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

Lecture 3:A lovely

Flan

Lecture 3:Alluvial

Fans

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

Alluvial Fan DefinitionAlluvial 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

Importance of Alluvial Fans to HumanityImportance 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

Importance of Alluvial Fans to HumanityImportance of Alluvial Fans to Humanity

Alluvial Fan StructureAlluvial 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)

Factors Affecting Alluvial Fan StructureFactors 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

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?

Alluvial Fan Flow ProcessesAlluvial 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

Alluvial Fan Deposition ProcessesAlluvial 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– Eventually, deposits fill available depositional

space– Create fan-like morphology at scale >> individual

flows (Parker, 1999, Fig. 4)

Alluvial Fan Deposition ProcessesAlluvial 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)

Channel AvulsionChannel Avulsion

• Occurs due to filling of present channel to point where banks overtopped

• Maybe– Nodal or random– Local or regional– Full or partial (bifurcation)

• http://www.geo.uu.nl/fg/palaeogeography/results/avulsions (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

Growth of Alluvial FansGrowth of Alluvial Fans

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

Alluvial Fan Long Timescale ProcessesAlluvial 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

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?

Hydrology of Alluvial FansHydrology of Alluvial Fans

• Triggering of streamflow & debris flow fan flooding events requires rainfall intensity-duration thresholds to be exceeded

(see GEOG 203, Lecture 1)

Hydrology of Alluvial FansHydrology of Alluvial Fans

• Fans often act as aquifers:– Often lake, springs at base:

Hydrology of Alluvial FansHydrology of Alluvial Fans

• Groundwater drainage modelling esp. important for mine tailings(pollutant pathways, concentrations)

Dating Alluvial FansDating Alluvial Fans

• Rock varnish microlamination (VML) Dating

• Rock varnish:– dark coating on subaerially exposed rock surfaces– world's slowest-accumulating sedimentary deposit: ~10m 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

Rock varnish microlamination (VML) DatingRock 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:– 12500 - 2800 yr BP

• Dating results indicate deposition during wet periods

Rock varnish microlamination (VML) DatingRock varnish microlamination (VML) Dating

http://www.vmldatinglab.com/

Alluvial Fan ModellingAlluvial 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

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?