Author(s): Kazuaki Ohtsuki; Takashi Kawasaki; Naoya Kitamura; Yasuo Nihei; Norio Onikura
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Keywords: Nonuniform suspended-solid transport; Sediment sampling analysis; Dam removal; Floods; Kumagawa River
Abstract: The effects of damming on sediment transport in downstream and shore zones have previously been discussed. Removing the dam can be the most fundamental countermeasure, but this has not been studied in detail, especially in the case of a full-scale dam removal. The Arase dam, which is located on the Kumagawa River, Kyushu Is., Japan, is currently undergoing demolition. This is the first high dam to be removed in Japan. Unusually, the Arase dam is relatively near to a river estuary, so the influence of the project could rapidly reach downstream and affect the tidal flats on the estuary. To assess the impact of the dam removal project on sediment transportation and deposition in the downstream region, we conducted primary surveys focusing on the transport of nonuniform suspended solids (SS) and the characteristics of the sediment. The results of two years of monitoring and 80 flood samples indicated that the flux of the SS and the coefficient b in the L-Q relation were larger than those reported in other rivers in Kyushu. The diameter of the SS was also remarkably smaller than that observed in other rivers in Japan. Comparing the results with those of a survey undertaken in 2002 before the decision had been taken to open the dam gates permanently showed that the size of the sediment particles on the tidal flats has increased and that result corresponds to the transport record for SS and the results of the analysis of the constituent elements of the sediment.
Year: 2013