6 Factors Affecting Structure External factorsAffect fan: Size, Slope, Hydraulics, SedimentTransport efficiency determines fan slope angle:drainage basin areaaverage annual rainfallsediment supplyIncised channels steeper than fan slopeYoung fans steeper than old fans
7 QuestionsWhy 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 flowdebris flowFlooding not predictablelocation unpredictablemay change rapidlly (“avulsion”)Flow spatial structure:sheetflow at apexthen channelized flowchannels only active over small part of fan
9 Deposition ProcessesDebris flow deposit creates long, thin topographic highSimilar process for channelised streamflows: constant deposition elevates bed
10 Deposition Processes Laboratory experiments: braided channels grow headwarddistal depositional lobesdeposition migrates up-fan, back-filling channelscycle at range of scalesfrequent avulsionUnstable at geomorphic & engineering time scales
11 Channel Avulsion Filling of channel until overtopped Maybe Nodal or randomLocal or regionalFull or partialOccurs at engineering timescalesc.f. New Orleans
13 Long Timescale Processes Over long timescales:Aggrade and prograde at decreasing rateAccumulate in topograhic lowsZones of subsidenceAt largest scales:Driven by tectonicsUplift causes formationFans ultimately subducted
14 QuestionsExplain 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 structureAre alluvial fans stable or unstable features over engineering and geomorphic timescales, and with what external factors are they tending towards equilibrium?
15 HydrologyTriggering requires rainfall intensity-duration thresholds to be exceeded
18 Dating Rock varnish microlamination (VML) Dating Rock varnish: coating on exposed rockthickness ~100 µmWell-preserved in arid regionsMicrolaminations:two types of layers: form micro-stratigraphyCarries climate recordyellow layers = dry periodsblack layers = wet periods
19 VML Dating Fan in Death Valley Units identified by: Age estimates: fan morphologyVMLAge estimates:yr BPDeposition during wet periods
21 ModellingAlluvial fan evolution difficult to study in the fieldModelling crucialRandom-walk modelsDiffusion modelsLaboratory 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?
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