The natural conditions that produce freezing floods depend on the geographical location of the river and the shape of the river channel. In alpine areas, rivers flow from low latitudes to high latitudes and the river channel shape is wide at the top and narrow at the bottom. There are more opportunities for serious flooding in places where the river channels bend and loop. This is because the lower section of the river freezes earlier than the upper section, and the upper section of the river thaws earlier than the lower section. Moreover, the thickness of the ice sheet is thicker in the lower part and thinner in the upper part. When ice appears in the lower section of the river, it blocks part of the upstream water and increases the water storage capacity of the river channel. When the ice melts and the river is opened, this part of the water stored in the channel is released sharply, causing peaks to pass downward, and more and more ice water accumulates along the way. , the ice peaks are increasing steadily. When the ice water in the upstream spreads downstream, it encounters narrower sections or corners of the river and forms ice dams, causing the water level in the upstream to rise. Whether ice floods are serious or not depends on the extent of the impact of river ice on water levels. Usually only when severe ice or ice dams appear in rivers will the water level rise suddenly and cause serious ice floods. In short:
1. Rivers with ice age.
2. The river section flows from lower latitudes to higher latitudes, with a clear north-south direction. The Yellow River in my country has freezing floods in its reaches in Ningxia and Shandong. Rivers in the Northeast will also experience freezing floods when the above conditions are met.
——Yellow River Flood
Flood is a common natural phenomenon in the Yellow River, a temporary rise in water caused by ice blockage. Because the Yellow River is located at a high latitude, the lower reaches of the river are usually frozen and closed in winter. When the ice thaws and the river opens in early spring, the ice and water rush down, the ice blocks the river, and the water level rises, forming a cold flood. This period is the cold flood season of the Yellow River. Because the latitude of the upper and lower reaches of the Yellow River differs by more than 3 degrees, the average winter temperature differs by 3 to 4 degrees Celsius. The upper section of the river freezes late and opens early, resulting in thinner ice; the lower section of the river freezes early and opens late, resulting in thicker ice. Generally, between January and February, when the temperature rises, the frozen upper section of the low-latitude river section first thaws and opens up. During the freeze period, the accumulated water in the river channel is released rapidly, forming a freezing flood. The peak flow increases along the way and the water level rises; but the lower section of the high-latitude river section Because the temperature is still low in this section, the ice is frozen. Under the action of water flow, the ice drum opens, forming the "Wukai River". During the "Wukai River", the thaw comes quickly, especially when the temperature rises sharply or the water level rises sharply, and large ice flakes surge down, which easily causes ice flakes. Sometimes a large number of ice flakes block narrow, curved, and shallow shoals, forming ice jams and ice jams. The dam causes the water level to rise sharply. Due to the plugging of ice and drift ice, the flow area is reduced and an ice dam can be formed. As the water level increases, water drums and ice will crack and the ice will overflow the embankment, causing disasters. The level of damage often exceeds that of floods. In some years, the temperature changes in the upper and lower river sections are not much different, and the frozen river is thawed in sections or on the spot to open the river, which avoids large-scale cold weather floods and the river opening is relatively smooth and smooth. This situation is generally called the "Wenkai River".
①Lingxun in the section from the Yellow River to Lanzhou
The section from the Yellow River to Lanzhou is the head end of the upper reaches of the Yellow River. Although the climate is cold and long, the Yellow River runs through the mountains of the Qinghai-Tibet Plateau. , the specific gradient of the river in each river section is very different, and the flow velocity also changes greatly. Therefore, some river sections have both flow and freezing; some river sections can only flow but not freeze; and some river sections often freeze under natural conditions, but the thermal and hydraulic power has been changed after the construction of the reservoir. Due to the conditions, several ice jams occurred upstream of the reservoir, and the downstream of the reservoir changed from frozen to unfrozen.
The section from the Yellow River to the Maqu River is 911.6 kilometers long. The first ice period generally occurs from mid-October to late November, with the earliest being August 21 (1960). The final ice date is generally from early April to late May of the following year, and the latest is June 29 (1956). It is the section with the longest freezing period in the whole river. For example, the average freezing duration of the Yellow River Station over the years is 160 days, with the longest being 193 days and the shortest being 128 days. The maximum ice thickness in the center of the river is 1.28 meters and the average ice thickness is 0.93 meters; the average freezing duration at the Maqu Station over the years is 105 days. The longest is 118 days and the shortest is 95 days.
The stretch from Maqu to Xunhua is 726 kilometers long. Due to the strong dynamic effect of the water flow, it can only flow but cannot be frozen. The first ice period occurs in late November, and the last ice period occurs in late February to early March.
The river section from Xunhua to Lanzhou is 211 kilometers long. The first ice date generally occurs in late November and the last ice date is in late February. In winter, the density of flow increases rapidly due to the sudden drop in temperature. There are local freezes above Lanzhou, and freezes are often found near the Lanzhou section. In the early 1960s, when the Yanguoxia Reservoir was built to store water and before the Liujiaxia Reservoir was built, the Liuyan River section experienced huge ice jam disasters for several years. The Lanzhou River section below the Yanguoxia Reservoir has begun to become unfrozen, but there are sparse currents. After the Liujiaxia Reservoir was put into use in 1968, it regulated heat and water volume, causing the ice flowing into the reservoir area upstream to melt. The temperature of the water released from the reservoir in winter was as high as 3 to 5 degrees Celsius, and the Lanzhou river section no longer suffered from flow and freezing.
②Ningxia Section Lingxun
The Ningxia Section of the Yellow River, from the entry at Nanchangtan in Zhongwei County to the exit at Mahuanggou in Shizuishan, has a total length of 397 kilometers and flows from south to north. The 135 kilometers from Heishan Gorge to Zaoyuan is a canyon river section with a river surface width of 200 to 300 meters. Due to the steep slope and rapid flow, the river can only be closed in cold winter years, so it is called an infrequently frozen river section.
It is more than 260 kilometers below Zaoyuan and the river is 500 to 1,000 meters wide. Due to its gentle slope, low flow speed and low temperature, it is a section of the river that is often frozen. In cold winters, the uppermost area is sealed to the lower riverside; in warmer winter years, it is frozen no further than the Zaoyuan.
After the Qingtongxia and Liujiaxia reservoirs were put into use one after another, due to the increase in flow and water temperature in winter, the unsealed section of the river was extended from Zaoyuan to Xintian for more than 20 kilometers; there are 40 to 90 kilometers of water under the Qingtongxia Dam. Although the river section is not frozen, more than 100 kilometers below Panchang are still frozen.
The Ningxia river section is not prone to flash flood disasters in general years. However, special circumstances can cause ice jams and water blockage in some river sections, causing flash flood disasters.
In the section from Shikong to Zaoyuan in Zhongning County, located 45 kilometers above the Qingtongxia Reservoir dam, three consecutive ice jams occurred from the winter of 1967 to the spring of 1968. The ice plug body is 16 kilometers long, the ice cover is 0.4 meters thick, and the ice flowers under the ice cover are as thick as more than 1 meter. The highest water level in the ice jam is 1.3 meters higher than the corresponding water level of 5,240 cubic meters per second during the smooth flow period, causing ice and flood disasters. The reasons for the disaster were: first, there was a lot of ice coming upstream, and a steady stream of drifting ice formed an ice jam in the Shikong River section from top to bottom; second, after the river was closed, the water coming upstream surged. On December 6, 1967, the Yellow River The gate of the diversion tunnel of the upstream Liujiaxia Reservoir failed, and the discharge flow increased by 300 to 750 cubic meters per second compared with the flow when the river was closed. As a result, the water drum above the stone void opened and a large number of ice flowers and ice blocks lurked under the Zaoyuan, forming a serious ice jam. Third, the Qingtongxia Reservoir has a high water storage level, which hinders the discharge of drifting ice. From late December 1967 to mid-January 1968, the water storage level in the reservoir was 13 meters higher than the natural water level. Therefore, the ice jam body cannot move downward, causing ice jam and flood disasters in the river section from Shikong to Zaoyuan.
③Inner Mongolia Section Lingxun
The Inner Mongolia Section of the Yellow River is located at the northernmost end of the Yellow River. It enters from Shizuishan City in Ningxia and La Monk Temple in Yikezhao League in Inner Mongolia, and ends in Yikezhao League. The main stream exits Yushu Bay from Mazha, Geer Banner, with a total length of 840 kilometers and a total drop of only 162.5 meters. The Huanhe River section is wide and has gentle slopes and twists and turns. Although it is located in the upper reaches, the river slope from Zhaojun Tomb to Toudaoguai is close to that of the Yellow River estuary. In the Inner Mongolia section of the Yellow River, there are few tributaries and they are all seasonal rivers that produce rainwater and floods. There is almost no water supply during the ice age. Therefore, most of the water in the Inner Mongolia section during the ice age comes from water above Lanzhou.
The Inner Mongolia river section is located at the northernmost end of the Yellow River Basin, with an altitude of more than 1,000 meters above sea level. It is far away from the ocean and is difficult for warm air to reach. It is often controlled by the Mongolian high pressure, and has significant continental climate characteristics, with severe and long winters. The temperature stays below zero for 4-5 months, and the lowest temperature can reach minus 35°C. The river freezes for 4-5 months, and most of the river sections are stable and closed. Since the river flows from south to north, the temperature in the upstream Gansu and Ningxia sections is high and the temperature in the Inner Mongolia section is low. Therefore, the flowing section of the river traces its source upward, while the thawed river gradually develops downward from the Gansu and Ningxia sections. Under natural circumstances, Inner Mongolia The freezing time of the river in Duanbaotou is often more than 20 days earlier than that of Lanzhou, but the time of thawing and opening of the river is more than a month later.
In winter, the river flow in Inner Mongolia is generally more than 400 cubic meters per second. The temperature in this section is low and the ice layer is thick. When the flood peak passes, the water drums and the ice cracks, causing large pieces of drifting ice. In addition, due to the influence of the river flow, the drifting ice is densely stuck in the ice dam, causing the water level of the river to burst and rise, which can easily cause a series of bank breaches. , flooding caused disaster.
According to incomplete statistics, during the period of thaw and river opening in the Inner Mongolia section, disasters of varying degrees have occurred every year in history, with an average of once every two years. After the Liujiaxia Reservoir on the upper reaches of the Yellow River was put into operation in 1968, the ice age adjusted the water volume and heat in the river, causing significant changes in the freezing floods in the Inner Mongolia section and greatly reducing the disaster. However, storm floods are complex and have many influencing factors. Under special circumstances, there is still the possibility of major storm flood disasters, and the threat of storm floods in the Yellow River has not been completely eliminated. Because the upstream reservoir is too far away from the downstream section of the river where flooding is serious, the adjustment and use of the upstream reservoir cannot fundamentally eliminate the threat of flooding in the Inner Mongolia river section. According to the water demand requirements for industrial and agricultural production, it is impossible to completely cut off the water during the thaw period. flow; and the reservoir's anti-crash storage capacity is limited, and it is impossible to store water for a long time. Therefore, at this stage, it is necessary to comprehensively apply a variety of flood prevention measures to reduce or avoid the occurrence of flash flood disasters.
④Lingxun in the lower reaches of the Yellow River
The Yellow River, from Taohuayu in Henan Province to the mouth of the sea, is called the lower reaches. It is 786 kilometers long, with embankments on both sides, and flows into the Bohai Sea in Kenli County, Shandong. The downstream river is wide at the top and narrow at the bottom. The river runs in the southwest and northeast directions. It is often hit by cold waves in winter. The average daily temperature differs 3~4°C between the upper and lower reaches of the river, and it is positive and negative alternately. The river flow is generally 200~400 cubic meters per second. rice. Due to natural conditions such as river course, meteorology, and hydrology, floods occur in the lower reaches every year, and floods and river closures often occur. Since the founding of the People's Republic of China, rivers have been closed more than 30 times. Most of them started from the mouth of the river and then moved upward section by section. In general, the total frozen length is about 400 kilometers, with the shortest being 40 kilometers and the longest being 703 kilometers. The river is opened from top to bottom, and ice water accumulates along the way, causing obvious peaks, and it is easy to be blocked in shoals, sharp bends or narrow river sections, forming ice dams, causing the water level in the river section to rise rapidly, threatening the safety of the embankment, and even Cause disaster.
The cold floods in the lower reaches of the Yellow River have historically been famous for their frequent breaches, serious hazards, and difficulties in prevention and control.
According to incomplete historical statistics, out of the 54 years from 1883 to 1936, there were 21 bursts during the peak flood season, with an average of two bursts every five years. Even in the early days of the founding of New China, two freezing flood bursts occurred in the Shandong estuary area of ??the lower reaches of the Yellow River. There have been sayings in history that "it is easy to plunder during late floods, but difficult to prevent during cold floods" and "if floods break out during cold floods, river officials are not guilty". After the founding of the People's Republic of China, the people in the lower reaches of the Yellow River, under the leadership of the party and the government, have overcome many serious severe storms and floods, reversed the dangerous situation of two major disasters in five years in history, and achieved great achievements in the 49 consecutive years of unresolved disasters.