Japan is subjected to harsh weather conditions, with a high overall annual rainfall, a rainy season, and concentrated torrential rains during the typhoon season. Moreover, rivers in Japan generally have a steep gradient and are characteristically subjected to sudden rises of water level that cause flooding, or the falling of water level over a short period when the rain ends, causing the water shortage. Although dams have been constructed according to river basin characteristics to handle flooding and water usage issues, the number of favorable dam sites in Japan is rather small. Thus, extensive dam upgrading, aimed at strengthening the function of existing dams, has recently been carried out. In this report, examples of past dam upgrading projects as well as the status of dam upgrading are reviewed. In particular, the technological issues of dam heightening and additional installation of discharge facilities are discussed for upgrading projects in which structural retrofitting is carried out while the dam is in operation. Moreover, the required measures are considered.

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