Sustainability and Life Span of Japanese Reservoirs-Statistic analysis of sedimentation condition in Japan

The sedimentation condition directly affected the sustainability of reservoirs which are as long as they offer sufficient water storage for their objectives such as electric power and water supply. Statistic analysis of sedimentation condition could give a measure of the sustainability and life span of reservoirs. In this study, we built a database of 421 reservoirs in Japan from 1994 to 2003. First, this paper describes the Mann-Kendall trend analysis of sedimentation in terms of total amount and annual average. Spatial distribution of sedimentation condition for each dam is presented by using geographic information systems (GIS). Most of 421 dams had a significantly increasing trend of total sedimentation with more than 2.0 in terms of Z test value. Most decreasing trend of total sedimentation are found at Chubu region, the west of Kyushu region and Tohoku region. For the annual average sedimentation, most of 421 dams show the significant decreasing trend, especially in the Chubu region (Higashiyama area and Tokai area), Kinki region and the northwest of Kyushu region. On the other hand, the significant increasing trend of annual sedimentation is presented at Hokuriku region, the west of Kanto region and the west of Shikoku region. The life span of all 421 dams are calculated by using a simple statistical method based on the Mann-Kendall trend analysis of sedimentation condition. The life span in Chubu region, Hokkaido region and Kyushu region had the shorter life span than other regions with 335, 473 and 559 years in average. In Chugoku region, there are eight dams that the life spans are over 5000 years. The relationship between the life span and watershed characteristics is estimated in this study. The average slope of watershed may have the high relationship with the sediment production. This study could give a direct information for the policy maker to conduct the best management on dams.