Descending to the Underground Shrine near Tokyo offers a cool escape from the intense summer heat. This impressive structure, part of the G-Cans flood management system, showcases colossal columns and a vast hall designed to channel excess rainwater and prevent urban flooding. As climate change increases rainfall intensity, Tokyo and other cities globally are investing significantly in drainage infrastructure to combat flooding, highlighting the urgent need for effective urban planning in response to climate challenges.
As you descend towards the Underground Shrine near Tokyo, an undeniable shift in temperature envelops you. Each step downward brings a refreshing coolness, a welcome relief from the sweltering Japanese summer, which stubbornly lingers even in October at a steamy 30 degrees Celsius.
Upon arrival at the shrine’s base, the oppressive heat of the outside world is momentarily forgotten. A delightful 20 degrees awaits, and the sheer magnificence of the shrine astounds. Spanning an impressive 177 meters in length, 78 meters in width, and towering 18 meters high, this cavernous hall could effortlessly accommodate the water from 66 Olympic-sized swimming pools, making visitors feel remarkably small in comparison.
With rising global temperatures, the intensity of rainfall is on the rise, overwhelming densely populated urban areas that struggle to manage such water influx. Cities like Tokyo, London, and Chicago are taking proactive measures against flooding, and Swiss cities are following suit with their own solutions.
A Hall Reminiscent of “The Lord of the Rings”
The sight of 59 colossal columns, each tipping the scales at 500 tons, is particularly breathtaking. These massive structures not only support the ceiling, as explained by the tour guide in Japanese, but also counteract the upward force of water pressure from below. The sheer number of columns is striking; fewer would suffice for ceiling support. Their grand appearance evokes imagery from the film “The Lord of the Rings,” and one could almost imagine the Fellowship led by Gandalf emerging from the shadows.
Yet, this awe-inspiring hall serves a critical purpose. It is part of the Metropolitan Outer Area Underground Discharge Channel, known as the “G-Cans.” This extensive 6.3-kilometer tunnel system, located on Tokyo’s outskirts, is designed to channel excess water during intense rainfall, preventing surface flooding in nearby communities.
Through five massive cylindrical shafts, rainwater from swollen rivers cascades into the depths, traveling through tunnels to the shrine, where it is pumped into the Edogawa River by Japan’s four largest turbines. This river ultimately guides the water to Tokyo Bay. Just this past June, the system was activated four times, surpassing its use for the entire previous year.
Although the G-Cans aren’t new, they rank among the largest systems globally. Constructed between 1993 and 2006 at a cost of approximately 1.3 billion Swiss francs, Tokyo has been proactive in addressing its flood risks. The region’s bowl-like geography and dense urban development have historically led to flooding issues. Since its inception, the system has been employed nearly 150 times, averting flood damages estimated at over 860 million Swiss francs. Nevertheless, future challenges loom large.
As temperatures continue to rise, the frequency and severity of heavy rainfall events are predicted to escalate. “As temperatures increase, so does the capacity of the atmosphere to hold water vapor,” explains Seita Emori, a climate scientist at the University of Tokyo and an author with the UN Climate Council. For every degree Celsius increase, the air can retain about 7 percent more moisture.
Emori highlights the expectation of unprecedented rainfall amounts due to rising temperatures, with more severe typhoons anticipated for Japan.
Urban areas like Tokyo were not constructed to handle such massive water surges. The densely built environment hinders water absorption, leading to flooding that incurs costs far greater than those for flood prevention. Consequently, Tokyo is working to enhance its drainage infrastructure.
A Billion-Dollar Investment
Japan is committing over 214 million Swiss francs by 2040 to bolster flood protection in the Tokyo metropolitan area. The expansive network of cisterns and water channels, mirroring the G-Cans, is currently engineered to handle rainfall rates of up to 75 millimeters per hour. However, the increasing frequency of typhoons bringing downpours of 100 millimeters per hour is testing the limits of Tokyo’s drainage capabilities.
Moreover, rising sea levels and the threat of tsunamis pose additional challenges. Tokyo’s flood defense strategy incorporates wide, undeveloped riverbanks that allow water absorption, along with dams and locks to prevent water from entering the city.
Tokyo is not alone in facing these challenges. Other urban centers are also investing in solutions for increased drainage capacity, both underground and above ground.
In London, for instance, a 150-year-old sewer system struggles to cope with current demands, originally designed for a population half its size today. Consequently, millions of cubic meters of untreated wastewater are discharged into the Thames annually, as noted by the Tideway London initiative.
To tackle this issue, the city is constructing a 25-kilometer-long tunnel known as the “Super Sewer,” which will redirect wastewater to treatment facilities outside the city and manage the surging volumes from heavy rainfall, thereby alleviating pressure on the aging sewer system.
Similarly, Chicago battles frequent flooding due to its swampy foundation. Short, intense rains have historically inundated the streets, exacerbated by the warming climate. This leads to wastewater contaminating Lake Michigan, the drinking water source for ten million residents.
Like Tokyo, Chicago has been enhancing its sewage infrastructure for years. The city is currently developing the McCook Reservoir, designed to be the largest wastewater reservoir globally, with a capacity of nearly 38 million cubic meters, dwarfing the Underground Shrine in Tokyo. Slated for completion in 2029, it will store excess rainwater for treatment and subsequent release into rivers.
However, early assessments indicate that the reservoir will not suffice in light of the anticipated climate change impacts that were not fully accounted for during its planning phase.
Increasing Heavy Rainfall in Switzerland
What about Switzerland? While the country may not experience typhoons, it has seen a 12 percent increase in precipitation from heavy rainfalls since 1901. With ongoing global warming, heavy and extreme precipitation events are projected to intensify, even as average rainfall may decline. Urban infrastructure must be equipped to absorb and manage significant volumes of water in short periods.
Swiss cities predominantly focus on de-sealing strategies rather than merely increasing drainage capacity. “Modern urban planning is essential for effectively managing large quantities of water,” emphasizes urban development experts, highlighting the need for innovative solutions in the face of climate challenges.