I. Introduction
Today's article will take you to review the summer floods in my country this year. The article is a bit long. Today's article starts with "a fire".
At about 19:00 on October 23, 2020, a forest fire broke out on Nan'ao Island in Shantou, Guangdong. With the help of dry northeasterly winds, the line of fire spread rapidly. The forest can be seen from the Sentinel-2 satellite image on October 25. Multiple fire spots in the district:
The open fire has been extinguished, but a large area of fire remains:
From the Fengyun 4 satellite cloud image on October 23, we can see a large clear sky area in the central and eastern part of China, and Typhoon "Shader" Traveling west in the South China Sea, this weather background indicates that China's mainland is gradually dominated by the winter monsoon. The dry and cold northeast monsoon moves southwards, and the hot and humid summer monsoon completely withdraws from China's mainland. The typhoon also affects the coastal areas of Hainan and southern Guangdong at most:
mentioned above The sudden forest fire on Nan'ao Island is related to the dry northeast wind blowing through Nanling. The summer wind is gone, and San-Xia can open its belly and start water storage on a large scale. On October 18, 2020, the reservoir area water level It is 175 meters above the station, which shows that San-Xia has completed the task of water storage from the summer and autumn flood control mode to the normal operation of power generation in winter and spring:
Although the summer monsoon has exited the mainland of China, its "stigma" on the land of China is clearly visible and still Not repaired.
2. Review of the main disaster-stricken areas
2.1 White Lake Farm
Anhui Baihu Farm is located in Baihu Town, Lujiang County, Hefei City, Anhui Province. It is a subsidiary of Anhui Baihu Farm Group Co., Ltd. It is a modern specialty integrating grain production, seed industry, rice industry, forestry and breeding. Large state-owned farm. The farm covers an area of 162 square kilometers, divided into two large polders, east and west, with 150,000 mu of standard fertile land, and more than 10,000 mu of water and woodland. Baihu was originally a sister lake of Chaohu. In 1953, the Baihu Farm was established here and the Baihu Farm was built by reclaiming the lake. It was later renamed Anhui Bai-Hu-June-Prison-Management-Administration-Sub-bureau. It is the largest crime-criminal-education-education-reconstruction base in Anhui Province and one of the super large farms in the country. (Source: Baidu Encyclopedia)
At 20:10 on July 18, 2020, the Anhui Provincial General Defense Department issued the Dongdawei flood storage instruction, and 65,000 acres of fertile fields were flooded, starting the seventh flood storage in the history of Baihu Farm. Three months have passed. From the satellite image on October 19, 2020, it can be seen that the water in Dongdawei has receded and a large piece of loess has leaked out, which is in sharp contrast with the farmland in Xidawei.
Although Dongdaxu showed bare soil on the optical image on October 19, 2020, it was still watery in the radar image of the same day:
this is because the water in Dongdaxu of Baihu Farm has just receded. From the satellite image on October 11, 2020, you can still see a large area of water, that is, it has been soaked in flood water for almost three months.
Since the flood has just receded, the dry loess on the surface of the land is actually very shallow, and the radar has a longer wavelength and better penetration, so it can detect wet soil with a large water content in the inner layer. This situation is also reflected in short-wave infrared, which has a much longer wavelength than visible light, so it also has a certain degree of penetration. From Sentinel-2 on October 23 and Landsat 8 on October 24, we can see that the combination of shortwave participation shows that the color of Dongdaxu is much darker than the simple visible light (red, green and blue) combination:
the characteristics of normal bare soil As shown in the figure below, it is also the Sentinel-2 image of Central Anhui on October 23. In the middle of the image is a large piece of bare land features after harvesting. Compared with the feature of the Dongdawei of Baihu Farm above, the feature in visible light (B4B3B2) The basic difference is not big, but there are still obvious differences in the presentation of shortwave.
The following picture shows the three-month flood period
in the Dongdawei of Baihu Farm: The picture below shows the Baihu Farm before the disaster:
2.2 Changzhou Township
On the evening of July 9, 2020, a breach occurred in Zhongzhouwei, Changzhou Township, Poyang County, and Changzhou Township was flooded and soaked in flood water for more than two months. During the period when the water was soaked by the flood, the content of sediment in the water was relatively high. As time passed, the sediment gradually settled and the water quality became clear:
Changzhou Township had already pumped out the flood with 62 pumps pumping water day and night. However, from the current satellite imagery, the tones of the features in Changzhou Township are not in harmony with the surroundings. Compared with the images of the same period last year, the NDVI level in Changzhou Township this year is significantly lower. This is due to prolonged flooding. , The crops have long been cut off.
From the radar image, compared with the same period last year, the image tone of Changzhou Township is obviously darker. Like the former Dongdawei of Wenbaihu Farm, the water content of the inner soil is almost saturated after prolonged flooding:
2.3 Wangjiaba
On July 20, 2020, the Wangjiaba gate opened for flood storage, and the Mengwa flood storage area sheltered the family. The flood inundated a large area of the Mengwa flood storage area and also reduced the flood peak for the downstream. Now the flood has faded, but the crops have basically been harvested. A large area of “bare soil” was left. Compared with last year’s NDVI, where crops were growing vigorously, a huge NDVI “hole” was formed in the Mengwa flood storage area this year:
because the Mengwa flood storage area has more floods than Baihu Farm and Changzhou Township The villagers in the disaster area have enough time to replant autumn crops. From the satellite images in October, there are already many green vegetation areas in the area compared to September, especially in the southwestern part of the area. The crops are more concentrated and contiguous:
2.4 Flood overview
The picture below is the Sentinel-1 satellite image of the Huai River Basin in mid-August this year and the same period last year:
The picture below is the Sentinel-1 satellite imagery of Chaohu Lake in mid-August this year and the same period last year:
The picture below is Poyang Lake in late July and last year Sentinel-1 satellite image during the same period: The
following figure shows the Sentinel-1 satellite image of Dongting Lake in late July this year and the same period last year:
3. The moment of flood discharge from the reservoir
3.1 San-Xia
In mid-August 2020, the Sichuan Basin suffered a record-breaking rainstorm. The Leshan Giant Buddha "washed its feet", Chongqing Ciqikou and Hongyadong were flooded, and the No. 5 flood peak in 2020 reached the San-Xia hub. The inflow flow reached 75,000 cubic meters per second. , San-Xia opened 11 flood discharge holes. After the flood faded, a large sludge appeared in front of the dam. Fortunately, the heavy rainfall in the Sichuan Basin and the heavy rain in July in the middle and lower reaches of the Yangtze River were staggered. If the heavy rains in the two regions meet at the same time, the floods this year will be It would be unimaginable.
3.2 Xiaolangdi
The Yellow River's water regime was generally stable this year, but there were also several flood peaks in August. Xiaolangdi smoothly cut the flood peaks, ensuring the safety of the downstream. The flood peaks of the Yellow River contain a large amount of sand. After the floods are cut off, the sand discharge holes at Xiaolangdi are fully fired, and the sediment flowing into the reservoir area needs to be washed out.
3.3 Xinanjiang Reservoir
Starting on the morning of July 7, 2020, the Xin'anjiang Reservoir, the "East China No. 1 Reservoir," opened its gates for flood discharge. For the first time in 61 years since the reservoir was built, 9 holes were fully opened. Compared with the Xiaolangdi of the Yellow River, the water quality of Xin'anjiang Reservoir was much clearer and the flood discharged I don’t know how many million bottles of Nongfu Spring can be filled.
3.4 Nierji Reservoir
From August 27th, within half a month, three typhoons successively "visited" the Northeast region, which was rare in history, causing severe precipitation in September in Northeast China. Rivers such as the Nenjiang and Songhuajiang Rivers expanded and some crops were flooded. The upper reaches of the Nenjiang River Flood release of Nierji Reservoir:
postscript
This year is a veritable year of wet water for the middle and lower reaches of the Yangtze River. In October, Poyang Lake and Dongting Lake should have entered the dry season of autumn and winter. However, the water surface of Poyang Lake and Dongting Lake in October this year is still vast, much larger than that of previous October. The high water level is bound to disturb the wetland ecosystem in the lake area and affect the growth of aquatic plants and aquatic plants.
The winter is about to enter, and migratory birds in Siberia will soon follow. The impact of this year's floods on their safe overwintering is still unknown.
END, that's all for today's introduction, once again paying tribute to the retrograde rescuers in the flood, and finally I wish everyone a happy life!
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