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River Meandering and Braiding River Meandering and Braiding Pierre Y. Julien 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. 1 1. River Meandering and Braiding Fine sediment Coarse o in suspension tan sediment o Equilibrium bedload o tan Point bar (a) (c) Thalweg Thalweg Fine Coarse Lateral migration 1 SF = 10° Sedimentation Sedimentation Erosion 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 Mississippi River 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 Meander Cutoffs • Lateral migration increases sinuosity 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 Avulsion H Before L Gravel-cobble- bedrock After Perching Natural H Floodplain Capture Levee 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 sin1 sin sin1 sin sin 1 1 2 c tan tan sin • Lane’s relationship: Пld=∞ or λ=0 and x =0 cossin 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 2G 1gds 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 19 20.
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