Analysis of Wave Characteristics in Osaka Bay for Three Major Typhoons Using WAVEWATCH III

Journal of Japan Society of Civil Engineers, Ser. B3 (Ocean Engineering), Vol. 75, No. 2, I_289-I_294, 2019.

Chathura MANAWASEKARA1, Yiqing XIA2, Mangala AMUNUGAMA3, Yoji TANAKA4, Katsuyuki SUZUYAMA5

1Member of JSCE, Disaster Prevention Division, Wave Analysis Dept., ECOH CORPORATION (2-6-4, Taito-ku, Tokyo 110-0014, Japan) E-mail:[email protected] 2Disaster Prevention Division, Wave Analysis Dept., ECOH CORPORATION (2-6-4, Taito-ku, Tokyo 110-0014, Japan) E-mail:[email protected] 3Technical Research Institute, ECOH CORPORATION (2-6-4, Taito-ku, Tokyo 110-0014, Japan) 4Member of JSCE, Disaster Prevention Division, Wave Analysis Dept., ECOH CORPORATION (2-6-4, Taito-ku, Tokyo 110-0014, Japan) 5Disaster Prevention Division, Wave Analysis Dept., ECOH CORPORATION (2-6-4, Taito-ku, Tokyo 110-0014, Japan) E-mail:[email protected]

Typhoon generated wave behavior in Osaka Bay under recent three major typhoons (T1820, T1821, and T1824) is simulated and discussed in the study. Simulation was conducted using third generation wave model WAVEWATCH III, and results showed that the impact areas for each typhoon can be explained by looking in to the sea wave properties enter into the bay area. Wave energy is represented with significant wave height related to each categorized components of wave frequency, and their contribution to the damages and critical conditions caused by each typhoon are discussed. Analysis shows that the longer period swell enters into the bay area for T1821 than that of for T1820. In contrary, effect of wind waves for T1820 is larger in North and West side of the bay while T1821 affect more in the Eastward side. In addition, wind data from two sources [Local Forecast Model (LFM) and Meso Scale Model (MSM)] by Jap- anese Meteorological Agency (JMA) were used in the study and the simulation effectiveness in related to wind source is also discussed in the study. However, results with newer and finer LFM wind data showed better agreement with observations in comparison to weaker MSM wind data.

Key Words : Typhoon Cimaron (1820), Typhoon Jebi (1821), Osaka Bay, WAVEWATCH III , LFM

1. INTRODUCTION are capable of causing variety of damages and poses a serious threat to properties specially in the coastal Extensive damage to coastal structures and facili- region. Therefore, typhoon generated wave heights ties caused by three big typhoons (T1820, T1821, play a major role in determine the design limitations and T1824) passing central Japan (in latter half of of coastal structures in most part of Japan. With year 2018) has drawn great attention, particularly number of economically important sites laying with regards to the reliability of existing safety around (Kobe port, Osaka port, Kansai International measures against typhoon-induced waves. The airport etc.), Osaka bay region plays a central role in large-scale economic loss from the halted operations economy of Western Japan. Moreover, even though in Kansai International Airport due to flooding by recent super-typhoons Cimaron and Jebi both made typhoon Jebi (T1821) has also emphasized the level their initial landfall in the southern part of To- of vulnerability present under passing typhoons. kushima prefecture before crossing Osaka Bay with During the annual typhoon season (generally from slightly varied paths, typhoon induced wave charac- May to October) at least about three typhoons hit the teristics and major damage areas appeared to be have mainland of Japan, and it is evident that typhoons are significant difference from each other. Therefore, it getting stronger in recent years. High waves, sudden is worth studying the typhoon-induced wave distri- rise of water levels and strong winds under typhoons bution in the wide and shallow sea area of the bay

I_289 Journal of Japan Society of Civil Engineers, Ser. B3 (Ocean Engineering), Vol. 75, No. 2, I_289-I_294, 2019. after entering through Kii-channel, and becomes the aim of the current research. In this study, we employed the third-generation wave model, WAVEWATCH III (WW3) to simulate the wave conditions under typhoon events T1820 (Cimaron), T1821 and T1824 (Trami). The applicability of WW3 in extreme wind situations has been addressed and confirmed in several recent studies1)2). LFM wind data provided by JMA were used as the main wind source for the study, and the main wave analysis and comparisons in the paper were conducted with wave simulation results using LFM winds. Additionally, a comaparison between results by LFM and MSM wind sources were done in the latter part of the paper. LFM, finer in resolution and the latest of two models, implemented in 2012 and provides data at every 1-hour interval. Recently, Kawaguchi et al.3) have successfully employed LFM winds in wave modelling under Typhoon Lan. A comparison between two wind sources and their results are discussed in the latter part of the paper.

Fig. 1 Computational domains (D1-D4) and data extraction points in D4 2. NUMERICAL MODEL

(1) Brief description of wave model In WW3 the generation, evolution and dissipation of ocean surface waves are predicted by solving the balance equation, DN  txk ),;,( S = (1) Dt  where, k is the wavenumber,  is the wave direction,  is the intrinsic frequency, x and t represent the space and time coordinates, respectively, = ()txkFN /,;,  is the wave action density spectrum, F is the wavenumber-direction spectrum, and S represents the net effect of source and sink terms for F. The latest version of WW3 (version 5.16) is em- Fig. 2 Typhoon path over mainland of Japan ployed using spherical coordinate system, and more detailed explanation on the numerics and physics of the model is available in the user manual and system documentation of WW34).

(2) Model set-up and parameters The input/dissipation source terms proposed by 5) Ardhuin et al. (ST4 in WW3) are applied in the current study with frequency and directional divi- sions of 35 and 36, respectively. Four nested (one-way) computation domains are used in the simulation with increasing resolution (from D1-D4 with grid resolutions 1/4⁰, 1/16⁰, 1/64⁰, 1/256⁰, respectively) when converging on the target area. A 0 minimum grid size of around 400m (1/256 ) is selected especially in order to represent critical topo- Fig. 3 Temporal variation wave height (above) and period (below) graphical properties effectively in the computation. at NW-Kobe for simulation and observation

I_290 Journal of Japan Society of Civil Engineers, Ser. B3 (Ocean Engineering), Vol. 75, No. 2, I_289-I_294, 2019. (a) (b) (c) (d)

T1820 T1821 T1824 T1820-T1821

Fig. 4 (a-c)Maximum sig. wave height by each typhoon and, (d) difference in maximum wave heights between T1820 and T1821

Wind data were input to WW3 at 20min time-interval and simulation output data were also recorded at the (2) Spectral analysis same time-interval. Graphical representation of each a) Wave spectrum at points domain area shown in Fig. 1 and the path of each In order to look into the composition of sea waves, typhoon over the area are as in Fig. 2. we extracted wave spectrum at points (Pt1-Pt4) in D4 shown in Fig. 1, at the peak time of NW-Kobe. Fig. 5 illustrates the frequency and directional spectrum at 3. RESULTS AND DISCUSSION Pt1 to Pt4 for T1820 and T1821. Even though the directional spectrum at Pt1 and Pt3 have distinct peaks between S-SW for both typhoons, it is evident (1) Validation of simulated wave that T1820 has higher level of energy travelling in Simulation results are compared with the obser- northward direction. The peak frequency lies around vations at NOWPHAS (Nationwide Ocean Wave 0.12Hz (wave period of ~8.3s) for both Pt1 and Pt3. information network for Ports and HArbours) data According to general classification, periods less than collection station located in Kobe (NW-Kobe). In the 8s (equivalently, frequency >0.125Hz) are consid- simulation T1820 reached the peak significant wave ered to be wind waves while rest to be considered height at NW-Kobe 08/24 00:20 while T1821 and swell. Thus, indicating under T1820 wind waves has T1824 reached peak at 09/04 14:40 and 10/01 00:40, been the cause for much of the impacts in the north respectively. Considering peak time as t=0 for each part of the bay. typhoon, temporal variation of wave height and pe- Concurrently, frequency spectrum of Pt4 shows riod for 9-hour period either side of the peak time are similar variations for both T1820 and T1821. How- depicted in . Even though, both T1820 and Fig. 3 ever noticeably swell component (frequency T1821 simulations slightly underestimate the peak <0.125Hz) of appears slightly higher at Pt2 for wave heights, overall, simulation results are in good T1821. Generally, with longer wave length and agreement with observation data. However, this higher wave periods swell tend to carry few times study does not account for the effect of sea level more energy than wind waves with corresponding change due to the storm surge caused by the typhoon. wave height. Therefore, we shed closer attention to Wave height is expected to increase with such rise in energy carried by each wave component through water depth, and can assume to be contributed to the discrepancy observation data. (a) (b) T1820 T1821 In addition, T1820 is slower in retreat in comparison to T1821, indicating that wave effect from T1820 last longer for the particular area. At the same time wave periods of 4s-8s are dominant for all typhoon events. Fig. 4(a-c) shows the spatial variation of the maximum significant wave height observed during each typhoon. Waves from T1820 are higher (c) (d) in comparison to T1821 throughout the Osaka Bay, T1820 T1821 especially in the Northern region around Kobe. In contrary, T1821 wave heights are slightly higher in eastern part of the bay around Osaka port area. Fig. 4(d) which shows the height difference between typhoons T1820 and T1821 further confirm this fact.

The wave by T1824 [Fig. 4(c)] also affect the east Fig. 5 Frequency and directional spectrum for T1820 [(a) and (c)] part of the bay, mainly under the influence of tail and T1821[(b) and (d)] at selected points at times when sig. wind, but not as significant as for T1820 or T1821. wave height at NW-Kobe reaches its peak.

I_291 Journal of Japan Society of Civil Engineers, Ser. B3 (Ocean Engineering), Vol. 75, No. 2, I_289-I_294, 2019.

basic spectral partitioning. categorization used by NOWPHAS as a reference. Frequency ranges of each category and the corre- b) Spatial distribution of wave energy sponding wave periods are listed in Table 1. Tem- Once the wave frequency spectrum from the sim- poral variation of frequency spectrum at each grid ulation =  (),)( dfFfE  is obtained (where, f is point (i, j) of the computational domain was esti- mated during the simulation. Fig. 6 demonstrates the frequency), significant wave height (Hs) can be es- spatial distribution of all-time maximum significant timated from, wave height [maximum of Hs,(i,j)] under each wave  f2 f1  H = 4 (), dfF  df − (), dfF  df (2) category, for the finest domain. s  f  f    0 0  As discussed above, in common classification, Since Hs is proportional to the surface wave en- waves in Ct1-Ct3 falls under swell while Ct4 con- ergy, with f0 as the minimum frequency of the sim- sidered as wind waves. It can be noted from the fig- ulation, Eq. 2 is applied to represent the surface wave ure that, Ct1 swell entering Osaka bay is significantly energy within a particular frequency range from f1 to larger for T1821 than other two typhoons. As long f2. In this study, we categorize wave frequency taking period swell is frequently associated with extensive

Ct1

T1820 T1821 T1824 T1821-T1820

Ct2

T1820 T1821 T1824 T1821-T1820

Ct3

T1820 T1821 T1824 T1821-T1820

Ct4

T1820 T1821 T1824 T1821-T1820

Fig. 6 Spatial distribution of maximum significant wave height obtained in the simulation for each typhoon (column 1~3), and the difference in maximum significant wave height between T1821 and T1820 (column 4).

I_292 Journal of Japan Society of Civil Engineers, Ser. B3 (Ocean Engineering), Vol. 75, No. 2, I_289-I_294, 2019.

into Kii-channel [Fig. 7(e)] due to the diagonal Table 1 Ranges of applied frequency categories course (over domain 4) of the typhoon wind direction Category f(Hz) range T(s) range significantly vary during the period it crossed over Ct1 0.041-0.066 24.28-15.07 Osaka Bay, when compared with T1820. However, Ct2 0.067-0.097 15.08-10.29 those higher winds of T1821 at the entry to Ct3 0.098-0.129 10.30-7.73 Kii-channel expected to contribute to the high Ct4 0.130-0.251 7.74-3.97 amount of long period of swell energy later enter in 6) to the Osaka Bay. Further, the tail wind (of speed up coastal flooding and damage , penetration of large to ~35m/s) mainly in NE direction produced as amount of swell poses a serious threat. Ct1 long pe- T1821 leaving the area, [Fig.7(g)] generates consid- riod reaches a wave height up to ~0.75m near Kansai erable amount of short period wind waves [Fig. 6 - International Airport (KIA) for T1821, nearly 50% Ct4) which later affect the Osaka port area. more than that for T1820. Sea level rise due to low Additionally, even though T1824 has taken a path surface pressure with the arrival with typhoon, further eastward to both T1820 and T1821, the long combined with the swell can cause flooding damage period swells (Ct1) entering in to the Osaka Bay as in KIA when T1821 hit Osaka Bay in September shows fairly similar distribution to that of T1820. 2018. For example, tidal records (during T1821) However, the maximum wave height inside the bay is from Tannowa (operated by JMA) also support this. caused by the Eastward tail wind of T1824 upon the In addition, wave height of Ct4 at the leeward side of exit of typhoon from the area [Fig. 6 – Ct4]. the airport for T1821 (wind waves) also shows higher Therefore, not only the wind-speed of the typhoon values in comparison to T1820. but also the course it takes has a significant effect on Categories Ct2 and Ct3 can be considered as a the distribution of ocean waves inside Osaka Bay. combination of young swell and wind waves. Most of the time the highest waves in storm seas consists of (3) Comparison of LFM and MSM wind fields waves in this region, and wave heights of T1820 are Both LFM and MSM wind data are available from significantly dominant in comparison to T1821. JMA, and their resolution and moist processes can be These can be caused by the strong (~30m/s) mostly identified as major differences between two models. northward wind over Kii-channel when T1820 LFM has a horizontal grid resolution of 2km while crosses over the region [Fig. 7 (a)-(c)]. As seen in MSM operates at a 5km grid resolution. In the study, Fig. 7, the course taken by T1820 has kept the wind typhoons were simulated using both wind sources in direction over the sea surface mostly northward for a order to assess their applicability in wave modelling. longer period than that for T1821. Even though, Fig. 8(a) illustrates the difference in maximum sig- T1821 has higher wind speed (35-40m/s) entering nificant wave height between LFM and MSM wind

T1820 T1820 T1820 T1820

(e) 2018/09/04 12:00 (f) 2018/09/04 13:00 (g) 2018/09/04 14:00 (h) maximum wind speed

T1821 T1821 T1821 T1821

Fig. 7 LFM wind speed (m/s) at 1hr interval when T1820 [(a)~(c)] and T1821 [(e)~(g)] reach Osaka Bay, and spatial distribution of maximum wind speed for (d) T1820 and (h) T1821 in domain 3 (typhoon course is marked in black dashed line)

I_293 Journal of Japan Society of Civil Engineers, Ser. B3 (Ocean Engineering), Vol. 75, No. 2, I_289-I_294, 2019.

(a) (b) WW3 is a useful tool in typhoon-induced wave (LFM) analysis, and LFM winds distributed by JMA are y=1.01x capable in producing effective wind field for wave simulation under extreme winds. The energy of long period waves (Ts>15s) entering y=0.82x (MSM) Osaka Bay under T1821 is higher in comparison to that of T1820. The effect by young swells and long period wind waves (7.7s

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