Booklet 7 - Flow Measurement Methods in Open Channels

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−the Q = AU relation uses a wet section (A) and the average velocity (U).Each of these methods will be discussed in detail in the sections that follow.1.7. Primary measurement device methodsMeasurements of continuous flows, in the case of an open channel flow, are performed using ahydraulic structure that establishes a unique depth-flow relationship. These hydraulic structures arecalled “primary measuring devices” and can be separated into two categories: control flumes and weirs.This method, which is most common, determines flow by means of calculation, based solely on ameasurement of the water depth upstream from the primary measuring device.Choosing a primary flow measurement device must be examined and designed on the basis of thefollowing:−−−−−a precise knowledge of the maximum and minimum flowrates that may be channeled to theprimary device. The flows must, depending on the primary device considered, be included in thelimits recommended in this guide;fluctuations in water depth, according to minimum and maximum flows, must be the extremelimits, in order to increase measurement precision, particularly if there are small variations in flow;the presence of solid material and possible obstruction;compliance with standards governing installation, conditions for approach to the primary deviceand loss of pressure due to inserting the device;the ease of calibration and testing equipment.Flumes are generally preferred over weirs, due to their numerous advantages, namely greater drops inpressure and their ability to self-clean.1.7.1. FlumesFlumes are usually prefabricated devices that are installed temporarily or permanently in a flowsystem.They can be a “flat-bottom” type. In the case of a flat-bottom flume, the shape of the side walls createsa contraction of the flow of liquid (ex. cutthroat flume). They can also combine vertical and sidecontractions (ex. Parshall flume).1.7.1.1 General descriptionA flume is essentially a specially-molded open section that creates a restriction in the flow area. Flumeswork according to the Venturi principle. By reducing the flow area, velocity increases and water depthchanges (1) .A flume usually has three sections: a converging section, throat section and diverging section.Page 16 of 223Booklet 7 - Flow Measurement Methods in Open Channels
Sizes vary according to the type and shape of flume. For practical purposes, to determine the absoluteflow of a flume, the calibration curves supplied by the manufacturer should be used.1.7.1.2 Advantages−−−−Minimal drop in pressure.Enables measurement in a large range of flow.The flowrate in flumes is usually high enough to prevent sedimentation; they are therefore selfcleaning.Provides a reliable measurement in free flow and submerged flow conditions.1.7.1.3 Disadvantages−−−−Installation is usually expensive.Installation requires extremely careful work.Requires a secure watertight base.Flow at the entrance must be evenly distributed, with little turbulence, to produce accuratemeasurements.1.7.2. WeirsA weir is one of the simplest and oldest structures used to measure flow. The weir’s large parts are easyto inspect, and any abnormalities can be detected or corrected immediately (2) .1.7.2.1 General descriptionA weir can be defined as a structure that is erected across an open channel to measure the discharge ofliquid.A thin-plated weir is a 3 to 6 mm (1/8 to 1/4 in) thick plate with a straight edge (crest), or a thick platewith a 45° angle edge that is used to reduce the thickness of the crest to the aforementioned sizes. Thisplate is placed across the flow and liquid is forced over the plate or into a V-notch in the weir.The edge over which water flows is called the crest.The height of the weir between the crest and the channel bottom is called the crest height.The height of the discharge over the crest is called the nappe.The air space under the nappe, downstream from the weir, is called the ventilation.Figure 1 shows the components of a weir.Booklet 7 - Flow Measurement Methods in Open Channels Page 17 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 and 16: Note: If effluents have different d
Page 17: This equation shows that water dept
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
Page 67 and 68: −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
Page 149 and 150:
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
Page 213:
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
Page 222 and 223:
REPLOGLE, John A., Meyers, Lloyd E.
Page 224 and 225:
TRIESTE, Douglas J., Evaluation of
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Booklet 7 - Flow Measurement Methods in Open Channels

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