Evaluation of River Restoration Techniques Using Woody Debris and Boulders Turally Generated From Flood Events

27 Mar Evaluation of River Restoration Techniques Using Woody Debris and Boulders Turally Generated From Flood Events

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Abstract

荒廃渓流であった北海道北部パンケオロピリカイ川に設置された構造物において，施工後の地形変化・構造物の機能及び耐久性・魚類生息環境について評価を行った．1998 年災害後に発生した倒流木と転石を用いて 3 タイプの構造物 (ログダム，デブリキャッチャー，巨石床固工) が設置され，施工後 8 年を経過して多様な環境が作り出されていた．施工区では落ち込み淵の増加がみられ，流速は緩急に富み，底質も多様な礫径分布となっていた．ほとんどの構造物では木材の腐朽は見られず，8 年以上構造物は維持されていた．魚類調査の結果，2004 年 6 月と 10 月のログダム区において対照区より著しく多いサクラマス幼魚とハナカジカが，10 月のデブリキャッチャー区において対照区より著しく多いサクラマス幼魚が捕獲された．ログダム直下の淵やデブリキャッチャーが捕捉した流木のカバーはサクラマスに，ログダム上流に形成された瀬はハナカジカに効果があったと考えられる．以上のことから，現地発生の倒流木と巨石を用いた構造物は多様な生息場を創出しながら防災機能を発揮しており，今後健全な河川環境を回復させるための工法の一つになると考えられる．

English Abstract

A stream that was severely damaged by heavy rainfall was restored using large woody debris and boulders from the site. The experimental site was a tural stream of the Horo i River system in northern Hokkaido, where intense rainfall triggered debris flow and flooding in 1998. Several types of woody structures were installed to stabilize sediment and gravel. A series of low check dams (log dams) constructed from large woody debris was installed along the main channel to prevent movement of new debris deposits. Complex grids of woody structures (debris catcher) were also constructed along alter ting sides of the shore to capture drifting wood. Large boulders were selected, attached to steel wires, placed along the bottom of the channel, and buried (boulder cluster). Construction was carried out in 2000, and changes in the geomorphological characteristics, debris movement, vegetation recovery, and influence on stream-dwelling fishes were monitored until 2008. Most of the log dams were functio l, and although evidence of minor lateral scouring was observed, no serious damage had occurred. The log dams were effective at creating deep plunge pools while maintaining a variety of water currents and substrates. The debris catcher captured drifting logs as well as provided rearing habitat for juvenile masusalmon (Oncorhynchus masou). The boulder clusters stabilized the stream channel, e bling tree seedlings to establish. Significantly more juvenile masu salmon were seen in June and October at the log dam section and in October at the debris catcher compared to the reference section. A similar result was observed for sculpin (Cottus nozawae) except in October. Generally, a large concrete dam is constructed after a flooding disaster, which seriously damages stream ecosystems. Given our results, we suggest that a combi tion of wood and boulder structures is an attractive alter tive to concrete dams from ecological, economical, and landscape perspectives.