Japan Geoscience Union Meeting 2023

Presentation information

[J] Online Poster

H (Human Geosciences ) » H-GM Geomorphology

[H-GM02] Geomorphology

Fri. May 26, 2023 3:30 PM - 5:00 PM Online Poster Zoom Room (6) (Online Poster)

convener:Hitoshi SAITO(Graduate School of Environmental Studies, Nagoya University), Junko Iwahashi(GSI of Japan), Thomas Parkner(University of Tsukuba, Faculty of Life and Environmental Sciences), Shintaro Takanami(Meiji University)


On-site poster schedule(2023/5/25 17:15-18:45)

3:30 PM - 5:00 PM

[HGM02-P05] Hierarchical Landform Classification and Sediment Transport Processes in the Upper Oi River Basin in the Akaishi Mountains, Central Japan

*Togo Iwase1, Hiroaki Tateno1, Toshihiko Sugai1 (1.Graduate School of Frontier Sciences, The University of Tokyo)

Keywords:mountainous river, sediment transport process, DEM, landform classification, roundness

There are many unknowns regarding sediment transport in mountainous areas, where sediment is produced. In particular, it is necessary to clarify the processes by which sediment produced on slopes is transported by rivers. Recent studies have discussed the relationship between slopes and mainstem rivers by analyzing the terrace composition near tributary junctions (Koiwa, 2005; Takahashi and Sugai, 2018; 2021). However, these studies have focused on mountain rivers where sediment transport by the main stem is dominant, and many unknowns still remain in the sediment transport process of rivers flowing through mountain areas where have large relief and sediment supply by tributaries is dominant. In recent years, aerial laser surveying has become widely used, and it has become possible to obtain detailed images of land surface morphology on a scale of a few meters. This suggests the possibility of capturing the sediment transport process at a finer spatial and temporal scale. In this study, we investigate the process of sediment production, transport, and deposition from tributaries and slopes to the upper reaches of the Oi River (Niken-goya to Sawarajima; 10.2 km), one of the most active sediment-producing areas in Japan, by means of hierarchical classification of landforms using fine-scale DEM, analysis of the present riverbed and terrace deposits, and measurement of the morphologies of tributary catchments.
Slope topography was classified into three categories: deep-seated gravitational slope deformations (DSGSDs), slope failure, and landslides. The geomorphic surfaces on the valley floor were classified into three categories: "main river geomorphic surface," "tributary geomorphic surface," and "talus". Tributary geomorphic surfaces were mainly distributed near the junction of the tributaries and the main-stem. Some of the tributary geomorphic surfaces develop lobate microtopographies and some are toe-cut by the main-stem and undercut of the tributaries, forming toe-cut terraces (Larson et al., 2015).
The longitudinal slope (Sp) of almost all of tributaries exceed the debris-flow dominated channel slope. The relationship between the catchment area of tributaries (At) and the area of tributary geomorphic surfaces (Ad) with toe-cut and lobe development was particularly strong. This suggests that the tributary geomorphic surface with lobes has not been dissected because of the younger age of its formation.
The roundness of the gravels in the modern main riverbed was higher than that in the tributary bed. The frequency distribution of the circularity of the gravels in the terrace gravel beds also differed depending on the bed level, and those with relatively high roundness were interpreted as mainstem sediments, while those with relatively low roundness were interpreted as tributary sediments. In all outcrops of the tributary geomorphic surfaces, a sedimentary structure was observed in which tributary sediments were placed on top of mainstem sediments.
The sediment supply, transport, and deposition processes from the slope to the river in the study area can be explained as follows. Debris flows triggered by slope failures is temporarily deposited near the junction and develop a tributary geomorphic surface. The temporarily stored tributary sediments are then eroded and transported by the mainstem, and some of them are re-deposited along the mainstem. Although absolute age samples have not been obtained in this paper, it is considered that the sediment transport process is common at least through the post-glacial period, when the climatic conditions are similar to those of the present, when debris flows dominantly occur.

References
Koiwa (2005) Geographical Review of Japan, 78, 433-454.
Larson et al. (2015) Progress in Physical Geography 39, 417-439.
Schumm (1956) Bulletin of Geological Society of America 67, 597-646. Takahashi and Sugai (2018) Quaternary International 471, 318-331.
Takahashi and Sugai (2021) Geomorphology 383, 1-18.