The reason for the formation of the Qiantang River spring tide is

1, date: August 16 to August 18 in the lunar calendar, the sun, the moon and the earth are almost in a straight line, so the tidal force (the difference between the gravity of the moon, the sun or other celestial bodies on the earth and the gravity of the earth, or the inertial centrifugal force generated by the earth's movement around the center of mass of the earth (the sun) and the moon (the sun) gravity.

2. Geographical location: It is related to the trumpet-shaped mouth of Qiantang River. Nearly 500,000 mu of land was reclaimed to the east of Zheshan on the south bank of Qiantang River, which blocked the estuary like a peninsula, making the section from Zheshan to the outside of Qiantang River look like a small belly bottle, and the tide is easy to enter but difficult to retreat.

The outer mouth of Hangzhou Bay is 100 km wide, and the outer 12 section is only a few kilometers wide. The riverbed in the east of the estuary suddenly rises and the beach is shallow. When a large amount of tidal water comes into the mouth of Qiantang River, it is too late for the tidal water to rise evenly due to the rapid contraction of the river surface, and it needs to be pushed forward wave after wave.

Layer upon layer. Secondly, it is related to excessive sediment deposition under the Qiantang River. Sediment deposition blocks and rubs the tidal current, which makes the front slope of the tidal current steep and gentle, thus forming a wave after catching up with the previous wave, and one wave overlaps with the other.

3. Wind direction: southeast wind is often blown along the coast, and the wind direction is generally consistent with the tidal direction, which is helpful for the tidal direction.

Extended data:

Tide-watching custom:

Every year on the 15th day of the eighth lunar month, the tidal bore of Qiantang River is the largest, and the tidal head can reach several meters. On August 18 of the lunar calendar, the water army inspected the Qiantang River and then observed it as a festival. In addition to the tide-watching festival around August 18 of the lunar calendar, there are quite large tides at the beginning and middle of each month in the lunar calendar, which can be used as a "tide-chasing tour".

Around the Mid-Autumn Festival, guests from all directions flocked to watch the wonders of Qian Jiangchao with unprecedented pomp. Or rush to the river bank at the high tide at the beginning of the year, hurting the tide watchers. The literature contains the poem "Poems of the Tang Dynasty in the South of the Yangtze River: View of Qiantang Confucius": "At first, it started to rain, but suddenly I saw the sea and wanted to float mountains. Ten thousand people retreat like soldiers, and the turbid waves are higher than the reading altar. " Haining Yan Guan Scenic Area, 50 kilometers away from Hangzhou, is the best place to watch the tide.

In ancient Hangzhou, the Phoenix Mountain and Jianggan area were the best places to watch the tide. Due to the change of geographical location, Yan Guan in Haining has been the first scenic spot to watch the tide since the Ming Dynasty, so it is also called "Watching the Tide in Haining".

"Qianjiang Tao Qiu" is famous at home and abroad, and it was popular as early as the Tang and Song Dynasties. On the day of tide-watching, especially in the days around August 18 of the lunar calendar, there were cars and people on the road.

Overlooking the bell mouth of Qiantang River, the tide forms turbulent waves, just like Malik galloping. When the sandbar of the riverbed near Ganpu is blocked, the tidal wave rises by three to five meters, and the tidal range is as high as nine to ten meters. There is indeed a trend of "the sky is full of turbid waves and the rivers and mountains are destroyed". Different places can enjoy different tidal scenes: the tower sees the "first-line tide", the babao sees the "confluence tide", and the old salt warehouse can enjoy the "ebb tide".

Historical origin:

Watching autumn tide in Qiantang has become a custom as early as the Han, Wei and Six Dynasties, especially in the Tang and Song Dynasties. According to legend, August 18 of the lunar calendar is the birthday of the tide god, so the tide peak is the highest.

In the Southern Song Dynasty, the imperial court stipulated that the navy division should be inspected on the Qiantang River on this day. Later, they became accustomed to each other, and August 18 gradually became a tide-watching festival. Pan Lang, a poet in the Northern Song Dynasty, wrote in Jiuquanzi:

Long memories of watching the tide, full of people vying to see the river. I suspect that the sea is empty and drums are beating all around. Xiang Tao, the frolic in the frolic, stands upright and keeps the red flag wet. I don't see the dream a few times, but I am still chilling. This poem is a true portrayal of the activities of "making waves" and "watching the tide" in those years.