FLUVIAL GEOMORPHOLOGY

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FLUVIAL GEOMORPHOLOGY

• Learning objectives
• Understand basic controls on flood generation and
  sediment delivery to rovers
• Define and measure the size and shape of river
  channels
• Explain how water and sediment moves in river
  channels
• Show awareness that understanding river channel
  behavior can be used to manage rivers in a more
  sustainable way

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Introduction

• Rivers vary greatly from source to mouth and
  between rivers
• Form and shape of river can be transformed
  overnight by a single large flood
• Rivers are dynamic adjust morphology spatially
  and temporally
• Shape the landscape
• Provide threat and resources to humans

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Catchment processes Runoff and river regimes

• Catchment runoff is controlled by
• Regional climate
• Catchment characteristics (soil, topography, land
  use, vegetation, geology)
• River regime seasonal variability in the water
  balance
• 4 major river regimes identified
• Snow and ice melt dominated
• Temperate oceanic environments
• Tropical, non-equatorial river systems
• Equatorial rivers
• Distinct differences also occur at these regional
  scales
• Flashy regimes
• Subdued regimes

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Figure 14.1
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Catchment processesSediment sources and delivery

• Variety of sources
• Hillslope erosion, gully erosion, landslides,
  floodplain erosion
• Arrives in pulses rather than continuous
• Temporary storage of sediment (slope bases and
  floodplains)
• Global scale differences
• Highest erosion sparse vegetation or very high
  rainfall
• Sediment yield per unit area is highest for small
  rivers
• Calibre of sediment varies
• Sediment yield effected by humans
• Poor agricultural practices, construction,
  deforestation

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Table 14.1
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Dominant discharge concept

• Rivers erode and receive sediment input during
  flood events
• flood that does the most geomorphological work
  the dominant discharge
• Large floods - have most potential to erode and
  transport
• Medium sized flood occur more frequently
• do more geomorphological work in the long-term
• Small floods - cannot mobilise coarse sediment
• In large rivers most sediment transport by
  floods occurring between twice each year and
  every five years
• Caution very simplistic concept
• Morphological change lags behind events
• Very active rivers change occurs more frequently

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River channel morphology measuring rivers

• Channel networks and slopes
• Horton (1945) stream ordering Strahlers (1957)
  modification
• Long profiles
• Important variables
• Main stream length
• Total stream length
• Drainage density
• These reflect the combined effects of
• topographic, geologic, pedologic and vegetation
  controls
• Channel planform
• 3 broad types braided, meandering, straight
• Intermediate types anastomising and wandering
• Common measured variable - channel sinuousity

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Figure 14.2
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Figure 14.4 Source Photo courtesy of Dominic
Habron
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Figure 14.5
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Figure 14.6
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• Channel cross section
• Requires straight section of river
• Measure bank full channel dimensions
• Flow resistance at minimum high conveyance
• Complicated by irregular channel sides merging
  into floodplain
• Channel boundary materials
• Particle size analysis
• Sediment mineralogy and roundness
• Riverine sediment well sorted but can be
  bi-modal
• Particle imbrication
• Tracers movement of bed material
• Sediment traps

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Figure 14.3
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River channel processesflow hydraulics

• 2 principal forces Gravity and frictional forces
• Mean water velocity in an open channel is
  estimated using the Manning Equation
• where V is mean water velocity in m s-1
• S channel bed slope (gradient in m per m)
• R hydraulic radius
• Mannings n is a measure of channel roughness
• Quantifying roughness is problematic
• Indices of bed grain size are usually used

V (m s-1)
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Hydraulic geometry

• The way in which water velocity, depth and width
  increase with a rise in water velocity
• Expressed as power functions of discharge
• Q is stream discharge in m3s-1, W is water width
  in metres, D is water depth in metres, V is water
  velocity in ms-1. Since wdv is equal to Q, the
  sum of the exponents b, f and m and intercept
  values a, c and k is equal to 1.

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Stream power

• One of the most important expression of channel
  flow
• Determines the rate of erosion, sediment
  transport and instability
• Stream power (W m-1)
• Q is stream discharge in m3s-1
• S channel slope
• ? water density

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Channel flow characteristics

• Isovels
• Lines of constant velocity
• Shows cross sectional velocity distribution
• Mid-channel greatest water velocities
• Thalweg
• Area of maximum flow
• Heliciodal flow
• Outward direction of flow at a meander
• Outside of meander bends become super-elevated
• Water flows inwards along the ned in return flow
  corkscrew motion

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Figure 14.7
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Sediment movement

• Washload transport
• Fine sediment in river flow (predominantly
  suspended sediment)
• Requires threshold of critical velocity (shear
  stress) to be crossed
• Bedload transport
• Almost entirely function of flow volume, velocity
  and turbulence
• Particles roll, slide and saltate at constant
  rate unless obstructed
• Increase in flow strength causes entrainment
• Dependent on channel slope, particle size, shape,
  fluid kinematic viscosity
• Fall in velocity causes deposition and hydraulic
  sorting
• Stream competence - max particle size a stream
  can carry
• Stream capacity - max volume of debris a stream
  can carry
• Sediment load is highly pulsed dependent on
  sediment supply

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River channelslinking processes and morphology

• Different elements of a channels morphology have
  different susceptibilities to change and may
  change over different timescales
• Negative feedback allows dynamic equilibrium
• Flood events
• Valley floor inundation (channel cannot adjust in
  time)
• Relaxation time time taken to return to
  original form
• Long profile
• Controlled by geology, inherited landscape,
  runoff variability

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Channel cross sections

• Relationship between downstream increase in
  discharge and channel morphology downstream
  hydraulic geometry (Leopold and Maddock, 1953)
• Larger channels are more efficient
• Boundary roughness decreases downstream offsets
  reduced channel slope
• Where wb, ab, vb are channel bankfull cross
  sectional width, depth and velocity values
• ONLY IN UNIFORM SEDIMENTS BED AND BANK
  ERODIBILITY AND SEDIMENT LOAD ARE ALSO IMPORTANT
  MORE COMPLEX

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Channel planform

• Studies have consistently demonstrated that
  channel slope and discharge are important
  variables
• Threshold slope - channels meander
• Higher threshold channels braid
• Critical threshold decreases with increasing
  discharge
• Relationship between planform stream power is
  complex due to many local variables
• Sediment calibre is important
• Coarse sediment mostly braided and wandering
• Important to understand channel planform response
  to management

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Figure 14.12
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Channel bed morphology

• Down stream fining of bed material particle size
  and rounding
• Perturbations are caused by tributary inputs and
  bank collapse
• Erosion and depositional bedforms
• Pool-riffle sequences
• Dunes and anti-dunes
• Pot-holes - caused by corrasion, cavitation and
  corrosion
• Bars variability in grain size
• Armoured layer coarse protective layer

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Short-term river channel changes

• Both slow and rapid channel changes
• Autogenic or allogenic
• Cross-sectional change
• Planform change
• Human induced change
• Straightening and embanking
• Reservoir construction, urbanisation, mining,
  land drainage, vegetation changes

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River management

• As previously demonstrated rivers are not
  static
• Hard engineering
• Traditionally management strategies - civil
  engineers
• Many dramatic failures
• revetments and bridges collapse
• return to natural dimensions after straightening
  and widening e.g. River Mississippi
• habitat loss and local extinctions
• Soft engineering
• Working with rather than against natural
  processes
• Geomorphologists have greater role

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• River maintenance
• Where human use of rivers does not allow natural
  processes
• e.g. dams water periodically released
  downstream
• Glen Canyon Dam, Colorado
• Building new river channels
• Where natural channels effect urban development
• Mirroring natural channel behaviour is
  preferred
• E.g. River Nith, Scotland old natural channel
  lost to valley floor mining for coal
• River restoration
• Where human activity resulted in ecological
  devastation
• Morphological diversity benefits ecological
  carrying capacity
• Reinstatement of meanders and riffle-pool
  sequences
• Need careful consideration of river regime
  equilibrium

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Summary

• Large variation in water and sediment carried by
  rivers
• Channel size and shape are characterised by
  cross-section and planform
• Channel water is subject to gravity and
  frictional forces
• Velocity has a large impact on stream power
• Threshold for sediment entrainment
• Bed morphology varies depending on bed material
• River channel changes can occur very rapidly
  causing problems for river management
• Fluvial geomorphology plays a very important role
  in modern river management