Advanced Drainage System in Ancient Beijing
The flood problem in Beijing, Chinese capital situated away from coast and major rivers, becomes more severe in the recent decades.
The cause could be multifactorial: The expansion of the urban footprint, the impermeable road pavement, the lack of maintenance and repair to the underground drainage infrastructure, and the damage done to the natural and manmade water system that historically served as reservoir. All these contribute to the frequent flood in the modern Beijing.
But Beijing was not like this before, and is not everywhere in Beijing like this today.
In July 2012, a super storm lashed Beijing that saw part of the Chinese capital inundation with dozens of casualties recorded, but the Round City (Tuancheng) at the centre of the storm was not affected by the flood at all.
The Drainage System in the Round City
The Round City is located in the heart of Beijing, between Northern Sea Park (Beihai Park) and Middle Sea & Southern Sea (Zhongnanhai, the central headquarters of the Chinese government).
There is no single drainage ditch on the ground, nor is any drain opening in the 276 metres long walls that encircle the Round City detected.
So why the area in not, and in fact never, flooded even for once?
The secret lies in its special pavers.
Each paver is in the shape of an inverted trapezoid, and the material that made the bricks has superb water absorption capacity, thus each brick acts like a mini-water reservoir.
Apart from that, there are also eleven waterholes scattered around the trees, and under each waterhole there is a vertical pit, while pits are linked with each other by brick arch culverts to form an underground drainage network.
The Drainage System in the Ancient Beijing
Beijing was constructed 600 years ago by the third Ming Emperor Yongle, based on the city planning model of old capital city Nanjing and the amendments made according to Beijing’s unique geographic setting, feng shui configuration and existing infrastructure condition.
When it came to the stormwater management, a comprehensive drainage system was set up right from the master plan stage.
In the inner city district, there were Inner City Moat, East Trench, West Trench and a water channel encircled the Forbidden City; in the outer city vicinity, Dragon Tassel Trench (longxugou) and Tiger Arch Trench (hufangqiao) along with Sanli River form a waterway network. All trenches were designed to allow the water naturally flow from higher terrain in the north towards the lower ground in the south, which reflects an advanced land survey technology, urban design skill and infrastructure construction quality.
Besides a highly effective drainage solution in the city’s microscope, most public buildings, including Altar of Heaven, Bell & Drum Terrace and various archways, have stormwater drainage pipes and gully pits installed, which efficiently prevented water from accumulating in the surrounding areas.
The Drainage System in the Forbidden City
As for the drainage design in the Forbidden City, that is even more sophisticated.
First of all, the site ground level falls nearly two metres from the north at Reduced Level (RL) 46.05m to the south at RL 44.28m, while the land in the middle is much higher than that in the east and in the west.
Secondly, the palace drainage system is linked to the moat that surrounds the Forbidden City, while the water in the moat is further connected to the ponds in the urban area and a river outside the city wall.
Thirdly, while the buildings are set on a multi-tiered platform that do not connect to the underground drainage ditches, such as the three major meeting halls in the Forbidden City, stormwater discharge points are installed.
Finally, stormwater is also distributed to a drainage system under the Forbidden City through numerous culverts and pits.
The construction of the Forbidden City and the city of Beijing is indeed a great architectural miracle.
Today’s Beijing is several times bigger in scope than the city when it was built 600 years ago. But does it mean the flood water in Beijing has to be several times deeper than the ancient city each time after a storm?