This document discusses stream gauging techniques used to measure stream discharge. It begins by explaining that stream flow represents the runoff phase of the hydrologic cycle and is the most important data for hydrologic studies. It then describes various methods for measuring stream stage including staff gauges, suspended wire gauges, automatic stage recorders, and bubble gauges. Common techniques for directly measuring stream discharge are also summarized, such as area-velocity methods using current meters and floats, as well as moving boat methods. Site selection criteria and types of stage data collected are also briefly outlined.

1. 1.3 Stream Gauging
Prepared by
Pradeep Kumawat
Assistant Professor
Civil Engg. Department
Late G. N. Sapkal COE

2. Stream Gauging
The most important phase of hydrological cycle is runoff.
Stream gauging It is the a technique used to measure the
discharge, or the volume of water per unit time, moving through
a channel or stream.
Stream flow representing the runoff phase of the
hydrologic cycle is the most important basic data for hydrologic
studies.
It was seen in the previous chapters that precipitation,
evaporation, transpiration, infiltration and evapo- transpiration
are all difficult to measure exactly and the presently adopted
methods have severe limitations. Interestingly, Run-off or stream
flow is the only part of the hydrologic cycle that can be
measured accurately.

3. Necessity of Discharge measurement:
 By measuring the discharge in river the irrigation water
can be distributed uniformly.
 By measuring flow in rivers flood warning can be issued.
 By measuring flow in canal quantity of loses can be known.
 The quantity of water flowing in the river helps for
future research.

4. Selection of Gauge site
• The bed and banks of the stream should be firm and stable so
as to ensure consistency of area- discharge relationship.
• The bed and banks should be free from vegetable growth,
boulders or other obstructions like bridge piers etc.
• There should be no larger overflow section at flood stage. At all
stages, the total flow is confined to a single channel.
• To ensure good consistency between stage and discharge there
should be a good control section far downstream of the gauging
site.
• The section should be straight and uniform for a length of about
10 to 20 times the width of the stream.
• The stream gauging station should be easily accessible.
• There should be no eddied or abnormality in the flow.

5. Stage Measurement
 Staff Gauge
 Suspended Weight Wire Gauge
 Automatic Stage Recorders
 Bubble Gauge

6. Staff Gauge
The simplest of stage measurements are made by noting the
elevation of the water surface in contact with a fixed graduated
staff.
The staff is made of a durable material with a low coefficient of
expansion with respect to both temperature and moisture. It is
fixed rigidly to a structure, such as an abutment, pier, wall etc.
The staff may be verticalor inclined with clearly
and accurately graduated permanent markings.

7. • The markings are distinctive, easy to read from a distance
and are similar to those on a surveying staff.
• Sometimes, it may not be possible to read the entire range of
water-surface elevations of a stream by a single gauge and
• In such cases the gauge is built in sections at different
locations. Such gauges are called sectional gauges.
• When installing sectional gauges, care must be taken to
provide an overlap between various gauges and to refer all
the sections to the same
common datum.

9. Suspended Weight Wire Gauge
• It is a gauge used to measure the water- surface elevation
from above the surface such as from a bridge or similar
structure. In this, a weight is lowered by a reel to touch the
water surface.
• A mechanical counter measures the rotation of the wheel
which is proportional to the length of the wire paid out. The
operating range of this kind of gauge is about 25 m.

10. Suspended Weight Wire Gauge

11. Automatic Stage Recorders
• The staff gauge and wire gauge described earlier are manual
gauges. While they are simple and inexpensive, they have to
be read at frequent intervals to define the variation of stage
with time accurately.
• Automatic stage recorders overcome this basic objection of
manual staff gauges and find considerable use in stream-flow
measurement practice.

12. Float type Gauge Recorder
• The float-operated stage
recorder is the most common
type of automatic stage recorder
in use. In this, a float operating
in a stilling well is balanced by
means of a counterweight over
the pulley of a recorder.
• Displacement of the float due to
the rising or lowering of the
water-surface elevation causes
an angular displacement of the
pulley and hence of the input
shaft of the recorder.

13. • Mechanical linkages convert this angular displacement to the
linear displacement of a pen to record over a drum driven by
clockwork.
• The pen traverse is continuous with automatic reversing when it
reaches the full width of the chart.
• A clockwork mechanism runs the recorder for a day, week or
fortnight and provides a continuous plot of stage vs time.
• A good instrument will have a large-size float and least friction.
• Improvements over this basic analogue model consists of
models that give digital signals recorded on a storage device or
transmit directly onto a central data-processing centre.
• To protect the float from debris and to reduce the water surface
wave effects on the recording, stilling wells are provided in all
float-type stage recorder installations.

14. Bubble Gauge
•In this gauge, compressed air or gas is made to bleed out at a very
small rate through an outlet placed at the bottom of the river. A
pressure gauge measures the gas pressure which in turn is equal to
the water column above the outlet.
•A small change in the water-surface elevation is felt as a change in
pressure from the present value at the pressure gauge and this in
turn is adjusted by a servo-mechanism
to bring the gas to bleed at the original rate under the new head.
The pressure gauge reads the new water depth which is
transmitted to a recorder.

16. The stage data
 The stage data is often presented in the form of a plot of stage
against chronological time known as stage hydrograph.
 In addition to its use in the determination of stream discharge,
stage data itself is of importance in design of hydraulic
structures, flood warning and flood-protection works.
 Reliable long-term stage data corresponding to peak floods
can be analyzed statistically to estimate the design peak river
stages for use in the design of hydraulic structures, such as
bridges, weirs, etc.
 Historic flood stages are important in the indirect estimation of
corresponding flood discharges. In view of these diverse uses,
the river stage forms an important hydrologic parameter
chosen for regular observation and recording.

17. Fig. Stage Hydrograph

18. STREAM FLOW MEASUREMENT
Stream flow measurement techniques can be broadly
classified into two categories as:
(i) Direct determination
(a)Area-velocity methods
(b)Moving Boat Methods
(c) Dilution techniques
(ii) Indirect determination
(a) Hydraulic, structures, such as weirs,
flumes and gated structures
(b) Slope-area method.

19. 1) Direct Measurement Methods
1.1 Area Velocity Method
• Approximate stream velocities can be determined by floats.
• The most commonly used instrument in hydrometry to
measure the velocity at a point in the flow cross- section is
the current meter.
• It consists essentially of a rotating element which rotates
due to the reaction of the stream current with an angular
velocity proportional to the stream velocity.
• Based on accuracy required, width of the stream is divided
into a number of vertical portions.
• In each portions, velocity is measured at one or more points
along the depth to get a representative velocity.

20. Velocity Measurement by Floats
A floating object on the surface of a stream when timed can yield
the surface velocity by the relation:
Vs = S/t
where, S = distance travelled in time t
Average velocity, Va = 0.85 Vs
Discharge is the product of cross-sectional area and velocity of
water, Q = Va * A
where, Q = discharge [m3/s] and A = cross-section of flow [m2]
This method of measuring velocities while primitive still finds
applications in special circumstances, such as (i) a small stream in
flood, (ii) small stream with a rapidly changing water surface, and
(iii) preliminary or exploratory surveys.

21. • While any floating object can be used, normally specially
made leak proof and easily identifiable floats are used.
• A simple float moving on the stream surface is called a
surface float. It is easy to use and the mean velocity is
obtained by multiplying the observed surface velocity by a
reduction coefficient .
• However, surface floats are affected by surface winds. To get
the average velocity in the vertical directly, special floats in
which part of the body is underwater are used. Rod float in
which a cylindrical rod is weighed so that it can float vertically,
belongs to this category.

22. 1.2 Moving Boat Method
 The measurement of velocity is an important aspect of
many direct stream-flow measurement techniques.
 A mechanical device, called current meter, consisting
essentially of a rotating element is probably the most
commonly used instrument for accurate determination of
the stream-velocity field.
• The present day cup-type instrument and the electrical
make-and-break mechanism were invented by Henry in
1868. There are two main types of current meters.
1. Vertical-axis meters
2. Horizontal-axis meters

23. 1. Vertical-Axis Meters
•These instruments consist of a series of conical cups
mounted around a vertical axis. The cups rotate in a
horizontal plane and a earn attached to the vertical axial
spindle records generated signals proportional to the
revolutions of the cup assembly.
•The Price current meter and Gurley current meter are
typical instruments under this category. The normal range of
velocities is from 0.15 to 4.0 m/s. The accuracy of these
instruments is about 1.50% at the threshold value and
improves to about 0.30% at speeds in excess of 1.0 mls.
•Vertical-axis instruments have the disadvantage that they
cannot be used in situations where there are appreciable
vertical components of velocities. For example, the
instrument shows positive velocity when it is lifted vertically
in still water.

25. 2. Horizontal-Axis Meters
 These meters consist of a propeller mounted at the end of
horizontal shaft. These come in a wide variety of size with
propeller diameters in the range 6 to 12 cm, and can register
velocities in the range of 0.15 to 4.0 m/s. Ott, Neyrtec and
Watt-type meters are typical instruments under this kind.
 These meters are fairly rugged and are not affected by oblique
flows of as much as 15°. The accuracy of the instrument is
about I% at the threshold value and is about 0.25% at a velocity
of 0.3 mls and above.
 The instruments have a provision to count. the number of
revolutions is a known interval of time. This is usually
accomplished by the making and breaking of an electric circuit,
either mechanically or electromagnetically at each revolution of
the shaft.
 The revolutions per second is calculated by counting the
number of such signals in a known interval of time, usually
about 100 s. Present-day models employ electromagnetic
counters with digital signal processing capabilities.

26. Fig. Horizontal-Axis Current Meters

27. 1.3 Dilution Technique
• Two dilution techniques are:
(1) the steady feed method and
(2) the instantaneous, point - source time indigenous
method
 For steady feed method, a solution of tracer material with
concentration C1 is injected at the constant injection rate QT.
 The tracer disperses laterally into the flow and tracer
concentration distribution is similar to as shown in figure.
 At some point X2 downstream, where the tracer material is
approximately uniformly mixed, the flow is sampled
continuously.

30.  If the tracer mixer has properties similar to the water, so that
there are no density gradients, vertical mixing is very rapid
due to turbulence of the flow.
 Theoretically, complete lateral mixing occurs at X but
practically it occurs between 20 to 100 times the channel
widths.
 By instantaneous injection method, a quantity of tracer w, is
injected, instantaneously at section X and time t0.
 The cloud of tracer disperses laterally and longitudinally as
it moves downstream.

32. 2) Indirect determination Methods
1. By Notches, Weirs, Venturiflumes and Spillways
• If the physical and hydraulic conditions at the site permit,
a fixed, un-deformable structure may be constructed to
measure river flow.
• A number of hydraulic structures are used to measure
flows in field conditions

33. Weirs:
• They are used to control upstream water level or for measuring
discharge or for both.
• They produce a critical relationship
between stage and discharge by obstructing channel flow.
• Weirs have a defined cross-section
and hence the computation of discharge is simple.
• The size and cost of the structures increase as the size of the
river increases.
• Site requirements consist of a reasonably straight approach
channel which should be free of excessive sedimentation, weeds
and other aquatic growth.

34. Weirs
• The structure should be rigid, water-tight, normal to the flow
direction, and should be capable of passing high flows without
any damage to its body.
• The stage-discharge relation at the site depends on the
geometrical characteristics of such a structure.
Q = Cd ∙ L ∙ H^n

35. Weirs

36. Venturi Flumes:
• A flume is a flow measuring structure formed by a constriction
in a channel.
• The constriction can be either a narrowing section of the
channel or a narrowing section in combination with a hump in
the invert.
• A unique stage-discharge relationship exists independent of the
downstream conditions.
For a rectangular flume, the discharge of an ideal fluid is
expressed as -
here, H represents the upstream energy and b is the typical width
dimension for the particular cross-section shape of the flume.

39. Flumes:
By introducing suitable coefficients, this equation can be generalized
to the following form –
Where, Cv = coefficient to take in to account the velocity head in the
approach channels, Cs = coefficient to take account of the cross-
section shape of the flume, Cd = coefficient for energy loss, and h =
depth of water, upstream of the flume, measured relative to the
invert level of the throat.

40. ADVANCED TECHNIQUES FOR GAUGE MEASUREMENT
(a) Electromagnetic method, and
(b) Ultrasonic method.