Lesson video

Hello, geographers, it's Mr. Robertson here and we are going to be continuing our fantastic unit of work today, "Rivers: What is special about them?" Isn't geography just the most fantastic subject? So far, we've learned all about rivers and the sort of land forms that are formed.

And last lesson we started to look at how the river changes over its course and journey as it moves from the source to the mouth.

Today, we're going to be looking at flooding rivers.

So, we are gonna need some equipment, aren't we? Geographers, have we got on our geography caps? And I really hope they're going to have a big brim and be very waterproof today.

Have we got on our waterproof or ruck sacks? And we definitely need a really good pair of wellies, because floods are going to be very wet.

Let's find out what we're going to be doing.

By the end of this lesson, you will be able to explain the reasons why rivers flood.

We've got four keywords today.

As always, I will say the keyword first and you are going to repeat it to me.

So let's begin.

Bank.

Fantastic.

Drainage basin.

Really good.

And deforestation.

Oh, well done.

That's quite a tricky one, isn't it? So what do these words mean? Well, let's find out.

A bank.

The sides of a river are called its banks.

A drainage basin is the area drained by a river and its tributaries.

Deforestation is where trees are chopped down by humans.

We're going to be using all these keywords today, and by the end of the lesson, I'm really confident you will know exactly what they mean.

So our lesson today, all about flooding rivers, is in two parts.

The first part of the lesson, our question is, what are the natural causes of river flooding? And the second part of the lesson is, how can humans cause flooding? Okay, geographers, are we ready for the first part of the lesson? Let's go.

Look at that photograph here.

Wow, what can you see there? That looks like an absolutely enormous flood, doesn't it? Look at the road carrying on and the water just there.

And look at those trees.

They look like they've been planted in the middle of a sea, don't they? And all that water looks all brown and muddy.

Goodness gracious me, that is quite a serious flood, isn't it? What is a flood? So, rivers flood where they can't hold all the water that is entering into them from tributaries, the soil, and the rock.

And that's what's happened here, isn't it? The water has overwhelmed.

The river can't hold any more water, and so it's flooded.

There's a diagram here to help us understand this a bit more.

That diagram is called a cross profile and it's a really exciting geography diagram.

The blue represents the river.

I want you to imagine that the water is moving towards you.

The brown represents the bed and banks of the river, and then the green is the vegetation.

Now, once the water rises above the top of the banks of the river, we call this a flood.

Here are the banks labelled for you.

There's the top of the banks.

You've probably stood on a river bank, looking down at the river.

Well, imagine if this river was rising as more and more water entered it, when the water came up to the top of the bank's where you'd be standing, then the river would be in flood.

So let's just check our understanding.

A river floods when the water goes over the top of its, A, bed, B, banks, C, mouth? What do you think, Geographers? Pause, the video.

Have a think.

Absolutely, geographers.

It's banks, isn't it? The top of the river is the banks.

And when the river floods, the water goes over the top of them.

The bed is the bottom of the river, isn't it? And the mouth is where the river enters a sea or lake.

Goodness gracious.

Look at that photograph there.

Can you see those really dark clouds? Can you see that rain looks like it's absolutely teaming down? I'll tell you what, I'm really glad I'm not standing there.

I'd be absolutely drenched, wouldn't we? So when we get really heavy rain or rain that just goes on for a very long time, that's a really common cause of flooding.

Because all of this rain, lots of water will enter the river from tributaries.

If you remember from our previous lessons, a tributary is a, it's a stream or smaller river that joins a main river and over the ground.

And when all of that water enters the river, it rises, the water level rises, and when it goes over the top of its bank, a flood occurs.

And so heavy rainfall, long lasting rainfall is a common cause of flooding.

Look at this photograph here.

Now, if you've been joining in our earlier lessons, you'll have seen this photograph before.

Notice the river there in the middle of the photograph, the river bends, that's the meander.

And on either side there's a flat area and that's called a floodplain.

You can see the labels pointing to the floodplain there.

Now, the word floodplain gives us a bit of a clue.

This is the area most likely to flood.

And that makes sense, doesn't it? Because imagine it's been raining really hard for a long time.

The water level in the river is going to rise up and when it goes over the banks, that's the first area that's going to flood.

And maybe sometimes you might have seen rivers flooding.

Maybe if you've got a train or you've driven, you might have driven along near a river and the floodplain has flooded and there's lots of sheets of water on it.

And there you've seen an example of a floodplain in flood.

We've got a diagram here, which, if you've been joining me with my earlier lessons, you'll see we've used this all the way through to help us understand how rivers work.

This is a diagram of a drainage basin and the orange dash line shows the drainage basin.

All the water and rivers and streams and tributaries all join into that drainage, that main river which then goes out into the sea or lake.

Now, the shape of the drainage basin can also encourage flooding.

I wonder if we could do some thinking at this point.

Can you think of a reason why the shape of the drainage basin may affect river flooding? Ooh, why might the shape affect it? Pause the video, talk to your talk partner, see what you come up with.

Ooh, lots of great ideas here.

I can hear people talking about the shape.

I can hear people mentioning something about the steepness of the slopes.

Let's find out if you are right.

Absolutely.

So steep slopes can cause flooding, because rainwater will run quickly downhill to the river during a storm.

Look at the photograph there.

You can see some really steep slopes.

Goodness me, I wouldn't want to try and climb those slopes.

They're really, really steep.

Well you can imagine, can't you, if it's raining really heavily or if it's been raining for a long time, the water is going to be flowing down those slopes really quickly into the river, and that is gonna make the river rise and create a flood over the top of its banks.

Here's the river at the bottom.

You can see it there, and you can see the floodplain either side of it.

That's the area where the flood is going to occur.

Okay, a question for you to check if we're under understanding the causes of flooding.

Look at this photograph again and I'd like you to talk to the person next to you and think about this question, why might this river be likely to flood? Look very carefully at the landscape.

Think about everything we've been learning.

Why might this river be likely to flood? Pause the video.

Have a think.

Well, done geographers.

I can hear some fantastic ideas here.

People talking all about the shapes of the hills around.

Well done.

Yes, this photograph, we've got some really steep valley sides and that means the rainwater is gonna run quickly downhill during a storm and this will make the river rise over the top of its banks and it's going to flood its floodplain, isn't it? So again, we've got steep valley sides, rainwater running quickly, creating a flood.

Nature is an amazing thing and there are ways in which we can reduce flooding using natural processes.

So if we look at this photograph, we can see some trees.

Now, trees are brilliant at collecting rainwater.

Think about under the soil there.

All the roots of those trees are going to be growing into the soil.

And of course trees are brilliant at collecting water.

So when it rains, rather than the water just running down the slopes and running off the ground, some of the water will soak into the soil and the trees will soak it up themselves and store it.

So having lots of trees in a drainage basin may reduce the risk of flooding because there will be less water running over the surface and more water will be collected and stored by the trees.

I love trees.

They are really amazing things, aren't they? Let's just check we've understood that point.

Lots of trees in a drainage basin can make flooding less likely, because A, they speed up water getting to the river, B, they collect and store water so less rain gets to the river, or C, they soak up water from rivers? Which of those answers makes the most sense? Pause the video.

Have a think.

Brilliant thinking, geographers, you are absolutely right.

It's B, isn't it? Trees can make flooding less likely because they collect and store water so less rain gets to the river.

The trees don't actually soak the water up from the rivers themselves do they? But their roots in the soil help to collect and store water to help reduce flooding.

Well done if you got that correct.

Okay, I've got a task now to practise and make sure we are really locking in this learning about the natural causes of flooding.

I've got a paragraph for you here and there are some missing words.

So I'm gonna start by reading you the paragraph and then your job is going to be to choose the correct missing word.

So, rivers flood when they can't hold all of the water that is entering into them during and after a blank word.

When water in a river rises over its flooding occurs, the flat area next to a river where flooding happens is called a.

Slopes can help cause a flood as rainwater will run quickly downhill to the rain during a storm.

This could make the river over the top of its banks, and our missing words are banks, steep, rise, floodplain, and storm.

Okay, geographers, can you find the missing words in the correct places so we can really understand how rivers flood? Off you go.

Fantastic thinking, geographers.

I'm going to read you this paragraph back and you can check if you put the words in the correct place.

Let's start.

Rivers flood when they can't hold all the water that is entering into them during and after a storm.

When water in a river rises over its banks, flooding occurs.

The flat area next to a river where flooding happens is called a floodplain.

Steep slopes can help cause a flood as rainwater will run quickly downhill to the river during a storm.

This can make the river rise over the tops of its banks.

Well done, geographers if you got that right, and hopefully you are really beginning to understand what flooding is and some of the natural causes of it.

So the second part of our lesson is this.

How could humans cause flooding? Let's go.

So we've talked about the natural causes of flooding.

Now we're going to talk about how humans can impact on this, because as a species, we can really make changes to the way the world works.

So, humans can make flooding more likely by changing the river or the drainage basin.

Look at this photograph here.

You can see we've got a river and we've got a village or small town.

I'd like you to look very closely and think about this question.

How have humans changed the river and its surroundings in the photograph? What have humans done to change and alter the river and its surroundings? Pause the video and have a think.

Oh, lots of interesting ideas.

Thank you very much.

Look, you might have noticed that on the left side of the river you can see trees, can't you? Which is quite natural.

But on the right side we've got lots of concrete and stone, haven't we? So humans have changed the way the river, the course of the river, and they've added in these big stone banks, haven't they? As opposed to the banks we might have in nature, which might be made of mud, sand, and rock.

We've also got a road, haven't we? And some houses.

And so we've got a hard surface of manmade materials rather than grass and trees that we might find in nature.

So there's been by humans, some big changes to the river and its surroundings.

Well done if you spotted some of those things.

Look at this photograph here.

I wonder if we know where this photograph is.

Absolutely.

It's the city of London, isn't it? That's the Gherkin skyscraper in the middle, a really famous one, and there are other famous landmarks there as well.

Now, building towns and cities can make flooding more likely and that's because cities are made of surfaces like concrete and that means that rainwater can't soak into the soil.

Look carefully at that photograph there.

If it rained really hard, all the rain is going to land on the buildings, isn't it? On the roads and the pavements and some of the town squares, they're made of concrete and stone, and water can't go anywhere there, it can't soak into that, so it's gonna run off it much quicker and into the water, and that means that the river is going to rise quicker.

We can see there's only very few trees and just a tiny bit of grass which could do anything to help soak up that water.

So towns and cities can make flooding more likely.

Rainwater will flow over concrete to drains and you can see a photograph of a drain there and I'm sure probably maybe on your way to school you might walk past drains like that and see them all the time.

And the point of a drain is that water will run off the road or pavements into the drain and that will quickly take it to the river.

The problem is when there's lots of water, if there's lots of rain or really heavy rain, it means lots of water is gonna be flowing into those drains and then into the river, which is gonna make it much more likely to flood, because the river, the water flows into the drains and then into the river, and that means the level of the water will rise very rapidly, which means we're more likely to get a flood.

Let's just check our understanding about that point.

I've got a sentence here for you.

I'd like you to choose which of these is the correct way of finishing it.

Building towns and cities makes rivers more likely to flood because A, water is collected and stored by the drains, B, water is collected and stored by the buildings, or C, water is moved quickly to the river by drainage systems? Which one is correct? Pause the video, geographers, have a think.

Brilliant, geographers.

It's C, isn't it? Water is moved quickly to the river by drainage systems. The drains and the buildings don't store water.

What they do is the water and the rain goes into them and then they are moved into the river.

And particularly, that happens really quickly.

And so that increases how a flood will happen.

Well done if you got that right.

Now, over time, many towns and cities in the UK have grown larger, and this actually makes flooding more likely in the UK.

Here's a photograph of a town called Chesterfield in Darbyshire, and this is a map from 1890.

You could see Chesterfield, can't you? In the middle, and around it we could see the buildings that make the town of Chesterfield.

Here is a photograph of Chesterfield today.

What do you notice is really striking about the difference between the two? Pause the video, talk to the person next to you.

What do you notice has changed about the city of Chesterfield between 1890 and today? Yes, you are absolutely right, geographers.

It's grown hugely hasn't it? Look, we've circled here where the town of Chesterfield was in 1890 and now look, it spread, hasn't it? All the way out.

You could see in the first map there's the village of Bloomington above it, it's totally separate, and now it's basically part of the same town.

So we have more buildings, we have more concrete, we have more roads and more houses, and this means we are more likely to get flooding.

Okay, let's check our understanding here.

True or false? Towns and cities have got smaller and this has increased the flood risk.

Is that true or false? Pause the video and have a think.

Well dome, geographers it's false, isn't it? Towns and cities have not got smaller, but why is it false? I'm going to give you two answers and I'd like you to choose the answer you think is correct.

Is it A, towns and cities have got smaller and this reduces flood risk, or B, towns and cities have got bigger and this has increased flood risk? Pause the video, have a think.

Well done, geographers, it's B, isn't it? Town cities have got bigger and this has increased the flood risk.

Think about those maps of Chesterfield we looked at and we could see how the town had got much bigger, hadn't it, since 1890? Ooh, look at that photograph there.

I always find pictures like that quite depressing because as I said earlier, I really love trees.

So this photograph shows something called deforestation, and deforestation is where humans chop down trees and that happens for a variety of reasons.

It might be to use the wood and it might be to use the land for farming or building on.

Deforestation is happening all over the world and you might have heard about it in other countries as well as this country.

Now, how might deforestation affect flooding? I wonder if you can think back to earlier in the lesson where we talked about what trees might be able to do, and then think if we cut down trees, what might the impact of that be? Pause the video, geographers, and have a think about this question.

Oh, brilliant thinking, geographers.

It's great to see you making those links in your learning.

So deforestation increases the risk of flooding.

That means there's going to be more floods because there are less trees to collect and store the rainwater during storms. Earlier in the lesson we mentioned, didn't we? How trees with their enormous roots can soak up and store water and slow the rainwater down, but if we cut the trees down, they can't store the water, can they? So more rain is going to run off the surface and more water will be put into the rivers, which will make them rise quicker and create more flooding.

Well done if you manage to think about that.

Let's just check our understanding again.

So deforestation can cause flooding because A, it is easier for the soil to collect and store rain water.

B, there are less trees to collect and store rain water, or C, it creates new rivers.

Which of those questions, which of those statements makes most sense? Pause the video.

Have a think.

Well done, geographers.

It's B, isn't it? Deforestation can cause flooding because there are less trees to collect and store rainwater.

Well done if you got that right.

Okay, so our final practise task is this.

You can see a photograph there, you can see a river in it as well.

We're going to bring all our thinking together about why a river might flood.

I want you to look at that picture carefully and think about two reasons why this river may be likely to flood, and your reasons could be related to how humans might cause flooding, which we've been talking about, or about the natural landscape, which we talked about in the first part of the lesson.

Two reasons why this river might be likely to flood.

Good luck, geographers.

Look forward to seeing what you'd come up with.

Oh, amazing thinking.

So I've had to go at labelling my photograph and I've put here that the river is close to the top of its banks, so it's already quite high and so that might cause it to flood if there's any more rain.

I've also noticed that there's a steep sided valley here, and so if there is rainwater, it's gonna flow to the river very quickly.

So that's a natural reason why there might be some more, why there might be some flooding.

I've also noticed that along the side here near the town, there's been some deforestation.

Trees have been cut down and that means there'll be less trees to store and collect rainwater, which is going to increase the flood risk.

And there's also been some houses because we've got a village here as well.

And that means there's going to be more concrete surfaces, which don't soak up water, and that means that rain is going to soak, is going to run off over the concrete and into the river.

And all of this together is going to mean that that river is much more likely to flood.

Really well done if you managed to think of some of those reasons, you are really beginning to understand what flooding is and how it's caused.

So let's summarise our learning today.

We've learned that rivers flood when they can't hold all the water that is entering into them from tributaries, the soil, and the rock.

We've also learned that heavy rainfall or long-lasting rainfall is a common cause of flooding.

We've learned that steep slopes can cause flooding as rainwater will run quickly downhill to the river during a storm.

And we've also learned that humans can make flooding more likely by deforestation and building tarmac and concrete surfaces.

Thank you so much for learning with me today, and I've really enjoyed learning more about flooding.

We're going to continue learning about flooding in our next lesson as well, where we're going to be thinking about how we might help prevent floods.

Well done, geographers, and I look forward to seeing you soon.