

PRINT VERSION MODULE


Module
Objectives 





Introduction
to measurement methods 




Guaging
by Current meters 





Special
Conditions 






Computation
of Discharge 






References 







Contributors 











MODULE
OBJECTIVES 

At the end of this module, you would
be able to
 Understand procedures of river discharge
by Velocityarea method
 Select number of verticals, spacing
between them to measure depth and velocity at each vertical
 Position the boat at verticals
 Determine average velocity at a vertical
 Apply corrections to discharge measurement
 Estimation of river discharge by Midsection
method







INTRODUCTION 





One may wonder
as to why a large number of stations located on various rivers across
India operate all the year round to measure river discharge and concurrent
water level. In order to steer clear of any doubt, let us visualize
the existence of a few structures that often open up before our eyes,
such as Dams, Barrages, Rail and Road Bridges, Canals, and many more.
Have you ever thought of extent of work involved in planning, design
and execution of these structures? Or, have you ever imagined impact
on us, if there were no such structures around us. Most likely, the
answer is negative. Without debating the role of these structures
for brevity, it is enough to stress here that proportion of all such
structures are largely dictated by analyses based on hydrometeorological
data collected at these stations. Secondly, since basin characteristics
and river regime both exhibit continuous changes, it is equally important
to gather data at judiciously adopted intervals. The Central Water
Commission under Ministry of Water Resources, Govt of India alone
maintains over 900 station (For details, pl visit: http://www.cwc.nic.in
).
Whilst a wide range of techniques to
record river discharge are available, in this module, our focus
will be pointed to river discharge estimation by velocityarea method
only. Alternative options, such as slopearea method, float method
etc. are available, and can be engaged at a particular site, if
velocity area method approach is limited by the geometric and hydraulic
characteristics of the channel, and by the facilities and instruments
available at the site.







DISCHARGE
COMPUTATION BY VELOCITYAREA METHOD 





As the name suggests,
in this method, discharge is computed by measuring river depths and
velocity at a number of regularly or irregularly spaced verticals
as shown in figure below. 












This set of information
is eventually integrated by midsection method to determine river
discharge. This satisfies the equation Q = A * V. Succeeding paragraphs
of this module elaborate these aspects in greater depth. 






SELECTION
OF VERTICAL INTERVAL AND NUMBER 





For rivers greater
than 10 m wide, it is recommended, in line with ISO 748 and other
practice, that at least 20 verticals be used and that the discharge
in any one segment does not exceed 10% of the total. Between 20 and
30 verticals is normally used. Uncertainties in stream flow measurement
are expressed as percentages. The percentage uncertainty of using
say 25 verticals is of the same order for all widths of river, irrespective
of the width of segments. 












For small rivers
less than 10 metres in width the following selection criteria are
recommended: 











Additional
Conditions 




Two additional verticals not included
in the above are required close to each of the two water's edges
(banks).




In all instances depths and velocities
made at the waters edge are additional to above.




The difference in water depth between
two adjacent verticals should not exceed 50% of the smaller.




The difference in velocity between nonzero
samples taken at the same proportion of depth, e.g. at 0.6D, in
adjacent verticals shall not exceed 50% of the smaller.







MEASUREMENT
OF WIDTH, HORIZONTAL DISTANCE OR POSITION IN THE HORIZONTAL 





The measurement of the width of the channel
and of individual segments or finding the position across the river
relative to a fixed reference, are obtained by measuring the distance
from or to a fixed reference point on the river bank. The technique
selected depends on the width of the channel and the method of deployment
used for gauging.




 Wading gauging
 Bridges
 Fixed cableways with bankside winch (unmanned
instrument carriage)
 Fixed cableways with cable car (winch and
cradle/manned trolley)
 If gauging is by boat, it includes following
techniques:



o Pivot point method
o Linear measurement methods
o Angular method, or using Sextant
o Stadia method, and
o Geographic positioning system (GPS)











Of several alternatives as listed above,
often used method in field is 'Pivot Point
Method' shown right; where position of a boat is fixed
with two rays merging at point M, in the middle of river. Here,
all other points, such as D, C, E and N are marked on the bank,
and interdistance between these points help estimate MD  the distance
of vertical/boat from a benchmark point D. By switching the position
of N, i.e. different CN values, boat position at other points can
be easily determined. In another approach, an officer is seen here
holding a sextant to determine the location of his boat along the
measuring section.













Another layout of 'Pivot Point Method'
shown here defines various positions of boats across the river by
holding flags at E1, E2  one by one. Either of the two methods
can be used in field according to site condition.












MEASUREMENT
OF DEPTH 





Once a team establishes its position
at a predefined vertical in the middle of river, their next target
is to measure river depth. The method of depth measurement during
gauging depends on depth and velocity and whether done by wading,
cableway, bridge or boat. Depth and position in the vertical are
measured by rigid rod or by a sounding weight suspended from a cable
provided that velocities are not too high. Regardless of what method
is opted for, at least two observations of depth are suggested at
each vertical and the mean of the two values used for area and discharge
computation.
Following instruments are commonly used in depth measurement:








o Wading rods
o Sounding rods
o Sounding reels and cables (including wet and airline correction)
o Echosounder






For a shallow river/stream, normally
wading or sounding rods is used to measure depth. For deeper rivers,
and where velocity is high, ecosounder is commonly used instrument.
The sounding transducer mounted underwater, releases bursts of ultrasonic
energy at fixed intervals and the instrument measures the time required
for these pulses of energy to travel to the stream bed and to be
reflected and return to the transducer. With the known propagation
velocity of sound in water, the sounder computes and records the
depth on a strip chart, dial, data logger or portable PC. Here,
in the picture, observer is seen measuring the depth by wading rod.












TYPE
OF CURRENTMETERS 





There are two types of current (point
velocity) meter which are used in India.




1.
Verticalaxis meter  cup/buckets



2. Horizontalaxis
meters  helical screw (impeller) 






Examples of both these types of meter
are shown right. The vertical axis cup or bucket type meter is the
most widely used current meter in India. Both types of meter have
their advantages and disadvantages. However, if they are maintained
well and deployed correctly then they should give satisfactory performance
and results.
The rotating element current meter operates on the proportionality
between local flow velocity and the local angular velocity of the
meter rotor. The relationship between velocity and rotor speed is
usually established experimentally by towing the meter at various
velocities through sensibly still water and recording the revolutions
of the rotor.
The calibration relationship is usually
of the form:




v = a + bn
where: v = water velocity (m/s)
n = speed of impeller (revs/s)
a, b = constants 


Optionally, a calibration chart can also
be used to read velocity against speed of impeller.









Generally for most Indian applications
it is recommended that an exposure time of 60 seconds be adopted.
If the velocities are very low and there are less than 20 counts
in fifty seconds the exposure time should be increased to 100 seconds.
Alternatively the time it takes to record
20 revolutions should be measured. In situations where the stage
is varying rapidly it is possible that the exposure time could be
reduced to 30 seconds.







SELECTION
OF CURRENT METER 






A limited number of current meters (often
2) are available for gauging at a particular station, usually a
standard larger diameter (100 to 125 mm diameter) and a smaller
diameter Pygmy meter. The larger diameter meter will be used for
bridge, cableway and boat measurement but for wading gauging the
meter chosen will depend on the depth of flow.





The selected current meter should normally
not be used in water less than four times the diameter of the impeller
because the registration of the meter is affected by its proximity
to the water surface and the bed. Thus a standard 100 mm diameter
meter should not normally be used when the depth of water across
the section is less than 0.4 metres. In addition, ISO 748 recommends
that the horizontal axis of the current meter is situated at a distance
not less than one and a half times the rotor height from the water
surface or three times the rotor height from the bed. In particular
no part of the meter should break the surface of the water. Where
only a small number of verticals (< 4) exist on the river margins
with a depth less than 0.4 m, it is usually not profitable to change
to a smaller meter.





The miniature (Pygmy) meter is best suited
for gauging in depths of less than 0.5 metres when the expected
velocity is less than the meter's maximum calibration velocity (usually
about 1 m/sec).





In general, meters should be selected
which will operate within their calibration range, and particular
consideration should be given to performance and the minimum speed
of response in rivers with very low velocities.





If a river is too deep or too rapid
to wade, the current meter is suspended from a boat, bridge or cableway.
A sounding weight is suspended below the current meter to keep it
stationary in the water. The weight also prevents damage to the
meter when the assembly is lowered to the bed provided the instrument
is handled carefully and the bed can be detected. The size of the
sounding weight used in current meter measurements depends on the
depth and velocity in the cross section. As a rule of thumb the
size of the weight in kg should be greater than 5 times the product
of velocity (m/sec) and depth (metres).







SELECTION
OF POINTS FOR VELOCITY MEASUREMENT 





Current meters measure the velocity of
water at a point. The measurement of discharge in open channels
requires the determination of mean velocity for each sampling vertical
across the measuring section. A number of methods are in use to
define the mean velocity in a vertical. Methods are usually defined
by the number of measurements taken in each vertical.













If the velocity distribution in a vertical
is close to the regular classical form then it can be assumed that
the mean velocity occurs at 0.6 of the depth (D) from the surface
i.e. 0.6D. The one (0.6D) and two point (0.2D & 0.8D) methods
are adequate for most routine fieldwork. The former is used for
depths less than 1.0 m and the latter for depths greater than a
1.0m, but for the latter also the 0.6D method may be used. In some
cases it is only possible to use the surface velocity method in
which case the surface velocity is multiplied to a coefficient similar
to that for a surface water float, say 0.85. Such coefficients should
be confirmed by estimating the mean velocity by another method.
Normally, for smoother velocity profiles,
average velocities are more or less same, and are independent of
methods used to determine it.
NOTE: In terms of reducing the overall
uncertainty in the discharge measurement it is better to use more
verticals than trying to measure more points in the vertical.







CURRENT
METER GAUGING BY WADING 





A measuring tape or tag line is stretched
across the river at right angles to the direction of flow. The positions
of successive verticals used for depth and velocity are located
by horizontal measurements from a reference marker (initial point)
on the bank usually defined by a pin or a monument.













The position of the operator is important
to ensure that the operator's body does not affect the flow pattern
at or approaching the current meter. The best position is to stand
facing one or other of the banks, slightly downstream from the meter
and at arm's length from it. The rod is kept vertical throughout
the measurement and the meter parallel to the direction of flow.
In very narrow channels, avoid standing in the water if feet and
legs would occupy a considerable percentage of the cross section;
stand on a plank or other support rather than in the water if conditions
permit.
Wading rods are usually marked in centimetres
and measurements made to the nearest 5 mm







GUAGING FROM CABLEWAYS 





Cableways are normally used when the
depth of flow is too deep for wading, when wading in a swift current
is considered dangerous or when the measuring section is too wide
to string a tag line or tape across it.
The operating procedure depends on the
type of cableway, whether it is an unmanned instrument carriage
controlled from the bank by means of a winch, or a manned personnel
carriage or cablecar which travels across the river to make the
observations.













In the case of the unmanned cableway,
the operator on the bank is able to move the current meter and sounding
weight and to place the current meter at the desired point in the
river by means of distance and depth counters on the winch. The
electrical pulses from the current meter are returned through a
coaxial suspension cable and registered on a revolution counter.
The manned cableway is provided with
a support for a gauging reel, a guide pulley for the suspension
cable and a protractor for reading the vertical angle of the suspension
cable.







GUAGING
FROM BRIDGES 





When a river cannot be waded, suitable
bridges may be used for current meter measurement and intervals
on the chosen side (upstream or downstream) marked in advance at
a small enough interval to allow sufficient vertical to be taken
when the width of flow is at a minimum.













Low footbridges can sometimes be used
on a small stream with rod suspension with extension rods. The procedure
in low velocities is the same as for a wading measurement but the
procedure for obtaining the depth in higher velocities should be
modified to eliminate errors caused by the water piling up on the
upstream face of the rod as follows:
 For each selected vertical, a point
is established on the bridge
 The distance from this point to the
water surface is measured by lowering the rod until the base plate
just touches the water.
 The rod is then lowered to the bed
and the reading again noted at the index point. The difference
in these readings is the depth of water in the vertical.




For road bridges care must be taken to
ensure that road traffic does not endanger the gauging team or other
road users. Particular precaution must be taken on narrow bridges
without pedestrian walkways. Warning signs should be set up at appropriate
distances on both approaches and the area of working clearly delimited
by marker cones. Additional precautions should be taken when the
section is subject to the passage of river traffic, with one team
member stationed as a lookout to give warning of approaching craft
with sufficient time to reel in the suspended equipment.







GUAGING
FROM BOATS 





Discharge measurements are made from
boats where no gauging cableways or suitable bridges are available
and the river is too deep to wade. The boat is held in place in
the measuring section either by fixing to a cable strung across
the river (the boat/cableway method) or by using an adequately powered
boat.













If the maximum depth in the section is
less than 3 metres and the velocity is low, rods can be used for
measuring the depth and supporting the current meter. Otherwise
cable suspension with a reel and sounding weight is used as for
bridge and cableway measurement.
Position in the cross section may be
fixed by using markers on the supporting cableway, by tag line from
the shore, or by the use of a variety of surveying methods based
on bankside flags. For a particular station, the precise method
of observation should be established in advance.
When a boat powered by a motor is used,
it is often difficult to maintain it exactly on the transit line
throughout the measurement of velocity. Where the position of the
boat can be established with precision at the beginning and end
of the velocity measurement, a correction to the observed velocity
can be made. CWC apply the following formula
V_{p}
= 0.064 + 0.98V_{o} + 0.98V_{d}
where :
V_{p} = True velocity in m/sec
V_{o} = Observed velocity with the boat drifting, and
V_{d} = Drift velocity in m/sec (Drift in metres / Meter
exposure time)




Personal safety is an important consideration
in boat gauging, and velocity of flow in relation to the power of
the boat will limit the conditions under which gauging is possible.
All members of the crew should wear serviceable life jackets. The
crew should always include one member specifically assigned to the
task of propelling, controlling and positioning the boat and that
person should have no other function. No gauging should be attempted
on any section less than 500 metres upstream from a weir, sluice,
waterfall or rapids unless special safety measures have been provided
(e.g. rescue vessel).







SPECIAL
CONDITIONS 





There are a number of conditions associated
with cableway, bridge and boat gauging which require additional
measurement and computation. These include cableway drag and consequent
depth corrections, corrections for oblique angle of flow and the
effects of rapid changes in stage during the gauging.







Drag
(Wet line / Dry line corrections) 





When measurements are made by suspending
the current meter in deep swift water, it is carried downstream
before the weight touches the bottom (Figure 1). The length of cable
paid out is more than the true depth. In order to obtain the corrected
depth, dry/air line and wet line corrections, which are functions
of the vertical angle p, are applied to the observed depth, where
the angle p is measured by a fixed protractor.









Figure 1
Definition sketch for dry line and wet line corrections






The recommended routine procedure
is as follows: 





 Measure the vertical distance from
the guide pulley on the gauging reel to the water surface using
the reel counter. This is (ab) the "airline" depth.
 Place the bottom of the weight at
the water surface and set the depth counter on the gauging reel
to read zero.
 Lower the weight to the bed. Read
the sounded depth (ce) and the vertical angle a
of the cable on the protractor.
 The airline correction (cd) is airline
depth (ab) * (sec p  1).
 Calculate the wet line length as (sounded
depth  air line correction) (de = ce  cd)
 The wet line correction for given
angle p is shown in Table below. The correction is applied to
wet line depth.





7. Wet
line depth minus wet line correction is true depth of water.








EXAMPLE
: AIR LINE AND WET LINE CORRECTIONS
The total length of a sounding line when
the sinker weight is touching the bed of a river = 7.55 m. The depth
from guide pulley to water surface = 3.0 m. The angle between the
vertical and the sounding line at the point of suspension i.e. p
= 20^{o} . What is the true depth of the vertical?
Solution:
ce = ae  ab = 7.55  3.0 = 4.55
Air line correction, cd = (sec a  1)
x ab = (sec 20 1) x 3 = (1.064  1) x 3.0= 0.19 m.
Wet line depth, de = ce  cd = 4.55  0.19 = 4.36 m.
Wet line % correction for p, 20 = 2.04% from Table
Wet line correction = 2.04% x 4.36 = 0.0204 x 4.36 = 0.09
True depth = ce  air line corr.  wet line corr. = 4.55  0.19
 0.09 = 4.27 m







OBLIQUE
ANGLE OF FLOW 





Current meter gauging requires the measurement
of the horizontal component of velocity perpendicular to the crosssection
at each point being sampled. It is not always possible to select
a measurement section which is at right angles to the direction
of flow, especially in the case of bridge measurement. In other
cases, flow across part of the section may approach it at an oblique
angle. It is necessary to obtain the component of velocity normal
to the cross section.
Propeller type meters on rod, held firmly
at right angles to the cross section will measure the component
velocity in such oblique flows and do not need correction. However,
cuptype meters and propeller meters on cable suspension align themselves
directly into the current and require correction by multiplying
the measured velocity by the cosine of the angle between the current
direction and the normal direction.








V_{corrected} = V_{measured}
x cos q 





where: q = the angle between the
direction of flow and the perpendicular to the crosssection. 




Rapidly
changing stage  Assessment of mean gauge height 





The mean gauge height corresponding to
the measured discharge is used in plotting the stage discharge
relationship or rating curve for gauging stations. An accurate determination
of the gauge height is therefore as important as the accurate measurement
of discharge. Where the change in gauge height during a measurement
is less than 0.05 m, the arithmetic mean of the gauge heights at
the start and end of the measurement can usually be taken as the
mean gauge height. However if the gauge height changes rapidly and
irregularly, the mean is obtained by weighting the gauge height
readings taken during the gauging by the corresponding measured
segment discharges that they represent.
The equation used is:










The observer must read the gauge
before and after the measurement and at intervals during the gauging. 




Rapidly
changing stage  Quick Approach 





Sometimes water level changes so rapidly,
especially during rising flood conditions, which it is difficult
to assign a mean gauge height to the gauging if the normal number
of intervals and exposure time is adopted. In these circumstances
it is legitimate to simplify the gauging such that it can be completed
in less than 30 minutes. The following simplifications may be made,
either singly or in combination.








 Reduce the number of vertical taken to about
15 to 18
 Reduce the velocity observation time to
about 30 secs.
 Use measurement only at 0.2 depth and multiply
the measured velocities by 0.87 (or an alternative value based on a
previous full gauging at the site) to obtain mean velocity in the vertical
 Use a presurveyed cross section to assess
depth at each vertical from the gauge height observed at each measurement
vertical and hence to set the meter at the measurement depth without
sounding.

COMPUTATION
OF DISCHARGE 





The computation of discharge can be attempted
by midsection method. In India, agencies involved in collection
of discharge observation activities use SWDES software  a freeware
for computation of discharge, storage of a range of hydrometeorological
data and its primary validation. This software estimates discharge
value following midsection method. Secondary and basin level data
validation and analyses are accomplished with HYMOS software.













Here below are steps enumerated for discharge
estimation by Midsection method for a river crosssection layout
shown here. Water edge begins at relative distance(RD) 7.0m on left
bank, and ends at RD 59m opposite bank. Schematic below displays
RDs & corresponding depth. Velocity was recorded at 0.60 at
each vertical.













 Segment Width: Since the midsection
method assumes that the velocity sampled at each vertical represents
a mean velocity in a segment, the segment width (and area) extends
from half the distance from the preceding vertical to half the
distance to the next. Here, in the current example, barring end
segments on either ends, all verticals are 5m apart




 Segment Area: Segment area is computed
as below
a_{i}
= d_{i} * (b_{i+1}  b_{i1}) / 2
where, b_{i+1}, b_{i1}
=distances from an initial (reference) point on the bank to
verticals i +1 and i  1, d_{i} = depth of flow at vertical
'i'. Let us denote first RD & its depth as b_{i1}
& d_{i1} respectively.
 Segment Velocity: Velocity at a point
is read from the appropriate current meter rating table for given
revolutions and time. Corrections as required are made for skewed
flow and drift. For measurements at 0.6d only, the vertical and
segment velocity are the same as the point velocity. For 2point
measurement at 0.2D and 0.8D the segment velocity is the mean
of the two velocities. However, when the cross section boundary
is vertical at the edge (e.g. bridge abutments and piers), the
segment velocity may not be zero and it is usually necessary to
estimate the velocity at the end segments as a percentage of the
velocity on the adjacent vertical because it may not be possible
to place the current meter close to the boundary




 Segment Discharge: Segment discharge
is the product of segment area and velocity. In algebraic terms
this is as follows:
q_{i} = v_{i}a_{i}
where q_{i} = discharge through segment i
v = mean velocity in vertical i
For first segment, a_{1} = 4m^{2},
v_{1} = 0.3,(refer to table next),
Hence, q_{1} = 4 * 0.3 = 1.2
m^{3}/sec or 1.2 cumecs




 River discharge is finally obtained
by summing up all segmental discharge
Q = q_{i}




 Wetted Perimeter is determined by
equation shown below




 A discharge computation sheet shown
here under presents calculations involved in estimation of discharge
and other parameters by midsection methods. This sheet can
be downloaded by users using the link available on Course webpage.
A tiny triangle in red at upper right corner of few cells contains
comments and hints that will guide users finish their assignment.
User may upload a complete assignment sheet for its evaluation
once this particular assignment is over.








REFERENCES 





 Design and Field
Manual of Hydrometry published under Hydrology Project Phase I
 World Meteorological
Organization Document on Stream Gauging by Conventional Current
Meter Method







CONTRIBUTOR 

Anup K Srivastava,
Director, National Water Academy 











































