Fluvial Geomorphology

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Fluvial Geomorphology

• Environmental Hydrology
• Lecture 20

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Fluvial Geomorphology

• Study of the role of running water in shaping the
  landscape

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Key concepts in fluvial geomorphology

• Characteristics of rivers as function of
  landscape position (geography)
• How rivers accomplish work
• How rivers respond to change

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Geomorphic Work
Running water exerts force ? ? Landscape offers
resistance
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Geomorphic Work

• Lanes Balance, Ward Trimble, Fig 9.20

Q S a Qs d
Where Q discharge Qs sediment discharge S
slope d median sediment size QS stream
power
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When does geomorphic work occur?
after Wolman, M. G. and J. P. Miller, 1960.
Magnitude and Frequency of Forces in Geomorphic
Processes. Journal of Geology, 68(1) 54-74.
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When does geomorphic work occur?

• see Ward Trimble, Fig 6.14

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Effective width, depth
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Geomorphic work expressed in channel
characteristics

• Channel dimensions (cross-section)
• Channel profile (longitudinal view)
• Channel pattern (plan view)

Winooski Falls, Photo by Jim Westphalen
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Channel dimensions
Geometry or cross-sectional shape of the channel
see also Ward Trimble, Fig 6.13

• Width, depth
• Bankfull width, depth (i.e. effective)
• Floodplain
• Terrace (abandoned floodplain)

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Helical flow creates variations in shear stress
across the cross section
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Channel dimensions
see also Ward Trimble, Fig 6.13
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What happens if force-resistance balance is
disturbed?
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Channel evolution
Graphic from A. Ward after Schumm
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Channel Profile
Longitudinal relationship between elevation and
distance downstream (slope)
Elevation
Distance downstream ?
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Channel Profile
Ward Trimble, Fig 6.8
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Channel Profile
Ward Trimble, Fig 6.27 (from Montgomery
Buffington, 1997. GSA Bulletin, 109, 596-611.)
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Channel Pattern
Plan (aerial) view of channel geometry
Meandering Stream
Braided Stream
Ward Trimble, Fig 6.10
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Channel Pattern
Sinuosity (S) Lc / Lv
where Lc channel length Lv valley length
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Helical flow induces meander formation
Cut banks
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Meander Geometry
Ward Trimble, Fig 6.11
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How do these concepts scale?
Graphic from A. Ward
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How do these concepts scale?
Hydraulic Geometry of stream channels
Ward Trimble, Fig 6.15