Offshore Breakwaters and Sills. Text

tfidranlicsResearcft Mhlingford

A REVIEI.IOF NOVEL SIIOREPROTECTION METIIODS

Volume 5 - Offshore breakwaters and sllls - Text

J Welsby T Eng J M Motyka BSc (Eng)

Report No sR 34 May 1987

RegisteredOffice: Hydraulics Research Limited, Wallingford, Oxfordshire OX10 8BA. Telephone:0491 35381. Telex: 848552 This report describes work carrled out by Hydraullcs Research on research lnto novel forus of shore protection. Prior to April 1985 this research was funded by the Department of the Envl,ronment (Water Dlrectorate) under contract nunber PECD7/7/O55. Since Aprll 1985 iE has been funded by rhe Minlstry of Agriculture, Flsheries and Food under contract number csi 1034, the nominated officer being Mr A Alllson. At the time of reportlng the Hydraullc Research nomlnated projecE officer was Dr S w lluntington. The report is published on behalf of the Department of the Environment and the Ministry of Agrlculture, Fisheries and l'ood, but any opinions expressed within lt are those of the authors only, and are not necessarlly those of the minlstries who sponsored the research.

published crown copyrtght 1987. by permlssion of the controller of Her Majesty's Stationery Offlce. ABSTRAGT

This Ls the fLfth l"n a series of reports on 1ov coat or novel forms of shore protectLon.

It covers the use of brealarater type structures rtose aln l"s to encourage beach stabLLLty. lte Etructures tbat are exanlned range from large gravity tSrpe breakvaters nhLch dlssl.pate and dlsperse wave energy through the snall sLlls shl.ch trip vaves at the toe of erLstlng seavalls and revetments.

CONTENTS Page 1 1 INTRODUCTION I

I.1 Purpose 2

OFFSITOREGRAVITY BREAKWATERS 5

2.I Structure type 10

2.1.1 Asphalt l0 2.1.2 Concrete 11 2.1.3 Rock L2

FLOATING BREAKWATERS L4

3.1 Structure type 15

3.f.1 A-frame 16 3.f .2 Flexible membrane 16 3.1.3 Hydraulle 16 3.1.4 Pneumatic L6 3.1.5 Pontoon L7 3.1.6 Porous 18 3.1.7 Scrap tyre 18 3.1.8 Tethered float 20 3.f .9 Turbulence generator 2L 3.1.10 Twin cylinder/1og 2L 3.1.11 Wavebarrier 22

SILLS AND PERCTTEDBEACT{ES 23

4.1 Structure type 24

4.1.1 Beach prisus 24 4.L.2 Sand isle 25 4.1.3 Calsson structures 25 4.L.4 Faggottlng 26 4. 1 .5 T,ongard tubes 26 4.L.6 Gablons 27 4.L .7 Sand bags 28 4.1.8 Sheet pile 29 4.L.9 Sandgrabbers 30 4.1.10 Sta-pods 30 4.1.11 Surgebreakers 31 4.1.12 Tinber 3r

ENVIRONMENTAL ASPECTS OF DESIGN 33

CONCLUSIONSAND RECOMMENDATIONS 35

REFERENCES 39

GLOSSARY 44 CONTENTSCONTID

FIGURES

1. Rubble-mound breakrrater 2. Sta-pod breakwater unLts 3. Rock breakwater, Rhos-on-Sea, North Wales 4. Two-cyllnder A-frane floating breakwater 'Goodyear' 5. type floatlng tyre breakwater tWave-mazet 6. type floating tyre breakwater 'wave-guardr 7. Schematic floatlng tyre breakwater after Harms (Ref 20) 1979

PLATES

1. Aerial vlew of Dengie flats showlng lighters in positlon 2. Rock breakwater, Rhos on Sea, North Wales 3. Sl11 incorporated ln sea wall design, East Wear Bay, Kent

APPENDIX(see separate volume)

I . SumrnarySheets INTRODUCTION This report, commissioned by the Ministry of Agriculture, Fl,sherLes and Food, ls the fifth in a series of reviews lnto novel or low cost methods of shore protection. It examines the potential of breakwaters and sinilar structures as a means of coastal defence ln a UK wave and tidal environment. The structures examLnedhere are designed to elther absorb, reflect or dlffract waves and by doing so lmprove beach condltlons ln their lee. They lnclude offshore free-standing breakwaters which are built on the sea bed, and are detached fron the coastline and those which float on the sea surface. Also included are structures known as nearshore breakwaters and sills which are essentially 1ow crested breakwaters bullt on Ehe lower part of the foreshore and used to retain beach fil1.

Interest in novel or low cost methods of shore protection has come about large]-y as the result of the increased cost of constructlng tradltional forms of coastal defence. In America the need for designs which are wlthin the means of private property owners promoted a major appraisal of shore protection nethods and devices which was carrLed out by the US Corps of Engineers. The Anerican "shoreline Erosion Control Demonstratlon Program "(referred to hereafter as the American programme) examined a wide range of structures. Many of the designs reviewed here are based on American experience but it cannot be stressed too strongl-y that they were often tested in conditions of snall tidal range (typically of the order of 1-1.5n) and of low to moderate wave actLvlty (wave heights typically less than 1.5n). Sinllarly, breakwaters ln Mediterranean countrles, eg Italy and Israel, have also been examined. These too are built in areas of low tidal range but with noderate to severe wave activlty.

At the other end of the spectrum we have also looked at massive, costly breakwaters whlch have been built in this country ln recent years. There is however a large gap ln our knowledge as to the behaviour of nediun sized structures and hence our assessment of the type of lnstallations that could be used in a IIK coastal cllnate is therefore largely subjective.

In Chapter 2 we describe the performance of fixed offshore breakwaters either butlt on the sea bed or connected to lt by piling. In this type of structure, the static forces such as mass and the dynamic forces such as wave action, are transmitted directly to the sea bed. The loads thus transmitted are largely compressive ones and hence structures in this cacegory require careful foundatlon design. The role of the floating breakwater and its ablliry to suppress wave activity ls examined in Chapter 3. In these structures htgh tensile forces are transmltted through the anchorage to the sea bed.

Nearshore structures such as low crested si1ls and other devices which dissipate wave energy at or near breaking or during uprush are covered in Chapter 4. Sills are often used in conjunctlon with beach nourishment schemes. However slnce essentially they are desLgned to retain beach levels, no distinctlon ls drawn between those structures which trap littoral drift and those which retain beach material placed artificially.

Environnental aspects are discussed in Chapter 5. Although breakwaters can be designed to withstand the most severe rrave conditions, their effectiveness Ln protecting the coastline is as yet imperfectly understood. The effect of such structures on adjacent coastlines can be danaging in that they can interrupt or cut off the natural supply of beach material. Anenity probleurs, such as reduced water exchange and additional pollution due to the collection of trash etc, may also be encountered with thls type of structure.

Chapter 6, the final chapter, is devoted to assessing the need for monitoring existing structures in a more positive fashlon. Conclusions and reconmendatLons about potential breakwater types are also included here.

Fina1ly, we are once again grateful to the publlshers of the "Shoreline Eroslon Control Demonstration Program", Moffat and Nlchol, Engineers of Long Beach, Callfornia for their pernission to publish excerpts f rorn the report.

1.1 Purpose The purpose of this review is to explaln the function of breakwater structures, the types and mat,erials most often used, where they are currently in use, and which of these come in to the "low cost shore protection" bracket.

Breakwaters are reviewed in this report under three headings; offshore, floating and Lnshore structures. The offshore types are usually of rubble mound construction although steel, tlmber, concrete caissons and even sunken shlps have been used. They are constructed normally parallel to the shoreline either singly or in series with a gap between each breakwater. Their main function is to dissipate rdave energy by turbulence, friction or by induclng the Idaves to break on their seaward faee. If the breakwater has a high degree of porosicy a certain amount of wave transmission nay take place. This is by no means always a bad thing and when such Btructurea are placed off bathing beaches they can give protection agalnst waves whlle maintaining the exchange of cl-ean water between the offshore area and the lntertidal zone.

Floatlng breakwaters act quite dlfferently having llttle effect on damplng large waves. They are nalnly used in sheltered waters to smooth out short period choppy naves. Fairly popular in parts of Canada, for lnstance, they can be found in ports or marinas sheltering snall craft. fron choppy seas or boat wash. Ilowever, they tend to collect trash and are thus not suitable for anenlty beaches. Even the most substantial of such structures do not reduce wave transmlssion to a degree, in our opinlon, to signlficantly effect beach changes.

The third type, whlch ls rather cheaper than a gravlty type offshore st.ructure, is the s111. Placed in t.he inter-tidal zone it. acts ln many ways slmilarly to the offshore breakwater. At low water the sill ls exposed on the lower foreshore and has ltttle effect other than to deflect ttdal drainage. Towards high tlde the s111 begi.ns to trip the incomlng waves and cause stilling in lts lee. At hlgh tide a sill ts ltkely to be submerged hence its capacity to build up the upper beach is somewhat limited. If placed too far up Ehe beach it acts more as a retalning wall or a revetment. The dimenslons, layout etc of sills therefore need very careful testlng to produce an efflcient deslgn. In the USA stl1s appear to have been used with some success. However, we belleve that this ls related t.o the fact thaE the snal1 ttdal range largely reduces an error of correct placenent, le the sill ls either wlthin the lntertidal zone or outside it. Many of the s111s which have Amerlcan patents (Sandgrabbers, Beach Prisms, Surgebreakers etc) are prefabricated concrete structures which have sufficlent poroslty to allow lrater to transport sand through thern. The sand then tends to settle out ln their lee causing some accretion. Wtth a low tidal range these structures can have relatively snall dlmenslons and be reasonably low cost. If "scaled up" to be effective in tidal ranges and wave heights of several metres, however, such structures are likely to be costly.

Perched beaches are often associated with the construction of sllls especially in areas of low ltttoral drift. Artificial feeding, le replenishing the beach sand landward of the sill from an outside source has the added benefit of not depriving the adjacent beaches of their littoral supply. To date all the perched beaches have been associated with sills of modest proportions. We do not think that their behavlour is well enough understood to scale up their performance to UK conditions. (In the one case where attenpfs have been nade in thl.s country to create a perched beach the att.empt has not been entlrely successful). OFFSEORECRAVITY BREAKI{ATBRS These are usually constructed parallel to the shorell.ne, their prinary funetlon belng to disslpate the energy of incident waves. Indeed, one problen with large breakwaters ls that providing there ls a plentiful supply of beach material, tombolos can form ln their lee. Tonbolos often act like long groynes cutting of the nat,ural supply of littoral drlft whlch may in turn cause erosion of the downdrift beaches. Alternatively tonbolos can be created deliberately. 0n the Adriatic coast of Italy for exanple an offshore submerged structure rdas constructed uslng sand filled bags together wlth groynes on the foreshore. Synthetic fabrlc bags filled with sand were then laid underwater from the seaward end of the groynes to the breakwater. These enclosed areas were then artlficially re-nourished. The function was to arrest erosion with the sand fllled diaphragms and trap littoral drift wlth the short rock groynes.

The ability of a breakwater to trap sand is a function of the wave and tidal conditlons, lts distance offshore, its length parallel to the shore, its porosity and, if nore than one breakwater is to be constructed, their spacing. Thus the height length, wave transmission characteristics etc, need to be examLnedcarefully if breakwaters are to be used as part of a coastal defence system.

There are good reasons for constructing offshore breakwaters with a low crest. This fundamental principle is in direct contrast to the one used in deslgning breakwaters to prevent harbour dist.urbance, where every effort is made to ensure a sufficiently hlgh crest elevation to prevent \ilave transmissLon. However subnerged breakwaters have several obvlous constructlonal di.sadvantages and may also be unacceptable fron an amenLty viewpoint. Offshore breakwaters are just as hazardous as natural reefs and need to be narked for navigation purposes.

trIhile breakwaters have not been widely used in the UK, a complex system of groynes and offshore breakwaters has reeently been constructed ln the Wlrral. One such system rdas constructed at Kingrs Parade, New Brighton in 1985. The breakwaters are subnerged to a depth of about 1 netre at mean high wafer. A11 consist of a sand core covered by a layer of rock. The crest is armoured wlth pre-caat concrete units known as reef blocks. The system is designed to increase beach levels in front of the high vertical wa1l at Kingrs Parade. The shoreline response is to be monitored over a number of years and presently it is too early for any comments to be made about their likely succeSs. Subnerged breakwaters are, of course, less expensive to build than the emergent type. Wave forces on the seaward face are snaller because more wave energy is transml.tted over such structures. The tripping of waves by the crest of such structures does mean that greater forces will be imposed upon the landward face. Therefore, submerged breakwaters need as careful testing and design as the euergent ones.

In areas with a large tidal range a subnerged breakwater may be relatively lneffective at high tide. Thls ts at the tine when incident wave actlvity is greatest and when the maximumof protectlon to the coastllne is desired. It ls therefore thought, that for naximun effectlveness these types should be restricted to areas of low tidal range (Ref 7). Although little is known about the effectiveness of this type of breakwater in the field, much useful lnformatlon ean be gained fron the research whieh has been carrled out in recent times (see Refs 2,7 & 14).

The choice of crest level will hinge upon a variety of factors including protectlon required, tldal range, storm levels etc. Loveless (Ref 36) suggests that from available evLdence it can be argued that a crest level around one metre below MHWSwould be sufficient in many cases of beach and coast protection.

The crest width of a submerged offshore breakwater should be a function of the design wavelength while the gaps between a series of breakwaters will depend nainly on the wave climate and the dtffraction patterns produced.

The most commonform of breakwater is the rubble mound structure. Providing the naterlal is freely avallable it is a relatively sinple operatlon to tip rock directly into posltion to form such a structure. Unsophlsticated breakwater types also have the advantage of being easily nodifled as construction progresses or as hydrodynamic conditions change.

For economic reasons breakwaters are often constructed hrith a steep seaward face and hence wave energy is also reflected seanards. Structures with a relatively flat seaward face together with a rough armour layer at the surface, allow wave energy to be dissipated over a large area and this deters seour at the toe by minimising wave backrush. It is sometimes more cost effective to bulld such structures as a series of segmented units. Thts allows some Iirave energy to be transmltted shorewards, helplng to maintain sediment transport and retard the formulation of toubolors. The water int.erchange helps to maintain environmental quality in their lee. Fixed offshore breakwaters have been used widely in countrles such as ltaly, Israel, Japan and the USA, generally in conditLons of low tidal range. They are mostly of the gravlty type and of rubble mound construction. Normally used to protect sand beaches fronted by a shallow sea bed, they are generally construct,ed parallel to the shoreline. A series of offshore breakwaters ln Singapore for exauple were speciflcally designed with their orientation parallel to the donlnant wave crests (Ref 50). The project nas designed to protect reclaimed land and the concept rras to lnstall offshore breakwaters or headlands at lntervals allowing crenulate shaped bays to form between them. Two types of rockflll structure were uttlised on this "1ow energy" coastline, gabions and soi.l mounds faced with layers of rock. Where the dominant direct.lon of wave actLon is quite different Eo the beach alignnent, placing then at an angle ln this way can help to develop a stable beach plan shape more quLckly between each breakwater unLt. Spaclng of Ehe headlands ln thls situation ls of course of prine lmportance.

The UK coastline is subjected to a severe vave climate and a large tidal range. To contend with this, structures in this country have therefore been large and expensLve to construct by comparlson with thelr foreign counterparts. Also, partly due to the high costs and the widely varylng hydrodynamlc conditlons from slte to site, no tlro breakwater structures have been desLgned in the same manner.

One of the relatively few breakwaters to be found off the llK coast ls at Rhos on Sea, North Wales. Thls roek breakwater whose general layout ls shown in Flgure 3, built up a two to three metre wlde sand berm on the seaward face of the structure. On the landward side shingle beach leve1s lncreased by two metres wlthln four years of construction. Slnce the structure is essentially t,here to provide additional protectlon against nave attack, it is clear t.hat such a system, fornlng on]-y part of the defence, cannot be classified ln the low cost category.

A similar sl-tuatlon exLsts on the Wirral coast, where sorne Lnnovatlve breakwater designs are undergolng trlal. At LeasoweBay, an area wlth a htgh tidal range and strong lnshore ttdal currents, wave action and tldal scour was eroding the beach and lt was decided to try to re-align the lrave approach and thus to reduce wave and tidal actlon. Two rubble mound breakwaters (Ref 4) were built ln 1982 in an area of sand foreshore, one shore connected and one detached. 80,000 t of rock were used on each breakwater. Beach levels ln the breakwater lee have built up. Ilere also the breakwaters provide only part of the defence system since the shoreline ls also protected by a rock revetment. Taklng the systen as a whole, the price of protecting thts frontage must be consldered to be high.

One of the most innovative systems tried ln the UK to date ls at Dengie ln Essex. This did not involve a htgh Eechnology approach but rather a pragmatie use of avallable materials. A nunber of disused lighters (barges) were bought by Angllan Water, half filled with silt, and topped off with gravel and towed out and sunk paralle1 to the shoreline. There were 16 llghters ln all, 19.2m long and sunk ln a llne parallel to the coast with 10rn gaps between each, givtng a tocal breakwater length of alnosE 460m. At present 2.5m above the bed they may well slnk into the nuddy sea bed naking thern less effective ln tlne - However, at an estimated cost (in 1983) of 8481000 this is a low cost scheme when relaLed to the cosE of refurbishing or replaclng the embankmentson-shore. Essentlally used to protect an area of saltlngs on a low flat coast, Lhe system ls seen as an experimental one. The performance of the system ls being monltored. Plate 1 shows an obllque aerial view of the sysEem and the intent,ion ls to take slnllar photographs at inEervals to assess the effect of the scheme ln halting saltrnarsh eroslon.

Breakwaters of a roughly simllar design were tested ln the Anerlcan progranme (Ref 55) buc with rattrer more llmlted success. The summary sheet describes the const.ructlon and performance of a concrete box system bullt at Kltts llummockin Delaware Bay. The breakwater, constructed of pre-edst concreEe boxes fi1led with sand, was 100n long and situated sone 230n from the shoreline at a depth of 0.3rn below MLW. Not surprisingly the sand in the boxes was quickly washed out. Had they been fllled with concrete or large aggregate they night have fared better. Purpose made concrete caissons are to be placed offshore of the Essex coasL at Sales Point. These will be filled wtth silt frorn the foreshore and capped with concrete.

The cost of breakwater construction on an open coastllne is llkely to be very high. In this context sand asphalt shows some promlse. Thls materlal, usually used as a core, can be laid rapidly and can be placed underwater wlthout losing its strength. It has some reslstance to wave attack and has the advanEage of being permeable. Use of sand asphalt, fill could improve the speed of constructlon but is unllkely to put it into the low cost category.

The "lighter" breakwater system used by Anglian Water may prove useful as coast Protection ln sheltered areas. Ilowever it ls unlikely to be accepEable in a harsh wave environment on open coast beaches. The draught of the lighters used at Dengie was small, of the order of 2.5n with the crests at mean water level. To protect an open coastline with relatlvely deep water close lnshore would require vessels with considerably more draught.

The design of an offshore breakwater can be complex and before developing an effective solution, the hydrodynamic regime of the area must be understood, ie waves, tides and sedinent transport.

Iilaves

lJaves generated by winds or moving vessels are always present on an open coast and represent the najor cause of erosion.

Reflection may occur on the seaward side of a breakwater, the degree depending upon the angle of the lncomlng rdave crest and the porosity of the structure. If lts seaward face is smooth and vertical, near perfect reflectlon will result. This would creare standing waves whlch could cause conslderable botton scour. Waves approachlng at an angle would generate choppy short crested seas whlch could also eause botton scour.

Diffraction occurs when the inconing rfaves paqs around the breakwater and energy is transferred along thelr crests to the quiet area in the structurets lee. The degree of diffraction determLnes whether tombolos are likely to form.

Tides

Tidal currents can be another cause of beach erosion. Breakwaters often include shore connections whlch help to reduce tidal scour. Erosion on the seaward face of the breakwater can be ninimised by suitable toe armouring.

Sedlnent

Waves breaking at an angle to the beach contours carry sediment along the shore in the direction of wave advance. Thts is termed littoral transport or drift.

The abtltty of the waves to transport sedinent is a function of the wave height to the power of 5/2 so a modest decrease Ln wave height ln the lee of a breakwater can have a najor effect on sediment movement. It should be obvious that accretion will depend on the amount of material available, and that trapping of sedinent at one point can lead to depletion further downdrift. The distance between the breakwater and the shore can have a slgnlficant effect on lts efficlency. Model tests Ln a wave flume can deteruine this optimal distance. Mlntkin (Ref 38) suggesrs thar the roe of the breaknater should be located at the seaward limit of the surf zone.

2.1 Breakwater materials A number of nat.erlals have been used in breakwater constructlon, the most commonbelng rock or concrete armour unlt.s. Bitumen may play an lncreasingly important role ln future years. At present lts use is largely as a grouting rnedlurnbut nith lts ease of handllng and its rnalleablllty, bit,unen could be very useful, for instance ln the constructlon of breakwaEer cores.

Steel, concrete, timber, rubber tyres and syntheEic materials have all been tested ln breakwater construction. Detalls of some of these design materials and how and where they have been put to use can be found among the summary sheets in the Appendix. In the followlng sub-sectlons lre have described these materials although not all can be used ln an open coast situatlon.

2.I.1 Asphaltic breakwaters

The conbinatlon of stone and asphalt has been used extensively ln llolland and Belglun for the construction of harbour walls and can be seen in various works ln the rDelta Projeetr. Asphaltic grouted roek breakwaters are impermeable structures and detalls of two such lnstallations, at Ijmulden and Iloek van Holland can be found among the summary sheets in the Appendix. Grouted structures have the advantage of allowing smaller rocks to be used than would normally be the case with 'free standing' rock armourlng. Details of the advantages and disadvantages of bitunlnoua constructlon can be found ln a site vislt report to Holland and Belgiun (Ref 39) and ln volume 4 in this series (Ref 57).

Among other types of asphaltic materlals ls tFixtonet a patented mlx of gap graded rock and bitunen. This form of permeable construction seems to be working well as breakwater material for example inslde the developlng harbour of Zeebrugge, Belglura. Here lt has been used as an armour layer, laid over a lean nix of sand-asphalt (blturnen and sand) whlch forms a fllter over the sand core of the breakwater. Laboratory tests at Delft, (Report No M1942) uslng regular waves have shown that rFixtonef, lal-d as a revetment on a 1 in 3 slope, can withstand wave heights of up to 2.65n (with a periodicity of 3 to 5 seconds) wirhout damage.

10 Other asphaltic nat.erials (such as grouted stone) can also withstand quite severe wave attack. The cost of asphaltic constructlon can be high, but it has potential as can be seen Ln nany coastal defences on the Continent. It is thought that costs could be reduced once the engineer becomes more experienced and less reliant. on the contractor for the deslgn of mixes etc.

A useful report on most aspects of the use of asphaltic products in hydraulic structures has been ltritten by the Technical Advisory Conmittee on I'later Defences in Ilolland, see Ref 53.

2.1 .2 Concrete

The Anerican test programme included a patented concreEe panel design called a Z-vaLL, consl.sting of rectangular, rel-nforced slabs held vertically and bolted to each other La zLg-zag fashion. A 30o long Z-waLI was built in the Great Lakes at Geneva State Park, Ohio at a distance of 30 metres from the shore (see sunnary sheet no 4). It consisted of 14 panels, each 1.8 metres high and 4.7 netres long and each weighing 6600 kg. The toe of the structure rrrasin about 0.9 n depth of water. The panels were lifted Lnto position by crawler crane and bolted to each other. No special preparation was made to the Lake bed before placing. A partial tombolo was formed and the structure maintained a falrly large accretl_on in its lee, despite loss of end panels. The easternmost panel broke off shortly after constructLon and when it was replaced further damage occurred. Uneven panel settlement danaged the edges of the panels causlng exposure of the reLnforceoent. The stablltty of this masslve structure left much to be desired, especially when one considers the snall water depth in which lt was placed.

A slnilar system of Z-vaLL breakwaters was built at Perre Marquette townshlp, L Michigan as part of another demonstration programue. Ilere three breakwaters rtrere butlt sone 15 metres from the shoreline. The panels were 1.8 metres htgh and 4.3 metres 1ong, weighing 5440 kg. A storm, wlth up to 3 metre breaking wavea, caused severe damage. Analysis indicated that the necessary deslgn changes would remove such structures from the low cost category.

On an open coaatline concrete panels would not resLst wave impact well and would tend to overturn, partlcularly the end panels. Erosion on the seaward would also be a najor problen. A systen of zIg-zag tinber breastworks has been used to protect the seaward face of a shlngle spit in the United Kingdom at Ilurst Castle. These, despite their perneability

1t suffered a sinilar fate to the Anerican structures. Their seaward faces were undermined and the panels rotated in a seaward direction. We would therefore, not generally recommendthe use of free-standing structures wlth vertlcal faces, due to the very high forces lnposed upon them by wave actlon.

2.1.3 Rock breakwaters

Dunped rock is useful as a means of constructlng lolt crested breakwaters. Provlding the wave cllmate is not, severe they can be of relatlvely slnple consLruction. Even so problems can be encountered if the hydraullc characterlstlcs and the llkely rate of seEtlement, are not assessed. In some sltuations the latter need not be a serlous problen, lf the structure has not been designed to fine limits. SetElement can normally be tolerated provided 1t takes place soon after constructlon and before the structure ls subject to severe wave attack.

As mentioned earller an offshore rubble mound breakwater nas const.ructed at Rhos on Sea 1n 198I to alleviate overtopping of the sea wall and flooding of the lord lying resident,lal area beyond. A detached offshore breakwater was opted for ln preference to ralslng the sea wall for several reasons: (a) a higher sea wall on the promenade would have been visually Lntruslve, (b) the breakwater would build up the beach ln lts lee, and (c) it would also a1low boats and fishing vessels shelter. The structure was posittoned at the low water nark and orlentated as shown ln Figure 3. It was destgned to prevent overtopplng by waves of up to 3m. While constructing this 240n long breakwater lt was notlced that waves were enterlng the lee side from the north and a rubble groyne (debris fron the sEructure) was bullt on the beach to prevent the beach material belng washed through. The st,ructure (see Plate 2) has been in existence for about five years and a substantlal amount of shtngle has accumulated in the lee of t,he groyne. A reasonable amount of fine sedinent has also collected ln the 1ee of the breakwaEer; beach levels too have inereased on lts seaward slde. Fears have been expressed that the breakwater may now be exacerbatlng the erosion downdrlft ln Colwyn Bay.

Another such structure was cested in the American programme, at Kitts Hummocksln Delaware Bay. Constructed of 250 to 55Okg quarry stone lt was stted 180n offshore at a depth of 2n at tow nater . 2.67n htgh the structure was built, half on fllter cloth and half on matstone. Founded on a soft mud botton the half on a matstone foundation (0.3rn thick) sank by about 0.15n while the other half showed no appreciable change. With no obvious displacement of the armouring

t2 during the nonltoring period about 0.3n of accretion took place in lts lee. Apparently the structure functLoned effectively, but at a eost (in 1979) of nearly $700 per metre it could not be considered low cost. Thts project demonstrates that not only the stabillty of the armouring needs careful design but that an assessment of the foundation material is equally lnportant. At present, design fornation is based largely on the result of hydraulic nodel tests of structures subjected to regular naves. llave conditions can vary widely fron site to site and clearly the energy distributlon needs to be reproduced correctly. The use of nodel random waves, representing reallstieally, the sea conditions at a specific site, can give much more accuracy in the test results. In a paper on randon wave physical nodel studies of low crest breakwaters, Allsop (Ref 1) nakes the followLng conclusions. The rate of overtopping is strongly dependent upon the significant wave height at the site, less so on the height of the breakwater above static water level. The degree of wave transmission is strongly dependent on the incident wave conditions, particularly the mean sea steepness. It is thus very important to model wave conditions very carefully when designing shore protective breakwaters.

13 3 PLOATING BREAKWATERS It has been demonstrated that offshore breakwaters in the tK are likely to be substantial structures. A cheaper way of reducing wave activlty ln special circumstances is by the use of floatlng breakwaters. These are tethered to the sea bed but are allowed to rise and fal1 with the tlde. Ilowever, unless they are extrenely large, floating breakwaters wlll only filter out high frequency waves, le those that have the least amount of energy.

Floatlng breakwaters or pontoons are used prinarlly to protect snall craft in harbours or other enclosed bodies of water, where the wind fetch and hence wave actlvity ls small. They can however be economical when water depths are large but where wave action is linited, ie sea lochs, deep estuaries, etc.

There are linits at which wave attenuation becomes insigntficant and at which polnt the structure begins to ride the waves. This is to some extent dependent on the size of the structure and a consensus of opinion suggests that a wave period of about 5 seconds is the upper limit for existing designs.

Although this type of breakwater ls used to suppress wave action in a nlld sea, it will still have to be able to withstand the worst conditions that are llke1y to occur, hence the anchorage and structural strength must be designed accordingly. It is therefore lnevitable that such a breakwater would be structurally overdesigned under nornal working conditions. As with all breakwaLers the design of a floating breakwater ls "slte-speciflc". Hales (Ref 19) suggests that waves attenuated by a floating structure do not usually exceed 1.2n with wave periods of 4 seconds or 1ess.

In order to achieve significant nave attenuat.ion, a floatlng breakwater will need to have a bean width of the same magnitude as the incident wave length and will need to have a submerged depth of the same magnltude as the incident wave height. This means that for a storm wlth signlficant wave heights of say 3 rnetres and zero crossing periods of 10 seconds the structure may have to be 3 metres deep and up to 100 metres wlde lf placed in l-0 metres of water. To be efflcient it would be massive and lnordlnately expensive to lnstall while the mooring forces would probably be so large as to pose serious design problens. It would also be well outside the low cost category.

Floating breakwaters can be constructed out of many forms of buoyant material such as polystyrene foam,

I4 foan filled rubber tyres, oil drums, hollow steel or concrete modules. I,ilith proper anchorage they can be used in deep or shallow lrater, in areas of sllty, sandy or roeky sea bed topography. Their greatest disadvantage ls the gradual loss of buoyancy with time as a result of siltatlon, marine grohrth, structural damage etc. They also tend to collect debris which also leads to a loss of buoyancy as well as rnaking then generally unsightly. Their greatest use is as protection in marinas and harbours where there are facllities to inspect, remove and repair them as and when necessary. A wide variety of floating breakwaters constructed of scrap tyres were revLewed in the first of rhls series of reporrs (Ref 40). For thls reason only a sma1l selectLon of these are included in the summary sheets found in Appendix 1. Included here also are floatLng caisson breakwaters, a few of which have been tested ln the UK.

It has to be stated at the outset that we consider such structures as an unproven means of coastal defence. Their hydraulic performance has been very variable. I,lhen structural failure begins it tends to be progressive and usually leads to the complete destruction of the system. In only a few of the case histories has there been assessment of their effect on adjacent beaches, usually because such changes were too snall to be measurable.

Anchorage 1s probably the most important aspect wlt.h all types of floatlng structure and will depend on the peak mooring eonditions, nater depth and type of sea bed encountered at the site. The breakwater is usually connected by wlre cable or llnk chaln Lo the anchor. Two of the most commonare the deadwetght anchor (usually a large concrete block) and the pile anchor (which can be expensive). Ordinary ship anchors and screlr anchors are sometioes used although they are lirnited to shallow depths and are difficult to install effectively in a firn sea bed.

Whichever type of anchorage system is used it must be substantial enough to prevent drag and can cost more than the structure it is anchoring.

A case in polnt rdas a floating tyre breakwater installed at Guilford, Connecticut. The maximumfetch here was sone 56kn and the site conditions obviously proved to be too exposed for the anchoring systen. Thts project highlighrs rhe dlfficulties rhat could be encountered if such a structure was sited on the open coast of the UK.

3.1 Structure type The types and designs of floating breakwaters vary widely as do the materials used. Below is a selection

15 of types presently in use or at the design stage.

3.1.1 A-frane rype

This concept comes from Canada where an abundance of tinber has pronpted research lnto designs using this locally available materlal (Ref 59).

(a) Twtn cylinder. Essentially an inverted trlangle suspended by pontoons fixed at each end of the basellne (see Figure 4). The frame itself is alumlnium with hol1ow aluminiun cylinders connected rigidly at elther end. sttting vertlcally through the middle of the triangle is a rigid tlmber wall. A floating breakwater such as this has been in service at Lund, British Columbia for several years.

(b) Four cylinder. A varlation of the above design was investigated by Brebner and Ofuya (Ref 9) in 1968. In their experiment they used two pairs of cylinders, (presumably for added buoyancy) one pair on each side connected either rigidly or by chain.

3.1.2 Flexible membrane

A number of laboratory tests have been conducted on various types of floating membranesor fluid fi1led bags but relatLvely few field tests have been carried out ln sheltered water and it is thought that this type of floatlng breakwater would have linited applicatlon on the open coast.

3.1.3 Hydraulic

This system attenuates the waves by discharging rrater under pressure through a submerged manl"fold in the direction of the lncoming rdaves. This creates a horizontal current and ensures partial or conplete breaking of the naves, dissipating a large part of their energy. Rao (Ref 45) suggests that thls type of breakwater could have inportant applicatlon in deep water naves. It is also thought, Ilerbich, Ziegler and Bowers (Ref 25), that it would be useful ln attenuating lraves near the coast. There is at the moment too 1itt1e field infornation to assess this type of structure for use in open coast situations. Ilowever, the slting of such structures, even if they could be shown to perforn well, would pose severe practical problems on the open coastline.

3.1.4 Pneumatic

Sonewhat sinilar to the hydraulic type, this concept was Patented in 1907. Wave attenuatLon in this case

16 is by the release of compressed alr through a subnerged perforated pipe vertically. Descrlbed by Brasher (Ref B) in 1915 Lr was re-analysed at the request of the Adniralty by Taylor (Ref 5l) tn 1943. Taylor (Ref 52) nodified rhe rheory in 1955 and his work was verifled by Kurihara (Ref 32) in 1958. Sherk (Ref 49) conducted further large scale experiments ln 1960 and lndicated that a nultiple nanifold system uay be more effectlve. One problen however could be that excessively large porf,er requirements may be necessary. Again there ls too little field infornation to assess this system for open coast sltuations.

3.1.5 Pontoon type

This is possibly the simplest form of floating breakwater and can consist of one or more, usually rectangular, pontoons floated onto the water, ballasted, Joined together and anchored. There are several different configuratlons includlng:

(a) Slngle type: where several are joined end to end.

(b) Double type: rows of caissons connected to float side by side, Joined rigidly to form two lines.

(c) Catanaran: this design consists of rectangular wooden modules (circa 13m long, 3n wide and 2n deep) fastened together to the required length and ballasted wlth concrete beaus. Woodendecking is usually fitted and the pontoon normally anchored by chain to concrete bloeks on the sea bed.

(d) Alaska type: so called because it was developed by the Alaskan Department of Public Works, it is constructed of twl-n pontoons built with light concrete (0.ln thick walls). The pontoons are connected with cross pontoon sections to form an open framework (circa t8m long, 6.5n wide and 1.5n deep). The hollows are filled with polystyrene foam for buoyancy. The pontoon breakwater at Sitka, Alaska is anchored by a combination of piles and concrete block connected by galvanised llnk chain.

(e) Sloping float rype: this forn of breakwater consists of a row of moored flat lnterconnected pontoons. Partial flooding allows their sterns to sink to the sea bed where they are anchored. Freeboard and angle of lncllnation is controlled by flooding a speclfied number of unlts. A nodification to this design is the addition of legs at the base to

L7 Lncrease the depth range by llfting the pontoons off the sea bed.

Sone of the above structures, notably the Alaska Eyper have been used with some degree of success ln partly sheltered waters, Ln areas of low to moderate tidal range.

3.1.6 Porous wa1l type

A perforated portable structure nas tested by Jarlan (Ref 27) in 1960. This idea whlch has only been tested, as far as is known, in the laboratory, consists of a rlgid steel plate fronted by a perforated plate. The incoming waves lnplnge on the front wa1l reflecting part of their energy and allowing the remainder t.o pass through the perforations. The breakwater being speciflcally designed to both dissipate and reflect incldent wave energy.

A variation of this concept is a floating breakwater consistlng of a horizontal array of open tubes.

3.L.7 Scrap Tyres

In general, structures incorporating scrap tyres can be used successfully to forn floating breakwaters where their low cost is a positive asset. Where they are used ln structures connected to the seabed or to the beach the flexing of indlvidual tyres and the deformation of the structure as a whole can cause problems with tyre to tyre connections and with anchorage. Such problens will be greatly nagnified if the structures are built withln the zone of wave breaking.

A number of fixed breakwaters rdere tested in the American programme, use being made of car and lorry tyres strung onto horizontal poles or stacked vertlcally over piles, see summarysheets. They performed reasonably well resulting ln some accretion in thelr lee. At FonEainbleau State Park, Lake Pontchartrain, Louisiana the tyre/tinber pile breakwater rras located only 15 metres from the shorellne at a depth of 0.3 metres below mean ti-de level. An incipient tonbolo began to develop and it was consldered that further accretlon could interrupt littoral transport, to the detriment of the downdrift shoreline. This like other similar structures had problens with tyre movement despite the sheltered nature of the site. Only one month after construction the tyres began fo sink into the sandy bed of the 1ake. The design wave height for this type of structure has been estimated at being less than 0.6 metres.

18 There appears to be llttle scope for using discarded tyres as offshore breakwaters in a harsh wave environment. Such structures have great permeabiltty and very large mounds of tyres would be needed to achieve a significant degree of wave damping. If the tyres were subjected to breaking wave forces then the tyre to tyre connections would have to be nade such as to prevent parting while at the same time the problerns of chafing would also need to be resolved. SLnce the tyres are pracfically neutrally buoyant, very strong anchorage would also be needed to keep the structure firnly flxed to the seabed.

A pilot scheme for reducing cliff and foreshore eroslon is taklng place currently on the Holderness coastline. This involves the use of scrap tyres filled with concrete and laid up to three tiers deep. Whtle we have reservatlons about schemes uslng tyres for halting or reducing shoreline recession we wlll be taking a great lnterest in lts performance.

Although there are nany combinations of tyre breakwater design they fall into four main categories:

(a) Goodyear type (Figure 5). The Goodyear Tyre and Rubber Conpany lnitiated research in 1974 into using old tyres as floating breakwaters. This research, Candle and Flscher (Ref 10), led to thern patentlng a modular building block design. This structure consists of a number of modules, each of 18 tyres, bound or bolted together to form an interlocked structure (eirca 2n by 2m and 0.75n deep). Transported like this to slte they are then assenbled as necessary lnto a pre-arranged pattern. Unwelded open llnk 13mn chain has been found to be best suited to thls type of construction while connecting materials vary as widely as heavy steel chain and conveyor belting. Prototype scale mooring llne forces rrere tested by Giles and Sorensen (Ref 17) in 1978 and Harms (Ref 20) in L979.

(b) Kowalskl type. A sinple nar-type floating breakwater nas tested by Kowalski (Ref 31) in L974, to determine the effectiveness of the breakwater in suppressLng waves and to lnvestigate the construction problems and durability of such a structure. The experiments concluded that a three-tyre deep nat had a wave suppression efficlency of. 7Q%. Ilowever, this was in waves with a significant height of 0.76n and a spectral peak density of the order of 2 seconds, not the conditions one would encounter on the open coast even in relatlvely caln weather.

19 (e) Wave guard type (Fig 7). Developed by Harrns and Bender (Ref 21) in 1978 thls wave guard or plpe-tyre structure consists of large logs such as telephone poles, and steel or relnforced concrete beams, onto which the scrap tyres are threaded. Each beam or 1og is interconnected longltudlnally by strlps of conveyor belting and the tyres are closely spaced to ensure a high spatial denslty, thus resulting in a snaller structure ln plan to attaln slnilar attenuatlon as the prevlous devLces. l'lodel tests ln fact showed that this type of structure offers a signlficantly greater degree of wave attenuatlon than the nore "porous" Goodyear concept. Anchorage, according to llarms and Bender, should include a tyre mooring damper (at least 5 tyres) at the breakwater end with an open f.ink, low carbon, 13nn dia anchor chain near the botron.

(d) Wave-mazetype (Ftgure 6). Patented by Stiff and Noble ln the early 1960rs thls design is usually made up of lorry tyres fllled wlth polystyrene to increase buoyancy. Design ls as shown ln Figure 6 wlth a top and bottom layer of horizontal tyres enclosing a layer of vertical tyres ln the centre in triangular fashlon. Noble (Ref 42) suggests that the width of the breakwater should be at least half of the length of Lhe waves to be attenuated. If wave heights exceed about 1.2n then addltlonal tiers should be added so that breakwater depth exceeds the wave height. The tyres are bolted together uslng conveyor beltlng as reinforclng washers.

Conveyor beltlng is used extensively ln this type of breakwater constructLon. It is highly abrasion resistant and lnert Ln sea rrater.

3.f.8 Tethered float type

Conceived initially, it ls thought, at the Scripps Institute of Oceanography, thls design consists of spherical floats connected to triangular modules of reinforced concrete (equllateral 6rn sided triangles) whlch are jolned together with flexible connectors at thelr apices to form mats about 30m wlde. Several mats can be Joined together to form the breakwater. These modules, some 0.5n thlck, are tethered to a franework resting on the sea bed which can be ballasted to prevent movement.

Ilales (Ref 19) presents thls and rnany other forns of floatlng breakwaters in more detail in his 1981 review.

20 3.1.9 Turbulence generator

This type of floating breakwater of whlch there are several designs, takes the form of a flat relatlvely thln surface floating franework.

(a) Seabreaker. Described by ltasler (Ref 24) it is a long (40n) rtgid ponroon of specialised design. Tested ln Stokes Bay ln rhe Solenr in 1971 tr remained on station for 2 years. Hasler contends that the structure attained up to 602 reductl,on at maximum design wave helght wtthln a maximum feteh of 5krn.

(b) Harris type. Developed for use at the port of Le Havre and described by llarris and Webber (Ref 23) /l the prototype breakwater was required to be effective in seas of up to 10n. A scale model one I tenth of the size of the prototype was built and lnstalled in Stokes Bay (around L97O) where wave heights of ln can be expected. Design consisted of two flat boons connected at intervals wlth cross members to form a rectangular surface floating plate with slots. Flve of these 9.6 by 10m units were Joined end-to-end to forn what looked like a giant ladder in p1an. Mooring rras by chaln at each corner of the ladder, supplernented by nylon lines to nullify shock loads. Each 50n long chain was connected to an anchor on the sea bed.

Performance, described by Harris and Thomas (Ref 22) 7n 1974, vas measured Ln terms of percentage energy reduction. The primary function of the breakwater rdas to inhibit the vertical component of orbital notLon wLth wave breaking and eddy format.l,on and its secondary purpose.

Breakwaters of this type have been used in the UK with varying degrees of success. It would seem that the structural problens associated with this type of deslgn have not as yet been entirely ellninated.

3.1.10 Twin cylinder type

This type of breakwater falls into two categorles.

(a) Twin-log type. In L964, Jackson (Ref 26) conducted a series of experiments to determine the most. eeononical system to attenuate O.6m waves to a height of about 0.15n. The tests were conducted Ln water depths ranging from 0.6n to 8m, with a maximumtidal range of 6.5n. Several tests were done and the effect of changing the angle of wave attack, lrater depth and wave period were checked

2L in a concrete flune. The nodel structure sinulated a twin-log floating breakwater made up of 1.2n diameter logs spaced at 1.7m centres. The flotation depth of the structure was 1.05n. The tests concluded that for an incident wave height of 0.6m and a perlod of 2 seconds the structure would provide sufficient protection for a small boat basin.

(b) Twin cylinder type. A variation on the above theme using two hollow cylinders (1.83n dia) connected rlgidly at intervals by structural supports was tested by Ofuya (Ref 43) in 1968 in the laboratory. Flotatlon effects were varied by fllling or partially fllling the cyllnders with water.

3 .1 .11 l.Iave barrler

Essentially a series of upright cylinders with counter weights beneath to hold them at the surface. Developed by Bowley (Ref 6) in L974 laboratory test s showed that buoy type systems such as thls can be deployed to attenuate waves.

Summary: As can be seen from the above the nunber and types of floating breakwaters are many and varied. They all however have the sane obJective, that is to obtaln danplng of high frequency naves. None of the designs would appear to have potential as a means of coastal defence, due to anchorage problems and low rrave attenuatl-on at high water heights.

It is fairly obvious that with the possible exception of the "Harris type" turbulence generator, none of these floating breakwaters would last very long in the harsh environment of the open coast of the UK. In the case of the "Harris type" turbulence generator it is not known whether the structure nas actually siEed.at Le Havre or, lf it was, how it fared.

Floatlng breakwaters are by no means maLntenance free and can qulckly collect flotsam. Therefore unless they are aceessible as they would be in sheltered waters they could become a malntenance llability.

Finally, adequate anchorage as mentioned above can be a problem and ln open waters could be lnsurmountable.

22 SILLS AT{DPERCEED BEACHES Stlls are essentially 1ow crested breakwaters constructed in the inter-tidal zone. Usually plaeed parallel to the shorellne they are used to protect the upper part of the beach, thelr function being to dissipate nave energy either by tripping the waves or through turbulence, and to promote the accretion of beach material. They have to be carefully designed so as not to cause scour of both the beach to seanard, by wave reflection, and to landward as a result of wave overtopping. These problens also occur with breakwaters, but to a lesser degree since they are sltuated in deeper water. Si1ls are generally free standing structures although this is not, always the case. A wave tripping device has been incorporated ln the toe bean of a sea wall in East Wear Bay, Kent. Shownln Plate 3, the idea ls for the sill to trap sand at the toe of t,he wall, and build up the beach. In actual fact although beaeh scour here has not been entirely eliminated it is clearly less serious than lf the sllls had not been used.

Sills are commonly used to protect an existing beach or one that has been bullt up by artlficial re-nourishment. In the latter case, t.he s111 and ral-sed beach is called a "perched beach system". The s111 can be constructed from a variety of materials but to hold a perched beach it has to be inpermeable to prevent the washlng out of the fill naterial. Materials such as quarry stone can be used but would require a filter cloth to be laid between the landward face of rhe sill and the beach f111.

At low water the sills are stranded on the foreshore and retaln the material on the upper beach, preventing lt fron being carried seaward in suspension via t.he dralnage runnels. It Ls not advlsable to construct high sills, this could result ln pondlng ln thelr lee, givlng rise to stagnant rf,ater conditions.

Screens or sills need to be designed carefully with regard to their position wlthin the inter-tidal zone. If they are too successful in trapping material they can build up an q.r_gof beach which can act as a barrier to llttoral drift. The alternative is that too often sills are placed too far offshore with the result that they produce no signlficant effect. More often than not ln the American programme sllls were bullt so low that they had ltttle trapping capacity and thus could not raise the beach sufficiently to reduce wave action at the back of the beach at high tide.

The deslgn of sl1ls varies greatly, ranging from slnple brushwood screens to couplex pre-cast concrete

23 units. The latter are patented st.ructures with seemingly somewhat exaggerated claims nade as to their ability to build up beaches. In the American programme, a number of these pre-cast units have been installed Ln areas of low tidal range and wirh moderate to low water activity. They performed lrith some degree of success but rarely up to thelr speciflcations.

In the American programme (Ref 55) the height of the sl1ls was one metre or so, not much less than the tidal range in some instances. When the sills were backfilled wlth sand because of thelr modest height the cost of nourishnent did not prove to be prohibltive. Sills in conjunctl-on wtth sand ft11 (perched beaches) can certainly lnprove the amenity value of the beach although because of their low height rarely dld they give protection to the defences at the top of the beach. They have not been tested in the conditlons typical of the UK coastline. Because of the large tidal range here the cost of such structures would be prohibitive. For example, a structure built ln a tidal range of 5n would require to be massive to produce rdave attenuation at high nater. Clearly nodel testing is requLred t,o optimise dinensions and deternlne its efficiency wlth respect to its position on the foreshore.

The Anerican programme of low cost demonstratlon devices was designed to a large extent with the private property olrner ln nind. They therefore would not expect a large degree of vandalism. This nust be borne in mind especlally with sills, whLch are usually placed in the inter-tidal zone and thus well withln reach of vandals.

4.1 Structure Lype 4.1.1 BeachPrisus

These are pre-cast reinforced concrete blocks triangular in cross-section and forned with recesses to allow rdater to pass through vertical slots. A line of prisms is held together with pre-stressed tie bars running through the apices of the structure.

Clains about their 1ikely performance can be found in the design brochure produced by the manufacturers. This states that under the rtght hydrodynanic condit,ions the system can rapidly butld up a beach and cover itself (with sand) in one or t\ro months. This breakwater design lras not tested in the American programme. The manufacturerst evaluation would appear to be somewhat optinistlc in vLew of the performanee of devices such as the surgebreaker which work on much the same design prlnciple.

24 Because the cross-sectlonal shape is an equilateral triangle the unlt is lnherently stable. The reinforeement running through the system would nake it difftcult for the structure to tr{lst or undergo differential settlement. The design sizes vary, but units can be up to 1.5 netres high wlth a weight of 435kg. At thls slze some 8 unl,ts would be needed for a st.ructure 3 metres long.

Transport of the pre-assembled prlsns by land or sea is usually by encloslng the two upper faces and ends with alrtight rubberised fabrie. This 1s then connected to a rnobile alr compressor whlch pressurises the cavities and the whole assembly moves llke a hovercraft. Alternatively they can be placed by tractor (at low water) or flat barge (at high water).

Thls type of unit may also be useful ln front of seawalls. Due to the action of storm or steep naves, beaches frontlng seawalls often recede. After storms sand whlch has been moved offshore returns to the beach areas. In calmer weather these concrete prisus, placed in a suitable position night well help tn restoring the upper beach.

4.L.2 Sandisle (a sand filled nenbrane) structure

A fairly nelr concept in offshore protection, a sand lsle ls basically a water tight membranewhich is taken to the sLte and firstly filled with water to glve it shape. A layer of coarse gravel ls next laid in the botton and the bag ftlled with sand. tle1l casings are lowered to the bottom and submersible pumps lnstalled at the botton of each. As the sand is punped in, the subnersible pumps draw out the water simultaneously. These structures have been tested on the south coast of the UK in water 14.3n deep.

Research is also golng on to Eest low cost methods of cementing the sand during or after constructlon, to convert lt into moderat.e strength "sandstone" whlch does not rely on hydrostatic pressures on the wall casing for stabillty. Sand isles ean also be connected together by walkways to provlde a nultl-cell structure.

The particular structure mentioned above and tested in Christchurch Bay suffered an lmperfect fabric seal and a serles of storms 1ed to ifs dest.ruction after one month.

4.1.3 Caisson structures

One caisson structure reviewed in this report was at Kltts llunmock, Delaware Bay. A line of concrete boxes

25 were laid on the seabed about 180n seawards of low water. Each concrete box was then fllled with sand. Not surprisingly sand was washed out of the boxes by wave actlon. Some settlement of individual units also took place.

Capplng the boxes rdlth a lean sand-cement mix mlght help such structures to survive the nild hydrodynarnic environment ln whlch they are placed. This type of construction would stand very little chance of success on the open coastllne, where any ftll, whether it be sand or shingle would be quickly washed out. Iilave reflection from the vertical concrete faces would lnduce scour and breaking waves could overturn the boxes unless they were of very large dl-mensions.

On the East coast of England at Dengie in Essex, plans are golng ahead to install concrete caissons seaward of an eroding saltings area. These purpose-made unlts will be placed at 20m spaclng and filled with silt. The tops will then be capped with weak concrete.

4.I.4 Fagotting

Brushwood fagotting has been used extensively, and with considerable success to reduce erosion of estuary banks. It has also been used on more open parts of the coastline, for example on the Dengie Peninsula, Essex. The salt marshes on Dengie Flat.s are eroding and wave energy is no longer disslpated by the marsh vegetation, allowing waves to overtop the embankments breachlng them in places. To regenerate the saltings Anglian l,later have enelosed areas of Ehe upper foreshore within a system of brushwood encloaures. These have tended to trap sllt and nud, but drainage channels have had to be cut to allow sediment borne in suspension to be spread evenly over the area.

Fagottlng was also used ln the American programme to form an offshore breakrrater at Fontainbleau State Park, Lake Pontchartrain, Louisiana. The so called dyke breakwater conslsted of two lines of brushwood posltioned seawards of the edge of the saltmarsh. The system is said to have performed well until the brushwood was washed out by wave action. This type of protection would have little success on an open coastline and indeed it is not designed for this purPose.

4.I.5 Longard tubes

These are a two-ply forn of flexible tube which are fllled by pumping ln a sand slurry, the water being drained off by means of a valve. The outer easing consists of high denslty polypropylene woven fabric whlch is sometimes coated for prot.ection with

26 sand-epoxy resln. The inner tube is an impermeable low denslty polyethylene filn. Longard tubes can be of substantial dinensionsr up to 1.75 metres dianeter and 60 metres in length. The tubes have been used successfully to build up sand beaches or to retain sand f111 and this is probably attrlbutable to their large dimensions.

A 61 netre long Longard tube was placed 61 metres offshore and in about I metre water depth at Basln Bayou, Florida, see strmnary sheet No 7. Before it was vandalised the breakwater caused a sand spit to form in lts lee. It was considered to be too close to the shoreline and there were fears that further accretlon night result in the formation of a tombolo, thereby starving downdrift beaches of sand. The Longard tube was vandalised some 8 months after installation and was replaced with a new one clad Ln aluninlum sheetlng. This proved unsuccessful and the tube and sheeting rrere removed because they had become a hazard to bathers. Thls type of structure has an estLmated deslgn wave heighr lintr of 1.5n (Ref 55). Ir is clearly not suited for open coast conditions.

4.I.6 Gabions

These are rectangular or mattressrrtype baskets filled with rock or large pebbles. The most commont,ype, Maccaferri gabions, consist of pVC coated galvani,sed wire which Ls woven into a hexagonal nesh. There are also gabions on the market whlch are constructed of galvanised wlres electrically welded into a rectangular mesh. Gabions made of tough plastic mesh are also available.

Revet.ments made of gabions have been widely used in this country to protect the upper parts of beaches. They are particularly effective on sandy upper foreshores where they tend to become covered with wind blown sand. When placed on the upper part of the tidal zone they are less affected by waves which would otherwl-se cause movement of the fill, settlement and abrasion of the wlres.

Where they have been used on the lower foreshore they have been subject to a htgh degree of wear and tear as well as corrosl,on (Ref 58). Plastic gabions have not as far as rre know been used ln the inter-tldal zone and are clearly not sulted for situations where they are exposed to direct wave attack. A11 the gabions can become damaged and the plastic gablons are very easily vandalised.

A gabion breakwater nas tested in the American progranme at Geneva State Park, Lake Erie. Built sone 18 netres fron the shoreline, it was approximately

27 1.8n high and stood in about 0.8n of rrater, see summary sheet No 5. In this area rrave heights of 1.3n are given as the maximun with a 0.5n wind set up occurring annually and increasing the depth of water in storm conditions. During the first year the structure trapped material ln lts lee, despite extensive damage. Ilowever, four months later the east end had become very badly danaged and much of the accretlon behind it had disappeared. Waves up to 1.3n hlgh were severe enough to have broken most of the baskets at the toe of the structure and the stone fill was then washed anay. Scour also resulted in some deformatlon of the structure.

A Guide for Engineers and Contractors has been produced by the US Arny Corps of EngLneers, summarising the infornatlon gained from the American programme. In thls report that the "wave height range" for this type of atructure is less than 1.5m (Ref 55). Clearly gabions are not suited as offshore breakwaters because of their short design llfe in a marine environment.

4.1.7 Sand bags

Sand-ftlled bags of varlous types and sizes have been test,ed as coastal defences ln the American programme. Sunmary sheets (in Appendix 1) describe their performance at, two such sites. The bags make for easy construction as offshore breakwaters but are not very resistant to damage by wave borne debris and are easily vandallsed. The design rrave height for such structures has been estimat.ed as being less than 1.5 metres and thus they can only be of use in sheltered rdaters .

Hessian or woven nylon bags can also be used, being filled with either sand or a sand/cement mortar and stacked in pyranid fashion, with rows of bags staggered for added stability. Typically, sills conslst of three tlers of bags with three forming the base, two the niddle layer and one bag forming the crest,.

From a structural vlewpoint both sand and mortar fllled bags act ln much the same manner. They need to abut well to each other to prevent wave penetration and scour of the beaeh. Light bags (weightng up to about 45kg) are easlly dlsplaced by wave action and hence larger bags are generally recommended in the Ameriean programme though these may be harder to fill and place. Belng relatively impermeable such structures are subJect to scour. The summary sheets ln Appendix 1 show that even where they build up beach levels or manage to retain an artificially nourished

28 beach the degree of protectlon ls insufflclent to prevent erosion of the upper beach.

Nylon sand bags are flLmsy and are llable to chenical- degradation under the actlon of UV ltght. Even when Ehey are not vandallsed Lheir design llfe is not likely t,o exceed one or two years. If thls type of bag is envisaged then a sand-cement nix should be used as f111, then tf the bag deteriorates the f111 lrill stand on lts own. It has been estimated that the maximumdeslgn wave height for sand bag sllls ls 1.5 metres or less.

Their performance has been described Ln an earller report (Ref 4f). Sufflce lt to say that we consider they have 1ltt1e potential as a means of coastal defence in the UK though they nay be useful as short term prot,ectlon. They are easy to install and quick to fill so they are useful for plugging temporary breachesr Saps ln sand dunes etc.

4.1.8 Sheet pile

Stlls can be constructed by simply driving in rows of tinber, concrete or sheet steel piles parallel to the shoreline. Such systems however, have dlstinct drawbacks when subjected to wave actlon. The scour is llkely to be of the same order of magnltude as the maxlmumincldent wave height (Ref 55). Such structures would also be extremely dangerous from an amenity point of view. The followlng example of a wooden sheet pile stll used ln the Anerican programme illustrates that thls forn of protectlon can nevertheless be effective ln nlld hydrodynanic conditLons.

At Slaughter Beach, Delaware a 100 metre long sill was formed of treated tinber planking (see spmmary sheet No 22). The planks were connected ln Longue and groove fashlon and each driven into the estuary bed to a depth of 2 netres. In the rnlld estuarial wave condlt,ions the ttnber piling retained the perched beach during the 9 nonth monltorLng period. There was no scour Ln front of the plllng and Lhere ltas no tendency for the wood to "float out". The performance was assessed as belng excellent "for low sl1ls in nild wave cllmates".

Conditions ln the United Kingdon are rarely conducLve to Ehe use of vertlcal faced structures situated ln shallow narer and subject to breaklng waves. Toe problems (scour, structural abrasion, loss of f111 through underminlng etc) are only too often encountered when seawalls are placed in shallow water and subJecE to wave attack. The use of vertical sills under euch conditions should be avolded.

29 4.1 .9 Sandgrabbers

These are patented prefabricated concrete units composed of hollow rectangular concrete blocks, somewhat slnllar to but larger than cellular buildlng blocks. They are placed in brickwork fashion wlth the hollows horizontal and are then tied together with U shaped steel rods threaded through the hollows. In prlnclple, rdaves carrylng thelr load of entralned sand pass through the blocks and in dolng so lose sufflctent energy to deposlt sand in their lee. As the openlngs ln the lower courses are filled in, wavea deposit sand on the seaward side.

The structures are certainly able to trap sand and enhance accretion, as can be seen from the fleld installatLons descrtbed l-n summarysheet No 26. At Basln Bayon, Florida, for example a 1.6m wide, 0.9n high, 73m long sandgrabber was conatructed ln shallow water with its crest Just above hlgh water level. In the flrst few months after construction 1t bullt up sand by jefttng it through the rectangular holes. Sand also accreted on the seaward side and then littoral transport seaward of the structure was re-established. In other field installations saEuration conditlons \rere not reached and the structures began to cause downdrlft erosion problens.

The current design does not incorporate any toe protect,ion and as a result there is likely to be uneven settlement. In a number of the installations, movementwas sufficiently large Eo pull the cable ties against the blocks hard enough to damage then. Progressive damage of thls klnd can ultinately lead to conplete collapse of the structure. It has been estlmated that the design wave height ls less the'n 1.5 metres. Scaling up such structures by slnply adding further tiers of units is noL feasible. The stresses inposed upon indlvidual blocks would be greater than in the present deslgn and hence the strength of the structure would be considerably reduced. The present unlt size is therefore not sultable for UK wave and tidal eonditions. Increasing the slze of lndividual blocks would allow a more robust design, especLally as the concrete compresslve strength could be lnproved. The stability of an increased Btructure slze under severe wave action would need to be tested for struclural strength and hydraulic performance.

4.1.10 Sta-pods

These are pre-cast concrete unlts (see Fig 2) which consist of four lnclined legs supportlng a cyllndrical trunk. They are about 1.7m high with a leg "spread" oE 2.7m, and weigh about 2000kg. They have not been tested ln this country.

30 A 29 netre long Sta-pod breakwater was built at Geneva State Park, Ohio as part of the Anerican programme. The units were placed in position by crane, about lg metres fron the shore and in about 0.9n of water, see summary sheet No 11. Although the units were laid out so as to overlap slightly the 0.6n dianeter trunks could not be placed closer than at 1.2m centres. Because of the high degree of perrneability they were not effective in screening out nave energy. The Sta-pods remained stable and undamagedduring the monltoring perlod despite some severe lrave activity.

The units would requlre some radLeal deslgn changes to make then more effectl,ve. They could perhaps be laid out ln parallel lines in order to reduce perneability. At present however they appear to have little potential for use ln UK wave and tidal condltions.

4.1.11 Surgebreakers

These are patented reinforced concrete modules which are triangular in sectlon. They have large vent holes to prevent the build up of nave pressures. Each unit weighs about 1700kg, is 2.lu long, 2.4m wide at the base and 1.2n high. These systems were not monltored over a long enough period in the AmerLcan programme to evaluate their performance. During the six rnonth period of nonltoring following Lts construction the adJacent beach showed little change.

At Basin Bayon where these were installed, the structures remained intaet with no observable change in alignment or any leaning of the units due to toe scour being apparent. It has been estimated that the maximumdesign wave helght is in excess of 1.5 metres. However, the structures rdere not tested to the llnit so the actual design wave height nay be considerably larger. The units were Lnstalled at some dlstance from the shorellne and because of their weight had to be airlifted into place. Increasing the size of the units to accommodateUK wave conditions and the larger ttdal range would pose problens in terms of lnstallatlon. The cost of construction and installat,lon would also be very hlgh. With a question mark as to thelr likely performance even in sheltered waters lt is difflcult to see their potential in UK condltions.

4.I.LZ Tinber

At Penhryn Bay on the North i.Iales coast an open tinber revetment was installed in 1980 sone 30n offshore and parallel to the existing stepped mass concrete sea wal1. It was shore connected at each side by conventional permeable vertical groynes, making it in effect a perched beach system. A wedge of rockfill,

31 of 250nn minimum size was placed against the tinber structure on its landward slde to absorb nost of the inconing nave energy. Inside this enclosure the upper beach was filled with smaller, well-graded granular material. The top of the tinber revetment is at MHWN, thus the structure is lmmersed at htgh tlde. A vislt in July 1986 showed thaE the beach level within the perched system had fallen sufficlently to expose the eheet steel- pile at the toe of the sea wall. It would appear that the permeable revetment has allowed the finer naterial to leach out while retaining the naJority of the larger boulders.

32 Ef,VIRONMENTAL ASPECTSOF DESIGN The hydraulic and structural aspects of design of breakwaters has been well documented and we would refer the reader to a recent report by Brampton and Smallnan (Ref 7).

Rat.her surprisLngly it has been found that breakwaters can accumulate beach materlal on the seaward slde as well as ln thelr lee. The amount of accretion is generally linired but it does add to the srability of both the beach and the structure ltself. The Rhos on Sea breakwater described earlier, accumulated a 3 to 4 metre sand berm along i.ts seaward toe within months of its constructLon. On the landward side the shingle beach increased by about 2 metres over a period of 4 years following constructi.on. The amount of landward accuuulation is now becoming a problern ln that boats can no longer moor so easily wlthin its shelter. A form of shlngle tombolo has now formed between the west end of the breakwater and the shoreline. It is ttkely, therefore that in the long term downdrift beaches w111 be starved of thelr littoral supply of shingle. Thts in fact may already be taklng place with erosion appearing to be on the increase in Colwyn Bay to the east of this breakwater.

Sills have been widely used in the USA wlth some degree of success on semi sheltered shorelines. Their purpose can be elther to retain an artlflcially nourished beach or aa a wave tripping device designed to trap suspended sediment. Usually no greater than 1-2 metres high, the expected increase ln beach levels to landward woul-d under ideal conditions be of the same order of magnitude providing an adequaEe supply of littoral materlal was available. Ilowever, Judgenent needs to be made as to where on the beach a si1l is placed. Too high up and it ls only reached by the tide for a linited perlod, too far offshore and it will give little accretion at the high water mark. Positloning is very dependent on the conditlons at a particular site and beach monitoring or model studies are a necessary part of the design.

Because of the severe wave cll.mate and large ttdal range experienced around most of the UK eoastline, silIs are rarely used. It is really only ln sheltered waters where waves are small that one can build a structure that can be cost effective. Mud flats and saltlngs have been successfully protected in this way by brushwood fencing but on the open coast the same degree of protection would require very large structures which would effectively be offshore breakwaters.

33 Reference 55 gives the following recommendations for sheltered rraters:

(a) use fixed breakwaters for shore protection only where the offshore slope is relatlvely flat and tidaL range is snall. If the water ls too deep at a distance of 60n or more offshore, consLder using a revetment at the shoreline or a floating breakwater instead;

(b) where the depth 60n offshore exceeds about lm and wave periods are short, a floating tyre breakwater may be more economical than a fixed one;

(c) use flxed or floatlng breakwalers where vegetation is to be cultivated as a shore stabilisatlon measure. Deslgn the structure adequacely to allow the vegetacion to become well established;

34 CONCLI'SIONSAND RECOU}IETIDATIONS 1. This report revlews the performance of structures whose prlnary purpose is to reduce wave energy at the shoreline and whose secondary purpose is to build up beach levels in their lee. Most of the breakwaters and s111s which have been reviewed were built in semL sheltered naters and in areas of low tidal range. They were all of nodest proportions being typlcally 1-2 metres high and placed in about 1 metre of water at high tide. Extrapolation of such designs to UK coastal eonditions, ie a high ltave energy clinate and a large tidal range, is theref ore subject,lve .

2. The najority of flxed gravity breakwaters around our coasts are very large and expenslve and even so they provide only partial protection. For example, on the Wirral coast an offshore breakwater which is sone 50 metres wide at the base and 6 metres or so high provides protectl-on, in conjunction with on-shore defences, against wave attack. Siroilarly at Rhos-on-Sea an offshore breakwater protects an existing sea wall against flooding. It is our opinion that cost will inhibir the bullding of such structures to provide protection for open beaches although they will probably continue to be used as additional defence in problem areas. To provide protectlon through the tidal cycle they have to be quite massive. The longitudinal breakwater, a commonsight off the Spanish and Italian coasts, can really only be economically viable in areas of low tidal range. Offshore breakwaters are normally const,ructed seawards of the 1ow water line and reduce rilave energy by breaking and by reflectlon. The energy which is transmltted shorewards is redistributed by diffraction. Experience has shown that a certain degree of wave transmission helps to malntaln an exchange of water between the beach and offshore. This is parttcularly important ln the case of amenity beaches and in such cases design requires some form of hydraulic modelling.

Tombolos can and often do develop 1n the wave diffraction zone. The siEing of such structures therefore requires site speciflc studies to ensure that accretion 1s not excessive and does not result in unacceptable erosion of the downdrift coast.

3. A" recent years some very large breakwaters have been construct,ed ln the IlK. While many do not fit into the low cost category their performance is a guide as to how structures of more modest proportions are llkely to behave. It has been found that even with the nost sophisticated designs the amount of beach build up ls modest and none of

35 the strucLures examined give "total protectlon" to the coast. Clearly, structures which are modest ln slze and which utght flt lnto the low cost, category are likely to provide a smaller degree of proEection and should be considered as a means of beach stablllsatlon, not as replacement for existlng coastal defences.

4. A number of unusual breakwaters have been tested off the Dengle Peninsula, Essex. Thls ls an area of sa1t. marshland subject to low wave conditlons and a moderate tidal range. A ser{es of barges have been placed in a llne on the lower foreshore and lnfllled. The breakwater system is deslgned to halt the eroslon of the edges of the salt marsh. The deslgn is certainly in the low cost category and initlal results are favourable. The adjacent coastllne ls being monltored regularly to see how effective the system ls ln danplng wave attack. Whtle the deslgn ls unllkely Co be sultable for open coast conditlons, it appears to have potential for senl-sheltered conditions.

5. Si1ls are low crested breakwater type atructures whlch are norrually constructed withln the intertidal zone. Their function is to trip lncldent naves, whose energy has already been reduced aa a result of wave attenuat.ion over the inshore sea bed. They are slted wlthin Ehe zone of wave breaking hence Ehe nater which passes over them usually conLains a large suspended sediment load. Effectlve sill designs allow the suspended sediment to settle out and build up inshore beach levels. Sllls constructed of artiflcial armour units have been tested i-n a recent fleld study ln the USA. Under conditlons of 1ow tidal range and low wave activity they have proved Lo be capable of building up nearshore bed levels and causLng some beach accretlon. Scallng up of such unlts to cope with the much larger tidal range in the United Klngdom would pose design problems and the structures would probably not be low cost. Thelr effecttveness in tripping rf,avesand causing local accretlon does however, suggest several possible uses. Many sea walls in this eountry are suffering from falllng beach levels and undermining of foundations. Si1ls of a modest size could be used to protect the toe of the wal1s without necessarily changlng the hydrodynamic conditlons except very locally. On a recent vlslt to the south-east coast of the Unlted Kingdon lt was noted (see Plate 3), that wave tripping devices have actually been lncorporated in the toe beam of a sea wall ln East Wear Bay, Kent. The beach scour in front of the sea wall has not been entirely eliminated but it is

36 clearly far less serious than if sills had not been used.

6. The use of more natural materlals ls also being examined in the fleld, though ln the context of groyne design. In Christchurch Bay a serles of groyne systems have been constructed using rock ttpped directly onto the foreshore. A low technology approach was considered to be approprlate and no attenpts ltere made to grade the rock, provide filter layers etc. l"lonitoring of the groynes has been carrled ouE aE several sltes by the Coast Protection Authority, Christchurch Borough Councll, wLth the co-operatlon of Ilydraullcs Research Linited. The groynes were found to absorb wave energy and helped stabilise an artificially placed shlngle beach. Settlement of these slnply constructed groynes has not been a problem, wlth the cresE being maintained by tipping sma1l additlonal quantitles of rock as and when required. We conslder that a sinilar rlow techr approach may also prove to be possible for breakwaters and sllls though these may be subJect to greaLer nave forces and nay suffer greater seEtlenent. Breakwaters or sl1ls constructed of rock would be nost approprlate in areas when a local supply of material is freely available. We consider that thls form of structure may have considerable potential for halting or slowing down foreshore erosion by reducing inshore wave actlvity. They could therefore be used in areas where the beach ln front of sea walls has been eroded t,o such a degree that groyning is insufficient to rernedy the situation (ie due to lack of littoral drift).

7. Floatlng breakwaters are not sulted for open coasE protection. They obvlously need to be positioned outside the breaker zone to attenuate incoming waves, slnce lrithln the breaker line energy is in any case rapidly dlsslpated by turbulence and bed frlctlon. Their location and dinensions would have to be carefully determined to be effective. Beam width for instance would have to be of the same rnagnit,ude as the lncident wave length, whl-ch for anythlng other than short waves would be lnordinately large. Water depth too 1s a llmiting factor in attenuating longer period rdaves, for example a structure situated ln 10 rnetres of \rrater and subject to a l0 second period wave would requLre a beam width of 90-100 metres. If Ln much deeper naLer the wave length ls proportionally greater and hence the bean width would need to be ln the region of 150-160 metres to obtain the same degree of attenuation.

37 Tidal- range poses logistlcal problens wlth respect to anchorage. With a large tidal range the floatlng breakwater requires a large amount of chain or cable to hold it at the surface at high water. At low tlde thls could result in the structure movLng off statlon t,aking up the slack in t,he anchortng cables.

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58. Welsby J and Motyka J M. "A revlew of novel shore protectlon methods, Vol 3. Gabions." Hydraulics Research Limlted, Report No SR 5, November 1984.

59. l{estern Canada Ilydraulics Laboratorles Limited. "Report on hydraulic ruodel studies of wave damping characteristics of "A" frame pontoon-type floating breakwater." UnnumberedReport, Port Coquitlan, Britlsh Colunbla, Canada, Nov L966.

60. I,IongP P. "Beach evolution between headland breakwaters." Shore and Beach, July 1981.

61. ZwambornJ A, FrommeG A tI and Fitzpatrick J B. "Underwater mound for the protection of Durban's beaches." Coastal Eng Conf, Vol II, 1970, p975.

43 8 GLOSSARY Alongshore - Parallel to and close to t,he shorellne

Beach - The area of sand or shingle from the mean low water line stretching landward to the coast (usually che limit of storm waves)

Coast - A strlp of land extending inlarrd fron the shoreline to the first major change ln t,he terrain

Dune - A hill bank ridge or mound of loose wind blown material, usually sand

Erosion - The removal of beach naterlals by wind wave or tidal action

Fetch - The area of water ln which waves are generated by the wind

Fllter cloth - A synthetic textile with openings that allow the passage of \{at.er but which prevents t,he passage of soil particles

Foreshore - The lntertl,dal area

Gabton - A wire basket, usually PVC coated for marine use, filled with stone

Groyne - A shore protection structure usually constructed perpendicular to the shoreline to trap littoral drift or retard erosion of the shore

Iligh water - The maximum elevation reached by each rlsing tide

Inpermeable - Doesnrt allow passage of appreciable quantltles of sand or waEer

Inrertidal zone - Usually fron mean low water to extreme high water

44 Leo - Littoral environmental observations. Usually daily observations on wind wave and current. conditions at a slte. An American Eerm, shown on the sunmary sheets

Llt,toral drlft - The sedirnentary maEerial moved in the shore zone under Ehe lnfluence of waves and currents

Littoral transport - The movement of lirtoral drift either parallel (alongshore) to the shore or perpendl-cular (onshore-offshore) co it

Llt,toral zone - An indefinlte zone extending seaward fron t.he shoreline to just beyond the breaker zone

Longard tube - A patented two-ply flexible tube. The outer ply is htgh density polypropylene woven fabrlc and the inner ply is an impermeable low density polyethylene film. The tube is filled with sand and comes in 0.25, 1.02 and 1.75n diameters l"lastic - Asphalt applted hot to seal voids in a rubble mound

Nourishment - The process of replenlshing a beach eit,her by naEural or artificial means

Perched beach - A beach or fillet of sand ret.alned above the normal proflle by a submerged dyke or sill

Permeable - Having openings large enough to permit the passage of appreciable quantlties of sand or waEer

Pile - A long section of tinber metal or concrete driven or jetted into the sea bed to serve as a support or protection

Revetment - A facing of stone concreLe etc built to protect an embankment or shore structure against erosion by waves or currents. Usually sloping

45 Riprap - A layer of stones, randomly placed, to prevent erosion or scour of a structure. Also the stone so used

Rubble - Loose angular waEerworn gtones on a beach or rough lrregular rock or concrete fragments

Rubble mound A mound of random-shaped or structure random-placed stones or concrete rubble protected wlth a cover layer of selected sEones or armour units

Sandgrabber - A patented perneable structure of hol1ow concrete blocks slmilar to but larger than commerclal building blocks. Tied together with U-shaped rods the structure ls placed on the beach face and rdaves wash sand through the hollow to build up on its landward side

Screw-anchor - A type of metal anchor, sometimes used with floating breakwaters, that screws into Lhe sea bed

Sheet pile - A pile, generally with a flat cross section, driven into the ground and meshed with like members to form a diaphragm or wa11 stll - A low offshore barrier type structure, designed to retain sand on lts landward side

Sta-pod - A concrete module with a vertical cyllndrical body and four legs. Placed close together they forn a permeable wave barrier

Surgebreaker - A patented breakwater system comprising triangular shaped concrete modules, L22mhigh with a base of 2.44m and 2.13n long. Set end to end in shallow water, openings in the blocks dissipate nave energy ln a profusion of water jets

46 Tonbolo - A bar or spit that connects or "ties" an island or breakwater t,o shore

I,lave-mazesystem - A patented f loating tyre breakwaLer sysEem

Z-waLL - A patented concrete breakwater system eomprising concrete slabs 1.83m high and 4.27n long set on edge ln zLg-zag fashion and joined with large hinge bolts

DDB 5/87

47

Figures

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Fitter ctoth

Typicotsection

Fig 1 Rubbte- mound breokwoter

Fig 2 Sto- pod breokwoter units Rhos-on-seo

Rubbte protection \,/

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Before construction 1980 1985 100m Beoch contoursin metres

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Tyre mooring domper

Plon view

Etevotion

'Wove-guord ' Fig 7 Schemotic f looting tyre breokwoterofter Horms ( Ref 20) 1979 Plates

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1. Aerlal view of Dengle flats showing lighters in positLon Rock breakwater, Rhos on Sea, North Waler 3. Si1l incorporated in sea wa1l design, East Wear Bay, Kent