Sediment-related
Disaster Caused by Typhoon 0310 Etau in Hidaka Region
of
Hokkaido,
Japan
Yasuhiro Murakami,
Osamu Shimizu, Hajime Sato, Takashi Yamada
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 basinfs inhabitants in order to reveal the actual magnitude of
the disasters. The storm-induced sediment solid 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
riverfs 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.