Types of bedforms and bar forms in rivers: dunes

Year 3 EARS 3072/GEOG 3430 Alluvial Flow...

Dunes: characteristics & scaling

•Dunes scale with flow depth….. ldune ~ 5-7Y•Dune height ~ 0.33Y•Dunes associated with macroturbulence….

’boils’ on the flow surface• So, since macroturbulence scales as:

•Tb = fU/Y ~ 5-7•the same as burst scaling?

Tb=fU/Y~5

Jackson, 1975, 1976

‘boil’flow

Dune-related macroturbulence interacting withthe flow surface - Jamuna River, Bangladesh

Flow over dunes

…using LDA

flow over dunes…..

Field Study:

Fraser River, British Columbia

Echo sounder trace

Downstream river flow

0

2

metres

compound dune

dune with ~10° leeside

50

downstream

vertical

lateral

ve

locit

y,

cm

s-1

ADCP field quantification: June 1999

flow

flow

flow

dunes ~ 2m high

so, ideas on how this causes dune scaling?

• suspension effects

BUT• gravel dunes• burst scaling - Strouhal law

St=fY/U ~ 0.2• so bedload flux is key?

suspension effects

Dinehart 1992

using PIV

- straight-crested 2D dune ..….concentrate on

two areas -

….examine the temporal characteristics of the

flow field over fixed dunes….

i) leesideii) upper stoss

I: flow separation, vortex shedding and its effects

ms-1

flow

downstream

upstream

mean

downstream

velocity

leeside:

downstream

velocity

mean

vertical

velocity

leeside:

vertical

velocity

animation: downstream velocity

leeside

m s-1

animation: vertical velocity

leeside

m s-1

upper stoss:

downstream

velocity

flow

upper stoss:

vertical

velocity

flow

downstream velocity

one frame at 15 Hz

vertical velocity

Reynolds stress

-0.05

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u, m s-1

-0.2 -0.1 0.0 0.1 0.2

v, m s-1

-20 -10 0 10 20 30 40

t R, N m-2

0s

0.27s

0.60s

1.00s

0.87s

1.13s

tR,

Dune Lee

0.45

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0.350.40

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u, m s-1

v, m s-1 t R, N m

-2

0.20 0.30 0.40 0.50 -0.2 -0.1 0.0 0.1 0.2 -20 -10 0 10 20 30 40

tR,

Dune Stoss

• Shear layer ‘flapping’

• Kelvin-Helmholtz instabilities shed along shear layer

• Ejections of fluid reach surface during flapping

• Ejections generate inrushes at downstream crest that have tR > 6-9x average tR

• link between separation zone dynamics and magnitude and location of downstream

sediment transport

summary & implications

~ 30m

upwellinglarge waves at

downstream edge

II: topology of dune-related macroturbulence –

some field observations

flow

flow

flow

Vortex ring interaction with flow

surface (after Sarpkaya, 1996)

vortex ring

Planform view of surface as vortex ring

approaches

flow

flow

Previous observations of dune-related

macroturbulence interacting with the

water flow surface

(after Babakaiff & Hickin, 1996)

Morphology of dune-related macroturbulent ejections

(after Müller & Gyr, 1983, 1996)

dune crest

Morphology of coherent vortices behind dunes

(after Nezu & Nakagawa, 1993)

Dune

Flow

Separated vortices

Dune

Cre

st

3-D

Inte

ractio

n

Low-speed fluid

Recirculation

Reattachment

Large-scale vortex

I: vortex approaching surface

DUNE-

GENERATED

VORTEX

Schematic of vortex interaction with a free

surface - based on field

& flume observations

II: vortex tip interacts with

free surface

Shear with mean flow

III: vortex leg interaction

with free surfaceVortex tubes developing

IV: vortex tube

development

from Patel, Lin and Yue

www.iihr/uiowa/projects/turbulentdune/

LES

simulations

Flow patterns one

grid cell below free

surface

Red: upwelling

Blue: downwelling

slipface angle(angle of repose ~ 30°)

Jamuna River

III: the influence of dune leeside angle

water surface

dune

leeside

flow

a LDA study

flow

flow

flow

flow

flow

flow in

the leeside

intermittent separation zone

He

igh

t a

bo

ve

be

d, y/Y

tot

% time flow reversal

flow

shear layer development from lower leeside

shear layers from other

changes in bed slopeintermittent separation

(‘transitory stall’)

‘wake’ stacking

sediment ejected into suspension

at ‘separation’ and ‘reattachment’ regions

flow deceleration

in leeside

sediment suspension along shear layers

from upstream dunes

large events may erupt at flow surface

• many natural alluvial dunes are ‘low-angle’ (flow resistance implications)

• dynamics dominated by intermittent separation and shedding

• control of sediment transport by separation zone/ejection-inrush dynamics

summary & implications

Ideas:

• they aren’t - they are part of a continuum?• wave instabilities of different size?• form controlled by different scales of CFS?• dunes evolve from ‘rogue’ ripples -therefore influence flow field?

Why are ripples and dunes separate forms?

Shear layer velocity gradients

& therefore turbulence?

References

Dunes:Babakaiff, C.S. and Hickin, E.J. 1996: Coherent flow structures in the Squamish River estuary, BC, Canada, In: Coherent Flow Structures, 321-342.Bennett,S.J. & Best, J.L. 1995 Mean flow and turbulence structure over fixed two-dimensional dunes….., Sedimentology, 42, 491-513Best, J.L. 2005 The fluid dynamics of river dunes: a review and some future research directions. J. Geophysical Research, Earth Surface, 110, F04S02, doi:10.1029/2004JF000218. Best, J.L. and Kostaschuk, R.A. 2002 An experimental study of turbulent flow over a low-angle dune, J. Geophysical Research, 107, C9, 3135-3153.*Jackson, R.G. 1976 Sedimentological and fluid dynamic implications of turbulent bursting… J.Fluid Mechanics, 77, 531-560.Muller, A. and Gyr, A. 1982 Visualization of the mixing layer behind dunes, In: Mechanics of Sediment Transport, 41-48Nelson, J.M. et al. 1993 Mean flow and turbulence fields over two-dimensional bedforms, Water Resources Research, 29, 3935-3953