Bombardem breakwaters

The ‘Bombardon’ Floating Breakwater
Rahul Jindal 11NA10028
Ajit Kumar 11NA30024
Under the guidance of
Prof. T Sahoo
Ocean Engineering and Naval Architecture
IIT Kharagpur 1

Contents
1. Introduction
1. Types of breakwaters
2. Development of Bombardon breakwater
3. Full scale floating breakwater
2. Fundamental aspects of
floating breakwater design
3. Principal Particulars
4. Base Principals
5. Mathematical Theory
1. Wave energy reflection
2. Pressure on barrier
3. Breakwater with gaps
6. Final Impact
7. Conclusion 2

Breakwaters are structures constructed on coasts as part of coastal
defense or to protect an anchorage from the effects of both
weather and longshore drift.
Purpose of Breakwater
 A breakwater structure is designed to absorb the energy of the
waves that hit it, either by using mass (e.g. with caissons), or by
using a revetment slope (e.g. with rock or concrete armour units).
To manipulate the littoral transport conditions and thereby to
trap some sand
3
Introduction

Introduction (Contd…)
Types of breakwaters
• Detached breakwater ( Breakwater be completely isolated from shore)
Headland breakwater
Nearshore breakwater
• Attached breakwater ( Breakwater can be connected to the shoreline)
Low crested structure Rubble mound structure
High crested structure Composite structure
• Using mass (Caissons)
• Using a revetment slope ( e.g. with rock or concrete)
• Emerged breakwaters
• Submerged breakwaters
• Floating breakwaters
4

Introduction (Contd…)
Types of breakwaters
5

 Floating breakwaters can be made where bottom connected
breakwaters is unfavourable due to poor foundation, deep water,
other environmental problems like water circulation and fish
migration.
 The engineering and subsequent construction of the
"Bombardon" floating breakwaters was an important episode in
the historical development of floating breakwater technology.
 It was build along the coast of Mulberry harbor for the D-day
invasion of France in June 1944.
 Conclusions drawn from "Bombardon" floating breakwater
development still hold true today and virtually all subsequent
floating breakwater development uses its finding till now.
6
Introduction (Contd.)

 Normandy harbour components
 Gooseberry : Corn cub block ships
 Mulberry : Floating outer breakwater ( Bombardons)
Static breakwater ( Gooseberries)
Concrete caissons (Phoenix)
Floating piers or roadways ( Whales)
Pier spuds ( Spuds)
7

Phoenix:
• Reinforced concrete caissons
• Once refloated, were towed across channel to
form Mulberry breakwater along with
Gooseberries
• Made on 6 different scales for various depths
• 146 caissons were built in 9 months
8
Gooseberries:
• Ships were sunk in order to
form interior breakwater type
to save the harbour from the
stormy sea.

Fundamental aspects of floating
breakwater design
 Buoyancy and floating stability
 Wave transmission
 Mooring forces
 Breakwater unit structural design
Floating breakwater design is a complicated and iterative process
due to the interdependency of each design factor.
10

Development of Bombardon Breakwater
11
• 1st model was tested in May, 1943, equipped with flexible sides.
• Proved floating breakwaters efficiency as good as fixed ones.
• Due to flexible sides, reflection of wave energy took place at antinodes.
• Largest floating structures ever built at that time.
• Number : 3
• Length : 200 ft
• Beam : 12 ft
• Draft : 16.5 ft
This breakwater was
not adopted because
of the vulnerability of
its fabric sides.

Development of Bombardon Breakwater
12
• The first rigid wall Bombardon model was tested in June,1943 and by the
end of August , sufficient data was assembled to establish the correctness of
theories applying to rigid sided type.
• Over 300 model tests of the rigid type were made before full scale design
were put in hand
Results:
• It was possible to construct a breakwater which would surpasses wave of
maximum size anticipated in operation ‘Overlord’ for an expenditure of
about 1.25-2.5 tones of steel per foot of breakwater frontage.
• Expenditure of one-
tenth of that required
by any other possible
method.

Full scale floating breakwater
13
• First test of full scale harbour took place at the beginning of April 1944 in
Weymouth bay.
• On the 1st and 2nd April an onshore gale was recorded with a wind of force 7
gusting up to force 8 resulting in a sea up to 170 feet long and 8 feet high.
• Sea corresponds to stress on the breakwater of nearly double the estimated
value.
• The height of waves reduced by approx. 2 feet in lee of breakwater.

Principal Particulars
Steel weight : 250 tons
Length/unit : 200 ft
Beam : 25 ft 1 inch
Depth : 25 ft 1.75 inch
Draft : 19 ft
Gap : 50 ft
Effective beam : <5 ft
at Waterline
14

Base Principles
The operation of the breakwater depends on four well known
principles :
• The maximum height, length and period of the waves in any given locality
are determined by its geographical configuration.
• The waves of sea are relatively skin deep.
• The amplitude of oscillation in an oscillatory system having a long natural
periodicity is small when subjected to a forced oscillation of relatively
short periodicity.
• The floating object may, under suitable circumstances, be design to have
long natural periods in each of its three modes of oscillation: Heave, Pitch
and Roll.
15

Mathematical Theory
• Wave Energy Reflection
• Marine or gravity waves were investigated on the assumption that the
motion of particles in system of uniform travelling waves is either circular
or elliptical.
• Coordinates of particles acted upon by system of travelling waves given as:
• From these , trajectory equation is determined , which found to be elliptical
in nature with major and minor axis represented as:
)cos(
sinh
)(cosh
wtkx
kH
Hyk
aX 

 )sin(
sinh
)(sinh
wtkx
kH
Hyk
aY 


,
sinh
)(cosh
2
kH
Hyk
a

kH
Hyk
a
sinh
)(cosh
2


Mathematical Theory
• Wave Energy Reflection
The breakwater must be designed in order to meet maximum period and the
depth must be enough in order to reflect the desired quantity of wave energy

Mathematical Theory (Contd…)
• As length to beam ratio of Bombardon breakwater was very small i.e it can
be considered as a thin beam.
• The GDE for beam vibration and its corresponding amplification factor is:
• ‘m’ be mass of breakwater and external force is train of gravity waves.
• is natural time period
• By making ‘m’ large, R small and increasing Q as much as possible, the
amplification factor can be made considerably less than unity.
• This results in very less motion of breakwater, the train of waves on
reaching wall will suffer total reflection and any water on lee side of wall
will remain unaffected by the passage and reflection of wave.
18
,
2
cos2
2
t
P
aRs
t
s
Q
t
s
m
E






 222
1
1























E
N
E
N
P
P
mR
Q
P
P
AF
R
m
PN 2

• The pressure on barrier
– Hydrostatic : due to water at rest
– Dynamic : due to motion of water
If incident wave is considered to be sinusoidal,
• The horizontal force exerted by waves has same frequency as that of incident waves.
• Only the reflected waves contribute to the external force.
• The natural frequency of barrier must not match that of incident wave frequency to
avoid resonance.
19
Resonance must be avoided in all three modes of
vibration: Heave, Pitch & Roll
Done by:
Increasing mass by using ballast water
Reduction in restoring force by using flexible sides

20
Here,
D : Draft
P : Wave pressure
Y : 0 at surface
• Pivoted at by mooring
lines
•The total linear amplitude at Y=0 is
a3.
2
1
KD
Ke
y 


Breakwater with gaps
• Bombardon breakwater was constructed as combination of several floating units.
• Units are positioned by using ships, which leads to certain gaps ( approx. 50 ft)
• A diffraction pattern of wave will immediately formed behind breakwater, which
will ultimately rebuild itself into transmitted wave.
• The energy passed through gaps must be included in calculating transmitted wave
amplitude.
21

Final Impact
• The ‘Bombardon’ floating breakwaters were designed for waves of
• Maximum wave height of 3.3 m
• Wave length of 45.7 m (5.6 s period)
• Unfortunately, the breakwaters were destroyed by an unexpected storm in
June 1944, ( worst storm in 40 years), just after 2 weeks of D-Day with
• Wave height of order of 4.6 m
• Wave length of 91m ( 8 s period)
22

Conclusion
• The floating harbour principle is one of considerable importance.
• There are many advantages of floating over fixed harbours
– The cost is to of fixed type.
– The area of sheltered eater may be readily enlarged by the addition of units or mere re-
sitting of moorings.
– No interference with underwater currents, results in freedom from silting and scour.
– Erection is very faster
– Floating harbour may be arranged to cater with temporary or seasonal requirement and then
units can be moved on or stored away in off season.
– Temporary floating breakwater can be utilized to protect fixed harbour works during its
erection phase
Although, at first floating breakwater appeared as rather startling innovation,
sceptics found many reasons before its trials why it could not work, yet, in
fact, it did work and it successfully accomplished its allotted task in the
invasion and liberation of Europe.
23
5
1
20
1

References
• Lochner, R., Faber, O. and Pennry, W.G., “ The ‘Bombardon’ Floating
Breakwater”, The Civil Engineer of war, Vol 2, (1948), Docks and Harbours,
The Institution of Civil Engineers, London, England.
• Tsinker, G.P., “ Marine Structures Engineering : Specialized Applications”,
published by Chapman & Hall, (1995).
24

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