Numerical simulations are used to study how countermeasure facilities for sediment-laden flood should be located. To verify the validity of the numerical simulation, river survey data are used. However, these are often obtained every few years, e.g., by LiDAR surveying, and the results are expected to reflect sediment transport due to multiple heavy rainfall events. It’s difficult to assume riverbed fluctuations due to sediment-laden flood from a single heavy rainfall event. Here, LiDAR surveys were conducted at high frequency in Yotagirigawa river, and the effect of surveying periods on the expression of riverbed change due to debris flow was analyzed. First, if the surveying wasn’t conducted before the debris flow, the decrease in riverbed-height could not be captured. Second, if the survey was not conducted immediately after debris flow, it wouldn’t be possible to capture the riverbed rise. These suggest that high-frequency surveying is necessary to validate the numerical simulation.

Volcanic eruptions can lead to the deposition of pyroclastic debris in mountainous regions, potentially triggering debris flows. According to Japanese law, the government must promptly conduct surveys when pyroclastic debris accumulates under specific conditions, one of which is the thickness of the pyroclastic deposits. Traditionally, this thickness is assessed visually from helicopters or through photo analysis instead of physical measurements. However, these methods are hindered by factors such as bad weather or nighttime conditions. Thus, using diverse survey methods is crucial for rapid evaluation. This study explores the feasibility of LiDAR (Light Detection and Ranging), increasingly popular in recent years, to measure the thickness of pyroclastic deposits. LiDAR is also capable of non-contact, automatic measurement day and night. Results confirm that LiDAR is capable of accurate measurements and is a promising alternative for measuring pyroclastic deposits.

Currently, regular inspections of erosion control facilities, aimed at understanding the decline in their functionality and identifying the extent and causes of structural damage, primarily rely on visual inspections based on inspection plans or UAV inspections. However, there are few cases where UAV inspections have been applied to slope failure prevention works. Therefore, this study investigates the applicability of UAV inspections for slope failure prevention facilities located in the southwest region, characterized by rapid vegetation growth due to warm conditions, which are generally disadvantageous for inspections. The results of the trial inspections conducted with UAVs in the summer dense vegetation period in Miyazaki Prefecture suggest that at least 60% of inspections are feasible. Additionally, based on trials using actual managed facilities and inspection results, a proposal for an efficient inspection operation flow using UAVs was made, highlighting potential cost reduction effects from combining UAV inspections.

To examine the impact of earthquakes on the occurrence of landslides caused by subsequent rainfall and its period, the number of rainfall events before and after the earthquakes, the number of landslide occurrences, and the periods of high intensity rain after the earthquakes were investigated for 12 large earthquakes. 9 of the 12 earthquakes increased the rate of landslides (number of rainfall events triggering landslides/number of total rainfall events) in the first and second year after the earthquake compared to pre-earthquake period. In the case of the Kumamoto earthquake, in the first year after the earthquake, areas with more significant seismic intensity at the time of the earthquake tended to have a larger increase in the rate of landslides than in the pre-earthquake period. 8 of the 12 earthquakes had a much smaller increase in the rate of landslides compared to the pre-earthquake period than the Kumamoto earthquake, and the increase tended to be in the areas with intensity levels of 4 or 5 lower at the time of the earthquake. The three earthquakes did not increase in the rate of landslides in the two years after the earthquake compared to the pre-earthquake period. In the case of the Kumamoto earthquake, many landslides were caused by torrential rains two months after the earthquake. On the other hand, the other earthquakes tended to occur landslides caused by torrential rains at least 3 months after the earthquake or to have less number of rainfalls and smaller rainfall magnitudes than the Kumamoto earthquake.

To prevent the outflow of nourished sand on the landside, two types of sand fence arrangements were tested on a natural beach, namely (1) a configuration that combined a straight and a transverse alignment, and (2) a configuration that combined a straight and a perpendicular alignment. The former configuration was superior in its function, although the latter configuration also worked satisfactorily. In addition, long-term field observations of the beach evolution during 15 years in response to a particular sand fence arrangement demonstrated that two rows of sand fences parallel to the shore, auxiliary employed on the landward side, also functioned well. The sand volume that each configuration can control was evaluated by assuming a suitable accumulation form around the sand fences. The configuration that combines a straight and a transverse alignment will effectively trap sand with under the assumption that the transverse alignment can form a wide dune parallel to the shoreline. Evaluation of the configuration that combined a straight and a perpendicular alignment could not be performed because sand accumulation forms around fences were too complicated and did not adhere to a suitable accumulation form. For the two rows of sand fences, a scalene triangle for the accumulation form was applied to the fences having low porosities located at the seaside. It was concluded that the evaluation could be performed with satisfactory accuracy. Then, based on the results obtained from these experiments and previous knowledge, suitable sand fence configurations for winds blowing obliquely to the shore, as well as for winds blowing normally to the shore, were proposed together with guidelines to determine dimensions of each element composing the fence configuration.