Chapter 47 Section 1 Tidal Disasters along the Coasts of Jiangsu, Shanghai and Zhejiang

There are tidal disasters along the coast of Jiangsu, Shanghai and Zhejiang provinces and cities in my country.Especially from northern Zhejiang to northern Jiangsu, because of the low and flat terrain, the tides in most areas are turbulent, and there are more and more severe tidal disasters. Subei and Zhebei are alluvial plains. In the 12th century, the coastline of northern Jiangsu was still in the vicinity of Yancheng County to Dongtai County. Later, due to the sediment deposition of the Yellow River and Huaihe River, in the 19th century, a large area of ​​sea area was turned into a plain in only 700 years, and the coastline moved eastward by 50 to 60 kilometers.In southern Jiangsu, in the 4th century AD, the coastline was around the line from today’s Jiading county government to Fengxian county government, and to the east was the vast ocean.Due to the sediment accumulation of the Yangtze River and Qiantang River, by the 12th century, the sea retreated, and the coastline was pushed eastward to Chuansha and Nanhui.That is to say, most of the land in eastern Hudong today was formed by silting up in the past seven or eight hundred years.The same is true for the area north of Xiaoshan in Shaoxing, Zhejiang.Due to the deposition of sediment in the Qiantang River and the removal of sediment by the tide, the coastline is also pushed northward, and the plain is also expanding.The altitude of these alluvial plains is very low, generally only a few meters, and in some places even as low as two meters.

The soil in the alluvial plain is soft and rich in organic matter and minerals, which is very conducive to the development of agriculture.So soon after it landed, people quickly developed it into high-yield farmland.Since the Song, Yuan, Ming, and Qing Dynasties, Songjiang, Jiaxing and other prefectures have become important grain-producing areas in my country, which is closely related to this.And because it is close to the sea and has superior conditions for producing sea salt, northern Jiangsu has long been one of the most important production bases of sea salt in my country.The same is true for the north and south of the Qiantang River.

However, along the coast from northern Zhejiang to northern Jiangsu, there are relatively large tides, which often submerge the newly silted land.Especially the tides in Hangzhou Bay and Yangtze Estuary.The Qiantang River estuary and the ancient Yangtze River estuary (before Chongming Island was formed) were both wide, each about 200 kilometers from the south end to the north end, while the mouths narrowed sharply, shaped like a trumpet, and there were sandbar uplifts at the bottom of the river.With such an estuary structure, when the back wave pushes the front wave, the front wave must rise rapidly and form a climax because the flow path is too narrow.Therefore, in ancient times, the Guangling tide on the Yangtze River estuary and the Qiantang tide on the Qiantang River were both galloping like ten thousand horses, very spectacular and famous all over the world.The Western Han Dynasty Meicheng "Qifa·Guantao" wrote about Guangling Chao.Both Qiantang tide and Guangling tide can rise several meters high.Typhoons often land in Jiangsu and Zhejiang.Depending on the wind, the tidal surge can reach a height of eight or nine meters.The highest tidal range of Qiantang tide measured in contemporary times is 8.93 meters.

Tides, under modern technical conditions, can be used to generate electricity.But in ancient times, apart from allowing people to appreciate its magnificence, it should be said that there were few benefits.It has caused great harm, ranging from destroying agricultural production and salt industry bases, to endangering people's lives, and even engulfing large tracts of land and turning them into oceans.On the north coast of Hangzhou Bay, due to the impact of severe tidal surges in history, the coastline has continuously moved inward.For example, Mount Wangpan, which was connected to the mainland during the Eastern Jin Dynasty, was an important place to station troops in the Eastern Jin Dynasty.After the continuous impact of the tide, all the nearby flat lands were reduced to the ocean. Although Wangpanshan did not sink, it was tens of kilometers away from the land and was isolated in the Hangzhou Bay.

In order to prevent tidal disasters, people along the coasts of Jiangsu, Shanghai, and Zhejiang built a great anti-tide project - seawall.In northern Jiangsu, it is called seawall; in Su, Song and Liangzhe, it is called seawall.These projects were first built in the Qin and Han Dynasties, and then continued to develop, expanding from short ponds to long ponds, from earthen ponds to stone ponds, and finally formed a coastal Great Wall, which stretches from Lianyungang, Jiangsu in the north to Shangyu, Zhejiang in the south.