The Metropolitan Area Outer Underground Discharge Channel (Japanese: 首都圏外郭放水路, Hepburn: Shutoken Gaikaku Hōsuiro), commonly known as Tokyo G-Cans, is the world’s largest underground floodwater diversion facility. Built 22 meters (72 feet) below ground level between 1993 and 2009, this immense structure spans 177 meters (581 feet) in length, 78 meters (256 feet) in width, and 25 meters (82 feet) in height. It features a massive water storage tank supported by 59 reinforced concrete pillars, each weighing 500 tons. Situated between Showa and Kasukabe in Saitama Prefecture, on the outskirts of Tokyo, the system was designed to mitigate flooding from typhoons and heavy rainfall.
The project includes five containment silos, each 65 meters (213 feet) tall and 32 meters (105 feet) in diameter, connected by 6.4 kilometers (4 miles) of tunnels located 50 meters (160 feet) underground. Completed in 2006, the G-Cans system is equipped with seventy-eight 10 MW (13,000 hp) pumps capable of discharging up to 200 metric tons (220 short tons) of water per second into the Edo River. As climate change accelerates, Tokyo continues to expand and enhance this critical infrastructure to manage increasing water volumes and maintain resilience against future flood threats.
G-Cans, originally known as the G-CANS PROJECT, is the name of a civic group focused on transforming the area around the Metropolitan Area Outer Underground Discharge Channel and the Shōwa Drainage Pump Station into a cultural and community hub. The group’s mission is to leverage these regional resources to foster regional development and community engagement. The “CANS” in G-Cans symbolizes the idea that “anything can be done with the ideas and actionable proposals of citizens.” While “G-Cans” is commonly used in English to refer to the flood management system itself, the term originally referred to this specific civic initiative.
Japan has a long history of contending with natural disasters, with floods posing a significant challenge, especially in the capital, Tokyo. The city’s vulnerability to heavy rainfall and typhoons, exacerbated by climate change, has led to the creation of an advanced flood management system—the Metropolitan Area Outer Underground Discharge Channel (MAOUDC), also known as Tokyo G-Cans. This underground network, located in Saitama Prefecture, stands as a testament to Japan’s engineering prowess and commitment to disaster preparedness.
Japan’s Battle with Heavy Rainfall
Japan experiences nearly double the global average rainfall, with Tokyo particularly affected during the typhoon season. The frequency of intense rainfall events has increased by 70% across Japan, a trend attributed to global warming by the Japan Meteorological Agency. This has resulted in significant economic damages, with floods in 2019 alone causing losses of 2.15 billion yen.
The MAOUDC: An Engineering Marvel
To address the growing threat of floods, Tokyo initiated the MAOUDC project in 1992, culminating in its completion in 2006 at a cost of approximately $2 billion. The system, located about 20 miles north of central Tokyo, features five massive vertical shafts connected by 6.3 kilometers of tunnels, all situated 50 meters underground. These shafts funnel excess water from smaller rivers into the larger Edo River, preventing urban areas from flooding during heavy rains.
At the heart of the MAOUDC is a vast water storage tank, often referred to as an “underground shrine” due to its impressive, cathedral-like design. 500-tonne pillars support this tank and house powerful pumps capable of moving 200 cubic meters of water per second. This system has been crucial in mitigating the impact of severe weather events, such as Typhoon Hagibis in 2019, which saw the diversion of over 12 million cubic meters of floodwater, preventing widespread damage in Tokyo.
A History of Flood Management and Future Challenges
Tokyo’s struggle with flooding is deeply rooted in its history, particularly following the devastation caused by typhoons in the post-war era. These events prompted significant investments in flood control infrastructure, culminating in today’s sophisticated system of tunnels, reservoirs, and levees. However, the increasing unpredictability and intensity of weather patterns due to climate change present new challenges that could overwhelm existing defenses.
Recognizing this, the Tokyo Metropolitan Government has revised its flood management strategies, raising the design standards for new projects. Current efforts focus on upgrading infrastructure to handle higher volumes of rainfall, with new projects being designed to manage up to 75 millimeters of rain per hour.
Implications for Global Cities and Future Strategies
The MAOUDC protects Tokyo and serves as a model for cities worldwide that face similar climate-induced challenges. The Tokyo Resilience Project, a 15-trillion-yen initiative launched in 2022, aims further to strengthen the city’s defenses against natural disasters. This includes expanding sewer capacities, increasing the volume of underground reservoirs, and employing artificial intelligence to enhance flood prediction and management.
Despite these advancements, experts warn that adaptation alone will not suffice. Japan, the fifth-largest emitter of greenhouse gases globally, has pledged to achieve carbon neutrality by 2050. However, progress has been slow, highlighting the critical need for comprehensive efforts to mitigate climate change alongside adaptation measures.
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