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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 – PowerPoint PPT presentation

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Title: Fundamental Concepts in Fluvial Geomorphology


1
Fundamental Concepts in Fluvial Geomorphology
Andrew Simon USDA-ARS National Sedimentation
Laboratory, Oxford, MS asimon_at_msa-oxford.ars.usda
.gov
2
Three Zones of the Fluvial System
3
Force, Resistance, and Form
4
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

5
Stream Types Schumm
6
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

7
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
8
Stream Power Thresholds
9
Time Scales in Geomorphology
Gradient
Cyclic time
Gradient
Graded time
10
Variables Change Dependency as a Function of Time
Scale!
11
Types of Equilibrium Thresholds?
12
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
13
Response to Disturbance
Compression of time scales following large-scale
disturbances natural or anthropogenic
1,000,000 years 100 years
14
Trends of Incision Channelization
15
Hydraulic Shear Stress Force and Resistance
16
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.
17
Bed-Level Response
18
Magnitude of Width Adjustments
Why are they so different ?
19
Basic Failure Types
20
Forces Affecting Soil Shear Strength
Shear surface
21
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
22
Changes in Width/Depth During Adjustment
23
Idealized Adjustment TrendsFor a given discharge
(Q)
t gVS Se
n tc d
24
This Will Also Be on The Test
Applied (Driving) Forces versus Resisting Forces
  • Hydraulic processes (bed, bank toe)
  • Geotechnical processes (bank mass)

25
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

26
Stages of Channel Evolution(just another
empirical model)
  • References
  • Stage I
  • Stage VI

27
Stage and Bed Material Yield
28
Stage and Suspended Sediment Transport
29
Stage of Channel Evolution
30
What Processes are Active?
31
Whats Happenin Here?
32
What Processes are Active?
Where do they change, and why?
33
What Processes are Active?
34
What Processes are Active?
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