Labyrinths are built underground, tunnels are stuffed with balloons… How do these cities resist floods?

Labyrinths are built underground, tunnels are stuffed with balloons... How do these cities resist floods?

“If you can’t even Tokyo, we all have to be careful.”

The heavy rain hit a historical record. On July 20, the Zhengzhou subway was flooded by floods, causing the tragedy of at least 12 deaths.

About two weeks ago, after Hurricane Elsa in the United States, the subway in New York City was flooded by flood water. The video shows people walking through the muddy flood water around their waists at the 157th Street subway station on Broadway and Manhattan.

Labyrinths are built underground, tunnels are stuffed with balloons... How do these cities resist floods?

In July of this year, the New York subway was flooded and people waded through the subway

“It was turquoise dirty water with rubbish floating on it,” said a local resident. “It’s really disgusting.”

The turbid torrent pours down the escalator, like a waterfall flowing into the subway; in the subway platform, rain gushes from the manhole cover… These shocking images also appeared in many other subway stations in New York.

With the increase in extreme climates around the world, similar subway accidents have not only occurred in Zhengzhou, China and New York, USA. When the rainy season in the northern hemisphere comes in July, how to avoid the subway becoming a vast ocean has become a common challenge in major cities around the world.

1. New York’s century-old subway, repaired amidst controversy

Even on a dry, sunny day, New York can pump 14 million gallons of water from the subway system every day.

And on July 7 this year, when the city’s rainfall for a month was released within two hours, many subway stations in Upper Manhattan and the Bronx were flooded by rain.

Sudden floods caused suspension along Line A for several hours. Due to negligence, many pumps did not arrange staff in advance, and hundreds of pumps in the subway system were overloaded.

Before the subway resumed operation, New York City’s drainage system had reached its limit.

The outside world quickly blamed New York’s creaky old facilities. As the subway with the most stations in the world, it has a history of nearly 117 years and is also one of the longest subway systems in the world.

In terms of annual passenger volume, New York has the busiest subway system in the Western Hemisphere and the eighth busiest subway system in the world, after Beijing, Shanghai, Seoul, Guangzhou, Tokyo, Moscow and Hong Kong.

Over the past few years, the 2012 Superstorm Sandy (Superstorm Sandy) caused the most damage to the New York subway.

At that time, a dozen subway tunnels and nine train stations in New York City were flooded. After the hurricane, some subways on Line A were suspended for several months and repaired for several years, causing billions of dollars in damage to the transportation system.

It is controversial that the subway staff at that time only used sandbags and plywood to protect the subway from water seepage.

After Hurricane Sandy, the U.S. federal government allocated 8 billion U.S. dollars to repair subways that were flooded during the hurricane and to protect subway platforms in flood-prone areas in the city.

Since then, the United States has also begun to invent and design more special appliances for subway waterproofing.

Flexible gates are used to protect low-lying subway stations and can be opened and used by one person in a few minutes.

According to the Verge report, the New York Metropolitan Area Transportation Department has also installed thousands of portable vent covers, as well as bulky, submarine-like waterproof arched gates.

New York also equips high-risk, low-altitude stations with “elastic tunnel plugs”-balloons made of a 32-foot-long special material, which is usually used to make space suits for astronauts.

Labyrinths are built underground, tunnels are stuffed with balloons... How do these cities resist floods?

New York subway waterproof facilities made of special materials

The new equipment is installed at more than 150 subway entrances and 2,200 sidewalks in New York. This has allowed New York to make great progress in the subway system since Hurricane Sandy hit, including after the hurricane, the disaster situation was much lighter than in 2012.

However, the US media “THE CITY” released a report in April this year that dozens of prevention projects in New York have not yet been completed-including several miles of protective walls erected around Upper Manhattan and Coney Island subway stations.

Klaus Jacob , a geologist at Columbia University in the United States, also said that many waterproof devices have small flaws. “When they are used according to the design and plan, the reduced water intake of the subway is very good,” Jacob said. “The problem is that many devices need to be activated on site in time.”

Moreover, these waterproof devices cannot solve the problem of groundwater infiltration-another major threat related to hurricanes. Many creeks in New York are buried in the ground, and floods from hurricanes will fill the creeks and flow into subway tunnels at any time.

For the century-old subway, New York still needs to be repaired from time to time, with different measures addressing different problems. For example, to repair underground drainage pipes, New York will use a new type of concrete embedded in an impermeable plastic film to replace the original old concrete.

2. How can the Japanese subways withstand the floods?

When the subway station in the United States was flooded once and repaired for ten years, and the flooding continued, Japan, with abundant rainfall and frequent floods, also learned a bitter lesson and adopted strict engineering measures for subway flood prevention.

The Japanese subway, which was built in 1927, was flooded many times in heavy rains. For example, in September 2000, under the influence of Typhoon No. 14, Nagoya’s single-day rainfall was as high as 428mm, Shin River burst its banks, and the entire Nagoya subway Flooded.

Japan is small and mountainous, and most of the cities are located on plains. The rivers are not long but the drops are large. Japan has a subtropical monsoon climate. During the rainy season or typhoon season, cities on the plains are vulnerable to flooding.

Over the past 10 years, over 97% of Japanese cities have experienced floods. According to statistics from the Ministry of Land, Infrastructure, Transport and Tourism of Japan, between 2009 and 2019, only 48 of the 1,741 cities, towns and villages in the country have not suffered floods.

According to historical precipitation records, the Tokyo flood control capacity designed by the Japanese Urban Planning Department can withstand up to 50 mm of precipitation per hour, especially in densely populated and built-up areas. If the rainfall exceeds 50 mm in one hour, the road is prone to water accumulation.

Local roads are flooded, and water-retaining engineering measures are the cornerstone to prevent the subway from being flooded.

Taking the Tokyo Metro as an example, the main water inlets are station entrances, vents and wellheads. Except for Gaodi Station, all station entrances are equipped with water baffles—aluminum alloy material, 35cm*2, with a height of 70cm.

If the surface water of the station exceeds 1m and the height of the water baffle is not enough, the subway station is also equipped with a fully sealed waterproof door to prevent the entrance of the rainstorm from invading. In recent years, Japan has invented folding waterproof doors and rolling shutter type waterproof doors to solve the shortcomings of excessively large areas. There are more than 500 such facilities in Tokyo Metro stations.

Vents are another hidden water hazard. For example, there are vents between older subway stations such as Nagoya Station and Kameshima Station. Each vent is equipped with an anti-flooding tester. If the water pressure exceeds 2m, the gate can be closed according to instructions.

According to the Ministry of Land, Infrastructure, Transport and Tourism of Japan, flood control projects have been constructed for 370 subway stations in low-lying areas in the Tokyo Metro area, including the installation of remote control switch facilities for road ventilators in subway stations. Whenever heavy rain hits, the moisture-proof panels at the entrances of these subway stations and the waterproofing facilities on the railway will also be activated.

Of course, flooding on the outside of the buried section of the subway tunnel is also terrible. These subway lines use a U-shaped retaining wall structure with reinforced concrete walls at both ends of the top. If the subway tunnel is under the river, a waterproof gate must be installed in the tunnel. Once the accumulation of water reaches a certain level, the water pump in the subway station will be turned on to drain the water automatically.

Early warning work before and after heavy rains is also extremely important. The “Japan Water Defense Law” stipulates that in the event of extreme weather, the government will advance weather warnings, temporarily suspend subway operations, and guide passengers to evacuate. These are all non-engineering measures for subway flood prevention.

Labyrinths are built underground, tunnels are stuffed with balloons... How do these cities resist floods?

Japanese subway flaps, waterproof doors

3. Drainage + tourism, Japan’s giant flood control project has become an international model

The urban flood control project is an urban infrastructure construction that the Japanese government attaches great importance to. Even during the post-war recovery period in the 1950s and 1960s, the Japanese government still used 6% to 7% of the national budget for disaster mitigation and risk relief.

Tokyo’s current flood control system took shape only after World War II. Tokyo has planned for underground drainage for decades and is constantly building it. Now there are dozens of dams, reservoirs and flood control embankments. The same is true for the subway flood control design.

If you cut Tokyo underground, you will see a labyrinth of drainage pipes crisscrossing with subway lines and natural gas pipelines. Among them, the “Outer Metropolitan Waterway” completed in 2006 is an international example of Japanese flood control projects, and it is also known as G-Cans. This project cost 2 billion US dollars and lasted for 13 years. It is located in Saitama, 32 kilometers north of Tokyo. In Kasugabe City, it is the world’s largest underground flood diversion facility.

G-Cans is connected to five major rivers in Japan through five huge vertical shafts, each of which can accommodate a statue of Liberty.

Labyrinths are built underground, tunnels are stuffed with balloons... How do these cities resist floods?

Guofang Waterway Project Outside the Capital Region

Whenever a river floods, water will flow into these 70-meter-high giant drainage shafts, through a 6.3-kilometer-long underground drainage pipe, into a 22-meter-deep giant reservoir, which is also known as the “underground palace.” .

After the storm subsided, the water in the reservoir was pumped into the Edo River at a total speed of 53,000 gallons per second, and finally into Tokyo Bay. This is equivalent to completely draining an Olympic-sized swimming pool every 12.5 seconds.

When there is no water, the underground palace is a popular tourist attraction and is even used as a movie and TV commercial.

However, in recent years, Japan’s high degree of urbanization, rapid industrialization, and large-scale water resources exploitation have caused the ground to sink and the urban drainage system has become more vulnerable.

In 2014, a Swiss Reinsurance company’s research on the risk of natural disasters showed that damage caused by earthquakes, tsunamis and extreme rainfall may make Tokyo and the port city of Yokohama the most risky metropolitan area in the world.

At the end of 2015, the Tokyo metropolitan area was severely damaged by strong typhoon rainfall. More than 670 million cubic feet of water flowed into G-Cans. The system’s four large drainage pumps took four days to clear the flood. The engine provides power similar to a Boeing 737 jet. The power of the aircraft when it is launched.

“Tokyo is facing dangers in all directions,” said Nobuyuki Tsuchiya, a flood-fighting expert and former civil engineering supervisor in Tokyo’s Edogawa Ward. “It’s hard to say, we have done enough.”

Labyrinths are built underground, tunnels are stuffed with balloons... How do these cities resist floods?

The working principle of the Guofang waterway project outside the capital area

4. Frequent extreme weather, Singapore and the United Kingdom set up flood control lines

When extreme weather around the world hits, it is not only Tokyo that is at risk. International cities such as New York, London and Bangkok are also becoming more vulnerable to heavy rains and floods.

Most cities are establishing assessment programs and new defense systems. In 2018, the U.S. Federal Government issued an assessment report: The flooding of the New York subway will become more frequent, and more than 20% of subway stations are at risk of storm surges.

If carbon dioxide emissions are not reduced, in the next ten years, New York will experience heavy rainfall every five years. Floods over 2 meters high are enough to inundate the ground floor of buildings. In the 1970s, such major floods would only occur every 25 years. Happened once.

London, UK, listed flooding as the number one threat to its climate change plan. One fifth of the city is located on the plain where the Thames converges.

Although London already has a very safe flood control embankment, there is a solid Thames flood gate in the east as a guarantee -this is a closable river gate that spans 520 meters across the Thames and protects 125 square kilometers of tides in central London. surge. But British planners believe that these safeguards may not be sufficient in the future.

In the UK, you will often see such a word-SUDS, which is the abbreviation of “sustainable drainage system”.

According to the definition of the British Environmental Protection Agency, “sustainable drainage system” includes a series of technologies for sustainable management of surface water and groundwater. For example, plant flowers and grass on the roof of the building to intercept rainwater; set up rainwater collection facilities in the courtyard to water the garden and flush toilets; pave the pavement with permeable concrete blocks, gravel or permeable asphalt, thereby partially replacing traditional sewers The function of drainage ditches; trenches are dug on the roadside and filled with rubble stones, which can reduce the flow rate and flow of the rainstorm.

In Singapore in Asia, the National Water Authority has developed a multi-pronged approach to stormwater management- mandating new development and reconstruction of flood protection facilities, development of old drainage infrastructure, and continuous monitoring and improvement of drainage systems . Specifically, it includes: developing and maintaining an 8,000-kilometer-long drainage network, systematically widening and deepening existing drainage ditches and canals, and regularly updating surface water drainage practices.

Even with good drainage infrastructure and systems, manual intervention during the last mile is particularly important. Due to poor maintenance of the pumping system, in 2018, too much water accumulated in the Bishan subway tunnel in Singapore.

Right now, almost every country is paying attention to Japan, wondering how its flood control system performs during typhoons and rainy seasons.

Singapore’s Lee Kuan Yew School of Public Policy Water Policy Research Institute Professor Torta Hada sighed: “If a preventive country like Japan can’t stand it, if it can’t even Tokyo, we all have to be careful.”

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