Small Hydraulic Strucutres

A113N AND DRAINAGE PAPER

FA0 IRRIGATION AND DRAINAGE PAPER

small hydraulic structures

by

d. b. kraatz

hydraulic engineer

and

i. k. mahajan

secretary, icid

prepared with the support of the international commission on irrigation and drainage

FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS Rome 1975

First printing 1975 Second printing 1982

The designations employed and the presentation of material in this publication do not imply the expression of any opinion whatsoever on the part of the Food and Agri~culture Organization of the United Nations concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries.

M-56 ISBN 92-5-100161 -8

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic. mechanical, photocopying or otherwise, without the prior permission of the copyright owner. Applications for such permission, with a statement of the purpose and extent of the reproduction, should be addressed to the Director, Publications Division. Food and Agriculture Organization of the United Nations. Via delle Terme di Caracalla. 00100 Rom'e, Italy.

TABLE O F CONTENTS

VOLUME I1

Page

PREFACE

6. WATER LEVEL AND VELOCITY CONTROL STRUCTURES

6. 1 Introduction 1 6.2 General Feat!ures of Checks o r Cross Regulators 1 6. 3 Checks with Fixed Overfall Cres t without Movable Controls 4

6r3.1 General 4 6. 3.2 Hydraulic design 8 6. 3. 3 Design example (diagonal weir) 16 6. 3.4 Design examples (duckbill weir) 16 6. 3.5 Check- slab structure (Mexico) 16

6.4 Checks Regulated by Stop Planks (Drop Bars ) o r Flash Boards 22

6.4.1 General 6.4.2 Drop-bar check s t ructure (Victoria, Australia)

6.5 Checks Equipped with Hand Operated Gates

6.5.1 General 6.5.2 Standard check (USBR) 6.5. 3 Check s t ructure made of sheet metal 6.5.4 Wooden checks 6.5.5 Por table checks 6.5.6 Radial gate check 6.5.7 The Romijn gate

6 .6 Hydraulically Automated Checks (Neyrpic)

6. 6 . 1 General structure and application 6. 6 .2 Range of standard gates available

6. 7 Semi Automatic Time Controlled Check 6 .8 Check Structures Combined with a Fal l , Drop o r Chute 6.9 General Features of Drops (or Fal ls ) and Chutes

Table of Contents Contld.

P a g e

6. 10 Ver t ica l Drops ( o r F a l l s )

6.10.1 Genera l 6. 10.2 Sarda-type fall (India) 6. 10. 3 Rectangular weir d rop with r a i s e d c r e s t 6 .10 .4 Ver t ica l check-drop (USBR) 6 .10 .5 YMGT-type d rop (Japan)

6. 11 Inclined Drops and Chutes

6 .11 .1 Genera l 6.11.2 Standing wave f lume fal l (India) 6. 11. 3 F l u m e type fall (CDO, Punjab, India) 6. 11 .4 USBR rec tangular inclined d r o p 6 .11 .5 Rubble cascade inclined d r o p

6.12 P iped Drops

6.12.1 Genera l 6. 12.2 Well d r o p regulator (U. S. S. R. ) 6. 12.3 Well type d rop (India) 6. 12.4 P i p e d r o p (India) 6.12.5 I n c l i n e d p i p e d r o p (U .S .A . ) 6.12. 6 Inclined pipe d r o p (U. S. S. R. )

6.13 F a r m Drop S t ruc tu re s

6 .13 .1 Genera l 6. 13. 2 Head wall d rop with g rave l basin 6 .13 .3 Cement block check and d r o p 6 .13 .4 Concre te check d rop 6 .13 .5 Wooden d r o p 6 .13 .6 P iped d rops 6. 13. 7 Sloping rock drop

7 . STRUCTURES AND DEVICES FOR WATER MEASUREMENT

Introduction Sha rp Cres t ed Measuring Wei r s The Romijn Broad Cres t ed Weir The Par shal l F l u m e The Standing Wave Measur ing F lume The Cut- throat F lume The Concrete (Cas t - in-Place) Trapezoidal Measur ing F lume Use of Culver t s a s Measuring Devices P r o p e l l e r M e t e r s Deflection M e t e r s The Dethridge Mete r The Constant Head Orif ice Turnout Calibrat ion of Measuring S t ruc tu re s

LIST O F REFERENCES

NOTATIONS AND SYMBOLS

Table of Contents Cont'd.

Page

LIST OF FIGURES

Page

Figure

6-1. - Flow through check structures: (a) f ree overflow; (b) submerged orifice flow

6-2(a) and (b). - Duckbill we i r s on distribution canals (Spain).

6-3. - Small duckbill weir installed in a concrete flume distribution system (Kiti Dam Project , Cyprus).

6-4(a). - Double duckbill weir for 480 l / s discharge capacity

6-4 (b). - Duckbill weir for 160 l/ s discharge capacity.

6-5. - Diagram of flow over diagonal, duckbill o r Z-type weirs.

6-6. - Determination of coefficient Im' for angles of ot grea te r than 45O.

.6-7. - Graph for determination of discharge over diagonal, duckbill, o r Z-type weirs (84)

6-8. - Determination of f rom known B and S and of (84).

6-9. - Standard diagonal check weir for capacities up to 500 l / s (84).

6- 10. - Duckbill weir , (Italy).

6- 11. - Standard duckbill weir design type "Giraudet" for capacities up to 1000 11s.

6-12. - Duckbill weir for 260 to 280 11s capacity (Spain).

6-13. - Check slabs in a channel stretch with steep grade. (State of St. Luis Potosi , Mexico).

6- 14. - Determination of spacing of check slabs

6-15. - Data for design of check slab s t ructures

6-16. - Stop plank grooves (54).

6-17. - Concrete check structure for average soil conditions (13).

6-18. - Small concrete check (33).

6- 19. - Ordinary flashboard check.

6-20. - Typical drop bar check structure (52).

6-21. - Drop b a r structure (Australia).

6-22. - Hand operated check gate ( F e r r a r a , Italy).

6.23. - Concrete check. (u. S. A. )

6-24. - Check structure made of sheet metal - dimensions

List of F igures Cont'd.

Figure

6-25. - Check constructed f rom prefabricated steel pa r t s (75).

6-26. - Single wall check' with side walls only for protection of banks (65).

6-27. - Double wall check (74)

6-28. - Portable check for f a r m ditches (46).

6-29. - Portable canvas check, sleeve type (13).

6- 30 (a). - Radial check gate (The Netherlands).

6-30(b). - Downstream view of radial gate check (The Netherlands).

6- 31. - Typical medium size upstream constant level gate, (NEYRPIC - AMIL).

Page

3 8

38

39

4 0

4 1

42

4 3

6-32. - Typical downstream constant level gate (NEYRPIC - AVIS). 4 5

6-33. - Typical downstream constant level gate for discharge through an orifice (NEYRPIC - AVIO). 4 6

6-34. - Diagrammatic layout of AMIL gate. 47

6-35 (a). - Diagrammatic layout of AVIS gate f rom 561106 to 901190. 49

6- 35 (b), - Diagrammatic layout of AVIO gate. 5 0

6-36 (a). - Basic draw- str ing check fitted with wing walls and bottom cut-off for use in an unlined ditch. 5 1

6- 36 (b). - Semi automatic check installed in an unlined ditch. 5 1

6-37. - Sarda type fall (U. P. ) 5 7

6-38. - Rectangular weir drop with ra ised c res t . 6 2

6- 39. - Rectangular weir drop - relationship between H(crt), discharge per met re width of c r e s t and coefficients 0.32, 0.36 and 0.40. 64

6-40. - Concrete vert ical check with 1 .5 f t drop. 67

6-41. - Concrete vert ical check with 3 .0 ft drop 6 8

6-42. - Drop structure in small flume channel (Cyprus). 7 4

6-43. - YMGT type drop - s i l l w a l l and stilling basin. 7 5

6-44. - YMGT type drop - symbols and notations for sill height, t rajectory of jet and dimensions of stilling basin 7 7

6-45. - YMGT fall - type 300. 8 3

6-46. - Details of a standing wave flume fall. 8 7

6-47. - Height of hump required to give proportionality for variation in discharge. 9 2

6-48. - Height of hump to attain bulk proportionality. 94

6.49. - Details of deflectors. 9 8

L i s t of F i g u r e s Cont'd.

' F i g u r e

6-50. - Standing wave f lume fal l .

P a g e

9 9

6-51. - Sketch of a f lume type fal l with a d rop of up to 0.90 m . 102

CDO type fal l (Punjab), hydraulic d r o p up to 1.00 m.

CDO type fal l (Punjab), hydraulic d r o p m o r e than 1 .OO m .

Rectangular inclined d rop (U. S. A. )

Design of USBR inclined drop.

Rubble cascade type fal l (India).

P i p e and s t ruc tu re s .

Well d rop regula tor (for c r o s s sect ions s e e F i g u r e 6-59).

Deta i l s of well d rop regulator ( a s shown in F igu re 6-58).

P i p e d rop spillway.

Well type d r o p

6-62. - Pipe d r o p without concre te outlet t rans i t ions .

6- 63. - P i p e d rop with concre te outlet t ransi t ion.

6- 64. - Check and pipe inlet. 149

6- 65. - Concre te outlet t rans i t ion (supplement to F igu re 6- 63). 15 1

6- 66. - P i p e d rop regula tor . 161

6-67. - P r e - c a s t concre te head wall d rop (60). 168

6-68. - Cement block check and d r o p s t ruc tu re (Canada). 169

6- 69. - Concre te check d r o p (U. S . A. ) 170

6-70. - Vert ica l wooden drop, d = 8".

6-71. - Vert ica l wooden drop, d = 12".

6-72. - P l a n for a cor rugated me ta l pipe drop.

6-73. - Steel b a r r e l drop .

6-74. - Sloping rock d rop s t ruc tu re (Canada).

7- 1. - Standard trapezoidal (Cipolletti) measu r ing wei r of 61 c m ( 2 ft) c r e s t length instal led a t a f a r m outlet.

7-2. - Diagram of f r e e d ischarge contracted weir showing position of staff gauge ups t r eam.

7-3. - P e r m a n e n t t rapezoida l we i r discharging under f r e e flow conditions

7-4. - Discharge over a suppressed rectangular weir p e r m e t r e of c r e s t length.

Lis t of Figures Cont'd.

Figure Page

7-5(a) and (b). - Small temporary V-notch weirs made of sheet metal (being used for studies on irr igation efficiency and water losses) . 20 1

7-6. - Example of a design for a 90° V-notch weir plate. 202

7-7. - Romijn broad-crested weir , sliding blades and movable weir c r e s t 204

7-8. - Romijn broad-crested weir, hydraulic dimensions of weir abutments. 205

7-9. - Values of Cd a s a function of the rat io Hcrt : Lcrt for the Romijn weir. 209

7- 10. - Approach velocity coefficient, Cv, fo r rectangular approach channel. 21 0

7- 11. - The Romijn movable measuring/regulating weir (drawing) (with supplement, Lis t of Materials) .

7-12. - Approach velocity coefficient, Cv, a s a function of the total head over the movable weir c res t (H,,~) in the stage - discharge equation

2 2 1 .5 Q = 5 C d C v ( j g ) 0 5 BtHcrt .

7- 13. - Small standard Parsha l l flume in operation.

7-14. - Plan and elevation of a concrete Parsha l l measuring flume showing component pa r t s (82).

7-15. - Diagram showing the ra te of submerged flow in l / s and in f t3 /s , through a 15.2. c m ( 6 inch) Parsha l l measuring flume.

7- 16. - Diagram showing the ra te of submerged flow, in 11s and ft3/ s, through a 23 c m (9 inch) Parsha l l flume. . 7- 17. - Diagram 'for computing the ra te of submerged flow through a 30. 5 cm (1 ft) Pa rsha l l flume (82).

7- 18. - Effect of submergence on Parsha l l flume - f ree discharge (81).

7- 19. - Section of a Parsha l l measuring flume illustrating the determination of the proper c res t elevation (82).

7-20. - Diagram for determining the head loss through the Parshal l measuring flume (82).

7-21. - Parsha l l flume of 152 c m (5 ft) throat width assembled frbm prefabricated sheet meta l par ts .

7-22. - Parsha l l flume of 183 cm (6 ft) throat width a t full discharge.

7- 23. - Commercially available Parshal l measuring flume.

7-24. - Standard concrete Parsha l l measuring flume - throat width 1 f t to 8 f t .

7- 25. - Standard concrete P a r shall flume.

7-26. - Sketch of Cut-throat flume (85)

Lis t of Figures Contld.

Figure

7-27. - Fina l design of a 61 cm (2 ft) rectangular Cut-throat flume (90).

7-28. - Cut-throat flume of 30.5 cm (1 ft) throat width, with automatic recording device, operating under f ree flow conditions.

7-29. - Generalized f r e e flow coefficients and exponents and St for Cut-throat flumes, in m e t r i c units.

7-30. - Installation of a Cut-throat flume.

7-31 (a) and (b). - Trapezoidal measuring flume with a ra ised bottom 3 cast in a concrete ditch. The discharge i s about 34 11s (1. 2 ft 1s ) a t a

submergence of about 70% (87).

7-32. - Trapezoidal flume for 1 f t i rr igation channels.

7-33. - Typical parallel flow cri t ical depth flume.

7-34. - Portable steel forin used to cas t trapezoidal concrete flumes in concrete ditches (8;').

7-35 (a) and (b). - Meter gate for pipe outlets (64).

7-36. - Sketch of pipe outlet with sliding gate for delivery control and measurement (88).

7- 37. - Rating curve for pipe outlet (8s).

7-38. - Sketch of a propeller mete r for open flows.

7-39. - Propel ler meter installed a t a pipe outlet

7-40. - Register of a propeller mete r .

7-41. - Range ability of a propeller mete r and the selection of meter diameter (+ 4% accuracy).

7-42. - Standard design of open type propeller mete r .

7-43. - Example of a deflection mete r with a pointer indicating against a fixed vert ical scale (Rajasthan, India).

7-44. - The Rajasthan channel flow meter in use .

7-45. - Commercially available deflection m e t e r .

7-46. - Sketch of the Rajasthan channel flow mete r .

7-47. - Sample calibration curve for 30 c m (1 2 inch) Rajasthan channel flow m e t e r .

Page

246

PREFACE

This publication i s the result of a joint effort by the Food and Agriculture Organ-

ization of the United Nations (FAO) and the International Commission on Irrigation and

Drainage (ICID) in producing a Handbook on small hydraulic structures and devices used

in open-channel irrigation distribution systems. There has been general recognition of

a need to review the abundant information and experience available on the subject and to

condense and dovetail them into a comprehensive and practical Handbook. Much basic

mater ia l for the Handbook has been generously provided by National Committees of the

ICID and by F A 0 projects and contacts in Member Countries, while complementary data

and information have been assembled from the extensive survey of the l i terature.

The scope of the Handbook i s confined a s the title suggests to small s t ructures

used a t the fa rm level in fields, and in networks with small discharges at the intakes,

such a s from small surface o r ground water resources . Such structures, having

capacities of l e s s than 1 cubic met re per second, and, indeed,many of them having

capacities of l e s s than 300 l i t r es per second, account for more than 70 per cent of al l

the hydraulic s t ructures installed in many irrigation networks.

In the past these small structures have not always received the attention they

deserve from planners and designers. It should be recognized that irrigation head

works, and other irrigation engineering works, however spectacular, would have little

value without an efficient distribution system (requiring small structures) extending

right down to the f a rmer s ' fields. The heavy investments normally involved in an

irr igation system can be justified, through conversion into cash benefits and the social

welfare of the rural population, only by paying full attention to the function and place of

each of the small s t ructures described in this Handbook.

xiv.

The Handbook i s published in three volumes. Volume I comprises Chapters

1 to 5. The types of small hydraulic s t ructures available, and their importance for

efficient distribution of irr igation water supplies a r e discussed in Chapter I.

Chapter 2 discusses the operation of irrigation systems and how this governs the

choice of the type of small hydraulic structure best suited to the purpose. Chapters

3 to 5 deal with small intake s t ructures , small flow-dividing s t ructures , and outlets

o r f a r m and field turnouts. Volume I1 comprises Chapters 6 and 7. Chapter 6

deals with small water-level and velocity control s t ructures (i. e . checks o r c ross

regulators, falls o r drops, and chutes) and Chapter 7 with small hydraulic s t ructures

and devices useful for measuring flow in irr igation networks. Volume 111, which will

be issued a t a la ter date, will cover small cross-drainage works, escapes and

miscellaneous s t ructures and will include a chapter on the detailed design of

gates.

F o r definitions of t e rms , reference should be made to the ICID Multilingual

Technical Dictionary on Irrigation and Drainage. Units of measurement a r e generally

expressed in the units f rom which the formulae, designs, tables and graphs have been

derived (and a r e thus best known in that system) but in certain cases i t has been

considered advantageous to convert English to Metric units for application in countries

using only the Metric system.

Since the Handbook attempts to assemble and describe many types of small

hydraulic structures which have proved successful in certain countries, and which

may be used elsewhere under similar conditions, i t i s hoped that i t will prove useful

to young engineers, technicians and extension workers involved in the remodelling

of existing irrigation systems o r in the design of new projects. It i s also hoped that

the Handbook will stimulate exchanges of ideas and information on techniques and

designs which have often been evolved in isolation.

The present edition i s a provisional version; i t i s intended that an updated

version covering Volumes I to I11 will be printed in final form at a la ter date.

Any comments o r further contributions which readers might like to offer will be

gratefully received and will be considered for incorporation in the next

edition.

xv.

Acknowledgments a r e due to the many who have assisted in the production of

this Handbook, some with systematic contributions, such a s the ICID National

Committees of :

Arab Republic of Egypt

Australia

Bulgaria

Canada

Colombia

Czechoslovakia

Republic of Korea

Malaysia

Mexico

Philippines

Ecuador

Federal Republic of Germany

Guyana

Hungary

India

Japan

Republic of China

Sri Lanka

Turkey

U. S. A.

U. S. S. R

and personnel of FA0 and individual contacts who have rendered valuable information

and advice, and to Mr. I. Constantinesco for his lucid eaiting of the manuscript.

Dated

Edouard Saourna Director Land and Water Development Division Food & Agriculture Organization of the United Nations

K. K. Framji Secretary General International Commission on Irrigation & Drainage

6. WATER LEVEL AND VELOCITY CONTROL STRUCTU'RES

6.1 INTRODUCTION

The water level and velocity control s t ruc tu res described in this chapter

comprise a group of engineering works installed in open channel irr igation networks

designed to regulate the water level in a channel, to control the quantity of water .

passing through it, to dissipate energy and enable water to be delivered accurately

and safely to the fields without causing erosion. Such s t ructures include checks o r

cross-regula tors , drops (o r fal ls) and chutes. F o r example, a check o r c ross

regulator will r a i s e the upst ream water level in a canal above i t s natural flow level

during periods of low discharge sufficiently to feed an offtake canal. A check will

a lso help to temporari ly absorb fluctuations of water supply in various sections of

the canal system, o r to control velocities and prevent breaches in the tai l reaches.

Drops (o r fal ls) and chutes narrow the difference in slope of the land and that

required for the canal. Generally, a drop o r a fall will be used to obtain a

reduction in slope over a short distance. When the distance i s greater and the

slope m o r e gentle, but st i l l s teep enough for the water to flow a t too high a velocity,

control may be achieved by employing chutes.

A. CHECKS OR CROSS REGULATORS

6 . 2 GENERAL FEATURES O F CHECKS OR CROSS REGULATORS

Checks o r c ross regulators may be fixed overflow weirs with no movable

controlling device, o r they may be provided with radial gates, slide gates, stop-

logs, checkboards (flashboards), o r combinations of these, o r include a device to

maintain a given upst ream water level. These s t ructures may be fixed o r portable

(the fo rmer should have provision for overflow) and they may be used in both lined

and unlined canals, ditches o r water courses. Where check gates a r e fitted, these

may be hand o r hydraulically operated (such a s the automatic Neyrpic gates) o r

equipped with automatic and t ime controlled re lease devices. Wherever possible,

and in the in teres t of economy, a permanent check should be combined with a drop

o r fall, o r a division box, o r a measur ing device located above o r below an intake,

outlet o r escape.

FIGURE 6.1. - Flow through check structures: (a) f ree overflow; (b) submerged orifice flow.

A check may be designed to function a s an overflow weir, a s an orifice o r a s

a combination of both. When a constant upstream water level i s desired an over-

flow type check i s normally used (see Figure 6. l ( a ) ). The flow over such a check

may be calculated f rom the equation:

where

Q . - 3 - discharge in m per second

C 2 .& = discharge coefficient ( m / a ) = 7P

B(t) = overf lowcres t length (m)

H = head o r water depth above the cres t , measured upstream (4 from the check (m)

Values of C a r e given below:

When the c r e s t length, B(t),' i s large, variations ifl discharge result in relatively

small changes in the upstream water level.

F o r m of Weir Cres t

/----a, broad cres ted with rounded edges, horizontal

A- sharp cres ted with aerated beam

rounded with vert ical upstream face and inclined L downstream slope

The above formula (derived f rom Poleni) i s valid only for f ree flow conditions.

The values of C a r e accurate enough for design purposes, such a s dimensioning

weirs f rom given discharges and water levels, but not for exact water measurement.

Also; a number of types of check s t ructures have been individually ra ted and flow

formulae developed for them, a s will be seen below. The use of checks for water

C

1.5 to 1 . 6

1 . 9

2 . 2

measurement would require them to be constructed to standard dimensions for

which ratings a r e available o r to be calibrated individually.

Check s t ructures mus t be designed to c a r r y the full design discharge of the

canal a t maximum water level . The velocity of flow through check s t ructures with

flashboards should not exceed 1 m / s because of the difficulty of placing and

removing the flashboards. Checks with gates can tolerate velocities greater than

1 . 5 m / s . F r o m the selected design velocity the required opening and the cor re -

sponding head l o s s a r e determined. The total head loss , h$) , through a check

structure can be estimated a t 0 . 5 of the difference in the velocity heads of the

upstream canal section and the check opening, thus:

When the water level i s to be controlled downstream from a structure, an

orifice-type check i s more desirable because of i t s more constant discharge (see

Figure 6. 1 (b) ). The discharge through an orifice may be determined f rom the

general equation:

1 - Q - - C A

(orf) ( '8 (c r t ) )

where

C = coefficient of discharge

A 2 (orf)

= a r ea of opening ( m )

and H = head causing flow (m) (4

The coefficient C ranges from 0 .6 to 0 . 8 , depending on the position of the

orifice relative to the sides and bottom of the structure and on the roundness of

the orifice edge. Fo r f ree discharge the head, H(crt), i s the upstream water

depth measured f rom the centre of the opening. Fo r submerged flow, the

effective head i s the difference between the upstream and downstream water

surface levels. Because of i t s head-di scharge relatianship, an orifice-type

check i s not so well adapted for upstream water level control since fluctuations in

flow result in relatively large upstream water level variations.

6 . 3 CHECKS WITH FIXED OVERFALL CREST WITHOUT MOVABLE CONTROLS

6. 3. 1 General

A fixed overfall cres t o r weir controls the water level at a given height

within relatively narrow limits. This height and the c res t length, B(t), a r e

determined in relation to the discharge to be passed over the weir c r e s t and to

the control requirements (e . g. maximum permissible level fluctuations, etc. ).

FIGURE 6. 2 (a ) and (b). - Duckbill we i r s on distribution canals (Spain).

The narrower the tolerances, the greater must be the c res t length. In distri-

bution channels the available width i s usually insufficient to accommodate a

t ransversal weir with a c res t long enough to pass the full supply discharge within

the level tolerances. Usual tolerances a r e in the order of 5 to 10 cm.

These conditions have led to the development of: the diagonal weir; the

duckbill weir; and the Z-type or other specially shaped weirs. Of these the

duckbill weir i s the most commonly used because i t i s , under most conditions,

the most economical one, providing optimum discharge capacity in relation to

length of structure and amount of construction mater ia l . Figures 6 - 2 to 6-4

show different types of duckbill weirs.

FIGURE 6-3. - Small duckbill weir installed in a -

concrete flume distribution system, (Kiti Dam Project , Cyprus).

The great advantage of fixed weir c r e s t s i s their simplicity in construction

and maintenance and their reliability in operation. Tampering i s almost

impossible. However their ability to t r ap silt so efficiently prohibits their use

where irr igation water i s permanently charged with silt. If the silt load i s small

o r temporary, siltation can be avoided by providing a flush opening in the weir a t

the floor of the structure. The provision of a gate i s advantageous on la rger

FIGURE 6-4 ( a ) . - Double duckbill weir for 480 l / s

discharge capacitv.

FIGURE 6-4 (b). - Duckbill wei r for 160 l / s d ischarge capacity.

s t ruc tu re s t o enable evacuation of the ups t r eam reach .

Hydraulic Design

The following d i ag rams and calculat ions a r e der ived f r o m the handbook

"Les ouvrages d 'un petit r e s e a u d ' i rr igat ion" p repa red by the Societe Genera le

d e s Techniques Hydro-Agricoles (SOGETHA) and published under the s e r i e s

"Techniques R u r a l e s en Afrique" by the F r e n c h Government in 1970.

The calculation of the d ischarge over a diagonal o r a duckbill we i r o r a

Z-type wei r i s based on the formula:

3 -

where m = discharge coefficient

B(t) = length of c r e s t

H ( c r t )

= water depth, i. e . head on the c r e s t ( s e e F i g u r e 6-5)

FIGURE 6-5. - Diagram of flow over diagonal, duckbill o r Z-type w e i r s .

The valu& of m depends on the shape of the c r e s t and the angle of 6 .

F o r 3 l a r g e r than 45' the values shown in F i g u r e 6-6 a r e used:

Diogond weir Duckbitl weir Z type weir

Unrounded crest: m : 0.34 0.32 0.31

Crest rounded I I ~ B ~ ~ ~ U I I I : m: 0.38 0.36 0.34

FIGURE 6-6. - Determination of coefficient ' m ' for angles of H grea t e r than 45O.

The graph, F igure 6-7, can be used to determine the discharge per m e t r e

of c r e s t and f r o m this the total length of c r e s t required.

F igu re 6-8 i s an aid for determining the angle f rom the required c r e s t

length. If oC i s below 45O i t i s recommended, for r ea sons of economy, that an

inclined weir be used r a the r than a duckbill wei r . Above 45O up to 70° the use of

a duckbill weir i s preferable . The end of the duckbill weir c r e s t i s fixed a t

40 c m independent of the bottom width of the canal. If the t ip of the duckbill i s

shaped semi-c i rcu la r with a d iameter of 40 cm, the c r e s t length will be

approximately 60 cm.

Example:

Given Canal discharge 150'1/s

side s lopes 1 . 5 : 1

bottom width 0.50 m

water depth 0 .40 m

Maximum water level variation 0. 13 m ,

i . e . H (4 = 0 . 1 3 m for 1 5 0 l / s

c r e s t height S = 0.40 - 0. 13 = 0.27 m

Available t r ansve r sa l c r e s t width B ( t )

= 1.30 m ( f rom Figure 6-7)

m = 0 .38 (rounded c r e s t , diagonal type )

m = 0 .36 (rounded c r e s t duckbill type)

Discharge ( I / s ) per m e t r e of c r e s t length

FIGURE 6-7. - Graph for determination of discharge over diagonal, duckbill, o r Z-type wei rs (84).

F r o m Figure 6-7 q = 80 l / s per m e t r e of c r e s t (diagonal type)

q = 75 l / s per m e t r e of c r e s t (duckbill type)

total length required (diagonal type)

0 F r o m Figure 6-8 the inclination of the weir @ i s found to be equal to 47 .

Therefore this would call for a duckbill weir ra ther than for a diagonal one.

Required length for a duckbill weir :

FIGURE 6-8. - Determination of [ from known B and S and of d (84).

Section A-A

4

Plon

Sect ion 0-0

Ronqe of suitoble dimensions for copocities u p t o 5 0 0 L / s

B = 0 .20 to 1.00

f = 0.20 to 1.00

y, = 0.10 t o 0.70lupstreom woter depth)

yCItl = 0 .05 to O.IS(difference between upstream

woter level ond crest level)

s = 0.10 to 0 .60

c = 0.15 (thickness of weir)

f = (width of ovoiloble upstreom woter surface)

9 B(t)C (crest length) = C X & o< = (ongle between weir crest and cross section

FIGURE 6-9. - Standard diagonal check weir for capacities

up to 500 l / s (84).

Off take canal-

S e c t ~ o n E-F

S e c t ~ o n C-D I I F A 0 - I C I D I

D U C K B I L L WEIR

P r c j e c t , R e g ~ o n , Country

Agency for A g r o r ~ a n Refo rm , C ~ c ~ l y

S e c r l o n A - 8 l t o l y

F ~ g u r e N o 6-10

+I -----I Section A-A

Section 8-8

Range of suitable dimensions for copocitlCs up to \ooo c/s

6 = 0 .20 to 1.00

f = 0.20 to 1.00

4 = 0 10 to 0.70 (upstream woter depth)

IfCrf, = 0 0 5 to 0.15 (difference between upstream woter

level and crest level)

3 : 0.10 to 0.60

C = 0.15 (thickness of weir)

f = (width of ovoiloble upstream woter surface)

L = (tot01 lenqth of crest : 0.40 + 2 A )

o< : (angle between weir crest ond cross section

of chonnel)

m = 1.5 f - 1.5s + 0.20

k = A s i n w

p : 2 . 5 f

t s f

FIGURE 6- 1 1. - Standard duckbill weir design, type IqGiraudet" for capacities up to 1000 l / s.

t

4 0 3 0

I B d

3 80 - 1.00 - Plan

+ t 0 15

0 59 I 3\

I 0-44 --'f-

I . - - . . . . . . . . . j0 15

lb " ' / Cross section

- 5 $ 5 per running metre

5 # 5 per running metre Concrete = 7.3 cbm

Iron = 25:5 kg

F A 0 - I C I D

DUCKBILL W E I R

tr . FOR 260 to 280 t / s C A P A C I T Y

Project , Region , Country

Detail A-A Spoin

Detail 6-6 Figure No 6 -12

F r o m F igure 6-8

Design Example ( ~ i a ~ o n a l Weirs )

F igu re 6-9 shows a s imple but efficient design for in situ construct ion of a

diagonal wei r in unreinforced concre te .

Design Examples (Duckbill Weir)

Standard designs for duckbill w e i r s have been developed in seve ra l countr ies

of the Medi ter ranean Basin. F igu re 6-10 shows a type used in Sicily.

Dimensions of capaci t ies between 110 and 370 l / s a r e given in the drawing. The

wei r i s usual ly combined with one o r m o r e f a r m out le t s . They a r e usual ly

provided with m a s k modules but a l so s imple me ta l ga tes a r e in u s e . The

s t ruc tu ra l design i s adapted to lined canals .

R Bouillon e t a l . (55) r epor t on a duckbill wei r of 24 m total c r e s t length.

With a head of only 8 c m the weir d ischarges 1, 100 l / s .

F igu re 6- 11 shows a s tandard design developed by the SOGETHA (84) for

l ined and unlined channels . It can be constructed ei ther in concre te o r masonry .

Dimensions shown a r e for unreinforced concre te .

F igu re 6-12 i l l u s t r a t e s a design a s developed in Spain.

6. 3. 5 1 /

Check Slab St ruc ture (Mexico)-

6. 3 . 5 . 1 Genera l

The check slab s t ruc tu re descr ibed he re in , developed and in use in

" Based on information provided by J. Ansherto Manobe Galvan, Depar tment of Small I r r iga t ion , Sec re t a r i a t of Hydraulic Resources , Mexico.

Mexico, i s an example of a check which controls depths and velocities in reaches

of field la tera ls or ditches which have steep grades, to enable water to be

delivered to the field through siphons ( ~ i ~ u r e 6-13). The slab i s adapted for

use in lined channels and can also be used a s a water measurement device.

FIGURE 6- 13. - Check slabs in a channt . etch with steep grade. (State of St. Luis Potosi, Mexico)

6 . 3 . 5 . 2 Structural and design characterist ics

The check slab structure consists of a slab made of concrete, wood,

o r other mater ia ls and placed a t convenient intervals ac ross the lined channel

sections in reaches of steep grades. Each check slab operates a s an independent

spillway where heads depend on the geometry of the slab.

There i s an orifice at the lower par t of the check slab drop to allow

drainage and evacutation of sediments.

The thickness of the slab may be 5, 10 o r 15 cm. The height of the

slab above the channel bed may be 20, 30 o r 40 cm. The section of the channel

may be rectangular, o r trapezoidal with side slopes of 1 : 1

F o r smooth flow and to facilitate operation of the spillway, a minimum

water cushion of 10 c m at the base of each check slab i s required. Under these

conditions, the spacing, X, of check slabs (in m e t r e s ) will be given by the

following formula (where s i s the grade):

FIGURE 6-14. - Determination of spacing of check slabs.

Thus, for example where H( slab) of the check structure i s 0.30 m, the spacing

will be :

0.20 0.30 - 0.10 - X = -

S S ( m )

F r o m this X can be determined for different grades :

X (met res )

Similar tables can be prepared for other values of H(slab) .

6.3.5.3 Application a s water measuring device

It has been proved by experiments that the check slab s t ructure can

a l so be used a s a water measuring device by designing i t according to the

following formulae:

where Q = discharge of the canal, in m 3 / s

B(t) = width of spillway slab a c r o s s the axis of flow,

in m e t r e s

H ( 4

= head on the spillway cres t , measured a t a distance not l e s s than five t imes i t s approximate value upst ream from the check slab structure, in m e t r e s

for a trapezoidal section p will be expressed a s

= 0.5 t 0.04 H(cr t )

P H(slab) T( slab)

and for a rectangular section:

= 0.6 t 0.01 H( cr t )

r" H( slab) T( slab)

where:

H(slab) = height of the check slab above the bed of the channel, in m e t r e s

. T(slab) = thickness of slab, in met res .

6.3.5.4 Numerical example

Design a suitable check slab structure for a canal with reaches

having grades of 0.0005 (length 100 m, joints a t 30 m interval ), 0.005 (length

90 m ) and 0.02 (length 90 m ) a s shown in Figure 6-15. Assume a trapezoidal

section with side slope 1 : 1, channel bed width 0. 30 m, lined with plain concrete,

discharge 0.074 rn3/ s and N (roughness coefficient) = 0.01 6

t Reoch A I Reoch B I Reoch C

I 1

FIGURE 6-15. - Data for design of check slab structures.

Using the above data, and Manning's Formula, the depths' and

velocities in the three reaches of the canal will be:

Reach

y (water depth of channel (m) )

Hydraulic conditions of the canal in reaches B and C show relatively

high velocities and, consequently, insufficient depth for lateral irrigation with

siphons. One solution to the problem would be reduction of the grade and intro-

duction of falls in the channel bed. An alternative i s to maintain the grades but to

introduce check slabs a t appropriate distances. The lat ter has been found

economical in Mexico.

While reach A has favourable hydraulic conditions and needs no check

structures, i t i s essential to apply such structures to reaches B and C.

Reach B

Assume a check slab with H(slab) = 0.30 m and T(slab) = 0.05 m,

then the maximum spacing of check slabs will be :

and B (t)

= 0.9 m .

This spacing may eventually be reduced to 30.00 m and so the check slabs may be

located a t the construction joints of the concrete lining.

Now H (4

and if C = $rfi

where = 0;30 m and T(slab) = 0.05 m

Then C = 1.477 + 7.885 H (4

F o r Q = 0.074 and

Therefore

This equation i s solved by tr ials , assuming different values of

H(crt), a s shown in Table 6- 1.

H being 0. 11 m, the channel will have the following c r o s s (4 section, etc.

B = 0.30 m

- *(ws)

- 1.12 m

TABLE 6-1

1 2 3 4 5 col 4 + 5

Reach C

In this reach the c r o s s section of the channel will be the same a s in

reach B, the difference being the spacing of check slabs due to the different slope.

Thus the maximum spacing in this case will be :

The hydraulic dimensions of this reach will be the same a s above

except that s = 0.02.

6.4 CHECKS REGULATED B Y STOP PLANKS (DROP BARS) OR FLASHBOARDS

6.4 .1 General

Stop planks o r drop b a r s a r e used in checks with capacities l e s s than

3 1 .5 m I s , where operational changes a r e infrequent. The water passes freely

over the top of the planks which a r e fixed horizontally, in slots, in the structure.

Flashboards should not be used in openings greater than 1.5 m wide o r with water

depths over 2 m . The guides o r grooves should be vertical. For stop planks of

thicknesses above 5 cm the groove design shown in Figure 6- 16 i s recommended.

Headed anchors welded to

-lc t ongle; for dio. and length,

L, Lonper leg of ongle -,

.. fi L n n 1 U U Y U

All dimensions in inches

Stop plank Dimensions Angle for Weight Anchor thickness

A B C groove per ft

1 1 1 2 - x 2 x - 2- 3 - 3

2 2 2 4 3. 62 8 dia. x 4

3 1 5 3 4 - 3 x 2 - x -

1 . 4 2 16

- dia. x 5 5.60

3 1 5 ,, 3 5 - 3 x 2 - x -

1 4 2 16

5.60 2 dia. x 5

FIGURE 6- 16. - Stop plank grooves (54).

Figures 6-17, 6-18 and 6-19 show simple concrete check structures for

small flows.

e ~ n ~ o n ~ e P U I I O J ~

P Y ~ O ~ W O O p u n P ~ I I * ~ l l v 6 n w o q I .q p l n o w 1111 4 1 1 0 3 101s 40 *.PIS

p u n s p u e u o O ~ U O J O ~ ) ~ -+ elf e d w p l n o q s e p ~ n o q q s q j pOpUOLUUlO3OJ # I U!OlIOq V i 4 I P UDeJ4SU*OP "1 d o l d l l W O ( l

1e.11 ~ Y I S J O J U I ~ J a q ~ 101 p e 4 n ~ 1 ) s q n s e q l n w UJOJ etJ) YO OIqOBMD OJD lDq4 SJDq UOJl

U W C O @ - . . e e l = , C C D ~ , C S I :a6 I ,CCO<Q ax,* I IP*PO*H

19 J . ,FCOZE I ~ , E a r a *IID**PIS

1; nY3 MI ,csor,gar,sz I I)(;- U n 3 9 IOADJ6 U J O d +

PA + JO ~d n3 c p u o e *IJW a I W o S El ~ u o w e o 4 ~ o . i 1

/ D l d / W JO l)UflOY/V O I D Y I X O J ~ ~ V e ~ n 4 l l ~ P I P U ~ U I Y ~ O ~ ~ Y

S311 U N Wfk9 11383N03

FIGURE 6-19. - Ordinary flashboard check.

6.4. 2 1 I Drop Bar Check Structure (Victoria, Australia) -

6.4.2 .1 General

Check structures for irrigation canals a r e currently being standard-

ized in Victoria, Australia. Under the prevailing conditions a trapezoidal weir

fitted with drop bars , a s shown in Figure 6- 20, has proved an effective and

economical means for controlling velocity at o r below 60 cm per second. The

structure maintains minimum water levels and depths of water for delivery

through f a rm outlets and minimizes loss of water when rain causes a shut-

down of irrigation. Under suitable conditions and with appropriate

modifications such checks may also be used to measure flows with a reasonable

degree of accuracy.

I' Based mainly on information f rom the Australian National Committee of ICID.

Prwa-t plmr* am4 CROS SECTION diopraghm

LONGITUDINAL SECTION

FIGURE 6-20. - Typical dkop bar check structure (52).

Design considerations

These checks a r e construtted with C6. trapezoidal shape to fit the canal

c ross section; this i s then divided bf piers - which may be precast - into

openings that a r e blocked by easily Handled timber drop bars, to the required

c r e s t height. The normal width of bpening between piers i s 183 cm. This

width gives an easily handled bar of 196 cm length with a cross section of 10 cm

height and 7 cm width.

The limit of discharge for these checks i s 1,400 11s per 183 cm

opening, but i t i s more usual to design for 850 11s per opening.

Downstream from the concrete apron, r ip-rap i s placed on the bed,

and batters for some 6 m. The r ip-rap on the bed i s dished in shape. End sills

a r e not provided a t the end of the concrete apron.

There i s some problem in adapting the trapezoidal regulator to small

channels. For regulation, checks should be evenly spaced where possible, and

a t such intervals that when a channel i s shut down, the wedge volumes a r e small

enough to be successively passed down the channel within a reasonable period.

Spclting of checks must be such that the drop in water level i s generally less than

one met re and the minimum water depth immediately downstream i s 30 cm.

F o r drops in water level of above 30 cm a s many openings a s possible

should be used to pass the flow, provided that the minimum depth of water below

the outer openings i s 60 cm.

F o r small drops in water level, the water s t reams through the open-

ings and the velocity energy i s not dissipated in the drop. In this case the length

of c res t spilling the water should not be greater than the channel water width

downstream, and water should not fall direct on to the concrete batter ; i t should

have a water cushion of some 60 cm in depth. The average channel velocity a t

the'downstream end of the structure i s limited to 60 cm per second.

The structure becomes very large relative to capacity, particularly

when i t i s required to provide a l a rge drop in water level, but with pre- cas t units

i t i s still fairly econdmical.

6 .4 .2 .3 Ratings for calculation and measurement

Because the previously used "Gibson" formula was considered to be

insufficiently accurate and because of the increased need to use drop bar checks

for flow measurements, model tes ts were undertaken in order to obtain more

exact ratings for f ree and submerged flow for a variety of structural arrange-

ments. The tested model check had openings of 61 cm width. Ratings were

based on the total upstream head applied to the full c res t length.

The tes ts showed that, for consistent and accurate results , special

attention must be given to the condition of the top ba r and, for f ree flow weirs, to

aeration of the nappe. It was found that if the top upstream edge of the c res t

drop bar became rounded to 12 m m radius, the discharge could be increased by

10%. Fo r more accurate measurement, i t i s recommended that a metal plate

with sharp edge be fixed to the upstream face of the top t imber bars .

Under f ree weir flow, non-aeration of the overfall nappe may increase

the coefficient of discharge by up to 7%. Different methods of providing aeration

were tested. The simplest arrangement suggested is the fixing of 7 .5 cm x 3 cm

timber wedge deflector s t r ips to the drop ba r guides, (Figure 6-21 (b) ). The

other method is to use an open mild steel section, with a slotted inside face, a s

I (a ) Port longitudinal section of typical structure I ( b ) and (c) Plan view of aeration arrangements

(d Dipstick measurement of total head (el , ( f ) and (9) Pier nose arrangements ( i Timber drop bars (ii) Drop bar guide (iii) Dsflector strip (iv) Slotted steel guide aerator ( v ) Average run-up on dipstick

(vi) Sharp upstream edge

I F A 0 - I C I D

DROP B A R STRUCTURE

Project, Region, Country Victoria, Austrolio

Fi,gure No. 6- 21

the drop bar guide, (Figure 6-21 (c) ).

As well a s these two important factors, the pier shape, position,

number of openings in the structure and number of drop ba r s inserted (i. e. c res t

height) also affect the rating. Thus i t was found necessary to prepare separate

ratings for three different structural designs because of differing pier nose

arrangements:

- 23 cm thick concrete pier with a mild steel flat plate and drop bar

slot at the upstream edge of the pier, ( ~ i ~ u r e 6-21 (e) )

- 23 cm thick concrete pier with square front face, except for 2.5 cm

chamfers on the edges, and drop bar slot 33 cm in from the upstream

edge - present standard,(Figure 6-21 (f) )

- 30 cm thick concrete pier with upstream edge rounded to a 15 cm

radius fofiowed immediately by the drop bar slot, (Figure 6-21 (g) ).

With each structural design, different ratings were also found to be

required for multi-opening structures, a s ratings differed between extreme outer

and inner openings and these in turn differed from single opening structures.

Separate ratings were also found to be required for inner openings

with six ba r s o r more in place (c res t height approximately 60 cm) and with five

ba r s o r l ess .

The various ratings a r e close for low heads, but for heads f rom 30 cm

up to 76 cm a s tested, there i s a variation f rom 670 up to 1170 between and within

ratings for different structure arrangements.

The multiple opening trapezoidal check i s the current standard used.

For accurate measurement this structure requires three tables, one for extreme

outer openings and two for inner openings related to number of drop bars in place.

If the top drop ba r s have sharp upstream edges, f ree flow has proper aeration,

a dipstick ( see below) i s used for upstream total head measurement and the

downstream gauge i s correctly located for submerged flow. The model tes t

ratings a r e considered to have a maximum probable e r r o r of + 2.5 70.

A detailed se t of tables for f ree and submerged flow and different

pier and bay arrangements i s given in a repor t "Rating of Drop Bar Structures"

by C. Kirkham, September 1967. This repor t also se ts out requirements for

accurate measurement.

6.4.2.4 F r e e flow weir formula

The tables for f ree flow weirs over drop bar regulators were plotted,

and an average curve was interpolated and related t o the bas ic weir formula:

where, Q = discharge in 11s

B(t) = full c r e s t length of opening in c m

H(crt) = total head in c m

C = coefficient related to head a s given below.

The coefficients C calculated for an average rating were a s follows:

This rating i s within - + 5 0/0 of the rating'tables for the current

standard structure for heads up to 76 cm. It ag rees within s imilar l imi ts for the

other two structure arrangements, except at heads over 60 cm the difference i s

up to 7 % in some cases .

6.4.2.5 Submerged flow formula

The submerged flow ratings were related to a coefficient C1 in the

formula :

where H ( 4

i s the total upstream head measured to the top of the drop b a r s and

HDR i s the difference between water levels upst ream and downstream, both

measured in feet.

The coe'fficient C1 var ies f rom 0.58 to 0.72 over the range of sub-

mergence and different structure arrangements tested, and i s smallest a t 40 %

submergence, where submergence = H(crt) - H(dr) , The C1 values were H(crt)

plotted against the submergence figures and an average C1 rating interpolated a s

follows :

70 submergence 0 10 20 30 4 0 50. 60 7 0 8 0 9 0

The average rating i s within + 5 % of the various ratings for the current standard

regulator and + 770 to+- 5% of the other arrangements.

6.4.2.6 Measurement of head

The common method of measuring upst ream head i s the use of a dip-

stick held vertically on top of the drop bar . The head i s taken a s the depth of

water !!run-up" on the bar a s shown in Figure 6-21 (d). This method i s liable to

e r r o r s if the dipstick' i s not of a standard width and i s not held vertically. The

water height should be taken to the mean of the fluctuations in run-up and not to

the highest water mark .

The model tes ts indicated that the use of a 5 cm dipstick i s a satis-

factory method of obtaining head and gives a head very close to the total head a s

indicated below:

.dipstick reading 19 c m 31.4 c m 46.5 c m 61.1 c m 75.5 cm

total head 18.5 30.6 45.7 61.0 76. 3

Because of this, the total head was used a s a basis for the rating tables.

The head may also be read f rom staff gauges set some 3 m upst ream

and close to the side of the channel.

F o r submerged flow conditions, the head difference between upstream

and downstream water levels i s taken f rom staff gauges se t upstream and down-

s t ream.

The downstream gauge in the model t es t s was placed immediately

downstream of the structure behind the pier in st i l l water. This location i s not

possible with some trapezoidal regulators and the tes ts indicated that a staff

gauge placed some 4 5 m downstream, close to the side of the channel in a back

flow section and facing downstream against the back flow, would give a close

reading.

6.5 CHECKS EQUIPPED WITH HAND OPERATED GATES

6.5.1 General

Gated checks a r e commonly used in channels where water level

adjustment i s required more frequently o r where the higher cost, compared

to stop-logs a r e justified (e. g. saving of labour). These checks a r e usually

fitted with hand-operated slide gates ranging f rom simple wooden shutters to

hand-wheel noprated adjustable orifice type gates (Figure 6- 22).

FIGUdE 6-22. - Hand operated check gate (Fe r . ,ra, Italy).

e wall, reinf. not shown

Extend cutof f

concrete as di

concrete deck

C.L. Pipe hondroil post connections.

Pipe hondrail post connection details (Handroil requlred when H,k is qreoter

than 16 inches)

Assembly pole guides w ~ t h flotheod bolls

Section A-A CONCRETE CHECK

The sil l of the gate i s usually made level with the bottom of the channel. Slide

gates a r e usually operated a s an orifice with the exception of weir,gates, such a s

the Romijn gate (see 6.5.7 and Chapter 7) which can be used a s an overfall weir.

Also, if checks a r e combined with a drop in the channel bed the gate may be

designed a s an overflow weir. The design of gates i s discussed in detail in

Volume 111 of this Handbook, while in this section discussion will concentrate on

the functional aspect of gates and the design of the supporting structure.

Checks may be stationary o r portable. A large variety of conventional

stationary checks exist, each of which have been developed to suit a given set of

conditions. These checks can be dimensioned a s outlined in section 6.2. A

few examples a r e described in 6.5.2 to 6.5.4 below and portable checks a r e

mentioned in 6.5.5.

6.5.2 1 / Standard Check (USBR)-

6.5. 2.1 General design features

This check has been developed by the USBR for upstream water level

control for maximum discharge capacities f rom 300 l/ s up to 2,400 l/ s.

The structure, (Figure 6- 23), consists of:

- an upstream approach of 3 m (10 feet) with gradually widening transition

f rom the width of the normal channel section to the width of the check

(across the axis of flow) and with the bed sloping down to the c res t of the

check

- a check wall with guides for a slide gate (the slide gate i s not shown in the

drawing), and wing walls

- a middle section with a pre-cast concrete deck with handrail on the down-

s t ream side

- downstream wing walls and a 3 m (10 ft) transition returning back to normal

channel section.

I' See also under drops.

TABLE 6-2 Dimensions of Standard Check (USBR)

L 1 ~ h e n a gate of specified height i s not available a gate with the next height available may be used with appropriate f r a m e height.

Str. No:

1

2

3

4

5

6

7

8

9

10

11

Max. Q

11s ft3/s

283 10

425 15

595 21

735 26

595 21

792 28

990 35

1190 42 II

990 35

1220 43

1190 42

Slide gate

Width Heighdl H(frame)

in f t

36 x 12 5

3 6 x 1 8 5

36 x 24 6 -

36 x 30 6

48 x 18 5

48 x 24 6

48 x 30 6

4 8 x 3 6 6

60 x 24 6

6 0 x 3 0 6

7 2 x 2 4 6

.Standard dimension

B~~ y w k ) L(s t r ) B(str) top X(vhr) 1 L ( ~ ~ ) ~ T T(ww)

( 4 T(CHW) T c o f f

in in

31 0" 1411 41 6" 101 6" - 24" 6 6'

31 o n 2011 41 6" 121 0" 51 6" 24" 6 6

31 o w 21 2" 51 0" 131 6" 61 3" 24" 6 6

31 o w 21 8" 61 0" 151 o n 71 0" 21 6" 6 8

4' 0" 2111 41 6" 131 3" 61 2" 24" 6 6

41 o w 21 3" 51 ott 141 9" 6111" 24" 6 6

41 0" 21 9" 61 0" 161 3" 71 8" 21 6" 6 8

41 0" 31 3" 71 O H 171 9" 51 7" 21 6" 7 8

51 0" 21 3" 51 ott 151 9" 71 5" 24" 6 6

51 OH 21 9" 61 0" 171 3" 51 511 21 6~ 6 8

61 o w 21 311 51 011 161 9" 7111" 24" 6 ' 6

Estimated Quantities

Conc- Reinf. Misc. r e t e s tee l metal

m kg kg 3

2.1 150 30

2.5 170 9 0

2.9 195 100

4.5 245 110

2.7 185 9 5

3.1 210 105

4.7 260 110

5.7 300 130

3.3 220 125

5.0 280 145

3 .4 240 125

The whole s t ructure i s of reinforced concrete except the gate.

Transitions a r e in ear th but the bed i s protected by a layer of coarse gravel.

The essential dimensions of the check a r e given in Table 6- 2 for 6

different standard structures. Size of structure and elkvation i s determined a s

follows. The c r e s t i s se t so that the top of the check wall (adjacent to the gate)

is a t control water level. This i s the level to be held by the check and i s usually

equal to the normal level of the water surface a t the check for design discharge.

F o r a known discharge 'Q1 a structure i s selected f rom Table 6-2 and i t s c r e s t

(a t elevation B in Figure 6-23) is se t so that the top of the check wall i s a t control

water level.

Excessive flow can pass over the concrete check wall.

Numerical example

Given Q = 575 l / s

Elevation A = 310.25 m

y1 = 45 cm

assume normal water surface = control water surface

control water surface = 310.25 + 0.45 = 310.70 m

Refer to Table 6-2. There i s no structure number for a Q of 575 l / s , so

select the next highest, Q = 21 f t3 /s = 595 11s. Structures Nos 3 and 5 both

3 have a maximum discharge of 21 ft / s . Select structure No 5 because i t has a

working head (h(wk)). which more nearly suits the given canal section and i t has

l e s s concrete than structure No 3.

Dimensions : h(wk) = 21 inches = 5 3 c m

Elevat ionB = 3 1 0 . 7 0 - 0 . 5 3 = 310.17 m

l leva ti on of B must not be higher than that of C)

Elevation C i s normally set 1. 5 to 3 c m lower than Elevation A to

allow for hydraulic head loss through the structure. Therefore, Elevation

C = 310.23 m.

Check Structure made of Sheet Metal

A prefabricated steel check structure i s shown in Figure 6-24. The steel

i s glass coated to reduce corrosion. The joints a r e bolted and sealed with a

special mast ic to help eliminate seepage. The data given below a r e for a

122 cm (4 ft) wide opening.

Model Dimension (ft) Arsa Wt. List ~ r i c e l / Number L F B E C ft lb $ us

4 ft wide

4W4442 4 4 4 4 2 160 1075 380

FIGURE 6-24. - Check structure made of sheet metal - dimensions.

FIGURE 6-25. - Check constructed from prefabricated steel parts (75).

6 . 5 . 4 Wooden Checks

Designs of small wooden checks are shown in Figures 6-26 and 6-27.

Shutter ,. , - f

F R O N T V l E W

I------̂. R E A R V l E W

Recommended Sizes

ft3/s A B C D E F

3 71011 21'611 31011 11611 21011 31011

6 91011 31011 31011 21011 31011 31611

9 1010" 31011 41011 21011 31011 31611

12 111011 31611 51011 21011 3lOl1 31611

16 1110" 480" 51011 21611 31011 4toll

FIGURE 6-26. - Single wall check with side walls only for protection of banks (65).

Minimum dimensions in inches

FIGURE 6-27. - Double wall check (74).

The structures can be improved by adding an apron at the downstream end. Aprons

can be made of almost any convenient material ranging from burlap sacks to con-

crete. The tables in Figures 6-26 and 6-27 give dimensions recommended for

various flow capacities o r opening widths. The capacities for the double wall

check in Figure 6-27 range from 0 to 1,000 11s.

6.5.5 Portable Checks

Often i t i s desirable to use a ditch in sections, filling sections lower down-

stream a s irrigation progresses. A ser ies of permanent structures for this

purpose would be costly ; but a portable dam o r stop in the form of a frame of

canvas, plastic o r butyl rubber, or a metal panel that can be driven into the earth

and across the ditch, can be used repeatedly to control flow. These devices a r e

applicable only to earth ditches. To permit the same control in lined systems,

slots for the checks can be cast into the sides of the channels at any desired

interval. Figures 6-28 and 6-29 show two simple designs for local manufacture.

Ditch bonk

AL L C O ~ O I bottom

Angle bar 4 0 x 40

e

Section A-A

1

FIGURE 6-28., - Portable check for farm ditches (46).

6.5.6 Radial Gate Check

The radial gate check i s used successfully for level control purposes in the

Netherlands. Its great advantages a r e that: the gate acts a s an overflow weir,

which requires l e s s frequent adjustment should the discharge of the channel

fluctuate; i t allows debris to pass the weir; and the gate can be lowered for

periodical cleaning of the upstream channel section. The gate i s operated with a

portable screw-thread bar and i s then kept in the required position by a chain on

FIGURE 6-29. - Portable canvas check, sleeve type ( 1 3 ) .

FIGURE 6-30 (a). - Radial check gate h he Netherlands).

FIGURE 6- 30 (b). , - Downst ream view of r ad ia l gate check (The Netherlands) .

each side. (See Figures 6-30 (a) and 6-30 (b).) Large gates a r e operated by a

fixed hoisting device.

6.5.7 The Romijn Gate

The Romijn gate i s a hand-operated broad crested weir used for: level con-

t rol a t intakes to distributing o r other subordinate channels; o r for level control

within a channel; o r a s a measuring device. The gate has been thoroughly

laboratory tested and rated for water measurement and is therefore discussed in

detail under Chapter 7 - Structures and Devices for Water Measurement in

Volume I1 of this Handbook.

6 .6 HYDRAULICALLY AUTOMATED CHECKS (NEYRPIC)

6 . 6.1 General Structure and Application

The need for more accurate water level control than i s possible with hand-

operated check gates has, among other needs, led to the development of the

Neyrpic automatic gates. Their operation re l ies entirely on the forces in the

system itself, such a s hydrostatic thrust and the weight of the device. The devices

to be discussed here a r e the standard AMIL, AVIS and AVIO gates.

The AMIL gate, a s shown in Figure 6-31, i s designed for constant level

upstream control. I t consists of a balanced radial gate with a float attached to the

leaf. The gate i s designed so that the forces acting oneit position the leaf to

. maintain the upstream water level at the height required. With the constant level

downstream gate (AVIS) a s shown in Figure 6-32 a float automatically positions the

gate leaf over the gate opening to maintain a predetermined and nearly constant

level downstream. Figure 6- 33 shows the AVIO gate, a variant of the downstream

level gate, which i s placed behind an orifice type outlet. The AVIO variant i s

required when the discharge of the supply canal i s large and the discharge to be

taken off i s small. I t i s more generally used on water offtakes that a r e controlled

f rom the level variations of a body of water such a s a storage pond. The choice

between the open and the orifice type gate i s solely determined by the maximum

level differential likely to occur between the upstream and the controlled levels.

With the constant level upstream gate the branch canal o r f a rm outlet i s placed

upstream of the gate and with the constant level downstream gate downstream of

the gate. (See also Chapters 2 and 3.)

FIGURE 6-31. - Typical medium size upstream constant level gate, NEYRPIC - AMIL. (1. Servo-tab, 2. float, 3. gate leaf, 4. hinge, 5. adjustable counterweight)

FIGURE 6-32. - Typical downstream constant level gate (NEYRPIC - AVIS)

FIGURE 6-33. - Typical downstream constant level gate for discharge through an orifice, (NEYRPIC-AVIO). (1. Opening, 2. gate leaf, 3 . adjustable counterweight, 4. hinge, 5. float, 6. chamber communicating with the downstream level. )

When comparing constant level gates with conventional gates the higher

initial cost of constant level gates has to be weighed against increased water use

efficiency in the entire irrigation system. Other aspe*cts to consider a r e the

labour saving automatic operation versus the increased attention necessary to pre-

vent jamming of and tampering with the device. Because of their relatively high

cost and susceptibility to clogging by debris, the constant level gates a r e especially

suited to hard surface lined canals o r flume irrigation systems. For a choice

between upstream and downstream control see Chapter 2.

6. 6.2 Range of Standard Gates Available

The information given in the following indicates the range of

available standard AMIL, AVIS and AVIO gates only. Fo r selection of size of

gate within each category the following data must be known:

- maximum r a t e of flow

- minimum head

- maximum head a t zero flow

- maximum head a t maximum flow.

F o r design and other complementary information reference should be

made to the abundant information available with the manufacturers.

AMIL gates

AMIL gates a r e described by the dimension indicated by IDt, which i s

approximately the width of the water surface, a s shown in Figure 6-34.

FIGURE 6-34. - Diagrammatic layout of AMIL gate.

The depth of water upstream i s 0.45 D ; the gate r i s e s to a maximum of 0.225 D ;

2 the a r e a of the passage through which the water passes i s about 0 . 2 D ; and the

2 a r e a of wetted section of canal immediately upstream from the gate i s 0. 35 D . Table 6-3 summarises the major pa ramete rs of small AMIL gates.

TABLE 6-3

Major P a r a m e t e r s of Small AMIL Gates

6. 6. 2. 2 AVIS gates

AVIS gates a r e identified in t e r m s of float radius r and bottom

sluice kidth C I , both in centimetres ( s e e Figure 6-35 (a) ). Two groups of

Type

gates a r e available, one for high heads and another for low heads. In the 3 capacity range below one m 1 s there a r e only two different s izes of the high head

type available, a s shown in Table 6-4.

Water depth

Y1

(cm)

3 6

4 0

45

50

56

63

71

D

(cm)

D 80

D 90 ,

D-100

D-110

D-125

D-140

D-160

TABLE 6-4

R

(cm)

63

63

63

63

90

90

90

Major P a r a m e t e r s of Small High Head AVIS Gates

Cross section

( cm)

a b c

85 65 60

95 50 45

106 56 50

118 63 56

132 71 63

150 80 71

170 90 80

Overall

(cm)

U V W

70 56 71

70 56 80

70 56 85

70 56 95

100 80 106

100 80 118

100 80 132

AVIS high head gate No

( r / c )

561106

711132

Approximate maximum discharge

(11 S)

190

250

330

420

570

770

1100

Overall dimensions

(cm)

A B C D E F R r

164 135 121 90 102 62 9 0 5 8

205 165 155 110 127 78 112 71

Approximate minimum

head loss a t maximum discharge

(cm)

5

6

7

7

8

9

10

Max. head

( cm)

Jrn

40

50

Sluice

(4

C H L

106 96 138.5

132 121 180

Approx. max. dis-

charge

( l / s )

800

1400

Appr ox: min. head loss a t maximum discharge

( cm)

6

7

downstream level

Sluice dimensions

FIGURE 6-35 (a). - Diagrammatic layout of AVIS gate from 56/106 to 90/190.

6.6.2.3 AVIO gates

- AVIO gates a r e identified in t e r m s of float - outside radius in c m and

sluice c r o s s sectional a r e a s ( h x L) in square dec imeters ( see F igure 6-35(b) ).

A s an example the float radius and sluice c ros s section of an AVIO 56/25 gate a r e 2 3 56 cm and 25 dm respectively. In the capacity range f rom zero to one m / s

t he re a r e 6 gate s i ze s available for high heads and 4 for low heads a s shown in

Table 6-5.

TABLE 6-5

Major P a r a m e t e r s of Small to Medium Size AVIO gates

Moximum Constant upstream level > downstreorn level 7

FIGURE 6- 35 (b)

Approx. Approx.

min. head Max. max. head Opening dis- l o s s a t

max. dis- (cm) (cm) charge charge

J~ h I., (11s) (cm)

142 25 25 80 14 140 32 32 110 16 180 40 40 280 2 0 . 90 40 80 410 12 224 50 50 49 0 26 112 50 100 750 15 280 63 63 900 3 3 140 63 125 1350 2 0 355 80 80 1500 4 0 180 80 160 2030 24

AVIO gate NO. -

High Low head head

281 6 36/10 451 16

45/32 5 6/ 25

56/50 71/40

71/80 90163

90/125

. - Diagrammatic layout

Overal l dimensions

( cm) -

A I.

85 60 65 35 50 28 105 75 85 45 63 36 135 90 100 55 80 45 135 90 100 55 80 45 165 115 130 7 0 1 0 0 56 165 115 130 7 0 1 0 0 56 2 1 0 1 4 5 1 7 0 9 0 1 2 5 71 210 145. 170 90 125 71 265 180 210 110 160 90 265 180 210 110 160 90.

of AVIO gate.

FIGURE 6-36 (a) . - Basic draw-string check fitted with side wing walls and bottom cut-off for use in an unlined ditch.

FIGURE 6- 36 (b). - Semi automatic check installed in an unlined ditch.

6.7 SEMI AUTOMATIC TIME CONTROLLED CHECK

The use of t ime controlled checks i s a s yet in the experimental stage and

there i s little field experience available. The method i s so far. confined to water

level control in fa rm distribution ditches. After allowing the water to r i se to a

predetermined level for a given t ime an automatic gate i s released, allowing the

water to flow on to the next check. A semi automatic portable check for use in

unlined ditches i s shown in Figures 6-36(a) and 6-36(b). Similar checks a r e

available for lined ditches. The check consists of a nylon reinforced butyl

rubber dam s ~ ~ , ~ o r t e d in a metal f rame designed to fit the ditch c ross section.

In the closed position the top edge of the flexible dam i s supported by a draw-

string threaded through grommets. A plastic covered steel cable i s used for the

draw-string. The draw-string i s released a t the end of the desired irr igation

period by a timing device. This device, which i s commercially available, con-

s i s t s of a wind-up spring, direct reading t ime indicator, a tripping mechanism

and an escapement re lease which i s operated by a small float. The t imer i s

mounted in a sealed casing. It operates a s soon a s water enters the ditch

immediately upstream from the check, when the timing mechanism i s tripped

into action by the rising float, Timers a r e available for two, five o r twelve

hour periods. Checks have been manufactured for 30, 35, 40, 45 and 50 cm

design water depths. Design details of the gates a r e given in Volume I11 of this

Handbook.

Humpherys (58) states that the portable draw-string check i s ideally suited

for use in an automatic cut-back furrow irrigation system, inathat the acreage

one i r r igator can manage can be increased up to ten o r more times, while

keeping run-off a t a minimum. (See also Chapter 2)

6.8 CHECK STRUCTURES COMBINED WITH A FALL, DROP OR CHUTE

Sometimes, it i s necessary to combine check s t ructures with falls, drops

o r chutes, particularly when i t i s necessary to control the upstream water level

and i t s velocity, in addition to achieving a reduction in grade and velocity down-

s t ream. Examples of checks combined with falls a r e given in part B of this

chapter, which follows.

6 . 9 GENERAL FEATURES OF DROPS (OR FALLS) AND CHUTES

Drops, o r falls, and chutes a r e control structures required at suitable

intervals in canals or channels which must have a more gentle slope than that of

the adjacent land, so a s to reduce the water level downstream, and reduce the

velocity of flow. They also provide for the safe dissipation of surplus energy.

Generally such a control structure i s called a drop, o r a fall, when the lowering

of the water level i s accomplished over a short distance. When the water i s

conveyed over long distances a t slopes which a r e still steep enough to maintain

high velocities (shooting flow), the structure generally used i s a chute. Chutes

may also be used on sloping land where a single drop, o r a ser ies of drops (i. e.

cascades), would be more expensive or otherwise undesirable.

In the case of main canals, branch canals o r sub-branch canals, which do

not directly irrigate any area, the site of a drop i s determined in consideration of

the cost of canal construction, including balancing cut and fill and the cost of the

structure itself. In the case of distributing canals, the falls a r e located so a s to

serve the commanded a r ea without having to build the canal banks too high. The

possibility of combining a drop with an intake, cross regulator, measuring

device, bridge o r some other canal structure must be given due consideration, a s

such combinations often result in economy and better regulation. Drops a r e

usually provided with a low crest wall, hump or check gate upstream to prevent

shooting flow in the upstream approach section.

In this Handbook drops a r e subdivided into three categories: vertical drops,

inclined drops and piped drops. Chutes a r e regarded a s being in the same

category a s inclined drops.

The choice between vertical and inclined drops i s governed mainly by the

difference in water level to be controlled by the structure, in other words, the

energy to be dissipated. However, local conditions, traditional practices, etc. ,

do not allow for generalization of the criteria for this choice on a world-wide

scale. The necessary drop in level and dissipation of energy can be achieved

either by one o r only a few large drops o r by several more small drops over the

same distance. The choice again i s much dependent on mater ia l and labour

available and the total cost of construction.

The inclined drop offers the alternative of dissipating energy through a

standing wave (hydraulic jump) whereas the shock of the overfall jet of the

vertical drop i s dissipated. Where the fall required i s considerable the whole

structure of the inclined drop requires l e s s mater ia l and labour input than the

wall and dissipator basin structure of the vertical drop.

Drops can be used to measure the quantity of water flowing over them. For

example, a vertical drop may be equipped with a calibrated weir section; and

inclined drops may be designed so a s to include a calibrated flume section.

Pipe drops a r e often found useful and economical where a drop can be

sited so a s to t raverse a road o r other crossing of an irrigation canal.

Drops in f a rm channels a r e basically of the same type and function a s those

in distribution canals, the only difference being that the drops in fa rm channels

a r e smal ler and simpler in construction and equipment. They a r e more often

provided with a check gate, which may be a simple slide gate o r a wooden shutter.

Both vertical drops and pipe drops may be employed, although vertical drops a r e

the mos t commonly used.

In the sections which follow in this chapter, exayples of'drops and chutes

applicable to main irrigation distribution systems a r e given in 6. 10 to 6.12, while

examples of drops for fa rm irrigation channels a r e described in 6.13.

6.10 VERTICAL DROPS (OR. FALLS)

6.10.1 General

Energy dissipation by a vertical drop i s usually resor ted to where the drop

i s small, although the interpretation of "small" differs in various par ts of the

world. According to the USBR standards, a small vertical drop i s one which

does not exceed 3 ft ( say 1 m ) , except where the canal i s lined with a hard

surface downstream of the structure, when the drop may be up to 6 ft (say 2 m).

In Australian usage a small vertical drop does not exceed 1.05 m from crest to

downstream bed (52).

The vertical drop structure generally incorporates a stilling basin and some

form of sill or baffle, o r both, combined with side wall arrangements, to dissi-

pate the jet. These structural arrangements should create a reverse rolling flow

at ground level to reduce scouring of the bed immediately downstream of the

structure. Rip-rap i s also usually placed on the downstream side to prevent

erosion. The dimensions of the stilling pool o r energy dissipator depend upon the

height of fall and the discharge over the crest.

Of a large number of designs available examples of three diverse designs

a r e described here, namely, the "Sarda" type used in India, the rectangular weir

drop proposed by SOGETHA (84), the standard USBR drop-check and the YMGT

type drop used in flumed systems in Japan. In addition to these i t must be pointed

out that extensive research has been carried out in the U. S. A. on small vertical

drop structures, which a re dealt with in detail in a special report under 'the

CUSUSWASH water management ser ies of Colorado State University (106).

6.10.2 Sarda Type Fall (India)

6.10.2.1 General

The Sarda type fall i s a vertical drop structure, developed on the

Sarda Canal Project in Uttar Pradesh (India) to replace the notch fall. It has been

tested by hydraulic experiments in the laboratory and by observations on the

prototype. It i s both simple and economical and therefore i s widely used in

Uttar Pradesh. It i s not adapted to flow measurement but i t may serve a s a

meter-fall when calibrated.

There a r e two types of Sarda fall, one with a vertical crest wall for

discharges below 15 m3/s , and another with a trapezoidal crest wall for

3 discharges above 15 m 1s. Only the vertical crest type i s described herein.

At f i r s t no depressed cistern flaring in the downstream wing walls

were provided. While the behaviour of this prototype was, on the whole, very

satisfactory, erosion of the banks downstream of the structure was noticed in

some cases . Model experiments were ca r r ied out at Bahadarabad Research

Station ( ~ t t a r Pradesh) to eliminate the defects,and the design cr i ter ia given

below follow the recommendations based on those experiments.

6.10.2.2 Structural design

The Sarda fall i s a ra ised c res t fall with vertical impact (F'igure 6- 37).

It consists essentially of upstream wing walls, a c r e s t wall, downstream

expansion and wing walls, an impervious floor and a cistern, and downstream

side and bed protection.

The upstream wing walls a r e generally shaped a s a segment of a

c i r c l e with a radius equal to 5 to 6 H(,,t) and subtending an angle of 60°, and

carr ied tangentially into the berm of the channel for a minimum of one met re .

The foundations of the wing walls should be laid on an impervious concrete floor

(a t the level of the downstream bed), which should be extended on both sides for

the purpose.

One o r two drainage holes (15 cm width and 15 cm height) should be

provided in the c r e s t at bed level to drain out the upstream bed during a closure

of the canal.

The top width of the c res t wall, (along the axis of flow) L (c r t )

i s given by L(crt) = 55 H(c-b) in met res

The bottom width (along the axis of flow) of the cres t wall, ( T ) ~ ~ ~

i s given by c-b

(T)bot = (min) q- in met res ,

where H (c-b)

i s the height of the c res t above the downstream bed level of the

canal.

The c res t breadth, B(crt),(across the axis of f1ow)is given by the

expression

B(cr t ) = B1 + Y1

The downstream wings a r e kept vertical over a 1;ngth varying f rom 5

to 8 t imes .f , and then stepped down to their required level. They '1 (Hdr)

a r e then flared, i. e. the water face i s gradually sloped from vertical to 1.5 : 1 o r

lea 0 1 5 x 0 1 5 Downstreom F. S. L.

Downstreom bed

Longitudinal section

(Al l dimensions ore in metres)

SARDA TYPE F A L L (U.P)

1 : 1. If the wings be discontinued after they attain the slope of 1 : 1, the warping

must be continued by the side pitching until i t has a slope of 1.5 : 1.

The total length of the impervious floor should be determined by

Bligh's theory, on the basis of which the safe hydraulic gradients for different

kinds of soils a r e given hereunder :-

Type of soil Hydraulic gradient

sand mixed with boulders - 1 1 9

to - and shingles, and loamy soils 5

light sand and mud 1 - 8

fine micaceous sand

coarse grained sand

The maximum hydraulic gradient will occur when the water i s headed

up to the top of the c r e s t on the upstream side with no flow on the downstream

1 side. The total length of floor required i s - H(c,b), dependihg on the type

s(H)

of soil. Of the total length, the minimum length of floor on the downstream side

in me t r e s should be 2 (water depth upstream + 1.2) + drop. The balance of

the total length may be provided under and upstream of the c r e s t wall.

Upstream of the c r e s t the uplift p ressures a r e more than counter- . . balanced by the weight of water standing on the floor. 4 Also the upstream floor i s

laid below the bed upstream of the fall and i s not subjected to flow, and thus a

thickness of 0. 3 m i s usually adequate for it. The thickness of the floor down-

s t ream i s generally 0. 3 m to 0.45 m .

F o r very small falls no cistern i s necessary, but when the hydraulic

drop warrants it, there should be one, according to the following parameters :

length of cistern, L (bas) = [ ~ ( d r ) ' H ( c r t ) ] 2

depth of cistern

The downstream bed pitching may be protected with dry brick about

20 c m thick rest ing on 10 c m thick ballast over a length th ree t imes y2. A

y2 curtain wall 35 c m thick and of depth equal to 2 , subject to a minimum of

0.5 m , and resting on 15 c m thick concrete, may be provided a t the end of the

pitching.

F o r downstream side protection after the wing walls, the side slope:

of the channel a r e pitched with 10 c m brick, ( i . e . brick on edge) for a length

equal to 3 y2. The pitching should rest on a toe wall having the same dimension^

a s the curtain wall.

6.10.2.3 Design formula

Under f ree overfall conditions:

F o r drowned (submerged) falls:

-2

H(,) i s determined f rom the formula,

H(crt) = (dr) + H ( s )


Data given:

B1 B2 = 1 .5 m

y1 & y2 = 0 . 7 5 m

H (dr)

= 0.90 m

- Maximum hydraulic gradient - 1 in 5

Then B(crt) = B1 + Y1 = 1 . 5 + 0 . 7 5 = 2 . 2 5 m

Top width of c r e s t wall = 0.55 = 0.62 m

1. 29 Bottom width of c r e s t wall - -

2 = 0 . 6 5 m

Increase i t to 0.80 m

Radius of upst ream wing walls should be 5H to 6H(crt) o r 1.80 m to 2.16 m . (4 Assume a radius of 2 m .

F o r calculating the length of the impervious floor, the maximum hydraulic gradient i s 1 i n 5.

Floor length = 5 . 1. 29 = 6.45 m say 6.5 m

Floor length required on downstream side

= 2 ( y 1 + 1 . 2 ) + H (dr)

= 2(0.75 + 1.2) + 0. 90

Provide a length of 5 m on the downstream side.

The remaining length to be provided on the upst ream side and under the c r e s t

= 6 . 5 - 5 = 1 .5 m

The thickness of the floor - 0 .3 m for the upst ream side and under the c r e s t and

0.45 m for the downstream side up to 2.5 m f r o m the toe of the c res t wall.

Length of cistern = r 5 . H(dr) . (4

- - 2.85 m 2 -

Depth of cistern - - 0'25 LH(dr) . H(crt)

2 - I 3

- - 0.25 (0. 324)3

Adopt 0.15 m

. Length of downstream bed pitching = 3 . 0.75 = 2.25 m

~ e n ~ t h of downstream side pitching = 2.25 m .

6.10.3 Rectangular Weir Drop with Raised Crest- 1 /

6. 10. 3.1 General characterist ics

The rectangular weir drop with ra ised c r e s t i s a simple structure for

use on small channels which may be constructed in concrete o r brick masonry o r

a combination of both. It does not measure flow and does not serve a s a check.

It requires little maintenance.

The design i s suitable for vert ical drops up to 7 m, for channel bed

widths of f rom 0.2 to one m e t r e and for full supply depths f rom 0. 1 to 0.7 m. It

consists of upstream bed and side protection, a c r e s t wall, stilling basin and

downstream bed and side protection. (See Figure 6- 38)

The width of the c res t ( ac ross the axis of flow) may be l e s s than the

width of the cistern by 0.10 m , but where there i s an offtake on the immediate

upst ream side of the drop the width of the c res t (in the case of a rectangular

channel) mus t be equal to the width of the channel.

' I Derived from SOGETHA (84)

Bank

Bank Bonk

Section A A

Bonk

Longiludinol section V

F A O - I C I D I R E C T A N G U L A R WEIR DROP

W I T H R A I S E D C R E S T


Developed by S O G E T A H (France)

Figure No. 6-38 C

6. 10.3 .2 Design procedure

F o r a given discharge, Q, hydraulic drop, H(dr), upst ream bed

width, B , and full supply depth, y , proceed a s follows:

Volume of basin, V = . *(dr)

m 3

150

Length of basin, L = 1 . 5 . H(dr) m

Area.of c ross section of the basin along the axis of flow = A = L ( Y 2 + H

2

(bas) m

Width of basin

Depth of basin i s f rom 0. 1 to 0. 3m

In the case of trapezoidal canal, the width of c r e s t - -

B(t) = - O . l o m I

In the case of a rectangular canal B (t) = B1

Discharge formula : 3 -

where C = 0. 36 for a vert ical upstream face of c r e s t wall and 0.40 for

an upstream face rounded off by the quadrant of a c i rc le of 5 to 10 c m radius.

C r e s t water depth, H(crt), i s determined f rom Figure 6-39 for a

given discharge per m e t r e and value of C.

Height of c res t over upst ream bed level, H(b-,) - - Y 1 - H m (4

Other dimensions of the structure a r e a s given in Figure 6-38.

6 .10.3 .3 Numerical example 1

Design a rectangular weir drop with the following data :

FIGURE 6- 39. - Rectangular we i r d rop - re la t ionship between H(,,.), d i scha rge p e r m e t r e width of c r e s t and coeff icients 0. 32, 0.36 and 0.40.

Volume of basin = v - - 200 . 0.80 = 1.07 m 3

150

Length of basin = L = 1 . 5 H ( dr)

= 1 . 5 . 0.80

C r o s s sectional a r e a of ' t h e basin along the axis of the floor = A = (0.50 + 0.10) . 1.20

= 0.72 m 2

Width of basin

Depth of basin = H(bas) = 0 . 1 0 m f

Width of c r e s t = B(t) = B (bas)

- 0.10

F r o m Figure 6-39, for C = 0.40 and q = 143 l / s ,

H(cr t ) = 0.19 m

. . H(b- c ) = 0.50 - 0.19 = 0.51 m

The design of the drop i s shown in Figure 6-38.

6.10.3.4 Numerical example 2

Design a drop in a rectangular channel with the following

data :

Volume of basin =. v = 50 . 0.50 = 0 . 1 6 7 m

3 150

Length of basin = L = 1 . 5 . 0.5 = 0.75 m

Depth of basin = 0.10 m

Area of the basin a l o n g t h e a x i s o f f l o w = A = ( 0 . 3 0 t 0 . 1 0 ) 0.75

Width of basin

Width of c r e s t = B(t) = 0 . 4 0 m

F r o m Figure 6.39 for q = 125 l / s , and m = 0.40

6.10.4 1 / Vertical Check-Drop (USBR) -

The structure described herein i s used along canals having steep t e r r a i

where functions of both a check and a drop a r e required. The maximum

allowable fall a t each drop i s 3 feet, (say 90 cm).

The structure (Figures 6-40 and 6-41) consists of: an upstream approach

of 1 .5 to 3 m (5 to 10 feet) ear th transition gradually widening f rom the normal

waterway section to the width of the check-drop (ac ross the axis of flow) and also

with the bed sloping down to the c r e s t of the check; upst ream bed gravel pro-

tection; check wall with guides for a gate f rame o r stop-logs and wing walls; a.

s t ructure with a pre-cas t concrete deck with a handrail on the downstream side;

and downstream reinforced concrete floor with cut-offs on either end; o r a

stilling basin and floor with cut-offs on either end, downstream wing walls and a

1 .5 to 3 m transition converging f rom the width of the check to the normal water-

" Based on information received through the US National Committee of the ICID.

precast concrete deck

Place grouting mortor as concrete as directed directed on top of asbestos

sheets to provide ,firm bearing surfoces for precast concrete deck.

Section D-D

Section 8-8

bock seat and hondwheel

+"x 6"onchor bolts with square

Plan of precosi concrete deck heods, hex, nuts ond cut woshers. Project 14.

flothead bolts Normal water surfoce

4 dio. x 5 headed anchors, weld to angle. Min. 3 onchors required

CONCRETE VERTICAL CHECK

WITH 1.5 FEET DROP

Section A-A Gate frome guide details

Notes: Outer face transverse bars to be continuous in walls ond floors. Thickness of concrete to vary uniformly between dimensions shown. Ploce grouting mortar'on top of asbestos sheets to provide firm bearing surfoces for precast deck.

Project, Region , Country U S A

Gate frame height measured from centre line of gote opening. I Figure No. 6-40

r

Precart concrete plonks, see &oils

$' oncha bdts a < project i

homftr on dl edger of & n & s

10 anchor Mi wiih s q w hcod, hex. nut, ond cut ~ S h t r

Reintorcement not shown

Plan required

Upstream eneqy level

Reinf. not shown

Stoplog guide details ( 2 required)

F A 0 - I C I D

CONCRETE VERTICAL CHECK

Section B-B WITH 3.5 FEET DROP

Longitudinal section Project , Region, Country U S A

Figure No. 6-41

way section with a bed sloping up f rom the floor to the normal bed of the

waterway downstream. The side slopes of the downstream transition a r e in ea r th

but the bed h a s a coarse gravel protection.

Overflow i s provided for over the check walls a t the inlet on the check with

a 0.45 m (1.5 f t) drop; however, the re i s no provision for overflow on the check

with a 0.9 m (3 ft) drop.

Tables 6- 6 and 6-7 give, for a given discharge, the essential dimension of

the various elements of the check- drop structure for 0.45 m (1.5 ft) and

0.90 m ( 3 ft) drop respectively.

F o r drops up to and including 0.45 m (1.5 ft) the structure shown on Figure

6-40 i s used. F o r drops greater than 0.45 m (1 .5 ft) and through 0.9 m ( 3 ft),

the s t ructure shown on Figure 6-41 i s used.

Numerical example 1 ,

Q 3 = 2 5 f t / s

E l A = 861. 10 f t (elevation of upst ream canal invert)

1 = 2 .00f t

v 1

= 1.6 f t / s (upstream canal velocity)

E l C = 860.00 ft (elevation of downstream canal invert)

v 2

= 1.6 ft / s (downstream canal velocity)

The fall, H (dr) '

i s equal to the difference between the upst ream and

the downstream energy levels . 2 - 1 - h = - - 1. 6L

V1 2 (32. 2) = 0. 04 ft;

2g

where g = acceleration due to gravity in feet per second per second

u p s t r e a m e n e r g y l e v e l = 861.10 + 2.00 + 0.04 = 863.14ft

downstream energy level = 860.00 + 2.00 + 0.04 = 862.04 ft

Note that unless the upst ream water depth and velocity a r e different f rom the

downstream water depth and velocity, HDR can be solved by simply subtracting

the downstream canal invert f rom the upst ream canal invert.

Since H(dr) i s l e s s than 1 . 5 ft, refer to Figure 6-40 and Table 6-6.

3 There i s no s t ructure for a Q of 25 ft 1 s . Consider structure No. 4 with a 3 3

maximum Q of 26 ft / s o r structure No. 6 with a maximum Q of 28 f t / s .

Select structure No. 6 because i t has a "h(wk)" dimension which m o r e nearly

suits the canal section and i t has l e s s concrete than structure No.4.

Assume normal water surface = control water surface

Control water surface = 861.10 + 2.00 = 863. 10 ft

Set E l B so that the top of the check wall is a t

control water surface elevation

(El B must not be higher than E l C)

Other dimensions a r e given in Table 6-6 and Figur.e 6-40.

C

Numerical example 2 .

Q = 30 ft3/s

E l A = 928. 60 ft (upstream canal invert)

E l D = 926.00 f t (downstream canal invert)

y2 = 2 . 2 0 f t

v 2 = 1 . 7 f t / s

L' When a gate of specific height i s not available, a gate with next greater available height shall be used with appropriate f r ame height.

TABLE 6-6 Dimensions of Cencrete Vertical Check with a 1.5 fee t Drop (USBR)

3 1 cubic yard = 27 cubic fee t

TABLE 6-7 Dimensions of Concrete Vertical Check with a 3 fee t Drop (USBR)

Max' NO.

St r . Max. H H L H L l'ws H ~ ~ l d~~ ww2 No. Q (dr) -1 (b-c) ( s t r ) L~~ o r ( T ) ~ Z B T R ~ B~~ EI A EI B EI c EI D

ft3/ s in (T)FR

Standard Dimensions

'CH h L B X L

(wk) ( s t r ) (str)top (vhr) wwl (T)- (T)ww2 1

in in in

Slide Gate

Width x F r m height 11 h t

in ft

- - - - -- -

Example only

Est imated Quantities

Conc. Re. steel Misc. meta l

Yd3 lb l b

Since y = y2 and v - 1 1 - v2' T d r )

= 928.60 - 926.00 = 2.60 ft.

Since Tdr) is grea te r than 1 . 5 f t , r e fe r to Figure 6-41 and Table 6-7.

Select s t ructure No. 6.

Assume normal water surface = control water surface

Control water surface = 928.60 + 2.20 = 930.80 ft

H~~ = 6. 00 ft, HSB = 0.67 ft

E l C = 930.80 - 6.00 + 0.67 = 925.47 ft

(E l C mus t not be higher than E l D)

See Table 6-7 for completed example.

6. 10.5 1 / YMGT Type Drop (Japan)-

6.10.5.1 General

Drops of rectangular notch type built in the past in Japan had suffered

f rom problems of flow turbulence on the downstream of the fall, abnormal waves

and overtopping a t the side walls and damage a t the bottom and side walls of the

canals .

In o rder to solve these problems the Yamagata Prefectura l Govern-

ment evolved a new design of drop for i t s land consolidation projects for use in

flumed distribution systems. The design i s based on the resul ts of hydraulic

model studies ca r r i ed out a t the Agricultdral Engineering ~ e s e a r c h Station,

Ministry of Agriculture and Fores t ry , Hiratsuka.

The structure, which has no provision for a check, i s suitable for

small canals, field channels, o r watercourses, and for discharges of l e s s than 3

one m / s. Figure 6-42 shows a drop s t ructure of s imilar design in use in

Cyprus.

L/ Based on information supplied by the Japanese National Committee, ICID.

Structural design

The YMGT type s t ructure i s a rectangular notch drop o r fall. It

consists of a sill wall and downstream stilling basin with the necessary t ran-

sitional bottom slope in the basin merging with the normal bed of the canal on the

downstream side. The foundations of the sill wall and the stilling basin a r e of

cobble stones, 15 to 20 c m in depth, set in cement. The sill wall and the floor

of the stilling basin a r e of reinforced concrete laid over a 5 cm layer of cement

over the cobble foundations. The fall i s connected to a prefabricated r e -

inforced concrete flume channel a s shown in Figure 6-42.

FIGURE 6-42. - Drop structure in small flume channel (Cyprus).

The standard design i s intended fo r flumed channels, but if the channels

a r e unlined, i t i s essential to have sufficient protection and suitable approach

walls both on the upstream and downstream sides.

Longitudinal 'section Section A tn

Plan

FIGURE 6-43. - YMOT typo drop - s i l l - w e l l and a t i l l i n g baain.

Design formulae

Brink depth, H(br) for a rectangular notch fall without sil l (elevated crest)

where Hc

where H (br)

= brink depth at the end of the notch

Hc = critical depth of flow a t the notch

9 = discharge per unit width through the notch

g = acceleration due to gravity = 9.81 m / s 2

Dimension of the jet trajectory

The trajectory of the jet can be calculated by the following

equation :

Without sill

With sill 1 -

where L(bas)l = horizontal length from sill to the point where the average of the upper and lower nappe meets the downstream water surface line

" : = specific energy corresponding to Hc

H(c-w12) = height between the sill line and downstream

water surface line

FIGURE 6-44. - YMGT type drop - symbols and notations fo r sill height, trajectory of jet and dimensions of stilling basin.

Angle of the jet

Without si l l

tan oc = 0.886

With sill

tan Ot

Velocity of the jet, V(jet)

where H(CEL-b) i s the height between the energy line a t the cri t ical depth over

the notch and the floor of the stilling basin.

Height of the si l l above the upstream bed level

The standard values of the height of the si l l above the upstream bed

level a r e given in Table 6-8.

TABLE 6-8

Stilling basin I

Depth of stilling basin

c (dr)

Length of the stilling basin

where L(bas) = total length of the stilling basin

L (bas)

= horizontal length from the sill crest to the point where the average of the upper and lower nappe meets the downstream water surface line

L(bas)2 = horizontal length from the point where the average ' of the upper and lower nappe meets the downstream

water surface line and the point where the average of the upper and lower nappe strikes the floor of the stilling basin.

.4

L (bas)3 = horizontal length of the stilling basin to dissipate the energy of the jet

4b .4 i s given by equation ( 3 ) or equation (4)

L (bas) L(bas) + L (bas)2

+ L (bas) 2,

TABLE 6-9

Standard Design of Small - Size Fa l l s (Dimensions and Mater ia ls ) in Flumed Canals

Quantity

Rein- P l a in forced ~~~t~~ con- F r a m e Cobble 11.011

. c re t e works stones b a r s m3 ,3 ,3 m2 m3 kg

0.021 7 .05 0.70 0.011 0.12 6.93 0.36 41.41

0.21 10.11 0.97 0.11 0.14 9.99 0.43 57.48

0.021 13.86 1.30 0.011 0.17 13.74 0.50 73.67

1.09 0.029 0.19 10.53 0.56 61.95

1.44 0.029 0.21 14. 37 0.64 80.71

1. 62 0.029 0.21 16.80 0.64 91.25

0.043 13.41 1.41 0.029 0.23 13.23 0.70 77.20

0.043 18.54 1.86 0.029 0.26 18.35 0.79 98.57

0.043 20.52 2.00 0.029 0.26 20.34 0.79 106.38

0.102 25.85 3.32 0.029 0.39 25.37 1.16 161. 63

0.102 32.59 4.04 0.055 0.42 32.11 1.27 447.94

0.102 39.48 4.77 0.055 0.46 39.00 1. 38 510. 62

4.30 0.195 0.50 32.38 1.49 360.36

5.12 0.195 0.54 40.17 1.61 544.37

5.62 0.195 0.54 46.03 1.61 598.23

0.324 48.78 6.27 0.296 0.67 48.22 2.00 662.60

0.324 59.76 7.40 0.296 0.71 59.20 2.14 998.39

0.324 73.34 12.12 0.296 0.81 72.77 2.43 1362.83

Type No.

) 350 )

400

1 500 )

) 700 )

800

1 1000 )

Dis- charge

m 3 ~ s

0.060

0.060

0.060

0 . 1 4

0. 14

0.14

0.225

0.225

0. 225

0.417

0.417

0.417

0.720

0.720

0.720

0.996

0.996

0.996

H (dr:

rn

0.30

0 .50

0.70

0.30

0.50

0.70

0 .30

0.50.

0.70

0.50

0 .70

1.00

0.50

0.70

1.00

0.70

1.00

1.50

Dimensions (cm)

H H L B (t-bflIl (c-b) (bas) '(t-bfll2 (TcW) L(baS) '(bas t) (OP) H(cw) (Tbas) B ( b a ~ f) (bas)

37 45 15 37 15 150 50 15 39.5 15 95 45

3 7 70 20 37 15 150 100 15 64.5 15 95 45

3 7 95 25 37 15 200 100 15 89.5 15 95 45

46 45 15 46 15 250 50 15 34 15 106 56

4 6 7 0 20 46 15 250 100 15 5 9 15 106 56

46 95 25 46 15 250 100 15 84 15 106 56

5 2 45 15 52 15 300 50 15 35.5 15 117 67

5 2 75 25 52 15 300 100 15 65.5 15 117 67

5 2 95 25 52 15 300 100 15 85.5 15 117 67

68 75 25 68 20 350 100 20 64.5 20 151 91

68 105 35 68 20 350 150 20 94.5 20 151 91

68 135 35 68 20 400 150 20 124.5 20 151 91

7 8 7 5 25 78 20 450 100 20 62 20 163 103

78 105 35 78 20 450 150 2 0 9 2 20 163 103

78 140 40 78 20 450 150 20 127 20 163 103

95 105 35 95 20 500 150 20 90 20 188 128

95 145 45 95 20 500 200 20 130 20 188 128

95 200 50 95 30 500 200 30 185 30 208 128

Connection of the stilling basin to the downstream bed of the canal

The downstream end of the stilling basin i s joined to the downstream

bed o r bottom of the canal by a slope of 1 : 4.

To avoid lengthy calculations, see Table 6-9 which has been prepared

to give the various dimensions of the different types of rectangular notch falls in

flumed canals a s well a s quantities of construction mate r ia l s required for them.

6.10.5.4 Numerical example 1

Design a YMGT type rectangular notch fall for an unlined canal,

without ra ised sill and with the following data:

s ides lopes , ( s s ) = 1 : 1

Design 2

- 1 hvl

- = 0.009 m 2g

Brink depth a t the notch, H(br) = 0.72 Hc

= 0 . 3 6 0 m

Depth of s t i l l ing bas in

Length of bas in

0.567

L(ba s ) . -

Angle of jet, oi

0.763

tan x = 0.886 [ 1

L = ( Y , + H (bas )

) cot 6(

= (0.873 + 0.433) cot 38 '5~ '

= 1 . 5 0 6 . 1.230 - - 1.606 m

. . Tota l length of bas in

= 2.5 (0.670 + 1.606)

= 5.690 m

Say 5.70 m

6 .10 .5 .5 Numer i ca l example 2

Design a YMGT type rec tangular notch fa l l with the following

da ta :


P lon

@ dio. 9 L.105 n = 2

C87-I

@ dio. L . 2 4 0 n . 3

Moteriols

Reinforcement bor

Reinforced concrete ' "ortor

Plain concrete

Frome work ,

~obble>l~nes

NO, ~i~ lenpth Unit N,,. Total Unit Total lenoth H~QM weight

1 I 3 2.27 8 18.16 1.04 18.89

0.70 2 UF 300 0.021 n? UF 350 901t. n?

0.12 4 UF 300 7.05 m3 UF 350 693 n?

0.36 n?

I ! !

Dio. 13 28.79 kg Dio. 9 12.62 kg

L

k160- - / -5 (All dimensions are in cen+imetres,l . @ dia. 9 Lz83n.2 - F A 0 - I C I D

dio. 13 C/C 20 k 6 5 -4

dio. 9 C/C 25 Y M G T F A L L - T Y P E 300

dio. 13 C/C 20 5 I5 project , Region, Country

J opon

Section A Section B Figure No. 6-45 -

H(dr) = 0.30 m

Canal is of flumed section

Height f rom bottom to top of flume section = 31.5 c m

Diameter of flume section = 31.5 c m

Design

Refer to Table 6-9.

3 F o r a discharge of 0.060 m / s and a hydraulic drop of 0.30 m , the

type 300 o r 350 may be adopted. The dimensions of the fall a r e a s hereunder :

q c - b ) = 45 cm

H(bas) = 1 5 c m

( Tcw) = 1 5 c m

H( cw) = 39.5 c m

L(bas) = 150 c m

Thickness of basin floor = 15 c m

The design i s shown in Figure 6-45.

6.11 INCLINED DROPS AND CHUTES

6.11. 1 General

The general features and applications of drops and chutes have been

mentioned in Chapter 2 and Section 6.9 of this chapter. There i s no basic

difference between an inclined drop and a chute. Small inclined drops and chutes

a r e usually rectangular in cross section, but trapezoidal sections a r e also

occasionally used where the whole length of the structure happens to be located in

a cutting. The energy dissipation in inclined drops and chutes i s usually effected

by the creation of a hydraulic jump at the toe of the structure, supplemented by

friction blocks and other energy dissipating devices. Inclined drops a r e often

designed to function a s flume measuring devices, notably the Indian Standing Wave

Flume.

Fo r the design of an inclined drop to be effective i t must be based on the

design discharge, depth at the inlet, shape, slope, roughness and length of the

channel (or chute o r flume). The slope of the channel section i s usually steep so

the control section of the flow will be at the inlet. The next, but most important,

step in designing inclined falls (and chutes) i s to compute the water surface profile

from the inlet to the bottom of the structure and to design the energy dissipation

system.

A number of standard designs of such structures have been developed and

examples described in this Section include the Standing Wave Flume Fall , the

Flume Type Fall (both from India), the Rectangular Inclined Drop (U. S. A. ), and

the Rubble Cascade Inclined Drop (India).

6.11.2 1 / Standing Wave Flume Fall (India)-

General

The standing wave flume fall described herein was developed at the

Central Water and Power Research Station, Poona, India, based on the results of

experiments which had started a s long ago a s 1.926. Later, the design of the

structure was standardized by the Indian Standards Institution.

The structure i s used when an appreciable fall of water level i s necessary

due to the surrounding topography (i. e. relatively steep slopes). It dissipates

energy efficiently and it can be designed to measure the flow of water passing

through it over a range from a few l i t res per second up to several hundred cubic

metres per second. Because of the inherent free flow conditions the measure-

L' Based on Indian Standard IS: 6062 - 1971

ment of flow requires only one gauge observation on the upstream side, (whereas

venturi flumes require two). Another advantage i s that i t demonstrates

favourable modularity relationships even with the deposition of sediment on the

upstream side.

The design cr i ter ia in respect of flow conditions in this type of fall

a r e confined to steady flows in open channels dependent only on the upstream head

and without consideration of submerged flows beyond modular limits.

In selecting the site for this flume particular attention should be paid

to the following points:

- straight channel long enough to accommodate the structure

- reasonably symmetrical and regular velocity distribution

- avoidance of super critical flow immediately upstream

- r i se in upstream water levels due to the measuring structure

- absence of conditions downstream which may affect flow conditions

in the controlling section (e. g. submergence by backwater effect

originated downstream from a check, o r silting).

6.11.2.2 Structural characteristics

The standing wave flume fall consists of an approach channel, a flow

measuring device and a downstream channel. . .

The flow in the approach channel should be free from disturbance and

i t s velocity should be distributed a s much a s possible over the cross-sectional

area, (which can be verified by measurements).

The measuring structure consists of an approach transition, a throat

with o r without a hump, an exit transition, a baffle and a platform (between the

glacis and cistern) and for better dissipation of energy and exit i t may have either

parallel sides o r expanding sides (see Figure 6-46). The entire measuring

structure should be rigid and watertight, for at least a length given by L(str), a s

shown in Figure 6-46. The structure should be set a t a right angle to the general

direction of flow.

The channel downstream of the measuring structure, i.e. of the con-

FIGURE 6-46. - Details of a Standing Wave Flume Fall.

trolling section, i s usually of no importance a s regards accuracy of measurement

provided that the fall has been so designed that i t cannot become submerged when

i t i s operating. The effect of a r i se in the water level on the downstream side due

to the possibility of silting would not normally be material , in so far a s rating i s

concerned.

6. 11.2 .3 Measurement of head

The water level upstream of the fall may be measured by any suitable

type of gauge installed in a stilling well.

The stilling well should be located so a s to measure the water level

upstream of the sill, where there i s no curvature of flow. This could be ensured 1

by locating the stilling well intake pipe a t a distance of 4 H ( m a 4 upstream of the

bell-mouth entrance where H I ( m a 4

i s the maximum value of upstream head over

the sill corrected for the velocity of approach. The stilling well should normally

be vertical and have a minimum margin of a t leas t 15 cm over the maximum water

level estimated to be recorded in the well. The well dimensions should be large

enough (say 60 cm x 90 cm) to permit the bottom of the well to be cleaned. The

diameter of the intake should generally be 10 cm.

Zero setting

Means for checking the zero setting of the head measuring device

should be provided, and should consist of a pointer with its point se t exactly level

with the si l l of the standing wave flume and be fixed perpanently in the approach

channel, o r alternatively in the stilling o r gauge well. The zero setting should be

periodically checked.

Head loss

The total head loss i s composed of losses in :

- approach transition

- exit transition

- friction in the structure

- hydraulic jump.

The losses in the approach and exit transitions depend on the degree

and gradualness of the fluming. They a r e expressed a s a fraction C of the

difference in velocity head of flow in the channel and the standing wave flume fall.

Accepted values a r e :

- 0.15 C for an approach t,ransition of cylinder quadrant type, and

- 0.3 C for an exit transition with a splay of 1 in 10, and

- 0.2 C for a hyperbolic type.

Loss due to friction i s usually small , and may be of the o rder of 0.015

to 0.03 m depending upon the size and the cri t ical velocity.

Loss due to hydraulic jump, h (1, ) j

i s given by :

where H(j)

= depth of flow before jump, and

H(j)2 = depth of flow after jump.

The entire measuring s t ructure mus t be finished with smooth (neat

cement finish) and t rue surfaces. The intersection of the upstream curve and the

hump a s well a s the downstream slope must form two parallel straight l ines a t a

right angle to the direction of flow.

6. 11.2.4 Design procedure and formulae

Approach transition

The radius of the side walls of the bell-mouth entrance should be

rn, where H' ( 4

i s the upstream head above the sill level of the throat

corrected for the velocity of approach. But when H' i s l e s s than 0. 3 m , the

radius may be ZH' (c r t )

f rom the throat. The curvature should continue until i t

subtends an angle of 60°, f rom where i t should be continued tangentially to mee t

the side of the upst ream channel. F o r smal ler head losses the radius of

curvature should be increased to 4.5 H' m . This curvature should continue (4

0 until it subtends an angle of 37 30' beyond which the wall should be continued

straight to meet the sides of the approach channel. The bed convergence should

begin on the same cross section a s the side convergence. The radius o f ,

curvature of the hump in the bed should be :

where R(hv) = the radius of curvature of the hump,

L(app) = length between the junction of the side wall with the bed of the upstream channel and upstream end of the throat measured along the axis, and

H(hu) = height of hump above upstream bed of the channel.

Throat

The sides of the throat should be vertical and their length should be

'. ~ { c r t ) '

1

where H (4 i s the upstream head above the sill level of the throat

corrected for the velocity of approach. The width of the throat may be calculated

from the following formula :

b

where Q . = discharge, m3/s

g = acceleration due to gravity

9f) = coefficient for friction having the following values: <

= 0.97 for Q from 0.05 to 0.30 m3/s ,

' 3 = 0.98 for Q from 0.31 to 1.50 m Is ,

B(t) = width of throat,

2 v 1

It should be noted however that too much constriction causes too much

head loss. Therefore the throat width should not be less than 1 . 5 ~ ; ~ ~ ~ ) and 1 /

fluming4should normally be restricted to 50 to 60%.

11 - width of controlling section ( Fluming = . 100 ) bed width of upstream channel

Hump

The stage discharge relation of a canal i s given by:

Q = cly;

where Q = discharge,

C1 = coefficient,

y1 = depth of water in the channel, and

x = an index varying from 1.5 to 2.0 a s given in Table 6- 10.

TABLE 6-10

Values of x

Shape of channel Value of x

1. Rectangular 1 . 5

2. Trapezoidal variable and increases with the flatness of the side slope

3 . Unlined canals with design side slopes 0.5 to 1 1 .6 to 1.7

4. Lined canals with side slopes 0.5 to 1

As compared to equation (4), in the case of a broad weir, Q i s

proportional to H I 5. As the exponent of y i s greater than the exponent of H I , 1

there will be draw-down at low supply levels and ponding near full supply levels,

provided the sill of the throat i s at the same level a s the channel bed. This can

be avoided by providing a hump in the throat. The height of the hump, H(hu),

required to give proportionality, that i s , the rate of change in y equal to the rate 1

of change in H (4 at a particular discharge, i s given by : 1 2

FIGURE 6-47. - Height of hump required to give proportionality for variation in diecharge.

where, H (4 = depth upstream over the sill of the throat, and

m = any particular fraction of discharge.

The height of hump required to give proportionality for a small

variation in discharge will thus vary according to the magnitude of the discharge.

Figure 6.47 gives the height of hump required for various values of m and x.

Where channels run with fluctuating discharge, proportionality i s not

obtainable for the whole range; i t i s then desirable to design the hump for

minimum e r r o r over the range of discharges chosen. This i s called the bulk

proportionality and in this case the height of hump, H(hu), required i s given by the

equation ':

Figure 6-48 gives the height of hump required to attain optimum

proportionality.

In the case of canals which run either full o r closed, a standing wave

flume fall which gives proportionality a t a full supply discharge i s desirable. In

the case of channels in which the discharge varies considerably, optimum

~ r o ~ o r t i o n a l i t y i s preferable.

Glacis

The glacis slope should be 2 : 1 and connected with the throat by a

curve of radius equal to 2 ~ ; ~ ~ ~ ) ~ tangential to the glacis and sill of the flume.

The downstream edge of the glacis should also be connected with a baffle platform

by a .curve of radius equal to H I (4 (see Figure 6-46), tangential to the glacis

and the baffle platform. The axial length of the glacis, including the curved

portion, should be equal to :

0.1 Y l 1.6 1.7 1.8 1.9 2.0

X VALUES OF X IN 0 C1 yl

FIGURE 6-48. - Height of hump to attain bulk proportionality.

where H(c-bpt) = the difference in levels of the crest and baffle platform.

Baffle platform

The baffle platform should be fixed at such a level that a standing wave

will form at the toe of the glacis. If the platform i s too high then hurdling will

occur. On the other hand, i f the platform i s too lo.w, surging will take place. In

the case of a fall with parallel sides, the level of the baffle platform may be

estimated by the following procedure.

The depth of water, y j , above the baffle platform in a parallel sided

fall i s f i rs t calculated from the following equation:

where

H(j)l = the supercritical flow depth at the toe of the glacis

immediately upstream of the jump,

= the discharge per unit width,

Cf = the coefficient of friction.

A level of the baffle platform i s assumed which will give y and 3

H Substituting the value of H in equation (8), (c-bpt). (c-bpt) H(j)i is

calculated. This in turn gives a value of y from equation (7). If the assumed 3

value does not tally with the one worked out as above, more trials a r e needed.

The level of the baffle platform i s then obtained by deducting y from the down- 3

stream water level.

In the case of a fall with expanding sides, the fall in water level,

A Z(exp), must f i r s t be converted into A Z (par)'

which i s the fall in water level

with parallel sides, by using the following equation:

where

(FR) = the flurning ratio : (see Figure 6-46), B1

y3 i s then estimated by t r ia l and e r ro r a s indicated above. With known A Z ( ~ x P ) '

a Z(par), and y the value of y which i s the depth of water above the baffle 3' 4'

platform in the case of a fall with expanding sides may be worked out by using the

equation:

To ensure that the standing wave will form at the toe of the glacis, a

baffle should be provided at the end of the baffle platform. The height of the

baffle, H(baf), i s given by :

where

Hc = the critical depth of flow above the sill of the flume at the controlling section,

that i s

The distance of the baffle from the toe of the glacis should be equal to

5' 25 H(baf)' If the baffle be fixed near to the toe of the glacis then water would

hurdle over the baffle a t supercritical velocity without forming a primary or

secondary wave and the energy would not be dissipated efficiently. The upstream

face of the baffle should be curved with a radius equal to, and ending at, two-

thirds of the H(baf). (Figure 6-50)

Expansion

The sides downstream of the baffle platform should expand hyper-

bolically to ensure uniform distribution of the flow downstream. The hyperbola

equation i s :

where

B (Y)

= width a t any distance y from beginning of expansion of the hyperbola,

Y = distance from beginning of .expansion of hyperbola,

B(t) = width of throat,

B2 = bed width of downstream channel, and

= length of cistern (see below).

Cistern

The depth of the cistern at i t s sides below the downstream water level

should be 1 .4 y2, and in the middle 1.75 y where y i s the depth of water in 2' 2

the channel downstream. The bed of the cistern a t the sides should not be higher

than the bed of the channel downstream.

* The recommended lengths of cistern for different soils follow the

following rules :

in shingle bed, y2

in good earth, 7 .5 y 2

in coherent sand, yz

The longitudinal profile of the cistern should be horizontal.

Deflectors

At the downstream end of the cistern deflectors of the following

dimensions should be constructed to ensure the formation of a positive bed

roller : 1

The height of each deflector should be equal to 12 of"the depth of water in

mid stream.

The gap between deflectors, X (def)

= H (def)

The length of each deflector, L (Clef)

= 4 H (def)

The width of each deflector, B(def) = H(def)

Fo r details see Figure 6:49.

SECTION L ~ ( d , f )

HEIGHT: DEPTH OF WATER IN MID-CISTERN

LENGTH L(d.f)=4 H(d*f)

BRl3QU-l B[.,O

OAP BETWEEN BLOCKS X

DISTANCE BETWEEN TWO ROWS : X ( d . ~ DEFLECTORS

FIGURE 6-49. - Details of deflectors.

6.11.2.5 Modular limit

For satisfactory functioning of the standing wave flume fall the ratio

of the depth upstream over the sill of the throat td the depth downstream over the

sill of the throat should not be less than 0.5.

6.11.2. 6 Maintenance

Adequate maintenance of %he measuring structure and the approach

channel i s important to ensure continual accurate measurements. The approach

channel, the gauge well and the connection to it must be kept clean and free from

sediment, and care must be taken during the cleaning process to avoid damage to

the structure.

Design example

A standing wave flume fall should be designed to satisfy the following

conditions :

STANDING WAVE FLUME - FALL Lonqitudinol section

- bulk proportionality between full supply discharge (FSD) and

1 FSD, and 3

- hyperbolic expansion in the cistern.

Data given

Bed width of canal (B and B2) - -

Side slope of canal - - Bed slope of canal - -

Manning IN' (metr ic) for canal - - Full supply depth in canal (y l and y2) =

Ful l supply discharge - -

F a l l in water level = fall in bed level

= H(dr) - -

Bed level of canal on the upstream side - -

On the bas is of the data given and the procedure set forth in 6.1 1. 2 .4

the design i s i l lustrated in Figure 6-50.

6.11. 3 1 / Flume Type Fa l l (CDO-, Punjab, India)

6.11.3. 1 General

The flume type fall described herein i s widely used in Punjab,

Pakistan, and in Punjab, Haryana and other states in India. It i s a mete r fall,

which i s simple and robust in construction and can con+eniently be built by local

labour in brick masonry.

Up to 1 .0 m e t r e drop, a glacis i s used on the downstream side and if

the drop exceeds 1 . 0 met re , the c r e s t ends in a drop wall. The structure i s

often combined with a bridge, an intake of a third-degree canal o r both.

6. 11.3.2 Structural design

Figure 6-51 i s a sketch of a flume type fall with a drop of up to

0.90 m.

" Central Design Office.

F.S.L.

Depth of floor below downstream bed = 7.5 cm

FIGURE 6-51. - Sketch of a flume type fall with a drop of up to 0 . 9 0 m.

Upstream approaches

1 .5 The radius of the upst ream side wall i s equal to 3.62 H

(c r t ) ' and i t

0 s t a r t s f rom the c r e s t side, the curve subtending an angle of 60 and continues

tangentially to 0.60 m beyond the surface width on either side (B1 + y l ) . The

3 channel discharge mus t be l e s s than 2.8 m 1s .

The horizontal length of the side curve, a s well a s the bed curve,

joining the c r e s t with the upst ream bed = = 3.74 H 1 .5

L ( a ~ ~ ) (c r t ) '

The radius of the bed curve- R . . L~

app + H~

- - (b- C )

Length of Throat

The length of the throat, L(t) = H(cr t )

Glacis

A 0. 60 m (2 ft) curve joins the c r e s t with the downstream glacis.

The glacis should have a slope of 2.5 to 1 for fal ls l e s s than 90 c m (3 ft).

Cistern

The length of the cistern should be y2 + H( dr)

The cistern floor should be 7.5 c m below the downstream designed bed

level of the channel for fal ls up to 1 m (3.28 ft) and 30 c m (1 ft) for fal ls above

1 m (3. 28 ft).

Downstream expansion

L ( e x ~ ) = 3 r B Z - B(t) o r length f rom the downstream -

edge of the c r e s t to the end of the cistern, whichever i s greater . In the case of

fal ls up to 1 m (3.28 ft) , the expansion should be in a curve with a radius of

expansion.

In the case of fal ls above one m e t r e (3.28 ft), the downstream expansion

will simply be diverged.

Upstream protection

The upst ream end of the curved floor in the approach should r e s t on a

masonry drop wall 35 c m thick and of depth equal to 0.33 y subject to a 1

minimum of 27 c m of deep masonry wall over 15 cm thick concrete. No other

protection either in the bed o r sides i s required.

Protection downstream

The side protection below the downstream expansion should be equal to

L(bas) and should consist of dry brick pitching 20 c m thick supported on a toe

wall of depth equal to 0. 5 y Z subject to a minimum of 27 c m of deep masonry wall

over 15 c m thick concrete. It i s preferable to lay roughened pitching. The bed

protection should consist of brick-bats of thickness depending on discharge a s

given below :

Up to 700 l / s 15 c m

700 1/ s to 1,400 l / s 23 cm

This bed protection mus t be laid horizontal (without depressions) a t

bed level, and be hand packed, and should extend up to a length equal to y + H 2 (dr)

beyond the downstream end of the side expansion.

Section of walls

Standard sections of wing walls and abutments a r e given in Chapter 3 .

The f ree board on the walls upstream of the c res t , without bridges, should be

15 cm, and with bridges they should be ra ised to the parapet level of the bridge.

The f ree board on the downstream walls should be 30 %m approximately, the exact

dimensions depending on the brick courses.

Gauges

Where there a r e no bridges, the bottom of the concrete coping on the

wall upst ream of the c res t should be accurately fixed a t designed full supply level. 1

With bridges the full supply level should be shown in the side wall by a 8 inch

s t r ip of steel embedded horizontally in the masonry joint for a distance of 60 cm,

start ing f rom the water surface edge, ( a s i t would ultimately be with a 0.5 to 1

slope). NO gauge walls should be provided in addition.

Bridges combined with falls

When a bridge i s required, the parallel sides of the flumes should be

made longer to serve a s abutments. The bed of the channel up to the upst ream

end of the abutments should be in masonry.

6. 11.3. 3 Design formula

- = 0.60 Fluming ratio, (FR) - B1

Depth of water over c r e s t

This i s worked out f rom the formula

1 .5 Q .= B(t) H(crt)

where Q 3 = discharge in m I s ,

B(t) = width of throat in m ,

qcrt) = height of upst ream full supply level over c r e s t in m.

3 3 F o r discharges of 0.014 m /s to 0.56 m / s C = 1.66

3 F o r dischar;es of 0.57 m / s to 1 .4 m 3 / s C = 1.68.

Numerical examples

Example 1

Design a CDO type fall with the following data :

B1 = B2 = 2.25 m

- 1 - Y z

= 0.56 m

Fluming ratio, (FR) = 60 per cent

. . B(t) = 2.25 . - 60 = l . 3 5 m 100

With Q = B(t) H(crt)

Length of c r e s t = 2 H = 0.8 m (4

Slope of g lac is = 2 . 5 : l

L 1 . 5

= 3 . 7 4 H = 3. 74 . 0.401.5 = 0.92 m ( ~ P P ) ( c r t )

Radius of ups t r eam side = 3. 62 H ~ . ~ = 3. 62 . 0. 4 0 1 a 5

= 0.90 m

Depth of c i s t e rn below bed leve l = 7 . 5 c m

Depth of c i s t e rn below c r e s t = 0 . 8 4 m

Length of g lac is = 0.84 . 2.5 = 2.10 m

Length of c i s t e rn - Y 2 + H ( d r )

= 0.56 + 0. 60

= 1.16 m

Length of expansion

Radius of expansion

The design i s a s shown in F i g u r e 6-52.

Example 2

Design a CDO type fal l with the following data :

Fluming r a t io = 60 p e r cent

Hydraulic drop -------- 060 m Plon Discharge- - - --- - -- -0-56 m3/s

/A// dimensions ore in metres /

Bank

Bank

m

S e c t i o n s No. 3 No. 2 No. I

Longitudinal sect ion

- F A O - I C I D

I C . D . O . T Y P E F A L L ( P U N J A B ) I H Y D R A U L I C DROP U P T O 1.00 m

Country , Region, Project 5 m hand pocked lndio ond Pokiston

C Floor 0.12 rn Concrete 0.15 rn Figure No. 6-52 I

Hydrodic drop -------- 1.20 m

Plon Discharge ------------. 0 5 6 n?/r

Section No. 4

Sect~on Nos l ond 2

Lonqitudinol Section

.. - .-

ncrete = 0.15 m

F A 0 - I C I D

C.D.O. T Y P E F A L L ( P U N J A B )

HYDRAULIC D R O P A B O V E 1 . 0 0 m

/ A / / o'imensions ore in metres l

Country, Region, Project lndio ond Pokiston

Figure No. 6-53

TABLE 6-10

Single Layer Reinforcement in Pool Floor for USBR Inclined Drop

H(dr) Q Lc r t -(t-chb) -bas

Max Max basw * h w

hVbas

2 / Pool Reinf. - Ybas T . T . U U +

h t t ' N0.0f weep

Quantities

Transv bars in walls & floors

4 @ 12" 4 @ 1 2 " 4 @ 1 2 " 4 @ 12" 4 @ 12" 4 @ 12" 4 @ 1 2 " 4 @ 1 z V 4 @ 12" 4 @ 1 2 " 4 @ 12" 4 @ 1 2 " 4 @ 1 2 " 4 @ 12'' 4 @ 1 ~ ~ ~ 4 @ 1 2 t 1 4 @ 1 2 " 4 @ 1 2 " 4 @ 1 1 1 ' 4 @ 11" 4 @ 1 0 1 ' 4 @ 1.2" 4 @ 12" 4 @ 12" 4 @ 1 0 t ' 4 @ 9" 4 @ 8" 4 @ 8" 4 @ 12" 4 @ 11" 4 @ 9" 4 @ 8" 4 @ 8" 4 @ 1 0 t ' 4 @ 9" 4 @ 12" 4 @ l o f1 4 @ 9" 4 @ 8" 4 @ 1 O t t 4 @ 9" 4 @ 8"

Conc. Reinf. Misc. s tee l Metal

(CU. yd) 31 (lb) ( lb)

Longit.

F loo r s

4 @ 72" 4 @ 7 + " 4 @ 7 9 4 @ 74" 4 @ 7+" 4 @ 7i" 4 @ 7 f " 4@7+11 4 @ 76" 4 @ 7 f " 4 @ 7+" 4 @ 7 f " 4 @ 7 " 4 @ 7 "

4 @ 7 Q " 4 @ 7 $ 4 @ 7 + " 4 @ 7 Z f ' 4 @ 7 " 4 @ 7 " 4 @ 7 " 4 @ 76" 4 @ 7 9 4 @ 7 9 ' 4 @ 7 " 4 @ 7 " 4 @ 7 " 4 @ 7 " 4 @ 7 + " 4 @ 7 i " 4 @ 7%" 4 @ 7 " 4 @ 7 " 4 @ 7 " 4 @ 6f" 4 @ 7f" 4 @ 7ftf 4.@ 7 " 4 @ 7 " 4 @ 7 " 4 8 7 " 4 @ 6 + "

i f s t r u c t u r e No. 5-3 indicates Q = 5 f3/s. Hdr = 3 ft .

2'4 @ 7; indicates 0 .4 in. d iameter b a r s on 7f in spacing.

3/1 cubic yard = 27 cubic feet -

Reinf .

Walls

4 @ 10" 4 @ 10" 4 @ 1 0 W 4 @ 10" 4 @ 10" 4 @ 10" 4 @ 1 0 M 4 @ 1 0 g t 4 @ 10" 4 @ 1 0 " 4 @ 10" 4@1O1I 4 @ 1 O t 1 4 @ 10" 4 @ 1 0 t 1 4 @ 1 0 ~ ~ 4 @ 1 0 " 4 @ 1 0 " 4 @ 1 0 1 ' 4 @ 10" 4 @ 1 0 " 4 @ 10" 4 @ 10" 4 @ 10" 4 @ 1 0 t ' 4 @ 10" 4 @ 1 0 f 1 4 @ 10" 4 @ 1 0 M 4 @ 10" 4 @ 10" 4 @ 1 0 1 ' 4 @ 1 0 1 ' 4 @ 8" 4 @ 8" 4 @ 10" 4 @ l o s t 4 @ 10" 4 @ 10" 4 @ 8" 4 @ 8" 4 @ 8"

60 = 2.25 . - = 1.35 m 100

H(crt) a s in example 1 = 0 . 4 0 m

Length of c res t = 2H(crt) = 0 . 8 0 m

L ( a ~ ~ ) = 0.95 m (as in example 1)

R(b- c ) = 2 - 9 0 rn (as in example 1)

Radius of upstream side = 0.92 m (as in example 1)

Depth of cistern = 0 . 3 0 m

Length of cistern = y2 + (Hdr) = 0.56 + 1.20

= 1.76 m.

The design i s shown in Figure 6-53.

6.11.4 1 / USBR Rectangular Inclined Drop-

6.11.4.1 General

The USBR has used standardized drop structures fo i many decades.

Figure 6-54 shows the most recently published design, which was revised in 1970.

The structure i s built entirely in reinforced concrete. Capacities, dimensions

and material requirements a r e shown in Tables 6- 10 and 6- 11.

TABLE 6-11

USBR Rectangular Inclined Drop - Dimensions

I' Based bn USBR Standards (1 07)

The rectangular inclined drop (Figure 6-54) coneists of: an upstream

10 ft earth transition converting gradually the normal side slope of the channel to

1 .5 : 1 a t the upstream end of the structure; an inlet; upstream headwalls;

control section; glacis and pool with chute blocks; expansion; downstream head-

walls; and an earth o r concrete lined transition on the downstream side. The

inlet provides a plank walk-way to operate the gate. Overflow i s provided for by

overflow walls, 'or weirs, built into the sides of the inlet.

The design of the reinforcement steel i s based on working s t resses of

24.000 pounds per square inch (psi). Monolithic concrete design i s based on a

compressive strength of 3,750 psi a t 28 days.

The standard drop structure i s easily designed, built and operated.

There a r e normally no erosion problems downstream i f the stilling pool i s

properly set with regard to the downstream canal water surface. Rocks should

not be allowed to remain in the stilling pool for long periods because of their

rolling, erosive affect on the concrete floor.


Refer to Figures 6-54 and 6-55 and Tables 6-10 and 6-11; the

calculations apply to a 20-11 model structure.

FIGURE 6-55. - Design of USBR inclined drop.

Top of structure = E l B + H = 1022.00 + 3.33 (F 1

= 1025.33ft

(The value of H i s taken f rom Table 6-1 1 . ) (F)

Lower E l B by 0.20 ft. (This will reduce the amount the s t ructure

extends above the canal bank f rom 0 .43 ft to 0.23 f t . )

Refer to Table 6- 10

With H(dr) = 10 ft, select Structure No. 20-11

E l C = downstream energy level = ( Y(bas) + hv (bas) )

= 1013.57 - 2.83 = 1010.74 ft

As a factor of safety against the possibility of unreliable downstream

canal water depths, lower E l C another 0. 10 ft.

Standard dimensions, except for L ( s t r )

and L(gla), which a r e

dependent on elevations B and C, a r e given on Figure 6-55 and Tables 6-10 and

3 6-11 for discharges f rom 5 to 40 ft / s .

To i l lustrate how to set elevations B and C, assume the

following :

v 1

= 2. 1 f t / s (upstream canal velocity)

H = 2.9 ft (height of canal bank) (b- Cbk)

Y2 = 1.50 ft

= 2. 1 f t / s (downstream canal velocity)

Refer to Figure 6-55.

The fall, H(dr) i s equal to the difference between the upstream and

downstream energy levels.

where g = acceleration due to gravity in f t / s L

Upstream energy level = 1022.00 + 1.50 + 0.07

= 1023.57 ft

Downstream energy level = 1012.00 + 1.50 + 0.07

(Unless the upstream canal water depth and velocity a r e different

f rom downstream conditions, H ( d r )

can simply be solved by subtracting El D

f rom E l A. )

E l B can be se t a s high a s E l A provided that the velocity through the

inlet does not exceed 3 f t / s , and provided further that the top of the structure

walls do not extend objectionably above the top of the canal bank.

Try a setting of El B = E l A = 1022.00 ft

Q - Q - - 2 0 Check v = - - - = 2.96 f t / s

A(x) by 1 4.5 (1.5)

and being l e s s than 3 f t / s i s satisfactory.

Check top of bank and top of structure

Top of bank = E l A t H (b- Cbk)

Top of check structure = E l A + HF

On the other hand the length of the structure is :

L ( s t r ) = 2 (El B - E l C)

use 22 ft 4 in

See Table on Figure 6-54 for completed example.

6.11.5 Rubble Cascade Inclined Drop

6.11.5.1 General

The rubble cascade type fall i s cheap and can be constructed where

stone i s easily available. It i s used for small discharges up to 560 l/ s (20 ft3/ s )

o r so and for fal ls up to 1. 5 m (5 ft). It has been installed a t a number of places

in India. +

6. 11.5. 2 Structural design

The fall consists of stone pitching upstream, c res t , downstream

glacis, cistern, curtain wall, and downstream bed and side pitching.

The length of the upst ream glacis i s limited to 5.0 ft. The length of

2 the c res t i s - H The downstream glacis has a slope of 1 : 8. The 3 (cr t ) '

c is tern i s 10.0 ft long and h a s i t s floor level 1 .0 ft below the downstream bed

level.

The details a r e shown in Figure 6-56.

Spoil bank I I

I L 7

. . ,-a. r

fss/ = I.5 : I Berm a t N. S. L. 545 .70 8.0' .

I Y

_519~61_J I :hing ! 1.0 thick downst

Long~tudinol section

Upstreom sectlon

Spoil bonk . Spoil bonk

scale 5 2;5 O 5 10 feet F.S.O. F S.L, F. S. D. B.L. Bed width Free Boord Bank width Bonk level

I F A O - I C I D

RUBBLE CASCADE TYPE FALL

Flume bed wdth to 35 ' o = 3.09 x L r N"' where o = 8.66 t t f r ~ = 3 - i H: (B.66)'" : 0.86

3-09 x 3.5

Length af crest 2/3H: 2/3 x 0.86'0.57 keep, j Crest level : 546.62- 0 86 : 545.76 Cistern to be 1.0' below downstreom bed level Cistern level ; 540.62 , Length of cistern = I 0 0 Upstream gloc!s 1 . 5 ond limited to 9.0 length Downsheom glocis I - 8 Cistern to l i f t r o l l = 1: 3

8 .66 f?/s 5 4 2 . 6 2

1.5' 545.12

4.5; 1.5

3'1 3' 548.12 N. S.L. 545-70

Project. Region, Country

India'

8.66 ft3/s 543.1;

1.5 541.65

4.5, 1.5

3'1 3' 5 44 .62

Downstream sectton A - B I Figure No. 6-56

The fall is flurned 75 to 80% of the upst ream bed width.

6. 11.5 .3 Design formula 3 -

Q = 3.09 33 H' (t) ( 4

where Q = discharge in f t / s

B(t) = width of throat in ft

H = height over c r e s t (c rt)

Numerical example

Design a rubble cascade type fall f rom the following data :

3 Q " = 8 . 6 6 f t / s

Bed width = 4.5 ft

Throat width to be 3 .5 ft.

Then Q = 3.09 B H 2 (t)

- - - 2 2 Length of cres t , L (4 - H

3 (c r t ) - - 0.86 3 -

Adopt 1. 50 ft.

The design i s shown in Figure 6- 56.

6.12 PIPED DROPS

6. 12.1 General

A piped drop is usually the most economical and practical choice when a

necessity for a drop in the canal water level happens to coincide with a road o r

FIGURE 6-57. - Pipe end structures: (a) triangular type; (b) extension wall; ( c ) pre-cast type; (d) impact dissipator (baffled outlet); (e) broken back; ( f ) subway; (g) diffuser., (52)

similar crossing of the canal. A piped drop may also be the most suitable and

economical (even without such crossings) compared to an inclined drop for small

canals (i. e. for small discharges). A piped drop i s usually equipped with a check

gate at i ts upstream end. Sometimes a grid i s installed to prevent the choking of

the pipe by debris. There a r e two main types of these drops - the well drop and

the inclined pipe drop. The application of these i s governed by topography and

soil and, in the ultimate analysis, by cost. In the well drop most of the energy

i s dissipated in the bottom part of the well, while in the inclined pipe the energy

dissipation takks place by the formation of the hydraulic jump, which forms either

in the pipe itself o r downstream of the outlet, depending on the outlet design, the

velocity of the water and the relationship between discharge and the pipe

characteristics.

Piped drops require an outlet which i s designed to dissipate the remaining

surplus energy and to adjust the outflow to normal channel flow conditions. A few *

designs a re shown schematically in Figure 6-57. It should be noted that these

3 designs have been developed primarily for capacities between 1 and 10 m 1s.

Yet some of them (as, for example, the baffled pipe outlet in Figure 6-57 (d) )

have also been standardized for smaller discharges, down to 200 11s. In the

following paragraphs three designs of well drops and two designs of inclined drops

a r e described.

6.12.2 1 / Well Drop Regulator (U. S. S. R. ) -

6.12.2.1 General

The early U. S. S. R. well drop regulators were constructed of re -

inforced concrete pipes, one metre long, with smooth ends.

The well was built of concrete cast in sit.u. The pipes had metal

covers at the joints. Shortcomings of these early structures were the numerous

pipe joints and the absence of an air discharge pipe. The latter resulted in a i r

getting into the downstream flow, which gave r i se to turbulence that damaged the

slopes and pipe joints. The structures also tended to choke with debris.

L / ~ a s e d on information supplied by A. T. Koshkina, E. P. Martin, A. V. Shatalova, D. D. Aliev and B.V. Kazarinov (U. S. S. R. ).


Grovel bed 10 crn Profile of the structure

(All dimensions are in cm) Country, Region, Project USSR

I I Figure No. 6-58

Carrying copocity

Section 1 - 1 Type of structure 85

SHPR P R - 6 0 - 2 5 0 250 1.12

Additional concreting Detoils P- 120 x 180 L is t o f detoils

9 0

1.20

Details SH-120

95

1.32

0 10 (U

I

s a I: V)

I00

1.43

toil TR-80 Detoils P-120x 18

Volume of moin works

Type of detoil

volume , m3

Weight, kg

*:

Number

ravel layer 10 cm

Detoil SHVOO-0- 4 8 0

Cost-in-situ concrete Concrete bed 10 cm

TR-60

0.54

1.350

Nome

Reinforced concrete detoils

Cast-in-ploce concrete

Gravel- fhling

Metal construction

Cross section 4 - 4

Grovel layer 10 cm

! All dimensions are in cm )

TR-60

027

675

Material

Concrete Reinforcement

Concrete M-200

Grovel

FA0 - IC ID

DETAILS OF WELL DROP - REGULATOR,

AS SHOWN IN FIGURE I

Country, Region, Project U S S R

Figure NO. 6-59

TP-80

1-03

2.575

2 . 1 1 2

C .-

m3

kg

m

m3

kg

Type of structure

SHPR-60-250

6.55 -- 608.40

1.2

9.6

167.7

SHVOO 0-480

1.05

2.630

P- 120x186

0.13

325

P- 180-3001

0.30

750

P - 80

0.371

928

0.04

100

6 1 2 1 6

?

SH- I20

0955

138

The new design provides for damping of energy in the downstream end

a s well a s discharge of a i r trapped in the pipe, and protection against choking

from debris.

6.12.2.2 Structural design

The well drop regulator (Figures 6-58 and 6-59) consists of a

rectangular well, a pipeline and a downstream apron. In the upper part of the

well there is an opening fitted with t rash bars and a metal gate. The well i s

formed by two welded lengths of channel cross section. The joints between the

various par ts a r e cement grouted and hydraulically sealed. There i s a

horizontal pipe at the lower part of the well. ' This pipe i s connected with the well

by means of concrete cast in situ, and a mastic packing to provide some mobility.

The diameter of this pipe s tar ts a t 60 cm and then expands to 80 cm.

An asbestos cement vertical pipe i s fitted in the 80 cm diameter par t

to discharge a i r trapped in the main pipe. At the end of the pipe there i s a

damper of cylindrical form with a diaphragm at the end.

The protecting walls of the well and the downstream parts of the

structure a r e fixed by means of triangular slabs and concrete cast in situ.

A trapezoidal downstream apron i s made up of and protected by ribbed reinforced

concrete slabs, placed on a packed gravel bed. All the pipe joints a r e of bell

and spigot type and the joints a r e packed with tow and mineral wool impregnated

with bitumen and cement grouted. The height of the protecting structures above

the downstream and upstream water levels i s 40 cm.


The hydraulic design of this structure has been arrived a t on the

basis of the results of laboratory tes ts on models. The design i s also based on

a hydraulic drop of 2.5 metres .

The discharge capacity of the drop, which varies f rom 1. 10 to

3 1.40 m / s according to different values of H(crt)9 i s given in Figure 6- 59.

The length of the protected downstream apron i s defined by the

formula:

where v = velocity a t the end part of the pipeline, m / s (PI,

"(flu) = assumed scouring or flushing velocity, 0.8 m / s

(for medium loam)

D(p)2 = diameter of the end part of the pipeline, m

2.4 = ' coefficient, determined from laboratory test results.

The length of the protected section of the apron obtained by the above

formula i s decreased by 20 to 30% due to the use of the pipe damper.

6. 12. 2.4 Numerical example

3 Assume a design discharge Q = 1.25 m / s , and a hydraulic drop,

H (dr)

= 250 cm.

It i s necessary to find the length of the protected section on the

downstream side.

The table of discharge capacity (Figure 6- 59) indicates that the

design discharge of the structure i s possible at a head, H(crt), of 92 cm.

The assumed .scouring velocity i s 0.8 m/ s (for medium loam). The

length of the protected section i s defined by the formula.

"(P) v

- 2.4 2 (P) 2 L(prot) - (PI, = 2.4 - 0:6

"(flu) 0.8 3

The length of the protected section i s decreased by 2570 due to the use

of the pipe damper.

Adopted L ( ~ r o t )

= 7 . 9 - 0 . 2 5 . 7.9 = 5 . 9 m

Ful l details of the design with tables of discharge capacities a r e

given in F igures 6-58 and 6-59.

6.12.3 Well Type Drop (India)

6.12.3.1 General design features

A cheap type of well drop consisting of two masonry wells connected

by -a rectangular b a r r e l o r an earthenware pipe has been evolved in India. The

drop (Figure 6- 60) consists of an upstream bed and stone side protection, a drop

well with a single notch to pass the discharge, an earthenware pipe and down-

s t ream well, and bed and side protection of stone pitching.

F o r drops f rom 4 to 6'ft the downstream well may be omitted.

Although this type of drop i s simple and cheap in construction i t i s

prone to choking by floating rubbish and requires regular supervision and removal

of the rubbish without delay. It i s widely used in small channels in some par t s of

India.


F o r discharges f rom 10 to 20 f t3 / s (280 to 560 l / s ) , the diameter of

the wells (both upst ream and downstream) i s 4.0 ft (1.20 m).

One notch i s provided for these small discharges and the formula for

f ree flow used i s :

3 - Q = 3.645 y; (L(No) + 0.4 . C y l )

where Q 3 = discharge of the canal in f t / s

1 = full supply depth upstream in ft

L (NO)

= length of notch in f t

Plan

Upstream water level 1 r

Woter level

Section A-A

Detail of qote

(All dimensions ore in cm unless otherwise specified)

Earthen bund

F A 0 - I C I D

P IPE D R O P SPILLWAY

Clay rawer w concrete pipe length =3 m : 4, = 30 cm


India

Longitudinal section Figure No. 6-60

C = 2 tan cx where 6 i s the angle made by the sides of the notch with the vertical.

In this equation there a r e two unknown parameters, L(NO) and C,

and i t can be solved i f values of Q, say, Ql and Q2 for any two values of y 1'

i. e. Y1. 1 and y a r e known. 1.2

Substituting and solving for C

- Q2 and L

(NO) - * 3 - 0 . 4 C Y l s 2

- 3.745 y

2 1.2

The head required between two wells to pass the required discharge

may be calculated from the following formula :

Numerical example

Data -

Q

Full supply level upstream

Full supply level downstream

Hydraulic drop

Full supply depth, upstream, y l

Full supply depth, downstream, y2

Bed width upstream, B1

Bed width downstream, B2

Free Board, (FB) = 2 0 f t

for Q 3 = 10 ft / s , Y1. 1 ' = l . l f t

Well diameter = 4 ft

Use 2. 0 ft diameter pipe, length - -

Area of pipe A, (P)

- -

. , Water level in the upstream well - -

Sill of drop = 496.10 - 1.65 - -

= 1. 65 ft; - y1. 1 y2

-

2 t a n e • . = 0 . 9 3 or tan = 0.465, say'0.50

say 2.00 ft

Top width of notch = L (NO)

+ ( 2 y tan& )

Invert level of pipe = 482.45

Invert level of upstream well = 482.45 - 1.00

= 481.45

Invert level of downstream well = 485.95 f t

For other details see Figure 6-60.

6. 12.4 Pipe Drop (Indta)

6. 12.4.1 General design features

The pipe drop described herein, which i s provided with a check, i s

a very simple and economical structure to use when a small tert iary channel, a

field channel o r a field watercourse has to negotiate an embankment. The

structure (Figure 6-61) consists of a masonry o r concrete apron around the inlet

of the pipe to prevent seepage, a pipe gate for checking purposes, an earthen-

ware or cement pipe, a stilling basin in concrete, o r brick or stone masonry and

downstream side protection of the embankment in r ip rap, and bed protection of

the channel. The corner or bend in the pipeline i s of large radius. The pipe

joints a r e made good with cement mortar . The stilling basin i s 1.2 to 1.5 m

long.

Table 6-12 gives the diameter of concrete pipes for discharges for

different hydraulic drops, H(dr). For high hydraulic drops, the length of the

stilling basin may be 3 m and for small ones 1.2 to 1.5 m . Its depth should be

equal to (0.10 m + D + 0.10 to 0.15 ) m. (P)

8045 Cement concrete 1:s: I emnt conuete I:S:IQ

k e m e n t concrete ;in I: 3 : s Longitudinal section

Half pkn ot top and half plan at bottom

Section of pipe Section through joint

Detoil of notch

Elevation of notch

WELL TYPE DROP

Project, Region, Country India

Figure No.6-61

TABLE 6-12

Discharge Capacities of P ipe Drops using Different S ize t of Concrete P ipes

H Diameter of pipe, cent imetres

(dr )

Numerical example

c m

30.0

Design a pipe drop with the following data:

Q = 200 l / s

H(dr) = 1 0 0 c m

B1l B2 = 0 . 7 m

YIS Y2 = 0 . 5 m

(as) = 0 . 5 : l

7 . 5 ' 1 0 . 0 12.5 15.0 17.5 20.0 22.5 25.0 27.5 30.0 Discharge Q (11s)

5 .6 10.7 17.5 26.0 36.4 48.5 62.4 78.0 '95 .5 114.8

F r o m Table 6- 12, for Q = 200 l / s and H(dr) = 100 cm, the diameter

of the concrete pipe, D ( ~ ) ,

i s over 29 cm. Adopt D(p) = 30 cm.

Make the stilling basin 3 m long, 65 c m deep and 0.7 m wide. Other

dimensions a r e a s shown in F igure 6- 61.

11 6.12.5 . Inclined Pipe Drop (U. S. A. 1-

11 Based on information supplied by G. N. Thorsky, (USBR. U. S. A. )

6. 12.5. 1 General design features

The pipe drop described here (made of reinforced concrete) i s used up

to a maximum hydraulic drop of 4 . 5 m (15 ft) and i s of non-meter type. The inlet

of the structure i s equipped with a check gate.

F rom Figures 6-62 to 6-65 i t will be seen that the structure consists

of a 10 ft (3 m) transition from the normal canal section at the upstream end

gradually changing to 1. 5 : 1 at the upstream end of the inlet structure. The inlet

structure i s fitted with a gate and a plank walk. The pre-cast concrete pipe f i rs t

inclines sharply downward, then only slightly and then slopes upwards. There i s

a concrete outlet transition and earth transition (10 ft long) with side and bed

protection equal to 2 .5 y2 o r 5 feet (1 .5 m) minimum. In the absence of a

concrete outlet transition, each transition must have coarse 'gravel protection on

both the bed and the sides a s shown in Figure 6-63. The inlet and outlet can be

accommodated to either an ear th canal o r a lined canal.

Dimensions of standard designs of the inlet part including the gate

s izes and pipe sizes a r e given in Table 6-13 and those of the outlet concrete

transition in Table 6-14.

Tables 6-15, 6-16, 6-17 and 6-18 show values of H2, H3, H4, L2,

3 L3, L4 for different hydraulic drops for 10 f t3 / s , 17 ft / s , 25 f t3 / s , and

34 f t3/s . Tables for other values of Q a r e apailable but a r e not included here . I

6. 12.5. 2 Design procedure

Tables 6-13 to 6-18 may be used for design purposes for discharges

up to 36 ft3/s for any given conditions. Pipe drops without a concrete outlet

transition a r e used for a maximum discharge of 22 f t3/s . For discharges

exceeding 22 ft3/ s , a concrete outlet transition i s required.

6.12.5. 3 Numerical example

Q = 1 0 f t 3 / s

Solve for H ( dr)

899.98 1.00 0.06

901.04 upst ream energy elevation

C

891.00 1.00 0.06

892.06 downstr earn energy elevation

901.04 892.06

8 .98 feet = H( dr)

Design check and pipe inlet and solve for elevations of pipe invert .

3 Refer to Table 6- 15 fo r Q = 1 0 f t / s , H

(IN) = 3. 25 ft,

and diameter of pipe = 24 inches.

3 Refer to Table 6- 13. There i s no structure for a Q bf 10 ft / s and

3 D(p) = 24 inches, so select the next highest, Q = 17 ft I s , which i s Structure

NO. 24-1.

900.98 = normal water surface = control w,ater surface

-3.25' =

897.73 = E l D (Figure 6-63)

F o r the computed H of 8.98 ft, go the the next highest H(dr) (dr)

of 9.00 ft (Table 6-15) and obtain H2, Hg, Hq, L2, Lg, L4.

Lonqi tudinol section

Normal water surface Original ground surface

EElevat ion of energy rs t ruc ture length r Precast concrete pipe

Normal water surface

---

Check and pipe inlet

b i t e r e d pipe bendsJ

~ o t h s .

The pipe slopes used will allow the substitution of 7° 36 precast concrete elbows for the mitered pipe bends.

The length of the earth outlet transition equals 3 pipe diameters (j minimum).

Precast concrete pipe shown. Other materials may be substituted

provided joints are rubber gasketed. I OUTLET. TRANSITIONS

Country , Region, Pro jec t

F igu re No. 6-62

Longitudinal sect ion

,--Original ground surface

Normal water surface Precast concrete pipe rElevation of energy

Earth transition L+L. ~ - - - i , + - ~ ~ ~ , - ~ l Normal water rE lev~ t i~n

Check and pipe

" i 5minimum) LMitered pipe bends

12 coarse gravel protection

Concrete transition

For outlet tmnsition details refer to Figure 4. I

provided joints are rubber gasketed.

Notes

The pipe slopes used will allow the substitution of 7 O 30' precast concrete elbows for the mitered pipe bends.

Precast .concrete pipe shown. Other materials may be substituted

I O U T L E T TRANSITION

FA 0- IC ID

PIPE DROP WITH CONCRETE

Country, Region, Project U S A

I F igure No. 6-63 I

Plan Typical slide Section B - B gate assembly

ly qote puides .

#4 0 8. Bend into hcodroll

4 0 12, Both w y s

or horizontolly with

obout rnid-Ienqth

Section A-A

& 16 bolts's f with squore heods, hex,'nuts ond cut woshers; project 4"

F A o - IC'ID

CHECK AND PIPE INLET

oround corners

Country , Region, Project continuous in ,walls ond floor U S A

Figure No. 6-64

Plan Section A-A

woll thickness to be some

. .

einforcement not shown

~onqitudinal section

into sidewalls

- 3 Lt4 tronsv. 12

Tronsversc bars continuous in wolb ond floor,

rpoced horizontally on structure C. L.

Section C-C

Reinforcemenl not shown

4~) - 3

FrGURE 6- 65. - Concrete outlet transition (supplement to Figure 6- 63).

TABLE 6-13

TABLE 6-14

Struc- t u r e No.

12-1 15-1 18-1 18-2 18-3 21-1 21-2 24- 1 24- 2 27- 1 30- 1 33-1 36- 1

Struc- Q D B L H B

t u r e (P) (OUT)l (OUT) H5 ( W W ) ~ H6 (Tft) L(ww) (Ttw) No.

12-1 5 12" 15-1 7 15"

6" 6"

18-1 10 18" 6" 6"

18-2 15 18" 6" 6" 6" 61-

18-3 21 18" 21-1

6" 6" 13 21"

21-2 6" 6"

28 21" 24- 1

6" 17 24"

6"

24-2 6" 6"

37 24" 27-1

6" 21 27"

6"

30- 1 4' 9" 31 0" 6" 6"

26 30" 9" 7 ' 6" 3' 0" 12" 6" 24" 33- 1

6" 31 33" 6" 6"

36- 1 36 36" 11" 9 ' 6" 5 ' 3" 31 3" 3' 6" 15" 6" 246!? 6"

Standard Dimensions

Q D B H L H T( sw) (FB) L (P) (IN) (IN) (IN) (IN) Or ( =hw'

(Tft) (hw) /

f t 3 / s

!' 1f a gate wlth a speclfled height IS not available, an available gate with the next g r e a t e r helght should be used wlth appropriate f r a m e h e ~ g h t .

5 12" 24" 21 6" 3' 6" 15" 6" 12" 4' 0" 6" 7 15" 24" 2 ' 6 1 , 3 1 6 , ~ 15" 6" 6" 12" 4' 0"

10 18" 2' 6" 2' 61, 31 68, 15" 6" 12" 4' 0" 15 180 21 69, 39 69, 4 t o v 21" 6" 6" 12" 4' 0"

2 1 18" 3' 0" 51 001 41 6" 3' 0" 6" 6" 12" 4' 0" 6"

13 21" 3 ' 0 " 3, 0,) 41 0" 15" 6" 6" 12" 4' 0"

28 21" 3' 6" 5 ' 3" 51 0" 3' 0 8 , 6" 12" 4' 0" 6" 17 24" 3' 0" 3' 3" 4' o w 15" 6" 6'1 , 12" 4' 0" 37 24" 4 1 0 " 51 6" 51 61, 3 t o 1 t 6" 6" 12" 4' 0" 21 2719 39 608 31 6" 4 ' 6 " 15" 6" 6" 15" 4 ' 4 " * 2 6 30" 31 6" 31 qt? 41 61, 15" 6" 15" 4' 4" 6" 3 1 334, 41 0" 4, 0" 41 6" 15" 6" 6" 18" 4' 9" 3 6 36" 4 , or! 4' 3" 58 0" 15" 6" 6" 18" 4' 9"

No.of walk planks

3 3 4 4 5 5 6 5 7 6 6 7 7

1 . 6 160 110 1 . 7 170 110

2 . 1 220 130

2.2 230 140

2.6 270 150 2.9 300 160

3.4 340 180

Est imated Quantities

Con- Reinf. Misc. Crete s teel Metal

(cu.yd) (lb) (lb)

Slide Gate 1.1 Width x Ht. F r a m e

B(ga) H(ga) H(frrn)

TABLE 6-15

Pipe Drops with Metre Bends Without Concrete Outlet Transition

3 Q = 10 ft / s Diameter of pipe = 24 inches H (IN) = 3.25 feet

Dimensions given in the Table are in feet

TABLE 6-16

Pipe Drops with Metre Bends Without Concrete Outlet Transition

Q = 17 f t3 / s Diameter of pipe = 30 inches H(IN) = 3 . 75 feet

Dimensions given in the Table a r e in feet

TABLE 6-17

Pipe Drops with Metre Benda with Concrete Outlet Transition 3

Q = 25 ft / s Diameter of pipe = 30 inches H (IN)

= 3.75 feet Dimensions given in the Table are in feet

TABLE 6-18

Pipe Drops with Metre Bends with Concrete Outlet Transition 3

Q = 34 ft / s Diameter of pipe = 36 inches H(IN) = 4.25 feet

Dimensions given in the Table a r e in feet

3 F o r pipe drops with discharges exceeding 22 ft / s , a concrete outlet

transition i s required.

Refer to Figures 6- 64 and 6: 65 and Tables 6- 17 and 6- 18.

Fo r the concrete outlet transition, see Figure 6-65.

Design procedure for the check and pipe inlet and the pipe drop i s the

same a s that given above for the pipe drop without concrete outlet transition. ,

6.12. 6 1 / Inclined Pipe Drop (U. S. S. R. ) -

6. 12. 6. 1 General

Ear ly pipe drop regulators used in the U. S. S. R. were built of a

number of one met re diameter reinforced concrete pipes with metal covered pipe

joints. Sudden water level fluctuations in the channels resulted in joint mis-

alignment and downstream scour.

The revised construction now used i s simple and reliable in operation,

and provides dissipation of energy downstream. This pipe drop regulator i s both

a drop and a check structure.

6. 12.6.2 Structural desipn

The structure (Figure 6-66) consists of an inlet siil, an inc lhed pipe,

a stilling basin and a downstream apron.

The inlet sill i s designed in the form of an inclined wall. A concrete

L' Based on information supplied by A. T. Koshkina, E. P. Martin, A. V. Shatalova, D. D. Aliev and B. V. Kazarinov.

I Threshold l c o n c r e t e ) ~ Road Section 1 - 1

Reinforced concrete I

Concrete, type 100, layer 6 cm

. . L c o s t -in- dace. Stilling basin--/ I/ . 1

I concrete, iype ioo ~ o c k - f i l l i n ~ - q - ' (E~o?) 4

Section 2-2

/- Prefab. concrete slabs

Reinforced concrete

L ~ ~ p e 100, cost -in - place, concrete loyer 6 crn

Concrete slobs

d

Cross section 4 - 4

PIPE DROP - REGULATOR

Type 100, cost-in-place concrete layer 10 cm

(All dimensions are in cm

threshold (the height of which i s equal to the difference between the water depth in

the channel and i t s critical depth at threshold) i s provided to prevent a drop of the

upstream water level.

The inclined pipe, consisting of bell and spigot jointed pipes, 5 m long,

60 cm or 80 cm in diameter, and the stilling basin, a r e constructed of reinforced

concrete. A small length of pipe with bevelled edges i s inserted where the pipe

levels off to the horizontal. The pipe joints a re packed with tow and mineral wool,

impregnated with bitumen and cement grouted.

A cylindrical stilling basin with a ring diaphragm at the end i s provided

at the end of the pipeline. Reinforced concrete slabs set on a gravel bed secure

the inlet and downstream parts of the structure in place. A rock-filled support i s

provided'at the inlet and at the end of the protected section.

These s t r tc tures a r e equipped with slide gates and screw jacks and the

gate frames a re secured to the inlets with bolts and rubber packing.

6. 12. 6. 3 Design formula

Maximum discharge capacity i s given by the formula:

where D ( P)

= diameter of the pipe.

This formula has been deduced on the basis of laboratory tests at the

Central Asia Scientific and Research Institute of Irrigation.

The head, Hc, with a f ree opening, according to laboratory test

data, i s assumed a s follows :

where C1 = 0.'8

and H' = specific upstream energy relative to the floor of the (bas)l stilling basin; equal to the height between the upstream

energy line and the floor of the stilling basin

H(j)l = the depth of flow with normal discharge at the beginning

of the hydraulic jump, and

q = the discharge downstream per me t r e mean width of the channel,

where s s = side slope.

Reciprocal depths downstream a r e calculated from Table 6- 19 using

11 formulas:-

and

where H(rec) l

= reciprocal depth of flow with a discharge, considered a s controlling, a t the beginning of the hydraulic jump

and H(rec)2 '= reciprocal depths of flow corresponding to discharge, considered a s controlling, at the end of the hydraulic

jump

H ' = specific upstream energy relative to floor of the (bas) stilling basin; equal to the height between the

upstream energy line and the'floor of the stilling basin.

C' and C" a r e coefficients whose values a r e given in Table 6- 19.

TABLE 6-19

1 / - according to Agroskin

It i s assumed that the velocity coefficient i s 0.8; the coefficient of

hydraulic jump submergence C(js) = 1 .1 .

The length of the downstream protected section i s calculated according

to the following formula, (deduced a t the laboratory) : 1 -

where q = B2 + ( s s ) Y Z

6. 12. 6 .4 Standard designs

The s t ructures a r e designed for maximum discharges of 0 .4 and 3 0.85 m / s for drops of 100 and 200 cm. Channel depths a r e assumed a t 60 and

80 c m according to pipe diameters 60 and 80 cm.

The height of the embankment above the upst ream and downstream

water levels i s 30 cm.

6.12. 6. 5 Numerical example (for type TPR-80- 100)

Data given

Pipe diameter, D ( ~ ) = 0 . 6 0 m

Hydraulic drop, H (dr)

= 1 . O m

Downstream bed width, B2 = 1 . 2 m

Downstream canal depth, y2

= 0 . 6 m

( s s ) = 1.5

Coefficient of hydraulic jump submergence, C = 1.1.

( j s )

It is necessary to calculate the discharge capacity and the length of

the protected section L ( ~ r o t )

According to the formula

The head a t the threshold of the structure

Hc = 0.86 D = 0.86 . 0. 60 = 0.52 m

(P)

The depth a t the beginning of the hydraulic jump:

1 = 0.8

A f i r s t approximation :

F r o m Table 6-19; for H ( ~ ) = 0.15 and for interpolation:

Taking into consideration the coefficient of hydraulic jump

submergence

'(j s ) = 1 .1

The length of the protected section

= 8 . 0 . 4 3 6 . 1 .1

= 3.85 m, Say 4.00 m.

6.13 FARM DROP STRUCTURES

General

Drops in farm channels a r e basically of the same type and function

similarly to those in distribution canals, the only differences being that the farm

drops a re smaller and their construction and equipment a r e simpler. They a r e

more often than not provided with a check gate, which may be a simple slide gate

or a wooden shutter. Vertical drops a r e the most frequently used. F a r m drops

should comprise: a cut-off wall, long and deep enough to prevent leakage and by-

passing of the water a t the flanks; an opening with slots for a check gate; and a

stilling pool with some form of end sill.

In 1971 the USDA Soil and Water Conservation Service published results of

field tests of 16 different types of farm irrigation drop- check structures (60).

Out of the conclusions drawn, the following five meri t quotation here.

"(1) Although there did not appear to be a consistent relationship between the

amount of scour and the end-sill height, visual observation indicated that there

was a greater degree of turbulence with the high sills.

(2) Structures having relatively wide basins performed better than those

with narrow basins. The narrow basins contracted and accelerated the flow,

resulting in higher exit velocities; the wide basins provided a larger flow area

and thus a lower velocity.

(3) With adequate tailwater depth, a trapezoidal stilling basin gave good

hydraulic performance: without sufficient tailwater, the performance was poor

and high velocity caused excessive downstream erosion.

(4) F o r relatively smal l s t ruc tu res and water depths, a non-aerated nappe

contributed to good stilling within the s t ructure .

(5) With adequate cut-off depth and head wall length, head wall s t ruc tu res

with a gravel-lined basin o r plunge pool were the mos t economical and the mos t

effective s t ruc tu res tested. "

6. 13. 2 Head Wall Drop with Gravel Basin

The s t ructure described in conclusion (5) quoted above i s perhaps the mos t

economical type of f a r m drop under al l conditions. The head wall o r cut-off wall

may be made of concrete o r masonry . The wall should be made considerably

s t ronger than in the s t ruc tu res having supporting walls in the direction of flow.

-Thus , for a masonry wall the thickness should be a t l eas t 30 cm, for unreinforced

concrete 20 c m and for reinforced concrete 10 cm. The width of opening required

may be calculated f rom the formula given in Section 6. 2. The length of the gravel

basin may be taken a s approximately 3 to 4 t imes the difference between the

ups t ream and downstream bed levels . The width should be about 1.5 t imes the

c r e s t length of the opening. Figure 6-67 shows this type of drop with a

head wall made of p re -cas t concrete.

FIGURE 6-67. - P r e - c a s t concrete head wall drop (60).

Cement Block Check and Drop

Figure 6-68 shows a design of a cement block check and drop a s developed

and used successfully in Canada. The length of the stilling pool i s about twice

the cres t height above the pool. This relatively short .distance i s compensated

for by the downstream gravel protection.

Wooden door

Flow Concrete - 8 x b i 16 block8

Wooden doo

6 x 8 x 16 blocks

Directions I. Dig down os shown by survey.

.2 . Stock blocks t o desired shope for correct locotion of 'woll ond h e i ~ h t of sill.

3. Pour concrete in cores of blocks-eoch row seporotely.

4. Pour remoining concrete for splosh ond floor. .

5. Any steel (spud links,etc.) in cores will

es ) greotly strengthen the structure. (A l l dimensions i n inch

FIGURE 6-68. - Cement block check and drop structure (Canada).

Concrete Check Drop

The structure shown in Figure 6-69 i s widely used in the U. S. A. where i t

i s usually made in pre-cast reinforced concrete with a wall thickness of 7.5 to

to 10 cm. The main dimensions are shown in the figure and in the tables below:

FIGURE 6- 69. - Concrete check drop ( U . S. A. ).

Capacity of Width of ditch in opening W

H C A

11 s cm cm * cm cm

60 30 3 0 15 60 170 60 30 15 6 0 230 75 38 15 6 0 280 90 4 6 2 0 75 400 105 46 ' 2 0 90

Drop (D) Length of Apron (L) -

rn

30 7 5 4 5 9 0 60 120 9 0 180

6. 13.5 Wooden Drop

Figures 6-70 and 6-71 show designs of wooden drops for discharge

capacities of 100 l / s to 250 l / s . These designs were introduced some 30 y e a r s

ago by the USDA Soil Conservation Service and they have proved suitable and

economical. The wood used for the s t ructure should be thoroughly impregnated

before assembly.

6. 13.6 Piped ~ r o ~ s

A simple pipe drop structure recommended by the USDA Soil Conservation

Service for f a r m irrigation systems i s shown in Figure 6-72. The figure also 3

provides the necessary data for design capacities f rom 45 11s (1 .6 f t /s ) to

105 11s (3.7 f t3 /s) . Protection by rip-rap, gravel o r concrete lining may be

required on erosive soils . The corrugated metal pipe may be substituted by a

concrete pipe. 1

The steel b a r r e l drop shown in Figure 6-73 i s a simple and cheap pipe drop

recommended by the Alberta Department of Agriculture, Canada.

The following formula and table may be used for design purposes :

where Q 3 = discharge in f / s

A = pipe a r e a ft 2

X

head differential

L = pipe length

C = head loss coefficient.

lide. See below

g-ODMin. I B I -

PLAN

subst~tuted here

Pressure treat oll lumber w ~ t h creosote and use cement c o o t ~ d n o ~ l s Corr~oge bolts moy be substl - tuted for no~ l s where ~ndlcoted, ~f deslred Upstreom jolnts moy be covered w ~ t h loth bottens to moke

DETAIL OF GATE GUIDE structure more water- t~ght WOODEN DROP STRUCTURE Use when drop serves

Bs Bottom w~dth of openlng FOR CHANNEL DEPTH

as a check b = Bose width of dltch 20 cm ( 8 inches ) d =Depth of water In d~tch H =He~ght of fall In woter surfaces L = Lengf h of opron Q= Cublc feet per second

OMETRIC VIEW

SECTION X- X

cooted nolls Corr~oge bolts may be substituted

DETAIL OF GATE GUIDE for nolls where lnd~cated ~f deslred Upstream joints moy be covered w ~ t h loth battens to make structure

Use when drop serves as a check

water surfaces

PLAN

ISOMETRIC VIEW OF water Surfoc* CONCRETE SLAB

(See no* No. (I)

r Top of Ditch Bonk -,

water Surfac*>

SECTIONAL ELEVATION ON CENTER LlNE

- - . . N O T E S

I SELECT A PIPE S I Z E THAT W I L L PROVIDE A GREATER CAPACITV THAN I S REQUIRED TO DISCHARGE THE NORMAL STREAM USE0 W E N IRRIGATING. TRY TO KEEP THE VELOCITY I I THE P l P E B E L W 3 FPS BASE0 ON I O R W L I R R l G A T l I G STREAM.

1. W E R THE CMlRUGATEO METAL P I P E OROP I S U S I D AT A D I T C H CROSSIIG. INCREASE WIDTH OF TOP OF DAY AND DIMENSIOI L2 BY 8 ' -0 '

3. THE OROP (H) FOR ANY SPECIFIC STRUCTURE C A I BE IICREASED 3 I ICHES BY PLAClIGrTHE TOP OF THE RISER PIPE 3 l n c n c s BELW THE TOP OF THE CONCRETE FLOOR OF THE INLET THE THICKIESS OF THE FLOOR SLAB ~OJICEIT TO THE PIPE s n o u L o BE INCREASED 3 INCHES TO MAKE A WATERTIGW CMlNECTlOI WITH THE PIPE. THE I N L E T TO THE P l P E SHOULD BE ROUNDED TO A 3 I I C H RADIUS TO SAVE FORMING AN0 IMPROVE THE EFFICIENCY OF THE INLET

U. THE DROP STRUCTURE I S FORMED BY CUTTING A STANDARD LEIGTH OF CORRUGATED WETAC P l P E w n l c n IS M~IUFACTUREO II MULTIPLES OF 2 FT. IX LEIGTH. 01 A u 5 O &#OLE AND WELDING THE C U i JOINTS TOGETHER TO F M l M A 90° BEN0 P l P E TO BE 1 6 GA CORRUGATED MET L JOINT BETWEEN HORIZONTAL A I D VERTIC&L PIECES OF P l P E TO BE BUTT WELOEO AND WATE!TlGIIT

5. SIX l N c n n r n o PLACEO RIP-RAP w r r BE s u s s r l r u r r o F ~ R c o w r f SLAB.

N O M E N C L A T U R E d - o E n n OF WATER IN o l i c n F - FREEBOARD I I DITCH 0 - DIAMETER OF P l P E R - LEIGTH OF VERTICAL P I P E ALONG CEMTER L l N E 12-LENGTHOFHORIZONTALPIPEALMGCENTERLIRE V - VELOCITY OF PIPE - FPS V - O I S C L R G E THROiJCH PIPE - C.F.S. II - DROP OF WATER SURFACE

T A B L E OF C O N C R E T E Q U A N T I T I E S O=IO' 0.15 CU.VDS. 0 - 1 2 " 0 . 2 6 CU VOS D=15. 0 2 9 C U Y O S

F A O - I C I D

CORRUGATED METAL PIPE DROP

Project, Region, Country U S A

Figure No. 6-72

FIGURE 6-7.3. - Steel barrel drop.

P ipe diameter inches

10

12

14

16

Values of C Concrete pipe Corrugated metal pipe

0.053 0 . 1 3 4

0.042 0 .107

0.034 0 .087

0.028 0 . 0 7 3

-6.13.7 Sloping Rock Drop

Another cheap drop design from Canada i s shown in Figure 6-74. It i s

suitable where greater falls, say 1. 5 to 3 m, a r e encountered, and where suitable

rock o r stone i s available locally. Gravel-fill between the rocks, or grouting,

improves greatly the durability of the structure.

." ,---Not under 12

-Ditch bed line

l+or more b c - I

Note: I. Grovel (if ovoiloble) should be

used to fill between rocks. 2.Rocks con be grouted.

Ditch crosa section

FIGURE 6-74. - Sloping rock drop structure (Canada).

. 7. STRUCTURES AND DEVICES FOR WATER MEASUREMENT

7.1 INTRODUCTION

7.1.1 Scope of this Chapter

The scope of this chapter i s restricted to the measurement of water in

irrigation systems and to methods of measurement which need only inexpensive,

but reliable and easily operated equipment. Almost any kind of obstacle that

partially res t r ic ts the flow of water in an irrigation channel can be used a s a

measuring device, provided that i t can be calibrated. However, the calibration

tests necessary to detelop accurate ratings can be rather costly and time

consuming and justifiable only where the calibrated device i s to be utilized for

a number of different purposes, o r in the case of large structures outside the

scope of this Handbook. For measuring small flows (say below 1000 11s) i t i s

nearly always preferable to use one of the numerous standard measuring devices

or ratings already de~eloped. In this chapter emphasis i s placed on standard

devices, which a r e defined as those which have been fully described, accurately

calibrated, correctly installed and have proved to be consistently successful in

operation. Before proceeding to the descriptions of these various measuring

devices i t i s appropriate to recall a t this stage the reasons for the measurement

of irrigation water and where in the system such measurement should take place.

Why Measure ?

For efficient water distribution

Increased demand on available water resources and ever increasing

irrigation development costs require that water be used economically and without

waste, and experience shows that this cannot be accomplished without water

measurement. Measurements serve to ensure the maintenance of proper delivery

schedules, to determine the amounts of water delivered, to single out anomalies,

and to estimate and detect the origin of conveyance losses.

For efficient water use at the farm level

More advanced knowledge of soil properties and soil moisture/ plant

relationships permits irrigation systems to be designed so that water can be

applied in the right amount and at the right time in relation to the soil moisture

status thereby obtaining maximum efficiency of water use and minimum damage

to the land. This knowledge can be utilized most effectively only by reasonably

accurate meagurement of the water applied.

For applied research

To establish cr i ter ia for efficient water use and management,field

t r ia ls and evaluation of existing irrigation a r e required for a number of ieasons

such as the evaluation of the efficiency of existing irrigation and to determine

intake rates, s t ream sizes required, length of furrows and border runs, water

losses, etc. Accurate water measuring devices a r e indispensable for such

t r ia ls and evaluations.

For socio-economic factors

Whether water be public or private property, water measurement '

i s an important means for implementing a distribution pattern to meet actual

requirements or legal rights or both, and for providing a reasonable basis for

estimating water charges. If the charge to the user i s based on the rate of

flow, then rate-of-flow measurements and adequate records a r e required.

Charges on the basis of volume necessitate a volumetric measuring device,

o r a rate-of-flow device combined with a time recording device. Ideally,

water flow should be measured at intakes from storage reservoirs, canal

headworks, at strategic points in canals and la terals and at delivery points to

water users .

7 .1 .3 Where to Measure

In the terminal distribution system facilities for water measurement may

be required, o r be desirable at intakes to lateral canals (distributaries, etc.)

o r at other bifurcation points. Clearly the most important point for measure-

ment i s the farm outlet (or turnout) which i s the meeting point between the

management and the water users .

The degree of need for a measuring device at the outlet varies according to

the delivery system employed. Delivery on demand usually relies upon the

measurement of water a s a basis for equitable distribution a s well a s for

computing possible water charges. Where water i s distributed by rotation

among fa rmers along a lateral (or distributary or "minor" canal) and where the

amount of water supplied to each farmer may be different, a measuring device at

the turnout i s required. On @e other hand, i f f a rmers along a la teral receive

water on the basis of area of land or crops irrigated measurement i s not entirely

necessary, but may still be desirable for other purposes, such a s improvement

of irrigation efficiency. Similarly, in all systems based on constant flow,

measurement i s not entirely necessary but may be advantageous.

Where several fa rmers share the water of each outlet and the flow in the

canal fluctuates considerably, each such outlet should be equipped with a

measuring device, ,even i f equitable distribution among outlets i s practised, so

that each group of fa rmers will know the flow available at any one time from their

respective outlet.

It follows that i f all the irrigation water from an outlet i s to be delivered

to one field (or farm) at a time, the measuring device on the outlet may be the

only one needed. But, i f the supply i s divided between two or more ditches i t

may be desirable to install some kind of simple measuring device at each offtake.

(For a comprehensive description of farm outlets see Chapter 5. )

7. 1 .4 Limitations

Water measurement i s a difficult problem in many irrigated areas: the

head available in the irrigation system may be too small to allow accurate

measurement; the varying water requirements on the farms and supply

variations cause fluctuations in the levels of the water in canals o r variations in

velocity, o r both; the presence of weeds and silt, the difficulty of maintaining

close tolerances during construction and many other factors may reduce the

accuracy of water measurement.

Considering that there may be a large number of outlets on an irrigation

scheme, the introduction of a delivery system based on water measurement a t

turnouts may require a large and costly operating organization which may involve

problems of personnel, recruitment, training, etc.

The cost factor i s particularly important where farm units a r e small or

economic returns low. In such cases, simple devices with less accuracy should

be selected (e . g. calibrated shut-off gates a s discussed in Section 7. 8).

7.1.5 Methods, Structures and Devices Available

The weir i s the most practical and economical device for water measure-

ment, provided there i s sufficient head available. The three most commonly

used sharp-crested weirs a r e discussed in Section 7.2.

Measuring flumes a r e extensively used in irrigation networks, where they

ape applicable to almost any flow condition. Their most significant advantages

a r e small head losses, reasonable accuracy over a large flow range, insensitivity

to velocity of approach, and little affected by sediment and debris transport. Of

this category of measuring structure the Parshall flume i s treated in detail in

Section 7.4, and the Standing Wave Flume (India) in Chapter 6 because of i t s

other main application a s a drop. Because of i ts future potential the Cut-throat

Flume, (on which experimental work has almost been completed) may become a

strong competitor to the Parshal l and other flumes, and i s described for free fall

conditions in Section 7.6. The cast-in-place Trapezoidal Flume i s also quoted

(7. 7), being a cheap and easily constructed device.

The use of the submerged orifice for the measyrement of water i s not

discussed here since i t does not offer any advantage over the use of weirs (when

sufficient head i s available) o r flumes (for small head losses). It i s however

discussed in reference (81).

Propeller meters a r e commercial flow measuring devices which have been

in use for a number of years and their advantages and limitations a r e discussed

in Section 7 . 9 . They a re particularly suited to systems where no head losses

can be permitted for water measurement and where water i s sold on a volumetric

basis.

For water measurement in small s t reams, particularly in field ditches and

furrows and where head losses must be very small, the Deflection or Vane Meter

has proved to be a useful device. Two types of such meters a r e presented in

Section 7. 10.

As pointed out ear l ier , the most important point of flow restriction in the

terminal portion of an irrigation system i s the farm outlet (or turnout). Many

outlets have been designed and calibrated to enable water measurement besides

the basic function of regulating the flow. Some of the more common ones a r e

the Flume Type Outlet, the Double Orifice Turnout, the Neyrpic Distributor, the

Meter Gate for culvert-type outlets, the Weir Box Turnout and the Dethridge

Meter Outlet. The e r ro r of these devices under operational conditions i s

usually within the 5 5 % range but may exceed + 107'. (A comprehensive

discussion of these outlets appears in Chapter 5. )

There a r e however a number of other outlets which have either not been

calibrated or a r e not suitable for water measurement. Where such structures

a r e used the installation of a separate standard type measuring device, located

some distance dowhstream of the outlet may be the best solution to obtain

sufficient accuracy in measurement without incurring further development or

construction costs.

Trends

The evolution of water measurement techniques and devices has pro-

gressed independently in many parts of the world, the result being an abundance

of types and designs, each one developed to suit certain local conditions.

However, many such devices could serve a s well in other areas . There i s

also scope for certain desirable features of one device to be integrated with

those of another device to improve overall performance. Refinement in

accuracy may be achieved by better calibration and by building structures more

exactly to standard dimensions. Structures may be further modified so a s to

become cheaper and easier to construct, such a s the Cut-throat Flume. Further

standardization and calibration of distribution and control structures could add

to economies in water measurement, such as for example the use of culverts a s

measuring devices.

i / 7.2 SHARP CRESTED MEASURING WEIRS-

7.2.1 Synopsis

Weirs a r e probably the most extensively used devices for the measurement

of the rate of flow of water in open channels. Weirs may be divided into: sharp

crested weirs, and broad crested weirs. In this section only sharp crested weirs

a r e discussed, Broad crested weirs a r e commonly incorporated in irrigation

structures but a r e not usually used to determine flow, with the exception of the

broad crested weir often known a s the "Romijn Gate", described in Section 7-3.

The types of sharp crested weirs commonly used for measuring irrigation

water a r e the :

- sharp crested contracted rectangular weirs

- I I I ' suppressed 11

- .I 1 " and sharp sided trapezoidal (Cipolletti) weirs

- sharp sided 90° V-notch weirs.

Each of these weirs has characteristics appropriate to particular operating

and site conditions. The Cipolletti i s perhaps the most frequently used type,

(Figure 7- 1). However, a considerable number of rectangular weirs may be

found in irrigation systems, notably at the farm level since they are simple in

construction and operation. The 90' V-notch weir gives the most accurate

results when measuring small discharges and i s particularly adapted to the

measurement of fluctuating flows. Measuring weirs require comparatively high

heads, considerable maintenance of the weir or stilling pool and protection of the

channel downstream of the crest. The accuracy of measurement i s comparatively

good. The selection of type and dimensions of the weir should in the f i rs t

instance be based on the expected rate of flow, o r the limits of the rates in the

case of fluctuating streams. Consideration should be given to the following.

(i) The head should be no less than 6 cm (0.2.ft) for the expected rate of flow

and should not exceed 60 cm ( 2 ft).

L / ~ h e information presented i s based largely on the US Bureau of Reclamation Standards (81).

(ii) For rectangular and trapezoidal weirs, the head should not exceed one-third

of the weir length.

(iii) ,The weir length should be selected.so that the head for design discharge will

be near the maximum subject to the limitations in (i) and (ii).

(iv) The crests should be placed high enough so that the water flowing over them

will fall freely, leaving an,airspace under and around the jets.

A weir, together with a turnout gate, operated with a free falling nappe and

without submergence, may be considered a s a semi-module. Any change in

upstream level results in a change of discF-rrge.

Calibration curves and tables have been developed for the standard type

weirs mentioned above and discharge through the weirs can be estimated readily

by reading the head recordedon a staff gauge against the table to obtain the

actual rate of flow.

FIGURE 7 - 1. - Standard trapezoidal (Cipolletti) measuring weir of 61 cm ( 2 ft) crest length installed at a farm outlet.

Hydraulic Properties

When the water surface downstream from the bulkhead i s far enough below

the crest so that air moves freely to the area below the nappe, the weir i s said to

have free discharge, when the rate of flow can be determined from only the .

upstream gauge stick and a knowledge of the weir size and shape. (Figure 7-2).

Point to rneosure depth H

Elevation of Shorp- crested weir

--A

FIGURE 7-2. - Diagram of f ree discharge contracted weir showing position of staff gauge upstream.

If the water surface in the downstream channel does not permit free

aeration around the nappe the discharge may increaseddue to low pressure. When

the water level rises above the elevation of the crest the flow i s considered to be

submerged; this may or may not affect the discharge rate to a measurable

degree, but dependable measurements under these conditions cannot be expected.

However, when the downstream water level rises above the weir crest a distance - of about 66 per cent o r more of the head on the crest, the degree of submergence

will appreciably affect the rate of flow through the weir notch. The rate of flow

can be determined under these submerged conditions provided that both the

upstream and downstream heads be measured and reference be made to sub-

merged flow tables. Submerged and non-ventilated flows are not desirable for

standard conditions and, except in unusual cases, should be avoided. In most

cases therefore weirs should be placed so a s to obtain ventilated and free-flow

discharge conditions.

If the weir notch be made of a relatively thin plate with a sharp upstream

edge and it be mounted on the supporting wall so that the water does not contact

the wall a s i t passes (i. e. i t "springs" past it), the weir i s called a sharp crested

weir. If the weir notch be mounted in a wall too thick for the water to "spring"

past it, the weir i s classed a s broad crested. Discharge coefficients and

discharge tables a r e usually obtained for broad crested weirs by calibrating the

weir in place. Most measuring weirs a r e constructed a s sharp crested weirs.

When the distances from the ends or sides of the weir notch to the sides of

the weir pool a r e great enough to allow the sheet of water a free and unconstrained

approach to the crest, the water will flow uniformly and relatively slowly toward

the weir ends. As the water from the sides of the channel nears the notch, it

accelerates and turns to pass through the notch opening. This turning effect

cannot occur instantaneously and a curved flow path o r contraction results with

the water "springing1' f ree to form a jet narrower than the weir opening. When

approach conditions allow contraction at both the ends and at the bottom of the jet

the weir i s called a contracted weir. For contracted conditions, the ends of the

weir should not be closer to the sides of the channel than twice the head on the

weir. For complete bottom contraction the weir crest should be placed no closer

than 2H from the bottom of the channel (Figure 7-2).

Setting of Weirs

The setting of weirs according to accepted standards i s a s important a s the

use of standard dimensions and shapes. Only then can the available rating tables

and graphs be applicable and individual calibrations be avoided.

Standard contracted rectangular weirs

The conditions and settings recommended for standard contracted

rectangular weirs a r e set forth below.

(i) The upstream face of the bulkhead should be smooth and perpendicular

to the axis of the channel.

(ii) The upstream face of the weir plate should be smooth, straight and

flush with the upstream face of the bulkhead.

(iii) The entire crest should be a level, plane surface with a sharp,

right-angled edge facing upstream. The thickness of the crest should

be between 1 and 2 m m (about 0.04 to 0.08 inches). Both ends of

rectangular weirs should be. truly vertical and of the same thickness a s

the crest .

(iv) The upstream corners of the notch must be sharp. They should

be machined or filed perpendicular to the upstream face, and free of

bu r r s o r scratches. Knife edges should be avoided because they a r e

difficult to maintain.

(v) The downstream edges of the notch should be chamfered i f the

plate i s thicker than the prescribed crest width (iii). This chamfer 0

should be at an an angle of 45 or more.

(4 The distance of the crest from the bottom of the approach channel

should preferably be not less than twice the depth of water above the

crest and in no case less than 30 cm.

( vii) The distance from the sides of the weir to the sides of the

approach channel should preferably be no less than twice the depth of

water above the crest and never less than 30 cm.

(viii) The overflow sheet (nappe) should touch only the upstream edges

of the c res t and its sides.

(ix) Air should circulate freely both under and at the sides of the

nappe .

(x) The measurement of head of the weir should be taken as the

difference in elevation between the crest and the water surface at that

point upstream from the weir which i s at a distance of four t imes the

maximum head on the crest . (A staff gauge i s usually installed here

having a graduated scale with zero placed at the same elevation as

the weir c res t . )

(xi) \ The cross-sectional area of the approach channel should be at

least eight t imes that of the overflow sheet a t the crest for a distance

upstream from 15 to 20 times the depth of the sheet,. (If the weir pool

i s ernaller than defined by these criteria, the velocity of approach may

. be too high and the staff gauge too low. )

7.2.3.2 Standard suppressed rectangular weirs

The standard suppre seed rectangular weir requires the same con-

ditions for accuracy of measurement as the contracted rectangular weir, except

for the conditions relating to side contraction. In the suppressed weir the sides of

the approach channel should be coincident with the sides of the weir, and should

extend downstream beyond the crest to prevent horizontal expansion of the nappe.

7.2.3. 3 Standard trapezoidal (Gipolletti) weirs

The standard trapezoidal weir, for which the discharge tables given

herein a re applicable, has a trapezoidal shape (see Figure 7-3) with the sides in-

clining at a slope of 1 (horizontal) to 4 (vertical). All conditions for accuracy

listed'in 7.2.3.1 for the contracted rectangular weir apl)iy to the trapezoidal weir.

FIGURE 7-3. - Permanent'trapezoidal weir discharging under free flow conditions.

7 .2 .3 .4 Standard 90° V-notch weirs

The c res t of the standard 90° V-notch weir consists of a thin plate,

the sides of the notch being inclined 45O from the vertical. This weir operates a s

a contracted weir and all conditions for accuracy stated for the standard contracted

rectangular weir apply again. The minimum distances of the sides of the weir

f rom the channel banks should be at leas t twice the head on the weir, and should be

measured f rom the intersection points of the maximum water surface with the

edges of the weir. The minimum distance f rom the notch to the pool bottom

should be a t l eas t twice the head on the weir, measured f rom the point (apex) of

the notch to the channel floor.

Because of the shape of this weir the head required for a small flow

through i t i s greater than that required with the other types of weirs with a long

horizontal c res t . This makes i t particularly suited to measure small flows with

high accuracy.

7 .2 .4 Hydraulic Formulae and Discharge Measurement

7.2.4.1 Standard contracted rectangular weirs

Numerous formulae have been developed for computing the discharge

of rectangular, sharp crested weirs with complete contraction. The most popular

and generally accepted one i s the Franc i s formula:

where Q = discharge in m3 per second

L = length of c res t in m

H = head in m or the vertical difference between the elevation of the weir c r e s t and the elevation of the water surface in the weir pool.

Equivalent in English units : 3

3 where Q = discharge in ft pe r second

L = length of c res t in ft

H = head in ft

TABLE 7 -1

3 11 Discharge of Standard Contracted Rectangular Weirs' (in m per sec)-

Head

H(cm)

.50 1.00 1.50 2.00 2. 50 3.00 3.50 4.00 4. 50 5.00

Length of Weir L (cm)

15.00 25.00 50.00 75.00 100.00 125.00 *150.00 175.00 200.00

.0001 .0002 .0003 .0005 .0006 .0008 .0009 .0011 .0013

.0003 .0005 .0009 .0014 .0018 .0022 .0027 .0032 .0036

.0005 .0008 .0017 .0025 .0033 .0042 .0050 .0059 .0067

.0008 .0013 .0026 .0039 .0051 .0064 .0077 .0090 .0103

.0011 .0018 .0036 .0054 .0072 .0090 .0108 .0126 .0145

.0014 .0023 .0047 .0071 .0095 . 0 118 .0142 .0166 .0190

.0017 .0029 .0059 .0089 .0119 .0149 .0179 .0209 .0239

.0021 .0036 .0072 .0109 .0145 .0182 .0219 .0256 .0293

.0025 .0042 .0086 .0130 .0173 .0217 .0261- -0305 .0349

.0029 .0049 .0101 .0152 .0203 .0254 .0306 .0357 .0409

TABLE 7- 1 (Cont'd.)

Head

H( cm)


15.00 25.00 50.00 75.00 100.00 125.00 150.00 175.00 200.00

TABLE 7-1 (Conttd.)

Head H( cm)

42.00 42.50 43.00 43.50 44.00 44.50 45.00

45.50 46.00 46.50 47.00 47.50 48.00 48.50 49.00 49.50 50.00

Lkngth of Weir L(cm)

15.00 25.00 50.00 75.00 100.00 125.00 150.00 175.00 200.00

.4583 .5834 .7085 .8336 .9587 ,4660 .5934 .7207 .8481 .9754 .4738 .6034 .7330 .8626 .9921 .4815 .6134 .7452 .8771 1.0090 .4893 .6235 .7576 .8917 1.0259 .4971 .6336 .7700 .9064 1.0429 .5050 .6437 .7825 .9212 1.0599

.5129 .6539 .7950 .9360 1.0771

.5208 .6642 .8075 .9509 1.0943

.5287 .6744 .8202 .9659 1.1116

.5366 .6847 .8328 .9809 1.1290

.5446 .6951 .8456 .9960 1.1465

.5526 .7055 .8583 1.0112 1.1640

.5607 .7159 .8712 1.0264 1.1816

.5687 .7264 .8840 1.0417 1.1993

.5768 .7369 .8970 1.0570 1.2171

.5849 .7474 .9099 1.0724 1.2349

1 / - Values determined partly experimentally, partly f rom the formula

Q = 3. 33 (L - 0.2H) H' and convertbd to me t r i c unit? (81)

Table 7- 1 gives the discharges of standard contracted rectangular

weirs for 9 different lengths and for heads ranging from 0 . 5 to 50 cm. The table

i s intended to be used for discharge measurements of standard rectangular weirs

but may serve a s well for their design. The discharge data may be interpolated

for other lengths of weir if their corresponding heads do not exceed one-third of

the crest length.

An improved. method for computing rates of flow through rectangular

thin-plated .weirs has been developed by Kindsvater and Carter. In their formula

they have introduced the effective coefficient of discharge, the effective weir

length and the effective weir head in order to take account of effects of relative

depth and width of approach channel and of velocity of approach. Since the

formula i s hardly ever used in the measurement of irrigation water with small

structures i t i s not elaborated on here, but reference may be made to (61) and (81).

7.2.4. 2 Standard suppressed rectangular weirs

For computation of discharge of the standard suppressed rectangular

weir the Rehbock formula and the Francis formula a r e commonly used. The

diagram shown in Figure 7-4 i s based on the Rehbock formula:

where

3 Q = d i scha rge inm per second

,u = discharge coefficient

L = length of weir crest in m

H = h e a d i n m

The discharge coefficient ,u i s determined from:

where

D = distance from the crest to the bottom of the approach channel in millimetr e s

H = head in mill imetres

Somplc calculation

Given: D = 6 0

L = 8 0

Measured: H = 38

Wanted: 9 = 7 P = 4 6 0 x 0 .80

0 = 368 1/r

1 I I I I I I I I l l

0 ' 2 3 4 5 6 7 8 9 1 0 @ 10 2 0 30 40 60

loo per m crest length @ I 0 0 2 0 0 4 0 0 6 0 0 8 0 0 1000

FIGURE 7-4 . - Discharge over a suppressed rectangular weir per metre of creet length.

According to Swiss standard (SIA No. 109)

H D should not be smal ler than H, ( - 6 1 ),

D

Dmin = 3 0 0 m m

Hmin = 25 m m

Hm, = 800 m m

The US Bureau of Reclamation (81) recommends that D should be a t leas t

2 t imes H, (D 2 Z H ) ,

Hmin = 60 m m (below this the nappe may not spring f ree of the c res t )

The US Bureau of Reclamation uses the Franc i s formula

where

Q 3 = discharge in f t pe r second

L = length of weir c r e s t in ft

H = h e a d i n f t

F o r discharge tables in English units reference may be made to (81).

Standard trapezoidal (Cipolletti) weirs

Taking the Francis formula a s a basis, Cipolletti has developed the

following formula for this type of weir :

where 3

Q = discharge in ft per second

L = length of the c res t in ft

H = head in ft o r the vert ical difference between the elevation of the weir c res t and the elevation of the water surface in the weir pool

TABLE 7-2

Discharge of Standard Trapezoidal Weir e (CIPOLLETTI) 1 / (in m 3 per sec) -

Head H( cm)


15.00 25.00 50.00 75.00 100.00 125.00 150.00 175.00 200.00

TABLE 7 - 2 (Contld.)

Head

H(cm)


15.00 25.00 50.00 75.00 100.00 125.00 150.00 175.00 200.00

TABLE .7- 2 (Conttd. )

1 / - Values determined partly experimentally, partly from the formula

Head H (4

P = 3. 367 LH and converted to metric units (81)

Length of Weir L (cmf

15.00 25.00 50.00 75.00 100.00 125.00 150.00 175.00 200.00

Metric equivalent :

where 3

Q = discharge in m per second

L = length of crest in m

H = h e a d i n m

Table 7-2 computed from this formula and partly from experiments

gives discharges over standard trapezoidal weirs of nine different c res t lengths

and for heads from 0.5 cm to 50 cm.

Discharge measurements using the Cipolletti weir and the above

formula a r e not a s accurate a s those obtained from rectangular weirs using the

Francis formula, but accuracy i s sufficient for general irrigation use.

The discharge figures may be interpolated for other lengths of weir

if corresponding heads do not exceed one-third of the c res t length.

7.2.4.4 Standard 90° V-notch weirs

Of the several well known formulae used to compute the discharge

over 90° V-notch weirs the formula recommended by the WMO (Ref. 61) i s

quoted here :

<

where

3 Q = discharge in m per second

g = acceleration due to gravity in m / s e c 2

Cd = coefficient of discharge

H = h e a d i n m

Cd i s a function of H and fluid property.

3 Table 7 - 3 gives discharges in m / s multiplied by 10 for heads from 5 to

38 cm.

TABLE 7-3

3 Discharge of 90° V-notch Weirs (in m / s x 10)

5 - Computed from the Formula Q = BJG~ H

15 (61)

Head Discharge Head Discharge Head Discharge m m 3 / s x 10 m m 1 s x 10 m ' m 3 / s x 10 3

For the English system of units the Cone formula, recommended by

the USBR, i s quoted :

where Q = discharge in second-feet

H = head in feet or the vertical distance between the elevation of the vertex or lowest part of the.notch and the elevation of the water surface in the weir pond.

Table 7-4 i e computed f rom the Cone formula for heads f rom 0.20 to

1.25 f t (61 to 380 mm).

TABLE 7-4

Discharge of 90' V-notch Weirs (in second-feet)

Computed f rom the Formula Q = 2.49 H 2.48

(81)

Head in Discharge Head in Discharge Head in Discharge feet in second- feet in second- feet in second-

feet feet feet

FIGURE 7- 5 (a) and (b). - Small temporary V-notch weirs made of sheet metal, (being used for studies on irrigation efficiency and water losses).

7.2;5 Construction of Measuring Weirs

Measuring wei rs may be temporary o r permanent. Temporary wei rs may

be portable. F o r ear th channels portable weirs may be made f rom sheet steel

cut approximately to the shape of the c ros s section of the channel but ra ther

l a rge r . , The weir opening in the sheet must be cut carefully ( see 7. 2. 3 on setting

of weirs). F igure 7-5 (a) and (b) shows portable 90° V-notch wei rs made of

3 m m and 5 mm sheet metal respectively and Figure 7-6 gives an example of a

suitable design.

FIGURE 7-6. - Example of a design for a 90° V-notch weir plate.

In lined channels temporary measuring weirs may be installed in a bulkhead

made of wood o r other material that has been sealed in place. Another possibility

i s to use existing structures, such as division boxes or checks for measurements

by temporarily substituting the gate with a weir plate.

Permanent measuring weirs may be constructed in almost the same way a s

check or drop structures (Chapter 6) and by applying standard proportions between

weir opening, bulkhead and weir pool as indicated above. Again for accuracy the

weir crest should always be formed of a thin plate of strong material such a s

sheet steel. Measuring weirs a r e sometimes built a s an integral part of f a rm

outlets, (an example of a design for this type i s given in Chapter 5).

Maintenance of weirs i s very important i f dependable measurements a r e to

be obtained over a long period of time. Maintenance involves :

- keeping the pool free from excessive deposits and weeds , - preventing leakage through and around the weir structure

- checking the elevation of the gauge in relation to the crest

- checking the condition of the crest and re-dressing i t if required.

1 / 7 . 3 THE ROMIJN BROAD CRESTED WEIR-

7.3.1 General

The Romijn weir was developed by the irrigation service in Indonesia a s a

regulating and measuring device for use in relatively flat irrigated regions

where the water demand i s variable because of different requirements during the

growing season.

A description of the weir was published for the f i rs t time in 1932 by D. G.

Romijn (1 34), after whom the structure has been named.

The Romijn weir consists of two sliding blades and a movable weir, crest

which a r e mounted in one steel guide frame, (Figure 7-7). The bottom blade,

" Based on information provided by M. G. Bos, Irrigation Design Engineer, Institute for Land Reclamation and Improvement, The Netherlands.

which i s locked in place under operational conditions acts a s the bottom

terminal for the movable weir. The upper blade, which i s connectdd to the

bottom blade by means of two ateel str ips placed in the frame grooves, acts a s

the top terminal for the movable weir. The movable weir i s connected by two

steel strips to a horizontal lifting beam. The horizontal weir crest i s

perpendicular to the water flow and slopes 1 : 25 upward in the direction of flow.

Its upetream nose i s rounded off in such a way that flow separation does not

occur.

The operating range of the weir equals the maximum upstream head

(Hcrt) which hae been selected for the dimensioning of the regulating structure.

Upper slide

Grooves

y Lcr? = 0.78 Hlcrtl'lmox

Zero level ------ of crest .

Movoble weir crest

Stobilizinp console

.-

FIGURE 7-7. - Romijn broad crested weir, sliding blades and movable weir crest.

Weir Abutments

The weir abutments a r e vertical and a r e rounded in such a way that flow

separation does not occur. There i s a rectangular approach channel to ensure

regular velocity distribution. The total upstream head over the weir (Hcrt) i s

measured in this approach channel at a distance of between two and three t imes

Hicrt)max upstream of the weir.

The dimensions of the abutments should comply with those indicated in

Figure 7-8.

Aeration groove-

Diverted flow

Side s l o v of c o n a l

FIGURE 7-8. - Romijn broad crested welr, hydraulic dimensions of weir abutments.

Undet certa* circumstances the radius ( r ) of the rounding-off of the

bbiutriients may be reduced, so that r >, Hot. This will happen, for

instance, if :

(a) the average flow velocity, v, in the undivided main canal i s low

so that there i s little danger of flow separation; in other words, 1 -

i f the Froude Number, F r = v ( %)', i s equal to or l ess than

0.10, where g i s the acceleration due to gravity, A i s the cross-

sectional a r ea of flow, and B i s the channel width at the free

water surface;

(b) the centreline of the weir structure i s parallel to or coincides

with the centreline of the undivided supply canal (in-line

structure);

(c) the water i s drawn directly from a storage basin.

'If several movable weirs a r e combined in a single structure, intermediate

piers should be provided so that two-dimensional flow i s preserved over each

weir unit, allowing the upstream head over the weir to be measured independently

per unit. The parallel section of the pier should therefore commence at a.point 1

located at a distance of H(crt) max upstream of the head measurement station and

extend to the downstream edge of the weir crest . P i e r s should have streamlined

noses, i. e. of semi-circular o r semi-elliptical profile (1 to 3 axes). To avoid

sharp curvatures at the cut-waters, the thickness of the intermediate piers

should be equal to o r more than 0.65 H I with a minimum of 0. 30 m . ( crt)max

Measurement of Head .I

To limit the effects of draw-down and to ensure that the energy loss between

the section of measurement and the upstream edge of the weir crest i s negligible,

the total upstream head over the weir ( H ~ , ~ ) must be measured at a point located

at a distance of between two and three times the total maximum energy head over

the weir upstream of the (imaginary) weir face. Since.the weir crest moves up

and down, a fixed staff gauge cannot be used to obtain a value for the upstream

head over the crest .

A variety of devices for measuring head requiring two readings for the

calculation of the upstream head have been developed, but these a re l e s s accurate

and more liable to lead to e r ro r s in the determination of Hcrt than a device that

requires one reading only. Of the latter type, the most simple and reliable i s a

staff gauge that travels up and down with the weir crest . Zero level of this gauge

coincides with the downstream edge over the weir crest (control section), so that

the upstream head over the crest equals the degree of immersion of the gauge.

Depending on circumstances, there a r e two ways in which the gauge can be

fixed to the movable weir :

(i) Where the water surface in the approach channel i s smooth (no waves),

where narrow intermediate piers a r e to be used, o r where no great

accuracy of gauge readings i s required, the gauge may be located in the

approach channel a s near a s possible to one of the abutments. A steel

beam i s then welded or bolted perpendicular to the lifting beam and

extended to the head measurement section. A second beam i s welded or

bolted to the movable weir 0. 15 m below crest level, and this i s also

extended to the head measurement section. The ends of the beams a re

connected via a steel or hardwood support to which the gauge i s mounted.

(ii) Where wave action in the approach channel makes i t difficult to make

gauge readings; o r where there i s a risk of the gauge o r i ts support being

damaged by floating debris, the gauge should be located in a rectangular

stilling or gauge well. In such circumstances the lifting beam should be

extended on either the left or right hand side of the guide frame to just

above the well. Attached to the end of the extended beam i s a vertical

support to which the gauge i s mounted. To ensure accurate gauge readings,

the length of this rectangular well a s measured from the face of the gauge

should be equal to or greater than two times the maximum depth to the

water surface in the well; i ts width should not be less than 0. 20 m.

If desired, the metre scale on the vertical staff can be replaced by a scale 3

in m / s or l i t res / s so that the immersion of the scale equals the weir discharge;

(i. e. any changes in the upstream head over the crest , and thus any changes in

the weir discharge caused by the vertical movement of the weir and/or a change

in water level in the approach channel, can be read direct without a time lag.)

Since the f i r s t gauge arrangement (i) i s liable to damage by floating debris

and to some extent by vandalism, i t i s advisable to use the latter arrangement

(ii) a s a standard solution.

If the rectangular gauge well i s used a s a stilling well to prevent

oscillations of the water surface caused by surging water and wave action, the

diameter of the inlet pipe o r slot width (D ) i s limited by the minimum cross - P

sectional a r e a of flow at the head measurement station (A,~,) a s follows :

The pipe o r slot should have i t s opening a t l eas t 0 .5 m below the lowest

c r e s t level and i t should terminate flush with and perpendicular to the boundary

of the approach channel.

7.3.4 Provision for F r e e Flow Conditions

The flow over the weir i s independent of variations in the tailwater head

over the weir c res t ( H ~ ~ ~ ) provided this downstream head does not r i s e above a

certain amount of the upstream head over the weir (Hcrt). If we assume sub-

cri t ical flow in the tailwater channel, the ratio Hdwl should not exceed 0. 66 to Hcr t

provide f ree flow (modular flow).

7.3.5 Hydraulic Proper t ies

The shape of the weir c res t a s introduced by Vlugter (136) in 1940 has the

following advantages over a truly flat and horizontal round-nosed weir :

a. the length of the weir in the direction of flow ( L ~ , ~ ) required to

produce a m o r e o r l e s s constant value of the discharge coefficient

(Cd) can be reduced, so that the rat io H { ~ ~ ~ ) ~ ~ , : Lcrt i s l e s s

than 0.78, which corresponds with a reduction of Lcrt by about 40%;

b. the loss of head due to friction above the weir c res t i s reduced,

resulting in a 4% higher discharge coefficient;

c. the movable par t of the structure i s smaller and thus more rigid;

d. both the steel structure and the weir abutments a r e more economical.

The general stage discharge equation for a broad-crested weir with

rectangular control section reads

where Cd i s the discharge coefficient, C;, i s the approach velocity coefficient,

g i s the acceleration due to gravity, and Bt i s the width (o r breadth) of the weir

a c r o s s the direction of flow.

The value of the discharge coefficient, Cd , has been determined in

laboratory tests , (Vlugter. Cohen, Groot). Variation of Cd values a s a function

of the rat io H' c r t . Lcrt

i s il lustrated i n e ~ i g u r e 7-9.

FIGURE 7-9. - Values of Cd a s a function of the rat io Hcrt : Lcrt for the Romijn weir.

ope rot in^ ronge of the movoble ROMIJN rneor~ring/re~uloting weir 1

F o r field s tructures with concrete abutments, i t i s advisable to use an

average value for Cd, equal to unity.

e l.06

1.05 U .- u 1-04 .- 'C "- u 1.03 8 U

1.02

g 1.01

5 1.00 .- 0 - 9 9

The maximum percentage e r r o r in Cd can be expected to be l e s s than 3%

if an average value Cd = 1.00 i s used.

- - I

.- +I 0 0

- *,*- - -

Averoge Cd reduction - due to friction/

I 1 I

I 1 - W q t i

Values for the approach velocity (Cv) a r e shown in Figure 7- 10 a s a

function of the rat io Cd Hcrt : (Hcrt + Hb-c ) where H (b- c)

i s the height

of the weir c r e s t above the bottom of the rectangular approach channel.

101 . 0'2 . 0'3 0.4 0.3 0.6

Rotio Heft/ Lcrt (dimensionlesr)

o Doto offer d a Groot (19721, = 0-60 m

Doto of ter Cohen (19531, Lcrt = 0-30 m

FIGURE 7- 10. - Approach velocity coefficient, Cv, for rectangular approach channels.

Limits of Application

The practical lower limit of Hcrt i s related to the roughness of the sloping '

weir blade, to the fluid properties, and to the accuracy at which gauge readings

can be made. The recommended lower limit i s 0.05 m or 0.12 Lcrt whichever

i s greater.

The width (or breadth) of the weir crest (Bt) should not be l e s s than

0. 30 m, nor l e s s than the maximum value of the total energy head ~ i ~ ~ ~ ) ~ ~ ~ over the weir crest .

The height of the weir cres t above the bottom of the approach channel

H(b-c) should not be l e s s than 0.15 m, nor l e s s than 0.33 HI (crt)max '

whichever i s greater .

In order to obtain a reasonably constant discharge coefficient, Cd, the

ra t io Hcrt : Lcrt should not exceed 0.78.

7.3.7 Commonly Used Weir Dimensions

It will be noted that al l dimensions given here of both the weir and i t s

abutments a r e related to the maximum value selected for the total energy head

over the weir c r e s t ( H I (c r t )max )

The loss of head (hc) required for modular flow i s also related to the total

energy head a s hc > 0.33 H' (crt)max '

Since a limiting factor in mos t relatively flat irr igation a r e a s i s the

available head for open canal and weir flow, the maximum value of Hcrt i s limited

to a practical value of approximately 0.47 m. The length of the weir c r e s t (in

the direction of flow), L(crt), consequently equals 0.60 m , of which 0.50 m i s

straight and sloping 1 : 25 upward in the direction of flow, while the remaining

0. 10 m forms the rounded nose, i t s radius being also 0. 10 m .

Theoretically, the width ( o r breadth) of the weir, Bt, which may be used i s

flexible over a relatively wide range ( see l imi ts of application), but differences

in this dimension should be limited in the in teres t of standardization of the

s t ructures of an irr igation project. It i s often preferable for Bt not to exceed

1. 50 m so that a central hand wheel can be mounted to move the weir in a simple

narrow (0.01 m ) groove arrangement. Drawings of constructional details a r e

given in Figure 7- 1 1.

7. 3 .8 Rating Tables for Standard Weirs

F o r the standard weir shown in Figure 7- 11 the following values for C d '

Hcrt and Hb-c apply :

Cd = 1.00

0.05 m ,( Hcrt 0.45 m

0 . 5 5 4 Hb-, t ( 0.95 m

/ 0 . 6 0 m + (Hcrt +Hb-c) < l . 0 0 m

Due to the regulating function of the movable weir , both the upst ream head (H,,~)

Ring

Welr toMe and

L50x100x8 Strip 100x8 @

Cond~tlons equ~ red from dellvery ex- workshop

I Surface of plates to be perfectly fiat

2 Bottom of scourlnq qote to joln onqle Iron @ correctly

3 Holes for bolts and locklnq wedges to be sufftciently overd~menstoned

(even ~f "red - ieoded*)

4 All metol work lo be twlce red - leaded

Lock~ng handle Deto~l top corner frome Detoils cross sectlon A-6

F A O - I C I D

THE R O M I J N M O V A B L E

M E A S U R I N G / R E G U L A T I N G W E I R

5 Eoch qote (measurement + scourtnq) to be provlded wtth 2 sets of

padlocks (copper and steel, 2 inches)

P r o j e c t . Reg~on, C o u n t r y

The Netherlonds / l n d o n e s ~ o

F~gure No 7-11

FIGURE 7- 11 SUPPLEMENT. - List of Materials.

B = breadth of approach canal

W = design freeboard

Remarks

Welded to f r ame

See drawing

Mark on ~ e ~ u i r e d Drawing Amount

1 2

1 a 2

2 1

3 1

4 2 t

5 1

6 2

7 2

8 4

9 2

10 1

11 1

12 1

13 1

14 1

15 2

16 2

17 1

18 1

19 1

20 1

2 1 1

2 2 1

2 3 1

Dimensions o r Breadth

Profi le Thickness Length

L50xlOOx8 W + 1950

L50xlOOx8 W t 1850

L50xlOOx8 B t 600

L50xlOOx8 B t 550

L50x100~10 B - 20

L80x12OxlO B - 10

128x10 150

100x8 925

45x1 0 925

38x6 W - 100

482 8 B + 100

665 8 B - 10

492 8 B + 100

W - 132 8 B + 180

50x10 B - 10

50x8 W + 476

50x8 W t 308

1 00x8 100

L50xlOOx8 150

Stem f3 32 c r 38 1 Steel housing with bronze nut )

Hand wheel 1 Wedge 1

Blocking wedge

and the height of the weir above the bottom of the approach channel (Hb-c) a r e

variable. Consequently, the Cv values range between the broken l ines shown

in Figure 7-12.

In irr igation practice i t i s confusing to work with several Cv values for the

same upst ream head. Therefore, the use of an average Cv value, a s a function

of the upst ream head only, i s advised. By using this average value, an e r r o r of

l e s s than 0.57'0 i s introduced in the ra te of flow, with a maximum when

Hcrt = 0. 29 m .

3 This discharge per m e t r e width (breadth) of weir cres t , ( q in m / s / m ) can

be calculated with the aid of Figure 7- 1 2 .

Values of q for each 0.01 m of Hcrt a r e presented in Table 7-5.

If no bottom slide i s used and the movable weir i s lowered behind a drop in

the channel bottom, the height of the weir c res t above the approach channel

bottom (Hb-c) i s l e s s than in the previous (standard) case. Consequently, the

approach velocity and thus the approach velocity coefficient (Cv) a r e significantly

higher.

F o r the standard weir with a length of weir c r e s t Lcrt, in the direction of

flow, of 0.60 m the values of Cd, Hcrt and Hb-c range between the following

value s :

Values of the ratio Cd Hcrt : (Hcrt t Hb- c ) thus range more widely than

before, a s do Cv values a s a function of Hcrt. Minimum and maximum possible

Cv values a r e shown again. By using the average Cv value, an e r r o r of 3.3%

i s introduced in the discharge.

In this context i t should be noted that the average accuracy of the discharge

measurement i s l e s s if the height of the weir c r e s t above the bottom of the

1.00 0 0.05 0.10 0 . 2 0 0 . 3 0 0 . 4 0 0 .45 0 .50

Upstream t'otal head over the weir crest /HCrt)

b 7

Note: I I I

The totol upstreom head over the welrfHcrt) should b e meosurtd - between 0 . 9 0 m ond 1.35m upstreom of t he foce of the weir

in o rectongulor opprooch chonnel whose the width equols t he width of t he weir fq) ond the woter depth equols

- f&t + be,). The flowwise length of t h e weir crest is Lot -0 .6 .

FIGURE 7 - 1 2 . - Approach velocity coefficient, C, , as a function of the total head over the - -

movable weir crest (HCrt) in the stage discharge equation 2 2 0.5 - C C 1.5

= 3 d v ( 7 ' ) BtHcrt

TABLE 7-5

Discharge per Metre Width (Breadth) of Weir Crest for the Romijn MeasuringfRegulating Weir

L~~~ = 0.60 m and. 0.60 m ,< (Hcrt + H ~ - ~ ) ,( 1-00 m.

Head Hcrt Discharge q Head Hcrt Discharge q Head Hcrt Discharge q m m3/ s / m m m 3 / s / m m m 3 / s / m

NOTE : The width (breadtkS of the weir (Bt) should be equal to o r greater than 0.30 m and greater than the total maxi'mum energy head over the weir ( H : ~ ~ ).

The total upstream head over the weir (HCrt) should be measured between 0.90 m and 1.35 m upstream of the weir face in a rectangular approach channel, the width of which equals the width of the weir ( B ~ ) and whose water depth equals (Hcrt + Hb-c).

The number of significant figures given in the column for the discharge should not be taken to imply a corresponding accuracy of the values given, but only to ass is t in the interpolatioti'dnd rounding off for various values of Bt.

approach channel (Hb-c) and the water depth in the approach channel

( H ~ ~ ~ + Hb-c) vary in such a way that the ra t io Hcrt : (H, ,~ + Hb-=) moves

in a wider range of values while Hcrt r ema ins constant.

The discharge pe r m e t r e width ( o r breadth) of the weir c r e s t (q) can be

calculated with the aid of F igure 7- 12. Values of q, in m 3 / s /m, for

each 0.0 1 m of Hcrt a r e presented in Table 7-6.

TABLE 7 -6

Discharge pe r Me t r e Width ( ~ r e a d t h ) of Weir C r e s t (q) for the Romijn MeasuringlRegulating ~ e k :

Lc r t = 0. 60 m and 0.20 m (Hcrt + Hb-c) 0.60 m.

Head Hcrt Discharge q Head Hcrt Discharge q Head Hcrt Discharge q m m 3 / ? / m m m 3 / s / m m m 3 / s / m

See Footnote Table 7- 5.

11 7.4 THE PARSHALL FLUME-

7.4.1 General Description

The Parshall flume i s a critical depth measuring device which may be

installed in a canal, ditch or furrow to measure the rate of flow of water. It i s a

particular form of venturi flume and i s named after i ts principal developer, the

late R. L. Parshall . The flume (Figure 7-13) has been standardized and

calibrated for a wide range of capacities in the United States.

FIGURE 7-13. - Small standard Parshall flume in operation.

The flume consists of three principal sections: a converging or contracting

section at i ts upstream end; leading to a constricted section or throat; and a

diverging o r expanding section downstream (Figure 7-14). ' The larger sized

flumes have an approach floor and wing walls a t the upstream end. The floor of

the converging section i s level, both longitudinally and transversely. The floor

of the throat inclines downward, and the floor of the diverging section slopes

upward.

L'Based on information in USBR Water Measurement Manual and USDA National Engineering Handbook Chapter 9 - Measurement of Irrigation Water,(81 and 82).

SEC n O N N-N

SECTION L-L

FIGURE 7 - 14. - Plan and elevation of a concrete Parshall measuring flume showing component parts ( 8 2) .

TABLE 7-7

Standard Dimensions and Capabi l i t ies of the P a r s h a l l F l u m e f o r Various T h r o a t Widths (W) f o r F r e e F l o w

Throa t Width W

1. E n g l i s h u n i t s

2. M e t r i c uni ts

6 in

15 .2 c m

9 in

22.9 c m

1 f t

30.5 c m

1 1 2 f t

45.8 c m

2 f t

61 c m

A

f t i n

c m

I I

B I C I

f t i n f t i n

c m I c m

1

3 f t 3

3 - 8 1 5 - 4 ~ 4 - 0 5 - 3 0 - 9 2 3 I

91 .5 c m 111.8 / 164.6 122 .0 7 . 6 , 22.9 5.1 7 . 6

4 f t

1 2 - 0 1 - 3 2

4 . 5 63.0 50 .8

2 - 1 0 i 1 - 3

86.4 / 38 .1

I 7 1 3 - 0 ' 4 - 4 3 2 - 0

0. 61

17.26

1 . 3

36.79

1. 6

45.28

2. 6

73.58

5 f t

1 5 2 . 5 c r n

6 f t

183.0 c m

-

D

f t in

c m

5

50 .4

1426

67.9

1922

85 .6

2422

103 .5

2929

1 - 3 5

44 .3

I I - l $ z - 6

5 7 . 5

1 2 - Q

4 - 4

132.2

4 - 8

142.3

91 .5 1 134.4 61.0 1 84.5

E

f t i n

c m

2 - 0

61.0

76.3

3 - 0

1 5 6 - 4 7 1 6 - 0 1 7 - $

I 194.4 1 1 8 3 . 0 / 230.3

91.5

I

F G K

f t in f t i n

7 3 - 2 4 - 7 5 2 - 6 3 - 0

96 .6 1 142.3 76.2 91 .5

7 1 I

3 - 0

91 .5

1 - 0 ' 2 - 0 1 3

3 0 . 5 6 1 . 0 7 . 6

1 - 0 ~ 2 . 6 1 3 I

3 0 . 5 1 76.2; 7 . 6

I I I 2 - 0 1 3 - 0 ) 3

3 - 0

91 .5 I

2

5 . 1

2

5 . 1

2

0 - 4 ~

11.4

1 0 - 4 2

1 1 . 4

0 - 9

5.1i 91.51

l

in

2 - 0

5 .1

5 .1

in

7 . 6 ' 2 2 . 9

N

f t i n

I 1 - 0 1 3 - 0 3

5 . 1 91 .5 7 . 6

c m . c m ) c m

2 - 0

5 .1

c m

2 1 3

5.11 7 .6

3

7 . 6

3

7 . 6

3

0 - 9

22.9

c m

1

3 - 4 / 4 - 1 % 3 - 0 1 3 - 1 1 - 3 - 0 1 2 - 0 1 3 - 0 3 I I I 101 .7 1 149.6 9 1 . 5 1 120.711 91.51 5 . 1 , 91.5 7 . 6

I

3 - 0 3

91.5j 7 . 6

7 .6

X Y

in

0 - 9

22.9

3 - 0 1 3 / 0 - 9

9 1 . 5 7 . 6 22.9 I

c m

0.15

4. 29

0.05

1 .42

0.09

2.55

0.11

2

5.1

0 - 9

22.9

24.6

696.2

3.9

110.4

8 . 9

251.8

16. 1

0 .42

11.89

3.11

F r e e - F l o w

Min imum

3

7.6

2

5.1

33.1

936.7-

455.6

Capaci ty

Maximum

1. f t3 / , 2.

3

7. 6

2

5.1

1. f t 3 / s 2. 11s

3

7.6

The flume has a number of significant advantages. It can operate with

relatively small head loss. This ability permits i t s use in relatively shallow

channels with flat grades. For a given discharge, the loss in head through a

Parshall flume i s only about one fourth that required by a weir under similar free

flow'conditions. The flume i s relatively insensitive to velocity of approach. It

also enables good measurements with no submergence, moderate submergence

o r even with considerable submergence downstream. Properly constructed and

maintained accuracies within f 270 for free flow and + 5% for submerged flow may

be obtained. The velocity of fiow i s sufficiently high to virtually eliminate

sediment deposition within the structure during operation. Another advantage i s

that there i s no easy way to alter the dimensions of flumes already constructed or

to change the device or channel in any way to obtain an unfair proportion of water.

A disadvantage of the flume i s that standard dimensions must be followed

within close tolerances in order to obtain reasonable accuracy of measurement.

This requires accurate construction and a high standard of workman ship which

makes the device relatively expensive. A further drawback i s that flumes cannot

be used in close-coupled combination structures consisting of turnout, control and

measuring devices.

The Parshall flume can be constructed in a wide range of sizes to measure

discharges from a l i t re per second to more than 100 m 3 per second. The width .

of the throat (W in Figure 7-14) i s used to designate the size of the flume. The

sizes discussed in this Handbook are limited to throat widths of from 15 cm (6

inches) to 183 cm (6 ft). This i s the size range especially suited to the measure-

ment of farm deliveries and the flow in relatively small streams and their

3 capacity range i s 11 11s (3.9 ft3/s) to 2.9 m 3 / s (103.5 ft 1s). The selection of

size of flume depends on the range of discharges to be measured. The ranges of

discharges and appropriate standard dimensions for various throat widths a re

shown in metric and English units in Table 7-7. Care must be taken to construct

the devices according to the structural dimensions given for each one, because the

flumes a r e not geometrically similar. For example, i t cannot be assumed that a

dimension in the 6-ft flume will be three times the corresponding dimension in the

2-ft flume.


Discharge through the Parshall flume can occur under either f ree flow o r

submerged flow conditions. To determine the rate of discharge, two depth gauges,

(Ha and Hb) a r e provided (Figure 7-14). Both gauges a r e set with zero points at

the mean elevation of the c res t of the flume.

When the correct relation between throat width and discharge i s chosen, the

velocity of approach i s automatically controlled. This control i s accomplished by

selecting a throat wide enough to accommodate the maximum flow to be measured

yet narrow enough to cause an increase in the depth of flow upstream. The result

i s a l a rger cross-sectional a r ea of the approaching s t ream and hence a reduction

in velocity.

F r e e flow

Under f ree flow conditions, the ra te of discharge i s dependent solely on the

length of crest , W, and the depth of water at the gauge point,Ha, in the converging

section, this being similar to a weir where only the length of c res t and head a r e

involved in computing the discharge. One of the important characterist ics of the

Parshal l flume i s i t s ability to withstand a relatively high degree of submergence,

over a wide range of backwater conditions downstream from the structure, without /

reduction in the indicated ra te of f ree flow. The s t ream passing through the

throat and diverging sections of the flume can flow a t two different stages:

(i) when the water a t high velocity moves in a thin sheet conforming closely to

the dip at the lower end of the throat (indicated by Q in Figure 7-14), and

(ii) when the backwater ra i ses the water surface to elevation S , causing a ripple

o r wave to form at o r just downstream f rom the end of the throat.

The relationship between gauge reading Ha , throat width W and discharge

Q a r e shown in Table 7- 8 in met r ic units. The equation which expresses this

relationship in English units for W from 1 to 8 feet i s :

where Q i s in cubic feet per second, and W and Ha in feet.

The equation for the 9 inch (22.9 cm) Parshal l flume (Table 7-8) reads :

1.53 Q = 3.07Ha

The equation for the 6 inch (15.25 cm) Parshal l flume (Table 7- 8) reads :

1.58 Q = 2.06 Ha

TABLE 7 - 8

F r e e Flow Discharge Values fo r P a r s h a l l Measuring F lume

2 2 . 8 6 c m

(0. 75 ft)

.0025

.0032

.0039

.0047

I 1 .0063

,0072 .0082 ,0092 .0102 .0112 ,0123 .0135 .0146 .0158

.0170

.0183

.0196 ,0209 .0222 .0236 .0250 .0264 .0279 ,0294

.0309

.0324

.0340

.0356

.0372

.0388

.0405

.0422

.0439

.0456

.0474

.0492

.0509

.0528

.0546

.0565

.0584

.0603

.0622

.0642

(5. 00 ft)

Disch

30.48 cn

(1. 00 ft

.0033

.0042

.0052

.0062

.0072

.0084

.0096

.0108

.0121

.0134

.0148

.0162

.0177

.0192

.0208

.0224

.0240

.0254

.0274

.0292

.0310

.0328

.0347

.0360

.0385

.0405

.0425

.0445

.0466

.0487

.0508 ,0530 .0552 .0574 .0597

.0619

.0643

.0666

.0690

.0714

.0738

.0762 ,0787 .0812 .0838

( 6 . 0 0 ft)

ge, Q, fo r

45 .72cm

' (1 .50 ft)

.0048

.0060

.0074

.0089

.0105

.0122

.0139

.0157

.0176

.0196

.0217

.0238

.0260

.0282

.0306

.0329

.0354

.0379

.0405

.0431

.0458

.0485

.0513

.054I

.0570

.Ob00

.0630

.0661

.0692

.0723

.0755

.0788

.0821

.0854

.0888

.0923

.0957

.0993

. 1029

. 1065

. 1101

. 1138

. 1176

. I 2 1 4

. I 2 5 2

1 .0726 .0805 .0887 .0971 . 1059 .1149

. 1242

. 1338

. 1436

.1537

.1640

. 1746

. 1854

.1965

.2078

.2194

.2311

.2431

.2554

.2678

.2805

.2934

.3065

.3198

.3333

.3471

.3610

.3752

.3895

.4040

.4188

.4337

.4489

.4642

.4797

.4954

Cont'd.

th roa t

00.96cm

(2. 00 ft)

-

.0116

.0137

.0159

.0182

.0206

.0231

.0257

.0285

.0313

.0342

.0372

.0402

.0434

.0466

.0500

.0534

.0569

.0604

.0641

.0678

.0716

.0755

.0794

.0834

.0875

.0916

.0958

. 1001

. 1045

. 1089

. 1133

. 1179

. 1225

. I 2 7 1

. 1319

. 1366

. 1415

. 1464

. 1513

. 1564

. 1614

. 1666

widths, W,

91.44cm

(3.00 f t )

.0169

.0200

.0232

.0266

.0302

.0339

.0378

.0418

.0459

.0503

.0547

.0593

.0640

.0688

.0738

.0789

.0841

.0894

.0949

. 1004

. l o 6 1

. 1119

. 1178

. 1238

. 1299

. I 3 6 1

. 1425

. 1489

. I 5 5 4

. I 6 2 0

. 1688

. 1756

. 1825

. I 8 9 6

. 1967

.2039

.2112

.2186

.2261

.2337

.2413

.2491

of -

121.92cm

(4. 00 ft)

.0348

.0395

.0444

.0495

.0549

.0604

.0661

.0720

.0780

.0843

.0907

.0973

. 1040

. I 1 1 0

. 1181

. 1253 .. 1327 . I 4 0 3 . 1480

. 1558

. 1638

. 1720

. 1803

. 1887

. 1973

.2060

.2149

.2239

.2330

.2423

.2516

.2612

.2708

.2806

.2905

.3005

.3107

.3210

.3314

Table 7-8 (Con t td . )

Cont'd.

. Head

Ha ( c m )

25.50 26.00 26.50 27.00 27.50 28.00 28.50 29.00 29 .50 30.00

30.50 31.00 31.50 32 .00 32.50 33.00 33.50 34.00 34.50 35.00

35.50 36.00 36.50 37:OO 37.50 38.00 38.50 39.00 39.50 40.00

40.50 41.00 41 .50 42 .00 42.50 43 .00 43.50 44.00 44.50 45 .00 45.50 46.00 46 .50 47.00 47 .50 48.00 48 .50 49.00 49.50 50.00

15 .24cm ( 0 . 5 0 ft)

.0440

.0454

.0468

.0482

.0496

.0510

.0525

.0539

.0554

.0569

.0583

.0599

.0614

.0629

.0645

.0661

.0677

.0693

.0709

.0725

.0742

.0758

.0775

.0792

.0809

.0826

.0843

.0861

.0878

.0896

.0914

.0932

.0950 ,0968 .0986 . l o 0 4 . 1023 . 1042 . 1060 ,1079

Q, for

45.72 c m

(1 .50 ft)

. I 2 9 1

. I 3 3 0 ' . I370 . 1410 . 1450 . I 4 9 1 . 1532 .1573 . 1615 . I 6 5 8

. I 7 0 0

. 1743

. 1787

. I 8 3 1

. I 8 7 5

. 1919

. 1964

.2010

.2055

.2101

.2148

.2194

.2241

.2289

.2337

.2385

.2433

.2482

.2531

.2580

.2630

.2680

.2731

.2782

.2833

.2884

.2936

.2988

.3040

.3093

.3146

.3199

.3253

.3307

.3361

.3416

.3471

.3526

.3581

.3637

22 .86cm

(0. 75 f t )

.0661

.0681

.0701

.0722

.0742

.0763

.0784

.0805 -0826 .0848

.0870

.0892

.0914

.0936

.0959

.0981

. 1004

. 1027

. l o 5 0

. l o 7 4

. l o 9 7

. I 1 2 1

. I 1 4 5

. I 1 6 9

. I 1 9 3

. 1218

. 1242

. 1267

. I 2 9 2

. 1317

. 1342

. I 3 6 8

. I 3 9 4

. I 4 1 9

. I 4 4 5

. 1471

. 1498

. 1524

. 1551

. I 5 7 7

. I 6 0 4

. I 6 3 1

. I 6 5 9

. I 6 8 6 . 1713

. I 7 4 1 . 1769

. I 7 9 7

. I 8 2 5

. 1853

Discharge ,

30.48

(1.00 f t )

.0863

.0889

.0915

.0942

.0968

.0995

. l o 2 3

. 1050

. 1078

. I 1 0 6

. I 1 3 4

. I 1 6 2

. I 1 9 1

. 1219

. 1248

. I 2 7 8

. I 3 0 7

. I 3 3 7

. 1367

. 1398

. I 4 2 8

. 1459

. 1490

. 1521

. 1552

. 1584

. 1616

. I 6 4 8

. I 6 8 0

. 1713

. I 7 4 5

. I 7 7 8

. 1811

. I 8 4 5

. I 8 7 8

. I 9 1 2

. I 9 4 6

. 1980

.2014

.2049

.2084

.2119

.2154

.2189

.2225

.2260

.2296

.2333

.2369

.2405

th roa t

60.96 c m

(2 .00 ft)

. 1718

. 1770

. 1823

. 1877

. 1931

. 1986 ,2041 .2097 .2153 .2210

.2267

.2325

.2383

.2442

.2502

. 2 5 62

.2622

.2683

. 27.44

.2806

.2869

.2932

.2995

.3059

.3123

.3188

.3253

.3319

.3385

.3452

.3519

.3586

.3654

.3723

.3792

.3861 ,3931 .4001 .4072 .4143 .4214 .4286 .4359 .4432 .4505 .4579 .4653 .4727 .4802 .4878

widths, W,

91 .44cm

(3 .00 f t )

.2569

.2649

.2729

.2810

.2892

.2975

.3058

.3143

.3228

.3314

.3401

.3489

.3577

.3667

.3757

. 3848

.3939

.4032

.4125

.4219

.4314

.4410

.4506

.4603

.4701

.4799

.4898 .

.4998

.5099

.5201

.5303

.5406

.5509

.5614

.5719

.5824

.5931

.6038

.6146

.6254

.6363

.6473

.6584

.6695

.6807

.6919

.7033

.7147 -7261 .7376

of - 121.92 c m

(4 .00 ft)

.3419

.3525

.3633

.3741

.3851

.3962

.4075

.4188

.4303

.4418

.4535

.4653

.4772

.4892

.5013

.5135

.5259

.5383

.5508

.5635

.5762

.5891

. 6021

.6151

.6283

. 6416

. 6549

.6684

. 6820

.6957

.7094

.7233

.7373

.7513 ,7655 .7798 .7941 .8086 .8231 .8377 .8525 . 8673 .8822 .8972 .9124 .9276 .9428 .9582 .9737 .9893

152.40 c m

(5. 00 ft)

.4267

.4400

.4535

.4672

.4810

.4949

.5090

.5233

.5377

.5522

-5669 .5817 .5967 .6118 .6270 .6424 . 6579 .6736 .6893 .7053

.7213 -7375 .7538 .7703 .7869 .8036 .8204 -8374 .8545 .8718

.8891

.9066

.9242

.9419

.9598

.9778

.9959 1.014 1.033 1 .051 1.070 1.088 1 .107 1.126 1.145 1.164 1.184 1 .203 1 .223 1 .242

182.88 c m

(6. 00 ft)

.5113

.5274

.5436

.5601

.5767

.5935

.6105

. 6277

. 6451

.6626

.6803

.6981

.7162

.7344

.7528

.7713

.7901

.8089

.8280

.8472

.8666

.8861

.9058

.9257

.9457

.9659

.9863 1.007 1.027 1 .048

1.069 1.090 1 .112 1 .133 1 .155 1 .176 1 .198 1.220 1 .243 1.265 1.287 1 .310 1 .333 1.356 1 .379 1.402 1.425 1.449 1 .473 1 .496

Table 7- 8 (Cont'd. )

Note: 1 . ' Table taken and converted into m e t r i c values f r o m P a r s h a l l , R. L. , Measur ing - w a t e r i n i r r igat ion 'channels , U. S.Dept.Agr., C i r . 843, p. 62, 1950 (out of print).

2. F o r Ha and W s e e F i g u r e 7-14 3. T o convert m3/ s into c u s e c s multiply above f igures By 35.3

Head H

( c a

50.50 51.00 51.50 52.00 52.50 53.00 53 .50 54.00 54.50 55.00

55.50 56.00 56.50 57.00 ' 57.50 58.00 58.50 59.00 59.50 60.00

60.50 61.00 61.50 62.00 62.50 63.00 63.50 64.00 64.50 65.00

65.50 66.00 66.50 67.00 67.50 68.00 68.50 69.00 69.50 70.00 70.50 71 .00 71.50 72 .00 72.50 73.00 73.50 74.00 74.50

,75.00

15.24cm (0.50 ft)

22.86 c m (0. 75 ft)

. 1882

. I 9 1 0

. I 9 3 9

. I 9 6 8

. I 9 9 7

.2026

.2056

.2085

.21.15

.2144

.2174

.2204

.2235 ,2265 .2295 . 2 3 2 6 .2357 .2388 .2419 .2450

.2481

.2513

Discharge , 30.48 c m (1. 00 ft)

.2442

.2479

.2516

.2553

.2591

.2628

.2666

.2704

.2743

.2781

,2820 .2858 .2897 .2936 .2976 .3015 .3055 .3095 .3135 t 3175

.3215

.3256

.3296

.3337

.3378 ,3420 .3461 .3503 .3544 .3586

.3628

.3671 , 3 7 1 3 .3755 .3798 .3841 .3884 .3927 .3971 .4014

Q, 45.72 c m (1.50 ft)

.3693

.3750

.3806

.3863

.3921

.3978

.4036

.4094

.4153

.4212

.4271

.4330

.4390

.4449

.4510

.4570

.4631

.4692

.4753

.4815

.4877

.4939

.5001

.5064

.5127

.5190

.5254

.5317

.5381 ,5446

.5510

.5575

.5640

.5706

.5771

.5837

.5903

.5970

.6036

.6103

fo r th roa t 60.96cm (2.00 f t )

.4953

.5030

.5106

.5183

.5261

.5339 ,5417 .5495 .5575 .5654

,5734 .5814 .5895 .5976 .6057 .6139 .6221 .6304 .6387 .6470

.6554

.6638

.6723

.6808

.6893

.6978

.7064

.7151

.7238

.7325

.7412

.7500

.7588

.7677

.7766

.7855

.7945

. a035

. a125

. a216 .4058 .6170 ,4102 6238 .4146 j : 6306 .4190 .6373 .4235 1 .6442 .4279 / .6510 .4324 .6579 .4369 1 .6648 .4414 , ,6717 .4459 / .6787

widths, W, 91.44 c m (3. 00 ft)

.7492

.7609

.7726

.7844

.7962

.a081

.a201

.a321 ,8442 .a564

. a686

.a809

. a932

.9057

.9181 ,9307 .9433 ,9559 .9686 .9814

,9943 1.007 1.020 1.033 1.046 1.059 1 .073 1.086 1.099 1.113

1 .126 1.139 1.153 1.167 1.180 1.194 1.208 1.222 1 .236 1.249

.a307

. a399

.8491

. a583

. a675

. a768

.8862

.8955

.9049

.9143

1.263 1.278 1.292 1.306 1.320 1.334 1.349 1.363 1.378 1.392

of - 121.92cm (4.00 ft)

1 .005 1.021 1 .037 1 .052 1.068 1.085 1 .101 1.117 1.133 1 .150

1.166 1.183 1.200 1 .217 1.233 1.250 1.267 1.285 1.302 1.319

1.336 1.354 1 .371 1.389 1 .407 1.425 1.443

-1.460 1.479 1.497

1 .515 1 . 5 3 3 1.552 1.570 1.588 1.607 1 .626 1.645 1 .663 1.682 1.701 1.720

1 .759 1.778 1.797 1.817 1 . 8 3 6 1 .856 1.876

15240 c m (5.00 f t )

1.262 1.282 1.302 1 .322 1.342 1 .363 1.383 1.404 1.424 1.445

1 .466 1.487 1.508 1.529 1.551 1.572 1.594 1.615 1.637 1.659

1.681 1.703 1.725 1.748 1.770 1.793 1.815 1.838 1.861 1 .884

1.907 1.930 1.953 1.977 2.000 2.024 2.047 2.071 2.095 2.119

1 8 2 8 8 c m (6. 00 ft)

1 .520 1.544 1 .569 1 .593 1.617 1.642 1.667 1.692 1.717 1.742

1.767 1.793 1.818 1 .844 1.870 1.896 1.922 1 . 9 4 8 1.975 2.001

2.028 2.055 2.082 2.109 2.136 2.163 2.191 2.218 2.246 2.274

2.302 2.330 2.358 2.386 2.415 2.443 2.472 2.501 2.530 2.559

2.143 2.167 .

192 2.216 2.240 2.265 2.290 2.314 2.339 2.364

2.588 2.617 2. 647 2.676 2.706 2.736 2.766 2.796 2.826 2.856

Submerged flow

In most installations, when the discharge i s increased above a critical value

the resistance to flow in the downstream channel becomes sufficient to reduce the

velocity, increase the flow depth, and cause a backwater effect at the Parshal l

flume. It might be expected that the discharge would begin to be reduced a s soon

a s the backwater level Hb exceeded the elevation of the flume crest; however,

t h i s i s n o t t h e case. Calibrationtests showthat the d i scharge isnot reduced

until the submergence ratio Hb : Ha , expressed as a percentage, exceeds the

following values :

Width of throat (W) Hb

F ree flow limit of - Ha

15.2 to 23 cm ( 6 to 9 inches) 60 %

30. 5 to 244cm (1 to 8 feet ) 70 %

The upper limit of the submergence ratio i s 95%. At this point the flume

ceases to be an effective measuring device because the head differential between

Ha and Hb becomes so small that any slight inaccuracy in either head reading

results in a large e r r o r in flow measurement.

Approach flow conditions

Experience has shown that Parshal l flumes should not be placed at right

angles to flowing streams, such as in turnouts, unless the flow i s effectively

straightened and uniformly redistributed before i t entefrs the flumes. Surges and

waves of any appreciable size should be eliminated. The water should enter the

converging section reasonably well distributed across the entrance width, and the

flow lines should be essentially parallel to the flume centreline. Also, the flow

at the flume entrance should be free of "white" water and free from turbulence in

the form of visible surface "boils" such a s might occur below a control gate.

Only then can the flume measure water a s intended.

Experience has also shown that i t i s better to provide standard conditions of

approach and exit than to t ry to estimate the effects of non-standard conditions;

such flow conditions cannot be described and evaluated in te rms of measurement

accuracy. Non- standard approach flow conditions should therefore be eliminated'

by deepening, widening, or straightening the flow channel, o r by resetting or re-

arranging the measuring station.

In locations where approach flow conditions have resulted in measurement

difficulties, and no upstream wing walls have been included in the original con-

struction, the curved wing walls shown in Figure 7- 14 should be considered.

Curved wing walls a r e preferred over straight 45O walls, although any

arrangement of walls, channel banks, o r other shapes that achieve uniformity

and smoothness in the approaching flow i s acceptable.

7 .4 .3 Discharge Measurement

F r e e flow conditions

When the flow i s free, o r when the submergence i s below the l imits quoted

in 7.4.2, the discharge may be read directly f rom Table 7-8, using the upstream

head Ha and the throat width, W, of the flume.

To i l lustrate the determination of the degree of submergence and ra te of dis-

charge, i t i s assumed that for a 2 f t flume the measufed heads (Ha and Hb) a r e 1

67 cm (2.2 ft) and 40 cm (1 .3 ft) respectively. The ratio of Hb to Ha i s 40 divided

by 67, o r 0. 6, o r 60%. Since this value i s l e ss than 70% (para. 7.4.2), f ree flow

conditions exist, and to find the discharge i t i s only necessary to use the one

measured head Ha = 67 cm. Referring to Table 7-8 for a 2 ft flume gives a

discharge of 768 11s.

Submerged conditions

When the ratio of the two heads Hb and Ha exceeds the l imit for f ree flow

conditions, i t becomes necessary to apply a negative correction to the f ree flow

discharge in order to determine the ra te of submerged flow.

Fo r throat widths of 15.2 cm (6 inches) and 23 cm (9 inches) the submerged

ra te of flow can be read directly f rom Figures 7- 15 or 7- 16 respectively.

Example:

Given 15.2 cm (6 inch) flume

measured Ha = 36.6 cm (1.20 ft)

measured Hb = 32.9 cm (1.08 ft)

Calculate percentage of submergence :

3 Refer to Figure 7- 15. The submerged flow i s 50.9 l / s o r 1.80 ft pe r sec.

E E E E E $ C, € C, € C, E C, 0 8 e C,

8 8 ~ ~ v u v v - o P h 0 t 0

C,

n t o i n ! C 9 cu cv c\r n3 rr) 3 v Q S 60

0 0.4 0.8 1-2 1-6 2.0 2.4 2.8' 3 2 3.6 4-0 Discharge in f t /s

1 k I I I I I I I I I I 0 10 2 0 30 40 . 50 60 70 80 90 100 110

ma/ s

FIGURE 7- 15. - Diagram showing the r a t e of submerged flow, in 11 s and in ft3/ s, through a 1 5 . 2 crn ( 6 inch) P a r shall measuring flume

E E E 15 o € 0 o o E F & E E E g o o o o t,

e 0

e 0 2 C,

0 o - v h c\r nl r r )

v 2 5 9 cu rr, rr, P 2 0 <o

P

0 0 5 1.0 1.5 2.0 2-5 30 35 40 4.5 5.0 5.5 6 0 Discharge in f t /s

FIGURE 7- 16. - Diagram showing the rate of submerged flow. in l / s and ft3/ s , through a 23 crn (9 inch) Parshall flume.

For flumes with throat widths between 1 and 8 ft, the submerged discharge

i s determined by using a correction diagram (Figure 7- 17). This diagram i s

for a 1 ft throat width and is made applicable to the larger fliunes by multiplying

the correction for a 1 f t flume by the factor (M) for the size of flume in use.

Thiebcorrection i s then subtracted from the f ree flow discharge f6r the measured

head (Ha), a s obtained from ~ a b l d 7-8. The factor M for various throat widths

i s tabulated below.

Throat width ft cm

Multiplying factor (M)

0.06 0.10 0.20 050 , 1.0 1.4 2.0 2.5 4 5 6 810 Correction in f i l s

FIGURE 7- 17. - Diagram fox computing the rate of submerged flow through a 30.5 cm (1 ft) P a r shall flume (82).

Example:

Find the submerged discharge through a 3 f t Parshall flume:

Given W = 91 .5cm (3 f t )

measured Ha = 64 cm (2 .1 ft)

measured Hb = 61 cm (2.0 ft)

Percentage of submergence:

2.0 - = 0.95 o r 95% 2.1

F r o m the correction diagram, Figure 7- 17:

3 Correction for a 1 f t throat width = 163 I / s (5.75 ft / s )

Since this correction must be made applicable to a 3 ft throat width,

multiply i t by the applicable value of M from the above tabulation -

Correction for a 3 ft throat width:

165 x 2.4 = 391 11s (13.8 f t3/s)

F rom Table 7-8 find the f ree flow for W = 91.5 cm and Ha = 64 cm

Q free flow = 1086 I / s (38.4 f t3/s)

3 Q submerged flow = 1086 - '391 = 695 l / s (24.6 ft 1s). .

Approximate determination of submerged discharges

Figure 7- 18 illustrates the effect of submergence on the discharge rates of

various flume sizes. For example a t 70% submergence only the .6 inch to 1 ft

flumes would be affected. The 6 inch flume would discharge 94% of the free

discharge rate. At a submergence of 80% the discharge from all flumes will be

affected to some extent. The free discharge values can be obtained from Table

7-8 a s shown before.

The graph may be useful in determining approximately the size of flume

required and the beet setting in the channel. The curves represent data obtained

during calibration tests and have a maximum deviation of + 7%.

50 60 70 80 90 100 Hb Submergence , , in per cent

FIGURE 7- 18. Effect of submergence on Parshall flume - free discharge (81).

Siting of Flumes

Generally i t i s advantageous to have the measuring flume conveniently near

the of diversion or regulating gate i f conditions bf operation require frequent

recording of discharge. On the other hand the flume should not be placed too

near the head gate, as *e disturbed water just downstream from the outlet may

cause surging and unbalanced flow; i t should best be located in a straight section

of the channel.

7.4.5 Selection of Flume Size

Following the selection of the site, information should be obtained on the

maximum and minimum flows to be measured, the corresponding flow depths,

the maximum velocity and the dimensions of the channel at the site. These

dimensions should include width, side slopes and depth, and the height of the

upstream banks with special reference to their ability to contain the increased

depth caused by the flume installation. With this information and the use of

discharge tables for standard flume dimensions the size and proper elevation of

the crest can be obtained. Examples a r e given below to assis t in the problem of

size and setting of the measuring flume a s covered by general field conditions

usually found in irrigation practice.

Example:

Given - A discharge of up to 566 l / s (20 second ft) i s to be measured in

a channel of moderate grade where the water depth i s 77 cm

(2.5 ft) and the channel banks a r e about 3 m (10 ft) apart.

Solution - This quantity of flow can be measured through several sizes of

flume, but for the sake of economy the smallest practical size

should be selected.

F i r s t le t i t be assumed that a submergence of 70% must not be exceeded in

order that the flow may be determined by the single gauge reading of Ha.

As a rule of thumb, the most economical flume size, W, i s from one-third

to one-half the width of the channel. Considering the 3 m (10 ft) channel width

the 4 ft (122 cm) flume seems to be the most practical, but the 3 ft and 2 ft

flumes should be investigated a s well.

4 ft (122 cm) flume

For this size and the given maximum discharge of 566 l / s (20 sec ft) the

head Ha i s found to be 35 cm (1.15 ft) from Table 7- 8.

FIGURE 7-19. - Section of a Parshal l measuring flume illustrating the determination of the proper crest elevation (82).

F o r a submergence of 70% the rat io of Hb gauge to Ha gauge i s 0.7; hence,

Hb for this condition of flow is 25 c m (0.81 ft). At 70% submergence, .the water

surface in the throat at the Hb gauge is essentially level with that a t the lower

end of the flume. Under this condition of flow, the water depth just below the

structure will be approximately the same a s before the flume was installed, that

i s 77 c m ( 2 . 5 f t ) . I nF igu re 7-19thedimension D r e p r e s e n t s t h i s d e p t h o f

77 cm. By subtracting Hb, o r 25 cm, f rom 77 cm, the value of X, o r 52 cm

(1.69 ft) i s obtained. This i s the elevation of the c r e s t above the bottom of the

channel. F o r this s ize of flume, set with the c r e s t a t 52 c m (1. 69 ft), the flow

of 566 11s (20 sec-ft) will be a t 7070 submergence, and the actual l o s s of head (L)

o r difference in elevation between the upstream and downstream water surfaces

will.be 13 cm (0.42 ft) a s determined by Figure 7-20.

2+4 Cm zr3 5 cm 183cm 152 5 crn 122 cm

91 5cm

61 cm

30 5cm

93 90 85 80 70 6 0 5 0 O M 5 0 0 2 0 0 3 0 0 4 0.06QO8OlO 0 1 5 0 2 0 0 . 3 0 0 4 0 0 6 0 0 8 0 1 0 Psrcento~e of submergence Loss of heod L ~n feet

FIGURE 7-20. - Diagram for determining the head loss through the Pa r sha l l measuring flume (82).

The depth of water upstream f rom the structure a t a flow of 566 11s (20 sec-ft)

will therefore be 90 cm (2.92 ft). It will be necessary to examine the freeboard

of the channel, a s well a s the effect of the r i s e of the water surface upon the flow

through the head gate, in deciding which size of flume i s the most practical.

3 f t (91.5 cm) flume

F o r this size and the given maximum discharge of 566 l / s (20 second-ft)

the head Ha i s found to be approx. 43 c m (1.39 ft) f rom Table 7-8. Again for

a submergence of 70% the rat io of Hb to Ha i s 0.7; hence the Hb for this

condition of flow i s 30 c m (0.97 ft). By reference to Figure 7- 19, the value of X,

o r the elevation of the c res t above the bottom of the channel, i s found to be 47 c m

(1.53 ft), and the actual loss of head through the flume (Figure 7-20) i s found to

be 16 cm (0.52 ft). The depth of water upstream for this size of flume will now

be 92 c m (3.02 ft).

2 f t (61 cm) flume

As before, find the Ha head in Table 7-8 for a f ree flow of 566 11s , (20 second-ft). F o r the 2 ft flume this head i s 55 c m (1.81 ft). At a sub-

mergence of 70% the value of Hb i s 39 c m (1.27 ft). By again referr ing to

Figure 7-19 the value of X o r the elevation of the c res t above the bed of the

channel i s found to be 77 - 39 o r 38 cm (1. 23 ft). F o r this size of flume dis-

charging 566 l / s (20 second-ft) a t a submergence of 70%, the actual loss of head

(Figure 7-20) i s 21 cm (0. 70 ft) and the depth of water upstream i s 97 c m

( 3 . 20 ft).

If i t i s found that the banks of the channel and entrance conditions through

the head gates a r e satisfactory, the 2 f t flume will be mos t economical because of

i t s small dimensions; however, when the width of the channel i s considered the

final selection may favour the 3 o r 4 f t flume because moderate to long wing walls

may be required.

In the above analysis of the three sizes of f lumes investigated, the actual

increase o r r i s e in the depth of water upst ream from the structure i s

considerably l e s s than the elevation of the c r e s t above the bottom of the channel

(X). F o r the 4 f t flume the c res t i s 52 cm (1.69 ft) above the channel bed, and

the r i s e in water upstream will be only 12 c m (0.42 ft).

This analysis shows further that a s the size of flume i s decreased, the

elevation of the crest becomes less , and the depth of water upstream from the

structure becomes greater for similar ra tes of discharge and like degrees of

submergence. It i s usually better to set the flume high rather than low, to

provide a margin of safety for variations of the water surface downstream. In

irrigation channels, especially those with earth banks and bottom, deposits of

sand or silt may change the downstream flow conditions, and weeds or other .

debris may likewise affect the degree of submergence.

Although the above analysis of the free flow data for the 4 ft flume shows

that i t would be necessary or desirable to lower the upstream water surface

elevation a s much a s possible, the effect of operating the flume at 95% sub-

mergence (or any other value between 70 and 95%) at the maximum discharge

might be investigated. Fo r example, a submergence above 70% would lower the

entire structure in the channel and because of reduced headloss could provide

more bank freeboard upstream.

Using the data from the above example, suppose that the maximum

discharge of 566 11s (20 second-ft) i s to be passed with a depth of 77 cm (2. 5 ft)

but with 95% submergence (instead of 70% a s previously).

From Table 7-8, Ha i s found to be 35 cm (1.15 ft) . For 95% submergence,

In Figure 7-19, X = D - Hb = 77 - 33 = 44 cm (1.41 ft)

Therefore, for 95% submergence the crest of the 4 ft flume should be set

a t 44 cmabove the bottom of the channel, a s compared with 52 cm for 7070

submergence. From Figure 7-20 the head loss i s found to be N 2 cm (0.077 ft),

a s compared with 13 cm (0.42 ft) for 70% submergence.

7.4.6 Deviation from Standard Dimensions

In principle, i f standard measuring procedures, tables and graphs a r e to

be applicable under all flow conditions, the Parshall flume has to be constructed

exactly according to standard dimensions a s given in Table 7-7. However, if

the flume i s never to be operated above the 7070 (60% for 6 to 9 inch flumes)

submergence limit (i. e . where enough head i s available and no backwater from

down stream is anticipated in the future) modifications of the standard design

downstream from the dip (e. g. different floor shape, stilling basin, wing walls)

should have no effect upon .discharge. With this submergence limit i t i s not

necessary to construct the portion of the flume downstream from the end of the

crest, shown as station 1 in Figure 7-19.

When only the upstream portion of the flume i s constructed, the flume i s

sometimes referred to a s the Montana flume. The crest of the Montana flume

should be set above the channel bottom in the same manner a s worked out in the

above examples. This will ensure that the flow profile over the crest section

i s not modified by backwater from the downstream channel. Erosion protection t

downstream from the flume may need to be considered.

In the case of submergence above the 70% (or 60%) limit, the effect of

modifications may cause measurements to be inaccurate when using standard

discharge tables. In such a case i t i s necessary to specially calibrate the

modified flume by the current meter or some other suitable method.

Construction

The Parshall measuring flume may be constructed of sheet metal, timber

o r reinforced concrete. Sheet metal flumes (Figures 7-21, 7-22) have proved

very satisfactory, but since the cost usually exceeds that of either wood o r

concrete, their use has been restricted to the smaller sizes. The most

common and practical sizes a r e those of less than 6 1 cm(2 ft).

Sheet metal flumes have the advantage of being portable, and they

can readily be reset and readjusted a s needed. They have a relatively

long life and a r e immune to fire hazards such a s those caused by ditch

cleaning.

FIGURE 7- 21. - P a r shall flume of 152 cm (5 ft) throat width assembled from prefabricated sheet metal parts.

FIGURE 7-22. - Parshall flume of 183 cm ( 6 ft) throat width at full discharge.

Commercially made flumes of this type (Figure 7-23) a r e available in a

wide range of capacities.

FIGURE 7- 23. - Commercially available P a r shall measuring flume.

Monolithic reinforced concrete flumes constructed in any of the previously

discussed sizes have proved satisfactory. Such flumes have the distinct

advantage of permanence and a re little subject to expansion or contraction, thus

ensuring uniformity of operation. They a r e not subject to fire and other hazards

a s a r e timber structures. Their principal disadvantage i s their relatively high

initial cost,

Standard designs, a s used in the U. S. A . , a r e shown in Figures 7-24 and

7 - 2 5 .

Where a number of flumes of the same size a r e to be built of concrete,

i t will be found economical and practicable to built portable knock-down forms,

preferably in sheet iron or plywood. It i s advantageous to construct the sides

of the flume f i r s t and after the concrete is set, to remove the forms and place

the floor. The floor i s screeded to proper grade by iron angles installed

at the changes in grade along the floor.

Ha gauge i f gauge H b gouge well well not prwided

7 gauge well (optional- if need*7

Flow -

. rods, C/C

Plon

gauge i f gauge well die. bolt set is not pravided-1 6 tn concrete

Dimensions, co~ocities and ouantities for vorious throat widths

' set icrete

. .

Flow Bottom - of chonnel-)

Dimensions

of flume m of chonnel

Provide suitable rip

Notes : The dimensions X x D depend upon the setting of the crest of the flume with reference to the bed of the channel and will be determined for each setting.

To obtain accurate discharge measurements,the f lum must be constructed exactly to dimensions listed in table ond given on plans.

Quontities given in table are for D =i and X = d d All reinforcing steel to beg&. rods placed ot centre

of sections. gouge not required unless Hb gouge reoding will

be over 70 % of the Ha gauge reading. The use of an Ha gouge well is optional. I f an

gouge well is not used, install the Ha gouge on the side woll of the flume,

For discharge tables, loss of head and setting of crest of flume with reference to the bed . of the chonnel. see Engineering Handbook and U.S.D.A. Farmers Bulletin No. 1683.

Free- Flow cawcitv

) stirrups 1 2 C / C A concrete opron to protect id ia . galvonized pipe 1: long rods l i c / c chonnel from scour ot flume i f gauge well provided outlet

Quantities

Concrete morto

Crest elevotion -rop or

Sectionol elevotion A-A

Ll*common woshers. Top of washers to be ot exact crest elevation

Sectionol elevotion 8 -6

Showing We gauge well (optional)

~f a fixed gouge is used, install 1 8 din. or lorger vitrified cloy pipe, j long. If removable gouge is used, d dio. pipe may be instolled

elbow. Set end of pipe with inside face of conc

Crest elevotion

2-6, i - i concrete base ,

Sectional elevation C-C

Showing Hb gouge well

F A 0 - I C I D

STANDARD CONCRETE PARSHALL

MEASURING FLUME

Throot width I foot to 8 feet -

Project, Regi0.n , Country U S A

Figure No. 7 - 24

FIGURE 7-25 . - Standard concrete Parsha l l flume. The water depth i s read on the staff gauge in the stilling well and i s converted to ra te of flow by reference to a rating table.

7 . 4 . 8 Maintenance

After a Parsha l l flume has been properly installed, periodic maintenance i s

required to ensure satisfactory operation. Moss may collect on the walls of the

entrance section and in certain channels debris may collect on the floor of the

entrance section and they should be removed. Walls of steel flumes may become

encrusted and the encrustation should be removed with a steel wire brush. Once

the walls have been scraped clean, applying asphaltic paint will add to the life of

the flume.

Commonly, Parsha l l flumes, o r any other type of flow measuring flumes,

placed in unlined channels will "settle" after being in operation for a period of

t ime. The levelness of the entrance floor should be checked after a few months

of operation, and again at the end of the season or year .

Either settling o r improper installation can cause a flume to tilt sideways.

If the settling i s minor, the discharge can still be estimated with fair accuracy

by measuring the flow depths on both sides of the flume. By employing the

average of the two readings when using the discharge equations o r rating tables.

the discharge can be determined.

Settlement occurs most commonly near the exit section because of channel

erosion immediately downstream from the flume caused by the jetting action of

the water. Use of the flow depths Ha o r Ha and Hb to obtain the discharge from

standard discharge tables will yield values less than the true discharge. Satis-

factory solutions to this problem include raising the lower end of the flume so

that i t i s level again o r placing a new level floor in the flume. Correction values

for settled "Cut throat flumest1 of a few sizes have been determined

experimentally and further research i s being carr ied out (83).

7.5 THE STANDING WAVE MEASURING FLUME

The standing wave measuring flume developed in India i s essentially a drop

which has been standardized and calibrated to serve for the measurement of flow.

It i s described in detail in Chapter 6, sub- section 6.1 1.2.

1 / 7.6 THE CUT-THROAT FLUME-

7. 6. 1 General

. The Cut-throat Flume has been developed recently to overcome some of the

shortcomings of other types of flumes already in existence. Figure 7-26 shows

the standard shape of this flume which was derived eGperirnentally. The flume

has a flat bottom and vertical walls, a s seen in Figure 7- 27. It can be operated

( a s the Parshal l flume) under both free flow and submerged flow conditions.

Since the flume has no longitudinal throat section the flume was given the name

"Cut-throat" by i t s developers (Skogerboe, Hyatt, Anderson and Eggleston).

Figure 7-28 shows a 1 ft flume in operation. Advantages of the flume as com-

pared with the Parshall flume a re a s follows.

Construction of the flume i s facilitated by the horizontal floor and removal

of the throat section.

Since the angles of convergence and divergence remain the' same for all

L' The description i s based mainly on reference (85).

flumes, the size of the flume can be changed by merely moving the walls in or out.

Therefore, ratings for intermediate sized flumes can be developed from the

ratings available. This i s extremely helpful when sizes other than those with a

rating a r e required o r a miatake i s made in the throat width during construction.

FIGURE 7-26. - Sketch of Cut-throat flume. (85)

If circumstances allow, i t i s preferable to have the cut-throat flume operate

under free flow conditions. This facilitates measurement and ensures a high

degree of accuracy. The following description i s limited to the free flow con-

dition. Fo r corresponding information on the submerged flow condition reference

(85) may be consulted. As to be expected there a r e similarities in the installation

and operation of the Parshall flume and the Cut-throat flume. As the former has

been described in considerable detail, discussion of the Cut-throat flume i s

limited to the essentials.

FIGURE 7-27. - Final design of a 61 c m ( 2 ft) rectangular cut-throat flume ( 9 0 ) .

FIGURE 7- 28. - Cut-throat flume of 30.5 cm (1 ft) throat width, with automatic recording device, operating under f ree flow conditions.

Determination of Discharge under F r e e Flow Conditions

F r e e flow through the Cut-throat flume i s given by the two formulae

3 where Q = flow ra te in m / s

C = f ree flow coefficient

Ha = upstream flow depth (measured a t a distance of - f rom the 9

throat, see Figure 7- 26 )

and

where C = f ree flow coefficient ( a s above)

K = the flume length coefficient

W = the throat width in m . ,

The values of n and K a r e obtained from Figure 7-29 for a given flume

length.

The discharge can then be calculated for any Ha by using the above two

formulae, provided f ree flow conditions exist in the flume, (c r i t e r i a for these

conditions a r e described in 7.6.3). F o r accurate d i s ~ h a r g e measurements, the

recommended ratio of flow depth to flume length (Ha : L) should be equal to o r

l e s s than 0.4. Increasing values of this rat io resul t in greater inaccuracies.

Example of flow calculation

A f ree flow rating i s needed for a Cut-throat flume of length, L, of 1. 22 m and

width, W, of 0.36 m. F r o m Figure 7- 29 the value of n i s found to be 1.75 and

the value K i s 3. 16. Then, using equation(2)the value of the f ree flow coefficient

C i s calculated.

C = K W 1.025

= 3.16 . 0.36 1.025

Now, knowing the values of n and C, the flow ra te through the flume can

be calculated for any value of Ha using equation (1).

Assuming Ha = 0 . 3 6 6 m

Installation of Cut- throat Flumes for Operation under F ree Flow Conditions

Criteria for location of the Cut-throat flume a r e identical to those already

described for Parshal l flumes (see 7.4.4). After the site has been selected i t i s

necessary to determine the design criteria:

- maximum quantity of water to be measured;

- depth of flow necessary to obtain this discharge;

- allowable head loss through the flume.

For design purposes, the head loss may be taken a s the change in water

surface elevation between the flume entrance and exit. The downstream depth of

flow will remain essentially the same after installation of the flume, a s i t was

prior to installation, but the upstream depth will increase by the amount of head

loss. ,The allowable increase in upstream depth may be limited by the height of

the canal banks upstream of the flume, and such condition may require an

increase in the flume size in order to bring the water level down to acceptable

limits.

<

The flume must be placed level in the channel, both longitudinally and

laterally, and be aligned straight with it.

The most important dimension i s the throat width, W. (As already

mentioned one of the principal advantages of this flume i s that an e r ro r in con-

structing the throat resulting in an e r r o r in the width can be taken into account by

writing new flow ratings, using equation ( 2 ) ) . If a Cut-throat flume i s to be

constructed in concrete, a steel angle can be placed at the throat cross-section

embedded in the concrete and this will fix the width correctly.

In the experience of the developers of this flume a transition structure

between the open channel and the flume i s not necessary. The only guidelines

to follow i s that the ratio of flow depth to flume length (Ha : L) should be 0.4 o r

1 I I I I I I I I

m m -4 m w I - - - -

0 -

0 0 0 0 0 0 22 0 0 N W

0 0

Transition submergence, St, in per cent

FIGURE 7 - 2 9 . - Generalized f r e e flow coefficients and exponents and St for Cut-throat flumes, in m e t r i c units .

l e s s a s already pointed out. Fo r the usual installations in channels of gentle

grade this will ensure that approach conditions will satisfy the conditions under

which the laboratory ratings were developed.

Measurements may be made in the flume by the use of a staff gauge o r

stilling well set at the specified location for Ha. The staff gauge must be

carefully referenced to the elevation of the flume bottom.

In order to ensure f ree flow conditions the ratio between the water depth at

the exit and at the entrance ( H ~ : H ~ ) should not exceed a certain limit, called the

transition submergence, St, which can be determined f rom Figure 7-29.

The procedure to follow for installing a Cut-throat flume to operate under

f ree flow conditions i s summarized a s follows :

(i) Determine the maximum flow rate to be measured.

(ii) At the site selected for installing the flume, locate the high water line

on the canal bank and ascertain the maximum permissible depth of flow.

(iii) Using equation ( I ) , calculate the depth of water that corresponds to the

maximum discharge capacity of the canal for the flume being used.

(iv) Place the floor of the flume at a depth @Ib) which does not exceed Ha

multiplied by the transition submergence St (Hb 4 Hast). Generally,

the flume bottom should be placed a s high in the kana1 a s grade and other

conditions permit to ensure f ree flow.

There i s no established rule for proportions between W and L or W and

Ha. Therefore, a s long a s further research resul ts a r e pending i t i s

recommended that the range of proportions which have been laboratory tested

be applied, which, adjusted to even met r ic values, a r e given in Table 7-9.

The procedure i s further illustrated by Figure 7-30 and by the following

two example s:

Example 1

.A Cut-throat flume of length L = 1. 22 m and throat width W = 0. 36 m i s

to be installed for f r e e flow operation (Figure 7-30). The maximum flow rate in

the channel is 0.200 m3/s .

Maximum water surface after flume installation

surface before

LOriginal canal bottom ,

FIGURE 7-30. - Installation of a Cut-throat flume.

The transition .submergence for this flume can be determined f rom Figure _

7;29 a s St = 68.2%. F r o m equations (1) and (2) the value of Ha that co r r e -

sponds to the maximum flow of 0.200 m 3 / s can be calculated:

n = 1.75 (Figure 7-29)

TABLE 7 - 9

Free Flow Calibrations for Selected Cut-throat Flumes,

Expressed by Throatwidth W x Flume Length L (85)

5 Discharge Q (m per second) Ha

TABLE 7-9 (Conttd.)

Discharge Q ( m 3 per second) H a

10 X 90 20 x 90 30 x 90 20 x 180 40 x 180 60 x 180 30 270 60 270 100 270 (cm) cm c m c m c m c m c m c m c m cm

.205 .019 .041 .060 .035 .071 .lo8 .052 . 107 .I80

.210 ,020 .043 .062 ,036 .074 .112 .055 . 1 1 1 . 187

.215 .021 .045 -065 .038- .077 .117 .057 .I15 . 194

.220 .022 .047 .068 .039 .080 .I21 .059 .119 . 201

.225 .023 .049 .071 .041 ,083 .I26 .061 . 123 .209

.230 .024 .051 .074 ,042 .086 . 130 .063 . 128 .216

.235 .025 .053 -077 .044 .089 . 132 .223 .135 .065

.240 .026 .055 .080 .045 .092 . 140 .067 .I37 .231

.245 .027 -057 .083 .047 .096 . 145 .069 . 141 .238

.250 .028 .059 .086 .049 .099 .150 .072 . . 146 .246

.255 .029 .061 .089 .050 .lo2 .155 .074 . 150 .254

.260 .030 .063 .092 .052 .lo6 ,160 .076 . 155 ,261

.265 .031 -066 .096 .053 .lo9 .165 .078 . 159 .269

.270 ,032 .068 .099 .055 .112 . 170 .081 . 164 .277 ,275 .033 .070 . 102 .057 .116 .I75 .083 .169 .285

.280 .034 .073 .186 .059 .119 .I80 .085 . 174 .293 .285 .035 .075 . 109 .060 . 123 .186 .088 .I79 .302

.290 .037 .078 .I13 .062 . 126 .I91 .090 . 183 .310

.295 .038 .080 .I16 .064 .130 ,197 .093 . 188 .318

.300 .039 .082 .120 .066 .134 .202 .095 .I93 . 327

.305 .040 .085 . 124 .067 . 137 ,208 .098 .I99 .335 . 310 .041 .088 .127 ,069 ,141 .213 .lo0 .204 .344 ..315 .043 .090 .131 . .071 .I45 .219 .lo3 .209 .353 .320 .044 .093 . 135 .073 . 149 ,225 . 105 .214 .361 .325 .045 .096 . 139 .075 . 152 .23 1 . 108 .219 .370 .330 .046 .098 . 143 .077 .156 .237 .I10 .224 .379 .'335 .048 .lo1 .I47 .079 .160 .243 .I13 .230 .388 .340 .049 .lo4 .I51 .08 1 . 164 .249 .116 .235 .397 .345 .050 .lo7 .I55 .083 .168 .255 . 118 .24 1 .40 6 .350 -052 .110 .159 .085 .172 .261 .121 .246 .416 .355 .053 .112 .I64 .087 .176 ,267 .124 .252 .425 .360 .054 . 115 .168 .089 .180 .273 .126 .257 .434 .365 .056 .118 . 172 .091 . 185 .279 . 129 .263 .444 .370 .057 . 121 . 177 .093 . 189 .286 . 132 .268 .45 3 .375 .059 . 124 . 181 .095 ,193 ,292 .I35 ,274 .463 .380 .060 . 127 . 185 .097 . 197 ,299 . 138 .280 .473 .385 .062 . 131 . 190 .099 .202 .305 .140 .286 .482 ,390 .063 . 134 . 195 . 101 .206 .312 .I43 .291 .492 .395 .065 .137 . 199 . 103 .210 ,318 . 146 .297 .502 .400 .066 .I40 .204 . 105 .215 .325 . 149 .303 .512 .405 .068 . 143 .209 . 108 .219 .332 .152 .309 .522 .410 .069 . 147 .213 . 110 .224 .339 .I55 . 315 .532 .415 .071 .150 .218 .112 .228 ,345 . 158 .32 1 .542 .420 .072 .153 .223 .114 .233 .352 .I61 .327 .552 .425 .074 . 157 .228 .I16 .237 .359 .164 .333 .563 .430 .076 .160 .233 . 119 .242 .366 .167 ,339 .573 .435 .077 . 163 .238 .121 .247 .373 . 170 .346 .584 .440 .079 .167 .243 .123 .251 ,380 . 173 .352 .594 .445 .080 .170 .248 .I26 ,256 .388 .176 .358 .605 .450 .082 . 174 .253 .128 .261 .395 .179 .364 .615

TABLE 7-9 (Cont'd.)

3 Discharge Q ( m per second) H a

10 x 90 20 x 90 30 x 90 20 x 180 40 x 180 60 x 180 3Cx 270 60 x 270 100 x 270 ( cm) cm cm cm cm cm c m cm cm c m .455 . 130 .266 . 402 .182 . 37 1 . 626 .460 . 1 3 3 .270 .409 . 185 .377 .637 .465 . 135 .275 .417 . 189 .383 .648 . 470 . 138 .280 . 424 . 192 .390 .659 .475 . I 4 0 .285 .432 .195 . 396 .669 .480 .142 .290 .439 . 198 . 4 0 3 . 681 .485 . 145 .295 .447 . 201 .409 .692 .490 . 147 .300 .454 .205 . 416 .703

The downstream flow depth, Hb, becomes

Hb = Hast = 0.377 . 0.682 = 0.257 m

Therefore the floor of the flume should be placed no lower than 0 . 2 5 7 m

below the high water line in the canal (Figure 7-30).

Suppose the logical Cut-throat flume size necessary to measure a maximum

discharge of 350 11s under f r e e flow conditions must be found. Presently, the

maximum flow depth in the channel i s 30 c m and the head loss i s not to exceed

15 cm. Under these conditions, the maximum downstream flow depth would be

30 c m and the maximum upstream flow depth 45 cm (30 + 15 = 45). The 3 0

submergence would be 67% ( 45 = 0. 67). F r o m Figure 7-29 i t i s found that the

only flumes with a transition submergence greater than 67% a r e those with a

length of 1.15 m and above. To select the proper flume size re fe r to Table 7-9.

Tentatively 'select the 40 x 180 c m flume and find the value of Ha which

corresponds to the given discharge of 350 11s. F o r this value the upstream

depth i s 54 cm, which i s greater than the allowable maximum upstream depth of

45 cm. Consequently a l a rger flume size i s necessary to satisfy the conditions

imposed. F r o m Table 7-9.it i s found that the 60 x 180 c m flume has an upstream

depth of 42 c m for a discharge of 0.350 m 3 / s , and since this value i s l e s s than

the res t r ic ted depth of 45 cm i t would be selected for use in this part icular

situation. A slightly smal ler flume size could be used, e. g. a throat width, W,

between 40 and 60 c m could be selected, which however would necessitate

preparation of a separate rating table. With known W and L the flume can be

dimensioned according to Figure 7-26.

7. 6.4 Maintenance

As f o r Parshal l flumes ( see 7.4.8).

Experience and resea rch have 'shown that, in many respects, trapezoidal

flumes a r e superior to the rectangular o r Parshall- type flumes, part icularly for

measuring smaller flows. The shape conforms to the normal shape of ditches,

particularly those that a r e lined. This minimizes the amount of transition

section needed a s compared to that required when changing f rom a trapezoidal

shape to a rectangular one and back to the trapezoidal. The trapezoidal shape i s

also desirable since the side walls expand a s the depth increases. This means

that one structure can convey a l a rger range of flow. Also, the entire range of

depth for a given range of discharge i s smaller. Another desirable feature of

the trapezoidal flume i s the flat bottom throughout ra ther than a dropped section

such a s with the.Parshal1 flume. The loss in head, i. e. total head loss , through

the trapezoidal structure, may be l e s s for comparable discharges.

These features make the trapezoidal flume particularly suited for

installation in concrete lined ditches. The flume i s usually put on top of the

lining, thus constricting the flow section to the extent required for f ree flow

conditions over the whole range of discharges up to the design dischargk of the

ditch (Figure 7-31 (a) and (b) ). The elevation of the flume floor above the ditch

bottom depends on the existing grade of the ditch; the lower the' grade the higher

the elevation.

FIGURE 7-31 (a) and (b). - Trapezoidal measuring flume with a raised bottom cast in a concrete ditch. The discharge i s about 34 11s (1.2 f t3 /s) at a submergence of about 70% (87).

A tentative standard for trapezoidal flumes, coded ASAE S 359 T, was

adopted by the American Society of Agricultural Engineers (AsAE) in 1972, see

(103). Two classes of flume a r e included. The f i rs t -c lass consists of f o u ~

experimentally calibrated flumes of short length relative to their flow capacity.

Some of their characteristics a r e given in Table 7- 10. Figure 7-32 shows a

standard design for the No. 1 flume. Flumes Nos. 1 and 2 a r e designed for use

in two standard lined ditch sections. Flumes 3 and 4 a r e recommended primarily

for use in unlined channels.

TABLE 7-10

Some Characteristics of the Standard Calibrated Trapezoidal Flume (Derived from ASAE Standard S 359 T)

I Width of approach Side slope of approach Range of Range of

Flume section (= bed section ( = side slope calibrated calibrated No. width qf ditch) of ditch) flow depth flow

cm (horizontal : vertical) cm l/ s

The second class of flume has a long enough thi-oat 'section to result in

parallel flow (in that section) to permit the discharge relationships td be

calculated by the solution of equations describing the conservatidn of energy

between the flume approach and throat sections, rather than by experimental

calibration. The size and shape of the flumes were also selected for use. in

ditches conforming to ASAE standard slipform lining. Submerged flow ratings

a r e not available for this class of flume since they will operate ilnder free flow

conditions in channels with the specified slope or a steeper one. Table 7- 11

gives some particulars of this class of flume which has been designed in 30

different types divided into five categories. Figure 7-33 illustrates a typical

parallel flow critical depth flume.

PLAN VlEW

Fitting for k l ~ 5 g +l'-@,'+A recorder w el I . 3'-10 g

THROAT SECTION

PROFILE VlEW END VIEW

FIGURE 7 - 3 2 . - Trapezoidal flume for 1 ft i r r igat ion channels.

TABLE 7-11

Some Characteristics of the Standard Parallel Flow Flume (Derived from ASAE Standard S 359 T)

Bed width of Range of Range of approach section Side slope of approach maximum

Flume maximum

code 11 (= bed width of section ( = side slope upstream measurable ditch) of ditch) flow depth flow

cm (horizontal : vertical) cm I/ 8

Each flume codp represents a group of 6 geometrically slightly different flumes ,

Trapezoidal flumes can be used to measure discharge with an accuracy of

+ 5% with free flow conditions. The accuracy i s however dependent on the - accuracy of the dimensions of the throat cross section, the stage of measurement

and the flume installation. Discharge e r rors will be approximately proportional

to throat area errors. For trapezoidal flumes with a wide throat width, the

discharge error approaches 1.5 times the error in stage *reading. For flumes

with a triangular throat section the discharge error wil1,pe about 2.5 times the

error in stage reading (103). Submerged conditions should be avoided but may

be necessary where head loss through the flume must be reduced to the minimum.

FIGURE 7- 33. - Typical parallel flow critical depth flume.

Although the standard flumes mentioned above may be used in unlined

ditches if cut-off walls a r e attached to each end, they have been designed

part icularly for concrete lined ditches. Installation i s bes t accomplished by a

steel form a s shown in Figure 7-34. If only a smal l number of f lumes a r e to

be instal led a l ighter form, using plywood and t imber , which a r e cheaper, may

be sat isfactory. Construction of a concrete flume without using a fo rm i s not

recommended i f s tandard rat ing tables a r e to be applied. As with a l l f lumes

the accuracy of measurement depends to a grea t extent on the precision of con-

struction. The throat section i s the control section and therefore the exact

dimensioning of this a r e a i s mos t important . F lumes in unlined ditches may be

built of galvanized steel sheet o r reinforced polyester res ins .

Complementary information such a s complete dimensions and rat ings a r e

given in the ASAE Standard (103) a s well a s in (59) and (87).

FIGURE 7-34. - Portable steel fo rm used to cas t trapezoidal concrete f lumes in concrete ditches (87).

7 . 8 USE O F CULVERTS AS MEASURING DEVICES

7 . 8 . 1 General

Numerous culverts a r e found in irr igation distribution sys tems a s well a s

in f a r m head ditches. They a r e useful for crossing water courses , roads o r

railway l ines and they a r e commonly placed through canal banks to divert water

into l a t e ra l s (pipe outlets), ei ther with a head gate placed a t the culvert inlet to

ROD

HOOK BOLT CLIP

SPIGOT BACK WITH PlPE ATTACHED

ANCHOR GROUT 7-71 SEATING' FACE c c l ,

< o a 0 1

I

' ,

FLAT BACK ATTACHED TO CONCRETE

NOTCH CUT IN LlFT STEM AT TOP OF LlFT NUT WHEN GATE IS AT POINT OF ZERO OPENING

f/ 1 GATE OPENING IS DETERMINED BY MEASURING DISTANCE - BETWEEN NOTCH ON STEM AND TOP OF LlFT NUT

6 MINIMUM TO INSURE COMPLETE SUBMERGENCE OF THE OUTLET PlPE AND A POSITIVE WATER MEASUREMENT IN THE WELL CONNECTED TO THE TOP OF PlPE

XJ BOTTOM OF 'DIFFERENCE IN WATER OUTLET DITCH

ELEVATION IN WELLS NOT GREATPR T H A N 1 R xcIZ

r I ON TOP OF PlPE

4'MIN f

INSTALL PIPE ON LEVEL GRADE

FIGURE 7 - 3 5 (a) and (b). - Meter gate for pipe outlets

(64) .

control the quantity of flow diverted to the lateral, o r without any control device.

If properly calibrated, culverts with and without control gates can be used for

discharge measurements.

Gated Culverts

A shut-off is, in most instances, required at a farm outlet because of the

manner in which the system i s operated. If properly calibrated the shut-off can

also serve as a means of measurement and there have been numerous attempts to

provide this combination.

A measuring gate used in the U. S. A. and in some other countries i s shown

in Figure 7-35 (a) and (b). It consists of a circular plate, operated by a screw,

which can shut off the outlet pipe. Two stilling wells, as shown, are fixed to the

outlet; one is connected to the canal and the other to the delivery pipe on the

downstream side of the gate. The difference in water levels in the two wells and

the gate opening is measured and the discharge obtained from tables derived from

standard calibrations. The head loss i s low but changes in either upstream or

downstream water levels alter the rate of flow so that periodic observations and

manual adjustments are necessary.

The flow through gated culverts may be estimated by using the formula :

where Q = discharge in 11s

C = coefficient = 0.7 for short culvert%. (such as those used for farm outlets)

A = area of orifice in cm 2

g = 981 cmfs 2

h = head in cm causing discharge through the orifice

Example:

Aasume orifice area = 300 cm2 and H = 40 cm

Tertiary canal

-Sliding iron gate

Farm ditch

I L = 300 cm (minimum I

FIGURE 7-36 . - Sketch of pipe outlet with sliding 'gate fdr delivery control and measurement (88)

A simpler form of gate and method of measurement i s used in the

installation shown in Figure 7-36. Calibration of this type of concrete pipe

outlet has been conducted by 'the State Hydraulic Works Department in Turkey.

The results a r e shown in Figure 7-37. After the water level in the supply canal

has been established by a canal eheck gate, the rate of flow through the outlet i s

determined by correlating i t s gate opening to the head which i s read from staff

gauges up and downstream of the pipe. Although accuracy i s relatively low, the

cost to provide this measuring facility i s negligible.

45 4 0 35 30 25 2 0 15 10 5 0

Gote opening (cm) - meorurad inclined

FIGURE 7-37. - Rating curve for pipe outlet (88).

7.8 .3 U n ~ a t e d Culverts

The discharge of a culvert i s dependent on effective head, i t s c ross section,

degree of submergence of inlet, pipe (or barrel) and outlet, shape of inlet, length,

slope, and roughness of the pipe (or barrel) . Of the basic flow conditions that

can exist, downstream conditions (f ree surface, o r submerged) usually control

the flow in culverts used in irrigation systems. So far only approximate dis-

charge formulae a r e available for free surface flow affected by downstream

conditions, and hydraulic computations a r e involved. Recent research on

culvert hydraulics conducted at the Colorado State University, Report No. 17 (96),

has provided the theoretical basis on which culverts can be accurately rated a s

flow measuring devices for three basic flow conditions. Rating tables have been

produced for a 30.5 cm (12 inch) diametric corrugated metal pipe for various

slopes, including horizontal, and pipe lengths of 1.5, 3 and 6 m (5, 10 and 20 ft),

and the report recommends discharge rating experiments be extended to a

variety of culvert sizes and lengths.

PROPELLER METERS

General

Propeller meters a r e commercial flow measuring devices used near the

end of pipes o r conduits flowing full (under gravity flow) o r as in-line meters in

pressurized pipe systems. The latter application will not be discussed here,

since pressure distribution systems a r e excluded from the scope of this Handbook

If used for gravity flow the meter i s also known a s an open flow meter . The

propeller rotates about a horizontal axle which i s geared to a totalizing head that

records the total number of cubic metres or cubic feet, passing the measuring

section. Some meters indicate instantaneous discharge a s well.


Hydraulic properties such as range of discharge, head loss and calibration

curves, vary slightly between manufacturers who usually furnish such data for

individual types. The data quoted below refer to one make which may be

generally representative of most of the available propeller meters .

FIGURE 7-38. -

FIGURE 7-39. - Propeller meter FIGURE 7-40. - Register of a installed at a pipe outlet. propeller meter.

The diagram in Figure 7-41 shows that the flow velocity should fall within

the range from 0.35 m / s (1.15 ft /s) to 2.5 m/a (8.2 ft /s) and that *e normal

flow velocity should preferably range between the limits 1.3 to 2.0 m / s (4.25 to

6.5 ftfs). If the velocity falls short of 0.35 m / s accuracy rapidly deteriorates.

Max. test flow

velocitv ronoe Min. test flow, 1 I

FIGURE 7-41. - Range ability of a propeller meter and the selection of meter diameter (t 4% accuracy).

,

Example of determination of required propeller diameter :

Given Maximum design flow ' = 0.5 m3/ s

Minimumdesignflow = 0.15m3/s

Normal flow rate = 0.4 m3/s

Solution - From Figure 7-41 it i s found that a propeller diameter of 600 mm

would best fit the given flows. Its permissible velocity range covers the

required maximum and minimum flows and the normal flow rate may be die-

charged within the normal velocity range.

F r o m the propeller diameter required, the suitable pipe diameter i s deter-

mined. The la t ter should be f r o m 1. 25 to 2.0 t imes a s large a s the propeller.

F o r normal velocity distribution in the measuring section the pipe upstream

of the meter should be straight for a length of 15 pipe diameters. Straightening

vanes may be required ahead of the device i f spiralling flow i s expected. This

may develop a t the entrance f rom the canal into the pipe.

F o r proper registrat ion the propeller must be completely submerged.

Therefore, if occasionally the pipe i s to be run only partly filled, a backwater has

to be created a t the outlet. This i s done by installing a submerged weir with a

c r e s t 10 c m (4 inches) above the top of the pipe. The face of the weir should be a t

l eas t 1 m e t r e plus 2 pipe diameters away f rom the pipe mouth ( see Figure 7-42).

The head losses of the mete r shown in Figures 7-38 to 7-40 a r e 10 cm

(4 inches) for flow velocities below 2.5 m / s ( 8 . 2 f t / s ) and 15 c m ( 6 inches) for

flow velocities f rom 2. 5 up to 3 m / s (10 f t / s ) .

Performance

Propel ler m e t e r s a r e used to a considerable extent in the U. S. A., Japan

and Australia, and some other countries have s tar ted to introduce them. They

may be used with advantage in systems where water i s sold on a volumetric basis

since flow volumes a r e given directly without computation. Other advantages

a r e small head losses , and independence f r o m external power. However,

propeller m e t e r s do suffer f r o m some distinct limitations :

- the m e t e r s a r e very susceptible to weeds ahd other debris in the flowing

water;

- suspended sediments may enter into the bearings and reduce the number of

propeller revolutions thus resulting in under - registrat ion;

- the m e t e r s must be submerged under al l flow conditions;

- manufacturers usually claim a degree of measuring accuracy which may be

obtained under controlled laboratory conditions (e.g. - f 2% accuracy) but

hardly under field conditions; apar t from e r r o r s introduced by debris and

bearing problems, considerable registration e r r o r s may be caused by faulty

installation. Under average field conditions the measuring accuracy may

be c loser to - + 5% than - + 2% and even l e s s .

- the serviceable life i s relatively short; Japanese sources claim a life of 5

Dia. 13 (@ 2 5 0

Front elevation or plane Section B-B

I. B = 1,200 mm for Dl 500 mm 5 = 1,500 mm for 4 3 6 0 0 mm

2 The half of the pipe is used for the attachment to the square box

3 The water vent and top is installed in the inside wolf i f indicated

4 Standard size of 61 ,B2 61 = 2 4 + 1,000 mm (min. 1,200 mm) B2 if Dl & 500 mm 5, = 5 0 0 mm

Dl 3 500mm 6, = 1,000 mm

Dia. 13 @ 2 5 0

Section A-A

F A O - l C l O

STANDARD DESIGN OF OPEN TYPE

PROPELLER METER

Project , Reqron, Country Unspeclfled , Jopon

F~gure No 7-42

to 8 years.

- propeller meters require continuous maintenance - purchase of spare par ts

may cause problems; the risk of damage i s high;

- propeller meters a r e relatively expensive to purchase.

7.9.4 Design Example

Figure 7-42 shows a standard design developed in Japan for low pressure

pipe systems. The design i s easily modified to fit into a pipe outlet to an open ditch.

7.10 DEFLECTION METERS

7.10.1 General

A deflection meter consists of a vane or rod dipped in the flowing water and

mounted on a horizontal spindle across the measuring section in a channel. The

deflection caused by the force of the flow against the vane or rod i s indicated on a

calibrated scale giving the instantaneous discharge. At least two systems of

indication a re in use: in the simplest one a pointer indicates the deflection on a

fixed vertical scale (Figures 7-43 and 7-44); a more advanced type consists of a

bubble glass tube attached to a scale that i s directly fixed on the top of the vane.

The discharge i s determined by reading the position of the centre of the bubble

against the scale (Figure 7-45).

FIGURE 7-43. - Example of a deflection meter with a pointer indicating against a fixed vertical scale (Rajasthan, India).

FIGURE 7-44. - The Rajasthan channel flow mete r in use

Deflection m e t e r s a r e usually manufactured commercially but a s shown

in Figure 7-43 and 7-44 can be constructed locally as well. The m e t e r s a r e

usually portable and may be easi ly moved f rom one station to another. The

bearings which keep the mete r in position a r e usually permanently installed in a

trapezoidal o r rectangular measuring section. Thus one mete r head can serve a

number of ditches of about the same flow capacities, provided these ditches a r e

equipped with permanent liner sections. Each meter handles about 1: 15 range of

flows in a given size of ditch and automatically compensates for different com-

binations of velocity and depth.

Under ideal conditions a measuring accuracy of + 2% may be obtained but in - practice the level of accuracy depends on local factors. Wind i s a major source

of inaccuracy and can produce large e r ro r s .

Since deflection meters a r e handy and easy to install, they have been used

successfully in field t r ia ls for irrigation efficiency and water management studies,

a s well as for water distribution control at the farm level.

FIGURE 7-45. - Commercially available deflection meter.

Elevotion End elevation

4nti-vortex Slot to just OllOw, boffk possoge of rod- _

Table of dimensions

_IZ. I 7-

Rods, needle bearings, lock nuts to be of brass

a,:*ionol elevotbn A-A Tube, 60f f le ind heodwoll to be of 245 G.I.

0

f .- 3 T -

Plon

FIGURE 7-46. - Sketch of the Rajasthan channel flow meter.

The mete r shown in Figure 7-44 i s commercially available in 16 standard

models. The smal les t model has a capacity range of 3 to 100 11s (0.1 to 3.5

ft3/ s ) while the l a rges t model i s applicable to a flow range f rom 40 to 850 l/s

(1.5 to 30 ft3/s) . The cost (in 1968) ranged f rom $595 to $615 for the m e t e r s

and f rom $43 to $83 for the l iner sections; i t i s c lear therefore that this type of

device i s economic only if one mete r can serve a s many measuring stations a s

po s sible .

7.10.2 The Rajasthan Channel Flow Meter

The Rajasthan Channel Flow Meter (Figures 7-43, 7-44 and 7-46) was

developed by the FAO/UNDP Pro jec t - Soil and Water Management Research and

Demonstration in the Rajasthan Canal Area, in 1970, a s a means of measuring

flows in irr igation efficiency t r ia ls .

Design c r i t e r l a for the mete r are : they should be operated with a negligible

head loss in muddy water containing some t rash ; they should be simple, robust,

capable of manufacture by village craftsmen and portable; the accuracy should be

of the order of 10% o r better and the indicating device should be simple to read.

On this basis four different s izes have been developed a s indicated below.

Meter Size c m inch

Measurement Range

11 s f t5/ s

A cost of 150 Indian rupees ($20) was quoted (in 1970) for the 30 cm mete r .

Field testing showed that in general deflections and discharge corresponded

well in the middle ranges of flow, with an accuracy of 10/o, while a t low o r

high flows resul ts were scattered. It i s concluded that the flow m e t e r appears

to be a cheap and useful measuring device, that with reasonable ca re in

manufacture, handling and installation, will attain measurements within + 570

accuracy l imits.

0 I 3 4 3 Flow in ft /s

0 0.01 0.02 0.03 004 005 006 0.07 0.08 0.09 0.10 0.11 L I I I 1 ' 1 I 1 I I 1 1

3 Flow in m/s

FIGURE 7-47. - Sample calibration curve. for 30 c m ( 1 2 inch) Rajasthan Channel flow me te r .'

7.11 THE DETHRIDGE METER

The Dethridge meter i s a self-integrating measuring device developed in

Austral ia . It i s designed particularly to fit f a rm outlets and i s therefore

discussed in detail in Chapter 5, F a r m Outlets, Section 5.5.

7 . 1 2 THE CONSTANT HEAD ORIFICE TURNOUT

The Constant Head Orifice Turnout i s a combined regulating and measuring

device that u se s an adjustable submerged orifice for the measurement of dis-

charge. The structure may be installed in canal intakes or (the most common

application) may serve a s a fa rm outlet (or farm turnout). A comprehensive

description of the structure i s given in Chapter 3, Section 3.4.

7.13 CALIBRATION OF MEASURING STRUCTURES

Calibration of a measuring -structure i s required in order to establish in

numerical values the exact relationships between water stage or gauge height

and discharge for any given water depth or opening (in the case of orifices).

Most of the standardized measuring weirs and flumes have been extensively

calibrated through laboratory o r field tests and the results a r e available in

published rating tables o r graphs. If the individual measuring structures be

built to these standard dimensions, these tables will be directly applicable with

a high degree of accuracy, say from + 170 to + 5%. However, if in actual

practice dimensions and materials of in-situ-built structures differ so much

from standard that application of standard rating tables would cause e r r o r s above

+ 5%, individual field calibration may be necessary in order to increase accuracy - to acceptable values. Calibration i s usually required where ordinary gates,

sluices o r other existing structures a r e to be used for water measurement.

In calibrating an individual structure, a se r ies of discharge measurements

a r e made covering the whole range of water depths or the whole range of gate

openings expected in the future and the working heads a r e recorded

simultaneously. From this information curves or tables have to be prepared.

Fo r calibration measurements the current meter method i s commonly used a s

described in many textbooks. Measurements a r e made in a rating section of

known dimensions. The rating section should be situated in a uniform channel

reach, f ree from disturbances caused by upstream conditions such a s bends,

waves and other distorting influences. A large number of current meter

readings a r e required to obtain a good match curve a s individual readings may

vary considerably.

Another calibration method that, where applicable, will yield accurate

results i s the use of a temporary weir o r flume of standard type installed up-

s t ream or downstream of the permanent structure to be calibrated. Fo r this

method, however, sufficient fall has to be available to ensure free fall conditions.

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Armco Steel Corporation, Metal Products Division. Armco Gates for Irr igation 1971 and Other Low Head Applications. Catalog G- 1057 1. Middle town, Ohio.

~ o h n s t o n , C. N. F a r m Irrigation Structures. University of California Agric. Exp. Station. Circular 362. 7945.

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Hydraulic Structures for Reclamation and Irrigation). Supplement to Bulletin No 9. Rome.

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Prac t i ces Seminar, NESA Region, New Delhi.

De L o s Rios Romero, D. F. Consideraciones Sobre Futuros Proyectos de Obras 1963 en l a s Zonas Regables. Ministry of Agriculture, Spain.

73. Robinson, A. R. and Hurnpherys, A. S. Water Control and Measurement on the 1967 F a r m . Irrigation of Agricultural Land. U. S. A. 1967.

74. USDA 'Soil Conservation Service - Engineering Handbook Section 11, Standard Structural Plans . Lincoln, Nebraska.

75. U. S. A. Water Control Corp. - Water-Control Irrigation Structures. Bulletin HW-103. Crystal Lake, Ill.

76. p r e s s , H. Stauanlagen und Wasserkraftwerke 11 Teil. Wehre, Berlin. 1959

Lis t of References Conttd.

77. Nugteren, J. Technical Aspects of Water Conveyance and Distribution Systems. 1971 P a p e r presented a t the F A 0 Seminar on The Effective use of Irrigation

Water a t the F a r m Level. Damascus.

78. F A 0 - Automated Irrigation - -Regulation of a Hydraulic System by Clement, R. 1971 Automated Irrigation - Irrigation and Drainage Paper No 5.

7 9 ICID - Eighth Congress on Irrigation and Drainage, Question 28.2. Reports for 1972 Discussion. Varna.

80. Thomas, C. W. World Prac t i ces in Water Measurements a t Turnouts. Journal 1960 of the Irrigation & Drainage Division. IR 2, June.

81. USDI Bureau of Reclamation - Water Measurement Manual. Second Ed. 1967 Denver, Col.

82. USDA Soil Conservation Service - National Engineering Handbook. Measurement of 19 62 Irrigation Water. Section 15 Irrigation. Chapter 9. Washington.

83. Tsu-Yang Wu. Effects of Settlement on Flume Ratings. Water Management 197 1 Technical Report No 12, Colorado State University, F o r t Collins

Colorado.

A 84. ~ e c r e / t a r i a t d lEta t aux Affaires ~ t r a n g k r e s - Techniques Rurales en Afrique.

1970 4. L e s 0uvrag.e~ dtun petit reseau d'irrigation.

85. Skogerboc, G. V. , Bennett, R. S. and Walker, W. R. Installation and Field Use 1972 of Cut-throat Flumes for Water Management. Water Management

Technical Report No 19, Coloradp State University, For t Collins, March.

86. Bennett, R. S. Cut-throat Flume Discharge Relations. Water Management 1972 Technical Report No 16. Colorado State University, F o r t Collins,

March.

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88. Ozal, K. and Ozsoy, E. Measuring Devices for F a r m Turnouts and Pipes. M.E. Technical University, Ankara, Turkey.

89. Worstell, R. V. The Snake River Auto-Start Siphon Tube. Agricultural 1971 Engineering, Vol 52, October.

90. ICID - Hydraulic Structures on Small Channels. Question 24, Seventh Congress 1969 on Irrigation and Drainage, Mexico City.


9 1. Ponzoulet, J. M. and Porcheron, R. Utilization d'un ordinatein pour l e calcul des 1967 / reseaux modernes d'irrigation - calcul pratique des debits a la

"demand" optimisation de s pa ramet res des canalisations. ICID Annual Bulletin.

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93. Canada - Alberta Department of Agriculture, Water Resources Division. Commonly used F a r m Irrigation Structures.

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Irrigation & Drainage Division, Vol 99, No IR 3: pp 237-338.

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Colorado, February.

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Conseil Superieur de llAgriculture Roumaine. Fonctionnement et rGgulation des canaux d'irrigation. P e r i m e t r e carasu.

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Symposium, November 1970. 20 p. USDA, U. S. A.

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Shipley, H. Development of Automation on Salt River Project . Journal of the 1970 Irrigation and Drainage Division, ASCE, June.

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ASCE - Operation and Maintenance of Irrigation and Drainage Systems. Journal 1973 of the Irrigation and Drainage Division, Vol 99, No IR 3, September.

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Butcher, A. D. Clear Overfall Weirs. 1922

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11 NOTATIONS AND SYMBOLS-

Area

Area of c ross section

Area of cri t ical section

Breadth o r width (usually a c r o s s the axis of flow)

Bed width of canal upst ream a fall, syphon, aqueduct, etc. , and in parent channel in case of intakes and outlets

Bed width of canal downstream a fall, syphon, aqueduct, etc. Bed width in the offtake channel below the intake o r watercourse below the outlet

Width of throat o r controlling section o r width of weir c res t ac ross the axis of flow

Width of inlet

Width a t outlet end

Width of gate opening o r sluice opening

Wi dth of stilling basin, cistern, etc.

'/ F o r terminology and definitions reference should be made to the Multilingual Technical Dictionary on Irrigation and Drainage published by the ICID in 1967.

Coefficient of discharge

Coefficient of roughness

Coefficient of submergence

Coefficient of submergence of hydraulic jump

Coefficient in Chezy's formula

Coefficient, approach velocity

Depth of canal

Designed depth of canal (if distinguished)

~ e ~ t h of canal upstream of falls, proportional distr ibutors o r divisors, syphons, aqueducts, etc. , and in parent channels of outlets and offtake channels

Depth of canal downstream of falls, etc. and depth of offtake channels below intakes and of watercourses below outlets

Depth of stilling basin

Diameter

Diameter of pipe

Discharge

Discharge intensity o r discharge per unit width

Discharge in the parent canal

Discharge of offtake channels o r outlets

Small increment in discharge

Distances and spacings

Efficiencies

Flexibility

F r e e board

Froude number

Height over har ding s

Head over cres t , etc.

Working head *

~ e a d - d u e to velocity of approach

Head loss

Height of gate opening

Height of c r e s t above upstream bed level

Height of c res t above bottom level of stilling basin

Height of c r e s t above downstream bed level

Height of upstream water level above soffits of orifices, pipes, etc.

Height of orifice above c r e s t o r bottom level of control section

Hydraulic drop

Depth of flow a t the beginning of hydraulic jump or super critical sequent depth

Depth of flow at the end of hydraulic jump or subcritical sequent depth

Critical depth corresponding to minimum energy

L -

Length

Length of c r e s t along the axis of flow

Length of glacis

Length of stilling basin

Length of pipe

Length of jump

Proportionality

Radius

Hydraulic radius

Ratio

Sensitivity

Shear s t r e s s

Slope (longitudinal)

Side slope

Thickness

Velocity

Critical velocity

von Karman's constant

Weights

Specific weight of fluid

.and and

econon

Small Hydraulic Strucutres

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