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Research group River and delta morphodynamics

River patterns

GEO3-4305, 2014

Dr. Maarten Kleinhans

m.g.kleinhans@uu.nl

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

2

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

3

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)

2

22

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