Natural Hazards 29 325 340 2003 2003 Kluwer Academic Publishers Printed in the Netherlands 325 Study on a Simplified Method of Tsunami Risk Assessment HIROAKI SATO River and Coastal Engineering Department The Newjec Consultants Inc 20 19 1 Shimanouchi Chuo ku Osaka 542 0082 Japan E mail satohr osaka newjec co jp HITOSHI MURAKAMI Department Ecosystem Engineering The University of Tokushima 2 1 Minami josanjima Tokushima 770 Japan E mail murakami eco tokushima u ac jp YASUNORI KOZUKI Department Ecosystem Engineering The University of Tokushima 2 1 Minami josanjima Tokushima 770 Japan E mail kozuki eco tokushima u ac jp NAOAKI YAMAMOTO Shikoku Research Institute Inc 2109 8 Yashima nishi Takamatsu 761 0192 Japan E mail nyamamoto ssken co jp Received 31 October 2000 accepted 15 February 2002 Abstract For the testing of the effect on the tsunami prevention facilities a simplified method for tsunami risk assessment was suggested without wave run up analysis This method is proposed using calculated offshore tsunami waveform and field reconnaissance such as the seawall height time necessary for residents evacuation and tsunami warning insurance Then two normalized values are evaluated one is the ratio of calculated maximum tsunami height to seawall height the other is the ratio of time between tsunami over topping and evacuation completion to total time required for evacuation These two values are used to qualitatively estimate the safety of residents and the effect of tsunami prevention facilities eliminating the necessity to compute complicated tsunami run up onshore Key words tsunami earthquake risk assessment prevention facilities evacuation 1 Introduction Many big earthquakes have occurred around the Nankai Trough which runs from east 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 the Nankaido suffer damage from tsunami disasters The government has taken some counter 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 evaluated in detail However location of the next earthquake cannot be specified 326 HIROAKI SATO ET AL Figure 1 Calculation domain Computation domain covers an area 515 168 75 km from Shikoku Island to Kii peninsula At Nankaido historical earthquakes have occurred around Nankai 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 the theoretical calculation the result values maybe change according to the different set up of the model Kawata and Koike 1994 If the next earthquake occurs in a different place from historical earthquakes more residents will suffer from the next tsunami because residents will act according to historical experiences In this study 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 areas where the tsunami will arrive very fast the breakwaters and seawalls should be constructed first because residents will have no time for evacuation If tsunami height is evaluated to be low evacuation facilities should be maintained first The government should grasp the effect of these measures and should decide which ones to adopt The safety of residents should be considered as the top issue In particular ease of evacuation becomes most important for reduction of damage to human beings Recently some research on evacuation has been reported Kawata and Koike 1996 Inoue et al 1996 and Shimada et al 1999 assumed evacuation velocity and routes Damage to human beings was estimated by using time series analysis based on the wave run up calculations However these methods take a long time and much calculation data and re calculation is required when STUDY ON A SIMPLIFIED METHOD OF TSUNAMI RISK ASSESSMENT 327 Figure 2 Discreet areas For easiness of considerations we divided computation domain into 16 area along coast of Nankaido other cases are supposed In this study a simplified method is proposed without calculations of wave run up This report is composed of 4 Sections In Section 2 using the shifted fault models along the Nankai Trough tsunami arrival time tsunami height and ratio of excess were evaluated In Section 3 a simplified method is proposed and tsunami risks are considered in detail From calculated waveform and field reconnaissance the overtopping risk and evacuation risk are defined Section 4 contains conclusions 2 Tsunami Risk Assessment Under the Shifted Fault Model 2 1 BASIC EQUATIONS AND CALCULATION DOMAIN Figure 1 shows the computation domain of the present study which covers an area 515 00 168 75 km The coordinate system is also illustrated in Figure 1 where the symbol 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 areas with equivalent distance along the coast of the Nankaido about 35 km as shown in Figure 2 The grid size for the numerical scheme is 1 25 km the minimum water depth is 5 0 m time step is chosen as 1 second for stability In addition offshore tsunami waves are calculated for one hour after the seabed displacement using two dimensional leap frog method Basic numerical equations are Equations 1 3 The second and third terms at the left side of Equations 1 and 2 can be neglected when the water depth is over 50 m MQ M M 1 M gH f 2 M N 0 1 t x H H x y NQ N N 1 N gH f 2 M N 0 2 t y H H x y 328 HIROAKI SATO ET AL M N 0 t x y 3 where t denotes time g is the gravity accelerator is water level lift from still water level h is water depth f is friction coefficient of ocean bottom M and N represents the discharge flux in x and y direction respectively is the vertical amount of seabed displacement which can be estimated by Manshinha Smylie theory 1971 by fault model Here is simply considered as the wave surface change In the above equations H and Q can be yielded as H h Q M 2 N 2 4 5 For boundary conditions full reflection is assumed on the landward side while non reflection is used for other sides Linear long
View Full Document
Unlocking...