The Terrifying Moment a 60 kg Manhole Cover Plummets from 6 Meters

Also in front of Shinjuku Station, Tokyo: ……
In addition to aging, the risk of occurrence has increased due to the growing size of typhoons and the increase in guerrilla downpours

A woman’s scream could be heard amidst the tremendous downpour,” said one witness. When I looked in the direction of the voice, I saw water violently spraying up from the manhole. It must have been about 15 meters high. The force of the water blew the manhole cover off,” said a pedestrian who witnessed the incident.

On August 21, sewage water blew out of a manhole in front of JR Shinjuku Station in Tokyo. The Tokyo Metropolitan Government has been working to improve facilities that can withstand 75 mm of rain per hour, but on that day, record-breaking short-time heavy rainfall information was announced for the neighboring Minato Ward area. It is believed that the design limit was far exceeded, and water that had nowhere else to go blew out all at once. An official from the Bureau of Sewerage of the Tokyo Metropolitan Government explained.

When a large amount of rainwater flows into a sewer pipe in a short period of time, the water level rises quickly and air pressure rises sharply, causing the ‘air hammer phenomenon. On that day, manhole covers were displaced and pavement was raised in five locations.

The air hammer phenomenon can generate as much as 6 tons of pressure at a moment’s notice. In Shinjuku, a manhole cover weighing about 60 kg fell down, blown away to a height of about 6 meters.

Similar phenomena have occurred all over the country.’ Junpei Tanaka, a resident of Higashiosaka City who suffered damage in July 2009, recalls.

“In the middle of a guerrilla downpour, ‘Bong!　Boom! I heard a loud noise during the guerrilla downpour. When I looked out from my apartment, I saw that the water had blown up from the manhole to the height of my room on the ninth floor. A lot of debris was also blown away and hit my car, which had to be scrapped.

The risk of air hammering is increasing due to the increase in the number of giant typhoons and guerrilla downpours. Mr. Naohide Oishi of the Japan Ground Manhole Manufacturers Association sounds the alarm.

There are about 16 million sewage manholes in Japan, and in the past 23 years, there have been 106 incidents of lids being blown off from torrential rains. Lids manufactured after 1991 have safety features, but there are nearly 6 million older types without them.”

As long as the old lids remain, it is important to stay away from manholes during heavy rain.

From the September 27 and October 4 combined issue of FRIDAY