On October 11th, the topic of “15-kilometer underground channel in the Forbidden City” trended on Baidu’s hot search. Chinese media reported that people got their first look at the process of directing rainwater into the Jinshui River and draining it outside the Forbidden City through the 15-kilometer underground channel.
The Forbidden City in Beijing, also known as the Palace Museum, was built in the fourth year of Emperor Yongle of the Ming Dynasty (1406 AD) and completed in the eighteenth year of Yongle (1420 AD). Covering an area of 720,000 square meters with walls approximately 3.4 kilometers long, it is one of the largest and best-preserved wooden architectural complexes in the world.
After the record of the Forbidden City not flooding for 600 years was broken in 2023, every heavy rain in Beijing has sparked people’s interest in the drainage system of the Forbidden City.
According to reports from the Chinese media on October 11th, workers recently sent robots to drill into the underground channel, revealing for the first time the hidden 15-kilometer-long channel beneath the magnificent golden tiles and red walls of the Forbidden City.
These underground channels connect branch canals and trunk canals, all leading to the Inner Jinshui River, which then merges with the moat around the city, draining rainwater outside the city. The scale of this underground system surpasses the visible Jinshui River on the surface, directing rainwater from over 90 courtyards and 720,000 square meters of the palace to the drainage system outside the Forbidden City.
The dense layout of buildings in the Forbidden City, with multiple layers of courtyards dividing them, and the high-pitched roofs of various halls all designed to swiftly drain rainwater.
The main buildings like the Hall of Supreme Harmony, the Hall of Central Harmony, and the Hall of Preserving Harmony have corresponding drainage systems under their bases. The artificial lakes within the Forbidden City, Taiye Pool and Houhai, also serve to regulate floodwaters, slowing down the flow to prevent floods caused by heavy rainfall.
The buildings of the Forbidden City incorporate various drainage holes. For instance, the three main halls mentioned earlier are situated on three-tiered terraces, each with a gentle slope of 3% to 5%, ensuring that water does not accumulate under the central palaces.
There are drainage holes at the base of the railings around the terraces, adorned with over 1,100 chimeric dragon heads designed for drainage, allowing rainwater to quickly flow down to the lower levels, creating a spectacular sight of “a thousand dragons spitting water” during heavy rains.
After the rainwater falls, it is channeled through visible trenches around the buildings, entering the Inner Jinshui River or flowing into the underground channels through various drains (money eyes, gutters), eventually emptying into the Inner Jinshui River and City waterways.
During the Ming and Qing dynasties, the underground channels were cleaned of blockages every March to ensure that the architectural wonders of the Forbidden City were not affected by floodwaters during the rainy season.
Through its ingenious drainage system, the grand architectural complex of the Forbidden City has withstood numerous sudden downpours unscathed for 600 years, displaying a remarkable miracle of not being flooded in storms.
These vast drainage systems are still functioning today. Ninety percent of the current rainwater drainage facilities in the Forbidden City are legacy structures built by ancient craftsmen, with modern constructions accounting for less than 10%.
In August 2023, due to heavy rainfall, severe flooding occurred on the west side of the Cining Palace courtyard. Chinese experts commented that as the residence of emperors during the Ming and Qing dynasties, the Forbidden City had remained unflooded for 600 years, and sudden waterlogging was not a good sign.
Subsequently, parts of the Forbidden City experienced multiple instances of flooding, which were resolved after workers cleared debris like large water bottles and plastic bags from the drainage channels to restore normal drainage functions.