Underneath Tokyo sits the largest man-made reservoir system in the world. 

In Kasukabe, Japan lies the world's largest underground floodwater facility. Massive man-made concrete caverns that make up the Metropolitan Area Outer Underground Discharge Channel, also known as the G-Cans project, sprawl under the cities surface-level infrastructure.  

Kasukabe Underground Flood Protection Surge Tank, Saitama

The large underground drainage system was built to prevent catastrophic flooding of the waterways that surround the city. Due to surges in rainy and typhoon seasons, the local geography is stricken by large flooding events each year. 

The city itself is located on the outskirts of Tokyo in the Saitama prefecture, but above ground, there is little that hints at just what lies beneath.

Engineering the reservoir

Construction on the project began in 1992, and spanned the course of roughly a decade and a half, with the project fully completed in 2006. While on paper the project contains only five containment silos, the significance of their size will give any onlooker pause.

Each silo is 213ft (65m) high and 104ft (32m) in diameter. These larger silos are then connected by 6.3km of large tunnels, all at a depth of 165ft (50m) below the surface. 

AMANO Jun-ichi/Wikimedia

Pillars weighing 500 tons each support the main reservoir, a concrete tank measuring 580ft (177m) long by 83ft (25.4m) high. Inside of this tank sit 78 10 MW pumps that are capable of transferring roughly 200 tons of water into the nearby Edo River each and every second. 

Staff at the facility are on constant alert, especially during Japan’s rainy and typhoon seasons from June to late October. The reservoir has helped reduce the number of homes affected by water damage in nearby areas by about 90%.

During dry periods, the surge tank is actually used as a tourist attraction. Visitors can tour the site for about 3,000 Japanese Yen, or roughly €25. While any jet-hopping civil engineer might be bursting to take a trip and see this underground marvel, it should be noted that the tours are only conducted in Japanese.

Scale model of pump gear mechanism at the Ryu-Q Kan

At this point though, it's natural to wonder what the city did before they built the Kasukabe Reservoir. How did a bustling town survive seasonal catastrophic flooding?

Initially, the infrastructure was sparse enough that the floods weren't considered to be a major issue. However, from the 1960s through the 1980s, much of the region's farmland had been replaced with housing and industrial facilities, significantly decreasing the opportunity for rainfall and snow melt to seep underground. 

The initial problem

When the suburbs of Tokyo were initially being developed, the sprawl occurred so fast that little thought was given to flood control. However, the city soon learned its lesson as it suffered numerous catastrophic floods, rendering a number of areas unbuildable. 

As the population grew and more and more of the ground was covered with non-porous concrete, engineers knew that something had to be done to remedy the situation. 

A diagram of the layout of the five different tanks and where they can outflow, with the primary outflow being the Edo River. Source: Edo River Office, Kanto Regional Development Bureau

After years of struggling with the floods, 1992 marked the beginning of the fightback against catastrophic flooding in the region due to the construction of the outer underground discharge channel tunnels. 

This massive system works through a system of switches that engineers can control during high rain or flooding events. Due to the series of tunnels that run throughout the city, connected to the underground reservoir, engineers can direct nearly all of the drainage from the city into the five large silo storage tanks. 

Kasukabe pump station and Ryu-Q Kan visitor center, Kasukabe Underground Flood Protection Tank (aka G-Cans), Saitama Prefecture

What the system accomplishes

The end goal of the system isn't just to hold water though. In flooding events that have a rapid onset, the first task of mitigating the flooding risk is handling the surge.

The underground reservoirs are built so large in order to handle large volumes of water that rapidly flow into the city during surges. The water that sits in the tanks can then be discharged more slowly into the Edogawa River, carrying it through natural channels into Tokyo Bay.

A look into the control room for the reservoir where engineers can determine inlet and outflow rates, otherwise protecting the city from catastrophic floods. Source: Nesnad/Wikimedia

The system is often viewed by other engineers as an interesting solution to large flooding events brought on by rapid population growth. The Kasukabe reservoir is regarded by many civil engineers as a successful solution to the problem of flooding. 

In fact, a similar flood facility is being built in Osaka Prefecture, which is scheduled for completion in 2044. And even more may be needed as global warming increases the prevalence and frequency of severe storms.

Water containment silo seen from the surge tank

According to Japan’s Meteorological Agency, the number of typhoons that threaten Tokyo each year has jumped 1.5 times in the past four decades.

But back to the reservoir being a tourist attraction. Why would tourists want to see massive underground concrete caverns? Well, the design of the underwater tanks gave them a sort of cathedral-esque look.

Due to the large support columns scattered throughout, the empty tanks present as elegant tranquil pieces of engineering sitting below the bustling city above.

An image of one of the holding tanks as part of the Kasukabe reservoir, with water still on the ground from recent flooding. Source: AMANO Jun-ichi/Wikimedia

For an animated demonstration of how the system works, take a look at this video. 

This article was written by Trevor English and first appeared in Interesting Engineering.com