The tendency of damage of stone walls in terrace fields in the northwestern Kumamoto City by the 2016 Kumamoto Earthquake and heavy rain in 2016 and 2006

The topographical features of farmland stone walls that were damaged by two heavy rains and the 2016 Kumamoto Earthquake were studied. Topographical analyses were performed on data for sites of the damaged farmland stone walls by QGIS. The topographical analyses focused on altitude, inclination angle and slope aspect. From altitude analysis results, the following is revealed. In lower-altitude districts, heavy rain was likely the cause of collapse. There is the possibility that collapses of stone walls caused by the earthquake occurred on farmland of every altitude except low-lying land. From inclination angle analysis results, the following is revealed. The collapses of farmland stone walls tended not to occur on steeper slopes by the heavy rain and the earthquake. There is the possibility that collapses of stone walls caused by the heavy rain and the earthquake occurred on farmland of every inclination angle. In addition, particularly in the earthquake, it was possible that the damage was concentrated in the area where damage from past earthquakes are concentrated and the geological boundary area. From slope aspect analysis results, it was thought that this factor was due to characteristics of the wind direction in the heavy rain and the seismic wave direction of the earthquake, not to topographical characteristics. In the heavy rain, more collapses tended to occur on the slope facing the wind direction when hourly precipitation was over 20mm. In the earthquake, more collapses tended to occur on the slope facing the direction in which strong acceleration acted.