Flooding Bang Goes the Theory

Here in Britain, we love talking about the weather

and over the past few months, there's been plenty to talk about.

But stoical though we are about the wind and the rain, this last winter

has been a powerful reminder of how water can wreak absolute havoc.

It all began in December,

when the British Isles were bombarded by storms.

Waves battered the coast, gale force winds uprooted trees

and torrential rain filled rivers to bursting point.

We witnessed some of the worst flooding we'd ever seen.

There's no wall, no defence - just nothing there.

We saw the flood waters right up to our gate.

We weren't prepared for this.

And it didn't end there.

Just when we thought we might get some respite,

the storms struck again.

The last 24 hours have seen the biggest waves

ever recorded in British waters.

We've never been flooded in 55 years and it's indoors now.

The water's rising an inch an hour, they tell me.

I'd have done anything to have saved my home.

This week, Bang puts flooding under the spotlight,

asking not only why does it happen and what can we do about it,

but what can we expect in the future?

Tonight, we'll be looking at what causes flooding,

from the humble raindrop to an epic storm surge.

'We'll look at what we can do to protect ourselves.'

There's literally water logging going on.

'And we'll see how technology can help us to deal with floods.'

It's wonderful to see it in action in real time.

-Fantastic.

-We can see live telemetry.

Files are being exchanged between the spacecraft and the ground.

This winter, the British Isles were struck by at least

12 major storms and some parts of the country received

more than twice the average winter rainfall.

This deluge provided us with some extraordinary scenes

of Britain under water, but sadly around 6,500 homes were flooded.

Back in January, I visited Rod and Holly

in the village of Thorney, in Somerset.

The flood water was so deep,

I had to don waders to get to their house.

-Hello.

-Hi. This is just horrendous, isn't it?

Just tell me how quickly the water came up.

Well, it came up quite slowly to start with

and we started to put all our furniture on bricks.

Then another hour or so, two hours, and we had to put it on

another brick and another brick and then some wood.

-Wow! Gosh, it's coming through the floor, isn't it?

-Yeah.

Has it changed the way you're thinking about this house?

Yes. Well, it has me. I feel very insecure.

And the hard thing, really, is the not knowing -

will it happen next year? Will it happen every year?

Yeah, absolutely, absolutely right. This was ten days' rain.

Am I going to be waking up every time I hear it raining, thinking,

"Gosh, we've only got six days of rain before we're flooded again."

It's completely untenable.

Sadly, Rod and Holly were flooded again

shortly after I left in January.

And for weeks after, they had several inches of water in their home.

It finally drained away at the beginning of March.

It's been an extremely traumatic time - not only for them,

but for people up and down the country.

And you can completely understand

that if you've been through all this,

you'd like some reassurance that it's not going to happen again.

So, what exactly is it that causes flooding?

And what can we do about it?

The total amount of water on earth remains fairly constant

and it moves in a cycle around our planet.

It's released by plants through transpiration

and evaporates from oceans and rivers.

And it returns to earth as precipitation.

Around 96% of it is held by the oceans.

1.7% exists as ice.

And 0.8% is ground water stored in rocks.

A mere thousandth of a per cent exists as water vapour in the atmosphere.

And while that may not sound like much,

it's still five times more water

than is held in all of the world's rivers.

So when weather events combine to bring a lot of water to one place,

our rivers, land and coastlines

can be inundated with more water than they can cope with.

But the weather isn't the only factor.

We also have to consider how that excess water

interacts with the land.

This is a model of a river in its catchment area or drainage basin.

You've got a natural landscape here upstream,

you've got trees and grass,

a town here and a village further downstream.

And, obviously, all the water is running to the sea in this

direction, so let's take a look at what happens if you get

heavy rainfall in this area.

So, take a look at what's happening.

Because this is natural landscape, a lot of water has been

absorbed by the soil.

But here, the river has still flooded in the flood plain.

But the thing is, this is supposed to happen.

A river and its flood plain are actually one natural single

highway of moving water.

As the water levels eventually decrease again, the flood plain

drains back into the river and all of the water goes towards the sea.

OK, so now let's look at what happens

if we alter the landscape in some way.

Imagine we've decided to build a big city in this nice

bit of land upstream of the town and village.

We're talking car parks, supermarkets, schools, roads etc.

So there's a lot of concrete covering up a lot of the soil.

And then we have a rainstorm.

I'm pouring the same amount of water,

but hardly any of it is being absorbed.

So what happens downstream of this city

is that this town gets completely flooded.

And what's clear is that it's not just about the amount

of rainfall at any given time -

it's about how the landscape is able to cope with that rain.

To make matters worse, building on flood plains has

increased by 12% in the last decade.

This town wants to be protected from floods and so they go ahead

and they build a big flood defence around their town.

So let's take a look at what happens

the next time a big rainstorm comes along.

Fairly quickly, it's evident what happens as a result.

The town is now protected because it's gone ahead

and built a big flood defence

but take a look what's happened downstream.

The village that was fine before

is completely flooded - and that's the crux of the problem.

The water has to go somewhere.

If you go ahead and make a change in one place,

that will inevitably impact somewhere else.

And you don't have to live next to a river for this to be a problem.

In our towns and cities, green gardens are disappearing

to make way for paving, decking and patios.

In London alone, seven Hyde Parks' worth of garden

have been paved over in the last ten years.

I've been responsible for a fair few garden revamps in my time

and paving and decking have indeed featured in them,

but it doesn't take a scientist to work out that paving will

absorb a lot less rain and water than a grassy surface.

But can paving over a small front garden like this one

really have an impact on flooding?

Well, according to the experts, yes.

In an hour's downpour, up to three bathtubs' worth of rain

could fall on this small driveway,

of which 5% will make its way into the ground.

The other 95% whooshes off as surface run off.

This can lead to more water than our drains can cope with.

And to make matters worse, this run off sweeps

pollution along with it, which ends up in our rivers.

But there are things we can do about it.

This front garden has been covered with a special porous paving,

which, they tell me,

lets the water naturally seep through to the ground below.

But we're not going to just sprinkle a little bit of water -

we're going to flood it.

The joints of the paving are filled with crushed stone,

as is the base layer beneath,

allowing the water to flow down to the bedrock.

Paving like this can make a difference

but it doesn't beat a real garden.

Here at the Susi Earnshaw Theatre School in London,

flooding was a regular occurrence -

until a scheme run by the Wildfowl and Wetlands Trust solved the problem.

Now rain water is caught at source,

channelled down this tarmac river and dealt with sustainably.

Andy, explain this project.

When it rains, the water overflows from the water butt,

and it ends up down here in our sustainable drainage system

and it spreads along this gutter that you can see in front of you -

you can see the water now from the recent rain - over the grassy strip.

All the sediments and big bits of pollution are trapped by the grass.

The grass turns the petrol and diesel into plant nutrients and then

any overflow makes its way into our Mediterranean gravel garden,

which is, essentially, a big storage feature for those big rainfall events.

So, not only are you solving the problem of the rain puddling up

in the parking area, you end up with a nice garden,

an educational facility and I'm glad to see you've got a pond there, as well.

Yeah, it's fantastic, isn't it?

Any overflow from the bog garden makes it into the pond,

overflow from there going into this rain garden there.

So what can this cope with in the way of rainfall?

We've designed this to deal with

all rainfall that a one-in-ten-year event will throw at the system.

That's quite a lot of rain.

One garden may not seem like much, but this scheme now involves

ten schools across the catchment.

And if businesses and home owners get involved too,

then these small things can start to have a big impact.

This will help us in our cities, but some of the worst flooding

this winter struck rural areas like the Somerset Levels.

And the question of what should be done there

brings us to the controversial subject of dredging.

Why was this not done ages ago? Why are you only doing this now?

Dredging is where the river beds are cleared of sediment.

The idea is that this creates more room in the river for excess water

and gets it to the oceans more quickly.

But it's not quite as simple as that.

So, imagine this is our flooded flood plain

with a river running through the middle of it.

And, as you can see, there's quite a lot of sediment

at the bottom of the river.

What would happen if we dredged the river and removed the sediment?

Well, if we do that, you can see that the water levels do drop a bit,

but it doesn't solve the problem.

And that's because the volume of water across the flood plain

is so much greater than the volume we've made available

by dredging the river.

Dredging couldn't have prevented the floods,

but increased capacity also means water can be moved more quickly.

And while we have to be wary of pushing the problem downstream,

research suggests dredging in Somerset could have

reduced the duration of the floods by draining the Levels faster.

But experts now believe that instead of focusing solely on rivers,

we should be looking at the entire catchment.

And here in Yorkshire is a scheme that's doing just that.

This is the town of Pickering, on the southern edge of the North York Moors.

And this river below me, running right through the centre of the town, is Pickering Beck.

Pickering sits in a catchment that begins in the steep hills

to the north of the town.

When the area gets a lot of rain, the water runs off the land

and into the rivers, which rush the water downstream.

By the time the water arrives here,

this river could already be at bursting point -

and when it meets an obstacle like this bridge,

it'll spill out, flooding the surrounding area.

Pickering has flooded four times in the last 15 years,

with the worst floods occurring in 2007.

To protect the town in future, a new project has been launched

using a combination of natural measures

and targeted engineering to hold excess water upstream.

What we're trying to do is bring that water back upstream

and the more we can hold it back in the upper parts

of the catchment, the smaller the flow hopefully will be

when it comes to the towns and the cities.

So, this is what we call a debris dam?

Yes, we refer to it as a large woody debris dam.

A porous dam. It's not sealed.

So, the fact there are gaps in it is absolutely crucial?

Very much so. We only want these to function

under high flow conditions.

It's when we get the heavy rainfall - that's when we want it to hold back that water.

So you're talking about holding water back -

is that in any way detrimental to the natural way the waterways flow?

No, we're recognising that the way we've managed

the land in the past has sped that flow of water off the land.

We've lost a lot of our natural wooded river systems.

These debris dams are a natural feature,

so we're trying to recreate that.

So, how much of the geology of this landscape

do you need to understand in order to know where to put the dams,

and that you're not causing more harm than good?

I worked with Durham University in applying models to see

what sort of difference this might make.

That sufficiently encouraged us to then go ahead,

with the understanding of the catchment, the hydrology

and the geology, to determine where we put these.

Although they only hold, individually,

a small amount of water, if you total that up over

hundreds of dams over the whole catchment, that's a lot of water.

There are 180 debris dams like this in the area

but that's just the beginning.

As well as slowing the rivers,

this scheme is also helping to hold water on the land.

Tom, we are in the catchment now, aren't we?

Yes, we're in the catchment of the Vale of Pickering.

The river's down there in the background just in the corner.

What are we looking at here, then? What are they planting?

This is an example of one of the other measures we're trying,

which is to plant woodland on farmland.

Not just anywhere, but targeting soils that we believe have

a high propensity to generate rapid run off.

Trees will help because they store water

and release it through transpiration,

but they can also have a positive impact on the soil.

On the left, this is land covered in forest.

Imagine each tree has a long, complex system of roots

and all of those roots are increasing the porosity of the soil.

You're also going to have lots of organic material -

dead, decaying plant material -

that's going to also increase the porosity of the ground,

and that means this soil is going to absorb a lot of water,

hold it for longer and also direct it deeper, down as far as the bedrock.

Now on the right, imagine this is farmed land

and imagine you're grazing this land so you've got

sheep constantly trampling on the ground here.

creating a compact layer at the surface.

That means that this soil is going to increase the risk of surface run off.

If I pour my coloured liquid in one position in our porous soil,

immediately you can see it permeating the soil

all the way down to quite a good depth.

Now, if I pour my coloured liquid on to the land

that's been farmed so you've got this thick compact layer of soil,

you can see immediately how much slower it travels

through that compact layer.

There is literally coloured water logging going on.

You get all of this water

having to go somewhere on the surface instead -

and that's when you get problems.

The final piece of the puzzle here at Pickering

lies further downstream.

Here, the Environment Agency are implementing a flood storage scheme

which will temporarily divert flood waters at high flow,

holding the excess back in these fields,

and preventing it from flowing down the river to the town.

The thing that impresses me most about this project

is that it looks at the whole catchment and its unique geology

and hydrology to come up with well-thought-out,

long-term solutions that are centred around working

with nature and not against it.

And it's projected this is going to reduce the flooding risk

in Pickering in any given year from 25% down to 4%.

Now, it's not going to solve extreme events,

but it is going to make a huge difference.

This winter, the river levels in Pickering did rise,

but the town didn't flood.

But, sadly, it's not just rain we have to contend with

when it comes to flooding.

There's another deadly source of water that threatens our islands -

storm surge.

'Another defence overwhelmed as the forecasts remain bad.'

On 31 January 1953,

Britain learned just how devastating storm surge could be.

The Great North Sea Flood inundated thousands of miles of coastline

in Britain, the Netherlands and Belgium,

claiming over 300 British lives.

There were virtually no warnings.

And many of those killed were drowned in their beds.

But what is storm surge and why is it such a threat?

A storm surge is a localised rise in sea level

which happens during storms

and one of the factors that causes this is low atmospheric pressure.

The atmosphere is constantly exerting pressure on earth

and here in the UK, we measure that pressure in millibars,

so, on average, at sea level, it's around 1013mb.

But that can vary depending on whether air is rising or falling.

In a storm, warm air rises

and, as it rises, it condenses and turns into clouds and rain.

But as it rises, it creates an area underneath it of low pressure.

That low pressure is exerting a weaker force on the sea below.

And we can see what effect that might have on the sea

by creating our own area of low pressure.

So, the water level

in this container and tube, at the moment,

is absolutely the same and that's because

the pressure on it is exactly the same.

But if we change that by sucking some of the air

out of the tube, you can see what happens.

As the pressure on that water changes, so the water level rises.

A one-millibar change in air pressure can lead to

a 1cm change in sea level.

So when air pressure dropped below 970 millibars in the North Sea last December,

that alone created the potential for a rise of up to 40cm.

In a storm, that rise in sea level, combined with strong winds,

can cause a dangerous storm surge.

And if that coincides with high tide,

the storm surge is even bigger.

It can become worse still when driven by north winds

into the smaller, shallower parts of the North Sea.

This caused the disaster of 1953 and threatened us again this winter.

But this time, we were ready.

The Thames Barrier was part of a range of flood defences

built in response to what happened in '53.

Its gates rotate to create a solid steel wall across the river,

each holding back up to 9,000 tonnes of water.

Just one of these central gates is the same width as Tower Bridge.

Just talk us through what's been happening here over the past few months.

I suppose the story really started on 6th December.

We saw a surge coming down the east coast of about three metres.

This is the highest level we'd actually seen for about 60 years

and as far as the barrier's concerned,

the largest tide in its 30 years' operational history.

Normally, we'd be closing about three to four times per year,

but we're up to about 50 closures so far in the last couple of months.

How do you make the decision to close the barrier?

We link in and work very closely with the Met Office, so we're able

to plot what actually is happening down the North Sea coast.

We also need to take account of rainfall

because the Thames catchment - the Readings, the Oxfords -

if it rains there, at some point, we're going to see that rainfall

pass us here in Woolwich on its way out to the sea.

We need to compute all of that in our computers here

and literally, if we're seeing that by closing the barrier

we can safeguard properties and people, than that is what we'll do.

We don't play Russian roulette with that. You have to get it right.

Without the barrier and its associated defences,

around 125 square kilometres of central London could be at risk

from flooding, an area that includes landmarks

like the Houses of Parliament and the London Eye

as well as 16 hospitals and 400 schools.

It makes you realise how vulnerable the city really is.

But the barrier also provides reassurance

that with the right knowledge, the right planning

and the right engineering, we can protect ourselves.

But decisions about how we should direct our resources

are being made harder by the uncertainty surrounding our future climate.

What we do know is that our planet is warming,

so I asked the Met Office Chief Scientist

how this might affect flooding.

There's a very simple link and it all goes back to basic physics -

that the warmer the air, the more moisture it can hold.

Then we take the weather systems that we've had

and those storms are now carrying that air that's holding

that little bit more moisture,

and when the storms arrive they wring out that moisture.

That means that the rainfall that we get from that system today

will be that little bit heavier.

So that's where we think climate change has made a real contribution

to the severity of the flooding that we've seen this winter.

What do you see as the biggest challenges that lie ahead for us?

We do need to get to grips with how often we're going to see

these sorts of events, because the investments that we

as a country need to make to protect our citizens

and our infrastructure is huge,

and we need to make that wisely.

More accurate data and better forecasting will be vital

for dealing with floods in future.

That's why I want to see how satellite technology can help.

During the recent flooding, we were able to gather vital information

from earth observation satellites,

which were built here at Surrey Satellite Technology.

One such satellite called upon in the recent floods was UKDMC2.

So, UKDMC2 is how above the horizon. We're tracking this pass.

It's a near overhead pass. You can see the red track.

The red line is where it's going to pass over.

And our ground station here down in Hampshire is now tracking the satellite.

It's moved into position to track UKDMC2

and it's receiving that information.

That's right. You can see on this screen here, this is all

-the information coming down.

-Fantastic.

We can see live telemetry.

Files are being exchanged between the spacecraft and the ground.

It's wonderful to see it in action in real-time.

And how long will it spend over any one particular spot?

When it's over the UK and we're getting data down,

it can be over us for a maximum of about 12 minutes.

You could be imaging for up to four to five minutes.

So, James, with the recent floods, how do you go about telling

the satellite to take images of the floods in question?

The mission planning system will produce a schedule file.

That tells the satellite where to take an image,

so it'll go over its target, it'll capture the image.

When it's back over our ground station, it'll download the image.

Nearby at DMC International Imaging,

a team of analysts are ready to process the images.

What are these blue lines around the globe, then?

Here, we've got the satellite passes for UKDMC2 in a 24-hour period.

So what are these orange dots representing?

These are the areas of interest to focus on for the flooding.

Can we have a look at one of the images?

Yeah, so here we've got the Oxford/Reading area.

Over here, we have a before picture and here we show

the image we got on 8th Feb, which shows the extent of the flooding.

That's desperate.

It really shows the extent of the flooding from these tributaries.

Claire, how do we use these satellite images to understand flooding better?

Here, we have the information products

created by the Environment Agency.

Here, they've got layers of satellite imagery, topography

and the Ordnance Survey map, and on top of that,

they have classified the flood from the satellite imagery.

And what's this image?

So this is also a DMCii satellite image.

It's on a different scale, so you can see

all of the Thames Valley, you can see the Somerset flooding.

It provides the bigger picture here.

It tells people how everything is actually linked up.

When it comes to flood mitigation, how can this data help us?

This will basically inform the decisions that are being made

about how to deal with these floods at the moment,

and all the risk areas can be identified.

By looking at this alongside previous satellite images,

you can see how things are changing over time

and then you can use that picture

to inform what may happen in the future.

And it's not just satellite images that are filling the gaps in our knowledge.

Satellites orbiting our planet at this very moment

are gathering vital information about the earth's natural systems.

Now this in NASA's Jason 2 satellite

and it uses a radar altimeter to bounce microwave pulses

off the surface of the earth, and by measuring how long it takes

for the pulses to come back, it can measure differences in sea level.

Satellites are also improving our knowledge of rainfall patterns.

In February 2014, an international satellite mission called

Global Precipitation Measurement was launched, and it can tell us

how much rain and snow falls around the globe every three hours.

This kind of data can be fed into models to help us monitor

and predict climate change and it can also lead to more accurate forecasting

of the conditions that lead to flooding.

I think it's safe to say that we haven't seen the last of flooding.

The storms will come again, the water will be back

and the defences and our ingenuity will be put to the test.

And what's clear is that there's no one solution.

We need to combat flooding with a combination of tools

from natural measures to engineering,

and we need to widen our gaze to look at entire catchments.

We also need to take responsibility

for the changes we've made to the landscape.

Some tough decisions with a long-term view lie ahead -

but armed with an ever-growing understanding of the nature of floods,

we can learn how to better prepare ourselves and mitigate their effects.

Bang is back in two weeks' time, looking at the railways

and how they'll cope as passenger numbers rise.

In the meantime, if you want to find out more about satellites,

check out the website at /bang for our careers guide.

And to learn more about flooding, follow the links

to the Open University.