地質を考慮した風化基岩における凍結融解による土砂生産量の推定方法

摘要

山地から海までの水系全体における土砂の動態を把握することは，防災や河川環境の維持を考えるにあたり重要である。そのためには，山間部の裸地斜面等からの土砂の生産，生産土砂の河道への流入，河道内での土砂の移動および堆積の一連の流れを的確に推定することが必要となる。現在では河床変動計算の技術がほぼ確立しており，流路における土砂動態を数値計算により推測することが可能となっている。しかしながら解析の実行にあたり境界条件として必要な，山間部の裸地斜面等における土砂生産の時期や量，粒径分布などの予測手法が確立されておらず，河床変動の将来予測を行うには限界がある。%The prediction of sediment production by freeze-thaw action is important for preventing sediment-related disasters and managing river environments. This study conducted a freeze-thaw experiment to investigate the destruction mode of various types of weathered bedrock (representing some of the many types of bedrock in Japan) and to develop destruction models for estimating sediment production. The experiment was conducted on bedrock samples of weathered granite (WGr), weathered granite porphyry (WGp), weathered shale (WS), weathered sandstone (WSa), and weathered rhyolite (WR). Freeze-thaw cycles caused the bedrock samples to increase in porosity and decrease in weight. When the porosities of WGr, WGp, and WS became greater than 0.43, 0.1, and 0.27, respectively, small pieces started to detach from the main sample bodies. Freezing and thawing caused only slight increases in the porosities of WSa and WR and in the production of fine sediment from their surfaces. During one freeze-thaw cycle, WSa and WR produced sediment amounts equivalent to depths of less than 1,0 and 0.2 mm, respectively. It is assumed that these depths are equivalent to the rock particle size. Using the experimental results, two simple destruction models were developed. A model was applied to WGr, WGp and WS. In this model, it was assumed that the porosity increased at a certain rate for each freeze-thaw cycle. When the porosity was greater than the threshold value, the weathered bedrock decomposed to sediment. An alternative model was applied to WSa and WR. In this case it was assumed that sand particles - the size of which was determined by the constituents of the weathered bedrock - were produced with each freeze-thaw cycle. Using this model, we performed a numerical simulation, and estimated the rate of sediment production. The simulation results were in good agreement with measured data.