LONG TERM CHANGES IN FLOODING AROUND GIFU CITY

Abstract. Heavy rainfall within a short period and consequent high river discharge are results in floods which subsequently creates huge infrastructure problems for areas, such huge economic deficits in production as well as damages to existing property and goods, even loss of human lives. In about hundred years ago the flat-land around the Kiso Three Rivers system (the catchment areas of the Kiso River, Nagara River, and Ibi River are called the Kiso Three River system) included Gifu City, Gifu Prefecture of Japan, was frequently damaged by inundation after heavy rainfall. Our research aims to evaluate flood risk changes by river improvement in the Kiso Three River system fin the past hundred years. For that reason, we simulated in several times on flood inundation in the large, medium and low scale case in the urban area of Gifu city with 2D flood inundation model, and compared the distribution of flood depth. The results, the flood inundation area in urban area for small scale flood disaster dramatically is reduced from 3.82 km² in the past time to 0.48 km² in the current time with the progress of river infrastructure improvement. However flooded area in urban area for large and middle scale disaster show the almost same. It is suggested that flood risks in urban area are dramatically improved for only small disaster.