Hydrodynamics and Morphodynamics of River Confluences

HYDRODYNAMICS AND MORPHODYNAMICS OF RIVER CONFLUENCES

Summer school: Day 2 – (29 July 2015) 11:30 – 12:15 (B)

Gökçen Bombar Ege University, Turkey

1 A confluence is the meeting of two or more bodies of water.

When Rivers Collide: 10 Confluences Around the World (the confluences were selected for their dramatic visual contrast)

Confluence of the Confluence of the Green and Colorado Rhone and Arve Rivers Rivers in Canyonlands National Park, in Geneva, Switzerland Confluence of the Jialing and Utah, USA Yangtze Rivers in Chongqing, China

http://twistedsifter.com/2012/04/confluences-around-the-world/ 2 River confluences : - fundamental elements of natural drainage networks, - play an important role in regulating the movement of sediment through river systems, - are habitats of high ecological value, - dynamics of mixing controls how tributary inputs of nutrients and food are dispersed within a main river, 3 Terminology Symmetrical Confluence Asymmetrical Confluence Y - Shaped

http://3.bp.blogspot.com/_KV-lA7YaO9U/SSGqh- http://twistedsifter.com/2012/04/confluences-around-the-world/ 4EhXI/AAAAAAAAC6o/cwyQO1ZMcws/s1600- h/IMG_7254.JPG

Bryan & Kuhn, 2002, Hydraulic conditions in experimental rill confluences and scour in erodible soils. 4 Influential Parameters :

tributary channel

Qst

inner bank

Q h sm Bm p-c

main channel outer bank post- confluence

5 Influential Parameters :

 discharge from tributary and main channel, Qt & Qm [discharge ratio, Qr = Qt / Qm ]

 sediment load, Qst , Qsm

 sediment characteristics, D50t , D50m , st , sm , rs

 angle between the two incoming streams and downstream channel junction angle, α,

 momentum flux ratios, Mr = ( r Qt Vt ) / ( r Qm Vm ) where Q, U, and ρ denote the flow discharge, mean flow velocity and density of water respectively, and the sub-indexes t and m refer to the tributary and main channel, respectively.

 velocity ratio, Vr = Vt / Vm

 height of bed discordance, Dz

6

Mosley (1976) An experimental study of channel confluences

• Flow patterns and processes in conflences, • Factors that control the scour depths • Equilibrium conditions

7  One of the most important issue, which has been the subject of a large scientific debate, concerns the * presence, and * number, of the secondary flow cells driven by the confluence.

 Mosley (1976) reported the presence of two counter-rotating helical flow cells downstream of the confluence, generated by the converging flows and separated by a shear layer.

8 Hydrodynamics at open channel confluences

Best (1987) Flow dynamics at river channel confluences: Implications for sediment transport and bed morphology

tributary channel

inner bank 3 1 main 5 post- channel 2 2 4 confluence 6 5 4

outer bank

9 A turbulence feature of particular interest has been the shear layer between two confluent flows.

This feature is analogous to the plane mixing layer that develops between interfacial shear flows. Over distance, mixing layers evolve by spreading laterally via vortex pairing.

10 Brown and Roshko (1974) On density effects and large structure in turbulent mixing layers. Mixing Layers:

Turbulent Mixing Layers are formed when two dissimilar streams of turbulent fluid travel parallel to each other. Commonly, the streams are initially separated by a thin, low angle splitter plate.

Figure shows a shadowgraph image of a mixing layer created by Brown & Roshko (1974).

These structures are caused by Kelvin-Helmholtz instability at the junction of the two streams.

channel width is 5 cm

11 Winant & Browand (1974) Vortex Pairing: the mechanism of turbulent mixing-layer growth at moderate Reynolds number.

Vortex Pairing:

Dye was injected between the two streams.

• unstable waves grow, • fluid is rolled up, • interact by rolling around each other, and • a single vortical structure was formed.

The downstream thickening of the turbulent mixing layer arises through sequential amalgamation of these vortices to form larger vortices.

https://www.youtube.com/watch?v=r5xV5Lt1EdI 12 University of Illinois, USA Kaskaskia River - Copper Slough confluence The field site near Champaign, IL ; measurement since 1989.

Kaskaskia River flow Copper Slough flow B = 7 m B = 8.5 m

D50 = 0.65 mm D50 = 3.5 mm

• Kenworthy, S.T. and Rhoads, B.L., 1995. Hydrologic Control of Spatial Patterns of Suspended Sediment Concentration at a Small Stream a = 600 Confluence. Journal of Hydrology, 168, 251-63. • Rhoads, B.L. and Kenworthy, S.T. , 1995. "Flow Copper Slough Structure at an Asymmetrical Stream Confluence." Geomorphology, 11, 273-293. • ...... Kaskaskia River • ...... • ......

The local path of the depth-averaged flow is often skewed in relation to the path of the channel centerline. July 1989 13  The cross-stream velocities consist of velocity components associated with helical motion, if present, and with skewness of the flow path in relation to the channel path.

 The definition of flow structure in natural rivers is contingent upon the frame of reference in which velocity components are presented.

Bathurst, Thorne & Hey (1977) Direct measurements of secondary currents in river bends

V : depth averaged velocity vector.

Vx and Vy : components of V on stream and cross flow axis.

vp and vs : components of the velocity at a particular depth in the flow column that are oriented parallel and orthogonal, respectively, to the V at that vertical.

vpy and vsy : projection of vp and vs on cross-stream axis, respectively.

14 vy = vpy + vsy

skewed helical flow motion

vs vsy

vy vx vpy Vy v p V v x

Bathurst, Thorne & Hey (1977) Direct measurements of secondary V currents in river bends 15 Rhoads & Kenworthy (1995) Field investigation at an asymmetrical Kaskaskia River flow - Copper Slough flow stream confluence West bank – East bank outer bank – inner bank Main channel - tributary channel At low stage Q = 0.56 m3/s - Q = 0.42 m3/s No sediment transport m t Mr = 0.55 < 1

Mixing interface

v y vpy vsy

• The mixing interface is positioned near the center of the channel immediately downstream from the confluence. • Weak but surface convergent cells occur on each side of the mixing interface. • The flow from the lateral tributary can not penetrate far into the confluence.

• Values of vsy are small 16 Effect of Momentum Ratio on the Shear Layer:

 Low momentum flux ratios (Mr < 1) indicates a dominan main stream and yield a shear layer close to the inner bank of the main channel,

 High momentum flux ratios (Mr > 1) lead to a shift of the shear layer towards the outer bank.

Lewis, (2014) Rates and patterns of temperature mixing at a small stream confluence under variable incoming flow conditions 17 Bed Discordance:

First models of junction dynamics have tended to assume that the confluent channels are of equal depth, a condition that may be found only rarely in natural junctions; more often, the depths of confluent channels are different.

The difference in height between the bed levels of the tributary and main channel and is highly influences the hydrodynamics of confluences.

This discordance is commonly the result of the bars – due to sediment transport - at the mouth of each confluent channel.

Concordant Bed Discordant Bed

18 Biron, Best & Roy (1996), Effects of bed discordance on flow dynamics at open channel confluences

a = 30 degree Bt = 8 cm ; Bm = 12 cm ; Bp-c = 15 cm LDA & white dye

* Seperation zone:

concordant discordant

Streak paths of spots of white water-based paint placed on the blue perspex flume door. a) The main channel flow is deflected away from the mouth of the tributary. The streamline curvature is associated with a seperation zone. b) Deflection is negligible 19 * Acceleration zone:

The existence of a seperation zone may reduce the available area through which the total discharge must pass, resulting in a flow acceleration zone.

At the discordant bed, absence of a flow seperation zone near the bed results in no marked flow acceleration zone near the corner.

* Upwelling flow:

Velocity vectors in the u-v plane near the tributary wall near the downstream junction corner: a) All the vectors remain parallel to the bed, b) A very strong vertical motion exits immediately downstream from the corner.

• Bed discordance clearly generates an upwelling of flow at the downstream junction corner. 20 Effect of sediment feeding from the tributary and the main channel on the equilibrium morphology of a river confluence

Experimental Facility

sediment feeder of the sediment feeder of the mean channel tributary channel

instrument car

tributary channel

control table main channel

21 Effect of sediment feeding from the tributary and the main channel on the equilibrium morphology of a river confluence CONFLUME

Control equipment: Data acquisition: • Tributary sediment feeding. • Water surface levels (ultrasonic sensor). • Movement of instruments in x, y, z, within • Bed topography (mini echo-sounder). specified intervals and speed. • Velocity (UVP and ADV - Vectrino). • Start and end the measurement session. • Camera. 22 Effect of sediment feeding from the tributary and the main channel on the equilibrium morphology of a river confluence

Q sm 0.6 0.2 0 Q st

0.5 A1 B1 C1

0.34 A2 B2 C2

0.17 A3 B3 C3

0.08 A5 B5 C5

0 A4 B4 C4

Q sm & Q st (kg/hr)(kg/min)

23 10A5 rep0.60 0.08

2 0 0.3 4

6 0.25 8

10 0.2

12 0.15 14

16 0.1

18 0.05 20

22 0 10010200A5 rep300 0.60400 500 0.08600 700 800 900

Effect of sediment feeding from2 the tributary and the main channel on the equilibrium morphology of a river confluence

1 0.3 4 2 0 6 0.3 4 0.25 8 6 0.25 10 0.2 8 12 10 0.2 0.15 14 12 0.15 16 14 0.1 18 16 0.1 0.05 1820 22 0.05 A1 20 0 C5 100 200 300 400A5500 600 700 800 900 22 C1 0 2 A1 0.60 0.50 10010200A5 rep300 0.60400 500 0.08600 700 800 900 C1 0 0.50 C5 0 0.08

2 2 2 2 1 0.3 10 0 00.3 4 0.3 0 0.30 0.3 4 4 0.35 4 6 20 6 6 0.25 6 0.25 0.25 0.25 0.25 8 8 8 8 30 10 0.2 0.2 0.2 10 0.2 0.2 10 10 40 12 12 12 12 0.15 0.15 0.1514 0.15 0.1550 14 14 14 16 60 16 0.1 16 0.1 16 0.1 0.1 0.1 18 20 18 18 18 70 0.05 0.05 20 0.05 0.3 0.05 0.0520 20 20 80 22 22 22 0 22 0 0 100 200 300 400 500 600 700 800 900 0 0 100 200 300 400 500 600 700 800 900 100 200 300 400 500 600 700 800 900 100 200 300 400 500 600 700 800 900 100 200 300 400 500 600 700 800 900

2

6 0.3 4 2 2 2 2 1 0.3 6 1 0.3 4 1 0.3 4 40 4 1 0.30.25 4 8 6 6 6 0.25 0.25 6 0.2 0.25 0.25 8 10 0.25 8 8 128 10 0.2 0.2 10 0.2 0.15 10 0.2 1410 12 12 12 0.15 12 0.1514 0.1516 0.1 14 0.15 14 14 16 18 0.1 16 16 0.1 0.1 16 0.05 18 60 20 0.1 18 18 0.05 18 20 22 0.05 0.05 0 20 20 100 200 300 400 500 600 700 800 900 0.05 22 20 0.2 0 100 200 300 400 500 600 700 800 900 22 22 0 22 0 100 200 300 400 500 600 700 800 900 0 100 200 300 400 500 600 700 800 900 100 200 300 400 500 600 700 800 900

2

9 0.3 4 10 2 2 10 3 0.3 3 0.3 80 6 60.3 6 0.3 4 4 0.25 20 8 20 0.25 6 6 0.25 0.25 0.25 30 10 0.2 30 8 8 0.15 0.2 40 12 0.2 0.2 0.2 10 10 0.15 40 14 50 0.15 12 12 0.15 0.15 0.15 50 16 0.1 14 14 60 0.1 18 60 0.1 16 0.1 16 0.1 70 100 0.05 20 0.05 18 18 70 0.0580 0.0522 0.05 20 20 0 0 100 200 300 400 500 600 700 800 900 80 100 200 300 400 500 600 700 800 900 22 22 0.1 0 0 0 100 200 300 400 500 600 700 800 900 100 200 300 400 500 600 700 800 900 100 200 300 400 500 600 700 800 900

2 2 0.3 2 12 4 6 0.3 2 2 4 6 0.3 120 4 12 0.3 62 4 9 0.3 6 4 0.25 6 0.3 6 0.25 84 6 6 0.25 8 0.25 6 0.25 8 10 0.2 10 0.2 8 8 0.25 8 10 0.2 0.2 12 0.05 12 10 10 0.2 0.15 0.15 10 0.2 12 12 14 14 12 0.15 0.15 12 0.15 14 14 16 0.1 16 0.1 14 0.15 14 16 16 18 0.1 18 140 0.1 16 0.1 0.05 16 0.05 18 20 18 20 0.1 18 0.05 0.052218 20 22 20 0.050 0 20 100 200 300 400 500 600 700 800 900 0.05 100 200 300 400 500 600 700 800 900 20 22 22 0 0 22 100 200 300 400 500 600 700 800 900 100 200 300 400 500 600 700 800 900 22 0 100 200 300 400 500 600 700 800 900 0 0 100 200 300 400 500 600 700 800 900

100 200 300 400 500 600 700 800 900 2 5 0.3 9 0.3 2 12b 10 4 2 9 0.3 15 0.3 102 6 4 12 0.3 20 0.25 4 0.25 9 4 0.3 0.25 8 6 15 0.25 6 30 6 0.25 10 0.2 8 0.2 208 0.25 0.2 8 40 12 10 0.2 10 0.2 0.15 25 0.15 10 0.2 0.1514 12 50 12 16 0.153012 0.1 14 0.15 14 0.1 60 0.1 0.15 18 14 16 0.1 35 0.05 16 0.1 70 20 16 0.05 0.05 18 0.1 1840 22 80 0 0.0518 0.05 100 200 300 400 500 600 700 800 900 20 20 0.050 0 20 100 200 300 400 500 600 700 800 900 100 200 300 400 500 600 700 800 900 22 22 0 0 100 200 300 400 500 600 700 800 900 22 100 200 300 400 500 600 700 800 900 0 100 200 300 400 500 600 700 800 900

2 10 2 12 0.3

14 4 0.3 12 0.3 4 20 5 26 0.25 12b 0.3 6 0.25 0.2530 12 4 0.3 108 8 0.2 0.25 40 106 0.2 10 0.2 15 0.25 128 12 50 0.15 Q = 0.6 kg/min 0.150.2 0.15 20 0.2 1014 14 sm 60 0.1 12 1625 0.15 16 0.1 0.10.15 70 14 18 0.05 1830 0.1 0.0580 16 0.05 20 20 0.1 35 0 18 22 100 200 300 400 500 600 700 800 900 22 0.05 0 Q = 0.5 kg/min 00.05 100 200 300 400 500 600 700 800 900 st 2040 100 200 300 400 500 600 700 800 900 22 0 100 200 300 400 500 600 700 800 900 0 100 200 300 400 500 600 700 800 900 2

15 4 0.3 10 15 0.3 6 2 20 0.25 8 14 45 0.3 0.25 12b 0.3 30 Q = 0.6 kg/min 10 0.2 106 sm 0.25 0.2 12 40 8 0.25 0.15 15 14 10 0.2 50 0.15 0.2 16 20 0.1 12 60 0.15 0.1 18 1425 0.15 0.05 Q = 0.08 kg/min 70 20 16 0.05 30 st 0.1 22 0.1 80 0 18 100 200 300 400 500 600 700 800 900 0 35 0.05 100 200 300 400 500 600 700 800 900 20 0.05

2240 0 100 200 300 400 500 600 700 800 900 0 100 200 300 400 500 600 700 800 900 10 2 18 0.3

18 4 0.3 20 0.25 6 30 2 0.25 0.3 8 0.2 14 4 40 10 0.2 6 50 0.15 0.25 12 8 0.15 14 60 0.1 10 0.2 16 0.1 70 12 0.05 0.15 18 80 14 0.05 20 0 16 0.1 100 200 300 400 500 600 700 800 900 22 0 18 100 200 300 400 500 600 700 800 900 0.05 20

22 0 2 100 200 300 400 500 600 700 800 900 21 2 4 0.3 21 0.3 4 6 0.25 6 8 0.25 8 10 0.2

10 0.2 12 0.15 12 14 0.15 14 16 0.1

16 0.1 18 0.05 18 20 0.05 20 22 0 100 200 300 400 500 600 700 800 900 22 0 100 200 300 400 500 600 700 800 900 10 24 0.3

20 0.25 30

0.2 40 Qsm = 0 kg/min 50 0.15 60 0.1

70 0.05 80 0 Qst = 0.5 kg/min 100 200 300 400 500 600 700 800 900

2 27 0.3 4

6 0.25 8

10 0.2

12 0.15 14

16 0.1

18 0.05 20

22 0 100 200 300 400 500 600 700 800 900

10 30 0.3 20 0.25 30

0.2 40

50 0.15

60 0.1

70 0.05 80 0 100 200 300 400 500 600 700 800 900 Qsm = 0 kg/min 24 Qst = 0.08 kg/min Effect of sediment feeding from the tributary and the main channel on the equilibrium morphology of a river confluence

A1 Qsm = 0.6 kg/min ; Qst = 0.5 kg/min A5 Qsm = 0.6 kg/min ; Qst = 0.08 kg/min 120 80 70 100 120 8060 80 7050 100 60 6040 80 30 40 50

20 sediment rate (kg/hr) sediment rate (kg/hr) 60 bedload feeding rate 40 20 bedload feeding rate ±5% ±10% 3010 40 ±5% ±10%

0 200 sediment rate (kg/hr) sediment rate (kg/hr) bedload feeding rate 20 0 3 6 9 12 15 18 21 24 27 30 33 0 3 6 9 12 15 bedload18 21 24 feeding27 30 rate 33 ±5% ±10% 10 time (hr) time±5% (hr) ±10% 0 0 50 0 3 6 9 12 15 18 21 24 27 30 33 10 0 3 6 9 12 15 18 21 24 27 30 33 time (hr) C5 time (hr) C1 Q = 0 kg/hr ; Q = 0.5 kg/min Qsm = 0 kg/hr ; Qst = 0.08 kg/min 40 sm st 50 10 30 40 5 20

30 sediment rate (kg/hr) sediment rate (kg/hr) 10 bedload feeding rate 5 bedload feeding rate 20 ±5% ±10% ±5% ±10%

0 0 sediment rate (kg/hr) sediment rate (kg/hr) 10 0 3 6 9 12 15 bedload18 21 24 feeding27 30 rate 33 0 3 6 9 12 15 bedload18 21 24 feeding27 30 rate 33 time±5% (hr) ±10% time±5% (hr) ±10% 0 0 0 3 6 9 12 15 18 21 24 27 30 33 0 3 6 9 12 15 18 21 24 27 30 33 time (hr) time (hr)

25 Effect of sediment feeding from the tributary and the main channel on the equilibrium morphology of a river confluence

A1 A5 C1 C5

0.35

20 0.3

40 0.25

60 0.2

80 0.15

100

0.1

120

0.05

140

0 100 200 t300 = 15 hr 400 500 600t = 14 hr700 800 900 t = 21 hr t = 30 hr 26 Thank you

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