Bank Erosion Model along Banks Composed of both Cohesive and Non-Cohesive Material Layers

Abstract

実河川では，粘着性土と非粘着性土が互層になっている河岸がよく見られる(例えば，インドネシア・カリマンタン島のセサヤップ川等)．本研究では，河岸の上層が粘着性土，下層が非粘着性土で構成された河岸の浸食過程の数理モデルを構築した．さらに，構築した河岸浸食モデルを移動一般座標系の平面二次元河床変動解析モデルに導入し，解析を試みた．その結果，粘着性土と非粘着性土が混在した河岸では，河岸の浸食速度を時間的に大きく変化することが明らかとなった．つまり，河床の洗掘が十分でなく，下層の非粘着性土が流れに露出していないときは，河岸の浸食速度は非常に遅くなる．一方，河床の洗掘が進み，非粘着性土が流れに露出し始めると，河岸浸食速度は急激に速くなる．このような浸食速度の急激な変化は，河岸の質に関する情報が無い場合は予測が難しく，河川近傍の住民の河岸浸食対策が遅れがちになり，大きな災害を引き起こす可能性が高い．

It is often observed banks which have the alternate layers between cohesive sediment layers and non-cohesive sediment layers in natural rivers (ex. the Sesayap River, East Kalimantan Province, Indonesia and so on). In this study, mathematical model of bank erosion process for banks with both the cohesive and the non-cohesive layers is developed. Furthermore, the model is installed into the horizontal two dimensional channel deformation model. The governing equations of the channel deformation model are written in boundary fitted moving general coordinate system. The results indicate that the horizontal shift speed of banks changes with time very much. When the bottom layer is not exposed to the flow, the speed of bank erosion is very slow. However, when bed near the bank is eroded with time and bottom layer starts to be exposed to flow, the bank erosion rate increases rapidly and it is considered that a severe disaster is caused. The calculated horizontal shape of bank lines is not so smooth and local curvature of bank lines becomes small.