Flow MeasurementFlow Measurement

PENGUKURAN DEBITPENGUKURAN DEBIT

Measurement ApproachesMeasurement Approaches

Direct methods (stream gaging)Direct methods (stream gaging)– Current-meter method (Current meter)Current-meter method (Current meter)– Floats (PELAMPUNG)Floats (PELAMPUNG)– Tracer-dilution technique (larutan)Tracer-dilution technique (larutan)– Ultrasonic method (metode ultrasonic) Ultrasonic method (metode ultrasonic) – Electromagnetic method (metode Electromagnetic method (metode

electromagnetic)electromagnetic)

Measurement ApproachesMeasurement ApproachesHydraulic Devices (Bangunan Hydraulic Devices (Bangunan hidrolik)hidrolik)– Weirs and notchesWeirs and notches– OrificesOrifices– FlumesFlumes

Indirect TechniquesIndirect Techniques– Slope-area methodSlope-area method– Contracted-opening methodContracted-opening method– Flow-over-structure methodsFlow-over-structure methods

Definition of TermsDefinition of Terms

Stage (TMA/h : tinggi muka air) : Stage (TMA/h : tinggi muka air) : Height of water surface above a Height of water surface above a fixed datumfixed datum

Rating (Lengkung): Relationship Rating (Lengkung): Relationship between stream stage and between stream stage and dischargedischarge

Alat Pengukuran Alat Pengukuran Alat ukur/sensor :Alat ukur/sensor :

Pelampung, Manual/Peischall,tekananPelampung, Manual/Peischall,tekananMetode Pencatatan: manual, Metode Pencatatan: manual, electronic,mekanik, elektromekanikelectronic,mekanik, elektromekanikAlat Perekam : pias/chart,komputer, Alat Perekam : pias/chart,komputer, data logger dsb.data logger dsb.

Stage Measurement

Gupta, Fig. 6.1

Typical Stilling Well-bangunan AWLR

Gupta, Fig. 6.2

Typical Bubble-gage

Gupta, Fig. 6.3

Measuring VelocityMeasuring VelocityCurrent metersCurrent meters– Cup-type (Price and pygmy)Cup-type (Price and pygmy)– Propeller typePropeller type– InductanceInductance

Velocity Distribution-distribusi v sungai

Chow, Fig. 2.3

Velocity Distributions

Chow, Fig. 2.4

Vertical Velocity Distribution

Rantz, Fig. 88

Computation of DischargeComputation of Discharge

Cross-section divided into 20-30 Cross-section divided into 20-30 sectionssections

Sections positioned such that no more Sections positioned such that no more than 5% of flow passes in one sectionthan 5% of flow passes in one section

Discrete integration based on one of Discrete integration based on one of two techniquestwo techniques– Mean-section methodMean-section method– Mid-section methodMid-section method

Mean-section Method-luas penampang

Gupta, Fig. 6.12

xxxxx

x WddVV

q

2211

Mid-section Method-luas penampang

Gupta, Fig. 6.11

qx V xWx 1 Wx1

2

dx

Measuring Widths

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Measurement Section

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Section Locations

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Velocity Measurement

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Stage-Discharge RatingsStage-Discharge Ratings(Kurva lengkung debit h vs Q)(Kurva lengkung debit h vs Q)If it is not possible to install a weir or If it is not possible to install a weir or flume (large channel), need to flume (large channel), need to develop a stage-discharge ratingdevelop a stage-discharge rating

3 basic types3 basic types– Simple stage-dischargeSimple stage-discharge

Control produces one-to-one ratingControl produces one-to-one rating

– Slope-stage-dischargeSlope-stage-discharge– Velocity-stage-dischargeVelocity-stage-discharge

Simple RatingSimple Rating

In general, ratings in natural In general, ratings in natural channels follow a logarithmic shape channels follow a logarithmic shape (power function)(power function)

Rating has an offset at the “point of Rating has an offset at the “point of zero flow”zero flow”

The point of zero flow can be The point of zero flow can be determined graphicallydetermined graphically

Determining Point of Zero Flow

Rantz, Fig. 141

Rating SectionsRating Sections

In natural channels, behavior of In natural channels, behavior of rating varies depending on flow raterating varies depending on flow rate

Separate rating sections are Separate rating sections are developed for:developed for:– Low flowLow flow– Intermediate flowIntermediate flow– High flowHigh flow

Rating-Curve Sections

Gupta, Fig. 6.28

Slope-Stage-Discharge RatingSlope-Stage-Discharge RatingIf control doesn’t exist, discharge is a If control doesn’t exist, discharge is a function of the stage and the slope of function of the stage and the slope of the water surfacethe water surfaceDischarge from rating curve is Discharge from rating curve is adjusted based on slopeadjusted based on slopeAuxillary station (usually upstream) Auxillary station (usually upstream) is used to determine drop in water is used to determine drop in water surface (fall)surface (fall)

Velocity-Stage-Discharge Rating Velocity-Stage-Discharge Rating If it is not feasible to use slope as a If it is not feasible to use slope as a parameter, can use velocityparameter, can use velocity

Velocity is measured at a point in the Velocity is measured at a point in the cross-section and used as an indexcross-section and used as an index

Velocity measured in one of four Velocity measured in one of four ways:ways:– Current meter, deflection meter, Current meter, deflection meter,

acoustic meter, electromagnetic meteracoustic meter, electromagnetic meter

Velocity-Stage-Discharge RatingVelocity-Stage-Discharge Rating( h vs v) ( h vs v)

Based on discharge measurements, Based on discharge measurements, develop ratings for:develop ratings for:– Area vs stageArea vs stage– Mean velocity vs stage and index Mean velocity vs stage and index

velocityvelocity

For recorded stages, ratings are used For recorded stages, ratings are used to determine area and mean velocityto determine area and mean velocityDischarge is the product of area and Discharge is the product of area and mean velocitymean velocity

WeirsWeirs

Definition: A regular obstruction Definition: A regular obstruction across the channelacross the channel– Two types based on longitudinal shapeTwo types based on longitudinal shape

Sharp crested: thin plate with sharp edgeSharp crested: thin plate with sharp edge

Broad crested: solid with flat center portionBroad crested: solid with flat center portion

– Various cross-sectional shapesVarious cross-sectional shapesRectangular, triangular (V-notch), Rectangular, triangular (V-notch), trapezoidal, parabolictrapezoidal, parabolic

V-Notch (Triangular/Sharp crested) Weir

WEIR –Broad-crested WEIR –Broad-crested

Cipoletti (Trapezoidal) Weir

WeirWeir Formula  Formula 

Q = CLH3/2 Q = CLH3/2

Q = Q = dischargedischarge

C = coefficient C = coefficient

L = length L = length

H = head over the H = head over the weirweir

C is usually between 2.6 and 3.2  

C Weir coefficient Sharp-crested – usually 3.1 or 3.2 Broad crested – usually 2.6 to 3

If it is critical, there are detailed references

See Figure 5.1 in book Embankments – 2.7 to 2.8

usually good

Sharp-crested Weir

Gupta, Fig. 6.20

Discharge Equation for Weirs

Gupta, Fig. 6.21

V2 C d 2g(H h)

Q 2

3CdL 2gH 3/ 2

End ContractionsEnd Contractions

If cross-section of weir doesn’t If cross-section of weir doesn’t extend across channel, end extend across channel, end contractions resultcontractions result

Effect of end contractions is to Effect of end contractions is to reduce the effective length of the reduce the effective length of the weirweir

Discharge formula is modified to Discharge formula is modified to account for this effectaccount for this effect

Broad-Crested WeirsBroad-Crested Weirs

If thickness of weir crest exceeds If thickness of weir crest exceeds 0.47H, weir is broad crested0.47H, weir is broad crested

On top of crest flow reaches critical On top of crest flow reaches critical depthdepth

Shape of weir greatly influences Shape of weir greatly influences discharge coefficient Cdischarge coefficient Cdd (0.85–1.1) (0.85–1.1)

Q 0.385CdL 2gH3 / 2

OrificesOrifices

Definition: Opening through which Definition: Opening through which water is discharged under pressurewater is discharged under pressure

Generally used at the side or bottom Generally used at the side or bottom of a tank or in a pipelineof a tank or in a pipeline

Characterized by size:Characterized by size:– Small: head h > 5d (d is diameter)Small: head h > 5d (d is diameter)– Characterized by shape, upstream edge:Characterized by shape, upstream edge:

Sharp edged or rounded cornersSharp edged or rounded corners

Orifices Q = C’A(2gH)1/2 C’ = coefficient

A = area of opening H = Head on orifice

Orifice

Gupta, Fig. 6.24

V Cv 2gh

Q CvC cA 2gh CdA 2gh

AssumptionsAssumptions

Steady state conditionsSteady state conditions

Frictionless flow along a streamlineFrictionless flow along a streamline

Incompressible fluidIncompressible fluid

Streamlines of jet normal to plane of Streamlines of jet normal to plane of orifice: velocity constant across orifice: velocity constant across orificeorifice

Corrections applied to account for Corrections applied to account for friction, edge conditionsfriction, edge conditions

Large OrificeLarge Orifice

If head on orifice h < 5d (diameter of If head on orifice h < 5d (diameter of orifice), velocity is not constant orifice), velocity is not constant through orificethrough orifice

Discharge equation is modified Discharge equation is modified further (e.g, Eq. 6.32 for a further (e.g, Eq. 6.32 for a rectangular orifice)rectangular orifice)

FlumesFlumes

Flume: A channel with a lateral Flume: A channel with a lateral constriction of the width (Venturi)constriction of the width (Venturi)

Parshall flume: A venturi flume with a Parshall flume: A venturi flume with a change in bed elevationchange in bed elevation

Typically installed in small channels Typically installed in small channels or in cases where overland flow is or in cases where overland flow is constricted to a small areaconstricted to a small area

FLUME-PARSHALLFLUME-PARSHALL

Parshall Flume

Chow, Fig. 4.6

Parshall Flume Ratings

Gupta, Table 6.5

Venturi Flume

Gupta, Fig. 6.27

Q 0.385Cd 2gWH 3/ 2

H h1 V1

2

2g