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Department of Civil Engineering The University of Queensland

Laboratory classes CIVL3140 Introduction to Catchment Hydraulics

Laboratory work is closely linked with the lecture program and is an INTEGRAL part of the fluid mechanics/hydraulics course. It is intended to draw to the student's attention the behaviour of real fluid flows. Four laboratory classes are scheduled for each student, running from weeks 3-6 and week 8-11 of the semester. Laboratory classes will run on monday to thursday 9weeks 3-6 & 8-11 only) depending on students’ timetables. ATTENDANCE Each student is required to attend all laboratory sessions. Each student will sign up for a group of 12 people, with 3 groups of 4 students per experiment. Timetables will be posted on the noticeboard in the Fluid mechanics laboratory at the beginning of each semester. Attendance is COMPULSORY and any unavoidable absence must be made up at the next available opportunity after consultation with the laboratory coordinator. [Note however that groups larger than 5 will be forbidden for safety reasons. Students must organise swaps before the end of the laboratory class period.] SAFETY REGULATIONS Adequate shoes or boots must be worn in the laboratory. Sandals or open footwear are NOT permitted. Strong clothing is recommended, incl. shorts and trousers. (No skirt, dress nor loose clothing.) The attached safety form must be completed and submitted to the TUTOR at the start of the FIRST lab session. ASSESSMENT The Laboratory assessment includes a combination of {a} Attendance, {b} Pre-Lab calculations, {c} Individual work during the afternoon, and {d} Group report. No individual reports are required to be submitted for this subject. The marks for the laboratory work and pre-lab calculations will be worth the Laboratory class component of the course. [1] Pre-requisite calculations : each student is required to perform INDIVIDUALLY the pre-lab calculations and submit them to the relevant tutor prior to the start of the experiment. (It will be assessed and later included in the group report.) [2] Pre-formatted report : each student group will complete one report during the experiment and it will be submitted for marking at the conclusion of the experiment. During the afternoon, there is a strong emphasis on group work. Each group (3-4 students) will complete a pre-formatted report during the experiment and submit it for marking at the conclusion of the experiment. The pre-formatted laboratory reports must be prepared to the format specified below and credit is given for them in accordance with the credit points of the subject. The final examination may have some question based on the experiments. Failure to satisfy in laboratory work will result in the failure in the subject : i.e. students who did not attend an experiment or who fail to complete a satisfactory laboratory report shall fail the subject. It is the responsibility of each and every student who miss a prac to organise swaps before the end of the laboratory class period (i.e. before end of teaching class period). [Practicals will run from weeks 3-6 and 8-

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11, with a break in week 7. No special Prac session shall be scheduled because of the large demand on the laboratory facilities and time-table classes with other activities. The first set of experiments may be dismantled at the end of week 6, while some major refurbishment works will take place in the laboratory from week 12. NO further laboratory session shall be organised.] PROCEDURE 1- At least 1 day before each scheduled lab session, each student is advised to visit the laboratory, inspect the apparatus, and prepare a preliminary sketch of it. Students are expected to read the information materials before commencing each experiment. It is important to start promptly at (or before) 2:00 pm since experimental work must usually be finished by 4 pm and laboratory classes must end no later than 4:50pm. (NOTE: the laboratory is open from 1:30 pm and students are strongly encouraged to start their experiment as early as possible; tutors are not needed to start the work). 2- During each afternoon, students perform one experiment during which experimental results are recorded on result sheets, white boards or computer spreadsheets and relevant graphs can also be plotted. Further analysis of experimental results and discussion with the supervisors are developed. At the end of the afternoon, students must have their work approved by the tutor before leaving. REPORTS At the beginning of each afternoon, each group will find a report form next to the experiment. The report is to be completed by the end of the afternoon. The students' names must be on the cover. The folder should be handed in for perusal and comment at the end of the laboratory session before 4:50pm. Comments A proper laboratory report requires a detailed description of the experimental work as well as of the calculations. Some points must be noted : - List of experimental data, e.g. atmospheric conditions, sizes of significant parts of the apparatus, etc. - Results. It is GOOD experimental practice to record and retain all data from an experiment. However, except when indicated in the instruction sheet, it will NOT be necessary to record tables of raw data, since the reduction of results to graphical presentation has been performed and agreed in the final hour of the laboratory session. Where possible students should ALWAYS compare their experimental results with those contained in the literature and should quote any references used. - Calculations. Sample calculations used in obtaining the required results from experimental data must be shown. - Discussions. Each instruction sheet includes some discussion questions; a satisfactory response to these questions is an essential requirement. SPECIAL NOTES REGARDING GRAPHS AND TABLES Graphs and tables are records of results, and these should be placed with the report. All graphs and tables must be prepared in such a manner that ALL titles, axis labels and scales, units and legends are easily legible.

Student Safety Declaration Form for Practical Class Work

This form must be completed by the Student and given to the Lecturer or Tutor during the first practical class. No experimental work should start until this form has been completed. Name: (Please Print) …………………………………………………………………….. Student Number: …………………………………………………………………….. Course Code: …………………………………………………………………….. YES NO • I have read and I understand the Occupational Health and Safety in the Laboratory guidelines (Undergraduate Student Edition)

• I am aware of my Workplace Health and Safety responsibilities

• The tutor has explained what personal protective equipment (PPE) is required for this course. I agree to wear it when required

• I understand that if I am not wearing appropriate PPE, I can be excluded from the laboratory for that class

• I agree to follow all safety procedures explained to me by the tutor

• I understand that I must not eat food or drink in the laboratory

• I understand that inappropriate conduct can result in the denial of further laboratory access

• I understand that all accidents, including ‘near miss’ incidents need to be reported to the lecturer or tutor immediately

• I understand that all faulty or broken equipment needs to be brought to the attention of my tutor immediately

• I am familiar with the emergency procedures for the laboratory and are familiar with the location of the eye wash and safety shower

• I understand the procedures outlined in this guideline regarding pregnancy Student Signature: ………...…………………………………………………………...

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CIVL3140

Department of Civil Engineering - The University of Queensland

Laboratory classes : CIVL3140 Introduction to Catchment Hydraulics Four practical studies are planned as part of the subject CIVL3140 Introduction to Catchment Hydraulics. These laboratory experiments are closely linked with the lecture program and they are an INTEGRAL part of the hydraulic engineering courses. It is intended to draw to the student's attention the behaviour of real fluids. Each experiment relates to the fluid mechanics of open channel flow. The basic concepts of critical flow conditions, subcritical flow and supercritical flow are experienced. Note that one experiment (broad-crested weir) is a smooth transition from an upstream sub-critical flow to a downstream supercritical flow (Fig. 1). The other (hydraulic jump) is the transition from an upstream supercritical flow to a downstream subcritical flow (Fig. 2). Fig. 1 - Sketch of a broad-crested weir overflow

Fig. 2 - Sketch of a hydraulic jump flow

CIVL3140

EXPERIMENT : BROAD CRESTED WEIR 1. AIM The purpose of this experiment is to investigate : - rapidly varying flow in the vicinity of a broad-crested weir. - flow transition between sub-critical and super-critical flows, and - the basic concepts of hydraulic controls, specific energy and Bernoulli principle applied to open channel flows.

2. INTRODUCTION TO BROAD-CRESTED WEIRS In hydrology, it is desirable to have long historic records of streamflows to predict the extremes : i.e. low-flow (or drought) and high flow (floods). Because quantifying flow rates is difficult, but measuring water-levels is easy, the strategy at a stream-gauging station is to measure water-levels continuously and calibrate a stage-discharge curve to convert heights into flow rates. A randomly selected stream location is unsatisfactory for gauging purposes because, for any given water height, the flow rate when a flood is rising is greater than when it is receding. An alternative is the construction of a weir in the stream channel to force critical depth conditions to occur. This results in a satisfactory monotonic relationship between flow rate and flow depth at the weir. In most practical situations, the approach flow velocity is small and the upstream flow conditions are sub-critical. However, on the immediate downstream side (provided the outlet is not "drowned" by some further downstream structure or hydraulic control phenomenon), the flow discharging from the weir crest will generally be supercritical. In crossing the weir, therefore, such flow must pass through CRITICAL flow conditions : i.e. the specific energy must reach a minimum. A weir is called "broad-crested" when the crest is long enough for the streamlines to be parallel to the crest invert, the pressure distribution to be hydrostatic and critical flow conditions to occur somewhere over the horizontal section of the crest. Besides investigating the position at which critical flow depth occurs (and if possible explaining why), it is useful to study the changes in specific energy that take place as the flow passes over the weir. If the energy loss is negligible in passing the sill crest, one should remember that the specific energy above the crest must be less than that upstream by the height of the step because specific energy is referenced to the local channel bed elevation (see definition of the specific energy).

3. PREREQUISITES 3.1 At least 1 day before the scheduled lab session, visit the laboratory, inspect the apparatus, and prepare a preliminary sketch of it. 3.2 Express the relationship between the specific energy, the discharge, channel width and depth in a horizontal rectangular channel. 3.3 Sketch a broad-crested weir flow in a horizontal rectangular channel. On your sketch, plot the free-surface profiles upstream, above and downstream of the weir. Indicate the flow direction. Sketch on your figure the variation of the pressure with depth at a section 1 (upstream of the weir), a section 2 (at the middle of the weir crest) and a section 3 (downstream of the weir). At sections 1, 2 and 3 , the velocity is assumed to be essentially horizontal and uniform. 3.4 In the horizontal rectangular channel, derive the relationship between the discharge, the upstream specific energy, the channel width and the weir height. 3.5 For a horizontal channel of rectangular cross-section, develop the dimensionless expression of the specific energy E/dc as a function of the dimensionless flow depth d/dc, where dc is the critical flow depth. Note : the resulting non-dimensional relationship is universally useful for studying any transition in any channel.

4. APPARATUS In the upper laboratory, three 3.2 m long rectangular horizontal channels are supplied with water from the constant head reservoir. The discharge is controlled by a valve mounted just above the head tank at the upstream end of the channel. Each channel are fitted with horizontal side rails and water depths are measured using mobile pointer-gauges. About halfway along the channels, a section of the floor is raised to form a weir platform (with a flat sill, about 42-cm long in the flow direction). The weir's upstream vertical face is rounded where it meets the horizontal sill, while the downstream face is fitted with a tapered downslope (ramp) that lowers the flow back to the original channel floor level

CIVL3140

in a smooth concave-upward curve. A profile of the weir is provided to assist in plotting the experimental flow profiles. At the downstream end of each channel, the normally free-fall outlet is fitted with an adjustable gate, which can be cranked downward into the outflow to force a hydraulic jump to form if it is desired to investigate when the weir becomes "drowned". Channel flow rates are measured by timing a specified "weight" of outflow in the channels 1 and 2. The outflow falls into a separate tank mounted on a weighing platform with a balance arm to which counter-weights can be added. Under steady-flow conditions, with the tank's lever-action outlet valve wide open, the water level in the tank rises until its outlet discharges at the same rate as the tank inflow. If counter-weights are THEN used to just exceed the effect of the tank contents, the balance arm falls to its "LOW" position. If the tank outlet is then closed, a timer may be started as soon as the balance arm moves to "HIGH". By THEN (quickly) adding a known EXTRA counter-weight to return the balance arm to "LOW", the timer may be stopped as soon as the next "HIGH" move indicates that the specified EXTRA water has entered the tank. In the channel 3, the water discharge is measured with a calibrated orifice meter. The calibration curve of the orifice is plotted next to the channel intake section. Comments Note how water depths in the channel are measured using pointer-gauges with vernier scales. Each vernier scale needs to be set to read zero when the pointer is at the current channel floor level, upstream of the crest. [One pointer-gauge may be set for use over the horizontal sill.] Remind yourself how to interpret the particular vernier scales. Check with your fellow group-members that you all read them the same way. Familiarise yourself with the outlet weighing system, noting the values of the counter-weights provided. Without water flowing, ensure that each member of your group understands the sequence of operations needed to measure flow rates using the apparatus and procedure outlined above. Figure out what circumstances would cause (a) the channel, and (b) the outlet tank, to spill over the lab floor.

5. REFERENCES CHANSON, H. (2004). "The Hydraulics of Open Channel Flow : An Introduction." Butterworth-Heinemann, Oxford,

UK, 2nd edition, 630 pages (ISBN 978 0 7506 5978 9).

6. Additional bibliography CHANSON, H. (1999). "The Hydraulics of Open Channel Flow : An Introduction." Edward Arnold, London, UK,

512 pages (ISBN 0 340 74067 1). CHANSON, H. (2006). "Minimum Specific Energy and Critical Flow Conditions in Open Channels." Journal of

Irrigation and Drainage Engineering., ASCE, Vol. 132, No. 5, pp. 498-502 (ISSN 0733-9437). GONZALEZ, C.A., and CHANSON, H. (2005). "Experimental Measurements of Velocity and Pressure Distribution

on a Large Broad-Crested Weir." Flow Measurement and Instrumentation, Vol. 18 (DOI 10.1016/j.flowmeasinst.2007.05.005) (ISSN 0955-5986).

HENDERSON, F.M. (1966). "Open Channel Flow." MacMillan Company, New York, USA. ISAACS, L.T. (1981). "Effects of Laminar Boundary Layer on a Model Broad-Crested Weir." Research Report No.

CE28, Dept. of Civil Eng., Univ. of Queensland, Brisbane, Australia, 20 pages. MILLER, D.S. (1994). "Discharge Characteristics." IAHR Hydraulic Structures Design Manual No. 8, Hydraulic

Design Considerations, Balkema Publ., Rotterdam, The Netherlands, 249 pages. MONTES, J.S. (1998). "Hydraulics of Open Channel Flow." ASCE Press, New-York, USA, 697 pages. NOVAK, P., MOFFAT, A.I.B., NALLURI, C., and NARAYANAN, R. (2001). "Hydraulic Structures." Spon Press,

London, UK, 3rd edition, 666 pages.

CIVL3140

Laboratory classes CIVL3140 Introduction to Catchment Hydraulics

Pre-requisite calculations

EXPERIMENT : BROAD CRESTED WEIR Date (Week and day) : Student's Name : Specific energy : E = Derivation :

Sketch of the broad-crested weir overflow

CIVL3140

Discharge : Q = Derivation :

Dimensionless specific energy : Edc

=

Derivation :

CIVL3140

EXPERIMENT : HYDRAULIC JUMPS 1. AIM The purpose of this experiment is to study the behaviour of a hydraulic jump and to quantify the characteristics of stationary hydraulic jumps (e.g. downstream flow depth, rate of energy dissipation) over a range of Froude numbers (see textbook, pp. 53-63 for the basic equations).

2. INTRODUCTION TO HYDRAULIC JUMPS A hydraulic jump is a stationary transition from an upstream supercritical flow to a downstream subcritical flow. Remember that the broad-crested weir induces the reverse flow transition : i.e., from an upstream subcritical flow to a downstream supercritical flow Under steady conditions, the forces tending to push it downstream must balance those opposed. If the jump is considered a free body, it is relatively easy to identify (and by experiment quantify) the pressure forces and momentum flux at the upstream and downstream ends of the control volume. Some hydraulic jumps appear like smooth (undular) waves with almost negligible turbulent activity, but most involve a vigorously tumbling flow region within which much kinetic energy is being "lost" (converted into heat). The latter type of jump is often employed as an energy "dissipator" (below a dam spillway, for example) to avoid scour of bed material in high velocity regions that might otherwise occur if the kinetic energy of the flow is not dissipated. 3. PREREQUISITES 3.1 At least 1 day before the scheduled lab session, visit the laboratory, inspect the apparatus, and prepare a preliminary sketch of it. 3.2 Sketch a hydraulic jump flow in a horizontal rectangular channel. On your sketch, plot the free-surface profiles upstream and downstream of the jump and indicate the flow direction. Sketch on your figure the variation of the pressure with depth at a section 1 (upstream of the jump) and a section 2 (downstream of the jump). Sections 1 and 2 are located far enough from the sluice gate for the velocity to be essentially horizontal and uniform. 3.3 Apply the continuity, momentum and energy equations to a hydraulic jump in a prismatic rectangular channel : In the textbook, find the dimensionless expressions of the downstream flow conditions (depth, Froude number) and rate of energy dissipation, as a function of the upstream Froude number Fr1 : 4. APPARATUS In the upper laboratory, three 3.2 m long rectangular horizontal channels are supplied with water from the constant head reservoir located about 2.5 m above floor level. The reservoir feeds a head tank which then feeds each channel with a supercritical flow through a vertical sluice-gate. Normally, the opening of this sluice-gate will have been preset to 20 mm for this experiments. The 0.45 m deep channels are fitted with horizontal side rails so that water depths upstream of the jump can be measured using a mobile pointer-gauge. At the downstream end of each channel, the free-fall outlet is controlled by an adjustable overflow gate, which can be cranked above the channel floor to force a hydraulic jump to form under steady conditions about 2 meters upstream of the channel outlet. Discharge measurements are made with either a 90-degree V-notch weir [The calibration chart for the V-notch weir is located on the side of the inlet tank] or a Venturi manometer [The discharge is related to the head loss by a calibration curve given]. Comments Investigate the discharge measurement system of the flume : + For the V-notch weir measurement, note how water levels are measured in the V-notch-weir tank by means of a hook gauge and vernier scale. The vernier scale needs to be set to read zero when the water level is at exactly the same level as the apex of the notch. In the time since this tank was last used, the vernier scale setting will probably have been tampered with, AND some water will have evaporated so that the tank water level is not at notch level. Check this last point by viewing the weir from its inlet side to see if the reflection of the V-shape in the undisturbed

CIVL3140

tank contents lines up with the weir itself. If the tank contents need topping up, a hose or bucket may be used. If too much is added, the tank has a valved outlet drain. Remind yourself how to interpret the vernier scale. Check with your fellow group-members that you all read it the same way. + For the Venturi measurement, note the location where the head loss measurement is made. Note that the head-tank contents are indicated by means of a manometer tube close to the tank's outlet and on the rear of the tank so that it may be viewed from the operating position for turning on the water supply valve. The valve must be set to supply water at exactly the same rate as it is being discharged through the sluice gate, which depends on the height of the water level inside the tank. Figure out what circumstances would cause (i) the channel, and (ii) the head tank, to spill over the lab floor.

5. REFERENCES CHANSON, H. (2004). "The Hydraulics of Open Channel Flow : An Introduction." Butterworth-Heinemann, Oxford,

UK, 2nd edition, 630 pages (ISBN 978 0 7506 5978 9).

6. Additional references CHANSON, H. (1999). "The Hydraulics of Open Channel Flow : An Introduction." Edward Arnold, London, UK,

512 pages (ISBN 0 340 74067 1). CHANSON, H. (2007). "Bubbly Flow Structure in Hydraulic Jump." European Journal of Mechanics B/Fluids, Vol.

26, No. 3, pp.367-384 (DOI:10.1016/j.euromechflu.2006.08.001) (ISSN 0997-7546). HAGER, W.H. (1992). "Energy Dissipators and Hydraulic Jump." Kluwer Academic Publ., Water Science and

Technology Library, Vol. 8, Dordrecht, The Netherlands, 288 pages. HENDERSON, F.M. (1966). "Open Channel Flow." MacMillan Company, New York, USA. MONTES, J.S. (1998). "Hydraulics of Open Channel Flow." ASCE Press, New-York, USA, 697 pages. NOVAK, P., MOFFAT, A.I.B., NALLURI, C., and NARAYANAN, R. (2001). "Hydraulic Structures." Spon Press,

London, UK, 3rd edition, 666 pages.

CIVL3140

Laboratory classes CIVL3140 Introduction to Catchment Hydraulics

Pre-requisite calculations

EXPERIMENT : HYDRAULIC JUMP Date (Week and day): Student's Name :

Hydraulic jump sketch

CIVL3140

Basic equations

Give the expression for the alternate depths as a function of the other depth and discharge, or as a function of the Froude numbers. Write the energy loss in terms of the energy equation either side of the jump.

[C]

[M]

[E]

CIVL3140

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EXPERIMENT : EQUILIBRIUM AND NON-UNIFORM FLOWS IN A LONG CHANNEL

1. AIM

The purpose of this experiment is to demonstrate the effect of open channel control features on gradually-varied flows,

and to quantify friction and backwater effects that occur differently in "steep" and "mild-sloped" waterways carrying

steady flows. The experimental procedure will include observations, modelling, prediction (numerical modelling) and

evaluation (comparison between data and computation).

During the afternoon, the students will be involved in physical modelling and experimental observations, numerical

modelling, and a comparison between physical and numerical model tests. Each experiment will be supported by an

audio-visual technical documentary ("Surfing the Dordogne" 2006).

2. UNIFORM EQUILIBRIUM AND GRADUALLY VARIED FLOW

2.1 Introduction to ungated flow

Flow enters the channel via a low-velocity stilling basin, then it gradually accelerates down the sloped channel.

Because the flow rate is steady, the flow depth gradually decreases with increasing velocity. As shear stresses gradually

increase with increasing velocity, the inertial acceleration is eventually balanced by frictional deceleration, yielding

uniform equilibrium flow of depth do. (Note that do is called the normal depth).

One might expect to measure normal depth somewhere near the downstream end of the channel, but before doing so,

we need to consider the effect of the overfall at the downstream end of the channel. If the channel slope is steep, the

uniform equilibrium flow regime will be supercritical, and information about the downstream "waterfall" cannot be

propagated upstream against supercritical flow. So near the outlet is a good place to look for do if the channel slope is

steep. However, if the slope is mild, normal flow will be subcritical, and critical depth must occur at or near, the

overfall as the free-falling nappe is an accelerating supercritical flow (see HENDERSON 1966, pp. 192-195).

"Backwater effect" from the hydraulic control (i.e. critical flow depth) at the overfall will be "conveyed" upstream to

the approaching flow in the form of a backwater curve. In this case, the best place to look for do is upstream of this

backwater curve i.e. probably near the middle of the experimental channel.

References : CHANSON (2004), pp. 100-106; HENDERSON (1966), pp. 38 to 44.

2.2 Uniform equilibrium flow conditions (normal flow)

At uniform equilibrium, the momentum equation along a streamline states the exact balance between the shear forces

and the gravity component. Considering a control volume, the momentum equation yields : τo * Pw * ∆s = ρ * g * A * ∆s * sinθ (1a)

where τo is the bottom shear stress, Pw is the wetted perimeter, ∆s is the length of the control volume, A is the cross-

section area and θ the channel slope. Replacing the bottom shear stress by its expression, the momentum equation for

uniform equilibrium flows becomes :

CIVL3140

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Vo = 8 * g

f * (DH)o

4 * sinθ (1b)

where Vo is the uniform (equilibrium) flow velocity and (DH)o is the hydraulic diameter of uniform equilibrium flows

(CHANSON 2004, pp. 69-77). The momentum equation for steady uniform open channel flow (Eq. (1a)) is rewritten

usually as: Sf = So (1c)

where Sf is called the friction slope and So is the channel slope defined as :

Sf = - ∂ H∂s =

4 * τoρ * g * DH

So = - ∂ zo∂s = sinθ

where H is the mean total head and zo is the bed elevation. Note that the definitions of the friction and bottom slope are

general and applied to both uniform equilibrium and gradually-varied flows.

2.3 Gradually-varied flow : backwater calculations

Considering an open channels flow and along a streamline in the s-direction, the energy equation written in term of

mean total head H is :

∂ H∂s = - Sf (2a)

where s is the distance along the channel bed and Sf is the friction slope :

Sf = τo

ρ * g * DH = f *

1DH

* V2

2 * g

where τo is the boundary shear stress, DH is the hydraulic diameter, f is the Darcy friction factor and V is the mean

flow velocity.

Equation (2) is the basic backwater equation. It can be applied to natural and artificial channels.

The backwater calculations are developed assuming [H1] a non-uniform flow, [H2] a steady flow, [H3] that the flow is

gradually varied, and [H4] that, at a given section, the flow resistance is the same as for an uniform flow for the same

depth and discharge, regardless of trends of the depth.

The gradually-varied flow (GVF) calculations do not apply to uniform equilibrium flows, nor to unsteady flows nor to

rapidly-varied flows (RVF). Further the last assumption [H4] implies that the Darcy, Chézy or Gauckler-Manning

equations may be used to estimate the flow resistance, although these equations were originally developed for uniform

flows only.

In man-made channels, simpler forms of the backwater equation are available. Assuming a hydrostatic pressure

distribution, the backwater equation can be expressed in term of the flow depth :

∂ d∂s * (cosθ - α * Fr2) = So - Sf (2b)

where So = sinθ, α is the Coriolis coefficient (or kinetic energy correction coefficient) and Fr is the Froude number

defined as :

CIVL3140

3

Fr = Q

g * A3

B

where A is the flow cross-section area and B is the open channel free surface width.

References : CHANSON (2004), pp. 102-106 and 275-288; HENDERSON (1966), pp. 38 to 44.

2.4 Backwater computations : standard step method (depth calculated from distance)

Assuming a hydrostatic pressure gradient, the mean total head equals :

H = zo + d * cosθ + α * V2

2 * g

where zo is the bed elevation, d is the flow depth measured normal to the channel bottom, θ is the channel slope, α is

the Coriolis coefficient, V is the mean velocity and g is the gravity constant.

Considering a gradually-varied flow, the backwater equation can be integrated between two stations denoted {i} and

{i+1} :

Hi+1 - Hisi+1 - si

≈ - 12 * (Sfi+1 + Sfi)

where s is the streamwise coordinate, and the subscripts i and i+1 refer to stations {i} and {i+1} respectively. It yields :

Hi+1 - Hi ≈ - 12 * (Sfi+1 + Sfi) * (si+1 - si)

If the flow conditions at station {i} are known, the total head at station {i+1} equals :

Hi+1 ≈ Hi - 12 * (Sfi+1 + Sfi) * (si+1 - si)

3. APPARATUS

Three long straight channels of uniform rectangular cross-section and smooth surface finish crosses the lower

laboratory, supported on elevated trusses. The channel characteristics are:

Channel (A) Channel (B) Channel (C) Length (m) : 20.15 12 12 Width (m) : 0.25 0.50 0.50 Remark : Elevated channel Elevated channel Lower channel Channel bed: Glass PVC PVC Bed slope So: 0.003 0.028 0.028

Water enters the channel from the roof-level reservoir, and falls from the channel outlet to the drains below. The

channel slope may be changed manually via a geared chain elevator, and control gates may be inserted at the channel

outlet and one intermediate position. (The slope will not be changed during the afternoon.) Flow rate is measured by

either a V-noth weir (Channel A) or an orifice meter (Channels B and C), and the flow depths are measured using

pointer gauges mounted on rails parallel to the bed. (Each pointer gauge should read zero on the bed. Since the flow

CIVL3140

4

surface is turbulent, flow depths are read when the tip of the pointer seems to be clear of the surface about half the

time.)

4. PREREQUISITES

4.1 At least 1 day before the scheduled lab session, visit the laboratory and inspect the apparatus. Find out the

downstream flow direction.

4.2 Read these instructions completely and complete the attached pre-requisite form.

4.3 Compute the critical flow depth and uniform equilibrium flow conditions (assuming roughness ks = 0.1 mm) for the

discharges and slope specified in the attached table.

Report your calculations in the attached pre-formatted sheet (that you will include with the group report).

5. TUTORLESS ACTIVITIES

Observe the location of the hinge at the upstream end of the channel's supporting truss. Note that the vertical members

of the truss are spaced 1 m apart. Count these members downstream from the hinge to a distance of 12 m (which is the

horizontal distance over which the channel slopes are measured). Determine how the channel slopes are set.

Measure the flow rate. In the narrow channel, the flow rate is measured at the downstream end with a V-notch weir.

With the wide channels, the discharge is measured with an orifice system which was calibrated on-site.

6. REFERENCES

CHANSON, H. (2004). "The Hydraulics of Open Channel Flow : An Introduction." Butterworth-Heinemann, Oxford,

UK, 2nd edition, 630 pages (ISBN 978 0 7506 5978 9).

HENDERSON, F.M. (1966). "Open Channel Flow." MacMillan Company, New York, USA.

Le Grand Angle (2006). "Surfing the Dordogne." DVD colour, SBS,7 October 2006.

PSE Library Call No. GV839.66.F8 S87 2006

CIVL3140 website : http://www.uq.edu.au/~e2hchans/civ3140.html

The tidal bore of the Seine river, France : http://www.uq.edu.au/~e2hchans/mascaret.html

7. Additional bibliography

CHANSON, H. (1999). "The Hydraulics of Open Channel Flow : An Introduction." Edward Arnold, London, UK, 512

pages (ISBN 0 340 74067 1).

CHANSON, H. (2004). "Mixing and Dispersion Role of Tidal Bores." in "Fluvial, Environmental & Coastal

Developments in Hydraulic Engineering", Balkema, Leiden, The Netherlands, Proc. Intl Workshop on State-of-the-

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5

Art Hydraulic Engineering, 16-19 Feb. 2004, Bari, Italy, M. MOSSA, Y. YASUDA and H. CHANSON Ed., pp.

223-232 (ISBN 04 1535 899 X).

CHANSON, H. (2005). "Tidal Bore Processes in the Baie du Mont Saint Michel (France): Field Observations and

Discussion." Proc. 31st Biennial IAHR Congress, Seoul, Korea, B.H. JUN, S.I. LEE, I.W. SEO and G.W. CHOI

Editors, Theme E.4, Paper 0062, pp. 4037-4046 (ISBN 89 87898 24 5).

KOCH, C., and CHANSON, H. (2006). "Unsteady Turbulence Characteristics in an Undular Bore." Proc. Intl Conf.

Fluvial Hydraulics River Flow 2006, Lisbon, Portugal, 6-8 Sept., Topic A1, R.M.L. FERREIRA, E.C.T.L. ALVES,

J.G.A.B. LEAL, and A.H. CARDOSO Eds., Balkema Publ., Taylor & Francis Group, London, Vol. 1, pp. 79-88

(ISBN 0 415 40815 6).

MONTES, J.S. (1998). "Hydraulics of Open Channel Flow." ASCE Press, New-York, USA, 697 pages.

NHK Japan Broadcasting Corp (1989). "Pororoca : the backward flow of the Amazon." Videocassette VHS colour,

NHK, Japan, 29 minutes.

CIVL3140

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Laboratory classes

CIVL 3140 Catchment Hydraulics

Pre-requisite calculations

EXPERIMENT : EQUILIBRIUM AND NON-UNIFORM FLOWS IN A LONG CHANNEL

Date (Day / week):

Student's Name :

Flow conditions for TWO experimental discharges and TWO channel widths

Q = 15 L/s Q = 25 L/s Units

Channel width : 0.25 0.50 m

Equivalent roughness height : 0.1 0.1 mm

Bed slope So : 0.003 0.028 --

Critical flow depth : m

Uniform equilibrium flow

Normal depth : m

Darcy friction factor : --

Relative roughness ks/DH : --

Reynolds number : --

Normal velocity : m/s

Froude number : --

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EXPERIMENT : FLOW THROUGH CULVERTS

1. AIM

The main purpose of this experiment is to compare and to understand the differences between the hydraulic

performances of two different types of culverts carrying the same flow rate: a box culvert (standard culvert) and a

minimum-energy-loss culvert (MEL culvert).

During the afternoon, the students will be involved in physical modelling and experimental observations, numerical

modelling, and a comparison between physical and numerical model tests. Each experiment will be supported by an

audio-visual technical documentary (APELT 1994).

2. INTRODUCTION TO CULVERTS

2.1 Presentation

Culverts are commonly used to carry stormwater flows under road (or railway) embankments. They are designed to

pass a specific flood flow rate which is associated with an acceptable natural flood level at the site.

[For both culvert models, the model-scale design flow rate is Q = 10 L/s, for which normal depth is known to be dD/S

= 38 mm.]

Basic design calculations are based upon the assumptions of free-surface flow and critical flow conditions in the

barrel, to maximise the discharge per unit width at the throat. However a culvert might operate also full depending

upon the flow rate and tailwater conditions.

If, at the design flow rate, the culvert's energy loss exceeds the natural energy loss, the normal flow will be disturbed,

and the free-surface elevation in the upstream flood plain may be significantly higher than normal flow conditions.

The difference between the upstream free-surface elevation with the culvert and at normal flow (i.e. in absence of

culvert) is called the afflux. Large afflux may (a) cause extensive flood damage to upstream properties, and/or (b)

cause the embankment itself to have to be raised (which may be expensive, particularly for long embankments).

2.2 Standard (box) culverts

A standard culvert is designed to pass waters at a minimum cost without much consideration of the head loss. The

culvert flow may exhibit various flow patterns depending upon the discharge, the upstream head above the inlet invert

(H1 - zinlet), the uniform equilibrium flow depth in the barrel do, the barrel invert slope θ, the tailwater depth dtw,

and the culvert height D (CHANSON 2004, pp. 440-454).

The flow patterns may be regrouped into two classes :

• Class I for free-surface inlet flow conditions, and

• Class II for submerged entrance.

Free-surface inlet flows [Class I] take place typically for :

H1 - zinlet

D ≤ 1.2

In each class, the flow patterns can be sub-divided in terms of the control location : i.e., whether the hydraulic control

is located at the entrance (i.e. inlet control) or at the outlet (i.e. outlet control) (CHANSON 2004, pp. 445-454).

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Minimum Energy Loss culverts

A Minimum Energy Loss culvert is a structure designed with the concept of minimum head loss. The flow in the

approach channel is contracted through a streamlined inlet into the barrel where the channel width is minimum, and

than it is expanded in a streamlined outlet before being finally released into the downstream natural channel. Both the

inlet and outlet must be streamlined to avoid significant form losses. The barrel invert is sometimes lowered to

increase the discharge capacity. APELT (1983) presented an authoritative review of the topic.

The basic concepts of MEL culvert design are : streamlining and critical flow conditions throughout all the culvert

(inlet, barrel and outlet) (CHANSON 2004, pp. 454-471).

Streamlining : the intake is designed with a smooth contraction into the barrel while the outlet (or diffuser) is shaped

as a smooth expansion back to the natural channel.

Critical flow conditions : as, in an open channel, maximum discharge per unit width for a given specific energy is

achieved at critical flow conditions, Minimum Energy Loss structures are designed to achieve critical flow conditions

IN ALL THE ENGINEERED WATER WATERWAY : i.e., in the inlet, at the throat (or barrel) and in the outlet. (At the throat,

the discharge per unit width q is maximum and it may be increased by lowering the barrel bed.)

Discussion

Because of their simpler geometry, box culverts usually cost much less than MEL culverts (APELT 1983). However

the energy loss (and hence afflux) of a box culvert is usually much higher than for a MEL culvert of equal flow

capacity (APELT 1983, CHANSON 2004). If the flow rate through a box culvert is reduced until its afflux is equal to

that of the fully flowing MEL culvert, the ratio of the flow rates gives the number of required parallel box culverts

needed to match the single MEL culvert's performance in terms of both total flow rate and afflux. It is not obvious

which combination would then yield the most economical solution.

For a culvert, design flow conditions will be a rare occurrence. When observing the performance of model culverts,

off-peak conditions may be just as important to study as the design flood. [The Clare Weir on the Burdekin River in

North Queensland failed at a flow rate much less than that for which it was "designed" : the failure was caused by

downstream undermining].

3. APPARATUS

Adjacent to the wind tunnel in the upper laboratory, and beneath the long channel experiments, a series of 3 complex

open channel structure, nominally 8 m long (width 1 m), is supplied with water from the low-level reservoir via a

black-handled valve mounted just above the head tank at the upstream end. Flow first enters a box-culvert model

channel (0.2 m deep and about 3 m long), and then passes into a MEL-culvert model channel (0.2 m deep and about 5

m long).

In the box-culvert channel, a trapezoidal embankment separates the upstream pool from the downstream pool, and

flow passes through this embankment via a horizontal duct called barrel (or throat) of rectangular cross-section (0.15

m wide, 0.107 m high and 0.50 m long). At the entrance to the culvert, flow is guided into the barrel by "wing walls"

angled at 45 degrees in plan and 0.3235 m long, and a symmetric arrangement guides the downstream expanding flow

(at the outlet).

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In the MEL-culvert flume, the same flow passes through a culvert with curved wing-walls and a lowered barrel floor.

(The "bridge" over this culvert is removable, to facilitate measurements.) The barrel length is 0.60 m, but the width is

only 0.10 m, maximum depth is about 0.17 m, and the bottom is 0.124 m lower than the upstream channel bed. The

upstream wing walls are circular in plan (tangent to the barrel). The outlet wing walls are nearly straight (except for

short transitions) and they diverge at a much smaller angle than the box culvert.

Note : From a top view, a MEL culvert sidewalls follow usually the shape of a Venturi meter (CHANSON 2004, pp.

467-469).

Water depths are recorded with glass manometers upstream of both culvert models, and also downstream of each (at

representative locations). Water surface elevations in the MEL culvert are measured by means of pointer gauges

mounted on rails parallel to the mean longitudinal bed slope : the channel floors of both culvert channels are slightly

sloped to represent the natural channel conditions (i.e.: So = 0.0035 in box culvert model).

Culvert flow rates are measured by means of an inlet bend-meter connected to a pressurised air-water U-tube, with a

calibration chart (indicating a manometric reading of 25.2 cm of water for a flow rate of 10 L/s).

At the downstream end of each channel, a geared or screw-operated gate acts as a control gate for raising the

downstream water level for each culvert.

4. PREREQUISITES

[1] At least 1 day before the scheduled lab session, visit the laboratory, inspect the apparatus, and prepare a

preliminary sketch of it.

[2] Read these instructions completely and complete the attached pre-requisite form.

[3] For the design flow rate (Q = 10 L/s), calculate the critical depths in both model culvert barrels and write your

results in the attached pre-formatted sheet (that you will include with the group report).

5. TUTORLESS ACTIVITIES

A. Note how water surface levels are measured using a pointer-gauge. Check that all of your group-members interpret

the vertical scale correctly.

B. A chart showing a longitudinal profile of the MEL culvert is provided. Values for width (notation : W in mm) and

bed elevation (notation : Zb in mm) are given. This chart shows also critical depths at every cross-section for Q = 10

L/s. Verify that critical depth in the barrel matches your pre-requisite calculation.

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6. GLOSSARY

Afflux : Rise of water level above normal level on the upstream side of a culvert or of an obstruction in a channel.

APELT : C.J. APELT is an Emeritus Professor in civil engineering at the University of Queensland (Australia).

Critical flow conditions : In open channel flows, the flow conditions such as the specific energy (of the mean flow) is

minimum are called the critical flow conditions. With commonly-used Froude number definitions, the critical flow

conditions occur for Fr = 1. If the flow is critical, small changes in specific energy cause large changes in flow

depth. In practice, critical flow over a long reach of channel is unstable.

McKAY : Professor Gordon M. McKAY (1913-1989) was Professor in Civil Engineering at the University of

Queensland.

Specific energy : quantity proportional to the energy per unit mass, measured with the channel bottom as the elevation

datum, and expressed in metres of water. The concept of specific energy, first developed by B.A. BAKHMETEFF

in 1912, is commonly used in open channel flows.

Streamline : is the line drawn so that the velocity vector is always tangential to it (i.e. no flow across a streamline).

When the streamlines converge the velocity increases. The concept of streamline was first introduced by the

Frenchman J.C. de BORDA.

Total head : The total head is proportional to the total energy per unit mass and per gravity unit. It is expressed in

metres of water.

7. REFERENCES

APELT, C.J. (1983). "Hydraulics of Minimum Energy Culverts and Bridge Waterways." Australian Civil Engrg

Trans., I.E.Aust., Vol. CE25, No. 2, pp. 89-95 (ISSN 0819-0259).

APELT, C.J. (1994). "The Minimum Energy Loss Culvert." Videocassette VHS colour, Dept. of Civil Eng.,

University of Queensland, Australia, 18 minutes.

CHANSON, H. (1999). "The Hydraulics of Open Channel Flows : An Introduction." Edward Arnold, London, UK,

512 pages (ISBN 0 340 74067 1).

CHANSON, H. (2004). "The Hydraulics of Open Channel Flow : An Introduction." Butterworth-Heinemann, Oxford,

UK, 2nd edition, 630 pages (ISBN 978 0 7506 5978 9).

8. Additional bibliography

CHANSON, H. (2006). "Minimum Specific Energy and Critical Flow Conditions in Open Channels." Journal of

Irrigation and Drainage Engineering., ASCE, Vol. 132, No. 5, pp. 498-502 (ISSN 0733-9437).

CHANSON, H. (2007). "Hydraulic Performances of Minimum Energy Loss Culverts in Australia." Jl of

Performances of Constructed Facilities, ASCE, Vol. 21, No. 4, pp. 264-272 (ISSN 0887-3828).

McKAY, G.R. (1970). "Pavement Drainage." Proc. 5th Aust. Road Res. Board Conf., Vol. 5, Part 4, pp. 305-326.

McKAY, G.R. (1971). "Design of Minimum Energy Culverts." Research Report, Dept of Civil Eng., Univ. of

Queensland, Brisbane, Australia, 29 pages & 7 plates.

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McKAY, G.R. (1978). "Design principles of Minimum Energy Waterways." Proc. Workshop on Minimum Energy

Design of Culvert and Bridge Waterways, Australian Road Research Board, Melbourne, Australia, K.F. PORTER

Ed., Session 1, pp. 1-39.

Minimum Energy Loss culverts and bridge waterways {http://www.uq.edu.au/~e2hchans/mel_culv.html}

The Minimum Energy Loss (MEL) weir design: an overflow earthfill embankment dam

{http://www.uq.edu.au/~e2hchans/mel_weir.html}

CIVL3140 website :{http://www.uq.edu.au/~e2hchans/civ3140.html}

CIVL3140

12

Laboratory classes

CIVL 3140 Catchment Hydraulics

Pre-requisite calculations

EXPERIMENT : FLOW THROUGH CULVERTS

Date (Day / week):

Student's Name :

Culvert geometry

Box culvert MEL Culvert Units

Upstream channel width :

Downstream channel width :

Barrel width :

Barrel height :

Barrel length :

Barrel flow conditions at design discharge

Box culvert MEL Culvert Units

Design discharge :

Critical depth in barrel :

Critical velocity in barrel

Formula : dc =