Rivers and Seas Collide Coast

This is Coast.

In the British Isles, we're familiar with wet weather

blown in from the wild seas.

One benefit of a temperate climate is our wonderful

labyrinth of rivers.

Giant waterways powered by rain, that all run to the coast.

As rivers and seas collide great estuaries emerge.

Making our mark on these colossal watery spaces

has taken centuries of struggle.

That's left a wealth of extraordinary stories

waiting to be discovered along our estuaries.

We're braving three of our greatest,

the Firth of Forth, the Thames and the Mighty Severn.

And we'll visit grand cities, too.

Tessa discovers how the pulling power of estuaries gave

the Victorian's a capital idea.

Turn the Thames into a giant self-flushing loo.

Where rivers surge into sea lochs,

Miranda swims with the fishes.

The Scottish salmon industry spawned a business worth around half a billion pounds.

But how did they crack the secret of farming a wild sea fish?

And Mark's on a rollercoaster ride under...

Ahhh...!

Way!

..and across the deadly tides of the Severn Estuary.

The story of how it was eventually crossed

is one of extraordinary ingenuity,

and also explosive tragedy.

We're here to explore what becomes of the coast

when rivers and seas collide.

I'm starting my estuary odyssey a pebble's throw

from Edinburgh on the Firth of Forth.

The scale of this seaway is staggering,

it's impossible to take the whole thing in.

What I could really do with is something tall to climb up.

So I can get a bird's-eye view.

Only the engineering marvel of the Forth rail bridge

does justice to the sheer spectacle of the estuary.

As we're coming up here

you can see the rivets on this bridge that hold it together.

6.5 million rivets, and every one of them has been painted by hand.

-This is it.

-This is it, Nick. Here we are on top of the Forth Bridge.

Up here, right in the middle of the Firth of Forth

you can get a real sense of the huge scale of this estuary.

I can see the Pentland Hills right over there,

there's the dark volcanic bump of Arthur's Seat

rising above the white buildings of Edinburgh.

Looking west, I can see all the way out to the open sea - the North Sea.

And looking inland, in this direction, there's even more.

Here's the Forth road bridge, arching over the water in front of me,

behind it Rosyth naval base,

and in the far distance I can just make out

Grangemouth power station oozing smoke into the sky.

This estuary is so huge that even from this incredible vantage point,

inland it just fades into invisibility.

The only way of actually getting a true sense of its size

is by looking at a map.

This is the mouth of the estuary

marked by this little island, the Isle of May, here.

In the other direction, 60 miles inland,

the water gets less and less salty, gets fresher and fresher,

until you reach Stirling here, where this estuary is born.

Starting at its birthplace, I'm flying the length of the waterway.

Will the change in wildlife help pinpoint the elusive spot where river becomes sea?

My guide's marine ecologist Stuart Clough.

And as we pass over Stirling,

the river's very beautiful seen from above

it's like a huge coiled rope.

You're in classic lower river territory here, erm,

lower fresh water river.

The place where the tide just starts to have its effect.

And even now the mud banks are starting to appear on the side.

And in those you've got all kinds of worms and shellfish

that live within those sediments, and they become food for birds.

It's a fantastic environment.

Is it possible to identify the point at which this river, the Forth,

ceases to be a river

and begins to be sea?

From a biologist's perspective, it's a continuum -

it changes all the time.

On the one hand it's a no-man's land

and on the other hand it's a diverse and rich place with masses of life.

Life is rich where rivers and seas meet.

And where we flock, so does the wildlife.

As we move into saltwater, the big hitters start to surface -

dolphins, seals,

and even whales have all been spotted here.

We're now over the sunlit seaside, aren't we, Stuart?

It's completely changed.

Absolutely, yeah. We're right out in the outer estuary now.

The freshwater influence is a long way behind us,

the beaches are sandy.

If we were down at sea level now

what kind of birds and so on would we be looking at?

Auks - like razorbills and guillemots and puffins.

You've got fulmar, you'll have kittiwakes, you'll have gannets -

real marine species, that you'd never find in the freshwater parts of the estuary.

At the edge of the estuary,

we get a box office view of the gannets of Bass Rock.

This swirling mass makes the most of food from the sea

and shelter from the land.

Where are we now?

We're just adjacent to the Isle of May -

very much the outer limit of the estuary.

We've flown the whole way from the freshwater of a river

to the saltwater of the open sea.

Over an extraordinary diversity of habitats

both human and natural - estuaries are worlds of their own.

Twenty million of us,

one third of the UK's population, live on an estuary.

Their flat shorelines are perfect for building,

so each of these coastal highways comes with its own gatekeeper.

Great cities surge up where mighty rivers plunge into the sea.

It's fitting that the country's capital crowns the most

hard-working waterway of all - the titanic Thames.

For centuries, Londoners have swallowed up the benefits

the estuary brings in.

The sea brought riches from abroad,

and the river supplies two-thirds of the city's drinking water.

But the Victorians found a new job for old Father Thames -

doing their dirty work.

Tessa's getting to grips with a grubby tale of triumph and tragedy.

The power of the tide gave an eminent Victorian engineer

an extraordinary idea -

turn the Thames into a giant self-flushing loo.

The tidal range of the river is huge - around eight metres.

This powerful ebb and flow, gave rise to an ingenious sewer plan.

Release excrement as the tide turns,

and let the outgoing flow flush London's waste way out to sea.

The city's relationship with the sea

spawned a sewer system that was the envy of the world.

Opened in 1865 by the Prince of Wales, this subterranean labyrinth,

elevated its mastermind Joseph Bazalgette

to become a hero of the Victorian age.

Bazalgette's master plan demanded a warren of waste pipes,

a network over 1,000 miles long

to carry the capital's raw sewage out to the Thames.

It took six years to build, constructed so well

it still forms the backbone of London's sewer complex.

Over 300 million bricks placed so precisely

they form water-tight tunnels.

You know how to treat a girl, don't you, Rob?

I do, I take them only to the best spots.

Impressive as this labyrinth is, it's only the means to a watery end.

The city's sewage still needed sweeping out to sea,

so it was piped towards the coast to pass the problem onto the tide.

The muck flowed downstream to arrive at the final

triumph of the entire system, the pumping station at Crossness.

This is staggering! It's like some sort of ballroom.

It's a real indication of the level of pride

they took in their work, the beauty is just breathtaking.

And these huge pumps are even named after members of the royal family.

The pumping stations were the final stage of Balzalgette's grand plan -

they pushed the sewage up into huge reservoirs,

to be stored until the tide began to turn.

When the tide started to ebb, they released the sewage into the Thames just there.

They relied on the surge of seawater to whisk Londoners' muck out of sight and out of mind.

This was Joseph Bazalgette's big tidal flush -

his plan to turn the Thames into one gigantic toilet bowl was complete.

Bazalgette was heralded as the city's saviour.

But is there a skeleton lurking in London's water closet?

Life may have been rosy for those in central London,

but it didn't smell so sweet for those living downstream.

Like a real-life toilet, the Thames is full of U-bends.

The waste wasn't clearing as fast as Bazalgette had imagined,

and the consequences turned out to be devastating.

It's the 3rd of September, 1878,

the pleasure steamer The Princess Alice

is on its way back to London crammed with passengers.

The day-trippers had been enjoying fresh air at the mouth of the estuary,

but returning to the city, near the sewage outlet,

the pleasure steamer was struck by disaster.

It collides with another boat.

Hundreds are flung into the river, many will be drowned.

But it's even worse than that.

Revealing the gruesome fate of those floundering in the estuary

is local historian Joz Joslin.

So the vessel's upended, and hundreds of people are in the water.

Yes. And lots of them are women and children,

and they're screaming,

and unfortunately it's not water that they're in,

they're actually in sewage, so there was no oxygen.

A lot of them died because there was no air to breathe.

So they're either being suffocated or drowning.

Or poisoned.

How revolting. And the majority died?

Yes, the majority died.

They said that every street in the east end of London had lost somebody,

because it was their Sunday school outings that were onboard the vessel.

The pleasure boat sank close to the sewage works,

and the timing could not have been worse.

The Beckton sewage outlet pipe carrying all of North London's waste

had just discharged its stinking load into the river.

Over 600 people choked to death in a toxic soup of human filth.

After the tragedy,

Bazalgette's sewage system came under the spotlight.

Members of the local historical society read the words of their forefathers.

"There had been an accumulation of black, greasy, filth along the shore.

"The filth settles on the steps as the tide goes down."

"The river in hot weather is very bad.

"In some places it smells so bad you cannot stand it."

A commission of inquiry delivered a damning indictment,

concluding "it is neither necessary or justifiable

"to discharge sewage in its crude state into any parts of the Thames".

The Pall Mall Gazette took Bazalgette to task, stating

"the natural man in him, puts off the evil day of having to admit failure".

Luckily for Bazalgette, the muck didn't stick,

but London did pull the plug on his big tidal flush.

In 1887, a new system started.

Now the solid human waste was pumped into vessels like this.

The excrement was shipped out to the open sea and dumped.

They were known locally as Bovril boats, amongst other things.

We used to call them... Well, never mind what we used to call them!

-What did you call them?

-No, I'm not saying.

They used to come and moor - they had moorings for them -

and they would take the residue of it.

Cos all the fluids were taken off,

so it was almost solid the stuff

that they took out - human detritus -

so that it wasn't into the river.

Sewage carrying ships didn't just do the dirty work of London -

they were once a common sight on our estuaries,

cleaning up Glasgow, Belfast and other coastal cities.

London's Bovril boats were finally pensioned off in 1998.

Balzalgette's tunnels still bring raw sewage here to the Crossness Works,

but now the solid matter's burnt off to make electricity.

The liquid sewage is treated -

it goes from this...

to this.

And the cleaned-up fluid?

It still gets the big tidal heave-ho,

and is discharged into the Thames, where the river and the sea collide.

Tide and traffic on the Thames flow two ways.

In deep waters at the estuary mouth, ships from around the world

come to unburden themselves on the docks at Sheerness.

But back in the 19th century a group of foreign stowaways

snuck off a ship and never left.

They set-up a secret community within the harbour walls.

This is the des-res of Britain's only colony...of scorpions.

But not the monster kind.

European yellowtail scorpions arrived here from Italy

on a masonry ship some 200 years ago.

Now the offspring of those Italian scorpions

have found a British admirer.

Hi, I'm Bex, and I'm a scorpo-holic.

I've been fascinated by scorpions since I was a teenager

and been hooked ever since.

I'm here to see Britain's only colony of scorpions,

but I've got to wait for the sun to go down.

It's properly dark now, so I'm going to see if I can find some.

I'm using a UV torch, cos scorpions glow under ultraviolet light,

and I think I've just spotted one.

Definitely an adult, probably out looking for something to eat.

It is pretty cool though, isn't it? Having scorpions in the UK.

They are a member of the spider family.

They have eight legs, not six.

They eat woodlice.

They are ambush predators, so they'll just sit and wait,

and then something will come past,

and they'll just jump out and grab it,

and subdue it with their claws rather than stinging it.

They don't generally use their stings, these ones.

Very happy that we found some.

This is a tiny, little, incy one.

He's so cute. But with tininess comes speed!

It's been a great night, we've seen loads of scorpions,

but I think I'll put this one back before...

it legs it.

Bye, little fella.

I think I'll leg it now, too.

We're on a journey to discover what becomes of the coast

when rivers and seas collide.

I'm exploring the Firth of Forth on Scotland's east coast,

where canny folk profited from their prime location -

ideal for seaborne business.

And with rich seams of coal for power, the population boomed.

With more mouths to feed, getting enough fresh food was tricky,

so they looked to the sea to preserve their provisions.

You'll find the evidence at St Monans.

Here, food processing created a curious landscape.

The shore is lined with lots and lots

of very strange grass-covered humps,

and what seems to be a ruined building over there

and up there a stone windmill.

The ruins of industrial activity reveal themselves the more you look.

This land was remodelled by people

making the most of one bounty from the sea

that isn't in short supply - salt.

Before refrigeration, salt was a valuable commodity -

preserving herring landed along the east coast.

Scottish salt was also exported to England,

turning a tasty profit for the salt works.

Those strange hummocks come in sets - each set of hummocks

is the ruins of a pan house.

Inside each of those pan houses

there was an iron pan about 6 metres by 3 metres,

with coal fires beneath it.

Sea water was pumped - probably using this windmill - from the sea

in pipes up to each pan house.

Once it had been boiled off in the pans, you had salt.

A rare film brings the enterprise back to life.

Salt works once flourished along Scotland's east coast.

The last operation at Prestonpans didn't close its doors until 1974.

It was the abundance of coal along this estuary

that made it a good site for boiling up sea water.

A sample of sea water stirs up a mystery,

right at the heart of this forgotten industry.

Out there is the sea full of salt.

And I can certainly taste it.

This little brook running into the sea,

doesn't taste salty, at all.

So, why is freshwater fresh

and why is sea water salty?

It's one of those brilliantly simple infuriating questions

that kids ask - why is the sea salty?

I'm enlisting the help of a grown-up.

Simon Boxall's from the National Oceanography Centre -

he should be able to work it out.

We've all swum in the sea, we know it doesn't taste like freshwater,

Simon, but why is it salty?

You have to go right back to the beginning-stage of the earth,

back several billion years.

And if you go back that far,

the earth was a completely different place -

it was full of volcanic eruptions, there was lots of steam around,

but, also, there was a lot of sodium in the rocks,

and that sodium was being hit by the hydrochloric acid

that was given off by these volcanic vents.

And if we take these two very harmful chemicals...

On the one hand, you've got the element of sodium - very reactive -

and on the other hand, you've got chlorine - very dangerous and very reactive.

You put the two together and you create...sodium chloride.

Which is the sort of thing you sprinkle on your chips.

Certainly isn't harmful, at all.

So you've got this hydrochloric acid pouring out of the volcanic vents,

meeting the sodium hydroxide which is lying around in the rocks on the sea bed

creating this stuff called sodium chloride, which is salt.

These ancient chemical reactions gave birth to our salty seas.

We can create those sort of primordial days.

We can actually take some hydrochloric acid -

the sort of stuff that came out of the vents of the volcanoes.

We've got some dilute sodium hydroxide -

which represents the stuff that was in the rocks.

And between us, if you want to, we can make salt.

We can take these two quite nasty chemicals

and produce something that's really more vital to life in many ways.

This is hydrochloric acid - it's very dilute.

And we're going to pop it into this vessel here.

OK, we're then going to add our sodium hydroxide.

At the moment, basically, the sodium and the chlorine are combining

and giving off heat - can you feel that?

It's warm! Really warm, wow!

We've effectively neutralised that acid, that sodium hydroxide,

and what we have in there now is basically water...and salt.

We've compressed billions of years of the earth's evolution

to make a miniature ocean.

Boiling off our DIY sea water leaves the prize ingredient.

So, here it is, our very own home-made salt.

White crystals that washed wealth in from the sea

to help feed an estuary.

The salty shoreline satisfies our appetite in surprising ways.

On sea marshes in Wales, sheep graze on grass made sturdy by regular salt baths.

This distinctly coastal vegetation gives the lamb a unique flavour.

But to explore the biggest effect our shoreline has on our waistline,

head north to Scotland.

A natural wonder plays out where rivers meet the sea,

like here, at Loch Carron.

Miranda is immersed in a secret life of the salmon.

I'm in the thick of Britain's favourite fish dish.

Our insatiable appetite for salmon

has spawned an industry in the UK

worth around half a billion pounds a year.

But farming these fish is a lot more difficult than you think.

Salmon are challenging to rear in farms because of their extraordinary life cycle.

They're born in freshwater rivers, then migrate to the sea.

To survive in saltwater, a salmon's body goes through radical changes.

That transformation is very tricky to manage in captivity.

To see why, I'm seeking out the wild fish.

Take a winter walk along a salmon river and there might be

delicate little eggs lurking deep in the watery gravel.

So why do salmon out at sea,

bother to come all this way to lay their eggs?

I'm meeting wild salmon expert Bob Kindness.

What a place for the salmon to come back to

and lay their eggs in the gravel here.

They have to be spawned in fresh water

because the eggs would not survive

other than in fresh water - they wouldn't survive in the sea.

So the salmon has to make that journey back from the feeding grounds,

usually back to its home river,

back to the area where it itself was born.

Once hatched, these freshwater fish face a big problem -

they need to get out to sea, where there's more food,

but their bodies are incapable of dealing with saltwater.

A jelly sweet experiment gives us a taste of their challenge.

This tiny jelly bear has a dense sugary body.

Soak him in water overnight and he looks like this...

all swollen up.

This plump bear is our happily hydrated fresh water salmon.

But if I add salt to create sea water,

the fresh water is sucked out of the sweet, and it shrivels.

Death by dehydration is the challenge a salmon faces

going from the river down into the sea.

To avoid dehydration salmon develop two defences,

their scales toughen to stop fresh water leaching out of their body.

Also their kidneys and gills adapt,

so they can cope with life in saltwater.

To see the scale of the transformation

we've anaesthetized two of the fish.

Gosh, you can really see the difference now, side-by-side.

The speckled brown tiddler is a freshwater salmon.

The silver beauty below, just a year older, is ready for the sea.

A salmon can turn around and go back

if its body isn't adapting to life in the seawater,

and some fish take years to make it out to sea.

That's OK in the wild, but how could fish farmers know

when it was safe to move their salmon to saltwater pens?

In the early 1970s, years of experiments finally paid off.

I've got a patent here applied for in 1971.

This is a scientific recipe for the crucial step in the growing of captive salmon.

The "fish equation" says that when the weight of the salmon, W,

times by 100,

divided by its length, L, cubed,

is equal to 0.8 -

it's ready for sea water.

Armed with the power to control nature,

salmon farming in Scotland rapidly became big business.

As the scale of production rose, the price of the fish started to plummet.

'The market shows every sign of expanding as the supply increases,

'and now, salmon is not just to be found in the exclusive fishmongers,

'but can be bought vacuum-packed in the supermarkets.'

Salmon stopped being a treat saved for special occasions,

now it was as cheap as chicken.

So how did fish farmers manage to mimic nature

with their production line process?

The salmon start life in freshwater tanks.

When they're big enough to satisfy the "fish equation"

they're sucked up and piped into trucks.

Trucks take them to boats, boats ferry them to saltwater enclosures

anchored at the sea end of the estuary.

The salmon are kept in these pens for up to 22 months,

but they are quite used to human company,

there are divers who regularly check on the fish

and the nets that hold them in.

The salmon are constantly watched.

CCTV operators check they're eating

and make sure they're not being eaten by seals.

Farmed fish carry more lice than wild ones,

so they're monitored for sickness too.

Our appetite for salmon makes all this effort profitable.

If our demand for these fish is such that we need to farm them,

then salmon pens will become an even more common site

where rivers and seas collide.

We're on a journey around three of our mightiest estuaries.

Where tideways bring bounty from the sea deep inland.

So imagine the potential of connecting two great estuaries

to form one super-highway.

That unifying vision excited the imagination of merchants

in the Firth of Forth.

In 1768, businessmen began to build their own waterway -

an epic canal joining the Forth to another great estuary, the Clyde.

What those early entrepreneurs couldn't know

was that their scheme to connect two estuaries would,

one day lead to a revolution in fibre-optic communication.

The story starts with an extraordinary discovery

on the waterway some 180 years ago.

Let's go back to that fateful day in 1834.

Young engineer John Scott Russell - this is him later in life -

was on the towpath of this canal.

Russell was watching a barge,

when suddenly its bow wave detached from the front of the boat

and sped off on its own.

It was a solitary hump of travelling water.

The engineer would become obsessed

trying to understand the solitary wave through mathematics.

But on first sighting, he had to act fast.

Russell jumped on his horse and gave chase to the wave.

Normally, waves peter out or break-up,

but this mysterious wave retained its shape and sped onwards.

After a chase of about two miles

the solitary wave still showed no signs of petering out.

Eventually it escaped Russell's pursuit altogether.

Russell would have recognised that this peculiar wave was

similar to the Severn bore.

That solitary wave is produced

when the tide collides with the River Severn,

sending a surge of seawater up the narrow funnel

of the Bristol Channel.

Not unlike the narrow channel of the canal.

So Russell experimented with canal-shaped water tanks.

Mathematician Chris Eilbeck thinks he can recreate Russell's

solitary wave.

-Keep our fingers crossed.

-I'm dying to see one!

You can see the wave there.

Look at that!

-You got it first time.

-Yeah.

Just a hump of water on its own.

You can feel the force of it when it hits the plate.

It's gone all the way back again.

They're amazingly stable it'll keep going for miles.

The canal's dimensions were perfect for keeping the wave stable,

deep enough for the wave not to steepen too sharply and break,

but shallow enough that it didn't flatten out.

Amazingly, these waves remain self-contained and intact,

even when they collide.

Three, two, one - go!

After the collision, they just re-appear on the other side.

Chris, are you saying, the waves didn't just bounce off each other

but went through each other?

Yes, that's correct.

They go through each other and re-appear on the other side.

-Undiminished?

-Yes, and mathematically exact.

Unbelievable.

These remarkable packets of travelling energy have been christened solitons.

Now is the time to scale our experiment up.

Let's try and re-create the full wonder of the soliton wave

on this 800ft long straight section of Scottish canal.

But it won't be easy.

The wave John Scott Russell chased back in 1834,

was created when a horse-drawn boat came to a sudden stop,

releasing its bow wave.

We're trying a more modern approach

a motor boat loaded with people to provide ballast, and to bear witness.

Rachel, you're at the front of the boat here with some top hats.

You're clearly expecting something to happen. What are the hats for?

We've got the hats as a tribute to John Scott Russell

who discovered the first soliton wave,

we're all going to be throwing them into the air with joy

when we re-create it, is the plan.

I'm very impressed by your optimism.

Solitons are rare, not easily produced.

The boat must gain enough speed to form a sizeable bow wave

then come to a sudden stop.

Hopefully, that'll release a soliton.

If it does, I'm going to chase it, just like Russell,

only I'll be racing it on foot, not on horseback.

I've run up mountains, and I've run a few marathons,

but I've never run against a wave before,

I'm not sure I'm going to have to.

Oh, my god!

Well, I beat it over a few hundred metres, but it's still going.

And I'm not going to be able to keep going as long as that wave.

-ALL:

-Hurray!

John Scott Russell designed special boats to ride his wave

at high speed along canals.

But sadly most people preferred a new form of transport - the railway.

Russell waved goodbye to his bright idea.

True to its nature though the soliton wasn't finished.

Today the solitary waves first seen on this canal

aren't only found in water - they travel as light.

Scientists have created soliton waves from tiny pulses of light.

Fired down the next generation of fibre optic cables,

the self-sustaining waves can carry communications

for thousands of miles -

travelling on endlessly between continents,

just as John Scott Russell's wave ran on out of sight of me.

-So his day has come.

-His day has finally come.

And if you were to meet him now, what would you say to him?

I would tell him that his idea has really come of age,

and everybody talks about him, everybody talks about the soliton.

It's a really big thing in science, so he'd be delighted, I think.

-A satisfied scientist.

-Yes.

On a journey around our estuaries we've arrived at the mighty Severn.

Here the Atlantic Ocean surges in to collide

with Britain's longest river.

The Severn estuary has a staggering rise of tidal water,

some 15 metres, in all.

The sea swirls in strange patterns here.

And its currents would wash the bodies of wrecked sailors

to the same spot again and again -

the village of Brean.

Those unknown souls delivered to the doorstep of Brean needed burial -

a sorrowful ritual remembered in song.

Folk singer and storyteller June Tabor recalls the Brean Lament.

The first thing that strikes you about being here,

it's timeless.

It could be any time between now and 200 years ago.

And the old timbers of this ship

going nowhere ever again.

The men who were on this ship -

did they survive?

# The waters they washed them ashore

# Ashore

# And they never will sail the seas no more

# We led them along by the churchyard wall

# And all in a row we buried them all. #

The song The Brean Lament describes

what happened quite commonly along this coast

when bodies were washed up.

You have to give them a burial, but not in the main churchyard -

in a separate sailors' graveyard.

It was quite strongly believed along this stretch of coast

that the sea might decide to reclaim their bodies as its own.

They didn't want the dead of the village being taken at the same time.

And, possibly as a way of appeasing the sea,

the sailors' boots were buried below the tide line,

so the sea would have something to take.

# We led them along by the churchyard wall

# Where all in a row we buried them all

# But their boots we buried below the tide

# On Severnside. #

The Severn estuary used to pose a fearsome challenge

on any journey between England and Wales.

The two countries were divided by this massive tear in our coastline.

Avoiding it meant a diversion deep inland.

Even so, only hardy travellers would brave the deadly waters.

Today, a concrete solution spans this vast channel.

But conquering the Severn was a bold venture fraught with peril,

as Mark is about to discover.

Dashing over the estuary from Wales to England commuters take

the elegant crossings their lives depend on for granted.

But imagine a world before this bridge was possible.

A world without steel cables,

without reinforced concrete,

when the sea reigned supreme.

That was a challenge faced by the Victorians to cross the River Severn.

The formidable collision of river and sea facing the early engineers

can still be experienced.

It's one of the most dangerous seaways in the world,

and I'm just a little bit excited.

The Severn Area Rescue Association is going to pit me against the ebb tide.

Cast off!

The power of the tide here is just extraordinary!

As the tide goes out it's like a maelstrom.

The waters were an immense challenge,

but by the 1840s crossing the river by boat was old hat.

An irresistible new force was spreading across Britain -

the railways.

Come hell or high water,

estuaries weren't going to stand in the way of progress.

The great Victorian engineer Isambard Kingdom Brunel

is a hero of mine - he'd already managed to cross the Avon gorge

with a mighty suspension bridge.

When his railway came to Bristol, he wanted to cross into South Wales,

and planned an even bigger suspension bridge.

Here are the preliminary sketches.

The biggest problem was the sheer scale of the span

that Brunel required - over 1,000 feet.

He left a little note in his notebook

which says "Is 1,100ft practicable?"

Brunel's bridge was never built,

but if taking trains over the water defeated the best brain of the age,

how about going underneath?

A tunnel - was that the answer?

Digging deep to create a railway under the water -

this was very bold, big thinking.

This is one of the original drawings of the tunnel from around 1887,

and you can see how the track comes down underneath the deepest part

of the Bristol channel here in The Shoots,

and gradually up to the Welsh side.

So what we've got here is around seven miles of railway track.

That passage under the estuary is now a vital link

between England and Wales.

Carrying over 250 trains a day.

Passengers are oblivious to a catastrophe that nearly sank

the tunnel before the first train ran,

and is a problem that still lurks below.

So here we go, I've been granted access to a shadowy water world,

few get to see.

It's great - we're just coming into the cutting,

the portal's ahead, and we're about to go under the sea.

Ah! Way!

Isn't that fantastic!

We're heading for the deepest point in the tunnel.

Just 50ft above us millions of gallons of water

are swirling around - the River Severn and the sea are in full flow.

Keeping the water out here is hard enough,

but can you imagine if there was a flood down here?

With an estuary hanging over their heads,

engineers knew there'd be seepage of seawater,

but it was freshwater that nearly drowned the project.

Nobody expected this - a raging torrent!

They'd broken through to an underground spring.

In October, 1879, water began to pour into the tunnel.

The workers fled for their lives.

The disaster struck when a shaft dug on the Welsh coast

cut into an underground river deep below the surface.

For four years the engineers made desperate attempts

to block the freshwater spring, but every effort proved futile.

And it's been flooding in at this alarming rate ever since.

If they couldn't stop the spring water, they'd have to live with it.

The only solution were pumps, massive ones like this

that pump the water out as fast as it comes in, right up to the surface.

Leighton Jenkins helps keep the tracks dry today.

So what would happen if the pumps actually failed?

Every second counts,

as soon as the pumps stop we'd have to inform the control

within 10 minutes to shut the tunnel itself,

and within 20 minutes we've got water coming up through the tracks,

so every second absolutely counts.

But have they ever failed?

No, not as far as I know, no. Not while I'm on a shift anyway.

The railways had proved irresistible,

with rival Victorian companies vying for routes,

by the time it was finished the tunnel already had a competitor.

In 1879, trains had started to roll over the estuary,

but the bridge's sturdy uprights -

always an obstacle to shipping - would ultimately prove its downfall.

Do you see, that's a tower where the railway bridge once crossed

the Severn estuary.

I've got a photograph that shows the stanchions

marching across the river - now totally destroyed.

The raging waters where river and sea smash together

would deliver a fatal blow to the rail bridge.

In October, 1960, the Arkendale carrying oil,

and the Wastdale laden with petrol

were heading for combustible collision.

The Arkendale was carried in by the surging tide.

That powerful current would drive it into the Wastdale

on a foggy night at Sharpness Docks.

As Alan Hayward knows.

They were coming up river intending to come into the docks here,

but they were accidentally swept past.

And then they collided and became in effect stuck together.

Disabled ships in thick fog,

carrying 600 tonnes of inflammable cargo

at the mercy of a swirling sea, propelled them to disaster.

They were desperate to separate from each other,

fighting by steering in different directions

but it just didn't work,

and they only had about four minutes before

they would reach the railway bridge.

The rail bridge across the Severn loomed out of the fog,

a collision with the ships carrying oil and petrol was now inevitable.

A lot of sparks would have been created

which ignited the petrol in one of the vessels.

The fuel of course spilt out over the river,

so the whole river became a mass of flame.

First mate Percy Simmonds was aboard one of the tankers.

His son Chris was 13 at the time.

I try to imagine that night and what he was going through,

and it must have been just terrible with the flames and everything.

I'm just sure he was determined to make it across this river somehow

and make it back to us.

Daylight and a low tide revealed wrecks of the fuel tankers,

smouldering on a sandbank.

Soon the first body was found.

They identified the body there,

and they you know let Mum know that, yeah, it was definitely Perce.

Chris's father Percy died along with four others

on that terrible evening.

The damaged bridge was too expensive to repair, it was demolished.

But each day, when the tide recedes, scars of tragedy are revealed.

Out there of course are two hulks buried now in the sands,

that have been washed over by countless tides.

But they're still there. They're there as monuments.

They're here as a reminder to all of us.

It's immensely humbling

to be next to such a vast body

of brooding water,

even on a calm day like this.

One can feel the power where rivers and sea collide.

Britain's mighty estuaries

have pushed engineers to breaking point and beyond.

In Scotland, there's a salty graveyard of overreaching ambition -

the Tay.

This immense river disgorges more water into the sea

than the Thames and Severn combined.

In the shadow of today's rail crossing, is a spine of stumps.

The footings of the first Tay Bridge.

On a stormy night in December, 1879, it collapsed -

60 passengers died.

And so did the myth of infallible Victorian engineers.

The blame was put on designer Thomas Bouch, lax maintenance,

and poor iron work.

Hard lessons learned just down the coast on the Firth of Forth.

Following the disaster at the Tay Bridge

that designer's plans for this crossing were thrown out.

The Forth rail bridge was beefed up

into a massive cantilevered skeleton built, not of iron, but steel.

This bridge is really a memorial, a masterpiece in steel,

to the poor souls who lost their lives in the Tay Bridge tragedy.

Surging waters urge us on to fresh endeavours.

And we're not alone in finding creature comforts

around the fringes of our great seaways.

The tide brings in the bounty that makes our estuaries brim with vitality.

Safe havens that offer boundless prospects.

Where rivers collide with the sea our coast comes alive,

and opportunity awaits.

Subtitles by Red Bee Media Ltd