Estimated Flood-prone Area Mapping on Landsat TM Image Based on Extreme-value Statistics of Osaka Bay Sea Level
Katsutoshi KOZAL, Rinya TAKAHASIII
Kobe Univ. Mercantile Marine
Keywords: Flood-prone area, return period value, Landsat TM
ABSTRACT
It is said that the damages caused by the tsunami was minimal in the Great Hanshin-Awaji earthquake occurred at January 17th, 1995. However, that earthquake certainly created an environment to reevaluate the present disaster prevention measures on the unusual sea level In general return period values of sea level for the period of 50 or 100 years are included in the design parameters of coastal structures such as breakwater according to its degree of importance. Return period values are parameters derived from extreme-value statistics and vary depending on the type of estimating method and the period of statistics.
An attempt is made to estimate high sea levels in Osaka Bay with the long return period based on the extreme-value distribution namely Gumbel and Frechet during the period from 1952 to 1994. The observed annual high sea levels agree better with the estimated high sea levels based on the Frechet distribution than the ones based on the Gumbel distribution for two tidal stations around the Osaka Bay. Estimated high sea levels with the 100 year-return period exceed well over 2 meters above the mean sea levels in Kobe and Osaka and they are mapped on the classified Landsat TM image. It became clear that the estimated flood-prone area on the Landsat TM image reaches about 78km the densely populated urban area between Kobe and Osaka.
1. INTRODUCTION
Since the hinterland facing Osaka Bay between Kobe and Osaka is easily influenced by tsunamis and high tides caused by typhoons, countermeasures against them have been frequently discussed for a long time. it is said that the damages caused by tsunami was minimal in the Great Hanshin-Awaji earthquake occurred on January 17th, 1995. However, that earthquake certainly created an environment to reevaluate the present disaster prevention measures on the unusual sea level. On the other hand, IPCC(Inter-governmental Panel for Climate Change) warned in 19951) that the average air temperature of the world will increase by 2 degrees and the mean sea level will raise by 50cm in year 2100, which creates 50 million more people living in the flood-prone area caused by high tides.
The purpose of this study is to contribute countermeasures against tsunamis and high tides by using extreme-value statistics of high sea levels of Osaka Bay. First of all return period values of high sea level for the period of 50 and 100 years were estimated based on the observed annual high sea levels at Kobe and Osaka for the period from 1952 to 1994. Return period values are parameters derived from extreme-value statistics and vary depending on the type of estimating
