- [Derek] This is the world's largest rainfall simulator,
located in Tsukuba, Japan.
Now, I know that it just looks like a warehouse with a lot
of sprinklers, but this building is incredibly important.
The science conducted here keeps tens of millions
of people safe, and it's only becoming more and more vital.
We were given exclusive access to tour the facility,
and they even let us experience what it's like
to stand under the most intense rainfall
ever recorded.
- [Petr] This is insane.
Nope, nope, nope, nope, nope, nope, nope, nope.
- Japan is a land of natural disasters.
Most people will think of earthquakes, tsunamis,
and volcanoes, but it also experiences massive typhoons,
which are like hurricanes
that pick up energy from the Pacific Ocean
and dump meters of rain on the islands.
Yakushima Island near the southern part of Japan is one
of the wettest places on Earth,
receiving up to 10 meters of rain every year.
For comparison, the Amazon rainforest gets only
about three meters of rain per year.
That's three times less than Yakushima Island.
Most of the time, rain isn't a problem.
The water evaporates or seeps into the soil
and is then absorbed by plants
or enters the underground aquifer systems.
But too much rain in a short period,
like after a typhoon can lead to serious problems.
Problems that scientists working
at NIED use the rainfall simulator to study
and hopefully prevent.
The giant warehouse is equipped
with 550 nozzles attached to the roof.
The scientists can control the intensity
of the rain from 15 millimeters per hour
to 300 millimeters per hour.
The most rainfall ever measured in one hour occurred
on the 22nd of June, 1947, when 305 millimeters
of rain fell on the town of Holt, Missouri.
So this warehouse can simulate the most intense rainfall
anyone has ever experienced.
In a way that makes it the rainiest place on Earth.
And honestly, I'm having a good time here
in sunny Australia.
So I sent Veritasium producer Petr in my stead.
- They're gonna turn on the rain really soon.
So I'm running out to grab my rain jacket, I'm ready.
Wow, this is so much rain.
All around me, there is exactly 300 millimeters
of rain falling every hour.
We've only turned it on about five minutes ago,
and there's just so much rain.
There's already puddles, this is genuinely, absolutely wild.
I don't think I've ever experienced anything
like this before.
- The nozzles at the simulator contain four holes
of varying diameters, so they can produce raindrops
of different sizes, and the size of the raindrop affects
how fast it falls.
You've likely experienced the kind of rain
with the big droplets that pelt your face
or walked through a drizzle
where small drops hover like mist.
Because the smaller the raindrop, the slower it falls.
There are two forces that act on a raindrop,
gravity and air resistance.
And the larger raindrops have a higher weight
to surface area ratio,
and so they have a higher terminal velocity,
so they will be falling faster.
A raindrop that's one millimeter in diameter falls
at only about two meters per second,
while a three millimeter raindrop falls
at six meters per second.
Air resistance is also why raindrops aren't shaped
like cartoon raindrops.
They are closer to spherical,
but a bit flatter on the bottom
where they encounter oncoming air.
If a raindrop gets too big,
it flattens out, caves in in the middle
and briefly resembles a little parachute
before it breaks up into smaller droplets.
Flooding is something that Japan takes very seriously
and for good reason.
In July, 2018, there were floods all over the country
due to the rainfall from Typhoon Prapiroon.
Some regions of the country received nearly two meters
of rain in just 10 days,
and the resulting floods were enough
that more than 8 million people had
to be evacuated from their homes.
Over 200 people died,
and the damage to property from the flooding was more
than 1 trillion yen, nearly $10 billion.
Tokyo, the capital city of Japan,
is very vulnerable to flooding.
There are more than a hundred rivers crisscrossing the city
of nearly 40 million people.
To prevent the rivers from overflowing, there are pipes
and tunnels under the city leading
to an enormous water storage tank.
In October, 2019,
typhoon Hagibis dumped over 200 millimeters of rain
onto the city in under 48 hours.
The underground system diverted 12 million cubic meters
of water and prevented an estimated $1.7 billion in damage.
But flooding isn't the only problem.
Japan is a very mountainous country with many towns
and villages situated in valleys.
So combined with the heavy rainfall,
this creates the perfect conditions for another hazard.
(man screaming)
The researchers at NIED have identified over 700,000 places
where landslides are a significant threat to homes.
But landslides are so complicated.
There are so many factors
that affect if a landslide will occur,
how large it could be, how fast it will move,
and how much damage it will do.
Not just the slope angle or the amount of new rain,
but the type of soil, the minerals present,
and the vegetation growing on top of the slope.
This is footage from a landslide
that occurred in Norway in June of 2020.
There's barely any slope angle here,
but the whole area sits on a layer of quick clay,
an incredibly unstable clay layer, which when exposed
to intense rainfall loses its structural integrity,
becoming a liquid.
In the case of this landslide, there were no casualties,
though several homes were swept out to sea.
The complexity of the physics
of landslides is why the work done
at the large scale rainfall simulator is so important.
The best way to minimize landslide damage is prevention,
and it's the same for cybersecurity.
Creating strong passwords, using anti-malware protection,
backing up your data and updating your operating system
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Petr filmed on location in Japan
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of the video, and now back
to the world's largest rainfall simulator.
- All right, I'm gonna try it without an umbrella.
I have a very nice, very nice jacket.
I'm not sure how it's gonna hold up, but we're gonna try.
(Petr screaming)
Oh my God, this is insane.
Nope, nope, nope, nope, nope, nope, nope, nope.
My jacket is waterproof, so I was fine.
But my jeans are absolutely soaked through.
- [Derek] To study landslides,
the whole warehouse has a secret, it moves.
- [Peter] The rainfall simulator can be
in one of five positions.
- Velocity is about one meter per one minute.
- One meter per minute.
Wow, it moves on these railway tracks.
Every time you move it, you need
to grab this gigantic wrench and undo these bolts.
This is the pipe that connects
all the water up to the sprinklers.
There's a number of these kind
of openings in the pipe along the line.
So when you move it,
you just connect it to a different part.
So this is the landslide testing facility?
- Yes, this is a 30 degree slope, you can see this.
This is a shallow landslide.
Now you can put the soil here, about one meter.
- [Petr] You would put all the sand and all the soil,
and then you'd bring the building over.
You'd move it over here.
And then you'd start raining on it.
- Yes.
- [Dr. Sakai] 20 degrees this slope, is 20 degrees.
- Right, so you have a 30, a 40, and a 20,
20, 30, 40. - Yes.
- [Derek] So what causes landslides?
Well, a slope will slide when the force
of gravity pulling it down becomes greater
than the force of friction holding it up.
- There's a misnomer out there
that water makes the materials slipperier, so it rains
and it soaks into the ground and it makes it slipperier.
That's not true, water's actually an anti lubricant
for many materials, including quartz,
which is our most common mineral in soils and on earth.
- [Derek] So water doesn't make soil slipperier,
but soil is porous.
There are grains and there are pores between those grains.
And as it rains, the water seeps into the soil,
the pores become filled with water,
and as it keeps raining,
the water pressure in the pores increases,
which decreases the friction between the grains.
For a slope that was already at risk of sliding,
this decrease in friction is what ultimately leads
to the slope breaking apart and sliding downhill.
- One thing that our mathematical models are not
very well developed at yet is determining
is it going to slide slowly
or is it going to slide or maybe flow rapidly?
That's a tough one, that's, so experimentation's a great way
of getting at that.
And you know, the scaler effects are huge.
So many people have a little model in their lab,
you know, a small scale flume,
but now you're dealing with materials, you know,
you're starting to get off scale.
A large scale stimulation device like they have in Japan
is really important.
- [Derek] So how can you prevent landslides?
Well, there are a few things you can do,
like using steel beams and mesh to anchor the slope.
Or you can dig up the top layer of the soil
to decrease the slope angle.
And when it's expected that a slope will slide
after too much rain, engineers drill holes into the slope
and place pipes to drain the water out.
Another solution is to create catchment zones
and diversion channels, big holes in the ground
that will catch or divert the landslide
before it hits a residential area.
Trees are incredibly effective at preventing landslides.
Not only do their roots provide great anchors,
they draw the water up from the soil to be evaporated away,
which effectively drains the soil
and decreases the water level.
(chainsaw operating)
Where steep slopes have been deforested
for the timber industry, landslides have become more common.
Over the last few decades,
there has been a tenfold increase in landslides
in the forests of British Columbia.
- Humans are really good at causing landslides
and we're shaping the land a heck
of a lot more quickly than geologic processes do.
So when we excavate over here or add more weight
or more landslide or more material over here,
and that triggers landslides very, very regularly.
Study in Seattle from Seattle, Washington a few decades ago,
showed that I think more than 85%
had at least partial human trigger, if you will.
- [Derek] While the focus of the work is primarily
on landslide study and prevention,
the simulator is also used to test
how drones fly in rainy and windy conditions.
It's also used to test self-driving cars.
The data is used to improve the hardware
and software of how the cars detect various objects.
The two main ways that self-driving cars detect other cars,
traffic lights and pedestrians is with cameras
or LIDAR sensors.
In both cases, rain can decrease the visibility
and accuracy of these sensors.
So testing them in a perfectly repeatable environment
helps engineers develop solutions
to these weather challenges.
- In Japan, the rainfall condition is changing.
And now in this century,
the rainfall condition is so many heavy rainfall
in so much short time.
- [Derek] As the world heats up due to humanity's addiction
to fossil fuels,
extreme weather events are becoming more common.
Compared to 30 years ago, the number of rain events in Japan
with an intensity of 50 millimeters per hour
have become 40% more common.
Rainfall at double
that intensity has become 70% more common.
Climate change will increase the rate of flooding
and the occurrence of landslides in the future,
which is why the work done
at the world's largest rainfall simulator is becoming more
and more important.
But I hope that humanity focuses its efforts
on addressing the root causes of climate change
and not just on the mitigation of its negative impacts.