0:00:09 > 0:00:11This is Coast.
0:00:39 > 0:00:42In the British Isles, we're familiar with wet weather
0:00:42 > 0:00:45blown in from the wild seas.
0:00:48 > 0:00:51One benefit of a temperate climate is our wonderful
0:00:51 > 0:00:52labyrinth of rivers.
0:00:53 > 0:01:00Giant waterways powered by rain, that all run to the coast.
0:01:01 > 0:01:04As rivers and seas collide great estuaries emerge.
0:01:12 > 0:01:15Making our mark on these colossal watery spaces
0:01:15 > 0:01:18has taken centuries of struggle.
0:01:18 > 0:01:21That's left a wealth of extraordinary stories
0:01:21 > 0:01:24waiting to be discovered along our estuaries.
0:01:26 > 0:01:28We're braving three of our greatest,
0:01:28 > 0:01:33the Firth of Forth, the Thames and the Mighty Severn.
0:01:35 > 0:01:38And we'll visit grand cities, too.
0:01:38 > 0:01:41Tessa discovers how the pulling power of estuaries gave
0:01:41 > 0:01:44the Victorian's a capital idea.
0:01:44 > 0:01:49Turn the Thames into a giant self-flushing loo.
0:01:51 > 0:01:54Where rivers surge into sea lochs,
0:01:54 > 0:01:56Miranda swims with the fishes.
0:01:58 > 0:02:03The Scottish salmon industry spawned a business worth around half a billion pounds.
0:02:03 > 0:02:07But how did they crack the secret of farming a wild sea fish?
0:02:09 > 0:02:12And Mark's on a rollercoaster ride under...
0:02:12 > 0:02:14Ahhh...!
0:02:14 > 0:02:15Way!
0:02:15 > 0:02:19..and across the deadly tides of the Severn Estuary.
0:02:20 > 0:02:23The story of how it was eventually crossed
0:02:23 > 0:02:27is one of extraordinary ingenuity,
0:02:27 > 0:02:30and also explosive tragedy.
0:02:31 > 0:02:34We're here to explore what becomes of the coast
0:02:34 > 0:02:37when rivers and seas collide.
0:02:42 > 0:02:46I'm starting my estuary odyssey a pebble's throw
0:02:46 > 0:02:49from Edinburgh on the Firth of Forth.
0:02:51 > 0:02:53The scale of this seaway is staggering,
0:02:53 > 0:02:57it's impossible to take the whole thing in.
0:02:59 > 0:03:03What I could really do with is something tall to climb up.
0:03:03 > 0:03:05So I can get a bird's-eye view.
0:03:11 > 0:03:14Only the engineering marvel of the Forth rail bridge
0:03:14 > 0:03:18does justice to the sheer spectacle of the estuary.
0:03:19 > 0:03:21As we're coming up here
0:03:21 > 0:03:24you can see the rivets on this bridge that hold it together.
0:03:24 > 0:03:276.5 million rivets, and every one of them has been painted by hand.
0:03:34 > 0:03:38- This is it.- This is it, Nick. Here we are on top of the Forth Bridge.
0:03:50 > 0:03:52Up here, right in the middle of the Firth of Forth
0:03:52 > 0:03:58you can get a real sense of the huge scale of this estuary.
0:03:58 > 0:04:00I can see the Pentland Hills right over there,
0:04:00 > 0:04:04there's the dark volcanic bump of Arthur's Seat
0:04:04 > 0:04:08rising above the white buildings of Edinburgh.
0:04:08 > 0:04:12Looking west, I can see all the way out to the open sea - the North Sea.
0:04:14 > 0:04:17And looking inland, in this direction, there's even more.
0:04:20 > 0:04:25Here's the Forth road bridge, arching over the water in front of me,
0:04:25 > 0:04:27behind it Rosyth naval base,
0:04:27 > 0:04:30and in the far distance I can just make out
0:04:30 > 0:04:33Grangemouth power station oozing smoke into the sky.
0:04:37 > 0:04:42This estuary is so huge that even from this incredible vantage point,
0:04:42 > 0:04:46inland it just fades into invisibility.
0:04:46 > 0:04:48The only way of actually getting a true sense of its size
0:04:48 > 0:04:51is by looking at a map.
0:04:55 > 0:04:56This is the mouth of the estuary
0:04:56 > 0:05:00marked by this little island, the Isle of May, here.
0:05:00 > 0:05:04In the other direction, 60 miles inland,
0:05:04 > 0:05:08the water gets less and less salty, gets fresher and fresher,
0:05:08 > 0:05:12until you reach Stirling here, where this estuary is born.
0:05:16 > 0:05:21Starting at its birthplace, I'm flying the length of the waterway.
0:05:22 > 0:05:29Will the change in wildlife help pinpoint the elusive spot where river becomes sea?
0:05:29 > 0:05:32My guide's marine ecologist Stuart Clough.
0:05:34 > 0:05:36And as we pass over Stirling,
0:05:36 > 0:05:39the river's very beautiful seen from above
0:05:39 > 0:05:40it's like a huge coiled rope.
0:05:40 > 0:05:43You're in classic lower river territory here, erm,
0:05:43 > 0:05:45lower fresh water river.
0:05:45 > 0:05:48The place where the tide just starts to have its effect.
0:05:48 > 0:05:52And even now the mud banks are starting to appear on the side.
0:05:52 > 0:05:55And in those you've got all kinds of worms and shellfish
0:05:55 > 0:05:59that live within those sediments, and they become food for birds.
0:05:59 > 0:06:01It's a fantastic environment.
0:06:01 > 0:06:04Is it possible to identify the point at which this river, the Forth,
0:06:04 > 0:06:07ceases to be a river
0:06:07 > 0:06:09and begins to be sea?
0:06:09 > 0:06:11From a biologist's perspective, it's a continuum -
0:06:11 > 0:06:13it changes all the time.
0:06:13 > 0:06:15On the one hand it's a no-man's land
0:06:15 > 0:06:19and on the other hand it's a diverse and rich place with masses of life.
0:06:23 > 0:06:26Life is rich where rivers and seas meet.
0:06:29 > 0:06:33And where we flock, so does the wildlife.
0:06:36 > 0:06:42As we move into saltwater, the big hitters start to surface -
0:06:42 > 0:06:44dolphins, seals,
0:06:44 > 0:06:47and even whales have all been spotted here.
0:06:50 > 0:06:53We're now over the sunlit seaside, aren't we, Stuart?
0:06:53 > 0:06:55It's completely changed.
0:06:55 > 0:06:59Absolutely, yeah. We're right out in the outer estuary now.
0:06:59 > 0:07:01The freshwater influence is a long way behind us,
0:07:01 > 0:07:02the beaches are sandy.
0:07:02 > 0:07:04If we were down at sea level now
0:07:04 > 0:07:07what kind of birds and so on would we be looking at?
0:07:07 > 0:07:10Auks - like razorbills and guillemots and puffins.
0:07:10 > 0:07:13You've got fulmar, you'll have kittiwakes, you'll have gannets -
0:07:13 > 0:07:18real marine species, that you'd never find in the freshwater parts of the estuary.
0:07:21 > 0:07:24At the edge of the estuary,
0:07:24 > 0:07:27we get a box office view of the gannets of Bass Rock.
0:07:29 > 0:07:33This swirling mass makes the most of food from the sea
0:07:33 > 0:07:35and shelter from the land.
0:07:36 > 0:07:38Where are we now?
0:07:38 > 0:07:40We're just adjacent to the Isle of May -
0:07:40 > 0:07:43very much the outer limit of the estuary.
0:07:43 > 0:07:48We've flown the whole way from the freshwater of a river
0:07:48 > 0:07:51to the saltwater of the open sea.
0:07:51 > 0:07:54Over an extraordinary diversity of habitats
0:07:54 > 0:07:59both human and natural - estuaries are worlds of their own.
0:08:06 > 0:08:08Twenty million of us,
0:08:08 > 0:08:13one third of the UK's population, live on an estuary.
0:08:16 > 0:08:20Their flat shorelines are perfect for building,
0:08:20 > 0:08:24so each of these coastal highways comes with its own gatekeeper.
0:08:29 > 0:08:35Great cities surge up where mighty rivers plunge into the sea.
0:08:35 > 0:08:39It's fitting that the country's capital crowns the most
0:08:39 > 0:08:44hard-working waterway of all - the titanic Thames.
0:08:47 > 0:08:50For centuries, Londoners have swallowed up the benefits
0:08:50 > 0:08:53the estuary brings in.
0:08:53 > 0:08:56The sea brought riches from abroad,
0:08:56 > 0:09:00and the river supplies two-thirds of the city's drinking water.
0:09:02 > 0:09:08But the Victorians found a new job for old Father Thames -
0:09:08 > 0:09:11doing their dirty work.
0:09:13 > 0:09:19Tessa's getting to grips with a grubby tale of triumph and tragedy.
0:09:20 > 0:09:25The power of the tide gave an eminent Victorian engineer
0:09:25 > 0:09:27an extraordinary idea -
0:09:27 > 0:09:32turn the Thames into a giant self-flushing loo.
0:09:34 > 0:09:39The tidal range of the river is huge - around eight metres.
0:09:39 > 0:09:46This powerful ebb and flow, gave rise to an ingenious sewer plan.
0:09:46 > 0:09:48Release excrement as the tide turns,
0:09:48 > 0:09:53and let the outgoing flow flush London's waste way out to sea.
0:09:55 > 0:09:57The city's relationship with the sea
0:09:57 > 0:10:00spawned a sewer system that was the envy of the world.
0:10:00 > 0:10:06Opened in 1865 by the Prince of Wales, this subterranean labyrinth,
0:10:06 > 0:10:09elevated its mastermind Joseph Bazalgette
0:10:09 > 0:10:13to become a hero of the Victorian age.
0:10:16 > 0:10:20Bazalgette's master plan demanded a warren of waste pipes,
0:10:20 > 0:10:23a network over 1,000 miles long
0:10:23 > 0:10:27to carry the capital's raw sewage out to the Thames.
0:10:27 > 0:10:32It took six years to build, constructed so well
0:10:32 > 0:10:37it still forms the backbone of London's sewer complex.
0:10:37 > 0:10:41Over 300 million bricks placed so precisely
0:10:41 > 0:10:44they form water-tight tunnels.
0:10:46 > 0:10:49You know how to treat a girl, don't you, Rob?
0:10:49 > 0:10:52I do, I take them only to the best spots.
0:10:52 > 0:10:57Impressive as this labyrinth is, it's only the means to a watery end.
0:10:57 > 0:11:00The city's sewage still needed sweeping out to sea,
0:11:00 > 0:11:05so it was piped towards the coast to pass the problem onto the tide.
0:11:05 > 0:11:09The muck flowed downstream to arrive at the final
0:11:09 > 0:11:14triumph of the entire system, the pumping station at Crossness.
0:11:24 > 0:11:28This is staggering! It's like some sort of ballroom.
0:11:35 > 0:11:38It's a real indication of the level of pride
0:11:38 > 0:11:42they took in their work, the beauty is just breathtaking.
0:11:42 > 0:11:48And these huge pumps are even named after members of the royal family.
0:11:51 > 0:11:56The pumping stations were the final stage of Balzalgette's grand plan -
0:11:56 > 0:11:59they pushed the sewage up into huge reservoirs,
0:11:59 > 0:12:03to be stored until the tide began to turn.
0:12:08 > 0:12:13When the tide started to ebb, they released the sewage into the Thames just there.
0:12:13 > 0:12:20They relied on the surge of seawater to whisk Londoners' muck out of sight and out of mind.
0:12:20 > 0:12:23This was Joseph Bazalgette's big tidal flush -
0:12:23 > 0:12:28his plan to turn the Thames into one gigantic toilet bowl was complete.
0:12:30 > 0:12:34Bazalgette was heralded as the city's saviour.
0:12:34 > 0:12:38But is there a skeleton lurking in London's water closet?
0:12:40 > 0:12:43Life may have been rosy for those in central London,
0:12:43 > 0:12:47but it didn't smell so sweet for those living downstream.
0:12:50 > 0:12:56Like a real-life toilet, the Thames is full of U-bends.
0:12:56 > 0:13:00The waste wasn't clearing as fast as Bazalgette had imagined,
0:13:00 > 0:13:04and the consequences turned out to be devastating.
0:13:05 > 0:13:08It's the 3rd of September, 1878,
0:13:08 > 0:13:12the pleasure steamer The Princess Alice
0:13:12 > 0:13:16is on its way back to London crammed with passengers.
0:13:16 > 0:13:20The day-trippers had been enjoying fresh air at the mouth of the estuary,
0:13:20 > 0:13:23but returning to the city, near the sewage outlet,
0:13:23 > 0:13:26the pleasure steamer was struck by disaster.
0:13:30 > 0:13:32It collides with another boat.
0:13:32 > 0:13:37Hundreds are flung into the river, many will be drowned.
0:13:37 > 0:13:38But it's even worse than that.
0:13:41 > 0:13:45Revealing the gruesome fate of those floundering in the estuary
0:13:45 > 0:13:48is local historian Joz Joslin.
0:13:49 > 0:13:53So the vessel's upended, and hundreds of people are in the water.
0:13:53 > 0:13:56Yes. And lots of them are women and children,
0:13:56 > 0:13:58and they're screaming,
0:13:58 > 0:14:00and unfortunately it's not water that they're in,
0:14:00 > 0:14:04they're actually in sewage, so there was no oxygen.
0:14:04 > 0:14:08A lot of them died because there was no air to breathe.
0:14:08 > 0:14:11So they're either being suffocated or drowning.
0:14:11 > 0:14:13Or poisoned.
0:14:13 > 0:14:15How revolting. And the majority died?
0:14:15 > 0:14:17Yes, the majority died.
0:14:17 > 0:14:20They said that every street in the east end of London had lost somebody,
0:14:20 > 0:14:25because it was their Sunday school outings that were onboard the vessel.
0:14:26 > 0:14:28The pleasure boat sank close to the sewage works,
0:14:28 > 0:14:32and the timing could not have been worse.
0:14:33 > 0:14:37The Beckton sewage outlet pipe carrying all of North London's waste
0:14:37 > 0:14:40had just discharged its stinking load into the river.
0:14:44 > 0:14:49Over 600 people choked to death in a toxic soup of human filth.
0:14:54 > 0:14:56After the tragedy,
0:14:56 > 0:14:59Bazalgette's sewage system came under the spotlight.
0:15:01 > 0:15:07Members of the local historical society read the words of their forefathers.
0:15:07 > 0:15:12"There had been an accumulation of black, greasy, filth along the shore.
0:15:12 > 0:15:16"The filth settles on the steps as the tide goes down."
0:15:16 > 0:15:18"The river in hot weather is very bad.
0:15:18 > 0:15:22"In some places it smells so bad you cannot stand it."
0:15:22 > 0:15:25A commission of inquiry delivered a damning indictment,
0:15:25 > 0:15:29concluding "it is neither necessary or justifiable
0:15:29 > 0:15:36"to discharge sewage in its crude state into any parts of the Thames".
0:15:36 > 0:15:40The Pall Mall Gazette took Bazalgette to task, stating
0:15:40 > 0:15:45"the natural man in him, puts off the evil day of having to admit failure".
0:15:46 > 0:15:50Luckily for Bazalgette, the muck didn't stick,
0:15:50 > 0:15:55but London did pull the plug on his big tidal flush.
0:15:55 > 0:15:59In 1887, a new system started.
0:15:59 > 0:16:04Now the solid human waste was pumped into vessels like this.
0:16:04 > 0:16:08The excrement was shipped out to the open sea and dumped.
0:16:08 > 0:16:12They were known locally as Bovril boats, amongst other things.
0:16:12 > 0:16:16We used to call them... Well, never mind what we used to call them!
0:16:16 > 0:16:19- What did you call them? - No, I'm not saying.
0:16:19 > 0:16:22They used to come and moor - they had moorings for them -
0:16:22 > 0:16:24and they would take the residue of it.
0:16:24 > 0:16:26Cos all the fluids were taken off,
0:16:26 > 0:16:28so it was almost solid the stuff
0:16:28 > 0:16:30that they took out - human detritus -
0:16:30 > 0:16:33so that it wasn't into the river.
0:16:34 > 0:16:39Sewage carrying ships didn't just do the dirty work of London -
0:16:39 > 0:16:42they were once a common sight on our estuaries,
0:16:42 > 0:16:46cleaning up Glasgow, Belfast and other coastal cities.
0:16:49 > 0:16:55London's Bovril boats were finally pensioned off in 1998.
0:16:56 > 0:17:03Balzalgette's tunnels still bring raw sewage here to the Crossness Works,
0:17:03 > 0:17:07but now the solid matter's burnt off to make electricity.
0:17:09 > 0:17:11The liquid sewage is treated -
0:17:11 > 0:17:13it goes from this...
0:17:13 > 0:17:15to this.
0:17:15 > 0:17:16And the cleaned-up fluid?
0:17:16 > 0:17:19It still gets the big tidal heave-ho,
0:17:19 > 0:17:23and is discharged into the Thames, where the river and the sea collide.
0:17:35 > 0:17:40Tide and traffic on the Thames flow two ways.
0:17:43 > 0:17:47In deep waters at the estuary mouth, ships from around the world
0:17:47 > 0:17:50come to unburden themselves on the docks at Sheerness.
0:17:53 > 0:17:58But back in the 19th century a group of foreign stowaways
0:17:58 > 0:18:01snuck off a ship and never left.
0:18:01 > 0:18:05They set-up a secret community within the harbour walls.
0:18:06 > 0:18:11This is the des-res of Britain's only colony...of scorpions.
0:18:14 > 0:18:15But not the monster kind.
0:18:18 > 0:18:21European yellowtail scorpions arrived here from Italy
0:18:21 > 0:18:24on a masonry ship some 200 years ago.
0:18:27 > 0:18:30Now the offspring of those Italian scorpions
0:18:30 > 0:18:32have found a British admirer.
0:18:35 > 0:18:38Hi, I'm Bex, and I'm a scorpo-holic.
0:18:38 > 0:18:41I've been fascinated by scorpions since I was a teenager
0:18:41 > 0:18:42and been hooked ever since.
0:18:44 > 0:18:47I'm here to see Britain's only colony of scorpions,
0:18:47 > 0:18:50but I've got to wait for the sun to go down.
0:18:52 > 0:18:56It's properly dark now, so I'm going to see if I can find some.
0:18:56 > 0:19:00I'm using a UV torch, cos scorpions glow under ultraviolet light,
0:19:00 > 0:19:03and I think I've just spotted one.
0:19:03 > 0:19:07Definitely an adult, probably out looking for something to eat.
0:19:08 > 0:19:11It is pretty cool though, isn't it? Having scorpions in the UK.
0:19:13 > 0:19:15They are a member of the spider family.
0:19:15 > 0:19:17They have eight legs, not six.
0:19:17 > 0:19:18They eat woodlice.
0:19:18 > 0:19:21They are ambush predators, so they'll just sit and wait,
0:19:21 > 0:19:23and then something will come past,
0:19:23 > 0:19:25and they'll just jump out and grab it,
0:19:25 > 0:19:28and subdue it with their claws rather than stinging it.
0:19:28 > 0:19:31They don't generally use their stings, these ones.
0:19:35 > 0:19:38Very happy that we found some.
0:19:38 > 0:19:39This is a tiny, little, incy one.
0:19:39 > 0:19:43He's so cute. But with tininess comes speed!
0:19:48 > 0:19:51It's been a great night, we've seen loads of scorpions,
0:19:51 > 0:19:55but I think I'll put this one back before...
0:19:55 > 0:19:56it legs it.
0:19:57 > 0:19:58Bye, little fella.
0:20:00 > 0:20:02I think I'll leg it now, too.
0:20:19 > 0:20:22We're on a journey to discover what becomes of the coast
0:20:22 > 0:20:24when rivers and seas collide.
0:20:27 > 0:20:32I'm exploring the Firth of Forth on Scotland's east coast,
0:20:32 > 0:20:36where canny folk profited from their prime location -
0:20:36 > 0:20:40ideal for seaborne business.
0:20:40 > 0:20:45And with rich seams of coal for power, the population boomed.
0:20:45 > 0:20:50With more mouths to feed, getting enough fresh food was tricky,
0:20:50 > 0:20:54so they looked to the sea to preserve their provisions.
0:20:56 > 0:20:59You'll find the evidence at St Monans.
0:21:04 > 0:21:08Here, food processing created a curious landscape.
0:21:11 > 0:21:15The shore is lined with lots and lots
0:21:15 > 0:21:18of very strange grass-covered humps,
0:21:18 > 0:21:21and what seems to be a ruined building over there
0:21:21 > 0:21:24and up there a stone windmill.
0:21:26 > 0:21:31The ruins of industrial activity reveal themselves the more you look.
0:21:35 > 0:21:37This land was remodelled by people
0:21:37 > 0:21:40making the most of one bounty from the sea
0:21:40 > 0:21:43that isn't in short supply - salt.
0:21:45 > 0:21:49Before refrigeration, salt was a valuable commodity -
0:21:49 > 0:21:53preserving herring landed along the east coast.
0:21:54 > 0:21:58Scottish salt was also exported to England,
0:21:58 > 0:22:01turning a tasty profit for the salt works.
0:22:03 > 0:22:08Those strange hummocks come in sets - each set of hummocks
0:22:08 > 0:22:09is the ruins of a pan house.
0:22:10 > 0:22:12Inside each of those pan houses
0:22:12 > 0:22:16there was an iron pan about 6 metres by 3 metres,
0:22:16 > 0:22:18with coal fires beneath it.
0:22:18 > 0:22:22Sea water was pumped - probably using this windmill - from the sea
0:22:22 > 0:22:24in pipes up to each pan house.
0:22:24 > 0:22:28Once it had been boiled off in the pans, you had salt.
0:22:31 > 0:22:34A rare film brings the enterprise back to life.
0:22:37 > 0:22:42Salt works once flourished along Scotland's east coast.
0:22:42 > 0:22:48The last operation at Prestonpans didn't close its doors until 1974.
0:22:51 > 0:22:55It was the abundance of coal along this estuary
0:22:55 > 0:22:58that made it a good site for boiling up sea water.
0:23:02 > 0:23:04A sample of sea water stirs up a mystery,
0:23:04 > 0:23:07right at the heart of this forgotten industry.
0:23:09 > 0:23:12Out there is the sea full of salt.
0:23:14 > 0:23:16And I can certainly taste it.
0:23:16 > 0:23:21This little brook running into the sea,
0:23:21 > 0:23:23doesn't taste salty, at all.
0:23:23 > 0:23:27So, why is freshwater fresh
0:23:27 > 0:23:30and why is sea water salty?
0:23:34 > 0:23:39It's one of those brilliantly simple infuriating questions
0:23:39 > 0:23:44that kids ask - why is the sea salty?
0:23:44 > 0:23:46I'm enlisting the help of a grown-up.
0:23:46 > 0:23:50Simon Boxall's from the National Oceanography Centre -
0:23:50 > 0:23:53he should be able to work it out.
0:23:53 > 0:23:56We've all swum in the sea, we know it doesn't taste like freshwater,
0:23:56 > 0:23:58Simon, but why is it salty?
0:23:58 > 0:24:01You have to go right back to the beginning-stage of the earth,
0:24:01 > 0:24:03back several billion years.
0:24:03 > 0:24:05And if you go back that far,
0:24:05 > 0:24:07the earth was a completely different place -
0:24:07 > 0:24:10it was full of volcanic eruptions, there was lots of steam around,
0:24:10 > 0:24:13but, also, there was a lot of sodium in the rocks,
0:24:13 > 0:24:16and that sodium was being hit by the hydrochloric acid
0:24:16 > 0:24:19that was given off by these volcanic vents.
0:24:19 > 0:24:21And if we take these two very harmful chemicals...
0:24:21 > 0:24:24On the one hand, you've got the element of sodium - very reactive -
0:24:24 > 0:24:27and on the other hand, you've got chlorine - very dangerous and very reactive.
0:24:27 > 0:24:31You put the two together and you create...sodium chloride.
0:24:31 > 0:24:33Which is the sort of thing you sprinkle on your chips.
0:24:33 > 0:24:35Certainly isn't harmful, at all.
0:24:35 > 0:24:39So you've got this hydrochloric acid pouring out of the volcanic vents,
0:24:39 > 0:24:43meeting the sodium hydroxide which is lying around in the rocks on the sea bed
0:24:43 > 0:24:47creating this stuff called sodium chloride, which is salt.
0:24:47 > 0:24:53These ancient chemical reactions gave birth to our salty seas.
0:24:55 > 0:24:58We can create those sort of primordial days.
0:24:58 > 0:25:00We can actually take some hydrochloric acid -
0:25:00 > 0:25:03the sort of stuff that came out of the vents of the volcanoes.
0:25:03 > 0:25:05We've got some dilute sodium hydroxide -
0:25:05 > 0:25:08which represents the stuff that was in the rocks.
0:25:08 > 0:25:10And between us, if you want to, we can make salt.
0:25:10 > 0:25:12We can take these two quite nasty chemicals
0:25:12 > 0:25:15and produce something that's really more vital to life in many ways.
0:25:15 > 0:25:18This is hydrochloric acid - it's very dilute.
0:25:18 > 0:25:20And we're going to pop it into this vessel here.
0:25:22 > 0:25:26OK, we're then going to add our sodium hydroxide.
0:25:26 > 0:25:30At the moment, basically, the sodium and the chlorine are combining
0:25:30 > 0:25:32and giving off heat - can you feel that?
0:25:32 > 0:25:35It's warm! Really warm, wow!
0:25:35 > 0:25:38We've effectively neutralised that acid, that sodium hydroxide,
0:25:38 > 0:25:42and what we have in there now is basically water...and salt.
0:25:44 > 0:25:49We've compressed billions of years of the earth's evolution
0:25:49 > 0:25:51to make a miniature ocean.
0:25:51 > 0:25:57Boiling off our DIY sea water leaves the prize ingredient.
0:25:59 > 0:26:03So, here it is, our very own home-made salt.
0:26:04 > 0:26:08White crystals that washed wealth in from the sea
0:26:08 > 0:26:10to help feed an estuary.
0:26:23 > 0:26:27The salty shoreline satisfies our appetite in surprising ways.
0:26:28 > 0:26:34On sea marshes in Wales, sheep graze on grass made sturdy by regular salt baths.
0:26:37 > 0:26:41This distinctly coastal vegetation gives the lamb a unique flavour.
0:26:44 > 0:26:49But to explore the biggest effect our shoreline has on our waistline,
0:26:49 > 0:26:51head north to Scotland.
0:26:54 > 0:26:58A natural wonder plays out where rivers meet the sea,
0:26:58 > 0:27:00like here, at Loch Carron.
0:27:03 > 0:27:08Miranda is immersed in a secret life of the salmon.
0:27:11 > 0:27:14I'm in the thick of Britain's favourite fish dish.
0:27:14 > 0:27:16Our insatiable appetite for salmon
0:27:16 > 0:27:18has spawned an industry in the UK
0:27:18 > 0:27:21worth around half a billion pounds a year.
0:27:22 > 0:27:25But farming these fish is a lot more difficult than you think.
0:27:27 > 0:27:32Salmon are challenging to rear in farms because of their extraordinary life cycle.
0:27:32 > 0:27:36They're born in freshwater rivers, then migrate to the sea.
0:27:38 > 0:27:42To survive in saltwater, a salmon's body goes through radical changes.
0:27:46 > 0:27:50That transformation is very tricky to manage in captivity.
0:27:50 > 0:27:53To see why, I'm seeking out the wild fish.
0:27:55 > 0:27:58Take a winter walk along a salmon river and there might be
0:27:58 > 0:28:02delicate little eggs lurking deep in the watery gravel.
0:28:02 > 0:28:04So why do salmon out at sea,
0:28:04 > 0:28:08bother to come all this way to lay their eggs?
0:28:08 > 0:28:11I'm meeting wild salmon expert Bob Kindness.
0:28:11 > 0:28:13What a place for the salmon to come back to
0:28:13 > 0:28:15and lay their eggs in the gravel here.
0:28:15 > 0:28:17They have to be spawned in fresh water
0:28:17 > 0:28:19because the eggs would not survive
0:28:19 > 0:28:22other than in fresh water - they wouldn't survive in the sea.
0:28:22 > 0:28:26So the salmon has to make that journey back from the feeding grounds,
0:28:26 > 0:28:28usually back to its home river,
0:28:28 > 0:28:31back to the area where it itself was born.
0:28:34 > 0:28:38Once hatched, these freshwater fish face a big problem -
0:28:38 > 0:28:41they need to get out to sea, where there's more food,
0:28:41 > 0:28:45but their bodies are incapable of dealing with saltwater.
0:28:47 > 0:28:50A jelly sweet experiment gives us a taste of their challenge.
0:28:51 > 0:28:56This tiny jelly bear has a dense sugary body.
0:28:56 > 0:28:59Soak him in water overnight and he looks like this...
0:28:59 > 0:29:01all swollen up.
0:29:01 > 0:29:06This plump bear is our happily hydrated fresh water salmon.
0:29:07 > 0:29:11But if I add salt to create sea water,
0:29:11 > 0:29:16the fresh water is sucked out of the sweet, and it shrivels.
0:29:16 > 0:29:20Death by dehydration is the challenge a salmon faces
0:29:20 > 0:29:22going from the river down into the sea.
0:29:24 > 0:29:28To avoid dehydration salmon develop two defences,
0:29:28 > 0:29:33their scales toughen to stop fresh water leaching out of their body.
0:29:33 > 0:29:35Also their kidneys and gills adapt,
0:29:35 > 0:29:38so they can cope with life in saltwater.
0:29:40 > 0:29:42To see the scale of the transformation
0:29:42 > 0:29:45we've anaesthetized two of the fish.
0:29:46 > 0:29:50Gosh, you can really see the difference now, side-by-side.
0:29:50 > 0:29:54The speckled brown tiddler is a freshwater salmon.
0:29:54 > 0:29:58The silver beauty below, just a year older, is ready for the sea.
0:30:03 > 0:30:06A salmon can turn around and go back
0:30:06 > 0:30:09if its body isn't adapting to life in the seawater,
0:30:09 > 0:30:12and some fish take years to make it out to sea.
0:30:14 > 0:30:18That's OK in the wild, but how could fish farmers know
0:30:18 > 0:30:22when it was safe to move their salmon to saltwater pens?
0:30:24 > 0:30:28In the early 1970s, years of experiments finally paid off.
0:30:31 > 0:30:35I've got a patent here applied for in 1971.
0:30:35 > 0:30:41This is a scientific recipe for the crucial step in the growing of captive salmon.
0:30:42 > 0:30:46The "fish equation" says that when the weight of the salmon, W,
0:30:46 > 0:30:47times by 100,
0:30:47 > 0:30:49divided by its length, L, cubed,
0:30:49 > 0:30:52is equal to 0.8 -
0:30:52 > 0:30:54it's ready for sea water.
0:30:56 > 0:30:58Armed with the power to control nature,
0:30:58 > 0:31:02salmon farming in Scotland rapidly became big business.
0:31:03 > 0:31:08As the scale of production rose, the price of the fish started to plummet.
0:31:10 > 0:31:13'The market shows every sign of expanding as the supply increases,
0:31:13 > 0:31:18'and now, salmon is not just to be found in the exclusive fishmongers,
0:31:18 > 0:31:22'but can be bought vacuum-packed in the supermarkets.'
0:31:23 > 0:31:26Salmon stopped being a treat saved for special occasions,
0:31:26 > 0:31:28now it was as cheap as chicken.
0:31:31 > 0:31:35So how did fish farmers manage to mimic nature
0:31:35 > 0:31:38with their production line process?
0:31:38 > 0:31:41The salmon start life in freshwater tanks.
0:31:41 > 0:31:44When they're big enough to satisfy the "fish equation"
0:31:44 > 0:31:47they're sucked up and piped into trucks.
0:31:51 > 0:31:55Trucks take them to boats, boats ferry them to saltwater enclosures
0:31:55 > 0:31:57anchored at the sea end of the estuary.
0:32:00 > 0:32:03The salmon are kept in these pens for up to 22 months,
0:32:03 > 0:32:05but they are quite used to human company,
0:32:05 > 0:32:09there are divers who regularly check on the fish
0:32:09 > 0:32:12and the nets that hold them in.
0:32:14 > 0:32:16The salmon are constantly watched.
0:32:16 > 0:32:18CCTV operators check they're eating
0:32:18 > 0:32:22and make sure they're not being eaten by seals.
0:32:23 > 0:32:26Farmed fish carry more lice than wild ones,
0:32:26 > 0:32:29so they're monitored for sickness too.
0:32:32 > 0:32:36Our appetite for salmon makes all this effort profitable.
0:32:37 > 0:32:41If our demand for these fish is such that we need to farm them,
0:32:41 > 0:32:44then salmon pens will become an even more common site
0:32:44 > 0:32:46where rivers and seas collide.
0:33:02 > 0:33:06We're on a journey around three of our mightiest estuaries.
0:33:09 > 0:33:13Where tideways bring bounty from the sea deep inland.
0:33:17 > 0:33:21So imagine the potential of connecting two great estuaries
0:33:21 > 0:33:23to form one super-highway.
0:33:25 > 0:33:30That unifying vision excited the imagination of merchants
0:33:30 > 0:33:33in the Firth of Forth.
0:33:33 > 0:33:38In 1768, businessmen began to build their own waterway -
0:33:38 > 0:33:44an epic canal joining the Forth to another great estuary, the Clyde.
0:33:48 > 0:33:50What those early entrepreneurs couldn't know
0:33:50 > 0:33:53was that their scheme to connect two estuaries would,
0:33:53 > 0:33:58one day lead to a revolution in fibre-optic communication.
0:34:03 > 0:34:05The story starts with an extraordinary discovery
0:34:05 > 0:34:09on the waterway some 180 years ago.
0:34:10 > 0:34:14Let's go back to that fateful day in 1834.
0:34:14 > 0:34:18Young engineer John Scott Russell - this is him later in life -
0:34:18 > 0:34:20was on the towpath of this canal.
0:34:22 > 0:34:24Russell was watching a barge,
0:34:24 > 0:34:29when suddenly its bow wave detached from the front of the boat
0:34:29 > 0:34:30and sped off on its own.
0:34:32 > 0:34:35It was a solitary hump of travelling water.
0:34:36 > 0:34:39The engineer would become obsessed
0:34:39 > 0:34:44trying to understand the solitary wave through mathematics.
0:34:44 > 0:34:47But on first sighting, he had to act fast.
0:34:47 > 0:34:51Russell jumped on his horse and gave chase to the wave.
0:34:53 > 0:34:56Normally, waves peter out or break-up,
0:34:56 > 0:35:01but this mysterious wave retained its shape and sped onwards.
0:35:01 > 0:35:03After a chase of about two miles
0:35:03 > 0:35:08the solitary wave still showed no signs of petering out.
0:35:08 > 0:35:12Eventually it escaped Russell's pursuit altogether.
0:35:12 > 0:35:16Russell would have recognised that this peculiar wave was
0:35:16 > 0:35:18similar to the Severn bore.
0:35:23 > 0:35:25That solitary wave is produced
0:35:25 > 0:35:28when the tide collides with the River Severn,
0:35:28 > 0:35:32sending a surge of seawater up the narrow funnel
0:35:32 > 0:35:34of the Bristol Channel.
0:35:34 > 0:35:37Not unlike the narrow channel of the canal.
0:35:38 > 0:35:43So Russell experimented with canal-shaped water tanks.
0:35:43 > 0:35:47Mathematician Chris Eilbeck thinks he can recreate Russell's
0:35:47 > 0:35:48solitary wave.
0:35:49 > 0:35:52- Keep our fingers crossed. - I'm dying to see one!
0:35:56 > 0:35:57You can see the wave there.
0:35:57 > 0:35:58Look at that!
0:35:58 > 0:36:00- You got it first time.- Yeah.
0:36:00 > 0:36:01Just a hump of water on its own.
0:36:01 > 0:36:04You can feel the force of it when it hits the plate.
0:36:04 > 0:36:05It's gone all the way back again.
0:36:05 > 0:36:08They're amazingly stable it'll keep going for miles.
0:36:09 > 0:36:14The canal's dimensions were perfect for keeping the wave stable,
0:36:14 > 0:36:18deep enough for the wave not to steepen too sharply and break,
0:36:18 > 0:36:21but shallow enough that it didn't flatten out.
0:36:23 > 0:36:27Amazingly, these waves remain self-contained and intact,
0:36:27 > 0:36:29even when they collide.
0:36:29 > 0:36:31Three, two, one - go!
0:36:34 > 0:36:37After the collision, they just re-appear on the other side.
0:36:37 > 0:36:40Chris, are you saying, the waves didn't just bounce off each other
0:36:40 > 0:36:42but went through each other?
0:36:42 > 0:36:43Yes, that's correct.
0:36:43 > 0:36:46They go through each other and re-appear on the other side.
0:36:46 > 0:36:49- Undiminished? - Yes, and mathematically exact.
0:36:49 > 0:36:51Unbelievable.
0:36:55 > 0:37:00These remarkable packets of travelling energy have been christened solitons.
0:37:01 > 0:37:04Now is the time to scale our experiment up.
0:37:05 > 0:37:09Let's try and re-create the full wonder of the soliton wave
0:37:09 > 0:37:13on this 800ft long straight section of Scottish canal.
0:37:17 > 0:37:19But it won't be easy.
0:37:20 > 0:37:24The wave John Scott Russell chased back in 1834,
0:37:24 > 0:37:28was created when a horse-drawn boat came to a sudden stop,
0:37:28 > 0:37:30releasing its bow wave.
0:37:32 > 0:37:34We're trying a more modern approach
0:37:34 > 0:37:39a motor boat loaded with people to provide ballast, and to bear witness.
0:37:40 > 0:37:44Rachel, you're at the front of the boat here with some top hats.
0:37:44 > 0:37:48You're clearly expecting something to happen. What are the hats for?
0:37:48 > 0:37:51We've got the hats as a tribute to John Scott Russell
0:37:51 > 0:37:53who discovered the first soliton wave,
0:37:53 > 0:37:55we're all going to be throwing them into the air with joy
0:37:55 > 0:37:57when we re-create it, is the plan.
0:37:57 > 0:37:59I'm very impressed by your optimism.
0:38:01 > 0:38:05Solitons are rare, not easily produced.
0:38:05 > 0:38:09The boat must gain enough speed to form a sizeable bow wave
0:38:09 > 0:38:12then come to a sudden stop.
0:38:12 > 0:38:15Hopefully, that'll release a soliton.
0:38:17 > 0:38:20If it does, I'm going to chase it, just like Russell,
0:38:20 > 0:38:23only I'll be racing it on foot, not on horseback.
0:38:25 > 0:38:28I've run up mountains, and I've run a few marathons,
0:38:28 > 0:38:31but I've never run against a wave before,
0:38:31 > 0:38:32I'm not sure I'm going to have to.
0:38:40 > 0:38:41Oh, my god!
0:39:11 > 0:39:17Well, I beat it over a few hundred metres, but it's still going.
0:39:17 > 0:39:21And I'm not going to be able to keep going as long as that wave.
0:39:21 > 0:39:23- ALL:- Hurray!
0:39:25 > 0:39:29John Scott Russell designed special boats to ride his wave
0:39:29 > 0:39:32at high speed along canals.
0:39:33 > 0:39:38But sadly most people preferred a new form of transport - the railway.
0:39:38 > 0:39:42Russell waved goodbye to his bright idea.
0:39:43 > 0:39:48True to its nature though the soliton wasn't finished.
0:39:50 > 0:39:54Today the solitary waves first seen on this canal
0:39:54 > 0:39:58aren't only found in water - they travel as light.
0:40:04 > 0:40:09Scientists have created soliton waves from tiny pulses of light.
0:40:11 > 0:40:14Fired down the next generation of fibre optic cables,
0:40:14 > 0:40:18the self-sustaining waves can carry communications
0:40:18 > 0:40:20for thousands of miles -
0:40:20 > 0:40:24travelling on endlessly between continents,
0:40:24 > 0:40:28just as John Scott Russell's wave ran on out of sight of me.
0:40:30 > 0:40:33- So his day has come. - His day has finally come.
0:40:33 > 0:40:36And if you were to meet him now, what would you say to him?
0:40:36 > 0:40:38I would tell him that his idea has really come of age,
0:40:38 > 0:40:42and everybody talks about him, everybody talks about the soliton.
0:40:42 > 0:40:46It's a really big thing in science, so he'd be delighted, I think.
0:40:46 > 0:40:48- A satisfied scientist. - Yes.
0:41:01 > 0:41:06On a journey around our estuaries we've arrived at the mighty Severn.
0:41:07 > 0:41:11Here the Atlantic Ocean surges in to collide
0:41:11 > 0:41:14with Britain's longest river.
0:41:14 > 0:41:18The Severn estuary has a staggering rise of tidal water,
0:41:18 > 0:41:21some 15 metres, in all.
0:41:25 > 0:41:28The sea swirls in strange patterns here.
0:41:31 > 0:41:34And its currents would wash the bodies of wrecked sailors
0:41:34 > 0:41:36to the same spot again and again -
0:41:39 > 0:41:41the village of Brean.
0:41:48 > 0:41:54Those unknown souls delivered to the doorstep of Brean needed burial -
0:41:54 > 0:41:58a sorrowful ritual remembered in song.
0:41:58 > 0:42:03Folk singer and storyteller June Tabor recalls the Brean Lament.
0:42:06 > 0:42:09The first thing that strikes you about being here,
0:42:09 > 0:42:11it's timeless.
0:42:11 > 0:42:15It could be any time between now and 200 years ago.
0:42:15 > 0:42:18And the old timbers of this ship
0:42:18 > 0:42:21going nowhere ever again.
0:42:21 > 0:42:24The men who were on this ship -
0:42:24 > 0:42:26did they survive?
0:42:31 > 0:42:36# The waters they washed them ashore
0:42:36 > 0:42:38# Ashore
0:42:40 > 0:42:45# And they never will sail the seas no more
0:42:47 > 0:42:52# We led them along by the churchyard wall
0:42:52 > 0:42:59# And all in a row we buried them all. #
0:43:01 > 0:43:04The song The Brean Lament describes
0:43:04 > 0:43:09what happened quite commonly along this coast
0:43:09 > 0:43:12when bodies were washed up.
0:43:12 > 0:43:17You have to give them a burial, but not in the main churchyard -
0:43:17 > 0:43:21in a separate sailors' graveyard.
0:43:24 > 0:43:28It was quite strongly believed along this stretch of coast
0:43:28 > 0:43:33that the sea might decide to reclaim their bodies as its own.
0:43:33 > 0:43:40They didn't want the dead of the village being taken at the same time.
0:43:40 > 0:43:46And, possibly as a way of appeasing the sea,
0:43:46 > 0:43:51the sailors' boots were buried below the tide line,
0:43:51 > 0:43:54so the sea would have something to take.
0:43:56 > 0:44:03# We led them along by the churchyard wall
0:44:03 > 0:44:10# Where all in a row we buried them all
0:44:12 > 0:44:20# But their boots we buried below the tide
0:44:24 > 0:44:29# On Severnside. #
0:44:42 > 0:44:45The Severn estuary used to pose a fearsome challenge
0:44:45 > 0:44:49on any journey between England and Wales.
0:44:50 > 0:44:55The two countries were divided by this massive tear in our coastline.
0:44:56 > 0:45:00Avoiding it meant a diversion deep inland.
0:45:01 > 0:45:06Even so, only hardy travellers would brave the deadly waters.
0:45:13 > 0:45:17Today, a concrete solution spans this vast channel.
0:45:19 > 0:45:24But conquering the Severn was a bold venture fraught with peril,
0:45:24 > 0:45:26as Mark is about to discover.
0:45:28 > 0:45:32Dashing over the estuary from Wales to England commuters take
0:45:32 > 0:45:37the elegant crossings their lives depend on for granted.
0:45:39 > 0:45:42But imagine a world before this bridge was possible.
0:45:42 > 0:45:45A world without steel cables,
0:45:45 > 0:45:47without reinforced concrete,
0:45:47 > 0:45:51when the sea reigned supreme.
0:45:51 > 0:45:56That was a challenge faced by the Victorians to cross the River Severn.
0:46:01 > 0:46:06The formidable collision of river and sea facing the early engineers
0:46:06 > 0:46:08can still be experienced.
0:46:10 > 0:46:12It's one of the most dangerous seaways in the world,
0:46:12 > 0:46:14and I'm just a little bit excited.
0:46:18 > 0:46:22The Severn Area Rescue Association is going to pit me against the ebb tide.
0:46:25 > 0:46:28Cast off!
0:46:28 > 0:46:31The power of the tide here is just extraordinary!
0:46:37 > 0:46:41As the tide goes out it's like a maelstrom.
0:46:41 > 0:46:45The waters were an immense challenge,
0:46:45 > 0:46:49but by the 1840s crossing the river by boat was old hat.
0:46:53 > 0:46:56An irresistible new force was spreading across Britain -
0:46:56 > 0:46:58the railways.
0:46:58 > 0:47:00Come hell or high water,
0:47:00 > 0:47:04estuaries weren't going to stand in the way of progress.
0:47:08 > 0:47:12The great Victorian engineer Isambard Kingdom Brunel
0:47:12 > 0:47:16is a hero of mine - he'd already managed to cross the Avon gorge
0:47:16 > 0:47:18with a mighty suspension bridge.
0:47:18 > 0:47:23When his railway came to Bristol, he wanted to cross into South Wales,
0:47:23 > 0:47:25and planned an even bigger suspension bridge.
0:47:25 > 0:47:27Here are the preliminary sketches.
0:47:28 > 0:47:32The biggest problem was the sheer scale of the span
0:47:32 > 0:47:36that Brunel required - over 1,000 feet.
0:47:38 > 0:47:41He left a little note in his notebook
0:47:41 > 0:47:44which says "Is 1,100ft practicable?"
0:47:48 > 0:47:51Brunel's bridge was never built,
0:47:51 > 0:47:56but if taking trains over the water defeated the best brain of the age,
0:47:56 > 0:47:59how about going underneath?
0:48:01 > 0:48:04A tunnel - was that the answer?
0:48:04 > 0:48:08Digging deep to create a railway under the water -
0:48:08 > 0:48:12this was very bold, big thinking.
0:48:13 > 0:48:18This is one of the original drawings of the tunnel from around 1887,
0:48:18 > 0:48:23and you can see how the track comes down underneath the deepest part
0:48:23 > 0:48:25of the Bristol channel here in The Shoots,
0:48:25 > 0:48:28and gradually up to the Welsh side.
0:48:28 > 0:48:32So what we've got here is around seven miles of railway track.
0:48:34 > 0:48:38That passage under the estuary is now a vital link
0:48:38 > 0:48:41between England and Wales.
0:48:41 > 0:48:44Carrying over 250 trains a day.
0:48:47 > 0:48:51Passengers are oblivious to a catastrophe that nearly sank
0:48:51 > 0:48:54the tunnel before the first train ran,
0:48:54 > 0:48:58and is a problem that still lurks below.
0:49:03 > 0:49:08So here we go, I've been granted access to a shadowy water world,
0:49:08 > 0:49:10few get to see.
0:49:12 > 0:49:14It's great - we're just coming into the cutting,
0:49:14 > 0:49:17the portal's ahead, and we're about to go under the sea.
0:49:23 > 0:49:25Ah! Way!
0:49:25 > 0:49:27Isn't that fantastic!
0:49:29 > 0:49:32We're heading for the deepest point in the tunnel.
0:49:36 > 0:49:39Just 50ft above us millions of gallons of water
0:49:39 > 0:49:45are swirling around - the River Severn and the sea are in full flow.
0:49:45 > 0:49:49Keeping the water out here is hard enough,
0:49:49 > 0:49:52but can you imagine if there was a flood down here?
0:49:54 > 0:49:57With an estuary hanging over their heads,
0:49:57 > 0:50:01engineers knew there'd be seepage of seawater,
0:50:01 > 0:50:05but it was freshwater that nearly drowned the project.
0:50:08 > 0:50:10Nobody expected this - a raging torrent!
0:50:13 > 0:50:17They'd broken through to an underground spring.
0:50:18 > 0:50:23In October, 1879, water began to pour into the tunnel.
0:50:25 > 0:50:27The workers fled for their lives.
0:50:30 > 0:50:34The disaster struck when a shaft dug on the Welsh coast
0:50:34 > 0:50:38cut into an underground river deep below the surface.
0:50:41 > 0:50:45For four years the engineers made desperate attempts
0:50:45 > 0:50:50to block the freshwater spring, but every effort proved futile.
0:50:53 > 0:50:57And it's been flooding in at this alarming rate ever since.
0:50:59 > 0:51:03If they couldn't stop the spring water, they'd have to live with it.
0:51:06 > 0:51:09The only solution were pumps, massive ones like this
0:51:09 > 0:51:14that pump the water out as fast as it comes in, right up to the surface.
0:51:17 > 0:51:22Leighton Jenkins helps keep the tracks dry today.
0:51:22 > 0:51:24So what would happen if the pumps actually failed?
0:51:24 > 0:51:26Every second counts,
0:51:26 > 0:51:29as soon as the pumps stop we'd have to inform the control
0:51:29 > 0:51:32within 10 minutes to shut the tunnel itself,
0:51:32 > 0:51:36and within 20 minutes we've got water coming up through the tracks,
0:51:36 > 0:51:37so every second absolutely counts.
0:51:37 > 0:51:39But have they ever failed?
0:51:39 > 0:51:42No, not as far as I know, no. Not while I'm on a shift anyway.
0:51:45 > 0:51:47The railways had proved irresistible,
0:51:47 > 0:51:52with rival Victorian companies vying for routes,
0:51:52 > 0:51:56by the time it was finished the tunnel already had a competitor.
0:52:01 > 0:52:07In 1879, trains had started to roll over the estuary,
0:52:07 > 0:52:09but the bridge's sturdy uprights -
0:52:09 > 0:52:14always an obstacle to shipping - would ultimately prove its downfall.
0:52:17 > 0:52:23Do you see, that's a tower where the railway bridge once crossed
0:52:23 > 0:52:25the Severn estuary.
0:52:25 > 0:52:28I've got a photograph that shows the stanchions
0:52:28 > 0:52:32marching across the river - now totally destroyed.
0:52:36 > 0:52:40The raging waters where river and sea smash together
0:52:40 > 0:52:43would deliver a fatal blow to the rail bridge.
0:52:47 > 0:52:53In October, 1960, the Arkendale carrying oil,
0:52:53 > 0:52:56and the Wastdale laden with petrol
0:52:56 > 0:53:01were heading for combustible collision.
0:53:01 > 0:53:05The Arkendale was carried in by the surging tide.
0:53:05 > 0:53:10That powerful current would drive it into the Wastdale
0:53:10 > 0:53:14on a foggy night at Sharpness Docks.
0:53:14 > 0:53:16As Alan Hayward knows.
0:53:17 > 0:53:21They were coming up river intending to come into the docks here,
0:53:21 > 0:53:23but they were accidentally swept past.
0:53:24 > 0:53:28And then they collided and became in effect stuck together.
0:53:30 > 0:53:32Disabled ships in thick fog,
0:53:32 > 0:53:36carrying 600 tonnes of inflammable cargo
0:53:36 > 0:53:41at the mercy of a swirling sea, propelled them to disaster.
0:53:43 > 0:53:46They were desperate to separate from each other,
0:53:46 > 0:53:49fighting by steering in different directions
0:53:49 > 0:53:50but it just didn't work,
0:53:50 > 0:53:53and they only had about four minutes before
0:53:53 > 0:53:55they would reach the railway bridge.
0:53:56 > 0:54:00The rail bridge across the Severn loomed out of the fog,
0:54:00 > 0:54:05a collision with the ships carrying oil and petrol was now inevitable.
0:54:11 > 0:54:13A lot of sparks would have been created
0:54:13 > 0:54:16which ignited the petrol in one of the vessels.
0:54:19 > 0:54:21The fuel of course spilt out over the river,
0:54:21 > 0:54:24so the whole river became a mass of flame.
0:54:27 > 0:54:31First mate Percy Simmonds was aboard one of the tankers.
0:54:31 > 0:54:34His son Chris was 13 at the time.
0:54:34 > 0:54:37I try to imagine that night and what he was going through,
0:54:37 > 0:54:40and it must have been just terrible with the flames and everything.
0:54:40 > 0:54:44I'm just sure he was determined to make it across this river somehow
0:54:44 > 0:54:47and make it back to us.
0:54:48 > 0:54:52Daylight and a low tide revealed wrecks of the fuel tankers,
0:54:52 > 0:54:56smouldering on a sandbank.
0:54:56 > 0:54:59Soon the first body was found.
0:54:59 > 0:55:01They identified the body there,
0:55:01 > 0:55:06and they you know let Mum know that, yeah, it was definitely Perce.
0:55:07 > 0:55:11Chris's father Percy died along with four others
0:55:11 > 0:55:13on that terrible evening.
0:55:13 > 0:55:19The damaged bridge was too expensive to repair, it was demolished.
0:55:19 > 0:55:25But each day, when the tide recedes, scars of tragedy are revealed.
0:55:25 > 0:55:29Out there of course are two hulks buried now in the sands,
0:55:29 > 0:55:31that have been washed over by countless tides.
0:55:31 > 0:55:34But they're still there. They're there as monuments.
0:55:34 > 0:55:37They're here as a reminder to all of us.
0:55:41 > 0:55:43It's immensely humbling
0:55:43 > 0:55:46to be next to such a vast body
0:55:46 > 0:55:48of brooding water,
0:55:48 > 0:55:51even on a calm day like this.
0:55:51 > 0:55:57One can feel the power where rivers and sea collide.
0:56:08 > 0:56:10Britain's mighty estuaries
0:56:10 > 0:56:14have pushed engineers to breaking point and beyond.
0:56:16 > 0:56:21In Scotland, there's a salty graveyard of overreaching ambition -
0:56:21 > 0:56:23the Tay.
0:56:28 > 0:56:31This immense river disgorges more water into the sea
0:56:31 > 0:56:34than the Thames and Severn combined.
0:56:35 > 0:56:39In the shadow of today's rail crossing, is a spine of stumps.
0:56:42 > 0:56:45The footings of the first Tay Bridge.
0:56:46 > 0:56:52On a stormy night in December, 1879, it collapsed -
0:56:52 > 0:56:5460 passengers died.
0:56:58 > 0:57:03And so did the myth of infallible Victorian engineers.
0:57:05 > 0:57:10The blame was put on designer Thomas Bouch, lax maintenance,
0:57:10 > 0:57:11and poor iron work.
0:57:15 > 0:57:19Hard lessons learned just down the coast on the Firth of Forth.
0:57:25 > 0:57:28Following the disaster at the Tay Bridge
0:57:28 > 0:57:32that designer's plans for this crossing were thrown out.
0:57:32 > 0:57:34The Forth rail bridge was beefed up
0:57:34 > 0:57:42into a massive cantilevered skeleton built, not of iron, but steel.
0:57:42 > 0:57:46This bridge is really a memorial, a masterpiece in steel,
0:57:46 > 0:57:50to the poor souls who lost their lives in the Tay Bridge tragedy.
0:57:56 > 0:58:00Surging waters urge us on to fresh endeavours.
0:58:02 > 0:58:06And we're not alone in finding creature comforts
0:58:06 > 0:58:09around the fringes of our great seaways.
0:58:11 > 0:58:16The tide brings in the bounty that makes our estuaries brim with vitality.
0:58:24 > 0:58:30Safe havens that offer boundless prospects.
0:58:30 > 0:58:34Where rivers collide with the sea our coast comes alive,
0:58:34 > 0:58:37and opportunity awaits.
0:58:40 > 0:58:43Subtitles by Red Bee Media Ltd