Semi-alluvial channels and sediment-flux-driven bedrock erosion
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Transcript of Semi-alluvial channels and sediment-flux-driven bedrock erosion
Semi-alluvial channels GBR 7, Tadoussac 2010
Semi-alluvial channelsand
sediment-flux-driven bedrock erosion
Jens M. Turowski
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
Gravel Bed Rivers 7, Tadoussac, Canada, September 2010
Swiss Federal Research Institute WSL
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 rock is exposed widely
Where alluvial cover is thin and is mobilised during floodsWhere bedrock
(walls, bed…) limits the dynamic evolution of the river
Semi-alluvial channels GBR 7, Tadoussac 2010
Objectives• Demonstrate the importance of sediment
in the dynamics of bedrock channels– In general, bedrock channels are semi-
alluvial!
Semi-alluvial channels GBR 7, Tadoussac 2010
Objectives• Demonstrate the importance of sediment
in the dynamics of bedrock channels– In general, bedrock channels are semi-
alluvial!
• 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 semi-
alluvial!
• 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 Climate History Humans
DischargeSediment Supply
Reach morphologyriver type
Reach variability
Local controls (variable)FloodsVegetationHumans- building projects- land use
Local controls (fixed)Substrate- lithology- jointing- weatheringValley morphology- sinuosity- width- depth- steepness 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
• Upstream supply– Water– Sediment
• Base level / uplift• Substrate
• Steady state channels...
Bedrock
Alluvium
Uplift
Incision
Qs
Sediment supply
Sediment discharge
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
Bedrock
Alluvium
Uplift
Incision
Qs
Sediment supply
Sediment discharge
Semi-alluvial channels GBR 7, Tadoussac 2010
End-member incision models• Possibility 1: Incision is of dominant importance
– Detachment-limited model
• Possibility 2: Transport is of dominant importance (alluvial rivers)– Transport-limited model
EUdt
dh
nmSkQE Erosion rate
Discharge
Slope
W
QU
dt
dh t 23
cbt WkQ
Bedload transport equation
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 behaviourKnickpoint propagationDetachment-limited: advection
Transport-limited: diffusionMany field examples.
Slide adapted from D. Lague
Few examples, but some.
Semi-alluvial channels GBR 7, Tadoussac 2010
Transient behaviourExample: Post-glacial gorge incision in the Alps(Valla, Van der Beek and Lague, JGR, 2010)
Detachment-limited Transport-limited
Some mixed form of behaviour….
Slide adapted from D. Lague Longitudinal distanceLongitudinal distance
Ele
vatio
n
Ele
vatio
n
Final profile
Original profile
Final profile
Original profile
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
100 10000.1
0.2
0.3
0.4
0.5
0.6
0.7
Exp
onen
t
Mean Concentration / ppm
100 10000.1
0.2
0.3
0.4
0.5
0.6
0.7
West East North
Exp
on
en
t
Sediment Concentration / ppm
Taiwan: Alluvial channels
Taiwan: Bedrock channels
Measure ofbank steepness
Mean sediment concentration
Mean sediment concentration
Exp
onen
t
Exp
onen
t
From Turowski et al., Geomorphology 2008
Semi-alluvial channels GBR 7, Tadoussac 2010
Tools and cover effectsTools effect
• Impacting particles remove rock– More particles = higher erosion
rates
Cover effect
• Particles cover and protect the bed– More particles = smaller erosion
rates
Impact marks on a marble surface (from Wilson, Thesis 2009)
Partly covered bed in a mountain stream in Taiwan
Semi-alluvial channels GBR 7, Tadoussac 2010
Tools and cover effects
Impact marks on a marble surface (from Wilson, Thesis 2009)
Partly covered bed in a mountain stream in Taiwan
0.0 0.5 1.0 1.5
Cover-dominated
Ero
sio
n r
ate
Relative sediment supply Qs/Q
t
Linear cover model Exponential cover model
Tools-dominated
Semi-alluvial channels GBR 7, Tadoussac 2010
Example: Erosion experiments• Sklar and Dietrich,
Geology 2001• Sediment in an erosion
mill
0 200 400 600 800 10000
5
10
15
20
25
30
35
Limestone Andesite Mudstone Exponential model Linear model
Sediment mass / g
No
rma
lize
d E
rosi
on
Ra
te
/ (g
/hr)
*(M
Pa
)2
• Demonstrate tools and cover effects and influence of grain size
Machine a Lavé,Attal et al. JHE 2006
Semi-alluvial channels GBR 7, Tadoussac 2010
Long-term landscape evolution• Cowie et al., Geology
2008• Field sites in Italy and
Greece• Clear evidence for
‘long-term’ tools and cover effects 0.0 0.2 0.4 0.6 0.8 1.0
0
1
2
3
4
5
Ero
sion
al e
ffici
ency
Relative Sediment Supply (a proxy for Qs/Q
t)
Rio Torto
Xerias
Torrente L'AlpaVoagris
Parabolic model
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
100 10000.1
0.2
0.3
0.4
0.5
0.6
0.7
West East North
Exp
on
en
t
Sediment Concentration / ppm
Taiwan: Bedrock channels
Measure ofbank steepness
Mean sediment concentration
Exp
onen
t
Steeper banks
From Turowski et al., Geomorphology 2008
Semi-alluvial channels GBR 7, Tadoussac 2010
Erosion at Lushui, Liwu
• Lateral erosion high for large floods• Vertical erosion high for small and medium flows
From Hartshorn et al., Science, 2002
Dry season
Typhoon Bilis
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
• Cumulative erosion at Lushui during 2005• Maximum incision at current terrace level in
quartzite (black line)
Not to scale of picture
From Turowski et al., ESPL 2008
Semi-alluvial channels GBR 7, Tadoussac 2010
Conceptual model• Transport capacity
scales ~linearly with discharge
• Model sediment supply with a power-law
200 400 600 800 1000
1
2
3
4
5
6
7
8
9
10
Qs > Q
t
Se
dim
en
t tra
nsp
ort
ra
te /
m3/s
Discharge / m3/s
Sediment supply Qs
Sediment transport capacity Qt
Qs < Q
t
Evacuation Deposition
cQQs
c
cct QQ
QQQQCQ
0
Exponent determines dynamics
Semi-alluvial channels GBR 7, Tadoussac 2010
Conceptual model• First possibility – λ>1 (Liwu River)
200 400 600 800 1000
1
2
3
4
5
6
7
8
9
10
Qs > Q
t
Sed
imen
t tra
nspo
rt r
ate
/ m3/s
Discharge / m3/s
Sediment supply Qs
Sediment transport capacity Qt
Qs < Q
t
Evacuation DepositionSmall and medium events evacuate sediment or incisethe thalweg
Large events deposit sediment
Field examples:• Liwu River (Hartshorn et al., Science 2002; Turowski et al., ESPL 2008)• Henry Mts (Johnson et al., JGR 2010)
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)
125 130 135 140 145 1500
1
2
3
4
Ts (
m)
125 130 135 140 145 1500.01
0.1
1
10
100 steady-state
I bed
/U
125 130 135 140 145 1500
20
40
60
80
Q*
125 130 135 140 145 1500.1
1
10
100
steady-state
Years
I ban
k /
(U c
os )
Discharge
Sed. thickness
Bed incision
Wall incision
Semi-alluvial channels GBR 7, Tadoussac 2010
Conceptual model• Second possibility – λ<1
200 400 600 800 1000
1
2
3
4
5
Qs > Q
t
Se
dim
en
t tra
nsp
ort
ra
te /
m3 /s
Discharge / m3/s
Sediment supply Qs
Sediment transport capacity Qt
Qs < Q
t
Deposition
Evacuation and erosion
Channel behaves essentially alluvial at low flow
Sediment evacuation and erosion during floods
Field examples• none yet, but many candidates…
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