All language subtitles for NOVA.S52E18.Superfloods.1080p.WEB.h264-BAE_track3_[eng]

♪ ♪

NARRATOR:
September 2024.

Torrential rain
hits North Carolina,

triggering flash floods
that surge down valleys...

MAN:
Holy...

NARRATOR:
...and destroy buildings.

MAN:
It just ripped it in half.

TRISHA PALMER:
Nobody in this part
of the country

has ever seen anything
like this before.

It was devastating.

(thunder rumbling)

NARRATOR:
One month later, in Spain,

a massive flood hits Valencia.

ANDRIES KLARENBERG:
Everything was flooded
with three feet of water,

which was pushing cars

down the streets,
into buildings.

MAN
(speaking Spanish):

NARRATOR:
The flood kills more
than 200 people.

HANNAH CLOKE:
Something went wrong
in this flood.

We know that the flood
forecasting and warning system

didn't work
as it should have done.

NARRATOR:
Then in Central Texas,

on the 4 of July 2025...

...deadly floods kill
over 130 people.

(rushing water)

Are flash floods
becoming more frequent?

MAN
(speaking Spanish):

NARRATOR:
And more devastating?

There was a cascading

and compounding effect
of this storm

which I fear is a harbinger
of things to come.

NARRATOR:
What can we do
to protect ourselves

from these deadly inundations?

MAN
(speaking Spanish):

NARRATOR:
"Superfloods,"

right now on "NOVA."

♪ ♪

♪ ♪

NARRATOR:
On the 4 of July

2025, an intense thunderstorm

in central Texas
generates floods that kill

more than 130 people, including

at least 39 children.

It's the deadliest
inland flooding

in the United States

in nearly half a century.

But this shocking disaster
isn't an isolated event.

In the preceding 12 months,

two other record-breaking storms

also produced
devastating floods:

one in North Carolina,

the other in Europe.

Altogether, the three storms
leave over 450 people dead.

What can we learn
from these disasters

to better
protect us in the future?

(woman shouting in Spanish)

♪ ♪

Asheville, North Carolina.

It's a popular tourist
destination.

Around 14 million people
visit the city every year

to enjoy its vibrant
arts and culture...

...historic buildings...

...and spectacular mountain
scenery.

Asheville's location,

over 250 miles from the coast
and 2,000 feet

up in the Blue Ridge Mountains

puts it far from the storm
surges of Atlantic hurricanes.

Which made many people think

it was relatively safe
from extreme weather.

(fishing line whizzing)

But that all changed
in the autumn of 2024.

♪ ♪

Wednesday, September 25.

Torrential rain
pummels North Carolina,

and it looks like
it's about to get worse.

For eight days,

meteorologists have been
monitoring a weather system

coming in from the Gulf.

PALMER:
By midday Monday,
we had what we call

a potential tropical cyclone.

Nothing is actually formed,
but something is going to form.

And already then
we were talking about

significant heavy rainfall.

NARRATOR:
By Wednesday morning,

the tropical cyclone
has been named Helene

and grown into a hurricane.

Its predicted track puts it
on a course

toward western North Carolina--
and Asheville.

The city sits at
the junction of two rivers:

the French Broad
and the Swannanoa.

On Wednesday afternoon,

the National Weather Service
issues

a worrying forecast.

PALMER:
We had a river forecast,

and our hydrologist put in there

the potential for
"ruinous flooding"

along the French Broad
and Swannanoa rivers.

He put in there
this has the potential to be

"once-in-a-lifetime" flooding.

"Consider relocating"
if you live

along the French Broad
and Swannanoa Rivers.

We were very concerned,
and quite frankly,

getting very scared
at that point.

NARRATOR:
But the trouble starts
even before Hurricane Helene

reaches North Carolina.

On Thursday afternoon,

the eye of Helene is still
500 miles away in the Gulf,

but in Asheville,
after two days of heavy rain,

the French Broad River
is already beginning to flood.

That night, at 11:10,
Helene makes landfall

on the Florida coast
as a category 4 storm.

Through the night,
it tracks north,

and in the early hours
of Friday morning,

dumps more rain onto Asheville
and the surrounding mountains.

Eight miles east of Asheville
lies the town of Swannanoa,

named after the river that
flows through it.

On Friday morning,
the overnight rain from Helene

swells the river,
and it rises rapidly.

Michael Burgin is at home with
his partner and 18 year-old son,

when water starts
flooding their street.

BURGIN:
It was around 7:30
in the morning,

I did get an alert on my phone
saying to evacuate.

At that time,

the water was about halfway
up our cars outside.

So evacuating
didn't seem possible.

Our idea was to go
up to the attic first.

NARRATOR:
But the water continues to rise.

BURGIN:
We were up in
the attic for a while.

The water started sloshing
into the attic a little bit,

and we didn't
want to get trapped.

So, we decided to swim down

into our kitchen.

NARRATOR:
With water nearly up
to the ceiling,

they try to escape
through the windows.

BURGIN:
We were going up
and getting air

and then swimming down

and trying
to get the windows open.

And neither
one of them would budge.

And then eventually my son was
able to break the window open.

We swam through the window
and then got up on our roof.

♪ ♪

NARRATOR:
14 miles downstream,
the floodwater flowing down

the Swannanoa,
reaches Asheville.

♪ ♪

CHRIS FABER:
I was on a corner unit
that basically

looked right up the river.

Early afternoon,

mid-afternoon is when it,
it started rising a lot faster.

When it started to flood
the first floor apartments,

everybody on
the second floor was like,

"Oh man, like, that's crazy."

And then
it hit the second floor.

(on camera):
Oh, my god!

(voiceover):
It kept coming and kept coming.

At first it was just
random debris, you know,

litter, trash, or some tires,

street signs.

By 1:00 or so,

we started to see box trucks,

and shipping containers,
and really quite large stuff.

♪ ♪

We could see these things
coming a ways away,

and it just
felt like slow motion.

It literally folded
a giant shipping container,

like tin foil.

It just wrapped around it.
Unbelievable.

It was pretty terrifying
because everything looked like

it was going to hit us,
and then kind of last minute,

it would whoosh its way out
into the river.

(on camera):
Oh, come on baby, miss us.

(voiceover):
Certainly,
the scariest one was seeing

entire buildings
coming down the river.

NARRATOR:
This isn't just a roof;

this is the entire
Asheville Tea Company building.

WOMAN:
It didn't hit us.

FABER:
Holy...

...it just ripped it in half.

♪ ♪

NARRATOR:
Upstream, in Swannanoa,
floodwater continues

to pour through
Michael's neighborhood.

BURGIN:
We were on the roof for a total

of about three hours.

We started hearing
foundations cracking,

It was very surreal
and very scary.

NARRATOR:
Eventually,
neighbors in kayaks

carry Michael
and his family to safety.

In the mid-afternoon,

the flood reaches its peak.

All along the Swannanoa river,

it has devastated communities.

On the morning of Saturday,

September 28,
the people of western

North Carolina wake up
to scenes of devastation.

FABER:
It looked like
a bomb had gone off.

I mean the destruction
is incredible.

Overturned cars
stacked on top of each other.

Incredible depth of just mud

that you had to kind of really

work your way to get through.

Streets that were
just ripped up.

It completely
changed the landscape.

NARRATOR:
On Michael's street,
everything is filled with mud.

BURGIN:
The water went down
remarkably fast.

Most things were destroyed,
obviously.

It just was really upsetting.

NARRATOR:
The flash floods
smashed through buildings,

swept away vehicles,
and destroyed roads

and bridges.

In the hills, the rain

triggered landslides that
buried homes and cut off roads.

As the search
begins for the missing,

people want to know:

why did this happen here
and now?

This was not the first time
Asheville flooded.

In July 1916, the tail end

of a hurricane hit western
North Carolina,

followed by another hurricane

nine days later
dumping heavy rain on the city.

The resulting flood
killed about 80 people.

It became known

as the Great Flood of 1916.

Despite its mountain location
far from the coast,

Asheville's geography actually
makes it vulnerable to floods.

Parts of the city
run along valleys,

close to the French Broad
and Swannanoa rivers...

...and the mountains around
the town help generate rain.

PALMER:
Western North Carolina,
upstate South Carolina,

it's got a unique feature called
the Blue Ridge Escarpment.

The terrain rises very rapidly.

So you have this air
that's being lifted up against

the Blue Ridge Escarpment
and as the warm air rises,

it condenses
and it starts raining,

and it just all starts

routing down from this creek
to this creek to this creek.

Eventually, the larger streams
start rising.

And this is where they live.

NARRATOR:
This landscape helped create
the Great Flood of 1916.

But the flood of 2024
was even worse.

In Asheville, the Swannanoa
River peaked over five feet

higher than
the level reached in 1916.

PALMER:
Nobody in this part
of the country

that's alive has ever seen
anything like this before.

It was devastating.

NARRATOR:
So why was this flood
so extreme?

A clue comes in the form
of another disaster.

On the other side of
the Atlantic.

Just a month after
the floods in North Carolina,

a deadly storm
hit Valencia in Spain.

Like Asheville,
Valencia is a popular

tourist destination...
but the landscape here

is different.

The city sits between
low-lying mountains and the sea.

Rain is rare.

The ravines that run

through the nearby towns
are usually dry.

But when big storms occur,
they have been known to flood.

October 29, 2024.

Meteorologists across Europe

are growing concerned about

an approaching weather system.

This was going
to be an extreme event.

This was going to be dangerous,
this was going

to be something that was going
to have a high impact

for the people in the region.

By the morning
of Tuesday the 29 of October,

a red warning had
been issued at about 7:00 A.M.

Red warnings mean
that there is a danger to life.

♪ ♪

NARRATOR:
On Valencia's coast,
a strong wind whips up

the Mediterranean sea,

but there's hardly any rain.

However, inland,
heavy rain is falling

in the hills,
and collecting in valleys.

Just south of Valencia,
a ravine,

called the Rambla del Poyo,

runs from the mountains
toward the Mediterranean.

Over half a dozen communities,

including the town of Paiporta,
line its banks.

More are dotted in
the surrounding flood plain.

Over 200,000 people
live along the ravine.

♪ ♪

Shortly after 2:00 P.M.
local time,

water flowing down the
Rambla del Poyo

reaches the town of Paiporta.

It carries a wave of debris.

Within a few minutes,
the dry ravine

becomes a powerful,
fast-flowing river.

In the afternoon, the rain in
the hills intensifies rapidly.

BENTLEY:
During the height of the storm,

one location recorded
771 millimeters of rain,

which is about
two and a half feet of rainfall,

falling over
a space of 14 hours.

NARRATOR:
Shortly after 5:00 P.M.,

flash flooding engulfs a town
near the top of the ravine.

At 6:28 P.M., in Paiporta,

the river overflows its banks
and rapidly floods the streets.

KLARENBERG:
All the neighbors,

in the entire area,

started bringing their cars up
out of the underground parking.

That was very strange.

NARRATOR:
Residents here
know what will happen

if the river floods basements.

(horns honking)

In towns along

the river's banks,

they race to save their cars.

KLARENBERG:
There was a lot of noise.
People shouting.

Every spare meter

of pavement, courtyard,

grass was filled with
the cars from the neighborhood.

DANIEL BURGUET
(speaking Spanish):

NARRATOR:
The storm that devastated
the communities

around the Valencia region
was unprecedented,

breaking the record for the most
rain to fall in Spain

during a 12 hour period.

(thunder rumbling)

What caused so much rain to fall
in such a short time?

The answer is a weather system

meteorologists call a
"cut-off low."

A cut-off low can form
when an area of cold,

low pressure air gets cut off
from the flow of the jet stream.

This high altitude rotating
mass of low-pressure air

can draw in moist air
from below.

The fact that they are cut off

from the main flow in the
atmosphere

means that they sit in one
location

and pump moisture into
the same region

for several hours to days,
even up to a week.

NARRATOR:
When cut-off lows interact with
a plentiful supply of moist air,

such as a warm ocean,
they can become dangerous.

That summer,

the Mediterranean sea reached
record high temperatures.

SHEPHERD:
You have cold air
in this cut-off low,

but the Mediterranean waters
are quite warm.

That leads to a very unstable
atmosphere

that can produce rising air,

condensation, and a lot of
rainfall ultimately.

NARRATOR:
Beneath the cut-off low, warm,
moist air at the surface

rises rapidly,
drawing in more warm air,

full of water vapor,
from the Mediterranean.

This rapid uplift
generates huge thunderstorms.

(lightning strikes,
thunder rumbling)

SHEPHERD:
When you have sustained
moisture flow

and the dynamics associated

with a cut-off low, that leads
oftentimes to extreme rainfall.

NARRATOR:
In North Carolina,

the heavy rain
that pummeled Asheville

in the days
before Hurricane Helene arrived

was also caused,
in part,

by a cut-off low.

SHEPHERD:
The convergence of many factors
produced a rainfall machine.

One of the factors

with this particular case
was a cut-off low.

The way that low was situated,

it allowed moisture to flow
into parts of Georgia

and North Carolina
essentially creating a funnel

of moisture
feeding these rain storms.

♪ ♪

NARRATOR:
While Hurricane Helene
was developing in the Gulf,

the cut-off low formed
over Western Tennessee.

This low-pressure system
sucked in air,

laden with water vapor,
from the Gulf and Atlantic.

As this rose over
the Blue Ridge mountains,

it triggered thunderstorms
and torrential rain.

Meteorologists call this a
predecessor rain event.

PALMER:
A predecessor rainfall event
is when you have rainfall

that is distinctly separate
from the tropical system,

but it involves moisture that is
being pulled up

from the tropical system,

so it saturates the ground
ahead of the storm.

And so by the time
the storm gets there,

everything is set up
for a disaster.

NARRATOR:
On Wednesday and Thursday,
before Helene arrived,

the cut-off low brought
heavy rain into North Carolina.

By Thursday afternoon,
when Helene was

still 500 miles away
in the Gulf,

the French Broad River in
Asheville

was already flooding.

But then the cut-off low
did something else that helped

turn a moderate flood
into a disaster.

When Hurricane Helene
made landfall,

the winds spiraling around
the cut-off low

pulled the storm toward it.

Downgraded to a tropical storm,

Helene barreled into
the Blue Ridge Mountains

and dropped even more rain
onto Asheville

and the surrounding area.

PALMER:
In portions of the
Black Mountains, we had

seven to ten inches
of rain leading up to Helene,

and then we had over
a foot of rain with Helene.

And as it just keeps raining,
keeps raining, keeps raining...

...again, it just set up for,

for just a
horrific, horrific event.

NARRATOR:
The extreme rainfall
in North Carolina, Valencia,

and Texas,
and the floods that followed,

had devastating consequences
for people and property.

Three historically devastating
floods in the space of a year.

Just a coincidence?
Or is a new pattern emerging?

SHEPHERD:
We know that all across
the United States,

the rate at which rain falls
is more intense.

And that's related to
a basic physics understanding.

As the atmosphere warms,
there's more water vapor

available to the storms.

(thunder rumbling)

NARRATOR:
For every degree Fahrenheit

that the atmosphere warms,
the air can hold

3% to 4% more water vapor.

Since pre-industrial times,

our atmosphere has warmed about

two degrees,
so it can absorb about

7% more water vapor
than it used to.

This means that,
in some storms,

heavy rain will be more intense
than in the past.

Rising sea temperatures

can also add more water vapor
to the atmosphere

because water evaporates faster
at higher temperatures.

SHEPHERD:
There is certainly

a relationship between warming
water temperatures,

perhaps in
the Mediterranean or the Gulf,

and the intensity and amount
of rainfall in cut-off lows.

NARRATOR:
The warm water in the Gulf
provided the energy

that raised Helene
to a category 4 hurricane.

Hurricanes get their fuel
from the oceans,

and that's a warm ocean.

And one of the things
that we're increasingly seeing

is warm sea surface
temperatures;

marine heat waves.

And so, these storms are
stronger as they're moving in

and they're producing
greater amounts of rainfall.

NARRATOR:
In July 2025,

high levels of water vapor
in the atmosphere above Texas

contributed to America's
deadliest inland flood

in nearly half a century.

Warm, moist air
from the Gulf moved

over central Texas,
feeding powerful thunderstorms

that soaked the western
Hill Country with heavy rain.

Floods swept down
the Guadalupe River

and surrounding creeks,

leaving death
and destruction behind.

SHEPHERD:
The Texas floods were
a combination of several

meteorological
or weather factors.

That region is known
as Flash Flood Alley.

It's close to the Gulf,
and there was ample moisture

from the Gulf--
a quite warm Gulf by the way--

and you had the remnants
of Tropical Storm Barry

that also was a moisture source.

As meteorologists,
we look for moisture

as a signature
of potential flooding.

And moisture was at
record levels.

It was certainly clear
to forecasters

that this was going to be
a flood event.

NARRATOR:
In Texas,

many weather stations reported
five to ten inches of rain

in less than six hours.

When that rain funneled
into the Guadalupe River,

it rose about 26 feet
in 45 minutes.

The result
was utter devastation.

(wood snapping)

The Texas
and other recent floods

may be a sign of things to come.

Recent studies indicate that,
as global temperatures rise,

the United States will see

an overall increase
in flooding;

especially in the east.

And there's another problem.

As the world's population grows,

more people are living
in areas that flood.

We've got a lot of pressure

to build new houses
all of the time,

and that, of course,

makes it very difficult

to protect those places
such as floodplains,

where we don't really
want people living,

because
they can be dangerous.

Sometimes floods don't
happen for, you know,

a hundred years at a time,

so people will build,

because they think
that there-- you know,

there's nothing
that's going to happen.

But actually,
it is a dangerous place.

NARRATOR:
It's a double threat.

To stay safe,
it's increasingly important

for people to
be aware of the risks.

(buzzing)

But the range of danger

that floods
and heavy rain present

depends a lot on the terrain.

(drone continues buzzing)

In the hills and mountains
around Asheville,

the biggest killer from Helene
was landslides.

The National Oceanic
and Atmospheric Administration

classified
the landslide deaths here

as fatalities from
fresh water flooding.

PHILIP PRINCE:
With this type of landsliding,

what we saw with Helene,
water is the key ingredient.

None of what we saw happen
is going to happen

without a tremendous amount of
water present in the soil.

(water surging)

NARRATOR:
Water from Helene
caused this slide.

Fortunately, no one was hurt.

♪ ♪

PRINCE:
It's absolutely the most
acutely dangerous natural event

that you see in the
Appalachian Mountains,

just because of the speed
with which it moves.

NARRATOR:
13 miles east of Asheville,

the storm left another tragedy
in its wake.

In the small community
of Craigtown,

13 people lost their lives

when a series of landslides
swept down the mountain,

destroying homes.

Geologists Jennifer Bauer
and Philip Prince

are helping compile
an inventory

of the more than 2,000
landslides in North Carolina

triggered by Helene's rain.

Today, they are
climbing to the top

of one of the
Craigtown landslides

to find out what started it.

Can you see anywhere

that might be a ground water
source up here?

Let's look under here.

NARRATOR:
This slide, like most
in North Carolina,

was a type of landslide
known as a debris flow--

a landslide made of soil, rock,
other objects, and water.

So this is what,
like, I don't know,

three feet long?

That's a good way
to concentrate

a lot of ground water
in one spot.

Yeah.
For sure.

NARRATOR:
Debris flows often begin
near the top of a mountain,

where the slopes are steepest.

Rain soaking into the soil
can accumulate as groundwater

high on the mountain.

Underground channels can then
funnel it to one spot.

This is a big hole in the ground

that groundwater
just comes out of,

and groundwater
can find its pathways

through all of the
loose boulders in the soil.

And probably shot out
through the soil right here.

PRINCE:
And with the added push

from that providing almost,
like, groundwater pressure

underneath the ground surface,

it's going to destabilize
the type of soil we have here.

NARRATOR:
The saturated soil here
triggered a debris flow

that traveled
about a mile and a half

before it hit Craigtown.

At approximately
25 miles an hour,

that would only take
about three and a half minutes.

Residents would have had little
warning of what was coming.

Across the southeast,
landslides were

one of Hurricane Helene's
biggest killers.

So, what can people do

to protect themselves
from them?

BAUER:
Watch the National Weather
Service warnings,

and if there's any wording
about landslides,

then to not be within
one of those

debris flow pathways or where
the landslides might start.

There are online landslide maps
available.

The North Carolina Geological
Survey has put some out

so that people can go online

and see if they're in
one of these areas.

And if they are,

they can then go
stay somewhere safe.

NARRATOR:
In Valencia,
the flatter terrain,

and more urban environment,

created a different danger.

Here, cars became death traps.

Safety advice says you should
never drive through flood water,

but for some in Valencia,
there was no way to escape it.

(rain pounding)

KAREN LOFTUS:
We were on the motorway.

It was raining very hard.

I've never been in rain

quite like it.

And the traffic
came to a standstill.

Then we started to see that

the water
wasn't going anywhere,

and it started to rise up
from the motorway.

After about five minutes,

the water started to come in
to the bottom of the car.

(water rushing)

The water continued to rise

to the point that
our car engine did cut out.

STEVEN LOFTUS:
Well, literally, the car
is now floating sideways.

(thunder rumbling)

NARRATOR:
The force of water
pressing around the car

makes it impossible
for Karen and her husband

to open the doors from inside.

KAREN LOFTUS:
Oh, god.

NARRATOR:
Luckily, they had lowered
the windows

before the engine cut out.

KAREN LOFTUS:
Within ten minutes,

I was having to
get out of my car window

into what was a muddy,
freezing cold torrent of water.

(water surging)

There were cars
starting to float,

and some people
had been able to get out

and some people
were still in their cars,

obviously very frightened
and scared.

There was a huge
articulated lorry,

and we just thought,
"we'll just try."

And we banged on the door
of this lorry,

and then this wonderful Moroccan
lorry driver drove us out.

♪ ♪

NARRATOR:
They were lucky.

(metallic clang)

Many others were not.

(indistinct radio chatter)

♪ ♪

The floods in Asheville,
Valencia, and Texas

were some of the worst
experienced

in the western world
in recent years.

In North Carolina,
the storm killed 107 people.

In Valencia, 228 died.

In Texas, more than 130.

Casualties like these

are very difficult to prevent,

because flash floods
happen so quickly.

But could there have been
more advanced notice,

and timelier warnings
to evacuate?

In the United States,

much of the necessary
infrastructure

is already in place.

Two days
before Asheville flooded,

the National Weather Service
computer models and forecasters

accurately predicted
that the flood could be

as big as the
Great Flood of 1916.

♪ ♪

But as the rain started
to fall,

they needed to forecast

where flooding would occur,

and when.

PALMER:
During the height of the event,

we had eight people here
overnight,

and then three of those eight

were issuing
flash flood warnings.

NARRATOR:
To help predict floods,

forecasters
in the United States

use a tool developed at

the National Severe Storms
Laboratory in Oklahoma.

The system, called FLASH,

calculates where heavy rain
will most likely cause a flood.

Across the country,

158 radars detect rain,

and tell forecasters

how much is falling.

But to predict a flood,

they need to know
where that water will go

when it hits the ground.

JONATHAN GOURLEY:
What is the condition
of the land surface

that the rainwater
is falling on?

A little bit will be
intercepted by the vegetation.

Some of it gets
infiltrated in the soils.

But in many urban areas

that are developed,

we don't have those soils
as much,

and buildings,
and in general,

they have much less capacity

to absorb some of that
incoming rainfall.

♪ ♪

NARRATOR:
The FLASH system contains a grid

with details of
the different terrain

in every square kilometer
of the country.

GOURLEY:
To run a simulation,

we have over
10 million grid points

across the United States.

At every grid cell,
we know the characteristics

of the land surface in
terms of the vegetation type,

the composition of the soil,
as well as the topography.

NARRATOR:
The system uses this information

to calculate
where the water will flow

and pinpoints areas
at risk of flooding.

The total time FLASH takes,

from detecting rain as it falls

to predicting a flood,

is just minutes.

GOURLEY:
The worst case scenario

would be
about 16 or 17 minutes.

Here on this left screen
is FLASH.

So, this is during
the height of Helene.

FLASH is really helpful
for us to determine

which areas are
more likely to need

a flash flood warning first.

Especially at
the beginning of an event.

Yellow is when

we're starting to
think about

maybe the base
flash flood warning.

The red to the purple
is when we're thinking about

looking at that catastrophic
or flash flood emergency.

NARRATOR:
For Helene,

the National Weather Service's
rainfall and flood forecasts

were pretty accurate.

But predicting a flood
is only half the challenge.

The next step is to make sure
people in danger know about it.

Here, Helene revealed

that the current system
has a critical weakness.

The main method authorities
use to inform people

is to send a wireless emergency
alert to their cell phones.

The system works by using

the country's
network of cell towers.

When there's
a threat of an emergency,

such as a flash flood...
(thunder booming)

...response officials
mark the danger zone on a map.

(beeping)

They then send
an alert message

to the nearby cell towers.

GPS coordinates
of the danger zone

are encoded in the message,

so modern cell phones
in or entering the zone

can trigger the alert,

which contains
a warning message

advising people what to do.

PALMER:
So, when we want to go to
issue a flash flood warning,

we use this system.

So, I can just start
drawing a polygon.

And if I want to send out

a Wireless Emergency Alert,

it has to be considerable
or catastrophic.

Once I'm happy with
everything I've typed in here,

I come down here to
"issue all."

(click)

NARRATOR:
When Helene unleashed its rain,

the team at this National
Weather Service office

issued more than
80 wireless emergency alerts

for flash flooding

in just two days.

But not all
the messages got through.

The problem was the wind.

Hurricane Helene's winds
were incredibly strong.

PALMER:
As these winds were
moving north,

it was a cascading
power failure.

Trees were
going down everywhere.

(crashing)

Power lines were being snapped.

So as the power grid
was falling,

communications
were going down,

just because power to the
cell phone towers was failing.

Once you don't
have the communications,

you can no longer get
the wireless emergency alerts.

NARRATOR:
In North Carolina,

the early forecast
of major flooding,

and the wireless emergency
alerts that did get through,

are thought
to have saved many lives.

(click)

In Texas,
the National Weather Service

also forecast the flooding.

The day before the floods,

it initiated a flood watch.

SHEPHERD:
We know that from a
meteorological perspective,

there were
ample weather warnings

in the days and hours
leading up to the event.

There are questions about
how that information was

channeled into warnings

and risk communication
for the public.

NARRATOR:
Flash floods happen so fast

there is little time
to take action.

Texas Hill Country
was no exception.

At 1:14 A.M.,

the National Weather Service
sent out a warning alert

to cell phones in
the area of Camp Mystic,

a Christian girls' summer camp.

About 45 minutes later,

the flood hit.

At Camp Mystic, at least 27
girls and members of staff died.

In total,
the Texas Hill Country floods

killed more than 130 people.

SHEPHERD:
We know that there were

not the type
of warning systems

that we often would expect

in this type of
complex terrain.

So, I think conversations
going forward

will be centered around

how adequate were

cell phone notifications.

Is there coverage?

Are weather radios needed?

Is some type of siren
or warning system needed?

These are all questions
that certainly should be asked.

NARRATOR:
And not only about Texas.

(crashing)

In Valencia,
there were problems with both

forecasting and warning people.

CLOKE:
We know that when we have

a good flood forecasting
and warning system

and that people know
what to do

when they receive
those warnings,

that lives are saved.

So something went wrong
in this flood.

We know that the flood
forecasting and warning system

didn't work as it
should have done.

NARRATOR:
In Valencia,

weather forecasters
saw the storm coming

well ahead of time,

and on the morning
of the flood,

issued a red alert.

But their forecasts

underestimated
the amount of rain.

They predicted that more
than seven inches of rain

would fall in 24 hours.

The highest actual amount

was 30 inches--

over four times as much.

GOURLEY:
One of the grand challenges

is predicting intense rainfall

that occurs over these really
small spatial scales.

We may have like a broad area

where we're anticipating

some heavy rain,

but what does that mean?

Which city is going to be
impacted?

And when I say heavy rain,

is it going to be a
catastrophic rainfall event

that's going to
annihilate the city,

or is it just going
to be a spring thunderstorm.

It's very difficult
to distinguish those two.

(lightning crashing)

NARRATOR:
When the storm turned out to be

one of the biggest
to ever hit Spain,

the emergency services
were overwhelmed.

(person exclaiming,
objects crashing)

(helicopter blades churning)

Unlike the United States,

Valencia didn't have
a system like FLASH

to automatically predict
where rain would cause floods.

Instead, it relied on
more traditional methods.

Around the Poyo ravine,

rain gauges in the hills

sent back readings of
rainfall every five minutes.

♪ ♪

On the ravine itself,

a sensor situated ten miles
upstream of Paiporta

measured the flow of water.

When the rain increased
in the afternoon,

the flow in the Poyo
started rising rapidly.

At 6:55 P.M.,

it reached 600,000 gallons
per second,

almost as much as
the average flow of water

over Niagara Falls.

Then, the water
swept away the sensor.

♪ ♪

Despite all
the gauges and sensors,

very little
of this information

appears to have reached
the emergency committee meeting

responsible for sending out

the wireless emergency alert.

(click)

(inaudible shouting)

At 8:11 P.M.,

two hours after the Poyo River
first broke its banks,

the cell phones of people
across the province

made an alarm sound

and displayed
a warning message.

(alarm tone)

KLARENBERG:
The first warning I got was

after the streets had flooded.

My mobile started

kind of aggressively buzzing.

Yeah, so, after...
after the fact.

KAREN LOFTUS:
Had we received the alert,

which we did receive,
four hours after the event,

of course we wouldn't have
traveled.

It would've been a folly
to travel.

NARRATOR:
Many of the victims had died

before the alert
was finally sent out.

♪ ♪

In Spain, an investigation
is under way

to find out what went wrong.

But timely warnings alone

aren't necessarily enough.

In North Carolina and Texas,

warning systems
were not always able

to get the messages through
to the people at risk.

♪ ♪

National Weather Service
advice

is to not rely
on just one technology.

If for whatever reason,

whether you don't have
your phones,

or there's not adequate
cell phone service,

we need redundancy,

we need other ways of
communicating the threat.

NOAA weather radio

is connected
directly to NOAA

and National Weather Service
warnings,

and so it will be triggered

irrespective of whether
there's cell phone coverage.

PALMER:
While cell phone towers

and cell phone
communications went down,

NOAA weather radio stayed up.

So, we always recommend
that folks have

a NOAA weather radio.

(person exclaiming,
water surging)

NARRATOR:
The disastrous floods in

North Carolina,
Valencia, and Texas

are a wake-up call to
authorities around the world.

SHEPHERD:
I'm often asked

if this is a harbinger
of things to come.

My response would be,

it's already here.

The atmosphere has warmed,

and the systems are
tapping into

elevated moisture supply,

and we're seeing
extreme rainfall events.

That's not my opinion,

that's what scientific
studies are showing.

NARRATOR:
Across the globe,

governments are taking action,

not only to improve
warning systems,

but also to protect
their citizens.

(rain pounding)

In Japan,

the annual number
of heavy rainstorms

has increased by 40%
over the last 30 years.

It's a serious problem
for the capital city, Tokyo.

(lightning striking)

When heavy rain hits,
parts of the city flood.

To channel water
away from rivers

that were at risk
of overflowing,

in the 1990s,
engineers began building

a massive underground
flood relief system.

Workers excavated

nearly four miles of tunnels
beneath the city.

♪ ♪

Giant subterranean silos

were designed to store
some of the water,

while a vast
cathedral-like reservoir,

60 feet high,
would hold the rest.

Yet, this extraordinary
underground project

is still not big enough to cope
with the increased rainfall

expected from global warming.

Work is now underway
to double its capacity.

♪ ♪

The increased risk of flooding

in many places

is forcing planners
and engineers

to rethink their designs.

CLOKE:
Often, new engineering
structures that go in

or the new plans
that are in place

are all based on the last
disaster that's happened.

But what we
really need to do

is imagine what other types of
flood might happen as well.

So that act of imagination,
the act of thinking of

through those scenarios
of the future,

is really important.

NARRATOR:
In Asheville,

communities
are attempting to rebuild.

It looks like they're
trying to reconstruct it.

We all had to leave.

There were tons and tons of mud
inside the building.

The first floor,
completely wiped out.

I was able to find
another place to live.

Some people left

and just didn't come back.

We actually still have

the window that we
swam out of,

it's just not attached to
the house anymore,

but this is the amount of space
we had to swim out of.

♪ ♪

It's been a struggle
in a lot of ways,

but we've had a lot of like

really skilled workers
working with us,

and a lot of great volunteers.

All over the world
people came to help people here.

The danger of
the flood happening again

is something
we have to take seriously.

We've applied for
the elevation program.

So, eventually, our house is
gonna be basically on stilts.

♪ ♪

NARRATOR:
The flood has brought this
Swannanoa community together.

BURGIN:
We know them all
so much better now.

We just want to stay in the area

and be around each other.

That's why everyone wants
to rebuild.

That's why
everyone wants to stay.

(water rushing)

NARRATOR:
Floods will always happen.

But we have the technologies
to make them less deadly.

CLOKE:
We're in the position now where

we have good weather forecasts,

and we have good flood
forecasting systems.

So we should be able to
provide warnings to people

to make sure that they have
the information they need

before floods hit.

SHEPHERD:
We know that we'll continue to
have extreme rainfall events

that will lead to flooding.

So, as we move forward,

we have to think carefully

about what type of warning
and dissemination systems

are needed--
that's a clear need

in terms of
risk communication

in Texas and in
places around the world.

♪ ♪

NARRATOR:
The science is clear:

with intense rainfall
increasing,

more frequent floods
are already here.

♪ ♪

♪ ♪

♪ ♪

♪ ♪

♪ ♪

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