CWR4101: Flow Measurement Chapter 7

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CWR4101 Flow Measurement Chapter 7
Dr. Marty Wanielista 407.823.4144 wanielis_at_mail.uc
f.edu www.stormwater.ucf.edu http//classes.cecs.u
cf.edu/CWR4101/wanielista
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Applications

• Streamflow velocities and flow rates
• Control devices for hydrograph peak attenuation
• Runoff measurement
• Low flow controls

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Circular Weir with Baffle
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Pond Discharge Control
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Streamflow Measurement Devices

• Direct Measurement (volumes over time)
• Indirect Measurements
• Stage or depth for velocity area or Manning
• Weir or orifice
• Pressure converted to depth
• Dilution via mass balance
• Flumes

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Streamflow

• 1. Velocity Area Method

Flow gage at one or more depths
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Staff Gage, Stage and Depth
Depth Gage
Water Surface
New Bottom
Old Bottom
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Example Problem 7.1 (page 271) For a particular
stream, estimate the flow rate (runoff for this
case) using the following data for velocities
measured at two depth (0.2 and 0.8 of the total)
and the cross-sectional area corresponding to the
velocity measurements.
Now you can do problem 7 on page 296
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2. Mannings Equation Method
Where n, the Manning rough coefficient, is given
in Table 7.3 on page 276. Q discharge (cfs) A
area (ft2) R hydraulic radius (ft) S slope
Now you can do problems 15 and 17 on pages 297
and 300, respectively.
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3. Weir
Weir

• Apply Conservation of Energy, or, in this case

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• Let z1-z2 y
• Then

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y
Weir
B
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• Let H equal the height above weir, then

Incorporating Cf friction loss over the weir
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Weir Equation

• Integrating

or
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or
Weir Equation

• Then

or
Where CW 3.33 (US) or 1.84 (SI), and
is dependent on P1/H (Figure 7.16 on page 297)
L/H (Figure 7.9 on page 280) Shape
(Figure 7.17 on page 297) roughness and
NR
Now you can do problem 10 on page 296
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Weir Equation

• For a rectangular weir not extended across the
  channel or one that has its ends contracted and
  negligible velocity head

Example Problem 7.3 (page 281) A channel is
known to produce a peak flow rate of 0.45 cms.
The upstream depth must not exceed 2.25 m. If
you have a 1.5 m wide end contracted rectangular
weir, how high should it be placed in the
channel so as not to exceed the 2.25 m upstream
depth?
Solve by trial and error
So
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Weir Equation

• Other types of weirs are also used
• 60o V-notch for low flow

where C60 1.43 (US) or 0.80 (SI)

• 90o V-notch for low flow

where C90 2.50 (US) or 1.38 (SI)
Now you can do problem 14 on page 297
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Z1
4. Orifice

• Apply Bernoulli Energy Equation

Z2
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Orifice Equation

• Let z1-z2 H

• Given the average velocity through the orifice we
  could state Q AV. Since we do not know this
  average velocity, V, or exact area, we
  incorporate a discharge coefficient

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Orifice Equation

• where
• Cd discharge coefficient (? 0.6)
• Ao orifice area
• H head above orifice center

note,
with Cv coefficient of velocity Cc
coefficient of contraction
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Example Problem 7.5 (page 284) For a 4-in.
orifice with a discharge coefficient of 0.60,
what head results when measuring a discharge of
0.87 cfs?
Cd 0.60, Q 0.87 cfs, d 4 in.
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Orifice Flow vs. Weir Flow

• 1. Weir Flow
  

H
Riser pipe
Riser pipe
2. Orifice Flow
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Orifice vs. Weir Flow

• What would be the minimum depth (H) for orifice
  flow?
• Area of flow over riser pipe APIPE

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Outlet Controls of Stormwater Ponds

• Refer to Figure 7.11 on page 284

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Mass Balance

• Mass in Mass out
• Use conservative substances Rodamine red, salt,
  natural isotopes, or other tracers.
• Development and do example problem 7.2
• C1V Q (SUM Ci (dt)i)
• Q Mass IN/ (SUM Ci (dt)i) EQU 7.4

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Flumes and others

• See table 7.5, page 278
• Selection table as a function of range of
  measurements
• Figure 7.8, page 280 for different shapes for
  flumes
• Write Energy Balances

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End Flow Measurements, Chapter 7
Dr. Marty Wanielista 407.823.4144 wanielis_at_mail.uc
f.edu www.stormwater.ucf.edu http//classes.cecs.u
cf.edu/CWR4101/wanielista