[A-HW22] Material transportation and cycling at the land-sea interface: from headwaters to the ocean

[AHW22-P06] Decreasing trend in suspended sediment concentration after the landslides in 2018 in a small suburban river

*SAI TUN AUNG SI¹, Shin-ichi Onodera¹, Sharon Bih Kimbi¹, Wanna Zaw¹, Mitsuyo Saito¹, Yusuke Tomozawa¹ (1.Graduate School of Advanced Science and Engineering)

Keywords:suspended sediment, sediment, water, turbidity

Sediments play a significant part in elements cycling in aquatic ecosystems. In July 2018, Higashihiroshima faced severe rainfall, triggering landslides and debris flows in its mountainous regions, notably impacting the Saijo River basin. In the Saijo River basin, surface water sediment pollution prevails, primarily attributable to intense rainfall during the rainy season. These sediments originate from upstream areas and surrounding agricultural land. This study aims to analyze the suspended sediment concentration (SSC) in the Saijo River following the 2018 landslide event, with a specific focus on understanding its trends and impacts on catchment hydrology. Samples were collected from the Saijo River, a tributary of the Kurose River, for this study. Daily water level measurements were conducted using a pressure probe (Onset HOBO) from 2019 to 2023, while sediment concentration measurements were carried out during torrential events from 2020 to 2023 by laboratory analysis. Turbidity data spanning September 2020 to December 2023 was measured in this investigation, employing a precise 10-minute temporal resolution. Applying linear regression analysis, the relationship between turbidity and suspended sediment concentration was obtained, thereby discerning the suspended sediment (SS) concentration. Precipitation data obtained from the Japan Meteorological Agency website was utilized to analyze variations in both discharge and sediment concentrations. Elevated suspended sediment concentrations were observed during periods of high rainfall events. Following flood events, a gradual reduction in suspended sediment concentration was noted over time. This declining trend in suspended sediment concentration was observed in this small suburban river after the 2018 landslide. The study area experienced annual rainfall amounts of 1664 mm, 1920 mm, 1123 mm, and 1255 mm in 2020, 2021, 2022, and 2023, respectively. The highest suspended sediment loads were found in 2020, regardless of whether it was a wet or dry year. This study significantly contributes to understanding post-landslide sediment dynamics in the Saijo River.
Acknowledgements: This study was supported by the APN Project (CRRP2019-09MY-Onodera).