When spring comes and flowers bloom, go to the mountains, but don’t neglect ice and flood prevention!

Some time ago, when people were immersed in the beautiful melody of the song "Climbing the Spring Mountain" and preparing to take the elderly and children to the suburbs to enjoy the spring outing, a piece of news that "the dam of Donggecuo Lake in Qinghai collapsed and caused a flood" went viral on the Internet. In the video circulated on the Internet, on the vast grassland, the poor sheep stood in the floodwater that submerged the sheep's legs and shivered, and the herdsmen who came and went rode horses to rescue them one by one. The weather was still very cold and there was no rain. Why did the lake overflow and then burst the dam?

Shepherds saving sheep (Source: Video screenshot)

01Are spring floods too "unseasonal"? Actually, they are very common

In the minds of most of us, floods often occur in July and August when rainfall is concentrated. In fact, floods can occur all year round, so floods in early spring or during the transition between spring and winter may be a bit "off-season" for the public's usual perception.

The flood in Maduo County, Qinghai, occurred in the chilly early spring, which inevitably caused panic and many rumors appeared on the Internet. According to official news, the cause of the dam collapse was the rising water level of Donggecuo Lake in Maduo County, Golog Prefecture, Qinghai Province, which caused the left guide wall of the lock chamber to be washed away and the flood pressure in the downstream river channel in Dulan County, Haixi Prefecture increased. No casualties occurred.

Historically, floods in winter and spring are not uncommon. For example, the so-called "off-season" floods occurred in Lijin County, Shandong Province, my country in February 1929, in Venezuela in December 1999, and in Paris, France in February 2018.

Flooding of the Seine River in Paris in February 2018 (Source: Xinhua News Agency)

02 Early spring warming poses great risks. How did the flood occur?

Due to the differences in rainfall time in various parts of my country, the flood season is not consistent, and is mostly concentrated from early summer to July and August. Although the flood season does not mean flood disasters, floods often occur during the flood season. The seemingly unusual "off-season" floods are generally divided into two categories:

Floods are a natural phenomenon caused by natural factors such as heavy rain, rapid melting of ice and snow, storm surges, etc., in which the amount of water in rivers and lakes increases rapidly, or the water level rises rapidly. They usually occur during the flood season. Due to differences in rainfall time in various parts of my country, the flood season is not consistent, and is mostly concentrated in early summer to July and August.

In this case, aren't floods in winter and spring abnormal? Don't rush to refute it. Floods are divided into the following types according to the season and cause:

① Snowmelt flood . It can also be called spring warming flood. In high altitudes and regions, there are many rivers formed by glaciers, and these rivers are generally long, even across different latitudes. When the climate warms, the melting of seasonal snow in the basin, the thawing of river ice or spring rain will cause the river water to rise. March to May each year is the peak season for this type of flood, which often occurs in Ili Prefecture, Tacheng, and Altay in northern Xinjiang, China. The impact of this type of flood is often greater, and has been increasing in recent years due to global warming.

The spring flood we are familiar with is one of the consequences of the surge in river flow in spring. The picture shows an urban park by the river during the spring flood (copyright image of the picture library, reprinting and using it may cause copyright disputes)

② Ice flood . It is mainly a phenomenon under special geographical environment when the northern rivers thaw in spring, which is related to the direction of the river. Some northern rivers flow from south to north in some sections, such as the Yellow River from Gansu, Ningxia to Inner Mongolia and some sections of Heilongjiang. When the ice thaws in spring, the upper reaches thaw first, and the floating ice flows downstream, while the lower reaches have not yet thawed, causing floating ice to block the water level and cause the water level to rise. In addition, floating ice cuts the embankment, which is more likely to break through the embankment and cause floods.

Ice floods usually occur in March and April, mainly in the mainstream of the Yellow River and the Sanjiang Plain in the northeast. When the ice in the upper reaches of the Yellow River melts and forms spring floods and flows to the lower reaches, it coincides with the blooming of peach blossoms along the coast, so it is also given a beautiful but dangerous name "peach blossom flood".

According to the terrain where it occurs, it can be divided into two types on a macro scale:

① Glacial flood : mainly occurs on the northern slope of glaciers and high-altitude areas, and the flood affects a large area; in China, it mainly occurs in the high-altitude areas of the northwest.

② Plain floods : mainly occur around rivers, and the affected areas are generally along the banks of the river. In China, they mainly occur in North China and Northeast China.

Melting glaciers (copyrighted images from the gallery, reprinting may lead to copyright disputes)

According to the occurrence area , it can be generally divided into three categories:

① Northwest type : The occurrence area is generally Xinjiang, Qinghai, Gansu and other places in the northwest, where the melting snow on the northern slope of the glaciers causes off-season floods;

② North China type : The occurrence area is generally the Yellow River Zhongwei in Ningxia-Linhe and Baotou sections in Inner Mongolia-Zhengzhou section in Henan-Kenli section in Shandong;

③ Northeast type : Heilongjiang River, Songhua River and Ussuri River sections in Northeast China.

The three types occur at different times. The North China type occurs first, generally from mid-to-late November each year to mid-to-late March of the following year. It is followed by the Northwest type, which occurs from March to May each year. The Northeast type generally begins in mid-April and lasts until early May.

03 Floods occur in any season, so don’t take it lightly

In recent years, with global warming, extreme temperature rise, high temperature and snowfall events have increased in various places, and the frequency of floods during unconventional flood seasons has increased. Snowmelt floods often occur in Northwest my country, especially in Xinjiang and Qinghai, and the public needs to take more precautions.

In terms of prevention and control, we need to do both human and technical defense. Specifically :

(1) In response to glacier-snowmelt floods, efforts should be made to increase the construction of relevant water conservancy projects , build large and medium-sized reservoirs according to local conditions, and construct control water conservancy projects on important rivers to maximize the conversion of glacier-snowmelt water into industrial and agricultural water ;

(2) Pay attention to strengthening the risk prevention and reinforcement of dangerous reservoirs, river bank protection and embankment construction , and conduct inspections of reservoir embankments and potential hazards in winter and spring;

Heilongjiang River ice breaking and flood relief site (Source: Xinhua News Agency)

(3) Continue to strengthen flood disaster monitoring, prevention, forecasting and early warning technologies , especially strengthen temperature monitoring of snow and ice melting in the northwest glacier area , make full use of modern meteorological satellite technology and " meteorological + " technology , establish information sharing and emergency linkage alarm mechanism between meteorological and water conservancy departments, predict, forecast and warn in advance when the weather suddenly turns warm or cold, respond and feedback actions in the first time, and avoid in advance. At the same time, the public should also pay attention to early warning information, have plans in mind, and do drills.

Author: Liu Hanbin, Senior Engineer, Shanxi Coal Geology Bureau

Reviewer: Zhang Ying , Researcher, Director of China Science Writers Association, Secretary General of Emergency Science Popularization Committee

Produced by: Science Popularization China

Produced by: China Science and Technology Press Co., Ltd., China Science and Technology Publishing House (Beijing) Digital Media Co., Ltd.