River Meandering and Braiding. Pierre Y. Julien. Department of Civil and Environmental Engineering Colorado State University Fort Collins, Colorado

Transcription

1 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 06 Objectives Brief overview of river and meandering characteristics, and riprap design:. River Meandering and Braiding;. Riprap Design; 3. Case Study; Gupo Bridge, South Korea.

2 . River Meandering and Braiding tan o o Fine sediment in suspension Equilibrium Coarse sediment bedload tan o Point bar (a) Thalweg (c) Thalweg Fine Coarse Lateral migration SF Sedimentation Erosion = 0 Sedimentation Erosion (b) (d)

3 Distance above stream bed (ft) Right bank Water surface Left bank Q = ft /s V = 4.9 ft/s C = 838 ppm - sand d m 50 = 0.0 mm - bed sediment S =.5 x Stationing across section (ft) Mississippi River Fine sand concentration mg/l < - 0 ft below LWRP > - 0 ft below LWRP > - 30 ft below LWRP 3

4 Old River Control Complex Mississippi River Outflow Channel 4

5 PGC Meandering Evolution of Natural River Fig.6. Free meandering patterns of Tanana River in Alaska 5

6 Slide 9 PGC 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, 8/9/00

7 Migration of the Mississippi River Examples of Natural Cutoffs Chute Cutoff Neck Cutoff Williams River, AK (Photo by N.D. Smith) Owens River, CA (Photo by Marli Bryant Miller 6

8 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

9 8 w + h Before After S + S z x Braided Anabranched Avulsion Before After River A River B Capture Before After High Low Aggrading Avulsing H L Anastomosing Island sand-gravel-cobble H L Anabranching Gravel-cobble- bedrock H L Perching Natural Levee Floodplain H L Braiding Bars Sand-gravel H L Shoaling Shoal Sand-silt Before After

10 Depth (m) 977 W W W W (km) Reference point Cross-section Previous situation Bare sand 0.5 (km) N Gorai Off-take Hardinge Bridge Ganges km Jamuna Tilly Aricha (Teota) Baruria Arial Khan Off-take Old Brahmaputra Dhaleswari Padma Dhaka Mawa Bhairab Bazar Upper Meghna Lower Meghna 9

11 Hydraulic geometry of the Rio Grande, NM Braiding Transition Meandering 0

12 Historic Artificial Meander Cutoffs Stages of meander cut-off construction for navigation on Wood Creek, NY in 8 th century are similar to design methods today. Clear natural channel. Clear cut channel and stockpile logs 3. Excavate ditch across meander neck 4. Dam old channel 5. New channel filled by stream flow (Sketches from New York State Museum)

13 Engineered Cutoffs Illustration of artificial cutoff construction (Figure 9. from Julien, 00) Engineered Meander Cutoffs Fly River in Papua New Guinea: meander cutoff showing abandoned channel and new straight river flow path (Rowland et al. 005)

14 . Riprap Design 3

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16 Q Top of bank F s a Downstream F s sin Fs Streamline Particle pathline drag component Sideslope gradient Bed Weight component 0 Downstream F D u A Streamline A Horizontal F s sin F s sin 0 View normal to sideslope F s a O' l Particle P O l 3 l Section A - A FL l 4 F D cos F s - a cos l 3 F D sin O l Particle P F - a s sin Section normal to section A - A Particle Stability SF0 l F s l F a s a cos l F cos l F 3 D 4 L SF 0 < Particles in Motion SF 0 = Incipient Motion SF 0 > Particles are Stable lfsa lfs a cos SF0' l F cos l F 3 D 4 L 5

17 Simplified Stability Ratio of τ θc (shear on embankment) to τ c (shear on horizontal surface) sin sin sin cos sin sin sin cos sin sin c c ld tan ld tan Brooks relationship: θ 0 =0 and П ld =0 sin sin sin sin sin c c tan tan Lane s relationship: П ld = or λ=0 and x =0 cossin sin sin cos sin x ld sin sin sin 0 0 c c sin sin sin Velocity Method V c K c K c 4h log tan ds G gd s 6

18 Riprap Design Riprap Thickness US Army Corp of Engineers 30 cm for practical placement At least the diameter of the upper limit of d 00 stone At least than.5 times the diameter of upper limit d 50 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 5 to 30 cm. 7

19 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 Riprap Failure 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 8

20 Gravel Filters Gravel filters should not be less than 5 to 3 cm ½ thickness of Riprap layer is a good guideline Suggested gravel filter gradation equations are shown to the right d d d d d 5 d 5 85 ( filter) 40 ( bank) 5 5 ( filter) 40 ( bank) ( filter) 5 ( bank) CHANNEL PROTECTION - RIPRAP 9

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