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.
It cost about 80 bucks all told and was a bit of an ordeal to organize (you need a fluent Japanese speaker), but was definitely an experience. It was a little disappointing in how limited our tour was. I get the sense that they've narrowed down the area you can wander since opening. Still, I've explored storm drains in the US quite a bit and this was a whole different level.
The tour is free, but yeah, I had to pay transportation for myself and the guide (who I arranged online after some googling). The actual site is fairly far from central Tokyo, so it's about a 20-30 dollar train fare.
The guide himself was free, a nice older gentleman who used to be some sort of government liason. He insisted on taking a taxi for the 2 miles from the train station to the tour though, which was pricey. But I didn't have much of a choice (despite a whole face-saving false choice he offered of walking vs taxi that was deeply emblematic of a certain strain of Japanese cultural weirdness).
He offered to walk or take a taxi, but when I said I was fine with walking he just kept repeating the same the question with increasing emphasis until I agreed to the taxi.
> The entire facility has quite a sci-fi aspect to it.
Which is likely why it's a common backdrop for many a Tokusatsu shows[0]. Every season of Kamen Rider[1] -- and likely Super Sentai[2] as well; I'm not sure since that's too juvenile for my taste -- is all but guaranteed to have at least one episode filmed there.
https://memorymulch.net/journal/2018/11/18/maoudc