Presentation on theme: "GEOG 361 Sedimentary & Ecological Flows: Process, Form and Management"— Presentation transcript:
1 GEOG 361 Sedimentary & Ecological Flows: Process, Form and Management Lecture 3:AlluvialFansLecture 3:A lovelyFlan
2 Alluvial Fan Definition Fan-shaped deposits of sediment(de Chant et al, 1999, Fig. 1).Due to change ingradientlateral constraint(base of mountains)resistant forces(river meeting ocean)Formation can benaturalhuman (e.g. mine tailings)Geomorphic & engineering relevance
3 Importance of Alluvial Fans to Humanity Scientific importance:Indicators of tectonic activityIndicators of palaeoclimate changeSocio-economic importance:Reservoirs of water, fossil fuelsPopular for human developmentBut high flood risk
5 Alluvial Fan Structure Plan view: fan-shapedRadial cross-section: concaveLateral cross-section: ~symmetric3D: ~ conic segmentCan merge to form “bajadas”Sediments fine distallySurface incised by many channelsStratigraphyHeavily hetergeneousMulti-scaleCharacteristics scale invariant:laboratory modelslargest natural fans (100’s km)
6 Factors Affecting Alluvial Fan Structure External factors: Climate, Tectonics, Topography, LithologyAffect fan: Size, Slope, Hydraulics, Sediment characteristicsMany attempts to determine relationships(e.g. Milana & Rusycki, 1999, Figs. 2, 8, 9B)Transport efficiency determines fan slope angle, affected by:drainage basin areaaverage annual rainfallsediment supply (secondary factor)Slope changes within fan stratigraphy indicate hydrological changes over timeIncised channels slightly steeper than general fan slopeYoung fans slightly 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 Alluvial 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 sameFlooding is not predictable using methods for riversFlow location unpredictableMay change rapidlly (“avulsion”)Flow spatial structure:Initial flow at apex of fan often sheetflowThen breaks down to channelized flowChannels only active over small proportion of fan at any one time
9 Alluvial Fan Deposition Processes Debris flow deposit creates long, thin topographic highSubsequent flows avoid this, deposit elsewhereEventually, deposits fill available depositional spaceCreate fan-like morphology at scale >> individual flows (Parker, 1999, Fig. 2).Similar process for channelised streamflows:constant deposition elevates bedchannels avulse to previously dry areasCreate fan-like morphology at scale >> individual flows (Parker, 1999, Fig. 4)
10 Alluvial Fan Deposition Processes Laboratory experiments show(Whipple et al. 1998, Fig. 5):flow forms multiple-thread braided channelsGrow rapidly headwardTransport sediment to depositional lobesDepositional centres then migrate up-fan, gradually back-filling channelsIncision/back-filling cycle occurs on range of scales & all parts of fanschannels avulse frequently, sweep across & re-grade fan surfaceFans built up as depositional features sweep across seeking lowest topographyThus unstable at geomorphic & engineering time scales(Parker, 1999, Fig 9)
11 Channel AvulsionOccurs due to filling of present channel to point where banks overtoppedMaybeNodal or randomLocal or regionalFull or partial (bifurcation)(mainly relevant to next week)Fans geomorphic timescale features……but avulsion occurs at engineering timescalesDestroys infrastructure, farmland, populated areasInevitable: levees etc. only delays eventc.f. New Orleans
13 Alluvial Fan Long Timescale Processes Over long timescales:Fans aggrade and prograde indefinitely……but at ever decreasing ratesAccumulate in topograhic lows, esp. coastal plainsBecome zones of subsidence through compaction by depositsAt largest scales:Fan formation and location driven by tectonicsTectonic subduction creates subsidenceAlluvial fan flows tend towards subduction zonesThus the fans ultimately become consumedTectonic uplift provides source of sediment and potential energy to form fans
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 Hydrology of Alluvial Fans Triggering of streamflow & debris flow fan flooding events requires rainfall intensity-duration thresholds to be exceeded(see GEOG 203, Lecture 1)
16 Hydrology of Alluvial Fans Fans often act as aquifers:Often lake, springs at base:
17 Hydrology of Alluvial Fans Groundwater drainage modelling esp. important for mine tailings(pollutant pathways, concentrations)
18 Dating Alluvial Fans Rock varnish microlamination (VML) Dating dark coating on subaerially exposed rock surfacesworld's slowest-accumulating sedimentary deposit: ~10m per 1000 yrsthickness typically ~100 µmparticularly well preserved in arid & semi-arid regionsMicrolaminations:observed when varnish shaved thin enough (5-10 µm) to see through with a light microscopedark layers in varnish thin section rich in Mn and Ba, but poor in Si and Alorange and yellow layers poor in Mn and Ba, rich in Si and Altwo types of layers intercalated to form microstratigraphyGrowing body of evidence indicates varnish microstratigraphy carries climate recordMn-poor yellow layers formed during dry periodsMn-rich black layers deposited during wet periods
19 Rock varnish microlamination (VML) Dating ~ 0.04 km2 alluvial fan in Death ValleySeven units identified on basis of:fan morphologyrock varnish coverageVarnish-based age estimates:yr BPDating results indicate deposition during wet periods
21 Alluvial Fan Modelling Alluvial fan evolution difficult to study in the field:rarity of significant eventsevents are too powerful to study safelyTherefore modelling is crucialRandom-walk modelsdescribe formation and spread of channels across fan surfaceassume quasi-random path for water and sedimentdriven into topographic lows by gravity but otherwise randomDiffusion modelsassume long term pattern of deposition diffusiveadopt boundary conditions defining sediment inputLaboratory modelsrelatively recent developmentrequire vertical exaggeration, dynamical scaling
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?