Natural Hazards 29: 325–340, 2003.© 2003 Kluwer Academic Publishers. Printed in the Netherlands.325Study on a Simplified Method of Tsunami RiskAssessmentHIROAKI SATORiver and Coastal Engineering Department, The Newjec Consultants, Inc., 20-19-1 Shimanouchi,Chuo-ku, Osaka, 542-0082, Japan, E-mail: [email protected] MURAKAMIDepartment: Ecosystem Engineering, The University of Tokushima, 2-1 Minami-josanjima,Tokushima, 770, Japan, E-mail: [email protected] KOZUKIDepartment: Ecosystem Engineering, The University of Tokushima, 2-1 Minami-josanjima,Tokushima, 770, Japan, E-mail: [email protected] YAMAMOTOShikoku Research Institute Inc., 2109-8 Yashima-nishi, Takamatsu, 761-0192, Japan,E-mail: [email protected](Received: 31 October 2000; accepted: 15 February 2002)Abstract. For the testing of the effect on the tsunami prevention facilities, a simplified methodfor tsunami risk assessment was suggested without wave run-up analysis. This method is proposedusing calculated offshore tsunami waveform and field reconnaissance such as the seawall height, timenecessary for residents’ evacuation and tsunami warning insurance. Then, two normalized values areevaluated; one is the ratio of calculated maximum tsunami height to seawall height, the other is theratio of time between tsunami over-topping and evacuation completion to total time required forevacuation. These two values are used to qualitatively estimate the safety of residents and the effectof tsunami prevention facilities, eliminating the necessity to compute complicated tsunami run-uponshore.Key words: tsunami, earthquake, risk assessment, prevention facilities, evacuation.1. IntroductionMany big earthquakes have occurred around the Nankai Trough, which runs fromeast to west of offshore Shikoku Island and Kii peninsula, Japan (here in Nankaido,Figure 1). The historical earthquakes have occurred at 100-to-150-year intervals,magnitudes have been near (or over) 8.0. Each time, residents living near theNankaido suffer damage from tsunami disasters. The government has taken somecounter measures against the next tsunami, for instance, breakwaters, seawalls,evacuation facilities and evacuation information.For counter planning against tsunami disaster, the tsunami risks should be eval-uated in detail. However, location of the next earthquake cannot be specified.326 HIROAKI SATO ET AL.F igure 1. Calculation domain. Computation domain covers an area 515 × 168.75 km fromShikoku Island to Kii peninsula. At Nankaido, historical earthquakes have occurred aroundNankai Trough.Tsunami arrival time and tsunami height will change with the fault location.Supposing the same fault model of Ansei earthquake in 1854 is utilized in thetheoretical calculation, the result values maybe change according to the differentset-up of the model (Kawata and Koike, 1994). If the next earthquake occurs ina different place from historical earthquakes, more residents will suffer from thenext tsunami because residents will act according to historical experiences. In thisstudy, using the shifted fault models along the Nankai Trough, tsunami arrival time,tsunami height and “ratio of excess” were evaluated in detail.The most effective counter measure differs from location to location. In areaswhere the tsunami will arrive very fast, the breakwaters and seawalls should beconstructed first because residents will have no time for evacuation. If tsunamiheight is evaluated to be low, evacuation facilities should be maintained first. Thegovernment should grasp the effect of these measures, and should decide whichones to adopt. The safety of residents should be considered as the top issue. Inparticular, “ease of evacuation” becomes most important for reduction of damageto human beings. Recently, some research on evacuation has been reported. Kawataand Koike (1996), Inoue et al. (1996) and Shimada et al. (1999) assumed evacu-ation velocity and routes. Damage to human beings was estimated by using timeseries analysis based on the wave run-up calculations. However, these methodstake a long time and much calculation data, and re-calculation is required whenSTUDY ON A SIMPLIFIED METHOD OF TSUNAMI RISK ASSESSMENT 327F igure 2. Discreet areas. For easiness of considerations, we divided computation domain into16 area along coast of Nankaido.other cases are supposed. In this study, a simplified method is proposed withoutcalculations of wave run-up.This report is composed of 4 Sections. In Section 2, using the shifted faultmodels along the Nankai Trough, tsunami arrival time, tsunami height and “ra-tio of excess” were evaluated. In Section 3, a simplified method is proposed, andtsunami risks are considered in detail. From calculated waveform and field recon-naissance, the overtopping risk and evacuation risk are defined. Section 4 containsconclusions.2. Tsunami Risk Assessment Under the Shifted Fault Model2.1.BASIC EQUATIONS AND CALCULATION DOMAINFigure 1 shows the computation domain of the present study, which covers an area515.00 × 168.75 km. The coordinate system is also illustrated in Figure 1 where thesymbol “” indicates the historical earthquake epicenters at the Nankaido offshore.In history, at the Nankaido, all serious earthquakes occurred within this area.For the case of computation, the computation domain is divided into 16 areaswith equivalent distance along the coast of the Nankaido (about 35 km, as shown inFigure 2). The grid size for the numerical scheme is 1.25 km; the minimum waterdepth is 5.0 m; time step is chosen as 1 second for stability. In addition, offshoretsunami waves are calculated for one hour after the seabed displacement using twodimensional leap-frog method.Basic numerical equations are Equations (1)–(3). The second and third terms atthe left side of Equations (1) and (2) can be neglected when the water depth is over50 m∂M∂t+ gH∂ζ∂x+ fMQH2+1HM ·∂M∂x+ N ·∂M∂y= 0, (1)∂N∂t+ gH∂ζ∂y+ fNQH2+1HM ·∂N∂x+ N ·∂N∂y= 0, (2)328 HIROAKI SATO ET AL.∂ζ∂t+∂M∂x+∂N∂y= 0, (3)where t denotes time; g is the gravity accelerator; ζ is water level lift from stillwater level; h is water depth; f is friction coefficient of ocean bottom; M and Nrepresents the discharge flux in x and y direction, respectively; ξ is the verticalamount of seabed displacement, which can be estimated by Manshinha–Smylietheory (1971) by fault model. Here, ξ is simply
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