[H-DS11] Geohazards in humid, tectonically active countries and their precursors

[HDS11-P03] Storage and reactivate of landslide-derived sediment after the 1973 Kotani’ishi disaster at Hokkaido, Japan

*Takashi Koi¹, Yasutaka Tanaka² (1.Center for Natural Hazards Resarch, Hokkaido University, 2.Ministry of Land, Infrastructure, Transport and Tourism Chubu Retional Development Bureau)

Keywords:sediment movement, seiment storage, rainfall history, vegetaion invasion, the 1973 Kotani'ishi disaster, Hokkaido

Heavy rainfall cause landslides in a mountain catchment, and a large amount of unstable sediment is supplied to fluvial systems. Although a part of the supplied sediment flows out of the catchment immediately after the landslide occurrence, most of it stays as stored sediment and moves to downstream during subsequent rainfall. Large-scale flooding events may cause debris flows due to the reactivation of such fluvial sediment storages, which may contribute to disasters. There are few studies focusing on the reactivation of landslide-derived sediments stored in fluvial systems on the medium- to long-term timescale, and to understand the dynamics of stored sediment for interdecadal timescale is an important issue in terms of disaster prevention. In this study, the time-series sediment movement was clarified from the occurrence of sediment production event on the several decades by comparison with the time-series aerial photographs, hydrological data and the timing of vegetation invasion in and around the fluvial channel. The study site is the Nakanosawa river at the Kotani’ishi district of the Shiriuchi town, Hokkaido, Japan. The Kotani’ishi recorded the highest hourly rainfall in the history of observation in 1973, and this heavy rainfall caused multiple landslides and debris flows in this area. After the 1973 disaster several check dams were installed by the Hokkaido prefecture. As a result of the interpretation of the time-series aerial photographs, there was no significant sediment discharge that reached catchment outlet from the 1973 event to 2008, when the latest aerial photographs were taken. On the other hand, according to the field survey conducted at the Kotani’ishi in 2020, reservoir of the erosion control dam installed near the catchment outlet was filled with sand. Thus, relatively large-scale sediment movements were expected to occur after 2009. From the timing of vegetation invasion to the river channel, it was found that moderate sediment movement continued from the upstream to the middle reach of the catchment, while there was minimal sediment movement in the downstream after the disaster until recent years. Due to the examining of hydrological data, a rainfall event that exceeded the rainfall intensity of the 1973 event occurred in 2013. Since no new landslides have been confirmed in the catchments since 2009, it was considered that a large amount of stored sediment in the river channel moved to the lower reach during the 2013 rainfall event. In this catchment, moderate sediment movements constantly occurred from the upstream to the middle reach. In addition, about 40 years after the landsliding event, high-intensity rainfall event caused large-scale movement of stored sediment and sediment discharge to catchment outlet. The large-scale discharge of sediment originating from the sediment produced several decades ago suggests the importance of facility development immediately after the disaster.