Many slope failures occurred in a valley on 30th June 1999 at Kannondai, Hiroshima city due to heavy rainfall. The slope failures in the valley can be classified into two types, upper slope failure and lower slope failure, according to their geologic, pedologic and topographic features for slide scars, and their setting in the valley. The upper slope failures occurred in head hollow between upper convex break of slope and lower one. The lower slope failures occurred in lower sideslope beneath the lower convex break of slope.
The process of the lower slope failure is presumed that the sand and humus layers above the weathered basement rock were saturated with water by the heavy rainfalls at the lower sideslope, and that the stability of the lower sideslope then decreased, which resulted in the regolith sliding on the basement rock. On the other hand, the process of the upper slope failure is presumed that heavy rainfalls induced piping by saturated through flow in angular gravel layer, and that regolith of the layer and other layers above the fresh basement rock at the head hollow had fallen off the fresh basement rock.
The existence of the four joint systems and the two types of weathering processes on the basement rock affect the geomorphic features of the slope failures and the landforms in the valley.
These two types of slope failures repeatedly occur at the same landforms, and their processes have been maintaining the landforms in the valley.