Semialluvial channels GBR 7 Tadoussac 2010 Semialluvial channels

Semi-alluvial channels GBR 7, Tadoussac 2010 Semi-alluvial channels and sediment-flux-driven bedrock erosion Jens M. Turowski Swiss Federal Research Institute WSL Gravel Bed Rivers 7, Tadoussac, Canada, September 2010 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

Semi-alluvial channels GBR 7, Tadoussac 2010 Some semi-alluvial channels

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 bedrock (walls, bed…) limits the dynamic evolution of the river Where rock is exposed widely Where alluvial cover is thin and is mobilised during floods

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… 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 • Steady state channels. . . • Upstream supply • Base level / uplift • Substrate – Water – Sediment supply Alluviu Qs m Bedro Sediment discharge ck Incision Uplift

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 Sediment supply Alluviu Qs m Bedro Sediment discharge ck Incision Uplift

Semi-alluvial channels GBR 7, Tadoussac 2010 End-member incision models • Possibility 1: Incision is of dominant importance – Detachment-limited model Discharge Erosion rate • Possibility 2: Transport is of dominant importance (alluvial rivers) Slope – Transport-limited model 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 behaviour Detachment-limited: advection Transport-limited: diffusion Knickpoint propagation Many field examples. Few examples, but some. Slide adapted from D. Lague

Semi-alluvial channels GBR 7, Tadoussac 2010 Transient behaviour Example: Post-glacial gorge incision in the Alps (Valla, Van der Beek and Lague, JGR, 2010) Detachment-limited Transport-limited Original profile Final profile Longitudinal distance Some mixed form of behaviour…. Slide adapted from D. Lague Elevation Original profile Final profile Longitudinal distance

Semi-alluvial channels GBR 7, Tadoussac 2010 More problems • Most incising streams are semi-alluvial

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 Exponent Taiwan: Alluvial channels Taiwan: Bedrock channels Exponent Mean sediment concentration Measure of bank steepness From Turowski et al. , Geomorphology 2008 Mean sediment concentration

Semi-alluvial channels GBR 7, Tadoussac 2010 Tools and cover effects Tools effect • Impacting particles remove rock – More particles = higher erosion rates Impact marks on a marble surface (from Wilson, Thesis 2009) Cover effect • Particles cover and protect the bed – More particles = smaller erosion rates 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

Semi-alluvial channels GBR 7, Tadoussac 2010 Example: Erosion experiments • Demonstrate tools • Sklar and Dietrich, and cover effects and Geology 2001 • Sediment in an erosion influence of grain size mill 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

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 Taiwan: Bedrock channels Exponent Measure of bank steepness Steeper banks Mean sediment concentration From Turowski et al. , Geomorphology 2008

Semi-alluvial channels GBR 7, Tadoussac 2010 Erosion at Lushui, Liwu Dry season Typhoon Bilis From Hartshorn et al. , Science, 2002 • Lateral erosion high for large floods • Vertical erosion high for small and medium flows

Semi-alluvial channels GBR 7, Tadoussac 2010 Typhoon Long-Wang Lushui Station before (July 2004) and after (December 2005) Taiphoon Long-Wang, 1 st October 2005 From Turowski et al. , ESPL 2008

Semi-alluvial channels GBR 7, Tadoussac 2010 Incision and cover Not to scale of picture • Cumulative erosion at Lushui during 2005 • Maximum incision at current terrace level in quartzite (black line) 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 powerlaw Exponent determines dynamics

Semi-alluvial channels GBR 7, Tadoussac 2010 Conceptual model • First possibility – λ>1 (Liwu River) Small and medium events evacuate sediment or incise 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) Discharge Sed. thickness Bed incision Wall incision

Semi-alluvial channels GBR 7, Tadoussac 2010 Conceptual model • Second possibility – λ<1 Sediment evacuation and erosion during floods Channel behaves essentially alluvial at low flow 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

Semi-alluvial channels GBR 7, Tadoussac 2010 Thanks! Any questions?