Channel Pattern-2011

Report
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
Single channels of varying
sinuosity
• Straight
• Meandering
Multiple channels of varying
sinuosity
• Braided
• Anastomosed
• Since patterns can be flow dependent, should
be described at some intermediate flow stage
Channel Types
(Thorne et al., 1997)
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
For single - row alternate bars and Fr  0.8; Fr  u
Alternate Bars
  5 wd C f
H b d  1.51C f w d 
1.45
for 6  w d  40
C f  gdS u 2  u* u 2
(Ikeda, 1984)
gd
Alternate Bars
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
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
(Schumm and Khan, 1972)
Single-row
alternate bars
(Point bar, Madison River, MT)
(Bridge, 2003)
(Bar tail, River Feshie, Scotland)
(Unit bars, P. Ashmore)
Double-row
alternate bars
(Braid bars, Saksatchewan River, CAN)
(Bridge, 2003)
(Braid bars, P. Ashmore)
Double-row
alternate bars
(Braid bars, Sunwapta
River, CAN)
(Bridge, 2003)
(Braid bar, Sagavanirktok River, AK)
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
channel thalweg length
Sn 
valley length
•
Braiding index
mean number of active channels or braid bars
BI 
cross  sectional transect
Controls on
Channel Pattern
Conceptual
(Knighton, 1998)
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.
http://www.berkeley.edu/news/media/releases/2009/10/05_meanders.shtml
Ingredients: (1) vegetation to
reinforce banks and prevent
erosion, and (2) sand to build
point bars and block off cut-off
channels and chutes
Controls on Channel Pattern
S  Q-b
Braided
Meandering
(Bridge, 2003)
Controls on Channel Pattern
Braided
S  Q-bDc
(Bridge, 2003)
Controls on Channel Pattern
w  D50b
Braided
Meandering
(Bridge, 2003)
Controls on Channel Pattern
Parker (1976) using channel
stability theory
Braided
S
d
 ; meandering stable
Fr
w
S
d
 ; braiding stable
Fr
w
Meandering
(Bridge, 2003)
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

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