Bouncing Up and Down... "Beware of Manholes During Heavy Rain"

Manhole Cover Weighing 40kg Opens in 41 Seconds Amid 50mm/hr Rain
Manhole Avoidance Needed in Heavy Rain Areas

In the early morning of the 9th, the drainage grate at Daechi Intersection in Gangnam-gu, Seoul, where many vehicles were submerged due to heavy rain, is swirling without its cover. [Image source=Yonhap News]

[Asia Economy Reporter Yoon Seul-gi] Manholes dislodged by heavy rain pressure are threatening public safety. Incidents have occurred where manhole covers are thrown above ground or people fall into open manholes, and experts emphasize that during heavy rain, the best course of action is to stay as far away as possible from flooded areas or avoid manholes.

On the 8th, when record-breaking heavy rain fell, an adult brother and sister went missing after falling into a manhole on a road in Seocho-gu, Seoul. The younger brother was found dead about 1.5 km in a straight line from the disappearance site. At the time, the two had taken shelter in a nearby apartment due to the heavy rain but went back out and fell into the manhole with an open cover they failed to see. In the Gangnam area that day, heavy rain of 100 mm per hour poured, and it is estimated they were swept away by the rapid current inside the manhole.

Earlier, the fire department deployed underwater robots to search for them but faced difficulties. Once someone falls into a manhole, they are quickly swept away by the internal rapid current, making rescue and search very difficult.

Manhole covers weigh between 40 kg and 160 kg, but sudden flooding causes water pressure to dislodge them from the ground. Even if locking devices are installed on manhole covers, they can still open due to water pressure.

In 2014, the National Disaster Safety Research Institute conducted an experiment recreating the frequently flooded area near Gangnam Station. The results showed that under 50 mm per hour heavy rain, a 40 kg manhole cover rose more than 26 cm into the air within 41 seconds. Under 20 mm per hour rainfall, the cover dislodged in 4 minutes and 4 seconds. When manhole covers open during heavy rain, people can be hit by the flying cover or fall into the manhole, leading to casualties.

Currently, there are a total of 624,318 manholes installed in Seoul. As localized heavy rain is expected to occur frequently in the future, there are calls for the Seoul Metropolitan Government to conduct comprehensive manhole safety inspections and take related measures to prepare for disaster situations.

Experts emphasize that in heavy rain situations like now, the safest option is to stay as far away as possible from flooded areas or avoid manholes. It is difficult to visually distinguish dislodged manhole covers, so there is a risk of falling in. If someone falls into a manhole, they should immediately call 119 or ask other pedestrians for help. Special caution is needed for manhole covers from which bubbles are rising. This phenomenon occurs when air inside the manhole is expelled due to rainwater pressure, creating a dangerous situation where the cover can suddenly pop up at any time.