Author(s): Menglu Qin; Takeyoshi Chibana
Linked Author(s):
Keywords: Alluvial fan rivers; Grain size distribution; Bedform configuration; Riverbed materials
Abstract: This study observed and characterized the bed materials of 20 alluvial fan rivers across central Japan and the Hokkaido island. These rivers were classified among the 3 typical basin geologies of Japan: granite rock basin, volcanic rock basin, and accretionary complexes basin. According to the measurements, it was found that the amount of aggregates larger than 2mm, correlates with the weathering characteristics of different rocks. Due to lesser boulder and cobble fractions, the bed materials in accretionary complexes rivers are finer than granite rock rivers and volcanic rock rivers. However, the fractional sand contents of alluvial fan rivers are mainly influenced by the total sediment yield from upstream. Only if the sediment yield is sufficiently high, sand can deposit on the bed surface. Moreover, the measurement showed that in alluvial fan rivers, high sand content generates the bimodal distribution of bed material. Therefore, this study proposed a simple method that can quantitatively estimate the characteristics of Grain Size Distribution (GSD), by utilizing sand content and standard deviation σG of gravel aggregates (>2mm). Finally, it was clarified the magnitude of bedform variability in accretionary complexes basin >> granite rock basin (high sand content rate) > volcanic rock basin. It suggests that the characteristics of GSD greatly influences the bedform variability, which is attributed to weathering characteristics of basin geology.
Year: 2020