Discharge Measurements in Canals
DISCHARGE MEASUREMENT OF A CANAL/DRAIN
AND CALIBRATION OF HYDRAULIC
STRUCTURES
A. Introduction
The key to conservation of water is good water management
and this cannot be achieved without measurement of surface and drainage flows.
The good water measurement practices facilitate accurate and equitable water
distribution. The regular discharge measurement can add the following advantages:
-
ü
Reliable and proper flow measurement data of the
system.
ü
Transparency in information and data sharing for
all stakeholders.
ü
Standard and uniform application of flow
formulae and discharge coefficients within Irrigation department.
ü
Proper water accounting and audit.
ü
Capacity building and development of knowledge
of the irrigation network for use by the irrigation Engineers.
B. Concept of Canal/Drain Discharge
Canal discharge is the volume of water flowing through a
cross section of a waterway per time unit. The unit used to measure canal
discharge is usually (ft3/sec-cusecs) or (m3/sec- cumecs). Symbol for canal
discharge is Q.
Stream flow records are primarily continuous records of flow
passing through a particular section of the stream. Stream flow data are
analyzed to determine the magnitude and variability of surface waters. These
records are input in planning, design, and operation of surface water projects
and are also used in design of bridges and culverts, flood forecasting systems,
and flood plain delineation. The sections where canal measurements are carried
out are known as canal gauging stations. A network of these stations is
established to collect data about surface water resources of a region. There
exists a relationship between stage and discharge at a section. This relation
is termed as stage-discharge relationship or rating curve.
The international Standard Organization (ISO) and World
Meteorological Organization (WMO) have brought out many publication related to
stream flow measurement; individual countries have their own standards.
C. Computation of Discharges of Canals/Drains
The two most significant flow regimes under which any open
channel constriction may operate are free flow and submerged flow. Other terms
for free flow are critical depth flow and modular flows, while other terms for
submerged flow are drowned flow and non-modular flow. The distinguishing difference
between the two flow conditions is the occurrence of critical velocity in the
vicinity of the constriction (usually a very short distance upstream of the
narrowest portion of the constriction). When this critical flow control occurs,
the discharge is uniquely related to the depth or “head” upstream of the
critical section. Thus, measurement flow depth at some specified location
upstream from the point of the critical condition is all that is necessary to
obtain the free flow discharge.
When the flow conditions are such that the downstream depth
is raised to the extent that the flow velocity at every point through the
constriction becomes less than the critical value, then the constriction is
operating under submerged flow conditions.
The following hydraulic formulae are frequently used for
computation of discharges at control points:-
Condition
of Flow
|
Formula
|
(i). Standing Wave formed and
gates are free
(Weir Type Free Flow Conditions)
|
Q = CB (H+Ha)3/2
Where
Q = Discharge in Cusecs
C = Coefficient of discharge = 3-3.4 for
canals and drains
= 3.3 to 4.8 for Barrages
B = Clear weir width at crest (feet)
H = Depth of water over the crest (feet)
Ha = Velocity head = V2/2g
(feet)
V = Average velocity of flow (ft/ sec)
g = Acceleration due to gravity = 32.2 ft/sec2
|
(ii). Standing Waves formed
when the D/S water level is above crest level
(All types of submerged flow
conditions)
|
Q = Cd A (h)1/2
Cd = Coefficient of discharge
for submerged flow condition = 5 to 7
A =
Flow Area (ft2)
h = working head = U/S Water Level
– D/S Water Level (feet)
|
Head Discharge
Relationships
The following hydraulic formulae are frequently used for
computation of discharges at control points.
Value of Coefficient and Exponents for Discharge Equations
Constriction
Type
|
Flow
Condition
|
Equation
|
Coefficient
|
Exponent
|
Reference
|
Sharp
crested weir
|
Free
flow
|
Q = Ce2/3
bc
h1.5
|
0.56-0.78
|
1.5
|
Boes
et.al 1987
|
Broad Crested Weir
|
Free Flow
|
Q = C B (H + ha)1.5
|
3.2
|
1.5
|
|
Open channel constriction
|
Free flow
|
0.73
|
1.5
|
Skogerboe
et. Al 1987
|
|
Open channel constriction
|
Submerged
|
nf=1.55
ns=1.0-1.5
|
Skogerboe
et. Al 1987
|
||
Orifice
|
Submerged
|
Qs=Cd
A [2g (hu – hd) ]0.5
|
0.60-0.80
|
0.50
|
Skogerboe
et. Al 1987
|
Orifice
|
Qs=Cd
A [2g (hu – hd) ]0.5
|
0.30-0.90
|
0.50
|
Skogerboe
et. Al 1987
|
|
Rectangular
control section
|
Free
flow
|
Q= Cd 2/3 [2/3 g ]0.5 bc
H1.5
|
0.1-1.0
|
1.5
|
Boes
et.al 1987
|
Orifice
|
Submerged
|
Q = C A
|
0.66
|
0.5
|
Boes
et.al 1987
|
Source:
IWMI (1997).
Channel constriction, broad crested weir and gated orifice
are used in Indus Basin System. Value of coefficients and exponent given in
above table are theoretical. These may vary due to field conditions variations.
Therefore actual head-discharge equations should be developed based on the
field measurements.
D. Arrangements for Discharge Measurements and Calibration
The steps for the calibration process are the following:-
ü
Planning
ü
Data collection
ü
Procedure equipment and staff
ü
Establish field procedures and work program
ü
Perform work program
ü
Analyze, evaluate and document results
ü
Prepare recommendations.
The calibration of an irrigation system is not an
undertaking that requires a major effort. It does require a well trained field
crew, survey and water measurement equipment and transportation. Even the date
collection step can be minimized as it is used more for planning and evaluation
than for conducting the actual field work.
Planning
It is a continual process that proceeds as the program advances.
Initially it involves the decision of what is to be done, how to do it, what
and who to do it, obtaining necessary personnel and equipment including
transportation and communication, providing any training or instruction required
to make the team more efficient and preparing the work program and making
adjustments as required.
Data Collection
Since calibration is the first step of developing a flow
monitoring program it is necessary to start at the beginning. Therefore, the
first step is to obtain a copy of the following data:
ü
System map
ü
Schematic diagram locating all structures
ü
As-built drawings or latest long-sections
ü
List of all sanctioned outlets and RD’s
(location)
ü
Location and datum for all bench marks along
canal/drain.
ü
Stage-discharge tables or curves available if
any.
Staff and
Equipment Requirements
The calibration requires two specialists; a surveyor and a
hydrographer. This constitutes the bare minimum and they should really have an
assistant to help with all of the work. As it is, the surveyor and hydrographer
would have to assist each other as much of the work requires two people.
The size of the crew depends on the speed at which the team
should work. A team for man, a 5-man survey crew, a 2-man hydrographer team, a
12-man flume measurement team, driver and a couple of casual laborers could
cover one or two canal circles a year. Reductions would obviously decrease the
work rate. A 6 to 7-man crew should be adequate at the division level; says
foreman, surveyor, hydrographer and 3 to 4 assistants. The number of assistants
depends somewhat on the number of portable flumes to be used. A 3-man crew
appears to be the minimum requirement.
Transportation and communication are required for two
reasons; (1) to move the team and its equipment along the canal and (2) to
continuously know what the discharge in the canal is in order to establish water
surface elevations at known canal discharges. Calibration should be made
throughout the normal range of flows.
A pickup truck would be the most practical transportation
unit that could be used. It has the capacity of carrying all the equipment
including the portable flumes or full a boat trailer. Other vehicles including
vans could be used but the field requirement must be transported carefully.
Field Procedures
A primary objective of this lecture is to make minimum use
of existing facilities for water measurement. This will require calibration and
rating of various hydraulic structures. The primary problem will be submerged
flow conditions. Unfortunately, in many cases, submergence may exceed 95
percent, making measurements less reliable. It is expected that gated and up
gated head works for distributaries as well other water control structures will
form the basis of measurement discharge stations. The outfall points of sub
drains branch drains and main drains would be best measurement sites for
drainage discharge.
Under the best of conditions hydraulic structures built for
the purpose of measuring the rate of low consist of a converging transition,
where subcritical flowing water is accelerated and guided to the crest without
flow separation; a crestt where accelerates to supercritical flow so that the
discharge is controlled, and a diverging transaction where the flow velocity is
gradually reduced to an acceptable subcritical velocity and potential energy is
recovered.
Under most field conditions, many hydraulic structures used
for the control of flows eliminate or ignore one or more of these parts. The
two most common divisions are; first, tail water conditions that submerge the
flow through the throat and eliminate modular flow and second, most canal head
works are set 90° to the streamlines in the senior canal. The latter often cause
non-uniform flow patterns which affect both the water and sediment discharge
patterns.
It is assumed that most of the existing stage-discharge
relations are based on the engineer’s field observations. In addition, changes
in channel control due to sedimentation or other reasons require a periodic
check on the stage- discharge relations.
E. Field Measurement Accuracy
The need for all calibration team members to work in a
professional manner cannot be over emphasized. We are referring to the
requirement that all work be accomplished in a meticulous manner includes neat
and detailed notes, accurate measurements, attention to details, checking and
rechecking of measurements, completion of all measurement, data collection and
entries on the appropriate forms.
Discharge measurements can be consistently be made in the
field with errors on the order of 2 or less if all measurements are carefully
made. It is important to know which measurements are the most critical. For
instance, an error of 0.010 feet (1/8th inch) in the water surface elevation
(the sill reference height) on an outlet results in an error of more than 2 in
the measurement of discharge. Fortunately, errors in measurements are not all
additive, some compensate each other.
Scale is an important concept to remember. This refers size
of the difference with respect to the total. For instance, 2 cfs in 200 cfs versus 2 cfs in 20 cfs or 0.01
inch in 12 inches and 1 inch in 50 feet. The most important dimension is the
water surface elevation. The water surface elevation, W.S. Elev., should be
read to the closest 1/6th or 1/8th inch depending on canal and water surface
movements.
Frequency of
Discharge Measurements
Initially the discharge measurements are made with the
frequency necessary to define the station rating, as early as possible, over a
wide range of stages. Measurements are then made at periodic intervals to
verify the rating or to define any changes in the rating caused by changes in
the stream channel. Monthly observations of canal/drain discharge are generally
sufficient, though daily data are needed to calculate the statistical
parameters of the discharge. Manual water discharge observations are still in
practice. However, in many countries even paper systems for continuous data
recording have been almost entirely replaced by largely automated electronic
logging, analysis and data transmission systems.
F. Location of Flow Measurements
In general, flows are measured at any point where the
conveyance system flows are divided between two or more channels. As an
everyday procedure, flows are not directly measured into watercourses. Flows
should normally be measured into all canals/drain, although there are cases
where it is not practical to measured flows in main or branch canals/drains
each time a small distribuaery is measured. The flow in the main canal would be
measured at the next downstream control structure and at outfall points in case
of drains.
Flow measurements are made in order to account for all water
coming into a system as well as its distribution throughout the system.
Although flows are not measured in watercourses/sub drains as a routine, they
should be measured when calibrating a distributary/main or branch drain.
G. Selection of Discharge Measuring Site
Discharge measurements by the current-meter method are made
from a boat, from a cable or bridge or by wading the stream. In irrigation
canals mostly the boat measurements are employed for the purpose. The
measurement need not to be made at the exact location of the gauge because the
discharge is normally the same as at the gauge throughout a reach of channel in
the general vicinity of the gauge. Discharge measurements of the highest
accuracy will be obtained at cross-sections having the following
characteristics:
ü
The velocity at all points is parallel to one
another and at right-angles to the cross-section of the stream.
ü
The curves of distribution of velocity in the
section are regular in the vertical and horizontal planes.
ü
The velocity is greater than 0.3-0.5 foot (10-15
cm) per second.
ü
The bed of the channel is regular and stable.
ü
The stream does not overflow the banks of the
channel.
ü
There is no aquatic growth.
It is usually not possible to find a site with all of these
characteristics. A satisfactory measurement can still be made at most sections
by increasing the number of observation points or by using special equipment.
H. Use of Current Meters
Current meters are velocity measuring devices that sample at
a point. Each point velocity measurement is then assigned to a meaningful part
of the entire cross section passing flow. The velocity-area principal is used
to compute discharge from current-meter data. Total discharge is determined by
summation of partial discharges. Data are usually determined over a useful
range of total discharges. These discharges are related to measured water
surface elevations related to a fixed head measuring device to provide a rating
curve. After full confidence in the rating is attained, the calibrated
head measurement
device and cross section may be used as a gaging station.
Price Current
Meter
Anemometer and propeller current meters are the most common
type used for irrigation and watershed measurements. These meters use
anemometer cup wheels or propellers to sense velocity. The Price current meter
and the smaller pygmy meter modification are the most common current meters in
use. These meters are rated by dragging them through tanks of still water at
known speeds. The reliability and accuracy of measurement with these meters are
easily assessed by checking mechanical parts for damage and using spin-time
tests for excess change of bearing friction. This type current meter does not
sense direction of velocity, which may cause problems in complicated flow where
backflow might not be readily apparent. For irrigation needs, this problem can
be avoided by proper gage station or single measurement site selection.
Acoustic Doppler
Current Profiler (ADCP)
Doppler type current meters determine velocity by measuring
the change of source light or sound frequency from the frequency of reflections
from moving particles such as small sediment and air bubbles. Laser light is
used with laser Doppler velocimeters (LDV), and sound is used with acoustic
Doppler velocimeters (ADV). Acoustic Doppler current profilers (ADCP) have also
been developed. These instruments measure average velocities k of cells of
selected size in a vertical series. Thus, they measure vertical current
profiles. ADCP measurements are becoming more frequent for deep flow in
reservoirs, oceans, and large rivers. Most of the meters in this class are
multidimensional or can simultaneously measure more than a single directional
component of velocity at a time.
The ADCP is a high performance water current profiler. It
measures water current with sound using a principle of sound waves. It receives
the velocity of water using a physical principle called Doppler shift.
The main external components of an ADCP are transducer
assembly and a pressure case. The transducer assembly consists of four
transducers that operate at a fixed, ultrasonic frequency, typically 300, 600,
or 1200 kilohertz (kHz). The transducers are horizontally spaced 90 degrees
apart on the transducer assembly; all transducers have the same fixed angle
from the vertical, referred to as a “beam angle,” that is typically 20 or 30
degrees. The transducer assembly may have a convex or concave configuration.
The pressure case is attached to the transducer assembly and contains most of
the instrument electronics.
When an ADCP is deployed from a moving boat, it is connected
by cable to a power source and to a portable microcomputer. The computer is
used to program the instrument, monitor its operation, and collect and store
the data.
The ADCP measures velocity magnitude and direction using the
Doppler shift of acoustic energy reflected by material suspended in the water
column. The ADCP transmits pairs of sort acoustic pulses along a narrow beam
from each of the four transducers. As the pulses travel through the water
column, they strike suspended sediment and organic particles that reflect some
of the acoustic energy back to the ADCP. The ADCP receives and records the
reflected pulses. The reflected pulses are separated by time differences into
successive, uniformly spaced volumes called “depth cells.” The frequency shift
(known as the ‘Doppler effect”) and the time-lag change between successive
reflected pulses are proportional to the velocity of the scatters relative to
the ADCP. The ADCP computes a velocity component along each beam; because the
beams are positioned orthogonally to one another and at a known angle from the
vertical (usually 20 or 30 degree), trigonometric relations are used to compute
three-dimensional water-velocity vectors for each depth cell. Thus, the ADCP
produces vertical velocity profiles composed of water speeds and directions at
regularly spaced intervals.
ADCP method of discharge measurement of Rivers is being used
in almost all the major river basins of the World and its viability and
correctness stands verified in research work published for the last ten to
twenty five years. ADCP(s) provide the capability to make accurate, rapid and
cost-effective measurements of river discharge. This method is recommended to
be used for calibration of structures of Indus Basin Irrigation System.
Optical Strobe
Velocity Meters
Optical strobe velocity meters developed by the U.S.
Geological Survey (USGS) and the California Department of Water Resources use
optical methods to determine surface velocities of streams (USGS, 1965). This
meter uses the strobe effect. Mirrors are mounted around a polygon drum that
can be rotated at precisely controlled speeds. Light coming from the water
surface is reflected by the mirrors into a lens system and an eyepiece. The
rate of rotation of the mirror drum is varied while viewing the reflected
images in the eyepiece. At the proper rotational speed, images become steady
and appear as if the surface of the water is still. By reading the rate of
rotation of the drum and knowing the distance from the mirrors to the water
surface, the velocity of the surface can be determined. The discharge rate of
the stream may be estimated by applying the proper coefficient to this surface
velocity and multiplying by the cross-sectional area of the flow section.
This velocity meter has several advantages. No parts are
immersed in the flowing stream. Moreover, it can be used for high-velocity
flows and for flows carrying debris and heavy sediment. The meter can measure
large flood flows from bridges. However, the meter measures only the water
surface velocity and is very dependent upon the selection of the proper
coefficient.
Whenever possible, current-meter gaging stations should be
located in straight, uniform stretches of channel having smooth banks and beds
of permanent nature. The station should be located far from flow disturbances
caused by turnouts and power stations. These flow disturbances will variably
affect the relationship of discharge to gage height. In many channels, these
conditions are difficult to find, and unusual care must be taken to obtain a
satisfactory location.
The changing nature of some rivers and canals may require
frequent current-meter measurements. Sand shifts may occur frequently, often
daily, and aquatic weeds may continue to grow and increase in area. To obtain
the gage-discharge relationship at stations on such streams, current-meter
measurements may be necessary two or three times weekly or perhaps daily if the
importance of equitable water distribution justifies such action. A rating
section consisting of a short-lined section in a straight stretch of channel
will ensure a meter station of unvarying dimensions if the sediment problem is
not serious.
Pygmy Meters
(Vertical Axis)
Pygmy meters are similar to Price meters in that both
contain a cup-type wheel mounted on a vertical shaft. The pygmy cup wheel is 2
in diameter, compared with 5 in for conventional Price meters. Thus, the
pygmy meter can measure velocities closer to flow boundaries. The contact
chamber is an integral part of the yoke and contains a single-revolution
contact only. The meter has no tailpiece, and no provision is made for cable
suspension. The rotational speed of the pygmy meter cup wheel is more than
twice that of Price meters. Consequently, use of the pygmy meter is limited to
velocities up to 3 or 4 ft/s. The pygmy meter was specially designed for use in
small, shallow streams. The smaller meter was necessary because a standard. Price
meter does not perform with sufficient accuracy when it occupies a good share
of the available stream depth. The pygmy meter may also be used in large canals
where the velocity of flow is low or near the edges of a canal to supplement
data taken farther out in the channel with a Price meter.
Propeller Meters
(Horizontal Axis)
In special situations, Reclamation has used meters of the
propeller type with horizontal axles. Hoff meters, Haskell meters, Ott meters,
and Neyrpic "Dumas" meters are examples. An assembly of eight Dumas
meters with appropriate handling equipment is shown on next page. In this case,
the equipment was mounted on a flatbed truck for positioning. These meters have
some advantages compared to the Price meters. They are less sensitive to
velocity components not parallel to the meter axis, they are smaller, and they
are more suited for mounting in multiple units.
Current meters must receive the best care during
transportation and use to ensure accurate velocity measurements. Particular
care should be taken when working near bridge piers and abutments, floating
debris, and also when measurements are being taken at irregular or unknown
sections and the meter is suspended on a measuring line. If the cups or blades
become bent or damaged, the results obtained from the rating curve for the
meter will be unreliable. After completing the measurements at a rating
station, the meter should be carefully cleaned. After each day's use, it should
be properly lubricated.
 |
| Velocity Measurement through Price Type AA Curret Meter |
 |
| Velocity Measurement through Digital Current Meter |
Propeller Meters (Horizontal
Axis)
and
Pygmy Meters (Vertical Axis)
The Price-type meters have special cases for storage when
the meter is not in use. For damage protection, the cup wheel should be
supported firmly on the resting pin that replaces the needle bearing while the
meter is stored or being transported. Meter damage has occurred because of
improper packing and careless handling in transportation. The meter case should
be substantial and rigid with properly fitted interior supports to prevent
movement and damage to the delicate parts.
In-Situ
Measurement
Currently there are no cost-effective, reliable options for canal/drain
discharge measurement, apart from in-situ methods. The conventional
current-meter method is most commonly used in gauging streams and is commonly
used world over.
Discharge measurements are made at each gauging station to
determine the discharge rating for the site. The discharge rating may be a
simple relation between stage and discharge or a more complex relation in which
discharge is a function of stage, slope, rate of change of stage, or other
factors.
The depth of flow in the cross-section is measured at
verticals with a rod or sounding line. As the depth is measured, observations
of velocity are obtained with a current meter at one or more points in the vertical.
The measured widths, depths, and velocities permit computation of discharge for
each segment of the cross-section. The summation of these segment discharges is
the total discharge of the site.
Types of Current
Meter Measurement
Current meter discharge measurements are classified
according to the type of equipment used and the nature of station:
ü
Wading measurement
ü
Cableway measurement
ü
Bridge measurement
ü
Boat measurement
Discharge measurement using current meter is accomplished by
measuring velocity and area. The depth of flow in the cross section is measured
at width stations with a rod or sounding line. As the depth is measured,
observations of velocity are obtained with the current meter at one or more
points in the vertical. The measured widths,
depths and velocities permit computation of discharge for each sub-area of the
cross-section. The summation of these sub-areas discharge is the total
discharge of channel.
Current meter method can be recommended to measure
discharges for calibration of structures of Indus Basin System. As most of the
measurements will be done on rivers, main canals and link canals boat
measurement will be used.
A current meter is an instrument used to measure the
velocity of flowing water. The principle of operation is based on the
proportionally between the velocity of the water and the resulting angular
velocity of the meter rotor. Velocity of water at that point is determined by
placing a current meter at a point in a stream and counting the number of
revolutions of the rotor during a measured interval of time.
The number of revolutions of the rotor is obtained by an
electrical circuit through the contact chamber of the current meter. Contact
points in the chamber are designed to complete an electrical circuit at selected
frequencies of revolution. Contact chamber can be selected having contact
points that will complete the circuit twice per revolution, once per
revolution, or once per five revolutions. The electrical impulse produces an
audible click in a headphone or registers a unit on a counting device. The
counting intervals are measured by a stopwatch.
Current meters generally can be classified into two main
types; those meter having vertical-axis rotors and those having horizontal-axis
rotors. The comparative characteristics of these two types are summarized
below:
Vertical-axis rotor with cups or
vanes
ü
Operates in lower velocities than do
horizontal-axis meters.
ü
Bearings or well-protected from silty water.
ü
Rotor is repairable in the field without
adversely affecting the rating.
ü
Single rotor serves for the entire range of
velocities.
Horizontal-axis rotor with vanes
ü
Rotor disturbs flow less than-do vertical-axis
rotors because of axial symmetry with flow direction.
ü
Rotor is less likely to be entangled by debris
than are vertical-axis rotors.
ü
Bearing friction is less than for vertical-axis
rotors because bending moments on the rotor are eliminated.
The vertical-axis current meter, the Price meter, type AA
which is extensively used by the US Geological Survey (USGS). The standard
price meter has a rotor, 5 inches in diameter and 2 inches high with six cones
shaped cups mounted on a stainless steel shaft. A pivot bearing supports the
rotor shaft. The contact chamber houses the upper part of the shaft an eccentric
contact that wipes a bead of solder on a slender bronze wire (cat’s whisker)
attached to the binding post. A separate reduction gear (pent gear), wire and
binding post provide a contact each time the rotor makes five revolutions. A
tailpiece keeps the meter pointing into the current direction.
In addition to the standard type AA meter for general use,
there is a type AA meter for low velocities, no pent gear is provided in it.
This modification reduces friction. The shaft usually has two eccentrics making
two contacts per revolution. The low-velocity meter is normally rated from 0.2
to 2.5 feet per second (fps) and is used when the mean velocity at a
cross-section is less than 1 fps.
In addition to the type AA meters, a Price pygmy meter is
used for shallow depths. The Pygmy meter is scaled two fifths as large as the
standard meter and has neither a tailpiece nor a pent gear. The contact chamber
is an integral part of the yoke of the meter. The pygmy meter makes one contact
each revolution and is used for rod suspension.
The detail procedure for current meter discharge measurement
is described in the following sections.
Accuracy of
Discharge Measurement
Accuracy of a discharge measurement depends, in part, on the
number of verticals at which observations of depth and velocity are obtained.
In general, the interval between any two verticals should not be greater than
1/20-th of the total width, and the discharge between any two verticals should
not be more than 5 percent of the total discharge. Observation verticals should
be located so as to best define the variation in elevation of the stream bed
and the horizontal variation in velocity. Fewer verticals are required on very
small streams, since the intervals between any two verticals must be greater
than the diameter of the current-meter propeller. The number of verticals
should be increased for the first few measurements at a new state.
Discharge accuracy depends on the reliability of the meter
rating, conditions of flow, and number of observations of depth and velocity
obtained. New and factory calibrated, or in good condition and well maintained
and passed the spin test current meters should be used for flow measurements.
Width Measurement
Channel width and the distance between verticals should be
obtained by measuring from a fixed reference point, which should be in the same
plane as the cross-section. Normally, the distance between verticals is
determined from a wire or graduated tagline temporarily stretched across the
stream or from semi-permanent marks painted on a bridge hand-rail or a
suspension cable.
Depth Measurement
A graduated rod or a drum-wire-weight system is used for
measurement of depth of flow. The effect of drag on a sounding wire may be
reduced by using a streamlined weight on the end of a fine wire. If the wire is
not normal to the water surface, the angle of departure should be measured with
a protractor.
The depth may also be measured with an echo (sonic) sounder.
The transducer is usually mounted on the boat, submerged about 1 foot below the
water surface, and the depth read from a strip-chart recorder. With previous
models of sonic sounders, regular calibrations were required under the same
conditions of salt content and water temperature as are encountered in the
measurement. However, with the latest versions (such as Raytheon DE-719 B) the
instruments are provided with compensatory adjustment devices, thus requiring
very calibration.
Depth may be read directly on a graduated rod set on the
stream bed if the measurement is done by wading. If the drum-wire-weight system
is used for measurement, the meter and weight are lowered until the bottom of
the weight just touches the water surface, and the depth dial reading is set at
zero. The weight is then lowered until its rests on the stream bed, and the
depth is read on the dial. Caution is necessary in alluvial streams to prevent
the weight from setting through soft bed material.
In order to increase the accuracy of the depth measurement,
the sounding weight may be equipped with an electrical device which signals the
moment the weight makes contact with the stream bed. If the weight on the
sounding line is not sufficient to keep the line perpendicular to the water
surface, the Angle between the line and the vertical should be measured to the
nearest degree with a protractor.
The angle should not exceed 30o. Methods of correcting the
observed depths for angle of sounding line are available. However, the accuracy
of the measurement is increased if sufficient weight can be used to maintain
the line in a nearly vertical position.
Two Point Method
In the two-point method of measuring velocities,
observations are made in each vertical at 0.2 and 0.8 of the depth below the
surface. The average of these two observations is taken as the mean velocity in
the vertical. This method is based on many studies of actual observation and
mathematical analysis. Experience has shown that this method gives more
consistent and accurate results than any of the other methods except the
vertical velocity curve method. The two point method is the one generally used.
The two point method is not used at depths less than 2.5
feet (75 cm) because the current meter would be too close to the water surface
and to the stream bed to give dependable results.
Six-tenth Depth
Method
In six-tenth depth method, an observation of velocity at 0.6
of the depth below the surface in the vertical is used as the mean velocity in
the vertical. Actual observations supported by mathematical analyses have shown
that 0.6 depth method gives reliable results and is used under the following
conditions.
ü
Whenever the depth is between 0.3-2.5 feet (75
cm).
ü
When large amounts of slush, ice or debris make
it impossible to observe the velocity accurately at the 0.2 depth (This
condition prevents the use of the two point method).
ü
When the meter is placed a distance above the
sounding weight which makes it impossible to place the meter at the 0.8 depth.
(This also prevents the use of the two-point method).
ü
When the stage in a stream is changing rapidly
and a measurement must be made quickly.
Computation of
Discharge
Discharge is computed either arithmetically or graphical
depending upon field procedure used to obtain the observation. There are two
arithmetic methods for computing the discharge, i.e. the mean section method
and mid-section method. Of these two methods, the mid-section method has less
procedural error; therefore, it is used for computing canal discharges.
Boat Measurement
Measurement of discharge from a boat is generally made
either on channels that can be readily spanned by a temporary cable of
sufficient strength to hold the boat in position while measurements are made,
or rivers where a cable cannot be stretched. In such rivers, the boat is
equipped with a facility (motor boat engine or anchor) to keep it temporarily
positioned in the stream. On channels where cable can be stretched horizontal
distances (widths) are measured by affixing graduations on the cable. On
channels without cable, the widths are either measured by the observer in the
boat or by an observer on the bank. For the purpose, points are fixed on the
bank(s) along the line of sight and perpendicular to it.
For actual stream flow measurement by boat, the other
equipment requirements are as indicated in the following:
Boat Improvement
Set
The boat improvement set consists of two aluminum fabricated
parts known as ‘boom’ and ‘horizontal stabilizer’. The boom extends beyond the
body of the boat a sufficient distance to eliminate any possible effect of the
boat on revolutions of the current meter cups. The boom is fixed by a cross-arm
called ’horizontal stabilizer’ which is in turn fastened to the boat sides.
The boom part consists of two structural aluminum channel
shapes, one telescoped within the other, to permit adjustment in length. The
boom has a pulley at the upstream and to guide the sounding cable and base on
the downstream end to the fix the real. The cross-arm called ‘horizontal
stabilizer’ constructed of structural aluminum channel, is fixed to the
gun-whales of the boat by J-bolts. At both ends of the arm, there are guide
sheaves under which the boat tagline is passed. There is also an arrangement to
clamp this arm with the stretched tagline to keep the boat stationary at a
desired position.
Sounding Reels
A sounding real has a drum for winding the sounding cable, a
crank and ratchet assembly and a depth indicator. Five types of reels are
available i.e. A, B.C, D and E. All reels are similar in that these are fitted
with a depth indicator, a treading sheave for laying the cable smoothly in a
single layer on the drum, electrical connections for two conductor cables and a
pawl and ratchet which can be used to hold the current meter and weight at any
desired elevation. All the reels are made largely of aluminum for lightness and
designed to operate under any load to the full strength of the cable ordinarily
used. A, B and E type reels have the same spacing of anchor studs, so that they
are completely interchangeable. D type reel is larger and is used for heavy
weights only.
A type reel, which is smallest of the four, plus a
detachable crank handle which is keyed to shaft when in use, has no brake,
whereas the other three are fitted with brakes and quick-releasing cranks.
Sounding Weights
and Accessories
Columbus-type fish weights suspended with cable are used to
sound depth as well as to suspend the current meter in the moving stream. These
are commonly known as C-type weights and maintain a steady position in water
flowing at high velocities. These are available is sizes of 15, 30, 50, 75,
100, 150, 20 and 300 pounds. The nose of each weight extends beyond the cups of
the current meter and hence affords protection against any damage. 15-pound
weight is a one-piece casting of gun-metal bronze. All the other sizes are cast
from lead and contain removable aluminum-alloy tail vanes. The shape of the
slot for the hanger-strap permits it to tip forward 15o and backward 5o from
the vertical. This limitation in the angles prevents the weight and current
meter from striking each other. Due to some variation in the quality of
material used in construction, it is necessary and describes that the weight be
balanced under water, regardless of level position it assumes when suspended in
air.
The hanger strip is used for carrying the current meter, its
lower end is fixed with the sounding weight and the upper end to the cable
through a connector. It is 1/8 inch thick, 1-1.5 ft long and made of steel. It
contains a hole threaded for a 3/8 inch hole at the opposite end. Three holes,
7/32 inch in diameter, are at 4,9,5,3 and 9.8 inches above the hole for weight
suspension. The lower two of these are used for supporting the current meter so
that its horizontal axis is 0.5 ft from the bottom of C-15 and C-30
respectively. The third hole is located so that the distance from the
horizontal axis of the meter to the bottom of the C-50 or heavier weight is
approximately one foot.
Timers and
Counting Equipment
In order to determine velocity at a point with a current
meter it is necessary to count the resolution of the rotor in a measured
interval of time. The equipment required is:
ü
Stop watch; and
ü
Headphone: Theses convert into individual sound
clicks, the electrical impulses resulting from each click (for 1 or 5
revolutions of the bucket wheel as the case may be) in the chamber of the
current meter. The resistance of head phone is normally 5-8000 ohms and it
operates on 11.5 volt dry cell.
ü
Automatic Electric Counter: This counter
automatically registers the revolutions up to 999 and has reset button.
However, this counter is not recommended with the contact wire chamber because
at low velocities the contact-wire wipes irregularly thereby sending several
signals to the counter for each revolution.
Float Method
The velocity of flow in a canal or canal/drain, and hence the
discharge may be determined approximately by the use of float. A stretch of the
canal, straight and uniform in cross-section and grade, and with a minimum of
surface waves, should be chosen for this method. Surface velocities only be
attempted on windless days to avoid wind-caused deflection of floats. Even for
best conditions, surface floats are diverted from a direct course between
measuring stations because of surface disturbances and cross-currents. In
addition to surface floats, which are immersed one-fourth or less of the flow
depth, rod floats, which are submerged more than one-fourth of the depth but
not touching the bottom should be used. In general, because a number of other
methods are easier and more accurate, this method should be used when other
methods are impossible or impracticable. Hence this method is not suitable for
calibrating the Indus Systems structures.
Following coefficients are used to convert surface velocity
to mean velocity.
Average
Depth of water (feet)
|
Coefficient
|
1
|
0.66
|
2
|
0.68
|
3
|
0.70
|
4
|
0.72
|
5
|
0.74
|
6
|
0.76
|
9
|
0.77
|
12
|
0.78
|
15
|
0.79
|
20 ft
|
0.80
|
Source: USGS (1960)
Pitot Tube Method
A pitot tube in its simplest form is an instrument
consisting of a tube with a right angle bend which, when partly immersed with
the bent part under water and pointed directly into the flow, indicates
velocity of flow by the height water rises in the vertical stem. The height of
rise (h in ft)) of the water column above the water surface, expressed in feet
and tenths of feet, equals the velocity head (V2/2g). The
velocity of flow (v) in feet per second, may thus be determined from the
relation
The simple form of pitot tube has a little practical value
for measuring discharges in open channels handling low velocity flows because
the height of water rises above the water surface is difficult to measure. Therefore it cannot be used for calibration
of Indus System structures.
Dilution Method
The color dilution method consists of injecting a known
concentration of a suitable dyne into the stream and measuring its
concentration after it has traveled far enough to thoroughly mix with stream.
But this method is also not practicable for calibration of structures in the
Indus Basin System.
Radioisotope
Method
The radio isotopic method is a variation of the dilution
method and uses a radioactive material as the tracer substance. A measure of
degree of dilution is obtained by counting gamma ray emissions from the
concentrated isotope solution, and from diluted solution consisting of the
stream plus the tracer, using Geiger counter.
This method is also not suitable for calibration of
discharge measurement points of Indus Basin System.
I. General Procedures and Precautions
Accuracy of measurement can be maintained by observing the
following precautions for Price meters (including the pygmy meter modification
of the Price meter):
ü
The meter should be spin tested before and after
completing measurements to assure that the meter has no error-causing damage.
With the shaft in a vertical position and the cups protected from air currents,
the cups should be given a quick turn to start them spinning. If the meter is
in proper adjustment and the bearings are free from foreign particles, the cups
should come to rest in not less than 3 minutes. If the length of spin is only
about 12 minutes, but the cup wheel comes to rest gradually, all flows except
those of very low velocities may be measured. If the length of spin is only
about 1 minute but the cup wheel comes to rest gradually, the meter may still
be used to measure velocities above 1 ft/s. If the length of spin is less than
1 minute, the meter should be reconditioned. Under laboratory controlled
conditions, rotation should continue for about 4 minutes. The manner in which
rotation ceases will help indicate the condition of the meter and should be
observed.
ü
The cross section of the stream should be
divided vertically into 20 or more segments. Very small streams and sections
with smooth, firm boundaries are exceptions, and a smaller number of stream
cross-section segments would be sufficient. A single vertical reading is used
if the distance between verticals is less than 1 ft. Horizontal divisions are
generally selected so not more than 10 percent, and preferably not more than 5
percent, of the discharge will occur between any two adjacent verticals.
ü
The stopwatch should be checked frequently and
kept in good condition.
ü
For low and irregular velocities, the period of
observation should be lengthened to obtain a more accurate average count.
ü
The current meter should be withdrawn from the
water between velocity readings to make sure that rotation is not being impeded
by debris or any other cause.
ü
The meter should be allowed at least 10 to 20
seconds to attain rotation speed before counting commences.
ü
The total operation of the meter at each
elevation of a vertical should consist of at least two consecutive periods of
at least 40 seconds. If significant differences are apparent in each period,
more readings should be taken.
ü
Measurements while wading should be done facing the
bank, standing just downstream from the tag line, and at least 18 in to the
side of the meter.
J. Method of Measurement
Depth sounding, either with a meter and rod assembly or with
a special sounding line and weight, should first be made at each of the permanent
measuring points. These depths should be properly recorded. Next, the mean
velocity at each of the measuring points should be determined with the current
meter by one of the methods listed in the following section. Velocity
measurements should be properly recorded.
Errors of velocity measurement will arise if the current
meter:
ü
Is placed closer to the boundary than 1-2 rotor
diameters
ü
Is used to measure velocities less than 0.5 ft/s
or out of the range of calibration. Overdriving the rotor can damage bearings
ü
Is not held steady in one position during the
time measurement
ü
Is used in significant waves, such as those
caused by wind
ü
Is used in flow which is not parallel to the
axis of the propeller meter or is oblique to the plane of the cup-type meter
ü
Is impeded by weeds or debris
K. Methods of Determining Mean Velocity
The following methods are used to determine mean velocity in
a vertical line with a current meter:
ü
Two-point method
ü
Six-tenths-depth method
ü
Vertical velocity-curve method
ü
Subsurface method
ü
Depth integration method
ü
Two-tenths method
ü
Three-point method
ü
One-point continuous method
The two-point method consists of measuring the velocity at
0.2 and then at 0.8 of the depth from the water surface and using the average
of the two measurements. High accuracy is obtainable with this method, and its
use is recommended. However, the method should not be used where the depth is
less than 2 ft.
The six-tenths-depth method consists of measuring the
velocity at 0.6 of the depth from the water surface and is generally used for
shallow flows where the two-point method is not applicable. The method gives
satisfactory results.
The vertical velocity-curve method consists of measuring the
velocities at enough vertical positions so that the velocity profile is defined
well enough to calculate a sufficiently accurate mean velocity. The method is
very accurate, depending upon the number of data points measured for profile,
but is time consuming and costly.
The subsurface method involves measuring the velocity near
the water surface and then multiplying it by a coefficient ranging from 0.85 to
0.95, depending on the depth of water, the velocity, and the nature of the
stream or canal bed. The difficulty of determining the exact coefficient limits
the usefulness and accuracy of this method.
The depth or traveling integration method is performed by
observing the velocity along a vertical line by slowly and uniformly lowering
and raising the meter throughout the range of water depth two or more times.
The method is not accurate and should be used only for comparisons or quick,
rough checks.
The two-tenths, three-point, and one-point continuous
methods are special procedures based on a relationship previously established
for the section between the true discharge and the velocities observed by these
methods. These methods are generally reliable for sections which undergo no
serious changes because of erosion, sedimentation, or other deformation. They
are discussed in detail in USGS (1965) and USGS (1980). Of the methods cited in
this section, the two-point method and the six-tenths-depth method are most
used in canal work.
L. Computing Discharge
The velocity-area principle is used to compute discharge
from current-meter data. Total discharge is determined by summation of partial
discharges. A partial discharge is the product of an average point or vertical
line velocity and its meaningfully associated partial area, expressed as:
The total discharge is then:
The measured and computed variables are as follows:
q = the
discharge in cubic feet per second (ft3/s) for a partial area
Q = total discharge
= the mean velocity associated with the partial area
a = partial area of total cross section
L1, L2,... Ln= distance to vertical measurement locations in feet from an initial point to vertical station
L = the distance in feet between consecutive vertical measurement stations
, ,... = the respective mean velocities in feet per second at vertical measurement stations
D1, D2,... Dn= the water depths in feet at verticals
n = the number of verticals related to the partial area
Q = total discharge
= the mean velocity associated with the partial area
a = partial area of total cross section
L1, L2,... Ln= distance to vertical measurement locations in feet from an initial point to vertical station
L = the distance in feet between consecutive vertical measurement stations
, ,... = the respective mean velocities in feet per second at vertical measurement stations
D1, D2,... Dn= the water depths in feet at verticals
n = the number of verticals related to the partial area
Simple Average
Method
Using the simple average of two successive vertical depths,
their mean velocity, and the distance between them results in:
The two hyphenated integers as a subscript denote that the
partial discharge, q, is for the area between two consecutive vertical
measurement points as numbered.
Midsection Method
In the midsection method, the depth and mean velocity are
measured for each of a number of verticals along the cross section. The depth
at a vertical is multiplied by the width, which extends halfway to the
preceding vertical and halfway to the following vertical, to develop a
cross-sectional area. The product of this area and the mean velocity at the
vertical gives the discharge for the partial section between the two halfway
points. A summation of the partial discharges gives the total discharge. The
formula for computing the partial discharge using the midsection method is:
The value, n,
with plus and minus after it denotes that the partial discharge, q, is
for the area between halfway back toward the previous vertical measurement and
halfway toward the next forward vertical.
The mean velocities are determined by any one of the methods
listed in section 12. For these two methods, the verticals do not need to be
equally spaced, but the verticals should be chosen such that:
ü
The error of computing the area between the
verticals does not exceed 3 percent when the bed is treated as straight lines
between the verticals.
ü
Except at the banks, the difference between the
mean velocities at the verticals does not exceed 20 percent relative to the
lower velocity of a pair of verticals.
Simpson's
Parabolic Rule
In this method, Simpson's parabolic rule is used twice to
compute discharge using the area velocity method. First, the area is computed
for three consecutive depths at velocity measuring stations using Simpson's
rule. Second, average velocity for the same three verticals is computed by the
rule. The discharge between the three verticals is the product of the average
velocity and area. Using Simpson's rule assumes both the vertical depths and
their corresponding average velocity vary parabolically. Natural riverbeds and
older earth-lined canal bottoms follow curved shapes rather than the typical
straight line geometry of hard-lined canal designs. Both vertical and
horizontal velocity profiles tend to be parabolic in either case. Using
Simpson's rule to obtain the area between three equally spaced consecutive
verticals or two consecutive partial areas results in:
where
is the distance between consecutive vertical velocity
measuring stations which are equally spaced across the flow section.
Using Simpson's rule to obtain the mean velocity of three
consecutive verticals or over two consecutive partial areas is expressed as:
The product of this velocity and the area from the previous
equation results in the relationship for the discharge through the two
consecutive partial areas, written as:
Simpson's parabolic rule method is particularly applicable
to river channels and old canals that have cross sections conforming in a
general way to the arc of a parabola or to a series of arcs of different
parabolas. Simpson's method requires equally spaced verticals. The simple
average and the midsection methods do not require equally spaced verticals.
Thus, these two methods are well suited to computing discharges in canals that
conform closely to their original trapezoidal rectangular shapes.
Typical discharge computations obtained by the midsection
method, and weir formula are attached in Table 1 and 2.
Weir Formula Free Flow Condition Date of Observation July 06,
2012 Time: 11:25
Q= L * C * H 1.5
Where:
L = Length of weir
C = Constant
H = Head on weir crest
Measured
Hydraulic Data
L = 21 ft
C = 3 - 3.4
H =
1.9 ft V = 4 ft per sec
H Total = 1.9 + 0.25 = 2.15 ft
B = 21 ft
C = 3.1
Computer
Discharge
Q = 3.1 x 3.15 x 21
= 205 cs.
Total Discharge
Measured = 205 cs.
Table 2: Discharge Measurement at Nawabshah East Main
Drain (NEMD) RD 0+00
Weir Formula Free Flow Condition Date of Observation July 06,
2012 Time: 11:45
Q = L * C * H 1.5
Where:
L = Length of weir
C = Constant
H = Head on weir crest
Measured
Hydraulic Data
L = 12 ft
C = 3 - 3.4
H =
2 ft
V = 3 ft/sec
= 0.14
H+
=
2.0 + 0.25 = 2.25 ft
Computer Discharge
Q = 3.3 x 3.13 x 12
= 3.3 x 3.15 x 12 = 124.74 cs
Discharge measured =
125 cs.
Table 3: Recorded Velocity Measurements Using Flow
Probe At Rd 159+700 Of Spinal
Date:
11 Sep 2012 Time of Observation 12:45pm Weather condition: Mild wind opposite
to flow direction
S
No.
|
Width
(ft)
|
Depth
(ft)
|
Area
(ft2)
|
Mean
Area
(ft2)
|
Velocity
(ft/Sec)
|
Discharge
(ft3/Sec)
|
1
|
0.0
|
0.0
|
0.0
|
0.0
|
0.0
|
0.0
|
2
|
3.0
|
5.0
|
15
|
7.5
|
1.6
|
12.0
|
3
|
7.0
|
6.5
|
45.5
|
30.25
|
2.2
|
66.6
|
4
|
10.0
|
7.4
|
74
|
59.75
|
2.4
|
143.4
|
5
|
10.0
|
8.9
|
89
|
81.5
|
2.4
|
195.6
|
6
|
10.0
|
10.1
|
101
|
95
|
2.6
|
247.0
|
7
|
10.0
|
10.3
|
103
|
102
|
2.8
|
285.6
|
8
|
10.0
|
11.5
|
115
|
109
|
3.4
|
370.6
|
9
|
10.0
|
10.7
|
107
|
111
|
3.4
|
377.4
|
10
|
10.0
|
11.1
|
111
|
109
|
3.5
|
381.5
|
11
|
10.0
|
10.5
|
105
|
108
|
3.5
|
378.0
|
12
|
10.0
|
10.3
|
103
|
104
|
3.5
|
364.0
|
13
|
10.0
|
9.5
|
95
|
99
|
3.6
|
356.4
|
14
|
10.0
|
10.5
|
105
|
100
|
3.3
|
330.0
|
15
|
10.0
|
10.7
|
107
|
106
|
3.1
|
328.6
|
16
|
10.0
|
10.5
|
105
|
106
|
3.0
|
318.0
|
17
|
10.0
|
9.9
|
99
|
102
|
2.8
|
285.6
|
18
|
10.0
|
9.0
|
90
|
94.5
|
2.8
|
264.6
|
19
|
10.0
|
7.5
|
75
|
82.5
|
2.6
|
214.5
|
20
|
10.0
|
6.2
|
62
|
68.5
|
1.9
|
130.2
|
21
|
10.0
|
2.3
|
23
|
42.5
|
1.3
|
55.3
|
22
|
0.0
|
0.0
|
0
|
11.5
|
0.0
|
0.0
|
5104.75
ft3/Sec
|
KPOD Gauge: 8.8 ft Total Discharge
M. Canal Discharge Curves
To rate a flow section, discharge measurements at a
current-meter station should be taken over a wide range of canal flows to
ensure accuracy in preparing velocity, area, and discharge rating curves
(section 1). Water is usually turned into the canals at gradually increasing
rates as demand increases during the irrigation season. Thus, measurements for
all flow stages in the canal often can be obtained during one season.
The canal bed at a well selected current-meter station is
generally permanent in character, and a permanent rating curve could be made if
not for sediment accumulations or for growths that occur in the canal during
the irrigation season. The sediment and the growths both decrease the discharge
capacity of the canal for all flow depths, and the effect is usually most
pronounced during the latter part of the irrigation season. This change in flow
capacity of the canal for a given depth of flow must be taken into
consideration when computing the quantity of water carried by the canal. If the
canal is cleaned during the season, the relationship of discharge to gage
height is again disturbed. The changing relationship of discharge to gage
height in irrigation canals caused by changing boundary conditions is the chief
source of error in flow measurements.
Gage Readings
To determine the quantity of water carried by a canal over a
period of time, the gage must be read at least twice daily. More than one
reading provides a means for checking the readings and also informs the canal
attendant of any unexpected changes in canal stage. More frequent readings are
needed when changes in stage are suspected or are made in the canal. The
readings should be taken by the canal attendant on regular rounds. The gage
should be read accurately, generally to the nearest hundredth of a foot.
Automatic water-stage recorders eliminate the need for numerous readings and
can increase the accuracy of the flow measurements.
Computations of
Discharges
Current-meter measurements made at several specific flows
can be used to obtain discharge, velocity, and area curves that apply to all
inclusive gage heights by plotting the appropriate data on cross-section or
graph paper (figure 10-18). Discharges, corresponding mean cross-sectional
velocities, and cross-sectional areas are plotted on the horizontal axis.
Corresponding gage heights are plotted on the vertical axis. Three separate
curves are drawn through these data points.
Figure 5: Typical discharge, mean velocity and area curves for a
canal.
The probable area curve is established first by drawing the
most probable line through the data points. Using this curve, the accuracy of
the area computations and of the flow depth measurements may be checked. Next,
the computed mean cross-sectional velocities are plotted, and a curve is drawn
through the points. This curve provides a check on the velocity computations
and helps detect changes in velocity that may occur in the canal because of
changing roughness or silting in the canal.
Finally, the discharge curve is drawn through the computed
discharge points. If flow conditions in the channel did not change resistance
significantly during the period needed for measurements over the full range of
canal flows, the curve will generally be easy to draw. If the relationship of
discharge to gage height was affected by growths or sediment deposits, one or
more additional discharge curves must be drawn. The number of rating curves
required for a cross-section location depends upon the degree of the flow
restric- tions encountered and the rate at which the restrictions developed.
These curves will generally be parallel to, but slightly displaced from, the
curve for the clean canal. For the periods when the change is in progress,
discharges may be estimated by proportioning between curves for the clean and
restricted conditions on a time basis.
N. Rating Table
From the rating curve, a rating table may be prepared for
each tenth or hundredth of a foot of gage height from zero to the maximum
height of water in the canal or stream. For canals affected by growths or
sediment, two or more such rating tables will be necessary, one for early in
the season when the canal is clean, and the other for late in the season when
growths or other restrictions are present. If the canal is cleaned during the
irrigation season, operating personnel should be instructed to switch to the
curves and tables for the clean canals.
Daily and Monthly Discharges
Discharges in acre-feet may be compiled from the daily gage
heights and the rating tables. From these tables, the monthly discharges and
the total amount of water delivered by the canal during the irrigation season
may be obtained.
Operation And Maintenance of Surface Drains
Operation And Maintenance of Surface Drains
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