河川流木の移動パターンに基づいた流木流出量の予測

摘要

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 57.1 km~2 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 2 years. 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 275 m to 7532 m. 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 5 years 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 2 years, but was underestimated for the other 3 years.%河川における流木は大雨により大量に流出し，橋梁を閉塞し氾濫を引き起こしたり，海に到達して漁業被害を引き起こすなど様々な災害の原因となるほか，海岸に堆積しゴミ問題を引き起こしている。そのため，流木災害の軽減対策として流木捕捉施設の設置，閉塞しにくい橋梁への改良などが行われてきたが，任意地点での流木流出量(通過量)の予測が不十分なために効果的な対策がとられていない現状にある。予測が可能になれば，そのようなハード対策に加えて，警戒避難体制を構築する際にも役立つことが予想される。海岸のゴミ問題についても，海への流木流出量が予測できれば，計画的な処理が可能になるはずである。