The estimation of woody debris flow based on the moving pattern of river woody debris

Hajime SATO, Tomoyuki ABE and Kazuhiro MINAMINO

We estimated woody debris flow based on its movement pattern by investigating woody debris along an 11.7�]km section of the upper Shiribeshi�]Toshibetsu River, in Hiyama District, Hokkaido. The watershed of the lower end of this channel covers 57.1km2 and contains the Pirika Dam. We measured the size of each piece of woody debris found along the channel, tagged it with numbered tape, and plotted it on a map. We then resurveyed the section and evaluated new woody debris similarly six times after main rain events over 2years. The proportion of woody debris that moved ranged from 1.9% to 56.1% and increased with the hourly maximum water table at an observatory. The woody debris that moved accumulated mainly on channel bars, and the mean distance moved during the rain events ranged from 275m to 7532m. The frequency distribution of the distance moved was an inverse�]J distribution. We postulated that obstacles like channel bars were distributed at the same intervals. The “P�]value", defined as the probability that woody debris was captured on each obstacle, decreased as the hourly maximum water table increased. We calculated the yearly woody debris flow to the Pirika Dam using the proportion of woody debris that moved and the “P�]value" for 5years and compared this to the volume of woody debris gathered at the dam. The estimated flow was similar to the volume of gathered woody debris for 2years, but was underestimated for the other 3years.