The heavy rain in Northern Kyushu District on July 5, 2017 caused a sediment disaster, resulting in the loss of many lives and damage to buildings. In this study, the primary causes (topography and geology) and trigger factors (rainfall) for the sediment disaster were spatially analyzed to examine factors contributing to slope failure. As a result, it was found that the number of slope failures was highest in metamorphic rock areas and the occurrence density of the landslides was highest in plutonic rock areas. In addition, the slope angle of the slope-failure source point was sizable in volcanic rock areas and many landslides occurred in the valley-formed areas. A rainfall analysis showed that the Akatani, Shirakitani, Sozu, Kita, Naragaya, Myoken, Katsura river basins and Ono, Ohi, Sata, Inaibaru river basins are different rainfall distributions, which significantly affected the slope-failure occurrence density.

The infiltration of rainfall into a slope surface may affect slope stability; thus, it is important to understand the amount of rainfall infiltration (hereafter referred to as the “infiltration capacity”) for a slope surface layer when evaluating slope stability. This research focuses on slope gradient, a factor affecting the infiltration capacity, and performs two types of water-spraying experiments using pit sand under the same conditions but with different slope gradients. In the first experiment, the surface flow rate and soil loss were measured using an earth-tank model with a horizontal distance of 0.5 m, depth of 0.1 m, and width of 0.2 m to form slope gradients of 2°, 20°, and 40° to clarify the effect of slope gradient on the infiltration capacity. In the second experiment, a water-spraying experiment that closely simulated natural rainfall was performed at a large-scale rainfall facility owned by the National Research Institute for Earth Science and Disaster Resilience (NIED), Japan. This experiment used an earth-tank model with a horizontal distance of 1.21 m, depth of 0.5 m, and width of 0.5 m to form slope gradients of 2°, 10°, 20°, 30°, and 40° with the aim of proposing a quantitative evaluation method for the relationship between the slope gradient and infiltration capacity. The results showed that the soil loss and infiltration capacity increased as the slope gradient increased in the case of the pit sand used in the experiments. This was confirmed to be due to the fact that an increased gradient allowed grains with diameters of <50 μm in the slope surface layer to flow out easily, thereby increasing the infiltration capacity. In addition, the relationship between the rainfall intensity and infiltration capacity revealed that the infiltration capacity varied depending on the rainfall intensity and slope gradient, which is unlike the relationship for constant values such as the permeability coefficient. Moreover, the research findings indicated a strong, positive linear relationship (R2 = 0.98) between the slope gradient and fitting factor Ic. Therefore, the relationship between rainfall intensity and the infiltration capacity could be expressed using the fitting factor Ic. This suggests the possibility of quantitatively evaluating the relationships between rainfall intensity, the infiltration slope gradient, and the infiltration capacity.