ISHIK2023
Title：A Study on the Flood Control Function of Sanpo-Gata in the Upstream of the Rokkaku River
発表者：大串浩一郎* ・ 千綿 友貴** （Koichiro OHGUSHI*, Yuuki CHIWATA **）
所属：*佐賀大学理工学部教授 **佐賀県県土整備部技師 （Professor, Faculty of Science and Engineering, Saga University ** Engineer, Land Development Department, Saga Prefecure）
キーワード：六角川, 三法方, 成富兵庫茂安, 数值解析, 治水機能
Keywords：Rokkaku River, Sanpo-Gata, Naritomi Hyogo Shigeyasu, numerical analysis, flood control function
Abstract
要旨：
佐賀県の六角川は白石平野を東に流れ有明海に注ぐ一級河川である、低平地を流れ、我が国最大の干満差を有する有明海に注ぐことから、感潮区間が約29km と非常に長い。したがって、六角川流域において満潮時で外水位が高い時の豪雨では、内水排除が困難となり、内水氾濫や外水氾濫が発生しやすい状況になる。 感潮区間が長く塩水が遡上してくる現在の武雄市橘町一帯を開発するため、1625年に肥前佐賀藩士の成富兵庫茂安が大日堰や石井樋等の施設を設けた、これらの施設は塩水の遡上を止めて真水を確保し、灌漑用水を確保しただけでなく、上流からの氾濫洪水を抑える治水の役割も担っていたと考えられている。本研究では三法方の施設がある武雄市橘町付近を対象に近年豪雨が発生した令和3年8月豪雨を対象として、洪水時における六角川本川及び支川の東川の流況を1次元不定流解析により再現し、東川及び支川の越流の様子を 2次元氾濫解析で再現した、横提がある場合とない場合の2つにケースの浸水深を比較することで三法方の治水機能の検討を行った。また、六角川と東川を繋ぐ生見川の治水効果の検証も行った。
Abstract：
The Rokkaku River in Saga Prefecture is a first-class river that flows eastward through the Shiroishi Plain and flows into the Ariake Sea. Since it flows on low-lying land and the tidal range of the Ariake Sea is largest in Japan, it is easily affected by tides and has a long tidal section. Therefore, in the Rokkaku River Basin, it is difficult to eliminate inland water during heavy rain at high tide and when the external water level is high, and inland flooding and external flooding are likely to occur. In order to develop the area of present-day Tachibana Town (Sanpo-Gata) in Takeo City, where the tidal section is long and salt water rises upstream, in 1625 Naritomi Hyogo Shigeyasu of the Hizen Saga Domain established facilities such as the Dainichi Weir and Ishii Gutter. These facilities are thought to play a role not only in water use by stopping salt water run-up to secure fresh water and allowing water to be taken in some villages, but also to play a role in flood control to suppress flooding from upstream. In this study, we reproduced the flow of the Rokkaku River main channel and the Higashi River by one-dimensional unsteady flow analysis during flooding due to the torrential rain in August, 2021. Moreover, two-dimensional flood analysis is executed for the overflow from the Higashi River and other tributaries. By comparing the inundated depth in the case with crossing embankment and that in the case without it, the flood control function of the Sanpo-Gata is discussed. We also examined the flood control function of the Ikumi River which connects thw Rokkaku River and the Higashi River..