Sediment-related Disaster Caused by Typhoon 0310 (Etau) in Hidaka Region of Hokkaido, Japan

Abstract

On August 9 and 10, 2003, Typhoon 0310 (Etau) hit the Saru and Appetsu river basins in the Hidaka region of Hokkaido. Mountain districts experienced disasters caused by washouts of sediment and woody debris produced by shallow landslides and debris flows. We conducted field surveys, aerial photography, satellite data analysis, and a fact-finding survey of the basin′s inhabitants in order to reveal the actual magnitude of the disasters. The storm-induced shallow landslide volume(excluding void volume) in the Saru Basin was estimated as 13 × 106 m3.The amount of resulting woody debris in the Saru River Basin was estimated as approximately 190 × 103 m3, and less than 10% of the debris reached the Nibutani Dam reservoir. The occurrence of shallow landslides tended to increase when the total rainfall exceeded 330 mm and also when the hourly rainfall intensity increased. The sediment budget for a small catchment (2.1 km2 in area) of the Saru River during the August 2003 storm revealed that approximately 100,000 m3 of sediment was generated by landslides, and 80% of this was stored within the catchment, especially in downstream higher-order channels; the budget also revealed that only 20% of the generated sediment was discharged from the catchment. The woody debris budget of the Appetsu River basin showed that 65,000 m3 of woody debris was newly produced from living trees on slopes or floodplains. An amount of 23,377 m3 originated from slope failures on mountain slopes or bank erosion; 10,604 m3, from riparian forests; and 12,278 m3, from old timber deposits along the river′s course. Approximately 30,000 m3 of woody debris was considered to remain upstream or to flow into the sea. In the Appetsu River basin, most shallow slope failures occurred during the period when the rain intensity reached 40 to 50 mm/h. There seemed to be no time lag between the occurrence of slope failures and the inflow of debris to the basin.