[Jiang Xiaoxi talks about Poyang Lake] The speed of "changing face" is amazing! Why does the lake water turn yellow in autumn and winter?

Original: Jiang Xiaoxi's science class

Hello, I'm Jiang Xiaoxi

Every year the dry season comes

Poyang Lake will stage a drama of "the sea turning into mulberry fields"

The same world was still shrouded in mist a few months ago

At this moment, I was as thin as a lightning bolt.

A while ago, the clear green lake

Once it reaches this point, it will turn yellow.

This speed of "changing face"

Xiao Xi couldn't help but recite a poem after seeing it.

The lake area shrinks after entering the dry season

This is not difficult to understand

But why

The color of the lake water turned yellow?

Come and find out with Xiaoxi

It turned out that the lake water carried a lot of silt.

They are the "behind-the-scenes characters" that make the lake water turn yellow

[Why is the lake water more turbid in autumn and winter? 】

Maybe someone thinks

The lake water should be more yellow during the flood season than during the dry season

Because there is a lot of rain, large amount of water and fast flow rate during the flood season

Lake water is more likely to become turbid

This understanding is biased.

We explain this from two aspects.

on the one hand

Because there is a lot of water during the flood season

Relatively speaking, the total amount of sediment is larger

But the sand sinks due to gravity

This causes the middle and bottom of the lake to become turbid.

The lake was deeper then.

The lake looks clearer.

With the coming of the dry season

The water level is gradually decreasing

The sediment content is increasing

Less water, more sand

The lake water is getting yellower.

on the other hand

April to September every year

The Yangtze River and Poyang Lake enter the flood season one after another

Water from the "Five Rivers" carries sediment into Poyang Lake

But the rising water level of the Yangtze River

This makes it difficult for the lake water to flow into the Yangtze River.

The flow rate slows down

The mud and sand will temporarily "stay" in the lake.

May to early September is the main sedimentation period

After September, it enters the dry season

The water and sand entering the lake from the "Five Rivers" are relatively small

The Yangtze River water level dropped first

The speed at which water from Poyang Lake flows to the Yangtze River begins to accelerate

The mud and sand are like riding on a speedboat

Being swept up and washed away by the water

The lake is in a state of flushing

The lake water naturally becomes more turbid.

(Scouring: refers to the phenomenon that water flow acts on the riverbed, causing soil and rock loss or erosion)

Where does the mud and sand in the lake come from?

“The Yellow River comes from the sky”

Water falls from the sky

It fell on the mountain and washed away the soil

Some of the sediment in the soil began to "migrate" with the water flow

Flowing into streams and rivers

There is "indigenous" sediment from the Poyang Lake Basin

There is also "Po Piao" silt from the Yangtze River

The "Five Rivers" converge into Poyang Lake

Their mud and sand also followed along the river.

After arriving at Poyang Lake

Because the terrain is slowing down

The sediment is deposited in the lake area

The sediments in the Poyang Lake system

We can call them the "natives" of Poyang Lake

The other is the "Po Piao" from the Yangtze River

Every flood season

The Yangtze River's water level is rising rapidly

Carrying the mud and sand of "Po Piao"

Come here to stay

Become a member of Poyang Lake

Although we are a family

But different characteristics

Due to differences in water volume and vegetation coverage

The sediment content and sediment transport capacity of the "Five Rivers" are also different

(Sediment transport capacity can be understood as the ability to transport sediment)

Among them, Ganjiang River has the largest sediment transport

Raohe is the smallest

But generally not high

The amount of sediment flowing into Poyang Lake is increasing every year.

Most of it is deposited in the lake area

Not transported to the Yangtze River

Although the sand content of the Yangtze River is not high

But the amount of water is very large

Therefore, the total amount of sediment transport is large

90% of them are concentrated in the flood season

[Small sediment, big effect]

First

A grain of sand is insignificant

However, after silting into piles, a large amount of organic matter is deposited

Aquatic plants, animals, and microorganisms grow and develop here

Fish thrive here

Complete the task of racial continuation

Second

Sediment can also absorb pollutants in water

Promote its degradation and transformation

Purify water environment

Third

Sediment is also a building material resource

From "sand" to "sand"

From the small substance that brings "water" to life

It has become a rock-solid "foundation of the building"

However, excessive sedimentation will also bring a series of negative effects

For example, the flood level increases during the flood season

Increased threat of flooding

Worse still, it causes the riverbed to change course.

Obstruction of shipping traffic

Squeezing fish living space and destroying the living environment, etc.

Excessive sediment is harmful

But illegal sand mining is not advisable

Illegal sand mining means "proliferation" and "disorder"

It destroys the ecosystem

Causes riverbed incision

This will affect the groundwater level and damage the embankment.

Possesses safety risks

therefore

Sand mining must be legal, systematic and scientific

[The impact of sluice gate construction on sediment scouring and deposition]

Movement of small sand

It will have a great impact on the ecological environment of the lake area.

Many friends can't help but ask

After the construction of Poyang Lake Water Conservancy Project

Will it affect the movement of sediment?

Speaking of which

Let's look at the experimental data released by two groups of relevant departments:

1. Location of the downstream section of the sluice site

Average annual sediment load

Before the sluice gate was built, it was about 5.6 million tons

After the sluice gate is built, it is estimated that the amount can be reduced to about 4.2 million tons

The average annual reduction is about 1.4 million tons

Annual average sediment content

Before the gate was built: 0.039kg/m³

After the gate is built, it is predicted that the pressure can be reduced to 0.029kg/m³

Annual average lake sediment discharge

The volume of water transported from the dam to the surrounding areas increased from about 6.8 million tons before the sluice was built.

After the sluice gate is built, it is estimated that the amount of wastewater can be reduced to about 5.5 million tons.

Annual reduction of about 1.3 million tons

The above data show that

Build a sluice to keep the water level from falling slowly during the dry season

The power of water to carry away sediment has weakened

The amount of sand in the downstream water will also decrease.

so

The construction of the sluice gate will not cause sedimentation downstream of the sluice gate

2. Poyang Lake area above the sluice gate

Predicted cumulative sediment volume by 2035

Before the sluice gate was built, it was about 12 million m³

After the sluice gate is built, it will be about 26 million m³

About 14 million cubic meters of sediment

The average annual increase is about 900,000 m³

3. The river channel below the sluice gate

Predicted cumulative sediment volume by 2035

Before the construction of the sluice gate, about 20 million m³

After the gate is built, about 22 million m³ of sediment will be flushed out. That is to say, the gate is expected to flush out about 2 million m³ of sediment, which is equivalent to about 130,000 m³ more per year.

so

After the sluice gate was built, the scouring and silting intensity of the lake area changed.

But the changes are not obvious

The impact is also small

Out of a long-term and rigorous consideration of the lake area

Whether it is siltation upstream of the sluice gate

Or downstream flushing?

After the gate is built, a certain river dredging method can be used.

and downstream embankment protection

For scientific maintenance

Respect and protect the sand

It will also bring endless wealth to nature and mankind

If you dig blindly

It will also resist silently in its own way

After listening to Xiaoxi's introduction

From now on, the little sand in your heart

Has it become extraordinary?

Protect every "little character" in Poyang Lake

Is to protect this "lake of clear water"