Secrets Of The Thames Barrier

By M@ Last edited 91 months ago

Last Updated 17 October 2016

Secrets Of The Thames Barrier
Looking up from the "Bull's nose" (upriver) side of the barrier.

The Thames Barrier was completed in 1982, since when it has protected London from flooding. We took a tour of the structure, gaining access to the river piers, the control centre and a little-known tunnel beneath the river...

What's beneath the Barrier?

A tunnel under the Thames, connecting the piers. The space is often used for training by the emergency services.

Wikipedia maintains a list of tunnels under the Thames. It is missing at least two. They're directly beneath the Thames Barrier and are used by maintenance teams to move quickly from pier to pier.

These are uniquely sensorial tunnels. A faint, musty odour pervades the air. Voices echo from unseen parts of the structure. A ship passes overhead, and the hum of its engines propagates down to this riverbed subway. Colour-coded pipes and conduits lace around bulkheads. Not many structures from the 1980s can be said to ooze character, but a walk beneath the Thames Barrier is truly atmospheric.

Each pier has a pine wood interior.

Could the barrier fail?

Above water, the Barrier comprises nine metal-hooded piers and two 'abutments' (additional piers without the sleek superstructure). All are built to last, with thick concrete hulls accessed by the heaviest steel doors we've ever opened. Redundancy is everywhere. The Barrier's function is so important to the capital that all systems have backups — including the service tunnel, which runs in duplicate. Each flood gate has a dedicated motor... and five spares. Should the National Grid go down, the Barrier has three backup generators that could operate for up to a month. It is designed to be bomb-proof and failure-proof. When a 3,000-tonne dredger hit the Barrier in 1997, the ship sank. The Barrier lost a ladder.

The Barrier's huge steel walls weigh 3,700 tonnes. This is one of the smaller non-navigable barriers, to be found at either end of the structure. Their resting position is above-water, unlike the main barriers, which lie on the river bed when not in use. The barriers are actually hollow, and maintenance teams can climb inside.

It needs to be tough. The Thames flood of 1928 killed 14 and made thousands homeless. Such an event today would cause billions of pounds worth of damage in central London, and knock out the Tube for weeks. With the Barrier in place, only a "once-in-1,000-years" mega-tide could inundate the capital.

Why has it closed so many times lately?

The Port of London Authority's control centre overlooks the barrier. From here, all shipping movements are monitored.

The Barrier has flexed its muscles with more regularity of late. Last winter, the steel wall was deployed on 50 occasions — more times than the previous 10 years put together. The vast majority of these closures were nothing to do with tidal flood waters from the North Sea. Instead, most were to manage water entering the Thames upriver, from rain runoff. If all this draining water were to meet an incoming tide, it could back up, damaging properties in west London. The Barrier holds the water in the wider reaches of the Thames until tidal conditions are right for its release. Last winter was one of the wettest on record, which mostly accounts for the increase in closures.

Underwater signposts.

Who looks after it?

The Environment Agency's distinctive control tower, which oversees closure of the Barrier.

Two operation centres oversee the Barrier from the Charlton shore. The Environment Agency (EA) is based in a peculiar concrete building immediately south of the river. You can see its green flag fluttering from the silver metal roof, which recapitulates the form of the hooded barrier. The EA has the keys to the Barrier itself, planning ahead for maintenance and closures. Sealing the Barrier is not as easy as simply pressing a button. The operation begins typically 12 hours before closure. Many river users need to be notified. Port of London Authority (PLA) boats must be put in place either side of the Barrier to alert and instruct approaching vessels. The closure process itself takes about 90 minutes, and is done in stages to minimise 'reflective waves' caused by the sudden imposition of 3,300 tonnes of metal blocking the river flow. At full closure, the river can be up to four metres higher on one side of the Barrier.

Can ships just pass through?

A tug pulls a barge of soil, probably from a Crossrail site, through the Barrier. The gap between piers is 61 metres, precisely and deliberately the same as the width through Tower Bridge.

Every vessel that passes through the barrier is required to seek permission via radio. Around four vessels per month fail to identify themselves, triggering various levels of response. A recent example saw a boat approaching the barrier faster than the 12 knots maximum. On interception, it turned out to be a perfectly legitimate vessel, but with an inebriated crew.

Traffic is monitored via another, more understated, operation centre to the east of the Environment Agency. Here the PLA monitors the 3,000-5,000 vessels that pass through the Barrier every month. "We're like the Air Traffic Control for ships," says Frank Jonathan Hart, the vessel traffic services supervisor manning the bridge on our visit. Frank shows us his suite of monitors. A trio of screens depicts the Thames from Teddington to Crayford Ness. The system uses a combination of marine radar and AIS to plot each craft's position and direction. You can have a play with a simple version yourself on the Marine Traffic website. The team also has access to thousands of cameras across London, including many on the river, which help identify any developing problems. The coastguard control centre is based in the same room as the PLA's team, making for an efficient operation.

What's the future look like?

A hydraulic arm.

The Barrier was originally designed to last until the 2030s, after which its effectiveness was expected to decrease. Although sea levels are rising by around 6mm per year, this is not as high as the 8mm predicted at the time of the Barrier's design, meaning that it should be effective for longer. Careful maintenance should now see the defences working until the 2070s. That said, there is always some uncertainty in climate prediction. The Environment Agency and other organisations are carefully watching and measuring, so that informed decisions can be made about the capital's future flood defences. The Thames Estuary 2100 report one step in preparing for that future.

With thanks to the Port of London Authority and the Environment Agency for granting access to their sites.