Evaluation of crush and abrasion phenomena in cobble gravels during transport in mountain rivers

Although the crush and abrasion phenomena associated with the downstream flow of riverbed gravel have long been studied, they still present a challenge. The crush and abrasion phenomena of gravel are not considered in river channel and erosion control plans in Japan. However, evaluating the crush and abrasion phenomena of gravel that flows down from mountain rivers is important from the viewpoint of the integrated management of sediment in a watershed. Thus, we conducted rotation crush and abrasion tests on nine cobble gravel rock types, 〜 200 mm in diameter, which are normally distributed in mountain rivers, and determined the characteristics of the weight reduction rate ( β r ) of cobble gravel by rock type and the grain size distribution of the sediment particles produced. Then, we conducted channel crush and abrasion tests to convert the crush and abrasion phenomena associated with the rotational movement of cobble gravel into those associated with the downstream flow of gravel in mountain rivers. The result of comparing the rates of weight reduction ( β r and β s ) of cobble gravel in both tests showed that the rate of weight reduction ( β r ) associated with rotation can be converted to the rate of weight reduction ( β s ) closer to the downstream flow of cobble gravel in mountain rivers by multiplying the conversion rate α ＊ with the rotation-associated crush and abrasion coefficient α r using Sternberg’s law.