Journal Article 多段砂州システムの周期的沖向き移動と岸沖漂砂・土砂量変動の関係に関する基礎的研究
Cross-shore sediment transport and sediment volume change during systematic migration of multiple sandbars

由比, 政年  ,  小椋, 太智  ,  松山, 正之

石川県千里浜・高松・七塚海岸に対する深浅測量データを基に,多段砂州の周期的沖向移動に伴う変動を抽出し,経験的固有関数(EOF)法に基づく解析を行って,砂州移動の固有モードや岸沖漂砂分布,土砂量変動の解析を行った.砂州の形成・移動・消失から成る地形変動は,EOF第1,第2モードの組合せで表現され,両モードが符号を変えつつ交互に支配的となる4つの型を1サイクル間に推移する.対応する岸沖漂砂分布は,地形変動に対する空間固有関数の空間積分と時間固有関数の時間微分の積により規定される.砂州移動に伴い,汀線での土砂輸送も岸向・沖向に周期変化して汀線付近の地盤高が変動する.前浜との土砂授受により断面内土砂量は周期変動を示すが,1サイクル経過すると元の状態に戻り,土砂量の長期変動には寄与しない.The characteristics of systematic net-offshore migration of the multiple bar system on the Chirihama and neighboring beach, Japan, have been investigated using a set of annual field surveys collected over 11 years. An EOF analysis has been conducted on the periodic movements of the sandbars, and the related cross-shore sediment transport and sediment volume variation were examined. The migration cycle of bars consisting of the successive generation, migration and degeneration were well reproduced by the combination of the 1st and 2nd eigen-modes, and these two modes become dominant alternatively. The principal modes of cross-shore sediment transport are shown to be characterized by the spatial integration of space eigen-function and the temporal derivative of the time eigen-functions. Corresponding to the cyclic movement of sandbars, the direction of sediment transport at the shoreline changes alternatively between seaward and landward directions. The sediment volume over the profile varies accordingly, but on the long-term the net-offshore migration of sandbars does not contribute to the sediment volume variation.

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