3.3.1. Drilling Settings

The sedimentary environments in different areas of Kaifeng may display significant differences due to a range of factors including variations of levees protection, flood intensities and terrain differences, therefore, the selection of representative sites is crucial. The core sites were located cognizant that: (i) the cultural layer should embody all cultural layers from the Warring States to the present time, reflecting the overall spectacle of the "city overlap city" landscape; (ii) the inhomogeneous sedimentary environments indicate that the core sites should be located along the major flood-path trend (i.e., from northwest to southeast). Based on these premises, three sedimentary cores (designated as ML, SZ and YZ) from the urban area together with one core (JM) from the suburbs were acquired through the deployment of a drilling machine (Figure 3).

**Figure 3.** The city walls of Kaifeng city in every dynasty and the core locations.

#### 3.3.2. Sampling and Physical and Chemical Analysis

In April 2012, the four 25 m-long cores (ML, SZ, YZ and JM) were obtained through the use of a corer and large drill in open air. They were then sampled continuously, mostly at 10 cm intervals with a few sandy samples at 20 cm or 30 cm intervals. A total of 861 samples were finally acquired, comprising 213 samples from SZ, 223 from YZ, 204 from ML, and 221 from JM. All samples were subjected to laboratory investigation which included grain size analysis, black carbon (BC) content analysis, and chemical elements (Cu, Zn, Pb, Cd, Al, P, As and Hg) content analysis to classify the various sedimentary cycles (representing flood events). Grain sizes were measured using a laser diffraction particle size analyzer (Mastersizer 3000, Malvern Co. Ltd., Malvern, UK). The black carbon content was measured using a TOC analyzer (Liqui II, Elementar Co. Ltd., Hanau, Germany); whilst the content of other chemical elements were measured using an ICP-MS (Xseries-2, Thermofisher Co. Ltd., Waltham, USA).

From the samples, carbon particles, animal bones, plant seeds and residue (a part of hand-selected uncarbonized plant or a single entity fragment of plant material) as well as clay specimens were diligently selected for dating using the Accelerator Mass Spectrometry of 14C (AMS14C) in the Institute of Archaeology and Culture, Peking University. A silty sample was selected for Optical Stimulated Luminescence (OSL) dating in the Digital Environmental Archaeology Laboratory, Institute of Geography, Henan Academy of Sciences. The 14C date for this research was calibrated using the computer program OxCal v4.2.3 with the IntCal13 atmospheric calibration curve [30]. The dating results are shown in Table 4 below.


**Table 4.** Results of annual survey of each borehole.

#### 3.3.3. Sedimentary Cycle Division

The number of sedimentary cycles is a basic indicator which can be used to identify the flood frequency in alluvial strata. The approach usually adopted for the classification of sedimentary cycles based on grain size, however, is sometimes not suitable for urban strata that has been deeply disturbed by human beings. For example, after a Yellow River flood, urban reconstruction work by local residents on the recent sediments and subsequent treasure hunting activities can lead to the disturbance of the normal sequence of sedimentary cycles, which means that classification based on grain size division alone is problematic. In this study, therefore, in addition to the traditional grain size cycle, two new indicators [31,32] were selected; namely, black carbon content (black carbon cycle) and chemical element content (element cycle). During the flood receding stage, residents' production activities and daily life resulted in accumulations of black carbon and other chemical elements (especially anthropogenic elements) near to the ground, the content of which is inevitably different from that occurring during the flood deposition stage. Based on this difference, sedimentary cycles can, therefore, be identified. With the aim of obtaining more accurate sedimentary cycles, this study comprehensively employed the cycle division results of the above three alternative indexes in order to obtain comprehensive cycles for the four cores.

#### **4. Results**

#### *4.1. Historical Geography Development of Urban Form*

The urban form used in this paper refers to the shape of the city, that is, the shape of the city formed by the enclosure of the city walls. Whether the walls of ancient cities were built or not reflects the development of cities to a certain extent. Therefore, "cities within walls" is one of the main characteristics of ancient cities in China [33]. At the same time, the change of the enclosure area represents the development scale and development level of the city [34,35]. According to the various historical developments of urban form, we can identify four main periods in the history of Kaifeng.

#### *Before 225 BC: "City-Guo" Mode*

Although the origin of Kaifeng is undocumented, it is known that a military fort existed near the southern border of the Wei kingdom between 770 BC and 476 BC. In 364 BC, Wei Huiwang moved the capital city from Anyi to Kaifeng.

From the Western Zhou to the Western Han dynasty, the capital city was named as West Cheng and East Guo by Yang Kuan [36]. It comprised a walled inner city (West Cheng) and a walled outer city (East Guo), but the inner-city layout of Kaifeng is unclear. An attempt to deduce the walls of the outer city (East Guo) was made by Wu et al. [37] (Figure 4).

**Figure 4.** The probable borders of the early city walls of Kaifeng before 225 BC.

The outer city wall of Kaifeng during the Warring States Period was almost square. Only two gates have been confirmed: Yi in the eastern wall and Gao in the western wall. The River Qushui crossed the northern part of the city [37,38].

In 225 BC Kaifeng was completely destroyed by General Wangben. He used the water of Honggou River to flood the entire city. Relics of the city were submerged to a depth of about 12–14 m [39].
