The Evolution of a Tie Channel

Joel C. Rowland & William E. Dietrich

University California - Berkeley

Source: Google Earth

Goal of Study

• Develop a conceptual model for the development of a self-formed leveed channel created by a sediment-laden current entering still water

What is a Tie Channel?

• Self-formed leveed channel

• Connect rivers to lakes

• Jet entering still water

• Bi-directional flow

• Stable

250 m

Fly River , Papua New Guinea

Tie channel

Global distribution of known tie channels

Raccourci Old River Tie Channel•65 km upstream Baton Rouge, LA

•Formed in 1851

2 km

Why Raccourci Old River ?

• Largest known tie channel

• Largely unaltered channel in naturally functioning floodplain

• Unprecedented documentation of channel developmentData sources:

• Historical records

• Hydrographic surveys (> 1880s)

• Aerial photographs (> 1940s)

• Satellite imagery

• ALSM data (Lidar)

• Long-term records for Miss River

• Field data

Talk Outline

• Channel Characteristics– Morphology– Sedimentology

• Conceptual Model– Levee growth– Channel widening

Channel Characteristics

Long profile of channel levees and width

Mississippi River

Oxbow lake margin

1 km

Channel bed from dam into lake

Distance from Channel Outlet (m)

Ele

vati

on

(m

)

-2000 -1500 -1000 -500 0 500 1000 15000

5

10Bed Elev.

mouth bar

Channel and levee growth

• Vertically accretes

• Channel widens

• Levee flanks broaden

Levees composed of sub-horizontal alternating layers

• Sand deposition: U* > Ws in channel, U* < Ws over levees

• Mud and organic deposition: U* < Ws everywhere

100

101

102

103

0

20

40

60

80P

erc

ent

100

101

102

103

0

50

100

Particle Diameter (microns)

Perc

ent Fin

er

data1data2data3data4data5

TC Levee Coarse

TC Levee Fine

TC Bed

Miss Susp Sed

Miss Bed

Incoming load sorted by tie channel processes

silt sand

clay

Model of Channel Formation

150 m

Jet Sedimentation

• Large “quasi-2D” turbulent structures

•Scale with jet width

•Sweep across newly forming channel advecting sediment to margins

150 m

Localized shear along inundated levee crests

?

Lake level < levee

crest

Large-scale advective transfer of sediments

Lake level > levee crest

• Unpaired levee crests heights

• Super-elevated bends

• Asymmetric levee x-sections

Concepts Watershed, 2005

Local advective transfer of sediments: Splays

• Locally erode crests

• Deposit on flanks

Widening by mass failure and narrowing by sediment drapes

Conclusions

• Channel selectively sorts and deposits incoming sediment

• Majority of levee sedimentation occurs during submerged/inundated conditions

• Splays redistributes sediments and broaden levees

• Channel width controlled by mass failures which are linked to levee height

Acknowledgements