Download - Channel Pattern
Channel Pattern
Outline• Description of channel pattern• Alternate bars• Channel pattern continua and evolution• Controls of channel pattern
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
(Thorne et al., 1997)
Channel Types
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
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)
Alternate bars in the Naka River, an artificially straightened river in Japan. Image courtesy S. Ikeda.
Alternate Bars
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
Tokachi River, Japan
Alternate Bars
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
(Bridge, 2003)
(Schumm and Khan, 1972)
(Point bar, Madison River, MT)
(Bar tail, River Feshie, Scotland)
Single-rowalternate bars
(Bridge, 2003)
(Braid bars, Saksatchewan River, CAN)
(Unit bars, P. Ashmore)
Double-rowalternate bars
(Bridge, 2003)
(Braid bars, P. Ashmore)
(Braid bars, Sunwapta River, CAN)
(Braid bar, Sagavanirktok River, AK)
Double-rowalternate bars
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
Channel Description
• Sinuosity
• Braiding index
lengthvalley
lengththalwegchannelSn
transectsectionalcrossbarsbraidorchannelsactiveofnumbermeanBI
(Knighton, 1998)
Controls on Channel Pattern
Conceptual
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
(Bridge, 2003)
Controls on Channel Pattern
S Q-b
Meandering
Braided
(Bridge, 2003)
Controls on Channel Pattern
S Q-bDc
Braided
(Bridge, 2003)
w D50b
Meandering
Braided
Controls on Channel Pattern
(Bridge, 2003)
Parker (1976) using channel stability theory
stablemeanderingwd
FrS ;
stablebraidingwd
FrS ;
MeanderingBraided
Controls on Channel Pattern
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
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