Following the heavy rains that recently inundated Beijing with such loss of life, the capital is to build 20 underground flood pools to relieve future storm flooding. They will be put under places known to susceptible to flash floods, low-lying roadways in particular.
A conventional surface flood pool is the land around a reservoir that is intended to be flooded in the event of extreme rains as the reservoir rises and backs up. The excess water is then run off by being released through the reservoir’s dam in subsequent days. Underground storm water storage works in much the same way, temporarily holding storm water until a city’s drainage system can handle it.
Flood pools are not only a common form of flood management, but, smartly managed, are also a potentially profitable source of municipal water supplies, irrigation, recreation facilities and fish or wildlife habitats, Beijing’s flood pools won’t bring such broad public goods as far as we can tell. They will have to be fitted in between all the other subterranean construction beneath a long-established city: sewer and water pipes, subway lines, building foundations and the like.
Modern underground storm water storage systems are often modular, so can be constructed as shape and space allows. Alternatively they can be built from large diameter pipes and threaded around obstructions. Their capacity is typically equivalent to a swimming pool and are easiest to retrofit to places like roads and parking lots that can be dug up and the systems installed under them.
Urban flooding is a worsening problem in China (and elsewhere) as global warming, urbanization and industrialization pose a growing triple threat to cities’ natural defences, defenses urban planners have anyway been concreting over with reckless abandon. It is, though, reasonable to ask why there hasn’t previously been more provision to deal with flash floods in the capital.
Hong Kong, for one, has underground storm water storage in several of its towns already and is planning to build more, notably in Happy Valley on Hong Kong Island which is to get an award winning, state of the art flood pool at a cost of HK$1 billion ($130 million). Hong Kong had the advantage of being able to build the lower -tech tanks it already has into its new towns from the outset, a luxury that Beijing doesn’t enjoy.
Urban planners are only now realizing that they have to make cities greener so they are less encouraging to extreme weather. All the newly constructed impervious surfaces such as roads, parking lots and rooftops, create run-offs that existing urban drainage, often old and inadequate, can’t handle. can’t handle. Not only is ground storage for rainwater scarce and run-offs from hard surfaces absent, ancient streams that could carry rainwater to rivers and ponds that could help it find its way to acquirers below have been filled in. As Beijing’s are expensively learning– the economic cost of the recent floods is estimated to be at least 2.3 billion yuan ($360 million), before the cost of installing the new storage tanks–they need to create the modern urban equivalent of something nature has provided naturally.