- If the Thames Barrier were to fail when it was needed,
the damage would be almost impossible to calculate.
The best estimates I've found say that the financial cost of London flooding
would be somewhere between £20 billion and £100 billion.
The Houses of Parliament,
every skyscraper in the city of London,
every building near the river would be unusable for weeks.
The central parts of the London Underground
would be out of action for months, possibly more.
That's before you get to the human costs.
Worst case, perhaps hundreds of thousands of people
who'd be forced out of their homes.
Which is why this barrier is designed to not fail.
To be clear, that still means it can have occasional glitches
or scheduled maintenance, or even have parts of it break entirely.
But if any of those things happen, it must still protect London.
And there's a whole team who work here full time
to make sure that it does.
- We aim to move each of the ten individual gates
at least twice a month to exercise them.
Today is one of our monthly test closures.
For test closures, we close just as the tide is finishing going out.
And that way, we can wait until the tide turns
and then we get equilibrium on both sides of the Barrier,
and therefore, we can reopen.
That avoids us having to be closed for the full tidal cycle.
- A couple of months ago,
the Barrier had a full high-tide test closure
and it is dramatic.
Normally, the gates just sit out of the way
on the bottom of the river,
but when they're needed, they're rotated up into place.
- Well, for the annual test closure,
because we're trying to maximise the differential,
the water levels on each side of the barrier,
we like to close early in the tidal cycle
to allow that differential level to build up
to a maximum of 4½ metres.
On a flood defence closure,
we close much later in the tidal cycle,
because we just want to stop the danger element of the tide
going up into Central London.
We try and be gentle with the river.
So, we start by closing the outer smaller gates.
And then, we slowly work across into the middle.
And sometimes, we'll bring the two 61-metre gates
in the centre of the river up a couple of metres below the surface,
before we finally bring them into the closed position.
And that avoids a reflective wave of the water
that's coming in and being stopped,
being pushed back downstream.
So, during a flood defence closure,
we don't have to stop the whole tide going into Central London.
We just need to stop the part of that tide
that would overtop the linear defences.
Normal spring tides that go into London each day
do not overtop the embankment walls, for example.
But if you've got a surge element on top of that,
that is what would go over the top of the walls in London,
and that's the element that we need to stop with the Barrier.
Once we've stopped the danger element of the tide
and got a differential,
we've effectively created a reservoir upstream of the Barrier.
We can then over-rotate the large gates
and allow a controlled flow of water upstream
into the reservoir that we've created.
And again, that reduces the amount of time
before we get equilibrium on both sides of the barrier and can reopen.
- The barrier cannot be allowed to fail when it's needed.
So, each gate can be moved by one of four separate motors.
That service tunnel I was in? There are two of them,
a few metres apart, identical,
with completely independent power and control systems.
The barrier has three connections to the UK power grid,
for both the north and south of the river.
And if the entire British national power grid fails,
which has never happened, but if it does,
the barrier has three separate diesel generators,
any one of which could operate the entire thing.
- We have a number of modes of operation,
so that if, when we're using Mode 1 operation,
we have a problem of a technical nature, we go to Mode 2.
Problem with Mode 2, we go to Mode 3.
We don't necessarily have time to sort out any technical issues,
but we've got other modes of operation that we can fall back on.
The barrier has to close when we need it to.
I always say that if the Prime Minister
was stood in the House of Commons in his wellingtons
and we said, "Sorry, PM, we'll get it right next tide,"
that would probably be fairly career-limiting(!)
- So, that's the technical resilience.
What about the physical construction?
Well, these piers are made of Portland cement,
so, they'll stand for a couple of centuries at least.
The steel gates are protected by what's called a sacrificial anode.
Corrosion in water like this is an electrochemical process.
So, if you connect a chunk of metal
that's more easily corroded to something like the gate,
that chunk of metal will take the corrosion damage instead,
and you can replace it fairly easily whenever you need to.
So, the gates will also be fine for a century or so.
What if a ship hits the Barrier?
Well, ships are designed to float.
Relatively speaking, they're not that solid.
These piers are thick concrete,
and each of the main gates is 3,000 tonnes of steel.
So, the ship's going to come off worse.
Collisions have actually happened several times.
In 1997, a ship called the Sand Kite hit one of the piers.
The only damage to the Barrier was some scratched paint
and a broken ladder, but the ship sank.
The trouble was that it sank where it hit
and sat broken on top of one of the open gates.
That would've blocked that part of the Barrier from closing.
The contingency plan, if there had been a flood tide,
would have been to close all the other gates
and use the force of the water trying to get through
to blast the wrecked ship clear.
But also, even if one gate was completely broken somehow,
the Barrier would still not technically fail.
The margin for error would be gone, but that's why the margin's there.
It would still hold back enough water to keep London safe.
- We have a series of forecast models
and we run what we call an ensemble forecast
that looks many tides ahead.
With any models, particularly forecast models,
you will always get a degree of error.
So, the forecaster here will make an adjustment to the forecast
and that data will be passed to the controller
from which to make a decision as to whether to close or not.
- So, the barrier is highly reliable and strong.
What if there's a really high tide,
a storm surge that's too much for this to cope with?
Well, the design odds of that are one in a thousand per year.
Not technically failure, just not being specified high enough.
The barrier was designed in 1970 for a 60-year life,
which would be 2030,
which must've seemed a very long way away then.
The good news is that with modern simulations and modelling,
the team here are confident that it'll last another 40 years at least.
But part of the reason for that is because those 1970s engineers
built in a huge margin of safety.
They didn't cut corners then, and that's given us breathing space now.
- The gates themselves,
with inspections and remedial works,
will probably be good to go until 2070,
but a lot of the supporting infrastructure,
the fire systems, the security systems, even the cranes on the piers
will all have to be updated in the years to come.
- But what if the tide went high enough to actually overtop the barrier?
Well, those gates are built strong enough that any water they can't hold back
can flow under into the low water on the other side.
Remember, they close the barrier early, when the river's at its lowest point,
so, yes, if some apocalyptic storm comes along,
the barrier should still work well enough.
If this was almost anything else,
you could say the redundancy was over-specified,
that requirements like this were ridiculous,
but with so much at stake,
the Thames Barrier is a marvel not just of engineering,
but of how you have to build when something cannot be allowed to fail.