More than 6000 landslides over 400 km2 were triggered by the Eastern Iburi Earthquake that occurred on September 6, 2018, in Iburi Sub-prefecture, Hokkaido. A large amount of sediment was transported downstream in gentle and hilly landscapes (< 500 m in elevation) by the landslides. The landslides occurred within two major land cover types : forested areas (FA) and logged areas (LA). Here, the characteristics of landslides within these different land cover types were evaluated based on GIS analysis and field investigation. A total of 1440 landslide scars were identified in an area of 18.9 km2 consisting of 87% FA and 13% LA. The ranges of the slope gradient within the two land cover types were identical, from 30 to 40°. The mean landslide area in LA at 2306 m2 (standard deviation [SD] : 1675 m2) tended to be greater than that in FA (mean : 1762 m2 ; SD : 1749 m2). Based on the field investigation, the mean depth of landslides in FA (1.5 m) and LA (1.4 m) were similar, while the mean estimated volume of the landslides was 3610 m3 in FA and 6359 m3 in LA. The mean runout distance of landslides in LA was longer at 231 m (SD : 168 m) compared to that in FA at 146 m (SD : 140 m). The short runout distance in FA was possibly associated with a reduction in sediment momentum through the presence of wood pieces in FA. The findings of our study suggest that vegetation cover and the resultant differences in landslide size and runout distance are important factors for evaluating potential earthquake-related disasters, and future planning in forest management.
Multi-disasters took place in the Mampituba River basin, southern Brazil, as a consequence of the passing of two cyclones, in July 2020. The first one was a bomb cyclone that caused several storm winds, damaging the rooftops and overthrow countless trees. The second was an extratropical cyclone that brought extreme precipitation, and triggered storm winds, debris flow, debris flood, woody debris flow, and floods. In the study area, there are two National Parks (Aparados da Serra and Serra Geral). The multi-disasters destroyed a river gauge station used as a part of an early warning system by the National Parks and destroyed partially a marked trail. The most affected municipalities were Praia Grande and Mampituba, where the damages were more related to the storm winds, floods, and debris flood. Furthermore, during the passing of the cyclones, many people were affected, and some of them should be moved to public shelter. Such multi-disasters occurred during the pandemic of COVID-19 in Brazil, which might increase the synergy of the simultaneous occurrence of such extreme events.
On July 4, 2020, heavy rainfall was observed in the southern part of Kumamoto Prefecture and the northern part of Kagoshima Prefecture. Due to heavy rainfall, floods and sediment disasters such as collapses and debris flows occurred predominantly in the Kuma River basin of Kumamoto Prefecture. We conducted field investigations at four sites in Ashikita Town and Tsunagi Town, Kumamoto Prefecture, where collapses and debris flows caused deaths. We also conducted field investigations in the Kawauchi River branch of the Kuma River, where vast amounts of sediment discharge caused damage to houses. The objectives of these investigations were to clarify the situation and mechanism of the disasters and to propose procedures to recover from the disasters. This report briefly summarizes the results of these investigations.