presentazioneg3-2013 vpetruzzelli pdf.ppt [modalit ... · g3-giornate giovani gnrac ferrara, 20...

G3-Giornate Giovani GNRACQuartiere Fieristico di Ferrara, 20 Settembre 2013

“ON THE USE OF LIGHTWEIGHT MATERIALS IN SMALL SCALE MOBILE-BED COASTAL PHYSICAL MODELS”

Valentina Petruzzelli

G3-Giornate Giovani GNRAC

Ferrara, 20 Settembre 2013

OUTLINE

� Objectives

� Introduction

� Methods

� Data analysis

� LIC/PoliBa tests

� LIM/UPC tests

� Results

� Concluding remarks

� Future developments



OBJECTIVESEvaluation of the morphodynamic response of different

granular materials in small-scale mobile-bed models

� Identify the intrinsic characteristics of the particles likely

to influence on their morphodynamic behaviour during

the tests

�Determine the suitability of lightweight particles to

perform models at smaller scale than those considered

as reliable

7p

L

m

LN

L= ≤ 14

p

L

m

LN

L= ≤

Scale effects negligible

(Hughes&Fowler, 1990)Scale effects acceptable

(Ranieri,1994)



INTRODUCTION�Physical models of sandy beaches are used for the

investigation of the coastal morphodynamic

processes





processes

�The larger experimental facilities are equipped with 30-m

wide wave makers and basins





processes

�The larger experimental facilities are equipped with 30-m

wide wave makers and basins

�Sediment dynamics modelling often

requires to represent segments of

shoreline larger than 300-450 m

�Necessity to set up 3-d physical

models at scales smaller than

1:10-1:15

Non-negligible scale effects



INTRODUCTIONRigorous downscaling would require the adoption of

several similitude laws based on the preservation in model

and prototype of specific dimensionless numbers:

Use of fresh water and natural sediments:Incompatibility among similitude criteria

Practical strategy:select only some relevant processes to be down-scaled

Scale effects



INTRODUCTION�A multitude of down-scaling methods proposed to

reproduce the nearshore morphodynamics(Le Méhauté (1970), Noda (1972), Kamphuis (1982), Dean (1973, 1985),

etc...)

�The most acknowledged approach for surf-zonemorphological models is the adoption of Froude’sand Dean’s laws (Dean&Dalrymple (2001))

�The compatibility between these criteria requires:

In case of sandy beaches and small-scales, possible choice

of too fine sand: cohesive effects could become relevant!

Sea waves

propagation

Suspended

sediment

transport



INTRODUCTION�Since the dependency of fall speed on density and

grain size, some authors considered the use of lightweight materials to minimize scale effects(Paul et al. (1972), Noda (1971, 1972), Kamphuis (1974, 1975, 1991), Aminti

(1983, 1998), Ranieri (2002), Henriquez et al. (2008), Grasso et al. (2009))

�Scales investigated: NL < 10

�Materials: Bakelite, PVC, Plexiglass, Crushed coal,

Pumice, Polyester, PMMA, etc...

ADVANTAGE: Fulfillment of more than one sediment

transport similitude law

DISADVANTAGE: Introduction of additional uncertainties (e.g., misleading particles accelerations, difficulties on bed-forms reproduction, bad simulation of the incipient motion condition)



METHODS�Step 1: Sediments Geotechnical Characterization

�Step 2: 2-d Froude's models at scale 1:100

�Step 3: 2-d Froude's models at scales 1:50÷1:15

Investigation limit: No sediment transport similitude laws were used!

Prototype-target conditions:Full scale experiments with planar sloping beaches and

irregular wave conditions (Dette&Uliczka,1987)

Flume dimensions: 324 m long 7 m deep 5 m wide



METHODS



METHODSGeotechnical Characterization� Grain size analyses� Settling velocity measurements� Direct shear tests� Hydraulic conductivity measurements� Density measurements� Capillary rise measurements� Angle of repose measurements

Preliminary tests:Pyrolusite

0.35 mm < D50 < 0.85 mm, ρ ≈ 2000 kg/m3, R.C. ≈ 60 cm

Zeolite 0.2 mm < D50 < 0.7 mm,

ρ ≈ 1500 kg/m3, R.C. ≈ 70 cm



METHODS

Flume dimensions: 1 m length; 0,40 m width; 0,35 m height


CIEMito wave flume

Model scale 1:100

Models scale 1:50 ÷ 1:15

LIC/PoliBa experimental set-up

LIM/UPC experimental set-up



METHODS


Model scale 1:100

Models scale 1:50 ÷ 1:15

Testing regimes



DATA ANALYSIS – LIC/PoliBA testsModels-prototype comparison of beach profiles

� Resins: instability of the initial beach profile, too high

mobility rates

� Melaminic plastic: floating of grain aggregates, too low

mobility rates, slight accretive behaviour

� Fine grained materials (Silica sands+Glass microspheres): resistance to mobilization due to scale effects +

apparent cohesion

� Anthracite: good mobility rates



� Different morphodynamic response depending on the profile region� Peculiar behaviour of Duroplast with respect to sand and anthracite

DATA ANALYSIS – LIM/UPC testsModel-prototype comparisons of beach profiles



DATA ANALYSIS – LIM/UPC testsTime-dependent evolutions of the shoreline position and mobility rates

� All materials show an erosive response and similar sedimentologicaltimes, although Duroplast shows a more marked behaviour



Anthracite 2 model at scale 1:15; LIM/UPC tests.

DATA ANALYSIS – LIM/UPC testsAnalysis of seaward beach slope behaviour

Prototype sand β-0.6~22°

LIM/UPC Sand β-0.6~21°

Models-prototype beach profiles comparison at scale 1:30; LIM/UPC tests.

Duroplast β-0.6~8°

Anthracite β-0.6~12°



Scale 1:30 Scale 1:50

� As the model evolution approaches to the equilibrium, the seaward

beach slope tends toward a steady value (typical of the material)

� Lightweight materials behaviour is inverted with respect to sand one

DATA ANALYSIS – LIM/UPC testsTime-dependent evolution of seaward beach slope behaviour



DATA ANALYSIS – LIM/UPC tests‘Morphological scale effects’ analysis

Mobility rates Shoreline displacementLIM/UPC sand

|A|/|A|P 1

SP 0

Better agreement with prototype

� Results of sandy models agree with Hughes&Fowler(1990) and Ranieri(1994)




Mobility rates Shoreline displacement

|A|/|A|P 1

SP 0


Anthracite

� Anthracite does not improve the models response with respect to sand




Mobility rates Shoreline displacement

|A|/|A|P 1

SP 0


Duroplast

� Duroplast show a better response at very small scales in terms of mobility, but not in

terms of shoreline displacement



RESULTS

� Lightweight materials spherical shaped, characterized by

low values of the repose and friction angles, show

instability of the beach profile and too high mobility rates

� Fine-grained and porous particles are not enough

mobilized in small-scale tests, due to the occurrence ofapparent cohesion within the emerged beach, its thickness

being smaller than the particles capillary rise value

� Materials likely to electrify for friction or which interact withwater are forced to mutual attraction and flotation

� Friable particles cause the occurrence of water turbidity

� Lightweight materials show mobility rates not consistentwith their fall speed



RESULTS

� Not all kinds of plastic particles have to be excepted,

because, in case they do not show a tendency to electrify,

they are likely to show good mobility rates

� Some doubts arise on low-density materials suitability toreproduce offshore zone sediment transport

� In terms of ‘morphological scale effects’:

• Sandy models results in good agreement with

Hughes&Fowler (1990) and Ranieri (1994)

• Anthracite granules show a bad response, both in terms of

shoreline displacement and mobility rates

• Duroplast at very small scales presents a better agreement with prototype than sands in terms of mobility,

although its more marked tendency to berm erosion



CONCLUDING REMARKS� Difficulties in identifying lightweight materials able to

improve sand morphological response in small-scalemodels in the whole beach profile, due to the occurrenceof different modes of transport in each region

� Intrinsic parameters other than settling velocity areparamount to assess the low-density materials reliabilityfor physical modelling, due to the influence given by:

• Shape• Grain size• Friction angle• Repose angle• Capillary rise• Porosity• Friability• Electrification



FUTURE DEVELOPMENTS

� Further investigations are needed to provide in depth knowledge about the use of lightweight materials in

small-scale mobile bed physical models

Confirm their effective use or lead to the conclusion of their not suitability

� Focus and deepen the aspects of correlation between the sediment intrinsic characteristics and the fluid-sediment interaction

Possibly derive specific sediment transport scaling laws suitable for low-density materials



THANK YOU FOR YOUR KIND ATTENTION!

“… the older Einstein is said to have

warned his son strongly of the

difficulties in dealing with sediment

transport processes.”

Vollmers (1989)

Any

Question

Valentina Petruzzelli, PhD

Assegnista di Ricerca presso

Università di Bologna

Centro Interdipartimentale per la

Ricerca Industriale (CIRI) Edilizia e

Costruzioni

Laboratorio di Ingegneria Idraulica

(LIDR)

[email protected]

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