Fundamental Concepts in Fluvial Geomorphology

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Fundamental Concepts in Fluvial Geomorphology
Andrew Simon USDA-ARS National Sedimentation
Laboratory, Oxford, MS asimon_at_msa-oxford.ars.usda
.gov
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Three Zones of the Fluvial System
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Force, Resistance, and Form
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Force and Resistance(Or what it takes to
initiate movement (erosion) of material)

• Think in terms of SPECIFIC PROCESSES
• On the stream bed
• Force/resistance
• On the stream banks
• Force/resistance

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Stream Types Schumm
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Continuity Equation

• Q w y V A V
• Q water discharge m3/s
• w flow width, in m
• y flow depth, in m
• V flow velocity, in m/s
• A cross-sectional area in m2

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Stream Power Proportionality

• gQS a Qsd50

g unit weight of water Q water discharge S
bed or energy slope Qs bed-material
discharge d50 median particle size of bed
material
Thus, streams are open systems with an ability to
adjust to altered energy inputs
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Stream Power Thresholds
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Time Scales in Geomorphology
Gradient
Cyclic time
Gradient
Graded time
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Variables Change Dependency as a Function of Time
Scale!
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Types of Equilibrium Thresholds?
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Why is Fluvial Geomorphology Important?This Will
Be on the Test

• gQS a Qsd50

g unit weight of water Q water discharge S
bed or energy slope Qs bed-material
discharge d50 median particle size of bed
material
Thus, streams are open systems with an ability to
adjust to altered energy inputs
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Response to Disturbance
Compression of time scales following large-scale
disturbances natural or anthropogenic
1,000,000 years 100 years
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Trends of Incision Channelization
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Hydraulic Shear Stress Force and Resistance
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Erosion Rate is a Function of Erodibility and
Excess Shear Stress
te (to-tc) or e k (to- tc) e erosion rate
(m/s) k erodibility coefficient (m3/N-s) to
boundary shear stress (Pa) tc critical shear
stress (Pa) (to-tc) excess shear
stress Critical shear stress is the stress
required to initiate erosion.
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Bed-Level Response
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Magnitude of Width Adjustments
Why are they so different ?
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Basic Failure Types
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Forces Affecting Soil Shear Strength
Shear surface
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Bank Stability The Factor of Safety

• Resisting Forces
• Driving Forces
• If Fs is greater than 1, bank is stable. If Fs
  is less than 1 bank will fail. (We usually add a
  safety margin Fsgt1.3 is stable.)

Factor of Safety (Fs)
Resisting Forces Driving Forces (gravity) soil
strength bank angle vegetation weight of bank
reinforcement water in bank
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Changes in Width/Depth During Adjustment
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Idealized Adjustment TrendsFor a given discharge
(Q)
t gVS Se
n tc d
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This Will Also Be on The Test
Applied (Driving) Forces versus Resisting Forces

• Hydraulic processes (bed, bank toe)
• Geotechnical processes (bank mass)

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Process, Process, Process
Use Form to Tell Us About Process

• Channel Evolution Models Use Form to Infer
  Process
• Schumm et al., 1984
• Simon and the Hupp, 1986 Simon, 1989

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Stages of Channel Evolution(just another
empirical model)

• References
• Stage I
• Stage VI

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Stage and Bed Material Yield
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Stage and Suspended Sediment Transport
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Stage of Channel Evolution
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What Processes are Active?
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Whats Happenin Here?
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What Processes are Active?
Where do they change, and why?
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What Processes are Active?
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What Processes are Active?