An underground cathedral protecting Tokyo from floods

Re:An underground cathedral protecting Tokyo from floods by bdamm on Friday November 30, 2018 @06:01PM (#18573540)

It's big, but I haven't seen any information about how the capacity of the system compares to the capacity of the rivers feeding into it to generate water. Would the system survive a Harvey-type event? Or is it meant to deal more with seasonal fluctuations to prevent runoff causing flooding? 100 tons per second doesn't seem like a lot to me, compared to runoff that can be generated during spring thunderstorms. It's impressive and no doubt Tokyo will benefit from the infrastructure, but really the main chamber does not look much larger than a dig for a large skyscraper.

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Re:An underground cathedral protecting Tokyo from floods by 240dl on Friday November 30, 2018 @07:03PM (#18573963)

In addition to the main chamber there are several pits that also store lots of water. Here's a photo I took on the tour that might show it better: https://static1.squarespace.com/static/5265796de4b0bacb22141...

Unfortunately I don't remember the tour specifics well enough to confirm the exact use case of the system.

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Re:An underground cathedral protecting Tokyo from floods by kweks on Saturday December 01, 2018 @01:54AM (#18575621)

From what I remember (and similar to other designs), the pits take the brunt of the flash flooding, before releasing into the chamber. The reservoir can hold the water for as long as required before dispatching it.

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Re:An underground cathedral protecting Tokyo from floods by twblalock on Friday November 30, 2018 @08:12PM (#18574311)

In the past few years there have been some large storms that missed Tokyo by a fairly small distance. The people who operate this system say that it would have been over capacity if those storms had hit Tokyo directly.

It's an impressive system but there are definitely still storms large enough to overwhelm it. But, combined with the other flood defenses in the area, it has made flooding less common in all normal weather scenarios, which is a big win compared to the situation before it was built.

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Re:An underground cathedral protecting Tokyo from floods by dmurray on Friday November 30, 2018 @07:23PM (#18574064)

The article talks about Tokyo being designed to handle 5 cm/hr of rain. That's extreme but not record breaking.

The rate given for the Edo Discharge Channel pumps in the article is 200 tons per second. At 5cm/hr, that's enough to drain a region 150km x 100 km, or the whole Tokyo metropolitan area.

There are other drains (like the Edo River itself) and other sources of rainfall, but based on that calculation 200 tons per second is a huge contributor to draining even a megacity.

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