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Research group River and delta morphodynamics

Alluvial Fans

Tjalling de Haas (t.dehaas@uu.nl)

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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19/46

Svalbard Utrecht

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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

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How did this fan form?

How did this fan form?

How did this fan form? How did this fan form?

Remember: big questions?

Which three main genetic types of alluvial fans

exist?

What is the difference between fluvial and

debris flow?

What determines whether a fluvial or

debris-flow occurs?