STREAM FLOW MEASUREMENT
Discharge (Ft 3 ./ Sec.)
This is a product of mean velocity by the cross-section area of flow
Mean Velocity
It is the average of velocities at the two segments.
Stage
It is the vertical depth of water at the gauging point. The stage is permanently fixed at the gauging point and should not be disturbed during the metering process.
Control
The control is a cross-section a reach of river channel that determines the relationship between stage and discharge` at the section and fall some distance up stream.
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Metering Section the metering section is the cross-section of the stream where the discharge is measured. the Discharge measured is plotted against the stage recorded.

STREAM FLOW MEASUREMENT

STREAM FLOW MEASUREMENT
Discharge Measurement Methods
 Float Method
 Area Velocity Method.
 Special Methods of Discharge Measurement (Dilution
Gauging)
 Empirical Formulae
 Hydraulic Model Studies
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STREAM FLOW MEASUREMENT
Float Method
 It is the most simple and quick method of surface velocity measurements. The distance travelled during the specific time by the surface flow is measured. If L is the distance moved by the float in T seconds. Then;
V
S
= L/T m/sec
 This surface velocity is multiply by reeducation factor (varies from 0.79 to 0.95) for calculating the average velocity of river.
 This method gives batter results where the flow is stream lined having impervious channel prism.
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STREAM FLOW MEASUREMENT
 The accuracy, ease and cost of discharge measurement depend upon the proper selection of measurement site. Therefore, selection of site is very important. A good metering section should fulfill the following requirements.
i.
The river reach should be fairly straight, on upstream and downstream of the selected cross-section upto at least 4 times the normal width of the river during floods or 0.8 Km, whichever is lesser.
ii.
The river bed and banks must be reasonably stable and free of vegetable, boulders.
iii. The reach of the river both upstream and downstream over a distance of 0.8 Km or 4 times of normal width during floods should be fairly uniform in cross-section at and below the high flood level and bed slops should not be subject to sudden changes.
iv. The site should preferably be away from bridges and other structures.
Which are likely to affect the flow of water.
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STREAM FLOW MEASUREMENT
v.
When a site is situated upstream of a confluence, its distance from the confluence should not be less than three times the maximum width of the channel or 0.8 Km whichever is greater. In case the site is situated downstream of the confluence the minimum distance be the same as on upstream.
vi.
Such site which is subjected to tidal influence, vortices formation, return flow, or any other local disturbances should be avoided.
vii.
The site should be easily accessible at all time of the year.
viii. The site should not be unduly exposed to wind.
ix.
At the selected site, water should flow in a single channel. It should not overflow the banks.
x.
The velocity should be greater than 0.3 m/s and less than 1.2 m/s.
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STREAM FLOW MEASUREMENT
Gauges and its Location
Gauges could be classified into following groups; i) Non-Recording Gauges ii) Recording or Automatic Gauges
Non-Recording Gauges
An observer is required for recording the gauge readings generally twice a day. Due to fluctuation the reading is not reliable. The gauges can be classified as under; a) Measuring Staff Gauges b) Weight Gauges c) Float Gauge d) Hook Gauge e) Pneumatic Gauge f) Crest Stage Gauge
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Staff Gauges
STREAM FLOW MEASUREMENT
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STREAM FLOW MEASUREMENT
Location of Gauges i.
Gauge should be installed upstream of the control, but within the range of its influence.
ii.
The base or support should be rigid and immovable, so that the elevation of the datum is unlikely to change. The section should be stable and uniform.
iii. It should be located where the greatest range of fluctuations in stage could occur.
iv. It should be located in a protected spot, where it may not be damaged by floating ice or debris.
v.
It should be easily accessible. Preferably the stream should flow in one channel only at the gauging site.
vi. The gauge should not be located upstream of the confluence with an other stream near enough to be affected by the back water level from that stream.
vii. The gauge should also not be located within the influence of back water of a dam or power plant, bridge etc. and the reach should be straight.
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STREAM FLOW MEASUREMENT
Discharge Measurement by Area Velocity Method
 This is a direct method of computing the discharge in a stream by measuring the velocity of flow and area of cross-section.
 As the depth and velocity of flow varies along the entire crosssection of the river, therefore the stream section is divided into a number parts.
 For each part, its area and velocity of flow through it is determined and discharge computed separately. By adding these
partial discharges, the total discharge of the river is obtained.
 In certain cases the stage may vary over time therefore, the stager measurement before and after the discharge measurement is inevitable.
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STREAM FLOW MEASUREMENT
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STREAM FLOW MEASUREMENT
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STREAM FLOW MEASUREMENT
 The width of the river is divided into about 20 sub-sections so that no sub-section has more than 10% of the flow.
 At each of the selected sub-division points, the water depth is measured by sounding and the current meter operated at selected points in the vertical to find the mean velocity in the vertical, e.g. at 0.6 depth (one-point method) or at 0.2 and 0.8
depths (two-point method).
 For each velocity measurement, the number of complete revolutions of the current meter over a measured time period
(about 60 s) is recorded using a stopwatch.
 The velocity in a sub section is calculated V = a + bN
 When velocities at all the sub-division points across the river have been measured, the stage is read again.
 Should their have been a difference in stage reading over period of the gauging a mean of the two stages is taken for discharge calculation.
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STREAM FLOW MEASUREMENT
Mean Section Method
 In the mean section method, averages of the mean velocities
in the verticals and of the depths at the boundaries of a section sub-division are taken and multiplied by the width of the sub-division, or segment.
Q = ∑q i
= ∑V.a = ∑ n i=1
(V i-1
+ V i
)
(d i-1
+ d i
)/
2
(b i
– b i-1
)
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STREAM FLOW MEASUREMENT
Mid Section Method
 In the mid section method, the mean velocity and depth measurement at a sun-division point are multiplied by the segment width measured between the mid points of
neighboring segments.
Q = ∑q i
= ∑V.a = ∑ n i=1
V i.
d i
(b i + 1
– b i-1
)/2
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STREAM FLOW MEASUREMENT
Problems in Gauging i.
Small Stream
The depth of flow may be insufficient to cover the ordinary current meter operation.
ii.
Mountain Torrent
Stream with steep gradient and high velocities can not be gauged satisfactory by this method and alternatives means be adopted (Dilatation Gauges).
iii.
Large Rivers
Across wide rivers there is difficulties in locating instrument accurately at the sampling points and in-accuracies may occurs.
Problem in locating the bed of the river may also arise in deep and fast flows.
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STREAM FLOW MEASUREMENT
Example of Velocity – Area Discharge Calculations
Distance b i
(m)
4.0
Depth d i
(m)
0.000
Velocity
V i
(m s-1 )
0.000
Mid Section Method
(b i+1
– b i-1
)/2
0.0
Q i
0.000
Mean Section method
(V i-1
+V i
)/2 (d i-1
+d i
)/2 b i
- b i-1
0.165
0.565
q i
5.0
0.466
9.0
1.131
0.300
4.0
1.493
0.343
1.435
3.0
1.477
12.0
1.740
0.357
3.0
1.864
0.358
1.867
3.0
2.005
15.0
1.993
0.358
3.0
2.140
0.356
2.025
3.0
2.163
18.0
2.057
0.353
3.0
2.178
0.347
2.057
3.0
2.141
21.0
2.057
0.340
3.0
2.098
0.343
1.981
3.0
2.038
24.0
1.905
0.346
3.0
1.977
0.343
1.829
3.0
1.882
27.0
1.753
0.341
3.0
1.793
0.327
1.753
3.0
1.720
Continued….
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STREAM FLOW MEASUREMENT
Example of Velocity – Area Discharge Calculations
Distance b i
(m)
30.0
Depth d i
(m)
1.753
Velocity
V i
(m s-1 )
0.314
Mid Section Method
(b i+1
– b i-1
)/2
3.0
Q i
1.651
(V i-1
+V i
)/2
Mean Section method
(d i-1
+d i
)/2 b i
- b i-1
0.318
1.676
q i
3.0
1.599
33.0
1.600
0.322
3.0
1.546
0.320
1.447
3.0
1.389
36.0
1.295
0.318
3.0
1.235
0.283
1.365
3.0
1.159
39.0
1.436
0.247
3.0
1.064
0.214
1.372
3.0
0.881
42.0
1.308
0.181
3.0
0.710
0.143
1.474
3.0
0.632
45.0
1.640
0.104
3.0
0.512
0.085
1.576
3.0
0.402
48.0
1.512
0.066
3.0
0.349
0.033
0.756
4.0
0.100
52.0
0.000
0.000
0.0
∑q i
=
0.000
20.610 m 3 s -1
∑q i
= 20.054
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STREAM FLOW MEASUREMENT
Dilution Gauging
 Constant Rate Injection Method
 This method of measuring the discharge in a stream or pipe is made by adding a chemical solution or tracer of
known concentration to the flow and then measuring the concentration of the solution downstream where the chemical is completely mixed with the stream water.
 Let c c
0
0
, c
1 and c
2 are chemical concentrations (e.g. g litre-1); is the ‘background’ concentration already present in the water (and may be negligible),
 c
1 is the known concentration of tracer added to the stream at a constant rate q, and c
2 is a sustained final concentration of the chemical in the well mixed flow.
 Thus Qc
0
+qc
1
=(Q + q) c
2
, whence:
Q = (c
1
– c
2
) / (c
2
– c
0
) q
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STREAM FLOW MEASUREMENT
Picture of Dilution Gauging
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STREAM FLOW MEASUREMENT
Dilution Gauging
 Gulp Injection
 An alternative to this constant rate injection method is the
.
 A known volume of the tracer V of concentration c
1 is added in bulk to the stream and, at the sampling point, the varying concentration, c
2
, is measured regularly during the passage of the tracer cloud.
Q = Vc
1
/ ∫ t2 t1
(c
2
– c
0
) dt
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STREAM FLOW MEASUREMENT
Dilution Gauging
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DISCHARGE MEASUREMENT (DILUTATION METHOD)
Example
A 25g/l solution of a fluorescent tracer was discharged into a stream at a constant rate of 10 cm3/s . The background concentration of the dye in the stream water was found to be zero. At a downstream section sufficiently far away, the dye was found to reach an equilibrium concentration of 5 parts per billion. Estimate the stream discharge.
Q = 10cm3/sec =10 x 10-6m3/sec
C1 = 25g/l = 25 x 10-3 kg/l
C2 = 5 x 10-9 kg/l
C0 = 0
Q = (C1-C2/C2-C0) q
Q = [25 x 10-3 – 5 x 10-9] x 10 x 10-6
[5 x 10-9 – 0]
Q = [25 x 10-3 – 5 x 10-9] x 10 x 10-6 = 50 m 3 /sec
[5 x 10-9]

STREAM FLOW MEASUREMENT
Discharge Calculation (Empirical Formulae) i) Rectangular Weir
Q = ⅔ c d
L √2g H 3/2 – With out Velocity of approach ii) For Triangular notch
Q=8/15 c d
√2g tan θ/2 H
L = Length of weir
5/2
H = head of water
V c d a
= velocity of approach
= Coefficient of discharge iii) Broad crested weirs
Q=CLH 3/2
C is coefficient of discharge and its values have to be determined for the range of head over the crest.
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STREAM FLOW MEASUREMENT
Discharge Observation by Empirical Formula
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STREAM FLOW MEASUREMENT
Discharge Observation by Empirical Formula
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STREAM FLOW MEASUREMENT
Stage – Discharge Relationship
 The establishment of reliable relationship between the monitored variable stage and the corresponding discharge is essential at all river gauging stations when continuous flow data record.
 This calibration of the gauging station is dependent on the nature
of the channel section and of the length of channel between the site of the staff gauge and discharge measuring site.
 The natural river conditions rarely remain stable over length of time and therefore, the stage – discharge relationship must be checked regularly and, particularly after flood flows and new discharge measurements should be made throughout the range of stages.
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STREAM FLOW MEASUREMENT
Stage – Discharge Relationship
The Stage - Discharge Relationship can be represented in three ways.
i.
Rating Curve ii.
Rating Table iii. Rating Equation (Q v/s H)
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STREAM FLOW MEASUREMENT
Stage – Discharge Relationship i.
The Rating Curve
 All the measured discharges, (Q) are plotted against the corresponding mean stages (H), on suitable arithmetic scales.
The array of points usually lies on a curve which is approximately parabolic in nature and a best fit curve should be drawn through the points by eye.
 At most of the gauging stations, the zero stage does not correspond to zero flow then suitable stage correction is to be applied.
 A typical rating curve of a river flow has been shown in the picture which is some what parabolic in character.
 The curve does not originate from the origin indicating certain
stage at the zero discharge, meaning there by that some correction to the stage needs to be applied for accurate discharge calculations.
 Impervious flows and well defined un-errodable banks are the positive indicators of a stranded quality Rating Curve.
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STREAM FLOW MEASUREMENT
Rating Curve (Picture)
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STREAM FLOW MEASUREMENT
Stage – Discharge Relationship ii. The Rating Table
 This is the simplest and most convenient form of the stagedischarge relationship for the manual processing of sequential stage recoded increases.
 When a satisfactory rating curve has been established, values of H and Q may be read from this curve at convenient intervals.
A rating table is constructed from this rating curve by taking
(Q) values with appropriate stage incremental values.
 From the table thus prepared discharge cloud be read for any stage-height.
This is very useful tool for discharge measurements during high floods.
 It may however, by clearly noted that stage-discharge relationship (Rating Curve and Rating Table) remained effective untill unless the flow within the banks of a river.
However, if the water speared out then the parameters changes and the relationship developed does not work.
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Rating Table
STREAM FLOW MEASUREMENT
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STREAM FLOW MEASUREMENT iii. The Rating Equation
 The rating curve being parabolic in nature can be represented approximately by following equation of the form:
Q α H b
Q =a H b
If Q is not zero when H = 0, then a stage correction, a realistic value of H0, for Q = 0 must be accounted for;
Q = a (H – H
0
) b
Log Q = Log a(H – H
0
) b
Log Q = Log a + b (H – H
0
) ---------------- (A)
‘a’ and ‘b’ are the constant depending upon the parameters of the metering section and the stage.
Now for measured value of Q and H for different heights.
Log Q1 = Log a+b (H
1
Log Q2 = Log a+b (H
2
– H
– H
0
0
) ------------------- (I)
) ------------------- (II)
All the values are known except ‘a’ and ‘b’
Now calculate the value of these constants from equation (I) and (II) above and by putting these in equation (A). The value of discharge various stage can be
33 calculated through computers.

STREAM FLOW MEASUREMENT
Stage – Discharge Relationship
Irr-Regularities and Corrections
 Discharge depends upon stage and water surface slop, but the
latter is not the same for rising and following conditions, stages due to non study flow (more in rising and less in falling).
 Therefore, two separate Rating Curve (Looped) can be produced and average of the two discharges may be taken for steady flow condition
.
 Non stable bed channel due to scouring and accretion negatively
contribute towards the results of the rating curve. Therefore either the channel prism should be rigged or the rating curve may be revised constantly particularly after the flood season.
 The other Irr-regularity may be caused by non uniform flow
generated by interference in the channel D/s of gauging site. Thus gauging site should so located which can not be effected by the channel confluence.
 Vegetable growth in the gauging reach will also interfere with Q various H relationship.
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STREAM FLOW MEASUREMENT
Looped Rating Curve
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STREAM FLOW MEASUREMENT
Stage – Discharge Relationship
i.
Velocity Area Method.
ii.
Logarithmic Extrapolation (Rating Equations) iii.
The Chezy Formula Q = AV = AC √(RS o
) iv.
The Manning Formula Q = AV = (AR 2/3 S o
1/2 )/ n
The value of n is as under
Concrete lined channel
Unlined earth channel
Straight, stable deep natural channel
Winding natural streams
Variable rivers, vegetated banks
Mountainous streams, rocky beds
0.013
0.020
0.030
0.035
0.040
0.050
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STREAM FLOW MEASUREMENT
Extension of Rating Curve
n
Log Q = Log b + n x Log (H – a)
Procedure
 Assume any value of “a” (Normally in the range of 0.1 – 0.6)
 Corresponding to this value of a, calculate the value of log(H –a)
 Plot the graph between log (H – a) and Log Q
 Depending upon the curvature of graph, assume different values of ‘a’ and repeat the same procedure again.
 Keep on assuming different values of ‘a’ till a single slope is obtained. (reasonability a straight line is obtained)
 Calculate the value of log (H – a) at the required value of ‘H’ for the value of ‘a’ at which graph is of single slope.
 Take offset from point log (H – a) to the curve, calculate the
37 value of Log Q and then determine the value of ‘Q’

STREAM FLOW MEASUREMENT
Extension of Rating Curve
We know that
Now
V = C √RS
AxV = AC√RS
Q = AC√R√S
R = A/P = B x D
B + 2D
For large rivers B is much greater than D. Hence same is neglected.
Thus R = B x D = D
Hence
B
Q = A (C√S)√D
Q = (C√S) A√D
Q α A√D
The graph between these two variables within the known value of stage will be a straight line which can be extended up to any stage height for calculating anticipated peak discharge Q.
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STREAM FLOW MEASUREMENT
Extension of Rating Curve
Procedure
 Calculate the values of A√D corresponding to the given data.
 Select suitable scale and plot the graph between “Q” and
“A√D”
 On the same graph paper, plot the graph between “H” and
“A√D”.
 Locate the point for which the value of discharge is to be calculated on the curve H – A√D.
 Take offset from this point to the H – A√D curve and then to the
Q – axis. Read the value from the scale for the required reading of the stage.
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TREAM FLOW MEASUREMENT
Usage of Rating Curve i.
Determination of available head (H) under different flow conditions (Q) at a dam sites for Hydropower Generation.
ii.
For obtaining a more accurate flood hydrograph for short intervals during a flood period, as discharge measurement at short interval is impossible during high flood.
iii. During floods, sometimes practical difficulties may prevent the observation of discharge, in such situations discharge can be obtained from the stage-discharge relationship.
iv. Extrapolation of stage-discharge relationship to determine the peek discharge (Q) for designing of any hydraulic structure.
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