Countermeasures against a landslide dam in Nagatono caused by Typhoon 12 in September 2011 －Countermeasures against a large landslide dam with a high head and steep slope

Abstract

An overflow channel and a sabo dam, which dissipates energy in the water flowing through the overflow channel and prevents foot scouring of the landslide dam, are currently being designed as countermeasures against the Nagatono landslide dam. However, the head of the overflow channel is as high as 80 m. Additionally, the dam has a steep slope of 1/2.5, and the dam's slope is curved. Because these conditions make it difficult to predict hydraulic phenomena, designing appropriate countermeasures is problematic. Therefore, we conducted hydraulic experiments to assess the efficacy of the proposed design and to confirm the effects of the overflow channel and the sabo dam. During experiments, a shock wave appeared on the slope and the curved section of the overflow channel due to the water's high velocity and drift current, but the water flow was controlled to safe levels using a raised revetment, the height of which was determined using the Nappe equation. When the length of the energy dissipater was determined based on the technical standard, more frequent overflow was observed on the wing of the sabo dam due to whirlpools caused by the drift current. Therefore, we concluded that, under these conditions, the energy dissipater needs to be longer. These results demonstrate the necessity for conducting hydraulic experiments during the planning and design stages to ensure the location and height of sabo dams are appropriate to ensure adequate reductions in water energy.