Booklet 7 - Flow Measurement Methods in Open Channels

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1.3.2. Continuous flow measurementContinuous flow measurements consist of a series of point-specific measurements at very close timeintervals (a few seconds apart), using instruments capable of recording values during the procedure.The advantage of this type of measurement is the fact that it can extend over an extended time period (afew hours to a few days) and reveals variations in flowrate that occurs during this period. Informationthat is obtained is therefore more complete.This type of measurement usually requires a temporary or permanent primary measuring device.1.4. Discharge system1.4.1. Closed conduitsA discharge is said to be in a closed conduit when the liquid is confined in a pipe and subjected topressure that is greater than atmospheric pressure.1.4.2. Open conduitsA discharge is said to be in an open conduit when the surface of the water, so-called waterline, is incontact with the air and subjected to atmospheric pressure only.Conduits can be natural, such as streams and rivers, or artificial, such as channels for navigation,irrigation, drainage ditches, sewer networks, etc.1.5. Flow principles in open conduitsStorm sewer and building drainage systems are open conduits. The flow area is in contact with the airand is subjected to atmospheric pressure only. Because pressure cannot be forced from one end of theflow channel to the other end, the flowrate depends on the gradient of the slope of the channel and thefrictional drag along the walls. Because the flow is stable and uniform (streamlined), there is always aprogressive drop in water level from the beginning to the end of the channel.In an open channel, the total energy contained in the flow is the kinetic energy present in the form ofvelocity (Head Velocity) and the pressure energy due to the difference in water depth. Which producesthe following equation:VTotal energy = + d2g2(8)whereVgdis the flowrate;is the gravitational constant;is the water depth measured from the channel bottom.Page 14 of 223Booklet 7 - Flow Measurement Methods in Open Channels
This equation shows that water depth varies when there is a variation in velocity, unlike closedconduits where changes in velocity produce changes in pressure. If flow increases and energy isconverted to velocity, water depth must fall. Inversely, when velocity declines, water depth mustincrease. A drop in water flow area is known as “drawdown”. On the contrary, in closed conduits,changes in pressure translate into changes in flowrate, but the water level remains constant.This flow can manifest itself in the form of three possible categories:uniform flow: the slope of the water line is parallel to the slope of the channel bottom;gradually varying flow: liquid experiences an acceleration or deceleration due to a moderatechange in the flow section or the slope of the channel bottom;critical flow: this type of flow is produced by a strong contraction created by a lift in thebottom and/or a narrowing of the width of the channel or by a sudden change in the slope of thechannel bottom.A Parshall flume is an excellent example of an instrument that measures changes in liquid depth whenvelocity increases. When water enters the flume, it accelerates in the converging section (approach) andreaches a maximum velocity when it moves through the throat section where the bottom slopes down;causing the water level to drop significantly.This variation in liquid depth is directly related to the amount of water flow (flowrate). Flow curvesand flow tables for standard Parshall flumes are prepared on the basis of these variations.1.6. Principles of flow measurement in open channelsA number of approaches can be used to measure the flow of liquid in an open channel:−−−consider only kinetics, that is, the mechanics of movement of a liquid without the need to examineunderlying forces. This is the principle of the depth/velocity method;basic information on known relations between gravity causing the flow and the inertial andviscosity forces that slow it down;quantitatively analyze changes in the physical or chemical properties of the liquid after solublesubstances have been added. This is the principle of the dilution method.Flow can be measured using a variety of methods:−−−use of the flow regime variations principle, by creating a critical flow with the help of a measuringinstrument known as a “weir” or “control flume”;measurement of the dilution rate of a tracer, the concentration of which is known at the intakepoint;the “volumetric tank” method, which consists of determining the time required to fill a containerof a known volume;Booklet 7 - Flow Measurement Methods in Open Channels Page 15 of 223
Page 1: Centre d’expertise en analyse env
Page 5: FOREWORDThis Sampling Guide for Env
Page 8 and 9: 4.4. Area/velocity method..........
Page 10 and 11: LIST OF ILLUSTRATIONSFigure 1 - Wei
Page 13 and 14: 1. GENERAL1.1. Definition of flowra
Page 15: Note: If effluents have different d
Page 19 and 20: Sizes vary according to the type an
Page 21 and 22: FIGURE 1 - WEIR COMPONENTSFlow is d
Page 23 and 24: 1.10. Reference measurementIf a mea
Page 25 and 26: FIGURE 2 - PALMER-BOWLUS - FLUME CH
Page 27 and 28: TABLE 1 - DIMENSIONS FOR PALMER-BOW
Page 29 and 30: 1.12. Choosing a measurement method
Page 31 and 32: FIGURE 4 - FLUME - FLUME AND FLOW C
Page 33 and 34: TABLE 3 - CHART FOR COMPARING THE M
Page 35 and 36: TABLE 4 - DIMENSIONS FOR STILLING W
Page 37 and 38: 2. DESCRIPTION OF FLUMESThe section
Page 39 and 40: 2.1.2. Operating principleA Parshal
Page 41 and 42: TABLE 5 - PARSHALL FLUME - STANDARD
Page 43 and 44: TABLE 6 - PARSHALL FLUME - RECOMMEN
Page 45 and 46: 2.1.7. PrecisionThe margin for erro
Page 47 and 48: channel at maximum flow (H c ), bef
Page 49 and 50: FIGURE 8 - PARSHALL FLUME - JUNCTIO
Page 51 and 52: FIGURE 9 - PARSHALL FLUME - EFFECTS
Page 53 and 54: 2.1.12. Discharge equation in a sub
Page 55 and 56: FIGURE 11 - LOSS OF HEAD - PARSHALL
Page 57 and 58: To monitor the total rise, follow t
Page 59 and 60: A Palmer-Bowlus flume, as experimen
Page 61 and 62: FIGURE 13 - SHAPE OF A PALMER-BOWLU
Page 63 and 64: TABLE 10 - PALMER-BOWLUS FLUME - ST
Page 65 and 66: Because some manufacturers do not a
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−use of the depth/flow table2.2.1
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For temporary systems, a simple ver
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Q is the flowrate in liters or US g
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FIGURE 15 - SHAPE OF LEOPOLD-LAGCO
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2.3.7. Measurement capacity of a fl
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2.3.11. Installation specifications
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2.4.2. Operating principleCutthroat
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TABLE 14 - CUTTHROAT FLUME - STANDA
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TABLE 15 - CUTTHROAT FLUME - RECOMM
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TABLE 16 - CUTTHROAT FLUME - VALUES
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TABLE 18 - CUTTHROAT FLUME - DISCHA
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Since a number of other cutthroat f
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where:Q is the flowrate in cubic me
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Example: To measure a 0.6 m 3 (21 C
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It consists of two sections: approa
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FIGURE 18 - H FLUME - PHYSICAL CHAR
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Although this type of flume can be
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TABLE 21 - H FLUME - STANDARD DIMEN
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TABLE 23 - H FLUME - RECOMMENDED MI
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CoefficientsandexponentsTABLE 24 -
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−erosion at the outlet of the flu
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2.5.13. Flume modificationTo obtain
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Example:For a rectangular weir wher
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−−−−−−−the presence o
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TABLE 25 - WEIR CALIBRATION - VOLUM
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FIGURE 20 - RECTANGULAR WEIR WITHOU
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TABLE 26 - RECTANGULAR WEIR WITHOUT
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Inadequate approach velocityThe app
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FIGURE 21 - RECTANGULAR WEIR WITH C
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The Kindsvater-Carter equation take
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3.3.5. PrecisionIn free flow condit
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TABLE 28 - RECTANGULAR WEIR WITH CO
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FIGURE 22 - TRAPEZOIDAL WEIR (CIPOL
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TABLE 29 - TRAPEZOIDAL WEIR (CIPOLL
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Length of contractionsIf side contr
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In addition to the information prov
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The discharge equation resulting fr
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3.6. Combined weirTo obtain a preci
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In addition to the information prov
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3.6.5. PrecisionIn free flow condit
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FIGURE 25 -COMPOUND WEIR - PHYSICAL
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TABLE 32 - COMPOUND WEIR - MEASUREM
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4.1.1.2 Characteristics of tracersT
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S gis the specific weight of the tr
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4.1.2.1.4 Disadvantages of the meth
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When selecting a sampling point, it
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4.1.2.2.8 Detection and measurement
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Dyes are well suited to flow measur
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4.1.2.3.8 Detection and measurement
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4.1.2.4.3 Preparation of radioactiv
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4.1.2.4.9 Preliminary testsBefore c
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4.1.3.1.3 Advantages of the method
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Testing procedures are as follows:T
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Use the following formula to determ
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The discharge equation is therefore
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• Choosing a tracerExamine the di
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Sampling must be carried out during
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−−contamination of sampling con
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Instantaneous injections1 2 3 4 5nP
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tracer is lost due to splashing, by
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The concentration of tracers is usu
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4.1.3.3.7 Causes of errorThe princi
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4.2.4. DisadvantagesThe principal d
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−If the dimensions cannot be meas
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4.3.7. DisadvantagesFlow measuremen
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4.3.8.2 Calculation of pumping time
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4.3.8.4 Number of testsWhen a pumpi
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−−−−−requires a number of
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4.4.9. Flow areaMeasurement of the
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Velocity measurements should be tak
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considering only the source of erro
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There are also other causes of bias
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12. GRANT, D. M., ISCO Open Channel
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36. INTERNATIONAL STANDARD ORGANISA
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CHANDRA, U., Aoki, P. E., Ramos e S
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HAMMER, Mark J., Water and Waste-Wa
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Resources Instrumentation, June 4-6
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REPLOGLE, John A., Meyers, Lloyd E.
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TRIESTE, Douglas J., Evaluation of
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Booklet 7 - Flow Measurement Methods in Open Channels

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