6Factors 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
7QuestionsWhy 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?
8Flow 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
9Deposition ProcessesDebris flow deposit creates long, thin topographic highSimilar process for channelised streamflows: constant deposition elevates bed
10Deposition 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
11Channel Avulsion Filling of channel until overtopped Maybe Nodal or randomLocal or regionalFull or partialOccurs at engineering timescalesc.f. New Orleans
13Long 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
14QuestionsExplain 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?
15HydrologyTriggering requires rainfall intensity-duration thresholds to be exceeded
18Dating 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
19VML Dating Fan in Death Valley Units identified by: Age estimates: fan morphologyVMLAge estimates:yr BPDeposition during wet periods