Environmental Damage from Urban Flooding (1990s–2020s) - May 1996 1990s: Urban flooding caused by torrential downpours became frequent in the 1990s, posing challenges nationwide in Japan. The summer of 1994 saw severe water shortages, prompting increased attention to rainwater reuse. In Tokyo, the Sumida Ward Office introduced a 1,000-cubic-meter underground rainwater storage tank, reusing over 30,000 tons of water annually. Additionally, the Tokyo Dome (designed by Takenaka Corporation) collected rainwater from its 36,000-square-meter roof, using 600 cubic meters per day for toilet flushing.

Environmental Damage from Urban Flooding (1990s–2020s) - May 1996 1990s: Urban flooding caused by torrential downpours became frequent in the 1990s, posing challenges nationwide in Japan. The summer of 1994 saw severe water shortages, prompting increased attention to rainwater reuse. In Tokyo, the Sumida Ward Office introduced a 1,000-cubic-meter underground rainwater storage tank, reusing over 30,000 tons of water annually. Additionally, the Tokyo Dome (designed by Takenaka Corporation) collected rainwater from its 36,000-square-meter roof, using 600 cubic meters per day for toilet flushing.

In Fukuoka City, Fukuoka Dome installed a 2,900-cubic-meter rainwater storage tank, utilizing approximately 55,000 cubic meters of rainwater annually for plant irrigation and toilet flushing. These initiatives aimed to balance preventing urban flooding with efficient resource use, though collaboration between local residents and businesses remained a challenge.

2000s: In 2001, Tokyo completed the "Metropolitan Outer Ring Floodway" (Underground Temple), consisting of a 6.3-kilometer tunnel and a massive pressure-regulating reservoir, capable of draining over 500,000 cubic meters of rainwater annually. This facility prevented flooding in approximately 20,000 households during the heavy rains of 2008, achieving significant results.

Osaka City formulated its "Basic Plan for Heavy Rain Countermeasures in Sewers" in 2004, developing drainage capacity to handle rainfall of 70 mm per hour. In 2009, it expanded rainwater storage facilities in the Yodo River basin, increasing total storage capacity to 50,000 cubic meters. Furthermore, collaboration with businesses and research institutions advanced flood prediction technology, contributing to disaster risk reduction.

2010s: Amid frequent extreme weather due to climate change, Tokyo promoted the "Tama River Flood Countermeasure Project." During Typhoon No. 15 in 2011, enhanced drainage pumps minimized damage in the Arakawa River basin. Furthermore, during the 2014 flash floods, Sumida Ward's underground storage basin activated, processing approximately 12,000 cubic meters of rainwater.

In Osaka City, the "Osaka Mega Rainwater Storage System," designed by Takenaka Corporation, began operation in 2015. With a storage capacity of approximately 100,000 cubic meters, it streamlined drainage into Osaka Bay. In Fukuoka City, the storage capacity of the Sannō Retention Pond doubled in 2018, enabling the reuse of 70,000 cubic meters of rainwater annually. This significantly mitigated damage during Typhoon No. 21.

2020s: Since 2020, Tokyo has advanced expansion work on the Metropolitan Outer Floodway. In 2023, new drainage pumps were introduced. Capable of discharging 45 tons of rainwater per minute, these pumps further reduced flood risks for residential areas along the Arakawa River.

Osaka City promoted its "Osaka Mega-Flood Countermeasures" in 2022, increasing the total capacity of its storage tanks to 100 million cubic meters. Furthermore, in collaboration with Daikin Industries, it introduced new rainwater filtration systems. The purified rainwater is now also utilized for urban greening projects.

In Fukuoka City, the "Smart Rainwater Storage System," developed with Daikin's cooperation, was introduced in 2023, achieving an annual reusable rainwater volume of 120,000 cubic meters. This has significantly mitigated flood damage during typhoons and reduced disaster risk.