G3-Giornate Giovani GNRAC

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1 Quartiere Fieristico di ON THE USE OF LIGHTWEIGHT MATERIALS IN SMALL SCALE MOBILE-BED COASTAL PHYSICAL MODELS Valentina Petruzzelli

2 OUTLINE Objectives Introduction Methods Data analysis LIC/PoliBa tests LIM/UPC tests Results Concluding remarks Future developments

3 OBJECTIVES G3-Giornate Giovani GNRAC Evaluation 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 Lp Lp NL = 7 NL = 14 L L m Scale effects negligible (Hughes&Fowler, 1990) m Scale effects acceptable (Ranieri,1994)

4 INTRODUCTION G3-Giornate Giovani GNRAC Physical models of sandy beaches are used for the investigation of the coastal morphodynamic processes

5 INTRODUCTION G3-Giornate Giovani GNRAC 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

6 INTRODUCTION G3-Giornate Giovani GNRAC 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 m Necessity to set up 3-d physical models at scales smaller than 1:10-1:15 Non-negligible scale effects

7 INTRODUCTION Rigorous 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

8 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-zone morphological models is the adoption of Froude s and Dean s laws (Dean&Dalrymple (2001)) Sea waves propagation The compatibility between these criteria requires: Suspended sediment transport In case of sandy beaches and small-scales, possible choice of too fine sand: cohesive effects could become relevant!

9 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: N L < 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)

10 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

11 METHODS

12 METHODS Geotechnical 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 < D 50 < 0.85 mm, ρ 2000 kg/m 3, R.C. 60 cm Zeolite 0.2 mm < D 50 < 0.7 mm, ρ 1500 kg/m 3, R.C. 70 cm

13 METHODS LIC/PoliBa experimental set-up Model scale 1:100 Flume dimensions: 1 m length; 0,40 m width; 0,35 m height LIM/UPC experimental set-up Models scale 1:50 1:15 Flume dimensions: 18 m length; 0,38 m width; 0,56 m height CIEMito wave flume

14 METHODS Testing regimes Model scale 1:100 Flume dimensions: 1 m length; 0,40 m width; 0,35 m height Models scale 1:50 1:15

15 DATA ANALYSIS LIC/PoliBA tests Models-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

16 DATA ANALYSIS LIM/UPC tests Model-prototype comparisons of beach profiles Different morphodynamic response depending on the profile region Peculiar behaviour of Duroplast with respect to sand and anthracite

17 DATA ANALYSIS LIM/UPC tests Time-dependent evolutions of the shoreline position and mobility rates All materials show an erosive response and similar sedimentological times, although Duroplast shows a more marked behaviour

18 DATA ANALYSIS LIM/UPC tests Analysis of seaward beach slope behaviour Models-prototype beach profiles comparison at scale 1:30; LIM/UPC tests. Anthracite 2 model at scale 1:15; LIM/UPC tests. LIM/UPC Sand β -0.6 ~21 Duroplast β -0.6 ~8 Anthraciteβ -0.6 ~12 Prototype sand β -0.6 ~22

19 DATA ANALYSIS LIM/UPC tests Time-dependent evolution of seaward beach slope behaviour 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

20 DATA ANALYSIS LIM/UPC tests Morphological scale effects analysis Mobility rates LIM/UPC sand A / A P 1 SP 0 Better agreement with prototype Shoreline displacement Results of sandy models agree with Hughes&Fowler(1990) and Ranieri(1994)

21 DATA ANALYSIS LIM/UPC tests Morphological scale effects analysis Mobility rates Anthracite A / A P 1 SP 0 Better agreement with prototype Shoreline displacement Anthracite does not improve the models response with respect to sand

22 DATA ANALYSIS LIM/UPC tests Morphological scale effects analysis Mobility rates Duroplast A / A P 1 SP 0 Better agreement with prototype Shoreline displacement Duroplast show a better response at very small scales in terms of mobility, but not in terms of shoreline displacement

23 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 of apparent cohesion within the emerged beach, its thickness being smaller than the particles capillary rise value Materials likely to electrify for friction or which interact with water are forced to mutual attraction and flotation Friable particles cause the occurrence of water turbidity Lightweight materials show mobility rates not consistent with their fall speed

24 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 to reproduce 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

25 CONCLUDING REMARKS Difficulties in identifying lightweight materials able to improve sand morphological response in small-scale models in the whole beach profile, due to the occurrence of different modes of transport in each region Intrinsic parameters other than settling velocity are paramount to assess the low-density materials reliability for physical modelling, due to the influence given by: Shape Grain size Friction angle Repose angle Capillary rise Porosity Friability Electrification

26 FUTURE DEVELOPMENTS G3-Giornate Giovani GNRAC 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

27 THANK YOU FOR YOUR KIND ATTENTION! G3-Giornate Giovani GNRAC Valentina Petruzzelli, PhD the older Einstein is said to have warned his son strongly of the difficulties in dealing with sediment transport processes. Vollmers (1989) Any Question Assegnista di Ricerca presso Università di Bologna Centro Interdipartimentale per la Ricerca Industriale (CIRI) Edilizia e Costruzioni Laboratorio di Ingegneria Idraulica (LIDR) valentin.petruzzelli@unibo.it