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: –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
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: –fan morphology –VML Age estimates: – yr BP Deposition during wet periods
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?