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Vol. 17 (SP. 1) (51)

THE INDIAN SOCIETY FOR HYDRAULICSJOURNAL OF HYDRAULIC ENGINEERING

PHYSICAL MODEL STUDmS ON STABILITY OF CONCRETE, ARMOUREDBREAKWATERS

by"

Manul, Subba Rao2,Kiran G. SbJrlal3, Prashanth J.4and Balakrishna Rao K.5

ABSTRACT

, As ~e breakwaterconstructionstartedmovingin tQdeeperwaters,the wave load on its armourincreased, resultinginusageofheavierarmourunitsinvitingenvironmentalandlogisticalproblems.Withtheadvancement

in the technology, concrete annour units have been extensively replacing the natural stones with benefits suchas saving in the materia! and cost while reducing loads on the seabed and helping in construction of environment

, etc. Tandembreakwaterandberm breakwatercouldbe the alternativesolutionsfor the abovesaid case.This '

paper presehts the two cases of physical model studies on the stability of concrete armoured structures, onewith tandem breakwater and another on berm breakwater. Both the models are subjected to monochromatic

, waves with varying characteristics.From the study it was observed that in case of tandem breakwater1l1ecrest width of submerged reef and the distance between breakwater and reef are the defming parameter whilein case of be(Ip breal-water the height of the berm and its width are the characteristic factors in reducing the

, breakwaterdamage.

KEYWORDS:, Break)Vater, Submerged reef, Tandem breakwater, Berm width, Berm recession, Bermbreakwaterand damage. '

INTRODUCTION

, ' ' Off late as the breakwater constructionstarted moving into deeper waters, the wave load on armourincrea.sed requiring heavier armour units and it resulted in environmental and logistic problems. With thedevelopment of concrete technology, the most logical solution was the use of concrete blocks instead ofnatural stones. Several major breakwaters like Sines breakwater in Portugal (1979), SaIaIah, in the Sultanate,of Oman, The Petoskey's breakwater, Montague U.S.A (2000) etc. have failed due to the lack of structuralintegrity of some of the ,armour units utilized in the building of the breakwaters and extreme wave loading duetocyclones. Hence, to avoid such failures, it was proposed to use innovative ideas such as tandem breakwaterand berm breakwater which reduce the wave loading on the armour. In case of tandem breakwater, it is the,submerged reef in front of the breakwater that breaks the steeper waves and dissipates the wave energy whileincase ofbern1 breakwater, it is the berm which plays the similar role thus permitting the lighter armour units.

-, Thesetwo structuresnotonlyreducethe waveloadsand limit the damageof the structurebut alsoreduce the

, ,overburden pressureon the soil below. '

,.' " ' Severalinvestigatorshaveevolvedtheguidelinestodesignsubmergedstructuresinfrontofmain breakwater..Gadreei aI. (1985) designed a submerged bund to break higher waves and dissipate energy while protectingthe re:vetmen~constructed at 100 m shoreward to retain land' behind it at Chennai port. Thus the requiredamour stone weight'of 15 tons for a conventional reclamation structure in a depth of 8 mwas dOWDsizedtoastone weight of 2 to 3 tons. Gadre et aI. (1989) designed a submerged breakwater at 80 m seaward to protecta damaged breakwater head of west breakwater at VeravaI port in Gujarat, India. This submerged structurebtoke the storm waves protecting the damaged breakwater which was repai,red later. Cox and Clark (1992)

'who coined the term tandem breakwater system, examined the effect of wave breaking, overtopping and,transmission on stability of the breakwater with the slope of IV: 1.5H. Cornett et aI. (1993) after conducting

1,2,3,4 Departmentof AppliedMechanicsand Hydraulics,Nartional Institute of TeChnologyKamataka,SurathkalP.O. Srinivasnagar,Mangalore- 575 025, Corresponding Authour's Email:gcmanu@yahoo.com

,Note: Written discussion of this paper will be open until 31st December2011

ISH JOURNAL OF HYDRAULIC ENGINEERING. VOL. 17. (SP. 1)2011

(52) PHYSICAL MODEL STUDIES ON STABILTY OF CONCRETEARMOUREDBREAKWATERS

VoL 17 (SP.1)

experimental investigation concludes that there may be an.optimum location for submerged reef of relativeheighthid > 0.6 whichprotectsthe innermainbreakwater.Shirlalet al. (2006)conductedstudyof stabilityof .

a 1 : 30 scaled model of breakwater defenced by a seaward submerged reef for varying characteristics ofregular waves. He airived at the optimum armour stone weight (30 gms), spacing (Xld=6.25-8.33) and crestwidth (B/d=O.6-O.7,B/Lo=o.035-0.045)of reef to protect the main breakwater.Hong-BinChen etal. (2007) ..

opined from their physical model study that the installation of submerged permeable breakwater in front of .

seawall is capable of reducing the wave run-up on the seawall efficiently. ..

The berm breakwater is normally constIucted with a berm that is allowed to reshape instead of constructing. . .

the reshaped profile directly. This is so because it has been considered cheaper to construct the breakwater . .with a reshaping berm, as it requires smaller size arinour stones. After reshaping by severe storms, several .breakwaters have been seen to achieve a stable profIle and they withstand later storms without significantfurther reshaping and damage (Torum et-al; 1999). This breakwater concept has been used in the constructionof breakwaters in several countries (pIANC MarCom WG 40,2003, O.J.Sayao, 1999). The design procedurefor the preliminary definition of structure cross-sections are available (Hall & Kao 1991; Van der Meer andKoster 1988; Torum et al. 1999,2003; PIANC MarCom WG 40, 2003). Hall and Kao (1991) performedbasic tests on berm breakwaters, studied the influence of wave height, period, spectral shape, number ofwaves, grading and rock shape on the profile reshaping. Torum et al. (1999,2003) developed an equation tocalculate the recession of the berm of berm breakwaters based on the wave height, period, and nominaldiameter of stones, gradation factor and depth factor. These design procedures have their own limitations .

because of wide range of armour stone size, gradation of armour stones, water depth, and seaward slope, crestheight and wave characteristics. Rao et al. (2007) conducted experiments on statistically stable berm breakwarer

model with armour stone weight ofW so=52 gms and berm width = 0.45 m and have showl1a stable profile forwave heightup to 0.14 m. .. .

OBJECTIVES OF STUDY

The objectiveof thepresentinvestigationthe following:. .

To studythe stabilityof tandembreakwatersystem,wavetransmissionat the reef structureand run-upand run~down at the main breakwater, subjected to varying wave climate while keeping .the constantcrest width of submerged reef and varying distance between the structures.. . .

To study the stability and recession of berm breakwater subjected to varying wave climate while keepingthe berm widthconstant; . .

METHODOLOGY OF EXPERIMENTAL WORK

The 1:30scalemodelofbreakwatersconstructedat about32m fromwavegenerator,on theflat bed of the:flume with concrete cubes will be tested for regular waves in a in a two dimensional wave flume for... .

Model Construction

Tandem Breakwater Model

The breakwater model of 1V:2H uniform slope is constructed with concrete cube of nominal diameter D ~ .equal to 0.0325 m as primary armoUrunit (i.e. the mean armourcube weight Wsoof 79.51gin),which isdeterminedusing Hudson's formula,for a designwaveof 0.1 m. A submergedreef modelofnahltal armour.stones with optimum weight of 30 gms of crest width (B) 0.3 m,height (h) of 0.25 m andU11ifonn slope of .

IV:2H with a nominal diameter DD50=o.0221m is constructed on the seaward side of t.l}ebreakwater at a.varying distances(X) of 2.5 m and 4 m as shownin Fig. 1 and breakwaterstabilityis investigated. . ..

Benn Breakwater Model

The model consist of 1.5:1 sloped benn breakwater made of concrete armour cubes weighing (W$0) 79.Sgms which is determined using Hudson's formula, for a design wave of 0.1 m. A horizontal benn of width (B)0.45 m is provided at a constailt depth of 0.425 m above the seabed. The crest width of the breakwater is keptas 0.15 m. In the present model, the primary layer is divided into three zones such as crest ward slope, berm

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Vol 17 (SP. 1) PHYSICAL MODEL STUDIES ON STABILTY OF CONCRBTB

ARMOURED BREAKWATERS(53)

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FIG.1TANDEMBREAKWATERMODELSET-UP

and toe ward slope, 'andthe armourunits in these regionsare colouredas grey,white and red respectively..Figure 2 shows the sectional elevation of the breakwater model studied. The armour layer thickness has beencalCulated using layer coefficient as explained in the CEM (2002). The seaward slope above the berm andbelow the berm is kept same(1.5:1). .

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Wave Flume

The wave flume.is 50 m long, 0.71 m wide, 1.1 m deep and has a 42 m long smooth concrete bed, Fig, 2. shows the sketch of the wave flume used in the present work. A bottom-hinged flap generates waves at one

.end of the deep chamber which is 6.3 m long, 1.5 m wide and 1.4 m deep. About 15 m length of the flume is. provided with glass panels on one side. The flap is controlled by an induction motor of 11kWand 1450 rpm.

. . This motor is regulated by an inverter drive (0-50 Hz) rotating with a speed range of 0- 155 rpm. Regular: wavesofbeight 0.02 m to 0.24 m, and periods 0.8 sec to 4 see can be generated with this facility.. . . . .. .

Model Testing

. ~ ..' The models are tested for its armour stability in a varying wave climate (Ref Table 1). The waves are sent. in short bUrst of five waves during the test so that the generator would be shut off just before the wave energyreflectedfromslopecouldreachthe waveflap.Capacitancetypewaveprobes are used to measuresthe waveheights:

. In case of tandem breakwater initially the newly constructed breakwater !;!Iopeis surveyed with the profiler,which is, the reference survey for comparison of subsequent surveys. 1Wo probes are used where, first probe

.measures the incident wave height (HI)at about 1m seaward of reef toe and second probe measures transmittedwave height (H,) after breaking over the reef at about 1 m from seaward of inner main breal.-water toe. Wavesare run until it appeared that no armour cubes would be moved further by waves of this height or 3000 wavesor the failUre of the structure whichever occurred earlier (Van der Meer and Pilarczyk, 1984). Damage level

ISH 10URNAL OF HYDRAULIC ENOINEERING, VOL. 17, (SP. 1) 2011

(54) PHYSICAL MODEL STUDIES ON STABn.TY OF CONCRETE

ARMOUREDBREAKWATERS

. W .' . .

(S) is calculatedas the ratio of area of erosion(Ae>to squareof nominaldiameterDD50of b.eakwat¥Iarmour -(Van der Meer, 1988).

VoL 17 (SP. 1)

In case of berm breakwater, a stable berm breakwater is dermed when recession of the berm (eroded bermwidth) is less than the initial berm width provided «R./B) <1 for a minimum storm condition of 3000 waves,or till the breakwater has failed, whichever occurred earlier is the limit for every test run, where "Reo"is therecessionof the berm and "B" is the initialberm widthprovided(Fig:3). -

-

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-- -. -

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FIG. 3 SKETCH SHOWING BERM RECESSION

RESULTS AND DISCUSSIONS

Case 1: Tandem Breakwater

Reef Spacing of 2.5 m

Influence of Deep Water Wave Steepness on Wave Run-lIp and Run-Down

Figures 4 and 5 reveal the influence of deep water wave steepness parameter tHi{gT2) on relative run-up(RjH) and run-down(RIB) respectively-for varyingwave climatein depths of waterof 0.3 01,0.35 m and0.401 i.e. increasing ranges of depth parameter (d/gT2).Both run-up and run-down decrease with the in~reasein Higf2. The results are"'compared with those for conventional breakwater. The In.aidn1umrun-up and run-down are respectively 1.34 times and 0.69 times the deep water wave height for the range of variables ~onsideredin the present study. For water depth of 0.3 01the relative run-up is from 11% to 14% a..l1d19% to 30% l~swhen compared with that for water depths of 0.35 01and 0.4 01respectively. While for 0.35 m water depth therelative run-up is from 9% to 18% less when compared with that for water depth of 0.401. Similarly for water -depth of 0.3 m the relative run-down is from 7% to 16% and 25% to 40%-less when compared with-that forwater depths of 0.35 m and 0.4 m respectively. While for 0.35 m water depth the relati ve run-down is from19% to 29% less when compared with that for water depth of 0.4 m. Considering all the ranges Hjgrz,

ISH JOURNAL OF HYDRAUUC ENGINEERING. VOL. 17,(SP. 1)2011 o

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TABLE-IWAVECHARACTERISTICS

SL No. I I Expression I.....

Variable Range

Tandem breakwater Berm Breakwater .1 Waveheight H 0.10,0.12,0.14,0.1601 0.10, 0.12, 0.14, 0.1601

2 Waveperiod T 1.5,2.0 & 2.5 sec 1.6 & 2.0 sec

3 Reef Crest widthl B 0.3m I 0.45 mBermwidth

4 Storm duration N 3000 waves 3000 waves

5 Angle of waveattack 9 90° 90°

6 Waterdepth D 0.3m, 0.3501& 0.401 a.37m, 0.401& 0.4301

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Vol. 17 (SP.1) (55)PHYSICAL MODEL STUDIES ON STABn.TY. OF CONCRETEARMOURED BREAKWATERS

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relative flll1~Upfor depths of water of 0.3 m (i.e..0.004 < dlgT2< 0.013), 0.35m(i.e. 0.005 < dfg'f2< 0.015)and 0.4 m (i.e. 0.006 < dIgT2< 0.018) are 47% to 48%, 39% to 41% and 25 to 35% lower than that forconventional breakwater. Similarly, for the same range of parameters, relative run-down are 37% to 58%,31to 49% and 15%to 29% lower thanthat for conventionalbreakwater. .

. .... .

Influence of Deep Water Wave Steepness on Transmission Coefficient. . .

. Figure 6 illustrates the variation of transmission coefficient ~ with the deep water wave steepness parameter(HjgT2) for varying relative reef height (hid). Kt decreases with an increase in Hjg~ and increase in relativereef height (hid). Kt increases with the increase in water depth and varies in the ranges of 0.4 to 0.55, 0.48 to

. 0.6~and 0.48 to 0.8 for depthsof waterof 0.3 m, 0.35 m and 0.4 m respectively.Whereas,the averagetrendshows a variation of Kt in the ranges ()f 0.46 to 0.48, 0.5 to 0.62 and 0.56 to 0.75 for the same depths. Thewave height attenuation achieved for the present configuration of the defenced breakwater is 20% to 60%. Itcan be inferred that with an increase in relative reef height (hid), wave damping increases and the influence of -

. wave steepness on ~ gradually reduces.

----

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.FIG. 6 VARIATIONOF K, WITH HJgrz .FIG. 7 VARIATION OF S WITHHJ9TZ

Influence of Deep Water Wave Steepness on Damage Level. . .. .. , . . . '. .

. . .. It is obs~rved that damage of the main breakwater occurs for depths of 0.35 m and 0.4 m for waves of 2 sec. period only, which are considerably smaller compared to single breakwater. The trends of damage level (5)

with varying wave steepness parameter (H /gT2) for increasing depths of water ()f 0.3 m, 0.35 m and 0.4 m. .. 0

and different wave periods of 1.5 see, 2 s~c and 2.5 see i.e. increasing ranges of depth parameter (d/gT2) are.shownin Fig 7. Fromthe figureit is seenthat,at 0.3 m depthof water,damagesto ~e inner main breakwateris nil. For 0.35 m and 0.4 m depths of water, small damages to the inner breakwater are seen. The maximumdanlage levels are from 65%.to 100% less for 2 sec wave period and from 82% to 100% less for a waveperiod of 1.5 see when compared with that of conventional breakwater.

ISH JOURNAL OF HYDRAUUC F.NOINEERINO. VOL. 17. (SP. 1) 2011

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(56) PHYSICAL MODEL STUDIES ON STABILTY OF CONCRETEAIU.iOURED BREAKwATERS

VoL 17 (SP.I) Vol.

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Influence of Deep Water Wave Steepness on Wave Run-Up and Run-Down

Figures 8 and 9 shows the influence of deep water wave steepness parameter (H ig'f2) on relativ~ nui-upo . .

(RjH) and run-down <R,/H) respectively, by best fit lines, for varying wave periods of 1.5 sec, 2.0 sec ~d2.5 sec and water depths of 0.3 m,0.35 m and 0.4 m. The results are compared with those ~orconventional .

breakwater. Tbe maximum run-up and run-down are respectively 1.73 times and 0.9 times the deep waterwave height for the range of variables considered in the present study. Both wave run-up and wave run-downdecreases with an increase in wave steepness. It also decreases with decreaSing water depth, i.e. range ofdepth parameter. For water depth of 0.3 m the relative run-up is from 20% to 21% and 27% to'31% less whencompared with that for water depths of 0.35 m and 0.4 m respectively. WbileforO.35 m water depth therelative run-up is from 13% to 18% less when compared with th~t for water depth of 0.4 m. Similarly forwater depth of 0.3 m the relative run-down is from 20% to 32% and 36% to 43% less when compared withthat for water depths of 0.3S m and 0.4 m respectively. While for 0.35 m water depth the relative run-down isfrom 17% to 20% less when compared with that for water depth of 0.4 m. Considering all the ranges Hjg'f2,relative run-up for depths of water of 0.3 m (i.e 0.004 < dIg'f2< 0.013),0.35 m (i.e. 0.005 < dIg'f2< 0.015)and 0.4 m (i.e. 0.006 < dIg'f2 < 0.018) are 31% to 58%, 11% to 47% and 4 to 39% lower than that forconventional breakwater. Similarly, for the same range of parameters, relative run-down are 40% to 47%, 22to 25% and 6% to 7% lowerthan that for conventionalbreakwater. .

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!njluence of Deep Water WaveSteepness on Transmission Coefficient .

Tbe variationof transmissioncoefficient~ with the deep water wave steepness parameter (Hjg'f2) for .

varying relative reef height (bid) is as shown in Fig. 10. From the figure it is observed that Ktdecreases withan increase in wave steepness parameter. As water depth increases, there is an increase in value ofK. indicatinglesser attenuation and varies in the ranges of 0.59 to 0.66,0.64 to 0.73 and 0.64 to 0.81 for depths of water .",of 0.3 m, 0.35 m and 0.4 m respectively. Also more attenuation is observed at depth 0.3 m and period of 1.5sec compai'ed to 2.0 sec and 2.5 sec. This is due to an increased relative reef height, attenuating the steeperwaveseffectivelyat thecrestofreef. .

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ISH JOURNAL OFHYDRAUUC ENGINEERJNG, VOL. 17.(SP. 1)2011

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PHYSICAL MODEL STUDIES ON STABILTY OF CONCRETE

. ARMOUREDBREAk'WATERS(57)

Influence of Deep Water Wave Steepness with Damage Level

The trends of damage level (8) with varying wave steepness parameter (Hlg'f2) for increasing depths ofwater orO.3 m, 0.35 m and 0.4 m i.e. increasing ranges of depth parameter (d/g'f2) are shown in Fig. 11. The

. damagedue to shorterperiodwavesof 1.5sec (i.e.highervalues ofHlg'f2) is seenon righthand side of thefigure whereas, damage aflonger period waves of2.5 sec (i.e. smaller values ofHlgT2) are seen on the lefthand .side and there is no damage for period of2.5 sec. More damage is observed for a wave period of2 secbecause of resonance of annour units resulting in increase in ro~king and displacement. For shallower depth(i.e. 0.004 d::; dlgP d::; 0.013) the damage progresses slowly as wave steepness increases. On the contrary,forrelatively higherdepths(i.e. 0.005dS d/gP d::; 0.015and 0.006d::; d/g'f2d::; 0.018)the damage levelincreases sharply with the increasing of wave steepness. This behaviour is commonly found for wave periodof 1.5 sec and 2 sec. Ranges of damage level S lies between 3.71 to 6.92 and 4.97 to 10.60 for wave periodsof 1.5 sec, and 2.0 sec respeG~ively.When comparing with the conventional breakwater for the wave period of

. 1.5sec, the maximumdamage leveldecreasestrom 35%to 41%, and it decreasestrom 34% to 50% for the. waveperiodof 2 secand no damageis foundforwaveperiodof2.5 SeC.But while comparingwith the reef

of crest width 0.3 m placed at a seaward distance of2.5 m(i.e. X/d 6.25 - 8.33), maximum damage level isftom 100% to 264% and 82% to 211% more for the wave period of 1.5 sec and 2 sec respectively.

. .

. Case 2: Berm Breakwater

Effect of Wave Period on Recession (R,tlD"s,)

. Figt1n"12 sbows the variation of recession with Ns for different water depths and position. For 0.37 mwater depth waves of period T = 1.6, 2.0 and 2.6 sec, shows the recess.ion for shorter wave period T = 1.6 secvaries for N. values up to 3.57. But for higher values ofN, the recession for longer wave periods T =2.0 secis higher compared to T = 2.6 sec. For 0.4 m water depth the recession for shorter wave period is slightly

lower for T = 1.6 sec and N. values up to 3.52. But for longer wave period (T= 2.0 sec), recession is higherthan the shorter wave period (T = 1.6 sec) and Ns value rema.ins almost same as that of shorter wave period.It is observedthat for higherwaterdepth i.e. 0.43 m; there is also linear increasein recession.But recessionforT =2.0 sec is slightly higher than that ofT = 1.6sec. Apart from linear variation of recession with Ns, itCUUlbe observedthat the variationof recessionfor lowerwater depth i.e. 0.37 m is more than that of higherwater depths of 0.4 m and 0.43 m. the variationof recessionis higher for 0.4 m comparedto 0.43 m waterd~. .

. -+- d/gTA2=o.009-0.015

d/gTA2=o.010-o.016

- d/gTA2=o.0l1-o.017

2.5 3 3.5 4

N.

FIG. 12 VARIATION OF RECESSION WITH STABILITY NUMBER FOR DIFFERENT WATER DEPTH

ISH1OURNALOFHYDRAULICENGINEERIN~ VOL. 17.(SP. 1)2011

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(58) PHYSICAL MODEL STUDffiS ON STABILTY OF CONCRETE

ARMOURED BREAKWATERSVol.l7 (SP. 1)

Effect of Storm Duration on Recession

The influenceof stormdurationon reshapingof berm is shownin Fig. 13.It showsthe recessionof berm '. .

for different wave heights against the number of waves for T = 1.6 see, berm width, B = 0.45 m and waterdepth in front of breakwater, d = 0.37 m. From the figure it is obseIVed that for the wave height, H = 0.1 m,most of the changesto.okplace during the fIrst 1000waves and the recession remains almost same for the .

number of waves more than 2000 and 3000 indicating the stability of the breakwater. When the wave heightwas H"" 0.12 m the recession of the berm was almost constant after 2000 waves. For the wave height,H = 0.14 m the recession was found increasing even up to 2000 number of waves. Though the rate ofrecession of the berm was reduced after 2000 number of waves, for the wave height, H = 0.16 ~, the recession- .was found to be increasing up to 2500 wav~s and recession the berm .reduced after 2500 waves, up to 3000waves. From the above discussion it can:be concluded that the stability of the breakwater is largely influencedby the storm duration. Also, as the wave height increases, the number of waves required to achieve stableprofile also increases. .

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FIG. 13 VARIATIONOF RECESSION WITH NUMBER OF WAVES FOR T=1.6 see & d=O.43 m

. d/gT"2=O.009-D.01.S

. , d/gT"2=O..OlO-O.016 .

-~ - d/gT"2=o.Oll-o.017'.

2.5 3.5 4

FIG. 14 VARIATION OF DAMAGE LEVEL (S) WITH STABILITY NUMBER (N.'FOR DIFFERENT WATER DEPTHS

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Vol. 17 (SP. 1) PHYSICAL MODEL S1UDIES ON STABU-IT OF CONCRETE

AR.\fOURED BREAKWATERS(59)

. Effect of Water Depth on Damage Level (S), ,

The variation of damage level (5) with N. for different water depths is shown in Fig. 14. It is observed thatthe variatiol1 of S with N is 1inear for all the water depths and it increases with the increase in N values. The. .zero damage wave heights are obtained corresponding to damage level 5 =2.3, from the graph. the zerodamage 1JVaveheights (in terms of Nt) are2.61, 3.27 and 2.61 for 0.37,0.40 and 0.43 m water depth respectively.But for higher values of Nt' the variation of S for 0.4 m water depth is the higbest and for 0.43 m water depth,it-is the least. And the variation of S for d =0.37 m is intermediate, the influence of berm was not there for0.37 m water depth, as the berm is locatedabovethe water depth. .,

CONCLUSIONs

Based on the Present experimental investigation, the following conclusions are drawn.

Tandem Breakwater, , .

1. 'The reef placed .at seaward distance of 2.5 m, resulted in relative run-up and run-doWn up to 48% and up.to 58% less.whilethereef placedat a seawarddistanceof 4 in, up to 58% andup to 47% less respectively .

whencomparedto that of the convention~ singlebreakwater; "

2. The reef in front of the breatcwater at 2.5 m distance, breaks steeper waves and transmits only 40% to 80%. of the waveheight,whilereef at 4 m distancetransmitsonly 56% to 81% of the waveheight.

3.. For reef placed at a seaward distance of 2.5 m, the breakwater damage is about 65% to 100% less, while. for reef at 4 m damageis about34% to 50% less respectivelywhencomparedto that of the conventional

breakwater.' '"

Benn Breakwl)ter

1. The stability of berm breakwater model studied is largely influenced by the storm duration. The model has. shown a stable profile after durationof 1000waves for the waveheightsof 0.10 m and 0.12 m.

'2. The recession of berm is largely influenced by the change in water level 'in front of the breakwater. Thedimensionless recession varied from 0.93 to 2.04 for the design wave height of 0.10 m and 2.32 to 2.72 forhigher wave height of 0.12 m. for the different water depths.

3. For the higher wave heights ofH=O.14 m showed a recession of6.36 and H= 0.16 m showed a dimensionlessrecession of 10.26 for d=O.37 m, Hence the recession is less with the use of cube as armour unit, for thethree cases of water depths studied.

, .

, 4.. The dimensionless run-up varied from 0.52 to 1.08 for the range of variables considered in the present. study.The variationof damagelevel (S) withNs for 0040m waterdepthis highest and0.37 m beingleast.

REFERENCES

, CEM Coastal Engin~ering Manual (2002). Fundamental of Design. EM 1110-2-1100 (Part-6), U. S. Army, Corps.of Engineers.. "

COfI)ett,A., Ansard, E. and Funke, E. (1993). Wave Transmission and Load Reduction using a Small TandemReef Breakwater",:, Physical Model Tests. Proceedings of ASCE Conference Waves'93 New OrleansUSA., .

Cox, 1. C. and Clark, G. R (1992). Design, Development of a Tandem Breakwater System for Hammond,, Indiana. Coastal Structures and Breakwaters, Thomas Telford Ltd."London;pp. 111-121.

Gadre, M. R., Somayaji, N. and Kale, A. G. (1985). Stones in Breakwaters and Seawalls. Proc., 1"1Nat.Conf. on Dock and Harbour Engg., fiT Bombay, Dec., 1, B-107 - B-112.

Garde, M. R, Poonawala, I. Z., Kale, A. G. and Kudale, M. D. (1989). Rehabilitation of Rubble Mound" Breakwaters. Proceedings of the Third National Conference on Dock and Harbor Engineering, KREC,

Surathk~l; Vol. 1, pp. 387-394.

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Hall, K. R. and Kao, J. S. (1991). The Influence of Armour Stone Gradation on Dynamically StableBreakwaters.Journal of CoastalEngineering,Vol. 15,pp. 333-346. .

Hong-Bin, Chen (2007). Wave Transformation between Submerged Breakwater and Stawall. Journal ofCoastal Research, SI SO (Proceedings of the 9th International Coastal Symposium), 1069-1074. Gold. .Coast, Australia, ISSN 0749.0208. . .

PIANC MarCom W G 40. (2003). State-of-the-Art of the Design and Construction of Berm Breakwaters.

. PlANC, Reportof WorkingGroup40 -MARCOM,Brussels. .. .. .

Sayao, O. J. (1999). On the Profile Reshaping of Berm Breakwaters. Coastal Structures, pp. 343"-351.

Shirlal, K. G., Subba, Rao, Venkata, G. and Manu (2006). Stability of Breakwater Defenced by a Seaward. SubmergedReef. OceanEngineering,An InternationalJournalof Researchand Development,Pergamon,ElsevierScienceLtd.NewYork,N.y:, U.S.A,pp.829-846. .. .

Subba, Rao, Subrahmanya, K., Ba1akrishna, Rao and Chandramohan, V. R (2007). Stability Aspects of NonReshaped Berm Breakwaters with Reduced Armour Weight and Varying Slopes. Journal of Waterways,Port, Coastal and Ocean Engineering, ASCE, USA, Vol. 134, No.2, March/April 2008, pp. 81-87.

Torum, A., Krogh, S. R, Bjordal, S., Fjeld, S., Archetti, R. and Jacobsen, A. (1999). Design Criteria and .

Design Procedure for Berm Breakwaters. Proc., Coastal Structures '99', Balkema, Rotterdam. pp; 331~341.' . .

Torum, A., Fran7.i~ka, K., Andreas, Menze (2003). On Berm Breakwaters, Stability, Scour, Overtopp.ing~

Journalof CoastalEngineering,Vol.49, pp. 209-238.. '., '. . .' .

Van der Meer, J. W. and Pilarczyk, K. W. (1984). Stability of Rubble Mound Slopes unde.r Random' Wave. .Attack. Proc., Coastal Engg. Conf., pp. 2620-2634. . 0 . . .'

Van der Meer, J. W. (1988). Stability of Breakwaters - Delft Hydraulics Developments. Proc., 3MNat. Com..

onDockandHarbourEngg.Dec.,K.R.E.CSurathkal,pp.889-899. . . . .

Van derMeer, J. W. and Koster (1988). CIRIA and CUR (1991), State of the Art Guide by PIANC (2003b).

ISH JOURNAL OF HYDRAUUC ENGINEER.lNG, VOL. 17. (SP. 1) 2011

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