[A-CG42] Water and sediment dynamics from land to coastal zones

[ACG42-P08] Quantitative estimation of sediment transport based on characteristics of grain-size components in Lake Kasumigaura

*Naofumi Yamaguchi¹, Takao Ouchi² (1.Global and Local Environment Co-creation Institute, Ibaraki University, 2.Ibaraki Kasumigaura Environmental Science Center)

Keywords:grain-size distribution, grain-size components, Lake Kasumigaura, sediment-transport pathway, sediment flux

Grain-size parameters of sediments have been commonly used as a possible clue to reconstruct their transport pathways and sedimentary processes. Sediments in natural environments including lakes, however, have frequently polymodal grain-size distributions, which make it difficult to interpret their representative parameters such as median diameter, sorting and skewness. The present study attempted to decompose polymodal grain-size distributions of bottom surface sediments in Lake Kasumigaura (Nishiura) and to examine the sediment transport pathways and sediment fluxes based on the analysis of the mixing proportions of grain-size component groups. We analyzed surface sediments of the lake bottom at 16 sites within a square with each side of approximately 4.2 km around the lake centre, and the grain-size distributions were separated into lognormal distributions. For decomposition of the grain-size distribution of each sample, we separated it into lognormal distributions by using expectation–maximization algorithm and determined the number of components based on the value of the Akaike information criterion. As a result of decomposition, the grain-size distributions of the present samples were separated into five or six common components. These components were classified into four common component groups and logratio analysis was applied to their mixing proportions. The logratio values of the finer component groups did not differ much, whereas the logratio values of the coarsest component group varied considerably between samples depending on the distance from the shore. Their spatial distribution indicates that sediments with the finer component groups are deposited relatively uniformly within the present study area, while sediments with the coarsest component group are predominantly supplied from the northeast and southwest coastal areas. It was also suggested that sediments with the coarsest component group may have arrived from the Ono River mouth area in the southwest of the study area. In addition, by assuming that the finest component group is invariant within the present study area, spatial differences in sediment fluxes of the coarsest component group were quantitatively estimated. It was found that the sediment fluxes of the coarsest component group at the sites closer to the littoral zone of the study site were about 3–6 times higher than that near the lake centre. The present study suggests that appropriate analysis of complex grain-size distributions may provide valuable information, such as quantitative sediment fluxes, which have previously been difficult to estimate from grain-size data alone.