CHAPTER 7 STRUCTURAL DESIGN OF BREAKWATERS · Chapter 7 Structural Design of Breakwaters CEI 451 Harbor, Navigation and Shore Engineering Dr. Hesham N. Farres 4 Rubble Mound Breakwaters

CHAPTER 7

STRUCTURAL DESIGN OF BREAKWATERS

Breakwaters are built for several reasons, one of those reasons are for determination of the harbor’s

water area, and to protect this area from the impact of the waves in order to protect the ships inside the

harbor from the action of these waves, especially during anchorage of these ships for loading and

unloading. Also breakwaters are built to protect natural protected harbor entrances or navigational

channels from the effect of wave impacts during their entrance (or exit), as well as the reduction of the

amount of dredging needed in navigational channels and harbor entrances. Breakwaters can be built

also for shore protection and to create calm areas for swimmers in recreational areas.

The following factors are taken into account in the planning of breakwaters

1. Breakwaters planning should lead to allowable wave heights within harbor water areas that

should be protected.

2. Breakwaters planning should not lead to creating water currents that can cause difficulties in

navigation through navigational channels or harbor entrances

3. Breakwaters planning should not lead to erosion or accretion on either side of port locations - or

at breakwater’s location. These locations may require additional artificial protection to prevent

the occurrence of this phenomenon.

4. Breakwaters planning should not result in entrance of currents loaded with sediments to the

port’s water area that could lead to sedimentation, and thus reduce designed water depths. These

accretions may require a lot of expensive dredging works, as well as to obstruction in ship

maneuvers within the port.

5. As breakwaters are one of the most expensive construction works, planning must be designed to

achieve economic costs by designing economic breakwater lengths and breakwater sections

without affecting the safety of breakwaters or the efficiency of the water area.

6. Planning of breakwaters is accompanied by the use of refraction and diffraction diagrams to

assure the safeness of the protected harbor water area from the effect of dangerous waves.

Structural Design of Breakwaters Chapter 7

CEI 451 Harbor, Navigation and Shore Engineering Dr. Hesham N. Farres

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Types of breakwaters

Breakwaters can be divided according to terms of construction and to how these breakwaters prevent

waves from entering the water area through the following types: -

1- Fixed breakwaters

a) Inclined breakwaters usually called rubble-mound breakwaters

b) Upright breakwaters

c) Composite breakwaters

2- Mobile breakwaters

a) Floating Breakwater

b) Hydraulic Breakwater

c) Pneumatic Breakwater

Rubble mound breakwaters consist of piles of rocks or stones that can consist of natural or artificial

concrete stones or both. Side slopes and stones’ weights are determined such that they do not move

from their positions under the influence of waves. As a result of the inclination of these types of

breakwaters in front of incident waves; water depths gradually decrease over these slopes that help in

breaking of the attacking waves.

Upright Breakwaters consists of vertical surfaces in front of incident waves; if attacking waves collides

with these surfaces, they do not break, but they are rather reflected and rebound again in direction of

the sea. As a result of the impact of waves on these vertical surfaces; breakwaters are designed to resist

the wave’s impact mainly by their weight.

Composite breakwaters have the advantages of both types; it consists of a rubble mound base over

topped by a wall with a vertical surface. In this case, the greatest effect of waves is on the upper part of

the breakwater.

Mobile breakwaters are usually built to protect a site for a temporary purpose, where the degree of

protection allows the presence of waves in the protected water area with heights larger than wave

heights typically allowed for fixed breakwaters.



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Floating breakwaters consists of floating objects on the surface of the water that are fixed to the sea

bottom such that they do not move much from their places. These objects consist of vertical surfaces in

front of incident waves, with a certain draft that is designed for a required degree of protection. These

breakwaters prevent part of the wave energy from entering the water area, while allowing part of the

energy located at depths greater than the breakwater’s draft to access the water area.

A hydraulic or water-jet breakwater is formed by-

forcing water through a series of nozzles mounted

on a pipe at certain depths not necessarily at depths

of the sea floor. These pipes are installed

perpendicular to the direction of the incident

waves. The jets create a surface current which

results in breaking of the incident wave.

Pneumatic breakwaters operate in a similar principle to hydraulic breakwaters, in this case; air pressure

inside pipes are forced perpendicular to the direction of the incident waves forming a horizontal surface

current that results in breaking of the incident wave. The vertical air current that comes out from the

perforated pipes are in the form of rising air bubbles; theses air bubbles spread in the water forming a

mixture of air and water whose density are less than the density of sea water, this low density mixture

rises to the water surface creating a horizontal current that stabilizes the incident waves.

The breakwater’s type is determined according to a variety of several factors including: -

1. The natural phenomena in the region in terms of wave properties, tides and water currents.

2. The bathymetry of the sea floor around the breakwater

3. Soil properties of the sea bed

4. The objective of the proposed breakwater

5. The cost of construction materials

6. Experiences in implementation, types of available equipment, and availability of workers.

7. Expenses of construction and comparison of operation costs and annual maintenance. Some

breakwaters require operation costs and continuous maintenance like hydraulic and pneumatic

breakwaters while others seldom need any like rubble mound breakwaters.



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Rubble Mound Breakwaters

These breakwaters consist of piles of either natural stones in the entire cross-section or composed of

both natural stones and artificial concrete blocks. Stones are placed in such a way that it prevents waves

from displacing these stones or blocks.

The formation of the breakwater’s cross-section

The breakwater’s cross-section is usually formed from three types of layers

The Core

The Armor Layer

The Secondary Armor Layer (The Filter)

The Core

This is the internal protection layer of the breakwater’s cross-section, it usually consists of natural

stones or artificial concrete blocks, their weights are usually small, that are not strong enough to resist

the impact of the waves, thus the core needs an outer layer to protect it; that layer is called the Armor

layer.

The Armor Layer

This is the outer protection layer of the breakwater’s cross-section which consists of layers of natural

stones or artificial concrete blocks with weights large enough to resist the effect of waves. Because of

the discrepancy in weights between the core and the armor layer and, hence, sizes of the core’s stones

and the stones used in the formation of a armor layer, the gaps that exist between the armor layer will



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allow the movement of core’s stones and the suction of theses stones out of the breakwater’s cross-

section.

Thus another layer named the Filter layer is placed between these two layers.

The Armor layer guarantees the protection of the breakwater’s cross-section from the sea side (the sea)

and from the lee side (the port or the shore). This layer is composed of natural stones or artificial

concrete blocks sufficient for their balance against the wave forces. The Armor layer may consist of a

single row layer or consist of several rows, but generally they are designed of two rows. Armor layers

are preferred to consist of several rows in order not to subject the internal layers (the Core or the Filter)

to any direct wave impact that could lead to the collapse of the breakwater.

The Secondary Armor Layer (The Filter or the Under-Layer)

This is the middle protection layer of the breakwater’s cross-section which consists of layers of natural

stones or artificial concrete blocks with moderate weights between core’s stones and armor’s stones.

They function as a filter to prevent the movement of core’s stones inside the core and their suction

through gaps formed between the armor layer and out of the breakwater’s cross-section and their

weights are determined accordingly. The secondary armor layer also may consist of a single row layer

or consist of several rows, but generally they are designed of two rows.

Advantages of Rubble Mound Breakwaters

1. Easiness of construction that do not require exceptional skills, as well as they do not require

unusual equipment.

2. Possibility of establishment over certain types of soil that may not be strong enough to carry

other types of barriers

3. Can be implemented over irregular sea beds

4. Durable

5. Maintenance is possible although it is costly in some cases

6. Can be constructed in any depth from the technical point of view.

7. Waves are not reflected on their surfaces providing a calm area within the water area.

Disadvantages of Rubble Mound Breakwaters

1. Requires huge amounts of construction materials, as well as labor, and requires an increase in

the amount of materials and labor with the increase in the water depth



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2. Requires an increase in the weights of natural stones or concrete blocks with the increase in the

wave heights causing difficulties in implementation.

3. Cannot be used for ship berthing, as their walls are not vertical

4. Requires continuous maintenance expenses to compensate for stones or blocks that move from

their positions.

It is not necessary that the stones weights are the same in each layer; they may vary such that the

largest stones are placed in the upper parts and up to a depth greater than the wave height measured

from the low water level.

Since the stones of the breakwater facing the sea side is directly exposed to the wave forces. Therefore

these stones are usually much larger than the stones placed in the breakwater’s lee side (the port or the

shore)

Yet the armor protection facing the sea side must be also applied over the breakwater’s crest and on

the lee side of the breakwater to the level of the low water unless a capping wall is to be constructed

over the breakwater.

During wave impact, the water level rises in front of the breakwater resulting in internal pressures that

could lead to the lifting of stones or blocks causing instability. Therefore a high degree of permeability

should be taken into account when designing breakwaters to facilitate the movement of water within

the body of the breakwater. For this purpose, it is preferably that stones of the main armor layer and

the secondary armor layer are to be placed randomly, this does not means to leave big gaps between the

blocks as this will help the core stones to be seeped out of the breakwater’s body. For example, if the

blocks used are in the form of cubes, they can be stacked next to each other or randomly placed, the last

method is much more preferred, taking into account during the implementation of these blocks

randomly; interference must be provided to prevent the formation of gaps.

Method of construction

The core is first constructed beginning from the beach where stones are thrown from Lorries; the

surface of this core is used as a road for Lorries to transport the stones or the concrete blocks



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Disadvantages of this method

1. The stone sizes must be able to resist the waves during the construction

2. It may be necessary to increase the core’s design breadth in order to facilitate the movement of

Lorries.

3. It may be necessary to use stones in graded sizes in the upper layer of the core to achieve a flat

surface to ease the movement of Lorries. This movement causes the upper surface layer to be

compacted, and it becomes necessary to remove the upper layer before implementing the

secondary armor layer in order to remove the compacted upper layer in order to maintain a

permeable structure.

4. It may be necessary to speed up the completion of the secondary and main armor layer to

protect the exposed core from any damage caused by wave forces or from losing any part of its

constituent materials.



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Advantages of this method

Despite all these disadvantages in the method of construction, this method requires a small amount of

large stones with respect to the amount of stones needed from small sizes. This makes it much more

preferred if construction materials are available with reasonable prices. (Note that during the bombing

of natural stone in quarries, high ratios of small stones are produced with respect to the small

proportion of large stones, which is required by this type of barrier).

Types of artificial blocks

These blocks are usually made of plain concrete, and in rare cases, made of reinforced concrete. These

blocks take different forms, each with its own specifications.

These blocks have been developed to deal with the possibilities in lacking some stone weights required

by the design, while others are made to achieve certain characteristics that cannot be achieved by using

natural stones.

The most commonly used types of artificial blocks are the following: -

1 – Ordinary Concrete Blocks

These are made in the form of cubes or parallelepiped (rectangular shaped)

2- Concrete Blocks with special forms

Examples of these blocks

Blocks with four branches called Tetrapods and usually made of ordinary concrete, and are

rarely made of reinforced concrete,

Blocks with six branches called (Haxapods)

Blocks that are rarely used called (Bipod, Stabit, Hollow N, Tripod, Hollow tetrahedron)

Modified cubes such as Grooved cube with a hole

Blocks with three vertical parallel cylinders three blocks

connected to each other at their middle height by a concrete slab;

these blocks are called Tribars

Blocks with a high porosity such as Akmons

Recent blocks that have been introduced such as Dolos and the

following shapes



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Design of Rubble Mound Breakwaters

There is no exact method for the design of rubble mound breakwaters, but certain equations derived

from experimental results are used as well as some general equations derived from experiences are

used. The designer generally takes into account the design of previous projects that was placed under

observation since its establishment. Generally, important projects are not executed unless hydraulically

experimental analysis is simulated taking into consideration all the natural phenomena that can face the

breakwater in nature.

The structure of breakwater can be divided longitudinally into two parts

Breakwater’s Head

This is the final part of the breakwater’s and its definition covers a minimum length of 50 m measured

from the end of the barrier in direction of the beach, if it is a connected breakwater, or in direction to

the other end if it is a separated or detached breakwater.

Breakwater’s Trunk

This is the part of the breakwater from its head to the beach if it is a connected breakwater, or to the

other head if it is a separated.



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These two parts (the trunk and head) are acted by different wave forces for the same wave properties,

as well as the requirements for the use of the surface of each may differ; thus each part must be

designed separately.

Due to the variation of the breakwater’s depths along the breakwater’s trunk, as well as the variation of

the design wave characteristics; as it is clear from the study of wave refraction; the breakwater’s trunk

is divided into several segments such that the design wave properties of each segment covers the worst

wave conditions on this length.

It is clear from the above mentioned that the sections of the breakwaters are not similar; the

breakwater’s structure consists of several different trunk sections as well as a section in the

breakwater’s head.

The rubble mound breakwater’s design includes the following: -

1. Determination of the quality and sizes (or weights) of the natural stones or artificial blocks to be

placed in different parts of the breakwater’s section.

2. Determination of the design levels of breakwater’s surface, as well as levels of the core and

secondary armor’s surface.

3. Determination of the breadth of the breakwater’s surface

4. Identification of breakwater’s side slopes, and determination of the thickness of both; the armor

layer and the secondary armor layer.

5. Investigation of the breakwater’s stability and the soil’s bearing capacity.

6. Calculation of the expected settlement of the breakwater’s structure in the soil, as well as the

expected settlement; due to the interference that occurs between the stones forming the section

with each other; under the influence of waves and the weights of the stones itself. This

settlement must be taken into account in the design, so that the actual levels of the breakwater

after settlement are identical to the design levels.

Determination of stone weights in the main armor layer facing the sea side

Natural stone weights or artificial block weights are designed such that they achieve stability in their

position in the breakwater’s section. The stones or blocks should be able to resist the acting waves from

moving any of them, whether by sliding or lifting them from their place. These weights can be



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determined such that no damage to any stone or block occurs, in this case the breakwater does not

require any maintenance work at all, or these weights can be determined such that the stones or blocks

are allowed to move with a certain relative amount of movement, in this case annual maintenance are

established to compensate for the amount of stones or blocks that are lost in the sea each year.

The equilibrium of the stones or blocks depends on several factors such as:-

1. The shape and weight of each of these blocks, and the specific gravity its material

2. The wavelength and wave height

3. The depth of water

4. The permeability of the structure

5. The height of the breakwater’s surface above the still water level, as well as the breadth of the

breakwater’s crest.

6. The location of the stone or block above the still water level

7. The angle of the acting wave on the structure

8. The inclination of the side slopes where the stones or blocks are placed

9. The method of placing these stones or blocks whether they are uniformly placed or randomly

distributed.

10. The number of layers forming the armor layer.



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Hudson’s Formula (called the “WES Formula” in some references)

W = γa H

3

KD Sr−1 3 Cot α (7-1)

Where:

W = weight of a single stone or block

H = significant wave height

KD = stability coefficient which depends very much on the shape of the block or the natural stone,

and the number of layers of these blocks and some other factors

= inclination of the side slopes with respect to the horizontal plane

(for example cot = 1.5 or 2)

a = specific weight of the armor material of this stone or block

Sr = Relative specific gravity of the material of this stone or block with respect to the kind of water

density the stones or blocks are placed

𝑆𝑟 = 𝜌𝑠

𝜌𝑤 (7-2)

s = mass density of the stone or block’s material

w = mass density of water (sw = 1.025 t/m3)

As stated before, the value of the stability coefficient (KD) depends on the shape of the block or the

natural stone, and the number of layers of these blocks, in addition to that, the value of the stability

coefficient is determined according to the location of these blocks; i.e. whether they are placed in the

head or the trunk of the breakwater. Smaller values of (KD) are usually taken for stones or blocks

placed in the head of the breakwater, while bigger values are taken for stones or blocks placed in the

trunk. Table (7-1) shows the different values of the stability coefficient (KD) that is commonly used in

the calculation of stone weights or blocks for various forms of protection. Table (7-2) shows the layers

porosity value for various forms of protection.

Determination of stone weights in the main armor layer facing the shore side

The design of the armor layer in the lee side of the breakwater; i.e. the side facing the shore side or the

harbor water area depends on the following conditions

1. The amount of water overtopping the breakwater’s crest

2. The height of waves acting on the lee side of the breakwater

3. The permeability of the breakwater’s section



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Table (7-1) Different design values of the stability coefficient (KD)

Type of stone or

block

Type of

placement

No of

layers

Trunk Head

Breaking Non

breaking Breaking

Non

breaking

Quarry stone

smooth rounded

Uniform 2 1.2 2.4 1.1 1.9

Non uniform 3 3.2 3.00 2.90 -

Quarry stone

Rough angular

Uniform 2 2 4 1.6 2.8

Non uniform 3 4.30 4.00 3.80 -

Parallelepiped blocks Non uniform 2 2.00 - - -

Cubes Non uniform 2 3.50 - - -

Modified cubes Non uniform 2 7.5 7.00 5.00 -

Tetrahedron Non uniform 2 2.00 - - -

Riprap Uniform 2 2.2 2.5 - -

Tetrapods Uniform 2 7 8 4.5 5.5

Non uniform 2 8.3 8.00 6.50 6.00

Quadripods Non uniform 2 8.3 8.00 6.50 6.00

Hexapods Non uniform 2 9.00 8.50 7.00 6.00

Tribars

Uniform 2 9 10 7.8 8.5

Non uniform 1 15.00 12.00 9.50 7.50

Akmon Non uniform 2 11.00 - - -

Dolos Non uniform 2 15.8 31.8 8 16

Stabit Non uniform 2 15.00 - - -

If the breakwater’s crest is above the still water level with an amount that prevents the overtopping; the

weight of the stones or blocks of the armor layer in the lee side of the breakwater’s trunk becomes

dependant on the wave height in the water area of the harbor or the shore side and dependant on the

permeability of the breakwater’s cross section.

If overtopping occurs; the weight of the stones or blocks of the armor layer in the lee side (from the

breakwater’s crest to the low water level) are similar to the stones or blocks of the sea side.



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If the wave heights are similar on the either sides of the breakwater’s trunk; i.e. the seaside and the lee

side; in that case the breakwater’s cross-section should be symmetric on both sides.

As for the breakwater’s head; the cross section should be similar on both sides for a distance not less

than 50 meters measured from the breakwaters end.

Table (7-2) Porosity value of layers and the shape factor KΔ

Type of stone or

block

Type of

placement No of layers Porosity % K ∆

Quarry stones Non uniform 2 38 1.00

3 40 1.00

Modified cubes Non uniform 2 47 1.10

Tetrapods Non uniform 2 50 1.00

Quadripods Non uniform 2 50 1.00

Hexapods Non uniform 2 47 1.15

Tribars

Uniform 2 54 1.00

Non uniform 2 47 1.13

Akmon Non uniform 1 55 -

Stabit Non uniform 2 52 -

Dolos Non uniform 2 60 1.3

Determination of stone weights in the secondary armor layer (the filter)

This layer acts as a filter layer to protect leakage of fine materials of the breakwater’s core The weight

of stones of the secondary armor layer (the filter) is usually taken as a ratio of that of the main armor

layer.

Wsec = W/10

Determination of stone weights in the core

The weight of stones of the core are usually taken as a value ranging from W/200 to W/4000 and not

less than 25 kg



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The Breakwater’s Toe

The waves acting on the breakwater can cause soil erosion in front of the breakwater, thus a protection

of the breakwater’s toe is needed to prevent leakage of fine materials of the breakwater’s core. This

protection is established by placing a horizontal armor layer below the main and secondary armor

layers of the breakwater’s cross section. This layer acts also as a filter layer to protect leakage of fine

materials of the breakwater’s core.

The weight of stones of the breakwater’s toe usually ranges from a value of 0.5 kg to 25 kg.

The width of the breakwater’s toe depends on several factors such as; the depth of water, the weight of

the stones used, and some other factors. Generally the breakwater’s toe should not be less than 0.35 m,



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but if the toe is subjected to the influence of waves and currents; in this case the width of the

breakwater’s toe should not be less than 1.50 m measured from the toe of the breakwaters side slope

The Geo-textile Layer

This is a layer formed from geo-textile materials; these materials are permeable fabrics used in

association with soil and have the ability to separate, filter, protect, or drain. They are typically made

from polypropylene or polyester. This layer acts as a filter to prevent leakage of fine materials of the

breakwater’s core.

The breadth of the breakwater’s crest

The breakwater’s crest must have a certain width in order to cover the following conditions:

1- The breakwater should be able to resist the waves, and to resist the rushing up of water mass on

the outer side slope; what is called “run-up”, as well as resisting the phenomena of overtopping

if run-up is allowed.

2- The breadth of the breakwater’s crest depends on the method of construction and quality of

equipment used.

3- If the breakwater’s crest is used for other purposes, such as a surface for a highway or railway;

therefore the breadth of the breakwater’s crest is determined according to those requirements.

4- The breadth of the breakwater’s crest should in any way not be less than the following values:

The significant wave height (Hs)

𝐵 = 𝑛 𝐾∆ 𝑊

𝛾𝑎

1 3

(7-3)

Where

B = breadth of the breakwater’s crest

n = number of layers of stones or blocks, usually taken 2 or 3

K ∆ = Shape factor, and its value is taken from Table (7-2)

W = Weight of the main armor layer’s stones or blocks

a = Specific weight of the material of the main armor layer’s stones or blocks

The level of the breakwater’s surface

Since the purpose of the construction of the breakwater is to protect the water area of the port from the

impact of the waves, it follows that level of the breakwater’s surface must prevent the overtopping of

http://en.wikipedia.org/wiki/Fabric

http://en.wikipedia.org/wiki/Soil

http://en.wikipedia.org/wiki/Polypropylene

http://en.wikipedia.org/wiki/Polyester



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water waves on the surface of the breakwater and its displacement to the water area. Overtopping is

allowed if it does not lead to the formation of waves in the water area to the extent that exceeds the

permissible limits

Thickness of the armor’s layer and the number of stones or blocks

The thickness of the above-mentioned layers is determined according to the following equation: -

𝑇 = 𝑛 𝐾∆ 𝑉1/3 (7-4)

Where:

T = thickness of the armor layer (sea side)

n = the number of stone layers inside each layer (2 or 3)

K = Shape factor (Table 7-2), if not given it can be assumed K = 1.1

V = Volume of one block = W/a

Number of stones per unit surface area

The number of stones or blocks per unit surface area of the above-mentioned layers is determined

according to the following equation: -

𝑁 = 𝑛𝐾∆ 1 −𝑃

100 𝑉−2 3 (7-5)

Where

N = number of blocks

P = porosity (the size of the gaps to the total volume) in percent, see Table (7-2)

Inclination of the breakwater’s side slopes

The choice of the inclination of the breakwater’s side slopes depends on both technical and economical

considerations. Although flat inclinations are more stable than steep ones and requires lighter stones;

yet these flat slopes increases the amount of material required for construction

Generally the maximum allowable slope is 5:4 (cot = 1.25), but it is preferably to take the maximum

value as 3:2 (cot = 1.5)



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Capping wall

Usually breakwaters are provided with a capping wall that is based on the secondary armor layer or the

core directly. This capping wall prevents the waves from reaching the surface of the breakwater’s crest

by reflecting the waves back to the sea. It is usually made of plain concrete cast in-situ

Combined Breakwaters



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Empirical Dimensions

References

1. Abidou, I. A. (1995),"Harbor Engineering and Coastal Structures", Textbook, Faculty of

Engineering, Alexandria University

2. Balah, A.M. (2010), Lecture notes, Faculty of Engineering, Ain Shams University

3. Coastal Engineering Manual (2006), U.S. Army Corps of Engineers CECW-EH Washington.

4. Mostafa, Y. S. (2010), "CEI 451 Harbor, Navigation and Shore Engineering", Lecture Notes,

Faculty of Engineering, Ain Shams University

CHAPTER 7 STRUCTURAL DESIGN OF BREAKWATERS · Chapter 7 Structural Design of Breakwaters CEI 451 Harbor, Navigation and Shore Engineering Dr. Hesham N. Farres 4 Rubble Mound Breakwaters

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