Tidal river - Kanchoukasen

In the downstream of a river that flows into the sea, the area where the water level and flow speed of the river fluctuate periodically with tide changes is called the tidal section, and a river that is influenced by such tides is called a tidal river. The tidal range is larger in large rivers with gentler gradients, and smaller in rivers with steeper gradients. In the Yangtze River, the influence of tides extends from the mouth to Hankou, about 1,000 kilometers upstream. Changes in salinity due to seawater intrusion are limited to the relatively downstream area, while changes in water level and flow speed extend much further upstream. A river where seawater intrudes and forms a saltwater wedge is called a tidal river. Near the mouth of a river, where the water level gradient is gentle, seawater intrudes from the mouth and flows upstream during high tide, and reverses direction and flows downstream during low tide. When a river mouth opens in a fan shape and the sea in front is shallow and has strong tides, a wall of water with a nearly vertical front runs up the river in a horizontal line of waves during high tide. This is called a tidal bore or tsunami. In the Qiantang River, which flows into Hangzhou Bay in China, the waves reach a height of about 10 meters and run up the river at a wave speed of 7 meters per second. The Seine River in France, the Severn River in the UK, and the Orinoco River in Venezuela in South America are also famous for tsunamis. Tidal energy is consumed in part by friction and viscosity, and is also used to carry water to gradually higher places. Even in the downstream where the water level gradient is gentle, a large amount of kinetic energy is required to run up the river against the current, so the degree of tidal influence becomes weaker the further upstream you go. In general, the duration of high tide is short and the duration of low tide is long. When the river flow rate is high, the duration of high tide is even shorter. It goes without saying that rising tides prevail during droughts. Fluctuations in flow speed or volume usually lag behind changes in water level caused by tides, and this time lag becomes greater the further upstream. In large rivers, there is a difference in water level between the two banks, and in the Northern Hemisphere, the right bank is higher than the left bank at both high and low tide. This is due to the effect of the Earth's rotation, and in the Hudson River in the United States, the difference is said to be 3 centimeters near the mouth of the river.

[Shigemi Takayama]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

海に注ぐ川の下流部で、潮汐(ちょうせき)変化につれて川の水位や流速が周期的に変動する範囲を感潮区間といい、そのような潮汐の影響を受ける川を感潮河川という。感潮の範囲は勾配(こうばい)の緩やかな大河になるほど大きく、勾配の急な川では小さい。揚子江(ようすこう)では河口から約1000キロメートル上流の漢口まで潮汐の影響が波及する。海水の侵入による塩分の変化は比較的下流部に限られ、水位や流速の変化はこれよりもはるかに上流側にまで及ぶ。海水が侵入して塩水くさびを形成する川を潮入り川という。水面勾配の緩やかな河口付近では、上げ潮のときには海水が河口から侵入して遡上(そじょう)し、下げ潮のときには向きを逆転して流下する。河口が扇形に開いていて、前面が遠浅で潮の強い海である場合、上げ潮時には前面が垂直に近い水の壁が横一列の波となって川を遡上する。これを潮津波または海嘯(かいしょう)といい、中国の杭州(こうしゅう)湾に注ぐ銭塘江(せんとうこう)では波高が10メートル前後に達し、秒速7メートルの波速で川をさかのぼるという。フランスのセーヌ川、イギリスのセバーン川、南米ベネズエラのオリノコ川なども海嘯で有名である。潮汐のエネルギーは一部を摩擦や粘性のために消費するほか、水をしだいに高い所に運ぶ仕事にも使われる。水面勾配の緩やかな下流部でも川の流れに逆らって遡上するには大きな運動エネルギーを必要とするから、感潮の度合いは上流部になるほど弱くなる。一般に上げ潮の時間は短く、下げ潮の時間は長い。川の流量が多いときには上げ潮の時間がさらに短くなる。渇水時に上げ潮が優勢になることはいうまでもない。潮汐に起因する水位の変動に対して流速または流量の変動は時間的に遅れるのが普通で、この時間的遅れは上流ほど大きくなる。大きな川では両岸の水位に差があり、北半球では右岸が左岸よりも満干潮時とも高い。これは地球自転の影響によるもので、アメリカのハドソン川では河口付近でその差が3センチメートルあるという。

［髙山茂美］

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