Flow in the Channel: Channel Hydraulics

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Flow in the Channel Channel Hydraulics
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Flow in the channel is a result of basin
processes and runoff generation
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Channel flow affected by

• Channel geometry
• Smooth trapezoidal channel
• Natural channel variability

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Stream Channel Geometry

• channel width w
• channel depth d
• cross sectional area a
• wetted perimeter Pw
• hydraulic radius r a/Pw
• channel gradient (slope) S

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FISRWG
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Velocity

• balance between
• driving force gravity
• resisting forces water turbulence, bed friction
  (roughness)

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Arkansas River near Salida, CO
August 2007
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Williams Creek cutoff, August 2006
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Velocity varies

• with distance from bed
• with distance from banks
• downstream
• over time

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Why Meanders?

• Random channel perturbations
• Development of secondary helicoidal flow cells
• Bedload content
• Bank erosion
• Minimum energy condition (equilibrium)
• Meandering dissipates kinetic energy of moving
  water along length of channel
• Meandering minimizes energy, efficient transport
  of sediment

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Helicoidal Flow

• Corkscrew motion
• In bends, water forced to outside of bend hits
  bank with force
• Water piles-up on outside of bend
• Forced downward along cutbank, back towards point
  bar
• Transfers and reverses motion around next
  alternating bend

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Williams Creek cutoff, August 2006
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Why Helicoidal Flow?

• Specifics are poorly understood
• Coriolis effect?
• Movement around obstacles?

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Convergent and Divergent Flow

• Convergent flow
• surface water flow is toward axis of channel
• maximum velocity near channel bottom
• scour dominant
• pools
• Divergent flow
• surface water diverges away from channel axis
• lt velocity
• gtdeposition
• riffles

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(S. Kite)
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(S. Kite)
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(S. Kite)
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(S. Kite)
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(S. Kite)
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Pecatonica River, WI
(Maher)
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Pecatonica River, WI Image Maher
Dates Knox, 2000
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Trush, McBain, Leopold (2000)