How is the “Cross Tide” formed?

On July 30, a video of two tidal waves in the Qiantang River colliding and forming a "cross tide" went viral. In the video, two surging tides formed two white lines, moving towards each other, meeting and separating, forming a cross shape on the river surface.

The reporter contacted the Hangzhou Forestry and Water Bureau, and the relevant person in charge blurted out: "It's so beautiful!"

But it is not seen in Hangzhou. The person in charge said, "Although the cross tide is beautiful, it only appears in the lower reaches of the Qiantang River and will not appear in Hangzhou."

This is because cross-tides require the magical help of nature.

In the lower reaches of Qiantang River, the river becomes wider and shallower, and the river is silted up, resulting in many sandbars. The sandbars that emerge from the water are called "Zhongsha". When the tide meets Zhongsha, it will be cut into two streams, namely the east tide and the south tide.

After passing the sandbar, the east tide and the south tide will cross each other like two brothers, and their power and magnificence are just like what Fan Zhongyan wrote: "Thunder gathers on the sea surface and waterfalls in the middle of the river." This is the cross tide.

Because the formation of sandbars in the river is full of uncertainty, the shape of the cross tide is also full of randomness. Sometimes it forms a "cross", sometimes it becomes a "person", or even a "D".

Cross tides usually occur near Jianshan, Haining. At present, cross tides can be seen in the Xincang section of Dingqiao, Haining, and Hangzhou Bay (there is a place called Big Gap 55 kilometers away from Hangzhou Bay).

For cross tides to occur, the river needs to be wide and the water needs to be shallow. Hangzhou, located in the upper reaches of the Qiantang River, has a narrow river channel and deep water, so cross tides do not occur.

The most common tide in Hangzhou is the "single-line tide", and you can also see the "returning tide".

The most common is the "one-line tide". In places without sandbars, the tide flows forward neatly, and the tide head is like a white line. When the tide encounters a T-shaped dam, it is like hitting a wall and is reflected back to form a "return tide". The "return tide" usually appears in places with T-shaped dams, such as the Beauty Dam, Jiuxi Water Gate, and Qibao No. 1 Dam.

Speaking of tide watching, in fact, there are about 120 days a year when you can watch it. Tides form from the first to the fifth day and the fifteenth to the twentieth day of every lunar month.

The two best times of the year to watch the tide are about to arrive - around the 18th day of the seventh lunar month is the Wangxun tide of the Qiantang River, which we often call the "Ghost King Tide"; and the 18th day of the eighth lunar month is the "birthday of the tide", when the tide is the highest in the year. Su Shi said, "The tide on the 18th of August is the most spectacular in the world."

The tide is spectacular but also dangerous. The person in charge reminded everyone that when watching the tide, they must pay attention to safety and watch the tide in a civilized manner.

Wang Qing, reporter of Orange Persimmon Interactive and Metropolis Express

Correspondents Ji Zhansheng and Xu Zhigang