Simulation of Storm Surges induced by Chanchu, Bilis and Kaemi in 2006 HsienHsien--WenWen LiLi1 ChengCheng--HanHan TsaiTsai2 YaoYao--TsaiTsai LoLo3

1 MinghsinMinghsin UniversityUniversity ofof ScienceScience andand TechnologyTechnology 2 NationalNational TaiwanTaiwan OceanOcean UniversityUniversity

2009/03/112009/03/11 This study uses the surge model reported in:

Hsien-Wen Li, Cheng-Han Tsai, Yao-Tsai Lo (2008) Numerical simulation of surges along the coast of , Natural Hazards 1.Average 3.6 typhoons affect Taiwan yearly

2. Surge height about 1 m for severe typhoon

3. Elevated sea level by the surge reduced river drainage ⇒ flooding in coastal lowland

4. Surge increased typhoon wave overtopping ⇒ flooding behind levee protected coast

5. Surge model can predict where and how high the surge will be.

6. This model uses typhoon parameters posted by the weather bureau in its hourly bulletin. Governing Equations: The spherical polar coordinates (r ,λ,φ) are ϕ used in this study , λ ∂ζ 1 ∂U 1 ϕ ∂ = −{ + ϕ(V cosϕ )} ∂t ρ R cosρ ∂ R cos ∂ ϕ ϕ λ λ λ ∂U 1 H ∂ p g H ∂ζ τρs −τ b λ = f V − a − ϕ + ∂ t R cos ∂ R cosϕ ∂ λ ϕ ϕ ϕ A 1 ∂λ2 U 1 ∂ ∂U + h [ + ( cos ϕ ) ] R 2 cos 2 ∂ 2 R 2 cos ∂ ∂ ϕ 1 ∂ U U 1 ∂ ϕ V U cos ϕ − a [ ( ) + ( ) ] ρ R cos ∂ H R cos ∂ H ρ ϕ ρ ϕ ∂ V 1 H ∂ λpϕ g H ∂ζ τ −τ = − f U − a − + s bϕ ∂ t R ∂ R ∂ ϕϕ ϕ ϕ ϕ A 1 ∂λ2 V 1 ∂ ∂ V + h [ + ( cos ϕ ) ] R 2 cos 2 ∂ 2 R 2 cos ∂ ∂ ϕ 1 ∂ U V 1 ∂ ϕ V V cos ϕ − a [ ( ) + ( ) ] R cos ∂ H R cos ∂ H Advantages of using spherical coordinates

1.Accurate bottom topography in shallow water is essential for surge model

2.No projection mapping necessary

3.No distortion in bathymetry τ , τ s = ρa CD W W b = ρ k U h U h

Wind stress, CD=0.003 Bottom stress, k=0.0025

In numerical calculations the Courant-Friedrich-Lewy stability condition is used,

∆t 1/ 2 < ∆s gH max Here we takeΔs (0.5’) as the shortest grid distance of Δλ in longitude, which equals 827 m at 27o N on the northern boundary of

this model. The maximum water depth in the model domain, Hmax, is 6644.5 m; therefore, we take the time step in the computation as = 2 sec. Typhoon model : Although typhoons have various shapes, they can be approximated as a circular storm on the ocean. Here we use the circular hurricane model of Jelesnianski (1965). Wind field 2/3 0 ≤ s ≤ E W=Wmax (s/E) -1/2 s ≥ E W=Wmax (s/E) Presusre field 1 s p = p + ( p − p ) ( )3 0 ≤ s ≤ E a o 4 ∞ o E 3 s p = p − ( p − p )( ) −1 s ≥ E a ∞ 4 ∞ o E s: distance from the center, E: radius of maximum wind Comparison of model typhoon wind Comparison of model typhoon distributions as a function of the pressure distributions as a function distance from the typhoon of the distance from the typhoon center, s, normalized by the center, s, normalized by the radius radius of maximum wind, E. of maximum wind, E. The simulation domain covers the area from 117 to 124 degs E and from 20 to 27 degs N.

Bottom topography around Taiwan a. Penghu (PH) b. Kinmen (KM)

Typhoon Chanchu Medium, 45 m/s max wind From Figure 1 we can see that there were large typhoon surges observed at Penghu (PH) and Kinmen (KM) before 4hr on May 18 and the calculated surges are small. Typhoon Chanchu of medium strength with maximum speed of 45m/s wandered in the South Sea before May 18 and moved into the model domain at 1hr on May 18. Thus at the beginning of computation the calculated surges are smaller than the observed. The maximum calculated surges at these two stations are larger than the observed. The reason is that Typhoon Chanchu became weak as it made , but model typhoon can not respond to the landfall effect of typhoon. a. Suao (SA) b. Gengfang (GF) c. Longdong (LD)

Typhoon Bilis Weak, 25 m/s Max wind d. Keelung (KL) e. Danshuei (DS) f. Jhuwei (JW)

Typhoon Bilis Typhoon Bilis is a weak one with maximum speed of 25m/s. These stations are Suao (SA), Gengfang (GF), Longdong (LD), Keelung (KL), Danshuei (DS) and Jhuwei (JW). Tidal data at Hsinchu (HC) and Matsu (MT) are not available. Typhooon Bilis made landfall near GF at about 22hr July 13 and moved offshore near Danshuei at about 1hr July 14. From Figure 2 we can see all the typhoon sureges are less than 0.5m. The discrepancies between the calculated surge and the observed at stations GF and KL are due to offshore wind in the model typhoon after Bilis made landfall. The observed surges at the beginning of computation are of the same phenomena as Typhoon Chanchu. a. Chenggong (CG) b. Fugang (FG) c. Dongshih (TS)

Typhoon Kaemi Medium, 38 m/s max wind d. Mailiao (ML) e. Penghu (PH) f. Kinmen (KM)

Typhoon Kaemi Typhoon Kaemi was of medium strength with maximum speed of 38 m/s. It made landfall near Chenggong at about 00:00 July 24 and moved offshore near Dongshih at about 4hr July 25. The comparisons of the observed surge with the calculated at stations near Kaemi’s track are shown in Figure 3. These stations are Chenggong, (CG), Fugang (FG), Dongshih (TS), Mailiao (ML), Penghu(PH) and Kinmen(KM). It can be seen that the calculated maximum surge heights are compatible with the observed at most stations. Conclusions 1. The calculated results are generally satisfactory 2. In the period after typhoons made landfall calculated results differed from the observed. However, the observed and calculated surges are all small, thus the flooding damage due to typhoon surge may be quite small. 3. Using the hourly warning bulletins issued by CWB the present typhoon surge model can be used for predictive purposes. 4. To perform surge prediction in the present grid area in a 24 hour period, the model needs 10 hours to complete, using a PC with Intel ® Pentium® 4 CPU 3.00GHz in the Linux environment.