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Research group River and delta morphodynamics
Alluvial Fans
Tjalling de Haas ([email protected])
How did this boulder get here?
Today
General intro on alluvial fans
Processes that act on alluvial fans
BIG QUESTIONS:
Which three (genetic) types of alluvial fans exist?
What is the difference between fluvial and debris
flow?
What determines whether a fluvial or debris flow
occurs?
Fan- or cone-shaped deposit of sediment
Develops at catchment-valley transition
What is an alluvial fan?
Catchment
Apex
ADL
IDL
Primary processes
Rock-gravity processes
Sediment without fluid transported by gravity force
Sediment-gravity processes
Sediment and fluid transported by gravity force
Fluid-gravity processes
Sediment moved by the force of water
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Rock gravity processes
Processes:
Rock fall
Rock slides
Granular flow
Fans:
Steep (20-35⁰; near angle of repose)
Restricted radial length (<0.5 km)
Sediments:
Coarse particles (cobbles – boulders)
Svalbard, Norway
Atacama, Chile
Sediment-gravity processes
Processes:
Debris-flow
Fans:
Low-steep (3-20⁰)
Sediments:
Wide range: mud - boulders
Fluid-gravity Sediment-
gravity
Rock-
gravity
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Debris-flow
Excess pore pressure
Fine particles (mud and fine silt) block pores
Water unable to escape
Mixture behaves as a slurry Bingham Liquid!!!
Debris-flow morphology
Levees
Lobes
Atacama, Chile 16/46
Atacama, Chile
17/46
Svalbard
18/46
Svalbard
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Real debris flow
https://www.youtube.com/watch?v=51C7vEAVbxk
Note:
Plastic flow behaviour
Large boulders in the flow
Large particle size range
Debris-flow fans
1. Dominated by levées
Steep slope (9-18⁰)
2. Proximal levées and distal lobes
Proximal steep slope (9-18⁰)
Distal lower-sloping (3-6⁰)
3. Dominated by lobes
Slope 3-8⁰
Blair and McPherson, 2009
27/46
Mars
Debris-flow fan deposits
Very poorly sorted
No stratification
Random clast orientation
No grading
Sometimes inverse grading
Atacama, Chile Atacama, Chile
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How did this boulder get here?
Atacama, Chile
Fluid-gravity processes
Processes:
Stream-flow
Hyperconcentrated-flow
Fans:
Low slope (2-5⁰)
Sediments:
Finest-grained fan types (dominantly gravel)
Svalbard, Norway
Sheetflood
Channel
Atacama, Chile
Svalbard Atacama, Chile
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Fluvial deposits
Moderately-well sorted
Stratification
Normal grading
Imbrication
Remember Debris-flow?
Very poorly sorted
No stratification
No grading
Random clast
orientation
Death Valley, USA
When fluvial or debris flow?
When fluvial or debris-flow?
Catchment is the key:
Slope
Drainage system
Lithology
■Production of Mud and fine Silt
■Fine sediment needed to maintain excess pore pressure
Secondary processes
Reworking by:
Water
■Winnowing of fines
■Rills, gullies and coarse lags
Wind
■Winnowing of fines
■Deposition of aeolian fines
Weathering
■Particle break-down
Tectonics
Atacama, Chile
Inactive surface
Active surface
800m
Sea
Atacama, Chile