Nat. Hazards Earth Syst. Sci., 12, 3799–3809, 2012 www.nat-hazards-earth-syst-sci.net/12/3799/2012/ Natural Hazards doi:10.5194/nhess-12-3799-2012 and Earth © Author(s) 2012. CC Attribution 3.0 License. System Sciences
Predicting typhoon-induced storm surge tide with a two-dimensional hydrodynamic model and artificial neural network model
W.-B. Chen1, W.-C. Liu2, and M.-H. Hsu3 1Supercomputing Research Center, National Cheng Kung University, 70101 Tainan, Taiwan 2Department of Civil and Disaster Prevention Engineering, National United University, 36003 Miaoli, Taiwan 3Department of Bioenvironmental Systems Engineering, National Taiwan University, 10617 Taipei, Taiwan
Correspondence to: W.-C. Liu ([email protected])
Received: 3 June 2012 – Revised: 15 November 2012 – Accepted: 15 November 2012 – Published: 21 December 2012
Abstract. Precise predictions of storm surges during ty- 1 Introduction phoon events have the necessity for disaster prevention in coastal seas. This paper explores an artificial neural network (ANN) model, including the back propagation neural net- Storm surges are as a result of strong tropical storms called work (BPNN) and adaptive neuro-fuzzy inference system typhoons in the northwestern Pacific Ocean. Abnormal sea (ANFIS) algorithms used to correct poor calculations with level rise due to storm surge is caused by strong winds and a two-dimensional hydrodynamic model in predicting storm atmospheric pressure disturbances (You and Seo, 2009). Tai- surge height during typhoon events. The two-dimensional wan is located on the west side of the Pacific Ocean (Fig. 1a). model has a fine horizontal resolution and considers the in- The country is often subjected to severe sea states that are in- teraction between storm surges and astronomical tides, which duced by typhoons and occur during the summer and winter can be applied for describing the complicated physical prop- seasons in either the South China Sea or the northwest Pa- erties of storm surges along the east coast of Taiwan. The cific Ocean near the Philippines, which result in the extensive model is driven by the tidal elevation at the open bound- loss of life and property. On average, Taiwan suffers three to aries using a global ocean tidal model and is forced by the four typhoons annually. Typhoons act more severely on the meteorological conditions using a cyclone model. The sim- east coast than on the west coast, because typhoons typically ulated results of the hydrodynamic model indicate that this approach Taiwan from the east. As a typhoon approaches model fails to predict storm surge height during the model Taiwan, its strong wind and low atmospheric pressure often calibration and verification phases as typhoons approached cause storm surges that can result in severe damage to coastal the east coast of Taiwan. The BPNN model can reproduce areas, especially on the low-lying lands near river mouths the astronomical tide level but fails to modify the predic- because of the double effects of the river floods by typhoon- tion of the storm surge tide level. The ANFIS model sat- brought rains and backward uplifting seawater floods from isfactorily predicts both the astronomical tide level and the storm surges. Therefore, it is necessary to develop a reliable storm surge height during the training and verification phases model to predict the height of typhoon-induced storm surge and exhibits the lowest values of mean absolute error and for coastal management and hazard mitigation. root-mean-square error compared to the simulated results at Several researchers have applied different numerical mod- the different stations using the hydrodynamic model and the els to predict storm surges. For example, Shen et al. (2006) BPNN model. Comparison results showed that the ANFIS applied an unstructured grid numerical model (Unstructured, techniques could be successfully applied in predicting wa- Tidal, Residual Intertidal, and Mudflat model – UnTRIM- ter levels along the east coastal of Taiwan during typhoon 3D) to simulate and predict the storm tide in the Chesa- events. peake Bay. The authors found that the use of an unstruc- tured grid provides the flexibility to represent the complex estuarine and coastal geometry for accurately simulating storm surges. Dietsche et al. (2007) adopted the ADvance
Published by Copernicus Publications on behalf of the European Geosciences Union. 636 637 3800 W.-B. Chen et al.: Predicting typhoon-induced storm surge tide 638 639 had the capability to accurately reproduce the statistical dis- tribution of water levels and the current in various locations and could be viewed as a reliable complementary tool. De Oliveria et al. (2009) used a neural network model to predict coastal sea level variations related to meteorological events in the southeastern coastal region of Brazil. The authors found that neural network models can efficiently predict the non-tidal residual and effectively complement the standard harmonic analysis method. You and Seo (2009) developed a cluster neural network model to predict storm surges in the Korean coastal regions. Bajo and Umgiesser (2010) devel- oped an operational surge forecast system based on a com- bination of a hydrodynamic model and an artificial neural network for the city of Venice. Filippo et al. (2012) used ar- tificial neural networks to train and translate the combined influence of meteorological and astronomical forcing to pre- dict sea level variations. The main objectives of this study were to apply an unstructured grid, two-dimensional hydrodynamic model (ADCIRC-2DDI) to simulate the astronomical tide level and storm surge height along the east coast of Taiwan. Two ar- tificial neural network models were subsequently adopted to improve the calculations of the hydrodynamic model. Three 640 Fig. 1. (a) Bathymetric map and locations of tidal gauge stations on the east coast of quantitative statistical measures, i.e. the mean absolute er- 641 Fig.Taiwan 1. and (a) (b)Bathymetric an unstructured map grid andin the locations modeling domain of tidal for gaugethe simulation. stations on ror, root-mean-square error, and peak error, were employed 642 the east coast of Taiwan and (b) an unstructured grid in the modeling to evaluate the prediction of the tide and storm surges un- 643 domain for the simulation. der the five typhoon events by a hydrodynamic model and an 644 ANN model, including the back propagation neural network 645 (BPNN) and adaptive neuro-fuzzy inference system (ANFIS) CIRCulation (ADCIRC) model to simulate the storm surge techniques.
response to two hurricane events26 that occurred in 1989 and 1999. Xia et al. (2008) simulated tropical cyclone storm- induced surge, inundation, and coastal circulation at the Cape Fear River estuary and adjacent Long Bay using the 2 Hydrodynamic model Princeton Ocean Model (POM) with a three-level nesting approach. Etala (2009) used the nested depth-averaged nu- 2.1 Storm surge model merical model for the simulation of storm surge at the Ar- gentinean Shelf. Rego and Li (2010) studied the storm surge The water surface elevations and circulation patterns are gen- of Hurricane Ike along the Texas–Louisiana coast using the erated using the ADCIRC-2DDI, which is a vertically aver- fully nonlinear Finite Volume Coastal Ocean Model (FV- aged two-dimensional, fully nonlinear, hydrodynamic model COM) with a high-resolution unstructured mesh. You et (Luettich, 1992). ADCIRC-2DDI uses the mass and momen- al. (2010) compared simulated storm surges using the two- tum conservation equations with the hydrostatic pressure ap- dimensional operational storm surge/tide forecast system (re- proximation. In the model, the baroclinic terms are neglected gion tide/storm surge model, RTSM, which is based on the and the hybrid bottom friction formulation is used. The lat- Princeton Ocean Model), and the three-dimensional regional eral diffusion/dispersion terms are enabled, which leads to ocean modeling system (ROMS) using the observed data the equality laws in primitive, nonconservative form ex- from 30 coastal tidal stations from three typhoons that struck pressed in a spherical coordinate system (Kolar et al., 1994): Korea in 2007. Recently, artificial neural network (ANN) models have ∂ζ 1 ∂UH ∂(V H cosφ) + [ + ] = 0 (1) been extensively applied to predict the storm surge and ∂t R cosφ ∂λ ∂φ tide variation and to resolve the issue of nonlinear relation- ∂U 1 ∂U 1 ∂U tanφ ships. For examples, Herman et al. (2007) combined a two- + U + V − ( U + f )V (2) dimensional hydrodynamic model and neural network mod- ∂t R cosφ ∂λ R ∂φ R els to predict the tidal levels and currents in the German 1 ∂ ps 1 τsλ τbλ = − [ + g(ζ − αη)] + Mλ + − North Sea coast. The authors demonstrated that the approach R cosφ ∂λ ρ0 H ρ0H ρ0H
Nat. Hazards Earth Syst. Sci., 12, 3799–3809, 2012 www.nat-hazards-earth-syst-sci.net/12/3799/2012/ W.-B. Chen et al.: Predicting typhoon-induced storm surge tide 3801
∂V 1 ∂V 1 ∂V tanφ + U + V + ( U + f )U (3) speed of any wetting fronts. The Courant number is less than ∂t R cosφ ∂λ R ∂φ R 0.5 to maintain numerical stability. More details of ADCIRC- 1 ∂ ps 1 τsφ τbφ 2DDI model can be found in Luettich et al. (1992), Kolar et = − [ + g(ζ − αη)] + Mφ + − R ∂φ ρ0 H ρ0H ρ0H al. (1994), and Westerink et al. (1994). where the vertically averaged momentum dispersion in the 2.2 Global tidal model longitudinal and the latitudinal directions can be expressed as follows (Kolar and Gray, 1990): A large fraction of the variance in many oceanographic vari- ables is due to tides. To simulate tidal propagation in the 2 2 Eh2 1 ∂ (U,V )H ∂ (U,V )H storm surge model, the driving tidal forces at the open bound- Mλ,φ = [ + ], (4) R2 cos2 φ ∂λ2 ∂φ2 aries are necessary. In the present study, a global ocean tidal model that was developed by Oregon State University, which and t = time; λ,φ = degrees longitude (east of Greenwich is is the TOPEX/Poseidon Global Inverse Solution (TPXO), is positive) and latitude (north of the Equator is positive), re- used to specify the open boundaries of ADCIRC for simu- spectively; U,V = the depth-averaged velocity in the longi- lating tidal propagation. TPXO is the current version of a tudinal and latitudinal directions, respectively; H = the to- global model of ocean tides, and it incorporates data from the tal height of the vertical water column (= h + ζ ); h = the TOPEX/Poseidon satellite. The tides are provided as com- bathymetric depth, relative to mean sea level (MSL); ζ = the plex amplitudes of the Earth-relative sea-surface elevation free surface elevation, relative to MSL; R = the radius of the for eight primary (M2, S2, N2, K2, K1, O1, P1, and Q1) and Earth; f = 2sinφ = the Coriolis parameter; = the an- two long-period (Mf and Mm) harmonic constituents. The gular speed of the Earth; ps = the atmospheric pressure at methods used to compute the model are described in detail the free surface; ρ0 = the reference density of water; g = the by Egbert et al. (1994) and Egbert and Erofeeva (2002). acceleration due to gravity; α = the effective Earth elasticity factor; Eh2 = the horizontal eddy viscosity; τsλ,τsφ = the ap- 2.3 Cyclone model plied free surface stress in the longitudinal and the latitudinal directions, respectively; τbλ,τbφ = the bottom shear stresses The meteorological driving forces underlying storm surges in the longitudinal and the latitudinal directions, respectively, consist of wind stress and an atmospheric pressure gradient. and η = the Newtonian equilibrium tide potential. Therefore, determining the wind field and the pressure field In the ADCIRC-2DDI model, the hybrid bottom friction of a tropical cyclone is indispensable for conducting cyclone formulation is used when the wetting and drying of elements surge calculations. is executed, because this expression results in being highly In the present study, the atmospheric pressure field derived dissipative as the water depth becomes small (Grenier et al., from cyclostrophic flow can be expressed as follows (Jeles- 1995). nianski, 1965): The bottom friction in the model is expressed as 1 r 3 = + − ≤ ≤ = = Pr Pc (Pn Pc) , for0 r Rmax (7a) τbλ Uτ∗ and τbφ V τ∗. (5) 4 Rmax 3 r −1 The hybrid bottom friction (τ∗) formulation, which pro- Pr = Pn − (Pn − Pc) , forr > Rmax, (7b) vided a depth-dependent bottom friction coefficient, can be 4 Rmax defined by the following formula: where Pc = the pressure at the typhoon center; Pn = the am- 2 2 1/2 bient pressure and was assumed to be equal to 1013 hPa; r = Cf(U + V ) τ∗ = , (6) the radius which is the distance from the typhoon center; and H Rmax = the radius of the maximum wind. Following Graham = [ + Hbreak θ ]γ /θ = and Nunn (1959), Rmax is represented by the following equa- where Cf Cfmin 1 ( H ) the bottom drag coef- −2 tion: ficient. The unit of Cf is kg m ; Cfmin = the minimum bot- tom drag coefficient that is approached in deep water when R = 28.52tanh[0.0873(φ − 28)] the hybrid bottom friction formulation reverts to a standard max + . / ( − P ) . + . V + . , quadratic bottom friction function; Hbreak = the break depth 12 22 exp 1013 c 33 86 0 2 f 32 77 to determine if the hybrid bottom friction formulation will behave as a standard bottom friction function or increase with where φ is the latitude of the typhoon center and Vf is the depth similar to a Manning’s type bottom friction function; forward speed of the typhoon. and θ and γ = dimensionless parameters which are given in The wind field comprises a rotational and a translational Sect. 4.2. component. At a distance, r, from the center of the typhoon, ADCIRC-2DDI is Courant-limited algorithmically due to the rotational wind speed, Vr, is described by the following its explicit feature, and it is also limited by the propagation equation (Young and Sobey, 1981): www.nat-hazards-earth-syst-sci.net/12/3799/2012/ Nat. Hazards Earth Syst. Sci., 12, 3799–3809, 2012 646 647 648 649 650 651 652 3802 W.-B.653 Chen et al.: Predicting typhoon-induced storm surge tide 654 655