This issue launches the publication of English language journals of the Japan Society of Erosion Control Engineering, which will be distributed through the internet as e-journals. The society intends to disseminate information on sediment hazards and SABO works, new SABO research and SABO technologies, and exchange SABO information in various places around the world. Thus, in this first issue, I review and introduce sediment hazards and SABO works in Japan.
A large scale landslide occurred in the Atsumari River Catchment, Minamata City, Kumamoto Prefecture on July 20, 2003. This type of landslide is called a “deep-seated landslide.” The debris flow induced by this landslide killed 15 people. The deep-seated landslide was caused by the rising of groundwater level due to heavy rainfall, hydrogeomorphological formation of the underground area prone to storage of groundwater, and deeply weathered volcanic rocks. Some debris-flow disasters induced by the deep-seated landslides have occurred around this disaster site, such as the disasters at Nishiuchitate in Ebino City, Miyazaki Prefecture in 1972 and at Harihara in Izumi City, Kagoshima Prefecture in 1997. These areas are underlain by quaternary volcanic rocks, andesite, and tuff breccia. The characteristics of the deep-seated landslides and the debris flow induced by it were examined based on a field study in the Atsumari River Catchment. On the basis of the topographical, geological, and hydrological data of the volcanic areas around these sites, this study describes a method of predicting potential deep-seated landslide sites.
Typhoon No.14 struck Japan in September, 2005, subjecting large areas to danger for many hours. The maximum hourly rainfall was not so great, but the total rainfall for 72 hours was 〉 500 mm in areas which suffered considerable damage. The typhoon caused 116 debris flows, 185 slope-collapses, and 30 landslides. Concerning human and building casualties, twenty-seven people were killed in Kagoshima, Miyazaki and Yamaguchi Prefecture, and two people went missing in Oita Prefecture. Additionally, 1,178 houses were completely destroyed and 3,692 houses were partly destroyed in Japan by the typhoon. These damages were due to comparatively heavy rainfall which continued for many hours. Large scale slope-collapses occurred in particular, around Miyazaki Prefecture due to heavy continuous rainfall (862 - 1,321mm).
We describe various management operations undertaken by the Japanese central government in response to sediment-related disasters caused by the Mid-Niigata Prefecture Earthquake of October 23, 2004. The operations included the implementation of emergency and permanent measures for stabilizing the landslide dam in Higashi-Takezawa district as well as policy development of a sabo (erosion and sediment control) master plan for the Imogawa River basin. The earthquake caused many landslides in the Imogawa River basin. The landslide that occurred in Higashi-Takezawa district formed a large dam that reached a height of 30 m, which imposed a risk of the water level rising high enough to overflow and cause the blockage to collapse. Therefore, sabo works were undertaken by the central government to plan and implement emergency countermeasures. The Yuzawa Sabo Office of the Ministry of Land, Infrastructure, and Transport was directly responsible for these emergency measures, which included pumping out water from the reservoir to reduce the water level and prevent the landslide dam collapsing, and constructing a temporary drainage channel in preparation for the coming snowmelt and rainy season. These emergency measures ensured the safety of the downstream area. The local headquarters established in the Yuzawa Sabo Office monitored the changing conditions in the area, and planned and implemented further preventive and recovery measures while consulting with experts. In addition, an advisory committee on the landslide dams in the Imogawa River basin was established, and ad hoc meetings were held to discuss emergency and permanent measures for stabilizing the dams and developing a sabo master plan for the Imogawa River basin. Helicopter and terrestrial monitoring took place during the snowmelt and rainy season after the earthquake. In addition, aerial photo surveys and aerial laser scanning surveys were conducted to follow up with the sediment yield and runoff processes. This monitoring contributes to the security of residents and construction workers, and to the revision of the sabo master plan.
On August 9 and 10, 2003, Typhoon 0310 (Etau) hit the Saru and Appetsu river basins in the Hidaka region of Hokkaido. Mountain districts experienced disasters caused by washouts of sediment and woody debris produced by shallow landslides and debris flows. We conducted field surveys, aerial photography, satellite data analysis, and a fact-finding survey of the basin′s inhabitants in order to reveal the actual magnitude of the disasters. The storm-induced shallow landslide volume(excluding void volume) in the Saru Basin was estimated as 13 × 106 m3.The amount of resulting woody debris in the Saru River Basin was estimated as approximately 190 × 103 m3, and less than 10% of the debris reached the Nibutani Dam reservoir. The occurrence of shallow landslides tended to increase when the total rainfall exceeded 330 mm and also when the hourly rainfall intensity increased. The sediment budget for a small catchment (2.1 km2 in area) of the Saru River during the August 2003 storm revealed that approximately 100,000 m3 of sediment was generated by landslides, and 80% of this was stored within the catchment, especially in downstream higher-order channels; the budget also revealed that only 20% of the generated sediment was discharged from the catchment. The woody debris budget of the Appetsu River basin showed that 65,000 m3 of woody debris was newly produced from living trees on slopes or floodplains. An amount of 23,377 m3 originated from slope failures on mountain slopes or bank erosion; 10,604 m3, from riparian forests; and 12,278 m3, from old timber deposits along the river′s course. Approximately 30,000 m3 of woody debris was considered to remain upstream or to flow into the sea. In the Appetsu River basin, most shallow slope failures occurred during the period when the rain intensity reached 40 to 50 mm/h. There seemed to be no time lag between the occurrence of slope failures and the inflow of debris to the basin.