Quantification of the woody debris in Appetsu river basin by Typhoon 0310, Etau

Abstract

We quantified the woody debris on the flood plain of the Appetsu River, watershed area is 287.9 km², by Typhoon 0310, Etau. This typhoon caused heavy damage, especially in the Hidaka and Tokachi districts of Hokkaido. We established 61 study sites in the river basin and investigated the size, species, and freshness of woody debris in a sampling plot in each study site. We used aerial photographs to estimate the volume of woody debris outside the study sites. The volume of woody debris totaled 46, 260 m³. The deposits were greatest in the upper river basin in the northern area. Of the total volume, 76% originated from landslides on mountain slopes or bank erosion, and the remaining 24% was from riparian forests. Of the woody debris, 15% were conifers and the remaining 85% were broadleaf trees. An estimated 74% of the volume came from trees that were living just before Typhoon 0310 struck, and the remainder was old fallen timber. Of the woody debris, 12% of the trunks had diameters under 10 cm, 57% were 10 - 30 cm, 28% were 30 - 50 cm, and 3% were over 50 cm.