1 KNE351 Fluid Mechanics 1 Laboratory Notes Broad-Crested Weir This booklet contains instructions and notes for the experiment listed above. Additional material relating to laboratory work will be delivered during the course. The expectations regarding lab work and reporting are described in a separate document,‘KNE351. FLUIDMECHANICS: Laboratory Method and Reporting’, which will also be circulated at the beginning of the course. It is expected that all students study these notes and complete the pre-lab component prior to the laboratory session. An overview of the laboratory equipment will be provided at the beginning of each session. A D Henderson 2 1. Learning Objectives 1. Observe and understand the behaviour of a real fluid flowing over a broad-crested weir, 2. Model this behaviour employing the Continuity and Bernoulli (Energy) Principles to predict the flow rate from depth measurements. 3. Evaluate these predictions by comparing with measured values and use Specific Energy to explain the changing nature of the flow over the weir. 2. Introduction The theory of non-uniform flow in channels is covered by the course text, by many other fluid mechanics texts, and by several web sites. The specific energy, E, is the energy at a channel cross-section referred to the base of the channel (in contrast to the Bernoulli equation, which is referred to a fixed horizontal datum). The expression given for E is actually an approximation valid for small bed slopes. You've measured the flume slope, and should examine this approximation in your report. A hydrostatic pressure distribution is assumed, and you should also examine the validity of this assumption. If the streamlines are not parallel, then the accelerative forces will modify the pressure - depth relationship. In general, two conjugate flows depths satisfy the specific energy equation for a given value of the specific energy. The greater depth is associated with subcritical flow, and the shallower depth with supercritical flow. At the critical depth the conjugate depths are equal, and the discharge for the given specific energy is a maximum. Broad crested weirs are used as a method of flow measurement in open channel flows. If the weir is sufficiently high and long, the free surface will drop to critical depth. If the height of the upstream flow is measured, then the flow rate can be determined. 3 3. Apparatus • Water flume comprising of pump, control valve, venturi and v-notch flow meters, downstream control gate. • depth gauges • 2 vertical water manometers • 2 total head tubes 4. Preparation Examine and sketch the layout of the channel and associated flow measuring equipment. Measure the channel width and note significant geometrical parameters of the nozzle venturi meter and V-notch weir. Note the directions of readings of all measuring scales. a. Measure the channel, weir dimensions, a.

1. 1
KNE351 Fluid Mechanics 1
Laboratory Notes
Broad-Crested Weir
This booklet contains instructions and notes for the experiment
listed above.
Additional material relating to laboratory work will be
delivered during the
course. The expectations regarding lab work and reporting are
described in a
separate document,‘KNE351. FLUIDMECHANICS: Laboratory
Method and
Reporting’, which will also be circulated at the beginning of the
course. It is
expected that all students study these notes and complete the
pre-lab component
prior to the laboratory session. An overview of the laboratory
equipment will
be provided at the beginning of each session.
A D Henderson
2
1. Learning Objectives

2. 1. Observe and understand the behaviour of a real fluid flowing
over a broad-crested weir,
2. Model this behaviour employing the Continuity and Bernoulli
(Energy) Principles to
predict the flow rate from depth measurements.
3. Evaluate these predictions by comparing with measured
values and use Specific Energy
to explain the changing nature of the flow over the weir.
2. Introduction
The theory of non-uniform flow in channels is covered by the
course text, by many other fluid
mechanics texts, and by several web sites.
The specific energy, E, is the energy at a channel cross-section
referred to the base of the
channel (in contrast to the Bernoulli equation, which is referred
to a fixed horizontal datum).
The expression given for E is actually an approximation valid
for small bed slopes. You've
measured the flume slope, and should examine this
approximation in your report. A hydrostatic
pressure distribution is assumed, and you should also examine
the validity of this assumption. If
the streamlines are not parallel, then the accelerative forces will
modify the pressure - depth
relationship.
In general, two conjugate flows depths satisfy the specific

3. energy equation for a given value of
the specific energy. The greater depth is associated with
subcritical flow, and the shallower
depth with supercritical flow. At the critical depth the conjugate
depths are equal, and the
discharge for the given specific energy is a maximum.
Broad crested weirs are used as a method of flow measurement
in open channel flows. If the
weir is sufficiently high and long, the free surface will drop to
critical depth. If the height of
the upstream flow is measured, then the flow rate can be
determined.
3
3. Apparatus
• Water flume comprising of pump, control valve, venturi and v-
notch flow meters,
downstream control gate.
• depth gauges
• 2 vertical water manometers
• 2 total head tubes
4. Preparation
Examine and sketch the layout of the channel and associated
flow measuring equipment.
Measure the channel width and note significant geometrical

4. parameters of the nozzle venturi
meter and V-notch weir. Note the directions of readings of all
measuring scales.
a. Measure the channel, weir dimensions, and v-notch angle.
b. Take zero readings for the manometers.
c. Prime the pump. Fill the flume and the v-notch weir (slowly)
and take a zero reading.
5. Measurement of Channel Slope and Point Gauge Traverse
Rails.
The horizontal datum line provided by a still water surface can
be used to determine the
channel bed slope and identify any departures from linearity in
the channel bed or point gauge
traverse rails.
a. Raise the gate and seal the edges with placticine.
b. Start the motor and run about 100 mm depth of water into the
channel. Close the valve
and stop the motor.
c. When the water surface is still, take a series of point gauge
readings on the water
surface at horizontal intervals of about 0.25 m over the whole
length of the channel.
Repeat this for the channel bed and aim for a precision of 0.2
mm.

5. 6. Detailed Measurements of Free Surface Profile at Two Flow
Rates
a. Lower the downstream gate all the way, open the control
valve, and obtain a high flow
rate by opening the control. Note you should have a region of
critical flow where the
depth is approximately constant on the weir crest. If this is not
the case, reduce the flow
rate.
b. Position the total head tubes adjacent to each other in the
flow to verify they measure
equal pressure (no head loss). Note that the reading is
unchanged by probe position.
c. Position the total head tubes upstream and downstream from
the weir (carefully to keep
them submerged). You should observe a small loss of total head
over the weir.
d. When the water level stabilizes, measure the water surface
over the weir using the
pointer gauge. Also measure the upstream depth and some 5 – 7
downstream depths over
the weir until the flow depth is approximately constant. Record
manometer readings and
v-notch readings.
e. Repeat the experiment with approximately half the flow
height over the weir crest (note
that H/Lw should be > 0.08 to avoid viscous effects.

6. 4
7. Effect of Downstream Condition on Flow
a. Slowly raise the height of the downstream gate in about 5
steps until the weir ‘drowns’,
and the upstream depth begins to change. At each gate setting,
record the manometer
readings, v-notch readings, the upstream depth (at one point
upstream where H stops
changing), and the downstream depth (one point only). Detailed
surface profiles are not
required.
b. Close the control valve and turn off the pump.
8. Analysis
a. Correct your measured profiles for variations in point gauge
traverse rails
b. Plot the two water surface profiles over the weir.
c. There are four ways of estimating flow rate. First, use the
venturi and v-notch manometer
measurements to calculate the flow rate.
d. Compare these values with the expected flow rate from the
formula in your prelab
calculations – note the discharge coefficient will not apply in

7. this case because the weir
has a smooth profile with low losses.
e. Estimate the flow rate assuming that critical conditions were
observed on the weir crest.
f. Calculate the critical depth for the actual flow rate and plot
this location on your water
surface profile.
g. Calculate the Specific Energy for each measurement point
based on the actual flow rate
and measured water depth and plot Specific Energy against
depth.
h. Calculate the Froude No. for each measurement point. Plot
Froude number vs distance.
i. From the total head measurements (total energy expressed in
m of water) upstream and
downstream of the weir, calculate the head (energy) loss across
the weir for the different
gate positions.
9. Discussion
a. Locate critical depth and label the sub-critical, critical and
super-critical flow regimes
(giving Froude Numbers) on your water surface profiles.
b. Compare your ideal and actual flow rates.
c. Does the flow gain or lose momentum as it passes over the
weir? Explain.
d. Does the flow gain or lose energy as it passes over the weir?
Explain.
e. What is a flow control?

8. f. What happens to the flow over the weir when the tail gate is
raised? Explain why
upstream head remains unchanged while the weir flow is not
submerged?
g. When is the head loss across the weir greatest? Explain why.
h. Errors between experiment and theory have 3 possible
sources;
i. inadequate theory (assumptions violated),
ii. errors in experimental measurement,
iii. calculation errors.
Which do you think are most significant in your
experiment, and why?
5
6
7
8

9. 1
KNE351- Fluid Mechanics 1
Prelab Exercise 3
The critical velocity Vc over a weir may be expressed in terms
of critical depth by c cV gy=
1 Apply the Steady Flow Energy Equation between an upstream
section and a section on the weir crest
where the flow depth is critical to derive Equation [1], which
describes volumetric flow rate Q in terms
of channel width b, upstream weir head H, and upstream
velocity V1 as shown above. Show all steps and
clearly state all assumptions. You may assume negligible
friction
3/23/2 2
12
3 2
V
Q b g H
g

10. æ öæ ö
= +ç ÷ç ÷
è ø è ø Equation [1]
A discharge coefficient is introduced to account for non-ideal
effects
3/23/2 2
1
,
2
3 2wd broad
V
Q C b g H
g
æ öæ ö
= +ç ÷ç ÷
è ø è ø
where ,
0.65
1 /wd broad w
C
H P
=

11. +
2 A 1.2-m high broad-crested weir is used to measure the flow
rate of water in a 10 m wide rectangular
channel. The flow depth well upstream from the weir is 1.8 m
(measured from the bottom of the channel
to the free surface). Determine the flow rate through the channel
and the flow depth on the weir crest.
Submit your derivation and calculations to the lab demonstrator
at the start of the experiment
KNE351. Laboratory practice and assessment guidelines Page
6 of 9
KNE351 Fluid Mechanics 1: Prelaboratory Exercise (Individual:
20% of Lab Mark)

12. Criteria High Distinction Distinction Credit Pass Fail Score
Theoretical Approach
Weighting 33.3%
Best engineering practice in terms of
theoretical approach used to solve
exercise.
Correct application of
relevant theory to solve
exercise.
Significant flaws in problem
solving approach.
/5
Analysis
Weighting 33.3%
Correctly calculated all required
variables and reported all of the results.

13. Calculations are neatly presented with
appropriate significant figures and units.
Correctly
calculated
most of the
required
variables
and
reported all
of the
results.
Correctly calculated most
of the required variables
and reported most of the
results.
Correctly calculated some
of the required variables
and reported some of the
results.
Partial / incomplete
results.
/5

14. Presentation of Graphical
Information.
Weighting 33.3%
Graphs (where appropriate) are well
presented with appropriate titles, units,
gridlines and scale.
Graphs are included.
Poorly presented / incorrect
graphs
/5
Comments:
Mark:
prelab3-2018.pdfAssessment-sheet.pdf