1

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.

2

1. River Meandering and Braiding

tan o

o

tan o

Thalweg

Sedimentation Erosion

= 10°SF

1

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

4

Old River Control Complex

Mississippi River

Outflow Channel

5

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

8

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

9

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

10

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

14

15

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

10

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

12

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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