Flooding of Parking Area: Heavy Rain Countermeasures for Underground Spaces Urgently Needed

Editorial

15:48 JST, October 3, 2025

Heavy rains that exceeded predictions have struck various regions in Japan, and the risk of flooding is heightened in parking areas, stores and other facilities located underground. Inspections must be conducted in every facility to determine whether preparations have been sufficiently made and measures must be taken.

On the night of Sept. 12, record-breaking heavy rain fell in the city of Yokkaichi, Mie Prefecture, resulting in flood damage to an underground parking area developed by the Land, Infrastructure, Transport and Tourism Ministry and a city-funded management company. The second basement level was completely submerged, while the first basement level was flooded up to a height of 1 meter, leaving 274 vehicles waterlogged.

There have been difficulties in removing the submerged vehicles from the parking area, and the issue of how compensation will be handled going forward also remains unresolved.

Dashboard camera footage from a car that narrowly escaped the underground area showed water rushing into it. Had the driver been trapped, it could have been life-threatening. It can be said that this situation was a reminder of how scary floods can be.

According to the ministry, electric-powered water barriers were installed at three car entrances, but two were already broken and the remaining one was rendered inoperable due to the flooding. Drainage pumps failed to operate because of a power failure.

What is problematic is that both the ministry and the management company were aware four years ago that the flood barriers malfunctioned, but they failed to repair them. This was because they could not decide on who should bear the repair costs, the ministry said.

The ministry has established a committee to examine the response to this incident. It is hoped that it will investigate the causes of the incident and work to prevent a recurrence.

There are underground shopping malls and parking areas near railroad stations in every region. Facility managers must examine the risk of flooding, further measures such as installing water barriers or sandbags and work to prevent electrical faults.

According to the Japan Meteorological Agency, the frequency of torrential rain events — defined as rainfall exceeding 80 millimeters per hour — has doubled compared to the 1980s.

In recent years, events where rainwater exceeds drainage capacity and overflows on the ground have also become more frequent. It is necessary to recognize that conventional measures are not able to deal with recent heavy rain events.

In September, the Tokyo metropolitan government revised its guidelines for measures against flooding in underground spaces for the first time in 17 years to address climate change. It provides a timeline of how relevant agencies should take action in underground malls during heavy rain and requires that evacuation drills be conducted.

Such spaces being located underground makes it difficult to assess conditions aboveground, and evacuation routes are limited. If flooded, water pressure could prevent doors from opening. Anticipating these unique underground challenges is crucial.

A sense of self-reliance is essential to protect oneself from floods. It is hoped that people will not forget to take action, such as, during heavy rain, not seeking shelter underground, staying alert to weather information and leaving underground areas if they notice something unusual.

(From The Yomiuri Shimbun, Oct. 3, 2025)