Simplicity in Braiding Rivers

Peter Ashmore

University of Western OntarioSéminaire GESTRANS, Grenoble, Nov 21, 2012

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 2003 + 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)

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:

1. Cross-section dimensions and hydraulic geometry

2. Bars and planform scaling (unit morphology)

3. Constraints on braiding intensity

4. Bed load transport and morphology

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

Regime geometry of wetted cross-section

Total wetted width

Mean depth

Ashmore 2001 & in press

Treatise on Geomorphology

2. Bars and planform scaling

Ashmore 2009 (and 1982, ’91)

Braiding is series of confluences and bifurcations – mean spacing (along channel)

scales with total discharge

Ashmore, 2001

Length of Confluence –Bifurcation units

Hundey & Ashmore 2009

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

Hundey and Ashmore 2009

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

“..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?

3. Regime braiding intensity

Braiding intensity varies with stream power

Ashmore 2009

Data from Ashmore flume experiments (1985) & Mosley (1981b)

TBI, ABI and ratio are all ‘regime’ states

Egozi & Ashmore 2009

TBI

ABI

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

Gardner PhD thesis 2009

988 990 992 994 996 998

1000

1001

1002

1003

Flow

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 1 2 3 4 5 6 7

Dimensionless stream power

Bra

id R

atio

Bertoldi et al in press

Egozi and Ashmore in press

ABI & braid ratio vary – predictable average values that vary with stream power relative to grain size

Bertoldi et al. 2009b

Egozi & Ashmore 2009

4. Sediment transport and active width

Bertoldi et al., 2009a

Bertoldi et al., 2008

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

Reduced to systematic relationship with dimensionless power

Bertoldi et al. 2009a

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

Ashmore et al. 2011

Variation with discharge in a reach

Variation of mean active width at ‘channel-forming’ flows

?

Bertoldi et al. 2009b

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?

…but it’s still complex!

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.

….

References• Ashmore, P.E., 1982. Laboratory modelling of gravel braided river morphology, Earth Surface

Processes and Landforms, 7, 201-225 • Ashmore, P.E., 1985. Process and form in gravel braided streams: laboratory modelling and field

observations. PhD thesis, University of Alberta.• Ashmore, P.E., 1991. How do gravel-bed streams braid? Canadian Journal of Earth Sciences,

28, 326-341• Ashmore, P., 2001. Braiding phenomena: statics and kinetics. In, M.P. Mosley (Editor), Gravel-

Bed Rivers V, New Zealand Hydrological Society, Wellington, 95-114.• Ashmore, P. 2009. The intensity and characteristic length of braided channel patterns. Canadian

Journal of Civil Engineering, 36, 1656-1666.(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 2009. Experimental analysis of braided channel pattern response to increased discharge, J. Geophys. Res., 114, F02012, doi:10.1029/2008JF001099.

• Ashmore, P. and Gardner, J.T. 2008. Unconfined confluences in braided rivers. Rice, S., Roy, A. Rhoads, B. (editors) River Confluences, Tributaries and the Fluvial Network. Wiley, Chichester, 119-143

• Ashmore, P. , Bertoldi, W. and Gardner, J.T. 2011. Active width of gravel-bed braided rivers. Earth Surface Processes and Landforms. 36, 1510-1521. DOI: 10.1001:esp2182

• Bertoldi, W., Ashmore, P. and Tubino, M. ,2009a . A method for estimating the mean bed load flux in braided rivers. Geomorphology 109, 330-340

• Bertoldi, W., Zanoni, L., Tubino, M., 2009b. Planform dynamics of braided streams. Earth Surface Processes and Landforms, 34, 547-557.

• Hundey, E. and Ashmore, P. 2009. Length scale of braided river morphology. Water Resources Research, 45, W08409, doi:10.1029/2008WR007521.

• Mosley, P.M., 1981. Semi-determinate hydraulic geometry of river channels, South Island, New Zealand. Earth Surface Processes and Landforms, 6, 127-137.

• Mosley, M.P., 1981. Scour depths in branch channel confluences, Ohau River. Report no. WS 395, Ministry of Works and Development, Christchurch, New Zealand.