1. Semi-alluvial channels GBR 7, Tadoussac 2010 Semi-alluvial channels and 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

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

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

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

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

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

7. 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?

8. Semi-alluvial channels GBR 7, Tadoussac 2010 Bedrock channels Various definitions… All rivers actively incising into bedrock Where rock is exposed widely Where alluvial cover is thin and is mobilised during floods Where bedrock (walls, bed…) limits the dynamic evolution of the river

9. 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!

10. 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

11. 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

12. Semi-alluvial channels GBR 7, Tadoussac 2010 Controls on channel morphology It‘s complicated… adapted from Schumm, River Variability and Complexity, CUP 2005

13. 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

14. Semi-alluvial channels GBR 7, Tadoussac 2010 Controls on channel morphology Upstream supply –Water –Sediment Base level / uplift Substrate Steady state channels... Bedrock Alluvium Uplift Incision QsQs Sediment supply Sediment discharge

15. 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 Bedrock Alluvium Uplift Incision QsQs Sediment supply Sediment discharge

16. Semi-alluvial channels GBR 7, Tadoussac 2010 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 Erosion rate Discharge Slope Bedload transport equation

17. 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…

18. Semi-alluvial channels GBR 7, Tadoussac 2010 Transient behaviour Knickpoint propagation Detachment-limited: advection Transport-limited: diffusion Many field examples. Slide adapted from D. Lague Few examples, but some.

19. 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 Some mixed form of behaviour…. Slide adapted from D. Lague Longitudinal distance Elevation Final profile Original profile Final profile Original profile

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

21. 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

22. 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…

23. Semi-alluvial channels GBR 7, Tadoussac 2010 Steepness of channel walls Taiwan: Alluvial channels Taiwan: Bedrock channels Measure of bank steepness Mean sediment concentration Exponent From Turowski et al., Geomorphology 2008

24. Semi-alluvial channels GBR 7, Tadoussac 2010 Tools and cover effects Tools 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

25. 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

26. Semi-alluvial channels GBR 7, Tadoussac 2010 Example: Erosion experiments Sklar and Dietrich, Geology 2001 Sediment in an erosion mill Demonstrate tools and cover effects and influence of grain size Machine a Lavé, Attal et al. JHE 2006

27. 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

28. 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

29. Semi-alluvial channels GBR 7, Tadoussac 2010 Steepness of channel walls Taiwan: Bedrock channels Measure of bank steepness Mean sediment concentration Exponent Steeper banks From Turowski et al., Geomorphology 2008

30. Semi-alluvial channels GBR 7, Tadoussac 2010 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

31. 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

32. Semi-alluvial channels GBR 7, Tadoussac 2010 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

33. Semi-alluvial channels GBR 7, Tadoussac 2010 Conceptual model Transport capacity scales ~linearly with discharge Model sediment supply with a power- law Exponent determines dynamics

34. Semi-alluvial channels GBR 7, Tadoussac 2010 Conceptual model First possibility – λ>1 (Liwu River) Small and medium events evacuate sediment or incise the 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)

35. 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

36. Semi-alluvial channels GBR 7, Tadoussac 2010 Conceptual model Second possibility – λ<1 Channel behaves essentially alluvial at low flow Sediment evacuation and erosion during floods Field examples none yet, but many candidates…

37. Semi-alluvial channels GBR 7, Tadoussac 2010 Conclusions Both incision and sediment transport are important! –Bedrock channels are semi-alluvial in general

38. 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)

39. 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

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