Riv Sys Bars

7/30/2019 Riv Sys Bars

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Bar patternsMSc course Fluvial Systems GEO4-4436

Dr. Maarten G. Kleinhans

Rakaia River, NZ, Geoff Leeming

Brahmaputra, Bangladesh, Google EarthGanges, India, Google Earth

Sunwapta River, USA


2/8

This lecture: main questions and points

why do bars form?

why do different bar patterns form?

what causes dynamics of bars?

Position of this lecture in the course

Flow, sediment transport and channel

morphodynamics

River patterns

Bifurcations and avulsion

From source to sink and in between

Cross-talk: main terminology

Tabular cross-stratification

lateral accretion surfaces

huh? stuff that migrating bars

and point bars leave

Mor phodynamics: Sedi ment ol ogy:

channel, bars and bends,levees

channel belt

Forced bar Point bar

Free bar Braid bar,mid-channel bar

Nonlinear transport andTransverse bed slope effect

Huh? thought spiral flowcaused meandering?

10/51

Bar pattern vs channel pattern

Bar pattern

interaction flow and sediment

ina channel

width-depth ratio!

Channel patterninteraction flow (and bars) with banks

outof the channel:bank erosion and floodplain formation

11/51

Alternate bars and bank erosion

low flow conditions!

Bridge (2003) Rivers and Floodplains

Instability of a plane bed

imagine

a plane bed with a perturbation

attracts flow

much more sediment transport (nonlinear)

perturbation grows!

leads tobraiding

cellular model

Murray Paola 1994


3/8

The sky is the limit?

bars grow increasingly fast from initial

perturbation

is there an end to this?

shear stress vectorversussediment transport vector

transverse slope effect(gravity)(Ikeda et al. 1981)

but: spiral flow effect(Struiksma et al. 1985; Talmon et al. 1995;

Kleinhans et al. 2008)

Transverse bed slope effect

transverseslope effect

spiral

flow effect

15/51

How to reduce transverse slope?

bendway weirsfixed layers

longitudinal dams bottom vanes16/51

Bar theory (1)(Struiksma et al. 1985)

flow and sediment interact

qs ~ mun (m=constant)

n>3 for theoretical reasons

n=3 for Meyer-Peter & Mueller

n=5 for Engelund & Hansen

slope effects on sediment:

downslope easier

transverse slope in bends >> river gradient

secondary flows in bends: upslope

sharper bends stronger secondary flow

17/51

Bar theory (2)

flow needs length to adapt to bed (bars)

relaxation length w:

imagine: momentum!

sediment too!

relaxation length s:

imagine: (transverse) slope effect bed cannot suddenly jump

18/51

Forced bars and free bars

Forced bars: stationairy,initiated and forced by channel curvature

Free bars: migrating,initiated by perturbation that grows(=unstable)


4/8

Braid bar types

Note misleading banklines:

now all these bars seem forced!

yet many braid bars are free

20/51

Damping and exciting bars (1)

Bar types:

forced bars: forced by flow curvature; staticposition

free bars: spontaneously develop andmigrate;initiated at perturbation (groyne, bend, tree)

Analogy:

spring-mass-damper

damper can also excite

21/51

Three regimes forced (alternating) bars

Overdamped

Underdamped

Excited

22/51

Narrow channel alternating barsw/h = 24

Q = 2,500 m/sAlternating bars

Free migrating bars

Time(Months)

X-coordinate (m)


5/8


6/8

31/51

Bend flow (2)

with:

s: longitudinal coordinaten: transverse coordinate

v: velocity (u for us)

z: elevation, s: surface, b: bedh: depth

R: bend radius

Koen

Blanckaert

Meandering??

Old theories and handbooks:

spiral / helical flow

Engineering since 1980s:bar instability modified by helical flow

33/51

Transverse slope effect

for infinite long bend analytical solution

Gravity acts on particles on transverseslope

particles pulled towards outer-bankcounteracts spiral flow

Shear stress vs sediment vector


Bar height

Bar length

Migration rate

36/51

Wider channels: braid bars

Narrow channel

Wide channel: stability of higher wavemodes


7/8



Braid bar morphology 3D fine grid

Bar tailsConfluence scour

Unit bars

Compound bars

Cross-bar channels

Tributary mouth bars

Bridge (1993)

Bed level Deposition/erosion Sed. transport


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Subsurface Subsurface + bed level + flow field

Barsformation

positive feedback: nonlinear sed transport

negative feedback: transverse bed slope effect

modification: spiral flow

pattern:

width-to-depth ratio of channel

forced vs free barsdynamics:

migration

sedimentation, bank erosion

bifurcation at bar, chute cutoff through bar

Open questions

bar shape and dynamics ( stratification)for different D and h?(and for different environments)

bar stabilisation?

when chute cutoffs (bifurcations)?

...

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