GG2021: Geomorphology

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GG2021 Geomorphology

• Fluvial Processes I

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Alluvial River meandering on its own floodplain
composed of transported sands/silts. (Upper
Mississippi)
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High bedload, variable discharge braided glacial
outwash stream, Flaajokull, Iceland
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Land forming - Lateral accretion in meandering
rivers builds floodplain
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River Channel Change River response to
base-level, climate change and a change in
flow/sediment supply balance Vertical
adjustment- River Terraces
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Rivers as Hazard Floods at Tewkesbury, July 2007
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Ephemeral Rivers

• INPUT
• Internally fed, convective storms dominant.
• Highly localised rainfall
• Temporally variable
• THROUGHPUT
• Horton overland flow dominant. No base-flow.
  Quick-flow dominant.
• Rapid initiation of surface runoff
• Transmission losses through channel seepage. Peak
  discharge may thus FALL downstream if runoff is
  localised.
• OUTPUT
• Flash Floods
• Steep rising and falling limbs of hydrographs
• Considerable flow variability
• FLOW OCCUPANCY 0-30
• INTERMITTENT STREAMS FLOW OCCUPANCY 30-100
• (Source Knighton)

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Ephemeral River System
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Perennial, Alluvial Rivers
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Perennial Rivers

• INPUT
• Internally fed (springs) external sources
  (mountainous headwaters, snowmelt)
• Larger headwater supply area for flow
• Greater flow reliability
• THROUGHPUT
• Saturated overland flow, sub-surface storm-flow.
  Base-flow Quick-flow.
• Longer response times to runoff
• Transmission loss through evaporation and
  artificial abstraction for human requirements.
  Otherwise Discharge INCREASE downstream due to
  tributary input.
• OUTPUT
• Seasonal floods
• Broader based hydrograph- longer rise/falling
  limbs
• Seasonably dependable.
• 100 channel occupancy (SourceKnighton)

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Channel Network Ordering The Strahler system
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River Regimes.Regime is the seasonal flow
variation of a river.
Complex 1 4-6 Hydrological phases eg 2 high/2
low flow phases per year. eg. May Snow-melt peak
summer convection storm peak
Durance Reuss
Rhine
Rhone Arve
Glacial snowmelt
Volga/ Dneiper
Winter rainfall
Congo
Rhone
Thames, Seine Weser
Atbara Blue Nile
Complex II Rivers which flow through distinct
relief/climate zones eg Nile, Ganges
SIMPLE REGIMES Single Peak
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Event flow in the river as shown by the
hydrograph (Knighton)
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Channel Width Function hydrograph form
Derivation of width function
Width function for R.Derwent at Chatsworth. Haw
Bridge R.Severn Predicted hydrographs
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Hydrograph Formation Role of network width
function

• The map shows a division of the Thames catchment
  based on geological regions and network width
  functions.
• Note the more permeable lithologies Chalk
  Limestone, have smaller numbers of channels.
• The Clay zones are impermeable and thus have high
  channel densities and large runoff reponse.

Smith Ward (1998)
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Channel flow hydrograph as a combination of
Hillslope response and network response (related
to network-width) (Knighton)
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Channel networks flow

• Mean annual flood in Trent Basin and link
  magnitude.
• Downstream flow accumulation on rivers Derwent
  Trent due to sub-catchment additions at tributary
  junctions.
• (Source Knighton)