Background:
In recent days, the causes of the Shouguang flood on August 19 have been frequently reported in the media, the Internet and wechat. Is it improper scheduling? An act of God? With professional sensitivity and a high sense of social responsibility, our team the water environment quickly organized the strength of the company,We gathered with many years of domestic and foreign large-scale flood risk management and numerical simulation experience of the wisdom of the team, to carry out research work. After 5 consecutive days of research, collection and analysis of public resources, model construction and simulation analysis, uphold a scientific, independent, objective and fair scientific research attitude, formed the 819 Shouguang flood causes of technical conclusions.
This report is a technical summary of the main basis and conclusion of the study. The report makes the team's professional judgment on the most critical flood causes, risk identification, and operation plan evaluation. Authoritative conclusions await the release of relevant management departments. Our real hope is that through professional work, we can rationally analyze the causes, sum up the lessons, and put forward the inspirations and suggestions arising from this incident that are generally applicable to many similar incidents. We hope to do our best to help strengthen the whole society's awareness of flood risk and change the situation of passive disaster relief which occurs frequently year after year.
I. Characteristics of major river basins
Basin topography
The main stream of Mihe originates from Tianqi Bay, Yishan, and the terrain decreases from southwest to northeast. Shouguang flood involved three reservoirs, Yeyuan reservoir is located in the upper main stream of the Mi River, black tiger reservoir and Songshan reservoir is located on the tributary, into the Mi River. Mi River around the city from upstream to downstream distribution of Linqu County, Qingzhou City, Changle County, Shouguang City. Mi River in the downstream through Shouguang City. See the picture below for the topography of the reservoir area.
2. Research methods
In order to assess the water situation and analyze the causes of the disaster, the research team formed a team of experts with rich experience in flood modeling at home and abroad, obtained relevant key basic data, and constructed a 1D&2D coupling flood model of the upstream reservoir and the middle and lower reaches of Shouguang. Among them, the upstream of Tanjiafang adopts the hydrohydrodynamic 1D model, and the total control confluence area is about 1850km2, including the Heihushan reservoir, Songshan Reservoir and Yeyuan Reservoir as well as the confluence area of tributaries in the upper reaches of the Mi River; In the lower reaches of Tanjiafang, 2D surface flood model was used to simulate the flood inundation process of Shouguang City and its downstream areas and flood risk assessment was carried out. The whole 2D surface model area was about 1600km2. The total area of the model is 3450 km2. The Precipitation data of the model was obtained from the Global Precipitation Measurement (GPM) radar of NASA in the United States with the interval of 30min. The ground elevation of the model is based on NASA's Shuttle Radar Topography Mission radar elevation data. The data accuracy is 30m*30m. The generalized diagram of the model is as follows.
3. Summary of main achievements
3.1 Model simulation results
(1) Inflow flow simulation of three reservoirs.
The process of inflow flow of the three reservoirs is simulated by using the hydrological model and the two-dimensional slope flow of the catchment area of the whole basin. The results of the two methods are similar. It is concluded that the maximum discharge of Yeyuan Reservoir, Hehushan Reservoir and Songshan Reservoir during 819 rainstorm is 1700m3/s, 650m3/s and 425m3/s, respectively, and the peak discharge occurs between 9 PM and 10 PM on August 19, 2018. The simulation process of inflow flow of the three reservoirs is shown in the figure below.
The simulation results of reservoir inflow flow of Yeyuan, Heihushan and Songshan 819 rainstorm
(2) Tanjiafang flow simulation results
Tanjiafang is an important node in the middle and upper reaches of Mihe River, with hydrological monitoring stations. The simulated maximum flow of Tanjiafang in this model is about 2250m3/s, and the time of occurrence is about 1:30 am to 2:30 PM on August 20, 2018. The simulation process of Tanjiafang is shown in the following figure. This process is derived from the simulation of the main characteristics of the basin and the analysis of hydrological experience. Due to the lack of reliable detailed data such as river section, reservoir level, measured water level and discharge in Tanjiafang, the model cannot be tested and there must be errors.
Flow process line of Tanjiafang (Model result)
The flow of Tanjiafang is 2250m3/s, which has exceeded the second-level orange warning standard of Shouguang flood control warning. It indicates that the flood control situation is very urgent.
(3) The flood situation in the middle and lower reaches of the Mi River
The dynamic analysis of the overall results of the model and the analysis of section flow indicate that the drainage flood flows through the center of Shouguang City to the east and west sides of the river. The statistical results of peak flow show that about 40% of the downstream section of Mihe Shouguang City flows westward, about 20% flows eastward, and the remaining 40% flows downstream, as shown in the figure below. Due to the lack of data on river section, embankment elevation and breach location, this result needs to be verified. According to the longitudinal section of the river, the plane topography and the trend of the river, the overflow mainly occurs in the main city of Shouguang slightly downstream slope is slow, the river curve is gradually more, even 90 degrees bend. It is estimated that the flood capacity of the river course and beach is attenuated due to few floods for many years.
Flood overflows in the Middle and lower reaches of the Mi River (Model Results)
3.2 Simulation diagram of flood inundation area
(1) Risk analysis
According to the measured discharge from the warehouse, the model simulated the flood inundation diagram of the downstream region (see the figure below). There is a flood inundation risk on the left and right sides of the Mi River, and the maximum inundation scope is near the streets of Los Angeles.
The areas with high flood energy represent the areas with high flood impact and high flood risk. Water depth and flow velocity determine flood energy, so the flood risk of inundation area in 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