PowerPoint Presentation · 2014-08-21 · sediment budgets and transfer through valleys and multiple basins reconstructing allogenic forcing accommodation space fluvial architecture

21/08/2014

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Faculty of Geosciences

River and delta morphodynamics

Introduction MSc course Fluvial Systems GEO4-4436

Dr. Maarten G. Kleinhans

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C D

B A

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Fluvial systems

geomorphology

sedimentology / geology

engineering

… in one course

→ challenge is to communicate

You cannot truly cross the borders of your own culture

until you master the language of another

(Chaim Potok, Wanderings)

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Four scales

Flow, sediment transport and channel

morphodynamics (morphodynamic loop)

repetition of earlier courses!

River patterns

Bifurcations and avulsion

From source to sink and in between 5

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Four scales (1)

Flow, sediment transport and channel

morphodynamics (morphodynamic loop)

steady uniform flow and backwater effect

equilibrium sediment transport

exner equation

required initial / boundary conditions

effects of changing boundary conditions

and time scale

basics of numerical modelling

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The morphodynamic system

also needed:

source of flow and sediment

initial valley shape

vegetation?

where it all ends: downstream

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flow

sediment

transport

morphology

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Four scales (2)

River patterns

floodplain formation

morphodynamic instability

bar patterns

river patterns

■ underlying physics (sketch)

■ necessary and sufficient boundary conditions

effect of changing boundary conditions

preservation, sedimentology

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Four scales (3)

Bifurcations and avulsion

physics of bifurcation stability

causes of avulsion

accommodation space

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Four scales (4)

From source to sink and in between

simple cases: mass conservation

sediment budgets and transfer

through valleys and multiple basins

reconstructing allogenic forcing

accommodation space

fluvial architecture

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What is the use of all this?

river engineering

river management

nature restoration

geological resources

sediments

water, oil

application to reconstruction

predict effects of global change

sustainable use of sinking/drowning deltas 11

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How do this course? approach

scales

logic

basic techniques

didactics

explicit clarification of terms

play: do yourself

discuss and provide constructive criticisms

reflection on different disciplinary approaches!

(it’s one world…) 12

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Cross-talk: main terminology

Morphodynamics: Quaternary Geology:

upstream boundary

conditions

climate

downstream boundary

conditions

base level, tides, waves

yet another term in the

sediment mass balance

tectonics / subsidence

wash load suspended load

bed + suspended bed

material load

bed load 13

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Course activities Lectures–tutorials

contribute! discussion, questions, calculations,

literature

Read many papers

Computer practicals

to get you started, finish reports at home

provides tools

for creative programming assignment

Delta project

turn one delta inside out:

explain/understand evolution

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Assessment The Matrix: matlab code: (1/3 grade)

5% for small exercises

18% for creative exercise

also presented in 5 min in ppt

The Delta Force: team project (1/3 grade)

turn delta inside out

presented in powerpoint + abstract

individual contributions indicated

The Final Frontier: exam (1/3 grade)

tests insight

STUDY the study guide!

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Literature and resources

Papers

Library, course website

Slides

Course website, make your own notes

Matlab

instructions

matlab help

Us, the lecturers

matlab code

geological and morphological data

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“This lecture: main questions and points”

main questions help you think

answer! your own summary = structure

Before lectures:

prepare! answer questions

about prescribed literature

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How to read literature?

title, abstract

figures + captions

what are you looking for?

write your own papers like this too

hints: http://www.lc.unsw.edu.au/olib.html

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Matlab programming

build up from simple exercises

build tools for data analyses (MSc thesis)

building a simple model helps to understand

existing complicated models

evaluation:

works? (does it run)

understandable? (comments)

does what it has to do? (RTFM)

http://plagiarism.org/?

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Your creative Matlab project

your own creative idea: anything fluvial

flow, particles, avalanches, avulsions..

see things as if for the first time

and ask the ‘stupid’ questions

analyse and plot (available) data

■ sediment transport / bedforms

■ experimental meandering

■ core data of Rhine-Meuse delta

model

■ 1D morphological model (as in Parker E-book)

■ meandering line or 2D cellular braiding

■ Connect other model code/output

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How conduct an investigation?

curiosity → question

hypothesis

method

results

evaluate hypothesis

tell the world

new questions

21/26

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How to make a ppt presentation?

structure

question, hypothesis, methods, results, discussion,

conclusion

kill your darlings

main message?

elevator pitch

figures not (much) words

yet keep core of creative idea!

simple language

practice 22

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Delta project

Choice from some well-documented deltas

unravel causes and evolution:

mechanisms

■ avulsion / mouth bar formation

■ subsidence, peat

boundary conditions

■ wash load

■ base level rise

how representative for other deltas

■ in record

■ wave/tide/fluvial dominated 23

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Present delta project

As in scientific conference:

On the basis of sound research

(not armwaving, not guessing)

2 page referenced abstract with figures

(densely written because scientists are busy)

Brief presentation

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Twisting the lion’s tail

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Physics and stamp collecting however complicated a system,

it must adhere to the laws of physics

e.g. mass conservation

Physics lead to fantastic patterns

mechanisms as explanations

typical cases/classes as helpful tools

Boundary / initial conditions

also cause patterns

e.g. climate, setting such as North Sea basin

forcings as explanations

type locations as helpful tools in reconstruction 26

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Mechanistic explanation (Machamer, Darden, Craver,

Philosophy of Science 2000)

reduce a phenomenon

to the workings of the underlying mechanisms

examples:

role of egg in pancakes

why can we preserve food

by salting or cooling?

key to researchable question:

unpeel your question! 27/26

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Logic

abduction

laws / generalisations major premises

mechanisms

causes / minor premises /

initial and boundary conditions (the world as it was)

effects / consequent

outcome (the world as it is now)

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Scales and explanatory elements explanation:

scale:

mechanisms generalisations causes

1.

river channel,

river reach

flow, sediment

transport

constitutive

(empirical)

relations

required

boundary

conditions

2.

river pattern,

channel belt

bar formation,

floodplain

formation

channel pattern

diagrams

necessary

conditions incl.

hinterland

3.

fluvial plain, river

displacement

bifurcation stability avulsion cycles boundary

conditions,

climate, base

level

4.

valley, delta

subsidence, peat

formation

Wave/tide/fluvial

dominated delta

shapes

Climate, tectonics

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Now what?

Collect + study indicated literature

quick repetition flow, sed tr and morph

learn matlab

new material,

new combinations of approaches

first matlab project, then delta project

professional scientific discourse

scientific presentations

exam: insight, combination, case? 30

8/21/2014

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Faculty of Geosciences

River and delta morphodynamics

Flow, sediment transport

and morphological change MSc course Fluvial Systems GEO4-4436

Dr. Maarten G. Kleinhans

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This lecture: main questions and points

quick review of basic concepts

and equations for

flow

sediment transport

morphology

What are physical concepts behind the

equations?

Sediment transport is nonlinear function of

flow force

so what? 2

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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

3

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Cross-talk: main terminology

Morphodynamics: Quaternary Geology:

upstream boundary

conditions

climate

downstream boundary

conditions

base level

yet another term in the

sediment mass balance

tectonics / subsidence

wash load suspended load

bed + suspended bed

material load

bed load 4

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flow

sediment

transport

morphology

The morphodynamic system

• Fluid mechanics

• Hydraulic roughness

• Bedforms

• Sediment transport

• Hydraulic geometry

• Bars, bends, islands

• Overbank

sedimentation

• Channel patterns

• vegetation

Some essential review of knowledge:

Remember 3rd years course?

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1. Flow: essential fluid mechanics

Steady uniform flow (normal flow) steady

1. Flow strength parameters

2. Turbulent flow uniform

Steady nonuniform flow

3. Sub- and supercritical flow

4. Backwater effect

0

t

u

0

x

u

0

x

u

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Big Questions about Flow

How do we describe flow strength?

Why would flow be nonuniform?

What determines flow resistance?

How account for complications of turbulence?

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Flow strength parameters

u = flow velocity (averaged over depth h)

Q = flow discharge:Q = uhW = uA

τ = flow shear stress: τ = ρghS

S = gradient, ρ = fluid density (1000kg/m3)

ω = stream power: ω = τu

many different symbols in use!

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Subcritical Supercritical

9/32

• slow

• downstream control

• Fr<1

• fast

• no downstr. control

• Fr>1

gh

uFr ghc Rivers?

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10 Bodewes & Leuven (2012), based on dataset Kleinhans

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Backwater effect subcritical flow: downstream roughness affects

upstream flow (e.g. dams, vegetation)

upstream distance over which water depth is

affected: adaptation length

h

S,i

BW : 63% adaptation

S

hBW

3%63

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Turbulence

Self-generated resistance of flow (at boundaries)

Reynolds number

laminar < 500

turbulent > 2000

uDuh Re,Re

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Effect of turbulence

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vertical flow velocity gradient: logaritmic profile

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Flow velocity over depth

z

u

dz

du

* )(ucity shear velowith 10

*

ms

0.40-0.38 Kármán von ofconstant

0

*

0*

**

ln

ln0for

ln:givesofnIntegratio

z

zuu

zu

cu

czu

uz

u

dz

du

z

z

z

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How to deal with turbulence? Turbulence is very complicated and far from solved

use semi-empirical equations

Note: [m/s]/([m/s2][m][m/m])0.5 dimensional

homogeneity

hW

WhR

gRS

u

ff

gRSu

empirical

2

88

Darcy-Weisbach law

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Friction I

friction scales with R and roughness length ks

Semi-empirical formula

90509065

1010

35.2,1,1

086.1log74.52.12log74.58

DtoupDDDk

k

R

k

R

f

s

ss

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Friction II

Use the log-profile to get the friction law

ss

ss

szz

k

RCandRS

k

Ru

e

xxandgRSu

k

Ru

ek

Ruu

zkande

Rzatuugiven

z

zuu

12log18

12log18

log

logln*

14.12ln

*33ln

*

33ln*

1010

10

10

0

0

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Roughness

hydraulic roughness + obstruction = flow resistance

form friction + skin friction = total friction

Bedforms grains

Vegetation

Engineering structures

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Vegetation

well-submerged vegetation

submerged veg with through-flow

through-flowed vegetation

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Steady uniform flow shear:

τ = ρgRS and

Turbulence is accounted for by:

Note: [C]=[m/s]/([m][m/m])0.5 = [m0.5/s]

Thus not dimensionally correct

uRSC 2

C

ug

sk

RC

12log18 10

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2. Sediment transport

Water flow results in sediment transport!

1. Sediments

2. Bedload sediment transport

3. Suspended sediment transport

4. Washload

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Sediments For example plot grain size against rel. cum. freq.:

Ways to measure: sieving, settling tube, lasers

Best method depends on application

Sample Grain Size Distribution (with Extrapolation)

0

10

20

30

40

50

60

70

80

90

100

0.001 0.01 0.1 1 10

Grain Size mm

Perc

en

t F

iner

D50 = 0.286 mm

D90 = 0.700 mm Dx is size such that x percent

of the sample is finer than Dx

Characteristics of sediment: Examples:

D50 = median size

D90 ~ roughness height

Mean and Standard deviation

Skewness/Kurtosis

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Sediment transport

Flow energy, τ = τ’+τ’’

dissipated by bedforms sediment transport

We use: ks’ – grain related roughness (skin friction)

C’ – grain related Chezy

τ’ – grain related shear stress

θ’ – grain related shields number

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The beginning of motion

Shields’ (1936) curve

*

50

:

''

u

note

gDs

Du**Re

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Bedload transport qb = c ∙ δ (c=celerity, δ=layer thickness)

c depends on u* ~ τ1/2

δ is related to τ

So: qb = f(τ3/2)

Include treshold for motion: qb = f(τ’-τc)3/2

Make dimensionless: φb = f(θ’-θc)3/2

φ = dimensionless transport rate

θ’ = dimensionless shear stress (skin friction)

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Bedload transport

Many (semi) empirical forms:

calibrated for limited sets of conditions

MPM

Parker Van Rijn

Ribberink

general form

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Bedload transport Plot sediment transport against shields:

sediment transport is more nonlinear

near beginning of motion

Up to power 16

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Suspended sediment transport Bagnold (1966)

Van Rijn (1984)

depends on sediment characteristics

and flow shear stress to the power of 1.5

total load predictor:

Engelund and Hansen (1967)

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Washload transport (Almost) no exchange between bed and

suspended sediment

Limited by the amount of upstream supply

29/4

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Nonlinear

qb = f(u3) Why? Remember τ = …

Nonlinearity of sediment transport

Now: Why do rivers exist at all?

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Big Questions about Bedforms

How do we know which bedform is stable?

When are bedforms in equillibrium? Are they

in nature?

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Bedforms

Sediment transport bedforms

organised sediment transport and bedform

friction

1. Bedform types

2. Bedform stability

3. Bedform dynamics

Kleinhans (2002)

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Bedforms

30 m

Dunes

Ripples on top of dunes

phenomenon at boundary between two materials

with different physical properties in transport

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Dunes in Parana river

34/41

Best (2005)

Parsons

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Bedforms and bed states

subcritical flow (Fr<0.8)

lower stage plane bed (no motion)

current ripples

current dunes

upper stage plane bed (sheet flow)

supercritical flow (Fr>1)

upper flow regime plane bed

standing waves

antidunes: migrate against the flow

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Dunes and Antidunes

36/41

1gh

uFr

1Fr

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Bedform stability (Van den Berg & van Gelder, 1993)

Emperical Diagram

)('

)(3'

'

12log18'

1''

90

90

31

250*

50

2

2

gravelDk

sandDk

k

hC

gDD

DsC

u

s

s

s

s

Temp

20

10.40 6

upper flow plane bed

because Fr>1!!

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Bedform stability Lines are not hard tresholds,

but gradual transitions

Physically based diagram

Bedforms depend

on flow and sediment

transport rate+mode

Note: bedforms are

often in disequilibrium

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Rivers during bankfull conditions

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Dune tracking

Migration of dunes = sediment transport

Can be measured by dune tracking:

Deviation from a triangle!

Calibration in practice:

cqb 21

60.055.0 withcqb

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Dunamics

Dune length

increases during and

after flood:

Dune height shows

hysteresis during

a flood:

0.0

0.9

1.8

0 7000 14000Discharge (m^3/s)

du

neh

eig

ht

(m)

flood 1982 flood 1988 flood 1995 flood 1997

1982

1988

1995

1997

0

30

60

0 7000 14000Discharge (m^3/s)

wavele

ng

th (

m)

flood 1982 flood 1988 flood 1995 flood 1997

1997

1982

19

95

1988

Wilbers 2004

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Prediction of duneheight Different Equilibrium predictors: f (h,τ,D), e.g.

Relaxation with adaptation time scale

Note: Bedforms are often NOT in equilibrium!

3.0

50

/

//

5.0

3.0

50

5.2-

Twith 3.7

25111.0

h

D

hh

Teh

D

h

cr

cr

T

van Rijn (1984) Julien & Klaassen (1995)

0.5 /

1 , 11 vR eqtimeT t T

t t t te

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Bedforms?

equilibrium form determined by

particle size (Bonnefille number)

mobility (Shields number)

flow regime (Froude number)

in nature often not in equilibrium

→ relaxed adaptation

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Morphodynamics

uniform steady sediment transport

→ no change!

needs gradient in sediment transport to change

morphology

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Modelling: big Questions

How are flow, sed transp and mass

conservation combined to model

morphodynamics?

How do rivers respond to changing boundary

conditions?

What are models (not) good at?

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Sediment continuity / mass conservation

Storage = Flux in – Flux out

This is the Exner equation Used in modeling/to predict changes in morphology

= porosity

= bed level

qb= transport rate

t = time

x = location transport gradient (out-in)

x

q

x

qq

b

outb,inb,

-t

storaget

p

p

1

1

bed level change

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Equilibrium? discharge

and sediment feeder

sediment bed

water depth

h0

slope i

grain flow thickness

hg

When IN = OUT YES!!

So when the slope

remains exactly the same

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flow

sediment

transport

morphology

Model the morphodynamic system

• Introduction

• Hydraulic roughness and

• bedforms

• Sediment transport

• Hydraulic geometry

• Bars, bends, islands

• Overbank sedimentation

• Channel patterns

• vegetation

First 1D modelling,

then 3D modelling

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1D modelling

1. Equations needed

2. Input values needed

3. Model

4. Case study: terrace crossing

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Input values

h S

Q,W

ks

Qb,s D50, D90, bed porosity: λ

Specification of flow:

Specification of sediment transport:

S

→ u(x), h(x)

→ S(x), (x)

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Information propagation

Q, H, z

51

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Numerics

step by step

direction of influence

water level in upstream direction

sediment in downstream direction

boundaries

see Parker e-book chapter 20

AgDegBW

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Morphological change

General rate of change:

1. Exner: /t~qb/x

2. So after sudden change, gradient (and thus qb/x) is large

3. Therefore morph change fast

4. But then gradient decreases and morph change less fast

5. (t) = equil +/- e-t

6. Exponential decrease or increase with representative T:

time

parameter

~63 % of change accomplished at T

T

Think about gradients!

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Case study

Netherlands, river Rhine

Holocene sea level rise

reduced supply of upstream sediment

Result:

In the downstream part: sedimentation (layercake)

In the upstream part: incision (formation terraces)

In between: terrace crossing

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Simple model result elucidates

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Compare with reality

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57 Spitsbergen, Svalbard, see http://blog.geo.uu.nl

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Modelling philosophy (1)

models not good at details in reality

must be specified in initial/boundary conditions

given uncertainty in both initial conditions and laws:

models cannot be verified (Oreskes et al. 1994)

so, models rarely predict well without calibration

■ how much of ‘physics-based’ explanation in calibration?

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Process, noise or boundary condition?

tim

e s

ca

le o

f p

he

no

me

no

n

minute

1 cm 1 m million km 1000 km 1 km

billion

years

1000

years

year

million

years

turbulence

earthquake

plate tectonics, mantle convection

ocean circulation

El Nino and La Nina

glacials and interglacials

storm events: river meandering, coastal erosion

ripples, dunes

coastal dunefields

river terrace forming

delta formation

formation of solar system

meteorite impact

length scale of phenomenon

boundary

condition

noise

noise

forcing

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Modelling philosophy (2)

models good at trends and behaviour!

comprehend results of complicated set of equations

manipulation: sensitivity to parameters

run scenarios for certain changes

mediate between theory (laws of physics) and nature

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Application

build a simple model in matlab

use (your own) model for delta project

understand modellers in practice

approach and value

uncertainties

■ initial and boundary conditions

■ numerics