Nicholas Crane explores the coast of south Wales, where the second highest tidal range in the world has had a huge impact on lives for thousands of years.
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It's a part of the coastline where one natural phenomenon dominates pretty much everything,
from defence to building regs.
The Welsh coastline has one of the most extreme tidal ranges in the world.
The extreme tides mean that twice a day, every day,
there's a vertical rise in sea level of between 12 and 14 metres.
That's an incredible 40 feet in old money, pretty much as high as that house over there.
And from time to time,
those tides create an awesome, almost unbelievable spectacle.
I'm standing on the banks of the River Severn.
It's the longest river in Britain,
much of it marking a border between England and Wales.
And although it's late at night,
there are people like me dotted along the river banks, anxiously watching, anxiously waiting to see
an incredible natural spectacle.
It's the Severn Bore. I've waited much of my life to see the Severn Bore.
It's one of nature's miracles, rather like the Northern Lights or a double rainbow.
If you ask the locals, they'll tell you the Bore has a mind of her own.
I'm sure there's something moving up there. The river's lost its shine.
All I can hear my own heart pumping like mad.
Breaking waves! There's breaking waves on the far side. Here it is, this is fantastic!
Must be about 200 metres away, and I can see the breaking wave already.
CHILDREN CALL: Look! What's that?
This placid river is suddenly being ripped up
and all the laws of nature have been thrown into reverse, broken,
because there's a great standing wave just MOVING...unstoppably
the wrong way. It's coming upstream.
-And it looks like something living!
And it's breaking on this side, too,
so we've got waves on both sides, this huge wall of surf that's
breaking out right close to me here and going out 15 metres, fantastic!
There are more waves following it.
My god, that huge wave!
Run quick, because the river goes through some bends further upstream,
if I'm quick I'll be able to cut the Bore off and meet it further up.
The Bore first appears some five miles inland from the sea
as the Severn suddenly bottlenecks.
Then it takes the best part of an hour
to follow the river's twists and turns to Minsterworth, where I am.
And in another five minutes or so, it should reach Minsterworth church.
CYCLE BRAKES SQUEAK
Made it. What's the time?
I reckon I've got about a minute at most
until the Bore comes creaming round that corner down there.
SOUND OF RUSHING WATER
Now I've seen it once,
I'm beginning to understand why thousands of years ago, the people who lived along this riverbank
looked at this wave with awe and a lot of incomprehension.
In fact, in ancient Welsh, this was just known as "the roaring wave".
From Minsterworth, the Bore continues relentlessly,
ripping at the river banks all the way to Gloucester.
But make no mistake about it,
what we're looking at isn't caused by the tide - the Bore IS the tide.
It's the raging sea 20 miles inland.
The entire river has been forced backwards,
and those waves taste of salt!
But if I'm to find out exactly what causes the Severn Bore,
at least part of the answer must lie 240,000 miles up there in the night sky,
because what I do know is that it's not Britannia that rules the waves,
it's the Moon.
Just like two kids spinning in a playground,
the Earth and Moon are in a constant pirouette around each other
and also around the Sun.
As it rotates around our planet, the Moon exerts a gravitational pull
on the greatest single mobile mass on Earth, the sea,
which is physically moved backwards and forwards to give us high and low tides.
Although much further away from Earth, the Sun also exerts its own massive gravitational pull.
So when, on occasions, the Sun, Moon and Earth align in a straight line,
the pull of the Sun and Moon together almost doubles the effect.
And it's no surprise that what we get on Earth is exceptionally high or "spring" tides -
exactly when we get the biggest bores on the River Severn.
So the Sun and Moon cause the tides,
but the Severn Estuary has the second highest tides not just in the UK, but in the entire world.
Second only to The Bay of Fundy in Canada. Why?
Well, the morning after the night before, the tide has gone into full reverse.
In search of answers, I've met up with oceanographer Chris Wooldridge.
Tide's now pouring back out to the Atlantic
and the buoy's being tilted over by the force of the water, isn't it?
It's beginning to go like a train, you know.
How does the shape of the Bristol Channel, the Severn Estuary, convert into this tidal wave further inland?
Because down here it looks fairly placid.
Well, the very shape itself - an ever-narrowing funnel -
plus the length of the basin, the length of the estuary,
and the gradient and shape of the seabed.
These are the unique factors that combine to trigger the Severn Bore.
This wall of water brought in from the Atlantic,
it's got to go somewhere.
It's been dragged across the ocean by the pull of the Moon, and the landmass wants to stop it.
But that tidal wave is going to run on, forced into an ever-narrowing funnel and forced up at speed.
So you've got the whole Atlantic Ocean squeezing up this funnel and then rushing upstream.
Uh-oh! WATER ROARS
Unlike the Northern Lights or a double rainbow, the Bore adheres to a strict timetable
and it's no surprise that crowds flock to see it, nor that many feel the urge to ride it. To tame it.
The beauty is that with two tides a day, there's a twice-daily rodeo.
What is amazing is that for hundreds, if not thousands of years,
people have apparently struggled to LIVE with these tides rather than running a mile.
Professor Martin Bell has spent 20 years examining evidence of this struggle
preserved deep in the thick tidal mud of the Gwent Levels, between the Severn Bridge and Cardiff.
Martin has promised to show me something incredible preserved in this mud. My first impressions?
Well, the mud-flats of the Gwent Levels don't have quite the instant appeal of the Valley of Kings
or even Salisbury Plain!
This grey, claylike material is laid down in layers, like layers of... cake icing, eh, except more skiddy?
Yes, and these sediments, banded sediments, incredibly, preserve human footprints.
-So you can see...
-Is that what these are?!
-Yes, these are actually Mesolithic human footprints.
-That is astonishing!
Made in the soft mud, but now semi-consolidated as the whole things become compressed.
As you see, they're quite small.
They are obviously mostly children of seven, eight, nine, probably.
So roughly when do these footprints date from?
7,000 years ago, at a time of very rapid sea-level rise, when the whole estuary was really inundated.
Previous to that, there'd been a bay
stretching between Pembrokeshire and Devon, but suddenly this funnel-shaped estuary opened up.
It's at that time that the huge tidal range, 14.8 metres, would've developed.
-That was the beginning of the Severn Bore?
-These children could've seen the first tides that caused the Severn Bore?
SOUNDTRACK: CHILDREN LAUGH...
Subtitles by Red Bee Media Ltd - 2006
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