1. i
Dedication
Dedicated to our Holy Prophet Muhammad (S.A.W) who is
always a great Reformer and a source of guidance in every
field of life.

2. ii
Acknowledgment
All our effort on this project and everything else are due to Allah‟s Blessings for his glory
and not our own. Many people have guided us for the completion of this project.
We are very thankful to Engr Muhammad Asif Aslam, Assistant professor, Department
of Civil Engineering UCE&T BZU Multan, for his guidance helping and improving text
of this project and also very thankful to Engr Shakeel Ahmad, Assistant Director
Monitoring, Haveli and Mailsi Canal Circle, for guiding, without which it would not have
been possible to complete this study.
We are indebted to M.Izhar Lab attendant Civil Engineering Department and Dr.Abuzar
Abid Department of Basic Science UCE&T BZU Multan for their assistance and giving
valuable guidance.

3. iii
Executive summary
Prediction of stage-discharge relation or a rating curve is of immense importance for
reliable planning, design, and management of most of the water resources projects.
Discharge rating curves may be simple or complex depending upon the river reach and
flow regime. These relations are typically developed empirically from periodic
measurements of the stage and discharge. These data are plotted on the graph to define
the rating curve for the stream. Measurement of discharge by this method involves a two-
step procedure. Once the stage-discharge relationship is established, the subsequent
procedure consists of measuring the stage and obtaining the discharge corresponding to
the stage from the stage-discharge relationship.
The rating curve on a man-made structure is always different with the rating curve made
on the natural streams because in the man-made channel, there is always a constant
discharge and on the natural streams, the discharge is changing with time. For new
gauging stations, many discharge measurements are needed to develop the stage
discharge relation throughout the entire range of stream flow data.
At Head Ambala, all the three canal passing through the Head are man-made canals. The
selected cross section of each canal is divided into segments and velocity by current
meter and discharge by mid-section method is measured, then the stage discharge data is
plotted on graph to obtain a rating curve and made best fit curve. The laboratory practical
is performed on a rectangular weir to obtain a rating curve between stage and discharge,
generate best fit curve and calibrate it.
The relation between stage and discharge can be affected by a number of factors that can
change shape and position of the rating curve. The factors that affect the rating curve
include changes in channel cross section, scour and silt, growth and decay of aquatic
vegetation, debris jams (an accumulation of logs and other organic debris which blocks
the flow of a stream), variable backwater, rapidly changing discharge, discharge to or
from over bank areas, including additional parameters, such as an estimate of the water
surface slope or the rate of change of the water surface at the gauge.
Aims and objectives
The primary aim of this report is a brief review of design, practice and problems of rating
curve. This report have great practical importance and a high degree of interest, for
improving the estimation of discharge from the stage. All evaluations of discharge are
strongly depend on stage discharge relationships, an improvement on discharge estimates
would obviously make hydrological models more reliable.

4. iv
The three main objectives of the study on the topic of rating curves can be outlined:
1. The first one is to gather information on current discharge then measure the water
level with gauges.
2. The second one to design the rating curve to improve empirically the discharge
records from gauging stations for different conditions, and how to extrapolate the
data beyond the measurement range.
3. The third point includes the difficulties in defining the stage discharge
relationship.
Conclusions
Thus, based upon the obtained results from the study, we concluded that,
1. The observations made from the study suggested that systematic and
continuous discharge data is not actually observed; instead its records are
made from converting the water level data to discharge by using a stage-
discharge relationship.
2. If discharge data are desired for a particular period (e.g. hourly, 15 minute, etc.),
discharges from these time periods are determined by interpolating between the
key stage height points and re-converted to discharge using the rating curves.
3. The zero discharge in the stream “Qo” is a hypothetical value that cannot be
measured in the field.
4. All discharge measurements in open channel cross-sections are not free of errors.
While it is not possible to predict this error exactly, an estimation of its likely
magnitude may be performed by analyzing the individual velocity measurements
that are required to estimate the river discharge. Incorrect or faulty values may
come into record due to instrumental, computational or copying errors.
5. Rating-curves count a number of practical applications in hydrology, hydraulics
and water resources management. For instance, hydrological rainfall-runoff
models are usually parameterized on the basis of concurrent observations of
rainfall and discharge; discharge observations in turn are generally derived from
water-level observation by means of a rating curve.
6. Typically a rating curve is a single log-linear equation.
The equation form is a power curve:
Q = K
ln Q = ln K + n ln D
Where
Q = flow as cfs and D = stage height in ft.
7. The Discharge equations are
a. Buch disty canal Q = 10.66
b. Sikandarabad disty canal Q = 22.07

5. v
c. Kachoor disty canal Q = 6.41
8. The rating curve of these canals is not straight line due to scouring. Silting,
growth of vegetation, back water curve, accumulation of debris, rapidly changing
discharge, over bank flow etc.
9. Peak discharge and low discharge cannot determine in the field due to changing
channel geometry, then extrapolate the rating curve and find peak and low
discharge.
10. The rating curve on a man-made structure is always different with the rating curve
made on the natural streams because in the man-made channel, there is always a
constant discharge and on the natural streams, the discharge is changing with
time.

6. vi
Table of Contents
1. Introduction............................................................................................................. 1
1.1 Description of Simple Rating Curves.................................................................... 2
2. Stage ......................................................................................................................... 4
2.1 Datum of Gauge ...................................................................................................... 5
2.2 Types of Stage / Flood categories........................................................................... 5
2.3 Stage data................................................................................................................. 9
2.4 Basic Requirements for Collecting Stage Data .................................................. 10
2.5 Sources of Stage Measurement Error................................................................. 10
2.6 Site Selection for the Installation Of gauge ........................................................ 12
2.7 Measurement of Stream Stage............................................................................. 13
3. Measurement of Discharge................................................................................... 20
3.1 Historical View of Discharge Measurement....................................................... 20
3.2 Basic Principle in Discharge Measurement........................................................ 20
3.3 Discharge Measurement Method......................................................................... 20
3.3.1 Direct Determination of Stream Discharge ........................................................ 21
3.3.2 Indirect Determination of Stream Discharge..................................................... 35
3.3.3 Hydraulics Structures / Devices........................................................................... 37
4. Rating curve .......................................................................................................... 48
4.1 Types of station control ........................................................................................ 49
4.2 Rating curves for steady uniform flow................................................................ 52
4.3 Rating curve for non-steady and non-uniform flow.......................................... 53
4.4 Difficulties in Defining Stage-Discharge Relationships..................................... 54

7. vii
4.5 Extrapolation of rating curve .............................................................................. 57
5. Calibration of flow measuring devices................................................................ 65
5.1 Lab practical for the calculation of co-efficient of discharge ........................... 66
5.2 Error During Calibration of Discharge Measuring Device .............................. 69
6. Visit to Ambala head Shujaabad canal sub division ......................................... 70
6.1 Buch Disty Canal................................................................................................... 70
6.2 Sikandar Abad Disty Canal ................................................................................. 72
6.3 Kachoor Disty Canal............................................................................................. 75
7 conclusions and recommendations………………………………………………78
7.1 Conclusions……………………………………………………………………….78
7.2 Recommendation…………………………………………………………………79
References...................................................................................................................... 80

8. viii
List of Figures
Figure 1: Example of a stage-discharge relation................................................................. 1
Figure 2: Height of the water surface above an established datum plane........................... 4
Figure 3: Over Flow the Lowest Natural Stream Bank Somewhere In the Corresponding
Reach................................................................................................................................... 6
Figure 4: Water surface is near or slightly above the top of its banks................................ 7
Figure 5: Flood Stage.......................................................................................................... 7
Figure 6: Buildings are expected to be inundated............................................................... 8
Figure 7: Roads are likely to be closed and some areas cut off.......................................... 8
Figure 8: Extensive flooding Structures may be completely submerged ........................... 9
Figure 9: Device used on ships to determine the depth of water...................................... 14
Figure 10: Vertical-staff gauges........................................................................................ 15
Figure 11: Weight is attached to the end of a steel measuring tape.................................. 15
Figure 12: Stilling well in the river bank.......................................................................... 16
Figure 13: Stage Sensor .................................................................................................... 17
Figure 14: float sensor consists of a tape or cable passing over a pulley ......................... 17
Figure 15: Gauge used to measure the water surface elevation from above the surface
such as bridge.................................................................................................................... 18
Figure 16: Graphical stage recorder.................................................................................. 18
Figure 17: Weight suspended on stranded insulated wire with depth markings and an
ammeter............................................................................................................................. 19
Figure 18: Floats: (a) surface float; (b) canister float for mean velocity; (c) rod float by
mean velocity. (Reproduced with permission from R.W. Herschy (ed.) (2009) stream
flow measurement 3rd edn. © 2009, by permission of Taylor & Francis, oxford.) ......... 22
Figure 19: Price Current meter ......................................................................................... 23

9. ix
Figure 20: Propeller Current Meter................................................................................... 24
Figure 21: Impeller Current Meter.................................................................................... 24
Figure 22: Wading Method for Open Channel ................................................................. 26
Figure 23: (a) Current meter in Culverts (b) Current meter in Bridge.................. 27
Figure 24: Cableway Is Placed Above the Water with Vertical Supports on Each Bank 28
Figure 25: Use of a boat on a stream ................................................................................ 29
Figure 26: Cross Section in Mid-Section Method ............................................................ 30
Figure 27: Cross Section Mean -Section Method............................................................. 32
Figure 28: Electromagnetic Method ................................................................................. 33
Figure 29: Ultrasonic Method........................................................................................... 34
Figure 30: Energy Profile Diagram for Any Open Channel............................................. 36
Figure 31: Sharp-Crested weir having sharp metal blade along the bottom and sides of the
crest................................................................................................................................... 39
Figure 32: Broad crest with no metal blade...................................................................... 40
Figure 33: X-section is reduced gradually........................................................................ 42
Figure 34: Parshall flume.................................................................................................. 43
Figure 35: Free Flow Conditions with Gates of Barrage Are Fully Raised...................... 45
Figure 36: Gates of Barrage Partially Down With Hydraulic Jump Flow Conditions ..... 46
Figure 37: Gates Of Barrage Partially Down With Submerged Conditions..................... 46
Figure 38: Head Regulator................................................................................................ 47
Figure 39: Representation of stage discharge relation...................................................... 49
Figure 40: Example of Section Control ............................................................................ 50
Figure 41: Example of Channel Control........................................................................... 50
Figure 42: Example of an artificial control....................................................................... 51
Figure 43: Example of shifting control............................................................................. 52
Figure 44: Stage-discharge relation for different hydraulic conditions............................ 53
Figure 45: For Permanent Control.................................................................................... 55
Figure 46: (a) Rating curve affected by Variable backwater (uniform channel) (b) rating
curve affected by Variable backwater (submergence of Low water control)................... 55
Figure 47: Rating curve affected by unsteady flow.......................................................... 56
Figure 48: Affected by scour and fill Affected by vegetation growth....... 56

10. x
Figure 49: Example of Low flow extrapolation................................................................ 58
Figure 50: Cross-sectional profile of channel................................................................... 59
Figure 51: Example of double logarithmic extrapolation of rating curve ........................ 59
Figure 52: Extrapolation based on stage-area/stage-velocity technique........................... 60
Figure 53: K versus gauge height ..................................................................................... 61
Figure 54: relation b/w stage and K.................................................................................. 62
Figure 55: Conveyances as f (h) ....................................................................................... 63
Figure 56: Slope extrapolation.......................................................................................... 63
Figure 57: Graph between Q_A and H*3/2 ...................................................................... 67
Figure 58: Graph between logQA and log H.................................................................... 68
Figure 59: Rating Curve of Buch Disty Canal.................................................................. 72
Figure 60: Rating Curve of Sikandar Abad Disty Canal .................................................. 74
Figure 61: Rating Curve of Kachoor Disty Canal ............................................................ 77
List of Tables
Table 1: Comparison of Principal Discharge Equation .................................................... 66
Table 2: Lab Practical for the Calculation of Co-Efficient Of Discharge ........................ 66
Table 3: Log Q and Log H Table...................................................................................... 68
Table 4: Stage and Discharge Buch Disty Canal at Head Ambala................................... 71
Table 5: Stage and Discharge Sikandar Abad Disty Canal at Head Ambala.................... 74
Table 6: Stage and Discharge Kachoor Disty Canal at Head Ambala............................. 76

11. xi
Glossary
A Stream's cross-sectional area
a & b Constants of the current meter
Distance from an initial point on the bank to verticals 4
Distance from an initial point on the bank to verticals 5
Distance from an initial point on the bank to verticals 6
, Distance from an initial point on the bank to verticals 5-6
Β Empirical coefficient
B Clear Water way
C Velocity of sound in water
Coefficient of discharge
Chezy roughness coefficient
D Depth of flow
d Average depth
Depth of flow at vertical 5
, Depth of flow at verticals 5-6
E1 U/S Total Energy Line (H + Va2/2g) – Crest Level

12. xii
E2 U/S Total Energy Line (U/S Water level + ha) – Bottom of Gates
g Acceleration
H Water surface elevation measured
H1 Upstream water surface elevation measured above the weir crest
H2 Downstream water surface elevation measured above the weir crest
h D/S Water level – Crest Level
Equilibrium depth
I Current in the coil
i Intercept on log axis (when log H=0)
j Antilog (i)
N No of sub areas
Ks Equivalent sand grain roughness
K Reduction factor
Conveyance
System constants
L Distance/Length
Ns Revolutions per second of the meter
n Manning‟s roughness coefficient
Q Discharge
Qs Submerged discharge

13. xiii
Q1 Free flow discharge under head H1
Discharge through segment 5
Discharge through segment 5-6
R No of revolutions per second
Hydraulic mean depth
S Distance traveled in time
S Frictional slope
Elapse time
V Mean velocity
Vavg The mean velocity in the given vertical (ft/s)
Mean velocities for small sub areas of the cross section
Component of flow velocity
Vs Surface velocity
The velocity at 0.2 of flow depth (ft/s)
The velocity at 0.6 of flow depth (ft/s)
The velocity at 0.8 of flow depth (ft/s)
V* Shear velocity
V Stream velocity
̅ Mean velocity in vertical 5
, Mean velocity in verticals 5-6

14. xiv

15. 1
1. Introduction
The relationship between the water-surface stage (the water level) and the flow discharge
in an open channel is known as stage-discharge Relation or rating curve, or also just
rating. These expressions are synonymous and they can be used interchangeably. These
measurements are used to produce a unique mathematical relation which allows, for a
particular location and usually for a period of time, continuous stage measurements are
converted into Discharge. The resulting rating curves are useful for interpolating or
extrapolating flow measurements and for modeling.
The rating curve is a very important tool in surface hydrology because the reliability of
discharge Data values are highly dependent on as a satisfactory stage-discharge
relationship at the gauging station. Although the preparation of rating curves seems to be
an essentially empiric task, a wide theoretical background is needed to create a reliable
tool to switch from measured water height to discharge.
The rating curve is an extensively used tool in hydrology to estimate discharge in natural
and artificial open channel. Since the early 19th century it is a common practice to
measure the discharge of streams at suitable times, usually by a current meter or other
methods. Meanwhile, the corresponding stage is also measured; a curve of discharge
against stage can then be built by fitting these data with a power or polynomial curve.
The traditional and simple way to gather information on current discharge is then to
measure the water level with gauges and to use the stage discharge relationship to
estimate the flow discharge. It is well known that direct measurements of discharge in
open channels is costly, time consuming, and sometimes impractical during floods.
Figure 1: Example of a stage-discharge relation

16. 2
Several methods have been proposed to improve data fitting, but generally they have not
adequately assessed the fundamentals of stage-discharge ratings. As a consequence,
several difficulties with stage-discharge ratings have been recognized. For example, in
some cases, the relation between stage and discharge is not unique. The water surface
slope, in fact, produces different discharges for the same stage.
There are many problems associated with the use of the rating curve, of which some are,
1. The assumption of a unique relationship between stage and discharge is, in general,
not always warranted.
2. Discharge is the rarely measured during a flood, and the quality of data at the high
flow end of the curve might be quite poor.
3. It is usually some sort of line of best fit through a sample made up of a number of
points sometimes extrapolated for higher stages.
1.1 Description of Simple Rating Curves
A rating curve is a two-parameter stage-discharge relation. Discharge is calculated from
the field velocity and cross sectional area.
To develop a proper rating curve, discharges must be measured at all representative
stages, using at least 10 to 12 points covering the range of low to high flows (Gupta,
2001). If there is a direct relation between discharge and gauge height, the discharge
rating is called simple relation. A simple rating may be only one curve but there is also a
compound curve consisting of three segments, one segment for the low, medium and then
for high water ranges (Kennedy, 1984).
A simple stage-discharge relation has a power form given by the following equation.
Where,
Q = discharge
d= gauge height
K, n = constants
When plotting this equation in log-log paper, the rating is transformed to a straight line.
A straight line is preferred because it can be extended or extrapolated, and it can be
described by a simple mathematical equation (Gupta, 2001).

17. 3
The resulting stage-discharge curve represents Q as a function of stage, datum correction,
channel slope and Manning‟s Coefficient (n). The procedure is costly and time
consuming, and dangerous or impractical during high floods. Thus typically, stream flow
rating only exists for limited station locations and with limited data at high flow
conditions.

18. 4
2. Stage
The stage of a channel is the height of the water surface above an established datum
plane. Water level is the level of the water surface profile with respect to some reference
point. The water-surface elevation for most rivers and streams is measured above an
arbitrary or predetermined gauge datum and is called the gauge height of the river or
stream. Gauge height is also known as stage although gauge height is more appropriate
when used with a reading on a gauge. Stage or gauge height is usually expressed in feet
(ft) (Potyondy and John, 1994).
River stage is an important concept when analyzing how much water is moving in a
stream at any given moment. Usually with the zero height being near the river bed, in the
river and is commonly measured in feet. For example, on a normal day when no rain has
fallen for a while, a river might have a stage of 2 feet (base flow conditions). If a big
storm hits, the river stage could rise up to 15 or 20 feet, sometimes very quickly.
Gauge-height records may be obtained by systematic observation of a non-recording
gauge, or with automatic water level sensors and recorders. Various types of transmitting
systems are frequently used to automatically transfer the gauge-height information from
remote gauging stations to office computers. A record of stream stage is useful in itself,
as in designing structures affected by stream elevations or in planning use of flood plains.
How high and how fast a river will rise during a storm depends on many things. Most
important, of course, is how much rain is falling. But also we have to look at other things,
such as the stage of the river when the storm begins, at what the soil is like in the
drainage basin where it is raining (is the soil already saturated with water from a previous
storm?), and at how hard and in what parts of the watershed the rain is falling.
Figure 2: Height of the water surface above an established datum plane

19. 5
Stream and reservoir stage are critical parameters in the computation of stream discharge
and reservoir volume, respectively. In addition, a record of stream stage is useful in the
design of structures that may be affected by stream elevation, as well as for the planning
for various uses of flood plains. This section describes equipment and methodology for
the observation, sensing, and recording of stage in streams and reservoirs.
2.1 Datum of Gauge
The datum of the gauge may be a recognized datum, such as mean sea level, or an
arbitrary datum plane chosen for convenience. The datum of the gauge may be either a
recognized datum, such as the North American Vertical Datum of 1988 (NAVD 88), the
National Geodetic Vertical Datum of 1929 (NGVD 29), or an arbitrary datum chosen for
convenience. NGVD 29 was the predominant datum used to establish lake and reservoir
gauges, and stream flow gauges, including those located in tidal zones or coastal areas;
however, with its inception, the NAVD 88 is currently the datum the United States
Geological Survey (USGS) recommends as the vertical datum for the USGS stream
gauging network. Where NAVD 88 exists, all gauges referenced to other datum should
be resurveyed or converted to NAVD 88. An arbitrary datum plane is usually used for
stream gauging sites where it is desirable for all recorded gauge heights to be relatively
low numbers (Thomas & Kennedy, 1990).
An arbitrary datum plane is selected for the convenience of using gauge heights of
relatively low numbers. To eliminate the possibility of minus values of gauge height, the
datum selected for operating purposes is below the elevation of zero flow on the control
for all conditions. A permanent datum must be maintained so that only one datum for the
gauge-height record is used for the life of the station. To maintain a permanent datum
each gauging station requires at least two or three reference marks that are independent of
the gauge structure. All gauges are periodically checked by running levels using the
reference marks to maintain a fixed datum. If an arbitrary datum plane is used, it is
desirable that it be referred to a bench mark of known elevation above mean sea level by
levels so that the arbitrary datum may be recovered if the gauge and reference marks are
destroyed (Thomas and Jackson, 1981).
2.2 Types of Stage / Flood categories
Flood categories are terms defined for each gauge location that describe or categorize the
observed or expected severity of flood impacts in the corresponding stream segment or
nearby stream. The severity of flooding at a given stage is not necessarily the same at all
locations along a stream due to varying channel/bank characteristics on portions of the
stream. Therefore, the stage for a given flood category is usually associated with lowest
water level corresponding to the most significant flood impacts somewhere in the reach.
Record flooding is flooding that equals or exceeds the highest stage or discharge at a

20. 6
given site during the period of record keeping. There are different types of stage some are
discuss below.
2.2.1 Bankfull Stage
An established gauge height at a given location along a river or stream, above which a
rise in water surface will cause the river or stream to over flow the lowest natural stream
bank somewhere in the corresponding reach. The term lowest bank is however, not
intended to apply to an unusually low place or a break in the natural bank through which
the water inundates a small area. Bankfull stages on streams with natural or manmade
high banks can be defined by the predominant vegetation line on the banks. The bankfull
stage on many streams is associated with the 2-year recurrence interval flood. Bankfull
stage is not necessarily the same as flood stage.
Figure 3: Over Flow the Lowest Natural Stream Bank Somewhere In the Corresponding
Reach
2.2.2 Action Stage
The stage which, when reached by a rising stream, represents the level where the
partner/user needs to take some type of mitigation action in preparation for possible
significant hydrologic activity. The type of action taken varies for each gauge location.
Gauge data should be closely monitored by any affected people if the stage is above
action stage.
Rivers typically at this level, the water surface is generally near or slightly above the top
of its banks, but no man-made structures are flooded; typically any water overflowing is
limited to small areas of parkland and marshland.

21. 7
Figure 4: Water surface is near or slightly above the top of its banks
2.2.3 Flood Stage
An established gauge height for a given location above which a rise in water surface level
begins to create a hazard to lives, property, or commerce. The issuance of flood
advisories or warnings is linked to flood stage. Not necessarily the same as bankfull
stage.
Figure 5: Flood Stage
2.2.4 Minor Flood Stage
Rivers minor flooding is expected at this level, slightly above flood stage. Few, if any,
buildings are expected to be inundated, however, roads may be covered with water,
parklands and lawns may be inundated and water may go under buildings on stilts or
higher elevations. Water will usually run all the way in waves during a minor flood.
Lifeguard structures and beach concession stands will usually be flooded, and may be
damaged by surf.

22. 8
Figure 6: Buildings are expected to be inundated
2.2.5 Moderate Flood Stage
Rivers inundation (over flow) at this stage. Roads are likely to be closed and some areas
cut off. Remove inhabitants to safer ground may be necessary. At moderate flood stage,
usually water overtops the natural embankments and begins flooding the land near the
river. Shoreline roadways and beaches will often be completely flooded out. High surf
usually associated with this level of flooding may pound some Oceanside structures like
piers, boardwalks, docks, and lifeguard stations apart. Beach houses may be damaged by
water and surf, especially if lacking stilts.
Figure 7: Roads are likely to be closed and some areas cut off
2.2.6 Major Flood Stage
Life threatening flooding is usually expected at this stage. Extensive flooding with some
low-lying areas completely inundated is likely. Structures may be completely submerged.
Large-scale evacuations may be necessary.

23. 9
Figure 8: Extensive flooding Structures may be completely submerged
Water surges over not only flooding the land near the river, but also man-made walls and
roads. Large and destructive waves pound weak structures to bits and severely damage
well-built homes and businesses. If major flooding occurs at high tide, impacts may be
felt well inland. If cities are at or below sea level, catastrophic flooding can inundate the
entire city and cause millions or billions of dollars in damage (such as occurred in New
Orleans during Hurricane Katrina).
2.2.7 Record Flood Stage
Rivers at this level, the river is at its highest that it‟s been since records began for the area
where the stream gauge is located. This does not necessarily imply a major flood. Some
areas may have never experienced major flooding, and thus record stage is in the
moderate category. Usually, record flooding at the coast is associated with Tropical
cyclones, but it may be associated with coastal storms, Nor'easters, seiches caused
by earth quakes or strong thunderstorms, or tsunamis. Destruction is often extensive and
may extend a far distance inland.
2.3 Stage data
The stage data is often presented in the form of a plot of stage against chrono-logical 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
waning and flood protection work, peek flood can be analysed statically to estimate the
design peek river stages for use in the design of hydraulic structures, such as bridge ,wire
etc. Historic flood stages are invaluable in the indirect estimation of corresponding flood
discharges. In view of these multifarious uses, the river stage forms an important
hydrologic parameter chosen for regular observation and recording. In stream gauging,
gauge heights are used as the independent variable in a stage-discharge relation to
compute discharges. Reliability of the discharge record is therefore dependent on the

24. 10
reliability of the gauge-height record, as well as the stage discharge relation. Elevation
records of lakes and reservoirs provide an index of lake-surface area and volume, as well
as the elevation of the lake or reservoir (K Subramanaya, 2001).
2.4 Basic Requirements for Collecting Stage Data
The collection of stage data, either manually or automatically, requires various
instrumentation, or components, established at a gauging site. For stage data to be useful
for their intended purposes, requirements for maintaining a permanent gauge datum and
meeting specified accuracy limits are important. The datum of the gauge may be a
recognized datum, such as mean sea level, or an arbitrary datum plane chosen for
convenience. An arbitrary datum plane is selected for the convenience of using gauge
heights of relatively low numbers. To eliminate the possibility of minus values of gauge
height, the datum selected for operating purposes is below the elevation of zero flow on
the control for all conditions. This section of the report provides definitions of the
components, as well as the basic accuracy requirements.
2.5 Sources of Stage Measurement Error
The measured stage of a stream or other water body at any given point in time is subject
to numerous sources of incremental errors. The combined effect of these errors should be
within the accuracy (U.S. Geological Survey; Reston Virginia, 2010).
2.5.1 Datum Errors
The gauge datum is described in a previous section of this report. Movement of a gauge
caused by uplift or settlement of the supporting structure can cause datum errors that can
only be detected by running levels. Gauge datum for reference gauges should be
maintained to an accuracy of 0.01 ft. (Rantz, 1982). which can usually be achieved by
running levels to established reference marks every 2 or 3 years. Where conditions are
not stable, levels may be required at more frequent intervals. Generally, gauges do not
need to be adjusted unless datum discrepancies exceed 0.02 ft.
2.5.2 Gauge-Reading Errors
Errors can result from inaccurate gauge readings, where it may be difficult to detect the
water line against a staff gauge because of poor lighting or very clear water. In other
instances, accurate gauge readings may be difficult to make because of water surge.
These errors can be reduced or eliminated by careful observation, and in the case of
surge, by averaging several observations. In almost all cases, read gauges to the nearest
0.01 ft.

25. 11
2.5.3 Stage-Sensor Errors
Stage sensors, such as floats, pressure transducers, and other stage-sensing devices, may
introduce gauge-height errors. The float in a stilling well may sometimes leak, the float-
tape clamp may have slipped, or small animals or snakes may rest on the float. In most
instances, problems with the float will cause it to float lower than originally set, causing
gauge readings to be too low. Stilling-well intake pipes may also become partly clogged,
where the stage inside the stilling well lags in time behind the actual stage of the stream.
2.5.4 Hydraulically Induced Errors
High velocity in the stream near the outside end of the intake pipes can cause draw down,
or sometimes buildup, of the water surface inside a stilling well. A similar condition can
occur when high velocity occurs near a bubble gauge orifice. For example, where a
sensor is located on the downstream or upstream side of a pier, the drawdown or buildup
can be very large during large flows, on the order of 0.5 ft. or more. This condition
should be investigated by making simultaneous readings of outside and inside auxiliary
gauges, or recorder readings, during periods of high stages and (or) high velocity. It can
also be checked by determining outside and inside high-water elevations. Hydraulically
induced errors can be reduced or eliminated through the use of an intake-static tube, or in
the case of a bubble gauge, an orifice-static tube. Relocating the intakes or orifice to a
zone of low velocity may also help. Where drawdown or buildup cannot be completely
eliminated, it may be necessary to develop an inside-outside gauge relation to use over
the effective range in stage for correcting inside gauge readings to represent the actual
outside gauge height.
2.5.5 Verification Errors
Stage readings require frequent and consistent verification to ensure that errors are
reduced or eliminated. Failure to perform proper verification standards can be the source
of undetected, and possibly significant, stage errors. Verification procedures include
frequent reading of independent auxiliary gauges, comparison of inside- and outside-
gauge readings, observation of high-water marks, redundant recording of peaks and
troughs by use of maximum/minimum stage-tape indicators (also referred to as Dahman
indicators), use of crest-stage gauges, and regular maintenance of gauge datum by
differential level surveys. These checks should be augmented as appropriate for unusual
field conditions. Hydrographers should notice and keep records of instrument
performance, including comparisons of recorded stages with the reference gauge reading,
and any applied corrections.

26. 12
2.5.6 Water Surface-to-Sensor-to-Recorder Errors
The communication link between the stream-water surface, the stage sensor, and the data
recorder can sometimes develop problems, or have inherent problems that result in
gauge-height errors. For instance, for a stilling-well-and-float system, the intakes may
become clogged, or excessive sediment may settle in the stilling well, or the float tape
may hang. These are major problems that usually result in a complete loss of data. More
subtle problems can also occur that are not so obvious, but may result in small gauge-
height errors. For instance, as the stage rises in a stilling well, the float tape that connects
the float to the data recorder via the float pulley is gradually transferred from one side of
the float pulley to the other. This shift in weight can cause the float to ride slightly higher
in the water, causing small positive errors in the recorded gauge height (Rantz, 1982)
describes and quantifies this and other sources of error, such as float lag and
submergence of the float-tape counterweight.
2.6 Site Selection for the Installation Of gauge
Choose the gauge site for stage measurement based on the following criteria:
1. If the stage record is to be used for computing stream flow, consider the requirements
for controls, rating curves, backwater, and other stream flow variables in selecting the
site, as well as the acquisition of stage data (Rantz, 1982).
2. Select the site so the intakes or orifice are in a pool, if possible, where stream velocity
is low and not subject to significant turbulence. If this is not possible, place the
intakes or orifice in a water zone, where they are protected from high velocity.
3. The gauge stilling well (if used) and the instrument shelter may be located on a
stream bank, bridge, dam, or other suitable structure, provided the other site selection
criteria are met as closely as possible. Do not place the gauge structure where it might
sustain damage during floods.
4. If the gauge is located at or near a bridge, make sure it is on the downstream side. If
this is not possible, and it must be located upstream of the bridge, then place it far
enough upstream to be out of the zone of drawdown caused by the bridge during
medium and high water.
5. Select the site where either a stilling well with intakes can be easily installed, or
where an instrument shelter can be installed for housing a bubble gauge. If a bubble
gauge is to be used, the site must provide suitable conditions to install the necessary
bubble tubing and orifice static tube. For bank installations, place the tubing
underground between the gauge shelter and the stream. For bridge installations, attach
the tubing to the bridge members and pier or piling. Firmly anchor the orifice static
tube in the stream, preferably in a zone of low velocity.

27. 13
6. Place the gauge intakes or orifice low enough to record the lowest expected stage. In
cold climates, place them below the frost line to protect them from freezing, if
possible.
7. The instrument shelter should be high enough to be above the 0.5 percent exceedance
(200 year) flood level, if possible.
8. Minimize the distance between the stream and the stilling well and (or) instrument
shelter.
9. The site should have a suitable location for one or more outside auxiliary gauges.
These could be staff gauges, wire-weight gauges, or tape-down reference points.
Make sure the auxiliary gauges are easily accessible and located in a position so that
accurate gauge readings can be easily made. They should be in the same pool as the
gauge intakes, or orifice, and should provide readings that are indicative of the
readings obtained through the intakes or orifice.
10. If the gauge site is for the purpose of measuring stage in a lake or reservoir and it is
near the outlet structure, then make sure the gauge intakes, or orifice, are located
upstream of the zone of drawdown of the outlet structure.
11. Make sure site conditions are such that an accurate datum can be maintained.
Appropriate reference marks and reference points should be located both on and off
the gauging structure to maintain accurate and timely level surveys of the gauge.
(Kenney, 2010).
2.7 Measurement of Stream Stage
The stage of a stream is defined as its water surface elevation measured above a datum.
This datum can be the mean-sea level (MSL) or any arbitrary datum independently to the
MSL. We still uses the traditional, basic stilling-well float system as a predominant
gauging station, modern electronic stage sensors and water-level recorders are now
commonly used. Bubble gauges coupled with non-submersible pressure transducers
eliminate the need for stilling wells. Submersible pressure transducers have become
common in use for the measurement of stage in both rivers and lakes. Furthermore,
noncontact methods, such as radar, acoustic, and laser methods of sensing water levels,
are being developed and tested, and in the case of radar, are commonly used for the
measurement of stage. Several telemetry systems are used to transmit stage data from the
gauging station to the office, although satellite telemetry has become the standard. These
telemetry systems provide near real-time stage data, as well as other information that
alerts the hydrographer to extreme or abnormal events, and instrument malfunctions.
Some stage measuring methods are given below, (Thomas et al., 1990).
2.7.1 Echo Sounder / Depth Finder

28. 14
Depth finder, also called echo sounder, device used on ships to determine the depth of
water by measuring the time it takes a sound (sonic pulse) produced just below the water
surface to return, or echo, from the bottom of the body of water. Sonic depth finders are
in operation on practically every important class of ship, naval and merchant, and are also
used on small craft.
Figure 9: Device used on ships to determine the depth of water
2.7.2 Staff Gauge
The staff gauge is either vertical or inclined. The standard USGS vertical-staff gauge
consists of porcelain-enameled iron sections 4 in wide and 3.4 ft. long and graduated
every 0.02 ft. The vertical staff gauge is also used in stilling wells as an inside gauge.
Vertical-staff gauges are set by leveling directly to the gauges. An inclined staff gauge is
used for an outside gauge and usually consists of a graduated heavy timber securely
attached to a permanent foundation. Inclined staff gauges built flush with the stream bank
are less likely to be damaged by floods, floating ice, or drift than are projecting vertical
staffs. Inclined-staff gauges must be individually calibrated by leveling to several points
along the length of the gauge, interpolating intermediate points, and marking these points
with a relatively permanent marking system.

29. 15
Figure 10: Vertical-staff gauges
2.7.3 Vertical Stand-Pipe
For surface water level, measurements are often made in a vertical stand-pipe installed
adjacent to a lake, river, or stream. The stream level (stage) is the same as the water
elevation in the vertical stand-pipe. A float and pulley is often used, but pressure
transducers, ultrasonic and resistive tape sensors work well also. Self-calibrating double
bubblers are accurate sensors for measuring water level, and have the added benefit of
keeping the sensor out of the measured liquid critical in corrosive environments.
2.7.4 Wetted Tape
This method is accurate for measuring water levels to depths up to about 90 ft. To use
this method, you must know the approximate depth to water in your stream. In this
method, a lead weight is attached to the end of a 100 ft. steel measuring tape. Eight to 10
ft. of tape end is dried and coated with carpenter‟s chalk before each measurement. The
tape is lowered into the stream until a part of the chalked section is below the water. The
contractor will align and note an even foot mark on the tape exactly at the top of the
casing or some other measuring point. Then, the tape is pulled up to read the mark where
the line is wet. He can determine the actual depth from the top of the casing to water level
by subtracting the wetted mark from the mark he held at the top of the casing.
Figure 11: Weight is attached to the end of a steel measuring tape
2.7.5 Stilling Well
One common approach is with a stilling well in the river bank or attached to a bridge
pier. Water from the river enters and leaves the stilling well through underwater pipes

30. 16
allowing the water surface in the stilling well to be at the same elevation as the water
surface in the river. The stage is then measured inside the stilling well using a float or a
pressure, optic, or acoustic sensor. The measured stage value is stored in an electronic
data recorder on a regular interval, usually every 15 minutes.
At some stream gauge sites, a stilling well is not feasible or is not cost effective to install.
As an alternative, stage can be determined by measuring the pressure required to maintain
a small flow of gas through a tube and bubbled out at a fixed location under water in the
stream. The measured pressure is directly related to the height of water over the tube
outlet in the stream. As the depth of water above the tube outlet increases, more pressure
is required to push the gas bubbles through the tube.
Figure 12: Stilling well in the river bank
2.7.6 Stage Sensor
A stage sensor is a device that automatically determines (senses) the vertical position of
the water surface. This may be a float riding on the water surface inside a stilling well. It
may be a non-submersible pressure transducer coupled with a gas purge bubbler orifice.
It may be a submerged pressure transducer coupled with an electronic cable to transmit
the vertical position of the water surface, and a venting tube to vent the submerged
transducer to atmospheric pressure. Or it may be an acoustic, radar, laser, or optical pulse
that reflects from the water surface to other instruments designed and calibrated for
measuring or recording the gauge height.

31. 17
Figure 13: Stage Sensor
2.7.7 Float Sensor
The float sensor consists of a tape or cable passing over a pulley, with a float in a stilling
well attached to one end of the tape or cable and a counter weight to the other. The float
follows the rise and fall of the water level, and the water level can be read by using an
index and graduated tape, or the pulley can be attached to a water-stage recorder to
transmit the water level to the recorder.
Figure 14: float sensor consists of a tape or cable passing over a pulley
2.7.8 Wire Gauge
It is a gauge used to measure the water surface elevation from above the surface such as
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 25m.

32. 18
Figure 15: Gauge used to measure the water surface elevation from above the surface such
as bridge
2.7.9 Stage Recorder
A stage recorder is a graphical, digital, or electronic device that automatically records and
stores gauge-height readings sensed by a stage sensor. Graphical (analog) recorders
produce a continuous chart of gauge height. Digital and electronic recorders generally
store gauge heights at predetermined time intervals, such as every 5 minutes, 15 minutes,
or 1 hour. Sometimes other uniform time intervals are used, as well as non-uniform time
intervals based on preprogrammed conditions.
Figure 16: Graphical stage recorder

33. 19
Gauge height retrieval is the means by which gauge height data are extracted from the
recorder. This may be simply by manually removing a chart or paper-punch tape, by
downloading the data from the recorder to a personal digital assistant (PDA) or field
computer, or by removing an electronic memory device from the recorder.
2.7.10 Electric Sounder or Electric Depth Gauge
An electric sounder or depth gauge is the most practical method for measuring the water
levels. It consists of a weight suspended on stranded insulated wire with depth markings
and an ammeter to indicate a closed circuit. Current flows through the circuit when the
end of the wire touches the water surface. Current is supplied by a small 9 or 12-volt
battery.
Figure 17: Weight suspended on stranded insulated wire with depth markings and an
ammeter
To collect a reading, the contractor lowers the electric wire or sounding line until the
needle deflects then reads the distance from the water to the top of the casing on the line.
He marks the reference point on the casing where he measured the depth. And, then he
uses a standard tape measure to measure the distance between the marks on the line.

34. 20
3. Measurement of Discharge
The science of water measurement in hydrology is called hydrometry. Stream discharge
is the rate at which volume of water passes through a cross sectional per unit time. In the
SI system it is expressed in units of cubic meters per second (m3/sec), although very
small flow it recorded in liters per second (L/s).
Measurement of discharge forms the most important data for engineers and hydrologist as
the peak discharge in the stream is very important in the design of any water resources
project. The measurement is required to develop hydrograph, Rating curve ,S-curve,
flood warnings, equal distribution of water supply and irrigation among the users, and
determination of annual and seasonal runoff.
3.1 Historical View of Discharge Measurement
The subject hydrometry is as old as human civilization.”Bibliography of Hydrometry” by
kolupaila is quite famous and comprehensive. The oldest hydrometic evidences are the
marking of flood stages of the river Nile cut in steep rock faces. Stage discharge
relationship was conceived long back in irrigation system to The Mughals in central Asia.
The famous Pitot tube for measuring the velocity in the river seine was proposed by
Henri De Pitot a French engineer in 1732.The price current meter which has been
extensively used through the world by measuring the velocity of river was initially
devised by T.G. in 1870 and subsequently redesigned by W.G. Price. A Parshall flume
which is extensively used in USA to measure discharge was developed by R.L Parshall in
1920. Ultrasonic method of velocity measurement was first reported by Swengel in
1995.Based on different hydraulic and electronic principles Hydrometry is developed
step by step .Many modern method and techniques have been developed.
3.2 Basic Principle in Discharge Measurement
A commonly applied methodology for measuring, and estimating, the discharge of a river
is based on a simplified form of the continuity equation. The equation implies that for any
incompressible fluid, such as liquid water, the discharge (Q) is equal to the product of the
stream's cross-sectional area (A) and its mean velocity (V), and is written as:
Q =
3.3 Discharge Measurement Method
Continuous measurement of stream discharge is very difficult. Direct measurement of
discharge is very time consuming and costly procedure. Two step procedures is followed.
First, the discharge in a given stream is measured and then in the next step the stage of

35. 21
the stream is observed. The observation of stage is easy, inexpensive, and continuous
reading can also be obtained. This method of discharge determination of streams is
adopted universally. Volumetric measurements are most appropriate for small flows,
dilution gauging for turbulent flows and artificial structures or natural control section for
permanent gauging sites. Stream flow measurement techniques can be broadly classified
into two categories. Under each category there are no‟s of methods, the important ones
are listed below.
3.3.1 Direct Determination of Stream Discharge
 Area velocity methods
 Electromagnetic method
 Ultrasonic method
3.3.1.1 Discharge by Velocity Area method
The most direct method of obtaining a value of discharge to correspond with a stage
measurement is by the velocity-area method in which the river velocity is measured at
selected intervals of known depth across a measured section of the river. Around 90%of
the world river gauging sites depends on this method (Shaw, 1994).
Measurement of Velocity
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 most probably used instrument for accurate
determination stream-velocity field. Approximates stream velocities can be determined
by floats (Chow, V.T and Maidment, 1988).
1. Velocity Measurement by Floats
A floating object on the surface of stream when timed can yield the surface velocity by
the relation,
=
Sometimes it is called float gauging. This method of measuring velocity while primitive
still finds application in special circumstances such as (1) small streams in floods (2)
small streams with rapidly changing water surface (3) preliminary or exploratory surveys.

36. 22
While any floating object can be used, normally specially made leak proof and easily
identifiable floats are used.
Figure 18: Floats: (a) surface float; (b) canister float for mean velocity; (c) rod float by
mean velocity. (Reproduced with permission from R.W. Herschy (ed.) (2009) stream flow
measurement 3rd edn. © 2009, by permission of Taylor & Francis, oxford.)
Measurement by floats gives only the surface velocity and correction factor must be
applied to gives the average velocity over a depth. A factor of 0.7 is recommended for a
river of 1m depth with factor 0.8 for 6m or greater (BS EN ISO 748, 2007). Specially
designed floats can be made to travel at the mean velocity of the stream. However surface
floats are affected by surface winds. To get the average velocity in the vertical directly,
the special floats in which the part of body under water is used. Rod float in which a
cylindrical rod is weighted so that it can float vertically, belong this category.
2. Velocity measurement by Current Meters
The most commonly used instrument in hydrometry to measure the velocity is the current
meter. There are mainly two types of current meters, cup type current meter (price current
meter) and the propeller type current meter. The principle involved in both the meters is
that the water passing through the rotating element of the meter makes it revolve due to
unbalance drag force acting on it and speed of the rotating element is directly
proportional to the velocity of water. Historically, Robert Hooke (1663) invented a
propeller type current meter to measure the distance travelled by ship. 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 meter.

37. 23
a) Vertical Axis Meters
These instruments consist of series of conical cups mounted around a vertical axis. The
cup rotates in horizontal plane and a cam attached to the vertical axis spindle records
generated signals proportional to the revolutions of the cup assembly. The price current
meter and Gurley current meter are under this category.
Figure 19: Price Current meter
The tail vanes (fins) will always align the meter along the direction of flow. The purpose
of the fish weight with a stream line shape at the bottom is to keep the meter cable as
nearly vertical as possible. The usual weights recommended are 10,15,25,35 and 50kg.
The recorded unit consists of headphone work by the operator and an electric circuit with
a battery and the wire of supporting cable. Each time the wheel of cups make one
revolution the electric circuit is closed and this causes a click in headphone to be heard by
the operator. The normal range of velocities is from 0.15 to 4m/s .The accuracy of these
instruments is about 1.50% at the threshold value and improves to 0.30% at speed excess
of 1m/s.
b) Horizontal Axis Meters
These meters consist of propeller mounted at the end of horizontal shaft. These come in
wide variety of size with propeller diameter in the range 6 to 12cm and can register
velocities in the range of 0.15 to 4m/s. Neyrtec and watt-type meters are typical
instruments under this category.

38. 24
Figure 20: Propeller Current Meter
These meters are fairly rugged and are not affected by oblique flows of as much as
15degree.the accuracy of the instruments is about 1% at the threshold value and is about
0.25% at a velocity of 0.3m/s and above. The current meter is so designed that its rotation
speed varies linearly with the stream velocity v at the location of the Instrument. A
typical relationship is,
v= a Ns + b
Typical values of a & b for standard size 12.5cm price meter(cup type) is a = 0.65 and b
= 0.003 smaller meters of 5cm diameter cup assembly called pygmy meters run faster are
useful in measuring small velocities. The values of meter constant for them a = 0.3 and b
= 0.003
Figure 21: Impeller Current Meter

39. 25
Velocity Measurement Methodology
a. Six-Tenths Depth Method
For shallow water depths, less than 2.46 ft (75 cm) for the larger current meters and 1.5 ft
(45 cm) for the small current meters, the Six-Tenths Depth Method is used. A single
current meter measurement is taken at a relative water depth of 0.6 below the water
surface, which means 0.4 relative water depth from the bed of channel and the resulting
velocity is used as the mean velocity in the vertical.
In irrigation canals, this method is commonly used at the first observation from each
bank, while the two point‟s method is used at all of the other verticals in the cross-
section. Accordingly, the first vertical from each bank has a low velocity so that the
discharge in each section adjacent to the left and right banks represents a very small
portion of the total discharge in the cross-section.
b. Three-Points Method
This method is used when the velocities in the vertical appear to be abnormally
distributed, such as having an unusual velocity distribution. The three point method
combines both the two point method and the six-tenths depth method. Therefore, current
meter measurements are taken at 0.2, 0.6 and 0.8 of the flow depth. The mean velocity, V
in the vertical will be,
Vavg =
( )
Current Meter Ratings
Usually, a current meter is calibrated in a towing tank. The current meter is attached to a
carriage that travels on rails placed on the top of the towing tank. Then, a series of trials
are conducted in the current meter is towed at different constant velocities. For each trial,
the constant velocity of the carriage is recorded, as well as the revolutions per second “R”
(rev/s) of the current meter. This data is plotted on rectangular coordinate graph paper to
verify that a straight-line relation exists; then, the equation is determined by regression
analysis. Equation of our current meters is,
V = 2.2048 R + 0.0178

40. 26
Methods of Employing Current Meters
a) Wading
1. The wading method involves having the hydrographer stand in the water holding
a wading rod with the current meter attached to the rod.
2. The wading rod is graduated so that the water depth can be measured. The rod has
a metal foot pad which sets on the channel bed.
3. The current meter can be placed at any height on the wading rod and easily
adjusted to another height by hydrographer while standing in the water.
4. A tag line is stretched from one bank to another bank which may be cloth or metal
tape.
5. This tag line placed perpendicular to the flow direction.
6. The zeros length on the tag line does not have to correspond with the edge of
water on one of the banks.
7. This tag line is used to define the location of the wading rod each time that a
current measurement is made (recheck measurement each time and check units).
8. The wading rod is held at the tag line.
9. The hydrographer stands sideways to the flow direction, facing towards one of the
banks
10. The hydrographer stand 5-10cm downstream from the tag line and approximately
50cm to one side of the wading rod.
11. During the measurement, the rod need to be held in vertical position and current
must be parallel with flow direction.
Figure 22: Wading Method for Open Channel

41. 27
b) Bridge
Many of the larger irrigation channels have bridges at various locations such as head
works, cross regulators, but they cannot be located at appropriate section for current
meter measurement.
1. However, culverts often prove to be very good location, with current meter
measurement usually being made on the downstream end of culverts where
parallel streams are more likely to occur.
2. Bridges often have piers, which tend to collect debris on the upstream face that
should be removed prior to undertaking current meter measurement.
3. Either a hand line or reel assembly may be used from bridge.
Figure 23: (a) Current meter in Culverts (b) Current meter in Bridge
In either case, weight is placed at the bottom of the line, which sets on the channel bed in
order that a line does not move as a result of water flow.
1. The current meter is then placed whatever location is required for each
measurement.
2. For a hand line assembly, the weight is lowered from the bridge to the channel
bed and reading is noted on the graduated hand line, then weight is lifted until it is
setting on the water surface and the difference on these two readings is noted is
recorded as water depth.
3. Afterwards current meter is placed at the appropriate location on the hand line in
order to make the velocity measurement.
4. If a weight heavier than 10-15kg is required in order to have a stable, nearly
vertical, cable line, then a crane and reel assembly is used.

42. 28
5. The reel is mounted on a crane designed to clear the hand rail of the bridge and to
guide the meter cable line beyond any interference with the bridge members.
6. The crane is attached to a movable base for convenience in transferring the
equipment from the measuring point to another.
c) Cableway
For very wide canal, rivers, with water depth exceeds 150cm a cableway is placed above
the water with vertical supports on each bank that a heavily anchored for stability.
1. The cable supports a car (box) that travels underneath the cable using pulleys.
This car carries the hydrographer and the current meter equipment.
2. The cable has markers so that location across the channel is known.
3. A hand line or cable reel assembly is used depending on the size of the weight
that must be used.
Figure 24: Cableway Is Placed Above the Water with Vertical Supports on Each Bank
d) Boat
For some very wide channels such as those encountered in subcontinent the installation
of cableway is a significant expensive. Consequently a boat is commonly applied instead
of cableway.
1. This method is not as convenient as wading method and it takes longer to make
measurements, but it sometimes it is best alternative.
2. Personal safety is the limiting factors in the use of a boat on a stream having high
flow velocity.

43. 29
Figure 25: Use of a boat on a stream
Discharge observation by using boat method
Usually, all of these conditions cannot be satisfied. Select the best possible reach using
these criteria and then select an appropriate cross-section. After the cross-section has
been selected, determine the width of the stream. String the tag line at the measuring
section by un-reeling the line as the boat moves across the stream. After a tag line has
been stretched without a brake across the stream, remove the sag by means of a block and
tackle attached to the reel and to an anchorage support on the banks.
Gauging Procedure for Current Metering
At the gauging station or selected river cross section, the mean velocities for small sub
areas of the cross section (Vi) obtained from point velocity measurement at selected
sampling verticals across the rivers are multiplied by the corresponding sub areas and
product summed to give the total discharge.
∑
Where n is the no of sub areas.
1. The estimate Q is the discharge related to the stage at the time of gauging,
therefore before beginnings of a series of current meters measurement the stage
must be readed and recorded.
2. The width of river is divided into 20 about sub-sections so that no sub-section has
more than 10% of the flow.

44. 30
3. At each of the selected sub division points the water depth is measured by
sounding and current meter operating at selected points in the vertical to find
mean velocity in the vertical e.g. at 0.6 depth (one point method) or at 0.2 and 0.8
depth (two point method)
4. For each velocity measurement the no of complete revolutions of the meter over
measured time period (about 60 sec) is recorded using stop watch. If pulsations
are noticed, then a mean of three such counts should be taken.
5. When velocity at all the sub-divisions points across the river have been measured,
then stage is read again.
Calculation the Discharge from Current Metering Data
a) Mid-Section Method
In the mid- section method, it is assumed that the velocity measured at each vertical
represents the mean velocity in a segment. The segment area extends laterally from half
the distances from the preceding vertical to half the distance to the next and from the
water surface to the sounded depth as shown by the hatched area in fig. the segment
discharge is then computed for each segment and these summed up to obtain the total
discharge. Referring to fig which shows diagrammatically the cross-section of the
streamed channel, the discharge passing through segment 5 is computed as,
̅
(( ) ( ))
Figure 26: Cross Section in Mid-Section Method

45. 31
̅ ( )
For the end segment 1 shown hatched the discharge can be computed as,
̅ ( )
And the end segment n as,
̅ ( )
The preceding segment at the beginning of the cross-section is therefore considered
coincident with vertical 1 and the next vertical at the end of the cross-section is
considered coincident with vertical 1.
b) Mean -Section Method
Segment discharges are computed between successive intervals. An example of one such
segment is shown hatched in fig. The velocities and depths for successive verticals are
each averaged, the segment discharge being the product of two averages.
Referring to figure the discharge passing through segment 5-6 is computed as,
( ) ( ) ( )
It will be noted that the depth of flow at vertical 1 is zero and problem of computing the
flow in the end segments does not arise in this method nor does it arise when the bank is
vertical and the velocity can be taken as approximately zero at the end vertical. The
computation is therefore carried out for the end segments is exactly the same way as for
the other segments. Nevertheless this facility does not give the mean section method an
overall advantage over the mid-section method, the latter being simpler to compute and
therefore quicker if the calculations are being performed manually. There is little
difference in time, however, if a pocket calculator is employed for the calculations.

46. 32
Figure 27: Cross Section Mean -Section Method
Problems with The velocity Area Method
a) Large Rivers
In wide rivers there is difficulty in locating the instruments accurately at the sampling
points and inaccuracies will occur. Problem in locating the bed of river may also arise in
deep and fast flows then gauging across such river take many hour to complete. Check
the readings of stage such as operation. In deep, swift flowing rivers heavy weights
according to the velocity are attached, but the force of the current usually causes a drag
downstream from the vertical. For detailed instructions on the gauging methods used in
large rivers using the moving boat method see (Herschy, 2009).
b) Shallow Rivers
The depth of flow may be insufficient to cover the ordinary current meter. Smaller
instruments known as pygmy current meter are used for Shallow River and low flow
gauging. They are attached to the graduated rod and operated by the gauge wading across
the section.
c) Upland Streams
Streams with steep gradient and high velocities cannot be gauged satisfactory by the
velocity area method and alternative means must be used (dilution gauging).

47. 33
3.3.1.2 Electromagnetic Method
The electromagnetic method is based on 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 carried a current I to produce a controlled vertical
magnetic field. Electrode provides at the bottom of the channel section measure the small
voltage due to flow of water in the channel. It has been found that signal output E will be
the order of mille volts and it is related to discharge as Q.
Q = ( ) n
Figure 28: Electromagnetic Method
This method is particularly for the rivers which undergoes considerable changes in their
cross sectional properties due to sedimentation, weed growth etc. Also in tidal channels
where changes rapidly with time both in magnitude and direction flow. Present day
commercially available electromagnetic flow meters can measure the discharge to an
accuracy of 3%. The maximum channel width that can be accommodated being 100m.
The minimum detectable velocity is 0.005 m/s (Subramanaya k, 2013).
3.3.1.3 Ultrasonic Method
This is essentially an area velocity method with the average velocity being measured by
using ultrasonic signals. This method was first reported by swengel (1995) since then it
has been perfected and complete system available commercially.
Consider a channel carrying a flow with two transducer A and B fixed at the same level h
above the bed and on either side of the channel. These transducers can receive as well as
ultrasonic signals. Let A send an ultrasonic signal to be received at B after an elapse time
.similarly let B send a signal to be received at A after an elapse time . If C = velocity
of sound in water.

48. 34
( )
Where;
L = length of path from A to B = Component of flow velocity in the sound path =
V
Similarly from figure we can see that,
Figure 29: Ultrasonic Method
( )
Thus,
( ) = = 2 v cos ( )
V = ( )
The specific advantages of the ultrasonic system of river gauging are
1. It is rapid and gives high frequency
2. It is suitable for automatic recording data.
3. It can handle rapid changes in the magnitude and direction, as in tidal rivers
4. The cost of installation is independent of the size of the rivers.

49. 35
The accuracy of this method is limited by factors that affect the signal velocity and
averaging of flow velocity
1. Unstable cross section
2. Fluctuating weed growth
3. High loads of suspended solids
4. Air entertainment
5. Salinity and temperature changes.
3.3.2 Indirect Determination of Stream Discharge
 Slope Area Method
 Rating Curve
3.3.2.1 By Slope Area Method
In slope area method the discharge is estimated by observing the water surface slope and
cross-section area. Accuracy of this method is less than velocity area method. When
magnitude of flows is high this method will be used.
A measurement reach is chosen for which three things are known (1) the cross sectional
geometry and properties at its ends (2) the value of Manning‟s n (3) water surface
elevation at its end. In the selected reach these three parameters are known. As far as
possible the length of reach should be such that the difference between the water levels at
the upstream and downstream gauges is not less than ten times uncertainty in the
difference.
Slope is computed from the gauge observations at either end of the reach, the
intermediate gauges are used to confirm that the slope is uniform throughout the reach.
The mean velocity is established by using known empirical formula which relates
velocity to the hydraulic mean depth, the surface slope corrected for the kinetic energy of
the flowing water and roughness characteristics. The discharge is computed as the
product of mean velocity and mean cross sectional area of the flow.
The resistance equation for uniform flow in open channel e.g. Manning‟s formula can be
used to relate the depth at either ends of the reach to the discharge. Fig shows the
longitudinal section of the flow in a river between sections 1 & 2.
The head at a section consist of water surface elevation and the velocity head. The head
loss is made up of two parts (1) frictional loss (2) energy loss due to expansion or
contraction. The frictional slope can be written as,

50. 36
( ) ( ) ( )
Where L is the reach length, k is the coefficient of energy loss its value is 1 for
contraction and 0.5 for expansion. According to Manning‟s formula the mean velocity in
reach 1-2 is calculated as,
( )
Figure 30: Energy Profile Diagram for Any Open Channel
Where R is the hydraulic mean depth and n is manning‟s roughness coefficient, S is the
frictional slope. If A is the cross sectional area then discharge Q is,
( )
The term 1/n A R2/3 is known as conveyance (Kc) of the channel and it depend upon
channel characteristics. As the flow in reach may not be truly uniform the average
conveyance of the reach is expressed as the geometric mean of the conveyances of two
end sections 1 and 2.

51. 37
√
The discharge can be calculated by,
Q=K√ =√
The slope area method can be used with some accuracy in open channel with stable
boundaries. This method is also used in alluvial channel including channels with over
bank flow or non-uniform flow cross section and uncertainty in large value of roughness
coefficient.
3.3.3 Hydraulics Structures / Devices
 Weirs, Notches
 Flumes
On small streams the flow can be measured with the help of hydraulic flow measuring
devices such as rectangular, triangular and trapezoidal weirs or flumes like venture flume
and Parshall flume. The discharge in all these cases is expressed as function of geometry
of the structure and some reference head. The discharge expression will also contain an
empirical coefficient.
These conventional structures are used in filed conditions also but their used is limited by
the ranges of head, debris or sediment load of the stream and the back water effects
produced by the installations. To overcome of many these limitations a wide variety of
flow measuring structures with specific advantages are in use.
The basic principle of governing the use of weir, flume or similar flow measuring
structures is that these structures produced a unique control section in the flow. At these
structures the discharge Q is the function of water- surface elevation measured at a
specified upstream location
Q = f (H)
Where H = water surface elevation measured from specified datum. Thus for example for
weirs
Q = K H n
Where H = head over the weir K & n = system constant

52. 38
Equation Q = f (H) is applicable so long as the downstream water level is below a certain
limiting water level known as the modular limit. Such flows which are independent of the
downstream water level are known as free flow. If the tail water conditions do affect the
flow, then the flow is known as submerged flow. Discharge under submerged condition is
obtained by multiplying a reduction factor to the free flow discharges. For example the
submerged flow over a weir is estimated by the Villmonte formula,
Qs = Q1 [1-(H2/H1) n
] 0.385
n = exponent of head in the free flow head discharge relation for rectangular weir n =1.5
An existing dam across a stream, a bridge opening or a causeway may also be used as a
means of determining discharge indirectly. In the case of dam the discharge can be
expressed as a function of length of dam and head flow over the dam. In the case of
bridge opening the discharge is the function of the area of flow at the constriction and
drop in water surface in near the bridge.
3.3.3.1 Weir
A weir is one of the simplest and oldest structures used to measure the flow. It is an
obstruction in an open channel which constricts the flow and causes it to fall over a crest.
Weirs consist of vertical plates; the top of the plate can be straight or notched. Weir
plates are available in fiber glass, aluminum, or stainless steel. A weir can be classified in
two broad categories.
a) Sharp-Crested Weir
Sharp-Crested weir has a sharp metal blade along the bottom and sides of the crest. The
top edge of the weir is thin. Various types include: triangular or V-Notch, rectangular,
and trapezoidal (Cipolletti). Sharp-crested weirs are most frequently rectangular,
consisting of a straight, horizontal crest. A V-Notch weir is better suited to low flow
streams. Rectangular weirs are able to measure much higher flows than V-Notch weirs.
Cipolletti weirs are less accurate than rectangular or V-Notch weirs.
Q = √

53. 39
Figure 31: Sharp-Crested weir having sharp metal blade along the bottom and sides of the
crest
I. For rectangular weir
Q = CBh1.5
Where
C = discharge coefficient, B = top width of weir or length of crest normal
to flow
II. V-Notch weir
Q = √ tan
b) Broad-Crested Weir
This weir has a long, broad crest with no metal blade. It may also include a ramp on the
front of the crest to reduce head loss. This type of weir is also called the long-throated
flume.
Q = b√ ( )

54. 40
Figure 32: Broad crest with no metal blade
I. Sluice
Q = b √
Mode of Operation
Weirs operate on the principle that an obstruction in a channel cause water to back up,
creating a head behind the barrier. The head is a function of flow velocity and flow rate
through the device. The discharge through weirs and flumes is a function of water level,
so water level measurement techniques must be used. Staff gages and float-operated
units used for flow measuring.
Weir Installation Requirements
The following points must be kept in mind for the installation of the weir:
1. The connection between the weir and the channel should be Water tight.
2. The weir should be ventilated, to prevent a vacuum from forming on the
underside of the nappy.
3. The height of the weir should be at least 2 times the maximum expected head of
liquid above the crest. This is necessary to lower the velocity of approach.
4. In a relatively large channel, water velocity approach should be less than 0.5
ft/sec.
5. The crest must be set higher than the maximum downstream elevation of the
water surface, otherwise, a submerged flow condition will occur.
6. A drop of about 0.5 ft (6") or more in the channel is needed to establish free-flow
conditions over the weir.
7. The head measuring point of the weir should be located upstream of the weir crest
at a distance of four times the maximum expected head of the weir.

55. 41
8. For a triangular or rectangular weir with end contraction, the minimum distance
of the sides of the weir from the channel banks should be at least twice the
maximum expected head on the weir.
9. Avoid deposition of gravel, sand, and silt above the weir so that accurate water
measurements can be obtained.
Discharge Computation Procedure
Flow rate is determined by measuring the vertical distance (water depth) from the crest of
the overflow part of the weir to the water surface in the upstream pool. The weir
calibration curve then translates this recorded depth into the rate of flow at the device.
Discharge tables for standard measurement devices are available from the online version
of the USGS water measurement manual or in the printed version of the manual.
Advantages of Weirs
1. Simplest device
2. Lower cost than flumes
3. Relatively easy to install
4. Very accurate when used properly
Disadvantages of Weirs
1. Operate with a relatively high head loss
2. Higher maintenance cost than flumes
3. Accuracy affected by approach velocity
4. Needs to be periodically cleaned
Type of Weirs General Comments
a) Rectangular Weir
Most widely used weir able to measure higher flows than V-Notch weirs.When sizing a
rectangular weir, a crest length of 1 foot is the minimum that should be considered.
b) V-Notch Weir
Better suited for low flows streams, has reasonable accuracy for flows up to 10cfs. Very
accurate in measuring flows less than 1cfs.

56. 42
c) Trapezoidal (Cipolletti)
Less accurate than V-notch or rectangular weirs, commonly used to measure high flows.
Offer a slightly wider range of flows than rectangular weirs.
3.3.3.2 Flumes
If at a certain reach X-section is reduced gradually, that part of the channel is known as
flume. Flumes are usually prefabricated devices that are installed temporarily or
permanently in a flow system. They can be a “flat-bottom” type. In the case of a flat-
bottom flume, the shape of the side walls creates a contraction of the flow of liquid.
Flume is the traditional method to measure the discharge in the agriculture system.
Normally, a flume consists of a converging section, a throat section, and a diverging
section. Flumes are designed with the idea of producing a critical depth in the flume
throat and creating a direct relationship between water depth and flow rate.
Figure 33: X-section is reduced gradually
Flumes are categorized in two main classes:
1. Long-Throated Flumes
Long-throated flumes are coming into general use because they can be easily fitted into
complex channel shapes as well as simple shapes (Replogle, 1975). Long-throated flumes
have many advantages compared to other measuring devices. Long-throated flumes are
more accurate, cost less, have better technical performance, and can be computer
designed and calibrated. Long-throated flumes are preferred over Parshall flumes.

57. 43
2. Short-Throated Flumes
Short-throated flumes control the discharge rate in a region that produces curvilinear
flow. The Parshall flume and the Cutthroat flume are the most common examples of this
type of flume.
The most popular flume design in use today includes the Parshall flume, the cutthroat
flume, and the trapezoidal flume. The long throated flume is the recommended choice for
most projects because of its simple design, easy installation and flexibility in placing
them.
Figure 34: Parshall flume
Discharge Computation Procedure
Flow rate is determined by measuring the vertical distance (water depth) from the zero
reference at the bottom of the flume to the water surface. The flume calibration curve or
charts then translates this recorded depth into rate of flow.
Discharge through a Parshall flume can occur for two conditions of flow. The first, free
flow occurs when there is insufficient backwater depth to reduce the discharge rate.
Under free-flow conditions a phenomenon known as the hydraulic jump or "Standing
Wave" occurs downstream from the flume. The second condition of flow is submerged
flow.
Instructions for Placing Flumes
1. Locate the high water line on ditch bank where the flume is to be installed.

58. 44
2. Select from discharge curves the proper depth of water or head (Ha) that
corresponds with the maximum capacity of the ditch. For example assume that a
1-foot flume is to be used and that the maximum discharge is 4.0cfs, therefore, the
depth of water on the crest Ha is 1.0 foot.
3. Place the floor of the flume at a depth not more than 70% of head. In general, the
floor of the flume should be placed as high in the ditch as the grade and other
conditions permit. For example, allow 70% submergence, then 0.7 x 1.0 = 0.7
feet. Therefore, set the flume crest not more than 0.7 feet below the high water
mark. The loss of head will be 1 feet minus 0.7 feet = 0.3 feet.
Advantages of Flumes
1. Self-cleaning to a certain degree
2. Relatively low head loss
3. Accuracy less affected by approach velocity than weirs
4. Lower maintenance cost than weirs
Disadvantages of Flumes
1. High cost
2. Difficult to install
Type of Flume General Comments
a. Parshall Flume
Most widely known and used flume for permanent installations. Available in throat
widths ranging from 1” to 50ft to cover most flows. Fairly difficult installation requiring
a drop in the conduit invert.
b. Cutthroat Flume
Similar to Parshall flume, except that flat bottom does not require drop in conduit invert.
Can function well with high degree of submergence. Flat bottom passes solids better than
Parshall flume.
c. Trapezoidal Flume
Developed to measure flows in irrigation channels. Principal advantage is ability to
measure wide range of flows and also maintain good accuracy at low flows.

59. 45
Discharge Calculation under Different Set of Flow Condition at a Barrage on River
There are three conditions of flow which are explained as below;
a) Free Flow Conditions with Gates of Barrage Are Fully Raised
Figure 35: Free Flow Conditions with Gates of Barrage Are Fully Raised
Formula,
Q = C’BE1
3/2
 Q = Discharge in cusecs
 C‟ = h / E1 C
 h = D/S Water level – Crest Level
 E1 = U/S Total Energy Line (H + Va2/2g) – Crest Level
 C = Coefficient of discharge = 3.8
 B = Clear Water way.

60. 46
b) Gates of Barrage Partially Down With Hydraulic Jump Flow Conditions
Figure 36: Gates of Barrage Partially Down With Hydraulic Jump Flow Conditions
Formula,
Q = C’B (E1
3/2
- E2
3/2
)
 Q = Discharge in cusecs
 C‟ = h / E1 C
 h = D/S Water level – Crest Level
 E1 = U/S Total Energy Line (U/S Water level + ha) – Crest Level
 C = Coefficient of discharge = 3.8
 E2 = U/S Total Energy Line (U/S Water level + ha) – Bottom of Gates
 B = Clear Water way.
c) Gates Of Barrage Partially Down With Submerged Conditions
Figure 37: Gates Of Barrage Partially Down With Submerged Conditions

61. 47
Formula,
Q = Cd A (2gH) 1/2
 Q = Discharge in cusecs
 Cd = 0.65
 A = Area (B X Opening of Gates)
 B = Total Clear Water way.
 H = U/S Water level - D/S Water level
 g = Acceleration due to gravity (32.2 ft2 / sec)
Discharge Calculations of Head Regulator
Figure 38: Head Regulator
Formula,
Q = C’ X B X (E1
3/2
- E2
3/2
)
 Q = Discharge in cusecs
 C‟ = h / E1
 h = D/S Total Energy Line = ((D/S Water level + hv) – Crest Level)
 E1 = U/S Total Energy Line = ((U/S Water level + ha) – Crest Level)
 E2 = U/S Total Energy Line = ((U/S Water level + ha) – Bottom of Gates)
 B = Clear Water way
 Gate opening = E1 – E2

62. 48
4. Rating curve
Hydrologists very often need to know flow rates in streams under many different
conditions but they do not have time or resources to go in fields and measure these flow
rates .Instead they rely on rating curves to give them this information. When the time and
energy is taken to directly measure stream flow at a given time in stream, the stage of the
water is usually recorded at the same time. Using these data points of water elevation
(stage) and the measured flow rate, hydrologist can produce a rating curve.
A Rating curve is a graph of discharge versus stage for a given point on a stream, usually
at gauging stations. The development of river discharge versus stage curves is a key step
in any Hydrographer work (Bailey, J.F. and Ray H.A., 1966).
Flow is the variable usually required for hydrological analysis but, continuous
measurement of flow on a river section is usually impractical or it is expensive. However,
stage can be observed continuously or at regular short time intervals with ease and
economy. A relation exists between stage and the corresponding discharge at river
section. This relation is termed a stage-discharge relationship.
The rating curve is an extensively used tool in hydrology to estimate discharge in natural
and in artificial open channel. In early 19th century, discharge at suitable section is
measured by using current meter or other methods (Rantz et al., 1982; ISO 1100-1, 1998;
SIMN, 1998). The stage is also measured; a curve of discharge against stage can then be
built by fitting these data with a power or polynomial curve. The direct measurements of
discharge in open channels is costly, time consuming, and sometimes impractical during
floods.
A rating curve is established by number observations of Stage and discharge over a
period of time. At many locations, the discharge is not a unique function of stage,
variables such as surface slope or rate of change of stage with respect to time must also
be known to obtain the complete relationship in such circumstances.
The rating relationship established is used to transform the observed stages into the
corresponding discharges. In its simplest form, a rating curve can be illustrated
graphically, as shown in simple rating curve Figure.

63. 49
Figure 39: Representation of stage discharge relation
4.1 Types of station control
The character of the rating curve depends on the type of the control which depends on the
geometry of the cross section, and the physical features of the river downstream of the
section. Station controls are classified into following types:
 Section and channel controls
 Natural and artificial controls
 Complete, compound and partial controls
 Permanent and shifting controls
4.1.1 Section and channel controls
When the control is such that any change in the physical characteristics of the channel
downstream has no effect on the flow at the gauging section, then such control is termed
as section control. In other words, any disturbance downstream the control will not be
able to pass the control in the upstream direction. Natural or artificial narrowing of the
cross section (waterfalls, rock bar, gravel bar) creating zones of acceleration are some
examples of the section controls.

64. 50
Figure 40: Example of Section Control
A cross section where no acceleration of flow occurs or where the acceleration is not high
to prevent the passages of disturbances from the downstream to the upstream, then such a
location is called as a channel control .The rating curve in such case depends upon the
geometry and roughness of the river downstream of the control. The length of the
downstream reach of the river affecting the rating curve depends on the normal depth
and on the energy slope S.
L
Where follows from manning equation,
So,
( ⁄ )
Figure 41: Example of Channel Control

65. 51
4.1.2 Artificial and natural controls
An artificial section control is one which has been constructed to stabilize the relationship
between stage and discharge and for which a theoretical relationship is available based on
physical modeling. Natural section controls include a ledge of rock across channel, the
brink of the waterfall, or a local constriction in width (including bridge openings). All
channel controls are natural.
Figure 42: Example of an artificial control
4.1.3 Compound controls or complex controls:
A complete control is one which governs the stage-discharge relation throughout the
entire range of stage experienced. However, station controls are the combination of
section at low stages and a channel control at high stages and are called compound or
complex controls.
4.1.4 Permanent and shifting controls
Where the geometry of a section and the resulting stage-discharge relationship does not
change with time, it is described as a stable or permanent control. Shifting controls
change with time and may be section controls such as boulder, gravel and sand riffles
which undergo periodic or near continuous scour and deposition.
1. Scour and fill in an unstable channel.
2. Growth and decay of aquatic weeds.
3. Over spilling and ponding in areas adjoining the stream channel.

66. 52
Figure 43: Example of shifting control
4.2 Rating curves for steady uniform flow
The most commonly used stage-discharge ratings treat the discharge as a function of the
stage. These ratings follow a power curve of the form given by Equation (Herschy, 1995;
ISO, 1998; Kennedy, 1984; Rantz et al., 1982b).
( ) (2)
Where Q is the discharge, h is the stage and C, a, α are calibration coefficients. “C” is the
discharge when the effective depth of flow (h-a) is equal to 1, and “a” is the gauge height
of zero flow, “α” is the slope of the rating curve. (h-a) is the effective depth of water on
the control.
Equation 2 is based on the Manning equation, which is used as the governing equation for
steady uniform flow problems.
( ) (3)
Where n is the Manning‟s roughness coefficient, is the bottom slope, A is the area and
R is the hydraulic radius. However, Equation 2 is a simplification of the Manning
equation, the conveyance function AR2/3 can be described by a simple power function of
the water height in wide rectangular cross section, following equation is used
(4)

67. 53
4.3 Rating curve for non-steady and non-uniform flow
The effect of non-steady and non-uniform flow on stage discharge curve will be
discussed in this section. Examples of hydraulic conditions with non-steady and non-
uniform flow which have an effect on the uniqueness of rating curve.
1. Scour and fill in an unstable channel
2. Growth and decay of aquatic(weed)growth
3. Formation of ice on the river
4. Variable backwater in a uniform channel
5. Rapidly changing discharge (for example when flood wave occurs)
Figure 44: Stage-discharge relation for different hydraulic conditions
The stage-discharge curves for the above hydraulic conditions are briefly discussed
below.
Permanent control (Figure (44.a))
A control is permanent if the stage-discharge relation does not change with time.

68. 54
Sand-bed channel (Figure (44.b))
The movement of sediment affects the conveyance, the hydraulic roughness and the
energy slope. This makes the determination of a stage discharge relation difficult.
Aquatic vegetation (Figure (44.c))
The growth of weed decreases the conveyance of the channel and changes the roughness
with result that the stage for a given discharge is increased.
Ice covers (Figure (44.d))
Ice in the measuring section increases the hydraulic radius and the roughness and
decreases the cross sectional area. The stage for a given discharge is increased.
Variable backwater (Figure (44.e))
If the control reach for a gauging station has within a weir or a dam, a diversion which
can increase or decrease the energy gradient for a given discharge, variable backwater is
produced.
Rapidly changing discharge (Figure (44.f))
At some stations, generally those of low energy slope, the stage discharge relation is
affected by the rate of change of the discharge. If the discharge is increasing rapidly, it
will be greater than that for zero rate of change and, conversely, if it is rapidly decreasing
it well be less.
4.4 Difficulties in Defining Stage-Discharge Relationships
A simple stage discharge relation depends upon stage only. In complex rating curve
additional variables such as the slope of the energy line are required to define
relationship. The need for a particular type of rating curve can be obtained by first
plotting the observed stage and discharge data on a simple orthogonal plot. The scatter in
the plot gives a fairly good assessment of the type of stage-discharge relationship
required for the cross section.
Examples of the scatter plots obtained for various conditions are illustrated below. If
there is negligible scatter in the plotted points then smooth single valued curve through
the plotted points than a simple rating curve is required. This is shown in figure,

69. 55
Figure 45: For Permanent Control
If scatter is not negligible then it further probing to determine the Cause of such higher
scatter. There are four different possibilities.
The station is affected by the variable backwater conditions arising due to high flows in a
tributary joining downstream. A smooth curve passing through those points having
normal slopes at various depths is drawn first. It can then be seen that the points with
greater variation in slopes from the corresponding normal Slopes are located farther from
the curve. This is as shown in Figures.
Figure 46: (a) Rating curve affected by Variable backwater (uniform channel) (b) rating curve
affected by Variable backwater (submergence of Low water control)
The stage discharge rating is affected by the variation in the local acceleration due To
unsteady flow. In such case, the plotted points can be annotated with the Corresponding
rate of change of slope with respect to time. A smooth curve (steady state curve) passing
through those points having the least values of rate of change of stage is drawn first. It