To investigate the effectiveness of closed-type check dams on retaining driftwood, we conducted a series of wood-laden debris flow flume experiments. In the experiments, we varied (1) the initial sedimentation condition of the check dam and (2) the length of input driftwood, investigating the amount of driftwood retained by the check dam and its temporal change. As a result, we found that the retention mechanism of the driftwood is more strongly governed by the difference in the initial sedimentation condition rather than by the difference in driftwood length. The ratio of retained driftwood to supplied driftwood was generally higher under the empty-dam condition, by which more than half of the driftwood was retained regardless of experiment cases. The ratio was notably higher under the empty-dam condition, with driftwood length longer than the spillway width. Under the filled-dam condition, on the other hand, the ratio showed relatively wider fluctuation among experiment cases, even when the driftwood length was longer than the spillway width. In light of the log behavior behind the check dam under all cases, driftwood was more certainly retained due to the resilient blockage structures formed by driftwood near the check dam spillway. In the case of the empty-dam condition, driftwood at the head of the debris flow that reached the check dam was covered and immobilized by the following sediment. Further efforts should be made to evaluate other factors, e.g., congestion level of log transport that potentially affect the effectiveness of closed-type check dams. Our findings suggest that maintaining the storage capacity of closed-type check dams is essential for utilizing existing structures for mitigating driftwood disasters.

On March 5, 2020, a slope failure was observed below the wing of the Maeirisawa No. 2 check dam in Shibukawa City, Gunma prefecture. To estimate the process of this failure, we conducted geological surveys and laboratory permeability tests on pumice and sand-gravel samples collected from the failed slopes. We also installed a time-lapse camera to monitor the slopes excavated for repair works. The investigation results revealed that a highly permeable pumice layer was interbedded within sand-gravel layers. Time-lapse images captured erosion in the exposed sections of the pumice layer during rainfall events, indicating that the layer was more prone to erosion than the sand-gravel layers. These findings suggest that the primary causes of the failure were the repeated erosion of the pumice layer and the subsequent collapse of the overlying and underlying sand and gravel layers due to rainfall.

On January 1, 2024, a large-scale earthquake (the 2024 Noto Peninsula Earthquake) with a magnitude of 7.6 struck the Noto region, Ishikawa Prefecture. This earthquake and the resulting tsunami and crustal movement caused extensive disaster to this region. Furthermore, the earthquake caused many slope failures, landslides, and river blockages, resulting in numerous casualties and damage to houses and infrastructure. The Japan Society of Erosion Control Engineering immediately established an investigation committee for these sediment related disasters, and an urgent investigation team was dispatched to the disaster area. The team visited to eight locations in the Noto Peninsula and conduct the investigation on February 11 and 12, 2024. The objectives of the urgent investigation are to comprehensively understand overall scope of the sediment related disasters, and identifying specific target areas for a more detailed investigation being carried out as a next stage. This disaster report preliminary summarizes the information on the sediment movement phenomenon triggered by the earthquake and the actual damages caused by the sediment movement obtained by the urgent investigation.