EDF
Direction
et
Recherches
des Etudes
ElectricitéEDF
Direction des Etudes et RecherchesElectricité
de France et EnvironnementService Applications de l'Electricité
Département Laboratoire National d'Hydraulique Groupe HydrauliqueMaritime 6,Quai Watier BP49 F - 78401 CHATOU CEDEX Phone: (33 1)30877252 Fax: (33 1)30 87 80 86 BENOIT M., DONNARS Ph.
MAST-2 RMBFM Project - FINAL REPORT INTERACTIONS IN THE STABILITY OF MAIN-ARMOUR AND TOE-BERM FOR A RUBBLE-MoUND BREAKWATER :
AN EXPERIMENTAL SruDY UNDER BOTH LONG -CRESTED AND SHORT -CRESTED WAVES
HE-42/96/002/A Volume 1 : Text,tables and figures
Related documents : see Volume 2 : Appendices
29 JAN
.1
9
96
B. CLAUDE, P.COURCIER and F. VINET have been participating in the laboratory experiments in the LNH multidirectional wave basin.
Abstract:
This study is concerned with one particular aspect of breakwater failure modes within the project Rubble Mound Breakwater Failure Modes (RMBFM) of MAST-2 European Research Program. A programme of laboratory experiments has been carried out in a multidirectional wave basin in order to investigate the interactions between the main-armour and the toe-berm for the stability of a rubble-mound breakwater.
The present experiments continue previous tests that have been performed in a wave flume at the University of Bologna in Italy. We focus here more closelyon three-dimensional effects, by considering the three following aspects :
- behaviour on trunk section and at round-heads (effect of breakwater's geometry) - wave obliquity (effect of angle of wave incidence)
- wave directionality (effect of angular spreading of wave energy)
These effects are investigated for two-main armour slopes and two wave steepnesses. The results of tests are presented, both for the trunk section and the round-head, and compared with other experimental results and existing design formulas.
This report is the final contribution of LNH to MAST-2 RMBFM Project. This project has been partly funded by the European Union under Contract MAS2-CT92-0042 and by the French Sea State Secretary (STCPMVN).
EDF
Direction des Etudes et RecherchesElectricité de France
Service Applications de I'Electricité
et
EnvironnementDépartement Laboratoire National d'Hydraulique
Groupe HydrauliqueMaritime
6,Quai Watier BP49 F -78401 CHATOU CEDEX Téléphone : (33 1)30 87 72 52 Fax: (33 1)30 87 80 86 BENOIT M., DONNARS Ph.
Projet MAST -2 RMBFM - RAPPORT FINAL INTERACTIONS ENTRE LA CARAPACE ET LA
BUTÉE DE PIED POUR LA STABILITÉ D'UNE DIGUE À TALUS ETUDE EXPÉRIMENTALE EN HOULE MONODIRECTIONNELLE
ET MULTIDIRECTIONNELLE
HE-42/96/002/ A Volume 1 : Texte, tableaux et figures
Docurnents associés: voir Volume 2 : Annexes.
2
9
JAN.
19
95
B. CLAUDE, P.COURCIER et F. VINET ont participé aux essais en laboratoire, réalisés
dans la cuve à houle multidirectionnelle du LNH. Résumé:
Cette étude conceme un aspect particulier parmi les modes de destruction d'une digue à talus à l'intérieur du projet européen "Rubble Mound Breakwater Failure Modes (RMBFM)" du programme de recherches MAST ..2. Une série d'expériences en laboratoire ont
été réalisées dans une cuve à houle multidirectionnelle dans Ie but d'examiner les interactions
entre la carapace et la butée de pied pour la stabilité d'une digue à talus.
Ces essais sont effectués dans la continuité d'essais en canal à houle réalisés
précédemment à l'Université de Bologne (Italie). L'attention est ici plus particulièrement portée sur les effets tridimensionnels, en considérant les trois aspects suivants :
- comportement en partie courante et au musoir (effet de la géométrie de la digue)
- direction de la houle (effet de l'angle d'attaque de la houle sur l'ouvrage)
- répartition directionnelle (effet d'un étalement angulaire de l'énergie de houIe)
Ces effets sont étudiés pour deux pentes de talus et deux cambrures de houIe. Les
résultats des tests sont présentés, pour la partie courante de la digue et Ie musoir, et comparés
à d'autres résultats expérimentaux ainsi qu'à des formules de dimensionnement existantes. Cette note constitue Ie rapport final du LNH pour sa participation au projet
RMBFM de MAST-2. Cette étude a été co-financée par l'Union Européenne (Contrat MAS2 -CT92-0042) et par Ie Secrétariat d'Etat chargé de la Mer (STCPMVN).
MAST -2 RMBFM Project - FINAL REPORT Titre:
INTERACfIONS INTHE STABll..ITY OF MAIN-ARMOUR AND TOE-BERM FOR A RUBBLE-MoUND BREAKWATER:
AN EXPERIMENTALSTUDYUNDERBOm LoNG-CRESTED
AND SHORT-CRESTED WAYES
Référence: HE-42196/0021 A Auteurs: BENOIT M., DONNARS Ph.
AID: E4201R(95) Attributs d'ordonnancement :
Type de rapport: NOTE TECHNIQUE Nombre total de pages 148
Autorisation de signalement: OUI Classement interne : OTE42M50
Mots clés :HOULE - DIGUE - DIGUE A TALUS - BRISE-LAMES -
STABILlTE-DlMENSIONNEMENT DE DIGUES - TESTS EN CUVE A HOULE
Document suivant la procédure AQ DER NON
Document concernant la sûreté nucléaire NON
Document concemant la disponibilité NON
Prédiffusion formalisée NON
Vérification indépendante NON
ResponsabIe de la vérification
Niveau AQ Niveau I NiveauIJ
Rédacteurs Chefde Projet Chef de Groupe Chef de Département
Indice Nom/Visa/Date Nom/Visa/Date Nom/Visa/Date Nom/Visa/Date
A M.BENOIT M.BENOIT C. TEISSON I-P. CHABARD
Ph. DONNARS
~/~,
l~II(,b
bÖ(l/qb~
I~
(lil" l1i"~
j/H-
~
~ Motif de révision : B M otif de révision :C
Diffusion: Indice: A , :I .mR. I~}M) Nbre d'exemplaires édités : 28
Rapport entier Page degarde + fiche de diffusion(P)
Page de garde+ Synthèse+fiche de diff(S) Rapport sans lesannexes (A)
Destinataires Nbre Destinataires Nbre Destinataires P/S/A
Unité Documentaire 1
Fonds AQ (IPN - SID) DER/AEE M.I-P. Allard P
MM Derive-Oddou : 1 LNH
MM Chabard-Nicollet 1 Aalborg Univ. (OK)
Pr. Burcharth 2 LNH/GHM MM Teisson 1 Participants RMBFM 10 MM Benoit 1 Donnars 1 STCPMVN Claude 1 MM Monadier-Piet 2 Courcier 1 Vinet 1 Circul. Agents GHM 1
Interactions in the stability of main-armour and toe-berm for a rubble-mound breakwater :An experimental study
under both long-crested and short-crested waves. Michel BENOIT and Philippe DONNARS (EDF - LNH) MAST 2 - RMBFM Project (Contract: MAS2-CT92-0042)- Final report (HE-42/96/002/A) Page 6
E
x
ecuti
ve
ab
str
a
ct
This study is concerned with one particular aspect of breakwater failure modes within the project Rubble Mound Breakwater Failure Modes (RMBFM) of MAST-2 European Research Program. A series of laboratory experiments has been carried out in a multidirectional wave basin in order to investigate the interactions between the main-arrnour and the toe-berm for the stability of a rubble-mound breakwater.
The present experiments continue previous tests that have been performed in a wave flume at the University of Bologna in Italy. We focus here more closelyon three-dimensional effects, by considering the three fóIlowing aspects :
- behaviour on trunk section and at round-heads (effect of breakwater's geometry) - wave obliquity (effect of angle of wave incidence)
- wave directionality (effect of angular spreading of wave energy)
These effects are investigated for two-main armour slopes and two wave steepnesses. The results of tests are presented, both for the trunk section and the round-head, and compared with other experimental results and existing design formulas.
Some important conclusions from the analysis are reported below: • stability of the armour layer (trunk section) :
The present results compare quite weIl with other experimental tests (Galland, 1993 ; Lamberti, 1994) and agree properly with the design formulas of Hudson and Van der Meer. However, some differences between present data and predictions from these above formulas are observed, highlighted and discussed.
• stability of the toe-berm (tronk section):
The formula from Gerding (1993) lies in acceptable agreement with present experimental results, although observed damage levels are slightly higher.
• interaction processes :
The major feature of the interaction process between main-armour and toe-berm is an increase of damage to the arrnour when the toe-berm is unstable. This may lead to a total failure of the armour if the toe-berm is sufficiently eroded to fail in providing static support to the armour. On the opposite, minor effect of main-armour on toe-berm stability was observed.In particular the toe-armouring process appears to occur only marginaIly and under precise conditions.
As the effects of interaction processes between main-armour and toe-berm appear quite weak (unless toe-berm stones are significantly lighter than their design value),it is proposed to use as a "rule of thumb" existing stability formulas to design both main-armour and toe-berm independently. The fuIl breakwater profile should then be tested in a wave basin with a correct modelling of local bathymetry in order to validate itsdesign.
This report is the final contribution of LNH to MAST -2 RMBFM Project. This project has been partly funded by the European Union under Contract MAS2-CT92-0042 and by the French Sea State Secretary(STCPMVN).
Interactions in the stability of main-armour and toe-berm for a rubble-mound breakwater : An experimental stud)'
under both long-crested and short-crested waves. Michel BENOIT and Philippe DONNARS (EDF - LNH)
MAST 2- RMBFM Project(Contract: MAS2-CT92-0042)- Finalreport(HE-42/96/002lA) Page 7
Synthèse
Cette étude conceme un aspect particulier parmi les modes de destruction d'une digue à talus à I'intérieur du projet européen "Rubble Mound Breakwater Failure Modes (RMBFM)" du programme de recherches MAST-2.Une série d'expériences en laboratoire ont été réalisées dans une cuve à houle multidirectionnelle dans Ie but d'exarniner les interactions entre la carapace et la butée de pied pour la stabilité d'une digue à talus.
Ces essais sont effectués dans la continuité d'essais en canal à houle réalisés précédemment à I'Université de Bologne (Italie). L'attention est ici plus particulièrement portée sur les effets tridimensionnels, en considérant les trois aspects suivants :
- comportement en partie courante et au musoir(effet de la géométrie de la digue) - direction de la houle (effet de l'angle d'attaque de la houle sur l'ouvrage)
- répartition directionnelle (effet d'un étalement angulaire de l'énergie de houle)
Ces effets sont étudiés pour deux pentes de talus et deux cambrures de houIe. Les résultats des tests sont présentés, pour la partie courante de la digue et Ie musoir, et comparés à d'autres résultats expérimentaux ainsi qu'à des formules de dimensionnement existantes.
Quelques unes des conclusions principales issues de l'analyse sont rappelées ei-dessous : • stabilité de la cara pace (partie courante) :
Les résultats de la présente étude se comparent assez bien à d'autres tests expérimentaux (Galland, 1993; Lamberti, 1994) et aux formules de dimensionnement de Hudson et Van der Meer. Toutefois, quelques différences entre les mesures et les prédictions de ces formules sont observées, détaillées et discutées.
• stabilité de la butée de pied (partie courante) :
Les résultats expérimentaux sont en bon accord avec la formule proposée par Gerding (1993), bien que les dommages observés soient en général quelque peu supérieurs.
• processus d'interaction :
La manifestation majeure de I'interaction entre la carapace et la butée de pied est l'augmentation des dommages à la carapace quand la butée de pied est instabIe. Ceci peut conduire à une destruction totale de la carapace si la butée de pied est suffisamment érodée pour perdre sa fonction du support statique de la carapace. A l'opposé, les effets de la carapace sur la butée de pied se sont révélés assez faibles.En particulier, Ie proces sus de consolidation de la butée de pied (par des bloes provenant de la carapace) ne semble se produire que de façon marginale et sous certaines conditions particulières.
Vu que les effets d'interaction entre la carapace et la butée de pied apparaissent relativement faibles (à moins que la butée soit très sensiblement sous-dimensionnée), il est proposé, comme règle de prédimensionnement, d'utiliser des formules de dimensionnement existantes, de façon découplée pour la carapace et la butée de pied. Le profil de digue complet doit ensuite être validé par des essais en cuve à houIe,avec une représentation correcte de la bathymétrie locale.
Cette note constitue Ie rapport final du LNH pour sa participation au projet RMBFM de MAST-2.Cette étude a étéco-financée par I'Union Européenne (Contrat MAS2-CT92-0042) et par Ie Secrétariat d'Etat chargé de la Mer(STCPMVN).
Interactions in the stability of main-armour and toe-berm for a rubble-mound breakwater :An experimental stud)'
under both long-crested and short-crested waves. Michel BENOIT and Philippe DONNARS (EDF - LNH)
Rubble-Mound Breakwater Failure Modes
MAST -2 : RMBFM Project
(contract MAS2-CT92-0042)
INTERACTIONS IN TUE STABILITY OF MAIN-ARMOUR AND
TOE-BERM FOR A RUBBLE-MOUND BREAKWATER
AN EXPERIMENTAL STUDY UNDER BOTH LONG-CRESTED
AND SHORT -CRESTED WAVES
-
FINAL REPORT
-Michel BENOIT and Philippe DONNARS
EDF - Laboratoire National d'Hydraulique (LNH)
6, quai Watier 78400 CHATOU
Interactions in the stability of main-armour and toe-berm for a rubble-mound breakwater :An experimental stud)' under both long-crested and short-crested waves. Michel BENOIT and Philippe DONNARS (EDF -LNH) MAST 2 - RMBFM Project(Contract: MAS2-CT92-0042)- Final report (HE-42/96/002/A) Page10
Interactions in the stability of main-armour and toe-berm for a rubble-mound breakwater :An experimental stud)'
under both long-crested and short-crested waves. Michel BENOIT and Philippe DONNARS (EDF -LNH)
MAST 2-RMBFMProject(Contract: MAS2-CT92-0042)- Final report(HE-42196/002lA) Page IJ
CONTENT
I
VOLUME 1 : TEXT -
TAB LES -FIGURES
PART I I.1
INTRODUCTION - SCOPE OF WORK 15
Overview of thestudy 15
1.1.1 General presentation of RMBFM project 15
1.1.2 Overview of work carried out at LNH within the RMBFM project 16
Some results of previousflume testsandintroduetion to basin tests 17
1.2
PART 11 EXPERIMENTAL SET-UP AND TEST PROCEDURE 18
11.1 Description of experimentalset-up 18
11.1.1 Description of the wave basin .18
11.1.2 Breakwater lay-out and cross-sections 18
11.1.2.1 Correspondence with previoustests performed at University of
Bologna 18
11.1.2.2 Cross-sectionsdefinition 19
11.1.2.3 Lay-out of the breakwater in the basin 19
11.1.2.4 Sectionsofinterest 19
11.2 Overview of test conditions 20
11.2.1 Choice of governingparameters 20
11.2.2 Overviewof test programme 21
11.2.3 Incidentwave conditions 21
11.2.4 Stonescharacteristics 22
11.2.4.1 Armourstones 22
11.2.4.2 Toe-berm stones 22
11.3 Experimental procedure for the tests 23
11.4 Calibration,measurement and analysisprocedures 23
11.4.1 Measurement of damage 23
11.4.1.1Bloc counting 23
11.4.1.2Photographs 24
11.4.1.3Opticalsensor 24
11.4.1.4Comparison ofdamagelevelsS and Nod 25
11.4.2 Wavesmeasurement andanalysis 25
11.4.3 Kinematicsnear the structure 26
Jnteractions in the stability of main-armour and toe-berm for a rubble-mound breakwater : An experimental study
under both long-crested and short-crested waves. Michel BENOJT and Philippe DONNARS (EDF -LNH) MAST 2- RMBFM Project(Contract: MAS2-CT92-0042)- Final report(HE-42/96/002!A) Page 12
PART 111 STABILITY OF TUE TRUNK SECTION 28
Ill.l Descriptive analysis of the stability of the trunk 28
m.l.1 Influence of wave direction and directionality (angular spreading) 28
m.1.1.1 Armour layer 28
m.1.1.1.a Armour slope 1:1.5 28
m.1.1.1.b Armour slope 1:2.5 29
111.1.1.2 Toe Berm 30
1I1.1.1.2.a Armour slope 1:1.5 30
m.l.l.2.b Armour slope 1:2.5 31
m.1.2 Influence of wave steepness 32
111.1.2.1 Armour layer 32
m.1.2.2 Toe Berm 33
m.1.3 Influence of toe-berm stonesweight 33
111.1.3.1 Armour layer 33
m.1.3.1.a Armour slope 1:1.5 34
111.l.3. 1.b Armour slope 1:2.5 34
111.1.3.2 Toe Berm 34
m.1.3.2.a Armour slope 1:1.5 34
11I.1.3.2.b Armour slope 1:2.5 35
m.2 Synthetic analysis of the stability of the trunk 35
m.2.1 Stability of the armour layer 36
111.2.1.1 Comparison of present resultswith other data sets 36
111.2.1.2 Comparison of present resultswith Hudson formula 36
11I.2.1.2.a Expression of Hudson formula 36
11I.2.1.2.b Comparison of test results with Hudson formula 38
m.2.1.2.c Uncertainty coefficientsin Hudson formula ..40
111.2.1.3 Comparison of present resultswith Van der Meer formula .42
m.2.1.3.a Expression of Van der Meer formula ..42
m.2.1.3.b Comparison of test resultswith Van der Meer formula .42
1I1.2.1.3.c Uncertainty coefficientsin Van der Meer formula .45
m.2.2 Stability of the toe-berm .47
111.2.2.1 Comparison of present resultswith other data sets .47
m.2.2.2 Comparison of present resultswith(modified)Gerding formula .48
m.2.2.2.a Expression of Gerding formula .48
m.2.2.2.b Comparison of test results with Gerding formuIa .48
m.2.2.2.c Uncertaintycoefficients in Gerding formula 50
111.2.2.3 Comparison of presentwith Gerding formuIa converted to
damage D 51
11I.2.2.3.a Expression of Gerding formuIa converted to damageD 51 m.2.2.3.b Comparison oftest results with Gerding formuIa
convertedto damage D 51
m.3 Particular analysisof interaction processes in thestabilityof the trunk 52
m.3.1 Analysisof failuremodes 52
Interactions in the stability of main-armour and toe-berm for a rubble-mound breakwater : An experimental stud}'
under both long-crested and short-crested waves. Michel BENOIT and Philippe DONNARS (EDF - LNH)
MAST 2-RMBFMProject(Contract: MAS2-CT92-0042)- Final report(HE-42196/002lA) Page 13
PART IV STABILITY OF THE ROUND-HEAD SECTION 56
IV.1 Descriptive analysis of the stability of the round-head 56
IV. 1.1 Influence of wave direction and directionality (angular spreading) 56
IV.1.1.1 Armour layer 56
IV.1.1.1.a Armour slope 1:1.5 56
IV.I.1.1.b Armour slope 1:2.5 57
IV.1.1.2 Toe Berm 58
IV.1.1.2.a Armour slope 1:1.5 58
IV.I.I.2.b Armour slope 1:2.5 59
IV.1.2 Influence of wave steepness : 59
IV.1.2.1 Armour layer 60
IV.1.2.2 Toe Berm 60
IV.l.3 Influence of toe-berm stones weight 61
IV.1.3.1 Armour 1ayer 61
IV.1.3.1.a Armour slope 1:1.5 61
IV.l.3.l.b Armour slope 1:2.5 61
IV.1.3.2 Toe Berm 62
IV.l.3.2.a Armour slope 1:1.5 62
IV.1.3.2.b Armour slope 1:2.5 62
IV.2 Detailed (angular) analysis of the stability of the round-head 63
IV.2.1 Detailed analysis for toe-berm stones T2 based on 6 sectors of 36° 63
IV.2.I.1 Armour layer with a slope of 1:1.5 63
IV.2.1.2 Armour layer with a slope of 1:2.5 64
IV.2.2 Detailed analysis based on 3sectors of 72° 65
IV.2.2.1 Armour layer with a slope of 1:1.5 65
IV.2.2.2 Armour layer with a slope of 1:2.5 66
IV.3 Particular analysis of interaction processes in the stability of the round-head 67
PART V CONCLUSIONS - FUTURE RESEARCH 68
ACKNOWLEDGMENTS 68
REFERENCES 72
TABLES FIGURES
I
VOLUME 2: APPENDICES
APPENDIX A : Analysis of wave conditions
Interactions in the stability of main-armour and toe-berm for a rubble-mound breakwater :An experimental stud)'
under both long-crested and short-crested waves. Michel BENOIT and Philippe DONNARS (EDF - LNH) MAST 2 -RMBFMProject (Contract: MAS2-CT92-0042)- Final report(HE-42/96/002lA) Page 14
Interactions in the stability of main-armour and toe-berm for a rubble-mound breakwater :An experimental stud)' under both long-crested anti short-crested waves. Michel BENOIT and Philippe DONNARS (EDF -WH) MAST 2- RMBFMProject(Contract: MAS2-CT92-0042)- Finalreport(HE-42/96/002lA) Page 15
PART I
INTRODUCTION -
SCOPE OF WORK
1
.
1
O
v
ervie
w
of the stud
y
•
1.1.1 General presentation of RMBFMproject:
The present study isconcemed with one particular aspect of breakwater failure modes within the project Rubble Mound Breakwater Failure Modes (RMBFM) of MAST-2 European Research Program(Contract MAS2-CT92-0042).
11 institutes have been participating in this project, fromDenmark,Germany,Italy, Spain,the Netherlands, the United-Kingdom and France :
Participant
Aalborg University(AU)
Delft Universityof Technology (DUT)
Centro de Estudiosde Puertasy Costas (CEPYC) FranziusInstitute - Universityof Hannover (PI) Delft Hydraulics (DH)
University of Bologna (UB) University of Roma(UR)
Danish Hydraulic Institute(DHI) Hydraulic Research Wallingford(HR) Technical University ofDenmark(ISVA) Laboratoire National d'Hydraulique(LNH)
Country Denmark The Netherlands Spain Germany The Netherlands Italy Italy Denmark United-Kingdom Denmark France No (Ol) (02) (03) (04) (05) (06) (07) (08) (09) (10) (11) The project iscoordinated and chaired byPr. Hans Burcharth, from the University of Aalborg (Denmark).
The project was started on January 1, 1993 for a total duration of 3 years. The Laboratoire National d'Hydraulique(LNH)joined the project in 1994,byasupplementary agreement N°2.
Interactions in the stability of main-armour and toe-berm for a rubble-mound breakwater :An experimental stud)' under both long-crested and short-crested waves. Michel BENOIT and Philippe DONNARS (EDF - LNH)
MAST 2 - RMBFM Project(Contract: MAS2-CT92-0042)- Final repon(HE-42/96/002!A) Page 16
The overall goal of the research project is to provide information on the failure processes of various component parts of a rubble-mound breakwater and to highlight the failure mechanisms,
with special attention paid to the three following items: Task 1:
Task 2:
Task 3:
Structural integrity of concrete armour units
Analysis of main failure modes for armour units made up of randomly placed large concrete armour units. Study of hydraulic instability and breakage,
including the correlation of some important types of concrete armour units. Analysis of concrete strength of prototype armour units in service and thermal stresses in Tetrapods.
Displacement failure modes
Study of the correlation (interaction) between important failure modes: scour -toe-berm stability - main-armour stability - wave-wall stability - rear-slope stability.
Design guidelines
Implementation of the results of items 1and 2 in reliability analyses on which basis a large step towards more complete engineering design guidelines for breakwaters can be taken.
1.1.2 Overview ofwork carried out at LNH within the RMBFM project:
This experimental study carried out at LNH within the RMBFM project aims to investigate the interaction between the main-armour and the toe-berm for the stability of a rubble-mound
breakwater.
Special attention ispaid to study the effects of :
- toe-berm characteristicsand behaviour on the stability of the main-arrnour,
- main-armour characteristicsand behaviour on the stability of the toe-berm.
The present tests are undertaken in a multidirectional wave basin (3D tests) and areaimed to continue previous tests that have been performed ina wave flume (2D tests) at the University of
Bologna(UB)in Italy. In present experiments, wefocusmore closelyon 3D effects,by considering
the three following aspects :
- behaviour at roundheads (effect of breakwater'send)
- wave obliquity (effectof angle ofwave incidence)
- wave directionality (effectof angular spreading ofwave energy)
The first itemisrelated to the geometryofthebreakwater whereasthe two lastones arerelated towaveconditions.
A programme of laboratory tests has been performed in 1995 (from April to September) in
order to investigate the three above mentioned items on a rubble-rnound breakwater model in a wave basin. This report gathers and presents the results from these tests, together with their interpretation andanalysis with respectto theabovementionedsubjectsof interest.
Interactions in the stability of main-armour and toe-berm for a rubble-mound breakwater :An experimental study under both long-crested and short-crested waves. Michel BENOIT and Philippe DONNARS (EDF - LNH)
MAST 2 - RMBFMProject(Contract: MAS2-CT92-0042)- Finalreport(HE-42196/002lA) Page 17
1.2
Some results of previous flume tests and introduction to basin tests
Lambertiand Arninti(1994)and Lamberti (1994)reported some resultsof the testsconducted in a wave flume at theUniversity of Bologna (VB) on the interaction between toe-berm and main-armour stability.They proposed a classification of structure failure modes based on the following points(Lamberti,1994) :
- the berm is more or less stabie than the armour layer.
- the berm does or does not stop and holdstones falling from the armour layer. - the berm does or does not provide staticsupport to the armour units.
In particular theyshowed that, for sufficiently wide toe-berms, the main feature of breakwater evolution could be a phenomenon of "toe-armouring" by stones falling down from the damaged armour. As these stones remain on the toe-berm, they increase its stability. Thisfinally results in lower damage levels to the toe-berm.
It may however be argued that such a "toe-armouring" process could be particular to normal wave attack on a breakwater trunk. Within the frame of present basin tests, it was therefore proposed to extend the field of interest towardsthree directions :
a. behaviour at roundheads: At a roundhead, the veloeities induced by the waves and the efforts on the stones are quite different from the case of normal incidence on a trunk. It is suspected that the stones falling from the armour layer will not be hold on the toe-berm the same way they are on the trunksection.They will thus probably be thrown out of the toe-berm and will not participate in the armouring process of the toe-toe-berm.
b. wave obliguity: Furthermore,under oblique wave attack,the long-shore transport process could disturb (or at least modify) the armouring process of the toe. It has been observed within MAST 1 G6-S project(Galland, 1993)that a 15 degrees angle of wave attack could lead to higher damage level to the toe than normal wave attack. Recent experiments on wave overtopping (Juhl and Sloth, 1994)also showed a maximum overtopping rate at an angle of incidence between 10 and20 degrees.
c. wave directionality: In addition, it was proposed to investigate the effect of angular spreading of energy (multidirectional waves) on the failure process. Short-crested waves are usually thought to give lower damage levels to the armour than long-crested waves. Recent experiments (Matsumiet al., 1994)however seem to indicate that multidirectional waves could lead to lower or higher damage at the head of the breakwater compared to unidirectional waves,dependingon the headsection and the mean angle ofwave attack.
The above raised comments and questionslie at the core of the present model testsin a wave basin under both long-crested(or unidirectional) wavesandshort-crested (multidirectional)waves.
The experimental set-up and test programme are described in Part 11of this report. The presentation and analysisofresults for the trunk section ofthe breakwater are given in Part 111.The same analysis is carried out for the round-head section in Part IV. Part V summarises the main findings andconclusions of the present study and gives some outlook to possible future research perspectives.
Interactions in the stability of main-armour and toe-berm for a rubble-mound breakwater .' An experimental stud)'
under both long-crested and short-crested waves. Michel BENOIT and Philippe DONNARS (EDF - LNH)
MAST 2 - RMBFM Project (Contract.'MAS2-CT92-0042)- Final report (HE-42/96/002lA) Page 18
PART 11
EXPERIMENTAL
SET-UP AND TEST
PROCEDURE
11.1 Description of e
x
perimental set-up
11.1.1 Description ofthe wave basin
The tests have been conducted in the multidirectional wave basin of Laboratoire NationaI
d'Hydraulique (LNH) in Chatou (France) (see Figure II.l). This facility is dedicated to physical
modelIing appIied to maritime and coastaI studies (harbour agitation, stability of maritime
structures, morphological evolution of the shore,...). It is also possible to generate complex and
unsteady current fields in addition to waves.
The overall dimensions of this basin are 54 m x 31 m x 1.3 m. The maximum water depth for
operational use of the basin is 0.80 m.
This basin is fitted out with a muItidirectionaI piston-type maker. The segmented
wave-maker is composed of 56 paddIes, being 0.40 m wide each. The tot al Iength of the wave-maker thus
reaches 22.4 m. Furthermore, the wave-maker can easiIy be displaced at various locations aIong the
main side of the basin. It can operate in the frequency range of 0.2 Hz to 2.0 Hz (Wave period
range: 0.5 s to 5 s).
During the experiments, vertical side-walls of 2 m length are set up at each side of the wave-maker in order to increase the work area (where waves may be considered as quasi-homogeneous)
by making use of "corner-reflection" method (Funke and Miles, 1987).
11.1.2 Breakwater lay-out and cross-sections
ll.1.2.1 Correspondence with previous tests performed at University of Bologna :
The tests performed at University of Bologna (UB) in a flume under random waves conditions are described in Lamberti and Aminti (1994) and Lamberti (1994).
Interactions in the stability of main-armour and toe-berm for a rubble-mound breakwater : An experimental study
under both long-crested and short-crested waves. Michel BENOIT and Philippe DONNARS (EDF -LNH)
MAST 2 - RMBFM Project(Contract: MAS2-CT92-0042)- Finalrepon (HE-42/96/002/A) Page 19
In the present basin experiments, it was initially intended to work with the same breakwater model. However,due to the so-called Biesel criterion which limits the angle of incidence for high frequency waves and by considering the frequency range of the LNH segmented wave-maker
(§ 11.1.1), it was decided to perform the tests at a larger scale than the one used for flume experiments at UB.
Compared to the previous tests performed at VB, the geometrie scale is thus multiplied by a factor of 1.32, so that the time scale becomes 1.15 times larger (using Froude similitude).
II.l.2.2 Cross-sections definition:
The definition of the breakwater cross-sections (see Figure 11.2for the slope of 1:1.5) is based on the tests carried out at VB. Based on preliminary conclusions of these experiments, the following choices have been made for the present experiments (Galland, 1994) :
- tests are performed with a flat bottom at a water depth of 0.45 m (d = 0.45 m).
- the ratio of water depth above toe-berm h, to design wave height Hsd is about 1 (htlHsdz 1)
- the width of toe-berm B, is constant over the whole test programme and is taken to be three times the diameter of design toe-berm stones Dn50t(design)(Bj
=
constantz3.Dn50t(design)- the thickness of the toe berm is constant and is taken to be that of two layers of design toe-berm stones Dn50t(design).
- two armour slopes are considered : 1:1.5(cotg a= 3/2 = 1.5) and 1:2.5 (cotga = 5/2 = 2.5).
We thus obtain various breakwater profiles, which are termed according to the classification proposed by Lamberti (1994):
•for design toe-berm stones :
DHNS (Deep water, High berm, Narrow berm,Steep armour slope) for the 1:1.5 slope. DHNM (Deep water, High berm, Narrow berm, Mild armour slope) for the 1:2.5 slope.
•for the lowest weight of toe-berm stones:
DHWS (Deep water,High berm, Wide berm,Steep armour slope) for the 1:1.5 slope.
DHWM (Deep water, High berm, Wide berm,Mild armour slope) for the 1:2.5 slope.
II.l.2.3 Lay-out of the breakwater in the basin:
The breakwater is not parallel to the wave-maker,but there is an angle of 15 degreesbetween them (see Figure 11.1).This orientation has been chosen in order to ensure a sufficiently high frequency limit for generated waves even for oblique incidences. Figure 11.3gives the relationship between the wave directions referred to the wave-maker and to the breakwater. In the following,we only use the direction of incidence referred to the breakwater :a 0° direction corresponds to normal wave attack, with wave crests parallel to the breakwater.
n.l.2.4 Sectionsof interest:
The breakwater used for the experiments consists in two half-breakwaters,each of them being composed of trunk and a head (see Figures 11.3 and 11.5).Each half-breakwater has the same armour unitsbut different toe-bermstones.
Interactions in the stability of main-armour and toe-berm for a rubble-mound breakwater : An experimental stud)'
under both long-crested and short-crested waves. Michel BENOIT and Philippe DONNARS (EDF -LNH)
MAST 2-RMBFM Project(Contract: MAS2-CT92-0042)- Final repon(HE-42/96/002lA) Page 20
Alm long test section is considered on each trunk section. In order to minirnize the repairing efforts, it is made use of wire mesh to fasten the armour layer in certain sections where damage would not be relevant for the scope of the tests.
The round-heads are divided in 6 angular sections (see Figure 11.3).
By this way,it is possible to test simultaneously(Galland, 1994):
- 4 sections (2 trunks and 2 round-heads)under normal waves, - 3 sections only under oblique waves(2 trunks, 1 round-head).
This means that for oblique wave incidence,only one of the two toe-berm stones types will be considered at the roundhead.
D.2
O
v
ervie
w
of test conditions
11.2.1 Choice of governing parameters
The choice of varying goveming parameters for the experimental tests is based on the analysis performed by Gerding (1993)and Lamberti (1994):
- the nominal diameter ofthe toe-berm stones [4 values]. The first value corresponds to
"design" value as determined by a conventional design approach, whereas the three other ones correspond to lower values which should lead to more unstable toe-berms. The characteristicsof stones used in the experimentsare given in§11.2.4.
- the slope of armour-mound [2 values : 1:1.5(cotga = 1.5) and 1:2.5 (cotg a = 2.5).]. The corresponding breakwater profileshave alreadybeen described in§11.1.2.2.
- the wave steepness [2values : Som=2% and Som= 5%]
The wave steepness is defined as: Som= HslLOm = Hs/1.56T m2. The lower value of steepness (som= 2 %) corresponds to "longer" waves and the higher value of steepness (som= 5 %) corresponds to "shorter" waves. As "longer" waves are thought to be more severe for the stability of the breakwater, most of tests are performed with the 2 %
steepnessfor incident waves.
- theangle ofwave incidence [2 values : ~ = 0° and ~ = 30°].The latter value ischosen to
ensure that the longshore transport process will occur under oblique attack. One test has also been conducted under a ~ = 10°angle of wave incidence to check whethersuch avalue couldlead to higher damage,as shown byGalland(1993)for toe-bermstabilityatconcrete armouredstructuresor byJuhland Sloth(1994)for wave overtopping.
- the angular spreading of energy [2 values: s=00 (unidirectional waves) ands= 15].The
spreading index s corresponds to the exponent in the directional model of spreading functionused for generatingdrive signalsforthewave-maker:
D(S)=
1..
cos2.S(S)Interactions in the stability of main-armour and toe-berm for a rubble-mound breakwater :An experimental stud)' under both long-crested and short-crested waves. Michel BENOIT and Philippe DONNARS (EDF -LNH) MAST 2-RMBFMProject(Contract: MAS2-CT92-0042)- Finalrepon (HE-42/96/002lA) Page 21
The short-crested case (s = 15) produces a directional sea-state with moderate angular spreading. The correspondingspreading function isplotted on Figure Il.4. The directional width definedas :
cr= Y2.(1-ml) with: mi =-Val+b?
where al and bIare the first Fourier coefficients of the spreading function, isequal in this case to
o
= 10 degrees.11.2.2 Overview of test programme
The test programme for experiments isbased on the choices presented in the previous section for the governing parameters.Due to the fact that the model breakwater is composed of two half-break waters (each with a different toe-berm stones size), the number of sections tested simultaneously is equal to 4 under normal wave attack and to 3 under oblique wave attack.
Due to the rather large number of governing parameters selected in this study, it was not possible to test all the combinations between each parameters,which would have resulted in: 4 (berm stones) x 2 (slopes)x 2.(steepnesses)x 2(wave directions) x 2(angular spreadings) = 64
This number as been reduced to 30 by considering only some of the above combinations. Because two half-breakwaters are tested simultaneously, the overall number of tests is finally equal to 15.Due to the fact that only three sections are tested under oblique wave attack,the total number of sections examined during the experiments isequal to 52 (instead of 15 x 4 = 60).
The detailsof testscharacteristicsare given in Table Il.1.
11.2.3 Incident wave conditions
The tests are performed under random unidirectional (long-crested)or multidirectional (s hort-crested)waves. For a given test,the target wave height is increased by successive steps: eachstep correspondsto a run.A test isthus composed of 7 runs.
The target wave characteristicsof the various runsof a tests are computed from the onesused at University of Bologna (Lamberti and Aminti, 1994)by using the scale factor of 1.32 for metrical quantities. The target wave characteristicsfor the present experiments (in terms ofsignificant wave height H, and mean period Tm)aresummarisedin Table Il.2.
A JONSWAP-type spectrum is used for the distribution of wave energy over frequencies.
Following the experiments at UB,we use avalue of the peak enhancement factor y = 5 for the tests atsteepnessSom= 0.05 ("shorter"waves) and a value of y = 1 for the tests at steepness som= 0.02 ("longer" waves). In this latter case, the JONSWAP expression reduces to a Pierson-Moskowitz spectrum.
Interactions in the stability of main-armour and toe-berm for a rubble-mound breakwater :An experimental study
under both long-crested and short-crested waves. Michel BENOIT and Philippe DONNARS (EDF - LNH)
MAST 2 - RMBFM Project(Contract: MAS2-CT92-0042)- Final report(HE-42/96/002lA) Page 22
11.2.4 Stones characteristics
Armour stones for the trunk sections have been scaled when compared to those used during flume experiments at VB, by using the scale factor of 1.32. In addition to that, a corrective factor has been applied in order to take into account the density difference between the stones used at DB and the ones used for present experiments.
11.2.4.1 Armour stones
The weight of armour stones was determined according to the one used at DB for flume experiments. The following characteristics were obtained :
Density : 2.55
Armour stones Nominal diameter Dn50: 2.91 cm Nominal weight W n50: 63 g
ilDn50 : 4.51
Same stones are used for the armour-layer both at the trunk sections and at the round-heads, whatever the weight of toe-berm stones is.
11.2.4.2 Toe-berm stones
The "design" value of toe-berm stones for trunk section (Tl) was deterrnined according to the one used at VB for flume experiments (21 g). Due to a difference in density between stones used at DB and during present experiments, a corrective factor was applied, leading to the following characteristics for the "design" toe-berm stones :
Density: Nominal diameter Dn50 : Nominal weight W n50: ilDn50 : Design toe-berm stones Tl (Trunk section) 2.72 2.58 cm 47 g 4.44
From the design value of toe-berm stones, the three other values (termed T2 to T4) are computed from the relationship proposed by Lamberti (1994) :
W (T)
=
Wn50t(TI) or K=
In (Wn50t(T1»)
n50t J 2Kj J 2 W n50t(Tj)
The values of Kj are chosen to be : • toe-berm stones Tl : KI
=
0 • toe-berm stones T2 : K2=
1.2 • toe-berm stones T3 : •toe-berm stones T4 : Wn50t(Tl)=
47 gr Wn5Ot(T2)=
Wn5Ot(T1)/ 2.3:::::20 gr Wn50t(T3)=
Wn50t(T1)/ 5.66:::::7.5 gr Wn50t(T4)=
Wn50t(T1)/ 22.6:::::2.2 grThe weight of toe-berm stones at the round-head are increased by about 25 %from the above values deterrnined for the trunk section.
The sorting index for the various toe-berm stones lies in the range: 2 <D85 / DIs <2.5. It must be emphasised that this sorting index is much larger (but also more representative of natural conditions) than the one used during the flume experiments at DB. Lamberti (1994) indicated that
Interactions in the stability of main-armour and toe-berm for a rubble-mound breakwater :An experimental stud)'
under both long-crested and short-crested waves. Michel BENOIT and Philippe DONNARS (EDF -WH)
MAST 2 -RMBFM Project (Contract: MAS2-CT92-0042)- Final report(HE-42196/002lA) Page 23
the mean sorting index in hisexperiments was equal to 1.1.This low value indicatesthat most of the blockswere quite identicalin hisexperiments.
The characteristics of the various stones used during the experiments are summarised in Table II.4.
ll.3
Experimental procedure for the tests
Asalready mentioned (cf. §II.2.3),each test iscomposed of 7 steps or runs with increasing wave height. Eachstep hasa duration of about 2000 wavesof target mean period Tm.
The drive signals for the segmented wave-maker are generated with this duration. The method of recycling shorter duration signalsto cover the test duration is thus avoided. The wave elevations
inthe basin are also recorded over that duration and the wave analysis procedures apply to the full recordedsignais.
There is no rebuilding of the breakwater between consecutive steps of a test: cumulative damage is observed and reported during the tests. At the end of a test,the breakwater isrepaired to itsinitial shape and a new test is started.
ll.4
Calibration, measurement and analysis procedures
The whole set-up of recording probes is shown on Figure 11.1.Refined measurements have been performed during the experiments in order to get a good description on the hydrodynamics
close to the structure.
11.4.1 Measurement of damage
During the experiments, two ways of evaluating the damage are used. They are described belowwiththeassociated methodsused to calculatesynthetic damage index.
11.4.1.1Bloc counting
Damageisfirst evaluated bycounting the number of units displaced from the armour-Iayer and the toe-berm. This is the standard method for measuring damage during the tests. For each displaced bloc,thestartingpositionand the final position have been printed inspecial tables bythe technicians in charge of the experiments. For each test section, the evolution of displaced units
numberNdisthusreported for eachsection of interest.
For the trunksections,the damageindexisgiven bythe number Nodof unitsdisplacedwithina
Interactions in the stability of main-armour and toe-berm for a rubble-mound breakwater :An experimental study
under both long-crested and short-crested waves. Michel BENOIT and Philippe DONNARS (EDF - LNH)
MAST 2 -RMBFM Project (Contract: MAS2-CT92-0042) - Final report (HE-42/96/002!A) Page 24
where I is the width of the trunk section where damage is observed.
In order to be able to distinguish between the stones, the following conventions have been adopted for painting the stones:
- for the main armour :the cover-Iayer ispainted by using 8 rows of different colours from the toe to the crest of the breakwater. The width of each row is approximately that of 2 armour-stones. The lower layer of armour stonesis not painted.
- for the toe-berm: a separate colour is used for the upper layer of each test section. Again the lower layer is not painted.
- the roundheads are divided into 6 angular sectors with a different colour combination for each sector.
Some pictures of the breakwater are given on Figures 11.5and 11.6,in order to illustrate the refined painting of stones,in particular at the round-heads.
11.4.1.2Photographs
Photographs have been taken at the end of each test, but sometimes also at the end of some steps in order to show the development of damage. To that end, four fixed frames have been set up for each half-breakwater : one in front of the trunk section and three around the head. By this way,
it was possible to take photographs exactly from thesame locationsat various dates(see Figure 11-6 at a round-head for example).
11.4.1.3Optical sensor
An optical sensor has also been used in order to get information of the profile of the breakwater and its changes after each step of a test. This procedure has however only been undertaken for several tests. It was not used systematically during the experiments and must be considered here as an optional approach for comparison.
By this way, it is possible to get a measure of the eroded area
Ae
in the breakwater profile. By thismethod, the damage level is
defined as:
S =__&_
~50
z
Eroded area Ae
One has to keep in mind that S
includes a measurement of settlement X
in addition to damage due to displacementsof bloes.
Measured breakwaterprofilesduringpresenttests aregathered in AppendixB(Volume 2).
Therelationship between Sand Nod isdiscussedin the next paragraph.
Interactions in the stability of main-armour and toe-berm for a rubble-mound breakwater : An experimental stud)'
under both long-crested and short-crested waves. Michel BENOIT and Philippe DONNARS (EDF - LNH)
MAST 2-RMBFMProject(Contract: MAS2-CT92-0042)- Final report(HE-42196/002JA) Page 25
H.4.I.4 Comparison ofdamagelevelsS and Nod:
An attempt to find a relationship between S andNod may be based on the following (crude)
assumptions:
•settlement isdisregarded
•the layer ofinterestin the breakwaterisconsidered to have a porosityE(E :::0.4to 0.6)
The volume of displaced material ina strip of width Dnsoisthen equal to :
V
=
Nod.D~SO but also to V=
Ae.Dnso=
S.D;so1 - E
Combining both expressionsleadsto : S = Nod/(l-E)
For rock layers,E lies in the range [0.4;0.5],which leadsto : S
=
[1.7 ; 2] Nod-Burcharth (1993) points that experience show that the above simple relationship is not so operational and dependson the slope angle.
Van der Meer(1993)notesthat"generallyS isabout twice Nod".
Lamberti(1994 ;his figure 5) made some comparisons in the tests performed at UB within this RMBFM project and found a correlation leading to : S::: 3.Nod
-Based on present experiments, similar comparison has been performed for damage to the armour layer,but for a limited number oftests. The results are plotted on Figure 11.7.From these data,the correlation appears to be : S::: 1.5.No
d-By combining, these different information and trends, it was decided to choose the relationship: S::: 2.Nod to convert damage level Nodinto damage level S.
11.4.2 Waves measurement and analysis
Twenty-one wave probes were used to record the wave characteristics both in front of the wave-maker and close to the breakwater (see Figure H.1 for their lay-out). Free surface elevations
are recorded by using resistive gauges, which have proven to be very reliable for wave measurementsin the laboratory (resolution of about 0.1 mm).These probes are checked every day and recalibrated every threeweeks. Theyusuallydo not exhibit anyvariation with time.
• Statistical (wave by wave)analysis : For each of these 21 probes,a statistical analysisby a Zero Up-Crossing method is performed on the whole time series. Various parameters
arethusobtained, such asmean period Tm,significant wave height H1/3, ...The results of analysis of all testsperformed aregiven in AppendixA.
• Incident wave characteristics : the array of probes 1 to 5 which lies close to the wave-maker isdevoted to the determination ofincident wave characteristics. It may be of linear type for unidirectional waves or bidimensional (using CERC configuration) for multidirectional waves. The wave data are analysed by using advanced methods for oblique and multidirectional reflection measurement, developed at LNH (Benoit and Teisson, 1994;Teisson and Benoit, 1994). This set of methods includes sophisticated
Interactions in the stability of main-armour and toe-berm for a rubble-mound breakwater : An experimental stud)' under both long-crested and short-crested waves. Michel BENOIT and Philippe DONNARS (EDF - LNH) MAST 2-RMBFM Project(Contract: MAS2-CT92-0042)- Final report(HE-42/96/002lA) Page 26
directional analysis methods with further refinements in order to take account for wave reflection :Maximum Likelihood Method(Isobe and Kondo, 1984),Bayesian Directional Method (Hashimoto and Kobune, 1987).
• Reflection measurement : Additional wave measurements are performed through an array of 6 probes (6 to 11) close to the structure. These data are analysed by using directional analysis methods as mentioned above,in order to get refined information on the reflection coefficient of the structure.
• Agitation along the breakwater : Wave probes 12 to 21 deliver information on wave heights and statistics along the breakwater. For each half-breakwater. two probes are located in front of the trunk section and three probes are located near the roundhead. These measurements were intended to give an overview of wave homogeneity along the breakwater, for various wave conditions.
11.4.3 Kinematics near the structure :
Two 3D current-meters were used during some of the tests in order to access the kinematics close to the toe-berm. This type of measurement seems especially important for explaining the behaviour ofthe stones at the roundhead,as recently attempted by Matsumi et al. (1994).
To that end, two three-dirnensional SENSORDATA SD-12 probes were used : one located above the toe berm in a trunk section and the other one in a roundhead.The probe at the round-head is mounted on a frame that is allowed to rotate around the center of the roundhead(see Figure Il.9). Through this device, it was possible to measure the velocity induced by the waves at various angular positions around the roundhead,butstill at thesame distance from the main-armour and the toe-berm. This probe directly delivered the radial, tangential and vertical components of the velocity.
These probes were calibrated in a wave/current flume equipped with a mobile computer-controlled carriage.
11.4.4 Calibration of the wave-maker
For the design of the experimental set-up (position of breakwater, length and orientation of wave-guides, wave-maker motions), we have made use of the code PHISAX, developed at LNH (Benoit, 1995). This software aimsto model a numerical wave tank and allowsto get map ofwave characteristics in the basin prior to the experiments. In particular, it isthus possible to check the homogeneity of the wave field alongthebreakwater. Figure Il.10 showsan example of simulation performed with the PHISAX model.
Before building the breakwater model in the basin, a series of tests have been performed in order to determine thestatisticsof generated waves.The drive signalsareindeed computed froma parametrie spectrum,which isdefined bythe significant wave height Hmü,the peak period Tpand the peakenhancementfactory. Equality isassumed between H, and Hmü.In order to get thetarget
Interactions in the stability of main-armour and toe-berm for a rubble-mound breakwater :An experimental stud)"
under both long-crested and short-crested waves. Michel BENOIT and Philippe DONNARS (EDF - LNH)
MAST 2- RMBFM Project(Contract: MAS2-CT92-0042)- Finalreport(HE-42/96/002lA) Page 27
characteristics of Table 1I.2 for the tests, it is necessary to determine a relationship between the simulated mean period Tm and the theoretical peak period T
p-To that end,a good number of wave signals have been created,simulated in the basin,recorded and analysed for normal incidence without any test-structure, The results of these tests are plotted on Figure 1I.8,together with regression lines fitted to the data.
By using these curves it isthus possible to get a more precise knowledge of the input peak period in order to get a given mean period in the basin. Table 11.3 shows the updated wave characteristics used for the computation of drivesignais.
Interactions in the stability of main-armour and toe-berm for a rubble-mound breakwater :An experimental stud)'
under both long-crested and short-crested waves. Michel BENOIT and Philippe DONNARS (EDF - LNH)
MAST 2 - RMBFM Project (Contract: MAS2-CT92-0042)- Final repon(HE-42/96/002lA) Page 28
PART 111
STABILITY OF TUE TRUNK SECTION
m.l
Descriptive anal
y
sis of the stabilit
y
of the tronk
In this section, we focus on the physical description of the tests and report some trends in the stability of the breakwater for the trunk section. The evolution of damage with increasing wave height is described and analysed in order to exhibit the influence of various goveming parameters.
111.1.1 Influence of wave direction and directionality (angular spreading)
We only consider in this chapter the tests carried out with the 2 % wave steepness and discuss the effect of incoming wave direction and directional spreading of wave energy. We split in the presentation the analysis of damage for the main-armour (§ IIL1. 1. I)and for the toe-berm (§ llI.I.I.2).
m.l.l.l Annour layer
III.l.l.1.a Armour slope 1:1.5
The results of tests I to 7 are gathered on Figure m.l on four separate graphs, corresponding to the four toe-berm stone sizes (Tl to T4). The damage index Nod to the main-armour is plotted as a
function of the measured incident significant wave height Hmo.
- Under long-crested waves (effect of wave direction) :
For the toe-berm stones Tl and T2 (upper plots of Figure III.I -tests 1,2 and 4) :damage on the main-armour appears to be of comparable level for incident directions 0° and 10°. At higher wave heights however, damage is higher for the 0° direction than for the 10°directions for both toe-berm stone sizes. Compared to the 0° direction, the 30° direction produces higher damage for the Tl toe-berm stones, but lower for the T2 toe-berm (same damage level as for direction 10°on this latter toe-berm). It isthus difficult to find a clear trend between these two toe-berm stones, one should note that the higher damage level observed on the main-armour for the toe-berm Tl (test 4) isnot observed on the toe-berm for the same test (Figure In.3).
Interactions in the stability of main-armour and toe-berm for a rubble-mound breakwater :An experimental study under both long-crested and short-crested waves. Michel BENOIT and Philippe DONNARS (EDF -LNH)
MAST 2- RMBFMProject(Contract: MAS2-CT92-0042)- Finalreport(HE-42/96/002lA) Page 19
For the toe-berm stones T3 and T4 (lower plots of Figure m.l - tests 6 and 7): only the 0°and 30°directions have been simulated.For both the toe-berm stones,corresponding to a somewhat less stabie toe-berm than T 1 or T2,the 0°direction clearly results in more severe damage on the main-armour than the 30°direction.Furthermore,the damage levels observed for the 0°direction for toe-berm stones T3 and for both 0° and 30° directions for toe-berm stones T4 are significantly higher than the ones observed for toe-berm stones Tl and T2.
- Under short-crested waves (effect of wave direction and angular spreading) :
The tests under short-crested waves have been performed for the toe-berm stones Tl and T2 only (upper plots of Figure III.1 - tests 3 and 5) : In both cases, it appears that damage are higher for the 0°direction than for the 30°direction. On the other hand -and this also a clear trend both for Tl and T2 toe-berm stones-, the short-crested waves cause higher damage level to the main-armour than the long-crested waves.This istrue both for the 0°and 30°directions of incidence.
- Summary of conclusions :
In spite of an unexplained higher damage level at a direction of 30° for long-crested waves and toe-berm Tl, the following observations may be emitted :
• in the range of tested values,the wave direction does not seem to have a significant effect on the stability of the main-armour as long as the toe berm is stabIe (Toe-berm stones Tl and T2).
• when the toe-berm is unstable(Toe-berm stones T3 and T4), damage levels are higher on the armour layer and the normal direction seem to be more severe than an oblique (30°) direction.
• damage on the main-armour is higher under short-crested waves (only one tested value of angular spreading) than under long-crested waves for the same incident wave height. This is a clear obervation from present experiments, which appears to be in some contradiction with other experimental test results. For instance, Thunbo Christensen et al. (1984) found from model tests on a breakwater armoured with quarry stones that uni-directional waves result in 30-40%more damage to the breakwater when compared to short-creted waves. Canel and De Graauw (1992) also concluded that short-crested waves result in an increase of the stability number (from 0 to 60%) for rock for the 1%damage level. Although no definite conclusion may be drawn from the rather low number of present experiments, this increase of damage under short-crested wavesappearsasquite clear trend.It is asumed to be due to a rather low
angular spreading of wave energy (o z 10°). Anyway, it is stressed that this aspect should
receive more attention through additional model tests.
III.1.1.1.b Armour slope1:2.5
The results of tests 8, 10, 11, 13, 14 and 15 are gathered on Figure III.2 on four separate graphs, corresponding to the four toe-berm stone sizes (Tl to T4). The damage indexNod to the main-armour isplotted as a function of the measured incident significant wave heightHmo.
- Under long-crested waves (effect of wave direction) :
For the toe-berm stonesTl and T2 (upper plots of Figure III.2 - tests8 and 11) :damage levels
on the main-armour are of thesame order of magnitude for both directions0° and 30°. For the toe-bermstonesT2,the 30°directionseemsa littlebit more severe than the 0°direction.
Interactions in the stability of main-armour and toe-berm for a rubble-mound breakwater :An experimental stud)' under both long-crested and short-crested waves. Michel BENOIT and Philippe DONNARS (EDF -LNH)
MAST 2 - RMBFM Project (Contract: MAS2-CT92-0042)- Final report(HE-42/96/002/A) Page 30
For the toe-berm stones T3 and T4 (lower plotsof Figure IIl.2 -tests 14ánd 15) :damage level to the main-armour lie quite closely for both incident directions 0° and 30° at low and medium wave heights. For higher wave heights however, the 0° direction seem to be more severe than the normal attack.
- Under short-crested waves (effect of wave direction and angular spreading):
The tests under short-crested waves have been performed for the toe-berm stones Tl and T2 only (upper plots of Figure IIl.2 - tests 10 and 13):In both cases,it appears that damage are higher for the 0° direction than for the 30°direction.As for the 1:1.5 slope,the angular spreading of wave energy results in more damage to the main-armour. This appear very clearly for the 0° direction, but it somewhat less pronounced for the 30° direction where damage levels are comparable to the ones observed under long-crested waves.
- Summary of conclusions :
We may conclude that the comments raised for thestability of the main-armour with a slope of 1:1.5 slope still apply, but with the following observations :
• the effect of wave direction is not very pronounced unless the toe-berm is unstable (Toe-berm stones T4).
• the increase of damage under short-crested waves in comparison to long-crested waves is strongly observed for the normal attack, butseemsto be quite feebIe for the 30°direction.
ill.1.1.2 Toe Berm
III.1.1.2.a Armourslope 1:1.5
The results of tests 1 to 7 are gathered on Figure 111.3on four separate graphs,corresponding to the four toe-berm stone sizes (Tl to T4). The damage index Nod to the toe-berm is plotted as a function of the measured incidentsignificant wave height Hmo.
- Under long-crested waves (effect of wave direction) :
The damage to the toe-berm Tl (upper-leftplot of Figure 111.3- tests 1, 2and 4) arevery low for the three directions of incidence.This stonesize corresponds to the"design" value for toe-berm stones and isproved here to actuallyresultina stabIe toe-berm. For thistoe-berm,the0° and 10° directionsproduce similar damage and the 30°direction asomewhat lower damage level.
The damage to the toe-berm T2 (upper-right plot ofFigure 111.3- tests 1,2 and 4) are clearly higher. Again, the 0° and 10°directions produce similar damage, whereas the 30° direction now shows anincreasein thedamage level to the toe-berm.
For the toe-berm stones T3 and T4 (lower plots ofFigure 111.3- tests 6 and7) : damage are increasingvery rapidly with waveheight.In particularthe toe-berm T4isalmostfullydestroyed by thewaves at the end ofthetest.For both toe-bermstonetypes, thenormalattack is moresevere for the toe-bermthanthe 30°direction.
- Under short-crested waves (effect of wave direction and angular spreading):
The tests under short-crested waveshave been performed for thetoe-berm stones Tl and T2 only (upperplotsofFigure IIl.3 - tests 3and5): In both cases andin thesame manner asobserved
Interactions in the stability of main-armour and toe-berm for a rubble-mound breakwater : An experimental study
under both long-crested and short-crested waves. Michel BENOIT and Philippe DONNARS (EDF -LNH)
MAST 2- RMBFMProject(Contract: MAS2-CT92-0042)- Finalreport(HE-42/96/002lA) Page 31
for the main-armour,damage are higher for the 0° direction than for the 30° direction. Short-crested waves also produce higher damage level to the toe-berm than long-crested waves, except for the toe-berm T2 and the 30°direction where unidirectional waves seem more severe.
- Summary of conclusions :
Conceming the stability of the toe-berm,the following observations may be drawn :
• The stability of toe-berm decreases with the weight of toe-berm stones, which is not really surprisingly, but seems to indicate that the toe-armouring process does not take place in a sufficient manner to stop the failure of the toe-berm. This will be discussed further in§III.I.3. • The normal direction seems to be more severe for the toe-berm than the 30° direction, if we
except the result of test 4 for toe-berm T2 under long-crested waves.
• Damage on the toe-berm is also higher under short-crested waves than under long-crested waves for the same incident wave height, if we again except the result of test 4 for toe-berm T2 under long-crested waves.
III.l.l.2.b Armour slope 1:2.5
The results of tests 8, 10, 11, 13, 14 and 15 are gathered on Figure lIlA on four separate graphs, corresponding to the four toe-berm stone sizes (Tl to T4). The damage index Nod to the toe-berm is plotted as a function of the measured incident significant wave height H
mo-- Under long-crested waves (effect of wave directtoni :
Damage to the toe-berm Tl (upper-left plot of Figure lIlA -tests 8 and 11) is stilllow for the 0° and 30° directions of incidence,but it is higher than the one observed for the 1:1.5 slope (see same plot of Figure 111.3).Although, this toe-berm Tl is made of "design" stones, it shows some damage at the end of the test. The 0° direction produceshigher damage than the 30°direction.
Damage to the toe-berm T2 (upper-right plot of Figure 11I.4- tests 8 and 11) is clearly higher than the one observed on the previoustoe-berm. As for the 1:1.5 slope, the 30°direction is a little bit more severe than the normal wave attack for the stability of the toe-berm.
For the toe-berm stonesT3 and T4 (lower plots of Figure 111.4- tests 14 and 15): damage is increasing very rapidly with wave height. Asfor the 1:1.5slope, the toe-berm T4 is almost fully destroyed at the end of the test. For both toe-berm stone types,the normal attack is more severe for the toe-berm than the 30°direction.
- Under short-crested waves (effect ofwave direction and angular spreading) :
The tests under short-crested waves have been performed for the toe-berm stones Tl and T2 only (upper plots of Figure 111.4- tests 10 and 13) :In both cases and in the same manner as
observed for the main-armour, damage are higher for the 0° direction than for the 30° direction. Short-crested waves also produce higher damage level to the toe-berm than long-crested waves,
only for the normal attack.For the 30°direction,long-crested wavesare clearly moresevere.
- Summary of conclusions :
Conceming thestabilityof the toe-berm,the following observationsmay be drawn :
• the various toe-berms are Iess stable for this slope of armour than the same ones associated with an armour slope of 1:1.5. This may be related to slight different hydrodynamical conditionsofwave breaking and run-down,resultinginstronger action to the toe-berm.
