the evolution of a tie channel joel c. rowland & william e. dietrich university california -...
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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