l§l
Hydraulics Research
Wallingford
SHORE PROTECTION BY OFFSHORE BREAKWATERS
A H Brampton Ph . D
J
V
Smallman Ph.D
Repor t No SR 8
July 1985
Registered Office:
Wallingford,
Telephone:
Hydraulics Research
Oxfordshire OX10 8BA.
0491 35381. Telex:
848552
Limited,
Thi s report describes work carri ed out wi t hin t he research programme
Commi s s i on
B
f or t he Mi n i s t ry of Agricultu re, Fi sheri es and Food .
The st udy
was carri ed ou t i n t he Coas t al Proces s es S ect i on of t he Mari t ime En gineering
Department of Hydraulics Res earch .
The departmen t al nom i n at ed of f i cer i s Mr A Alli son .
The Company's nom inat ed
project of f i cer is Dr S W Hun t i n g t on .
The report i s publi s hed on behalf of MAFF but any op inions exp res s ed are not
necessari ly t hose of t he Mini s t ry .
C
Crown copy ri ght 19 85
ABSTRACT
This report reviews the information available for the design and use of
offshore breakwaters in shore protection.
As an introduction to the subject
the physical processes occurring in the lee of an offshore breakwater are
described with reference to natural examples.
of case histories,
This is followed by a survey
and mathematical and physical modelling techniques for
offshore breakwaters.
Some of the methods which are available for the
design of a breakwater system are reviewed.
the design process are described,
Possible future developments in
and the areas in which further research on
the effects of offshore breakwaters is required are highlighted.
CONTENTS
Page
E XECUT I VE S UMMARY
1
I NTRODUCTION
1
2
OFFSHORE BREAKWATERS - THE PHYS I CAL PROCE S S E S
2
2.1
Na t ural E xamples
2
2.2
Phy s i c al pro c es s es
4
3
4
LITERATURE STUDY
6
3.1
Rev iew o f c a s e h i s t o r ies
6
3.2
Phy s ic a l mod el s t u d ies
10
3.3
Ma thema t ical mo del s tudies
16
DE S I GN METHODS FOR
AN
OFFSHORE BREAKW ATER S YSTE M
19
4. 1
Develo ping the init ia l d es ign
19
4.2
Metho d s for improvin g the brea kwa t er design
22
5
FUTURE DEVELOPMENT
26
6
CONCLUS IONS AND RECOMMENDATI ONS
28
7
ACKNOWLE DGE MENT S
29
8
RE FE RENCES
30
FI GURES
PLATES
EXECUTIVE SUMMARY
1.
These paragraphs summarise a repor t on t he role of offshore
breakwaters in shore pro tec tion .
Commission B
2.
-
The study was under taken for MAFF -
Marine Flood Protection , during 1984 and 1985.
The objective o f t he report is t o evaluate the effectiveness o f
offshore breakwaters on reducing wave action a t the main line o f sea
defence works .
This evaluation also includes an assessmen t of t he
pot ential of such structures in encouraging improvement of natural
beaches .
3.
As an introduc tion to t he subject , the physical processes occurring
in the nearshore zone in the lee of an o ffshore breakwater are
described with reference to natural examples .
4.
A major part o f t he study comprises a li terature review of case
histories of o ffshore breakwater s .
I t was found that whilst there is
much experience of design and building these struc tures in areas o f
low tidal range , for example in I t aly and Japan , less information is
available on sites where the tidal range is appreciable .
I t is in
such areas , in par ticular in the UK , where continual monitoring and
appraisal of t he offshore breakwaters and t heir effect s would provide
useful guidelines for future design .
S.
The report also describes the mathematical and physical model
techniques which may be used to assist in the design of offshore
breakwaters .
At present mat hematical models may be used to predict
trends in beach plan shape due to changes in nearshore wave climate ,
and also to optimise t he layout o f an offshore breakwat er system ,
taking into account both diffrac tion and over topping .
Physical
models may be employed bot h to assess t he hydraulic performance o f an
offshore breakwater , and to represent the effect s of an offshore
breakwater system on an adjacent beach .
6.
In addition the repor t suggest s areas where fur ther research into
modelling techniques would be valuable .
1
INTRODUCTION
This report, commissioned by the Ministry of
Agricul ture, Fisheries and Food, assesses the role of
offshore breakwaters and similar structures in sea
defence applications around the UK coast line .
Although such structures can be used on their own to
protect a coas t , it has been assumed in this s tudy
t hat tneir principal role will be to reduce wave
action, and its harmful effects, at an existing coast
line. Wave heights are reduced both directly, by
reflecting or absorbing the wave energy incident o n
t h e structure, and indirect ly by improving t he beach
within the area sheltered by the structure .
As will be explained later, it is comparatively
straightforward to calculat e some of the benefits
provided by, say, a detached surface piercing
breakwater .
In contrast other aspects, including the
prediction of the changes in the beach, both in
profile and plan shape, presen t much more difficult
and challenging t asks .
In Chapter 2 the general background of the developmen t
of artificial breakwaters is discussed .
This includes
examples of naturally formed coastal formations which
demons trate the effects of such struc tures on a beach,
as well as a description of the physical processes
involved .
This is followed in Chapter 3, by a review of relevant
literature and case histories, mainly based on
overseas experience, but also including a preliminary
appraisal of three offshore breakwaters recently
constructed along the UK coastline (at Rhos-on-Sea and
at eit her end of Leasowe Bay on the Wirral) .
This
review gives some general guidance on t he design of
such struc tures, although most of the examples cit ed
are on coasts with a very much smaller tidal range
t han is t ypical around the UK coas t .
Following from these general guidelines, Chapter 4
deals with the methods available for designing both
submerged and emerged (surface piercing) breakwaters .
The chap ter also reviews the role of bot h physical and
mathematical modelling as aids to design . Although
there is already a considerable amoun t of research
relevant to such design techniques, many problems
remain, especially in connect ion with submerged and
permeable structures .
Possible future developments in the design of offshore
breakwaters, and the research required, are discussed
in Chapter s. This includes discussion on topics such
as the effects on a beach caused by a permanent
alteration to the wave climate and the role of beach
l
nourishment in combination wit h t he cons truct ion of
offshore breakwaters.
The final chapter summarises the conclusions and
recommendations of the study.
2
OFFSHORE
BREAKWATERS
THE PHYSICAL
PROCESSES
2.1
Natural
examples
Along many sandy coast s around t he world, especially
in the Mediterranean Sea, natural offshore islands and
reefs have caused accumulations of beach material in
t heir lee. Often t he effect is simply to produce a
local increase in the beach widt h resulting in a bulge
or cusp in the sheltered area. However when an island
is sufficiently close to a shoreline, in comparison to
its length, the resulting beach plan shape can be much
more spect acular.
In such circumstances, a narrow ' neck ' of beach
material can form between the shoreline and t he
island, and this feature is generally referred to as a
' tombolo ' . The word is of I talian derivat ion and is
used t here to describe any low lying sandy area near
the coast and would be perhaps more correctly
t ranslated into English as ' links ' .
In coastal
engineering , however, t he t erm is reserved for the
specific feature described above .
Tombolos can be almost perfect ly symmetric resembling
a wine glass stem in plan shape, or distorted to some
extent either by obliquely incident waves or by the
shape of the offshore island. Two very impressive
examples demonstrating these points can be found on
the Pakist an coast and are shown in Figs 1 and 2 .
Occasionally more complicated shapes occur such as t he
double (or even treble) t ombolo as illustrated in
Fig 3 which shows a famous example at Orbe tello on the
west coast of It aly .
It is also worth noting how varied the size of
tombolos can be. At one extreme it is simple to
creat e a tombolo in the laboratory, or to find
examples of similar size on t he shores of lakes and
ponds. Typical examples on open coast s range from
t ens to hundreds of metres in lengt h , but t he examples
on the Pakistan coast are several kilometres long,
which is exceptional . The formation of these two
tombolos, however, has been assisted by rapidly
changing land levels , due to local seismic activity.
2
Th e re are als o a surp r i s ing number of s imilar f ea tures
around the northe rn and wes t e rn coas t s of the Uni t ed
Kingdom. A good examp le of a beac h cusp behind an
o f f s ho re reef i s seen i n Pla t e 1, which s hows part of
the Do r s e t coa s t .
It i s intere s t ing to no te the
und e rwater e xt e n s i on of the cusp wh ich resemble s an
'unde rwa t e r' t ombolo.
This ap pears to be a common
f ea t ure in UK wa t e r s , presumably as a result of t he
la rge t i dal range. Fu rthe r e xamp le s of subme r ge d or
'low-wa t e r ' tombo lo s can be f ound at St Mi c ha e l's
Mount in Cornwall, and at Burgh I s land i n Sou t h De von.
Fully eme r ged o r 'hi gh -wat e r' t ombolo s are le s s easy
t o f i nd alt hough there are good examp le s at Llanddwyn
I s land on Anglesey and at Langne s s on the I s le of Man.
It als o s eems likely that bo t h Hugh Town, I s le s of
Sci lly a nd S t Ives i n Cornwall are part ially bu i lt o n
s u c h tombolo s . Tombolo s can also f o rm o n s hingle
beaches, and t he Por t land end o f Che s i l beach i s a
good e xamp le.
The d i s t inct ive p lan shape of eme rged t ombolo s c a n
only p e r s i s t , howev e r , i f there is no ne t t alongs hore
d r i f t of be ach ma t er i a l.
In t he p r esence of such a
d r i f t , the area shelte red by the i s land gr adually
f i lls f r om the updr i f t s id e ; one half o f the t ombolo
i s thus obs cured alt hough the coa s t of t en r e t a i ns i t s
indented shape o n t he downd r i f t s i d e . A good e xample
of t h i s s i t u a t i on is provided by t h e c o as t at
Har t lepool (Fi g 4 ) . The downd r i f t bay o f t e n prov i d e s
a natural s i t e for a harbour, a s in th i s case .
Such
bays have a t t racted c onside rable i n t e r e s t in recent
years as coas t a l engi nee r s have t r ied to a r t i f ic i a lly
c re a t e s i mi lar shapes , e i ther to reduce e r o s i o n o r
p r ovide she lte red har bour s i t e s . As a res ult , such
f e a tures have been d e s c ribed by a plet hora of t e r ms
inc lud ing ze ta, log-s pi ral, c renula t e or
headland -bay s . Th i s i n t e r e s t is we ll demons t r a t e d by
the la rge number of refe rences revi ewed i n Chap t e r }.
There i s no doubt that both t ombolo s and headland bay
beaches are usually very s table as well as
ae s th e t i cally p leas i n g natural coa s t li ne f o rmat i o n s .
I t i s therefore n o t surpris ing that when faced wi t h
e r o s i on p ro blems, e ngineers i n a reas s u c h as the
Med i t e rranean Sea tried to emula te nature, and p r o t e c t
Thi s
c oa s t s w i th a r t i f i c i a l o f f shore i s land s .
d e c i s i on was undo ubtedly i n f luenced by a supply of
s u i ta b le ro c k, and the relative ly unsoph i s t i ca t e d
cons t ru c t i on me t hods re quired.
Des i gn o f such
o f f s hore breakwa t e r s , however , was based on emp i r i c a l
c r i te r i a , e xp re s s ing f o r e xamp le , the ra t i o o f
b re akwat e r length to t h e d i s t ance o f f s h o r e . Fu r the r ,
t he s e s t ru c tures we re u s ed a s a unive rs al p anacea for
coa s t e r o s i on, wh atever i ts cau s e , and of ten the
e f f e c t achi eved was les s than ideal. The rema i nde r
o f t h i s chap ter i s theref o r e devoted t o a d e s c r i p t i o n
3
of the hydraulic processes which occur in the vicinity
of a nearshore island or breakwater .
2.2
Physical
processes
The response of a beach to an offshore island is
brought about by the j oint action of waves and the
currents which they create as they break . To explain
the processes involved , it is sufficient to consider
the effect of an island on an initially straight
coastline , with normally incident waves .
In such a
situation , the breaking waves produce a current system
similar to that shown in Fig Sa .
It will be observed that close to the shoreline ,
currents are driven into the lee of the island from
either side . These currents are created by two
separate mechanisms , namely the oblique angle of the
breaking waves and the wave height gradient. The
former mechanism is well known and has been described
by many authors (see , for example , Muir-Wood and
Fleming , Ref 1) . The generation of shore parallel
currents by an alongshore variation of breaking wave
heights , is a less widely publicised phenomenon .
Briefly , however , breaking waves produce within the
surf-zone an increase in the mean water level , known
as set-up . Set-up increases with increasing breaker
height , and so in a situation where there is a marked
alongshore variation in breaker height , a current
flows laterally as a consequence of the varying mean
In the case of an
water level within the surf zone .
offshore island, this mechanism thus creates a flow
into the shadow z one from either side .
Referring again to Fig Sa , it should be noted that an
inward flow sometimes also occurs along the rear face
of the island . This is created as waves , diffracted
around each tip , break and decrease in height as they
travel further into the sheltered zone . The
mechanisms involved here are the same as along the
coast , but since the water depth is greater the
currents are much smaller . Nevertheless it is quite
easy to demonstrate in physical models the four-cell
system shown in Fig Sa .
Given these currents , and the agitation caused b y
breaking waves which brings sediment into suspension ,
it is not surprising that beach material is swept into
the lee of the offshore island .
In consequence there
is almost invariably a broadening of the beach behind
the structure as shown in Fig Sb .
Following the formation of such a beach cusp , a
similar current patter� to that described previously
will exist , although the currents are reduced in
Indeed they may be too weak to move
intensity .
4
material any further seawards, and the beach may
stabilise in this configuration . Such situations are
not uncommon on the UK coastline, and a good example
is provided by Carr H ouse Sands, sou th of Hartlepool
(see Fig 4). A variety of fact ors may explain the
failure of such a beach ' cusp ' to develop into a full
tombolo, including the dimensions of the island
compared to its offshore distance (bot h of which
should be expressed as multiples of the leng t h of the
incident waves) .
In areas of large tidal range, such
dimensions and lengt hs vary with water level, and the
hydraulic processes are also affec ted by tidal
current s .
I t should also be not ed that Fig Sb indicates some
erosion of the shoreline on either side of the shadow
z one . This erosion is explained by beach material
being displaced laterally t o form the cusp, but it is
worth making the point t hat some shoreline recession
may take place even if extra beach material is
supplied artificially in combination with the
construc tion of the offshore breakwater . The ultimat e
development o f the shoreline, for the simple geomet ry
being considered here, is a full tombolo as shown in
Fig Se . Here there are no currents, and in
consequence the plan shape of t he beach is in
equilibrium (for the part icular incident wave
condition) .
It should be noted that in this situation, wave crest s
s trike t he t ombolo at an angle which would cause
currents to flow ' outwards ' , ie away from the shadow
zone towards the open coast on either side . This
mechanism is exactly counter-balanced by t he
alongshore variation of wave �eights, which tends to
produce an ' inward ' flowing current .
The balance between these two currents generating
mechanisms (which in the first instance acted toget her
to modify the beach plan shape) is described in a
paper by Komar (Ref 2), although in the context of
beach cusps rather than tombolos . As will be seen in
t he literature review, there has been further research
int o the t heory of t ombolo formation based on the
ideas in that paper .
If the predominant wave direction produces an
alongshore drift, the beach will develop rather
differently than in the simple situations considered
so far. This situation is illustrated in Fig 5d, and
it should be noted that the current driving the
sediment along the beach is diverted seawards wit hin
the shadow z one . As a consequence an offshore
breakwater can function as an extremely efficient
groyne . While this may be the intention, t he
5
cons equen t i a l downd ri f t erosion i s a pot ent i a l
dang e r .
So f a r t h i s de s c r i pt i on of t h e i n t e rac t i on be tween an
o f f s ho r e i s land and a beach has concen t ra t ed on the
changes in beach plan shape .
Howeve r, there i s
usually a corres ponding change i n bea ch pro f i le s ,
e s pe c i a l ly d i re c t ly behind the b reakwa t er whe re the
g r e a t l y re duced wave c li ma t e re s u l t s in s te e pe r beach
s lo pe s . Ac companyi ng this change i n beach pro f i l e s ,
i t is of t e n repo r t e d that ma t e r i a l is t ranspor ted
onshore, i e t o the u pper beach from the s eabed be l ow
the low wat e r ma rk.
However, i t is s u b s t a n t i a l ly more
d i f f i cu l t t o ant i c i pa t e chang es in beach pro f i l es than
i n plan shape , and much more work is requi red b e f o r e
reliable pre d i c t i ve mode l s become avai lable.
3
3.1
LITERATURE SURVEY
Review of ca s e
histories
Muc h o f the design of new o f f shore brea kwa t e r s i n
count ries such a s Ja pan , I t a ly a nd Israe l , i s ba s ed o n
e xpe ri ence gained f rom o b s e rving t h e pe r f o r mance of
earlier s t ru c t u r e s .
In this s e c t i on of t he r e po r t we
wi l l review case h i s t o r i e s whi ch de t a i l t he e f f e c t s of
var i ou s breakwa ter sys t ems on t he coas t l in e .
A number o f f i e ld s t udies of t he e f f ec t s o f o f f s h o r e
breakwa t e r s have been c a r r i e d o u t in Ja pan.
Toy o shimo
(Re f 3) pre s e n t s d e s ign c r i t e r i a f or de tached
breakwa ter sy s t ems whi ch are ba s ed on t he i r observed
per f o rmance ove r a number o f year s . These des ign
recommenda t ions have been pu t into pra c t i s e wi t h s ome
success in Ja pan. As an e xample , Toyosh imo r e po r t s o n
t h e e f f ec t of a s y s t em o f t h r e e de tached breakwa t e r s
o f f t he Kaike c oa s t , Japan (ma ximum t i d a l range les s
In t h i s area whi c h had pr eviously
t han 1m) .
e xpe rienced s e r i ou s coas t a l e r o s i on pro b lems t he
i ntroduc t i on of the o f f s hore breakwa t e r s led t o sand
r e t u rn i ng to t he c oa s t and the subsequent f orma t i on o f
t omb o l o s in t h e l e e o f t he s t ru c t u re s . T o yosh imo (Re f
4 ) subseque n t ly repo r t s on t he add i t ion of f u r t he r
breakwa te r s t o t he s ame s tr e t c h o f coas t line between
197 4 and 19 8 1 and e xamine s t he f or e s hore vari a t i on due
to the compl e t e breakwa t e r s ys t em.
He concludes tha t
b o t h s ho r e l ine and s eabed have reached a s ta b le
pos i t io n i n con t ra s t t o the s evere e r os i on obs erved
pri o r to the const ru c t i on of t he breakwa t e r s .
Many e xample s o f t he u s e of breakwa t e r s i n shore
pro t e c t i on off the Italian coas t are provided by
Cor t emegi l ia et al (Ref 5) .
In pa r t i cu lar they
e xamin e the e f f ec t of man-made s t ruc ture s on the
l i t t o ra l zone , s h owing t h a t the measures c a r r i ed out
o f t e n res t ore to e qu i l i b r ium s i t e s whi c h we re
6
previ ously being se r i ousl y eroded. Amongst the areas
quo t ed is Rivabella beach whe r e , si nce the
c onst ruc t ion o f a syst em o f eme r ge d det ached
b re a kwa te r s, t ombolos have formed in t he lee of t he
b a r r i e r s t o the extent that breakwat e rs are now jo ined
t o the coast t hu s i ncreasing si gnifi c ant ly t he beach
area. C o r t emiglia et al also i nd i c a t e some of t he
l e ss favourable aspe c t s o f such a breakwa te r syst em.
These include er osion o f downd r i ft bea c hes,
degrada t i on in wat e r qua l i t y due to li mited wa te r
e xchange and the crea t ion o f ri p cu rrent s in t he gaps
be tween t he breakwat e r s. A suggested a l t e rnat ive t o
eme r5ed, le surface pi e rc ing breakwat ers whi ch would
overcome some of t he se unwe l come e ffe c t s are submerge d
bar r i e r s. De t a i ls of physi ca l mod e l tests conducted
b y Ami nti e t a l (Ref 6 ) t o e xamine t he use o f
subme rged ba rriers fo r sho re pr o t e c tion a r e gi ven i n
se c t i on 3 .2 be low.
Fr ied (Re f 7) d e scrib es the hist ory of t he br eakwa t e r
system whi c h pro t ect s t he Tel-Avi v coast in Israe l.
The Te l -Avi v coast is chara c t e r ised by narrow sandy
beaches;
the seabed i n the nea r sho re sha llow wat e r
has a numbe r of ro ck outcrops and fu rther out i n
dee pe r wa t e r t he seabed i s sandy. It was found t hat
whi lst d uri ng the summer mo nt hs (maximum tidal range
0. 2m) a reasona bly wide beach was ma intained, the
wint e r st orms red uced the wid t h of t he bea c h to ze r o
i n c e r t a i n a reas. In order t o prevent t h i s erosion
and at the same t i me incre ase the beach a rea, a se r i e s
of five o ffsho re breakwa t e r s, e i t her de t a c hed or shore
conne c t ed we re bui l t . The scheme was suc c e ssfu l l y
c ompleted i n 1 9 6 8 w i t h the fo re sho re ha ving been
t ransformed to a wide sa ndy beach. Tombolos fo rmed in
the lee of the breakwa t e rs expand ed st ead i ly and
reached an e qu i l i brium st a t e a ft e r th ree yea r s. Fri e d
re po r t s that fo l lowing t hi s su ccess i t was pro po se d
t hat further pr o t e c t e d beaches should b e c re a t ed on
A syst e m of fou r
the Te l -Aviv coast by sim i l a r means.
deta ched b reakwa t e r s was designe d , wi t h t h e a i d o f a
mobile bed physi c a l mod e l (fu r t her detai l s o f whi ch
are g i ven i n 3 .2 ) , and const ruct ion of these compl e t e d
in 1976. The add i t iona l st ructures also proved t o be
suc c e ssful in tha t the e xi st ing beach was
sub st an t i a l ly widened wit ho u t causing any pe rmane nt
e r osi on to the adjacent sho re .
The c a se h i st o r i e s given above i nd i ca t e t h e use o f
conve n t i onally bu i l t b reakwat e r syst ems in areas o f
low t idal range t o pro t ect and improve e xi st i ng
bea ches. Evidence of t he use o f less convent i onal
offshore st r uc t u r e s, eg compo sed of t yres or
sand-fi l le d bags, for pr o t e c t i on o f i nland or
she l t e red si tes i n t he USA wi th va r ying degrees o f
suc cess i s pre sen t ed i n Low Co st Shore P r o t e c t ion
(Ref 8 ) .
7
Another novel means of coast protection is given by
Zwamborn et al (Ref 9) who detail the mobile bed model
tests, construction and subsequent monitoring of an
underwater mound acting as a submerged breakwater in
Durban, South Africa . The mound, composed of spoil
from dredging operations in Durban harbour, was used
to protect nearby beaches which had been gradually
eroded over a number of years. Model tests were
performed to design and position the mound so that it
would remain stable and have a beneficial effect on
the beaches in its lee . Monitoring of the completed
underwater structure has shown it to be reasonably
stable and successful in providing protection for
Durban ' s shoreline .
Finally we turn our attention to offshore breakwaters
recently constructed in the UK with the aim of
providing coast protection .
It should be restated
here that most of the experience gained in designing
and building offshore breakwaters is in areas whose
tidal range is very much lower than that generally
found in British coastal waters . The spectacular
results in which wide sandy beaches and tombolos are
created in countries such as Israel and Italy should
not be anticipated in this country . Detached offshore
breakwaters in the UK should be viewed as a means for
reducing wave action, and its consequent detrimental
effect on the coastline, in the lee of the barrier .
The use of offshore breakwaters around the British
Isles for coast protection is a relatively recent
development . Rubble mound breakwaters have been built
at Rhos-on-Sea and Leasowe Bay, The wirral, within the
last five yeas and a system of detached and shore
connected breakwaters at Kings Parade, New Brighton
has recently been completed . As all the breakwaters
mentioned are relatively new constructions, only a
limited amount of literature on the design materials
used and their subsequent effect on the shoreline is
available . An assessment of their performance so far
is largely based on visits to all three sites made
during August 19 8 4. It should be emphasised that it
is too early to judge the long term response of the
shoreline to the structures and only initial
impressions of their effects are given .
The rubble mound breakwater at Rhos-on-Sea (maximum
tidal range approximately 7m) was constructed with the
intention of alleviating frequent flooding of the
residential area behind the existing seawall (see Fig
6a) . A detached offshore breakwater was used at this
site in preference to an alternative measure requiring
the height of the seawall to be increased to a level
Prior
which was considered to be visually intrusive .
to construction of the breakwater flooding behind the
seawall could occur on almost every occasion when
8
gales be twe en north and eas t coincided with high t i d e ,
the w o r s t c on d i t i ons a r i s i ng with nor t h-ea s t e r l y g a l e s
(Owen, Re f 10). T h e breakwat e r i s pos i t i o ned at t h e
low water mark facing e a s t -n o r th-east and w a s d e s igned
and c ons tructed t o prevent ove rtopping o f the s eawa l l ,
and c o n s e quent f looding, f o r a 3 m wave o n a 100 yea r
wa t e r leve l.
Dur ing con s t ru c t ion of the breakwa t e r it
noted
that
wa ves f r om a n o r t h -ea s t e r l y d i re c t ion
was
we re entering the gap at t h e northern e nd of t he
breakwat e r and running a long the s eawa l l . As t h i s was
t houg h t t o be unde s i rable, armour s t one reje c t ed a s
uns u i t a b l e f o r the b reakwa ter w a s used t o make a
g r o yne nort h o f the breakwa ter.
Since c ompl e t i on o f
t he breakwa t e r and gr o yne, material h a s be gun t o
a c c r e t e in the l e e o f t h e s t ructures (s ee P l a t e 2) .
In pa r t i cu l a r i t can be s ee n b y comparing beach
pr o f iles f r om surveys done in Fe brua r y 1980, Ju l y 1984
and Sept embe r 198 6 , Fig s 6 and 7 , that a reasona b l e
quan t i t y o f s ed iment h a s accumulated i n the lee of the
breakwat e r and a l s o a ve ry large amount of s h ingle ha s
accumulated in the lee of t he gro yne . The beach
pr o f i l es show a charac t e r i s t i c s t ee pen i ng s i nce the
cons t ru c tion of the breakwa t e r with beach leve ls at
the t oe of t he s e awa l l being increased b y as much a s
I t is also wo rth not ing that ma t e r i a l has a l s o
2m.
b e e n d e po s i t ed at t h e o f f s hore t o e o f the breakwat e r .
The o f f shore breakwa t e r s a t Leas owe Ba y whic h we re
compl e t e d i n s umme r 1 9 8 2 a r e shown in P la t e 3. A f u l l
report o f the his t or i ca l deve l o pment o f Leas owe Ba y
and t h e i nve s t iga t i ons made b e f o r e the breakwa t e r s
we re des igned and cons t r ucted is given i n Ba rber a n d
Dav ies (Ref 11). T h e wave c l ima t e i n t h i s a r e a i s
chara c t e r i s e d b y a h i g h t i dal range (ma ximum t i d a l
range a pproxima t e l y 8m) w i t h s t rong t idal currents
c lo s e to exi s t i ng revetment s . The s ys t em at Lea s ow e
Ba y c o ns i s t s o f t w o r u b b l e mound breakwat e r s wh ich a r e
e merged at a l l s t a t e s o f t h e t id e ( t h e e a s t e rn-mo s t
breakwat e r i s s ho r e connec t ed ) and a f i shtai l shaped
g r o yne (s e e P l a t e 3 ) . The s c heme was devised t o
i ncrease beach levels loca l l y and t o r e t a r d f u r t h e r
e r o s ion o f t h e North Wirral coa s t line.
Si nce
cons t ruct ion has been compl e t e d beach leve ls in the
lee of t he s t r uctures have increa s ed, pa rt icu lar l y
adjacent t o t h e s ho r e c onne c t ion of t h e ea s t e rn-m o s t
b r e akwat e r and i n the vi c i n i t y o f t he f is h t a i l g r o yne .
Ob s erva t i ons made at the s i t e ind i ca t e that there are
s t i l l s t r ong t i dal currents be tween the two break­
wa t e r s . The s t one a prons a t t he breakwat e r end s s e em
t o have been s uc ce s s f ul in d i s s i pa t ing t he energy o f
wave s breaking over the roundhead and n o local s cour
i s appa rent.
Beach l eve ls hav e a l s o i n c re a s e d o n the
s eaward s ide o f the breakwa t e rs .
The s ys t em o f g r o yne s and o f f s ho re b reakwaters a t
Ki ngs Pa rade, New Br i gh t on, wa s compl e t e d in Spr ing
9
1985.
The breakwat e r layou t is s hown in Fig 8 . The
breakwa t e rs a re des i gned to be subme rged to a d e pth o f
l m at mean h i g h wat e r . All of t he m cons i s t o f a sand
c o re c ove red by a laye r of be d s t one which i s armoured
w i t h s pe c i ally des i gned pre cas t re infor ced c on c r e t e
uni t s , known as r e e f blo cks , whi ch s tand on c on c r e t e
pads cemented ont o the unde rlying a r mo u r rock . The
bre akwat e r s ys t e m at Ki ngs Parade i s i nt ended to
i ncrease e xi s t i ng be ach levels and to ext end the
s h o reline t o jo in t he ends of t he breakwa t e r s a t
lo c a t i ons 1, 2 and 3 (s ee Fi g 8). The lo cation 1 and
3 b reakwat ers i n t e r s e c t t he major wave t ra i ns and
current d i re c t i on s , wh ils t the lo ca t i on 2 b r eakwa t e r
i s t o as s i s t in moving t h e shoreli ne s eaward s . The
b r eakwa t e r a t lo cation 4 is a wave screen pr i ma rily
i n t e nded as a coas t pro t e c t i on mea s u re .
In a d d i t i on t o the well known o ffsh o re breakwat e rs a t
Rho s -on-Sea and The Wi rral t here i s a s c he me unde rway
a t Deng i e Fla t s , Es s e x, i ntended t o preven t , o r a t
lea s t r e t a r d , fu rther e r o s i on o f t he s alt fla t s i n
t h i s are a . T he d e s i g n fo r the b reakwa t e r a t this s i t e
c o ns i s t s o f placing a number o f l ig ht e r s offsho re,
s e pa r ated by gaps of s pe c i fi ed width, and s i nking
them.
Before such a s c he me was pu t into ope ra t ion an
i nve s t i gat i on i nt o i t s pro bable e ffect ivenes s was
condu c t e d by Pe t hick (Ref 12). A d i s cus s ion of t he
me t hods used by P e t h i ck i s g i ven in s e c t i on 3 . 3 . Work
is unde rway at t he s i t e to place the lig ht e r s so it i s
t oo early t o make a n y comme nt s on t h e i r pe rfor mance i n
preve n t i ng fu rt her e ro s i on .
At all o f the s i t e s o f offs ho re b r eakwat e r s i n t he UK
that have been included he re it is int ended tha t t he
s ho r e li ne res po ns e should be mon i t ored over a number
of year s . This s hould provide u se ful expe r i ence t o
a i d wi t h futu re des ign o f such s t ru c t u re s .
3.2
Phys i ca l mod e l
s tud ies
Ove r the yea rs many au thors have r e po r t ed t he result s
o f phys i cal model t e s t s d e s igned t o a s s e s s t he
pe r fo r ma nce o f offs hore b reakwa t e r s . Such phys i c a l
model s tudies ma y be broadly d i vided into two areas .
The fi rs t c ompr i s e s model t e s t s concerned �i t h the
mea s u rement o f t he e ffe ct of pa r t i cula r b reakwat e r
c haract e ri s t i c s (eg c r e s t height, breakwater width,
angle o f face s lo pe ) on a n a pproaching wave t ra i n .
The s econd area i nclud e s e xpe r i ment s intended t o
o b s e r ve t he c hanges mad e t o beach plan a nd profile b y
t h e mod i fi e d wave c li ma t e res ult ing fro m the pres e nc e
of d e t a ched o ffs hore b r eakwa t er s .
In reviewing t he e ffec t of breakwa ter charac t e r is t i c s
o n a n inci dent wave t rain one of t he mai n poi n t s t o b e
c on s i d e red i s the height o f i t s cre s t re lat i ve t o the
10
s t i l l wat e r leve l . A subme rged breakwate r , whe re the
cre s t i s a t or be low t he s t i l l wa ter leve l , affords
prot e c t ion t o t he s hore area in its lee b y a t t e nuat ing
t he inc ident waves as t he y pas s over t he b a r r i e r . T he
s ubme rged s t ructure absorbs s ome of t he wave ene rgy by
causing t he waves t o break premature l y .
Some of t he
remain ing energy is refle c t e d and s ome t ransmi t t e d
s horewards .
Subme rged bar r i e rs a r e genera l l y l e s s
e xpe ns ive to cons truct and mai nt a i n t han a
c onvent i o nal eme rged b reakwa t e r whose c r e s t may b e
above t he hig he s t e xpe c t ed t idal leve l . T h i s i s
be caus e the wave fo rces o n a submerged s t ruc ture wi l l
b e l e s s t han those on a c onve n t i onal s t ru c t ure a s t he
wa ve s do not break d i r ec t l y on t he b a r r i e r . T he r efore
t he subme rged barrier o ffe rs a pot en t i a l ly e c onomic
s o lut i on i n s i t u a t i ons whe re c omplete pr o t e c t i on fr om
waves is not ne c e s s a r y or d e s i ra b l e .
Howe ver in
pla c e s w i t h an appreciable t idal range a s ubmerged
breakwa t e r may be re l a t ively ineffe c t ive at hig h tide
and s i mply a c t as a c onvent i onal eme rged breakwa ter a t
l o w t i de .
Consequent l y i t s hould be bu ilt to
w i t hs tand t he fo rce o f waves breaking d i r e c t l y on i t .
T he r e fo re i t i s sugges ted b y various a u t ho r s (eg
Johnson et a l , Ref 1 3 , Raman et a l , Re f 14 ) t ha t fo r
maximum effec ti vene s s t he u s e of submerge d barriers be
re s t ricted t o a r eas o f l ow t idal range .
Muc h research has been done in recent yea r s w i t h t he
o bjec t o f inve s t i ga t i ng t he pe rfo rma nce and offering
des ign gu ide l i ne s for subme rged barri e r s . Jo hnson,
Fuc hs and Mor i s on (Re f 13) present a rev i ew of e a r l y
wo rk i n the fi eld t og e t he r wi th r e s u l t s o f t he i r own
regular wave t e s t s whe r e t he wave s were no rma l l y
inc iden t o n submerged rec tangular ba r r i e rs .
The i r
phys i c a l mod e l t e s t s conce n t ra t ed o n t he e ffe c t o f
barrier he ig ht and wi d t h, relat ive dept h and po s i t i o n
o f t he breakwa ter on t he t r ansmi s s i on coefficient
(transmit te d wave he i g ht /incident wave he i g ht ) .
The
conclus i ons reached by Johnso n et al as a r e s u l t o f
t he i r t e s t s ma y b e s ummar i s ed as fo llows:
(i )
(i i)
(i i i)
A barrier of g iven rela ti ve he ight (bar rie r
he ight/water de pt h) i s more e ffe c t ive in
dampi ng (lowe r t ransm i s s i on coeffi c ient) s t e e p
waves t han flat waves . The e ffe c t o f wave
s t e epness i s sma l l when t he c r e s t of t he
barri e r i s above s t i l l water leve l .
Fo r a given re l a t ive barrier dime ns ion and
wave s t ee pne s s , t he t ransmi s s ion c o e ffi c ient
is great e s t for large r e l a t i ve d e p t hs .
A wide ba rrier (rel a t ive t o i n c i de n t
wave lengt h) ha s a be t t e r dampi ng effe c t t han a
narrow barrier e s pe c i a l l y for s t e e per wa ves .
11
(iv)
The transmission coefficient is lower if the
barrier is situated a wavelength or more
seaward from the normal wave breaking point.
The wave damping effect of a submerged dyke was later
considered by Nakamura, Shiraishi and Sasaki (Ref 15).
They present the results of extensive physical model
tests as a series of graphs (illustrating experimental
trends) which are intended for practical use in
determining the effect of submerged dykes on regular
incident waves . From the graphs it can be seen
whether the dyke will cause the incident wave to
break, and also allow the transmission coefficient and
changes in wave height and length for breaking and
non-breaking waves to be determined .
Milne-Dick and Brebner (Ref 1 6) considered the
modification made to the energy spectrum of an
incident wave by both solid (thin and rectangular in
cross section) and permeable (composed of horizontal
nested tubes) submerged barriers . The experimental
values of transmission coefficients for these
rectangular barriers in infinitely deep water were in
agreement with those calculated by Dean (Ref 1 7 )
provided the incident wave trough never falls below
the breakwater crest . Milne-Dick and Brebner also
found that for all the breakwaters they tested 36% 6 4% of the energy transmitted was at a higher
frequency than the incident wave . For a permeable
breakwater their results indicated that there was a
clearly defined minimum transmission coefficient
whereas for an impermeable structure of the same size
there was no such well defined minimum . This was
thought to be due to breaking and turbulence
eradicating fluctuations .
Dattatri, Raman and Jothi-Shankar (Ref 1 8) extended
the work of Milne-Dick and Brebner (lac cit) to look
at permeable and solid breakwaters of rectangular,
triangular and trapezoidal section . Raman et al (Ref
14) found that for all the sections tested the effect
of the breakwater position and dimensions were similar
to those of Johnson et al given in (i) to (iii) above .
In contrast to the results of Milne-Dick and Brebner,
Raman et al found that there was very little
difference in the behaviour of the transmission
coefficients for permeable and impermeable
breakwaters . However it should be noted that Dattatri
et al used rubble mound breakwaters for their
permeable tests whereas Milne-Dick and Brebner used
nested tubes . Dattatri et al also recommend that the
best design for an offshore breakwater has a sloping
seaward face with a vertical backface, this is in
agreement with experiments performed by Kabelac (Re£
1 9 ) on the sediment trapping ability of such
structures .
12
Ami n t i , Lambe r t i and L i be ratore (Ref 6) also pre sent
t he re sult s of a se ri e s of e xpe rime n t s conce rned wi t h
b reakwa t e r shape and roughness. The i r tests we re
carried ou t w i th t he b re akwater sec t i ons on bo t h fi xed
and mob i le beds w i t h normally inc ident re gu lar wa ve s.
The results for t he mob i le bed mode ls appear to be
inconclusive and t he authors are continuing t he test
programme.
For t he fixed bed models t he aut ho rs
pre sent a se ries of resu lt s and comment on t he effec t
of t he barriers on t he waves, the velo c i t y fie ld nea r
t he barri e r, se t up and run up and t he effe c t of
barrier c hara c t e r i st i c s. The y pu t forward d e si gn
sugge sti ons for limi t i ng b o t h offsho r e t ranspor t and
sc our a t t he landward toe whi lst st i ll maintaini ng
reasonab le wa ter exc hange in t he le e o f t he
bre akwa t e r .
In areas whe re t he t i da l range i s appr e c i a b le i t i s
more usual for offsho re b reakwa t e r s t o be const ruc t ed
so t he i r c re st is above t he hi ghest expe c t e d stages o f
t he t i d e . Howeve r , o n b o t h physi cal and e conom i c
grounds t he re wi ll a lways e xi st t he possi b i li t y o f
some incident waves ove r t o ppi ng the st ructure. As t he
he i ght o f t he br eakwa t e r c rest a bove t he wa t e r level
(free board) dec rease s t he probabili t y of ove r t o pping
Se veral authors have condu c t e d physi cal
incre ases.
mo del t e sts in an e ffo r t t o rela t e the rate of
ove r t o pping and t he wave t r ansm i ssi on coeffi c i e n t
(transmi t t ed wave he i gh t /incident wave he i ght ) t o t he
br eakwa t e r and incident wave c harac t e r i st i c s. God a ,
Takeda a n d Ho r i ya (Ref 2 0 ) and Go da (Ref 2 1) give an
empi ri cal fo rmula for pre d i c t ing wave t ransm i ssi on,
Se e li g (Re f 22) c ompa r e s t he se va lu e s wi t h
e xpe rimental result s fo r a smoo t h i mpermea ble ba rri e r
subje c ted t o b o t h regula r and i rregula r waves.
The
formula due to Goda is found to provide a reasonable
e stima t e fo r pred i c t ing t he transm i t ted wave he i ght in
i r r e gular wave s.
The results of Se e li g's e xpe r ime n t s
also indi cate t hat fo r e me r ged breakwa t e r s t he
t r ansm i ssi on coeffi c i ent i nc re ases as t he i ncident
wave hei ght increase s for a fixed breakwa ter
confi gura t i o n.
The result s for regula r waves sho w
t ha t a high pe rcentage of t he t ransmi t t ed wave ene rgy
i s at a hi ghe r fre quency t han t he incident wave and
t hi s is i n line wi t h the o bservat i ons made by
Milne -Dick and Brebner (Ref 16) for subme rged
barriers.
In i rregu lar wave s change s in t he spe c t ral
sha pe pr oduced by the b reakwa t e r s were very sma ll.
Allsop (Re f 23) gi ves t he re sults o f a numbe r o f
random wave test s condu c te d t o me asure t he r a t e o f
wave ove r t o pping and t he coeffic ient o f wave
t ransm i ssion.
Allsop rela t e s t he wave o ve r t oppi ng t o
the dimensi onless free board parame t e rs ,
fr eeboard /si gn i fi cant wave he i gh t and freeboard/dept h
of wa t e r at t oe st ru c ture. He conclude s t ha t t he r a t e
o f ove r t opping is st rongly dependent o n t he
13
significant wave height but less so on the freeboard,
whe reas t he waves t ransmi t t ed b y overt opping are
d e pendent on t he mean sea ste e pness.
The se cond a rea in whi c h physi cal mod e l t e st s have
been used is in t he inve st ig a t i on of t he c hange s
i nduced in bea c h plan and profile by t he int roduc t i o n
o f a n offsho re breakwat e r.
Some of the ph ysi cal
mo de lling has been done to d e sign a syst em of offshore
breakw at e rs to me e t t he requi rement s of a spe c i fi c
si t e .
Fo r e xample , Fried (Ref 7 ) d e t a i ls t h e three
d imensional movea ble bed hydraulic model u sed to
d e sign the breakwat e rs off t he coast of Te l-Aviv in
I srae l. The physi cal model was bui lt in acco rdance
with an e xt e nsi ve su rve y of the proposed si t e . T hi s
i nclud e d pre pa r i ng t he model sand t o ma t c h the
d i st r i bu t i on of grain si ze i n the pr o t o t ype .
The
mo del was cali b ra ted w i t h respect to t he
sed iment o logi cal pro c e sse s wh i ch had been mea sured i n
t h e v i c i ni t y of t wo e xi st i ng breakwa t e rs fu rthe r along
the same st re t ch of coast li ne.
In prac t i ce t he t i me
scale was found by compa ring the rate of t ombolo
formula t i on in the mod e l and for the e xi sting
b r eakwate rs.
Fried suggest e d t hat two t ime scales
were necessa r y, one fo r t he initial st age of t o mbolo
format i o n, when t he beach chang e s are due t o shi ft ing
of lo cal sand and longshore t ra nspo r t , and for t he
develo pment of t he t ombolo due t o onshore-o ffsho re
sand t ransport .
Once these scales had been adopted a
number of d i fferent desi gns were t ested and t he c ho se n
d e sign re-test e d t o g a i n an appreciation o f
sed ime ntolog i cal proce sses w hen the pr oposed
c o nst ruc t i o n schedule was corre c t ly reproduced in t he
Fr i ed concludes by comparing t he impleme n t ed
models.
breakwa t e r scheme w i t h the mo del t e st s and find s that
the ac tual pe r formance of t he breakwat e r system was i n
li ne wi th t h a t pre d i c t ed by t h e mod el.
Sa t o and
Tanaka (Re f 24) present the result s of hydraulic model
t ests int ended t o find so lu t i ons to the pro blem of
ma i nt a i n i ng t he a r t i ficial beac hes at Ito and Suma i n
Osaka Bay, Ja pan. Re c ommend at i ons based on t he
physi cal mo del studi e s are made for c hang es t o t he
e xist i ng layout of offsho re breakwa t e r s and g r o ynes i n
order t o pro t e c t t h e beac he s and promo te wa ter
e xchange a t bot h si t e s.
A mo re general se t of physical mo del stu d i e s t o
d e t e rmine t he be a c h chang e s due t o a de tached
breakwa t e r a re pre se nt e d in Rosen and Vajda (Re f 2 5).
Th e y review the work of a number of aut ho rs and gi ve
r e sult s fr om their own mobile bed physi cal mode ls.
T he t e st s conducted were on breakwa t e r s whose wid t h
and crest height we re fi xed but whose le ng t h and
d i st a nc e from the shoreli ne we re varia ble. All t he
br eakwa ters t e sted were po si t i o ned pa rallel to t he
shor eline and subje c ted to no rmally incident waves of
14
va r i o us d ifferent ste e pne sses (de e p wat e r wave he i ght /
deep wa t e r wave l eng t h ) . The i r r e su l t s ind i ca t e t ha t
the signifi c ant pa rame t e r s c harac t e r i si ng t he
d i me nsi ons of a spi t o r t o mbolo a r e the rel a t i ve
l e ng t h of t he breakw at e r (c o mpa red t o t he d i stance
fro m the o r i g inal sho r e line ) , the re lat i ve d i stance
f ro m the orig in al sho re l ine (c o mpared w i t h t he
posi ti o n of t he b reake r l i n e ) a nd the rela t i ve c r e st
h e i g ht (a bove mean sea le ve l c o mpa red w i t h incident
Based on the i r resu l t s and o t he r
wave h e i gh t ) .
obse r va t i ons Rosen and Va jda propo se and d i sc u ss a new
e xplana t i on of whe n sed i ment o l ogical and mo rpho logi cal
e qu i l i br i um is rea c he d .
Mimura, Shimi zu a nd Ho r i kawa (Ref 2 6 ) g i ve the r e su l t s
o f a se r i e s o f mod e l t est s per formed t o study t he
i nfluence of a d e t a c he d b reakwate r on c oa st a l change .
The physi cal mode l resul t s are also c o mpared wi t h
t hose fro m a 'one -l i ne ' mathema t i ca l mod e l (d e t a i l s o f
t h i s w i l l be given i n se c t i on 3.3). As i n t he
e xpe r i ment s conducted b y Rosen and Vajda (loc c i t ) a
mob i le bed mode l was use d , the waves being norma l l y
inc i dent o n a breakwa t e r l ying pa rall e l to the c oast .
Mimura e t al me a sured qua nt i t i e s relat i ng t o t he wave
a nd current fie l d s, sand t ra nspor t and b o t t o m
t opo graphy i n o rder t o asse ss t he e ffe c t of a de tached
breakwa t e r on wave s, nea rsho re cur rent , sed i ment
t ransport and t opog raphi c c hange. They concl uded t ha t
t h e po si t i ons of e r o si on a n d a c c r e t i o n a ppear a l ong
t he ma i n c o u r se of t he curre nt, t he sand being
t ra n spo rt e d b y currents which d e ve l o ped sho rewa r d s o f
t h e breakwater. The eroded sand fro m a long t he
sho rel i ne fa r fr o m the b reakwa t e r was t ranspo r t e d i nt o
t he shadow zone o f the breakwa t e r and t he sand scoured
fro m t he si d e s o f the breakwat e r was carried into t h e
o ff shore region w i t h t he current .
I t would see m fro m t he l i t e ra ture r e vi ewed in thi s
sec t ion t hat t here are ce r t a i n a reas in the physica l
mod e l l ing of offshore breakwa t e r s whi c h ha ve rece i ve d
ve ry l i t t l e a t t e nt i o n .
In par t icular a se r i e s o f
mode l t e st s could u se fu l l y b e conduc ted t o invest iga t e
t he effe c t o f a breakwat e r o n t he beach pla n and
profile whe re the wave s were not norma l l y i nc i d e n t on
Si mi l a r l y no physi cal mode l t e st s
t he struc ture.
appear t o have been done, e xc e pt for pa r t i c u la r sit e s,
on the effec t o f o ffshore breakwa t e r s whi c h a r e not
po si t i o ned pa ra l l e l to the o ri g ina l sho r e l i ne . C l ea r l y
t he re i s scope for fu r t he r physic a l mod e l stud i e s t o
i mpro ve o u r unde rstand i ng o f general pro c e sses
invo l ved in beach c ha ng e s cause d by the in t roduc t ion
o f a n o ffshore breakwa t e r .
15
3. 3
l1athematical
model studies
The prediction of the long term evolution of a
coastline due to changes in the nearshore wave
climate is a complex mathematical problem.
In recent
years research has been carried out by a number of
authors to produce mathematical models capable of
predicting the response of a shoreline to the action
of waves or the introduction of a man-made structure .
The simplest of the mathematical models is the so
called ' one-line ' method where the beach is
represented by a single contour and the model deals
only with changes in the plan shape . This idea was
first put forward by Pelnard-Considere (Ref 2 7) and
has subsequently been developed by many coastal
engineers. Ozasa and Brampton ( Ref 2 8 ) employ and
extend the basic one-line method to model changes due
to wave action in a beach backed by a sea wall . They
include a new formula for alongshore transport of
sediment which extends that developed by Komar and
Inman (Ref 29) £o situations where the wave height
varies along the beach .
(Normally alongshore sediment
transport on a beach is caused by the oblique breaking
waves but in cases where the waves are diffracted, for
example by an offshore breakwater, the gradient of
alongshore wave height must also be considered) .
Ozasa and Brampton also point out £hat a n expression
for the rate of onshore-offshore sediment transport
may be included in the basic equations but in practise
such a rate is difficult to quantify . The
mathematical model results of O zasa and Brampton are
compared with those from a physical model study of a
bay in Japan . The nearshore wave heights and
directions for the mathematical model are taken from
measurements made in the physical model, agreement
between the mathematical and physical models was
generally found to be quite good.
In the case of modelling a prototype beach the
one-line model could still be used, the nearshore wave
information being derived from a refraction model .
In
extending the method further to the case where an
offshore breakwater was present, the input data would
need to be obtained from a mathematical model of the
nearshore zone, taking into account both refraction,
diffraction and overtopping. This would clearly add
further complexities to the problem. N.imura, Shimizu
and Horikawa (Ref 2 6) employ the one-line me£hod to
model the effects of a detached breakwater on
shoreline change. They use the formula for alongshore
sediment transport rate developed by Ozasa and
Brampton (loc cit) but with different values of the
coefficients determining the balance between the forms
due to oblique wave incidence and variation in
breaking wave height. The new values of these
16
coeffi c i e n t s were derived from Kra us, Ha r i ka i and
Kubota (Ref 3 0 ) . Again the input wave condi t i ons t o
t he ma thema t ical mode l were taken from a physi c a l
mode l of t h e same breakwa ter l a yo ut .
The result s fr o m
the ma thema t i c a l and physic a l mode l s were compared but
the two se ts were found t o d i sagree in t he lee of t he
b reakwa t e r and in t he shore line re g i on some distance
from the breakwa t e r . Thi s d i sagreeme n t was t hought t o
be d ue to ne g l e c t ing the cross-shore t ranspo rt i n t he
region away from t he breakwa t e r .
In the l e e of t he
breakwa t e r the dispa r i t y in the resul ts was thought to
be d ue to assuming a c onstant value o f t he depth a t
which l ongsh o r e -t ranspor t i s thought t o be neg l i g i ble.
Var i a t i on i n the waves and curre n t s around the
de tache d breakwa t e r wi ll ch ange the c r i t i c a l de pt h o f
sand movement and so i nva lidate the c onstant de pth
I t i s suggested b y Mi mura e t al that
assumpt i o n .
fur ther research is needed t o deve l o p a more rat i on a l
mod e l for a ssessing c ha ng e s i n bo t t om t o pography over
a wide range of cond i t io ns.
The authors me n t i oned t h i s fa r have only compa red t h e
o ne -l i ne ma t he ma t i c a l mode l with a c o r r e sponding
physi cal mod e l .
Brampto n , Franco and No l i (Re f 3 1)
use t he one -l i ne me thod t o pre d i c t future shorel i ne
e vo lut i on and the effe c t o f po ssi ble c onst r uc t i on
The
works on a s t re t c h of coast l i ne in Italy.
ma themat i ca l mod e l is c a l i b rated b y using sur vey d a t a
ga t hered fo r the si te b e t ween t he Ri ver Ar no a nd
Vi a re gg i o ha r bo ur si nce 19 3 4 . The ma thema t i cal mod e l
was st arted w i t h the obse rved sho re l i ne cond i t ions i n
19 34 and compa r i sons wi t h o t her yea r s surve y data
(i nclud i ng 19 8 3 ) was found t o be sat i sfa c t o r y . The
mode l wa s then use d, und e r ce rtain assumpt i ons, t o
pre d i c t sho r e l ine charges up t o the year 2 0 3 4 .
Ba sed
on these re sul ts the autho rs make recomme ndat ions fo r
coast pro t e c t i on works to prevent further coast
e r o si on d ue to long shore d r i ft .
The r e fore i t woul d se em tha t whi l st a one -li ne me t h o d
w i ll not provide i nfo rma t ion on chang i ng beach
pro fi l e s, if ca reful ly e mplo yed i t can be useful i "
pre d i c t i ng evo lut ion o f t he pl an shape o f a beach.
Wh ere the movement of se d i ment onshore-offshore i s
si gni ficant the two l i ne theory o f Ba kker (Re f 3 2)
provides a more sophist i cated approa ch.
The two-l i ne
me thod a l lows for the beach t o be repre sented b y two
(non pa r a l l e l ) c on t o urs.
The advantages afforded by
the i nc lusi on of onsh o r e -o ffsho re sed i ment t ranspo r t
and c hang e s i n beach slope ma y not b e ful l y rea l i sed
a s such effec ts a re d i fficul t t o quan t i fy.
If st i l l fur the r informa t i on on the pr ofi le and plan
of a beach are requi red then the n-l i ne me t hod o f
P e r l i n a nd Dean (Ref 3 3 ) c o ul d be consi d e r e d . Howeve r
a l thoug h the me thod c l a i ms t o give a reasona b l e
17
approximation to bathymetric changes due to wave
climate and the introduction of shore perpendicular
constructions, Perlin and Dean point out several
features which they feel could be improved . In
particular, their method makes no provision for water
level fluctuations in the nearshore zone . The
longshore and onshore-offshore sediment transport are
given by rather simple empirical formulae derived from
laboratory tests. A number of adaptations would also
be required to the basic model to include the effects
of refraction and diffraction by offshore structures
on the nearshore wave climate.
Finally, a mathematical model to investigate the
efficiency of an offshore breakwater as a means of
reducing wave erosion at a salt marsh site is given by
Pethick (Ref 12), see also section 3. 1. The proposed
breakwater consisted of twenty lighters which would be
floated into an optimum location for coast protection
and then sunk . The aim of the research done by
Pethick was to decide on the best position and spacing
of the lighters to prevent further erosion of the
maximum possible stretch of coastline . The wave
conditions at the breakwater were calculated from
deep-water wave conditions using a simplified model
for predicting the changes in wave height and
direction due to refraction .
The refraction model
used does not take into account energy losses due to
the frictional effects of the shallow beach slopes or
muddy seabed and it will overestimate the wave heights
at the seaward side of the breakwater . The
modifications made to the waves by a breakwater with
several gaps are modelled using the work of Penny and
Price (Ref 3 4 ), for diffraction, and Nakamura,
Shiraishi and Sasaki (Ref 1 5), for overtopping .
It
should be noted that the diffraction theory of Penny
and Price is f or breakwater gaps which are ' wide '
relative to the incident wavelength. Treating the
gaps between the lighters as 'wide ' will have led to
the wave heights shorewards of the breakwater being
overestimated. The combined effect of diffraction and
overtopping at the breakwater is calculated by adding
the diffraction and overtopping coeff icients as
suggested by Goda, Takeda and Moriya (Ref 20) and
Treloar and Nagle (Ref 3 5 ) . Pethick uses the results
of laboratory tests to relate the wave heights at the
coast to the erosion rates of the salt marshes .
The
results from the mathematical model for a number of
breakwater gap spacings were verified against a
physical model, the agreement between the two was
found to be reasonable .
Several different breakwater
alignments were then tested using the mathematical
model and the optimum position and spacing of the
lighters was found . As stated in section 3.1, the
placing of the lighters in the positions calculated
using the mathematical model has been started but it
18
is obviously too early to make any assessment of their
performance.
From the literature it can be seen that although there
are some very good mathematical models we are not yet
at the stage of having a complete model for predicting
the effect of an offshore breakwater on shoreline
evolution.
Certain aspects of the mathematical
modelling, for example, calculation of overtopping
coefficients and onshore-offshore sediment transport
rates, could usefully receive more attention .
4
DESIGN METHODS
FOR AN OFFSHORE
BREAKWATER
SYSTEM
This chapter discusses the various problems faced by
the designer of an offshore breakwater system, and the
methods which are available to help solve them.
Probably the greatest of these problems are
encountered at an early stage, namely deciding whether
the use of one or more offshore breakwaters should be
considered as a possible solution to a particular
problem, and then how to draw up a first tentative
UnliKe the design of a sea wall, or a groyne
plan.
system, the engineer will not normally have the works
either of his neighbours or his predecessors to guide
his initial thoughts.
The next section of the report
therefore discusses these first steps.
Despite the lack of experience of offshore breakwaters
in British waters, there, nevertheless, are a variety
of methods which can be used to improve an initial
design, both in cost and hydraulic efficiency.
Indeed, as evidenced by the preceding chapter, there
is possibly as much literature relevant to offshore
breakwater design as far more conventional coastal
defence methods such as beach nourishment or groyne
systems.
Section 4.2 briefly summarises the
techniques by which the design of potential breakwater
may be refined.
4. 1
Developing the
initial design
As with all coast protection methods, it is unlikely
that a universal method for using offshore breakwaters
will ever be developed. Each potential site is likely
to have its own distinctive characteristics which will
dictate the eventual solution adopted.
Indeed in some
circumstances the use of offshore breakwaters may not
be appropriate.
It is therefore worthwhile to first
try to identify the types of problems that may be
solved efficiently with one or more offshore
breakwaters.
19
The immediate impact of such a structure will be to
reduce wave heights in its lee, and to slow alongshore
drift. These two effects themselves suggest
applications for offshore breakwaters.
If there is a
localised, relatively short stretch of coast, or
coastal defence, which is experiencing problems, an
offshore breakwater may well offer an appropriate
solution. For example, if a particular part of a sea
wall is subject to overtopping or undermining,
reducing the wave heights will bring an immediate
improvement, without having to increase the sea wall
height or extend its toe protection. Both of these
latter options can be expensive and unpopular,
especially on amenity beaches.
Similarly, problems often arise at the boundary
between a sea wall and an unprotected stretch of
coast.
In such areas an offshore breakwater can at
least spread erosion from over a wider frontage, and
prevent the sea wall from being attacked.
It is also
possible that breakwaters may be deliberately designed
to produce an efficient terminal groyne, perhaps in
connection with a beach nourishment exercise. Such an
application is, of course, fraught with potential
danger and could have a dramatic effect on adjacent
stretches of coastline if not carefully designed.
Because of the lack of experience of such structures
in British waters, it is likely, in the immediate
future, that the role of breakwaters may be largely
limited to these localised types of problem. However,
in other countries they have been used more
extensively in order to change the nature of a whole
stretch of coastline. An array of breakwaters built
parallel to the shoreline can be used to completely
arrest a nett alongshore drift of material and
simultaneously reduce the intensity of the wave
climate on that coast.
In view of the foregoing, the most likely applications
for offshore breakwaters in the UK in the immediate
future are when the alternative would be the
rebuilding, raising or extending a sea wall over a
stretch of a few hundred metres.
It is also possible
that offshore breakwaters may be considered either as
terminal groynes to retain a beach nourishment, or in
the place of conventional groynes where it is
important to return a healthy beach, say in an area
where tourism is of great commercial value.
Having identified a potential site for a breakwater
installation, the engineer is then faced with deciding
on the height, length (parallel to the shore),
orientation and offshore distance of a suitable
structure. Except for rather special situations, it
is likely that the optimum orientation for a
20
shore-p r o t e c t ion b reakwa t e r w i ll be parallel t o t he
coastline . T h i s will, in g e ne rally p r o t e c t t he
g re a t e st frontage of coast for a given stru c t u re
leng t h .
In some case s i t m a y be p o ssi b l e t o p lace t h e
breakwa t e r s such that t h e dominant wave d i re c t i on i s
t aken into a c c ount . Howeve r, such a move should be
g iven ve r y c a re ful consi d e r a t ion, as it ma y lead t o
the beach i n t he lee o f t h e st ructures being e xp o sed
t o oc casional la rge sto rms from o t her than the
dominant wave d i re c t ion.
It is also like l y, for t he maj o r i t y o f appli ca t i ons,
that t he breakwat e r wi ll be su r fa c e p i e rc i ng at a l l
sta t e s of t h e t i de , if i t i s t o pe rfo rm
sat i sfac t o ri ly. Th i s general rule may perhaps be
rela xed p r ovided suffi ci ent model t e st i ng i s c a r r i e d
o u t , or i f the o b j e c t i s simply to prevent scour a t
t he base o f a sea wall. The use of " su bmerged c r e st "
breakw a t e r s designed t o i mprove a beach i s me nt i oned
again la t e r i n this se c t i on.
Fo r all o ther purpose s,
howe ver, i t is vi r t ually e ssen t i al fo r t he br eakwa t e r
to have a n " eme rged c r e st " .
Ha ving sa id that, i t i s not e sse nt i al that the c rest
should be so high that i t i s never overt opped by
wave s. Most of the e xa mp les d e sc r i be d i n the
worldwide l i ter ature, and those alrea dy bu i l t in t he
UK, a re d e si gned t o allow some wave ene r gy t o pass
over t he m i n seve re storms. Alt hough t h i s has some
obvi ous i mp a c t on t he desi gn of b o th the c re st and
landward slo p e of t he st ruc t u r e , t he e xt ra c o st of
pr ovid ing a heavi er c onst ru c t i on the re is outwe i g he d
b y the much grea t e r e xp en se of e xt ending the crest t o
a muc h h ighe r, and p o t ent i all y unsight l y level.
The rema i ning d imensions of t he breakwa t e r (system)
are the len g t h of t he st ruc t u r e , pa rallel to t he
sho re , and i t s d i stance o ffsh o re .
Although these
d imensions a re fundamental t o a su ccessful breakwa t e r
design, t he re are st i ll no unive rsa lly a c c e p t e d
met hod s fo r d e t e rmining them.
Clearly the di stance
offshore w i l l be i nfluenced by t he l i ke l y st ructure
he ight rela t ive to the d i ffe rent t id a l levels, a nd b y
t he mate r ial a nd c onst ru c t i on cost s.
Gene ral advice
on t he ra t i o o f leng t h and offshore d i st ance i s
available, see fo r e xample Ref 4 8 , whe re i t i s
recommended t ha t the breakwa t e r should be further
o ffsh o re than its own le ngth to avoid the format i on o f
a t ombo lo , wh i c h could have drama t i c e ffe c t s o n t h e
ad j acent coast if there is a n y ne t t alongshore d r i f t .
Conve rsely i f a b reakwa t e r is p laced t oo fa r offsh o r e ,
then a p a r t from the g r e a t c onst ruc t i on c o s t , t here i s
a danger that i ts shelt e r i ng e ffe c t w i ll be 'd i lu te d ',
i e , spaced ove r such a w ide front age b y the effe c t s o f
d i ffr act i on t h a t i t s e ffe c t w i ll b e hardly n o t i ceable .
21
An approximat e met hod of calculating suitable
dimensions is t o assume that the structure wil l a t
l east partially prot ect the coast line for
approxima t e l y 2 to 3 times its own length, provided i t
is placed about 1 . 2 5 - 1 . 5 t imes its own length
offshore . Thus if it is wished t o prot ect a 2 5 0m
stre tch of frontage , t he likely first design for a
breakwater would be a structure approximat e ly lO Om
long and about 1 2 5 t o 1 5 0m offshore. Of course t his
is only a very simple and crude method of obt aining a
first layout.
The fol lowing sect ion describes how
t his first idea may be improved.
Before t hat , however, it is worth discussing the
possibl e use of submerged crest breakwaters . C learly
such structures have obvious advantages in cost over
ful ly emerged breakwat ers, and t hey have become
popular in recent years in I t aly.
Here there has been
a growing dissatisfaction wit h t he visual impact of
emerged breakwaters, and also the negligible tidal
range often leads to t he water between such
breakwaters and the coast becoming virtual ly stagnan t
and heavily pollut ed .
Submerged crest breakwat ers
overcome bot h t hese problems and a lso tend t o
e liminat e t h e s trong rip currents which somet imes
occur be tween emerged breakwaters during even moderat e
wave act ivity .
However , their effect on the upper beach, in t erms of
improving coastal stability is rather l ess certain.
C er t ainly intertidal outcrops of rock can oft en
produce a loca l ly wider beach , even in front of a sea
wal l. Despi t e this, there is no accep t ed me thod of
designing an artificial structure to produce a similar
effect. For the time being, therefore, the engineer
must either rely on physical mode l tests, or be
prepared to experiment, perhaps based on obs ervation
of natural features, such as shown in Pla t e 1 .
4.2
Methods for
improving
breakwat er
design
Once an initial breakwat er layout has been de t ermine d ,
there are a varie ty of mathematical and physical
mode l ling t echniques which can be used to improve t hat
design, both in t erms of construction costs and
hydraulic performance . These t echniques can be
broadly divided int o t wo areas , name ly optimising the
breakwat er cross-section and modifying t he plan shape
of the breakwater layou t . These t wo subj ect areas are
now considered in t urn.
The vast majority of offshore breakwat ers that have
been built as shore-prot ec t ion struc tures around the
world, have been constructed using randomly placed
22
rock. It i s l i ke l y that t h i s popula r form of
co nst ruction wi l l a l so be favoured in the UK
e spe c i a l l y on the western and no r the rn coast s where
land rock can be obt ained from local sources.
Indeed
even on t he eastern and south ern coasts, imported rock
from Scand i navia is a l ready be ing consi dered and used
a s an a l t e rna t i ve to conc re t e .
Apa rt from u sua l l y being the cheape st a n d simpl e st
for m of const ruc t i on, rock mound breakwa t e r s a l so
offe r hyd rauli c benefi t s, such a s r oug hne ss and
pe rmeabi l i t y wh ich some a l t e rnat ive breakwa t e r t y pe s
lack.
Ro ck b reakwa t e r s can be bu i l t e i the r usi ng land
based plan t , a s in two o f the UK example s already
inst a l le d , o r b y using mar i ne based pl ant, a s a t t h e
recent harbour deve l o pment at Ramsg a t e i n Ke nt. Thi s
lat ter o pt i o n may we l l have d i st i n c t advantages in
are a s where it i s i nconvenient t o di srupt t o urism or
whe re acce ss from the land is d i ff i c u l t .
Ph ysi cal mod e l l i ng t o t e st t h e st a bi l i t y and
pe r f o rmance of a breakwa t e r ha s be come a we l l a c c e p t e d
technique i n t he last decade or s o , and i s u sua l l y o f
g r eat va lue t o a designer. No t only c a n such t e st ing
avoid over o r und e r -designing a st ructure, bu t such
mod e l s a l so c l a r i fy some of the d i f fi c u l t i e s l i ke l y t o
be encount ere d dur ing breakwa ter const r ucti on.
It i s
worth maki ng the point he re that a c onsiderable amount
of ex pe r i ence has been a c cumu la ted from t est i ng
breakwat e rs o ver the last few yea rs, and i t i s o ften
po ssi ble t o provide much usefu l advice t o a d e signer
o n th e basi s o f this exper ience. This ma y, in some
case s, e ven obvia t e the need for spe c i fic model
stud i e s, a l t hough such stud i e s a r e usu a l l y worthwh i le
at lea st in the l a t t e r stages of design.
Physical mod e l s can also be used to assess the i mpac t
o f a stru c t u re o n a beach, and t o d i re c t l y measure,
sa y the redu c t i on of ove r t o pping of an e x i st i ng
seawa l l by the const ruc tion o f a shore pro t e c t ion
breakwa t e r . Re search has a l so been carried out whi c h
e n a b l e s a de signe r t o assess the durabi l i t y of rock in
the ma rine envi ronme nt ( Re f 3 6 ), and this can c lea r l y
be o f gre a t impo r tance since o ffsho re breakwa t e r s have
to endure in a pa r t i cu larl y h o st i le area, pre fe ra bl y
wi thout the need fo r regu l a r ma intenance.
The subje c t
a re a o f test i ng rock breakwaters is now r a t h e r large ,
and b e yond the sc o pe o f t h i s present r e po r t .
Fo r
fu rthe r, mo re d e t a i led d i scussi on and a ssi stance t h e
r e a d e r i s refe r red t o Powe l l and Al l so p ( Re f 37).
Despi t e the gene ral prevalence of rock b reakwat e r s,
there are, howeve r , some a l t e rnat ive const ruc tion
te chn i qu e s whi c h should be consi d e red. Fo r exampl e , a
rece nt subme rged c rest breakwa t e r near New Br ig h t on ,
o n the Wirra l i n Me rseyside, h a s been bui l t using
23
concrete units designed to dissipate wave and tidal
energy through turbulence created by the units . Wor k
has also been carried out in Italy to develop another
type of interlocking concrete unit also designed for
use in a permeable offshore breakwater . This
development has been initiated by pressure in Italy to
stop the use of and quarrying for rock for aesthetic
reasons.
It has also been suggested that breakwaters may be
built using sand or other granular material, held in
place by a layer of bituminised rock. This form of
construction clearly requires little expensive
material but will not be either as rough or as
permeable as a rock mound .
In consequence , the
breakwater faces would probably need to be of a much
flatter slope .
Finally , it is conceivable that an offshore breakwater
could be built using concrete caissons. Although this
would minimise the amount of material required to
create such a structure , the highly reflective front
face would be an obvious disadvantage on a granular
Such a structure would therefore require very
beach.
careful consideration , and could only be recommended
in special circumstances. For all the alternative
types of structure , however , the use of physical
models can be expected to be useful in improving the
original , conceptual design.
Turning now to the optimisation of the plan layout o f
an offshore breakwater , it is again possible to use a
physical model , and indeed this is often essential if
several breakwaters are being proposed , or if the
coastline is particularly complex .
Such models allow
the designer considerable versatility in testing
different breakwater configurations . For example they
can be used to look at the effects of different
roundheads at the ends of the breakwater , or to
investigate the value of a causeway or a similar
structure linking the breakwater to the coast.
In
addition , a physical model with appropriately scaled
bed material is at present the most reliable method of
assessing the effects of a breakwater on the nearshore
z one . There are still some difficulties with mobile
bed modelling techniques , particularly with unwanted
scale effects if the prototype bed material is sand.
However , in exploring the response of the beach to
varying tidal levels and changes in wave climate ,
caused by the introduction of an offshore breakwater ,
a mobile bed p hysical model is certainly a useful
In addition , such models are also valuable in
tool.
allowing local people the opportunity to visualise the
s t ructures in re la t ion t o t he exi s t ing coas t l i ne .
This may be important s ince such b reakwa ters of ten
24
cause concern regarding t he i r v i sual and a me n i t y
a spe c t s.
Howeve r , suc h physi cal mod e l te sts can be grea t l y
a ssisted o r even replaced by rela t iv e l y c heap
ma thema t i ca l mo d e l l ing me thod s . Admi t t ed l y the
calcula t i on of wave cond i t i ons behind a b reakwa t e r i s
not st raightfo rward, si nce it i s te chni ca l l y c o mplex
to mod e l wave d iffra c t ion ove r a slo ping sea be d ,
wh ich i t se l f i s l i ke ly t o adjust i n shape.
Nev e r t h e l e ss, suc h mod e l l i ng can be use fu l l y used t o
test the se nsi t i vi t y of t h e pe r fo rma nce o f a
breakwat e r to var i a t ions in leng t h, o ffsho re d i stance
and even c r e st heigh t . By c o mb i ning suc h t e st i ng wi t h
pa ra l l e l calculat ions o f t he va r i a t i on i n co st o f t he
st ruc ture, i t i s possib l e to d e t e r mine a configu ra t i o n
w h i c h gives g o o d value for money.
De t a i led
d e sc r i pt i ons of so me of the t e c hniqu es ava i l a bl e fo r
t hi s t ype of mo d e l li ng ma y be found in Re ferences 3 8 ,
3 9 , 4 0 and 4 1.
Ma t he ma t i cal mo d e l s are a l so avai lable t o study t he
e ffect o f a breakwa t e r on t he long term e vo lu t i o n of
t he coast l i ne , but a t present t he se model s a re
re str i c t ed t o rathe r coarse spat ial and t i me
resolut i o n.
The r e fo r e the local changes in beach
confi gu ra t i on, for exampl e , in t he lee of the
breakwa t e r , will be ve ry poo r l y represented, but the
o ve r a l l response o f the c o a st at some di stance away
wi l l be accura t e l y model led. F u r ther d i sc u ssi ons on
t h i s i ssu e ma y be found in Brampt o n, Fra nco and No l i
(Re f 3 1) .
Fina l l y, in mo st si t ua t i ons the suc c e ss o f a n o ffshor e
breakwa t e r i s l i ke l y t o d e pend o n i t s e ffe c t o n
beaches i n i t s l ee .
If the i n st a l l a t i on o f a
breakwa t e r h a l t s or even reve r se s the t re nd o f fa l l i ng
beach leve l s a t the mai n l i ne of defence works, or
prevent s fu r ther downd r i ft e ro si on whic h p revio usly
t hreatened t o ou t flank these wor ks, t hen i t wi l l
proba b l y be judged a succe ss.
It fo llows, t he re fo r e ,
t h a t i t would be val ua b l e t o eva lua t e and co mpa re t h e
e ffec t o f d i ffe rent breakwa t e r ar rangeme n t s o n bea c h
sta b i l i t y, i n t he long t e r m, a s par t o f t he design
proce ss.
Th i s, unfo r t una t e l y, is pr ese n t l y beyond t he
sco pe of ma thema t i cal mode l l i ng and indeed would be
expensive t o a c hi eve e ven u si ng large physic a l mode ls,
given the variab i l i t y of t i da l cond i t i ons, wave
h e i gh t s, pe r i o d s and dire c t ions expe r i enced a round t he
UK c oa st s.
Expe r i ence has shown howeve r , that o ffsh o r e
b reakwa t e r s i nva r i a bl y i mprove b e a c h leve ls i n t he i r
di rect l e e, and fo r a considerab le d i stance o n e i ther
In t hi s se nse t he y have been mo re suc c essfu l
si de.
than sea wa l l s and i ndeed man y g r o yne syste ms. The
25
quant i fi cat ion o f t he benefit to beach leve l s ,
howeve r , i s a rather d i st ant obje c t i ve and i s
d i scussed in t he fol low ing c hapte r .
5
FUTURE
DEVELOPMENT
The aim o f t hi s c hapt e r i s to i d e nt i fy t he e xt r a
i n format ion t ha t i s re qui red b y the desi g ne rs of an
offsho re b reakwa t e r , and t he fut ure deve lopment work
t hat is needed to provide t hat informa t i on .
Possi b l y the m o st impo r tant r e qui rement i s a
d i st i l l at ion o f t he e xpe r i e nce of de signing and
b ui l ding such st ruc t ur e s, t og e t he r w i t h an app r a i sa l
o f the suc c e ss achieved and any d i ffi cul t i e s
e nc o unt ered .
Al thoug h t he e xi st i ng l i t e ra t ur e ,
revi ewed in C ha pt e r 3, c o nt ains much useful
i nfo rma t i on t he number of a r t i c les conce rned is qui t e
large and the y a r e va ri able i n content and qua l i t y .
Sec o nd l y t here a re o n l y a sma l l numbe r of offsho re
bre akwa t e r s whi c h have been i n st a l l ed i n a re a s of
hi g h, or even mode rat e , t i dal range and i t is the se
st ruc t ures which a re likely to be of grea t e st interest
t o e ng i ne e r s in t he UK .
At t he t i me o f writ ing t hi s r e port , a survey o f
b reakwa t e r s in It aly i s b e i ng c a r r ied o ut and wi ll
hopeful l y be ava i lable in late 1 985 . Thi s survey wi l l
ho peful ly form pa rt of a 'd a t a ba se ' fo r offshore
breakwat e r s , whi c h c o uld b e expanded t o a muc h wide r
range of coast lines wor ldwi de .
Suc h a data base should not o n l y i nc lude d e t a i l s o n
d e sign enginee ring , and c oastal re sponse , but also on ,
sa y, naviga tion, amenit y, and e c o logical aspec t s . The
rece nt l y bui l t breakwat ers on t he Wi r ra l , fo r e xampl e ,
have become a safe hig h-wa ter r o o st for wad ing b i r d s,
whi l st at Rho s-o n-Sea local boat own e r s have used t he
calm wa t e r in the b r eakwa t e r lee for moor ing (and are
now conce rned b y decreasing wat er d e pt hs as t he beach
accre t e s) .
In t he Med i t e r ranean, some b r eakwa t e r s
have b e e n provided w i t h a smoo t h c a p to a l l ow
sunba thi ng or an offsho re pr omenade . On the o t he r
hand, pro po se d breakwa t e r s i n t he UK c o ast st imul a t ed
imme d i a t e o b j e c t i ons from local fishermen and concern
has also been e xpressed about t he dang e r s t hat low
c r e st breakwa t e r s may pose to beach use r s .
Al tho ug h t he r e a re onl y a few ma n -made st r uc t ure s
a r o und t he UK c oast that can be inc luded in such a
review , consi d e rat ion should be g iven t o the e ffe c t o f
nat ura l offsho re i slands, reefs and banks whi c h mi ght
give va l uable suppleme ntary data .
Thi s appr a i sa l m a y be pa r t ic ula r l y valuable i n
a ssessing t he po t e nt i a l o f subme rged crest breakwa t e r s
26
wh i c h may
be u s e f u l
lower p a rt
of
in i m p r ov ing beach leve l s
t h e i n t e r - t idal z on e .
p ro t o t y pe t e s t i ng o f
man-made may w e l l
Ano the r a re a wh i c h has
b r e akwa t e r ' s
i n t e ra c t i on wi t h ,
f lows .
is
Med i t e r ranean coun t r i e s ,
an almost
t h a t would
f rom wave
a t t e n t i on
a nd e f f e c t s
on
is
a
t id a l
h a r d l y s u rp r i s i ng l y ,
howeve r ,
result
it
a nd
is
In many a reas
is
p o s s i b le
that
f r om s h e l t e r i n g t he coas t
act i v i t y c o u l d be d imi n i s he d by a n
unexpe c t ed i n c r e a s e i n t i d a l f l ows b e h i nd
breakw a t e r .
Thi s
when a proposed
l a rge
e f f e c t ive
c o n s i de ra t i on at a l l i n
unt ouched s u b j e c t a re a .
a r ound t he UK c o a s t ,
bene f i t s
s ome
s t ru c t u re s .
r e c e i ve d s cant
g i ven no
the
b o t h na t ur a l a nd
be a r e l a t ive ly cheap and
me t h o d f o r inve s t i g a t i n g such
This
Indee d ,
such s t ru c t u re s ,
in
is
t he
a p a r t i cu l a r l y i mp o r t ant
site
lies
topic
i n o r n e a r an e s t ua r y o r
t id a l i n let .
S ome f u r t h e r cons i d e ra t i o n a l s o nee d s
t h e s p e ci f i ca t i o n o f
c a l cu l a t i ng
t i da l
the e f f e c t
c o n d i t i on s .
of
a breakwa t e r
Al t hough mo de ls
p r o p ag a t i on w i t h
t o be g i ve n t o
leve l and curre nt when
o n wave
f o r c omb i n i ng wave
tidal currents
are
already
in u s e ,
as
a re mode l s f o r c a l c u l a t ing wave d i f f ra c t i on a r ou n d a
breakwat e r ,
it
is
ra t h e r expens i v e t o u s e
t h e s e mo d e l s
t oo of t e n .
Ideally ,
t h e r e f o re ,
i nc i de n t
wave
e a r l y s t ag e
a c ce p t able
a f ew i mp o r t ant
combina t i on s
to
red uce
l e ve l .
the
l e ve l s ,
comp u t a t i onal e f f o r t
How t h e s e
c omb i na t i ons
chos e n t o g i v e an i nd i c a t i o n of
p e r f o rmance of
t i de
and
s ho u l d b e i d e n t i f i ed a t
a breakwa t e r i s
the
an
to a n
c a n be
ove r a l l
s t i ll
an o p e n
q ue s t i o n .
D u r i ng t h e p re v i o u s
mo d e l s
to assess
chap t e r ,
b reakwa t e r was d i s c u s s e d .
been
carried out
the u s e o f
t h e e f f e c t ivene s s o f
S u f f i c i ent
p hy s i ca l
an o f f s h o re
t e s t s have n o w
t o g i ve g o o d adv i c e o n breakwa t e r
c r o s s -s e c t i on d e s i g n a l t hough s o me
f u r t h e r wo rk i s
r e q u i r e d o n d e f i n i ng t h e cha rac t e r o f wave s
ove r t o pp i ng a b r eakwa t e r ,
is
o r p a s s i n g t h rough
p e rmeable l ik e mos t rock b re akwa t e rs .
much g r e a t e r need
g u i d an c e
for a series
of
in p la n s h a p e l a y ou t s .
tests
if
should
but
t o g i ve a be t t e r unde rs t a nd i ng o f
also on
the
bas i c h y d r o d ynam i c s o f a n o f f s h o r e b r e akwat e r .
i nve s t i ga t i on
W a l l i ng f o r d ,
of
this
t y p e i s s ho r t ly
i n comb i n a t i o n wi th
to b e
An
s ta r t e d a t
the deve l o pment o f
p a ra l l e l ma t h ema t i c a l mode l l ing s t udy whi ch w i l l
h op e f u l ly
b e a b l e t o ext end s p e c i f i c m o d e l t e s t s
m o r e genera l i s e d
27
s i t u a t i on s .
it
is a
t o g i ve
These t e s t s
n o t o n l y b e c a r r i e d o u t wi t h a m o b i l e b e d ,
a r i g i d bed
it
The re
to
a
Conside rabl e re search has already been unde r t aken int o
the me t ho d s o f m a themat i ca l l y mode lling o f f shore
breakwater s, for exampl e , as desc r ibed in Re f 4 1.
These mode l s, howeve r , are gene r al l y both d i f f i cul t t o
underst and and use i n a coastal engi ne e r i ng context .
Some f urther deve lopme n t i s needed to make these
mode ls, or at least the i r re sul t s, simp ler t o appl y .
Fi na l l y, t h e r e i s n o doubt t h a t much f undame nt a l wo r k
i s requi red in pred i c t i ng t he response o f a gra nular
beach to a perma nent a l t e rat i on in wave c l imat e .
In
t he sho r t t e rm , valuable guid anc e could be gained f r om
beach prof i les t aken, over a long t imesc a l e , be f o re
and a f t e r a breakwa t e r has been const ruc t ed .
Ove r a
much l onger t i me it may be po ssi ble t o d e ve lop a
t heoret i cal f r amewo rk t o pre d i c t such change s, but
t h i s is a major task and it may take several decade s
t o c ompl e t e .
6
CONCLUSIONS AND
RECOMMENDATIONS
( i)
Thi s re po r t reviews the role o f o f f shore
breakwa t e r in shore p r o t e c t ion around the UK
coast l i n e . As an i nt r o d uc t i on t o t he sub j e c t
t he physi cal p r oce sse s have been di scussed and
i l lust rated w i t h r e f e rence to nat ural
exampl e s.
( i i)
An ext ensive surv e y of the pub l i shed
l i t e ra t ure on o f f sho re breakwa t e r s and t he i r
e f f e c t s o n the nea r shore zone has been
pre se n te d .
This review co ve rs case hi st o r i e s,
and ph ysi cal and mathema t i c al model
technique s.
It w a s f ound that whilst there
are a number o f c a se h i st o r i e s for l o c a t i o n s
wi t h a small t ida l rang e, expe r i e nce o f
o f f sho re breakwat e r s in areas o f high t i d a l
range i s v e r y limi t e d . The re por ted c a se
h i st o r i e s were supplemented by o b se rva t io n s
a n d d i scussi ons w i t h local e nginee ring st a f f
a t o f f sh o re bre akwa t e r si t e s in the UK .
( i ii)
( iv)
The si t ua t i o ns i n wh ich of f sh o re bre akwa t e r s
c o ul d b e a viable a l t e rna t i ve t o e sta bli shed
co ast pr o te c t i on me t ho d s have been reviewe d .
I t was suggested that the i r use i n the UK, in
the near f ut ure, would ma inl y be f o r
a l le v i a t ing l o c a l i sed problems, rather than
pro t e c t and deve l o p long st re t c he s of
coast line a s in, f or examp l e , It aly and
Ja pa n.
The amount of general guidance ava i lable f o r
the design o f plan layout f o r a system o f
o f f shore breakwaters i n a n a rea o f a ppr e c i ab l e
Some gene ral
t id a l ra nge i s ve r y limi t ed .
28
des ign gu idel ines have been given in the
repo r t , but it is rec ommended t ha t phys i c a l
and ma t hema t ical model t e s t s should be
unde rtake n at the d e s ign s t age to o pt imi s e the
breakwa t e r a r rangemen t .
( v)
7
Informa t i on on t he des ign o f a breakwa t e r
pro fi l e i s far more read i l y ava i lable.
Howeve r , i t i s s t i l l cons id ered advi s ab l e t o
conduct phys i ca l mod e l s tudies t o t e s t t he
s ta bi l i t y and hyd ra u l i c pe r formance o f the
d e s i gn.
( vi)
Exi s t i ng mathema t i c a l and phys i ca l mod e l s can
be used t o improve the bas i c d e s ign of a
breakwa t e r s ys t em.
However , b o t h modell ing
t echni ques a r e s t i l l capable of being e xtended
in order t o g ive a be t t e r r e present a t i on of
In the long t e rm
t he phys ical problem.
research i s requ i re d t o i nves t igate both t h e
phys ic a l proces s o f t he re s ponse o f granu l a r
beache s t o a pe rmanent change in wave c l ima t e ,
a s caused b y the int roduc tion o f a n o ffs ho r e
breakwa t e r, a nd to e s t ab l i s h a the o r e t i ca l
framewo rk for pred i c t ing long t e rm c hanges .
( vii)
The des ign o f an o ffs hore b reakwa te r sys t em i n
t he UK could b e aided cons iderably b y
a ppr a i s a l o f the pe rformance o f exi s t i n g
o ffs ho r e breakwa t er s . I n order t o do this
cont inual mon i t o r i ng o f b o th the s t ructures
and the i r env i r onmental e ffe c t s wou l d be
nec e s s a r y.
ACKNOWLEDGEMENTS
The authors wou l d l i ke t o t hank Mr G B No rwo od of t h e
Bo r oug h o f Co lwyn , Mr C D Dav is o f t h e Met ropo l i t an
Bo roug h o f Wi rral and Mr A M Mas call forme rly o f
Ang li a n Wat er ( C o l c he s t e r Div i s i on ) fo r t he i r
ass is tance.
29
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Co a s t a l Eng i ne e ring �,
s lo p e wave equa t i on .
1985 .
Shore Pro t e c t i on
DDB
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10
20
30
40
50
60
70
80
90
100
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Dist a n ce f ro m
Fig 7
R h o s - o n - Sea p ro f i les
prom e n ade ( m )
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p
a
r
a
d
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e
K i ngs
Offs hore brea k waters
0
400m
Wallasey
L i ve r poo l
Bir kenhead
Fig 8
Ki ngs
Parade brea k water layout and
locat ion
Pl ates
Plate 1
Offshore ree f , Dorset Coast
( P hotograph copyright Colour Library International)
Plate 2
Offshore breakwa t e r , Rhos on Sea
Plate 3
Offshore breakwaters , Leasowe Bay
( P hotograph copyright Sealand Aerial Photography)