1. Simplicity in Braiding Rivers
Peter Ashmore
University of Western Ontario
Séminaire GESTRANS, Grenoble, Nov 21, 2012

2. Thanks to: UWO students Roey Egozi, Tobi Gardner, Beth Hundey
Many lab, field and data assistants
BOKU: Helmut Habersack and students
U. Trento visitors to Sunwapta 1999 and Chris Paola
Walter Bertoldi – it’s all his fault 
Jim Chandler – Loughborough University
….and John Shaw & Gary Parker
Funding provided mainly by:
Natural Science and Engineering Research Council of Canada (NSERC)
CFI-Canada Foundation for Innovation (flume construction costs)

3. The ‘threshold’ separating braiding from other morphological types leads to idea that braided rivers are ‘different’ even though we recognise a continuum of channel patterns.
Visual appearance suggests complexity.
…… but a series of observations suggest that there is order and structure in braiding channel pattern, morphology and kinetics across a range of braiding rivers which lead to some core relationships describing and predicting braiding morphology and processes:
Cross-section dimensions and hydraulic geometry
Bars and planform scaling (unit morphology)
Constraints on braiding intensity
Bed load transport and morphology

4. Hydraulic geometry (e. g
Hydraulic geometry (e.g. depth) of anabranches and confluences follow the same scaling
Physical model
Sunwapta R., Canada + Mosley (1981a) data Ohau R. NZ
Ashmore & Gardner 2008

5. Regime geometry of wetted cross-section
Total wetted width
Ashmore 2001 & in press
Treatise on Geomorphology
Mean depth

6. 2. Bars and planform scaling
Ashmore 2009 (and 1982, ’91)

7. Braiding is series of confluences and bifurcations – mean spacing (along channel) scales with total discharge
Ashmore, 2001 7

8. Length of Confluence –Bifurcation units
Hundey & Ashmore 2009 8

9. Results from flume show linear relationship between length and width (two possible regression results).
Hundey and Ashmore 2009 9

10. And bigger (larger Q) rivers have longer average length
Comparison with field data suggest scaling very similar to pool-pool spacing in meandering channels or alternate bars and very similar to “incipient braids”
And bigger (larger Q) rivers have longer average length
But if channels are complex and have range of sizes, how is length scaled with total river discharge? – relates to braiding intensity and minimum size and number of active channels 10

11. 3. Regime braiding intensity
“..this consecutive branching process,…, perpetuates itself until the equilibrium or regime state is reached.”
Yalin and da Silva 2001
but what is this state and how does it develop?

12. Braiding intensity varies with stream power
Data from Ashmore flume experiments (1985) & Mosley (1981b)
Ashmore 2009

13. TBI, ABI and ratio are all ‘regime’ states
Egozi & Ashmore 2009 13

14. Channel pattern is developed over time by progressive migration of only a few active channels (usually 2 or less)
Morphology and dynamics controlled by one or two major active channels and related bifurcation / confluence / switching
Egozi and Ashmore 2009

15. Gardner PhD thesis 2009

16. Flow Note middle portions of bed where most change is occurring.
Difference maps demonstrate history of single channel migration rather than simultaneous channel activity.
Change is restricted to active areas
Time frame is 11 hours ( …fyi Petros) at one hour intervals
NEEDS SCALE BAR

17. ABI & braid ratio vary – predictable average values that vary with stream power relative to grain size
Bertoldi et al. 2009b
Egozi & Ashmore 2009

18. 4. Sediment transport and active width
Bertoldi et al., 2009a

19. Flux increase seems to depend more on active width than on bed stress changes – and there seems to be a predictable function for mean active width
Bertoldi et al., 2008

20. Reduced to systematic relationship with dimensionless power
Bertoldi et al. 2009a

21. Field observations (repeat daily cross-section surveys during daily melt hydrograph sequence), Sunwapta River, Alberta – takes us back to transient shifting of activity
Ashmore et al. 2011

22. Variation with discharge in a reach
Variation of mean active width at ‘channel-forming’ flows ?
Ashmore et al. 2011

23. Which brings us back to regime for ABI/TBI and possible approximations relating active width (and bedload flux) to observable (TBI) and inference to ABI?
Bertoldi et al. 2009b

24. …but it’s still complex!

25. Braiding rivers have average ‘regime’ morphology for:
individual channels, local features and total channel cross-section dimensions
Scale of unit features such as bars and confluence-diffluence
Complexity of braiding network (braiding intensity) – both total and ‘active’
Well-defined relationship for total sediment (bed load) flux
Variation in active width ‘at a station’ and for overall variation in dimensionless stream power - which also relates to bed load transport
Braiding rivers regime morphology and there is a continuum of morpho-dynamic characteristics determined mainly by sediment mobility and river size (total discharge) – and they have internal regime relations between channel size, braid length and complexity, active width and bedload flux. ….

26. References
Ashmore, P.E., Laboratory modelling of gravel braided river morphology, Earth Surface Processes and Landforms, 7,
Ashmore, P.E., Process and form in gravel braided streams: laboratory modelling and field observations. PhD thesis, University of Alberta.
Ashmore, P.E., How do gravel-bed streams braid? Canadian Journal of Earth Sciences, 28,
Ashmore, P., Braiding phenomena: statics and kinetics. In, M.P. Mosley (Editor), Gravel-Bed Rivers V, New Zealand Hydrological Society, Wellington,
Ashmore, P The intensity and characteristic length of braided channel patterns. Canadian Journal of Civil Engineering, 36, (Invited paper for a special issue in honour of Prof. S. Yalin)
Ashmore, P. In press. Morphology and dynamics of braided rivers. In: Schroder, J. Jr., E. Wohl (Eds.) Treatise on Geomorphology. Academic Press, San Diego.
Egozi, R., and P. Ashmore Experimental analysis of braided channel pattern response to increased discharge, J. Geophys. Res., 114, F02012, doi: /2008JF
Ashmore, P. and Gardner, J.T Unconfined confluences in braided rivers. Rice, S., Roy, A. Rhoads, B. (editors) River Confluences, Tributaries and the Fluvial Network. Wiley, Chichester,
Ashmore, P. , Bertoldi, W. and Gardner, J.T Active width of gravel-bed braided rivers. Earth Surface Processes and Landforms. 36, DOI: :esp2182
Bertoldi, W., Ashmore, P. and Tubino, M. ,2009a . A method for estimating the mean bed load flux in braided rivers. Geomorphology 109,
Bertoldi, W., Zanoni, L., Tubino, M., 2009b. Planform dynamics of braided streams. Earth Surface Processes and Landforms, 34,
Hundey, E. and Ashmore, P Length scale of braided river morphology. Water Resources Research, 45, W08409, doi: /2008WR
Mosley, P.M., Semi-determinate hydraulic geometry of river channels, South Island, New Zealand. Earth Surface Processes and Landforms, 6,
Mosley, M.P., Scour depths in branch channel confluences, Ohau River. Report no. WS 395, Ministry of Works and Development, Christchurch, New Zealand.