Here the boat approaches the Barrier in the distance.
Apparently the tide can get above the top of this sign.
The Barrier is set up as a series of "gates" that normally lie at the bottom of the river, so that ships can pass through. When a dangerous tide is expected, the gates are raised to hold back the water. We were lucky enough on our tour to see one of the gates raised for servicing (needless to say, the system doesn't work well unless all of the gates are raised together).
The aluminum (I refuse to say "al-you-MIN-ee-um") domes contain the hyraulics to raise the gates.
Here's a closer shot of the raised gate. The yellow arm is in the raised or "defensive" position.
The piers sit between two gates. Here's one gate in the raised position.
I just think these things are cool to photograph.
Near the bottom of this photo, you can see the circular nature of the gates. Each gate is essentially a cylinder that's been cut in half lengthwise. To raise the gate, the cylinder is rotated.
The raised gate from upstream, gives you a better sense of the cylindrical design.
I don't think I've explained this well, so let's have the official version with this animation:
http://www.environment-agency.gov.uk/homeandleisure/floods/117704.aspx
(be patient, it starts out kind of slow, but the drama builds)
Along the cruise, you pass this abandoned old ferry boat. Legend has it that this is the ship referred to in the song "Ferry Cross the Mersey"
Cost: £8.50
Getting there: Once you're in Greenwich, easy to hop on the boat. You can also board the same boat right at Westminster Pier
Kid-Friendliness: Pretty low, unless the kids are into engineering works
Italian Tourist Factor: Very Low Risk
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