1. Rain and flood in flood season

The Yangtze River basin belongs to the subtropical monsoon climate zone, and both southwest monsoon and southeast monsoon can enter, which provides favorable conditions for the formation of heavy rain. The Yangtze River is abundant in rainfall, with an average rainfall of1.057mm for many years. There are four main rain areas: (65,438+0) Wuyishan rain area, with annual rainfall of 65,438+640 mm, and the earliest rain period is from March to June. (2) In the rainy area of Nanling, the annual rainfall is about 1400 mm, and the rainy period is late, from April to June. (3) The annual rainfall in Ya 'an rainy area of Emei Mountain is 1000mm, and the rainy season is from July to August. (4) In the rainy area of Hanjiang River, the rainy season is the latest, from August to September, and even extended to 10, with an annual rainfall of about1000 mm.. From March to April in normal years, the rain belt in the Yangtze River basin moves from southeast to northwest, and the rainy season in the middle and lower reaches is earlier than that in the upper reaches, and Jiangnan is earlier than Jiangbei. The rainfall distribution decreases from southeast to northwest, and the rainfall in the middle and lower reaches is more than that in the upper reaches.

The Yangtze River is a rainy and flooded river, and the flood variation law generally corresponds to the rainstorm. The flood season in the middle and lower reaches is earlier than that in the upper reaches. Generally speaking, Poyang Lake water system and Xiangjiang River in Dongting Lake are the main flood seasons from April to June, Dongting Lake resources and Lishui River are from May to July, and the upstream tributaries are from July to September. In case of autumn flood, there will be a big flood in 65438+ 10. The annual maximum water level and maximum discharge of control stations in the main stream of the Yangtze River generally appear in June-September, with the most frequent in July-August.

The annual runoff of the Yangtze River is 889 billion cubic meters, of which the upstream runoff accounts for 47%, Dongting Lake accounts for 2 1% and Poyang Lake accounts for 17%.

Cuntan to Yichang Three Gorges section, the main stream of the Yangtze River, is about 660 kilometers long and covers an area of about 6,543,800 square kilometers. If Wujiang River is not included, the area is 56,000 square kilometers, accounting for 5.6% of the area above Yichang. Although this area accounts for a small proportion, the flood in this section is often fierce and often plays a role in capping the flood peak in Yichang. For example, 1982, the maximum flood peak in Yichang is 52400cms, and the maximum flood peak in the Three Gorges area is 27900cms. Generally, the direction of rainstorm causing interval floods moves from the center of rain area of Minjiang River, Tuojiang River and Jialing River in Sichuan to the southeast or from west to east to the Three Gorges area, and also from the center of rain area in northeast Sichuan and the upper reaches of Hanjiang River to the south to the Three Gorges area, which is consistent with the flow direction of the Yangtze River. The rainstorm in the Three Gorges is usually two to three days, which is also the main reason why the floods in the Three Gorges area are often superimposed on the rising section of the upper reaches of the Yangtze River or near the flood peak. The Three Gorges reach of the Yangtze River is a rainstorm-prone area in the middle and lower reaches of the Yangtze River, and flood forecasting in this reach is of great significance to flood control and dispatching in the middle and lower reaches of the Yangtze River.

2. Major rainstorm areas in the basin and relevant rainstorm extreme data.

The 24-hour maximum rainstorm in the Yangtze River Basin is 822 mm (1August 4, 960) in Chaoqiao, Rudong County, Jiangsu Province, and the three-day maximum rainstorm is 1076mm,1July 3 -5, 935 in Wufeng County, Hubei Province.

3. Flood characteristics of main tributaries

Rainstorms and floods in the Yangtze River can be divided into whole basin and local ones. The former has a wide rainfall range and a long history, with the rainy season overlapping upstream and downstream, and the movement direction of the rain area is roughly the same as that of the main stream, resulting in serious floods in the main and tributaries, such as 1954 and 1998. The latter is the rainstorm in some reaches of the main stream and some tributaries, such as the middle reaches 1935 and 1996, the upper reaches 198 1 year, and the Hanjiang river 1983.

Flood characteristics of the Yangtze River:

(1) The flood peak is high and lasts for a long time. The measured maximum peak flow of the main stream is 92,600 cubic meters per second (1954 Datong Station), and the investigated maximum peak flow reaches 1 10000 cubic meters per second (1860, 1870 Zhicheng); The measured maximum flow of major tributaries such as Khan Award and Jialing River exceeds 40,000 cubic meters per second, and the investigated maximum flood peak flow exceeds 50,000 cubic meters per second. A flood process lasts about 20-30 days in Pingshan and Yichang, more than 50 days in Hankou and Datong stations, and generally lasts about 10 day in each tributary. Due to the high peak height and long duration, the total flood volume is very large, such as Hankou Station1954, the maximum flood volume in 60 days is 322 billion cubic meters, and Datong Station is 42 10 billion cubic meters, and the flood peak volume is much larger than other rivers.

During the flood season (May-65438+1October), with Datong as the control station, about 50% is above Yichang, 44% is in the middle reaches and less than 5% is in the lower reaches. With Hankou as the control station, Yichang and above account for 66%, Dongting Lake water system accounts for 23.9%, Hanjiang River accounts for 7%, and Qingjiang River accounts for less than 2%.

(2) The flood is relatively stable with little interannual change.

(3) Low sediment concentration and large sediment transport. Yichang has an average sediment concentration of 1.2kg/m3 for many years, but it has a large amount of water, with an average annual sediment discharge of 530 million tons. The average sediment concentration of Datong Station for many years is 0. 157Kg/m3, and the average sediment transport for many years is 470 million tons.

4. Historical flood description and peak number

(1) 1998 flood

During 1998, a basin-wide flood occurred in the Yangtze River basin, which was second only to 1954. Its main features: (1) The whole basin floods followed the floods of Wuhu River in Poyang Lake system, Yuanjiang River, Lishui River and Xiangjiang River in Dongting Lake system, and the floods in the middle and upper reaches of the Yangtze River occurred one after another, resulting in another whole basin flood since 1954. (2) The number of floods is mostly affected by rainfall. There have been many floods in Poyang Lake and Dongting Lake water systems, and there have been eight flood peaks in the upper reaches of the Yangtze River. (3) The magnitude of flood is large. Compared with this year's flood of 1954, the incoming water of Yichang Hydrological Station in the upper reaches exceeds 1954 in July and August, and the maximum flood volume on 30th and 60th days is equal to or greater than 1954, with a frequency of about 80 ~ 100. The flood volume in the middle reaches is second only to 1954. If flood reduction is not considered, the maximum flood volume of Hankou in July and August, 30 days and 60 days is greater than 1954 flood. Downstream Datong is slightly less than 1954. (4) The flood peak water level is high. The water levels in the Yangtze River, Dongting Lake, Poyang Lake Shashi-Luoshan and Wuxue-Jiujiang reach exceeded the historical record by 0.55-1.25m for many times, and the highest water level in Shashi reached 44.95m, 44.84m and 45.22m respectively for three times, and the highest flood peak level exceeded the historical maximum water level by 0.55m. (5) The flood lasted for a long time. Since mid-June, the water level of the Yangtze River has exceeded the alert. This year, the water levels of Shashi, Jianli, Luoshan, Hankou and Jiujiang in the main stream of the Yangtze River exceeded the warning water level for 57 days, 82 days, 865 days, 438+0 days, 84 days and 94 days respectively, and the water levels of Jianli-Luoshan and Wuxue-Jiujiang reached the highest level in history for more than 40 days. (6) In the same period of history, the highest water level appeared in the middle and lower reaches of the Yangtze River, Xiangjiang River in Dongting Lake and Ganjiang River in Poyang Lake from June to March. In the flood season, the main control stations in the middle and lower reaches of the Yangtze River have exceeded the warning water level since June 24, and the first flood peak appeared in the upper reaches of the Yangtze River in early July, about half a month earlier than normal. On July 4th, Jianli, Wuxue and Jiujiang reached the highest water level in history.

(2) 1954 flood

In the case of heavy rain in the whole basin, heavy floods occurred in other parts of Ganjiang River and Poyang Lake water system in early June and early July. Serious floods will occur continuously in the Dongting Lake water system in late May, late June, mid-July and late July, with Taoyuan Station reaching19200m3/s (May 26th),17800m3/s (June 27th) and 17800 m3/s(7. Xiangjiang River also experienced continuous floods at the beginning, middle and end of June. On June 30th, the maximum flood peak discharge at Xiangtan Station reached 1.83 million cubic meters per second, which was close to the measured maximum flood. Lishui and Zishui also experienced major floods. The Hanjiang River also experienced a moderate flood at the end of July and the beginning of August, and the maximum inflow of Hankou-Hukou tributary reached 13582 cubic meters per second. Under the above circumstances, the water level of rivers and lakes rose rapidly, with Hankou Station exceeding the warning water level (26.30m) on June 25, and the highest water level 193 1 in July was 28.28m m. In the case of the overall high water level in the downstream, there were four flood peaks at Yichang Station in the upstream from June 25 to September 6, and the maximum flood peak flow reached 6600 on August 7. Jingjiang flood diversion is divided into three times, with a total flood volume of 65.438+0023 billion cubic meters, a water level of 44.67 meters in Shashi, 33.95 meters in Chenglingji, 29.73 meters in Hankou (the measured maximum peak flow is 76 100 cubic meters per second) and 33.95 meters in Chenglingji. The maximum flow of Chenglingji Station 108900 m3/s and Hankou Station 1 14 183 m3/s.

(3) The principle of flood peak numbering

1) scope: temporarily limit the main flood in Yichang-Datong section of the Yangtze River.

2) Numbering standard: within the above range, Yichang will be numbered only when there is a flow of more than 50,000 cubic meters per second or the flood peak water level of the main control station exceeds the warning water level, otherwise it will not be included in the numbering sequence;

3) Under the above-mentioned scope and standards, the numbers shall be numbered according to the time serial number of the observed flood peak;

4) When the numbered flood peak of the downstream section is the result of the downward propagation of the numbered flood peak of the upstream section, the number of the upstream flood peak should be used, otherwise the flood peak of the downstream section should be numbered separately.

Principle 4) means: let's just let the upstream flood peaks merge, that is, let a certain flood peak merge into a certain flood peak in a certain river section. When there is a variable peak with a small leaf peak under the upstream flood peak, if all the downstream reaches meet the numbering standard, new numbers should be allowed to be inserted in the downstream reaches in chronological order.

Hydrological characteristics of the yellow river basin ... the rainstorm in the lower reaches of the yellow river is more frequent in July, followed by July. Rainfall in the upper reaches of the Yellow River

Features: large area, long duration, low rainfall intensity and strong continuous rain; There is only a few days of rainfall at a time ... the rainfall in the middle reaches of the Yellow River.

Features: High intensity, short and frequent duration, and the rainfall area is smaller than the upstream, but the rainstorm area larger than 50mm is larger than the upstream.