河川の土砂運搬作用を考慮した河床縦断面形のシミュレーション

Numeral simulation of longitudinal river profile considering fluvial sediment transportation

谷川 晋一; 三箇 智二*; 安江 健一

Tanikawa, Shinichi; Sanga, Tomoji*; Yasue, Kenichi

従来の河川地形変化のモデル(拡散過程のモデル)は、特に下流域における土砂運搬量を適切に表現できていなかった。この問題を解決するため、河川の土砂運搬能を、河床堆積物の粒径と河川流量の関数として下流側へ増大させる数理モデルを提案し、これに基づくシミュレーションプログラムを開発した。さらに本研究では、仮想の河川における河床縦断面形発達のシミュレーションを実施し、開発したプログラムの有効性を確認した。また、合流による河床上昇の問題を解決できることも確認した。本シミュレーション手法は、河川における地形発達過程の理解の一助になるものと考えられる。

Existing simulation methods of long-term fluvial processes, represented as conventional difference scheme of diffusion equation, cannot control about the expression of sediment transportation satisfactorily, particularly around confluences in middle and lower reach of the river. To address this, we newly developed a numeral model and simulation program of fluvial processes, in which sediment outflux is distributed to multiple grids with the formula that enlarges the fluvial transport efficiency downstream as a function of grain-size and drainage area (named "transport distance increment curve"). This formula can control the concavity of longitudinal profile. With this model, analyses about expressions of geomorphic development in longitudinal river profile were carried out. As a result, it was confirmed that, with accurate settings of parameters, this model can yield features of river profiles, considering sediment transportation around confluences. It is believed from the above that this model enables the expression of fluvial topography as seen in nature, by translating the concept of long-term fluvial process into parameters.