Estimation of the behavior of coarse woody debris in an intense storm in August 2016 and its budget for the Tottabetsu River Catchment, Eastern Hokkaido

Takuya KUDO, Norihiro NAGANO, Akira MATSUOKA, Tomoya HAYAKAWA, Takanori KAMIJO, Yohei MATSUYAMA, Nobutomo OSANAI and Mio KASAI


In recent years there has been an increasing concern about the wood in debris flows and floodwaters, which results in extensive destruction of properties and infrastructures along river courses. In order to prevent the damage, appropriate installment of measures to control the outflow of the wood is necessary, with a reliable budget of coarse woody debris underpinned by solid understanding of its behavior. This study presents the budget for the Tottabetsu River catchment (153.4km2) in Eastern Hokkaido for a record high rainfall brought by Typhoon No. 10 in August 2016. There were no control measures for woody debris in the river before the event. The budget was estimated by the combination of field investigation, an analysis of airborne LiDAR data obtained before and after the event, and an interpretation of a series of aerial photograph images. The storm intensively eroded riverbeds and banks, producing a large amount of woody debris. The wood, some of which remained jammed or captured in sediment deposits along the course after the event, was originated from both hillslopes and riparian forests. The outflow was estimated at 58,000m3 from the Tottabetsu River in total, which was 70% of the inflow into the course. Dividing the catchment into units based on stream order, the proportion of the outflow of the wood ranged from 0.90 to 1.00 for the mountainous and from 0.17 to 0.86 for the alluvial parts. The outflow was positively correlated with catchment sediment yield for each unit. The outcomes in this study will help plan the control measures of coarse woody debris through the channel network, not only for the Tottabetsu River but also elsewhere with a range of catchment sizes.

Key words

coarse woody debris, budget of coarse woody debris, wood behavior