INVESTIGATIONS ON STATICALLY STABLE, 
SLOPING, RUBBLE MOUND COASTAL STRUCTURES 
b y 
Prasanthi Dilukshi Mirihagalla 
A thesis submitted to University of Moratuwa 
for the Degree in Master of Engineering 
073027 
University of Moratuwa 
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Research supervised 
by 
Professor S.S.L.Hettiarachchi 
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DEPARTMENT OF CIVIL ENGINEERING 
UNIVERSITY OF MORATUWA 
MORATUWA 
SRI LANKA 
September 2000 
7 3 0 2 7 
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ABSTRACT 
The ability to predict the level of reflection, transmission, run-up and run-down for 
various types of coastal structures plays an important role in the assessment of their 
hydraulic performance. These parameters together with the hydraulic, geotechnical and 
structural stability of the individual components and of the structure as a whole 
determine the overall performance of the structure. The porosity and permeability of 
the structure too has a significant influence on the hydraulic performance and the 
economics of construction. This study has done a literature review and presents the 
results from a study of the hydraulic performance of a wide range of structures used in 
harbour and coastal engineering. The results of two detailed hydraulic model 
investigations of trapezoidal layered breakwaters at scale 1:20 (tested at LHI). The 
results are compared with a model investigation done on a homogeneous breakwater at 
scale 1:40 (tested at Imperial College, London). The investigations were designed to 
obtain a full profile of the energy dissipation characteristics of the structures tested, 
including the damping of waves as they propagate through the structure. The results are 
discussed in the context of the importance of porosity and permeability of wave 
absorbing structures, their application in practice and further research. 
Key words Breakwaters, Reflection, Transmission, Dissipation, Porosity, Permeability, 
Scale Effects 
i 
ACKNOWLEDGEMENTS 
My sincere gratitude goes to the research supervisor Professor S.S.L. Hettiarachchi for 
introducing m e to the area Statically Stable Sloping Rubble Mound Coastal Structures and 
for the guidance given to m e in many ways. I am also grateful for h i m for the valuable 
criticism, advice and c o m m e n t s , encouragement given and handl ing the tedious task of 
correcting the study. I am thankful for h i m for arranging me to work at Lanka 
Hydraulic Institute (LHI), Katubedda, Moratuwa, for my mode l investigations. 
G u i d a n c e and encouragement given wi th respect to publ ishing and presenting research 
papers are appreciated. I am thankful for h i m for giving m e an opportunity to get an 
exposure t o the industry by permitt ing m e to attend conferences , seminars and 
workshops . 
I a m grateful t o the Vice Chancel lor , D e a n o f the Engineering Faculty and the Senate 
Research C o m m i t t e e , Head, Department of Civil Engineering, Director, Postgraduate 
Studies o f the University of Moratuwa, for funding this project and giving m e an 
opportuni ty for research. 
I wish to thank the management and staff o f Lanka Hydraulic Institute (LHI) for their 
u tmost corporation and making available their facilities at a reduced cost. My special 
thanks g o to Mr. Jayantha Rajapakse for the support given in my mode l testing. 
I thank Peter Hyllested of the Dan i sh Hydraulic Institute, Denmark, for he lp ing m e to 
sort out the wave gauge distance calculations for the measurement o f wave reflection 
through e-mail communica t ions . 
My heartfelt gratitude goes to Dr. (Mrs.) Premini Hettiarachchi for the assistance, ideas, 
c o m m e n t s , criticism and advice given. I am thankful for her for he lp ing m e in many 
ways. 
I thank my parents a n d my husband for the encouragement given and standing by m e 
throughout . I thank my father for b inding this thesis. My thanks g o to my friends 
Rarnesh Perera and Subashi Pooliyadda for the support given. 
ii 
Declaration 
This thesis is a report of research carried out in the Department of Civil Engineering, 
University of Moratuwa, between March 1999 and September 2000. Except where 
references are made to other work, the contents of this thesis are original and have 
been carried out by the undersigned. The work has not been submitted in part or whole 
to any other university. This thesis contains 90 pages. 
Supervisor 
Prof. S.S.L.Hettiarachchi 
Department o( Civil Engineering, 
I Iniversity of Moratuwa. 
Sri Lanka. 
P.D.Mirihagalla 
Department of Civil Engineering, 
I Jniversity of Moratuwa. 
Sri Lanka. 
in 
CONTENTS 
Abstract [ 
Acknowledgement u 
Declaration i u 
Contents i v 
Summary of Tables and Figures 
1. Introduction 1 
1.1. Background 1 
1.2. Objective of the study 2 
1.3. Guide to the thesis 3 
2. Design issues and review of literature 4 
2.1. Design issues 4 
2.1.1. Harbour design 4 
2.1.2. Classification of harbour and coastal structures 5 
2.1.3. Breakwater design 5 
2.1.4. Analysis of wave reflection, transmission and dissipation 6 
2.1.5. Influence of porosity and permeability with respect to stability 7 
2.1.6. Importance of wave reflection 8 
2.1.7. Prediction of wave reflection 9 
2.2. Review of literature 11 
2.2.1. Non porous sloping structures 11 
2.2.2. Rock armoured sloping structures 12 
2.2.3. Concrete armoured sloping structures 12 
2.2.4. Porous sloping protection to vertical structures 14 
2.2.5. Armoured porous trapezoidal structures 14 
2.2.6. Field investigations on wave reflection 15 
iv 
3 . Exper imenta l p r o g r a m m e - D e s i g n a p p r o a c h a n d detai ls 25 
3 .1 . Relevance of large scale hydraulic models 25 
3 .2 . Investigating energy dissipating characteristics 25 
3 .3 . Structures investigated 26 
3.3.1. Rock Armoured Trapezoidal Layered Breakwater 27 
3.3.2. Concrete Armoured Trapezoidal Layered Breakwater 27 
3.4 . Experimental Set-up 27 
3.5 . Test condit ions and parameters measured 28 
3.6. Instrumentation and data acquisition 28 
4 . D i s c u s s i o n o f results 35 
4 .1 . Approach to the discussion 35 
4.2 . Rock armoured trapezoidal layered breakwater 35 
4 .3 . Concrete armoured trapezoidal layered breakwater 38 
4.4. Comparison between rock armoured and concrete armoured 40 
trapezoidal layered breakwaters 
4.5. H o m o g e n e o u s trapezoidal breakwater 41 
5 . C o n c l u s i o n s a n d r e c o m m e n d a t i o n s 64 
Re ference 67 
A n n e x e A 70 
A n n e x e B 79 
V 
S U M M A R Y OF TABLES A N D FIGURES 
TABLE DESCRIPTION PAGE 
2.1 Damage reported to seawalls (Thomas and Hall, 1992) 17 
2.2 Structural configurations investigated 18 
2.3 Prediction equations for wave reflection, based on experimental data 19 
2.4 Prediction equations for wave reflection, based on field investigations 22 
3.1a Prototype characteristics of armour material for rock armoured and 30 
concrete armoured (secondary layer) trapezoidal layered breakwaters 
3. lb Prototype characteristics of core material for rock armoured and 30 
concrete armoured trapezoidal layered breakwaters 
4.1a Prototype conditions investigated experimentally on rock armoured 44 
trapezoidal layered breakwater - scale 1:20 
4. lb Percentage of wave energy dissipation at the sections where wave 44 
transmission was measured - rock armoured trapezoidal layered 
breakwater-scale 1:20 
4.2a Prototype conditions investigated experimentally on concrete 45 
armoured trapezoidal layered breakwater - scale 1:20 
4.2b Percentage of wave energy dissipation at the sections where wave 45 
transmission was measured - concrete armoured trapezoidal layered 
breakwater-scale 1:20 
4.3a Effect of wave period on wave transmission and reflection for the 46 
rock armoured trapezoidal layered breakwater 
4.3b Effect of wave period on wave transmission and reflection for the 46 
concrete armoured trapezoidal layered breakwater 
4.4a Equivalent prototype conditions investigated experimentally on 1:40 47 
scale models of homogeneous trapezoidal breakwaters - (tests 
conducted at Imperial college, London) 
4.4b Percentage of wave energy dissipation computed from model 47 
measurements corresponding to the sections where wave probes were 
located. Scale ratio 1:40, water depth = 8m (prototype) 
vi 
FIGURE DESCRIPTION PAGE 
2.1 Types of rubble mound breakwaters 23 
2.2 Relationship between block stability and voids ratio 23 
Willock (1981) 
2.3 Reflection performance of rock protection to existing seawall 24 
Allsop and Hettiarachchi (1989) 
3.1 Prototype breakwater cross-sections of the structures investigated 31 
3.2 Lanka Hydraulic Institute (LHI) - Laboratory Flume 32 
3.3 Experimental set-up for 1:20 scale model of Rock Armoured 33 
Trapezoidal Layered Breakwater tested at LHI 
3.4 Experimental set-up for 1:20 scale model of Concrete Armoured 34 
Trapezoidal Layered Breakwater tested at LHI 
4.1 Coefficient of Transmission vs. Steepness for Rock Armoured 48 
Trapezoidal Breakwater at scale 1:20 
(Regular waves) 
4.2 Coefficient of Transmission vs. Steepness for Rock Armoured 49 
Trapezoidal Breakwater at scale 1:20 
(Random waves) 
4.3 Coefficients of Transmission and Reflection vs. Steepness for Rock 50 
Armoured Trapezoidal Breakwater at scale 1:20 
(Regular waves) 
4.4 Coefficients of Transmission and Reflection vs. Steepness for Rock 51 
Armoured Trapezoidal Breakwater at scale 1:20 
(Random waves) 
4.5 Coefficient of Reflection vs. Iribarren Number for Rock Armoured 52 
Trapezoidal Breakwater at scale 1:20 
4.6 Coefficient of Transmission vs. Relative Distance for Rock Armoured 53 
Trapezoidal Breakwater at scale 1:20 
(Regular waves) 
vii 
4.7 Coefficient of Transmission vs. Relative Distance for Rock Armoured 54 
Trapezoidal Breakwater at scale 1:20 
(Random waves) 
4.8 Coefficient of Transmission vs. Steepness for Concrete Armoured 55 
Trapezoidal Layered Breakwater at scale 1:20 
(Regular waves) 
4.9 Coefficient of Transmission vs. Steepness for Concrete Armoured 56 
Trapezoidal Layered Breakwater at scale 1:20 
(Random waves) 
4.10 Coefficient of Transmission vs. Relative Distance for Concrete 57 
Armoured Trapezoidal Breakwater at scale 1:20 
(Regular waves) 
4.11 Coefficient of Transmission vs. Relative Distance for Concrete 58 
Armoured Trapezoidal Breakwater at scale 1:20 
(Random waves) 
4.12 Coefficients of Transmission and Reflection vs. Steepness for 59 
Concrete Armoured Trapezoidal Breakwater at scale 1:20 
(Regular waves) 
4.13 Coefficients of Transmission and Reflection vs. Steepness for 60 
Concrete Armoured Trapezoidal Breakwater at scale 1:20 
(Random waves) 
4.14 Coefficient of Reflection vs. Iribarren Number for Concrete 61 
Armoured Trapezoidal Breakwater at scale 1:20 
4.15 Coefficient of Reflection vs. Steepness for Homogeneous Trapezoidal 62 
Breakwater at scale 1:40 
4.16 Coefficient of Transmission vs. Steepness for Homogeneous 62 
Trapezoidal Breakwater at scale 1:40 
4.17 Details of the 1:40 scale model - Homogeneous Trapezoidal 63 
Breakwater 
viii