[A-CG48] Water and sediment dynamics from land to coastal zones

[ACG48-P08] Tidal sediment transport and its change in a brackish lake as suggested by grain-size component analysis: a case study from Lake Hinuma, Japan

*Naofumi Yamaguchi¹, Ayaka Miura² (1.Global and Local Environment Co-creation Institute, Ibaraki University, 2.Department of science, Ibaraki University)

Keywords:Grain-size distribution, Lacustrine sediment, Sedimentary process, Sediment transport

Sediment transport in brackish lakes connected to the sea is complicated by flood tidal currents as well as inflow rivers and waves. It is generally difficult to directly observe such sediment transport. The characteristics of sediment grain size in the bottom surface layer, which reflects its source and sedimentary process, is an important clue for estimating sediment transport trend; however, the grain-size distributions are complex and difficult to interpret where there are multiple sources and sedimentary processes. In the present study, sediment supply and transport into the lake were investigated through a detailed analysis of the complex grain size distributions in the bottom surface sediments of brackish lake. The study target, Lake Hinuma, is a brackish lake located in the central part of Ibaraki, Japan. It is connected to the sea through the downstream of Hinuma River and is flooded at high tide. In the present study, surface sediments from the lake bottom were collected at a total of 21 sites in the offshore area. In addition, columnar samples up to 34 cm in depth were collected at three of the sites using a gravity-type columnar sampler. The columnar samples were divided into 1 cm depth-wise sections and used for grain size analysis. Grain-size distributions of these samples were measured, and the obtained distributions were separated into lognormal distributions using the EM algorithm. The number of components to be separated was determined by AIC and BIC based on Yamaguchi (2023). An unchanging component was estimated and a logratio analysis was applied to the mixing proportions of each component. The grain size distributions of the lake bottom surface sediments and columnar samples were separated into five or six lognormal distributions with common characteristics. The grain size distributions of the lake bottom surface sediments and columnar samples were separated into five or six log-normal distributions with common characteristics. The mixing proportion of the finest-grained component (mean grain size: ca. 9.6 phi) varied little between samples. As a result of logratio analysis with the finest-grained component as the normalised component, the logratio values of the finer-grained components varied little within the lake and in the vertical direction, whereas the values of the coarser-grained components varied significantly between samples. The spatial distribution of the coarser-grained components of the bottom surface sediments suggested that the sediments were supplied and transported by tidal currents from Hinuma River. An increase in the coarser-grained components was observed towards the top of the columnar samples at the site near Hinuma River, suggesting an increase in sediment transport by tidal currents in recent years. Further investigation of more survey sites may enable more detailed sediment transport pathways and their changes to be estimated.