According to this Tokyo is now prepared to handled 50mm/hour rainfall. There are plans to increase this to 75mm/hour level, but that will take around 20 years.

According to this Tokyo is now prepared to handled 50mm/hour rainfall. There are plans to increase this to 75mm/hour level, but that will take around 20 years. http://www.japantimes.co.jp/news/2015/09/21/national/tokyo-high-risk-devastating-floods-experts-say/ http://www.japantimes.co.jp/news/2015/09/21/national/tokyo-h... Handling for example 10 inches (25cm) of rain in half day is quite a challenge on buildup area. Think about a quite small plot of 100m x 100m. From that you get already 1001000.25 = 2500m3 of water and in cities you have square kilometers of paved area where the water needs to be taken somewhere. In somebody is interested in how they are handling this in Tokyo, here's some information: https://japangasm.wordpress.com/2012/01/12/the-underground-wonder-of-tokyo/ https://japangasm.wordpress.com/2012/01/12/the-underground-w... and http://edition.cnn.com/2012/10/31/world/asia/japan-flood-tunnel/ http://edition.cnn.com/2012/10/31/world/asia/japan-flood-tun...