Semi-alluvial channels
GBR 7, Tadoussac 2010
Semi-alluvial channels
and
sediment-flux-driven bedrock erosion
Jens M. Turowski
Swiss Federal Research Institute WSL
Gravel Bed Rivers 7, Tadoussac, Canada, September 2010
With thanks to: D. Lague, N. Hovius, C. Stark, J. Barbour, D. Rickenmann,
M.-L. Hsieh, M.-J. Horng, M.-C. Chen, H. Chen, A. Wilson, A. Beer, A.
Badoux, all of you who wrote great papers, and many others
Semi-alluvial channels
GBR 7, Tadoussac 2010
Some semi-alluvial channels
Semi-alluvial channels
GBR 7, Tadoussac 2010
Some semi-alluvial channels
Semi-alluvial channels
GBR 7, Tadoussac 2010
Some semi-alluvial channels
Semi-alluvial channels
GBR 7, Tadoussac 2010
Some semi-alluvial channels
Semi-alluvial channels
GBR 7, Tadoussac 2010
Some semi-alluvial channels
Semi-alluvial channels
GBR 7, Tadoussac 2010
Questions
• How do these different types of channel form?
• What is the influence of the sediment on channel
morphology?
Semi-alluvial channels
GBR 7, Tadoussac 2010
Bedrock channels
• Various definitions…
All rivers actively
incising into
bedrock
Where bedrock
(walls, bed…) limits
the dynamic
evolution of the river
Where rock is exposed
widely
Where alluvial cover is
thin and is mobilised
during floods
Semi-alluvial channels
GBR 7, Tadoussac 2010
Objectives
• Demonstrate the importance of sediment
in the dynamics of bedrock channels
– In general, bedrock channels are semialluvial!
Semi-alluvial channels
GBR 7, Tadoussac 2010
Objectives
• Demonstrate the importance of sediment
in the dynamics of bedrock channels
– In general, bedrock channels are semialluvial!
• Convince you that some widely used
bedrock incision laws are incorrect
Semi-alluvial channels
GBR 7, Tadoussac 2010
Objectives
• Demonstrate the importance of sediment
in the dynamics of bedrock channels
– In general, bedrock channels are semialluvial!
• Convince you that some widely used
bedrock incision laws are incorrect
• Argue that sediment-flux-dependent
incision can account for channel forms and
morphology
Semi-alluvial channels
GBR 7, Tadoussac 2010
Controls on channel morphology
• It‘s complicated…
Upstream controls
Lithology
Tectonics
Local controls (fixed)
Substrate
- lithology
- jointing
- weathering
Valley morphology
- sinuosity
- width
- depth
- steepness
History
Climate
Discharge
Sediment Supply
Reach morphology
river type
Humans
Local controls (variable)
Floods
Vegetation
Humans
- building projects
- land use
Reach variability
Base level
Length
Downstream controls
adapted from Schumm, River Variability
and Complexity, CUP 2005
Semi-alluvial channels
GBR 7, Tadoussac 2010
Controls on channel morphology
• Steady state channels…
– Fixed point in dynamics
– Local controls only on morphology
• Need to understand steady state to understand
dynamic behaviour
Semi-alluvial channels
GBR 7, Tadoussac 2010
Controls on channel morphology
• Steady state channels...
• Upstream supply
• Base level / uplift
• Substrate
– Water
– Sediment
Sediment supply
Qs
Sediment discharge
Uplift
Semi-alluvial channels
GBR 7, Tadoussac 2010
Controls on channel morphology
• The stream has two jobs to do:
– Transport the supplied sediment
– Incise the bedrock at the uplift rate
Sediment supply
Qs
Sediment discharge
Uplift
Semi-alluvial channels
GBR 7, Tadoussac 2010
End-member incision models
• Possibility 1: Incision is of dominant importance
– Detachment-limited model
dh
U  E
dt
Discharge
Erosion rate
E  kQ S
m
n
• Possibility 2: Transport is of dominant importance
(alluvial rivers)
Slope
– Transport-limited model
Bedload transport equation
Qt
dh
U 
dt
W
Qt  kbW    c 
3
2
Semi-alluvial channels
GBR 7, Tadoussac 2010
Problems
• Detachment-limited and transport-limited
models are inconsistent with each other
• Neither of the models adequately
describes field data
Picture just for your entertainment…
Semi-alluvial channels
GBR 7, Tadoussac 2010
Transient behaviour
Detachment-limited: advection
Knickpoint propagation
Many field examples.
Transport-limited: diffusion
Few examples, but some.
Slide adapted from D. Lague
Semi-alluvial channels
GBR 7, Tadoussac 2010
Transient behaviour
Example: Post-glacial gorge incision in the Alps
(Valla, Van der Beek and Lague, JGR, 2010)
Detachment-limited
Transport-limited
Original profile
Final profile
Longitudinal distance
Some mixed form
of behaviour….
Slide adapted from D. Lague
Elevation
Elevation
Original profile
Final profile
Longitudinal distance
Semi-alluvial channels
GBR 7, Tadoussac 2010
More problems
• Most incising streams are semi-alluvial
Semi-alluvial channels
GBR 7, Tadoussac 2010
More problems
• Most incising streams are semi-alluvial
• In many environments, bedrock incision occurs
due to the impact of moving particles
Semi-alluvial channels
GBR 7, Tadoussac 2010
More problems
• Most incising streams are semi-alluvial
• In many environments, bedrock incision occurs
due to the impact of moving particles
• The effect of sediment flux on incision rates has
been demonstrated both in the laboratory and in
the field (tools and cover effects)
• Sediment-flux-dependent incision models may
be an alternative…
Semi-alluvial channels
GBR 7, Tadoussac 2010
Steepness of channel walls
Taiwan: Alluvial channels
0.7
Exponent
Exponent
0.6
0.5
0.4
0.3
Taiwan: Bedrock channels
0.2
0.7
0.1
100
1000
0.6
Mean Concentration / ppm
Exponent
Exponent
Mean sediment concentration
Measure of
bank steepness
From Turowski et al., Geomorphology 2008
0.5
0.4
0.3
West
East
North
0.2
0.1
100
1000
Sediment Concentration / ppm
Mean sediment concentration
Semi-alluvial channels
GBR 7, Tadoussac 2010
Tools and cover effects
Tools effect
• Impacting particles remove rock
– More particles = higher erosion
rates
Impact marks on a marble
surface (from Wilson, Thesis 2009)
Cover effect
• Particles cover and protect the bed
– More particles = smaller erosion
rates
Partly covered bed in a
mountain stream in Taiwan
Semi-alluvial channels
GBR 7, Tadoussac 2010
Tools and cover effects
Cover-dominated
Erosion rate
Tools-dominated
Impact marks on a marble
surface (from Wilson, Thesis 2009)
Linear cover model
Exponential cover model
0.0
0.5
1.0
Relative sediment supply Qs/Qt
1.5
Partly covered bed in a
mountain stream in Taiwan
Semi-alluvial channels
GBR 7, Tadoussac 2010
Example: Erosion experiments
• Demonstrate tools
• Sklar and Dietrich,
and cover effects and
Geology 2001
• Sediment in an erosion influence of grain size
mill
35
Limestone
Andesite
Mudstone
Exponential model
Linear model
Normalized Erosion Rate
2
/ (g/hr)*(MPa)
30
25
20
15
10
5
0
0
200
400
600
Sediment mass / g
800
1000
Machine a Lavé,
Attal et al. JHE 2006
Semi-alluvial channels
GBR 7, Tadoussac 2010
Long-term landscape evolution
5
Parabolic model
4
Erosional efficiency
• Cowie et al., Geology
2008
• Field sites in Italy and
Greece
• Clear evidence for
‘long-term’ tools and
cover effects
Xerias
3
Rio Torto
2
1
Torrente L'Alpa
0
0.0
0.2
0.4
Voagris
0.6
0.8
1.0
Relative Sediment Supply (a proxy for Qs/Qt)
Semi-alluvial channels
GBR 7, Tadoussac 2010
Cover/tools effect and channel dynamics
• Asymmetry of erosion
between channel walls and
floor
– Cover effect inactive (less
active) on walls
• High sediment flux – cover
effect dominates – increased
erosion on the wall
• Low sediment flux – tools
effect dominant – increased
erosion on the floor
Semi-alluvial channels
GBR 7, Tadoussac 2010
Steepness of channel walls
0.7
0.6
Taiwan: Bedrock channels
Measure of
bank steepness
Exponent
Exponent
0.5
0.4
Steeper banks
0.3
West
East
North
0.2
0.1
100
1000
Sediment Concentration / ppm
Mean
sediment concentration
From Turowski et al., Geomorphology 2008
Semi-alluvial channels
GBR 7, Tadoussac 2010
Erosion at Lushui, Liwu
Dry season
Typhoon Bilis
From Hartshorn et al., Science, 2002
• Lateral erosion high for large floods
• Vertical erosion high for small and medium
flows
Semi-alluvial channels
GBR 7, Tadoussac 2010
Typhoon Long-Wang
Lushui Station before (July 2004) and after (December
2005) Taiphoon Long-Wang, 1st October 2005
From Turowski et al., ESPL 2008
Semi-alluvial channels
GBR 7, Tadoussac 2010
Incision and cover
Not to
scale of
picture
• Cumulative erosion at Lushui during 2005
• Maximum incision at current terrace level in
quartzite (black line)
From Turowski et al., ESPL 2008
Semi-alluvial channels
GBR 7, Tadoussac 2010
Conceptual model
C Q  Qc  Q  Qc
Qt  
0
Q  Qc

• Model sediment
supply with a powerlaw

Qs  cQ
10
3
Sediment transport rate / m /s
• Transport capacity
scales ~linearly with
discharge
Sediment supply Qs
9
8
7
6
5
4
Qs < Qt
Qs > Qt
Evacuation
Deposition
3
2
Sediment transport capacity Qt
1
200
400
600
3
Discharge / m /s
Exponent determines dynamics
800
1000
Semi-alluvial channels
GBR 7, Tadoussac 2010
Conceptual model
• First possibility – λ>1 (Liwu River)
3
Sediment transport rate / m /s
10
Small and medium
events evacuate
sediment or incise
the thalweg
Sediment supply Qs
9
8
7
6
5
4
Qs < Qt
Qs > Qt
Evacuation
Deposition
Large events
deposit sediment
3
2
Sediment transport capacity Qt
1
200
400
600
800
3
Discharge / m /s
Field examples:
• Liwu River (Hartshorn et al., Science
2002; Turowski et al., ESPL 2008)
• Henry Mts (Johnson et al., JGR 2010)
1000
Semi-alluvial channels
GBR 7, Tadoussac 2010
Dynamic model: SSTRIM
• This behaviour has been shown to occur in dynamic
models of channel geometry (SSTRIM, Lague, JGR 2010; also
Howard, in Rivers over Rock, 1998)
80
Q*
60
40
Discharge
20
0
125
130
135
140
145
150
130
135
140
145
150
135
140
145
150
140
145
150
Sed. thickness
Ts (m)
4
3
2
1
0
125
Bed incision
Ibed/U
100
1
0.1
Ibank / (U cos )
0.01
125
Wall incision
steady-state
10
130
100
10
1
0.1
125
steady-state
130
135
Years
Semi-alluvial channels
GBR 7, Tadoussac 2010
Conceptual model
• Second possibility – λ<1
3
Sediment transport rate / m /s
5
Channel behaves
essentially alluvial
at low flow
Sediment transport capacity Qt
4
3
2
Qs < Qt
Qs > Qt
Sediment evacuation
and erosion during
floods
Deposition
Evacuation and erosion
1
Sediment supply Qs
200
400
600
800
3
Discharge / m /s
Field examples
• none yet, but many candidates…
1000
Semi-alluvial channels
GBR 7, Tadoussac 2010
Conclusions
• Both incision and sediment transport are
important!
– Bedrock channels are semi-alluvial in general
Semi-alluvial channels
GBR 7, Tadoussac 2010
Conclusions
• Both incision and sediment transport are
important!
– Bedrock channels are semi-alluvial in general
• Using sediment-flux-dependent incision laws, we
can predict
– Conceptually different channel types
– Width and slope scaling of natural channels (not
demonstrated here)
Semi-alluvial channels
GBR 7, Tadoussac 2010
Conclusions
• Both incision and sediment transport are
important!
– Bedrock channels are semi-alluvial in general
• Using sediment-flux-dependent incision laws, we
can predict
– Conceptually different channel types
– Width and slope scaling of natural channels (not
demonstrated here)
• A single representative flood is not sufficient to
describe channel dynamics
Semi-alluvial channels
GBR 7, Tadoussac 2010
Thanks!
Any questions?