The Assassin (Sha ren zhe Tang Zhan)

In this episode...

This is the largest water transfer project in the world!

...an engineering megaproject

that's sending water across China

to where it's desperately needed.

I think it's really amazing that all this water

slowly goes under this huge yellow river.

And the groundbreaking innovations from the past...

Wow. That is a long way up.

It's quite hard unless you know what you're doing,

and I'd imagine things can go wrong very easily.

...that make the impossible possible.

captions paid for by discovery communications

Beijing, China, is one of the world's largest cities,

home to over 21 million people.

But beneath the surface, there's a problem.

This giant city is seriously lacking in drinking water.

Beijing is actually

one of the most water-scarce cities on the planet.

The groundwater is so depleted,

it's actually causing subsidence.

In some areas of the city,

the earth is actually starting to collapse.

Rainfall in the city has been decreasing since the 1950s,

and the problem is only going to get worse.

The south of China has a much higher rainfall than the north,

so could it be possible to transport water

across the country from the south to the north?

Moving enormous quantities of water

across one of the world's biggest countries

seems impossible.

But engineers have come up with something truly spectacular.

The south-to-north water diversion project

is the largest and most ambitious in the world.

Once complete, the entire network will be

over 2,700 miles long,

more than the distance between New York and L.A.

Three routes... eastern, western, and central...

Will connect rivers from the south of China to the north

and transfer over 11 trillion gallons of water each year.

Of the three routes, the eastern has been completed,

and the western has yet to be started,

but the central is the most challenging.

On the central route,

the water must flow over 870 miles

entirely by gravity, with no pumps.

At the start, the Danjiangkou reservoir

will store over 7 trillion gallons of water.

The 5.5-mile-long Shahe aqueduct

will transport the water at 100,000 gallons per second.

It will then travel under the yellow river

and arrive 15 days later in Beijing.

China's water diversion project is so massive,

it takes days to drive from one end to another.

I'm following a central route from Danjiangkou to Beijing.

New York-based architect Wendy Fok

is traveling from the south to the north

to see how this seemingly impossible project

is being built.

It's such a huge challenge.

How do you move water

thousands of kilometers across a whole country?

How do you cross mountains, rivers, valleys,

and all these types of terrain and keep water flowing downhill?

How do you manage the flow of water over such large distances?

Wendy's journey along the central route

starts in the south

at one of the largest manmade lakes in China,

the Danjiangkou reservoir.

It's beautiful here.

The water here will travel over 850 miles north to Beijing.

Every drop of this water in this reservoir

is going to be heading north,

so in order to contain this much water,

you need a very big dam.

The original dam has been here since the 1950s,

but it's not big enough for this project.

When this dam became part

of the south-north water diversion project,

more water had to be contained.

So to do this, they had to raise the dam by 15 meters.

Raising a dam is not easy.

Old concrete and new concrete are not easy to join.

And the dam's location raises the stakes even higher.

There's approximately 700,000 people that live here,

so it's vital for the engineers

to make sure that a dam does not break.

To find a solution to raise the dam safely,

engineers had to look to the past.

I'm really excited to see this.

This place is part of engineering history.

Engineer Dan Dickrell is in

the Sierra Nevada mountains in California

to check out a construction

that could help inspire the engineers in China.

Wow!

This is the O'Shaughnessy dam.

It's 430 feet tall and 900 feet long.

When it was completed,

it was the second-biggest dam in the world.

Wow!

What an amazing view.

But the dam wasn't always this high.

The original dam, built in 1923,

created the Hetch Hetchy reservoir,

which provided water to the residents of San Francisco.

But within a year, the project hit a snag.

More water was required,

which needed the reservoir to become bigger.

The only way to do that was to raise the dam.

Wow. This is actually quite impressive.

I'm lucky to have access to this space.

Only people that maintain this dam are allowed in this place.

The problem with raising a dam is,

you have to put new concrete on top of old concrete.

Trying to get it to bind together,

there's a problem, because the new concrete

will shrink as it cures, as the water leaves that mixture.

That shrinkage can cause stress

in between the old concrete and the new concrete,

creating a poor bond.

A second issue... a larger dam allows more water to be stored,

but the pressure of that water behind this dam,

trying to force its way out, toppling the dam...

Those forces are tremendous.

At the time, no dam had ever been raised 85 feet,

but water engineer Michael O'Shaughnessy

was determined to build a revolutionary dam

that could achieve the impossible.

The original dam he built was 345 feet tall.

And that was big, but he was a smart guy

and he realized it might need to be bigger in the future.

His design needed to be adaptable.

So, O'Shaughnessy came up with a pioneering design adaptation

to his original dam that allowed it to be raised in the future.

O'Shaughnessy included a series of five-foot steps on the dam.

What this did is it increased the surface area,

and I'm able to bond

between the old concrete and the new concrete

to be as strong as possible.

Building a series of giant steps on the original dam

dramatically increased the surface area,

helping the new concrete to stick.

Engineers took O'Shaughnessy's trick of increasing adhesion

one step further

by scoring and notching the surface of the concrete,

allowing for a tighter bond

between the new and the old material.

The result is an incredible feat of engineering.

The overall reservoir capacity increased by 75 percent,

to an overall volume of over 530 billion liters of water.

More than 80 years after the raising of the dam,

it still provides water to the 2.5 million citizens

of the San Francisco area.

The Danjiangkou dam in south China

is 12 times longer than the O'Shaughnessy dam,

but engineers have still been able to raise it

by almost 50 feet.

This is one of the longest dams I've ever seen.

This dam is huge, and you could really see

the new and the old dam being joined together.

It is so big that the whole thing

actually feels like it's wrapping all around us.

It's amazing.

And with the sun setting, it's beautiful.

Joining the old and new concrete together

was the dam engineers' biggest challenge.

And like the O'Shaughnessy dam in California,

the secret was to increase the bonding area.

First, the existing concrete is blasted

to make thousands of grooves,

which, like the steps on the dam in California,

increase the size of the surface.

Then, thousands of steel rods are drilled into the dam

to help anchor the new concrete as it's poured on top.

It took four years for the dam

to reach its new height of over 577 feet.

But increasing the volume of water in the reservoir

by more than 3 trillion gallons creates a new challenge.

With too much water pressure,

the bottom of the dam would erode away

and the dam itself would tip over.

So to solve the high water pressure

and the base of the dam from eroding,

engineers had to install a huge grout curtain

on the base of the dam.

A grout curtain prevents the weight of water

from seeping underneath the dam's base.

Waterproof grout is injected beneath the dam,

forming a giant underground wall, or curtain, 213 feet deep

that holds the soil under the dam in place.

The engineers succeeded in raising the dam

and filling the reservoir.

But this is just the beginning.

The water now has an 870-mile-long journey north

through some of the country's most challenging terrain.

So digging a very long, very deep tunnel

in this very soft ground is a huge engineering challenge.

China's south-to-north water diversion project

will use three routes