The number of underground facilities for temporarily storing rainwater to protect urban areas from torrential rainfall has more than tripled over the past 10 years, reaching about 60,000, it has been learned.
It is said that the private sector is a driving force for constructing such facilities in recent years as urban areas have frequently experienced "inland water overflow," in which sewers overflow due to torrential rain.
The central government also provides a system to promote the construction of large-scale rainwater storage facilities in preparation for an increasing number of disasters caused by intense rainfall. The system's adoption, however, has been sluggish as there are high hurdles to apply.
According to the Land, Infrastructure, Transport and Tourism Ministry and the Association for Rainwater Storage and Infiltration Technology in Tokyo, around 1960, due to roads being paved and residential land development, rainwater that did not seep into the soil started to flow into sewer systems and other facilities, causing floods. To deal with the problem, construction of rainwater storage facilities started above and below ground to catch rainwater and then discharge it into sewer systems and rivers.
Local governments have also encouraged private business operators to build such facilities in accordance with the City Planning Law, and construction of small and midsize facilities has been spreading in the underground areas of houses, buildings, parking lots and other places in urban areas.
According to the association, the number of these facilities increased to about 60,000 in fiscal 2019 from about 17,000 in fiscal 2009, partly because localized heavy rains frequently occurred.
The annual number of newly built facilities has been as many as 4,000 to 5,000 since fiscal 2013.
The ministry revised the Sewerage Law in 2015 to promote the construction of large-scale underground rainwater storage facilities.
Under the revised law, when a local government designates a certain area by ordinance, businesses that build such facilities in that area can receive a subsidy of up to 66% of their construction costs from the central and local governments.
However, the subsidy system has been used only for two areas: around JR Yokohama Station, designated by the Yokohama municipal government; and around Keio University's Shonan Fujisawa Campus, designated by Fujisawa, Kanagawa Prefecture.
The ministry states that based on its own analysis this system involves enacting ordinances and expenditures by local governments and that these are "high hurdles for them."
During Typhoon No. 19 in October last year, the 6.3-kilometer-long Metropolitan Area Outer Underground Discharge Channel, a type of underground rainwater storage facility in Kasukabe, Saitama Prefecture, captured about 11 million tons of rainwater and discharged it into the Edogawa river, contributing to damage mitigation.
Prof. Hiroaki Furumai, an urban engineering specialist at The University of Tokyo, said: "Constructing large-scale underground rainwater storage facilities is an effective countermeasure against flooding, but in urban areas, building such facilities is achieved only in urban redevelopment plans or on similar occasions. To promote construction of more facilities, it is necessary to establish a system for the public and private sectors and local residents to exchange information."
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