[A-CG45] Water and sediment dynamics from land to coastal zones

[ACG45-08] Sediment yield and discharge during heavy rainfalls and subsequent elevated sediment discharge in mountainous watersheds

★Invited Papers

*Taro Uchida¹ (1.University of Tsukuba)

Keywords:Sediment yield and discharge, Mountainous watershed, Sediment management

In Japan, the mountainous areas occupy about 70% of the national land. In mountainous areas, various sediment transport phenomena, such as landslide (shallow landslide, deep-seated landslide etc.), earth flow, surface erosion, debris flow, sediment-sheet flow, bedload, suspended load and so on. Landslides and debris flows rarely occur during rainfall that occurs less than once every few years, and are concentrated during rainfall that occurs once every several decades to hundreds of years. appear. On the other hand, the amount of sediment that moves due to landslides and debris flows is several orders of magnitude larger than the amount of sediment that moves due to other processes, likes surface erosion, so the amount of sediment that moves in mountainous areas fluctuates extremely from year to year. Furthermore, once a large-scale rainfall or earthquake causes a number of landslides or debris flows, the sediment transport capacity of mountain river during single event is insufficient, leaving a large amount of sediment in and around mountain rivers. Therefore, in recent years, it has been clarified that the elevated sediment discharge will continue for several to several decades thereafter. So, the sediment dynamics in mountainous areas are as follows: (1) The amount of sediment movement increases sharply as the scale of rainfall increases, and there is a highly non-linear relationship between the amount of rainfall and the amount of sediment movement, (2) the environment related to sediment dynamics changes on a time scale of several years to decades, which is not found in rainfall-runoff response and sediment movement in downstream rivers. Therefore, it is important to quantitatively evaluate the characteristics of (1) and (2) in order to deepen the understanding of sediment movement in the system from mountain to ocean and to carry out appropriate and sustainable sediment management. Based on the above, in this presentation, I will reorganize from the viewpoints of (1) and (2), focusing on the recent research conducted by the authors, and discuss the possibility and issues of quantitative evaluation.