筑後川下流低平農地域の洪水緩和機能の評価
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概要
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The flood mitigation function of agricultural land in the Chiyoda district of Saga Prefecture, Japan, has been quantitatively evaluated using numerical simulations. This district is located on a flat low－lying area along the lower reach of the Chikugo River, characterized by paddy fields with a network of creeks and drainage to the tidal compartment of the Chikugo River. Generally speaking, agricultural land has a function, such as that of a reservoir, which temporarily stores up storm flow during floods. This flood storage effect for agricultural land, which is known as the flood mitigation function, is one of the many important functions for country's conservation that has been in effect since the beginning of agriculture itself. First, a paddy field basin model was constructed based on the present condition of the Chiyoda district and an upland field basin model and an urbanized basin model were introduced under the assumption that paddy fields of the Chiyoda district would be converted into upland fields and urbanized areas. Second, the difference in flood runoff characteristics caused by land uti｝ization, the creek area ratio, and the existence of optimal control of drainage facilities, was evaluated using the basin models from the viewpoint of flood mitigation. Finally, an indicator to estimate the flood mitigation function of agricultural land in a flat low－lying area in a large scale view, that was proposed by the authors in the previous paper, was examined in the Chiyoda dist,rict. It was concluded that the flood mitigation function of a flat low－lying agricultural area was evaluated by the macro－indicator MI, consisting of three sub－indicators, i.e. 1. for the land utilization, lp for the creek area ratio, and lr for the optimal control of drainage facilities.クリーク低平農地域の洪水緩和機能を総合的に表現するマクロな指標の策定を目的として, 佐賀県神埼郡千代田町に選定したクリーク水田流域を対象に数値実験を行い，前報（平松ら, 1999）の仮想流域での計算結果との比較を交えながら, その洪水流出特性を検討した. 流域地目指標Iαやクリーク面積率指標Iβ, 最適管理指標Iγの値は, 流域条件や降雨条件の違いによって当然変化してくると考えられるが, 本論文で示した洪水緩和指標MIを用いることで, クリーク低平農地域の洪水緩和機能をマクロに評価することができるとの確信を得た. 得られた結果を要約すると以下のようである. すなわち, ①農地の洪水緩和機能は水田の場合に特に効果的であり, また, 畑地も水田にほぼ匹敵する洪水緩和機能を有する, ②クリークのもつ洪水緩和機能はその面積比率で評価できる, ③適切な排水施設管理によって洪水緩和機能はさらに高まる, ④クリーク低平農地の洪水緩和機能は（1）式でおおむね評価できる．
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