ENGINEERING UTILITIES 2

BUILDING WATER FLOW CALCULATIONS SAMPLE PROBLEM 1:

A triangular weir has a vertex angle of 60°. The head
FLOW METER of overflowing water at the weir is maintained 700mm. If the
A flow meter is a device used to measure the weir coefficient is 2.1, determine the discharge of water from
volume or mass of a gas or liquid. Flow meters are referred the weir.
to by many names, such as flow gauge, flow indicator, liquid
meter, flow rate sensor, etc. depending on the particular
industry. However, they all measure flow. Open channels,
like rivers or streams, may be measured with flow meters. Or
more frequently, the most utility from a flow meter and the
greatest variety of flow meters focus on measuring gasses
and liquids in a pipe. Improving the precision, accuracy, and
resolution of fluid measurement are the greatest benefits of
the best flow meters.
B. RECTANGULAR WEIRS
OPEN CHANNEL Suitable for a wider range of flows and easier to
An open channel is a waterway, canal or conduit in construct compared to V-notch weirs.
which a liquid flows with a free surface.

OPEN CHANNEL FLOW METER

An open channel flow meter - is used to measure
the flow rate of fluids in open channels or partially filled
pipes, such as rivers, streams, or wastewater channels. Unlike
closed pipe systems, open channels allow fluid to flow freely PROS:
and are typically influenced by gravity.  Easier to construct and calibrate compared to V-notch
weirs.
FLOW METERING METHODS USED IN OPEN CHANNEL  Suitable for a wider range of flow rates and depths.
1. WEIRS  Provides accurate measurements for moderate to high
A weir is a structure placed across an open channel flows.
to measure flow by the height of the water over the weir. CONS:
 Not as sensitive to low flows compared to a V-notch
COMMON TYPES OF WEIRS weir.
A. V-NOTCH WEIRS  Flow range, the performance can be affected by
Typically used for small flows. The shape allows sediment or debris, especially at low flow rates.
precise measurement with relatively small changes in flow.
PROS: FLOW RATE FORMULA FOR RECTANGULAR WEIR
 More sensitive to low flows, making it ideal for Q=2/3C √2gh
applications where precise measurement at low flow
rates is crucial.
 Provides highly accurate measurements for small to
moderate flow rates due to the shape's geometric
properties.
 Easier to maintain calibration in low-flow conditions.
CONS:
 Not suitable for high flow rates; its accuracy diminishes
at higher flows.
 More complex to construct and calibrate accurately
compared to rectangular weirs.
 Performance can be affected by debris or sediment SAMPLE PROBLEM 4:
accumulation in the notch. A suppressed rectangular sharp crested weir has a
weir length of 500mm. If water is overflowing on the weir
FLOW RATE FORMULA FOR V-NOTCH WEIR with a constant head of 200mm, what is the discharge/ flow
rate of water at the weir?

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 ENGINEERING UTILITIES 2
C. TRAPEZOIDAL WEIR  SUBMERGED FLOW
A trapezoidal weir is a type of flow measurement occurs when the water surface downstream of the
structure used in open channel flow to measure the flow rate flume is high enough to restrict flow through a flume,
of water. It has a trapezoidal shape, characterized by a wider submerged flume conditions exist. A backwater buildup
top (the crest) and narrower base, with side slopes. This effect occurs in a submerged flume. For a flow calculation, a
design allows for a gradual transition of water over the weir, depth measurement both upstream and downstream is
which can help reduce turbulence and provide more accurate needed.
flow measurements.
PROS: FREE FLOW FORMULA
 Improved Flow Measurement Accuracy
 Versatility Q=C Haⁿ
 Reduced Erosion:
 Easier maintenance where:
 Cost-Effective Q = Flow rate (cubic feet per second or cubic meters per
 Enhanced Hydraulic Performance second)
C = is the free-flow coefficient for the flume (specific to the
SAMPLE PROBLEM 3: flume size and design)
Find the discharge through a trapezoidal notch Ha - is the head at the primary point of measurement (ft)
which is 1.2m wide at the top and 0.45m at the bottom and is n = Exponent (specific to the flume size and design)
0.3m high. The head of the water on the notch is 0.225m.
Take Cd=0.60.

2. FLUMES
SAMPLE PROBLEM 2:
These are specially shaped channels designed to
Consider a 6 ft Parshall flume operating under free
control and measure the flow.
flow condition with a head, Ha = 3.5 ft. What is the flow rate
through this Parshall flume?
TYPES OF FLUMES
A. PARSHALL FLUMES
Widely used due to their accuracy and ease of
installation. They are particularly effective for medium to low
flows.

FREE FLOW VS. SUBMERGED FLOW

 FREE FLOW
occurs when there is no “backwater” to restrict flow
through a flume. Only the upstream depth needs to be
measured to calculate the flow rate. A free flow also induces
a hydraulic jump downstream of the flume.

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 ENGINEERING UTILITIES 2
SUBMERGED FLOW FORMULA B. H-FLUMES
The Submergence Ratio (S) of a Parshall flume is a Used for varying flow conditions and provide
comparison of upstream (Ha) and downstream (Hb) water accurate measurements over a wide range of flow rates.
levels to determine if downstream conditions are affecting
(slowing) the discharge out the flume.

The point at which downstream flow conditions

affect the discharge out of the flume is termed the
Submergence Transition (St). SAMPLE PROBLEM 6:
You are tasked with determining the flow rate in an
H-flume. The flume has a discharge coefficient of 0.60 and a
length constant of 1.8. The depth of water at the throat of
the flume is measured to be 0.80 meters. The reference
depth for correction is 0.10 meters, and the correction factor
exponent is 0.5. Determine the flow rate .

C. TRAPEZOIDAL FLUMES
Trapezoidal flumes offer an ideal measurement
solution for flows with significant debris or solid content.
SAMPLE PROBLEM 5: Named for its trapezoidal cross-section, this flume also has a
Using the Parshall flume flow equations and Tables flat bottom that makes it easy to fit into most standard
1-3, determine the flow type (free flow or submerged flow) irrigation channels. As a result, measuring flow rates in
and discharge for a 36-inch flume with an upstream depth, irrigation ditches and channels is the most common
Ha of 1.5 ft and a downstream depth, Hb of 1.4 ft. application for trapezoidal flumes. Measurements remain
accurate at high or low flow rates.

APPLICATIONS:
 Water Management: Used in irrigation systems to
manage water distribution efficiently.
 Environmental Monitoring: Helps in measuring flow
rates in rivers and streams for ecological studies.
 Industrial Uses: Employed in wastewater treatment
facilities to monitor discharge.

BENEFITS OF USING TRAPEZOIDAL FLUMES

 Cost-Effective: Relatively low construction and
maintenance costs compared to other flow
measurement devices.
 Adaptability: Can be designed to fit specific site
conditions and requirements.
 Simplicity: Easy to install and operate, with minimal
moving parts.
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 ENGINEERING UTILITIES 2
D. PALMER-BOWLUS FLUME
Named after its inventors, Harold Palmer and Fred
Bowlus, the Palmer-Bowlus metering flume is widely used in
open channels or pipelines that are not under any pressure.
This is accordance with ISO 4359.
This flume is normally installed in a "U" shaped
channel fed by a pipeline such as storm drains and sewers.
This convenient flume requires little redesign or special
modification of circular conduits for installation.

APPLICATIONS:
 Irrigation & Agriculture
 Environmental Monitoring
 Wastewater Management
 Hydraulic Engineering
 Industrial Applications

3. CURRENT METER
Instruments that measure the velocity of the water flow.

COMMON TYPES OF CURRENT METERS

1. MECHANICAL CURRENT METERS
Measure velocity by the rotation of blades or cups in
the flow.
2. ELECTROMAGNETIC CURRENT METERS
Use electromagnetic principles to measure velocity
without moving parts.

4.STAGE-DISCHARGE RATING CURVES

Involves measuring the stage (water level) of the
channel and using established curves that relate stage to
discharge. This method requires periodic calibration and can
be used in conjunction with weirs or flumes.

5.ULTRASONIC AND ACOUSTIC SENSORS

These use sound waves to measure water level and
flow velocity. They can be non-contact methods and are
suitable for a variety of conditions.

6. HYDROSTATIC PRESSURE SENSORS

Measure the pressure exerted by the water column
to determine the depth and thus the flow rate, often used in
combination with other methods.

7. FLOW GAUGES AND INTEGRATED SYSTEMS

Combine various sensors and technologies to
provide comprehensive flow measurement and monitoring,
often including data loggers and telemetry systems for
remote monitoring.

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