Different methods of water flow measurements.pdf

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Topic:
Different methods of
flow measurements

DIFFERENT METHODS OF
FLOW MEASUREMENTS
Water is the most valuable commodity in
irrigated agriculture. Accurate measurement of
irrigation water provides efficient use of this
scarce natural resource. Measurement of water is
needed in several aspects of irrigation water
management.

Main purpose:-
• Amount of water delivered to the land.
• Test the wells for their yield in meeting the crop
water requirement.
• Estimate the conveyance and other losses.
• Detect the origin of these losses for remedial
action.

Measurement of flow in
open channels
grouped into 2 :-
1)direct discharge method
2)Velocity area method

1)Direct/ volume method
a) Bucket and Stopwatch method :-
A very easy method to estimate discharge is to simply
measure the time it takes to fill a container of a known
volume.
 This method only works for systems with fairly low
flow volume.
Its main limitation is that the discharge must fall from a
pipe or ditch, in such a way that the bucket can be placed
underneath it to capture all the discharge.
 Any size bucket can be used as long as it does not fill up
too fast to get an accurate measurement.

Taking the Measurement
1) Locate the site’s discharge pipe. If discharge occurs
via a channel, then a temporary dam may need to be
placed across the channel with the discharge directed
through a single outlet pipe.
2) Place the container of a known volume (e.g., a 1 or 5
gallon bucket) directly under pipe. All of the discharge
should flow into the container.
Note:- The 5-gallon line on the bucket may need to
be measured and marked ahead of time.
3) Using a stopwatch, time how long it takes to fill the
container.
4) Repeat this process three times to obtain an
average.

2) Velocity area method
Used to determine the discharge rate of water in a pipe or
open channel by multiplying the cross sectional area of
flow by the average Velocity of water.
Discharge rate=area*velocity
Q=discharge rate m3/sec
A=area of cross section of channel m2
V=velocity of flow m/sec
Sub types under this method :-
i) Float method
ii) Current meter method
iii) Tracer methods.

i) FLOAT METHOD
If discharge from the site flows through an open ditch
or channel, another fairly simple method to use is the
float method.
This method requires the measurement and
calculation of the cross-sectional area of the channel
as well as the time it takes an object to “float” a
designated distance.
This is the least accurate method of those presented
in this guide but does provide a reasonable estimate.

Equipments needed
•Markers (flagging tape, cones, etc.)
•Timer (stopwatch)
•Float (an orange or wood float)
•Measuring tape
•Paper and pencil for record keeping
procedure
Measure 30 m length in a straight channel and mark the up
stream and down stream points .
 Allow the wooden float to float on the running water well
above the upstream point

Note the time when it touches the upstream point and let
this be the initial time .
Then note the time when it reaches the down stream point
which will be the final time.
Repeat the procedure several times and find out The
average time taken for the float to cross the 30m distance.
Velocity=length of channel/average time taken by float

ii) Current meter method
Is a small instrument containing a
revolving wheel or vane that is
turned by the movement of water.
 The number of revolutions of the
wheel in a given time is noted and
corresponding velocity is recorded
from a calibration table or graph.

iii) Tracer method
Is use full in measuring the flow of water in the stream
with no regular shape.
 Also be used in any open channel or pipe.
The method consists of injecting a dye called tracer of
known concentration in upstream and measuring the
dilution in the down stream.
Discharge (Q) = C2-C1/C0-C1*q
Q= discharge (cusec) q=tracer injection rate
C1=tracer conc. Prepared
C2= tracer conc. In down stream
C0= initial tracer conc. In up stream

Direct discharge Weirs
•Weirs are used to measure flow in an irrigation
channel or the canal outlet at the source.
•A weir is a notch in metallic sheet / concrete
structure through which the irrigation stream to be
measured is made to flow.
The commonly used weir can be
1) Rectangular
2) Trapezoidal
3) Triangular

1)Rectangular weir
•It is used to measure comparatively larger discharge.
•The discharge through rectangular weirs may be
Computed by the “FRANCIS” formula.
Suppressed rectangular weir : Q =0.0184LH3/2
Contracted rectangular weir : Q = 0.0184 ( L- 0.2H3/2)
Q =discharge
L = length of crest (cm)
H = head over the weir (cm)

2) Trapezoidal weir
Used to measure the medium
to larger discharges
Computed by the formula
Q= 0.0186 LH3/2
L = length of crest
H= head over the weir

3) Triangular weir
Commonly used to measure small and
medium size streams accurately.
 It is V shaped metallic notch fixed across
the flow .
 Computed by using formula
Q= 0.0138H5/2
Q= discharge (lit/sec)
H= head (cm)

ORIFICE
An orifice is a horizontal or vertical
opening with a closed perimeter through
which water flows.
 They may be operated under free flow or
submerged flow condition.
Free flow orifice plates can be used to
measure comparatively small streams like
the flow into the border strips ,furrow or
check basins.

In submerged flow orifice , plastic scale may be
fixed on the upstream face of the orifice plate
such that the zero of the plastic scale coincides
with the center of the orifice.
Calculated by using the formula
Q = 0.61* 10-3 a √2gh
Q= discharge,
a= area of the cross section of orifice,
g= acceleration due to gravity
h=depth of water over the centre of the orifice

Measurement of Flows by the Flume Method
•Flumes are specially shaped sections that are
installed into a channel to restrict the channel
cross-sectional area.
•This restriction results in an increased velocity and a
change in the level of discharge flowing through the
flume.
•The head of the flume is measured and used
to calculate the flow rate.
• Like weirs, specific equations are used depending on
the type of flume installed.

Flumes are generally used where weirs
are not feasible.
Flumes can measure higher flow rates
than weirs and are better suited for
flows that contain sediments.
As with the weir, flumes have specific
installation requirements and an expert
should be consulted if it is to be used for
flow measurement.

Parshall
Themostwidelyknownandusedflume.
 Itwasdesignedinthe1920'sbyR.L.Parshallinordertoprovideameasuringdevicewhich
didnothavethelimitationsofweirsandexistingflumedesigns.
Parshallflumescanalsooperatesatisfactorilywithapercentageofsubmergence,however
tworeadingsmustbetakenatover70%submergence,andacorrectionapplied.
Thereareseveralfirmsthatmanufacturepre-fabricatedParshallflumes.

Itis openchanneltypemeasuringdevicethatoperateswithasmalldropinhead.
Itconsistsof3sectionsconverging,throatanddivergingsectionsandmeasuring
plastictubecalledstillingwellsinconvergingandthroatsections.
Itismadeofmetalsectionsofdifferentthroatwidthviz.,7.5,15&30cm&canbe
usedtomeasureflowassmallas 0.3to100lps.

Cut-throat Flumes
• The cutthroat flume was developed by
researchers at the “Utah Water Research
Laboratory” in the 1960's.
•This is a flat-bottomed flume having only an
entrance and an exit section, and no throat
section.
• These flumes can be used in situations similar to
those in which Parshall flumes can be used,
however, they are not as widely used, and would
probably have to fabricated in the field or at the
station.

•Can operate either in free flow or in
submerged condition.
• It should be Installed in a straight section
of the channel & not near the gate
because of unstable and surging effects
which might results from the gate
operation.

Different methods of water flow measurements.pdf

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