Lesson video

Hello, my name's Miss Gilyeat, and I'm your geography teacher for today.

In today's lesson, we are going to be learning about the causes of river flooding, which is Lesson 9 in the River's topic.

Our lesson outcome for today is that you can describe the human and physical causes of river flooding.

We have got five keywords for today's lesson.

The first keyword is prolonged rainfall, which is continuous rainfall that lasts for longer than expected.

Relief is the physical shape of the land in an area, including its height and its steepness.

Geology is the physical structure of an area and what rocks and minerals exist there.

Deforestation is the complete removal of trees from an area.

And urbanisation is the increase in the proportion of people living in towns and cities rather than in the countryside.

We've got two learning cycles.

So to start off with, we're gonna learn about what the physical causes of flooding are and then we're gonna have a think about what the human causes of flooding are.

So let's start off with learning cycle one.

Now, a river floods when it cannot hold all the water that is entering the river channel.

Okay, so let's just make this clear by looking at a diagram.

So here is a diagram of a cross section of a river.

So if you imagine if you've cut a river in half and you are looking at it side on.

Okay, so the two parts of the river which are on the side are called the banks of the river.

The bottom of the river is called the bed.

Okay, so the banks are the side.

Now, when a flood happens, the river bursts its banks and spreads onto the surrounding land.

So it can't hold all the water that is entering it so the water flows over the top of the banks, and you can see an image of a flood of where that has happened.

Now it's really hard to actually see where the river is in that photograph because the area has flooded so much.

Now, there is often a flat area beside a river called a floodplain, and this is the area which is most likely to flood.

So if you have a look at my picture, we can see the river channel flowing through there.

Either side of that river channel, there's very flat land, and that's called a floodplain.

Okay, let's check our understanding.

A river floods when the water goes over the top of its, A, bed, B, banks, or C, mouth? The answer is banks.

Well done if you got that right.

Now there are many physical, which are natural causes of flooding.

So even if humans didn't exist on this planet, and if there were still rivers, these rivers would flood.

Okay, sometimes they would flood, I'm not saying they always would, but there are natural reasons to why rivers flood, so let's have a look at some of them.

Prolonged rainfall, geology of the catchment, the relief of the catchment and heavy rainfall.

Okay, so we're going to have a look at these physical causes of flooding in a little bit more detail.

Now, heavy rainfall is exactly what it says on the tin, it's a lot of rainfall, which is very heavy.

Okay, now heavy rainfall can lead to surface runoff when the ground is not able to absorb the water quickly enough, which is infiltration, so it runs off into the river.

Okay, so if you imagine a really heavy downpour, the ground can't absorb all of that water that's going in so instead it pulls on the surface, so that surface water, and eventually that surface water will flow into the river.

Okay, if the water enters the river really quickly then it is more likely to flood.

And sometimes this type of flooding is called flash floods because they can happen super, super quickly.

Prolonged rainfall is rainfall which happens over a long period of time.

This can saturate the ground, reducing infiltration, and increasing the volume of surface runoff flowing directly into rivers, which can lead to river flooding.

So if you imagine if rainfall happens for a really long period of time, eventually the ground won't be able to soak any more water up.

Okay, it's saturated.

If it becomes saturated, no more water can flow into it, infiltration, and therefore the water stays on the surface as surface water and eventually that water will also flow into the rivers, yeah, leading to river flooding.

So a way to imagine that if you imagine a sponge which is already saturated with water, it can't hold anymore so no more water goes into it so instead it goes into the river.

Okay, let's check our understanding.

Which two types of rainfall can lead to rivers flooding? Okay, we've got an answer from Sam first of all.

So, "Prolonged rainfall can saturate the ground, increasing the volume of water flowing into rivers, which can lead to river flooding." Well done, Sam, spot on.

And we've got an answer from Jun, "Heavy rainfall leads to surface runoff, so it runs off into the river." Well done, Jun.

He's also correct.

Now the relief, which is if we remember is the shape and the height of the land of the drainage basin, may also encourage flooding.

So I want you to think how can the relief, so the shape and the height of the land, how might that affect river flooding? Have a quick chat with the person you are sat next to.

Now, on steep slopes, water moves quickly over the surface, leading to rapid surface runoff.

So if you imagine a steep valley like this, the water is going to very quickly go down the sides of the valley and enter the river channel.

Okay, if the river valley was a little more gently sloping, the water flows much slower to the bottom of the valley and therefore it is less likely to flood.

Okay, so this increases the risk of river flooding because the water reaches rivers and streams much faster, raising water levels quickly.

So if we have a look on the slide there, we can see a diagram of some mountainous areas with very steep slides.

Okay, the water runs down, we get a rapid surface runoff, that means a lot of water enters the river channel at a very quick rate, and that can increase your chance of flooding.

Now, geology, which is the study of rocks, so the rock type can affect the risk of flooding as well.

Now, some rocks do not allow water to pass through them.

These are called impermeable rocks.

Okay, so granite is a good example of this.

Water can't flow through it because it's not got any cracks or it's not got any holes in it.

Now, that means if an area of land is mostly a type of rock, which doesn't allow water to pass through it, it's very hard for the water to soak down into the groundwater, okay? And therefore you get increased surface runoff.

Okay, so more of the water pulls over the top because it's not able to sink into the ground, therefore increasing the risk of flooding, okay? Right, let's check our understanding.

Which of these physical factors increases the risk of flooding? A, light rain, B, rocks which don't allow the water through, C, gentle slopes, or D, prolonged rainfall.

You can have more than one answer for this.

Okay, so the answer is B and D.

Well done if you got that right.

Let's have another quick check.

So looking at that photograph I've got on the slide there, why might this river be likely to flood? Have a look at the surrounding area, why might it be likely to flood? Okay, so steep valley sides mean rainwater will quickly run downhill to the river during a storm.

This can make the river rise over the top of the banks.

Okay, so you've got steep valley sides in that photo.

Well done if you got that right.

Okay, so we're onto our first task for today's lesson.

What I'd like you to do is finish that paragraph filling in the words from the missing words box at the bottom.

Okay, so pause the video and have a go at that first task.

Okay, let's check our understanding.

So 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 cause a flood as rainwater will quickly go downhill to the river during a storm.

This can make the river rise over the top of its banks, so well done if you filled that in correctly.

Okay, so we're moving on to our second learning cycle for this lesson, which is looking at the human causes of flooding.

Now, there are many human causes of flooding.

So some of these include deforestation, urbanisation, and agriculture.

Now, there are a couple more as well, but these are the ones that we are going to focus on for today's lesson.

So first of all, if we have a look at this photograph here, we can see a river.

Now there's very obvious evidence in that photo that humans have impacted the landscape.

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

So looking at that, how have humans changed the river and its surroundings in the photograph? Have a quick chat with the person you're sat next to.

Okay, so I'll point some out to you.

So for example, we can see there that humans have built on the landscape.

You've got a footpath, you've got a road, you've also got buildings.

Now that's a lot of concrete, and we're gonna learn about in a minute why that can affect flooding.

As well we can see that the bank of the river has been altered by humans.

So it looks like on the right-hand side of the river bank, it's a wall, it's part of the footpath, but that has been built, which is actually suggesting that humans have altered not only the surrounding area, but actually the river channel as well.

In the very far back of the photograph, there is evidence of a field.

Now, it might not be super obvious, but actually humans have altered that landscape too.

Naturally, it's more likely that that area would have been wooded.

So we're gonna think about these different factors and think about how they can increase the risk of flooding.

So first of all, urbanisation has led to an increase of people living in towns and cities.

Urbanisation can make more flooding likely or can make flooding more likely, sorry.

Now, cities are made up of surfaces, like concrete, which can't absorb water.

So if you think last time that you saw a heavy rainfall period, it's very likely that there would've been puddles on the ground.

And that's because water can't infiltrate, so it can't soak into concrete, so puddles form.

Now, therefore water flows more quickly into rivers and drains, raising the flood risk especially during prolonged rainfall.

Okay, so those puddles, that surface water can flow into rivers very quickly.

Rainwater will flow over concrete to drains, which can quite quickly take it to the river.

Lots of water entering the river quickly will make it more likely to flood.

So lots of cities and towns and where people live have drains in them.

Now, the reason that we have drains is so that we don't get as many puddles.

Okay, and we don't get surface flooding, which is when the water just stays on top of the concrete.

Now, that can be dangerous because it can mean that the roads becoming unsafe to drive on if they've got massive puddles on them.

It can also cause damage to buildings so we've put drains in place to drain away this water.

However, those drains act as a very quick route for water to get back to the river, which is good for getting water out of the way of the surface, but it means that they're quickly getting water to the river and therefore that can increase the risk of the river flooding.

Okay, let's check our understanding.

Building towns and cities make 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? The answer's C.

Well done if you got that right.

Towns and cities in the UK have grown larger over time.

This has made flooding more likely across the UK.

Okay, so for example, if we look at Chesterfield in 1890, so this is an example of an old map of Chesterfield and compare it to Chesterfield today, we can see how much the town has expanded by.

Okay, now you might think, "Well, how does that relate to flooding?" Now all of that new land which has been built on means that that is less land that is in infiltratable.

Okay, so more and more areas have got concrete on them, which can't absorb the rainwater as quickly.

Okay, if all of that area was fields or woodland, much, much water would be stored on the ground in vegetation rather than pooling on the surface or going into drains and therefore into the river quickly.

Oka, true or false, towns and cities have got smaller and this has increased flood risk? That is false, towns and cities have got bigger and this has increased flood risk.

Now, deforestation is where humans chop down trees.

They do this to use the wood or to use the land for farming or building on.

Okay, so deforestation is becoming more and more of a problem.

It can affect ecosystems, but it can also affect flooding.

So I want you to have a quick think, how do you think chopping trees down might make flooding worse? Deforestation increases the risk of flooding as there are less trees to collect and store rainwater during storms. So if you have a look at that picture on the right, you can see a leaf there, which is collecting a few water droplets.

Now, obviously just a couple of water droplets isn't enough to reduce the risk of flooding, and if you just got rid of one leaf, then that's not going to affect it.

However, if you've got hundreds of trees with thousands of leaves on all collecting that water, then chopping trees down does have a great impact on the amount of water which reaches the floor.

So therefore, more rainfall falls directly onto the soil and flows as surface runoff towards the river.

So less water is stored on vegetation, more water hits the floor, okay, and therefore more water flows quickly into the river, which increases the flood risk.

Let's check our understanding.

Deforestation can cause flooding because, A, it is easier for the soil to collect and store rainwater, B, there are fewer trees to collect and store rainwater, or C, it creates new rivers? The answer's B.

Well done if you got that right.

Now finally, agriculture, which is another word for farming, can also increase the risk of flooding.

Overgrazing and farming using heavy machinery can compact the soil.

So overgrazing is when the land has had too much livestock, so things like sheep, cattle, using the land.

Okay, so they've eaten too much vegetation off the land, there's too many animals for that area.

Okay, and farming using heavy machinery, which is basically if we have a look at the picture there, we can see heavy machinery.

Okay, so using less people and more machines to do the farming for them.

And if you imagine if you've got big heavy tractors going onto the soil, it can compact it so it can make it harder, okay, and more dense.

Now, overgrazing and heavy machinery compacting the soil means that less water can absorb into the ground, okay, or less or less water can infiltrate really because it's become more dense, okay, it's become harder.

If less water can infiltrate, so can less can soak into the ground, that means that more flows on the top so it stays on the surface and therefore more surface water means more surface runoff, which means it's more likely to enter the rivers quickly.

Okay, so that can also increase the risk of flooding.

Okay, so we're on to our final tasks for this lesson.

First of all, I'd like you to name the different causes of flooding from the pictures and then put them into the correct column of the table, so human and physical causes of river flooding.

And then finally what I'd like you to do is label the picture to show two reasons why this river may be likely to flood.

Your reasons could be human or physical reasons, and it might be worth writing a little sentence to go with each one to say how it makes the flooding more likely.

Okay, let's check our answers.

So we've got agriculture, geology, heavy or prolonged rainfall, deforestation, urbanisation, and relief.

And the human causes are agriculture, deforestation, and urbanisation.

And the physical causes are geology relief, heavy and prolonged rainfall.

So well done if you got that right.

Okay, for this picture, these are the ones that I got.

The river is already quite close to the top of its banks.

There is a steep sided valley there, so rainwater can quickly flow into the river.

Deforestation has happened in some areas, so therefore there's less vegetation to collect the rainwater.

And houses have been built, which will add concrete surfaces to the drainage basin.

So well done if you spotted all of those human and physical reasons.

Okay, we've got a summary for today's lesson on here.

Floods occur when a river receives more water than the channel has capacity to hold and its banks are overtopped.

Heavy and prolonged rainfall are the main physical causes of flooding.

Other physical causes of flooding are the relief and geology of the drainage basin.

Deforestation, urbanisation and agriculture are three human causes of flooding.

Right, that's it for today's lesson.

Well done, you've done a fantastic job, and I'll see you next time.