# Experiment 6 Weirs And Open Channel Flow Measurement

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Engineering
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Oregon State University
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Winter 2021 Department of Civil and Environmental Engineering CE 362L
1
Experiment 6: Weirs and Open Channel Flow Measurement
Introduction
The purpose of this experiment is to investigate weir applications in an open channel. Flow rate
measurements are used to determine the discharge coefficient for a triangular weir. The
behavior of this device is then compared to that of a Parshall flume. Each student should
analyze the provided data and upload a completed worksheet in D2L before the deadline.
Theory
Weirs allow engineers to measure flow rate in open channels such as small streams or storm
sewers. The following equation (derived from Bernoulli’s equation) relates weir head (H) and weir
geometry to channel flow rate for triangular (or v-notch) weirs:
Q
ideal
=
8
15
tan
2
2g
( )
1/ 2
H
( )
5 / 2
(Equation 1)
A discharge coefficient is employed to account for idealistic assumptions in deriving Equation 1:
Q
actual
= C
wt
Q
ideal
(Equation 2)
Where is the angle of the v-notch, H is the distance from the water surface to the v-notch, and
C
wt
is the coefficient of discharge of the weir. Typical values of C
wt
range from 0.58 to 0.62. See
A Parshall flume is another device used for measuring flow rates in open channels. Working under
similar principles as the Venturi meter, measurements of water elevation through a constriction
allow calculation of discharge. Although Parshall flumes are more complicated than a weir, they
have the added benefit of lower energy losses and less sedimentation downstream of the device.
The procedure to determine flow rate through a Parshall flume is outlined in the Appendix to this
experiment.
In this experiment we will be measuring flow rate with both of the devices above and then
comparing how they operate under the tested conditions.
Apparatus
This experiment will be conducted in the flume with a triangular weir and Parshall flume insert
to measure flow rate. Flow will additionally be measured using an Omega paddle wheel
flowmeter when possible (Q
o
). For the V-notch, the paddle wheel flowmeter works for the first
3 measurements, but then stops working. A 3-D Doppler probe called the Vectrino is used to
measure the velocity of the water at a given position.

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Winter 2021 Department of Civil and Environmental Engineering CE 362L
2
Procedure
Two flow regimes are used for this experiment; a medium flow rate for the Parshall Flume part
and a low flow rate for the V-notch. The experiment will be conducted with at least five (5) flow
rates for each structure. Analyze the triangular weir and Parshall flume for each:
1. Record the width of the Parshall flume throat and the angle of the triangular weir.
2. Turn on the pump and adjust the flow rate as necessary. Record the flow rate after the
system has reached a steady state.
3. Read the measurements for the Parshall flume.
4. Repeat steps two and three for four additional flow rates.
5. Adjust the flow rate to a low flow rate. Record the flow rate after the system has reached
6. Insert the triangular weir. Record the height measurement after the system has reached
7. Repeat steps five and six for four additional flow rates.
8. Turn off the pump.
Appendix
Table 1. Discharge equations for Parshall Flumes for various throat widths.*
*Table from Houghtalen et. al. 2017. ‘Fundamentals of Hydraulic Engineering Systems, 5
th
ed.’

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Winter 2021 Department of Civil and Environmental Engineering CE 362L Experiment 6: Weirs and Open Channel Flow Measurement Introduction The purpose of this experiment is to investigate weir applications in an open channel. Flow rate measurements are used to determine the discharge coefficient for a triangular weir. The behavior of this device is then compared to that of a Parshall flume. Each student should analyze the provided data and upload a completed worksheet in D2L before the deadline. Theory Weirs allow engineers to measure flow rate in open channels such as small streams or storm sewers. The following equation (derived from Bernoulli’s equation) relates weir head (H) and weir geometry to channel flow rate for triangular (or v-notch) weirs: Qideal =   8 1/ 2 5/2 tan (2g) (H ) 15  2  (Equation 1) A discharge coefficient is employed to account for idealistic assumptions in deriving Equation 1:  Qactual = CwtQideal (Equation 2) Where  is the angle of the v-notch, H is the distance from the water surface to the v-notch, and Cwt is the coefficient of discharge of the weir. Typical values of Cwt range from 0.58 to 0.62. See Section  10.8.2 in your book for more information. A Parshall flume is another device used for measuring flow rates in open channels. Working under similar principles as the Venturi meter, measurements of water elevation through a constriction allow calculation of discharge. Although Parshall flumes are more c ...
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