Semi-alluvial channels GBR 7, Tadoussac 2010 Semi-alluvial channels and sediment-flux-driven bedrock erosion Jens M. Turowski With thanks to: D. Lague,

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


Some semi-alluvial channels










Questions• How do these different types of channel form?• What is the influence of the sediment on channel

morphology?


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


Objectives• Demonstrate the importance of sediment

in the dynamics of bedrock channels– In general, bedrock channels are semi-

alluvial!




alluvial!

• Convince you that some widely used bedrock incision laws are incorrect




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


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


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



• Upstream supply– Water– Sediment

• Base level / uplift• Substrate

• Steady state channels...

Bedrock

Alluvium

Uplift

Incision

Qs

Sediment supply

Sediment discharge



• 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


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


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…


Transient behaviourKnickpoint propagationDetachment-limited: advection

Transport-limited: diffusionMany field examples.

Slide adapted from D. Lague

Few examples, but some.


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


More problems• Most incising streams are semi-alluvial


More problems• Most incising streams are semi-alluvial• In many environments, bedrock incision occurs

due to the impact of moving particles


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…


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


Exp

onen

t

Exp

onen

t

From Turowski et al., Geomorphology 2008


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


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


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


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


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


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


Exp

onen

t

Steeper banks

From Turowski et al., Geomorphology 2008


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


Typhoon Long-Wang

Lushui Station before (July 2004) and after (December 2005) Taiphoon Long-Wang, 1st October 2005

From Turowski et al., ESPL 2008


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


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


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


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)


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


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


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…


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)




• 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


Thanks!

Any questions?

Documents

Semi-alluvial channels GBR 7, Tadoussac 2010 Semi-alluvial channels and sediment-flux-driven bedrock erosion Jens M. Turowski With thanks to: D. Lague,