FIG. 1 Distribution map of topographic elevation and water system in Zhengzhou

FIG. 2 Elevation (m) along Jialu River (Jiangang Reservoir-Beijing-Hong Kong-Macao Expressway)

1.2 Analysis of rainfall events on July 20

According to the rainfall data obtained from the public account of "Ecohydrological Remote Sensing" team, the temporal and spatial distribution of rainfall processes at five national meteorological stations from west to east in Zhengzhou City during 15 hours from 20:21 on July 18, 2021 (FIG. 3).

Before the one-hour extreme rainstorm occurred in Zhengzhou, from 20:00 on July 18 to 15:00 on July 20, high-intensity rainfall occurred at Songshan Station, Gongyi Station and Xinmi Station located in the upper reaches of Zhengzhou's main city, with the cumulative rainfall reaching 515mm, 464.5mm and 476mm respectively (FIG. 4).

Zhengzhou Station is located in the northeast of the main city, the heavy rain mainly concentrated from 15:00 to 17:00 on the 20th, the maximum hourly rainfall reached 201.9 mm, and the maximum 24h total rainfall exceeded 622 mm.

July 20 The rainstorm center gradually developed from the southwest mountainous area to the downtown area of Zhengzhou in the northeast. As a result, the water level of the urban river rose rapidly due to the rainstorm runoff in the mountainous area and the safe discharge of the reservoir, and the drainage conditions of the urban river were unfavorable in the event of a heavy rainstorm in the urban area.

FIG. 3 Distribution map of national meteorological stations in Zhengzhou

(Photo credit: Ecohydrology Remote Sensing Frontier public account, "Ecohydrology Remote Sensing" team, Faculty of Geographic Sciences, Beijing Normal University)

FIG. 4 7.20 rainstorm hour by hour and cumulative rainfall at the National Rainfall Measuring Stations in Zhengzhou and Western China (Map: Water-friendly environment)

(Data source: Ecohydrological Remote Sensing Frontier public account, "Ecohydrological Remote Sensing" team, Faculty of Geographic Sciences, Beijing Normal University)

1.3 The return period of heavy rain in Zhengzhou on July 20

How many years does the July 20 rainstorm happen?

The frequency or recurrence period of heavy rainfall is determined by hydrological statistical analysis. This is a very specialized research area under the hydrology discipline.

The recurrence period of hydrological events (such as heavy rain) is an expression of quantifying the probability of random events based on the analysis of random samples, so that comparison, analysis and decision can be made in application and research.

The study of rainstorm in cities is still basically limited to deduce the rainstorm formula and the range below one occurrence in 100 years through the observation records of stations.

Due to the high safety requirements of important water conservancy such as reservoirs, the application of extreme events in water conservancy is very common. For example, the design standard of the Three Gorges Dam is once in ten thousand years, and the design standard of some dykes in the Netherlands is once in hundreds to 100,000 years.

Because of the sensitivity and importance of frequency results, the analysis of extreme frequencies is not only calculated by ranking several years of observation data. Extreme event frequency analysis requires extensive research from historical surveys, archaeology, flood remains, etc.

For extreme frequencies, on the other hand, the accuracy of the results cannot be measured, but are the best estimates based on current knowledge and science. The July 20 rainstorm in Zhengzhou shattered too many perceptions.

Exactly how many years it happens needs to be studied by academic experts in hydrometeorology.

However, from the perspective of the two extreme events of "75.8 Zhumadian heavy rain" and "July 20 floods in many places in South China", these really challenge the analysis of extreme events in hydrology statistics.

According to the report of Academician Zhang Jianyun, the combination of topographic conditions and special climate in this region may be the main reason for the extreme rainfall.

Therefore, these extreme events may need to be classified compared with conventional events due to their different causes and engineering objectives.

2. Storm flood risk analysis

2.1 Storm water simulation

In order to assess the rainstorm water situation and inundation risk in Zhengzhou city and analyze the causes of flood disaster, this study constructed a 2D hydrological and hydrodynamic flood model for the main urban area of Zhengzhou city. The model range was 1057.8km2, including the main urban area and the main watercourse of flood discharge and drainage in Zhengzhou city.

The model rainfall data of Zhengzhou's national meteorological stations were analyzed and sorted by the "Ecohydrology Remote sensing" team of the Faculty of Geographical Sciences of Beijing Normal University according to the hourly observation data of China's surface meteorological stations. The hourly rainfall data of Zhengzhou Station from 20:00 on July 19, 2021 to 0:00 on July 21, 2021 were selected to enter the model calculation. The total rainfall is about 640 mm, the maximum 1-hour rainfall is 201.9 mm, and the runoff coefficient is 0.9. The surface elevation of the model is based on NASA's Shuttle Radar Mission radar elevation data. The accuracy of the data is 30m*30m.

The maximum water depth distribution map was drawn based on the results of the flood model (FIG. 5). The water depth risk map mostly matches the news description (see the legend, where the blue level represents the water depth).

The results show that the overall water accumulation situation in Zhengzhou city is severe. The maximum water depth in large areas exceeds 0.25m required for urban waterlogging prevention and control, and the maximum water depth in some areas is even more than 2m.

In the northeast of the city, Jinshui District and Huiji District, the water accumulation is the most serious in the low terrain; In the main urban area, the area from Zhengzhou North Railway Station to Zhengzhou Station, around Putian West Railway Station and along Zhengzhou Metro Line 5 is wide and the water depth is also large.

Further statistics show that the area with the maximum water depth of more than 0.25m is 479.1 km2, accounting for 45.3% of the total area, among which, the area of 0.5~2m is 272.4 km2, accounting for 25.8%; The area above 2m reaches 116.0 km2.

FIG. 5 Distribution map of flood inundation depth in Zhengzhou

Table 1. Statistical table of flood water area in the main urban area

Figure 6. Area proportion of inundation depth of waterlogging model water in main urban area

2.2 Analysis of flood water quantity

According to the "Zhengzhou Sponge City Special Plan (2017-2030)" issued by Zhengzhou Municipal Government in 2017, in terms of water security, the standard is that the return period of waterlogging control design in urban areas and aviation cities should be once in 50 years. In Zhengzhou, the design rainstorm volume of 24h in 5-year and 50-year recurrence periods is 119.0m and 199.4mm, respectively.

Table 2 Design rainstorm rainfall in Zhengzhou City

According to the above characteristic rainstorm rainfall analysis, the water displacement of pipe network (including sponge facilities) under the design capacity accounted for 19% of the total rainfall of this extreme rainfall, the water displacement of once-in-50-year waterlogging prevention system (including pipe network) accounted for about 32%, and the proportion of exceeding the standard rainwater reached 68%. A large amount of rainwater beyond the capacity standard of waterlogging prevention system will form rapid surface slope confluence. Flow to low-lying areas.

The water balance analysis and percentage results of each part within the model range of 1057.8km2 in the main urban area are shown in Table 3 and Figure 7:

Table 3 Water balance analysis table of model analysis

FIG. 7 Water proportion analysis diagram of Zhengzhou main urban area

2.3 Inundation risk analysis

The areas with high flood energy represent the areas with high impact and high risk.

Water depth and flow velocity determine flood energy, so the flood risk in the inundation area of the study area is evaluated according to the maximum water depth and maximum flow velocity of ground inundation obtained by the model.

The formula of risk assessment factor F is as follows:

F=Hmax*Vmax

The flood risk distribution map of the main urban area of Zhengzhou is calculated (FIG. 8). It can be seen that the high risk area of inundation is mainly located in Jinshui District of Zhengzhou.

Jinshui District is located in the lowest terrain of the main urban area, and the water system in the district is connected horizontally and horizontally, which is the main flood travel section of the urban area. The main flood discharge rivers, including Jialu River, Dongfengqu River, Xiong 'er River, Jinshui River and Wei River, all converge in the Jinshui District, resulting in continuous accumulation of water in the area and high water level, far exceeding the safety storage capacity of waterlogging.

In addition, there are also middle and high inundation risk areas on both sides of the middle and upper reaches of Jialu River, Jinshui River, Shalihe River and Qilihe River, etc. The rainstorm in Zhengzhou City leads to the river in the city far exceeding its drainage capacity.

The inundation water depth in most areas is greater than 0.5 meters, even 1-2 meters, and the flow rate is greater than 0.8 meters/second, which is a great risk of water flow and seriously affects the safety of the city.

Figure 8 Flood and inundation distribution map in the main urban area of Zhengzhou City

2.4 represents the process of point inundation water level

During the rainstorm, Zhengzhou Fuwai Hua Central Vascular Disease Hospital (referred to as Fuwai Hospital) due to the rainstorm water, water and power supply, roads flooded, traffic blocked, resulting in nearly 5,000 people trapped.

Zhongmou, east of the city where Fuwai Hospital is located, is at the lowest terrain of Zhengzhou, downstream of the confluence of Jialu River, Wei River, Longhu and other water bodies. After the rainstorm, the river surge led to a large amount of water backfilling, resulting in a fast rise in the road water level and a high rise, but the retreating speed was slow due to the topography.

Through simulating the actual rainfall in Zhengzhou Station from 15:00 on July 20 to 21:00 on July 20, 2021, the results showed that the water level in the area around Fuwai Hospital could rise about 2.25m in 6 hours.

FIG. 9 Water situation and water accumulation map of Zhengzhou Fuwai Hospital

(Credit: Thepaper.cn)

FIG. 10 Curve of simulated water change process (Zhengzhou Fuwai Hospital)