- [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 and software regularly. Something that's particularly useful is a virtual private network
like today's sponsor, NordVPN. NordVPN hides your IP address and helps protect your online identity. Petr filmed on location in Japan and he used NordVPN to safely
access public wifi hotspots. And I am currently in Australia, and I've been using
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like about NordVPN is that their speeds are blisteringly fast. So it's effortless to just
keep it turned on all the time. High speeds are particularly
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locations to watch content that's only available in those regions. For viewers of this channel, use the link nordvpn.com/veritasium for a huge discount on a two year plan, plus an extra four months for free. So I'll put that link
down in the description. There's a 30 day money back guarantee, so it's entirely risk free. Just go to nordvpn.com/veritasium. So I wanna thank NordVPN
for sponsoring this part 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.
