Emergency response and permanent measures for large landslide dams triggered by the 2004 Mid-Niigata Prefecture Earthquake in Japan

Abstract

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.