Meandering and Braiding

Pierre Y. Julien

Department of Civil and Environmental Engineering Colorado State University Fort Collins, Colorado

River Mechanics and 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.

1 1. River Meandering and Braiding

Fine sediment Coarse o in suspension  tan  sediment o  Equilibrium bedload

o tan 

Point (a) (c) Thalweg Fine Coarse Lateral migration

1 SF  = 10° Sedimentation Sedimentation Erosion

(b) (d)

2 Right bank Water surface Left bank 12 250 500 750 10 1000 8 1500 2000 2500 3000 6 4000 3500 5000 4 3 Q = 31 600 ft /s V = 4.9 ft/s

Distance above stream bed (ft) 2 C m = 838 ppm - sand d 50 = 0.20 mm - bed sediment S = 1.5 x 10 -4 0 0 100 200 300 400 500 600 700 800 900 Stationing across section (ft)

er iv R ippi iss ss Mi

Fine sand concentration

0 100 200 300 400 500 mg/L

< - 20 ft below LWRP

> - 20 ft below LWRP

> - 30 ft below LWRP

3 Old River Control Complex

Mississippi River

Outflow Channel

4 PGC1

Meandering Evolution of Natural River

Fig.6. Free meandering patterns of Tanana River in Alaska

5 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 Migration of the

Examples of Natural Cutoffs

Chute Cutoff Neck Cutoff

Williams River, AK Owens River, CA (Photo by N.D. Smith) (Photo by Marli Bryant Miller

6 Natural Cutoffs

• Lateral migration increases of the channel until two bends connect

• Sedimentation occurs where the bends connect – Neck cutoff – Abandoned channel

7 + w h Aggrading After High Low Before

+ S

1 Shoaling H z S Shoal L x Sand-silt Braided

Braiding H Bars L

Sand-gravel

Anabranched Anastomosing H

Island L sand-gravel-cobble

Anabranching H Before L Gravel-cobble- bedrock

After Perching Natural H Capture L River B River A

Avulsing After Before After Before

8 1977 0 -4

-8 Jamuna -12 Old Brahmaputra 1977 W -16 Bhairab -20 Hardinge Bridge Bazar 0 Tilly G D ange Aricha ha -4 0 2.5 s lesw (km) Gorai (Teota) ari -8 Off-take Baruria Dhaka Upper -12 1978 Meghna P -16 1979 ad ma -20 0 Arial Khan Mawa Off-take

Depth (m) -4 W -8 -12 Lower 1981 0 20 40 km Meghna -16 N -20 0 -4 1984 -8 -12 1985 -16 W -20 0 2.5 5.0 7.5 10 W (km) Reference point Cross-section Previous situation Bare sand

9 Hydraulic geometry of the Rio Grande, NM

1935 1972 1992

Braiding Transition Meandering

10 Historic Artificial Meander Cutoffs

12345

• 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 (Sketches from 5. New channel filled by stream flow New York State Museum)

11 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)

12 2. Riprap Design

13 14 Q

eam Weight component str wn Do     0 1 Streamline Particle pathline drag component A

Top of bank Downstream   FD Hori zontal Fs sin 1 u  line Fs sin 1 F a eam A  s   Bed Str 1  Fs sin 0 Fs Sideslope gradient View normal to sideslope

Particle P FL FD sin  l 4 l Particle P O' 3

Fs a F cos  F 1 - a 2 sin  l2 O l3 D s  2 O l1 Fs 1 - a  cos  l1 Section A - A Section normal to section A - A

Particle Stability

•SF0<1 – Particles in Motion

•SF0=1 – Incipient Motion

•SF0>1 – Particles are Stable l F a SF  2 s  0 2 l1Fs 1 a cos   l3FD cos  l4 FL

2 l2 Fsa  l1Fs 1 a cos  SF0'  l3FD cos  l4 FL

15 Simplified Stability

• Ratio of τθc (shear on embankment) to τc (shear on horizontal surface)  sin sin   sin cos sin sin   sin cos 2 sin 2   sin 2   c   1 0  1 0 1 0 1  2 2 sin 2  c 1 ld tan 1 ld tan

• Brooks’ relationship: θ0=0° and Пld=0 2 2 c sin1 sin  sin1 sin  sin 1     1 2  c tan tan sin 

• Lane’s relationship: Пld=∞ or λ=0 and x =0 cossin sin   sin cos  sin 2  x  1 0 c    1 1 2 2 2 2  sin 2  1 ld sin   sin 1  sin 0 c

Velocity Method

Vc  Kc 2G 1gds  4h    Kc  log  tan  ds 

16 Riprap Design

Riprap Thickness US 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.

17 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

18 Gravel Filters

• Gravel filters should not be less than 15 d50 ( filter) to 23 cm  40 d50 (bank) • ½ thickness of d ( filter) Riprap layer is a 5  15  40 good guideline d15 (bank) • Suggested gravel d ( filter) filter gradation 15  5 equations are d85 (bank) shown to the right

CHANNEL PROTECTION - RIPRAP

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