Lecture

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: