事例研究　CASE STUDY

洪本により発生した倒流木と巨石を用いた河川再生工法の評価

高山 裕將^１）３）・柳井　清治^１）４）・白川　北斗^２）

１）北海道工業大学環境デザイン学科　〒006-8585　北海道札幌市手稲区前田7条15丁目4-1
２）北海道大学大学院水産科学院　〒041-8611　北海道函館市港町3-1-1

Hiromasa TAKAYAMA¹⁾³⁾, Seiji YANAI¹⁾⁴⁾, Hokuto SIRAKAWA²⁾ : Evaluation of river restoration techniques using woody debris and boulders naturally generated from flood events. Ecol. Civil Eng. 14(2), 139-154, 2011.

1) Departmentof Environment Design, Hokkaido Institute of Technology, 4-1, 7-15, Maeda, Teine, Sapporo, Hokkaido 006-8585 Japan
2) Graduate School of Fisheries Science, Hokkaido University, 3-1-1, Minato-cho, Hakodate, Hokkaido 041-8611, Japan

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 natural stream of the Horonai 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 alternating 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 functional, 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 masu salmon (Oncorhynchus masou). The boulder clusters stabilized the stream channel, enabling tree seedinsgs 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 combination of wood and boulder structures is an attractive to concrete dams from ecological, economical, and landscape perspectives.

Key words: debris catcher, flooding, log dams, masu salmon and restortion

摘要

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.

2010年12月8日受付, 2011年10月8日受理
* e-mail: h-takayama@ndsinc.co.jp
3) 現所属：日本データサービス(株)　〒065-0016　北海道札幌市東区北16条東19丁目1-14
Present Address : Nippon Data　Service Co., Ltd. 1-14, kita16-higashi19, higashiku, Sapporo, Hokkaido 065-0016, Japan
4) 現所属：石川県立大学生物資源環境学部環境科学科　〒921-8836　石川県石川郡野々市町末松3丁目308番地
Present Address : Department of Environmental Sciences, Ishikawa Prefectural University, 1-308, Suematsu, Nonoichi, Ishikawa 921-8836, Japan