1. GEOG 361 Sedimentary & Ecological Flows: Process, Form and Management
Lecture 3:
Alluvial
Fans
Lecture 3:
A lovely
Flan

2. Alluvial Fan Definition
Fan-shaped deposits of sediment
(de Chant et al, 1999, Fig. 1).
Due to change in
gradient
lateral constraint
(base of mountains)
resistant forces
(river meeting ocean)
Formation can be
natural
human (e.g. mine tailings)
Geomorphic & engineering relevance

3. Importance of Alluvial Fans to Humanity
Scientific importance:
Indicators of tectonic activity
Indicators of palaeoclimate change
Socio-economic importance:
Reservoirs of water, fossil fuels
Popular for human development
But high flood risk

4. Importance of Alluvial Fans to Humanity

5. Alluvial Fan Structure
Plan view: fan-shaped
Radial cross-section: concave
Lateral cross-section: ~symmetric
3D: ~ conic segment
Can merge to form “bajadas”
Sediments fine distally
Surface incised by many channels
Stratigraphy
Heavily hetergeneous
Multi-scale
Characteristics scale invariant:
laboratory models
largest natural fans (100’s km)

6. Factors Affecting Alluvial Fan Structure
External factors: Climate, Tectonics, Topography, Lithology
Affect fan: Size, Slope, Hydraulics, Sediment characteristics
Many attempts to determine relationships
(e.g. Milana & Rusycki, 1999, Figs. 2, 8, 9B)
Transport efficiency determines fan slope angle, affected by:
drainage basin area
average annual rainfall
sediment supply (secondary factor)
Slope changes within fan stratigraphy indicate hydrological changes over time
Incised channels slightly steeper than general fan slope
Young fans slightly steeper than old fans

7. Questions
Why are alluvial fan depositional areas, which are potentially so hazardous, also so attractive for human habitation?
What are the key factors that determine alluvial fan morphology and stratigraphy and how do they each control the fan characteristics?

8. Alluvial Fan Flow Processes
Distinction can be made between fans formed by:
Depositional stream flow (wetter)
Debris flow (drier)
Mixture of stream & debris flow (composite)
Structure of fans is much the same
Flooding is not predictable using methods for rivers
Flow location unpredictable
May change rapidlly (“avulsion”)
Flow spatial structure:
Initial flow at apex of fan often sheetflow
Then breaks down to channelized flow
Channels only active over small proportion of fan at any one time

9. Alluvial Fan Deposition Processes
Debris flow deposit creates long, thin topographic high
Subsequent flows avoid this, deposit elsewhere
Eventually, deposits fill available depositional space
Create fan-like morphology at scale >> individual flows (Parker, 1999, Fig. 2).
Similar process for channelised streamflows:
constant deposition elevates bed
channels avulse to previously dry areas
Create fan-like morphology at scale >> individual flows (Parker, 1999, Fig. 4)

10. Alluvial Fan Deposition Processes
Laboratory experiments show
(Whipple et al. 1998, Fig. 5):
flow forms multiple-thread braided channels
Grow rapidly headward
Transport sediment to depositional lobes
Depositional centres then migrate up-fan, gradually back-filling channels
Incision/back-filling cycle occurs on range of scales & all parts of fans
channels avulse frequently, sweep across & re-grade fan surface
Fans built up as depositional features sweep across seeking lowest topography
Thus unstable at geomorphic & engineering time scales
(Parker, 1999, Fig 9)

11. Channel Avulsion
Occurs due to filling of present channel to point where banks overtopped
Maybe
Nodal or random
Local or regional
Full or partial (bifurcation)
(mainly relevant to next week)
Fans geomorphic timescale features…
…but avulsion occurs at engineering timescales
Destroys infrastructure, farmland, populated areas
Inevitable: levees etc. only delays event
c.f. New Orleans

12. Growth of Alluvial Fans

13. Alluvial Fan Long Timescale Processes
Over long timescales:
Fans aggrade and prograde indefinitely…
…but at ever decreasing rates
Accumulate in topograhic lows, esp. coastal plains
Become zones of subsidence through compaction by deposits
At largest scales:
Fan formation and location driven by tectonics
Tectonic subduction creates subsidence
Alluvial fan flows tend towards subduction zones
Thus the fans ultimately become consumed
Tectonic uplift provides source of sediment and potential energy to form fans

14. Questions
Explain how channel avulsion produces alluvial fans which are much larger than the flow features which create them, and why fans formed from streamflow and debris flow are so similar in structure
Are alluvial fans stable or unstable features over engineering and geomorphic timescales, and with what external factors are they tending towards equilibrium?

15. Hydrology of Alluvial Fans
Triggering of streamflow & debris flow fan flooding events requires rainfall intensity-duration thresholds to be exceeded
(see GEOG 203, Lecture 1)

16. Hydrology of Alluvial Fans
Fans often act as aquifers:
Often lake, springs at base:

17. Hydrology of Alluvial Fans
Groundwater drainage modelling esp. important for mine tailings
(pollutant pathways, concentrations)

18. Dating Alluvial Fans Rock varnish microlamination (VML) Dating
dark coating on subaerially exposed rock surfaces
world's slowest-accumulating sedimentary deposit: ~10m per 1000 yrs
thickness typically ~100 µm
particularly well preserved in arid & semi-arid regions
Microlaminations:
observed when varnish shaved thin enough (5-10 µm) to see through with a light microscope
dark layers in varnish thin section rich in Mn and Ba, but poor in Si and Al
orange and yellow layers poor in Mn and Ba, rich in Si and Al
two types of layers intercalated to form microstratigraphy
Growing body of evidence indicates varnish microstratigraphy carries climate record
Mn-poor yellow layers formed during dry periods
Mn-rich black layers deposited during wet periods

19. Rock varnish microlamination (VML) Dating
~ 0.04 km2 alluvial fan in Death Valley
Seven units identified on basis of:
fan morphology
rock varnish coverage
Varnish-based age estimates:
yr BP
Dating results indicate deposition during wet periods

20. Rock varnish microlamination (VML) Dating

21. Alluvial Fan Modelling
Alluvial fan evolution difficult to study in the field:
rarity of significant events
events are too powerful to study safely
Therefore modelling is crucial
Random-walk models
describe formation and spread of channels across fan surface
assume quasi-random path for water and sediment
driven into topographic lows by gravity but otherwise random
Diffusion models
assume long term pattern of deposition diffusive
adopt boundary conditions defining sediment input
Laboratory models
relatively recent development
require vertical exaggeration, dynamical scaling

22. Questions What is the key hydrological role of alluvial fans?
How does VML dating allow the stratigraphy of alluvial fans to be interpreted?
What is the most effective way of modelling alluvial fans?