Heavy rainfall which we had never experienced occurred in the northern part of Kyushu Island on July 2012. A record-breaking rainfall was observed at several stations. This heavy rainfall caused sediment disaster, river flooding and inundation that claimed many casualties and missing in Ooita, Kumamoto and Fukuoka Prefecture. In Taketa City, Ooita prefecture, two people died and more than 250 houses were inundated due to the flooding of the Tamarai River which flows through the Taketa City. Especially, the water level rose rapidly and reached about 2.5m from a top of a levee in the Haitabaru area, downstream part of the Tamarai River. A field observation and a numerical simulation were carried out in the Tamarai River Basin in order to find out the reason why the water level had risen like that. Results from calculations showed that the increase in the water level due to a decrease of just 10 percent in the cross-sectional area of the river was estimated to be 3m. Because of heavy rainfall, a large amount of driftwoods were produced in the upstream area and accumulated at the river bridge, New Azou Bridge. As a result the cross- sectional area of the river decreased and the rapid increase in the water level occurred.

In the yosasa river, that a flood occurred by a heavy rain in the August, 1998, the large-scale improvement was performed. The authors observed the changes of the topography of the river-bed to examine a response by the impact of the construction.

As a result, I confirmed the following: 1) Sands and gravels began to flow from the river-bed, then accumulated in the wide place of the river and in the inside of the bend. 2) In several places, the gravels of the river-bed disappeared, and bedrock cropped out. 3) A plant grew in the place that water did not flow through, and the landscape of the river changed. 4) The change of the topography of the river-bed is slowly, because the amount of gravels and sands flowing from the upper basin is little.

Unprecedented heavy rainfall occurred in Yamaguchi and Shimane Prefectures on July 2013. These regions suffered severe damage brought by floods during the rainfall. In order to investigate the characteristics of the damage and the mechanisms of land erosion by the floods, post-disaster survey and numerical simulation were conducted for Abu River in Yamaguchi along which three railway bridges were washed out and soil in agricultural lands was intensively eroded. The results of the field observations and the two-dimensional flow analysis indicate that the overflow at a meandering point of the river caused land erosion, and the strong flows were induced at the points where one of the railway bridges was washed out.