ ~~ __ ~~~_~_~~____ 105Computing depth indicator ~~~~~_~~~~ _~~~~_~~ __ ~_ ~ __ ~~ ~~_~_~~~~ ~ 105Handline ~_~_~~~~~~~~~~~~~~~~_~ 106Handline in use from a bridge ~~~~~~~~~~~_~~_~~~__ ~~~_ 107Handl ine reels ~~ ~~_ ~_~~ ~_~_ ~~_ ~~ ~~_~ ~_ ~~_ ~_ ~~_~~ 108Sounding weight with compass and sonic transducer ready for as-

sembly ~~_~ __ .__ ___ ~_~ __ ~__ _ ~_~~~~ 109Sonic measuring assembly ~_~~ __ ~~~~~~~~~ __ ~~~. ~__ 109Tag~line reels ~~~~~~~__ ~~ 111Sitdo 'n cable car ~~_~~~~~~~~ ~~_~_~ ~~~~~~~~~~~~~~_~~_~__ 112Standup cable car ...........•................................ JJ 2Cable-car puller for follower.brake cable cars, left; for standard

cars, right ~~_~~~~~~~~~~~~~~~~~~~~~~~ ~ 113Power-operated cable cars ~~~~__ ~ __ ~~~~_~~~~_~ __ ~~ __ ~~~_~_~~~~ 114Sitdown cable car with Canfield reel clamped to Side of car ~~~~~_ 115Portable reel seaton sitdown~type cable car ~~_~_~_~~~_~_~_~~_~. 116Carrier (bank-operated) cableway ~~_~~~~_, ~~~~~~~~~~_~~ __ ~ _ 117Type-A crane with three-\"-'heel base ~_~_~~~~~~~~~_~__ ~ __ ~_~_~~ 118Type-A crane with four-wheel base with boom in retracted position 119Bridge board in use ~~~,.~~~~__ .~~~~~~~~_~ __ ~ .. ~~_~~~~~~~~., ~ 120Truck-mounted crane used on the Mississippi River _~~~~~~~~~~~ 121Horizontal·axis boat tag·Hne reel without a brake .••........... 122

XII

FIGURE 79,80,8L82,83,84,85,86,87.88,89,

90,9L92,93,

94,

95,96,97,9R99,

100,

lOL102,

103,104,105.106,107,lOR109,110,11L112,

113,114,115,116,

117.

118,

119.

CONTENTS

Horizontal-axls boat tag-line reel with a brake ~~~~~~~~~~~~~~~~ 123Vertical~axis boat tag-line reel ~ ~_~ ~ ~_~ ~ ~_ 124Boom and crosspiece for use on boats __ ~ ~ ~~ 125Measuring equipment set up in a boat . 126Gasoline-powered ice drill . . __ 127

Collapsible reel support and ice~weight assembly 128Velocity~azimuth_depthassembly ._ 129Automatic counter (left) and headphone (right) 130Ice creepers for boots and waders . _ _ __ _ 131Typical vertical-velocity curve ~~~~~~~~~~~~~~~~~_~~ 133Relation of surface~velocity coefficient to distance from vertical

wall of a smooth rectangular channel ~~~ ~~ ~~~~~~~__ ~~~~~~ __ ~~~ 137Measurement of horizontal angle with measurement~notesheeL_ 143Measurement of horizontal angle with folding foot~rule~~ __ ~~~~~~~ 144Wading measurement using top-setting rod ~~ ~~~~~ _~ ~~ ~~~_~~ ~~_ 145Ice rod being used to support current meter for a discharge mea-

surement, top; ice drill being used to cut holes, bottom ~ ~~~~~~ ~ 152Method of computmg meter settings for measurements under ice

cover ~ ~ ~ ~~ ~~ ~ ~ ~ ~ ~~~ ~ ~ ~ ~~ ~ ~~ ~~ ~ ~ ~ ~ ~ ~~ ~~ ~~ ~~ ~ ~ ~ ~ ~~ ~~ ~ ~ ~ ~~ ~ ~~ ~ 153'fypical vertical~velocitycurve under ice cover __ ~~~~ __~~~~~~~ __ ~ 154Part of notes for discharge measurement under ice cover ~~.~~~~~ 156Determining position in the cross section, stadia method ~~~~_~~~ 157Determining position in the cross section, angular method ~~_~__ 157Position of sounding weight and line in deep, swift water~~~~~~~~ 160

Sketch ofgeometry of relation of actual to measured vertical anglewhen flow direction is not normal to measurement section ~~~ _ 166

Computation of discharge-weighted mean gage height ~~~~_ 172Discharge-measurement notes with discharge adjusted for channel

storage ~.' ~ ~~ ~~ ~ ~ ~~ ~ ~~ ~ ~ ~ ~ ,. ~~ ~~ ~ ~ ~ ~~ ~ ~~ ~ _~ ~ ~~ ~~ _~ _~~ ~ ~~ ~ ~ .. _~ 178Sketch of stream with markers _~ ~~~~~ __ ~~~~~~~~ ~_~~~~~_~~_~~_~ 185Diagram of velocity vectors ~ ~~~~~~~~~~~~~~__ ~~~~~~~~ ~.~~~~~_ 186Sketch of boat showing equipment ~~~~~ _~ __ ~~~~~~~ __ 188Component propeller~typemeter ~ __ ~~~~~~~~ .~~~~~~_~~_ 189Control panel of rate indicator and counter ~~~~~_~~~~~~~~_~~~~~ 190Sample tables of meter rating,Lb, and sine a _~_~~~~_~_~~_~~~~~ 191Sonic sounder and control panel ~~~~~_~~~~~~_~~~~~~_~~~~~~~~_ 194Boat with mounted vane assembly ~~~~~ _~~~~~~~~__ ~~~~~~~~_ 196Detailed view of vane~mountingassembly 196Definition sketch of midsection method of computation

superimposed on a facsimile of a sonic~sounder chart ~~ ~~._ ~~~_~ 205Sample

FIGURE 120.121.122.123.

124.

125.

126.

127.

128.

129.130.131.132.

133.

134.135.136.137.138.

TABLE '.2.3.

4.

5.

6.

7.

8.

9.

CONTENTS XIll

Schematic diagram of the fiuorometeL •••.••..•...•••••••.•.••• 224Mariotte vessel (constant+head device) ~_~__ ~ 232Floating siphon (constant-head device) ~ ~~ +_+ __ 234Pressurized constant-rate injection tanks for injection of dye into

streams .•...•.. .• __ • _•• ._.__•.•••..••_..••_._....... 235Sample analysis sheet used for computing discharge by the

constant+rate-injection method of dye dilution ~ 244Dye-injection rate of various stock dye concentrations related to

estimated stream discharge __ ~ ~ __ ~ ~ 248

Schematic diagram of conductance meter used in salt-clilutionmethod of measuring discharge ~ ~ ~ 253

Sample computation of discharge using salt dilution in thesudden-injection method ~ ~ ~~_~__ ~ ~ 256

Curve of relabve conductance versus time for the ionic wave in thesample problem ~ ~ ~_~~ ~_~_~~ 257

Portable weir-plate sizes ~ ~~ ~ 264

Working drawing of a modified 3-in Parshall flume ~+~ 267Modified 3+in Parshall flume installed for measuring discharge __ 268&hematic sketch oflongitudinal water-surface profile during pul+

sating flow +_+ ~~ __ ~ . ~ _~__ ~ 269

Discharge hydrograph at Santa Anita Wash above Sierra MadreWash, CaJiL, April 16, 1965, and plot of discharges computedfrom observations of flow ~_ __ _ 271

Definition sketch of a slope-area reach __ ~ __ ~ 276Definition sketch of an open-channel contractlOn 278Graphical presentation of the Bernoulli equation in culvert flow 280Classification of culvert flow ~_~." ~ 282Idealized sketch of a bend (plan view) ~ ~_~ ~ __ ~ __ 283

TABLES

Page

Range of velocities that can be measured with optIcal currentmete'••••••••• _._ ••••..••_.•__.•••_••••...••••_••••__ •__••. 92

Coefficients for standard vertical-velocity curve ~ +~++ 133Current+meter and velocity-measurement method for various

depths (wading measurement) ~ ~ ~_~~ ~_ 145

Velocity-measurement method for various meter suspensions anddepths _~ ~~ ~ +_~_ __ __ ____ ___ 148

Air-correction table, giving difference, in feet, between verticallength and slant length of sounding line above water surface forselected vertical angles " " " . 161

Air-correction table, giving difference, 10 meters, between verticallength and slant length of soundmg line above water surface forselected vertical angles ••..••_._•••__ .•••.•.•..••_._••__ ._. 162

Wet-Ilne table, giving difference, in feet, between wet-line lengthand vertical deptb for selected vertical angles ~ 164

Wet+line table, glving difference, in meters, between wet-linelength and vertical depth for selected vertical angles ++ __ +_ 165

Degrees to be added to observed vertical angles to obtain actualvertical angles when flow direction is not nonnal to measure-ment section _"_._+ 167

XIV

TABLE

CONTENTS

10. Summary table for setting the meter at O.8.depth position in deep,swift streams __ ~ ~ __ ~ 168

lL Registration errors, in percentage of stream velocity, caused byvertical motion of current melcr __ ___ _ 181

12. Spacing of verticalhline markings, in inches, on the sonichsounderchart for various combinations of chart speed and range distance 195

13. Temperature-correction coefficients for Rhodamine WT dye _ 22714. Rating table for 3-io modified Parshall flume 26615. Characteristics of types of culvert flow 281

CONVERSION FACTORS

lFacton for cclnvenlng Inch-pound to meU'1c umts are shown W four Slgmficant figures Hn_vcr, In the text themetTle equIvalent!;, where shown, are earned only to the numberof~'lgmfkantfigurescons",.1..enl with Ihe valuesfor the English unlls.]

fnch·pound

acresacre-ft (acre-feet)acre-ft/yr (acre feet per year)ft (feet)ftlhr (feet per hour)ft/s (feet per second)rels (cubic feet per second)in (inches)lb (pounds)mi (miles)mF (square miles)oz (ounces)

Multiply b.y_

4.047xIO~

1.233x 103

1.233 x 10:13

MEASUREMENT AND COMPUTATIONOF STREAMFLOW

VOLUME 1. MEASUREMENT OF STAGE ANDDISCHARGE

By S. E. RANTZ and others

CHAPTER I.-INTRODUCTION

PURPOSE OF THE MANUAL

The purpose of this manual is to provide a comprehensive descrip-tion of state-f-the-art standardized stream-gaging procedures,within the scope described below. The manual is intended for use as atraining guide and reference text, primarily for hydraulic engineersand technicians in the U.s. Geological Survey, but the manual is alsoappropriate for use by other stream~gagingpractitioners, both in theUnited States and elsewhere.

SCOPE OF THE MANUAL

The technical work involved in obtaining systematic records ofstreamflow is discussed, in two volumes, in accordance with the fol-lowing six major topics:Volume 1. Measurement of stage and discharge

a. Selection of gaging~station sitesb. Measurement of stagec. Measurement of discharge

Volume 2. Computation of discharged. Computation of the stage-discharge relatione. Computation of daily-discharge recordsf. Presentation and publication of stream-gaging data

In order to make the text as broadly usable as possible, discussionsof instrumentation and measurement are aimed at the technician,and discussions of computational procedure are aimed at the juniorengineer who has a background in basic hydraulics.

Many of the procedures for determining discharge that are dis-cussed in volume 2 require specialized instrumentation to obtain fielddata that supplement the observation of stage. The descriptions ofsuch specialized equipment and associated observational techniques

2 MEASUREMENT OF STAGE AND DISCHARGE

are given in appropriate chapters in volume 2 so that the reader mayhave unified discussions of the methodologies applicable to each typeof problem in determining discharge.

In general the authors have attempted to prepare a manual thatwill stand independently-references are given to supplementarypublished material, but the reader should find relatively few occa-sions when there is pressing need to consult those references. Thereare three notable exceptions to that statement.1. The subject of indirect determination of peak discharge (v. 1, chap,

9) is treated here only in brief because of space limitations; thesubject is treated fully in five reports in the Geological Surveyreport series, "Techniques ofWater-ResQurces Investigations." Thefive reports are named in the reference section of chapter 9.

2. Among the methods discussed in this manual for computing thedischarge of tidal streams are four mathematical techniques forevaluating the differential equations of unsteady flow (v, 2, chap.13). The four techniques are given only cursory treatment becausea detailed description of the complex mathematical techniques isconsidered to be beyond the scope of the manual.

3. The processing of streamflow records by digital computer (v. 2,chap. 15) is a subject that is given only generalized treatment here.It was not practicable to include a detailed description of each stepin the sequence of operation of an automated computing systembecause of space limitations, and also because the particulars ofeach step are somewhat in a state of flux in response to continualimprovement in storage and access procedures.

STREAMFLOW RECORDS

Streamflow serves man in many ways. It supplies water for domes-tic, commercial, and industrial use; irrigation water for crops; dilu-tion and transport for removal of wastes; energy for hydroelectricpower generation; transport channels for commerce; and a mediumfor recreation. Records of streamflow are the basic data used in devel-oping reliable surface-water supplies because the records provide in-formation on the availability of streamflow and its variability in timeand space. The records are therefore used in the planning and designof surface~waterrelated projects, and they are also used in the man-agement or operation of such projects after the projects have beenbuilt or activated,

Streamflow, when it oCCurs in excess, can create a hazard-floodscause extensive damage and hardship. Records of flood events ob-tained at gaging stations serve as the basis for the design of bridges,culverts, dams, and flood-control reservoirs, and for fiood~plain de~lineation and flood-warning systems.

1. INTRODUCTION 3

The streamflow records referred to above are primarily continuousrecords of discharge at stream-gaging stations, a gaging station beinga stream-site installation so instrumented and operated that a con-tinuous record of stage and discharge can be obtained. Networks ofstream-gaging stations are designed to meet the various demands forstreamflow information, including inventory of the total water re-source. The networks of continuous-record stations, however, areoften augmented by auxiliary networks of partial~recordstations tofill a particular need for streamflow information at relatively lowcost. For example, an auxiliary network of sites, instrumented andoperated to provide only instantaneous peak-discharge data, is oftenestablished to obtain basic information for use in regional flood-frequency studies. An auxiliary network of uninstrumented sites formeasuring low flow only is often established to provide basic data foruse in regional studies of drought and of fish and wildlife mainte-nance or enhancemenL

GENERAL STREAM-GAGING PROCEDURES

After the general location of a gaging station has been determinedfrom a consideration of the need for streamflow data, its precise loca-tion is so selected as to take advantage of the best locally availableconditions for stage and discharge measurement and for developing astable stage-discharge relation.

A continuous record of stage is obtained by installing instrumentsthat sense and record the water-surface elevation in the stream. Dis-charge measurements are initially made at various stages to definethe relation between stage and discharge. Discharge measurementsare then made at periodic intervals, usually monthly, to verify thestage-discharge relation or to define any change in the relationcaused by changes in channel geometry and (or) channel roughness.At many sites the discharge is not a unique function of stage; vari-ables other than stage must also be continuously measured to obtaina discharge record. For example, stream slope is measured by theinstallation of a downstream auxiliary stage gage at stations wherevariable backwater occurs. At other sites a continuous measure ofstream velocity at a point in the cross section is obtained and used asan additional variable in the discharge rating. The rate of change ofstage can be an important variable where flow is unsteady and chan-nel slopes are flat.

Artificial controls such as low weirs or flumes are constructed atsome stations to stabilize the stage-discharge relations in the low-flow range. These control structures are calibrated by stage and dis-charge measurements in the field.

The data obtained at the gaging station are reviewed and analyzed


by engineering personnel at the end of the water year. Dischargeratings are established, and the gage-height record is reduced tomean values for selected time periods. The mean discharge for eachday and extremes of discharge for the year are computed. The dataare then prepared for publication.

SELECTED REFERENCE

Carter, R. W., and Davidjan, Jacob, 1968, General procedure for gaging streams: U.S.Oeol. Survey Techniques Water-Resources Inv., book 3, chap. A6, p. 1-2.

CHAPTER 2.-SELECTION OF GAGING-STATIONSITES

INTRODUCTION

The general location of a gaging station is dependent on the specificpurpose of the streamflow record. If the streamflow record is neededfor the design or operation of a water project, such as a dam andreservoir, the general location of the gaging station obviously will bein the vicinity of the water project. The selection of a gaging¥stationsite becomes complicated, however, when the station is to be one of anetwork of stations whose records are required for study of the gen¥eral hydrology of a region. Such studies are used to inventory theregional water resource and formulate long-range water¥development plans. In that situation, attention to hydrologic princi-ples is required in selecting the general locations of the individualstations in the network to ensure that optimum information is ob-tained for the money spent in data collection.

A discussion of the design of gaging-station networks is beyond thescope of this manual and for the purpose of this chapter we willassume that the general location of a proposed gaging station hasbeen determined. The discussion that follows will be concerned withthe hydraulic considerations that enter into the selection of the pre-cise IDeation of the gage to obtain the best locally available conditionsfor the measurement of stage and discharge and for the developmentof a stable discharge rating.

CONSIDERATIONS IN SPECIFIC SITE SELECTION

After the general location of a gaging station has been determined, aspecific site for its installation must be selected. For example, if theoutflow from a reservoir is to be gaged to provide the streamflow dataneeded for managing reservoir releases, the general location of thegaging station will be along the stretch of stream channel betweenthe dam and the first stream confluence of significant size

2. SELECTION OF GAGING-STATION SITES 5

downstream from the dam. From the standpoint of convenience alone,the station should be estahlished close to the dam, but it should be farenough downstream from the outlet gates and spillway outlet to allowthe flow to become uniformly established across the entire width ofthe stream On the other hand the gage should not be located so fardownstream that the stage of the gaged stream may be affected by thestage of the confluent stream. Between those upstream anddownstream limits for locating the gage, the hydraulic featuresshould be investigated to obtain a site that presents the best possibleconditions for stage and discharge measurement and for developing astable stage-discharge relation. If the proposed gaging station is to beestablished for purely hydrologic purposes, unconnected with the de-sign or operation of a project, the general location for the gage will bethe stretch of channel between two large tributary or confluentstreams. The same consideration will apply in the sense that the gageshould be far enough downstream from the upper tributary so thatflow is fairly uniformly established across the entire width of stream,and far enough upstream from the lower stream confluence to avoidvariable backwater effect. Those limits often provide a reach of chan~nel of several miles whose hydraulic features must be considered inselecting a specific site for the gage installation.

The ideal gage site satisfies the following criteria:1. The general course of the stream is straight for about 300 ft (ap-

prox. 100 m) upstream and downstream from the gage site.2. The total flow is confined to one channel at all stages, and no flow

bypasses the site as subsurface flow.3. The streambed is not subject to scour and fill and is free ofaquatic

growth.4. Banks are permanent, high enough to contain floods, and are free

of brush.5. Unchanging natural controls are present in the form of a bedrock

outcrop or other stable riffle for low flow and a channel constric-tion for high flow-or a falls or cascade that is unsubmerged atall stages (chap. 3).

6. A pool is present upstream from the control at extremely lowstages to ensure a recording of stage at extremely low flow, andto avoid high velocities at the streamward end of gaging-stationintakes during periods of high flow.

7. The gage site is far enough upstream from the confluence withanother stream or from tidal effect to avoid any variable influ-ence the other stream or the tide may have on the stage at thegage site.

8. A satisfactory reach for measuring discharge at all stages is a vail-able within reasonable proximity of the gage site. (It is not nec-

6 MEASUREMENT OF STAGE AND DISGHARGE

essary that low and high flows be measured at the same streamcross section.}

9. The site is readily accessible for ease in installation and operationof the gaging station.

Rarely will an ideal site be found for a gaging station and judgmentmust be exercised in choosing between adequate sites, each of whichhas some shortcomings. Often, too, adverse conditions exist at allpossible sites for installing a needed gaging station, and a poor sitemust be accepted. For example, all streams in a given region mayhave unstable beds and banks, which result in continually changingstage-discharge relations,

The reconnaissance for a gaging site properly starts in the officewhere the general area for the gage site is examined on topographic,geologic, and other maps. Reaches having the following pertinentcharacteristics should be noted: straight alinement, exposed consoli-dated rock as opposed to alluvium, banks subject to overflow, steepbanks for confined flow, divided channels, possible variablebackwater effect from a tributary or confluent stream or from a reser-voir, and potential sites for discharge measurement by current meter.The more favorable sites will be given critical field examination; theyshould be marked on the map, access roads should be noted, and anoverall route for field reconnaissance should be selected.

In the field reconnaissance the features discussed earlier are inves-tigated. With regard to low flow, a stable well-defined low-water con-trol section is sought. In the absence of such a control, the feasibilityof building an artificial low-water control is investigated. If a site on astream with a movable bed must be accepted-for example, a sand-channel stream-it is best to locate the gage in as uniform a reach aspossible, away from obstructions in the channel, such as bridges,which tend to intensify scour and fill. (See page 377.) Possiblebackwater resulting from aquatic growth in the channel should alsobe investigated. If the gage is to be located at a canyon mouth wherethe stream leaves the mountains or foothills to flow onto an alluvialplain or fan, reconnaissance current-meter measurements of dis-charge should be made during a low-flow period to determine wherethe seepage of water into the alluvium becomes significant. The sta-tion should be located upstream from the area of water seepage inorder to gage as much of the surface flow as possible; the subsurfaceflow or underflow that results from channel seepage is not "lost" wa-ter, but is part of the total water resource.

With regard to high stages, high-water marks from major floods ofthe past are sought and local residents are questioned concerninghistoric flood heights. Such information is used by the engineer inmaking a judgment decision on the elevation at which the stage-

2. SELECTION OF GAGING-STATION SITES 7

recorder must be placed to be above any floods that are likely to occurin the future. The reoorder shelter should be so located as to be shel-tered from waterborne debris during major floods. Evidence is alsosought concerning major channel changes, including scour and depo-sition at streambanks, that occurred during notable floods of the past.That evidence, if found, gives some indication of changes that mightbe expected from major floods of the future.

The availability of adequate cross sections for current-meter meas-urement of discharge should also be investigated. Ideally, the meas-urement cross section should be of fairly uniform depth, and flow linesshould be parallel and fairly uniform in velocity throughout the crosssection. The measurement section should be in reasonable proximityto the gage to avoid the need for adjusting measured discharge forchange in storage, if the stage should change rapidly during a dis-charge measurement. However a distance of as much as 0.5 mi (ap-prox. 1 km) between gage and measuring section is acceptable if sucha distance is necessary to provide both a good stage-measurement siteand a good discharge-measurement site. Low-flow discharge meas-urements of all but the very large streams are made by wading. Forflows that cannot be safely waded, the current meter is operated froma bridge, cableway, or boat. It is most economical to use an existingbridge for that purpose, but in the absence of a bridge, or if themeasuring section at a bridge site is poor, a suitable site should beselected for constructing a cableway. If construction of a cableway isnot feasible because of excessive width of the river, high-water meas-urements will be made by boat when safe to do so. The cross sectionused for measuring high flows is rarely suitable for measuring lowflows, and wading measurements are therefore made wherevermeasuring conditions are most favorable.

Consideration should be given to the possibility of variablebackwater effect from a stream confluence or reservoir downstreamfrom the general location of the required gaging station. Withoutknowledge of stage and discharge at a potential gage site and ofconcurrent stage at the stream confluence or reservoir, the engineercan only conjecture concerning the location on the stream wherebackwater effect disappears for various combinations of dischargeand stage. A safe rule is the following: Given a choice of severalacceptable gaging sites on a stream, the gaging site selected should bethe one farthest upstream from the possible source of variablebackwater. Ifit is necessary to accept a site where variable backwateroccurs, a uniform reach for measurement of slope should be sought,along with a site for the installation of an auxiliary gage. If a gagingstation must be placed in a tide-affected reach, the unsteady flow thatmust be gaged will also require an auxiliary gage, but in addition line


power must be available to insure the synchronized recording of stageat the two gages. The availability of line power or telephone lines isalso a consideration, where needed for special instrumentation or forthe telemetering unite that are often used in flood-forecasting andflood-warning systems.

In cold regions the formation of ice always presents a problem inobtaining reliable winter records of streamflow, However in regionsthat are only moderately cold, and therefore subject to only moderateice buildup, forethought in the selection of gage sites may result instreamflow records that are free of ice effect. Gage sites that aredesirable from that standpoint are as follows:1. Below an industrial plant, such as a paper mill, steel mill, thermal

powerplant, or coal mine. llWaste" heat may warm the watersufficiently or impurities in the water may lower the freezingpoint to the extent that open-water conditions always prevail.

2. Immediately downstream from a dam with outlet gages. Becausethe density of water is maximum at a temperature of 4°C, thewater at the bottom of a reservoir is commonly at or near thattemperature in winter. 110st outlet gates are placed near thebottom of the dam, and the water released is therefore approxi-mately 4°C above freezing. It would take some time for thatwater to lose enough heat to freeze.

3. On a long fairly deep pool just upstream from a riffle. A deep poolwill be a tranquil one. Sheet ice will form readily over a stillpool, but the weather must be extremely cold to give completecover on the riffle. At the first cold snap, ice will form over thepool and act as an insulating blanket between water and air.Under ice cover the temperature of the streambed is generallyslightly above the freezing point and may, by conduction andconvection, raise the water temperature slightly above freezing,even though water enters the pool at O"C. That rise in tempera-ture will often be sufficient to prevent ice formation on the riffle.

After the many considerations discussed on the preceding pageshave been weighed, the precise sites for the recording stage gage andfor the cableway for discharge measurements (if needed) are selected.Their locations in the field are clearly marked and referenced. Themaximum stage at which the low-water control will be effectiveshould be estimated; the intakes to the stage recorder sbould be lo-cated upstream from the low-water control, a distance equal to atleast three times the depth of water on the control at that estimatedmaximum stage. If the intakes are located any closer than tbat to thecontrol, they may lie in a region where the streamlines have verticalcurvature; intake location in that region is hydraulically undesirable.

The gaging station on the Kaskaskia River at Bondville, Ill., shown

2. SELECTION OF GAGING-sTATION SITES 9

in figure 1, satisfies most of the requirements discussed in this chap·ter. Low-flow measurements are made by wading upstream from thecontrol. The bridge site provides accessibility, convenience to powerlines, and a good location for an outside reference gage, which isshown on the downstream parapet wall of tbe bridge.

Up to this point there has been no discussion of specific site locationfor crest-stage gages. Those gages provide peak-discharge data only.Where possible they should be installed upstream from road culverts,which act as high-water controls. The specific site for a gage is at adistance of one culvert width upstream from the culvert inlet In theabsence of such control structures, the crest-stage gage should beinstalled in a straight reach ofchannel that can be utilized in comput-ing peak discharge by the slope-area method (chap. 9).

SELECTED REFERENCES

Carter, R. W" and Davidian.. Jacob, 1968, General procedure for gaging streams: U.s.Geoi. Survey Techniques Water-Resources lov., book 3, chap. A6, p. 2-3.

World Meteorological Organization, 1974, Guide to hydrometeorological practices (3dcd.], WMO·no. 168, p. 3.I-3.4, 3.13--3.16.

FIGURE I.-Gage, concrete control, outside gage on bridge, and an engineer making awading measurement, Kaskaskia River at Bondville, Ill.


CHAPTER 3.-GAGING-STATION CONTROLS

TYPES OF CONTROL

The conversion of a record of stage to a record of discharge is madeby the use of a stage-discharge relation. The physical element orcombination of elements that controls the relation is known as acontrol. The major classification of controls differentiates betweensection controls and channel controls. Another classification differ-entiates between natural and artificial controls. (Artificial controlsare structures built for the specific purpose of controlling the stage-discharge relation; a highway bridge or paved floodway channel thatserves incidentally as a control is not classed as an artificial control.)A third classification differentiates between complete, partial, andcompound controls.

Section control exists when the geometry of a single cross section ashort distance downstream from the gage is such as to constrict thechannel, or when a downward break in bed slope occurs at the crosssection. The constriction may result from a local rise in thestreambed, as at a natural riffle or rock ledge outcrop, or at aconstructed weir or dam; or it may result from a local constriction inWidth, which may occur naturally or be caused by some manmadechannel encroachment, such as a bridge whose waterway opening isconsiderably narrower than the width of the natural channel. Exam-ples of a downward break in bed slope are the head of a cascade or thebrink of a fa]]s.

Channel control exists when the geometry and roughness of a longreach of channel downstream from the gaging station are the ele-ments that control the relation between stage and discharge. Thelength of channel that is effective as a control increases with dis-charge. Genera]]y speaking, the flatter the stream gradient, thelonger the reach of channel control.

A complete control is one that governs the stage-discharge relationthroughout the entire range of stage experienced at the gaging sta-tion. More commonly, however, no single control is effective for theentire range of stage, and so the result is a compound control for thegaging station. A common example of a compound control is the situ-ation where a section control is the control for low stages and channelcontrol is effective at high stages. The compound control sometimesincludes two section controls, as wen as channel control. In that situ-ation the upstream section control is effective for the very low stages,a section control farther downstream is effective for intermediatestages, and channel control is effective at the high stages.

With regard to complete controls, a section control may be acomplete control if the section control is a weir, dam, cascade, or falls

3. GAGING-STATION CONTROLS 11

ofsuch height that it does not become submerged at high discharges.A channel control may be a complete control if a section control isabsent, as in a sand channel that is free of riffles or bars, or in anartificial channel such as a concrete~lined floodway.

A partial control is a control that acts in concert with anothercontrol in governing the stage-discharge relation. That situationexists over a limited range in stage whenever a compound control ispresent. As an example consider the common situation where a· sec-tion control is the sole control for low stages and channel control issolely operative at high stages. At intermediate stages there is atransition from one control to the other, during which time sub~mergence is Hdrowning out" the section control. During that transi~tion period the two controls act in concert, each as a partial controL Ineffect we have a dam (low-flow control) being submerged by tailwater;before complete submergence (channel control), the upstream stagefor the particular discharge is dependent both on the elevation of thedam and on the tailwater elevation. Where the compound controlincludes two section controls, the degree of submergence of theupstream section control will be governed by the downstream sectioncontrol, during a limited range in stage. When that occurs, each of thesection controls is acting as a partial controL A constriction in chan-nel width, unless unusually severe, usually acts as a partial control,the upstream stage being affected also by the stage downstream fromthe constriction.

ATTRIBUTES OF A SATISFACTORY CONTROL

The two attributes of a satisfactory control are permanence (stabil-ity) and sensitivity. If the control is stable, the stage-dischargerelation will be stable. If the control is subject to change, thestage-discharge relation is likewise subject to change, and frequentdischarge measurements are required for the continual recalibrationof the stage-discharge relation. That not only increases the operatingcost of a gaging station, but results in impairment of the accuracy ofthe streamflow record.

The primary cause of changes in natural controls is the high veloc-ity associated with high discharge. Of the natural section controls, arock ledge outcrop will be unaffected by high velocities, but boulder,gravel, and sand-bar riffles are likely to shift, boulder riffles being themost resistant to movement and sand bars the least resistant. Of thenatural channel controls those with unstable bed and banks, as foundin sand-channel streams, are the most likely to change as a result ofvelocity- induced scour and deposition.

Another cause of changes in natural controls is vegetal growth. Thegrowth of aquatic vegetation on section controls increases the stage


for a given discharge, particularly in the low-flow range. Vegetalgrowth on the bed and banks of channel controls also affects thestage-discharge relation hy reducing velocity and the effectivewaterway area. In the temperate climates, accumulations of water-logged fallen leaves on section controls each autumn clog the in-terstices of alluvial riffles and raise the effective elevation of all natu-ral section controls. The first ensuing stream rise of any significanceusually clears the control of fallen leaves.

Controls, particularly those for low flow, should be sensitive; thatis, a small change in discharge should be reflected by a sign/icantchange in stage. To meet that requirement it is necessary that thewidth of flow at the control be greatly constricted at low stages. In anatural low-water control such constriction occurs if the control is ineffect notched, or if the controlling cross section roughly has a flatV-shape or a flat parabolic shape. Those shapes will ensure that thewidth of flow over the control decreases as discharge decreases. Gen-erally speaking, a low-water control is considered to be sensitive if achange of no more than 2 percent of the total discharge is representedby a change of one unit of recorded stage. For example, in the U.S.A.,stage is recorded in units of hundredths of a foot; therefore, for thelow-water control to be regarded as sensitive, a change in stage of0.01 ft (0.003 rn) should represent a change of no more than about 2percent of the total discharge.

In the interest of economy a gaging station should be locatedupstream from a suitable natural control (fig. 2). However, wherenatural conditions do not provide the stability or the sensitivityrequired, artificial controls should be considered. The artificial con-trols are all section controls; it is not feasible to pave or otherwiseimprove a long reach of channel solely for the purpose of stabilizingthe stage-discharge relation.

ARTIFICIAL CONTROLS

An artificial control is a structure built in a stream channel tostabilize and constrict the channel at a section, and thereby simplifythe procedure of obtaining accurate records of discharge. The artifi-cial controls built in natural streams are usually broad-crested weirsthat conform to the general shape and height of the streambed. (Theterm "broad-crested weir ," as used in this manual, refers to any typeof weir other than a thin-plate weir.) In canals and drains, where therange of discharge is limited, thin-plate weirs and flumes are thecontrols commonly built. Thin-plate weirs are built in those channelswhose flow is sediment free and whose banks are high enough toaccommodate the increase in stage (backwater) caused by the instal-lation of a weir (fig. 3). Flumes are largely self cleaning and can

3. GAGING·STATION CONTROLS 13

therefore be used in channels whose flow is sediment laden, but theirprincipal advantage in canals and drains is that they cause relativelylittle backwater (head loss) and can therefore be used in channelswhose banks are relatively low. Flumes are generally more costly tobuild than weirs.

Flumes may be categorized with respect to the flow regime thatprincipally controls the measured stage; that is, a flume is classed aseither a critical·flow flume or a supercritical-flow flume. The Parshallflume (fig. 4) is the type of critical-flow flume most widely used.Supercritical-flow flumes are used in streams that transport heavyloads ofsediment that include rocks that are too large to pass througha critical-flow flume without being deposited in the structure. Ofthose flumes the trapezoidal supercritical-flow flume (fig. 5) is themost widely used by the U.s. Geological Survey.

Artificial controls eliminate or alleviate many of the undesirablecharacteristics of natural section controls. Not only are they physi-cally stable, but they are not subject to the cyclic or progressivegrowth of aquatic vegetation other than algae. Algal slimes thatsometimes form on artificial controls can be removed with a wirebrush, and the controls are self cleaning with regard to fallen leaves.In moderately cold climates artificial controls are less likely to beaffected by the formation of winter ice than are natural controls. Theartificial control can, of course, be designed to attain the degree ofsensitivity required for the gaging station. In addition, an artificial

FIGURE 2,~Gage and natural control, LIttle Spokane River at Elk, Wash.


control may often provide an improved discharge-measurement sec wtion upstream from the control by straightening the original angular~ity of flow lines in that cross section.

In canals or drains, where the range of discharge is limited, artifi-cial controls are usually built to function as complete controlsthroughout the entire range in stage. In natural channels it is gen-erally impractical to build the control high enough to avoid sub-mergence at high discharges, and the broad-crested weirs that areUEually built are effective only for low, or for low and medium, dis-charges. Figures 6 and 7 illustrate broad-crested weirs of two differ-ent shapes; the crests of both have a flat upward slope from the centerof the stream to the banks, but in addition, the weir in figure 7 has ashallow V-notch in the center for greater sensitivity.

FIGURE 3.-Rectangular weir on Trifolium Drain 23 of Imperial Irrigation DistrIct,near Westmorland, Calif. (VIew looking downstream,)


The attributes desired in an artificial control include the following:1. The control should have structural stability and should be perma-

nent. The possibility of excessive seepage under and around thecontrol should be considered, and the necesssry precautionsshould be taken for the prevention of seepage by means of sheetpiling or concrete cut-off walls and adequate abutments.

2. The crest of the control should be as high as practicable to possiblyeliminate the effects of variable downstream conditions or tolimit those effects to high stages only.

3. The profile of the crest of the control should be designed so that asmall change in discharge at low stages will csuse s measurablechange in stage. If the control is intended to be effective at allstages, the profile of the crest should be designed to give astage-discharge relation (rating curve) of such shape that it canbe extrapolated to peak stages without serious error.

4. The shape of the control structure should be such that the passageof water creates no undesirable disturbances in the channelupstream or downstream from the control.

FIGURE 4,-Four·foot Parshall flume discharging 62 fills (1.76 m~/s) under free-flowconditions. Scour protection is required with this height of fall. (CQurtesy U's-Bureau of Reclamation.)


5. If the stream carries a heavy sediment load, the artificial controlshould he designed to be self-cleaning. Flumes have that attri-bute; broad-crested weirs can often be made self-cleaning by adesign modification in which the vertical upstream face of theweir is replaced by an upstream apron that slopes gently fromthe streambed to the weir crest.

Artificial controls are often built in conformance with the dimen-sions of laboratory-rated or field-rated weirs or flumes. The questionarises whether to use the precalibrated rating or to calibrate in placeeach new installation. There are two schools of thought on the sub-ject. In many countries, the precalibrated rating is accepted, and dis-charge measurements by current meter or by other means are madeonly periodically to determine if any statistically significant changesin the rating have occurred. If a change is detected, the new rating isdefined by as many discharge measurements as are deemed neces~

.-l"~_:-_>, . '.'~'lh;t~fJIA$'~;...FIGURE 5.-Flow through a 3-ft trapewidal supercritIcal,tlow flume showing transition

from subcritical to supercritIcal flow.


sary. In other countries, including the U.s.A., the position is takenthat it is seldom desirable to accept the rating curve prepared for themodel structure without checking the entire rating of the prototypestructure in the field by current-meter measurements, or by othermethods of measuring discharge. The experience in the U.s.A. andelsewhere has been that differences between model and prototypeinvariably exist, if only in approach-channel conditions, and thesedifferences are sufficient to require complete in place calibration ofthe prototype structure. In place calibration is sometimes dispensedwith where the artificial control is a standard thin-plate weir havingnegligible velocity of approach.

CHOICE Of AN ARTIFICIAL CONTROL

Cost is usually the major factor in deciding whether or not anartificial control is to be built to replace an inferior natural controLThe cost of the structure is affected most by the width of the streamand by the type or condition of bed and bank materiaL Stream widthgoverns the size of the structure, and bed and bank material governthe type of construction that must be used to minimize leakage underand around the structure.

If an artificial control is to be used, the type and shape of thestructure to be built is dependent upon channel characteristics, flowconditions, range of discharge to be gaged, sensitivity desired, andthe maximum allowable head loss (backwater).

FIGURE 6.-Concrete artificial control on Mill Creek near Coshocton, Ohio.


CHOICE BETWEEN WEIR AND FLUME

As a general rule a weir is more advantageous for use as a controlstructure than a flume for the following reasons.1. Weirs are usually cheaper to build than flumes.2. A weir can be designed to have greater sensitivity at low flows

with less sacrifice of range of discharge that can be accommo-dated, than is possible with a flume. Sensitivity is increased fora weir by notching the crest or by shaping the longitudinal pro-file of the crest in the form of a flat V or catenary. Sensitivity atlow flow for a flume is usually attained by narrowing the effec-tive width (converging sidewalls), or by using a trapezoidal crosssection (narrower width at low stages), or by doing both; thosemeasures significantly reduce the capacity of the flume.

3. The high water end of the stage-discharge relation for a weir canbe extended beyond the stages at which the weir is effective as acontrol, with more confidence than can be done for a flume. Inother words the stage-discharge relation for a flume becomescompletely uncertain when any part of the structure upstreamfrom the stage-measurement site is overtopped; the stage-discharge relation for a weir becomes completely uncertain onlywhen the bank or abutment is overtopped and flow occursaround the end(s) of the weir.

If a flume is installed with the expectation that it will be overtoj}-

-~_.~;,---

FIGURI.: 7.-Artificial control on Delaware River near Red Bluff, New Mex., with shal-low V notch in the broad-erested weir.


ped by floodflows, it is advisable to install a nonrecording stage gagein a straight reach of channel, upstream or downstream from theflume but beyond the influence of the flume structure. Simultaneousobservations of stage at the flume and in the unobstructed channelcan be used to obtain a relation between the two stage gages. Astage-discharge relation for the site of the nonrecording gage can thusbe derived from the flume rating, up to the discharge at which over·flow starts at the flume. If for some reason, such as one of thosediscussed on page 273, high-flow discharge measurements are notavailable, the stage-discharge relation for the nonrecording gage canbe extrapolated to flood stages; that extrapolation can then be usedwith the stage relation to define the overflow portion ofthe dischargerating for the flume.

However, in the following situations the use of a flume is moreadvantageous than the use of a weir.1. If a heavy sediment load is carried by the stream, a weir will trap

the sediment; accumulation of the sediment in the approachreach will alter the stage-discharge relation by changing thevelocity of approach. Flumes however are usually self cleaningby virtue of their converging sidewalls and (or) steep floor. Theytherefore are more likely to maintain an unchanging stage-discharge relation. However, if the size of the sediment is notlarge, it is often possible to make a broad-crested weir self clean-ing by including in its design an upstream apron that slopesdownward from crest. An apron slope of 1 vertical to 5 horizontalwill usually result in the transport of small sediment over theweir.

2. Weirs are usually not suitable for use in steep channels where theFroude number (VIVid) is greater than about 0.75. Best resultsare obtained with weirs where the velocity head is a very minorpart of the total head, and that is not the case in steep channelswhere the velocity head becomes excessively large. Further-more, as explained above, weirs act as sediment traps on steepstreams.

3. Flumes create less backwater for a given discharge than do weirs.Consequently it is more advantageous to use a flume if thechannel has low banks. However for most natural streams it isimpractical to design any type of structure to act solely as ametering device for the entire range of stage that may be experi-enced. In other words, for most natural streams sporadic over-bank flooding is to be expected, with or without a control struc-ture in the stream. Consequently the advantage of reducedbackwater for a flume is usually realized only where the flow canbe controlled to give some preassigned value of maximum dis-


charge. That situation occurs only in diversion canals or In nat-ural streams having a bypass floodway.

CHOICE BETWEEN CRrrICAL-FLOW FLUME ANDSUPERCR1'rICAL-FLOW FLUME

After it has been decided for a particular site that the use of a flumeis more desirable than the use of a weir, a decision must be made as towhether to use a critical-flow flume or a supercritical~ftow flume.

Both types of flume will transport debris of considerable size witb-out deposition in the structure, but if the transported rocks are exces-sively large, they may be deposited at or immediately upstream fromthe critical-depth section of a critical-flow flume. That will cause achange in the discharge rating of the flume. Therefore where thatsituation is likely to occur, a supercritical~flow flume should beselected for use.

If a critical-flow flume will pass the transported sediment load, thattype of flume should be selected for use because the discharge ratingfor a critical-flow flume is more sensitive than that for asupercritical-flow flume.

SU~I-'IMARY

The artificial controls recommended for natural streams are as fol-lows:

Light load, small-size sediment _Medium load, medium-size sedimentHeavy load, large-size sedimenL _

RITQ",mended slructurp

_~ Weir.___ ~ Parshall flume.

________ Trapezoidal supercritlcal-flowflume.

The adjectives used above-light, medium, heavy, small, andlarge-are relative. Observation of both the study site and of theoperation of control structures installed in an environment similar tothat of the study site will provide the principle basis for the selectionof the optimum type of control for use. In short, do not use a flumewhere a weir will do; if the use of a flume is indicated, do not use asupercritical-flow flume where a critical-flow flume (Parshall flume)will do.

The above recommendations also apply to canals or natural chan-nels whose flow is controlled to limit the maximum discharge to somepreassigned value. However, there is one exception; if the banks havelittle freeboard at the maximum discharge before the installation ofan artificial control, a flume installation is generally preferable to aweir, in order to minimize the backwater caused by the structure.


DESIGN OF AN ARTIFICIAL CONTROL

Having decided on the type of artifical control to be used-weir,Parshall flume, or trapezoidal supercritical-flow flume-the next stepis to design the structure. A standard design will usually be used,although channel conditions may make it necessary to make minormodification of the standard dimensions of the structure selected. Thefour factors-channel characteristics, range of discharge to be gaged,sensitivity desired, and maximum allowable head loss(backwater)-must be considered simultaneously in the precise de-termination of the shape, dimensions, and crest elevation of the con~trol structure, However, two preliminary steps are necessary.

First, the head-discharge relations for various artificial controls ofstandard shape and of the type selected are assembled. Several suchrelations are to be found in hydraulics handbooks (King and Brater,1963; World Meterological Organization, 1971). In addition, head-discharge relations for artificial controls that were field calibratedwill usually be available in the files of water-resources agencies inthe area-some are given in chapter 10, The head-discharge relationsthat are assembled need only be approximately correct, becausechannel conditions at the site of the proposed control will seldommatch those for the model control.

The second preliminary step is to determine an approximatestage-discharge relation for the anticipated range in stage in theunobstructed channel at the site of the proposed control. That may bedone by the use of an open-channel discharge equation, such as theManning equation, in which uniform flow is assumed for the site anda value of the roughness coefficient is estimated. The reliability of thecomputed stage-discharge relation will be improved if one or moredischarge measurements are made to verify the value of the rough-neSs coefficient used in the computations. The purpose of the compu-tations is to determine the tailwater elevation that is applicable toany given discharge after an artificial control is installed.

The next step is to consider the lower discharges that will be gaged.The tailwater elevations corresponding to those discharges are usedto determine the minimum crest elevation permissible for the pro-posed artificial control at the lower discharges, under conditions offree flow or allowable percentage of submergence. If a flume is to beinstalled, the throat section should be narrow enough to ensure sen-sitivity at low discharges_ If a weir is to be installed and the stream isof such width that a horizontal crest will be insensitive at low dis-charges, the use of a flat V crest is recommended-for example, onewhose sides have a slope of 1 vertical to 10 horizontaL The sensitivitydesired is usually such that the discharge changes no more than 2percent for each change in stage of 0.01 ft (0.003 m). For a weir or


critical-flow flume (Parshall flume) it is also desirable that theminimum discharge to be gaged have a head of at least 0.2 ft (0.06 m)to eliminate the effects of surface tension and viscosity.

Stage-discharge relations for the several controls under considera-tion are next prepared for the anticipated range in discharge. A struc-ture is then selected that best meets the demands of the site in actingas a control for as much of the range as possible, without exceedingthe maximum allowable backwater (head loss) at the higher stagesand with minor submergence effect and acceptable sensitivity atlower stages. In other words, a high crest elevation minimizes sub-mergence but maximizes backwater effect which may cause or aggra-vate flooding; a low crest elevation maximizes submergence butminimizes backwater effect; and where flumes are concerned, theattainment of the desired range of discharge may require some sac-rifice of sensitivity at extremely low discharges. The engineer mustuse judgment in selecting a control design that is optimum for thelocal conditions. For example, to design a flume whose throat is suffi-ciently wide to accommodate the desired range ofdischarge, it may benecessary to relax the 2-percent sensitivity criterion at extremely lowflows~ in that situation the engineer may accept a sensitivity suchthat a change in stage of 0.01 ft (0.003 m) results in as much as a5-percent change in discharge.

From a practical vie'wpoint the use of artificial controls is limited tostreams with stable channels. Artificial controls seldom operate satis~factorily in sand channels having highly mobile beds. The transportof sediment as bedload continually changes the characteristics of theapproach channel, and the control itself may be buried or partiallyburied by the movement of sand dunes.

SELECTED REFERENCES

Carter, R. W., and Davidian, Jacob, 1968, General procedure for gaging streams: U.S.GooI. Survey Techniques Water-Resources lnv., book 3, chap. AG, p. 3-5.

Corbett, D. M., and others, 1943, Stream~gaging procedure: U.S. Geoi. Survey Water-Supply Paper 888, p. 109~ 115.

King, H. W., and Brater, E. F., 1963, Handbook ofhydrauJics [5th cd.]: New York,McGraw~Hill, 1373 p.

World Meteorological Organization, 1971, Use of weirs and flumes in stream gaging:WMO·No. 280 Technical Note No. 117, Geneva, 57 p.

CHAPTER 4.-MEASUREMENT OF STAGE

GENERAL

The stage of a stream or lake is the height of the water surfaceabove an established datum plane. The water-surface elevation re-

4. STAGE MEASUREMENT 23

ferred to some arbitrary or predetermined gage datum is called tbe"gage height." Gage height is often used interchangeably witb themore general term I'stage," although gage height is more appropriatewhen used to indicate a reading on a gage. Stage or gage beight isusually expressed in feet and hundredths of a foot, or in meters andbundredths or thousandths of a meter.

Records of gage heigbt are used with a stage-discharge relation incomputing records of stream discharge. The reliability of the dis-charge record is therefore dependent on the reliability of the gage-height record as well as on the accuracy of the stage-discharge rela-tion. Records of stream stage are also useful in themselves for suchpurposes as the design of structures affected by stream elevation andthe planning of flood-plain use. The gage-height record of a lake orreservoir provides, in addition to elevations, indexes of surface areaand volume of the water body.

A record of stage may be obtained by systematic observations of anonrecording gage or by means of a water-stage recorder. Special-purpose gages that do not give a complete record of stage are dis-cussed on pages 74-78. The advantages of the nonrecording gage arethe low initial cost and the ease of installation, The disadvantages arethe need for an observer and the lack of accuracy of the estimatedcontinuous-stage graph drawn through the plotted points of observedstage. For long-term operation the advantages of the recording gagefar outweigh those of the nonrecording gage, and therefore the use ofthe nonrecording gage as a base gage is not recommended. However,at a recording-gage station, one or more nonrecording gages should hemaintained as auxiliary gages for the operation of the station (p.53-54). Telemetering systems are often used to transmit gage-beigbtinformation to points distant from the gaging station (p. 54-59).

DATUM OF GAGE

The datum of tbe gage may be a recognized datum, such as meansea level, or an arbitrary datum plane chosen for convenience. Anarbitrary datum plane is selected for the convenience of using rela-tively low numbers for gage heights. To eliminate the possibility ofminus values of gage height, the datum selected for operating pur-poses is below the elevation of zero flow on a natural controL Wherean artificial control is used, the gage datum is usually set at theelevation of zero flow.

As a general rule a permanent datum should be maintained so thatonly one datum for the gage-height record is used for the life of thestation. An exception occurs at gage sites where excessive streambedscour, after installation of the station, results in low-flow stages hav-ing a negative gage height. In that situation a change in gage datum


to eliminate the negative numbers is recommended, to avoid possibleconfusion involving the algebraic sign of the gage heights. Anotherexception occurs when channel changes at a station make it impracti-cal to maintain the station at the existing site. It may then be neces-sary to move the station a distance, such that there is significant fallin the water~surfaceelevation between the old and new sites, eventhough discharges at the two sites are equivalent. In that situationthere is generally little to be gained by establishing the datum at thenew site at the same sea-level elevation as the datum at the originalsite. That is especially true if the station is moved downstream adistance such that negative gage heights would result from use of theoriginal datum. When datum changes are made for whatever reason,a record of the change should be a part of the published station de-scription.

In any event, when a station is established, it should be assumedthat a permanent datum will be maintained at the site. To maintain apermanent datum, each gaging station requires at least two or threereference marks; that is, permanent points of known gage-height ele~vation that are independent of the gage structure. The datum at eachgaging station is periodically checked by running levels from thereference marks to the gages at the station.

If an arbitrary datum plane is used, it is desirable that it be re-ferred by levels to a bench mark of known elevation above mean sealevel, s~ that the arbitrary datum may be recovered if the gage andreference marks are destroyed.

NONRECORDING STREAM·GAGING STATIONS

On page 23 it was mentioned that a record of stage could be ob-tained by systematic observation of a nonrecorcling gage. The advan~tages (low initial cost) and disadvantages (need for an observer, lackof accuracy) of a nonrecording gaging station were briefly discussed.On pages 53-54 the use of nonrecording gages as auxiliary and asreference (base) gages at recording-gaging stations will be discussed,Chapter 15 (p. 559-560) will describe the manner in which an ob-server's gage readings are used to compute a record of stage at anonrecording gaging station. This section of the manual describes thevarious types of nonrecording gages.

Of the five types to be described, the two most generally used atnonrecording stations are either the staff or wire~weight gage. Atsuch stations the nonrecording gage is usually read twice daily by anobserver, and additional readings are made during periods of rapidlychanging stage. The observer systematically records and reports hisreadings to headquarters. The record book and report cards shown in

4, STAGE MEASUREMENT 25

figure 8 are used by the U.S. Geological Survey, the book being thepermanent record of nonrecording-gage readings. The weekly reportcard serves as an interim report of the observations made. The weeklyreport cards are read promptly on their arrival at the headquartersoffice so that any problems or difficulties that arise can be handledwithout delay.

On each routine visit to a l1onrecording stream-gaging station, thehydrographer also visits the observer to enter in the stage-recordbook the gage reading(s) that the hydrographer has made. At thattime he also inspects the record book to check for discrepancies in theobserver's readings. Such visits by the hydrographer are important; ifthey are not made the observer tends to feel that no one pays anyattention to his work, and he may become less conscientious about hisgage re