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

2. Definition Fan-shaped deposits of sediment Due to change in
gradient
lateral constraint
resistant forces
Natural or human

3. Importance
Scientific importance
Socio-economic importance

4. Importance

5. Structure fan-shaped concave ~symmetric Merge  “bajadas”
Fine distally
Channel incision
Hetergeneous
Scale invariant

6. Factors Affecting Structure
External factors
Affect fan: Size, Slope, Hydraulics, Sediment
Transport efficiency determines fan slope angle:
drainage basin area
average annual rainfall
sediment supply
Incised channels steeper than fan slope
Young fans 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. Flow Processes Formation by: Flooding not predictable
stream flow
debris flow
Flooding not predictable
location unpredictable
may change rapidlly (“avulsion”)
Flow spatial structure:
sheetflow at apex
then channelized flow
channels only active over small part of fan

9. Deposition Processes
Debris flow deposit creates long, thin topographic high
Similar process for channelised streamflows: constant deposition elevates bed

10. Deposition Processes Laboratory experiments:
braided channels grow headward
distal depositional lobes
deposition migrates up-fan, back-filling channels
cycle at range of scales
frequent avulsion
Unstable at geomorphic & engineering time scales

11. Channel Avulsion Filling of channel until overtopped Maybe
Nodal or random
Local or regional
Full or partial
Occurs at engineering timescales
c.f. New Orleans

12. Growth

13. Long Timescale Processes
Over long timescales:
Aggrade and prograde at decreasing rate
Accumulate in topograhic lows
Zones of subsidence
At largest scales:
Driven by tectonics
Uplift causes formation
Fans ultimately subducted

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
Triggering requires rainfall intensity-duration thresholds to be exceeded

16. Hydrology
Fans often act as aquifers

17. Hydrology
Groundwater drainage modelling

18. Dating Rock varnish microlamination (VML) Dating Rock varnish:
coating on exposed rock
thickness ~100 µm
Well-preserved in arid regions
Microlaminations:
two types of layers: form micro-stratigraphy
Carries climate record
yellow layers = dry periods
black layers = wet periods

19. VML Dating Fan in Death Valley Units identified by: Age estimates:
fan morphology
VML
Age estimates:
yr BP
Deposition during wet periods

20. VML Dating

21. Modelling
Alluvial fan evolution difficult to study in the field
Modelling crucial
Random-walk models
Diffusion models
Laboratory models

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?