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NARRATOR: Flash floods strike and take their victims...

...by complete surprise.

Powerful walls of water destroy everything in their path.

Of all extreme weather conditions,

floods cause the greatest number of deaths.

(Cries out)

At any one moment,

12,000 billion tonnes of water are suspended in the atmosphere,

waiting to fall as rain.

Below, more than 37 million billion cubic metres

make their way across the planet,

relentlessly flowing downwards.

Most of the time, we can harness and manage its power.

But when the world's water runs out of control,

it becomes a flood.

And nothing can stop it.

The most dangerous floods are unexpected flash floods.

They often happen in the world's driest places.

When thunderstorms hit the deserts of the south-western United States,

the rainwater they bring can kill without warning.

Las Vegas is the second-driest city in America.

It's surrounded by desert.

Sherry Swensk has been a weather forecaster here for 15 years.

Our climate is known for just being hot and dry and unforgiving.

Overwhelmed by floods?

That's nuts!

But on 9th August 2003,

huge thunder clouds gathered over Vegas

and unleashed a year's worth of rain in less than an hour.

That day, that much rain on our hard desert dirt,

it hit and it ran.

Flood-control basins designed to protect the city were overwhelmed.

The detention basins were doing their job.

They were keeping up as fast as they could.

Unfortunately, it wasn't fast enough.

That day, Angela Stitcher set off

to pick up her two young children from nursery school.

As she drove under a highway bridge,

she had no idea the flood was heading straight towards her.

There wasn't really any standing water when I got there,

nothing to cause any concern.

But the water just came out of nowhere. It was like a tidal wave.

I was just a sitting duck. There was nowhere I could go.

The water was more than a metre deep and moving at 40km/h.

The water was actually coming up to my window.

There was a man standing on top of a U-Haul truck,

yelling at me to get out of my car.

MAN: Get out of the car now!

The sheer force of the flood starting moving her car.

So I climbed up on top of my car.

I think, at this point, fear really started to set in.

And then, as I lay there, I just realised, 'OK, this is it.

This is my time.'

I've accepted it. I've said my goodbyes.

I pictured the faces of my kids

and I just laid there and I closed my eyes, thinking,

'OK, I'm ready to go.'

The next thing I know, there was a guy shaking me.

And I was like, 'Wow! Where did he come from?!'

A police rescue helicopter braved the nearby powerlines to rescue her.

It was like, 'Oh, hi. I'm glad you're here.'

They reached Angela just in time.

Before they even got me up in the helicopter,

my car washed away.

So they made it not a moment too soon.

The man on the truck decided his only option was to jump.

Luckily, he wasn't hurt.

He also escaped the flood.

More than 60 people had to be rescued from the water that day.

Even the fire department had to be airlifted to safety.

The power of moving water is tremendous,

because it moves quickly and it gathers speed.

That six inches of water that's moving down a road or a street

can gain 30 miles an hour in speed.

It's very powerful, and can become really dangerous.

Just 15cm of water is enough to sweep you off your feet.

That depth can exert a third of a tonne of pressure.

That's enough to push a car around.

A flow of about half a metre can move heavy vehicles,

like this fully loaded cement mixer.

And the steeper the slope, the more powerful the flow.

*

Northern California has one of the highest rainfall totals in the US.

An extensive system of dams and levees

channels snowmelt and rainwater down from the Sierra Nevada mountains

to the orchards and vineyards of the Sacramento Valley.

These giant levees were built in the early 1900s

to protect this valuable land and the people who live here.

Almost a hundred years later, during the winter of 1996,

they were tested to the limit.

Unusually warm weather was melting snow in the mountains.

Heavy storms dropped over a metre of rain.

As the reservoirs reached capacity,

water manager Steve Onken started to worry.

By December 30th, most of the reservoirs in Northern California

were full,

and we were releasing or spilling water

because the inflow was greater than we could store.

There was just too much rain, too much run-off,

and we were headed for a flood-type situation.

As the pressure built up behind the dams,

the hydro-engineers released more and more water.

They assumed the levees downstream would keep it contained.

But they were wrong.

It was New Year's Eve in Marysville, California,

and Mike and Mary-Anna Otorino were on holiday

during one of the wettest winters on record.

Water managers were trying to keep the nearby dam from overflowing.

As the river level rose, the Otorinos were unconcerned.

We honestly didn't think there was gonna be a problem for us, no,

because we just felt the levee is going to hold

and it's doing what it's designed to do.

We could count on the levee.

But when rising levels put too much pressure on the levees,

one of them collapsed.

As the water swept out across the California flood plain,

the authorities were powerless to stop it.

The water spreads out very quickly.

If you're in the path of the water, you're in trouble.

The Otorinos were staying

at a friend's house close to the levee break.

Police warned them they had 15 minutes to get out.

They left just before dark.

So we started down the road and the road looked fine

until we got to the corner.

As soon as we made the turn,

the headlights from the car shone upon the water

and there was literally a river of water

going across the roadway.

The Otorinos decided they could cross safely.

That was a big mistake.

It just got higher and higher and higher,

and at that point, the car stalled.

As the water kept going up and up,

we realised, 'This is definitely serious. This is not good.'

(ENGINE STALLS)

We then had to evacuate and get on the roof of the car.

And then it started to lap at our feet on the roof.

And this all happened within 20 minutes, so it rose very rapidly.

Because of the force of the water,

we were now holding on to each other.

They hung on till daybreak,

when a rescue helicopter spotted them

standing on top of their submerged car.

After spending nearly 13 hours in the water,

seeing that helicopter come back and stop over us,

you couldn't believe the feeling that we felt.

The downdraft from the helicopter blades

pushed them into the flood water.

I started sinking and sinking,

and all of a sudden, I felt an arm grabbing my arm,

and turned me around, and all of a sudden there was a halter around me.

It was the fastest ride I have ever been,

I was just so excited and delighted, it was amazing.

The Otorinos were lucky to survive their ordeal.

In the US, 60% of all flood fatalities die in their cars.

Weather and storms and water,

these are things that have an impact on people

and can take away things in a matter of seconds.

The faster water flows, the more dangerous it becomes.

Even the slightest increase in speed can turn a creek into a deathtrap.

(SCREAMS)

This woman in Turkey became trapped

when she tried to drive across a flooded river.

(PEOPLE TALK ALL AT ONCE) (WOMAN SCREAMS)

(PEOPLE SCREAM)

Somehow, the driver escaped with her life.

Floods can be incredibly destructive.

When flood waters swept right through the middle

of a small English village,

the devastation was terrible.

The coast of Cornwall ranks among the most beautiful in the world.

Here, rugged cliffs meet the waters of the Atlantic,

and give century-old villages shelter from ocean storms.

One is the village of Boscastle, home to just 800 people.

Every summer,

holiday-makers are drawn to Boscastle's medieval harbour,

which sits at the bottom of a huge natural funnel.

Trixie Webster serves the tourists at the Harbour Light's teashop.

The building had been in my family for 50 years.

It was a 14th-century building

which was one of the most photographed buildings

in the whole of Cornwall.

Graham King is curator of the Museum of Witchcraft in Boscastle.

It's totally magical.

It's something about the wildness of the sea,

the very steep valley,

it's a combination of all these things somehow blend together.

Part of Boscastle's picturesque charm

comes from its gently meandering rivers.

But this also means that any rain which falls here

is channelled straight through the village.

In August 2004, these gentle streams became torrents.

*

August 2004 - the height of the holiday season in Boscastle, England.

Trixie Webster was serving tea to a full house.

Down to this level.

It was just at midday,

this big black cloud came over.

There was now an 11km-wide storm at the top of the valley

above the village.

The river started rising at quite a horrendous rate,

and it went right up to the top of the riverbanks,

which is very unusual.

The waters started flowing down the main street.

Boscastle was now at the mercy of a flood.

It just changed into something that's terrifying.

You can't imagine that's the same river and the same place,

but of course it is.

MAN: It's gonna break the bridge.

People struggled to get through the water,

even though it was just a few centimetres deep.

Then, things went from bad to worse.

From a car park at the top of the village,

some of the cars began to drift downhill.

When the first car came down,

we didn't even know if there was someone in it or not,

and we couldn't do anything about it.

It was a horrible, horrible feeling.

If anybody had fallen in at that point,

there would have been no coming back

because to wash cars away as it did,

cars and vans just bobbing along like dinky toys,

that's quite powerful.

The flood literally threw the cars at the buildings.

Trixie Webster's 600-year-old teashop began to crumble.

Water just burst through the door

and burst through the windows at the back.

Every car that came down hit it.

And it just disappeared.

It was there one minute and it was gone the next.

The building didn't stand a chance. It just disintegrated.

Over 1.5 billion litres of rainwater flowed through Boscastle that day.

With almost a thousand lives at stake,

rescuers launched the biggest airlift since World War II.

After two hours, the flood began to subside,

exposing the devastation it had left behind.

Just the destruction, the mess, it was quite amazing.

It's quite incredible how much the river had changed the landscape.

It was like a bombsite. Everything was...

It was terrible - cars piled one on top of the other,

the building across from me had trees in every window.

It was very difficult to take on board, really.

With 84 vehicles lying wrecked at the harbour entrance,

the search for victims got underway.

Dozens of cars had been swept through the village.

Rushing water can even be powerful enough to move ten-tonne boulders.

But incredibly, the flood hadn't claimed a single life.

The Caribbean coast of Venezuela.

When the Spanish conquistadors arrived here in the early 1500s,

they dared to build towns where the local Indians did not -

crammed onto a tiny flood plain between steep mountains and the sea.

Professor of engineering Carlos Anatias

now understands the risk his ancestors took.

Not knowing this menace, we decided to build in the middle of the way

that the water normally takes to come down to the sea.

And every once in a while,

the water comes and produces a lot of disasters.

When extreme rain fell in 1999,

30,000 people paid with their lives.

(DISTANT HUBBUB)

The coastal resort of Los Corales in Venezuela

was home to Justino Bravo.

(MAN SPEAKS SPANISH)

In December 1999, an unusually fierce storm hit the coast.

As it approached the city, Justino began filming.

(JUSTINO SPEAKS SPANISH)

At the nearby airport,

Mandy Donahue arrived for a holiday with some friends.

The rain was really coming down.

I, not having travelled, just sort of thought,

'This is what it's supposed to look like. It rains here.'

But this wasn't normal.

Over the next few days, there was two years' worth of rain.

The steady sort of drumbeat of the rain was like...

It just droned on and on. You just sort of stop hearing it

because it's so constant.

In all, there was a metre of rain.

It ran down from the mountains behind the town,

gathering volume and power all the way.

The water coursed through the streets,

surrounding Justino Bravo's apartment building.

(JUSTINO SPEAKS SPANISH)

The flood water was demolishing

everything that stood between it and the sea.

But water wasn't the only problem.

On the slopes above Los Corales,

the rainwater has dislodged soil and rocks.

Now, the floods were thundering down at up to 96km/h,

carrying uprooted trees, huge chunks of debris

and boulders the size of school buses.

They smashed their way through buildings.

(JUSTINO SPEAKS SPANISH)

Justino Bravo's apartment was also in the line of fire.

(JUSTINO SPEAKS SPANISH)

Mandy Donahue could only watch as the flood waters reached her hotel.

The hotel was right on the beach,

and so everything in the city was sort of flowing towards us.

The water began to have a disastrous effect

on poorly built shantytowns clinging to the mountain slopes.

We would hear these sloughing sounds.

We realised that what we must have been hearing

was the sound of these communities built up the hill

that were now in rubble at the bottom of the hill.

This swooshing sound was the sound of these neighbourhoods

being wiped away.

When the flood waters finally subsided,

Los Corales was buried under tonnes of gigantic boulders.

Along 50km of coastline,

the water and debris had smashed thousands of buildings into the sea.

30,000 people were killed.

But only a thousand bodies were ever found.

The rest just disappeared into the water.

150,000 people lost their homes.

Los Corales was literally washed away.

Professor Carlos Anatias has studied how something as simple as rain

can cause such devastation.

At the beginning, you have small sediments, small particles,

but as it continues to rain heavily,

you have larger pieces of soil that come down with the water.

And then, this kind of thick mixture of solid and liquid

could move anything that it could find on the way.

Especially, it could transport rocks, heavy rocks.

Because it is so dense, it could lift them easily.

The geography of Venezuela's northern coast changed overnight.

The hillsides are scarred where the soil was washed away.

An incredible 1.4 million cubic metres of debris

was swept to the bottom of the slopes,

in places up to ten metres deep.

Flash floods like this might strike only twice in a century,

but in some parts of the world,

slow-rising floods can be just as scary.

And they happen every year.

Slow-rising floods can take days to develop,

and are much more predictable than flash floods.

But there's no way to stop them once they start.

Every spring, the water level in the Mississippi

gradually rises until the riverbanks can no longer contain it.

Satellite images reveal the huge scale of the floods

that sweep over the Midwest every year.

In 2008, heavy spring rains caused the levels to rise

more than four metres above normal.

The Mississippi and its tributaries overflowed their banks

and spread out over the flood plain.

Most people got out of the way,

but the damage to property, crops and livestock came to $2.5 billion.

As the water fanned out across eastern Iowa,

it could have filled 1.5 Olympic-size swimming pools every second.

When it reached Cedar Rapids,

it inundated 23km sq of the city.

The flood took resident Lindsay Janacek completely by surprise.

I've lived in Cedar Rapids all my life around the Cedar River

and when I thought about a flood,

I was prepared for maybe inches of water.

Instead, there was three metres of water.

As it flowed into downtown Cedar Rapids,

it surrounded 1,300 city blocks.

It demolished highways and railway bridges,

and wrecked more than 5,000 homes.

Lindsay's house was one of them.

Oh, my God!

I knew it was going to be bad,

but when I walked in, I didn't realise it would be that bad.

Oh, God! It's full of water.

And shoes.

I never would have imagined walking into my home

and seeing the ceiling torn down and the walls black with mud.

This is the stove. It turned it over completely.

It took appliances and turned them completely over,

and moved the dressers across the room and...

I'm at a loss for words, to be honest.

It's been my home, it's part of who I am.

The Mississippi is the largest river in the US.

At the mouth of its 3,700km-long course,

the Mississippi fans out into a delta.

And right in the middle is the city of New Orleans.

In 1927, the river burst its banks in a catastrophic flood.

It killed 500 people and inundated nearly a million homes.

Many were in the poorest Ninth Ward

where authorities dynamited the old levees

to prevent flooding elsewhere in the city.

After that flood, the army corps of engineers

built an elaborate system of dykes and levees

to protect the city.

But it isn't just river water that threatens New Orleans.

There are also storm surges from hurricanes that slam into the coast.

The city is much more vulnerable to floods from the sea

because the protective wetlands around the city

have started to dry up and shrink.

80% of New Orleans is now below sea level.

The lower Ninth Ward is on the eastern side of New Orleans.

Here, the only thing that separates the residents

from the water five metres above them is this wall.

Robert Green lives here,

and he'd never thought of the nearby canal as a threat.

We grew up loving the water, we grew up fishing,

you know, so it never was a sense of danger.

He's always lived on Tennessee Street

in the heart of the neighbourhood.

A lot of children, a lot of families,

a lot of fun going on.

Right now, you would have nothing but noise from children.

Robert used to take care of his three granddaughters.

My grandkids were always with me.

If you saw me, you saw them.

That's how they was - three sisters together

with the old man to play with sometimes.

Tennessee Street was just two blocks away from the flood wall.

In August 2005, Hurricane Katrina churned across the gulf

towards the Louisiana coast.

It pushed a 3.5-metre storm surge up the Mississippi Delta,

testing the dykes and canals that protected the city.

One of those canals channelled water

straight towards the lower Ninth Ward.

*

As Hurricane Katrina raged around them,

Robert Green and his family had no choice

but to stay in their home and take their chances.

As time passed on, the weather changed,

I was listening to the wind, listening to the rain,

and the water started coming through.

The water from the hurricane storm surge was beginning

to lap over the top of the flood walls.

Another lower Ninth Ward resident, Ernest Edwards,

knew this was a worrying sign.

You could see the water rising.

It came up to the top of the wall,

and I told my son, 'It's time to go get on a bridge. It's time to run.'

We didn't have much time.

As he got to the bridge, water was pouring over the flood walls

and eroding the foundations.

The lower Ninth Ward was starting to fill up.

At about four o'clock in the morning,

my brother said, 'Robert, we have water in the house.

We have to get to the roof.'

Just minutes after they reached the roof,

the full force of the storm surge smashed through the flood wall

and swept towards Tennessee Street.

It knocked Robert Green's house clean off its foundations.

It literally pushed us into the middle of the street.

Literally, our house floated from 1826 Tennessee Street

down the street and hit up against a tree.

I just felt this was it.

The impact threw his three-year-old granddaughter off the roof.

She was lost in the dark rushing water.

There was nothing I could have done to save my granddaughter.

There was no sense in trying to reach for her, she was gone.

She was washed away.

The odds were stacked against us,

the hole in the levee and the water coming through

was so great that it just flushed everything away.

As dawn broke, thousands of lower Ninth Wall residents

were stranded on their roofs.

Robert and his two surviving granddaughters were among them.

The sun came out, the clouds were gone,

the wind was gone, the rain was gone,

and in the distance, we heard a boat.

It was Ernest Edwards.

He found Tennessee Street submerged in 5.5 metres of water.

All the trees along that street had people in them.

Now to stop loading up and start riding.

I didn't stop riding

till seven o'clock that night, picking up people.

Ernest Edwards literally saved our lives

and probably 200, or even more than that, people.

But there were many people in the lower Ninth Ward

who couldn't be saved.

It was a sad day.

A whole lot of them died down there.

It was a sad, sad, sad situation.

The wall at the lower Ninth Ward wasn't the only one that failed.

A total of 50 flood defences were overwhelmed.

Over 700 people died,

and 370,000 were left homeless.

Damage was estimated at $80 billion,

making it the costliest flood in recorded history.

The power of floods is unpredictable and deadly.

No matter where they come from or why they begin,

floods are almost impossible to control.

So next time it rains, keep your eyes on the sky.

The world's deadliest natural disaster could be on its way.

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