The Riddle of the Tides 1 Coast

Coast is home.

We're back to explore the most endlessly fascinating shoreline

in the world - our own!

The quest to discover surprising, secret stories

from around the British Isles continues.

This is Coast.

For as long as we've gazed

from our island shores over the seas,

we've struggled to solve

the mystery of our tides.

Twice a day, like the chest of a sleeping giant,

the sea heaves up and down,

re-drawing the shape of our island home.

The effect of the two tides varies around the coast.

In the Bristol Channel,

we have one of the greatest surges of water in the world.

It creates the remarkable Severn Bore.

Over in East Anglia in the south-east corner of England,

the tides are relatively weak.

Down here on the south coast, the opposite is true,

because the tides get forced up and down the English Channel

around a promontory called Portland Bill.

You get huge standing waves there. It's really scary.

Why does the sea behave so differently around our coast?

We're here to explore The Riddle Of The Tides.

My tidal odyssey takes me to the North West,

and a city that sits by the sea.

In Liverpool, I'm on the trail of a forgotten genius,

who made a machine to calculate the tides anywhere, any time.

Look at that, lots of brass, cast iron, steel axles,

absolutely stunning, isn't it?

But my journey begins on tidal rapids. The Menai Strait...

..a narrow ribbon of wild water.

Mariners have always been at the mercy of the tides.

Trying to master those turbulent waters

was a great voyage of discovery.

I'm setting sail on this 19th-century-style schooner...

Shall we put it up, Scott?

Yes, go for it.

..to see how salty seadogs began to tackle the riddle of the tides.

Good work-out, isn't it?

It's all hands on deck as we rush to set sail

before the tide turns against us.

Proper old ropes that takes the skin off your hands.

A tricky passage awaits along some of Britain's most treacherous waters.

Navigating the Menai Strait isn't for the faint-hearted.

We're racing to make it through The Swellies -

the tidal surge around the island of Anglesey.

Any misjudgement of the tides here could wreck the boat on jagged rocks.

It's a real worry for the skipper, Scott Metcalfe.

Quite a few people have come to grief.

There's a lot of rocks around here, there's rocks on this side,

and certainly there's rocks on the other side,

the Cribbin Rock, which is quite a nasty one.

If you get the timing wrong

you can get swept onto

one of the rocks, basically.

And this is a, you know, a historic vessel.

How important were tides back in the days before motors?

Very, very important.

I don't know if you can see those two white posts,

we should have those basically in line.

So, you line the two white posts up,

steer for the posts, and that gets you through the deeper channel.

People talk about The Swellies

as if it's some kind of white-knuckle fairground ride.

What are The Swellies?

That's just this stretch of water between the two bridges, basically.

It is the fastest flowing part. This is the most treacherous part.

Scott makes sure to navigate The Swellies at slack water -

the brief period when the tidal flow is weakest.

For sailors, reading the mood of the sea is a matter of life and death.

Since the earliest times,

mariners have known that the moon drives the tides,

but how, exactly?

And why are there two tides a day?

Sailing with me is Tom Rippeth from Bangor University.

Tom, can you explain to me

why it is that we get two tides every 24 hours?

We've got a simple model, here, Nick.

If you'd just like to hold that.

Yes.

This is obviously the Earth,

and then here we have the moon

and, erm, the Earth and moon,

basically, orbit around each other in space,

and there's two forces acting, really.

There's one force which is the moon's gravitational pull,

and another force which is the centrifugal force,

which is pulling the water away from the planet.

The Earth's motion and the moon's gravity make the tides.

To see how, imagine our planet completely covered in water.

One bulge in the sea is caused by the moon's pull.

There's an opposite bulge because water gets pushed out

by centrifugal force, as the Earth whizzes through space.

The Earth also rotates, once every 24 hours.

Measure the sea level at a single point

and it rises as the Earth spins, and then falls again,

and sea level rises again 12 hours later, so two high tides a day.

But our world isn't completely submerged,

the shape of the coastline and cliffs on the sea bed,

like the Continental Shelf,

disrupt the flow of water, changing the height of our tides.

Our tides go up and down at the edge of the Continental Shelf,

and that generates tidal waves.

So, we're not talking about a gradual rising

and falling of water every, what, six hours, roughly?

Absolutely not. We're talking about waves

which will travel down one coastline and travel up another coastline,

so, for instance, down the east coast of England,

we'll see big changes in the height of the tide,

but also in the timing of the tide,

so you might have low water in the north,

and you might have high water in the south.

So, our tides aren't simple.

They travel in massive waves,

which makes it hard to predict the sea-level.

Tom's wave tank shows how the tide behaves differently,

depending where you are.

Here we've just put three examples on the Irish Sea.

We've got Liverpool, here, which has very large tides,

and if you move elsewhere in the Irish sea, we actually see

places where it's high water at Liverpool

and it can be low water elsewhere.

So, you can have high water at different times

-in different parts of the coast?

-That's right.

-It's a very complex system, isn't it?

-Absolutely, very complex.

Tides are further complicated by our craggy shoreline,

which makes predicting them very tricky.

But later I'll discover a remarkable machine,

created to crack the puzzle.

All around our coast, businesses run to the rhythm of the sea,

especially the Port of Liverpool.

The mouth of the Mersey yawns wide open into the Irish Sea.

As the tide rushes in, the estuary swallows a vast deluge of water.

The flood brings in seafood for the wildlife of the marshes.

The tide also carries in cargo ships - big ones.

They do a dangerous dance over sandbanks

that can only be cleared at high water.

It's a race against the tide twice a day.

No wonder Liverpool has always kept a close eye on the tides.

They've been measuring the rise and the fall of the sea here

for over 250 years. It's the longest tidal record in the UK.

Sailors watched the water so closely

to try and work out what it's going to do next.

Ray, as a Mersey skipper, do you carry tide tables on your boat?

I do, yes, it's here right now, it's like a Bible.

We have one of them all the time.

-This is your Bible?

-It certainly is.

Time and tide wait for no man.

But mariners did have to wait an awfully long time

to get truly accurate tide tables.

The riddle of the tides turns out to be

much, much harder to crack than you'd think.

There's more to predicting tides than the pull of the moon.

You've got to add in the gravity of the sun,

account for multiple elliptical orbits,

the tilt of the Earth. The complexity goes on.

What about the depth of the sea, the shape of the coast?

Over centuries, the best brains solved pieces of the puzzle,

but before computers, tidal maths was too complex

to be worked out in your head.

So, calculating machines had to be invented.

In the 1940s, all that effort to solve the riddle of the tides

finally reached its high water mark here in Liverpool

with the construction of a mechanical brain.

The cogs and wheels of tide-predicting machines

used to whirr away inside Bidston Observatory,

on a hill overlooking the mighty Mersey.

This site was once the nerve centre for global tide tables.

Most of the British Empire ports

relied on the calculations done at Bidston.

But now the machines that crunched the numbers

are a bit crunched themselves.

Deep in storage at National Museums Liverpool,

the tidal prediction machines are in bits.

Now, for the first time in years, one of the mechanical brains

is about to be re-assembled.

Wow, look at that.

Lots of brass, cast iron, steel axles,

absolutely stunning, isn't it?

-At the time, this was state of the art.

-It was indeed, yes.

When these wheels rotated, they could forecast the future,

the future of the sea.

But how?

While the original machine is carefully pieced back together,

I'm heading to the museum on Liverpool's quayside,

where Alan Bowden has something to show me.

So this is a model of a tide prediction machine.

It's absolutely beautiful, but what are the main principles

driving the computations, the predictions that it's making?

It's actually quite a complex set of mathematical equations

which depend on a number of variables,

and on this little model we've only selected four variables.

For instance, we've got the impact of the moon,

which is the principal component on the earth's tides,

we've got the impact of the sun,

and then we have two other variables here, for instance,

we have the eccentricity of the moon's orbit,

and then on this one here we have the effect of the sun -

it's higher in summer, lower in winter.

Adjustments must also be made to take account of local variations,

like the shape of the coast.

So this wire is the processor, this is the thing that amalgamates

the readings from different variables and converts them to a line...

And converts them to a line which gives us high tide

and the low tide and the points in between.

The full-scale machine had 42 variables

and took one and a half days

to run a year of tide predictions for one port.

Liverpool became the world centre for tidal prediction

thanks to one man - Arthur Doodson.

He devoted his life to improving

and perfecting tidal prediction machines at Bidston Observatory.

But Arthur also needed workers to operate them -

people he called computers.

So you worked in the basement?

We did. One of the machines was down here.

Arthur Doodson's daughter-in-law

worked on the wheels of tidal fortune here for 44 years.

Valerie Doodson retired from Bidston, but now she's back.

-Wow. Is this it?

-This is where it all happened for the years

that we operated the Doodson-Lege tide predicting machine.

It was situated in this room facing this wall,

but with a space behind it,

cos we needed to get at the back to set it up.

Well, that's an example of setting up the machine.

One person set it up and another person checked the information.

And who are these people?

These are the tidal computers

in the early part of the 1960s, and that's me, but don't tell anyone.

Oh, wow! Wonderful.

So what's this card here, Valerie?

This is the setting card for Penang.

But that's in Malaya?

That's correct, yes. For 1965,

and it's predicting the high and low water.

The cards are very neatly filled in, aren't they?

Very important.

One test we had when we came for interview was a handwriting test.

If your handwriting didn't meet the requirement,

you didn't get the job.

I have subsequently been called a perfectionist,

because mistakes were not tolerated.

During its heyday, Bidston prepared tide tables

for ports across the British Empire.

Their work was crucial during the Second World War.

The Atlantic Wall has been penetrated.

Indeed, the computers even predicted low tide for the D-Day landings,

where avoiding submerged Nazi sea defences was vital to success.

Now we have a solid foothold on Fortress Europa,

men and material are poured onto the newly-won beachheads

with every favourable tide.

By the late 1960s, new electronic computers had taken over.

The role of the mechanical machines and their operators

has largely been forgotten.

But now, after years of hibernation,

the machine that predicted tides in the Second World War is reborn.

This is absolutely wonderful.

It's a little bit more exciting than looking at a modern circuit board.

Looking back from an age in which calculations are conducted

invisibly from within modern computer software,

this incredible piece of mechanical hardware is a reminder

that maths is beautiful, it's elegant,

that it decodes universal mysteries.

Without maths and without this very ingenious machine,

we couldn't have solved the riddle of the tides.

Ingenuity feeds the industry of Liverpool.

The docks were built to trap precious seawater behind their gates,

because at low tide, the water rushes away from the city.

Nearby, that leaves Antony Gormley's Iron Men

gazing wistfully after the retreating seas.

The Isle of Anglesey sits snugly next to mainland Wales.

Between them lies the Menai Strait,

a straitjacket for the surging tide.

With nowhere else to go,

the water must speed up to make it through the rocky channel.

Fast-flowing water floods the strait with food

that makes this the ideal location for fattening up mussels.

Sea farmers collect their mussels in specially designed boats,

which do a merry dance to harvest their crop.

Whatever their craft,

all around the Anglesey coast, sailors respect the raging tide.

Even the bigger boats seek shelter.

They hide behind sturdy sea walls.

But I'm not hiding.

For me, the final riddle of the tides is how to tame them.

I'm about to take on the great surge of the Atlantic tide

as it squeezes around the ferocious rocks and reefs just off Anglesey.

And I'm going to be in a boat not much bigger than a matchstick.

Nigel Dennis was one of the first men to kayak right around Britain.

He knows the waters here are amongst the most challenging we have.

The tide creates powerful surges in the sea.

The water races on, carrying kayaks with it for fun.

These tidal races are a test of both skill and stomach.

Now it's my turn.

I'm a beginner, Nigel.

OK, this already looks moderately serious to me.

You're going to be stretched a little bit today.

So how much paddling have we done?

Well, I've done quite a lot of what I call canoeing

-on inland waters and rivers.

-Kayaking, kayaking, this is.

This is something very different, isn't it?

This is kayaking, this is for the ocean.

And have you done anything in tides, moving water?

I've done a little bit, Nigel, a little bit.

But I can tell this is way beyond anything I've experienced before.

Right, OK. So the tide's going to be pushing us towards the rocks,

towards the race, and we're going to drop down through the water.

-OK, I'll give you a hand with your boat.

-OK.

So, Nigel, in this little moment of calm, well, it's not really calm,

all these things are relative for me,

but can you just tell me what tides mean to a kayaker, a sea kayaker?

Well, it's really important that kayakers understand what it means.

You can go around the corner on a calm day and end up in a tide race,

and people won't have the skill, or the power,

to get out of the flowing water,

so they'll actually be sucked straight through the race.

So you need skills, power, but also a deep knowledge of how tides work?

I mean, we call it seamanship, really.

Some people have a natural understanding

and other people never learn.

THEY LAUGH

And just when I'm thinking I've got the hang of it,

the tide trips me up.

OK. Just give us your boat, just hop back in...

Just plonk yourself back in.

Good! Well done.

How did you get to the flat water and then capsize?

HE LAUGHS

Lack of concentration.

The rocky outcrops don't just produce swirling waters.

They also create a curious feeling of claustrophobia,

which adds to my anxiety.

Caught between two emotions, fear and exhilaration.

Are we going into the tide or with it, are we going with the tide?

Yes, the first tiny bit of tide, you can just see it up ahead.

You call it a tiny bit of tide but I can see white horses...

The first heart in mouth moment.

A tide race off one of the most dangerous coasts in Britain.

It's kind of exciting. A real thrill.

But I'm sweating buckets trying not to turn upside down.

These waves are so big that in the troughs I can see nothing but sea.

We're working with the tide, not fighting against it,

and I can really feel its full force pushing me onwards.

Just keep paddling, you're doing really well.

Nice and relaxed, that's good!

My battle against the tide was a one-off.

I'm just happy to have made it through in one piece.

But all around our coast, every minute of every day,

the tides rule the rhythm of people's lives.

I've just discovered how tricky tides can be.

After capsizing and an awful lot of paddling,

I'm back on the beach taking in one of nature's great free shows,

the ebb and flow of this vast body of water,

whose restless motion is driven by the heavens.

It's awe-inspiring.

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