Introduction 1. Channel and bar patterns · Introduction 1. Channel pattern classification • bars...
Transcript of Introduction 1. Channel and bar patterns · Introduction 1. Channel pattern classification • bars...
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Research group River and delta morphodynamics
River patterns
GEO3-4305, 2014
Dr. Maarten Kleinhans
www.geo.uu.nl/fg/mkleinhans
flow
sediment
transport
morphology
This course: the morphodynamic system
• Introduction
• River flooding
• Hydraulic roughness and
• bedforms
• Sediment transport
• Mixture effects
• Hydraulic geometry
• Bars, bends, islands
• Overbank sedimentation
• Channel patterns
• vegetation
Introduction
1. Channel pattern classification
• bars
• channels
• channel pattern prediction
2. What really determines bar pattern?
3. What really determines channel pattern?
BIG QUESTIONS
What determines bar pattern?
What determines channel pattern?
1. Channel and bar patterns
Many classifications and
phenomenologies…
A few theories…
Meandering:
initiating from alternating bars (vertical)
initiating from bend instability (planform)
braided river
transitional
meandering river
Alternating bars
W/h < 20 no bars
20 < W/h < 30 stable alternating bars
W/h > 30 dynamic alternating bars
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Andre Ermolaev 8/30
9/30
Kleinhans & van den Berg, cond. Acc ESPL
After Nanson & Knighton (1996)
Channel pattern stability diagram
42.0
50, 900Dtv
van den Berg 1995
Explanation??
Why valley stream power?
Why median grain size?
The power of patterns
Streampower? ω = τu = ρgQS/W
Van den Berg (1995):
potential specific valley-related streampower
where
Braided separated from meandering by
12/30
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Kleinhans & Van den Berg (2010, ESPL)
14/30
• No human impact
• No entrenchment
or confinement
• Data from literature
• Google Earth
• Mean annual Q
indept. of morph
• Valley gradient
unpolluted by
sinuosity
The data
Kleinhans & Van den Berg (2010, ESPL)
Why does it work??
Deliberate misprediction of channel width
wide shallow river
→ narrower, deeper, higher ω
→ braided river
Bank strength!
→ floodplain formation and destruction
(see review paper Kleinhans 2010)
16/30
2. Explaining bar patterns
Bar pattern
interaction flow and sediment
in a channel
width-depth ratio!
Channel pattern
interaction flow (and bars) with banks
out of the channel:
bank erosion and floodplain formation
Ships don’t like bars...
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Forced bars and free bars
Forced bars: stationairy,
initiated and forced by channel curvature
Free bars: migrating,
initiated by perturbation that grows
(=unstable)
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
Bar theory (2)
flow needs length to adapt to bed (bars…)
relaxation length λw:
imagine: momentum!
sediment too!
relaxation length λs:
imagine: (transverse) slope effect
→ bed cannot suddenly jump
22/51
Damping and exciting bars (1)
Bar types:
forced bars: forced by flow curvature; static
position
free bars: spontaneously develop and migrate;
initiated at perturbation (groyne, bend, tree…)
Analogy:
spring-mass-‘damper’
damper can also excite
23/51
Three regimes for free (alternating) bars
Overdamped graphics: after Mosselman et
al. 2006
Underdamped
Excited
24/51
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Bar theory (3)
from theory:
(just accept this…
full derivation in Struiksma et al.!)
bar damping length LD
bar length Lp
Bar theory (4)
so, most important parameter (spring-damper!) is
Interaction Parameter: λs/λw
IP depends mostly on W/h
and a bit on friction and on the slope effect f(θ)
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2
h
W
C
gf
w
s
Bend flow
conservation of momentum AND
logarithmical flow velocity profile
helical (spiral) flow
bed shear stress towards inner bend
inner-bend bar
main flow forced towards outer-bend
transverse movement of momentum
and net transverse flow velocity
Think! infinitely long bend?
Bend flow (2)
with:
s: longitudinal coordinate
n: transverse coordinate
v: velocity (u for us)
z: elevation, s: surface, b: bed
h: depth
R: bend radius
Koen
Blanckaert
Transverse slope effect
Gravity acts on particles on transverse
slope
particles pulled towards outer-bank
counteracts spiral flow
Transverse bed slope effect
Schuurman, Marra & Kleinhans 2013, JGR
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How to reduce transverse slope?
bendway weirs fixed layers
longitudinal dams bottom vanes
Underdamping overshoot!
Struiksma et al (1985)
Adaptation and overshoot (Delft3D)
Examples: a bit overshoot and much excitation
profiles along outer-bend bank
straight bend straight long bend straight
bit overshoot
excitation
Damping and
exciting bars (2)
three regimes:
overdamped
■ just forced bars in
reaction to curvature
underdamped
■overshoot superimposed
on forced bars
unstable, exciting
■ free bars grow spatially;
‘spread like a disease’
■damping length negative
Wider channels: braid bars
Narrow channel
Wide channel: stability of higher wave modes
mode m=2, etc.!
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Is this getting chaotic?
chaotic forcing
violin:
■horsetail on string
river:
■many perturbations
■ turbulence
dominant modes
violin:
■ length of the string, +overtones
river:
■width
Andre Ermolaev
Andre Ermolaev
Andre Ermolaev
Andre Ermolaev
3. From bar pattern to channel pattern
Width-depth ratio!!
remember hydraulic geometry:
width depends on bank strength
(depth depends on width and roughness)
■vegetation
■cohesive sediment
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Courtesy Jim Best
Yep!
John Bridge
Remember: bank strength!?
Imagine a river in sand without mud or
vegetation
θc,banks < θc,bed (slope of bank!)
initially: θ >> θc
so, erosion of bank toe and collapse of
bank!
eroded sediment deposited on bed
shallowing
continue until θ = θc in bankfull conditions
‘threshold channel’
Bars and bank erosion
pools between the bars:
velocity high
bank deeply undercut
celerity and size of bars:
wide river: fast, small bars
→ everywhere bank erosion
→ straight planform
narrow river: slow, large bars
→ alternating bank erosion
→ meandering planform
Alternate bars and bank erosion
low flow conditions!
Bridge (2003) Rivers and Floodplains
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bank sedimentation
floodplain sedimentation
But, does it really work like this?
Ongoing work!
Wout van Dijk PhD finished
Filip Schuurman PhD study ongoing
work in Delft, Illinois and Japan (with us)
Effect of mud in rivers
Wout van Dijk, Wietse van de Lageweg
Meander migration
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1977 Ganges River 1985
1989 1997
1999
Simulation
bend instability
Sun et al 2001
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Camporeale et al 2007
These models
look better
than they are!
Nevengeul van de Vecht,
Junne, bij Dalfsen
One pattern explanation interaction between pools (bars…) and banks
stronger banks
→ narrower channels
→ slower, alternating bars
→ meandering
weaker banks
→ braiding
Bank strength derived from
self-formed floodplains
vegetation
So far state of the art… much work in progress!
Major points
1. What determines bar pattern?
2. What determines channel pattern?
Let’s overheat the brain...
Example calculation for 18th century Rhine
river upstream of Lobith:
S_valley = 0.00016, S_channel = 0.00013
Qmaf = 5580 m3/s, Qbf = 3370 m3/s
D50 = 2 mm, D90 = 8 mm
Width = 520 m, depth = 5 m
calculate:
θ’, Fr, Re, λBW, ωpv, LD, Lp, λs, λw, IP