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Seminar Coastal MorphodynamicsSeminar Coastal Morphodynamics

IMAU, Utrecht University, 2008IMAU, Utrecht University, 2008

2Huib de Swart

Prof. dr. Paolo Blondeaux

Prof. dr. Giovanna Vittori

Dpt. of Environmental Engineering

University of Genoa, Italy

Lecturers:

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General objective of this seminarGeneral objective of this seminar

Discuss physical processes that cause theDiscuss physical processes that cause the

presence of undulations of the sea bottompresence of undulations of the sea bottom

Horizontal length scale ~ 10 cm ; height ~ 2 cm

Generation timescale ~ hours

Due to: waves

Relevance: wave prediction, estimation of sediment transport

Example 1: Ripples at the beach

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Example 2: Tidal sand waves

Horizontal length scale ~ 500 m; height ~ 2 m

Generation timescale ~ years

Due to: tides

Relevance: e.g, buckling of pipelines, navigation

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Sand waves in the Marsdiep

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Example 3: Sand ridges on the outer and inner shelf

NetherlandsNetherlands

NorthNorth

SeaSea

BritainBritain

Tidal sand ridgesTidal sand ridges Shoreface-connected sand ridgesShoreface-connected sand ridges

Horizontal length scale ~ km; height ~ 5-10 m

Generation timescale ~ 100 years

Due to: tides and storm-driven currents

Relevance: sand mining, coastal stability

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Research questionsResearch questions

1. Which mechanisms are responsible for

the formation and maintenance of

rhythmic topography in coastal seas?

2. Can we predict the characteristics of bedforms?

• spatial pattern

• migration speed

• height

3. What is the response of bottom patterns to

- human interventions (e.g., extraction of sand)?

- sea level changes?

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1. Collection and analysis of field observations

(identify phenomena+ describe behaviour)

Research approaches:

Problems:

• lack of data

• selection of spatial + temporal resolution

• selection of spatial + temporal extent

• what is transient/nontransient behaviour?

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Advantages: data obtained under controlled conditions

Problems: link to reality (scaling problems)

2. Collection and analysis of laboratory data

C. Paolo

Univ. of Minnesota

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Model is proposed, field data -> parameter values,

next predictions

Advantages: fast and flexible

Disadvantage: yields little insight in basic physics

3. Use of empirical or semi-empirical models

Knaapen & Hulscher (2005)

Example: Regeneration of sand waves after dredging

(Seto Inland Sea, Japan)

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4. Use process-oriented numerical models

Examples:

DELFT3D-MOR

MIKE21 (Denmark)

Telemac (France)

These models use physical laws for water motion,

sediment transport and bottom evolution

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Advantages: based on physics, results often reliable and useful

Disadvantages: slow, difficult to gain insight

Example: morphodynamic simulation Dutch coastal zone

(near IJmuiden) with Delft3D

Lesser et al

(2004)

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-Based on simplified descriptions regarding the

• geometry

• physical processes involved

-Designed to gain understanding about the basic physics

4. Use idealised process-oriented models

Example 1: model for tidal sand waves

sea surface

bottom

tidal current

x

z

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Advantages: fast + flexible, suitable for sensitivity studies

Disadvantages: how to choose relevant processes?

link with reality

Example 2: Model for shoreface-connected sand ridges

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Idealised models are successful in explaining the formation

of many different types of bottom patterns

Two types of models:

1. Template models

-pattern in the water motion forces the pattern in the bottom-no feedback bottom water motion

Example of template pattern:

submarine longshore bars

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2. Self-organisation models

-bed forms develop as inherent (or free) instabilities

of the coupled water-coupled system

-feedback water motion and erodible bottom is crucial

spatial/temporal scales of bedforms are

uncorrelated with those of external forcing

STABLE

decay

UNSTABLE

growth

side view:side view:

brown: bottombrown: bottom

blue: waterblue: water

yellow: sand fluxyellow: sand flux

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2.2. Demonstrate that many bottom patterns in Demonstrate that many bottom patterns in

coastal seas emerge as coastal seas emerge as inherent instabilitiesinherent instabilities

of the coupled water-bottom systemof the coupled water-bottom system

(focus on sand ridges, sand waves and ripples)(focus on sand ridges, sand waves and ripples)

3.3. Behaviour of bedforms can be analysed withBehaviour of bedforms can be analysed with

mathematical methods (stability analysis)mathematical methods (stability analysis)

Specific objectives of this seminarSpecific objectives of this seminar

1.1. Discuss structure of models that simulateDiscuss structure of models that simulate

water motion + their feedbacks with sedimentwater motion + their feedbacks with sediment

transport and morphology in coastal seastransport and morphology in coastal seas

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Structure of morphodynamic models:Structure of morphodynamic models:

water motion bottom evolution

sediment transport

We need to

- solve the flow structure

- find expressions for transport of sediment

(due to the water motion)

- evolution of the bottom

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water motion bottom evolution

sediment transport

Four steps to explore

morphodynamic self-organisation:

22.. Find an equilibrium state (without bedforms)

1. Formulation of a model :

4. 4. Perform nonlinear (stability) analysis

finite-amplitude behaviour of bars and ridges

3. 3. Perform linear stability analysis

dynamics of small perturbations, arbitrary scales, do they grow?

each perturbation -> growth rate

mode with the largest growth rate: the preferred mode

=> spatial pattern, migration speed, growth time scale

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growth growth rate versus versus

longshore wavenumberlongshore wavenumber

spacing: 7.6 km

timescale: ~1000 yr

migration: 2 m/yr

spatial patterns of modes

light colours: crests

dark colours: troughs

Example of model output

(here: shoreface-connected sand ridges)

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Overview of this seminar:

9 oral sessions (default: Monday 11.00-13.00)

9 practise periods (Wednesday 9.00-11.00)

Specific time schedule:

www.phys.uu.nl/~deswart/morpho08_seminar

Credits (3.75 ECTS) will be based on

* presence at oral lectures

* answers to exercises during practise periods

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• questions about theory

• simulations with numerical codes

Oral sessions

• based on recent key literature

• read 1 paper/session as preparation

Practise periods

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Session 2: Modelling sediment transport

Literature:

Huntley, D.A. and A.J. Bowen 1990. Modeling sand transport on continental shelves.In: Modelling Marine Systems 1, 222-254.

(29-09-2008)

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Session 3: Introduction boundary layer theory (06-10)

• Internal friction and coastal morphodynamics

• Laminar versus turbulent flow

• Vertical structure of oscillatory currents and streaming

Literature:

Mei, C.C. et al, 2007. The applied dynamics of ocean

surface waves, section 9.2. World Scientific.

sea surface

residual

circulation cells

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Session 4: Tidal sand ridges

Literature:

Huthnance, J. 1982. On one mechanism forming

linear sand banks. Est. Coastal Shelf Sci. 14, 77-97.

(13-10-2008)

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Session 5: Sand waves

Literature:

Besio, G., P. Blondeaux, M. Brocchini and G. Vittori, 2004.

On the modelling of sand wave migration.

J. Geophys. Res. 109 C04018.

(20-10-2008)

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Session 6: Shoreface-connected sand ridges

Literature:

Trowbridge, J. 1995. A mechanism for the formation and

maintenance of shore-oblique sand ridges on storm-dominated

shelves. J. Geophys. Res. 100 (C8), 16071-16086.

(27-10)

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Session 7: Turbulent boundary layers under sea waves

Literature:

lecture notes of G. Vittori

(10-11-2008)

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Session 8: Turbulent bottom boundary layers II

Literature:

lecture notes of G. Vittori

(17-11)

Computed shear stress at the wallversus dimensionless time t

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Session 9: Sand ripples under sea waves

Literature:

lecture notes of G. Vittori

(24-11)

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Session 8: Turbulent bottom boundary layers II

Literature:

lecture notes of G. Vittori

(17-11)

eddy

viscosity

height

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Session 9: Two equations models

Literature:

lecture notes of G. Vittori

(date: ….)

Computed shear stress at the wallversus dimensionless time t