BASIC HYDRAULIC PRINCIPLES OF OPEN-CHANNEL FLOW

[Pages:25]BASIC HYDRAULIC PRINCIPLES OF OPEN-CHANNEL FLOW

By Harvey E. Jobson and David C. Froehlich

U.S. GEOLOGICAL SURVEY Open-File Report 88-707

Reston, Virginia 1988

DEPARTMENT OF THE INTERIOR DONALD PAUL HODEL, Secretary

U.S. GEOLOGICAL SURVEY Dallas L. Peck, Director

For additional information write to:

Chief, Office of Surface Water U.S. Geological Survey 415 National Center 12201 Sunrise Valley Drive Reston, Virginia 22092

Copies of this report can be purchased from:

U.S. Geological Survey, Books and Open-File Reports Box 25425, Federal Center Denver, Colorado 80225

CONTENTS

Page

Abstract ................................................................ 1 Introduction ............................................................ 1 Part I - Basic principles of hydraulics for an ideal fluid .............. 2

Lesson 1 - Fluid properties ....................................... 2 Lesson 2 - Forces on submerged objects ............................ 8 Lesson 3 - Similitude and dimensional analysis .................... 17 Lesson 4 - The energy equation for an ideal fluid ................. 27 Part II - Steady uniform flow of real fluids in open channels ........... 34 Lesson 5 - Velocity profiles ...................................... 34 Lesson 6 - The energy equation applied to real fluids ............. 43 Lesson 7 - Flow resistance ........................................ 51 Lesson 8 - Computations for steady, uniform flow .................. 58 Part III - Advanced principles of steady flow ........................... 61 Lesson 9 - Flow in channel sections with variable roughness ....... 61 Lesson 10 - The momentum principle ................................ 66 Lesson 11 - Specific energy ....................................... 71 Part IV - Gradually varied flow in open channels ........................ 79 Lesson 12 - Determination of flow resistance in open channels ..... 79 Lesson 13 - Classification of water-surface profiles .............. 97 Lesson 14* - Local energy losses in natural channels ............... 104 Lesson 15 - Water-surface profile computations .................... 107 Part V - Discharge computations for rapidly varied flow ................. 116 Lesson 16 - Rapidly varied flow at constrictions .................. 116 Lesson 17 - Flow through culverts ................................. 124 Lesson 18 - Flow over weirs ....................................... 132 References .............................................................. 145 Answers to problems ..................................................... 147

ILLUSTRATIONS

Page

Figure 1-1. Free body diagram of fluid element ......................... 3 2-1. Definition sketch of the pressure prism for vertical rectangular gate ......................................... 8 2-2. Definition sketch of the pressure prism for a submerged gate 4 feet wide ......................................... 9 2-3. Definition sketch of the pressure prism to compute the total force of the water on a 4-foot wide gate ........... 10 3-1. Diagram illustrating flow through constriction, model, and prototype ................................................ 17 4-1. Definition sketch of steady flow through a streamtube ...... 28 4-2. Definition sketch for the flow of an ideal fluid in an open channel .................................................. 28 4-3. Definition sketch for the example problem of flow down a ramp ..................................................... 30 5-1. Definition sketch for sheet flow over a wide inclined plane of width W ............................................... 35 5-2. Diagram illustrating velocity distribution in a fully developed, rough turbulent flow .......................... 38

111

ILLUSTRATIONS

Page

Figure 6-1. 6-2. 7-1. 9-1.

10-1.

11-1. 11-2. 11-3. 12-1. 12-2.

12-3. 13-1. 13-2. 14-1. 15-1. 15-2. 16-1. 16-2. 16-3. 17-1. 17-2. 17-3.

18-1. 18-2. 18-3.

18-4. 18-5. 18-6.

Definition sketch of the energy diagram for open-channel flow ..................................................... 43

Diagram illustrating typical flow through a bridge constriction ............................................. 45

Diagram illustrating flow in a prismatic open channel ...... 51 Definition sketch for cross section with overbank flow ..... 61 Diagram illustrating the control volume for applying the

conservation of momentum equation for the flow past a pier ..................................................... 67 Definition sketch for the specific energy curve ............ 71 Diagram illustrating flow changes from subcritical to supercritical ............................................ 73 Diagram illustrating critical flow at the outlet of a box culvert .................................................. 74 Diagram illustrating eight types of roughness found in sand-bed channels ........................................ 93 Diagram illustrating the effect of size of bed material and Froude number on the bed form and Manning's n for a range of flow conditions with sands of 0.28- and 0.45-millimeter median diameter in an 8-foot wide flume .................................................... 94 Graph showing the relation of stream power and median grain size to the bed form ............................... 95 Sketch illustrating gradually varied flow in an open channel longitudinal scale greatly reduced .............. 97 Definition sketch of surface profiles of varied flow ....... 99 Sketch illustrating the energy grade lines at a local obstruction .............................................. 104 Definition sketch for water-surface profile computation .... 108 Graph showing the variation of friction slope with distance along the channel ............................... 109 Sketch illustrating the theoretical water-surface and energy profiles through a contracted opening ............. 116 Sketch illustrating the streamlines for flow through a sharp-edged opening ...................................... 119 Graph showing the coefficients for type 1 opening, vertical embankments, and vertical abutments ............. 121 Definition sketch of culvert flow .......................... 124 Diagram illustrating the classification of culvert flow .... 125 Graph showing the base coefficient of discharge for types I, 2, and 3 flow in box culverts with square entrance mounted flush in vertical headwall .............. 127 Definition sketch for a contracted, sharp-crested weir ..... 132 Definition sketch for a broad-crested weir ................. 132 Graph showing the coefficients of discharge for full width, broad-crested weirs with downstream slope = 1:1, and various upstream slopes .............................. 134 Definition sketch of a V-notch (triangular) sharp-crested weir ..................................................... 137 Definition sketch of a Cippoletti (trapezoidal) sharp-crested weir ....................................... 139 Diagram illustrating various other sharp-crested weir profiles ................................................. 139

IV

TABLES

Page

Table 1-1. 6-1.

12-1. 12-2. 12-3. 15-1. 15-2. 17-1.

Mechanical properties of some fluids ........................ 4 Kinetic energy correction coefficients for natural

channels .................................................. 44 Values of the Manning resistance coefficient ................ 86 Equations for resistance based on bed-material size ......... 91 Sediment grade scale ........................................ 92 Criteria used to select friction slope equation ............. 110 Computation sheet for backwater analysis .................... 112 Discharge coefficients for box or pipe culverts set

flush in a vertical headwall; types 4 and 6 flow .......... 129

UNIT CONVERSION

Data listed in this report are defined in the inch-pound system of units. A list of these units and the factors for their conversion to International System of units (SI) are provided below.

Abbreviations of units are defined in the conversion table below or where they first appear in the text. Symbols are defined where they first appear in the text.

Multil inch-ound unit

To obtain SI unit

foot (ft) inch (in) pound (Ib) slug

slug per cubic foot

(slug/ft 3 )

0.3048 0.02540 4.448 14.59

515.4

slug per foot second (slug/s ft)

pound per cubic foot (lb/ft 3 ) pound per square foot (lb/ft 2 ) square foot per second (ft 2 /s) pound per square inch (lb/in 2 ) pound per cubic inch (lb/in 3 ) pound per inch (Ib/in) pound second per square foot

(Ib s/ft 2 ) foot per square second (ft/s 2 ) cubic foot per second (ft 3 /s) degree Fahrenheit (?F)

47.88

157.1 47.88 0.09290

6,895 271,400

175.1 47.88

0.3048 0.02832 3C = ................
................

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