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Experiments in fluvial geomorphology using an EM1 Flume
Introduction
The following exercise makes use of the EM1 Flume in the Department of Geography’s John
B. Thornes Laboratory. This document is an instruction manual that should allow you and
your group to work through two exercises and answer a series of questions on a self-guided
basis.
You must read the Lab Rules document before entering the lab and do not begin until either
Dr Daniel Schillereff (Teaching Fellow) or Dr Francis O’Shea (Laboratory Technician) have
introduced the equipment and the activity. Please wear a lab coat and disposable gloves for
the duration of the practical and wash your hands at the end of the class. Also, remember
that neither food nor drink is allowed in the lab.
Powered by a motorised propeller, the flume is able to realistically simulate hydrological
flows -- drag, lift and turbulence, for example -- under variable conditions (depth, velocity).
The flume also comes with colour-coded sediment that can be used to test the physical
processes controlling sediment transport and various styrofoam inserts that can be inserted
to test their influence on flow behaviour. The following exercises require you to explore all
of these concepts.
Warnings before use – students, please read all of these:



Ensure there is ample water in the reservoir at the start of the practical. Never run
the motor dry.
If any small objects drop into the flume, stop it immediately by hitting the PWM
knob (see schematic below) and retrieve them. Do not let them enter the
propeller.
Do not reach into the propeller shaft or the intake duct while the Emflume1 is
turned on. Moving parts can cause serious physical injury.
It is essential that you read to the end of each exercise before you start it to ensure you fully
understand what the task requires. You are encouraged to take photographs at regular
intervals, both to document progress and as a form of data collection.
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PMW knob
Figure 1. The major components of the EM1 flume. Please note in particular the upstream gate, the
downstream valve and the tilt crank. Although they are not labelled here, also find the pitot tube
(vertical, curved glass tube) and the channel bed slope adjuster (opposite end to the PMW knob).
Flume Initial conditions
Ensure the drainage tube is closed (the lever at the bottom of the flume should not be
pointing out). Turn on the sink tap and fill the lower reservoir to the ¾ mark - Francis or
Daniel will assist. Make sure both the upstream gate and downstream valve are fully
opened. Set the Emflume1 working section to a bed slope of 0°. Start the pump (socket) and
use the controller on the left-hand end of the flume to set a Pulse Width Modulation (PWM)
of 40%. Let the flow stabilise; this may take several seconds.
1.Exploring sediment transport and bedform development in an open
channel
1.1
The equipment:
Ruler
Stop-watch
1.2
The experiment:
(i)
Lower the upstream gate to a height of 3 cm above the black/white
measuring tape.
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Question 1: Describe what happens to the flow entering the working section of the flume
as the upstream gate is narrowed. Why is this?
(ii)
(iii)
Close the downstream valve almost all the way (try closing it fully and then
open it one half-turn). The water velocity should decrease such that the
movement of individual sediment particles should be traceable.
Measure the water depth in the working section. Slightly close or open the
downstream valve until you have achieved a water depth of 6 cm.
Figure 2. Modes of sediment transport.
Question 2: Sketch the flow dynamics visible through the side-on view of the channel.
Think about vertical changes in flow velocity and think back to the lectures on fluvial
geomorphology as to why these patterns may occur.
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Question 3: As a group, spend 60 seconds carefully watching the movement of individual
particles. Which modes of sediment transport discussed in Andreas’s lectures can you
observe (Figure 2)? List them below, using the correct technical terms.
Extra credit if you can spot saltation! (Try slightly raising and lowering the upstream gate
and watch for a further 60 seconds.)




(iv)
(v)
(vi)
(vii)
Set the upstream gate to a height of 5 cm. Close the downstream valve
almost all the way (try closing it fully and then open it one half-turn). The
water velocity should decrease such that the movement of individual
sediment particles should be traceable.
Flow velocity should have decreased, meaning sediment will have begun to
accumulate on the channel bed. (Discuss as a group and look back to your
lectures if you are unsure why!)
Allow sediment transport and deposition to progress for three minutes (use a
phone or the stop-watch). Examine the bedforms that have begun to appear.
Manually send a slug of sediment into the propeller shaft and up into the
working section. Do this by reaching into reservoir -- not near the propeller! - and pushing a handful of sediment towards the propeller. Carefully watch
the evolution of the bedforms under these conditions of greater sediment
supply.
Question 4a: Sketch the bedform(s) you can see from both a lateral and planview
perspective. Do they remind you of any depositional features from other settings that you
covered in the lectures?
Question 4b: The particles are colour-coded by grain size. Comment on any patterns in
grain size you observe in the material deposited on the channel bed.
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Question 4c: Think back to the lecture series. Try and classify the bedforms you can see
and explain the mechanism(s) leading to their formation.
Question 4d: Watch the bedforms evolve for another three minutes. To what extent do
the bedforms migrate? If they do, comment on the dynamics involved (i.e., the direction
of travel, where the sediment is sourced and delivered, does the entire bedform migrate
at the same rate.
2. Vertical velocity distribution in an open channel
2.1 The equipment:
Ruler
Pitot tube (this is an instrument that measures pressure from which flow velocities of
fluids can be determined)
Calculator (the scientific calculator function on a smartphone should be able to perform
the square root operation)
2.2. The experiment
(i)
Open the drainage valve fully and raise the upstream gate until most of the
sediment has been flushed into the reservoir.
(ii)
Return the flume to its initial conditions. Wait for the flow to stabilise and
identify the measurement point at 40 cm (in effect, 10 cm from the upstream
gate). Check the depth of flow (h) is 6 cm.
(iii)
Carefully move the pitot tube to this point by loosening the black screws.
Perform a test measurement by lowering the pitot tube into the water (opening
facing ‘up-stream’) and note what happens. Make sure everyone in your group
fully understands why the pitot tube is useful and the answer given for Question
5.
Question 5: Describe briefly the physical association between velocity of the flow and the
pitot tube measurements. If velocity increases, what will happen in the pitot tube? If
velocity decreases, what will happen in the pitot tube?
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(iv)
(v)
You now need to measure the velocity head (hv). This is the fluid pressure
measurement of the pitot tube, reflected by the water level in the tube. Measure
this by placing the short ruler into the water adjacent to the tube and recording
the distance from the base of the tube to the maximum water level. Take care
when lowering and raising the pitot tube!
Measure the velocity head (hv) of the surface flow, at 0.5h (in other words, half
the water depth from the surface) and as near the bottom of the channel (as
close as you can without risking damage to the tube).
(vi)
Enter your numbers into Table 1, ensuring your units are correct -- and convert
them where necessary.
(vii)
The velocity of flow (V) can be calculated using the follow equation, where g =
9.8 m s-2:
𝑉 = √2𝑔ℎ𝑣
Table 1.
Water depth (h)
(m)
Surface
Velocity head (hv)
(cm)
Velocity head (hv)
(m)
Velocity (V)
(m s-1)
0.5h
Bottom of the
channel
Question 6a: Draw a diagram presenting the vertical velocity distribution of the open
channel. The channel bed and surface should be labelled as well as the direction of flow
and the different (or similar) flow velocities at each measured water depth based on the
length of arrows (i.e., longer arrows represent more rapid water flow).
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Question 6a: Offer an explanation (or explanations) as to why the profile follows that
shape.
3. Investigating Manning’s coefficient for uniform open channel flow
The channel and bed morphology of a river channel means flow dynamics evolve along the
length of a river. This introduces complexities when attempting to calculate flow. As a result,
the conventional approach is to assume uniform flow conditions, which can be found in
individual reaches. Uniform flow is classified when the following are constant: flow rate,
channel slope and cross-sectional shape of the channel.
A number of formulae have been developed to compute uniform flow, the most widely used
of which is Manning’s equation (developed in the late 19th-century by an engineer, Robert
Manning). Refer to the fluvial geomorphology lectures for more information.
(�
)2⁄3(�
)1/2
𝑉=
which can be re-written as
𝑛
(�
)2⁄3(�
)1/2
𝑛=
𝑉
in order to calculate n.
Where R is the hydraulic radius, S is the channel bed slope and the parameter n is the
Manning Roughness Coefficient. Don’t worry if you are unsure how to determine all the
parameters; this exercise will enable you to do so!
3.1 The experiment
(i)
Set the channel bed slope to 1 by cranking the channel bed slope knob (opposite
end to the PMW knob)
(ii)
As you follow the next steps, enter the numbers into Table 2.
(iii)
Uniform flow depends on constant channel size and shape. To calculate the
hydraulic radius (R) of a channel, two numbers are required: the cross-sectional
area (A) (width x depth) and the wetted perimeter (P) (the total length of the
bed and sides in contact with fluid) -- see Figure 3.
(iv)
Now calculate the hydraulic radius, using the equation R = A / P
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(v)
At the same measurement point as Exercise 2, re-measure the water depth of
the channel (h). Also re-measure the velocity head (Vh) at a depth of 0.6h. (Note:
as a general rule, this is the depth at which average flow velocity for a crosssection occurs.)
X
Y
Figure 3. Schematic to assist in calculating cross-sectional area (A) and wetted perimeter
(P). Cross-sectional area is width * depth, so X * Y. Wetted perimeter is the total length of
the bed and sides in contact with fluid, so (X + X + Y).
(vi)
Calculate the average flow velocity (see Exercise 2)
(vii)
Calculate the discharge (m3 s-1) as Q = V x A
(viii)
Calculate the hydraulic radius (R)
(ix)
Calculate the Manning coefficient (n)
(x)
Complete Table 2.
Table 2.
Channel bed slope (S) =
PWM (%) =
Flow depth (h) =
Velocity
head
(Vh) (m)
Velocity
(m s-1)
CrossWetted
Discharge
sectional
Perimeter Q (m3 s-1)
area
(A) P (m)
2
(m )
Hydraulic
Manning
Radius (R) Coefficient n
(m)
Question 7. What is your Manning coefficient n for the channel reach?
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4. Evaluating the hydrodynamics of a weir
4.1 The equipment:
Ruler
Pitot tube
A broad-crested weir
Weirs are walls or obstructions across a stream or a channel which block flow of water. They
are extremely important in the design of hydraulic and civil engineering projects involving
flood control and water heads in reservoirs and hydroelectric plants. They are also used to
measure discharge in channels, known as measuring weirs.
Weirs have a striking influence on flow dynamics, shown by the schematics in Figure 4.
Figure 4. Flow over a broad-crested weir
4.2 The experiment
(i)
Return the flume to its initial conditions and wait for the flow to stabilise.
As you follow the next steps, enter the numbers into Table 3.
(ii)
Carefully insert the weir into the flume at the 30-cm mark (i.e., 20-cm from the
upstream gate). It is a tight fit; take your time. Make sure it is lying flat against
the channel bed and that the slope is facing upstream.
(iii)
Measure the height of the weir (P)
Question 8. Briefly describe the flow dynamics across the weir. What happens in terms of
water depth, flow velocity and flow dynamics just in front of and just behind the weir?
5SSG2023 PG: ESP&L 2016-17
Small-group Laboratory Activity Week 9: EM1 Flume
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(iv)
Measure flow depth, h1, upstream of the weir far enough not to have the
drawdown effect (before the point where the flow angles downwards.)
(v)
Use the pitot tube to measure the velocity head at 0.6h at this spot
(vi)
Calculate the average flow velocity (see Exercise 2)
(vii)
Calculate discharge Q (see Exercise 3)
(viii)
Use a pitot tube to measure the maximum velocity head at the location where
water flows over the weir. Do your best to repeat steps (vi) and (vii) for the flow
over the weir. Carefully consider where the maximum discharge is going to be
and what the cross-sectional area and wetted perimeter are.
(ix)
Repeat (v) – (viii) 10-cm downstream of the weir.
(xi)
Complete Table 3.
Table 3.
Upstream
flow
depth H1 (m)
Upstream
velocity head Vh1
(m)
Upstream
velocity V1 (m s-1)
Cross-sectional
area A1 (m2)
Discharge Q1 (m3
s-1)
Flow depth over
the weir h2 (m)
Weir
velocity
head Vh2 (m)
Downstream flow
depth h3 (m)
Downstream velocity
head Vh3 (m)
Weir velocity V2
(m s-1)
Weir A2 (m2)
Downstream velocity
V3 (m s-1)
Downstream A3 (m2)
Discharge Q2 (m3
s-1)
Downstream
Discharge Q3
Question 9. Comment on the difference between calculated discharges (Q) upstream,
over the weir and downstream
Question 9. In what way(s) do your findings show the use of weirs to engineers and
hydrologists?