Eroding landscapes:
fluvial processes
Sediments and
bedrock erosion
Mikaël ATTAL
Acknowledgements: Jérôme Lavé, Peter van
der Beek and other scientists from LGCA
(Grenoble) and CRPG (Nancy)
Marsyandi valley, Himalayas, Nepal
Lecture overview
I. Field testing of fluvial erosion laws: do sediments matter?
II. How do sediments modulate bedrock erosion rates?
III. What does control sediment characteristics in bedrock
rivers?
General form: fluvial incision laws
m n
E = KA S .f(qs)
 Stream Power Law(s) (laws 1, 2, 3): f(qs) = 1
 Laws including the role of the sediments: 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.
I. Field testing of fluvial incision laws (1)
Excess shear stress model (law 4): Lavé & Avouac, 2001
V
D
Basal shear stress:  τ = ρ g R S, where R = WD / (W+2D)
 τ = ρ g D S, if W >10D.
Fluvial erosion law: E = K (τ - τc)
Fluvial incision along Himalayan rivers
MFT
Fluvial incision measured using terraces
Comparison between fluvial incision (terraces)
and excess shear stress (channel geometry)
Shields stress (non-dimensional):
τ* = τ / (ρs – ρ)gD50
[Lavé and Avouac, 2001]
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
S
LH
HHC
TSS
Modified from Lavé & Avouac, 2001
Lavé & Avouac, 2001:
maximum fluvial erosion
rate in the HHC zone for
6 main Himalayan rivers
S
LH
HHC
TSS
Lavé & Avouac, 2001:
maximum fluvial erosion
rate in the HHC zone for
6 main Himalayan rivers
Modified from
Lavé & Avouac,
2001
I. Field testing of fluvial incision laws (2)
Using the transient response of the landscape
 All laws predict similar steady-state
topographies (concave-up profile, etc.),
 Predicted transient response to a disturbance
depends on the fluvial incision law chosen.
Transient response of fluvial systems
Detachment-limited law (SPL, laws 1, 2, 3)
(2002)
Transport limited law (law 5)
(2002)
Transient response of fluvial systems
Detachment-limited law (SPL, laws 1, 2, 3)
(2002)
Transport limited law (law 5)
(2002)
Transient response of fluvial systems
Detachment-limited law (SPL, laws 1, 2, 3)
(2002)
Transport limited law (law 5)
(2002)
Transient response to tectonic disturbance
(Whittaker et al., 2007a, b, 2008; Cowie et al., 2008, Attal et al., 2008)
Fiamignano,
Italy
Xerias,
Greece
0
600 km
http://www.phys.uu.nl/~gdevries/maps/maps.cgi
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
Erosion efficiency f(Qs)
Italy closer to DL end-member, Greece closer to TL end-member
(Cowie et al., 2008)
Qs/Qc
0
Fiamignano,
Italy
Xerias, Greece
SEDIMENTS DO MATTER!
1
II. How do sediments modulate bedrock erosion rates?
Role of sediment: the “tools and cover” effects (Gilbert, 1877)
Cover
Tools
Experimental study of bedrock abrasion by saltating particles
Sklar & Dietrich, 2001
Role of sediment: the “tools and cover” effects
Sklar & Dietrich, 1998, 2004
E
Qs
W Lf
 Qs 
1  
 Qc 
1998: theoretical
E = ViIrFe
2004: mechanistic
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.
Role of sediment: the “tools and cover” effects
Turowski et al., 2007
Erosion efficiency f(Qs)
Maximum bedrock erosion
for sediment supply = Qc
(“dynamic cover effect”)
Sklar &
Dietrich
0
Qs/Qc
Turowski et al.
1
Sediment SUPPLY / Qc
Sediment SUPPLY ≤ Qc
 Qs/Qc = Sediment SUPPLY / Qc
Sediment SUPPLY > Qc
 Qs/Qc = 1
Role of sediment: the “tools and cover” effects
Effect of grain size? (Sklar & Dietrich, 2004)
But very simplistic model: bedload is made of only 1 grain size!
Role of sediment: the “tools and cover” effects
Effect of grain size? Bedload is made of a wide range of grain sizes
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)
Sediment mobility in bedrock rivers
Cellular Automata model
Movement probability
0.05 0.05
0.05
0.2
0.2
0.05 0.05
0.05
Courtesy Rebecca Hodge,
University of Glasgow
Role of sediment: the “tools and cover” effects
Erosion efficiency f(Qs)
Hodge et al., work in progress
Lower erosion rates for
higher sediment supply
because of increasing
likelihood of jams
Sklar &
Dietrich
Turowski et al.
0
Qs/Qc
1
Sediment SUPPLY / Qc
Sediment SUPPLY ≤ Qc
 Qs/Qc = Sediment SUPPLY / Qc
Sediment SUPPLY > Qc
 Qs/Qc = 1
Characterizing sediment mobility in the field
Calder River, Renfrewshire
Characterizing sediment mobility in the lab
Schmeeckle et al.
Modelling sediment
motion…
http://www.markschmeeckle.com/
Ideally, we would include pebble
and bedrock abrasion in such
models. But the computer that
can do that efficiently doesn’t
exist yet…
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)
http://projects.crustal.ucsb.edu/nepal/
What does control the
characteristics of
sediments in (1)?
(2)
 (2) Characteristics of
the source of sediment
(location, amount, grain
size distribution,
Sediment
lithology)
mobility
(1)
(3)
 (3) Transport and
abrasion processes
along the channel
III. What does control sediment characteristics in bedrock rivers?
Pebble abrasion during fluvial transport
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
Pebbles are
abraded during
fluvial transport
Each pebble is reduced in
size and gets more rounded
Marsyandi River, Nepal
III. What does control sediment characteristics in bedrock rivers?
Common pebble abrasion processes:
Chipping
Crushing
Cracking
Grinding
Splitting
These processes
reduce the size of
pebbles and tend
to make them
more rounded
Marsyandi River, Nepal
III. What does control sediment characteristics in bedrock rivers?
Pebble abrasion during fluvial transport
Not necessarily,
because fresh
material is
added along the
river course in
mountain
rivers (≠
alluvial rivers)
Downstream fining?
III. What does control sediment characteristics in bedrock rivers?
Pebble abrasion during fluvial transport
If the 2 rock
types are
eroded at the
same rate
A
Rock-type content in bedload
(coarse fraction, > ~1 mm)
100%
B
Change in rock type
proportion?
50%
0% A
B
Distance downstream (km)
III. What does control sediment characteristics in bedrock rivers?
Pebble abrasion during fluvial transport
If the pink rock
type is more
resistant than
the orange one
A
Rock-type content in bedload
(coarse fraction, > ~1 mm)
100%
B
Change in rock type
proportion?
50%
0% A
B
Distance downstream (km)
Experimental study of pebble abrasion during fluvial transport
Scale 1/5 model
The « machine
a laver »
… on its frame
The circular flume
Piping
suspended
1.35m above
the ground
Non-abrasive floor condition
 physical laws of pebble abrasion
Abrasion = f (pebble size, velocity, lithology, amount of sediment)
Videos: the flume in action…
Experimental study of pebble abrasion during fluvial transport
Differences in pebble abrasion rates can be up to a factor 200!
Pebble abrasion rate (% / km)
Attal and Lavé, 2006
Colchen et al., 1986, modified
III. What does control sediment characteristics in bedrock rivers?
Field study: the sediments of the Marsyandi river (Himalayas)
Size distributions
Rock type proportions
DOMINANT
LITHOLOGIES:
Limestone
Gneiss
Schist
Sandstone
& Schist
Measurement sites:
Gravel bar
Source of sediment
Shear stress (N/m²)
Gravel bar D50 (cm)
Lavé and
Avouac, 2001
Increase in
grain size due to
change from
moraine-type source
(a) to landslide-type
source (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
(a)
(b)
Distance from source (km)
“Source” and
“transport” effects
Attal and Lavé, 2006
% Area
Lithologies exposed
Gneiss and granite
Schist
Distance from source (km)
Sandstone
Quartzite
Limestone
% Weight
Resistant rock types (Quartzite,
Gneiss + Granite) are
overrepresented with respect to
poorly resistant rock types (Schist,
Sandstones) – “Abrasion” effect
Gravel bar content
Distance from source (km)
Attal and Lavé, 2006
III. What does control sediment characteristics in bedrock rivers?
Red Deer River, Alberta,
Canada (Parker, 1991)
After a few hundreds of km
of transport, Quartz
becomes the dominant rock
type in bedload
Kali Gandaki - Narayani, Nepal
(Mezaki and Yabiku, 1984)
%
(km)
Downstream
III. What does control sediment characteristics in bedrock rivers?
Angular pebbles, varied rock types
TO SUMMARIZE
Rounded pebbles, resistant
rock types dominant
Pebbles are
abraded during
fluvial transport
What happens to the other rock types?
They are turned into sand, silt 
transported to sedimentary basins, mostly
in suspension
Marsyandi River, Nepal
III. What does control sediment characteristics in bedrock rivers?
Perfectly rounded quartz
pebbles on the Isle of
Arran
The ideal model of fluvial erosion and landscape evolution?
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:
- characteristics of the
sources of sediment (2),
http://projects.crustal.ucsb.edu/nepal/
- fluvial transport law (3),
(2)
- law of pebble abrasion
during fluvial transport (3),
- law of bedrock abrasion
due to impacts by moving
particles,
(1)
(3)
- particles tracking, from
hillslopes to rivers, from
mountain range to basins.