Download - Channel Pattern
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Channel Pattern
Outline• Description of channel pattern• Alternate bars• Channel pattern continua and evolution• Controls of channel pattern
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Description of Channel Pattern• Defined by the nature of channel splitting around
braid bars or islands, and the sinuosity of channel segments
• Since patterns can be flow dependent, should be described at some intermediate flow stage
Single channels of varying sinuosity
• Straight• Meandering
Multiple channels of varying sinuosity
• Braided• Anastomosed
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(Thorne et al., 1997)
Channel Types
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Bridge (2003)
1. Bed evolves towards a statistically constant geometry composed of alternate bars
2. Channel responds to alternate bars by inducing bank erosion and channel widening
3. Bed adjusts to sediment fluxes and thalweg wanders
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Alternate Bars
2*
2
45.1 406for 51.1
5
;8.0 and bars alternate row-singleFor
uuugdSC
dwdwCdH
Cwd
gduFrFr
f
fb
f
(Ikeda, 1984)
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Alternate bars in the Naka River, an artificially straightened river in Japan. Image courtesy S. Ikeda.
Alternate Bars
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Alternate bars in a flume in Tsukuba University, Japan: flow turned low. Image courtesy H. Ikeda.
Alternate bars in the Rhine River between Switzerland and Lichtenstein. Image courtesy M. Jaeggi.
Alternate Bars
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Tokachi River, Japan
Alternate Bars
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Channel Response to Alternate Bars
• As water level drops, highest parts of the bars become emergent
• Bar tail, riffle, and head become recognizable
• Cause localized flow diversions, and localized erosion and deposition
• Deposition on convex banks inside of river bend point bars
• Deposition in mid-channel braid bars
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(Bridge, 2003)
(Schumm and Khan, 1972)
(Point bar, Madison River, MT)
(Bar tail, River Feshie, Scotland)
Single-rowalternate bars
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(Bridge, 2003)
(Braid bars, Saksatchewan River, CAN)
(Unit bars, P. Ashmore)
Double-rowalternate bars
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(Bridge, 2003)
(Braid bars, P. Ashmore)
(Braid bars, Sunwapta River, CAN)
(Braid bar, Sagavanirktok River, AK)
Double-rowalternate bars
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Channel Pattern
• Defined by the nature of channel splitting around braid bars or islands and the sinuosity of channel segments
Single channels• Straight• Meandering
Multiple channels• Braided• Anastomosed
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Channel Description
• Sinuosity
• Braiding index
lengthvalley
lengththalwegchannelSn
transectsectionalcrossbarsbraidorchannelsactiveofnumbermeanBI
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(Knighton, 1998)
Controls on Channel Pattern
Conceptual
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Controls on Channel Pattern
Braudrick & Dietrich (2009) hypothesize that meandering rivers also require:1) bank strength from either cohesive material or vegetation, 2) overbank flows to attach bars to their floodplains,3) fine sediment to fill the downstream end of bars and chutes.
Ingredients: (1) vegetation to reinforce banks and prevent erosion, and (2) sand to build point bars and block off cut-off channels and chutes
http://www.berkeley.edu/news/media/releases/2009/10/05_meanders.shtml
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(Bridge, 2003)
Controls on Channel Pattern
S Q-b
Meandering
Braided
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(Bridge, 2003)
Controls on Channel Pattern
S Q-bDc
Braided
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(Bridge, 2003)
w D50b
Meandering
Braided
Controls on Channel Pattern
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(Bridge, 2003)
Parker (1976) using channel stability theory
stablemeanderingwd
FrS ;
stablebraidingwd
FrS ;
MeanderingBraided
Controls on Channel Pattern
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Channel Pattern and Stream Restoration
• Bed topography and flow redirection are the primary controls on channel pattern, as conditioned by boundary composition
• Indices can be used to facilitate design of stable stream patterns, though these must be combined with sediment transport relationships
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Channel Pattern
Conclusions• Alternate bars play an important role in
the evolution of straight channels• Channel pattern defined by channel
splitting around bars and the sinuosity of channel segments
• Discharge, slope, sediment load, etc., all interact with river flow to modify channel pattern