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Flare-Shaped Revetments: installation experience&

results on sensitivity of design to climate change

lsao lrie, Tokyo CoDoo, (IDEA Consultants, lnc.), lrieTcodoo@aol.comYosh i h i ro Hamazaki, (KOBE STEE L, LTD), hamazaki.yos h i h iro@cc.kobelco.comKeisu ke M urakami, (U n iversity of miyazaki), keis uke@cc.m iyazaki-u.ac j pTomotoki Toyoda, (ATSC lnc.), toyoda.tomotoki@kobelco.com

lntroductionThe safety of water front will be highly upgraded if no wave overtopping is expected even onrough sea. This paper presents on the development and installation experience of Flare-Shaped Revetments (Flare-Shaped Stmctures) with no wave overtopping, and also the resultson sensitivity of design to climate change. Most of the existing coastal protective structuresare designed by the expected overtopping rate method, for instance, obtaining the timeaveraged overtopping rate for Rayleigh distributed waves. Actual coastal waves, however,include wave groups composed of high waves and unexpected high rate of overtopping can besometimes observed. When the landward spaces are utilized by more fragile objects such aswalkers, automobiles, and residences, those exceptionally high wave overtopping is notallowed, and some devices on the performance of structures have been required.

Performance of the Flare-Shaped Revetments

Figurel. Upright type Figure 2. Wave absorbing type Figure 3.Flare-shaped typeIn the above figures, the state of wave overtopping of 'Flare-Shaped Revetments' (Fig.3) iscompared with that of conventional 'Upright type' (Fig.1) and 'Wave absorbing type' (Fig.2)under the waves of l8cm in height, 3.2s in period, 6 :0.22 in wave steepness and l/10 in bedslope. As explained later, the front face of the 'Flare-Shaped type' is curved so that thecentrifugal force of water mass running up the front face may decrease linearly from thebottom to the top. Thus, no wave overtopping takes place basically because the centrifugalforce at the tip is almost zero. on the other hand, both'Upright type'(Fig.l) and'Waveabsorbing type' (Fig.2) have a vertical front face and thus the water mass running up the wallwill be spread landward as well as seaward whether the front part is covered by wavedissipating blocks or not. Figure 4, for instance, is an example of wave overtopping of 'Wave

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absorbing type' in the field and this type of overtopping is frequently seen on many roads,where the emergency waming, 'Close to traffic'is given.

Figure4. Wave absorbing type FigureS.Offshore casting, Flare-shaped

Figure 5 shows how the 'Flare-Shaped type' protects from wave overtopping: all the watermass running up is thrown back to offshore, securing the high level safety of hinter land.

Designing the Flare-Shaped Revetments

The Shape DesignThe word 'Flare' in this paper comes from 'Naval Architecture' and means the bow of shipspushing aside waves. The front face of the Flare-Shaped type is curved so that the centrifugalforce of water-mass running up the front face may decrease linearly from the bottom to thetop. As shorvn in Fig.6, assume that the spray particles 5.Omm in diameter has been injected atthe angle of elevation 0 and have the effects of wind of q (m/s). The spray particles exposed to

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offshore ward i landward:

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Figure 6. Location of spray landing(X)(Left) injected frorn Flare face at angle of 0(After Kamikubo)

the wind will be retum back onshore and will be landed on the crown level of the Flare at thedistance x (m) from the tip of the Flare. This landing location is evaluated by changing g andwind velociry q (n/9 as shown in the left of Fig.6. From the figure, the condition that thewater particles fall offshore side of the Flare tip (x=0) is 0 = n 14 under the wind speed ofl5m/s representing the ordinary storms. The Flare shape thus obtained using the abovecondition is shorvn as (A) in Fig.7. The Flare shape is further evaluated though hydraulicexperiments with respect to wave reflection as well as wave run up and the curve @) inEig.7,where the ratio of height to depth of the curve is selected tobe 2'.1, has been selected.

Designing the cross sectionThe coast of the present design is located in Osako Port in Hiroshima Prefecture in the Seto

An ejection angle

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Inland Sea. It was hit by heavy typhoon two times (in 1999 and 2004) and the coastal areawas suffered by heavy wave overtopping. Various reconstruction measures including waveabsorbing revetments as well as offshore breakwaters were reviewed and finally, Flare-shapedRevetments were selected because of lower crown height, keeping good view and especiallylow cost. The present paper introduces the major points of designing.

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curves of wave overtopping rateline) and by Goda(Broken line)

Design conditionsSince the coast of Osako Port is located in Hiroshima prefecture of the Seto Inland Se4 waveclimate is rather modest, howeveq the wave attack combined with storm surges results inheavy disasters. The input data for design conditions are as follows: wave height; IIo:1.86m,wave period;T:5.22s, the muximum high-water level; +4.5m above C.D.L, sea bottom slope;1/10, allowable overtoppingrate; q:0.01m3 /rnls, sea bottom elevation at the foot; h:fl.Sm.

Determination of cross sectionDesign water depth at the foot: h:3.0m, Wave steepness: HJL":O.044

The ratio of water depth at the foot to offshore wave heighFh/Il"':1.61

Non-dimensional allowable rate of overtopping=ql Jrg(Ho')z:8.90 X l0-oFigure 8 is the standardized experimental curves of wave overtopping rates for Flare-ShapedRevetments shown in solid lines, and also the curves for upright sea wall by Goda in brokenlines on the condition of rt/L.=0.036, where actual wave steepness was H"/L.:0.044. Inorder to determine the crown height of Flare-Shaped Revetment ho,the above two parameters,hlIJ.' andJzs@o')3 were utiliz"a itt fig.A and the result is h"l4' = 0.75, thus, the crownheight from the sea water level is: ho:1.4m, and thus the crovm height of the Flare-ShapedRevetment is determined: fu: l.4m+4.5m=6.0m. The crovm height, ho =6.0m is lower thanexisting upright revetment (16:6.5m, see Fig.ll). Cross section thus determined was testedexperimentally with actual wave steepness(:0.044) in order to confirm the actual overtoppingrate as well as the stress working on the structural members.

Confirmation by hydraulic experimentsCross section determined so far has been tested by hydraulic experiments, and has beenconfirmed that the rate of overtopping is q:g.gOOrm'/m/s for hydrographic design conditionswhere allowable condition is q:6.9f

-3/m/s. Wave pressures are also checked and confirmed

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that the pressure(shock pressure) is 5 times higher than the ordinary pressures on uprightbreakwater given by Goda's wave pressure formula. This dynamic wave pressure wzm used asthe static pressure in confirming the stability of Flare-Shaped Revetments as well as durabilityof members (see Fig.l0).

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