Interdecadal-timescale storage and reactivate of landslide-derived sediment of fluvial systems in Hokkaido, Japan

Heavy rainfall and earthquake 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 situation of vegetation invasion in and around the fluvial channel. The study sites are the Kotani'ishi and the Okushiri Island, 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 three catchments. In the Okushiri Island, landslides occurred intensively on the island due to the 1993 southeast-off Hokkaido earthquake (M_W 7.7). From the results of time-series aerial photography interpretation, there was almost no sediment movement along the fluvial channel for decades after the sediment production event in the both sites. 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 basin exit was filled with sand. Thus, relatively large-scale sediment movements were expected to occur after 2009, when the latest aerial photographs were taken. 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 reported 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. These results indicates that the constant movement of stored sediment in the channel was not remarkable in the both study sites, however, it was suggested that the high-intensity rainfall event directly caused reactivation of the stored sediment even after several decades from the landslide occurrences.