lecture
TRANSCRIPT
Sediments and Sediments and bedrock erosionbedrock erosion
Mikaël ATTALMikaël ATTAL
Marsyandi valley, Himalayas, Nepal
Acknowledgements: Acknowledgements: Jérôme Lavé, Peter van Jérôme Lavé, Peter van der Beeder Beek and other scientists from LGCA k and other scientists from LGCA
(Grenoble) and CRPG (Nancy)(Grenoble) and CRPG (Nancy)
Eroding landscapes: Eroding landscapes: fluvial processesfluvial processes
Lecture overviewLecture overview
I. Field testing of fluvial erosion laws: do sedimentI. Field testing of fluvial erosion laws: do sediments matter?s matter?
II. How do sediments modulate bedrock erosion raII. How do sediments modulate bedrock erosion rates? tes?
IIIII. What does control sediment characteristics in bedrock I. What does control sediment characteristics in bedrock rivers?rivers?
E = KAmSn.f(qs) Stream Power Law(s) (laws 1, 2, 3): f(qs) = 1
Threshold for erosion (law 4), slope set by necessity for river to transport sediment downstream (law 5), cover effect (law 6), tools + cover effects (law 7).
Similar predictions at SS: concave up profile with power relationship between S and A.
Different predictions in terms of transient response of the landscape to perturbation.
Laws including the role of the sediments: f(qs) ≠ 1
General form: fluvial incision laws
I. Field testing of fluvial incision laws (1)
Basal shear stress:
Fluvial erosion law:
V
D
Excess shear stress model (law 4): Lavé & Avouac, 2001
τ = ρ g R S, where R = WD / (W+2D)
τ = ρ g D S, if W >10D.
E = K (τ - τc)
Fluvial incision along Himalayan rivers
MFT
Fluvial incision measured using terraces
[Lavé and Avouac, 2001]
Comparison between fluvial incision (terraces) and excess shear stress (channel geometry)
Shields stress (non-dimensional):
τ* = τ / (ρs – ρ)gD50
E = K (τ* - τc*)Independent
measurements: E from terraces and τ
from channel geometry.
τc* value used = 0.03
See Buffington and Montgomery, 1997, for extensive description of the critical shear
stress concept.
Important role of lithology
Modified from Lavé & Avouac, 2001
TSSHHCLHS
Lavé & Avouac, 2001: maximum fluvial erosion rate in the HHC zone for 6 main Himalayan rivers
Modified from Lavé & Avouac, 2001
TSSHHCLHS
Lavé & Avouac, 2001: maximum fluvial erosion rate in the HHC zone for 6 main Himalayan rivers
All laws predict similar steady-state topographies (concave-up profile, etc.),
Predicted transient response to a disturbance depends on the fluvial incision law chosen.
I. Field testing of fluvial incision laws (2)Using the transient response of the landscape
(2002) (2002)
Detachment-limited law (SPL, laws 1, 2, 3) Transport limited law (law 5)
Transient response of fluvial systems
(2002) (2002)
Transient response of fluvial systemsDetachment-limited law (SPL, laws 1, 2, 3) Transport limited law (law 5)
(2002) (2002)
Transient response of fluvial systemsDetachment-limited law (SPL, laws 1, 2, 3) Transport limited law (law 5)
http://www.phys.uu.nl/~gdevries/maps/maps.cgi
Fiamignano,Italy
Xerias,Greece
0 600 km
Transient response to tectonic disturbance
(Whittaker et al., 2007a, b, 2008; Cowie et al., 2008, Attal et al., 2008)
Fiamignano, Italy Xerias, Greece
Transient response to tectonic disturbance
(Whittaker et al., 2007a, b, 2008; Cowie et al., 2008, Attal et al., 2008)
Fiamignano,Italy
Xerias, Greece
Transient response to tectonic disturbanceItaly closer to DL end-member, Greece closer to TL end-member
(Cowie et al., 2008)
SEDIMENTS DO MATTER!
Ero
sion
eff
icie
ncy
f(Q
s)
Qs/Qc
0 1
Sklar & Dietrich, 2001
Role of sediment: the “tools and cover” effects (Gilbert, 1877)
Experimental study of bedrock abrasion by saltating particles
Tools
Cover
II. How do sediments modulate bedrock erosion rates?
Sklar & Dietrich, 1998, 2004
c
s
f
s
Q
Q
LW
QE 1
Role of sediment: the “tools and cover” effects
2004: mechanistic
1998: theoretical
E = ViIrFe
Vi = volume of rock detached / particle impact,Ir = rate of particle impacts per unit area per unit time,Fe = fraction of the river bed made up of exposed bedrock.
Turowski et al., 2007
Role of sediment: the “tools and cover” effects
Ero
sion
eff
icie
ncy
f(Q
s)
Qs/Qc0 1Sediment SUPPLY / Qc
Sediment SUPPLY ≤ Qc
Qs/Qc = Sediment SUPPLY / Qc
Sediment SUPPLY > Qc
Qs/Qc = 1
Sklar & Dietrich Turowski et al.
Maximum bedrock erosion for sediment supply = Qc (“dynamic cover effect”)
Effect of grain size? (Sklar & Dietrich, 2004)
Role of sediment: the “tools and cover” effects
But very simplistic model: bedload is made of only 1 grain size!
Effect of grain size? Bedload is made of a wide range of grain sizes
Role of sediment: the “tools and cover” effects
At low flow: bedload is motionless and protects bedrock from erosion.
During floods, the smallest particles will be put in motion ( tools) while the largest might remain
motionless ( cover): difference in sediment MOBILITY will affect
bedrock erosion
Not only size will affect sediment mobility: interactions between particle will do it as well (e.g.
patches, gravel-bars)
Movement probability
0.05 0.050.05
0.05 0.050.05
0.2 0.2
Sediment mobility in bedrock rivers
Courtesy Rebecca Hodge, University of Glasgow
Cellular Automata model
Hodge et al., work in progress
Role of sediment: the “tools and cover” effects
Ero
sion
eff
icie
ncy
f(Q
s)
Qs/Qc0 1Sediment SUPPLY / Qc
Sediment SUPPLY ≤ Qc
Qs/Qc = Sediment SUPPLY / Qc
Sediment SUPPLY > Qc
Qs/Qc = 1
Sklar & Dietrich
Turowski et al.
Lower erosion rates for higher sediment supply because of increasing likelihood of jams
Calder River, Renfrewshire
Characterizing sediment mobility in the field
Schmeeckle et al.
Characterizing sediment mobility in the lab
http://www.markschmeeckle.com/
Modelling sediment motion…
Ideally, we would include pebble and bedrock abrasion in such models. But the computer that can do that efficiently doesn’t exist yet…
What does control the characteristics of sediments in (1)?
(2)
(3)(1)
http://projects.crustal.ucsb.edu/nepal/
(2) Characteristics of the source of sediment (location, amount, grain size distribution, lithology)
(3) Transport and abrasion processes along the channel
III. What does control sediment characteristics in bedrock rivers? Bedrock erosion in (1) will depend on sediment characteristics in (1):
- what proportion of sediment is bedload? ( tools and cover)- what is the grain size distribution of the bedload? ( for a given flood, what will be tools, what will be cover, and how efficient the tools will be)- what is the lithologic content of the bedload? ( how efficient the tools will be)
Sediment mobility
Sediments entering the channel are usually angular
Marsyandi River, Nepal
III. What does control sediment characteristics in bedrock rivers? Pebble abrasion during fluvial transport
Angular pebbles in the river
Marsyandi River, Nepal
Pebbles are abraded during fluvial transport
Each pebble is reduced in size and gets more rounded
III. What does control sediment characteristics in bedrock rivers? Pebble abrasion during fluvial transport
Common pebble abrasion processes:
Marsyandi River, Nepal
Chipping Crushing
Cracking
Splitting
Grinding
These processes reduce the size of pebbles and tend to make them more rounded
III. What does control sediment characteristics in bedrock rivers?
Downstream fining?
Not necessarily, because fresh
material is added along the river course in
mountain rivers (≠
alluvial rivers)
III. What does control sediment characteristics in bedrock rivers?
Pebble abrasion during fluvial transport
Change in rock type proportion?
If the 2 rock types are
eroded at the same rate
A
B
Distance downstream (km)A B
Rock-type content in bedload (coarse fraction, > ~1 mm)
100%
50%
0%
III. What does control sediment characteristics in bedrock rivers?
Pebble abrasion during fluvial transport
Change in rock type proportion?
If the pink rock type is more
resistant than the orange one
A
B
Distance downstream (km)A B
100%
50%
0%
Rock-type content in bedload (coarse fraction, > ~1 mm)
III. What does control sediment characteristics in bedrock rivers?
Pebble abrasion during fluvial transport
Experimental study of pebble abrasion during fluvial transport
Scale 1/5 model
The « machine a laver »
The circular flume
… on its frame
Piping suspended 1.35m above the ground
Non-abrasive floor condition
Videos: the flume in action…
physical laws of pebble abrasion
Abrasion = f (pebble size, velocity, lithology, amount of sediment)
Differences in pebble abrasion rates can be up to a factor 200!
Attal and Lavé, 2006
Pebble abrasion rate (% / km)
Experimental study of pebble abrasion during fluvial transport
III. What does control sediment characteristics in bedrock rivers?
Col
chen
et a
l., 1
986,
mod
ifie
d
DOMINANTLITHOLOGIES:
Limestone
Gneiss
SchistSandstone & Schist
Measurement sites:
Gravel barSource of sediment
Field study: the sediments of the Marsyandi river (Himalayas)
Size distributionsRock type proportions
Increase in grain size due to change from moraine-type source (a) to landslide-type source (b)
Distance from source (km)
Gra
vel b
ar D
50 (
cm)
She
ar s
tres
s (N
/m²)
Lavé and Avouac, 2001
(a)
(b)
Increase in grain size due to drop in shear stress the river is less likely to move large particles supplied from hillslopes and upstream
Attal and Lavé, 2006
“Source” and “transport” effects“Source” and “transport” effects
Gravel bar content
Distance from source (km)
% W
eigh
t%
Are
a
Lithologies exposed
Distance from source (km)
Resistant rock types (Quartzite, Gneiss + Granite) are
overrepresented with respect to poorly resistant rock types (Schist, Sandstones) – “Abrasion” effect
Attal and Lavé, 2006
Gneiss and graniteSchist
Sandstone
QuartziteLimestone
Red Deer River, Alberta, Canada (Parker, 1991)
After a few hundreds of km of transport, Quartz becomes the dominant rock type in bedload
%
(km)Downstream
Kali Gandaki - Narayani, Nepal (Mezaki and Yabiku, 1984)
III. What does control sediment characteristics in bedrock rivers?
TO SUMMARIZE
Marsyandi River, Nepal
Pebbles are abraded during fluvial transport
Angular pebbles, varied rock types
Rounded pebbles, resistant rock types dominant
What happens to the other rock types? They are turned into sand, silt
transported to sedimentary basins, mostly in suspension
III. What does control sediment characteristics in bedrock rivers?
Perfectly rounded quartz pebbles on the Isle of
Arran
III. What does control sediment characteristics in bedrock rivers?
The ideal model of fluvial erosion and landscape evolution?
(2)
(3)(1)
http://projects.crustal.ucsb.edu/nepal/ - characteristics of the sources of sediment (2),
- fluvial transport law (3),
- law of pebble abrasion during fluvial transport (3),
- law of bedrock abrasion due to impacts by moving particles,
- particles tracking, from hillslopes to rivers, from mountain range to basins.
Sediment characteristics strongly influence bedrock erosion rates. To better understand and predict how these characteristics evolve along rivers, the ideal model would need to include: