4.2.1. The Daliang City of the Warring States and the Honggou flood in 225 BC

This was the first time in history that Kaifeng was destroyed. From the results of this study, the ancient ground (cultural layer) of Daliang City is 10–15 m below the present ground level, which is consistent with the relevant archaeological findings [25,39]. The chasm began in 360 BC and was built in about twenty years. The Honggou diverts the water of the Yellow River from Xingyang, where it passes through Xingze and Putianze to Daliang, where it joins the Huaihe River system. At that time, the Daliang City Wall was very tall and was known as the "Seven Gaps City". General Wangben of the Qin State failed to capture the city, and then proceeded to dig the Honggou to the west which resulted in the entrapment of Daliang City by flood water for three months. Finally, a section of the soil-based city wall to the south of the West Wall was immersed and collapsed, and the Wei State was destroyed. One hundred years after Daliang City was destroyed, Sima Qian came to Xunyi (renamed from Daliang) only to be confronted with a shattering scene, the "Ruins of Daliang" [43]. From this we can see that the damage caused by the Honggou flood to the Daliang City was very serious.

**Figure 10.** Dating framework of the drilling cores since the Warring States. Gray-black layers were formed under reduced environment or human activities, indicating flood receding periods or cultural layers. In other words, gray-black layers are an important reference indicator for determining sedimentary cycles.

4.2.2. Dongjing City in Northern Song Dynasty and the Yellow River Flood in 1387AD

This Yellow River flood caused the sediment deposited in the drilling area of the SZ and YZ cores to be within 2 m in 1399 AD, and once again, Kaifeng city was buried as it was too late to take mitigating action. From the Qin to Yuan Dynasties, the common ancient ground (cultural layer) in Kaifeng was buried 9–13 m below the present ground level, a finding which is basically consistent with the relevant archaeological discoveries [25,39]. As Kaifeng city in the Jin and Yuan dynasties was rebuilt on the basis of Dongjing City, its magnificence and influence were far less than that of Dongjing City, and the two dynasties lasted for a relatively short time. After 12 years (1399 AD), it was silted up again, so the ancient city flooded by the Yellow River in 1387 AD could still be regarded as Dongjing City.

4.2.3. Kaifeng City in Late Ming Dynasty and the Yellow River Flood in 1642 AD

In the seven floods of Kaifeng, the flooding of the Yellow River in 1642 AD was the most serious. According to statistics, of the 370,000 people who were in Kaifeng at the time, only 30,000 survived the flood [39]. According to the results of this study, the thickness of the flood deposit is 2.3–3.8 m, the depth of the surface culture layer in Kaifeng city is between 3.5~8 m of the late Ming dynasty,

and the depth of the suburban area is greater than that in the urban area. It was not until 5 years later (i.e.1647 AD) that the Yellow River breach was completely blocked. It should be pointed out that under the ground of Kaifeng city in the late Ming dynasty, there still existed the surface of the Ming dynasty which was flooded and silted by the Yellow River in 1399 AD and 1461 AD, and the thickness of sediment was 1–2 m. Whether there were ruins of Kaifeng city in these two periods underground needs further investigation. The authors believe that the two floods (i.e., 1399 AD and 1461 AD) were close to the Yellow River flood in 1387 AD, and the sedimentary thickness is not too large. On the basis of the silted ruins and broken walls, the houses were rebuilt, and the city walls and horse paths were heightened in turn. Kaifeng city may not have existed during this period, as only the cultural layer remains.
