Departmental Bulletin Paper 豪雨に対する谷池ならびに下流域の被災リスク低減に関する研究

吉迫, 宏

(1)  , pp.79 - 133 , 2017-03-28 , 国立研究開発法人 農業・食品産業技術総合研究機構 , National Agriculture and Food Research Organization (NARO)
ISSN:2432-7883
Description
本稿は,ため池における管理実態の解明や各種の機能診断・評価手法の検討を通じて,ため池の豪雨に対する施設と下流域の被災リスクを管理により低減する方策を解明した。受益農家や集落が実施する管理作業の実態は,石川県珠洲市のため池を事例としてアンケート調査と現地調査に基づいて明らかにした。また,目視点検を踏まえた機能診断手法として,堤体の漏水経路を1m 深地温の測定結果を基にして,堤体中の地温分布のシミュレーションによる逆解析から推定する手法を提案した。豪雨によるため池の被災リスクの評価と低減策は,広島県東広島市のため池を事例として洪水流出モデルによるため池貯水位予測モデルを作成し,貯水位を指標として検討した。減災対策のうち,水位の低下管理は流域面積の大きなため池では被災リスクの低減効果は限定的な効果に留まるのに対し,洪水吐の簡易改修は安定的に被災リスクを低減することを明らかにした。ため池群の洪水緩和効果による下流河川流域の被災リスクの評価と低減策は,広島県椋梨川上流域を事例としてため池群を組み込んだ広域洪水流出モデルを作成し,下流河川の基準点における水位を指標として検討した。ため池群を活用して被災リスクの低減を図るためには,降雨ピーク前の空き容量確保が重要なことを明らかにした。ため池の利水容量の転用による洪水調節容量の創出は,広島県東広島市のため池を事例として取水に伴う貯水率の減少度合と降雨による回復度合を観測水位から求め,検討期間中の貯水率変化を簡便に予測する手法を考案して検討した。受益水田の転用・転作が進んでいるため池では,かんがい期間と洪水期間が競合する期間においても利水容量を転用できる可能性が高いことを明らかにした。棚田における土壌流出の評価を踏まえたため池集水域管理の検討は,島根県出雲地方における棚田の土壌流出の観測結果に基づいて観測田(耕作田/遊休田)の土壌流出実態を明らかにした上で,土壌流亡予測式における水稲・水田の作物係数C と保全係数P を算出して行った。土壌流亡予測式の係数比較から水稲作の維持は土壌流出の抑制に有効であることを明らかにし,貯水池への土砂流入抑制による貯水容量の維持に資する,棚田を含むため池集水域の管理について考察した。
In this study, we investigated management measures to reduce the risk of heavy rain disaster to irrigation pond facilities and downstream areas by clarifying the actual condition of irrigation pond management and discussing various methods of functional diagnosis and evaluation.We made a case study on the actual condition of management conducted by farmers and local community members who benefit from irrigation ponds in Suzu City, Ishikawa Prefecture through questionnaire survey and field investigation. We then proposed a functional diagnosis method based on visual checks; this method estimates leakage paths in a dam from the inverse analysis of simulated ground temperature distribution in the dam on the basis of ground temperature measurements at a depth of 1 m.We evaluated the risk of heavy rain disaster to irrigation ponds and discussed mitigation measures for the case of irrigation ponds in Higashi-hiroshima City, Hiroshima Prefecture by developing a reservoir level prediction model based on a flood runoff model and using the reservoir level as an indicator. We found that among the disaster mitigation measures, controlled water level reduction in an irrigation pond having a large catchment area has only a limited effect on disaster risk reduction, whereas minor modifications to spillways can reduce disaster risks on a stable basis.We evaluated disaster risk reduction in downstream river areas resulting from flood mitigation using a group of irrigation ponds and discussed mitigation measures for the case of the upper river basin of the Mukunashi River in Hiroshima Prefecture by developing a regional flood runoff model incorporating a group of irrigation ponds and using river water levels at downstream reference locations as an indicator. We found that it is important to secure an available capacity before experiencing peak rainfall in order to reduce disaster risks by using a group of irrigation ponds.To create an extra capacity of flood control by diverting the service water capacity of irrigation ponds, we studied the case of irrigation ponds in Higashi-hiroshima City, Hiroshima Prefecture and deduced, from the observed water levels, decrease in the percentage of storage due to withdrawal and increase in the percentage of storage due to rainfall. We then created and discussed a simple method to predict changes in the percentage of storage during a given investigation period. We found that in irrigation ponds serving water to paddy fields where diversion and crop changeover have progressed, it is likely that service water capacity can be diverted even during an overlapping periodof irrigation and flood control.In studying the management of irrigation pond catchment areas in consideration of soil runoff from terraced paddy fields, we observed soil runoff from terraced paddy fields in the Izumo district of Shimane Prefecture to clarify the state of soil runoff from the observed cultivated and idle paddy fields, and then calculated the cropping factor C and the conservation practice factor P in the soil loss equation for the studied paddy and paddy fields. From the comparison of factors in the soil loss equation, we found that preserving rice cultivation is effective for suppressing soil runoff. We then discussed the management of irrigation pond catchment areas including terraced paddy fields in terms of maintaining reservoir capacity by suppressing the flow of deposit into reservoirs.
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