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School of Civil Engineering/Linton School of Computing, Information Technology & Engineering
Lecture 10: Threshold Motion of Sediments
CEM001 Hydraulic Structures, Coastal and River Engineering
River Engineering Section
Dr Md Rowshon Kamal
H/P: 0126627589
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School of Civil Engineering/Linton School of Computing, Information Technology & Engineering
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School of Civil Engineering/Linton School of Computing, Information Technology & Engineering
The topics we cover under ‘River Sediment Transport’ are:
1. Threshold motion of sediment
2. Design of channels in erodible material
3. Modes of sediment transport
Content
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School of Civil Engineering/Linton School of Computing, Information Technology & Engineering
• Deposition and filling of reservoir in hydropower plants.
• Blockage of water inlets (river navigation) in irrigation schemes.
• Wear and tear of water turbines.• Attach organic and toxic to sediment particles
hence effect water quality.• Problems from old polluted sediments on
ecosystems of the water course.
Why Do We Study ‘River Sediment Transport’?
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School of Civil Engineering/Linton School of Computing, Information Technology & Engineering
Bed load
- Particles roll and slide along bed with occasional jumps into the main stream.
Sediment transport mechanisms
Depend on U, s and d50
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Saltation load
- Particles bounce or hog along the bed due to the impact of bouncing particles.
Sediment Transport Mechanisms (con’t)
School of Civil Engineering/Linton School of Computing, Information Technology & Engineering
Suspended load
- Particles suspend in water due to the turbulent velocity fluctuations.
- Wash load can be considered as a part of suspended load.
Sediment Transport Mechanisms (con’t)
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School of Civil Engineering/Linton School of Computing, Information Technology & Engineering
Initiation of Particle Motion & Erosion
Low discharge (or velocity)
No particle movement i.e. flow condition is similar to a fixed bed
Discharge ≈ Certain value
Random motion of individual particles i.e. initiation of sediment transport. Condition is known as incipient motion/ threshold of motion or critical motion
Discharge > Certain value
Appreciable sediment transport
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School of Civil Engineering/Linton School of Computing, Information Technology & Engineering
Initiation of Particle Motion & Erosion (con’t)
Threshold of Motion
- Movement of single particle,- Movement of few particles,- Movement of bed,- Sediment transport rate tends to be zero.
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Average Boundary Shear Stress (τo)
W
x
FLOW
W sin
y1y1
y2
A
τ
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School of Civil Engineering/Linton School of Computing, Information Technology & Engineering
Average Boundary Shear Stress (τo)
Force acting in the direction of flow
xP
xAg
xP
W
Area
Forceo
sin)(sin
sinWF
gRSP
gASo
Hydraulic radius, R = A/P
SsinSometimes S is denoted by So
P=2y1+y2
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School of Civil Engineering/Linton School of Computing, Information Technology & Engineering
Shear Velocity (u*)
gRSu o
*
It has the units of velocity (m/s), however it is not a physical velocity
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School of Civil Engineering/Linton School of Computing, Information Technology & Engineering
Shields Analysis (1936)
Particle will begin to move when the combined drag (FD) and lift (FL) moment equals the weight (G) moment.
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School of Civil Engineering/Linton School of Computing, Information Technology & Engineering
Shields Analysis (1936) con’t
Taking moment about ‘Point of Contact’,
cos)( 211 bba
sin22 ba
sin33 ba
From geometry
231 GaaFaF LD
Combining and multiplying by cos/G
tan)(
32
21 G
GbF
b
bbF
L
D
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School of Civil Engineering/Linton School of Computing, Information Technology & Engineering
Shields Analysis (1936) con’t
tan)(
2
21 Gb
bbFD For simplicity, we
can assume 1/ GFL
tan1GFD Where
At high Reynolds numbers pressure forces >> viscous/skin friction force.
FD will act through the centre of the particle (b1=0, α1)
tan
tanGFD
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21 bb
b
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School of Civil Engineering/Linton School of Computing, Information Technology & Engineering
Shields Analysis (1936) con’t
Let G = Submerged weight (N.B. G/ in lecture notes),
gd
G s )(6
3
6/4
Let FD = Drag force,24
22B
wDD
UdCF
*2UU B
dUCD
*
223 42
1 DC
gdG s3
4 )(
2*
23 UdFD
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School of Civil Engineering/Linton School of Computing, Information Technology & Engineering
Shields Analysis (1936) con’t
gdG s3
4 )( 2*
23 UdFD
SCF
tanGFD
tan)1( 5
2*
gds
UCritical value = 0.056
)(Re)1( *
2* f
gds
U
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School of Civil Engineering/Linton School of Computing, Information Technology & Engineering
Shields Diagram
)1(
2*
sgd
uFs
0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.10
0.11
1 10 100 1000
Particle Reynolds Number, Re*
Sh
ield
s E
ntr
ain
me
nt
Fu
nc
tio
n,
F S
du*
0.056
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School of Civil Engineering/Linton School of Computing, Information Technology & Engineering
Shields Parameter for Larger Particles
056.0)1(
2*
sgd
uFs
400*
duGoverning conditions
smg /81.9 sm /1014.1 26 65.2s
906.02*
d
U 6* 104.456 dU
mmd 619
School of Civil Engineering/Linton School of Computing, Information Technology & Engineering
Manning’s and Strickler’s Formulae
n
SARQ o
2/13/2
6/1041.0 dn
Manning’s formula
Strickler’s formula d is in metres
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Example 01
An irrigation canal is to be constructed to pass 3.0m3/s along a line having a slope of 0.01. The sides are to be banked and protected with grass (this tells us that we don’t need to worry about the stability of the banks). Calculate the minimum width of canal if the bed material consists predominantly of:
i) Gravel d75 = 50.0mmii) Gravel d75 = 4.0mm
(N.B. This is the d50 size i.e. 75% of the particles by weight are smaller. The bed tends to become ARMOURED with this size of particle)
Smaller particles at top of bed are transported away – leaving the larger particles which then protect the smaller particles beneath d75 is used because it takes into account armouring)
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Thank You
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