I said in an earlier post I'd tell you about my visit...
Described as one of the world's largest underground discharge channels, 50 metres below ground; the system comprises a tunnel of 6.3km long /10m wide, 5 huge shaft/silos, and a pressure settling water chamber. Plus pumps, aircraft engines (to move the water) and other assorted equipment. The shafts are large enough to accommodate the Statute of Liberty and even the Challenger space shuttle & vary a bit in size, the largest 2 being 71m high & 31.6m wide.
So, the larger Tokyo area is in a large shallow basin and floods fairly frequently. The flood facility (my words the actual title is very long) has a flash control room which monitors the 5 rivers flowing through the area. When the 4 contributing rivers flood, the water is diverted into nearby shafts; and when they reach a prescribed point, the water flows through the tunnel & other shafts finally reaching shaft #1 which is part of the underground settling chamber. This chamber is 177m long, 78m wide & 18m high. It has 59 concrete pillars (7m long, 2m wide, 18m high) which act as baffles and quieten the water, stopping it surging, before its release into the Edogawa river which takes it to Tokyo Bay.
The machine that dug out the shafts and the tunnel is still parked on site and is the same sort of circular excavator used for the Auckland train tunnels and also the UK-France Channel tunnel.
There are 4 differing tours you can take the most strenuous involving climbing shaft 1. Given my previous 3 weeks of long days, I elected to stick with the easy tour of the chamber. Once I got there I wished I had been more adventurous, but I have a vague recollection that photo opportunities may not have been available on the other tours and they did monitor us closely so we couldn't take photos on the 110 steps in and out of the chamber.
It's very well set up for visitors with a gallery, movie theatre, models and photos. The authorities have been very surprised at the level of interest both from foreign & national visitors.