Abstract
In recent years, frequency of flood occurrence has been increasing all over the world because of global climate since 1990s as one of the reasons. In Kochi Prefecture. amount of precipitation has been changing irregularly and widely, and number of disasters has increased since 1994. There were two heavy downpours in Kochi prefecture after 1994. The cause of unusual heavy downpour is not contingent phenomenon but rather drastic global climate change. The total amount of damage caused by the downpour in Kochi City in 24th September 1998 was about 68billion yen. Flood control project cost about 33 billion yen was carried out by the government grants to change the year probability of occurrence from 1/30 years to 1/50 years at Kokubu River. The rainfall data from 1994 to 2005 were compiled andanalyzed to know the fluctuation tendency and evaluated the year probability of occurrence. According to the result of analysis and evaluation, 1/50 years probability of occurrence found to be occurred in 1/20 years. As a result. it can be said that the safety of flood control has fell down and potentiality of flood occurrence has been increased at Kokubu River.
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