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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
G3-Giornate Giovani GNRAC
Ferrara, 20 Settembre 2013
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)
G3-Giornate Giovani GNRAC
Ferrara, 20 Settembre 2013
INTRODUCTION�Physical models of sandy beaches are used for the
investigation of the coastal morphodynamic
processes
G3-Giornate Giovani GNRAC
Ferrara, 20 Settembre 2013
INTRODUCTION�Physical models of sandy beaches are used for the
investigation of the coastal morphodynamic
processes
�The larger experimental facilities are equipped with 30-m
wide wave makers and basins
G3-Giornate Giovani GNRAC
Ferrara, 20 Settembre 2013
INTRODUCTION�Physical models of sandy beaches are used for the
investigation of the coastal morphodynamic
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
G3-Giornate Giovani GNRAC
Ferrara, 20 Settembre 2013
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
G3-Giornate Giovani GNRAC
Ferrara, 20 Settembre 2013
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
G3-Giornate Giovani GNRAC
Ferrara, 20 Settembre 2013
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)
G3-Giornate Giovani GNRAC
Ferrara, 20 Settembre 2013
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
G3-Giornate Giovani GNRAC
Ferrara, 20 Settembre 2013
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
G3-Giornate Giovani GNRAC
Ferrara, 20 Settembre 2013
METHODS
Flume dimensions: 1 m length; 0,40 m width; 0,35 m height
Flume dimensions: 18 m length; 0,38 m width; 0,56 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
G3-Giornate Giovani GNRAC
Ferrara, 20 Settembre 2013
METHODS
Flume dimensions: 1 m length; 0,40 m width; 0,35 m height
Model scale 1:100
Models scale 1:50 ÷ 1:15
Testing regimes
G3-Giornate Giovani GNRAC
Ferrara, 20 Settembre 2013
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
G3-Giornate Giovani GNRAC
Ferrara, 20 Settembre 2013
� 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
G3-Giornate Giovani GNRAC
Ferrara, 20 Settembre 2013
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
G3-Giornate Giovani GNRAC
Ferrara, 20 Settembre 2013
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°
G3-Giornate Giovani GNRAC
Ferrara, 20 Settembre 2013
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
G3-Giornate Giovani GNRAC
Ferrara, 20 Settembre 2013
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)
G3-Giornate Giovani GNRAC
Ferrara, 20 Settembre 2013
DATA ANALYSIS – LIM/UPC tests‘Morphological scale effects’ analysis
Mobility rates Shoreline displacement
|A|/|A|P 1
SP 0
Better agreement with prototype
Anthracite
� Anthracite does not improve the models response with respect to sand
G3-Giornate Giovani GNRAC
Ferrara, 20 Settembre 2013
DATA ANALYSIS – LIM/UPC tests‘Morphological scale effects’ analysis
Mobility rates Shoreline displacement
|A|/|A|P 1
SP 0
Better agreement with prototype
Duroplast
� Duroplast show a better response at very small scales in terms of mobility, but not in
terms of shoreline displacement
G3-Giornate Giovani GNRAC
Ferrara, 20 Settembre 2013
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
G3-Giornate Giovani GNRAC
Ferrara, 20 Settembre 2013
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
G3-Giornate Giovani GNRAC
Ferrara, 20 Settembre 2013
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
G3-Giornate Giovani GNRAC
Ferrara, 20 Settembre 2013
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
G3-Giornate Giovani GNRAC
Ferrara, 20 Settembre 2013
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)