GEOG 361 Sedimentary & Ecological Flows: Process, Form and Management http://www.geog.ucsb.edu/~jeff/wallpaper2/page2.html Lecture 3: Alluvial Fans

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

Importance Scientific importance Socio-economic importance

Importance

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

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

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 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

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

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

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

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

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

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 Triggering requires rainfall intensity-duration thresholds to be exceeded

Hydrology Fans often act as aquifers

Hydrology Groundwater drainage modelling

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 Dating Fan in Death Valley Units identified by: Age estimates: fan morphology VML Age estimates: 12500 - 2800 yr BP Deposition during wet periods

VML Dating http://www.vmldatinglab.com/

Modelling 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?