Lesson video

Hello, geographers.

My name is Mrs. Homigo.

I'm looking forward to teaching you today.

I hope you're gonna learn lots and enjoy the lesson.

Let's get started.

Today's lesson looks at factors that affect the flood risk.

By the end of the lesson, you'll be able to explain the physical and human factors that affect flood risk, and understand why flood risk is increasing.

There are four key words for today's lesson.

Flood risk, relief, urbanisation, and climate change.

Flood risk is the probability of a flood event occurring and its potential impact.

Relief is the physical shape or features of the land's surface.

Urbanisation, this is the increase in the proportion of people living in cities and towns and often has an impact on natural land.

Climate change are the long-term shifts in temperatures and weather patterns that are often linked to human activity.

There are two parts to today's lesson.

What factors affect flood risk? And why are flood risks increasing? Let's begin learning cycle one.

So let's just think now.

Flood risk is the probability of an area experiencing a flood event and how severe the consequences of that flood might be.

Some areas will have a much higher flood risk than others.

Flood risk is affected by physical factors such as precipitation, geology, and relief; and human factors: urbanisation, deforestation, and agriculture.

And we're gonna have a look at each of these in a little bit more detail.

Capacity is the maximum amount of water a river can hold without the water overflowing its banks.

You can see there, it's shown on the diagram.

Rivers flood when their capacity to hold water is exceeded.

So there is too much water in the river channel to stay within the banks.

Floods usually follow heavy rain, but different factors affect how much rainfall in a drainage basin enters a river, and how quickly it gets there.

So every drainage basin will be different and every river will have a slightly different capacity.

Let's have a think about how precipitation affects the flood risk.

Rainfall intensity refers to how much rainfalls in a given period.

So if we have a high rainfall intensity, it means that lots have of rain has fallen in that period.

This will lead to surface runoff.

The ground isn't able to absorb water quickly enough through infiltration, and so more water will flow a surface runoff, and the risk of flooding increases.

Rainfall duration refers to how long the rain lasts.

So if we have prolonged rainfall, so rain over a very long period of time, the ground will become saturated.

It will have to absorb so much rain that it is now saturated.

Once the ground is saturated, infiltration slows down and may stop, and so more water will flow a surface runoff directly into the rivers and surface runoff increases the risk of rivers flooding.

Geology and soil type.

So impermeable rocks and soils do not allow water to pass through.

There is no infiltration or percolation.

Instead, the water will flow as surface runoff.

This means that more water returns more quickly to the river and the flood risk increases.

Relief, on steep slopes, water moves quickly over the surface.

It doesn't infiltrate in the same way as it would if the slopes were more gentle.

Therefore, we have a much more rapid flow of surface runoff, and this increases the risk of the river flooding 'cause the water reaches the rivers much more quickly, and raising water levels.

Right, let's have a quick check.

Which of these physical factors increase the flood risk? Is it a, light rain; b, impermeable rocks; c, gentle slopes; or d, prolonged rainfall? Pause the video and make your decision.

Hopefully, you identified impermeable rocks and prolonged rainfall.

So impermeable rocks stop infiltration and percolation happening, and so increased surface runoff; and prolonged rainfall leads to the saturation of the soil, and so more water flows as surface runoff.

Human land use can also increase the flood risk.

Urbanisation, agriculture, and deforestation will all have an impact on the natural processes and the way in which water returns to the river.

Let's have a look first urbanisation.

So in urban areas, more land is covered by impermeable surfaces.

So if you think of towns and cities to you, there are many more surfaces, such as concrete and tarmac, and they're covered with buildings.

Now these surfaces do not let infiltration take place.

So water flows as surface runoff.

So surface runoff increases.

And water flows more quickly into rivers and drains, and raises the flood risk.

And this is a particularly a problematic following heavy rainfall when there is a lot of water to return to the river.

Agriculture: overgrazing an intense farming using machinery can harm and compact the soil.

So push it together.

And soil can often be left bare after harvest.

If the soil is damaged and compacted, infiltration rates will be reduced.

And if there is no vegetation on the soil, then surface runoff will increase.

Therefore, water is flowing more quickly into rivers and it increases the flood risk.

Finally, deforestation.

By removing all the trees, we are stopping rainfall being intercepted by leaves and branches and being absorbed by the roots.

Therefore, more rainfall is falling onto the soil and flowing as surface runoff.

Therefore, more water is flowing more quickly into the river which leads to a greater risk of flooding.

True or false then? Human land uses can increase the impermeable surfaces, which increase the flood risk.

Pause the video and make your decision.

And can you explain why you've chosen what you do? I hope you said true and your explanation would've been something like this.

Urbanisation increases impermeable surfaces as the land is covered by concrete, tarmac, and buildings.

This reduces infiltration and increases surface runoff, which increases the flood risk.

Well done.

So let's have a go at our first task.

Can you explain how one human and one physical factor increase the flood risk? Alex is thinking this through.

He says, "I could use rainfall intensity, relief or geology as my physical factors and urbanisation or deforestation as my human factors." Pick which one you want to choose and have a go and explaining how it increases the flood risk.

Come back when you finished.

Your answers may look something like this.

For our human factor, we've chosen urbanisation.

Urbanisation leads to an increase in impermeable surfaces such as concrete and tarmacs used for roads, buildings, and pavements.

This prevents water from infiltrating into the soil.

Instead, rainfall flows a surface runoff, which increases the volume of water flowing rapidly into rivers.

This increases the flood risk.

And then for our physical factor, I've picked rainfall intensity.

Heavy intense rainfall can saturate the ground, making it unable to absorb more water, and this leads to rapid surface runoff that fills rivers quickly and increases the flood risk.

Well done if you've had answers that were similar to those.

Let's move now to our second learning cycle.

Why are flood risks increasing? This line graph shows us the number of flood events over the last 70 years.

We can see that the general trend is that there is an increase in the number of flood events.

The flood in risk is increasing.

There were below 25 in 1950, and there are over 150 in 2023.

But why is this? Let's think why the flood risk is increasing.

Human land uses such as urbanisation, deforestation, and agriculture are increasing as populations increase.

More of these land uses increases the surface runoff.

And if we think back to our storm hydrograph, increasing the surface runoff leads to a shorter lag time because this is the quickest way for water to return to the river.

So the time between the peak rainfall and the peak discharge will be very short.

This makes it more likely that the river's capacity will be exceeded, which increases the flood risk.

As more people are living closer to rivers due to the pressure on space, we need to manage them.

So we need to try to stop them flooding.

However, managing them upstream can increase the flood risk downstream.

Some strategies that people might use is to straighten the channel, which increases the speed at which the water moves downstream.

Therefore, if we have a lot of water in the river, it may cause problems and increase the risk of flood further downstream.

Flood defences along the banks.

For example, concrete walls might infiltration, so therefore the discharge of the river will increase.

There'll be more water in the channel, which will lead to a greater chance of the capacity being exceeded and the river flooding.

You can see here that both of these will reduce the lag time and increase the peak discharge, which increases the risk of the river capacity being exceeded and increases the risk of flooding.

This image here is of a floodplain, the flat land next to the river in the lower course.

More and more floodplains, particularly in urban areas, are being built on, again, due to the increasing demand for land.

And this heightens the flood risk, so increases the flood risk.

They are natural spaces for rivers to flood out on during re high flows.

So anything that built on a floodplain has a high flood risk.

And by building on this floodplain, we are also increasing the number of impermeable surfaces, roads, buildings, pavements, which again in reduce the infiltration rates and increase surface runoff.

So this too adds to that increase in the flood risk.

Check for you now.

There is three spaces.

What these human land uses are increasing? This is because they increase and reduce the, so try to link the three human land uses to the storm hydrograph on the slide.

Hopefully you manage to remember those three human land uses that are on increasing due to population increase.

Our urbanisation, agriculture and deforestation.

And they cause a problem with the flood risk.

This is because they increase the surface runoff and reduce the lag time, and those two things lead to a greater risk flooding.

Let's have a look now at how climate change affects flood risk.

Climate change is leading to more extreme weather events, so more intense and frequent rainfall storms. This is likely to lead to more saturated ground, so more surface runoff.

So more water reaching the river channel leading to higher discharge.

Warming temperatures is leading to an increased melting of glaciers and ice caps, which will also contribute to a higher discharge and a greater chance of the river channel exceeding its capacity.

Earlier snowmelt means that more water is flowing into the rivers.

It may also mean that the ground becomes saturated, so any rainfall falling then will flow a surface runoff.

Again, contributing to the river discharge and increasing that flood risk.

And changing rainfall patterns, so it may mean that you're getting heavier rain at different times of the year, which can lead to problems and the capacity of the river being exceeded.

Let's have a check now.

Which of the following are ways in which climate change is increasing the flood risk? Warmer temperatures lead to earlier snowmelts; increased glacial melting leads to a lower discharge; storms are becoming more intense and frequent.

Pause the video and make your decision.

Hopefully, you said warmer temperatures lead to an earlier snowmelt and storms are becoming more intense and frequent.

b, increased glacial melting doesn't lead to a lower discharge.

It leads to a higher discharge.

Threats from an increased flood risk affect people and the environment.

And you can see there the devastating effects of floods across natural areas of vegetation and on people's homes and businesses.

Threats from an increased flood risk affect people.

They may be made homeless or their homes may be damaged.

They could even be killed or injured by the flooding.

Jobs may be lost as businesses are destroyed.

Flood water can contaminate water sources, and so there isn't clean water for people to drink, which can lead to illness and other problems. It may cause a disruption of transport and power supplies, and it may lead to food shortages.

If fields are no longer able to be used for growing crops or if stores of food are damaged, that will lead to a rise in food prices, which may mean that some people are unable to access the food that they need.

Threats from an increased flood risk also affect the environment.

They can lead to a damage to ecosystems, a loss of habitats or food sources.

So interrupting the natural ecosystems. It could damage crops and agricultural land.

Pollution, so it may mean that it causes pollution to wash into the rivers, or it may be that the flood water is polluting water sources.

A loss of vegetation, and there may be the death of animals if their habitats are destroyed.

Let's have a check now.

Which of these threats from flood risk directly affect people? Loss of habitats, disruption of power supply, homelessness, and damage and loss of crops.

Pause the video and come back when you've made your decisions.

I hope you said, the disruption of power supply, homelessness, and damage and loss of crops.

Remember, that can lead to food shortages and rising food prices.

Task B for you.

Can you explain how these factors are increasing the risk of flooding? a, building on floodplains; and b, more intense rainfall due to climate change.

Pause the video and come back when you've had a go.

For a, your answer may have included: building on floodplains reduces the natural space for rivers to expand during heavy rainfall causing water levels to rise more quickly.

It also creates more impermeable surfaces like roads and buildings, which prevents infiltration and leads to greater surface runoff.

More water reaches the river more quickly, shortening the lag time and increasing the discharge, leading to a higher flood risk.

Well done.

Now let's look at b.

Your answer may have included: climate change causes more frequent and intense rainfall events, which increases the river discharge has more water enters the rivers.

This higher discharge means there is a greater chance rivers will exceed their capacity and this increases the flood risk.

Well done.

Okay, let's have a look at the summary for today's lesson.

Flood risk is affected by physical factors, such as geology, precipitation, and relief.

These factors reduce infiltration, increase surface runoff, and decrease rivers lag time.

Human land uses can also increase the flood risk.

For example, urbanisation, deforestation, and agriculture.

Climate change is also increasing flood risk in a range of waste.

Flood risk poses threats to people and the environment.

Well done today.

I hope you now feel confident to explain how physical and human factors affect the flood risk, and that you also understand why the flood risk is increasing.

I look forward to seeing you all again soon.