Semi-alluvial channels and sediment-flux-driven bedrock erosion

1. Semi-alluvial channelsandsediment-flux-driven bedrock erosion Swiss Federal Research Institute WSL Jens M. Turowski 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

2. Some semi-alluvial channels

3. Some semi-alluvial channels

4. Some semi-alluvial channels

5. Some semi-alluvial channels

6. Some semi-alluvial channels

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

8. 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. Objectives • Demonstrate the importance of sediment in the dynamics of bedrock channels • In general, bedrock channels are semi-alluvial!

10. 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. 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. It‘s complicated… Controls on channel morphology adapted from Schumm, River Variability and Complexity, CUP 2005

13. 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. Upstream supply Water Sediment Base level / uplift Substrate Controls on channel morphology • Steady state channels... Sediment supply Alluvium Qs Sediment discharge Bedrock Incision Uplift

15. Controls on channel morphology • The stream has two jobs to do: • Transport the supplied sediment • Incise the bedrock at the uplift rate Sediment supply Alluvium Qs Sediment discharge Bedrock Incision Uplift

16. 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 Discharge Erosion rate Slope Bedload transport equation

17. 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. Transient behaviour Detachment-limited:advection Knickpoint propagation Many field examples. Transport-limited:diffusion Few examples, but some. Slide adapted from D. Lague

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

20. More problems • Most incising streams are semi-alluvial

21. More problems Most incising streams are semi-alluvial In many environments, bedrock incision occurs due to the impact of moving particles

22. 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. Steepness of channel walls Taiwan: Alluvial channels Exponent Taiwan: Bedrock channels Mean sediment concentration Exponent Measure ofbank steepness From Turowski et al., Geomorphology 2008 Mean sediment concentration

24. 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. Tools and cover effects Impact marks on a marble surface (from Wilson, Thesis 2009) Partly covered bed in a mountain stream in Taiwan

26. Example: Erosion experiments • Demonstrate tools and cover effects and influence of grain size • Sklar and Dietrich, Geology 2001 • Sediment in an erosion mill Machine a Lavé, Attal et al. JHE 2006

27. 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. 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. Steepness of channel walls Taiwan: Bedrock channels Measure ofbank steepness Steeper banks Exponent Mean sediment concentration From Turowski et al., Geomorphology 2008

30. Erosion at Lushui, Liwu Dry season • Lateral erosion high for large floods • Vertical erosion high for small and medium flows Typhoon Bilis From Hartshorn et al., Science, 2002

31. Typhoon Long-Wang Lushui Station before (July 2004) and after (December 2005) Taiphoon Long-Wang, 1st October 2005 From Turowski et al., ESPL 2008

32. 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

33. Conceptual model • Transport capacity scales ~linearly with discharge • Model sediment supply with a power-law Exponent determines dynamics

34. Conceptual model • First possibility – λ>1 (Liwu River) Small 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)

35. 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. 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…

37. Conclusions • Both incision and sediment transport are important! • Bedrock channels are semi-alluvial in general

38. 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. 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. Thanks! Any questions?