Management of a typhoon-induced landslide in Otomura (Japan)

Late in January 2004 slope instability evidence such as cracks and subsidence appeared on a retaining wall along National Highway 168, near Otomura (Nara Prefecture, Japan). This road plays a strategic role as a long distance route for passenger vehicles and trucks, therefore detailed investigations and constant surveillance have to be carried out in order to manage the induced risk situations. Six months later, on August 10th, a large landslide occurred due to heavy rainfalls related to typhoons #10 and #11 that hit Japan on the first week of August. Field and aerial surveys of the site were carried out soon after the appearance of the first geomorphologic evidence of landslide movements, and a monitoring system was immediately set up. Landslide displacements have been measured since the early stage of movement and road traffic was strictly controlled in order to minimize possible damage. This paper illustrates the effects of landslide activation and the investigations carried out in order to assess landslide hazard and predict the time of failure. Suitable methods for risk management oriented to increase the public safety and including risk control and crisis mitigation acts are also discussed.