特集: 長良川河口堰の影響に関するモニタリング評価
FEATURE: Evaluation of Monitoring on Impacts of Estuary Barrage in Nagara River
原著論文 ORIGINAL PAPER

長良川河口堰下流部の溶存酸素濃度の動態

中村由行¹⁾・ 藤野智亮²⁾

1)港湾空港技術研究所海洋・水工部　〒239-0826 横須賀市長瀬3-1-1
2) 東洋建設（株）　〒730-0032広島市中区立町2-23　野村不動産ビル

Yoshiyuki NAKAMURA¹⁾ and Tomoaki FUJINO²⁾: Dynamics of dissolved oxygen concentration in the lower reaches of rivermouth barrage of the Nagara River Ecol. Civil. Eng. 5(1), 73-84, 2002.

1) Port and Airport Research Institute, 3-1-1 Nagase, Yokosuka, Kanagawa 239-0826, Japan
2) Toyo Construction Co. Ltd, 2-23 Tatemachi, Naka-ku, Hiroshima 730-0032, Japan

Abstract:Dynamics of dissolved oxygen (DO) concentration in the bottom layer in the lower reaches of a rivermouth barrage of the Nagara River was investigated. The tidal action is a primary component governing the temporal variation of the daily-averaged DO concentration of the bottom layer of the Nagara River estuary and river flow gives disturbances to this primary mode. When we have a big flood in summer season, DO concentration in the bottom layer of the estuary increases rapidly during a rising stage of the discharge, but after the peak discharge it becomes very low because of the intrusion of anoxic saline water from the inner part of Ise Bay. Such fundamental feature remains unchanged before and after the construction of the Nagara Barrage. After the construction, however, DO concentration of the bottom layer remains low even during the spring tide due to weaker vertical mixing with the Nagara Barrage in addition to the neap tide period. The change in the pattern of DO concentration in the estuary is caused by the difference in mixing regime associated with tidal action. The mixing regime in the spring tide changed from a well-mixed condition to a partially-mixed condition.

Key words: anoxic water, dissolved oxygen, river discharge, rivermouth barrage, tidal cycle

摘要

潮汐作用と河川流量という物理作用から,長良川河口堰下流部の貧酸素水塊形成の基本的な過程を整理した.堰運用前後に共通した特徴は,底層 DO の変動が大潮・小潮のサイクルに応じた変動を主体とし,河川流量変動に伴う撹乱が重なって生じている点である.大規模出水時には,底層DO濃度は増水時に一時的に増加するが,流量が低減した後には極端な高塩分化に伴った貧酸素化が生じることが多い.これは,伊勢湾奥部に形成された貧酸素水塊の遡上によって生じる.堰運用前後の違いは,主として混合形態の差によって生じている.堰の運用前では,大潮時には強混合であり,混合が活発化し貧酸素化が解消されていた.しかしながら,運用後には大潮時でも緩混合的であり,鉛直混合は不十分で,表層・底層の水質に差が見られ,貧酸素化の回復が不十分である.

2001年8月8日受付，2002年5月21日受理
1) e-mail: nakamura_y@pari.go.jp