Transcript STREAM FLOW MEASUREMENT

STREAM FLOW MEASUREMENT
Discharge (Ft3./ 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.
Metering Section
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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;
VS = 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
Requirement of Metering Station

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
Requirement of Metering Station
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)
ii)
Non-Recording Gauges
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)
b)
c)
d)
e)
f)
Measuring Staff Gauges
Weight Gauges
Float Gauge
Hook Gauge
Pneumatic Gauge
Crest Stage Gauge
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STREAM FLOW MEASUREMENT
Staff Gauges
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STREAM FLOW MEASUREMENT
Location of Gauges
i.
ii.
iii.
iv.
v.
vi.
vii.
Gauge should be installed upstream of the control, but within the
range of its influence.
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.
It should be located where the greatest range of fluctuations in stage
could occur.
It should be located in a protected spot, where it may not be
damaged by floating ice or debris.
It should be easily accessible. Preferably the stream should flow in
one channel only at the gauging site.
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.
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
Picture of Current Meter
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STREAM FLOW MEASUREMENT
Cableway System (Current Meter Measurement)
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STREAM FLOW MEASUREMENT
Measurement of Velocity with current meter (The Procedure)

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
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calculation.
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 = ∑qi = ∑V.a = ∑n i=1 (Vi-1 + Vi)/ (di-1 + di)/2 (bi – bi-1)
2
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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 = ∑qi = ∑V.a = ∑n i=1 Vi.di (bi + 1– bi-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
bi (m)
Depth
di (m)
Velocity
Vi (m s-1)
(bi+1 – bi-1)/2
Qi
4.0
0.000
0.000
0.0
0.000
9.0
12.0
15.0
18.0
21.0
24.0
27.0
1.131
1.740
1.993
2.057
2.057
1.905
1.753
0.300
0.357
0.358
0.353
0.340
0.346
0.341
Mid Section Method
4.0
3.0
3.0
3.0
3.0
3.0
3.0
Mean Section method
(Vi-1+Vi)/2
(di-1+di)/2
bi - bi-1
qi
0.165
0.565
5.0
0.466
0.343
1.435
3.0
1.477
0.358
1.867
3.0
2.005
0.356
2.025
3.0
2.163
0.347
2.057
3.0
2.141
0.343
1.981
3.0
2.038
0.343
1.829
3.0
1.882
0.327
1.753
3.0
1.720
1.493
1.864
2.140
2.178
2.098
1.977
1.793
Continued….
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STREAM FLOW MEASUREMENT
Example of Velocity – Area Discharge Calculations
Distance bi
(m)
Depth
di (m)
Velocity
Vi (m s-1)
(bi+1 – bi-1 )/2
Qi
30.0
1.753
0.314
3.0
1.651
33.0
36.0
39.0
42.0
45.0
48.0
52.0
1.600
1.295
1.436
1.308
1.640
1.512
0.000
0.322
0.318
0.247
0.181
0.104
0.066
0.000
Mid Section Method
3.0
3.0
3.0
3.0
3.0
3.0
0.0
∑qi =
Mean Section method
(Vi-1+Vi)/2
(di-1+di)/2
bi - bi-1
qi
0.318
1.676
3.0
1.599
0.320
1.447
3.0
1.389
0.283
1.365
3.0
1.159
0.214
1.372
3.0
0.881
0.143
1.474
3.0
0.632
0.085
1.576
3.0
0.402
0.033
0.756
4.0
0.100
∑qi =
20.054
1.546
1.235
1.064
0.710
0.512
0.349
0.000
20.610
m3 s-1
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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 c0, c1 and c2 are chemical concentrations (e.g. g litre-1);
c0 is the ‘background’ concentration already present in the
water (and may be negligible),

c1 is the known concentration of tracer added to the stream
at a constant rate q, and c2 is a sustained final
concentration of the chemical in the well mixed flow.

Thus Qc0+qc1=(Q + q) c2, whence:
Q = (c1 – c2) / (c2 – c0) 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
‘gulp’ injection, or integration method.

A known volume of the tracer V of concentration c1 is
added in bulk to the stream and, at the sampling point, the
varying concentration, c2, is measured regularly during the
passage of the tracer cloud.
Q = Vc1 / ∫t2t1 (c2 – c0) 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.
Solution: Given data:
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 m3/sec
[5 x 10-9]
STREAM FLOW MEASUREMENT
Discharge Calculation (Empirical Formulae)
i)
Rectangular Weir
Q = ⅔ cdL √2g H3/2 – With out Velocity of approach
ii)
For Triangular notch
Q=8/15 cd √2g tan θ/2 H5/2
L = Length of weir
H = head of water
Va = velocity of approach
cd = Coefficient of discharge
iii) Broad crested weirs
Q=CLH3/2
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
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positive indicators of a stranded quality Rating Curve.
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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STREAM FLOW MEASUREMENT
Rating Table
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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:
Qα Hb
Q =a Hb
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 – H0)b
Log Q = Log a(H – H0)b
Log Q = Log a + b (H – H0) ---------------- (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 (H1– H0) ------------------- (I)
Log Q2 = Log a+b (H2– H0) ------------------- (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
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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 as shown in the figure.
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
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various H relationship.
STREAM FLOW MEASUREMENT
Looped Rating Curve
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STREAM FLOW MEASUREMENT
Stage – Discharge Relationship
Extension of Rating Curve
i.
Velocity Area Method.
ii.
Logarithmic Extrapolation (Rating Equations)
iii.
The Chezy Formula Q = AV = AC √(RSo)
iv.
The Manning Formula Q = AV = (AR2/3 So1/2)/ n
The value of n is as under
Concrete lined channel
0.013
Unlined earth channel
0.020
Straight, stable deep natural channel
0.030
Winding natural streams
0.035
Variable rivers, vegetated banks
0.040
Mountainous streams, rocky beds
0.050
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STREAM FLOW MEASUREMENT
Extension of Rating Curve
ii) Rating Equation
Q = b(H – a)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
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value of Log Q and then determine the value of ‘Q’
STREAM FLOW MEASUREMENT
Extension of Rating Curve
iii) Chezy’s Method
V = C √RS
AxV = AC√RS
Q = AC√R√S
Now
R = A/P = B x D
B + 2D
For large rivers B is much greater than D. Hence same is neglected.
Thus
R= BxD =D
B
Q = A (C√S)√D
Q = (C√S) A√D
Hence
Q α A√D
We know that
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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