GG2021: Session 10

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GG2021 Session 10

• Alluvial Fans

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Black Mountain Fan Death Valley
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Quaternary Fans, SE Spain Durcal
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Fan Aprons Piedmont or Bajada Panamint Mts,
Death Valley
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The Alluvial Fan System

• Flow sediment yield generated in the catchment.
• The fan volume, area and morphology are
  determined by sediment yield flow from the
  basin area. (Q/Qs).
• As a depositional landform the fan will be
  affected by changes in flow of sediment yield, as
  well as tectonics which may lead to changes in
  fan or basin channel gradients.

Source Rachocki
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Alluvial fan W.China (Nasa)
Intense braided flow- deposition over wide area
of fan.
Braided flow
Concentrated flow Scour more likely
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Black Mountain Fans Death Valley, US
Catchment Flow confined by valley sides.
Fan Apex Proximal Zone
Fan Toe Distal Zone
Basin Flow laterally unconfined
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Causation of deposition of alluvial fans
Badwater Fan, Death Valley
Transport Bedload transport rate determined by
Stream Power/Unit width of flow QS/w
Stream Power/unit width Q.S- /w Also Depth -,
Friction Mean Velocity Thus Deposition
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Copper Canyon Fan Death Valley, USA

• Fan gradient is determined by
• Calibre of deposited sediment
• Fan building Process
• Debris flow- steep
• Fluvial flow- less steep

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Morphological Relationships

• Relations of drainage basin area(Ac) and fan
  area (Af) for different locations.
• Af aAcn
• (n gradient)
• Locations of lines reflect variations in
  catchment sediment yield size.
• Relations of fan gradient and drainage basin
  area. Note larger area, lower gradient.
  Material carried further?

Area
Gradient
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Proximal- Distal fan facies variation on Spanish
fans (Harvey 1997)

• Spatial variations in fan deposition reflect
  proximal-distal variations in depositional
  process and sorting mechanism.
• Dominance of debris flow in proximal area gives
  way to sheet-flood fluvial deposits over the
  distal fan surface. Transition often mid-fan.
• Distal Environment often sands/silts sheets
  dominating over gravel deposits.
• Usually down-fan DECREASE in clast size.

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Wells Harvey Fan development in the Howgill
Fells, Lancashire (1987)

• Conceptual model showing
• a) changes in water-sediment ratio of flows
  produced by different catchments
• b) temporal sequence of flow types
  deposition in typical fan building events.
• Main controls
• Total drainage area contributing runoff
• Part of basin contributing sediment
• Channel gradient in the catchment

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Fans Process Horseshoe Park Fan, California
Recent event on the fanbraided flow deposits
Apex- deposition of coarse bedload at basin
outlet as flow loses power rapidly
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Fan Deposits

• Sieve Deposit
• Course material is deposited on the fan surface
  by flow from apex channel.
• Flow is then sieved through the coarse lag
  deposit. This process causes deposition of finer
  grades behind the sieve lobe.

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Fan Deposits

• Fanglomerate up-tilted by tectonics in death
  valley area.
• Note the coarse fluvial fan deposits.
• Fluvial fan deposits are often relatively poorly
  sorted due to rapid deposition of bedload

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Schumm Aggrading Entrenching Fans
Dispersed flow- mainly widespread deposition at
apex and mid-fan
Entrenched, incising flow at apex, deposition
shifted down-fan to distal zone.
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Schumm Weaver Laboratory Fans

• A apex incision results in deposition near
  toe.
• B channel backfills and locus of sedimentation
  (intersection point) moves up-fan
• C channel obliterated and flows from drainage
  basin now spread over fan- braided flow at apex
  and deposition- increasing gradient
• D oversteepening of apex until inherant
  stability threshold is exceeded and downcutting
  begins.
• E channels in topographical lows slowly backfill
  and raise relative elevation of this sector
  until,
• (F) deposition returns to the apex.

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Small Fan Death Valley

• 1. Some dissection at apex fluvial incision.
• 2. Fan largely constructed of debris flow
  deposits.
• Note indicative lobate, hummocky surface of
  debris flow deposits

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Cucamonga Fault Fan Note some dissection.
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Scott Erskine Fan incision in Australia
Basin area plotted against fanhead slope for
trenched untrenched fans reveals THRESHOLD
discriminant line.
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Threshold behaviour on fans Scott Erskine

• Cyclic behaviour on Australian fans Aggradation
  increases apex gradient until the threshold for
  sediment entrainment for the material passed.
  Subsequent flows then start to entrench. This
  process reduces fan channel gradient until
  deposition begins again.

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Intrinsic Slope threshold (Geomorphic threshold)
Barranco del Muerto Spain At threshold apex
gradient - unincised
Barranco del Muerto Fan During high intensity
flow event the slope threshold has been passed
apex incision (entrenchment) has occurred. Fan
is dissected.
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Dissected Quaternary Fan- Central Alps Climate
change induced dissection will give large scale
incised channels.
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Himalayan Fan Himachal Pradesh, India.Glacial
retreat and reduction in sediment supply
tectonic tilting
Fan terrace-old
Fan terrace-younger
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Alluvial Fans and environmental change
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Fan Entrenchment

• INTRINSIC FACTORS TIMESCALE
• Variation within storm event (Denny,
  1967) Single event
• Intra-storm scour-fill (Beaty, 1974) Single
  event
• Declining flood water stage (Blackwelder,
  1928) Single event
• Debris flow/fluvial flow alternation (Beaty,
  1974) Single event
• Shifting locus of flow/deposition to local
  topographic low Single/mulitple
• Gradient increase at apex beyond stability
  threshold
• (Schumm, Schumm Weaver) Multiple/single
• Capture of fan feeder channel by adjacent channel
  (Hooke) multiple
• EXTRINSIC FACTORS
• Increase in storm magnitude due to increase mean
• Precipitation/variability (Eckis) Multiple
• Decrease volume of flood water due to reduction
  rainfall Multiple
• Overgrazing/human activity changing runoff
  production Multiple
• Decreased sediment supply due to lowered relief,
  exposure
• of resistant lithology Multiple
• Downcutting over a cycle of erosion
  (Eckis) Prolonged
• Base level lowering (Blissenbach,
  1954) Prolonged

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Bull MacFadden Tectonics fan segmentation

• BASIN UPLIFT VS CHANNEL DOWNCUTTING.
• 1. Active
• Rate uplift gt rate downcutting piedmont
  deposition
• - fan that receives deposits from fan head
• 2. Slight
• Rate uplift lt rate downcutting gt piedmont
  erosion
• - entrenched fan with old soils on fanhead
• 3. Inactive
• Rate uplift ltlt rate downcutting lt piedmont
  erosion
• - dissected piedmont terraces

Younger deposits
Younger deposits
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Doornkamp Mukang (1985)

• Fan- basin morpho-tectonics in China
• A incised fans develop below in front of
  older, higher fans (intense uplift, downwarping)
• B similar to (a) but less intense tectonics
  Imbricated fans
• C sequence of buried fans- basin intermittently
  downwarped.
• D Compound Type C process followed by uplift
  and river incision in previous area of fan
  building. Exposes old fan development.