Fundamental Concepts in
Fluvial Geomorphology
Andrew Simon
USDA-ARS National Sedimentation Laboratory,
Oxford, MS
asimon@msa-oxford.ars.usda.gov
National Sedimentation Laboratory
Three Zones of the Fluvial System
Force, Resistance, and Form
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
Stream Types: Schumm
Continuity Equation
Q = w yV =AV
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
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
Stream Power: Thresholds
Gradient
Time Scales in Geomorphology
Graded time
Gradient
Cyclic time
Steady time
Graded time
Variables Change Dependency
as a Function of Time Scale!
Types of Equilibrium: Thresholds?
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
Gradient
Response to Disturbance
Graded time
Cyclic time
Compression of time scales following large-scale
disturbances: “natural” or anthropogenic
1,000,000 years = 100 years
Trends of Incision: Channelization
Hydraulic Shear Stress; Force and
Resistance
to = g R S
to = mean boundary shear stress
g = unit weight of water
R = hydraulic radius = A / 2y + w
S = channel gradient
t* = to / [ (gs – gw) * d]
t* = dimension less shear stress
gs = unit weight of sediment
d = characteristic particle diameter
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.
Bed-Level Response
Magnitude of Width Adjustments
Why are they
so different ?
Basic Failure Types
Forces Affecting Soil Shear Strength
Cohesion
– chemical bonds
between particles
Normal load – weight of
bank increases friction
Friction interparticle roughness
Matric suction –
apparent cohesion
Pore-water pressure –
reduces effective
friction
Bank Stability – The Factor of Safety
Factor of Safety (Fs) =
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 – Fs>1.3 is
stable.)
Resisting Forces
soil strength
vegetation
reinforcement
Driving Forces (gravity)
bank angle
weight of bank
water in bank
Changes in Width/Depth During
Adjustment
Idealized Adjustment Trends
For a given discharge (Q)
t
gVS
Se
n
tc
d
This Will Also Be on The Test
Applied (Driving) Forces
versus
Resisting Forces
• Hydraulic processes (bed, bank toe)
• Geotechnical processes (bank mass)
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
National Sedimentation Laboratory
Stage I. Sinuous, Premodified
h<hc
Stages of Channel Evolution
(just another empirical model)
hc = critical bank height
= direction of bank or
bed movement
h
Stage I. Sinuous, Premodified
h<hc
Stage IV. Degradation and
Widening
h>hc
Stage II. Constructed
Stage III. Degradation
hc = critical bank height
h<hc
h<hc
terrace
floodplain = direction of bank or
bed movement
h
h
h
h
slumped material
Stage II. Constructed
h<hc hc = critical bank height
= direction of bank or
floodplain
Stage IV. Degradation and
Stage VI. Quasi Equilibrium
Widening
Stage
Aggradation and Widening
Stage
III.V.Degradation
h<hc
h>hc
h>h
h<h
c c
terrace
terrace
terrace
bed movement
h
h
h
h
h
bank
bankfull
slumped
material
References
aggraded material
slumped material
Stage IV. Degradation and
Widening
Stage III. Degradation
Stages I, II
h>hc
h<hc
Stage VI. Quasi Equilibrium
Stage V. Aggradation and Widening
primary
terrace
h<hc
h>hc
knickpoint
Stage III
Stage IV
terrace
terrace
Stage V
top bank
h
h
plunge
bank
precursor
h
pool
knickpoint
h
direction o
bankfull
f flow
slumped
slumped material
secondary
material
knickpoint
aggradation
oversteepened reach
aggraded
material
aggraded
material zone
Stage VI. Quasi Equilibrium
ening
h<hc
aggraded material
•Stage I
•Stage VI
Stages I, IIterrace
primary
h
knickpoint
bank
National
Stage III Sedimentation Laboratory
Stage IV
bankfull
Stage VI
aggraded material
BED-MATERIAL YIELD, IN T/YR/MI
2
Stage and Bed Material Yield
1000
100
10
Raw data
Median values
1
0
1I
II
2
III
3
IV
4
V
5
STAGE OF CHANNEL EVOLUTION
VI
6
7
Stage and Suspended Sediment
Transport
SLOPE OF SUSPENDED SEDIMENT
RATING RELATION
3.0
2.5
2.0
1.5
1.0
0.5
West Tennessee sites (Simon, 1989a)
Goodwin Creek and Toutle River sites
1I
II
2
III
3
IV
4
V
5
STAGE OF CHANNEL EVOLUTION
VI
6
Stage of Channel Evolution
Colorado
New Mexico
Animas River
Estes Arroyo
Pump Canyon
Reach 5
La Plata River
Horse Canyon
Aztec
Flora Vista
Gobernador
Canyon
Reach 4
Reach 3
Farmington
Bloomfield
Kirtland
Reach 1
Reach 2
Cañon Largo
Stage of channel evolution
III
IV
V
Armenta Canyon
VI
Kutz Canyon
San Juan River
0
20
Gallegos Canyon
kilometers
What Processes are Active?
What’s Happenin’ Here?
What Processes are Active?
Where do they change, and why?
National Sedimentation Laboratory
What Processes are Active?
National Sedimentation Laboratory
What Processes are Active?
National Sedimentation Laboratory