Schematic map of the location of the Qiantang River estuary (Qiantang River spring tide)

Every August 18th of the lunar calendar, people come to Haining, Zhejiang, to watch the tides in droves. At this time, the shore was crowded with people and thousands of people were moving around. People were anxiously waiting for the arrival of that exciting moment. After a while, I suddenly saw the crowd commotion, and I saw a white line in the distance, approaching from far away; in an instant, the tide stood on the wall, like a tall wall of water, roaring over, making a thunderous roar, deafening . This is really: "The turbulent waves from the sky are emptying, and the rivers and seas are destroyed." This is the world-famous Qiantang River tide. The raging and spectacular Qiantang tide has always been known as "a wonder of the world". People usually call this kind of tide "swelling tide", and some also call it "raging tide". The phenomenon of tidal surge is also seen in many estuaries around the world. Such as the Amazon River in Brazil, the Seine River in France, etc. The spring tide of Qiantang River in my country is also world-famous.

Schematic diagram of the location of the Qiantang River estuary

Why does such a spectacular surge occur?

First of all, this is related to the shape of Hangzhou Bay where the Qiantang River flows into the sea, and its special topography. Hangzhou Bay is trumpet-shaped, with a big mouth and a small belly. The Qiantang River channel narrows and rises sharply from the west of Kanpu, causing the capacity of the river bed to suddenly shrink. A large amount of tidewater crowds into the narrow river channel, and the tide head is blocked. The tide behind it advances rapidly, forcing the tide head to stand steeply and break. There was a roar and a thrilling and spectacular scene appeared. However, there are many trumpet-shaped river estuaries, but there are only a few estuaries that can form tidal waves. Why is the Qiantang tide honored to be included among these few?

Qiantang River surges

After research, scientists believe that the generation of tidal waves is also related to the ratio of the speed of the water flow in the river to the speed of the tidal wave. If the two speeds are the same or similar, and they are evenly matched, it will be beneficial to the generation of tidal waves. If the speeds of the two are very different, Far away, even though there is a trumpet-shaped estuary, it cannot form a tidal wave.

Also, tidal surges can form at the mouth of a river, which is related to the tidal range at its location. Because Hangzhou Bay is on the west coast of the East China Sea, and the tidal range of the East China Sea is greater on the west coast than on the east coast. After the Pacific tidal wave enters the East China Sea from the northeast, it is deflected to the right by the geostrophic deflection force during the process of moving south, making the tidal difference between the two sides greater than the east coast.

Hangzhou Bay is located in a place where the Pacific tidal waves rush straight from the east. It is also the location with the largest tidal range on the west coast of the East China Sea, so it is uniquely blessed by nature. Therefore, various reasons came together to cause the Qiantang River to surge.

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