BEHAVIOUR OF DRIFTWOOD IN THE SARU RIVER DURING TYPHOON NO.10

Abstract

Typhoon Na.10 attacked the Pacific side of Hokkaido on August 9, especially in Iburi and Hidaka sub prefectures. The flood generated large amounts of driftwood and caused bridge washout in the Saru River basin. Based on an onsite survey conducted immediately after the typhoon, we examined the driftwood in the Saru River. We studied the distribution of locations where driftwood beached in the river channel, the volume of such driftwood, the distribution of driftwood origin in the river channel, reasons for driftwood originating there. The influence of driftwood to the rivers structures (bridge piers etc) is also investigated. It became clear that the driftwood generated from the river channel and the mountain area occupied 20% and 40% of all driftwood, respectively as a result of investigation. The generating places of the driftwood from the river channel were the flood plain at the inner bend and the eroded outer bank shore.