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

transectsectionalcross

barsbraidorchannelsactiveofnumbermeanBI

(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

stablemeanderingw

d

Fr

S ;

stablebraidingw

d

Fr

S ;

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