Chapter 3.pdf

Hydrometric Measurement
HWRE-3122
Mengistu .Z (MSc in Hydraulic Engineering )
Lecturer @ Hydraulic and Water Resources Engineering
department
Mizan Tepi university
Email: mengistu.zantet@gmail.com
mengistuzantet@mtu.edu.et
P.O.Box: 260
Tepi, Ethiopia
15-Dec-22 1

3.1 General aspects of Hydrometric Measurement
1) Introduction
2) Components of stream flow measurement
3) Flow measuring site selection
4) Measurement techniques
5) Flow velocity measurement
6) discharge computation
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3.2 INTRODUCTION
 It was seen that precipitation, evaporation and
evapotranspiration are all difficult to measure exactly
and the presently adopted methods have severe
limitations.
In contrast the measurement of streamflow is the
only part of the hydrologic cycle that can be
measured accurately.
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Cont..
A stream can be defined as a flow channel into which
the surface runoff from a specified basin drains.
Streamflow is measured in units of discharge (m3/ s)
occurring at a specified time and constitutes historical
data.
The measurement of discharge in a stream forms an
important branch of Hydrometry, the science and
practice of water measurement
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Measurement of River Flows
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lecturer@ Hydraulic and water
resources Engineering Department 5
 The sections where river measurements are carried out
are known as stream gauging stations.
 A network of these stations is established to collect
data about river flows of a region.
 Stream flow records are the continuous data of flow
passing through a particular section on the stream.

Necessity of stream flow measurement.
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 For effective and efficient water resource assessment
 For management and proper river basin planning
 For the development of flood forecasting system.
To establish stage discharge relationship.

Generally,
Continuous stream flow records are necessary for:
In the design of water supply systems
 In designing hydraulic structures
 In the operation of water management systems
In estimating sediment or chemical loads of streams
To determine the magnitude and variability of surface waters.
7

2) Components of stream flow measurement
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3.Location of stream measurement
 The location of gauging sites primarily depends upon
the purpose of data collection.
 If the site is needed for a specific project, the
general location will be in the vicinity/near of the
project.

Cont..
If a network of gauging stations is to be established to
study the general hydrology of a region and
For planning and design of various projects, careful
planning is required to identify locations so that
optimum information is obtained for the resources
deployed in the data collection.
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4.Stream flow measurement techniques
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Stream flow measurement techniques can be broadly
classified into two categories as
1) Direct determination and
2) Indirect determination.

1.Direct flow measurement techniques
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A. Area-velocity methods
B. Dilution techniques
C. Electromagnetic method, and
D. Ultrasonic method.

2.Indirect flow measurement techniques
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a) Hydraulic structures, such as weirs, flumes ,gated
structures and
b) Slope area method Measurement

A) Area-velocity methods
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This is the direct methods computing the discharge in
stream by measuring its velocity and area of flow .
Since velocity vary for entire river channel it is
important to divided the channel in to different section
and the summation of this partial discharge will give river
discharge

Cross-sectional Area (A)
In wide and/or irregularly shaped channels, the cross-
sectional area is divided into several segments.
 The area of each segment is the product of the
width(b) of the segment and its average depth (h)
1. Rectangle area = b* d
2. Trapezoidal area = (
𝑏1+𝑏2
2
) *d
3. Triangle area =
𝑏∗𝑑
2
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The areas of the segments are summed to determine the
total cross-sectional area
A= 𝒂
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Flow Velocity computation (V)
The flow velocity in stream can be measured by
1) Surface and subsurface floats
2) Pressure instruments and
3) Current meter
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1) Surface and subsurface floats
A floating object on the surface of a stream when timed can yield
the surface velocity by the relation V=
𝑆
𝑡
where S = distance travelled in time t.
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Finally , flow velocity is
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The surface velocity(Vs) is equal to 1.2(average
Velocity, V) i.e. Vs = 1.2V and Flow velocity
from channel is V = 0.8 Vs.

Restriction of floats
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Used when high accuracy is not required
 Used at uniform cross-section and free of surface disturbances
The test section of the channel length is measured at the beginning,
midpoint and end
A float is released at sufficient distance upstream to attain the
stream velocity before it enters the test section
 The time when the float passes each section is recorded and the
average velocity is determined

2) Pressure instruments
The velocity head of the moving water is converted in
to pressure head with the use of certain devices ,called
pressure devices.
The pressure head can measured and velocity calculated
by equating their pressure head(in meters of water) to
𝒗𝟐
𝟐𝒈
A pitot is example of this kind of instrument, but its use
is restricted only to pipes or experimental channel 21

3) Current meter
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Current meter is the best instrument for measuring the velocity of
natural stream.
V = a + bN where N is the rotation of the propeller (revs per
sec) a and b are coefficients determined by calibration in an
experimental flume.

Calibration
The relation between the stream velocity and
revolutions per second of the meter as is called the
calibration equation
Typically the vales of a and b for diameter size (cup
type) 12.5 cm is a = 0.65 and b= 0.03 and for cup
diameter of 5cm is a=0.3 and b=0.003
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Field Use
The velocity distribution in a stream across a vertical
section is logarithmic in nature.
In a rough turbulent flow the velocity distribution is
given by
where v = velocity at a point y above the bed, v* = shear
velocity and ks= equivalent sand-grain roughness
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Generally velocity is obtained from
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 In shallow streams of depth up to about 3.0 m, the velocity measured at 0.6 times
the depth of flow below the water surface is taken as the average velocity v in the
vertical
 In moderately deep streams the velocity is observed at two points; (I) at 0.2 times
the depth of flow below the free surface (v0.2) and (ii) at 0.8 times the depth of flow
below the free surface (v0.8).
 In rivers having flood flows, only the surface velocity (vs) is measured within a
depth of about 0.5 m below the surface. The average velocity v is obtained by using
a reduction factor K as
 The value of K is obtained from observations at lower stages and lie in the range of
0.85 to 0.95

Sounding Weights
Current meters are weighted down by lead weights
called sounding weights .
Sounding weights come in different sizes and the
minimum weight is estimated as
where W = minimum weight in N, v = average stream velocity in
the vertical in m/s and d = depth of flow at the vertical in meters.
Advantages: to enable them to be positioned in a stable
manner at the required location in flowing water 26

Mean and Mid section methods
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Finally , Discharge is
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The cross-sectional area of flow is then multiplied by the average
velocity to get the flow rate

Example#1
The data pertaining to a stream-gauging operation at a
gauging site are given below.
The rating equation of the current meter is
v = 0.51 Ns + 0.03 m/s where Ns is revolutions per
second.
Calculate the discharge in the stream.
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Solution
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Eample#2
• The following data were collected for a stream at a
gauging station. Compute the discharge.
Rating equation of current meter: v = 0.3 N + 0.05, N = rps,
v = velocity, (m/sec), Rev.-Revolutions, Sec time in seconds. 31

Solution
The discharge in each strip, ΔQ = (b*d) V, where V is
the average velocity in each strip,.
In the first and the last strips (near the banks) where
the depth is shallow = 0.6d, and in the other five
intermediate strips (with deep water), V =0.2d
+0.8d/2.
Width of each strip, b = 3 m, mean depth of strip = d,
and the total discharge, Q = Σ ΔQ = 20.6 cumec,
32

Cont.…
Depth
from one
end of
water
surfaces
(m)
Depth
d
in(m)
Immersing current meter below water
surfaces
Average
velocity in
strip
v(m/s)
Discharge in
strip
Q= (b*d)*v
Where
b=3m
Depth=x*d in m
(x=0.2,0.6 and
0.8)
Revolution R Time(t)
seconds
N=R/t
rps
V=0.3N+0.05 (m/s) Vav
=(v1+v2)/
2
3 1.4 0.84 12 50 0.24 0.122 0.122 0.52
6 3.3 0.66 38 52 0.73 0.269 0.233 2.16
2.64 23 55 0.42 0.176
9 5 1 40 58 0.69 0.257 0.236 3.54
4 30 54 0.56 0.218
12 9 1.8 48 60 0.80 0.290 0.259 7
7.2 34 58 0.59 0.227
15 5.4 1.08 34 52 0.65 0.245 0.238 3.85
4.32 30 50 0.60 0.230
18 1.8 0.76 35 52 0.67 0.251 0.234 2.68
3.04 30 54 0.56 0.218
21 5.4 1.08 18 50 0.36 0.158 0.158 0.86
Total =20.60
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Exercise #1
The following data are obtained from the current meter
gauging of a stream, at a gauging station.
Rating equation of current meter: v = 0.2 N + 0.04, where N = rev./sec, v =
velocity (m/sec). Compute the stream discharge. 34

Exercise #2
The following data were collected for two verticals in a
stream at a gauging station.
Rating equation of the current meter; v = 0.7 N + 0.03
where N = rev./sec, v = velocity (m/sec).
 Compute the discharge in the elemental strips by
(i) the mid-section method (ii) the mean-section method
35

B) Dilution techniques or chemical
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Tracer dilution discharge measurements rely on the
conservation of mass law.
 A tracer of known concentration is injected into a
stream at a constant rate for a predetermined period of
time upstream from the site of interest, which usually is a
gagging station.

cont..
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Concentrations of the tracer are measured before,
during, and after the tracer passes the site.
Typically, the tracer concentrations rapidly increase,
remain steady, and then decline rapidly at the downstream
section.
 This form of tracer-dilution discharge measurement is
called the constant rate method. And Other method is

Steady state condition
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Tracers characteristic
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 It should not be absorbed by the sediment, channel
boundary and vegetation.
 It should not chemically react with any of the above
surfaces and also should not be lost by evaporation.
 It should be non-toxic.
 It should be capable of being detected in a distinctive
manner in small concentrations.
It should not be very expensive.

Cont..
The tracers used are of three main types
1) Chemicals (common salt and sodium dichromate are
typical)
2) Fluorescent dyes (Rhodamine-WT and Sulphur-
Rhodamine B Extra are typical)
3) Radioactive materials (such as Bromine-82, Sodium-
24 and Iodine-132).
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Example#1
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A 25 g/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
Using constant rate injection Methods
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Example#2
A 500 g/litter solution of discharge was used chemical
traces dosed at a constant rate of 4 litter per second
and down stream section ,the equilibrium concentration
was measured as 4 part per million (ppm) .
Estimate the discharge in stream
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Solution
Given C1= 500g/l=0.5kg/l
C2=4ppm=4 ∗ 10−6
Qt=4litter per second=4 ∗ 10−3
𝑚3
/s
C0= 0
Q= 499.996~=500m3
/sec
44

Exercise #1
A 200g/litter solution of common salt was discharge
in to stream at constant rate of 25 litter per second
,the background concentration was found to be
10ppm the salt concentration was found to reach on
equilibrium values of 45 ppm.
Estimate the discharge in the stream
45

Length of Reach for injection
The length of the reach between the dosing section and
sampling section should be adequate to have complete mixing of
the tracer with the flow.
This length depends upon the geometric dimensions of the
channel cross-section, discharge and turbulence levels.
An empirical formula suggested by Rimmar (1960) for
estimation of mixing length for point injection of a tracer in a
straight reach is
15-Dec-22 46

where L = mixing length (m), B = average width of the stream (m), d =
average depth of the stream (m), C = Chezy coefficient of roughness and g =
acceleration due to gravity.
 The value of L varies from about 1 km for a mountain stream carrying a
discharge of about 1.0 m3/s to about 100 km for river in a plain with a
discharge of about 300 m3/s.
 The mixing length becomes very large for large rivers and is one of the
major constraints of the dilution method.
 Artificial mixing of the tracer at the dosing station may prove beneficial for
small streams in reducing the mixing length of the reach
47

Example#1
It is proposed to adopt the dilution methods of stream
gauging hydraulic properties at average flow are as
follows
Determine safe mixing length that has to be adopted
for stream section 48
Width (m) Depth(m) Discharge (m3/s) Chezy coefficient
45 2 85 20 to 30

Solution
Given B= 45m, given
g=9.81 m/s ,Known
d=2m , given
C= 30 ( for safe length) given
Required: Length of reach (L)
substituting the parameter all above in the equation
L= 10974.45m~10.975km~11km is safe length
49

Advantages of Dilution techniques
 The discharge is estimated directly in an absolute
way.
 It is a particularly attractive method for small
turbulent streams, such as those in mountainous areas.
 It can be used as an occasional method for checking
the calibration, stage-discharge curves, etc. obtained by
other methods.
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C) Electromagnetic method
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The method is based on the Faraday’s principle that an emf is
induced in the conductor (water in the present case) when it
cuts a normal magnetic field.
Large coils buried at the bottom of the channel carry a current I
to produce a controlled vertical magnetic field
Electrodes provided at the sides of the channel section measure
the small voltage produced due to flow of water in the channel.

Cont..
It has been found that the signal output E will be of
the order of millivolts and is related to the discharge Q
as
where d = depth of flow, I = current in the coil, and n,
K1 and K2 are system constants.

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Cont..
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Advantages of Electromagnetic method
1) It gives the total discharge once it has been calibrated,
2) Suited for field situations where the cross-sectional properties
can change with time due to weed growth, sedimentation, etc.
3) Best for tidal channels where the flow undergoes rapid changes
both in magnitude as well as in direction.
4) can measure the discharge to an accuracy of ±3%, the
maximum channel width that can be accommodated being 100
m. The minimum detectable velocity is 0.005 m/s.

Disadvantages of Electromagnetic method
The method involves sophisticated and expensive
instrumentation and
Has been successfully tried in a number of installations
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D) ULTRASONIC METHOD
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This is essentially an area-velocity method with the average
velocity being measured by using ultrasonic signals.
The method was first reported by Swengel (1955), since then
it has been perfected and complete systems are available
commercially.
 Consider a channel carrying a flow with two transducers A
and B fixed at the same level h above the bed and on either side of
the channel

Cont.…
These transducers can receive as well as send ultrasonic
signals. Let A send an ultrasonic signal to be received at B
after an elapse time t1 Similarly, let B send a signal to be
received at A after an elapse time t2.
 If C = velocity of sound in water,
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cont..
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procedures of ultrasonic method
Thus for a given L and G, by knowing t1 and t2, the
average velocity along the path AB, i.e., v can be
determined.
It may be noted that v is the average velocity at a height
h above the bed and is not the average velocity V for
the whole cross-section
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Advantages ultrasonic method
It is rapid and gives high accuracy.
It is suitable for automatic recording of data.
It can handle rapid changes in the magnitude and
direction of flow, as in tidal rivers.
The cost of installation is independent of the size of
rivers. 60

Disadvantages ultrasonic method
The accuracy of this method is limited by the factors
unstable cross-section,
 fluctuating weed growth,
 high loads of suspended solids,
 air entrainment, and
 salinity and temperature changes.
•
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15-Dec-22

A) Flow-Measuring Structures
The basic principle governing the use of a weir, flume
or similar flow-measuring structure is that these
structures produce a unique control section in the
flow.
 At these structures, the discharge Q is a function of
the water-surface elevation measured at a specified
upstream location
where H = water surface elevation measured from a specified
63

where
H = water surface elevation measured from a
specified datum.
Thus, for example, for weirs, Eq. (4.20) takes the
form
where
H = head over the weir and
K, n = system constants
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15-Dec-22

The various flow measuring structures can be broadly
considered under three categories:
a) Thin-plate structures
b) Long-base weirs
c) Flumes
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Slope-Area Method
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Cont..
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Example #1
During a flood flow the depth of water in a 10 m wide
rectangular channel was found to be 3.0 m and 2.9 m at
two sections 200 m apart. The drop in the water-surface
elevation was found to be 0.12 m. Assuming Manning’s
coefficient to be 0.025.
Estimate the flood discharge through the channel.
70

Solution
Using suffixes 1 and 2 to denote the upstream and
downstream sections respectively, the cross-sectional
properties are calculated as follows:
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Cont.
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Over all calculation is below in the table
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Example#2
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During the high flows, water surface elevation of small stream
were noted at two section A and B, 10km apart ( A is upstream of
B) . The appropriate eddy loss coefficient are 0.3 for gradual
expansion and 0.1 for gradual contraction. These Elevation and
other salient Hydraulic properties are given below
Estimate the discharge in the stream ,assuming meanings
roughness coefficient n as 0.02
Section Water surface elevation (m) Area of cross- section (m2) Hydraulic Radius(m)
A 104.771 73.293 2.733
B 104.500 93.375 3.089

Solution
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15-Dec-22

THE END
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