This article is from WeChat official account:Bring science home (ID: steamforkids), author: Shen mengxi, title figure from: vision China
This summer, the most eye-catching thing for everyone, apart from the sporadic cluster of COVID-19, may be the flood.
Town in the Flood Picture/AFPBBB News
Since the beginning of the plum, the rain in southern China has hardly stopped. As a result, almost all southern provinces and cities have to face the threat of flooding. So, how did the threatening floods come about?
The practical one-sentence explanation is: this is what we humans “seeked”
In our geography class, we may all learn the phrase “every great city is on the bank of a big river”. We can actually start this story from the beginning of human settlement. After human beings ended the cave era, when they came out of the cave, they would all face a choice: where to live? We have no way of knowing whether they quarreled over where to live, but from the results we see now, they all chose to live by the river without exception. Why did they make this choice?
We all know that rivers generally originate in mountains. The tops of those high peaks, the ice covering the topsThe layers melt every summer, and the melted snow flows down the slopes, forming streams. In the process of flowing downwards, streams will converge in valleys to form mountain rivers.
The snowmelt on the top of the mountain is one of the sources of river water. Picture/unsplash
Mountain rivers continue to flow along low-lying places. During the flow, the mountains along the way will also collect rainwater into valleys due to rain, so these mountain rivers have more and more water.
Rainwater is supplied to the river along the dotted line in the figure, and at the same time it also carries a large amount of sediment into the river, which will become the downstream farmland Fertilizer
In addition, the rainwater flowing down the hillside will continue to wash the hillside, carrying the rocks, sand and soil on the hillside into the river water. When the river water finally rushes out of the mountain pass and comes to the mountain front area, The terrain opened up suddenly. The valley that originally bound the river disappeared, and the river can flow freely in the mountain piedmont area. At this time, their flow rate will naturally decrease, and their carrying capacity will naturally decrease, so at this time, most of the sediment they carry Zi precipitated in the piedmont zone, forming alluvial fans and piedmont plains.
Here, there are abundant groundwater resources, and the sedimentation of sediment brought by rivers also makes the soil of the piedmont plains extremely fertile. These fertile soils are very important for agriculture.High-quality resources can greatly increase crop yields in ancient times without any agricultural knowledge. Therefore, on the one hand, it is convenient to get water from the river, on the other hand, the soil is fertile and suitable for farming. The ancient humans naturally found a slightly higher place on this mountain front plain to settle down.
Large farmland on alluvial fan Picture/NASA
Beijing built on the plain before the mountains
For the plains along the lake and the coastal plains, it is actually similar. When large rivers carry a huge amount of sediment from the land to the lake/sea, they face the resistance of the lake/sea water, the flow velocity suddenly decreases, and the water carrying capacity of these reduced flow velocity also rapidly decreases, so the sediment Precipitating in the estuary area, accumulating to form a huge estuary delta, this is the case in China’s Pearl River Delta, Yangtze River Delta, and the Yellow River estuary.
Picture of the Pearl River Delta Plain with dense river network/USGS
The growing Yellow River enters the sea, and the farmland is also expanding. Original image/USGS
On these plains, the rivers are abundant, the water is convenient, and the land is fertile and suitable for cultivation. Naturally, they began to attract ancient human settlements. As a result, settlements began to take shape in the higher terrain along the river. There were more and more fields around the settlements and more and more people to feed. The settlements gradually escalated, from villages to market towns to big cities. So now, let’s see, basically all major cities are built along the river.
On the non-mountainous and non-delta plains, if we look at the satellite map, we will see a curved lake similar to the picture below.
Oxyoke Lake on the plains of the middle and lower reaches of the Yangtze River
In fact, this was not a lake a long time ago, but an ancient river. We call this kind of lake an oxbow lake. When the river rushes out of the mountain pass and flows on a wide plain, it will continue to bend. When it is bent to the extreme, the two extreme curved ends may be connected by a flood. Once connected, the river will be The bend will be cut and straightened, and the original old curved river will be abandoned and a new straight river will be adopted.
On the satellite image, you can see the swing of the river over time and the formation of the oxbow lake image/youtube
The river is constantly oscillating on the plain through this process of intercepting and straightening, so the soil on the entire plain is actually formed by the river’s deposition for a long time, so the soil on the plain is fertile and very suitable for farming. So people consciously or unconsciously began to settle on this large plain. Once settled, cities and villages were built along the river and along the lake.
More importantly, when choosing to live along the river, ancient humans did not choose to live in a safer place that is particularly far away from the river. In many cases, they would choose to live next to the river.
The ancient city of Liangzhu 5300 years ago, people live next to the river
When there is no human settlement, the river water from the river can flow freely on the flat ground. When the water volume is small, the river will naturally accumulate a slightly higher natural dike, and the river water flows in the natural dike. Once the water volume increases, the river begins to flow over the natural embankment, and the river flows over the natural embankment. Because the river suddenly widens, the flow rate suddenly drops, and the large amount of soil carried by the river settles on the outside of the natural embankment, which forms a floodplain.
Pictures of floodplains and natural dikes/see watermark
The sediments in the floodplain are all muddy substances with the smallest grain size in the river water. After precipitation, large tracts of silt are formed. This silt is rich in nutrients and is very suitable for farming. The ancient humans did not understand why the crop growth was the best here, but since they discovered this, they must reclaim the floodplain area on a large scale, and they will also settle and live near the floodplain. Unfortunately, in the event of heavy rain, the amount of water in the river will inevitably increase, and the crops and villages on the floodplain will be inundated. The ancient humans built dikes to prevent flooding. As a result, the dikes became higher and higher. , The river in the dam is flowing more and more urgently. These rapids broke through some unreliable dams in the process of flowing, and then rushed down at the breach, flooding the neighboring farmland and villages, which formed a devastating flood.
Submerged floodplain picture/see watermark
So in fact, the summary is that the formation of flood disasters actually originated from the competition between man and nature. In the long-term life of mankind, the floodplain area along the river has been continuously developed or simply surrounded by lakes and rivers for farmland. However, once it encounters heavy rains for several days and the river flow increases, these rivers will in turn compete with humans for land. The nature of the flood.
City in Flood Map/Yahoo News
Of course, from my personal point of view, the floods that we humans have suffered in the process of moving from ancient times to today’s civilization is a process of unconsciously confronting the laws of nature. The predecessors have not discovered this kind of The laws of nature, so the losses suffered are especially huge, but we have now mastered some of the laws of nature, what we should do is to make good use of these laws of nature, and continue to move forward while coexisting in harmony with nature as much as possible.
This article is from WeChat official account:Bring science home (ID: steamforkids) , author: Shen mengxi