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The underground cathedral protecting Tokyo from floods

Source(s):  British Broadcasting Corporation (BBC)

By Diego Arguedas Ortiz


If Japan is a pilgrimage destination for disaster and risk-management experts like [Cecilia Tortajada], this is one of its main temples. The floodwater cathedral hidden 22 meters underground is part of the Metropolitan Area Outer Underground Discharge Channel (MAOUDC), a 6.3 km long system of tunnels and towering cylindrical chambers that protect North Tokyo from flooding.


The Channel sucks in water from small- and mid-size rivers in Northern Tokyo and moves it to the bigger Edo River, which can handle the volume with more ease.

When one of these rivers overflows, the water falls to one of five enormous 70-meter tall cylindrical tanks spread across the Channel’s length. Each of these tanks is big enough to accommodate a space shuttle or the Statue of Liberty and they are interconnected through a 6.3km long network of underground tunnels. As the water approaches the Edo River, the ‘floodwater cathedral’ Tortajada visited reduces its flow, so the pumps can push it to the river.


Based on historical rainfall records, the city planning authorities designed Tokyo’s defences to withstand up to 50 millimetres of rain per hour, particularly in areas were people and property are concentrated. But what was considered normal fifty years ago is not anymore.  

As in other parts of the world, the number of days with heavy precipitation have increased in the past 30 years says the Japanese Meteorological Agency, a sign that patterns are changing. Some estimates suggest that over the 21st Century, rainfall in Japan could increase by 10%. In the summer, that number could go up to 19%.


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  • Publication date 29 Nov 2018

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