GEOG 361Sedimentary & Ecological Flows: Process, Form and Management

http://www.geog.ucsb.edu/~jeff/wallpaper2/page2.html

Lecture 3:Alluvial

Fans

DefinitionDefinition

• Fan-shaped deposits of sediment

• Due to change in – gradient– lateral constraint – resistant forces

• Natural or human

ImportanceImportance

• Scientific importance

• Socio-economic importance

ImportanceImportance

StructureStructure

• fan-shaped• concave• ~symmetric

• Merge “bajadas”

• Fine distally• Channel incision• Hetergeneous

• Scale invariant

Factors Affecting StructureFactors 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

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?

Flow ProcessesFlow Processes

• Formation by:– 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

Deposition ProcessesDeposition Processes

• Debris flow deposit creates long, thin topographic high

• Similar process for channelised streamflows: constant deposition elevates bed

Deposition ProcessesDeposition 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

Channel AvulsionChannel Avulsion

• Filling of channel until overtopped

• Maybe– Nodal or random

– Local or regional

– Full or partial

• Occurs at engineering timescales

• c.f. New Orleans

GrowthGrowth

http://www.archatlas.dept.shef.ac.uk/Environmental_change/Environmental_change.htm

Long Timescale ProcessesLong 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

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?

HydrologyHydrology

• Triggering requires rainfall intensity-duration thresholds to be exceeded

HydrologyHydrology

• Fans often act as aquifers

HydrologyHydrology

• Groundwater drainage modelling

Dating 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

VML DatingVML Dating

• Fan in Death Valley

• Units identified by: – fan morphology

– VML

• Age estimates:– 12500 - 2800 yr BP

• Deposition during wet periods

VML DatingVML Dating

http://www.vmldatinglab.com/

ModellingModelling

• Alluvial fan evolution difficult to study in the field

• Modelling crucial

• Random-walk models

• Diffusion models

• Laboratory models

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?