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The Futuristic Underground Tunnel System Protecting Tokyo from Floods

Almost every country on earth has been affected by climate change, in one way or another. Japan, for instance, has been ravaged by massive floods that have inflicted heavy damage on Tokyo, the capital city, as well as some other parts of the country. Sprawling concrete metropolises around the world are particularly susceptible to flooding, due to the lack of soil, vegetation, and rain-absorbing trees that can soak up the excess water after a bout of torrential rainfall. 

Climate change has led to an increase in the severity of weather occurrences in recent years. Concrete being a terrible sponge of rainwater, flooding has become a major pain point for many metropolitan cities, including Tokyo, which is located on a flat floodplain of soft, alluvial soil in a monsoonal climate with regular typhoons. Rising oceans have also made the Tokyo metropolitan region vulnerable to storm surges, while years of pumping groundwater has caused some parts of the city to sink 15 feet over the last few decades, wide areas of the metropolis now sitting below sea level, barely protected by aging dykes.

The low-lying areas of downtown Tokyo had been ravaged by terrible floods since the 1950s and 60s, when severe typhoons and heavy rainfall destroyed large parts of the city. Residents who grew up in Tokyo during that period remember floods being a regular part of their day-to-day lives. Some even recall seeing large, freshwater fish swimming in the flooded streets of Tokyo, ready to be caught by adventurous residents and neighborhood kids for their next meal. 

These climatic disasters have only gotten worse in recent years, with historic floods that previously only happened once in 200 years now becoming much more common. Experts believe that these developments can be attributed largely to the effects of rising average temperatures and changing patterns of rainfall, both caused by global warming. 

The heat island effect – wherein heavily urbanized areas experience higher temperatures than outlying regions – might also be partly responsible for the intense, localized downpours that regularly submerge Tokyo in more than 100mm of rain in less than an hour. 

Official studies conducted by the Japanese government have found that without an effective anti-flood system, Japan might spend ¥148 billion ($1.4 billion) on disaster cleanup costs every year. This system was finally commissioned by the government after a series of catastrophic floods that left dozens dead and submerged 30,000 homes in 1991. It took over a decade to complete the one-of-a-kind, underground flood prevention project that today protects the city from similar disasters that could potentially claim more lives. 

A World-Class Anti-Flood System

The Metropolitan Area Outer Underground Discharge Channel (Kasukabe, Saitama , Japan).By Dddeco, is licensed under CC-BY-SA

The Japanese government spent seventeen years (and $2.6 billion) to construct the so-called “underground Parthenon” that now protects Tokyo from regular flooding. A gigantic, subterranean flood system (arguably the world’s most advanced system of its kind) helps redirect the rainwater away from the city, whenever Tokyo experiences a torrential downpour (or typhoon) that might potentially cause flooding.

The project, known as the Kasukabe flood tank, comprises a network of massive tunnels, huge pillars, enormous pumps, and gargantuan water tanks that together help divert floodwaters away from the 35 million inhabitants of Tokyo. The city (and its suburbs) is crisscrossed with almost a hundred rivers and canals, which tend to overflow after a heavy thunderstorm or typhoon. The Kasukabe anti-flood system redirects this overflowing water into underground silos and tunnels, finally discharging into the Edogawa River. 

The Kasukabe system is more than five storeys deep and large enough to accommodate two entire football fields. Overall, the central tunnel stretches over an area of 6.5 kilometers underground. Comprising numerous large pillars weighing more than 500 tonnes each, the reservoir is in some parts deep enough to hold the Statue of Liberty. 

The soaring pillars that support the structure also help break the momentum of the water as it flows through the tunnels, on its way out into the Edo River. This is important because the tank can release accumulated floodwater into the river at a rate equivalent to a 25-meter swimming pool being discharged every second with the power of a jet engine. Managing the force of the flowing water, with the help of the 59 pillars in the central tank, is essential for the stability of the structure.

The construction of the Kasukabe flood tank has led to a 90 percent decrease in the number of homes and businesses affected by water damage between the months of June and October, the season of rains and typhoons in Japan. When not in use, the anti-flood system is open to tourists and visitors, chiefly for the purpose of spreading awareness about the importance of disaster management.

The Japanese government collaborated with the Japan Institute of Wastewater Engineering Technology (JIWET) to construct the Kasukabe system. Officially titled the Metropolitan Area Outer Underground Discharge Channel, the massive installation comprises a 580-foot long cathedral-like underground surge tank, 213 flow-regulating silos, and several kilometers of interconnected tunnels that can pump about 200 tons of water per second. 

The operation room managing the entire underground drainage infrastructure is used to safely control the flow of water through each inflow facility. Like the human body, the operation room acts as the brain of the entire system while the pumps serve as its heart, sending blood to the various parts and organs. The operation room – staffed by engineers, analysts, and mechanics – consolidates and directs the functioning of the different parts to ensure safety and optimal efficiency. 

Construction began in 1992 and was completed in 2006, and since then the Kasukabe anti-flood system has protected the Tokyo Metropolitan Area from huge financial setbacks every year during the rainy season. The excess water from minor rivers in and around the city is collected via five concrete containment silos, connected to a long central tunnel. 

Kasukabe G-Cans First Shaft.By Keihin, is licensed under CC-BY-SA

Each of the containment silos is 30m wide and 70m high, making the reservoir large enough to handle the flooding from even the most devastating calamities. Designed to absorb the water volume of such incredible floods, these silos angle the incoming water to flow down the circular walls in a vortex, to reduce the impact of the crashing water after a heavy bout of rain. An underground tunnel, situated underneath a major Tokyo thoroughfare known as Highway 16, then carries the collected water to the surge tank in Kasukabe. 

Once the weather has reverted to normal, the stored water is pumped into the Edo River, from where it can safely be released into Tokyo Bay without the risk of further floods. The huge pumps used to push the water through the tunnels were reportedly modeled on rocket pumps. 

In effect, one can say that Tokyo has built a whole new river 50m below the city, just to contain the flood water. Powerful turbines (modified from jet aircraft engines) are used to operate the massive anti-flood system, along with its many interdependent parts. While it has benefited the country as a whole, the system has been most beneficial to those living and working in the Nakagawa Basin, a low-lying area below the level of the Edo River. 

Since it was inaugurated, this impressive feat of engineering has been studied extensively by engineers around the world. The underground surge tank, that resembles a cathedral or a temple, is also a popular item on the itinerary of many visitors to the city of Tokyo. 

The guided tours of the reservoir gained greater popularity after the surge tank was featured extensively in various types of domestic and international media, including video games, films, and advertisements. For instance, a 2006 television commercial for the British automobile brand Land Rover used the Kasukabe system as a primary filming location. Because the space seems to resemble a temple, some TV shows and movies have also used it as a setting to film mystic scenes. 

However, foreign visitors need to hire the services of a local translator as the guided tour is, so far, only offered in Japanese. In all, there are seven tours per day, and they take place every day of the week during the dry season. Each session lasts about fifty minutes in total. For foreign visitors, one of the most notable attractions is that the last Hunger Games movie (Mockingjay – Part 2) used the reservoir and surge tank for some of the underground Capitol scenes. 

Aftereffects of Installing the Anti-Flood System

After more than a decade in operation, the artificial river under Tokyo is proving to be extremely effective. Since the facility was built, the amount of flooding (and the resulting damage) has almost halved, even under the same weather conditions as before. 

The soil excavated – when the tunnels and silos were being dug – has been used to construct larger levees or floodbanks along the rivers. Over the last few years, the levees that regulate water levels and keep the rivers from flooding have been strengthened and broadened for better protection. 

The new floodbanks have been especially designed to ensure that they won’t collapse even under massive strain, such as during an earthquake or when they’re submerged underwater during a great flood. This has further strengthened Tokyo’s protection against floodwater, since natural disasters such as earthquakes are, unfortunately, a common occurrence in the region. According to a study on natural disaster risks conducted in 2014 by the Swiss Re insurance firm, Tokyo (along with its neighboring port city of Yokohama) forms the riskiest metropolitan area on earth, threatened constantly by extreme rainfall, typhoons, earthquakes, and tsunamis.  

According to Nobuyuki Tsuchiya, former head of civil engineering for the flood-prone Edogawa Ward in Tokyo, who currently works as an independent anti-flooding expert and consultant, “Tokyo faces dangers on all sides.” Even with the impressive anti-flood system underneath, he does not believe that the city is doing enough to ward off the potential danger posed by flooding. 

Super Typhoon Nabi was a Category 3 typhoon in the western Pacific when the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite captured this image on September 2, 2005, at 11:55 a.m. Tokyo time. It had sustained winds of around 200 kilometers an hour (160 miles per hour) and was located roughly 1,000 kilometers (600 miles) from Saipan and Okinawa at that time. It was predicted to gather strength and make landfall at the southern end of the Korean peninsula early on September 7. However, the range of possible storm tracks as of September 2 takes in possibilities ranging from Shanghai on Asian mainland to Kyoto on the southern end of the Japanese Islands.

Devastating storms and heavy typhoons wreaked havoc across greater Tokyo in late 2015, causing a record 670 million cubic feet of water to drain into the underground facility. The four large pumps (powered by rocket engines) took almost five days to release the water into the Tokyo Bay and clear the deluge. 

Despite the unimaginable scale of the engineering that has taken place below the ground, there is relatively little awareness among the residents of Tokyo about the subterranean ‘Parthenon’ that is protecting them from another calamity like the one that ravaged the city in 1991. 

Still, if properly replicated and implemented in the other urbanized, flood-prone regions of the world, this system could be of much use to the global economy, saving assets and lives that would make it well worth the investment. The avant-garde technology with its pressure-adjusting water tanks has already caught the attention of flood control experts in South Korea, China, and other Asian countries.

The onset of more frequent and intense rainstorms in Tokyo (partly the effect of global warming) has, in recent years, forced many officials to consider whether or not the region’s anti-flood protections are strong enough. Over the last three decades, there has been a 30 percent increase in the incidence of rainfall measuring more than two inches an hour, according to estimates released by the Japan Meteorological Agency. 

The government may soon need to build larger and more durable anti-flood systems in other parts of the country, but a major challenge lies in Japan’s deteriorating public finances. Work on the Kasukabe facility began in the early 1990s, when the country was investing heavily in large public-works projects.

But with the spiraling costs of supporting an aging population and a cumulative government debt more than twice the size of the national economy, the current Japanese administration may struggle to fund such ambitious (if necessary) flood-control projects. 

Still, it is undeniable that the Kasukabe facility has greatly benefited the Tokyo metropolitan region since it became operational. In the low-lying areas of the city, where the underground facility has done the most to reduce flood damage, local industry has flourished, enabling the construction of new shopping malls and large e-commerce distribution centers. 

Adaptation to a major climate risk can never be one-dimensional, however, and investment in infrastructure must be paired with public risk awareness programs, zoning and land use initiatives, and the development of disaster survival skills among local residents and business owners. This would involve familiarizing them with evacuation protocols, flood hazard maps, sewer check valves, and much more, so that they can protect themselves in the event of a disaster that cannot be controlled through manmade technology and infrastructure alone. 

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