0:00:25 > 0:00:29The Thames barrier is a direct result of the 1953 floods.
0:00:29 > 0:00:33It's one of the largest and most expensive defences of all,
0:00:33 > 0:00:37designed to protect London from surge tides until 2030.
0:00:37 > 0:00:39It's absolutely gigantic.
0:00:39 > 0:00:41That's what it takes to stop
0:00:41 > 0:00:44the capital city drowning.
0:00:45 > 0:00:51Beautiful too. It's easy to forget how utilitarian engineering can look absolutely stunning.
0:01:01 > 0:01:06With the high tide mark creeping up by 60cms a century, even this has its limitations.
0:01:09 > 0:01:16Engineers are looking at extending its use until 2100 by raising the estuary walls by half a metre.
0:01:16 > 0:01:19And another option, for the distant future,
0:01:19 > 0:01:23is an even bigger barrier further out into the estuary.
0:01:26 > 0:01:31There'll always be pressure to defend the nation's capital.
0:01:31 > 0:01:35But, outside London, the costs are less easy to justify.
0:01:36 > 0:01:41Continuously replacing rock armour is like signing away a blank cheque to the sea.
0:01:41 > 0:01:46So, in the Black Water Estuary, they're thinking about coastal defence in a whole new way.
0:01:49 > 0:01:53I'm cutting across the fields at Abbotts Hall Farm in Essex
0:01:53 > 0:01:57to look at an unconventional approach to coastal defence.
0:01:57 > 0:01:59Just down here, on the edge of the estuary,
0:01:59 > 0:02:02they're cutting holes in the sea wall.
0:02:05 > 0:02:08Punching through the old defences seems like madness
0:02:08 > 0:02:14but it's like opening a safety valve to take the pressure of erosion out of the estuary.
0:02:14 > 0:02:19This allows the water inland to create inter-tidal salt marshes -
0:02:19 > 0:02:23nature's defence against the power of the sea.
0:02:26 > 0:02:28What these salt marshes do,
0:02:28 > 0:02:31they take the energy out of the water. The currents slow down
0:02:31 > 0:02:36as they move through creek networks and the plants jut into the water
0:02:36 > 0:02:39and take the wave energy out of the wave.
0:02:40 > 0:02:44Repairing old defences is a costly job.
0:02:44 > 0:02:47In the long run, creating man-made salt marshes can be cheaper.
0:02:49 > 0:02:56But acres of farmland have to be given over to the buffer zone, so the scheme's not without critics.
0:02:56 > 0:03:00In people's minds it probably does feel like we're giving up.
0:03:00 > 0:03:03But we're not. We're trying to work with nature,
0:03:03 > 0:03:07so when we have another 1953-type event, we're prepared for it.
0:03:07 > 0:03:11We have areas like this for water to be stored in terms of the flooding.
0:03:11 > 0:03:15You're talking about a coastline that's changing all the time,
0:03:15 > 0:03:18a map of the UK that could look quite different in 100 years.
0:03:18 > 0:03:21It's interesting, the word "coastline".
0:03:21 > 0:03:25I think what happened was, you know, we mapped our coastline
0:03:25 > 0:03:31and put a line on it and it's a snapshot of what it looked like when that person mapped that coast.
0:03:31 > 0:03:35So it's a very hard perception to change in people's minds
0:03:35 > 0:03:38that it isn't fixed, it's a very dynamic place.
0:03:38 > 0:03:42Coastlines are probably the most dynamic environments in the UK
0:03:42 > 0:03:43and it's constantly changing.
0:03:54 > 0:03:56It may be a bitter pill to swallow
0:03:56 > 0:04:01but we can't realistically protect every inch of our coastline.
0:04:02 > 0:04:06Concrete walls destroy the sense of wonder that draws us to the coast.
0:04:06 > 0:04:10But, without them, we face a precarious future
0:04:10 > 0:04:13in the face of advancing seas.