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Introduction to flow measurement & current meter gauging

John FenwickNational Institute of Water & Atmospheric Research Ltd

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Streamflow measurement - definition

Volume of water - including transported material - flowing past a given point (through a cross-section) in a given period of timeUnits are

l/s or m3/s normalml/s for very small flowsm3/hour or dayfeet3/smillion gallons/dayacre/feet head ............

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Streamflow measurement -techniques

(in approximate order of usefulness)

Current meter gaugingvolumetricdilutionweirsslope-areavelocity head rodsothersthe future ............

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Volumetric gauging

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Volumetric - applicationsCalibration of small weirs at low flow

folded metal “spout” or similarTemporary dam or diversionculvert outletstanks, reservoirs, surge chambers, pondsLimitations

fall available & size of containertime to fill not too short

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Volumetric... continued.

General techniquestart flow into a calibrated container, starting a stopwatchwhen level reaches calibration mark, stop the watchcalculate flow by dividing volume by time to fill

variationsdivert in, divert out, measure volumeetc.

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Volumetric gaugingExcellent if conditions rightusually needs small flowsaccuracy depends on

time & volume resolutioncalibration of container

Possible to use large “containers”

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Estimation of a flowPick a cross-section with uniform flowUse formula Q= Depth x Width x VelocityEstimate/measure widthEstimate mean depth (measurements)Estimate mean velocity (floats, etc)Multiply 3 valuesUse result appropriately

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Dilution gauging

Capable of excellent resultsrequires some set-up timesHas to have complete mixing of tracer and flowcomplementary to current meter

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Constant-rate injection

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Dye gauging

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Slope area

Uses the Manning formula V= R2/3 Sf

1/2 /nQ = Area x Vgood for estimating flood peaks which cannot otherwise be measuredsome uncertainty in estimating n

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Velocity head rodsUses V2 = 2gHmeasures mean velocity in verticalcan be difficult to read accurately

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V-notch weirs

Need good hydraulic conditionsprecise dimensionsportable units can be useful

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Acoustic profilers

Measure velocity profiles and depthssuitable for larger rivers, so far ...expensive!

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Acoustic profilersAdvantages

speed (2-3 mins/run; normally x 4)no tagline required

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Bottom trackingThe key development for gaugingBottom-tracking measures width and corrects for a curved trackNo tagline required

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Rapid resultsLaptop displays data as collectedlarge amount of data collectedSome unmeasured areas:

transducer depthblanking distancebottom interferenceextrapolated - power law or constantsides - estimated & extrapolation

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ResultsGood results in deeper, “smoother” flows -Clutha floods, Tekapo canal, tidal flowsSome lost data and differences in rougher flows - Rakaia Gorge low flowlarge flows OK e.g 100,000 m3/s (Kaiparaharbour outflow)Limitations:Depth - mostly >3m

Weed, logs, fish, steep slopesrough water can be a problemmoving bottom - need Diff. GPS

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Current meter gaugingThe most versatile methodwell established and researchedcurrent meters can be accurately calibrateddepth and width measurement relatively easyrelatively quick and simple

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Current meter -disadvantages

Time takenOK except for changing flowscalculation takes time

meters subject to damagehigh velocities cause errorscan put people in dangerous situations

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Current meter technology

Current meters - rotating element – tried, true and accepteddepth measuring equipment - rods, winches, sounderselectromagneticthermalpressureacoustic reflectionradar

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Current meter gauging - an art and a science

Science, OK. Art???it’s more than just taking readingshave to make judgements on how best to make the readings represent what you seestreams are dynamic, have ever-changing flow patternswe represent complex parameters with straight lineswe have to cope with less than ideal conditions

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Site selectionSuit purpose uniform reach with normal velocity distributionstraight & 5 x widthleast turbulenceadequate depths and velocitiesunobstructed, no weed, etc.contained in channelmodify if necessary/possible

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Site selectionmodify section if necessary/possible

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Width measurement

perpendicular to flowgood quality tape, etcpermanent markingsfixed reference pointsaccurate to 0.5 %watch for systematic errors

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Depth measurementWading rods - resolve to 3 - 5 mmreels with revl’n counters -resolve to 1 cmtake a mean by several readingswatch “bow wave”, etc

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Current meters

Precision instrumentsimperative they stay in calibrationspin tests essential to check this

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Choice of current meter

Minimum recommended depthsMin and max velocity rangesweed can be a problem with bucket wheelsprops better for turbulence

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Velocity sampling in vertical

Typical, but not universalmean estimated at 0.60.2 & 0.8 bettermore points even better

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Horizontal velocity distributionGenerally follows shape of bottomneeds sufficient verticals to define; >20no more than 10% in any one vertical - sounds easy?Vary spacing - more in middle where more flowdefine changes in grade

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Avoid obstructions to the meterStand downstream and away from meter1/2 metre rule

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Time of velocity measurement

velocity at a point pulsates over time due to eddieswe need to measure over at least 40 seconds to get representative value

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Measure horizontal angles

Can be a significant systematic erroruse protractor on card

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Methods for larger flows

Colombus weightsreels with depth countersboat, bridge, cablewaydifference in meter setting calculation

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Other errors - vertical angles

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Vertical angles - cnt’d.

Add tags above meter to measure cdgood approximate solution

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Other methods ......

Wading rods also possible off bridge

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Edge assessmentsNecessary to estimate ungauged areas at the sidesthese should be smallcan’t ignore otherwise biasestimate mean V in side section as % of V at last verticalremember constant decline to edge is 50 %100 % only possible if deeper & faster at edge

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Calculation

q1 = (b2 -b1) x (d1 + d2)/2 x (v1 + v2)/2

total discharge obtained by adding the discharges for all segments.

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Mid-section method

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safety

Rule of thumb is:depth x vel no greater than 1

wear lifejackets above crotch levelgo with the flow, bounce acrosswatch for tree debris

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Gauging is “inaccurate” because....

Measurements are not repeatablethe entire flow cannot be measured instantlyonly some portions of the flow are sampled

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Quantifying gauging “inaccuracy”

AccuracyPrecisionRandom errorSystematic error

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Error typesSYSTEMATICconstant biasmust be eliminatede.g. stretched tape, blunt pivot, techniquecannot be reduced by taking more samplescannot be estimated

RANDOMscattermust be minimisede.g. short time turbulence, technique assumptionsminimised by taking large enough samplescan be estimated by use of statistics

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Current meter gauging - an art and a science

Science, OK. Art???it’s more than just taking readingshave to make judgements on how best to make the readings represent what you seestreams are dynamic, have ever-changing flow patternswe represent complex parameters with straight lineswe have to cope with less than ideal conditions