特集：標津川再生事業の概要と再蛇行化実験の評価 FEATURE:Shibetsu River Restoration Project
原著論文 ORIGINAL PAPER

標津川蛇行復元における２way河道の流況と河道変化

渡邊康玄^1)*・ 長谷川和義²⁾・ 森　明巨²⁾・ 鈴木優一¹⁾

1)独立行政法人北海道開発土木研究所河川研究室　〒062-8602　札幌市豊平区平岸1条3丁目
2) 北海道大学大学院工学研究科　〒060-8628　札幌市北区北13条西8丁目

Yasuharu WATANABE^1)*, Kazuyoshi HASEGAWA²⁾, Akio MORI²⁾ and Yuichi SUZUKI¹⁾: Changes of flow regime and river configuration in restoration project of the Shibetsu River. Ecol. Civil. Eng. 7(2), 151-164, 2005.

1) Civil Engineering Research Institute of Hokkaido, Hiragishi 1-3, Toyohira-ku, Sapporo 062-8602, Japan
2) Division of Environment and Resource Engineering, Hokkaido University, N13 W8, Kita-ku, Sapporo 060-8628, Japan

Abstract:The Shibetsu River originally had a number of meanders, and a wetland extended at its lower reaches. Cut-off works to straighten the meandering river course began in 1953. As a result, safety against flooding was improved, the wetland disappeared and farmland development in the river basin progressed. In recent years, however, there have been increasing demands from local residents for the creation of a better river environment. Restoration of the natural river environment has therefore been promoted since 2000. Since it was considered to be difficult to ensure safety against flooding if only the old meandering river course was restored, it was ultimately decided that the old river would be restored while maintaining the present straightened river course. Water was reflowed into the restored meandering river course on March 18, 2002. The meandering river experienced six floods during which annual maximum daily discharge exceeded the average annual maximum daily discharge rate. Eight field studies were conducted to visually observe changes in the channel and flow conditions. And to quantitatively understand changes in landform, five cross-section surveys were performed. These investigations showed that bank erosion, other than the expected bank erosion at the bifurcations, occurred in many places because of meander bends and flow deviation due to sandbars. The erosion resistance of banks differed markedly between the pre-improved channel and the excavated channel. Therefore, it is essential to take this behavior into consideration when predicting channel migration. It is also important to gain a thorough understanding of the areas to be affected by the plan through the examination of relevant data, including the centerline of the channel, bed configuration and flow conditions.

Key words: channel variation, field observation, restoration, river environment, Shibetsu River

摘要

近年の河川環境への意識の高まりを受け，北海道東部の標津川において河川環境の復元事業が策定された．河川環境復元の方法は様々検討されたが，日本では蛇行復元の先例が無く，河川工学的にも生態学的にもどのような現象が生じるかが不明確であった．このため，本格的な河川環境復元に先立ち，技術的な問題の把握と解決のため，標津川に直線化された河道を残しつつ旧川である三日月湖を接続させた試験区間を設け，現象の把握を行うこととした．本論文は，この試験区間における１年間の河道の変化について追跡し，現象の解明を試みたものである． 　河道の変化について，洪水前後に実施した現地踏査と河道の横断測量により把握を行った．その結果，事前に室内実験で水理模型実験を行い想定された以外の現象が確認され，今後の復元計画へ反映させる必要のある事項が把握された．すなわち，河岸侵食が湾曲部外岸側以外にも生じる可能性が存在し，流れの状態によっては内岸側も侵食が生じることを確認した．また，河岸が裸地の場合と植生で覆われている状況とでは，侵食の進む速さが大きく異なり，植生で覆われていることによる河岸の耐侵食性が高まることが確認された．
　河岸侵食と密接に関係するといわれる偏倚流速を簡易に算出する方法により求め，実際の侵食量と比較を行った結果，おおむね両者の傾向は一致するが，河床に形成される砂州の影響が大きく，河岸侵食を考える場合には砂州の形成をも検討に取り入れる必要性を明らかにした．また，この結果を用いた河道形状の将来予測手法の適用性を明確にした．

2003年12月11日受付，2004年8月25日受理
* e-mail: y-watanb@ceri.go.jp