STREAMFLOW MEASUREMENT
THE VELOCITY-AREA METHOD
VOLUME 1
JULY 2002
STREAMFLOW MEASUREMENT
THE VELOCITY - AREA METHOD
VOLUME 1
COMPILED:
M.C.BRIGGS
J.C.CAMERON
L.J.TEPPER
TABLE OF CONTENTS
INTRODUCTION ................................................. 1
1.0 METHODS OF STREAMFLOW MEASUREMENT ........................ 2
1.1 Velocity - Area Method.................................. 2
1.2 Float Gauging........................................... 2
1.3 Slope - Area Method..................................... 2
1.4 Stage - Fall - Discharge Method......................... 2
1.5 Weirs and Flumes........................................ 3
1.6 Dilution Method......................................... 3
1.7 Moving boat Method...................................... 3
1.8 Volumetric Measurement.................................. 3
1.9 Continuously recording Flow Meters...................... 4
1.9.1. Mechanical ......................................... 4
1.9.2. Ultrasonic ......................................... 5
1.9.3 Acoustic Doppler .................................... 5
1.9.4 Electromagnetic .................................... 6
1.10 Flow Measurement Units................................. 6
2.0 THE VELOCITY-AREA METHOD OF STREAM FLOW MEASUREMENT ..... 10
2.1 Spacing of Verticals................................... 10
2.1.1 Segments of equal flow ............................. 10
2.1.2 Bed profile. ....................................... 10
2.1.3 Equidistant ........................................ 10
2.2 Measurement of Velocity................................ 11
2.2.1 Vertical velocity curve method ..................... 12
2.2.2 The one-point or six-tenths method ................. 15
2.2.3 Two-point method ................................... 15
2.2.4 Three-point method ................................. 15
2.2.5 Five point method .................................. 16
2.2.6 Six point method ................................... 16
2.2.7 Surface Velocity Measurement ....................... 16
2.2.8 Bed Velocity Measurement ........................... 17
2.3 Computation of Current Meter Measurements.............. 17
2.3.1 Mid-section method ................................. 17
2.3.2 Mean section method ................................ 19
2.3.3 Velocity – Depth Integration method ................ 20
2.3.4 Velocity – Contour method .......................... 21
2.4 Procedure for Measurement of Discharge by Current Meter 22
2.4.1 Selection of gauging site. ......................... 22
2.4.2 Current meter measurement by wading ................ 23
2.4.3 Current meter measurement from cableways ........... 25
2.4.4 Current meter measurement from bridges ............. 28
2.4.5 Current meter measurement from boats ............... 28
2.4.6 Other current meter methods ........................ 29
2.4.6.1 Two-tenths depth method......................... 29
2.4.6.2 Sub-surface velocity method..................... 30
2.4.6.3 Integration method.............................. 30
2.4.6.4 Interpolation method............................ 30
2.4.7 Sounding weights.................................. 30
2.4.7.1 Sounding weight hanger bar...................... 31
2.5 Special Problems in Streamflow measurement............. 31
2.5.1 Depth corrections for sounding line and weight ..... 32
2.5.1.1 Positioning the Meter in the Vertical........... 32
2.5.2 Oblique flows (Angled Flow) ........................ 39
2.5.3 Pulsations in flow ................................. 40
2.5.4 Mean gauge height for current meter measurements ... 42
2.5.4.1 Discharge weighting............................. 42
2.5.4.2 Time weighting.................................. 43
2.5.5.1 Large Streams................................... 44
2.5.5.2 Small streams................................... 44
2.5.6 Correction of discharge measurement for storage... 46
2.5.7 Correction of discharge measurement for travel time
........................................................ 47
2.5.8 Measurement of discharge with sections of dead water 49
2.5.9 Measurement of discharge with sections of reverse flow
......................................................... 52
2.5.10 Measurement of discharge with variable backwater .. 52
2.5.11 Measurement of discharge with tributary inflow .... 54
2.5.12 Overflow (out-of-bank flow) ....................... 54
2.5.13 Velocity measurement to a vertical wall ........... 55
2.6.1 Human Error ........................................ 59
2.6.2 Instrument Error ................................... 59
2.6.3 Method Error ....................................... 60
2.6.4 Sounding Error ..................................... 61
2.6.5 Width Errors ....................................... 62
3.0 CURRENT METERS .......................................... 64
3.1 Cup-type current meter................................. 64
3.2 Propeller - type current meter......................... 65
3.3 Rating of Current Meters............................... 68
3.4 Care of Current Meters................................. 68
3.5 Maintenance and repair of the Gurley Current Meter..... 69
TABLE OF TABLES
TABLE
TABLE
TABLE
TABLE
TABLE
TABLE
TABLE
TABLE
TABLE
TABLE
1 - COMMON METRIC UNITS AND SYMBOLS ............................ 8
2 - COMMON CONVERSION FACTORS .................................. 9
3 - CURRENT METER - ADOPTED MINIMUM DEPTH SETTING ............. 11
4 - VERTICAL VELOCITY CURVE - STANDARD CO-EFFICIENTS .......... 12
5 - AIR-LINE CORRECTION - PERCENTAGE TYPE ..................... 34
6 - WET-LINE CORRECTION - PERCENTAGE TYPE ..................... 35
7 - AIR-LINE CORRECTION ....................................... 35
8 - WET-LINE CORRECTION ....................................... 36
9 - VELOCITY CO-EFFICIENTS IN VICINITY OF A VERTICAL WALL ..... 56
10 - CURRENT METER PERFORMANCE ................................ 59
TABLE Of FIGURES
FIGURE
FIGURE
FIGURE
FIGURE
FIGURE
FIGURE
FIGURE
FIGURE
FIGURE
FIGURE
FIGURE
FIGURE
FIGURE
FIGURE
FIGURE
FIGURE
FIGURE
FIGURE
FIGURE
FIGURE
FIGURE
FIGURE
FIGURE
FIGURE
FIGURE
1 - CONTINUOUSLY RECORDING CURRENT METER ...................... 5
3 - VERTICAL VELOCITY CURVE METHOD ........................... 14
4 - TYPICAL VERTICAL VELOCITY CURVE .......................... 14
5 - THE MID-SECTION METHOD OF COMPUTING MEASUREMENTS ......... 18
6 - MID-SECTION METHOD WORKED EXAMPLE ........................ 18
7 - THE MEAN-SECTION METHOD OF COMPUTING MEASUREMENTS ........ 19
8 - MEAN-SECTION METHOD - WORKED EXAMPLE ..................... 20
9 - THE VELOCITY-DEPTH INTEGRATION METHOD .................... 21
10 - THE VELOCITY CONTOUR METHOD OF COMPUTING MEASUREMENTS ... 22
11 - SCHEMATIC ARRANGEMENT OF AN UNMANNED CABLEWAY ........... 27
12 - "TATURA" UNIVERSAL HANGER BAR ........................... 33
13 - POSITION OF THE SOUNDING WEIGHT IN A DEEP SWIFT STREAM .. 34
14 - CONVENTIONAL CURRENT METER .............................. 40
15 - COMPONENT PROPELLER METER ............................... 40
16 - DISCHARGE FIELD SHEET - CORRECTION FOR OBLIQUE FLOW ..... 41
17 - TIME & DISCHARGE WEIGHTED MEAN GAUGE HEIGHT COMPUTATION . 45
18 - CORRECTION OF DISCHARGE MEASUREMENT FOR STORAGE ......... 49
19 - CORRECTION OF MEASUREMENT GAUGE HEIGHT FOR TRAVEL-TIME .. 50
20 - CORRECTION OF DISCHARGE MEASUREMENT FOR DEAD WATER ...... 51
21 - CORRECTION OF DISCHARGE MEASUREMENT FOR REVERSE FLOW .... 53
22 - VELOCITY ADJUSTMENT AT A WALL – SAMPLE CROSS SECTION .... 56
23 - VELOCITY ADJUSTMENT AT A WALL. SAMPLE MEASUREMENT ....... 57
24 - GAUGING WEIGHT ZEROING LINE ............................. 62
25 - CUP TYPE CURRENT METER .................................. 66
26 - PROPELLOR TYPE CURRENT METER ............................ 67
STREAMFLOW MEASUREMENT
INTRODUCTION
Streamflow is the combined result of all climatological and
geographical factors that operate in a drainage basin.
It is
the only phase of the hydrological cycle in which water is
confined
in
well-defined
channels,
which permit accurate
measurements to be made of the quantities involved.
Good water management is founded on reliable streamflow
information and the final reliability of the information depends
on the initial field measurements.
The hydrographer making
these measurements has the responsibility of ensuring raw data
of acceptable quality are collected.
The accuracy and
subsequent usefulness of the published data depends entirely on
the quality of the field measurements and the reliability of the
stage-discharge relation over the entire life of the station.
Measurements are not an end in themselves but are an integral
part of the development of a stage-discharge relation.
There are many different uses of streamflow data, such as water
supply, irrigation, flood control, pollution control, energy
generation and industrial water use.
The importance placed on
any one of these purposes may vary from area to area as well as
from state to state.
The emphasis for any one need may also
change over a period of time.
Discharge measurements made at each gauging station determine
the stage-discharge relation for that site.
This may be a
simple relationship between stage and discharge, or a more
complex one in which discharge is a function of stage slope,
rate of change of stage or other factors.
Initially the
discharge measurements are made at various stages at the station
in order to define the discharge ratings. Measurements are then
made at periodic intervals, generally monthly to verify the
rating or to define any changes in the rating caused by changes
in stream channel conditions.
Streamflow, or discharge, is defined as the volume rate of flow
of water. Discharge is expressed in megalitres per day (ML/d.)
-1-
1.0 METHODS OF STREAMFLOW MEASUREMENT
A summary of various methods of
discussed in the following section.
streamflow
measurement
is
1.1 Velocity - Area Method
A discharge derived by current meter is equal to the summation
of the products of the partial areas of the stream cross-section
and their respective mean velocities.
Since most methods of stream gauging utilise a velocity-area
computation it could be inferred that all are velocity area
methods.
Throughout this manual, however, the velocity-area
method infers the use of a current meter.
1.2 Float Gauging
Float gauging is basically a direct Velocity-Area method of
determining instantaneous flow. The mean velocity is calculated
using surface velocities as indicated by a floating object timed
over a pre-determined length of channel and at different
positions across the channel.
Velocities obtained will usually
be greater than the mean velocity in the vertical and are
therefore subject to a correction factor.
The area for any
gauge height is obtained from a plot of the mean cross-section
of the length of channel over which the velocities are measured.
The discharge is derived from the sum of the products of
corrected velocity multiplied by the area.
Generally, this method is used only when the flow is either too
fast or too slow to use a current meter.
It is only a fair substitute for current meter gauging but,
given accurate field data, it can be a more accurate means of
computing a high stage discharge than using empirical formula.
1.3 Slope - Area Method
The discharge is derived from the measurement of the slope of
the water surface, the cross-section of the channel over a
fairly straight reach, and by selecting a roughness coefficient
for the channel boundaries.
1.4 Stage - Fall - Discharge Method
If variable backwater exists at a site, the energy gradient is
variable at a given stage and the discharge rating cannot be
defined by stage alone.
This is most commonly caused by a
downstream confluence or structure.
The discharge under these
conditions is a function of stage and the slope of the energy
gradient. This situation is that of a natural flood where the
flow maintains a stable wave profile as it moves down the
channel. This type of wave often produces a loop rating.
-2-
1.5 Weirs and Flumes
The relation between stage (or head) and discharge over a weir
or through a flume is established from laboratory or field
calibration.
The discharge is derived from this rating
equation.
When shallow depths and low velocities are encountered in a
stream it is sometimes impossible to carry out a satisfactory
discharge measurement using a current meter. In this situation
a portable weir plate or flume is a useful device for measuring
the flow.
Each weir or flume should be supplied with
installation procedures and a theoretical rating to aid in
compiling
the
stage-discharge relation from actual field
discharge measurements.
1.6 Dilution Method
A tracer liquid is injected into the channel and the water is
sampled at a point downstream where turbulence has mixed the
trace uniformly throughout the cross-section.
The change in
concentration between the solution injected and the water at the
sampling station is converted into a measure of the discharge.
1.7 Moving boat Method
A current meter is suspended from a boat that traverses the
river normal to the stream flow. The component of the velocity
in the direction of the stream is computed from the resultant
velocity and the angle of this resultant. The discharge is the
sum of the products of the subsections of the stream crosssectional areas and their respective average velocity.
1.8 Volumetric Measurement
The most accurate method of measuring small discharges is the
volumetric method.
Observing the time required to fill a
container of known capacity or the time required to partly fill
a calibrated container to a known volume does this.
The
equipment required is a calibrated container and a stopwatch.
Calibration can be achieved by weighing the container with
varying amounts of water in it, noting the level in the
container and then using the following formula.
V = W2 – W1
where
V = volume of water in container, in litres
W2 = weight of container with water, in kilograms
W1 = weight of empty container, in kilograms
-3-
A container can also be calibrated by adding known volumes of
water, by increments, and noting the levels on the container.
Volumetric measurements should be made where the flow is
concentrated into a narrow stream or at an artificial control
where the flow is confined to a notch or narrow width of a
shaped weir crest. Sometimes it is necessary to place a trough
or funnel against the artificial control to carry the water to
the calibrated container.
The measurement should be carried out three or four times to be
certain that errors have not been made and that the results are
consistent.
It is good practice to then mean all the accepted
readings.
1.9 Continuously recording Flow Meters
There numerous instances where continuous flow data is required
and the option of using a conventional stage / discharge rating
or a calibrated hydraulic structure is not possible or
practical.
Such
instances
may
require
continuous
flow
associated with the operation pumps, channel flow, sewerage or
in manufacturing and production type environments. In addition,
velocity measuring devices can be used in larger waterway
applications where tail water or tidal effects limit the use of
a more conventional approach.
1.9.1. Mechanical
Continuously
recording flow meters, such as the Sparling and
Davis-Shepherd types (see figure 1) are used by the irrigation
and water supply districts of this state.
These meters are designed to continuously record the flow in
pipelines and open channels.
They are suspended, facing the
centre of flow, in a pipe of known cross-sectional area or in
the centre of a stream with flow brought through a tube. Water
passes through the pipe, and the rotational speed of the
propeller is known to be proportional to the average flow
velocity.
A simple gear train links the propeller to the
register, reading directly in standard volumetric units.
The two basic requirements for correct operation are that the
pipe must always remain full and the flow must exceed the
minimum rated range.
-4-
FIGURE 1 - CONTINUOUSLY RECORDING CURRENT METER
1.9.2
Ultrasonic
The ultrasonic (acoustic) velocity sensor is a device that
utilises acoustic transmission to measure the average velocity
along a line between one or more opposing sets of transducers.
The velocity of flow is determined from the travel time of sound
pulses moving in both directions along a path diagonal to the
flow. In practice, the application of these sensors is limited
to “pipe full” flow situations or in streams where there is
always sufficient depth of water, above and below the sensors,
to facilitate optimum operating conditions.
1.9.3 Acoustic Doppler
The acoustic doppler system measures velocity magnitude and
direction using the Doppler shift of acoustic energy reflected
by material suspended in the water column.
Systems using the
Doppler principle range from a small single sensor, which can be
installed in a pipe or concrete channel, to a portable system
which is used from boat to measure river flows.
The latter
application is generally safer and quicker than the more
conventional boat gauging using a current meter.
The accuracy
and reliability of these measurements is also consistent with
the results obtained with a current meter.
-5-
1.9.4
Electromagnetic
The discharge is found by measuring the electromagnetic force
(EMF) produced by a moving conductor (the flowing water) through
a magnetic field produced by a coil around the flow conduit.
This application requires a pipe full condition for satisfactory
operation.
1.10 Flow Measurement Units
Streamflow or discharge is defined as the volume rate of flow of
water.
The flow rate unit that is generally used by Thiess
Services is the Megalitre per day (ML/d).
This flow rate is derived from the MEGALITRE (ML) which is
equivalent to 1,000,000 litres.
A megalitre is best explained
by using a diagram. This is illustrated below:
One of the reasons this unit was chosen is that the premetric
unit, the acre-foot, is very similar to a Megalitre.
A flow
rate of 1 ML/d for 24 hours will cover an area of 1 hectare to a
depth of 0.10 metres.
For a discharge measurement, the cross-sectional area is
measured in square metres and the velocity is expressed in
kilometres per day (km/d) to obtain the discharge in Megalitres
per day.
Many other authorities, generally non-irrigation, do not use the
ML/d but rather the cubic metre per second (m3/sec).
Area is
measured in square metres and the velocity is expressed in
metres per second.
The relationship between the ML/d and the CUMEC is:
1 m3/sec (cumec) = 86.4 ML/d
The relationship between Km/d and m/sec. is:
l m/sec = 86.4 Km/d
When using the discharge unit, ML/d, and the velocity unit,
Km/d, care should be taken when transposing these units into
flow formulae because, almost without exception, these formulae
require the units to be in cumecs and m/sec.
-6-
For example:
The formula for discharge through a submerged orifice is:
Q = Cd x A x 2 x g x h. in cumecs
To determine
becomes:
the
theoretical
discharge
in
ML/d
the
formula
Q = 86.4 x Cd x A x 2 x g x h.
Similarly, where computations involving velocity are carried
out, a conversion from Km/d to m/sec should be made prior to use
in the formulae.
A table of common metric units and their symbols is listed as
Table 1 and frequently used conversions is listed as Table 2.
-7-
ITEM
UNIT
SYMBOL
Length
Millimetre
Metre
Kilometre
mm
m
km
Area
square millimetres
square metres
square kilometres
Hectare
mm2
m2
km2
ha
Volume
cubic metres
Litre(1)
Kilolitre
Megalitre
m3
L
kL
ML
Flow Rate
megalitres per day
cubic metres per second
litres per minute
litres per second
ML/d
m3/s
L/min
L/s
Velocity
metres per second
kilometres per day
m/s
km/d
Mass
Kilogram
Gram
Milligram
Tonne
kg
g
mg
T
Density
kilograms per cubic metre
tonnes per cubic metre
kg/m3
t/m3
Force
newton, kilonewton, meganewton
N, kN, MN
Pressure,
stress
pascal, kilopascal, megapascal
(also metres head of water)
Pa, kPa, MPa
Energy
Joule
newton metre
J
Nm
Power
Kilowatt
kW
Temperature
degree Celsius
0C
TABLE 1 - COMMON METRIC UNITS AND SYMBOLS
-8-
TO
CONVERT
INTO
MULTIPLY
BY
: Millimetres
: Metres
: Kilometres
: Inches
: Feet
: Miles
: 0.0393701
: 3.280840
: 0.621371
: Square metres
: Square Kilometres
: Hectares
: Square yards
: Square miles
: Acres
: 1.19599
: 0.386102
: 2.4710538
:
:
:
:
Cubic metres
Cubic metres
Litres
Megalitres
:
:
:
:
Cubic Yards
Cubic feet
Gallons
Acre feet
:
:
:
:
1.30795
35.3147
0.219969
0.8107132
:
:
:
:
:
:
:
Litres per second
Litres per second
Cubic metres per second
Megalitres per day
Megalitres per day
Megalitres per day
Cubic metres per second
:
:
:
:
:
:
:
Gallons per minute
Millon gallons per day
Cubic feet per second
Cubic feet per second
Gallons per minute
Cubic metres per second
Megalitres per day
:
:
:
:
:
:
:
13.198155
0.0190053
35.3147
0.4087346
152.75642
0.011574
86.4
LENGTH
AREA
VOLUME
FLOW RATES
MASS
: Kilogram
: Tonne
: Pound
: Ton
: 2.20462
: 0.984207
: Kilometres per day
: Kilometres per day
: Feet per second
: Metres per second
: 0.037973
: 0.011574
: Kilogram per cubic metre
: Tonnes per cubic metre
: Pound per cubic foot
: Pound per cubic foot
: 0.062428
: 62.428
: Kilopascal
: Pound per square inch
: 0.145038
VELOCITY
DENSITY
PRESSURE
TABLE 2 - COMMON CONVERSION FACTORS
-9-
2.0 THE VELOCITY-AREA METHOD OF STREAM FLOW MEASUREMENT
The velocity-area method used for the determination of discharge
in open channels requires the measurement of stream velocity,
depth of flow and the distance across the channel between
observation verticals. The velocity is measured at one or more
points in each vertical, by current meter, and an average
velocity determined in each vertical. (See section 2.2.) The
discharge is equal to the summation of the products of the
partial areas of the stream cross-section and their respective
average velocities. The discharge obtained is normally used to
establish a relation between stage (water level) and stream
flow, which is referred to as the stage-discharge relation.
2.1 Spacing of Verticals
In order to determine bed shape and horizontal and vertical
velocity
distribution
accurately,
an
infinite
number
of
verticals would be necessary, however, for practical reasons
only a finite number is possible. The cross-section is divided
into segments at a sufficient number of locations across the
channel to
ensure an
adequate
sample
of both velocity
distribution and bed profile.
The spacing and number of
verticals are crucial for the accurate measurement of discharge
and for this reason between 20 and 30 verticals should be used.
This applies to streams of all widths, except where the channel
is so narrow that this would be impractical.
Verticals should be spaced on the basis of the following
criteria and will depend largely on the flow conditions, the
geometry of the cross-section and the width of the stream.
2.1.1 Segments of equal flow
For streams having a variable velocity distribution, or a
significant variation in the horizontal velocity distribution,
it is advisable to space the verticals to achieve segments of
equal flow over the required distance rather than segments of
equal widths.
2.1.2 Bed profile.
For streams having abnormalities in the bed profile, the
verticals are spaced to make allowance for depressions or
obtrusions and general irregularities of the bed.
2.1.3 Equidistant
For very wide rivers, over 300 metres, it is sometimes
convenient to make the verticals equidistant.
A general rule
for current meter measurements is to make the width of the
segments less as the depth and velocities become greater.
Irrespective of which criterion is used, the spacing of the
verticals must be arranged so that no segment contains more than
10% of the total flow.
If the stage is steady the ideal
- 10 -
measurement is one having no segment with more than 5% of the
total flow.
2.2 Measurement of Velocity
The current meter measures velocity at a point. To carry out a
discharge measurement at a cross section requires determination
of the mean velocity in each of the selected verticals.
The
mean velocity in a vertical is obtained by observing the
velocity at many points in that vertical but it can be
approximated by taking a few velocity observations and using a
known relation between those velocities and the mean in the
vertical as per Table 4 or as calculated from previous
observation records.
The following Table sets out the Thiess Services standard as
adopted
against
the
International
Standard,
for
minimum
recommended depth settings for gaugings.
It was necessary to
vary from the International Standard to allow for measurement of
small flows that are often encountered in Victoria. The adopted
standard is based on the assumption "For a current meter to
perform at an acceptable level for wading measurements, the
minimum distance from the horizontal axis through the current
meter or propeller to the bed should never be less than the
width of the cup or the diameter of the propeller"
ADOPTED Thiess Services STANDARD FOR:
CUP WIDTH OR
MINIMUM DEPTH RECOMMENDED
PROPELLER
FOR
DIAMETER
1 Point
2 Point
Method
Method
GURLEY-PYCMY
20 mm
5 cm
10 cm
OSS-PCI
50 mm
12 cm
25 cm
GURLEY
50 mm
12 cm
25 cm
100 mm
25 cm
50 cm
120 mm
30 cm
60 cm
125 mm
31 cm
62 cm
OTT-C31
OSS-B1
SIAP
OTT-C31
OSS-B1
TABLE 3 - CURRENT METER - ADOPTED MINIMUM DEPTH SETTING
- 11 -
The more commonly used methods of determining the mean velocity
in the vertical are:
2.2.1 Vertical velocity curve method
In this method velocity observations are made in each vertical
at a sufficient number of points, distributed between the water
surface and the bed, to effectively define the vertical velocity
curve.
The mean velocity is obtained by measuring the area
between the curve and the ordinate axis with a planimeter and
dividing the area by the length of the ordinate axis. (See
Figures 2 and 3 for an example).
The number of points required depends on the degree of
curvature, particularly in the lower part of the curve, and
usually varies between six and ten (See Figure 4). Observations
should normally include velocities at 0.2, 0.6 and 0.8 of the
depth from the surface so that the results from the curve can be
compared with various combinations of reduced point methods, and
the higher and lower points should be located as near to the
water surface and bed as possible. (Refer to sections 2.2.7 and
2.2.8.)
This method is valuable in determining coefficients for
application to the results obtained by other methods, but is
generally not adapted to routine discharge measurements due to
the extra time required to collect field data and to compute the
mean velocity.
Table 4 shows average ordinates taken from the standard vertical
velocity curve.
Ratio of observation depth to
depth of water
Ratio of point velocity to
mean velocity in the vertical
0.05
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0.95
1.160
1.160
1.149
1.130
1.108
1.067
1.020
0.953
0.871
0.746
0.648
TABLE 4 - VERTICAL VELOCITY CURVE - STANDARD CO-EFFICIENTS
- 12 -
Murray River below Rufus River Junction.
Measurement No. 87/8
1
2
3
Ch = 8m
Area = 90
Ch = 14m
Area =
112
Ch = 20m
Area =
138
Ch = 26m
Area =
194
Ch = 32m
Area =
236
Ch = 38m
Area =
244
Ch = 44m
Area =
262
Ch = 50m
Area =
238
Ch = 56m
Area =
226
Ch = 62m
Area =
242
Ch = 68m
Area =
230
Ch = 74m
Area =
212
Ch = 80m
Area =
198
Ch = 86m
Area =
168
Ch = 92m
Area =
188
Ch = 98m
Area =
182
60
Ch = 104m
Area = 190
Ch = 110m
Area = 184
4
1
2
3
4
1
2
DEPTH IN METRES
3
4
1
2
3
4
1
2
3
4
1
2
3
40
80
20
40
Velocity in Km/Day
0
80
20
40
60
0
80
20
40
60
FIGURE 2 - VERTICAL VELOCITY CURVE METHOD – GRAPH REPRESENTATION
OF VELOCITY DISTRIBUTION
- 13 -
Murray River below Rufus River Junction -
Measurement No. 87/8
300
250
200
150
100
Area = 21 687
∴Discharge = 21 687 Ml/d
50
120
10
20
30
40
50
60
70
80
90
WIDTH IN METRES
FIGURE 3 - VERTICAL VELOCITY CURVE METHOD
FIGURE 4 - TYPICAL VERTICAL VELOCITY CURVE
- 14 -
100
110
2.2.2 The one-point or six-tenths method
In the one-point method a single velocity observation is taken
0.6 of the depth below the surface and the value obtained is
accepted as the mean for the vertical.
(See Table 4.)
This
method is generally used under the following conditions.
a) Whenever the depth precludes multiple observations for the
meter being used.
b) When the distance between the meter and sounding weight is
too great to permit placing the meter at the 0.8 depth. (This
prevents the use of the two-point method.)
c) When the stage is changing rapidly and a measurement must be
made quickly.
2.2.3 Two-point method
Velocity observations are made in each vertical at 0.2 and 0.8
of the depth below the surface and the average of the two
readings is taken as the mean for the vertical. Here again this
assumption is based on theory and on the study of vertical
velocity curves and experience has confirmed its essential
accuracy. The World Standard suggests a minimum depth of 0.75 m
for this method, however we have adopted lesser minimum depths,
(provided bed condition are suitable), to adequately cater for
the measurement of predominantly small shallow streams. (Refer
Table 3).
Overhanging vegetation that is in contact with the water, or
submerged objects such as large rocks and aquatic weed growth
that are in close proximity, either upstream or downstream, to
the vertical will distort the vertical velocity curve.
Where
that occurs, this method will not give a reliable mean velocity
value and an additional reading at 0.6 of the depth should be
made. A rough test of whether or not the velocities at 0.2 and
0.8 are sufficient for determining the mean vertical velocity
is, "that the 0.2 depth velocity should be greater than the 0.8
depth velocity", but less than twice as great”.
2.2.4 Three-point method
Velocity observations are made in each vertical at 0.2, 0.6 and
0.8 of the depth. The mean velocity is calculated by averaging
the 0.2 and 0.8 depth observations and then averaging that
result with the 0.6 depth observation.
This method is used when the velocities in the vertical are
abnormally distributed.
The velocities can also be obtained
from this equation.
- 15 -
V =
V 0.2 + 2*(V 0.6) + V 0.8
4
2.2.5 Five point method
Velocity observations are made in each vertical at 0.2, 0.6 and
0.8 of the depth and as close to the streambed and the surface
as practical. (Refer sect ions 2.2.7 to 2.2.8)
The mean velocity is obtained from this equation
V =
V surface + (3*V 0.2)+ (3*V 0.6) + (2*V 0.8) + V Bed
10
2.2.6 Six point method
The six-point method should be used in situations where a
distorted vertical velocity distribution is known or suspected,
for example, where there is aquatic growth.
Velocity observations are made in each vertical at 0.2, 0.4, 0.6
and 0.8 of the depth and as close to the streambed and surface
as practical. (Refer sections 2.2.7 and 2.2.8).
The mean velocity is obtained from this equation
V =
V surface + (2*V 0.2)+ (2*V 0.4)+ (2*V 0.6)+ (2*V 0.8)+ V Bed
10
2.2.7 Surface Velocity Measurement
The setting for all surface velocities is based on
which states "The horizontal axis of the current meter
placed at a depth of not less than one and a half
rotor height from the water surface and no part of
shall break the water surface."
I.S.O 748
should be
times the
the meter
In suspension measurements, the zone between the water surface
and a depth of 0.15 m, current meters can give erratic results,
so for uniformity a depth of 0.35 m below the water surface has
been adopted as standard for all surface velocities. (This depth
caters for Gurley and OTT current meters.)
If a surface
velocity is used to calculate the mean velocity in a section of
a natural channel, a co-efficient of 0.85 or 0.86 may be adopted
if no co-efficient has been developed from previous field
surface velocity measurements. These are the reciprocal of the
values in Table 4.
- 16 -
2.2.8 Bed Velocity Measurement
The setting for bed velocities is based on I.S.O 748, which
states.
"The current meter should be placed at a distance of
not less than three times the rotor height from the bed".
In suspension measurements this distance is also dependent on
the size of the sounding weight, so, for uniformity when using
different sounding weights and current meters a distance of 0.35
m from the bed has been adopted.
2.3 Computation of Current Meter Measurements
The computed discharge is the summation of the products of the
partial areas of the stream cross-section and their respective
average velocities.
The formula Q =
Where
(AV) represents the computation
Q = total discharge
A = individual partial cross-section area
V = mean velocity of the flow normal to the partial area.
2.3.1 Mid-section method
In this method it is assumed that the velocity sampled at each
vertical
represents
the
mean
velocity
in
a
particular
rectangular segment.
The area extends laterally from half the
distance from the preceding observation vertical to half the
distance to the next and vertically from the water surface to
the sounded depth, as shown by the hatched area. (See Figure 5).
The segment discharge is then computed for each segment and
these are summed to obtain the total discharge. (See Figure 6)
- 17 -
1, 2, 3,
,n, Number of verticals; b1, b2, b3,..., n,
distance from initial point;
d1, d2, d3,
n, depth of flow at verticals; , average
velocity in verticals.
FIGURE 5 - THE MID-SECTION METHOD OF COMPUTING MEASUREMENTS
FIGURE 6 - MID-SECTION METHOD WORKED EXAMPLE
- 18 -
2.3.2 Mean section method
The mean section method differs from the mid-section method in
its computation procedure.
Segment discharges are computed
between successive verticals.
The velocities and depths at
successive verticals are each averaged.
The section extends
laterally from one observation vertical to the next. Discharge
is the product of the average of two mean velocities, the
average of two depths and the distance between observation
verticals. (See Figure 7.) A worked example is shown in Figure
8.
Experiments have shown that the mid-section method is slightly
more accurate, however the mean-section method has been adopted
as the standard by the Thiess Services.
Segments. 1, 2, 3, ...,n, Number of verticals; b1, b2, b3, ...,
n, distance from initial point; d1, d2, d3, ...n, depth of flow
at verticals; , average velocity in verticals.
FIGURE 7 - THE MEAN-SECTION METHOD OF COMPUTING MEASUREMENTS
- 19 -
HYDROGRAPHIC SERVICES
Conditions : Weather:
Cloudy
Stream:
Steady
Time
14:5 15:21
E.S.T.
9
Gauge
0.982 0.982
Height
Rec Height 0.982 0.982
Mean Gauge Height: 0.982
DISCHARGE MEASUREMENT
Stream: ALBERT RIVER
Meas. No:
Station: HIAWATHA
Date: 5.3.84
Point of Measurement: .
Party:
Method:
Meter A1559
Weight:
No:
Wad Rod No:
Tape No:
REMARKS
OBSERVATIONS
Time Distan Depth Vert
Adj
Revs
ce
Ang Depth
Waters Edge
Stream Temp:18.5
Sample No:
EC @ 25 deg:
Weighted Mean:
Counter No:
Time
Seconds
0.5
0.00
WE
RB
0.8
0.14
4
41.0
1.2
0.15
8
43.0
1.6
0.14
10
45.0
2.0
0.12
8
48.5
2.4
0.12
6
57.5
2.8
0.13
6
47.0
3.2
0.14
4
52.5
3.6
0.10
4
48.0
4.0
0.18
2
40.0
4.4
0.13
2
42.0
4.6
0.20
5.0
0.18
2
4
4
48.0
55.0
49.0
Velocity
Point Mean Mean
Vert Sect
0.0
3.25
6.5
9.05
11.6
12.65
13.7
12.05
10.4
8.60
6.8
7.50
8.2
6.70
5.2
5.45
5.7
4.70
3.7
3.40
3.1
3.65
4.2
4.90
5.6
Area
Area
COMPUTATIONS
Mean
Adj Horiz
Depth Wid Ang
Width
Discharge
0.021
0.07
0.3
0.068
0.057
0.142
0.4
0.516
0.056
0.14
0.4
0.708
0.051
0.1275
0.4
0.615
0.048
0.12
0.4
0.413
0.050
0.125
0.4
0.375
0.054
0.135
0.4
0.362
0.048
0.12
0.4
0.262
0.056
0.14
0.4
0.263
0.062
0.155
0.4
0.211
0.033
0.165
0.2
0.120
0.076
0.19
0.4
0.372
Disch.
4.285
0.612
FIGURE 8 - MEAN-SECTION METHOD - WORKED EXAMPLE
2.3.3 Velocity – Depth Integration method
The velocity-depth integration method is a graphical method of
computing discharge.
If sufficient velocity observations have
been made in the verticals, a curve of mean velocity and depth
of flow (area of vertical velocity curve) may be drawn over the
cross-section.
The area of this curve represents the total
discharge. The areas contained by the curves should be measured
by planimeter. (See figure 9.)
- 20 -
FIGURE 9 - THE VELOCITY-DEPTH INTEGRATION METHOD
2.3.4 Velocity – Contour method
The velocity – contour method is a graphical method of computing
discharge.
If sufficient velocity observations have been made
in the verticals then the procedure is as follows:
1. Vertical
vertical.
velocity
distribution
curves
are
drawn
for
each
2. Interpolate the curves for convenient intervals of velocity
e.g. 20 km/d, 40 Km/d etc.
3. Curves or contours of equal velocity (isovels) are drawn.
(See figure 10).
4. Starting from the maximum, the areas enclosed by successive
velocity contours are measured by planimeter and plotted on a
diagram with the ordinate indicating velocity and the abscissa
indicating the corresponding area enclosed by the respective
velocity contour.
The summation of the area enclosed by this
curve represents the total discharge.
- 21 -
FIGURE 10 - THE VELOCITY CONTOUR METHOD OF COMPUTING
MEASUREMENTS
2.4 Procedure for Measurement of Discharge by Current Meter
Current meter measurements should be classified in terms of the
means used to traverse the river during the measurement. These
are normally by wading, cableway, boat or bridge.
The actual
method used depends mainly on the depth of flow and the
velocity.
No matter which method is used or how the current
meter is suspended, the principles of measurement described in
the previous section are the same.
2.4.1 Selection of gauging site.
For a gauging station, the selection of the site is often
dictated by the needs of water management or by the requirements
of the hydrometric network.
In fulfilling water management
needs there is little or no freedom of choice in selecting
gauging sites, and frequently records need to be obtained under
extremely adverse hydraulic conditions. This is often the case
where spot measurements are required at specified locations.
Ideally a site is selected to meet the design requirements that
will provide reliable stream flow data. It is often useful, if
possible, to inspect potential sites during different flow
conditions. Generally however, the aim is to select a reach of
stream containing the following characteristics:
a. The channel should be straight and of uniform cross-section
and slope in order to avoid abnormal velocity distribution.
b. The depth of water in the selected reach should be sufficient
to allow effective immersion of the current meter.
c. The measuring site and the reach upstream should be clear and
unobstructed by trees and other obstacles so that the field of
view enables floating debris to be seen in sufficient time to
permit the removal of the instrument from the stream.
- 22 -
d. The bed of the reach should not be subject to changes during
the period of a measurement. (i.e. siltation or scouring etc.)
e. All discharges should be contained within a well-defined
channel having substantially stable boundaries with well-defined
geometric dimensions.
f. To avoid disturbance of the flow the site should be remote
from any bend or natural or artificial obstruction.
g. The gauging site should be kept clear of aquatic growth.
h. Sites at which vortices, reverse flow or dead water occur
should be avoided.
i. A measuring section with converging and more so diverging
flow over an oblique measuring section should be avoided as it
is difficult to make allowance for the systematic errors that
can occur.
j. The orientation of the reach should be such that the
direction of the flow is normal to that of the prevailing wind.
k. To facilitate gauge reading during a measurement and to avoid
the effect of storage between the measuring site and the gauges
the measuring site should be situated relatively close to the
gauges.
1. The measuring site should be situated in close proximity to
the gauging station to avoid the effect of tributary inflow.
m. The measuring site should be readily accessible, where
possible, to provide safe passage at all stages of flow and in
all conditions for personnel and vehicles.
n. Permanent markers should be established at each measuring
section to facilitate the repetition of levels or soundings.
o. The distance from the measuring section to the station gauge
should be accurately defined to evaluate measurements and
identify any change in cross-section.
p. When the length of straight channel is restricted, it is
recommended that for current meter measurements the straight
length of channel upstream of the measuring section should be
twice the length of that downstream. (Generally, one hundred
metres should be regarded as the minimum straight length
upstream).
2.4.2 Current meter measurement by wading
Current meter measurements by wading are preferred against other
methods because they are generally less time consuming and
permit the selection of the most suitable site for a particular
stage. They also allow more control over the gauging procedure.
This is particularly the case in the selection of a cross- 23 -
section which may not be at the usual station measuring section
and when selecting verticals and measuring depths.
If natural
conditions for measuring (with respect to depths and velocities)
are not ideal the section may be modified to provide acceptable
conditions.
After any modifications, flow must be allowed to
stabilise before starting the gauging.
A measuring tape or tagged line is strung across the river at
right angles to the direction of flow.
Using a minimum of 15
verticals, the spacing is determined so that no segment contains
more than 10% of the total discharge (See section 2.1.) Usually
an approximate discharge can be obtained for this purpose from
the stage discharge curve, the current rating table or from
previous measurements. The position of successive verticals are
located by horizontal measurements from a reference marker
(initial point) on the bank.
The gauging starts at the waters
edge, where depth and velocity may or may not be zero. At each
chosen vertical the depth is measured and the value used to
compute the setting/s of the current meter (usually 0.6 or 0.2
and 0.8 depth).
After the meter is in position the rotor is
allowed to adjust to the stream velocity before the revolution
count is started.
This may take only a few seconds where
velocities are over 25 Km/d but a longer period is necessary for
slower velocities.
A revolution count is then taken at each
selected point for a minimum of 40 seconds, but where the
velocity is known to be subject to short period variations or
pulsations it is advisable to continue the observations for at
least 60 seconds. Start the stop watch simultaneously with the
first signal or click, counting zero not one. End the count on
a convenient number given in the column heading of the meter
rating table. (i.e. 2, 4, 6, 8, 10, 15, 20 .... 100, 150). Read
the time to the nearest half second.
Revolutions may also be
counted over a fixed time period (40 seconds, 60 seconds, 80
seconds etc) using a counter, however this can lead to an error
in the observation at low velocities because only full
revolutions are counted.
Consideration must also be given to the direction of flow,
because it is the component of velocity normal to the
measurement section that must be determined. (See section 2.5.2
on oblique flows).
Tail fins are provided to assist this
process and must be used when appropriate.
Limitations on wading are imposed by the combination of depth
and velocity and by the quality of foothold on the bed.
The
advisability of wading must be judged by the operator at each
site.
The position of the operator is important to ensure that the
operator’s body does not effect the flow pattern at or
approaching the current meter.
The best position is to stand
facing one or other of the banks, slightly downstream of the
meter and at arm's length from it.
The rod is kept vertical
throughout the measurement with the meter parallel to the
direction of flow. Avoid standing in the water if feet and legs
would occupy a considerable percentage of the cross-section of a
- 24 -
narrow stream. In a small stream where the width permits, stand
on a plank or other support rather than in the water.
2.4.3 Current meter measurement from cableways
Cableways are normally used when the depth of flow is too deep
for wading, when wading in a swift current is considered
dangerous, or when the section is too wide to string a tape or
tagged line.
There are two basic types of cableway:
a. Those with the instrument carriage controlled from the bank
by means of a winch, either manually or electrically operated.
(See figure 11).
b. Those with a manned personnel carriage.
The general gauging procedure is similar for both except that in
the case of the non-manned cableway the instrument carriage,
suspended from the top cable, moves the current meter and
sounding weight across the stream between the cable supports.
The operator remains on the bank and operates the gauging winch
that is provided with a depth counter for placing the current
meter at the desired position.
Tags can also be placed on the
sounding line as an aid in determining depth.
The electrical impulses from the current meter are returned
through the core conductor of the suspension cable and
registered on a counter or by an audible signal.
The gauging procedure is as follows:
a. The waters edge is identified in relation to a permanent
initial point (i.e the operational post) on the bank by means of
a tagged endless wire which is also used for spacing the
verticals.
b. The current meter and weight are lowered at the first
vertical until the predetermined displacement mark on the weight
touches the water surface and then the depth counter is set to
zero. (See section 2.6.4) If a tagged line is used, the depth
is read directly off the tags.
c. Scour is likely to be less on the upstream side.
The advantages of using the downstream side are:
a. Vertical angles are more easily measured on the downstream
side as the sounding line will move away from the bridge.
b. The flow lines of the stream may be straightened out by
passing through a bridge opening with piers.
Utilising the upstream or downstream side of a bridge for a
current meter measurement should be decided on site for each
- 25 -
individual bridge. This is done after considering the above and
the physical conditions at the bridge such as walkway locations,
traffic hazards and accumulation of debris.
The meter is controlled by a gauging winch mounted on a bridge
crane or bridgeboard.
A hand line may be used with small
weights.
c. The current meter assembly is then lowered until the weight
touches the streambed and the sounded depth is recorded.
d. The velocities are measured at the selected depths in the
vertical.
e. The current meter should be checked between verticals for
obstruction or damage, particularly if there is a sudden
variation in the velocity indicated by the counter, or through
the audible signal.
The channel, upstream from the section,
should be watched closely for any debris that could damage the
meter.
f. If measurements are made where the river is deep and swift,
the sounding weight may not be sufficient to maintain the
suspension cable in the vertical.
In this instance, the angle
of divergence from the vertical is measured by protractor, and
the soundings corrected to obtain the actual vertical depth.
(See section 2.5.1.)
g. The remoteness of the observer
horizontal angles difficult to determine
therefore essential that cableways are
possible location at right angles to
errors caused by undetected horizontal
angles are encountered the procedure in
adopted.
- 26 -
from the meter makes
from a cableway. It is
installed in the best
the flow, to minimise
angles.
If horizontal
Section 2.5.2 should be
FIGURE 11 - SCHEMATIC ARRANGEMENT OF AN UNMANNED CABLEWAY
- 27 -
2.4.4 Current meter measurement from bridges
Although cableways are generally preferred to bridges for
current meter measurements, highway or railway bridges can often
be used to advantage. Bridges rarely offer the right conditions
for stream gauging but measurements from them may be necessary
where suitable sites for wading or for a cableway are not
available. As contracted sections, piers and other obstructions
effect the distribution of velocities it is necessary to use a
larger number of verticals and more velocity observations in
each vertical, especially, close to the bridge piers and banks.
No set rule can be given for selecting the upstream or
downstream
side
of
a
bridge
for
obtaining
discharge
measurements, however the advantage of each are set out below.
The advantages of using the upstream side of the bridge are:
a. The hydraulic conditions of the upstream side of the bridge
opening are usually more favourable.
b. Approaching debris can be seen and avoided more easily.
Another method that may be utilised is the "Side Suspension". A
temporary cross-line is erected upstream or downstream of the
bridge with the gauging equipment attached by pulleys and
operated from the bridge.
A wire or rope is attached to the
pulley system to aid in traversing the sounding equipment. The
advantages of this method are:
a. Discharge measurements can be made at a distance from debris
build up or the effects of piers or other obstructions.
b. The cross line can be erected at right angles to the flow to
eliminate potential angular corrections that may be required if
the bridge were to be used.
2.4.5 Current meter measurement from boats
Where the river is too wide for a cableway installation and too
deep to wade, discharge measurements are made from boats. Boat
use is limited by high water velocity during floods, especially
as personal safety must be of primary consideration.
Note:
"Gaugings made on any streams are to be made in accordance with
the requirements of the Maritime Services Board Regulations or
any applicable local regulations".
A tagged line is used to span the river at the measuring
section. The tagged line serves the dual purpose of holding the
boat in position during the measurement and locating the
verticals laterally.
If there is any likelihood of traffic on
the stream, one alert person must be positioned on the bank to
lower the line to allow traffic to pass safely.
- 28 -
The procedure for
and cable is the
cableway once the
for traversing the
gauging from a boat using a sounding weight
same as that for measuring from a bridge or
tagged line has been erected and the method
boat has been determined.
The advantages of boat gauging are:
a. Flexibility in the selection of measuring sites.
b. Close proximity of the observer to the current meter allows
an accurate evaluation of conditions at each vertical and quick
repair to gauging equipment.
The disadvantages of boat gauging are:
a. Difficulties in erecting a tagged line
streams or streams with snags on the bed.
in
debris
laden
b. Vertical angle of the sounding line is difficult to determine
due to the position of the observer directly behind the sounding
line and the limited length of dry line.
c. If the section is subjected to wind action and stream
velocities are less than 25 km/d boat measurements are not
recommended.
2.4.6 Other current meter methods
There are several other methods that are sometimes used to
measure discharge using current meters.
These methods are
useful when a full current meter gauging is inappropriate
because time is limited, high velocities are experienced or
stage is changing rapidly. A loss of accuracy, however, must be
expected. These methods are only described in brief as they are
not in general use in Victoria.
2.4.6.1 Two-tenths depth method
In this method the velocity is observed at 0.2 of the depth
below the surface and a coefficient applied to the observed
velocity to obtain the mean in the vertical.
When it is
impossible to obtain soundings, a standard cross-section at the
site is used to compute the 0.2 depth.
A measurement made by this method is normally computed by using
the 0.2 depth observations (without coefficients,) as though
each were a mean in vertical.
The result obtained is then
divided by the area of the measuring section to give the mean
value of the 0.2 depth velocity. The plotting of the true mean
velocity versus the mean 0.2 depth velocity for each measurement
will give a velocity relation curve for use in adjusting the
mean velocity for measurements made by the 0.2 depth method. If
too few measurements have been made to establish the vertical
velocity curve, the coefficient to adjust the 0.2 depth to the
mean velocity is about 0.87.
- 29 -
2.4.6.2 Sub-surface velocity method
In this method the velocity is observed at some arbitrary
distance below the water surface.
The distance should be at
least 0.6 m, and preferably more in deep swift streams to avoid
the effects of surface disturbances.
The sub-surface velocity
method is used, when it is impossible to obtain soundings and
the depths cannot be estimated with enough reliability to even
approximate a 0.2 depth setting. Coefficients are necessary to
convert the observed velocities to the mean velocity in the
vertical.
The coefficients are determined by measuring the
depths
after the stage has receded sufficiently.
The
coefficients to be used with the sub-surface observations can
then be computed by obtaining vertical velocity curves at the
reduced stage of the stream.
2.4.6.3 Integration method
In this method the meter is lowered to the streambed and then
raised to the surface at a uniform rate.
The total number of
revolutions and the total elapsed time are used to obtain the
mean velocity in the vertical. This method cannot be used with
a vertical axis current meter as the vertical movement of the
meter effects the motion of the rotor.
The accuracy of the
measurement is largely dependent on the hydrographer maintaining
a uniform rate of movement of the meter.
2.4.6.4 Interpolation method
This method has been devised for use on wide rivers where there
is limited time available.
The velocity is measured in only
three verticals and the average velocity is interpolated for the
remaining verticals or sub-sections.
2.4.7 Sounding weights
If a stream is too deep or swift to wade, the current meter is
suspended in the water by cable from a cableway, boat or bridge.
The sounding weight is suspended below the current meter to keep
it stationary in the water and in an approximately vertical
position. The weight also prevents damage to the meter when the
assembly is lowered to the streambed to measure depth.
The
sounding weights now commonly used are the Columbus type or
their equivalent.
These weights are stream lined, with tail
fins, to align them with the current and to cause minimum
interference to the flow. Each weight has a vertical slot and a
drilled horizontal hole to accommodate a weight hanger bar and
securing pin.
Some sounding weights have a ground sensor that
produces a signal when the weight touches the bed. The distance
between the centre-line of the current meter and the bottom of
the weight must be considered when setting the meter at the
velocity observation points.
- 30 -
Columbus sounding weights are manufactured in a variety of
sizes.
These are 7, 15, 23, 34, 45, 68, 75, 100, 125 and 150
kilograms.
The size of the sounding weight used in current meter
measurements depends on the depth and velocity in the crosssection. As a rule of thumb the size of the weight in kilograms
should be greater than five times the maximum product of
velocity and depth in the cross-section, divided by 86.4.
The formula is:
Sounding weight required (kg) =
5 x Velocity x Maximum Depth
86.4
For example,
The maximum velocity at a gauging station is estimated at 225
kilometres per day and the maximum depth to be measured is
estimated at 4.7 metres.
Therefore: Sounding weight required
=
=
5 x 225 x 4.7
86.4
61 kg
So, a sounding weight of 68 kilograms would be required
measure the estimated maximum discharge at this site.
to
2.4.7.1 Sounding weight hanger bar
The "Tatura" universal hanger bar has been adopted as
standard hanger bar for use within Thiess Services.
It
designed to overcome the shortcomings of existing hanger
sounding weight and current meter combinations and has
following advantages.
the
was
bar,
the
a. A single hanger bar with suspension pin configurations to
suit sounding weights from 15 to 68 kilograms.
b. A sounding and meter setting accuracy of ±0.01 metres.
c. Eliminates the need for a thread within the sounding weight
d. Compatible with Gurley, OTT and Oss current meters.
e. Allows a comparison between
sounding line if required.
the
winch
counter and tagged
Figure 12 shows a detailed drawing of the hanger; bar and pin
arrangements.
2.5 Special Problems in Streamflow measurement
When current meter measurements are carried out under adverse
conditions normal procedures must sometimes be altered.
The
most common of these procedures are covered below:
- 31 -
2.5.1 Depth corrections for sounding line and weight
When suspended current meter measurements are obtained in deep
swift water, the current meter and the sounding weight may be
carried downstream for a certain distance before the weight
touches the bottom. In such cases, corrections must be applied
in order to determine the correct depth and the depth of the
current meter setting.
Figure 13 shows the position assumed by the sounding line when
the weight is just off the bed of the stream, and supported by
the line only. It can be seen that from the length of the line
AF, the distance AE and the difference between the length of EF
and BC must be deducted in order to determine the depth BC,
(assuming the stream bed is horizontal.) Both these corrections
are functions of the vertical angle and are given in tables 5
and 6.
The values in these tables are based on the assumption that the
drag force on the weight in the comparatively still water near
the bottom can he neglected and that the sounding line and
weight are designed to offer little resistance to the flow of
water.
The uncertainties in these assumptions are such that
significant errors may be introduced if the vertical angle is
more than 30 degrees. If the direction of flow is not normal to
the measuring cross-section, the corrections in the table will
be too small due to underestimation of the angle.
2.5.1.1 Positioning the Meter in the Vertical
The conditions that cause errors in sounding also cause errors
in placing the current meter at selected positions in the
vertical.
The amount of drift experienced by the meter is not
constant but varies with depth.
The wet-line tables therefore
are not strictly applicable when setting the meter at the
observation depths.
- 32 -
Material: STAINLESS STEEL 304
GENERAL NOTES: This hanger bar can be used with
weights from 15kg to 68kg.
Tagged winch lines must be adjusted from 400mm bar.
Some weights must be reslotted and drilled to fit this
new bar.
10mm ∅ hole
6mm ∅ hole
To suit both Gurley & Ott Meter set with
axis ≈ 0.35m above bottom of weight
6mm ∅ hole
To suit both Gurley & Ott Meter set with
axis ≈ 0.3m above bottom of weight
6mm ∅ hole
To suit both Gurley & Ott Meter set with
axis ≈ 0.2m above bottom of weight
9mm ∅ 3/8" BSF
threaded
for 68kg with sensor
9mm ∅ 3/8" BSF
threaded
for 68kg without sensor
9mm ∅ 3/8" BSF threaded
for 22kg
15kg
FIGURE 12 - "TATURA" UNIVERSAL HANGER BAR
- 33 -
FIGURE 13 - POSITION OF THE SOUNDING WEIGHT IN A DEEP SWIFT
STREAM
Air-Line Correction
Vertical angle
(degrees)
Correction
%
Vertical angle
(degrees)
Correction
%
4
6
8
10
12
14
16
0.24
0.55
0.98
1.54
2.23
3.06
4.03
18
20
22
24
26
28
30
5.15
6.42
7.85
9.46
11.26
13.26
15.47
TABLE 5 - AIR-LINE CORRECTION - PERCENTAGE TYPE
- 34 -
Wet-Line Correction
Vertical angle
(degrees)
Correction
%
Vertical angle
(degrees)
Correction
%
4
6
8
10
12
14
16
0.06
0.16
0.32
0.50
0.72
0.98
1.28
18
20
22
24
26
28
30
1.64
2.04
2.48
2.96
3.50
4.08
4.72
TABLE 6 - WET-LINE CORRECTION - PERCENTAGE TYPE
AIR LINE CORRECTION
AIR
LINE
in
METRES
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0
8.5
9.0
9.5
10.0
VERTICAL ANGLE (DEGREES)
4
6
8
10 12 14 16
CORRECTION IN CM
18
20
22
24
26
28
30
35
1
2
2
3
4
5
5
6
7
8
8
9
10
11
12
12
13
14
15
15
3
5
8
10
13
15
18
21
23
26
28
31
33
36
39
41
44
46
49
51
3
6
10
13
16
19
22
26
29
32
35
39
42
45
48
51
55
58
61
64
4
8
12
16
20
24
27
31
35
39
43
47
51
55
59
63
67
71
75
79
5
9
14
19
24
28
33
38
43
47
52
57
61
66
71
76
80
85
90
95
6
11
17
23
28
34
39
45
51
56
62
68
73
79
84
90
96
102
107
118
7
13
20
27
33
40
46
53
60
66
73
80
86
93
99
106
113
119
126
133
8
15
23
31
39
46
54
62
70
77
85
93
100
108
116
124
131
139
147
155
11
22
33
44
55
66
77
88
99
110
121
132
143
154
155
176
187
198
209
220
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
1
1
1
2
2
2
2
3
3
3
4
4
4
4
5
5
5
6
1
1
2
2
3
3
4
4
5
5
6
6
7
7
8
8
9
9
10
1
2
3
4
6
7
8
9
10
11
12
13
14
16
17
18
19
20
21
22
2
3
5
6
8
9
11
12
14
15
17
18
20
21
23
24
26
28
29
31
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30
32
34
36
38
40
TABLE 7 - AIR-LINE CORRECTION
- 35 -
WET - LINE CORRECTION
WET
LINE
IN
METRES
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
VERTICAL ANGLE (DEGREES)
4
6
8
10
12
14
16
18
20
22
1
1
2
3
4
4
5
6
6
7
7
9
9
10
11
12
12
13
14
14
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
2
3
4
5
6
8
9
10
12
13
14
15
17
18
19
20
22
23
24
26
2
3
5
7
8
10
11
13
15
16
18
20
21
23
25
26
28
30
31
33
2
4
6
8
10
12
14
16
18
20
22
24
27
29
31
33
35
37
39
41
2
5
7
10
12
15
17
20
22
25
27
30
32
35
37
40
42
45
47
50
24
26
28
30
35
CORRECTION IN CM
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
3
3
3
3
3
1
1
1
2
2
2
3
3
3
4
4
4
4
5
5
5
6
6
6
1
1
2
2
3
3
4
4
5
5
6
6
7
7
8
8
9
9
10
10
3
6
9
12
15
18
21
24
27
30
33
36
38
41
44
47
50
53
56
59
4
7
11
14
18
21
25
28
32
35
39
42
46
49
53
56
60
63
67
70
4
8
12
16
20
24
29
33
37
41
45
49
53
57
61
65
69
73
78
82
5
9
14
19
24
28
33
38
42
47
52
57
61
66
71
76
80
85
90
94
7
13
20
27
33
40
46
53
60
66
73
80
86
93
99
106
113
119
126
133
TABLE 8 - WET-LINE CORRECTION
A workable system to overcome the problem is to set the meter at
factors of the wet-line length rather than the true depth.
As the largest vertical angle is normally at the full depth, by
factorizing the wet-line length EF (Figure 13) the true settings
will be more closely approximated than if wet-line correction
tables and the true depth are used.
Eg. using the 0.2 & 0.8 method.
If the observed wet-line depth = 20 m, the meter is placed at a
measured depth of 4.00 m for the 0.2 position and 16.00 m for
the 0.8 position.
The procedure for applying depth
meter positions is as follows:
corrections
and
determining
a.
Measure
the
vertical
distance
A
B
from
the
guide
pulley/traveller to the water surface.
This will give the
vertical distance to be used with the airline correction table
(Table 7).
This table will only be used when depths are read from a counter
on the gauging winch and not when depths are read from a tagged
sounding line.
- 36 -
b. Align the displacement mark on the weight with the water
surface and zero the depth counter on the gauging winch
c. Lower the sounding weight to the bed of the stream. Read and
record the sounded depth DF and the vertical angle .
These
tables have also been produced in an expanded form for use in
the field (See tables 7 and 8.)
d. With the aid of the tables compute and record:
(i) the air correction, D.E. from Table 7 (if applicable)
(ii) the wet line depth EF = DF - DE
(iii) the wet line correction for length EF and angle
Table 8
from
(iv) add both corrections together and subtract them from the
sounded depth DF, this will give the actual depth. BC.
Example 1
a. In gauging a deep swift stream from a cableway, the total
depth measured by the gauging winch counter from the guide
pulley to the surface was measured as 3.00 m. The observed depth
of water (after zeroing the counter with the displacement mark
at water surface) was measured as 4.55 m and the vertical angle
was 200.
Find the true depth and the current meter positions
using the 0.2 and 0.8 depth method.
The distance from the
centre line of the current meter to the bottom of the weight was
0.30 metre.
From Figure 13.
AB = 3.00 m
=200
DF = 4.55 m
Air-line correction, DE, from Table 7, for 200 and 3.00 m
Therefore DE = 0.19 m
Wet-line depth EF = DF-DE = 4.55 - 0.19 = 4.36 m
Wet-line correction from table 8 for 200 and 4.36 m
Therefore Wet-line correction = 0.09 m
Therefore true depth BC
= 4.36 – 0.09
= 4.27 m
b. To place the current meter at the 0.8 depth position in the
vertical.
0.8 x wet-line = 0.8 x 4.36 = 3.49 m.
- 37 -
Therefore the current meter should be raised and set at a depth
of (3.49 + 0.30 + 0.19) = 3.98 m on the counter. (Airline
correction = 0.19)
To place the current meter at the 0.2 depth position in the
vertical.
0.2 x wet-line depth = 0.2 x 4.36 = 0.87 m.
Therefore the current meter should be raised and set at a depth
of (0.87 + 0.30 + 0.19) = 1.36 m on the counter.
Example 2
In gauging a deep swift stream from a cableway the depth
measured using a tagged sounding line was 4-36 m and the
vertical angle was 200.
= 200
EF = 4.36 m
Wet-line correction from table 8 for 200 and 4.36 m = 0.09 m
Therefore the true depth BC
= 4.36 – 0.09
= 4.27 m
The current meter positions in the vertical as per example 1
minus the airline correction. I.e. 0.8 depth setting = 3.79, 0.2
depth setting = 1.17.)
USE OF METER DEPTH SETTING TABLES
These tables can be used under the following circumstances.
1. If no significant drift angle is observed, tables can be used
directly for the setting of the meter in the vertical whether
using tags or a depth counter.
The meter position relative to
the bottom of the weight is taken into account on individual
tables. (Check use of correct table).
2. If significant drift angle
carried out utilizing a counter,
is
observed
and
depthing
is
Procedures "A" or "B" may be used.
(A) The tables
procedure.
may
be
used
by
complying
with
the
following
1. Subtract the airline correction fro the counter reading to
derive wet-line length.
2. Refer to the appropriate meter setting table and ascertain
the appropriate counter reading.
3. Add the airline correction to the table result.
- 38 -
(B) Calculations may be made manually as follows.
1. Subtract the airline correction from the counter reading to
derive the wet-line length.
2. Multiply the wet-line length by the required meter setting
factor e.g. 0.2 or 0.8 then
3. Add distance of meter above weight bottom and
4. Add the airline correction.
The procedures outlined in this section should be carried out
with care to avoid magnifying the errors involved. The two main
difficulties in measuring from a cableway are:
a. Employing an adequate sounding
sounding line in a vertical position.
weight
to
maintain
the
b. Measuring the vertical angle.
2.5.2 Oblique flows (Angled Flow)
Oblique or angled flow is flow that is not at 900 to the
measuring section.
The angle of flow is that angle between a
0
line at 90 to the section and the direction of current.
To
eliminate errors introduced by such angles it is necessary to
obtain the component of the velocity normal to the crosssection.
To make the correction for oblique flow for a rod
supported meter, the current meter is rigidly held in line with
the direction of flow.
(Tail fins must be used to ensure
correct alignment.)
(See Figure 14).
For a current meter
suspended from a line, the meter will automatically take up the
direction of the current. The velocity in the direction of the
current and the angle of deflection are measured. The measured
velocity, when multiplied by the cosine of the angle of the
current, gives the velocity component normal to the measuring
cross-section. For small angles of less than 8 0 the correction
is negligible.
The assumption is made that the point velocity corrections apply
to the whole vertical or between velocity points in the
vertical.
When a correction has to be applied for oblique flow, the
velocity observations on the measurement field sheet should be
adjusted by the cosine of the angle of flow to the normal. The
field sheet does not allow for this adjustment to velocities, so
the correction is applied to the section width by booking the
angle in the "horizontal angle" column. The resulting discharge
in the section is the same but because incorrect area readings
are used to obtain the discharge potential problems exist for
users of the data. However, unless the field computation sheet
is modified the corrections should be booked in this way and
appropriate comments made in the remarks section.
A worked
example is shown in Figure 16.
- 39 -
Propeller type current meters with a "component propeller"
measure the velocity component normal to the measuring section
in oblique flow up to an angle of 450, depending on type, and no
correction is necessary if the current meter is held rigidly at
right angles to the measuring section. (See Figure 15).
FIGURE 14 - Conventional
Current Meter
FIGURE 15 - Component Propeller
Meter
CORRECTION FOR OBLIQUE FLOW
2.5.3 Pulsations in flow
The phenomenon of pulsation has an effect on the measurement of
point velocities and therefore on current meter gauging in
general.
The velocity at any point in the stream is
continuously fluctuating with time even when the discharge is
constant and when the surface is apparently smooth and free from
surges and eddies.
This pulsation is caused by secondary
currents developed by hydraulic conditions upstream of the
gauging site.
For example; obstructions in the approach
channel, by surging produced at riffles or rapids being
continued through pooled reaches, or by the acceleration of the
water at bends. Generally, the velocity at any point changes in
cycles that can vary from a few seconds to possibly more than 1
hour.
Lower velocities have a pulsation of a greater magnitude and
should therefore be observed over a longer period than higher
velocities.
Whilst it is customary to observe velocity at a
point for a period of 40 to 120 seconds, it should be recognized
that this time range is not long enough to ensure the accuracy
of a single point velocity observation.
However, because the
pulsations are random and because observations are made at in
excess of 16 verticals, usually with two observations being made
in each vertical, there is little likelihood that the pulsations
will bias the total discharge measurement in each stream.
Longer observation periods should not be used if there is a
likelihood of a significant stage change during the measurement
period. Longer observation periods also increase the operating
cost when a large number of gauging stations are considered.
- 40 -
THIESS SERVICES
FIGURE 16 - DISCHARGE FIELD SHEET - CORRECTION FOR OBLIQUE FLOW
- 41 -
2.5.4 Mean gauge height for current meter measurements
The mean gauge height of a discharge measurement represents the
mean stage of the stream during the measurement period.
The
mean gauge height is one of the two co-ordinates used in
plotting the discharge-rating curve for gauging stations.
An
accurate gauge height is as important as an accurate measurement
of the discharge.
There is no difficulty in determining the
gauge height that corresponds with the measured discharge during
constant or nearly constant stream stage.
If the change in
stage is minimal and the discharge distribution is uniform the
arithmetic mean of the start and end gauge height can usually be
taken as the mean gauge height. However, measurements are often
made during periods when the change in stage is neither uniform
nor slight and the discharge distribution is not uniform.
The correct gauge height is obtained by computing the weighted
mean gauge height which requires additional observations of
stage between the start and end of the measurement.
The
readings are made at regular intervals or as required depending
on the rate of change of stage.
The assistance of a gauge
reader is usually necessary for obtaining the readings and
noting the time of observations. The weighting is done by using
partial discharge, time, or a combination of both as the
weighting factor.
Studies have shown that discharge weighting tends to over
estimate the mean gauge height, whereas time weighting tends to
under estimate the mean gauge height.
Ideally both methods
should be used and the result averaged, however either method
may be used if the average result is considered unsatisfactory.
Where weighting has been used the method adopted should be
indicated on the measurement note sheet.
A description of the
two methods follows. (See Figure 17)
2.5.4.1 Discharge weighting
In this process the partial discharges measured between
observations of gauge height are used with the mean gauge
heights for the periods when the partial discharges were
measured. The formula used to compute the mean gauge height is:
H =
q1 x h1 + q2 x h2 + q3 x h3.....+qn x hn
Q
where
H = mean gauge height in metres.
Q = total measured discharge in megalitres/day
q1, q2, q3, qn = discharge measured during the time interval
1,2,3,n in megalitres per day.
h1, h2, h3, hn = average gauge height during the time interval
1,2,3, n in metres.
- 42 -
Figure 17 shows the computation of a discharge weighted mean
gauge height
This has been done using a standard form.
2.5.4.2 Time weighting
In this process the arithmetic mean gauge heights between
observation times are used with the duration of those time
periods. The formula used to compute the mean gauge height is:
H =
t1 x h1 + t2 x h2 + t3 x h3.....+tn x hn
T
where
H = mean gauge height in metres.
T = total time for the measurement in minutes
t1, t2, t3, tn = duration of observation times, in minutes,
between stage reading.
h1, h2, h3, hn = average gauge height, in metres during the time
period 1,2,3,n
The computation of the weighted mean gauge height using both
methods is shown in Figure 17.
2.5.5 Rapidly changing stage
When
extremely
rapid
changes
in
stage
occur
during
a
measurement, the weighted mean gauge height is not truly
applicable to the discharge measured.
Measurements made under
these conditions should be completed more rapidly than those
made under constant or slowly changing stage to reduce the range
in stage. Shortcuts in the measurement procedure usually reduce
the accuracy of the measured discharge, therefore procedures
during rapidly changing stage must be optimized to minimise the
combined error in measured discharge and mean gauge height. The
reduction in measurement time makes it possible to obtain a
gauge height value that is representative of the measured
discharge.
The procedure for measuring discharge on large and small streams
also varies due to the different behaviour of flood and peak
flows on each type. The procedure to be followed for each is:
- 43 -
2.5.5.1 Large Streams
During periods of rapidly changing stage on large streams the
discharge measurement time may be reduced by modifying the
standard measurement procedure in the following way:
a. Use the 0.6 depth method (See section 2.2.2.). The 0.2 depth
method (See section 2.4.6.1) or the sub-surface method (See
section 2.4.6.2) may be used if placing the meter at the 0.6
depth
creates
vertical
angles
requiring
time
consuming
corrections, or if the vertical angle increases because of drift
collecting on the sounding line.
b. Reduce the velocity observation time.
c. Reduce the number of verticals taken.
d. By taking velocities at every second vertical only. This is
a valuable method in cases of changing sections.
An accurate
profile and area is obtained with little extra time.
By incorporating the above procedures a measurement can be made
in 15 to 20 minutes.
The standard error using a 25 second
observation period and the 0.6 depth method with velocity
observations at 16 verticals is 8%.
The error created when
using the short cut method is generally less than the error
caused by shifting flow patterns during rapidly changing stage.
2.5.5.2 Small streams
To obtain discharge measurements of flash floods on small
streams, advance warning of the event is required to enable the
hydrographer to be on site and prepared before the stream starts
to rise. The measurement procedure is as follows:
a. Use 6 to 10 verticals in the measurement cross-section. The
actual number depends on the width and uniformity of the crosssection.
b. Current meter observations are commenced as soon as the
stream starts to rise and are continued until the flow returns
to near normal.
After completing one traverse of the crosssection, the next traverse is started immediately in the
opposite direction, and observations are continued back and
forth.
c. Single velocity observations should be taken in each vertical
using the 0.6 depth method (See section 2.2.2) or the 0.2 depth
method (See section 2.4.6.1).
d. Staff or auxiliary gauge readings at the measuring section
are made at the start and finish of each traverse. Intermediate
readings are made at a minimum of every third vertical and the
time is recorded for each.
- 44 -
FIGURE 17 - TIME & DISCHARGE WEIGHTED MEAN GAUGE HEIGHT
COMPUTATION
- 45 -
When the stream has receded the stream bed elevations in the
vertical are checked to determine if any changes have occurred.
The most reliable results are obtained where the streambed is
relatively stable. Because of the rapid change of stage that
occurs during each traverse, the conventional computation
procedure cannot be applied.
The alternative to follow is to construct an individual relation
of mean velocity to stage for each observation vertical.
The
mean
velocity
is
obtained
by
applying
the
appropriate
coefficient to each observed value.
For each vertical, mean
velocity is plotted against stage and each point is identified
by time. A single smooth curve is fitted to the points, but a
scatter may indicate the need for a curve for the rising limb of
the hydrograph and another for the falling limb of the
hydrograph.
With this information a stage-discharge relation can be
constructed for the station. The section widths are known, the
mean velocities are known and for any stage the corresponding
depths are known.
These data can then be used in the
conventional
manner,
to
compute
the
total
discharge
corresponding to a selected stage.
By repeating the operation for several stages, a stage discharge
relation for the entire stream is constructed.
If necessary,
one for the rising and one for the falling limb of the
hydrograph can be compiled.
2.5.6 Correction of discharge measurement for storage
If a discharge measurement is taken some distance from the gauge
during a change in stage, the discharge passing the gauging
station control will not be the same as the discharge at the
measuring section due to the effects of channel storage between
the two sites.
Adjustment for channel storage is made using a figure obtained
by multiplying the channel surface area by the average rate of
change in the reach to the measured discharge.
A reference
gauge set at the measuring site is required to enable the
necessary calculations to be carried out.
The water surface
elevation at the measuring section and at the gauging station is
determined before and after the measurement to compute
h.
If the measurement is made above the control, the adjustment
will be added for a falling stage and subtracted for a rising
stage and conversely if the measurement is made below the
control the adjustment will be subtracted for a falling stage
and added for a rising stage.
The adjustment for storage is separate and distinct
adjustments required for changing stage and variable slope.
- 46 -
from
The formula for storage adjustment is:
h x 86.4
t
Qg = Qm ± W x L
Where
Qg =
Discharge going over the control in megalitres per
day.
QM =
Measured discharge in megalitres per day.
W =
Average width of the stream between
section and control, in metres.
L =
Length of reach between the measuring section and
control, in metres.
measurement
h =
Average change in stage in the reach, L, during the
measurement in metres.
t =
Elapsed time during the measurement in seconds.
A worked example of adjustment for storage is shown in figure
18. The adjusted discharge figure is the one used for defining
the stage - discharge relation.
2.5.7 Correction of discharge measurement for travel time
It is also possible to approximate the effect of storage by
computing the time of travel of the flood wave between the
measuring section and the control and then adjusting the gauge
height for the travel time to correspond to the measured
discharge.
Adjustment can be made by applying a correction to the observed
gauge at the gauging station control using the following
formula.
S =
R x L x 1440
1.3 V
Where S = Stage difference at the control gauge in metres.
R = rate of change of stage,
=
=
L = length
kilometres
of
reach
between
difference (in metres)
time (minutes)
metres/minute
measuring
site
and
gauges,
in
V = mean velocity of measurement, in kilometres per day.
If the measurement is made upstream from the control, the stage
difference (S) will be subtracted from the control gauge heights
- 47 -
on a falling stage and added on a rising stage. Conversely, if
the measurement is made downstream from the control, the stage
difference (S) will be added to the control gauge height on a
falling stage and subtracted on a rising stage.
A worked example is shown in Figure 19.
height is the figure used for defining
relation.
The adjusted gauge
the stage–discharge
STORAGE
When the measuring section is remote from the gauging station.
Measuring section
1.2 Km
Direction of flow
Gauging Station/Control
QG =
Discharge going over the control = ?
ML/d
QM =
Discharge measured.
ML/d
= 7780
Stage at control during measurement
∴Change in stage at control
=
2.20m at 15:00 hours
=
1.80m at 17:10 hours
=
-0.40m
Stage at measuring section during measurement=
3.62 m at 15:00 hours
=
3.14 m at 17:10 hours
∴Change in stage at measuring section
=
-0.48 m
h =
Average change in stage
=
(0.40 + 0.48) = 0.44
2
t =
Elapsed time during measurement
=
=
2 hours 10 min
7800 seconds
L =
Distance between measuring section and control
=
1200 metres
W =
Average width between measuring section and control
=
65.2 metres
The measuring section is upstream of the control and the stage
is falling, therefore the correction will be positive.
USING:
QG
= QM + W * L *
h*86.4
t
Ml/d
- 48 -
∴QG
= 7780 + 65.2 * 1200 * 0.44 * 86.4
7800
∴QG
= 8161 ML/d
FIGURE 18 - CORRECTION OF DISCHARGE MEASUREMENT FOR STORAGE
2.5.8 Measurement of discharge with sections of dead water
When discharge measurements are carried out in natural channels,
flow conditions may vary with stage and create less than ideal
conditions.
These problems can be overcome by utilizing
alternative measuring sites where better conditions predominate.
However, alternative sites are not always available and these
problems have to be identified and compensated for within the
discharge measurement.
One such problem is an area of no flow
commonly called dead water, adjacent to one or both of the
stream banks.
If a discharge measurement has to be carried out and this
problem is evident, the following procedure should be adopted.
a. The complete cross-section should be gauged from waters edge
to waters edge.
This includes all sections where dead water
occurs.
b. Soundings should be taken at regular intervals through the
section of dead water.
c. Velocity observations
first detected.
should
commence
from
where
flow
is
d. The discharge measurement is then carried out as normal.
This procedure has been adopted so that the bed profile, area
and mean velocity for a particular stage can be accurately
identified.
This is essential when measurements are compared
for rating purposes to validate measurements and to extrapolate
rating curves.
Figure 20 shows the booking procedure to be
followed.
- 49 -
FIGURE 19 - CORRECTION OF MEASUREMENT GAUGE HEIGHT FOR TRAVELTIME
- 50 -
DEAD WATER
No velocity
3m
No velocity
No velocity
Dead water between 3.0 and 9.0
metres
4m
6m
8m
Left Bank
10m
To gauge this section correctly, the complete cross-section must
be sounded and the point where flow starts should be where
readings commence.
THIESS SERVICES
FIGURE 20 - CORRECTION OF DISCHARGE MEASUREMENT FOR DEAD WATER
- 51 -
2.5.9 Measurement of discharge with sections of reverse flow
When discharge measurements are carried out in natural channels,
flow conditions may vary with stage thus creating less than
ideal conditions.
These problems should be overcome by having
alternative measuring sites where better conditions predominate.
However, alternative sites are not always available and these
problems have to be identified and compensated for within the
discharge measurement.
One such problem is reverse flow
adjacent to one or both of the stream banks.
If a discharge measurement has to be carried out and this
problem is evident, the following procedure should be adopted,
a. The complete cross-section should be gauged from waters edge
to waters edge.
This includes all sections where reverse flow
occur.
b. The sections where reverse flow starts and ends should be
identified and noted in the remarks column.
c. The sections with reverse flow should be gauged as normal.
d. The sections where reverse flow ends and normal flow commence
should be kept as small as possible to minimise errors.
e. When the measurement has been completed, the total discharge
of the sections with reverse flow should be subtracted from the
total discharge of the sections with normal flow.
f. The mean velocity is calculated from
sectional area and the adjusted discharge.
the
total
cross-
This procedure has been adopted so that the bed profile, area
and mean velocity for a particular stage can be accurately
identified.
This is essential when measurements are validated
and compared for rating purposes.
Figure 21 shows the booking
procedure to be followed.
2.5.10 Measurement of discharge with variable backwater
Several factors can cause scatter of discharge observations
about the stage discharge relation at a station.
Backwater is
one of these factors.
The velocity is retarded resulting in
higher stage for the same discharge.
Backwater is caused by constrictions such as narrow reaches of a
stream channel or artificial structures downstream such as dams
or bridges or downstream tributaries.
All these factors can
increase or decrease the energy gradient for a given discharge
and can cause variable backwater conditions. Regulated streams
may have variable backwater most of the time, whilst other
streams will have only occasional backwater from downstream
tributaries or from the return of over bank flow.
- 52 -
3.0
4.5
5.0
Left Bank
6.0
7.0
Right Bank
9.0
8.0
Reverse flow between
3.0 and 7.0 metres
HYDROGRAPHIC SERVICES
DISCHARGE MEASUREMENT
Meas. No: 83/7
Weighted Mean: 1.953
COMPUTATIONS
16:50 3.0
N.D.
NO
VEL
REVERSE
4.5
0.80
REVERSE
5.0
1.40
REVERSE
6.0
1.60
17:00 7.0
1.70
17:04 8.0
1.74
9.0
1.68
9.5
1.70
10.0
1.70
17:15 10.5
1.66
11.0
1.60
11.5
1.50
12.0
1.30
12.5
0.80
17:25 13.0
N.D.
5
6
8
8
6
6
NO
NO
6
6
15
15
20
20
20
20
20
20
20
20
25
20
20
15
10
8
NO
43.0
50.0
42.0
46.0
43.0
51.0
VEL
VEL
42.0
47.0
47.0
54.0
41.0
47.0
50.0
55.5
42.5
44.0
48.0
50.5
50.5
42.0
54.5
58.5
47.0
53.5
VEL
NO FLOW
NO FLOW
FORWARD
FORWARD
FORWARD
FORWARD
FORWARD
FORWARD
FORWARD
FORWARD
FORWARD
FORWARD
FORWARD
FORWARD
FORWARD
FORWARD
FORWARD
FORWARD
FORWARD
FORWARD
1.952 1.948
Counter No:
Area
Disch
arge
Velocity
Point
Waters Edge
L/B
1.958
Mean Gauge Height: 1.954.
Time
(Seco
nds)
Distan
ce
Vert
Angle
Adjust
ed
Depth
Revs
Meter TR2073
Weight: 23Kg
No:
Tape No:
OBSERVATIONS
Wad Rod No:
REMARKS
Time
Method: SUSP 2P
1.958
17:1 17:2
5
5
1.952 1.948
Sample No:
Adjust
ed
Horiz.
Angle
Width
30m u/s of gauges
Depth
Point of
Measurement:
Party: FY/GR
Date: 24/10/1983
17:04
Mean
Depth
Station: DARTMOUTH
Stream Temp: 7
EC @ 25 deg: 50
Mean
Vert
Mean
Sect
Stream: PLURRY RIVER
Conditions : Weather: Fine
Stream:
Falling
Time
16:5 17:00
E.S.T.
0
Gauge
1.960 1.958
Height
Rec Height 1.960 1.958
0.0
7.5
7.7
11.8
10.8
8.8
7.6
0.0
0.0
9.0
8.2
19.2
16.8
29.0
25.4
24.0
21.6
28.0
27.2
25.0
23.8
29.4
29.0
22.0
15.6
13.0
9.4
3.80
0.600
0.400
1.5
2.280
9.45
0.550
1.100
0.5
5.198
9.75
1.500
1.500
1
14.625
4.10
1.650
1.650
1
4.30
1.720
1.720
1
6.765
28.868
7.396
13.30
1.710
1.710
1
22.743
22.60
0.845
1.690
0.5
19.097
25.00
0.850
1.700
0.5
21.250
25.20
0.840
1.680
0.5
21.168
26.00
0.815
1.630
0.5
21.190
26.80
0.775
1.550
0.5
20.770
24.00
0.700
1.400
0.5
16.800
15.00
0.525
1.050
0.5
7.875
5.60
0.200
0.400
0.5
1.120
159.409
130.542
7.6
11.3
8.2
0.0
8.6
18.0
27.2
22.8
27.6
24.4
29.2
18.8
11.2
0.0
Disch.
FIGURE 21 - CORRECTION OF DISCHARGE MEASUREMENT FOR REVERSE FLOW
- 53 -
Many sites can be operated by utilizing the stage-fall-discharge
method using the control gauge, at which stage is measured
continuously, and current meter measurements made occasionally.
An auxiliary reference gauge should be installed some distance
downstream from where stage is measured continuously. When the
gauges are set to the same datum, the difference between the two
stage records is the water surface fall and provides a measure
of the water slope.
Precise time synchronisation between the
gauge sets is very important if stage changes rapidly or when
fall is small.
Reliable records can usually be computed when
the fall exceeds 0.1 metre.
Under backwater conditions the fall measured between the two
gauge sets is used as a third parameter and the rating becomes a
stage-fall-discharge relation.
2.5.11 Measurement of discharge with tributary inflow
If a discharge measurement is made at a site some distance from
the gauging station control it is necessary to determine whether
there is additional inflow between the measuring site and the
station control. When a significant inflow has been identified
it must be measured, or if the flow is insignificant compared to
the mainstream flow it may be estimated. This discharge is then
used to determine the actual flow at the gauging station.
If
inflow occurs upstream of the gauging station and downstream of
the measuring site then the tributary inflow is added to the
measured discharge.
If an inflow occurs downstream of the
station control and upstream of the measuring site then the
tributary inflow is subtracted from the measured discharge.
2.5.12 Overflow (out-of-bank flow)
Streams with large overflow or out of bank flow present many
complications
in
stream
flow
measurement
and
in
the
determination of the stage-discharge relation, particularly
during rising and falling stage.
It is possible to establish
separate discharge rating curves for flow in the main channel
and in the overflow area with the total discharge being the sum
of these. There are two distinct types of overflows.
a. Anabranch flow
Anabranch flow is flow that is separate to, but derived from the
mainstream.
If anabranch flow is encountered it should be
measured first as this is generally where the greatest change in
discharge is occurring. Mainstream flow in this case generally
changes far less and can be measured later or estimated from a
mainstream only stage-discharge relation by extrapolating the
curve from earlier measurements.
No significant loss in
accuracy occurs.
Auxiliary gauges should be placed at the anabranch to assist in
establishing a stage-discharge relation separate from the
mainstream. The gauges should also be set to the same datum as
the main stream gauges.
- 54 -
b. Flood plain flow
Flood plain flow is part of the mainstream flow that has spilled
onto the adjoining flood plain.
Any stage change in the flood
plain is reflected by a similar stage change in the main stream.
2.5.13 Velocity measurement to a vertical wall
It is usually necessary to estimate the velocity at an end
vertical as some percentage of the adjacent vertical because it
is not possible to measure the velocity accurately with the
current meter close to a vertical wall.
Laboratory tests
suggest that the mean velocity in the vertical in the vicinity
of a smooth sidewall of a rectangular channel can be related to
the mean velocity in the vertical at a distance from the wall
equal to the depth.
When velocity observations are taken at a distance from a
vertical boundary, which is less than the depth, the results
should be treated with caution.
In many cases we are required
to take velocity readings within this area.
For this reason
Table 9 has been produced to enable an estimation of the
velocity, within this area and at the vertical boundary,
relative to the adjacent observed velocity.
Example 1
The last
from the
i.e., at
boundary.
actual velocity observations were taken at a distance
vertical boundary equal to the depth at the boundary,
chainage 21.5 m that is a distance of 4.0 m from the
(See Figure 22)
Given a depth at the wall of 4.0 m and a mean velocity of 62.64
Km/d at chainage 21.5 m.
To calculate the mean velocity at 23.5 m.
Ratio of distance to depth at known velocity
= 4/4 = 1
Ratio of distance to depth at required vertical
= 2/4 = 0.5
From Table 9 the multiplication factor = 0.95
The Mean Velocity at chainage 23.5 m
= 0.95 x 62.64
= 59.51 Km/d
- 55 -
TABLE 9 - VELOCITY CO-EFFICIENTS IN VICINITY OF A VERTICAL WALL
FIGURE 22 - VELOCITY ADJUSTMENT AT A WALL – SAMPLE CROSS SECTION
- 56 -
Waters Edge
L/B
Conditions : Weather: Fine
Stream Temp:18.5
Sample No:
Stream: Steady EC @ 25 deg:
Time E.S.T.
12:00 13:20
Gauge Height 1.735 1.735
Rec Height
Mean Gauge Height: 1.735
Weighted Mean:
Counter No:
Disch
Adj
Widt
Horiz
.
Wid
Mean
Dept
Area
COMPUTATIONS
Vel
Time
Sec
Revs
Vert
Angl
Adj
Dept
Dept
h
Dist
Time
HYDROGRAPHIC SERVICES
DISCHARGE MEASUREMENT
Stream: GOULBURN RIVER
Meas. No: 1
Station: MADDOGS
Date: 29.12.87
Point of Measurement: 100m d/s
Method: W
Party: JB RC
Meter 5973
Weight:
No:
2
Wad Rod No:
Tape No:
REMARKS
OBSERVATIONS
Point Mean Mean
Vert Sect
0.0
12:00 2.5
3.5
0.50
15
50.0
17.9 17.9
4.5
1.20
6.0
2.30
12:20 8.5
3.70
30
40
90
50
90
60
40
60
40
60
50
60
50
60
90
60
40
60
42.0
43.0
42.0
49.5
42.0
95.0
45.0
45.0
41.0
41.0
48.0
61.0
49.0
93.0
90.0
93.0
43.0
98.0
41.3
53.5
54.8
58.0
54.8
76.3
51.2
76.3
56.1
83.6
59.8
83.6
58.6
79.8
57.4
79.8
53.5
71.6
8.95
0.250
0.250
1
2.238
32.65
0.850
0.850
1
27.753
51.90
2.625
1.750
1.5
136.238
60.98
7.500
3.000
2.5
457.313
64.65
9.750
3.900
2.5
630.338
66.80 10.375
4.150
2.5
693.050
70.78
8.400
4.200
2
594.510
70.45
8.500
4.250
2
598.825
68.90
9.000
4.500
2
620.100
65.58
8.860
4.430
2
580.995
61.03
8.260
4.130
2
504.067
57.90
4.050
234.495
48.50
4.000
194.000
47.4
56.4
65.6
11.0
4.10
63.8
13.5
4.20
15.5
4.20
17.5
4.30
19.5
4.70
13:00 21.5
4.16
23.5
4.10
59.5
24.5
4.00
56.3
13:20 25.5
4.00
40.7
69.9
71.7
69.2
68.6
62.6
Disch
FIGURE 23 - VELOCITY ADJUSTMENT AT A WALL. SAMPLE MEASUREMENT
To calculate the mean velocity at chainage 24.5 m
Ratio of distance to depth at known velocity 4/4 = 1
Ratio of distance to depth at required vertical = 1/4 = 0.25
From table 9 the multiplication factor = 0.90
The mean velocity at chainage 24.5 m
= 0.90 x 62.64
= 56.38 Km/d
To calculate the mean velocity at chainage 25.5 m (wall).
Ratio of distance to depth at known velocity
= 4/4 = 1
- 57 -
5273.919
Ratio of distance to depth at required vertical
= 0/4 = 0
From Table 9 the multiplication factor = 0.65
The mean velocity at chainage 25.5 m
= 0.65 x 62.64
= 40.72 Km/day
It may however have been possible, without undue risk of damage
to the meter, to take velocity observations at a distance from
the vertical wall of half the depth at the boundary, i.e., at
chainage 23.5 m.
If this is the case it should be done bearing in mind what is
written in the first paragraph of this section.
Example 2:
Using Figure 22, if a current meter observation was taken at
chainage 23.5 m the resultant actual mean velocity would be in
the vicinity of 59.5 Km/d. (as obtained by calculation in the
previous example). The velocity at chainages 24.5 m and 25.5 m
would then be calculated using Table 9 in the following manner.
To calculate the mean velocity at chainage 24.5 m.
Ratio of distance to depth at known velocity
= 2/4 = 0.5
Ratio of distance to depth at required vertical
= 1/4 = 0.25.
From Table 9 the multiplication factor = 0.947
The mean velocity at chainage 24.5 m
= 0.947 x 59.5
= 56.4 km/d
To calculate the mean velocity at chainage 25.5 m (wall).
Ratio of distance to depth at known velocity
= 2/4 = 0.5
Ratio of distance to depth at required vertical
= 0/4 = 0
From Table 9 the multiplication factor = 0.684
- 58 -
The mean velocity at chainage 25.5 m
= 0.684 x 59.5
= 40.7 Km/d
2.6 Errors in Streamflow Measurement
There are three main sources of error that
accuracy of a discharge measurement. They are:
can
reduce
the
2.6.1 Human Error
Human errors are those made by the observer in reading
instruments or tags, making biased observations by reading high
or low consistently or by actually booking incorrectly.
Many
factors such as weather conditions, inadequate training, mental
attitude and equipment condition contribute to such errors.
These errors can be reduced and their effect minimised by proper
training, by creating and maintaining high morale through
involvement and by appropriate maintenance and development of
equipment
2.6.2 Instrument Error
The condition and type of instruments used and their calibration
effect the accuracy of the discharge measurement.
The
instruments used in making discharge measurements include the
current meter, depth and revolution counter, stop watch, depth
indicator and width indicator.
It is difficult to evaluate instrument error but investigations
generally indicate an error of less than 1% if all instruments
are in good condition.
Current meters are usually accurate to
within 2% of a standard rating providing the meter is undamaged
and the velocities being measured are within the limitations of
the meter. (See Table 10)
Gurley
Pygmy
OTT.
C31
OSS.
OSS.
SIAP
PC1
PC1
B1
Prop
Prop
Prop
Prop
Prop
Prop
Prop
TABLE
MIN. VEL.
2.6 Km/d
1.3 Km/d
1
2.6 Km/d
3
4.8 Km/d
1
2.2 Km/d
3
3.0 Km/d
2
3.5 Km/d
4
3.5 Km/d
1
4.3 Km/d
10 - CURRENT METER PERFORMANCE
- 59 -
MAX.
526
109
432
860
172
518
865
345
865
VEL.
Km/d
Km/d
Km/d
Km/d
Km/d
Km/d
Km/d
Km/d
Km/d
It is important that if suspect or damaged bucket wheels or
propellers are used, appropriate notes should be made on the
measurement and that the meter be rated in that condition in
order to derive correct velocities.
The meter must then be
repaired and re rated prior to further use.
All equipment should be treated carefully, be well maintained
and regularly checked for malfunction. Typical checks that can
be carried out are:
(a)
Stop watch checked against wrist watch
(b) Rev. counters checked visually and against watch
(c) Depth counters checked against tags
(d) Tags checked against a tape
(e) Observe and listen for an irregular beat in counter or headphones that could indicate contact maladjustment or failure.
2.6.3 Method Error
(a) Velocity Error
This is the error due to the assumption that the mean of the
point velocity observations taken equals the mean velocity in
the vertical. This error is minimised by increasing the number
of observations in the vertical.
(b) Pulsation Error
This error is due to the assumption that the mean of the point
velocity observations taken does not vary with time. This error
is minimised by increasing the observation time period.
(c) Velocity - Depth Error
This is the error due to the assumption that the velocity and
depth vary uniformly from one observation to the next.
This
error is related to the number of verticals used in the
measurement.
Assuming that 100 verticals would result in no error, a study
carried out by the U.S.G.S indicates that the error in two out
of three cases wou1d be less than:
4.5% for 10 verticals
3.0% for 15 verticals
2.5% for 20 verticals
1.2% for 40 verticals
- 60 -
2.6.4 Sounding Error
Sounding
errors.
errors
can
be
due
to
method,
human
and
instrument
For a wading measurement, there should not be any error in the
depth sounding.
An error, however, in meter setting at a
partial depth can occur when the wading rod must be lifted off
the bed to set the meter.
As a result the rod may not be
replaced in the same position.
Ideally the wading rod should
not be lifted off the bed during observations in the vertical.
Cableway and endless wire sounding errors will arise if the
equipment is not properly maintained or due care is not taken.
Errors may be due to:
(a) Buoyancy of the meter and weight, causing the main support
cable to rise and the horizontal section of the meter support
(elseworth) cable to sag, when the weight and meter are
completely immersed after zeroing the depth counter.
For
example a 45 kg gauging weight displaces approximately 5.5 kg of
water resulting in an error in the vicinity of + 40 to 60 mm if
sounding by depth counter. This effect is minimized by zeroing
the depth counter when a pre-determined calibration mark on the
weight corresponds with the water surface and by tensioning the
main support cable to specification.
It should be noted here that the light "Endless Wire"
configuration as used at many gauging stations may be subject to
greater error due to buoyancy than is the case with the heavier
cableway.
If soundings are made from an endless wire using a
depth counter, a check must always be made on the buoyancy
effect of the weight and meter at the particular site before
commencing a measurement in order to ascertain the zeroing
point. Cableways on the other hand if strained to specification
should be more consistent therefore allowing pre-calibration and
permanent marking of weights.
The following method may be used to check the buoyancy effect on
the soundings mark using a depth counter from an endless wire,
cableway or boat:
1. Mark two points on the suspension cable at a known distant
above the bottom of the weight. (Say 1.0 and 2.0 m).
2. With all equipment ready to commence gauging, including
correct tensioning of the traversing cable, lower the gauging
weight and meter assembly into the water until the predetermined
mark (2.0 m) corresponds with the water surface level. (Ensure
the weight is not on bed).
3. Zero the depth counter.
4. Raise gauging weight/meter assembly one metre and ensure that
the second mark and counter agree.
- 61 -
5. Raise the gauging weight and meter assembly until the depth
counter reads negative 2.0 metres.
6. Note where the water surface cuts the weight in line with the
hanger bar and mark if possible.
FIGURE 24 - GAUGING WEIGHT ZEROING LINE
(For depth counter use)
This point becomes the zeroing point for that weight at that
particular installation or for that particular boat.
Note: The above error source only applies when sounding are made
by depth counter.
2.6.4 Cont: (b) The uplifting force applied to the weight by the
river bed in order to activate any mechanical sounding mechanism
such as a bed feeler. This again causes the main support cable
to rise and the horizontal section of the meter support cable to
sag.
(c) Drift of the meter and cable that has not been compensated
for by measuring the drift angle.
(d) Inaccuracies
being horizontal
be made in line
angles not being
in zeroing the counter due to the weight not
(hence the suggestion that any calibration mark
with the hanger bar), or turbulence and drift
measured.
(e) Inaccuracies in determining the bed due to poor sounding
conditions such as moving sand bed, soft bed and steep banks.
(f) Incorrect observations by the observer.
(g) Incorrect booking of observations.
The error due to (c) is covered in section 2.5.1.
Increasing
the weight and so reducing the drift angle can reduce this
error.
This correction however may increase the error due to
(a) and in some cases could increase the overall error.
2.6.5 Width Errors
Width errors can be both instrument and human.
due to:
- 62 -
Errors may be
(a) Mismarking or misinterpretation of marks on the traversing
cable.
(b) Malfunctioning or incorrectly calibrated width counters.
(c) Distances not being measured from the standard distance
datum.
This gives errors in individual measurements where
depths must be taken from a cross-section and when combining
measurements for extrapolation at a particular vertical.
Note: In order to minimise width errors the booker must check
distances
with
the
observer
periodically
throughout
the
measurement and at the end of the measurement.
- 63 -
3.0 CURRENT METERS
The current meter is still the most common instrument used to
determine velocity.
The principle is based upon the relation
between the water velocity and the resulting angular velocity of
the rotor.
By placing a current meter at a point in a stream
and counting the number of revolutions of the rotor during a
measured time interval, the velocity of the water at that point
can be determined.
The number of revolutions of the rotor is
obtained by various means, depending on the design of the meter,
but normally this is achieved by an electric circuit through the
contact chamber. In all types of design the electrical impulse
produces a signal which either registers a unit on a counting
device or an audible signal.
A stop watch or an automatic
timing device measures intervals of time.
Current meters can generally be classified into two main types,
those meters which have vertical axis rotors and are commonly
known as cup type meters (Figure 25) and those which have
horizontal axis rotors and are commonly called propeller - type
meters. (Figure 26).
Comparative tests of the performance of vertical axis and
horizontal axis meters under favourable conditions indicate that
almost identical results will be obtained.
3.1 Cup-type current meter.
The cup-type current meter consists of a rotor revolving about a
vertical shaft and hub assembly, bearings, main frame, a contact
chamber containing the electrical contact, tail fin and a means
of attaching the instrument to rod or cable suspension
equipment.
The rotor is generally constructed of six conical
cups fixed at equal angles on a ring mounted on the vertical
shaft. This assembly is retained in the main frame by means of
an upper shaft bearing and a lower pivot bearing.
The contact
chamber houses the upper part of the shaft and an eccentric
contact that wipes a platinum wire attached to the binding post.
A separate reduction gear, wire and binding post provide a
contact each time the rotor makes five revolutions. A tailpiece
keeps the meter pointing into the current.
Vertical axis current meters do not register velocities
accurately when placed close to a vertical wall.
When held
close to a right bank vertical wall the cup-type meter will
under register because the slower water velocities near the wall
strike the effective concave face of the cups. The converse is
true at a left bank vertical wall.
The cup-type meter also
under registers when positioned close to the water surface or
close to the streambed.
- 64 -
The characteristics of this type of meter are summarized below:
a. Robust instrument requiring little specialised maintenance.
The rotor is replaceable in the field without affecting
performance.
b. The bearings are well protected from silty water by virtue of
the fact that they operate in air pockets.
c. A single
velocities.
rotor
serves
throughout
the
entire
range
of
3.2 Propeller - type current meter
The propeller-type current meter consists of a propeller
revolving about a horizontal shaft, ball bearings in an oil
chamber, the body containing the electrical contact, a tailpiece
with or without a vane and a means for attaching the instrument
to suspension equipment. The meter may be supplied with one or
more propellers, which differ in pitch and diameter and may be
used for various velocity ranges. Also available are component
propellers which automatically compensates for the velocity
projection normal to the measuring section for angles up to 450
and velocities up to 260 km/day. However each propeller must be
checked to ascertain the component it will measure.
The characteristics of this type of meter are summarized below:
a. The propeller disturbs flow less than the cup-type meter
because of axial symmetry with flow direction.
b. The propeller is less likely to become entangled with debris
than the cup-type meter.
c. Bearing friction is less than for vertical shaft
because any bending moment on the rotor is eliminated.
rotors
d. A propeller-type current meter is not so susceptible to
vertical currents as cup-type meter and therefore give better
results when used for boat measurements.
- 65 -
FIGURE 25 - CUP TYPE CURRENT METER
- 66 -
FIGURE 26 - PROPELLOR TYPE CURRENT METER
- 67 -
3.3 Rating of Current Meters.
In order to determine the velocity of the water from the
revolutions of the rotor of the current meter, a relation is
established between the angular speed of the rotor and the speed
of the water that causes it to turn. This relation is known as
the current meter rating.
The usual method of rating a current meter is to tow it through
still water in a rating tank and observe the time of travel and
the number of revolutions in a given distance.
The number of
revolutions per second and the corresponding velocity are then
computed.
When these two quantities are plotted against each
other an equation is derived by establishing a line of best fit
"through" all the plotted points. A rating table is prepared by
using this equation.
Thiess Services uses the Hydrological Services rating tank at
Liverpool, NSW to rate each current meter. Meters are re rated
every two years or more often if required.
3.4 Care of Current Meters
To ensure reliable observations of velocity are obtained it is
necessary to maintain the current meter in good condition. Good
maintenance practices may be summarised as follows,
a. Before and after each discharge measurement, examine the
meter cups or propeller, pivot, bearing and shaft for damage,
wear and faulty alignment.
b. Clean and oil meters after use. (See section 3.5)
c.
Clean
the
meter
immediately
after
each
measurement,
especially if it is used in sediment laden water. For cup type
meters the surfaces to be cleaned and oiled are the pivot
bearing, pentagear teeth and shaft, cylindrical shaft bearing
and thrust bearing at the cap. (See section 3.5)
d. After oiling and adjusting, spin the rotor to make sure that
it operates freely.
Identify and correct the trouble if the
rotor stops abruptly.
e. Record the duration of spin for a cup-type meter.
A
significant decrease in the duration of the spin indicates that
the pivot or pivot bearing require replacement.
f. Keep the pivot and pivot bearing separate when the meter is
not in use.
g. Replace worn pivots
h. Limit on-site repairs to minor damage only.
This is
particularly the case with a propeller where small changes in
shape can effect the rating. In cup-type meters minor dents in
the cups can often be straightened
- 68 -
i. Repairs to badly sprung yokes, bent yoke stems, misaligned
bearing tailpieces and propellers should be carried out in the
workshop.
j. Damaged plastic propellers should be replaced.
k. If a suspension measurement is being carried out, check the
meters balance on the hanger bar and the alignment of the rotor
when the meter is on the hanger bar.
Note: The balance should be observed with the meter submerged.
The calibration and maintenance of vertical axis type current
meters is presented in detail in a U.S.G.S. publication by
"Smoot and Novak".
A copy of this publication is available in
the appendix.
Also available are manuals for each propeller
type meter, however these are not as detailed as the U.S.G.S.
publication.
3.5 Maintenance and repair of the Gurley Current Meter.
This section has been prepared by Andy Keep to provide
information on variations to recommended procedure, as described
by "Smoot and Novak" which have of necessity become accepted
practice within Thiess Services.
"The current meter is undoubtedly the Hydrographers tool of
trade.
It is the one indispensable item upon which the
information he provides will ultimately depend.
An inaccurate
current meter, whether rendered so by neglect, damage, incorrect
adjustment or any other cause, must yield incorrect data.
At
best this will result in delay, wasted effort and consequent
expense and at worst could have far reaching effects involving
considerable cost, inappropriate action and inconvenience in
many quarters."
The current meter is more prone to the development of error due
to neglect or inexpert operation than virtually any other item
that is used in conjunction with it.
This comparatively
delicate instrument should be treated at all times with the care
and respect that it undoubtedly deserves and it is the aim of
this section to ensure that this is the case.
Referring now to Messrs. Smoot and Novak's "Calibration and
Maintenance of Vertical Axis type Current Meters", pages 1 to 8
contain many interesting facts and useful information which any
person involved in the operation of the Gurley meter would do
well to read and digest.
Points made in the Introduction on
Page 1 are pertinent, and are worthy of even greater emphasis.
Under the heading "Description of the Small Price Current
Meter", sub-heading "Pivot", page 5, it should be stated that
not many, if any, pivots currently in use in Commission meters
are of stainless steel. Many are instead made of silver steel,
whilst others have been made form 3/16" diam. high tensile steel
- 69 -
bolts. Both these materials are liable to rust, therefore care
must be taken to remove the pivot, wipe it dry and either apply
a penetrating oil or spray it with C.R.C., Formula 4 or similar
fluid, immediately after the measurement is concluded.
Some
operators then prefer to place the oiled pivot in its rack in
the meter box, which appears preferable to replacing it in the
meter, the more so if the party is to travel some distance
before the meter is next used.
Then, under sub-heading "Pivot Bearing", page 5. Tungsten
carbide, the material of which the pivot bearing is made, is
second only to diamond in its degree of hardness. It can be refaced only using precision equipment which, paradoxically,
employs a small, high-revving arbor of soft material to restore
the bearing surface.
Those who feel moved to clean out the
pivot bearing of their meter with a needle or a long pin need
have no fears that they will damage the surface.
Tungsten
carbide is, however, an alloy of steel and is therefore
vulnerable to rust, so the pivot bearing must also be oiled or
sprayed immediately after use.
"Binding Posts", page 5.
The type of brush contact described here was fitted to all
SR&WSC Gurley meters prior to the late 1950's, at which stage it
was identified as the reason for signal difficulties in some
streams, mainly in the north of the State, due to the formation
of silver nitrate on the contacts through electrolytic action.
The stainless steel-silver combination was then replaced by a
platinum wire that solved the problem.
"Assembly and Disassembly of the Small Price Current Meter".
A few points in this section are worthy of comment, e.g. Step 3.
The letter "S" mentioned here seems to have been replaced by the
letter "5" on most Gurley meters operating in Victoria.
Step 4. Everybody should now know why there is a hole in the
shaft of the Gurley meter, and hopefully the question will not
be asked again for some years.
Unfortunately there are also
grooves and scores on some of these shafts where pliers have
been used with rather too much enthusiasm in the absence of a
tightening pin. There is absolutely no need to screw this shaft
in as if it was a cylinder head bolt. It will remain in place
if tightened with just enough tension to make sure that it is
right home, and if this recommendation is followed there will be
no need to get out the Stillsons when it next requires to be
removed.
Step 9.
Here again, there is no need to apply the
contact chamber cap with unnecessary force.
The cap, in its
original condition is knurled, indicating that it is to be
tightened by hand, and as the thread is very fine, it should be
possible to obtain ample pressure to seal the joint without the
aid of pliers.
"Disassembly".
- 70 -
The authors have made two extremely valid points in this
section, which should be carefully observed to avoid damage as
described.
A sound alternative practice is to commence
disassembly by removing the contact chamber complete with cap.
"Inspection and Repair of Current Meters".
It will be noted that the phrase "should be replaced with a new
one" keeps recurring throughout this section, which we may take
as an indication that the authors are blissfully unaware of the
financial
drought
which
has
quite
recently
overtaken
Hydrographic activities in this State.
It is probably fair
comment to suggest that if the advice offered here were to be
taken literally, the only original part remaining on some
current meters may be the contact chamber cap. Fortunately for
the continuation of our activities it has been proved repeatedly
that, contrary to the recommendations contained in this section
of the text, almost any component of a Gurley current meter can
be satisfactorily reconditioned or repaired without any serious
detrimental effect to the original sensitivity of the meter, and
with minimal effect as regards it's ratings, which of course
must always be re-established in such cases.
Rotor and Shaft Alignment
The text specifically describes procedures for identifying this
fault.
Should the shaft be bent in its thinner top section as
will be more commonly the case, it is best straightened by
gripping the full length of this section in a 3 jaw drill chuck
and exerting gentle pressure in the appropriate direction, from
time to time checking alignment by operating the drill
momentarily.
Alternatively a 1/8* diameter hole of sufficient
length to fully accommodate the narrow portion of the shaft may
be drilled in a metal block in which the narrow section can be
inserted for full support, and pressure exerted to restore
alignment, the shaft being rotated by hand in order to check on
progress.
A bend in the heavier section of the shaft may be dealt with in
much the same manner, however as the repair must be perfect it
is strongly suggested that it be placed in experienced hands.
Eccentricity, presumably meaning a buckle, in the bucket wheel
can usually be corrected but repairs must not be attempted
whilst the wheel is still fitted to the meter or a bent shaft
will probably result.
Remove the wheel from the meter, then
remove its hub assembly and mount the wheel between two nuts on
the threaded end of a long 3/8" diameter precision bolt.
The
buckle can then be removed using both long nose and standard
pliers.
Sprung Yoke
A sprung yoke is usually indicated if the screwed sleeve, which
raises the cup bearing from the pivot, appears to screw down
1/8" or more before it lifts the wheel assembly. A check can be
carried out using a straight piece of 3/16" diameter steel rod.
- 71 -
Strip the yoke, and insert the rod through the pivot hole so
that its other end passes through the hole vacated by the
contact chamber. If the rod is located centrally in the contact
chamber hole, all is well.
If not, the yoke may be either
sprung or twisted, and the position of the rod in the hole will
indicate which way the yoke must be bent to correct the
situation.
Controlled leverage should be used rather than
impact, in other words don't belt it back into line with the
hammer.
Damaged Cups.
This is a common problem which can and should receive regular
attention.
Techniques and tools which can be used to re-shape
damaged cups are available in each centre. Statements have been
made that minor damage to the bucket wheel of the Gurley meter
has negligible effect on the rating. The authors of the text do
not appear to agree, and as accuracy is a prime consideration
the risk should not be taken when with a little effort it can be
avoided.
Contact Chamber.
The text is self-explanatory and deals with the component and
its contents quite fully, therefore little additional comment is
necessary.
It may be worthy of mention that the upper bearing
can be re-bushed very easily and at little expense if an undue
amount of wear should occur at that point.
Pivot and Bearings.
It seems from the text that the meter pivot should be changed at
an earlier stage of wear than some operators may have believed.
There is said to be ample evidence to suggest that a worn pivot
has quite minimal effect on sensitivity in velocities between
about 15 Km/day and 40 Km/day, and no effect above that stage,
but this should not be used as an excuse for failing to change
the pivot when wear exceeds the specified limit. Whereas use of
a worn pivot will do little harm to the pivot bearing,
persistence with a worn pivot bearing will rapidly destroy the
pivot, and in cases where a meter appears to be consuming pivots
at a fast rate the pivot bearing should be immediately suspected
and inspected.
A quick and usually accurate check can be
carried out without fully dismantling the meter by feeling the
bearing surface with the point of a needle or a long pin.
If
roughness or pitting can be felt, the bearing should be
replaced.
Here again,
Hydrographic
procedure is
of damage to
the authors do not appear to favour long-standing
practice of replacing the cup bearing, although the
simple and can be followed with absolutely no risk
any component.
Lubrication.
- 72 -
Again a self-explanatory text which requires little elaboration.
There are numerous lubricating fluids which have proved most
satisfactory in this regard, notably penetrating oils such as
Penetrene, Three-in-One, Burrsthred etc. and, more recently,
spray can products C.R.C., Formula 4 etc. which also dispel
residual water and assist electrical efficiency.
Spin Tests.
Earlier papers provided by the makers, W. & L.E. Gurley, are
recalled in which the practice of spin-testing meters is
heartily condemned as pointless, indicative of nothing and
damaging to the meter.
The authors obviously do not agree,
however it should be noted that later in this section they also
describe the meter test recommended by Gurley in the papers
referred to, i.e. by rotating the yoke and watching for movement
in the bucket wheel.
No opinion is therefore put forward on
this matter, and the choice of test is left to individual
operators.
Routine Cleaning and Oiling of Current Meters.
This section is quite comprehensive and contains much useful
information that should be noted and observed.
General
Bearing wear both above and below the bucket wheel is promoted
by the undesirable practice of allowing the wheel to spin freely
in the wind. There are two reasons (a) the "wear per rev" rate
of any bearing increases as the rotation rate increases, and (b)
the effective weight of the wheel assembly in such a situation
is greater than when immersed, as it lacks the buoyancy effect
produced by immersion, thus the load on the lower bearings,
normally many tonnes/sq. cm., is increased.
Do not allow the
meter to spin uncontrolled during a pause in the measurement.
Lower it into the water or lay it gently on its side.
Apart from the obvious indications of shaft bearing wear, other
signs are (a) uneven signal duration or missed signal impulses,
and (b) a buzzing or vibration when the meter is spun briefly.
The effect of this wear on the rating can be minimised by
keeping the bearing well lubricated until repairs can be done.
NEVER dump the meter assembly on the ground or subject it to any
other form of suddenly arrested downward motion. See note above
re. phenomenal load which normally exists at the support point,
then imagine how many times that load will be multiplied by any
such treatment.
- 73 -
REFERENCES
W.M.O. Manual on Stream Gauging
HERSCHY, R.W. Streamflow Measurement
I.S.O. 748 Liquid Flow Measurement in Open Channels
I.S.O. 100 Establishment and Operation of a Gauging Station
PUBLIC
ORKS
DEPARTMENT,
Measurement and Rating
W.A.
Bulletin
No.
1
Discharge
U.S.G.S. Calibration and Maintenance of Vertical - Axis Type
Current Meters
OTT. Bulletin No. HLe 120/4 - OTT C31 Current Meter
HYDROLOGICAL SERVICES. Instruction Manual - OSS
Meter.
- 74 -
B1
Current