1
Introduction to flow measurement & current meter gauging
John FenwickNational Institute of Water & Atmospheric Research Ltd
2
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 ............
3
Streamflow measurement -techniques
(in approximate order of usefulness)
Current meter gaugingvolumetricdilutionweirsslope-areavelocity head rodsothersthe future ............
4
Volumetric gauging
5
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
6
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.
7
Volumetric gaugingExcellent if conditions rightusually needs small flowsaccuracy depends on
time & volume resolutioncalibration of container
Possible to use large “containers”
8
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
9
Dilution gauging
Capable of excellent resultsrequires some set-up timesHas to have complete mixing of tracer and flowcomplementary to current meter
10
Constant-rate injection
11
Dye gauging
12
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
13
Velocity head rodsUses V2 = 2gHmeasures mean velocity in verticalcan be difficult to read accurately
14
V-notch weirs
Need good hydraulic conditionsprecise dimensionsportable units can be useful
15
Acoustic profilers
Measure velocity profiles and depthssuitable for larger rivers, so far ...expensive!
16
Acoustic profilersAdvantages
speed (2-3 mins/run; normally x 4)no tagline required
17
Bottom trackingThe key development for gaugingBottom-tracking measures width and corrects for a curved trackNo tagline required
18
Rapid resultsLaptop displays data as collectedlarge amount of data collectedSome unmeasured areas:
transducer depthblanking distancebottom interferenceextrapolated - power law or constantsides - estimated & extrapolation
19
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
20
Current meter gaugingThe most versatile methodwell established and researchedcurrent meters can be accurately calibrateddepth and width measurement relatively easyrelatively quick and simple
21
Current meter -disadvantages
Time takenOK except for changing flowscalculation takes time
meters subject to damagehigh velocities cause errorscan put people in dangerous situations
22
Current meter technology
Current meters - rotating element – tried, true and accepteddepth measuring equipment - rods, winches, sounderselectromagneticthermalpressureacoustic reflectionradar
23
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
24
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
25
Site selectionmodify section if necessary/possible
26
Width measurement
perpendicular to flowgood quality tape, etcpermanent markingsfixed reference pointsaccurate to 0.5 %watch for systematic errors
27
Depth measurementWading rods - resolve to 3 - 5 mmreels with revl’n counters -resolve to 1 cmtake a mean by several readingswatch “bow wave”, etc
28
Current meters
Precision instrumentsimperative they stay in calibrationspin tests essential to check this
29
Choice of current meter
Minimum recommended depthsMin and max velocity rangesweed can be a problem with bucket wheelsprops better for turbulence
30
Velocity sampling in vertical
Typical, but not universalmean estimated at 0.60.2 & 0.8 bettermore points even better
31
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
32
Avoid obstructions to the meterStand downstream and away from meter1/2 metre rule
33
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
34
Measure horizontal angles
Can be a significant systematic erroruse protractor on card
35
Methods for larger flows
Colombus weightsreels with depth countersboat, bridge, cablewaydifference in meter setting calculation
36
Other errors - vertical angles
37
Vertical angles - cnt’d.
Add tags above meter to measure cdgood approximate solution
38
Other methods ......
Wading rods also possible off bridge
39
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
40
Calculation
q1 = (b2 -b1) x (d1 + d2)/2 x (v1 + v2)/2
total discharge obtained by adding the discharges for all segments.
41
Mid-section method
42
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
43
Gauging is “inaccurate” because....
Measurements are not repeatablethe entire flow cannot be measured instantlyonly some portions of the flow are sampled
44
Quantifying gauging “inaccuracy”
AccuracyPrecisionRandom errorSystematic error
45
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
46
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