Numerical modelling of wave overtopping of Cubipod armored mound breakwaters with low crest ...

102 Pages·2017·5.5 MB·English

Numerical modelling of wave overtopping of Cubipod
armored mound breakwaters with low crest freeboard
Brecht De Vos
Internal supervisors: Prof. dr. ir. Peter Troch
Prof. dr. ir. Andreas Kortenhaus
External supervisor: Prof. Josep R. Medina
Master's dissertation submitted in order to obtain the academic degree of
Master of Science in Civil Engineering
Laboratorio de Puertos y Costas
Chair: Prof. Josep R. Medina
Departamento de Ingeniería e Infraestructura de los Transportes
Universidad Politecnica de Valencia
Department of Civil Engineering
Chair: Prof. dr. ir. Peter Troch
Faculty of Engineering and Architecture
Ghent University
Academic year 2016-2017
Preface
I would like to thank Prof. Troch for placing me in contact with Prof. Medina, for
making the exchange possible and assisting me whenever I had questions.
I would like to thank Prof. Medina for allowing me to participate in the ESBECO project
currently being conducted in the Laboratory of Ports and Coasts at the Polytechnic
University of Valencia. Also his guidance during the writing of this thesis was greatly
appreciated.
Special thanks to Mapi, Gloria, Jorge and Pepe for their help with the experiments, for
sharing their knowledge while making this thesis and creating a very pleasant
atmosphere in the laboratory.
Finally, I want to express my gratitude to my parents for allowing me to seize this
opportunity to have an international experience. Of course, I can’t forget to thank my
entire family for their endless support.

Departamento de Ingeniería e Infraestructura de los Transportes.
ETSI Caminos, Canales y Puertos (ETSICCP)
Content

Chapter 1 Introduction ..................................................................................................... 1
Chapter 2 Literature review ............................................................................................. 2
2.1 Low-crested breakwaters ....................................................................................... 2
2.2 Breaking conditions ................................................................................................ 3
2.3 Damage ................................................................................................................... 5
2.3.1 Failure modes ................................................................................................... 5
2.3.2 Measuring damage ........................................................................................... 7
2.4 Hydraulic stability of low-crested structures .......................................................... 9
2.4.1 Rock armour units .......................................................................................... 10
2.4.2 Artificial units ................................................................................................. 21
2.5 Hydraulic stability of Cubipods ............................................................................. 26
2.6 Overtopping .......................................................................................................... 32
2.6.1 Empirical formulae ......................................................................................... 32
2.6.2 Tolerable overtopping .................................................................................... 38
2.7 IH-2VOF ................................................................................................................. 40
2.7.1 Introduction.................................................................................................... 40
2.7.2 Program structure .......................................................................................... 42
2.7.3 Calibrating the breakwater model ................................................................. 46
Chapter 3 Experimental setup ........................................................................................ 47
3.1 Equipment for experiments .................................................................................. 47
3.1.1 Wave flume .................................................................................................... 47
3.1.2 Wave generator.............................................................................................. 48
3.1.3 Wave dissipation system ................................................................................ 49
3.1.4 Overtopping measurements .......................................................................... 49
3.1.5 Wave height registration................................................................................ 50
3.1.6 Photo cameras and video cameras ................................................................ 51
3.2 Experiments .......................................................................................................... 52
3.2.1 Design ............................................................................................................. 52
3.2.2 Construction ................................................................................................... 53
3.2.3 Calibration of the wave gauges ...................................................................... 54
3.2.4 Tests without breakwater model (regular and irregular) .............................. 55
3.2.5 Tests with breakwater model (regular and irregular) .................................... 55
3.3 Data processing ..................................................................................................... 60
3.3.1 Incident wave height ...................................................................................... 60
3.3.2 Analysis of the waves ..................................................................................... 61
3.3.3 Porosity & damage ......................................................................................... 61
3.4 IH-2VOF ................................................................................................................. 62
3.4.1 Mesh ............................................................................................................... 62
3.4.2 Geometry ....................................................................................................... 64
3.4.3 Characteristics ................................................................................................ 64
Chapter 4 Results ............................................................................................................ 65
4.1 Laboratory tests .................................................................................................... 65
4.2 IH-2VOF ................................................................................................................. 70
4.2.1 Regular wave comparison without model ..................................................... 70
4.2.2 Regular wave calibration with model ............................................................ 74
4.2.3 Regular wave tests ......................................................................................... 78
Chapter 5 Conclusion ...................................................................................................... 82
Chapter 6 References ..................................................................................................... 84
Annex A: Wave flume with wave gauge locations ...................................................... 89
Annex B: LPCLab Excel Output example regular wave ............................................... 91
Annex C: Mean overtopping discharges ..................................................................... 93
List of figures
Figure 1: Types of low-crested breakwaters .................................................................... 2
Figure 2: Breaker types (Galvin, 1968) ............................................................................. 4
Figure 3: Failure modes .................................................................................................... 5
Figure 4: Heterogeneous packing ..................................................................................... 6
Figure 5: Cubipod armour unit ......................................................................................... 7
Figure 6: Transition heights (van der Meer, 1990) ......................................................... 13
Figure 7: Reduction factor r (van der Meer, 1990) ....................................................... 14
D
Figure 8: Breakwater geometry (Vidal et al., 1995) ....................................................... 15
Figure 9: Stability graphs for different sections at start of damage (Burger, 1995) ...... 17
Figure 10: Cross-section according to Burger (1995) ..................................................... 18
Figure 11: Detail roundhead layout (Kramer and Burcharth, 2004) .............................. 19
Figure 12: Assembled data (Kramer and Burcharth, 2004) ............................................ 20
Figure 13: Cross-section breakwater (Muttray et al., 2012) .......................................... 21
Figure 14: Overview of displaced armour units (all tests) (Muttray et al., 2012) .......... 22
Figure 15: Stability number Ns at start of damage: overall damage (top left), seaward
slope (top right), crest (bottom left) and rear slope (bottom right) (Muttray et al.,
2012) ............................................................................................................................... 24
Figure 16: Stability number Ns at start of damage (Muttray et al., 2012) ..................... 25
Figure 17: Influence of toe berm on double layered breakwaters (Vanhoutte, 2009) .. 27
Figure 18: Influence of toe berm on single layered breakwaters (Vanhoutte, 2009) .... 27
Figure 19: Linearized dimensionless damage using qualitative K (Vanhoutte, 2009) .. 28
D
Figure 20: Linearized dimensionless damage using quantitative K (Vanhoutte, 2009) 28
D
Figure 21: Linearized equivalent dimensionless armour damage in function of stability
number (Gómez-Martín, 2015) ...................................................................................... 29
Figure 22: Measured stability numbers of double-layer cube and Cubipod armour
layers (Gómez-Martín, 2015).......................................................................................... 30
Figure 23: Measured stability numbers of single- and double-layer Cubipod armour
layers (Gómez-Martín, 2015).......................................................................................... 31
Figure 24: Conventional cross-section with parameters ................................................ 32
Figure 25: Design graph (Goda, 1985) ............................................................................ 33
Figure 26: Comparison of EurOtop (2007) formula with EurOtop II (2016) formula ..... 36
Figure 27: Cross section (Gómez-Martín and Medina, 2008) ........................................ 36
Figure 28: Measured versus estimated overtopping rates (Gómez-Martín, 2008) ....... 37
Figure 29: IH-2VOF Preprocessing interface .................................................................. 42
Figure 30: New geometry and mesh with Coral ............................................................. 43
Figure 31: Calculation progress ...................................................................................... 43
Figure 32: Wave gauge registrations .............................................................................. 44
Figure 33: Overtopping analysis ..................................................................................... 45
Figure 34: Visualisation of the model ............................................................................. 45
Figure 35: 2D Wave flume .............................................................................................. 47
Figure 36: Wave generator ............................................................................................. 48
Figure 37: Wave Paddle .................................................................................................. 48
Figure 38: Piston ............................................................................................................. 49
Figure 39: Servomotor .................................................................................................... 49
Figure 40: Original wave dissipation system .................................................................. 49
Figure 41: Final dissipation system layout ..................................................................... 49
Figure 42: Wave gauge locations near wave paddle ...................................................... 50
Figure 43: Wave gauge locations near breakwater model ............................................ 51
Figure 44: Video camera locations ................................................................................. 51
Figure 45: Breakwater cross-section .............................................................................. 52
Figure 46: Drawn cross-section ...................................................................................... 54
Figure 47: Placement of the core ................................................................................... 54
Figure 48: Placement of the filter layer .......................................................................... 54
Figure 49: Placed armour units ...................................................................................... 54
Figure 50: Top view armour layer .................................................................................. 54
Figure 51: LASA-V software ............................................................................................ 60
Figure 52: LPCLab 3.7.1................................................................................................... 61
Figure 53: Displayed mesh .............................................................................................. 63
Figure 54: Mesh quality .................................................................................................. 63
Figure 55: Wave height comparison at the toe .............................................................. 66
Figure 56: H* at the toe in function of H at deep water (for hs=20 cm) ............ 67
m,inc m,inc
Figure 57: H* at the toe in function of H at deep water (for hs=25 cm) ............ 67
m,inc m,inc
Figure 58: Mean wave overtopping discharge q in function of relative freeboard
................................................................................................................. 68
F(cid:1)i(cid:2)gu/r(cid:4)e(cid:5) 5,9(cid:7):(cid:8) (cid:9)M∗ean wave overtopping discharge q in function of incident mean wave
height at the toe ............................................................................................ 69
Figure 60: Mean w(cid:4)a(cid:5)ve,(cid:7) o(cid:8)v(cid:9)e∗rtopping discharge q in function of mean wave period Tm . 69
Figure 61: H and T comparison - Sensor at deep water ............................................. 71
m m
Figure 62: H and T comparison - Sensor 5 ................................................................. 71
m m
Figure 63: H and T comparison - Sensor 10 ............................................................... 72
m m
Figure 64: H and T comparison - Sensor 11 ............................................................... 72
m m
Figure 65: H and T comparison - Sensor 12 ............................................................... 73
m m
Figure 66: Influence of β ................................................................................................. 74
Figure 67: Influence of α ................................................................................................. 75
Figure 68: Influence of porosity ..................................................................................... 76
Figure 69: Overtopping comparison of case E with laboratory...................................... 77
Figure 70: Mean overtopping discharge comparison for hs=20cm ............................... 80
Figure 71: Mean overtopping discharge comparison for hs=25cm ............................... 80
Figure 72: Comparison of overtopping for all tests ........................................................ 81
List of tables
Table 1: Breaker classification according to Galvin (1968) .............................................. 3
Table 2: Empirical coefficients a and b by Allsop (1983) ................................................ 10
Table 3: Summary of test conditions (van der Meer, 1998b) ........................................ 12
Table 4: Damage levels with damage level parameter S according to Vidal et al., 1995.
d
........................................................................................................................................ 16
Table 5: Coefficients by Vidal et al. (1995) ..................................................................... 16
Table 6: Summary of test conditions (Vidal et al., 1995) ............................................... 16
Table 7: Different types of tests (Burger, 1995) ............................................................. 18
Table 8: Summary of conditions by Kramer and Burcharth (2004) ................................ 19
Table 9: Summary of conditions by Muttray et al. (2012) ............................................. 22
Table 10: Incident wave heights resulting in damage levels (Vanhoutte, 2009) ........... 26
Table 11: Coefficients a and b for straight slopes (Owen, 1980) ................................... 33
Table 12: Tolerable overtopping according to Eurotop II (Van der Meer et al., 2016) .. 39
Table 13: Regular wave characteristics hs=20cm (20 waves per test) ........................... 56
Table 14: Regular wave characteristics hs=25cm (20 waves per test) ........................... 57
Table 15: Irregular wave characteristics (1000 waves per test) .................................... 58
Table 16: Mesh characteristics ....................................................................................... 62
Table 17: Initial material characteristics ........................................................................ 64
Table 18: Porosity measurements .................................................................................. 64
Table 19: Wave characteristics of cases without model ................................................ 70
Table 20: Initial characteristics ....................................................................................... 74
Table 21: Average overtopping discharges .................................................................... 75
Table 22: Average overtopping discharges .................................................................... 75
Table 23: Porosity combinations for calibration ............................................................ 76
Table 24: Resulting overtopping discharges ................................................................... 77
Table 25: Wave parameters introduced into the IH-2VOF model for hs=20cm ............ 78
Table 26: Wave parameters introduced into the IH-2VOF model for hs=25cm ............ 79
Chapter 1 Introduction
This thesis was made during the start of a project called ESBECO which is an acronym
for “EStabilidad hidráulica del manto, BErma y COronacion de dique en talud con
rebase y rotura por fondo”. The translation of this title is “Hydraulic stability of armour
layer, toe berm and crest of breakwaters with overtopping and breaking wave
conditions”. Breakwaters with a significant amount of overtopping in breaking wave
conditions are not abundantly studied in literature. The majority of design rules and
formulae are based on small-scale experiments, conducted in non-breaking wave
conditions and either negligible overtopping or enormous overtopping (low-crested
structures). Nowadays, breakwaters in breaking or partially breaking conditions (H >
s
0.4 h) and with relevant overtopping (0.5 < R /H < 1.0) are becoming important due to
c s
the desire of reduced visual impact and the imminent consequences of climate change.
Both overtopping and hydraulic stability will be investigated during the project.
The objective of this thesis was to provide a numerical model that represents the wave
flume in the laboratory and that is able to achieve comparable results obtained from
2D small scale physical tests.
In the second chapter a literature study is presented of the different aspects related to
the subject. Breaking wave conditions, damage quantification, hydraulic stability of
low-crested structures and Cubipod armour units, overtopping formulae and the
numerical software IH-2VOF will be discussed.
The experimental setup, characteristics of the experiments and the methodology to
analyse the data will be elaborated in chapter three.
In chapter four, the results from the tests conducted in the laboratory and executed
with the software will be compared and discussed.
Finally, in chapter five a conclusion will be formulated about the obtained findings and
recommendations for future research will be given.

1
Chapter 2 Literature review
2.1 Low-crested breakwaters
Low-crested structures differ from conventional breakwaters in that way that they
allow some or even severe overtopping. Hydraulic stability of armour units on the
seaward slope can be higher than for non-overtopped structures since a lot of wave
energy can pass over the structure. Armour units on the crest and the rear slope,
however, are subjected to wave attack resulting in a lower hydraulic stability.
Three types of low-crested breakwater can be distinguished according to Van der Meer
and Pilarczyk (1990). Dynamically stable reef breakwaters are mounds of stones
without core or filter layer which are allowed to reshape when subjected to wave
attack. Statically stable low-crested breakwaters have a smaller freeboard compared
to conventional breakwaters and are not allowed to undergo changes of the cross-
section. Finally, statically stable submerged structures have a crest below the still
water level. In this thesis, only the second category is dealt with so the other types are
not discussed. (Figure 1)

Figure 1: Types of low-crested breakwaters
2
2.2 Breaking conditions
When waves propagate from deep to shallow waters, various transformations that can
occur are refraction, shoaling, diffraction, dissipation due to friction & percolation,
breaking, additional growth due to the wind, wave-current interaction and wave-wave
interaction.
The surf zone can be defined as the region between the seaward boundary of wave
breaking and the limit of wave uprush. In this zone, wave breaking is the dominant
hydrodynamic process.
Waves approaching the coast experience a decrease in wave length L and possibly an
increase in height H, causing the steepness H/L to increase with decreasing water
depth. Waves break as they reach a limiting steepness, which is a function of the
relative depth d/L and beach slope tanβ.
Galvin (1968) proposed four types of breaking waves: spilling, plunging, collapsing and
surging waves. The wave may be classified in one of these groups in function of the
breaker or surf similarity parameter, ξ , defined as:
0
(cid:21) Eq. 1
(cid:20)
(cid:4)(cid:12) (cid:22)
(cid:11)(cid:12) = (cid:14)(cid:15)(cid:8)(cid:16)(cid:17) (cid:19)
(cid:18)(cid:12)
In this equation, α represents the angle of the beach or bottom, H denotes the wave
0
height in deep water and L is the wave length in deep water conditions. The surf
0
similarity parameter can be seen as the ratio between the steepness of the bottom
slope and the wave steepness.
Table 1: Breaker classification according to Galvin (1968)
Breaker type Surf similarity parameter range
Spilling ξ < 0.5
Plunging 0.5 < ξ < 3.3
Collapsing/Surging 3.3 > ξ
In spilling breakers, the wave crest becomes unstable and runs down the shoreward
face of the wave, creating a foamy water surface. This tends to occur for waves with a
3