Over the years, the public and private sectors in Tokyo and elsewhere in the country have implemented a host of measures, from building flood control-related infrastructure, including reservoirs, dams, dikes, and superlevees, to introducing water-resilient housing and educating residents about disaster preparedness.

In Kita Ward, the old red Iwabuchi sluice gate is a reminder of past efforts by Tokyo to help prevent flooding of the Sumida River, while a new blue version has replaced it. Discharge channels, though differing in design to the Kasukabe cistern, also exist in Osaka Prefecture and other areas.

But in recent years, Tokyo increasingly faces risks posed by climate change, Tsuchiya said, pointing to the challenges of the sudden, heavy downpours, known in Japan as "guerrilla rain."

Data from the Japan Meteorological Agency showed that the average annual heavy rainfall measuring 50 millimeters per hour or more for the 10 years from 2015 to 2024 has increased by about 1.5 times as compared to the decade starting in 1976, the first decade for which data is available.

While the agency says the metropolitan area can withstand rainfall rates of 50 mm per hour, Tsuchiya pointed out that the hourly precipitation level in Japan, hit by typhoons seasonally, now sometimes reaches up to 100 mm.

 Most recently, Typhoon Hagibis wreaked havoc on seven prefectures and disrupted the Rugby World Cup in 2019, although Tokyo escaped its wrath.

A key flood-regulating mechanism closer to central Tokyo is the Kanda River Ring Road No. 7 Underground Regulating Reservoir facility.

The 4.5-km-long tunnel is being expanded to link channels for overflow from Shirako and Kanda rivers and carry floodwaters into Tokyo Bay by fiscal 2027.

The Tokyo metropolitan government has drawn up a roadmap to make the capital, home to around 14 million people, resilient to various major risks -- storms and floods, earthquakes, volcanic eruptions, pandemics, and power shortages, as well as communications failure.

While Kasukabe's hidden water infrastructure continues to do the job of keeping Tokyo and its vicinity from being inundated, disaster management experts say the facility alone is not enough for sustainable river management.

Tsuchiya stressed the need to balance disaster management with future-focused measures, such as encouraging real estate operators to build raised houses, and cited the "room for river" concept introduced in Europe.

"Flooding does not happen just anywhere. It always occurs at the same places, so we can prepare for it," Tsuchiya said, labeling water disasters as largely "man-made."

"Water management is not only about managing water at one time but it is about how to take care of water so that 100 years or more from now, we can have a prosperous future," he said.