River Meandering and Braiding - Colorado State University
Transcript of River Meandering and Braiding - Colorado State University
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Pierre Y. Julien
River Meandering and Braiding
Department of Civil and Environmental Engineering
Colorado State University
Fort Collins, Colorado
River Mechanics and Sediment Transport
Lima Peru – January 2016
Objectives
Brief overview of river and meandering characteristics, and riprap design:
1. River Meandering and Braiding;
2. Riprap Design;
3. Case Study; Gupo Bridge, South Korea.
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1. River Meandering and Braiding
tan o
o
tan o
Thalweg
Sedimentation Erosion
= 10°SF
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Thalweg
Lateral migration
Sedimentation Erosion
(a)
(b)
(c)
(d)
Fine Coarse
Equilibrium
Point bar
Fine sediment in suspension
Coarse sediment bedload
3
250
750
3000
4000
Right bank Left bankWater surface12
10
8
6
4
2
00 100 200 300 400 500 600 700 800 900
Stationing across section (ft)
Dis
tanc
e ab
ove
stre
am b
ed (
ft)
= 31 600 ft /s = 4.9 ft/s = 838 ppm - sand = 0.20 mm - bed sediment = 1.5 x 10
Q V S
C d
m50 -4
3
3500
500
1000
15002000
2500
5000
Mississ
ippi Rive
r
< - 20 ft below LWRP
> - 20 ft below LWRP
> - 30 ft below LWRP
Fine sand concentration
0 100 200 300 400 500 mg/L
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Old River Control Complex
Mississippi River
Outflow Channel
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PGC1
Meandering Evolution of Natural River
Fig.6. Free meandering patterns of Tanana River in Alaska
Slide 9
PGC1 Numerical model evaluated the hydrodynamics of the location and recommendations were made to construct 4 dikes on the right bank. After construction, the problem was immediately converted from unmanageable to a manageable situation. It is anticipated that proposed numerical stdies will similarly identify a manageable solution.Phil Combs, 8/29/2002
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Migration of the Mississippi River
Examples of Natural Cutoffs
Williams River, AK
(Photo by N.D. Smith)
Owens River, CA
(Photo by Marli Bryant Miller
Neck Cutoff Chute Cutoff
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Natural Meander Cutoffs• Lateral migration
increases sinuosity of the channel until two bends connect
• Sedimentation occurs where the bends connect– Neck cutoff– Abandoned
channel
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w +h
Before
After
S+
Sz
x
1
Braided
Anabranched
Avulsion
Before
After
River ARiver B
Capture
BeforeAfter
High
Low
Aggrading
Avulsing
H
L
Anastomosing
Island sand-gravel-cobble
H
L
Anabranching
Gravel-cobble- bedrock
H
L
PerchingNatural
LeveeFloodplain
H
L
Braiding
Bars
Sand-gravel
H
L
Shoaling
Shoal
Sand-silt
Before
After
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Hardinge Bridge
Gorai Off-take
TillyAricha
Baruria
Arial Khan Off-take
Mawa
Bazar
0 20 40 km
(Teota)
Bhairab
Old Brahmaputra
Jamuna
Dhaleswari
Ganges
Padma
Lower Meghna
Upper Meghna
Dhaka
0 2.5 5.0 7.5 10W (km)
0-4
-8-12
-16
-20
0-4
-8-12
-16-20
0-4
-8-12
-16-20
0-4
-8-12
-16
-20
Dep
th (
m)
1977
1979
1981
1985
1977
1978
1984
W
W
W
0 2.5(km)
N
Reference point Cross-section Previous situation Bare sand
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1935 1972 1992
Hydraulic geometry of the Rio Grande, NM
Braiding Transition Meandering
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Historic Artificial Meander Cutoffs
• Stages of meander cut-off construction for navigation on Wood Creek, NY in 18th century are similar to design methods today
1. Clear natural channel
2. Clear cut channel and stockpile logs
3. Excavate ditch across meander neck
4. Dam old channel
5. New channel filled by stream flow
1 2 3 4 5
(Sketches from
New York State Museum)
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Engineered Cutoffs
• Illustration of artificial cutoff construction
(Figure 9.21 from Julien, 2002)
Engineered Meander Cutoffs
• Fly River in Papua New Guinea: meander cutoff showing abandoned channel and new straight river flow path
(Rowland et al. 2005)
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2. Riprap Design
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A
Au
F
F sin
F sinF a
FBed
Top of bank
Sideslope gradient
Streamline
View normal to sideslope
Horizontal
s 1
s
D
s 1
s 0
F sins 1
F
F cos
F 1 - a cos
O'
OF a
1
2 3
4
2
s D
s
LParticle P
Section A - A
1
3
F sinD Particle P
O
F 1 - a sin2s
Section normal to section A - A
Downstream
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Downstre
am
Streamline drag component
Particle pathline
Weight component
Q
l
l
l
l l
l
Particle Stability
LD
ss
LDs
s
FlFl
aFlaFlSF
FlFlaFl
aFlSF
43
212
'0
432
1
20
cos
cos1
coscos1
• SF0<1– Particles in Motion
• SF0=1– Incipient Motion
• SF0>1– Particles are Stable
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Simplified Stability
• Ratio of τθc (shear on embankment) to τc (shear on horizontal surface)
• Brooks’ relationship: θ0=0° and Пld=0
• Lane’s relationship: Пld=∞ or λ=0 and x =0
21
20
2
2
201
2
01
sin
sinsin1
tan1
cossinsinsin
tan1
cossinsinsin
ldldc
c
21
2
sin
sin1
c
c
21
2211
sin
sin1
tan
sinsin
tan
sinsin
c
c
0
21
222
01
sinsinsin1
cossinsinsincos
ld
x
Velocity Method
tan4
log
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sc
scc
d
hK
gdGKV
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Riprap Design
Riprap ThicknessUS Army Corp of Engineers
• 30 cm for practical placement • At least the diameter of the upper limit of d100
stone • At least than 1.5 times the diameter of upper
limit d50 stone, whichever is greater. • If riprap is placed under water, the thickness
should be increased by 50%.• If it is subject to attack by large floating
debris or wave action it should be increased 15 to 30 cm.
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Riprap Failure
• There are four main types of riprap failure: particle erosion, transitional slide, riprap slump, and sideslope failure.
• The four types of riprap failure are shown in the figure to the right.
• The most common failure type is particle erosion from flow
Gradation of Riprap
• Well graded riprap scours less than uniform size riprap due to the process of armoring
• Suggested Riprap gradation from USACE is shown to the right
• Riprap with poor gradation may be used, but a “filter” layer is required
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Gravel Filters
• Gravel filters should not be less than 15 to 23 cm
• ½ thickness of Riprap layer is a good guideline
• Suggested gravel filter gradation equations are shown to the right
5)(
)(
40)(
)(5
40)(
)(
85
15
15
15
50
50
bankd
filterd
bankd
filterd
bankd
filterd
CHANNEL PROTECTION - RIPRAP
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