Effective Flood Control Through Integrated and Collaborative Dam Operation at Three Dams in the Upper Nabari River

Heavy rain with Typhoon 18 threatened the Nabari River Basin, Kansai region, with inundation early on the morning of October 8, 2009. The Nabari River is a tributary of the Yodo river basin that contains Osaka and Kyoto and runs through Nabari City, which is a residential zone a commutable distance from Osaka city. In the upper reaches of the Nabari, there are three multipurpose dams – Shorenji Dam, Hinachi Dam, and Murou Dam – that are operated by the Kizugawa Integrated Dam Control and Management Office (KIDCMO), a branch office of the Japan Water Agency (JWA). Since it rained heavily downstream from the three dams, regular operation of the dams complying with given flood control regulations appeared unable to save Nabari City from inundation. The JWA and Ministry of Land, Infrastructure, Transport and Tourism (MLIT) therefore conducted collaborative operation of the three dams to avoid inundating the city. In this case, flood control operation of the three dams commenced at an early stage before inflow reached defined flood discharge in consideration of the water level of the Nabari River, rainfall conditions, and the capacity of reservoirs. During operation, discharge from dams was changed in a timely manner and appropriately through collaborative operation of the three dams in order to maximize the effectiveness of all flood control capacities of reservoirs based on the latest rainfall forecast technology and runoff analysis. The use of improved rainfall forecast technology and runoff analysis models enabled effective application of flexible operation protocols. It is estimated that this operation has resulted in a 1.5 m decrease in the water level at the Nabari design control point and saved approximately 1,200 households from inundation. Considering recent climate change, it is possible to have extreme rainfall more often. The proof of the adaptability of this flexible operation is quite meaningful not only for flood damage mitigation in the downstream but also for future prospects of flood control by dams.