Wall segments from the 10 metre diameter tunnel that links the river intake shafts across the basin. On the other side of the road, the tops of the outlet gate mechanisms are visible on the bank of the Edo River 江戸川.
Finishing up, we called another taxi and then made our way through the flat patchwork of streets to Minami-sakurai Station 南桜井駅.
Sampling random items from the ubiquitous vending machines in Japan is irresistible to me, particularly when they offer hot drinks, so while we waited on the platform for the next train to Ōmiya Station 大宮駅 I tried my luck with a hot coffee called Kirin FIRE キリン ファイア. I'll admit, the deciding factor was the cool looking little can: a yellow metallic diamond crimped tin with a red flame logo. Naturally, it tasted as watery and lacking in depth as coffee in a can should.
After some additional snaps and discussion of the system we finished our tour of the underground chamber and ascended once more to the light of day. Back across the field to the top level of the main control building for a debrief and survey, then goodbyes with staff and a wander around the facility.
At the side, various items from the tunnelling and construction phase are on display, like this giant cutting head repurposed as a compass rose.
At the far end of the regulating tank are four giant impellers, the largest of their kind in Japan, each powered (via an impressively large gear reducer) by a modified version of an aircraft gas turbine. At full output, even with all of the silencing systems, it must sound unsettling.
The massive drainage shafts take in water via overflow levees from the No. 18 Water Channel 第18号水路 (shaft two), Naka River 中川 and Kuramatsu River 倉松川 (shaft three), Koumatsu River 幸松川 (shaft four) and Otoshifurutone River 大落古利根川 (shaft five), which then flows to the first shaft and into the tank where the pumps regulate the water flow to keep the level within a safe operational range. It's expelled via six subway tube-sized sluiceways into the Edo River 江戸川, making its way into Tokyo Bay 東京湾 and keeping it safely out of the Naka River 中川 and Ayase River 綾瀬川 basin.
The system has proved its effectiveness, consistently saving huge areas from inundation during even the most extreme rain events.
When it isn't going to be active, the tank is cleared of silt and debris using earthmoving equipment lowered through the first shaft. It's an amazing piece of engineering and an impressive experience in person.
Each of the 59 pill-shaped columns weighs about 500 tonnes. In addition to their obvious function of supporting the roof and the 22 metres of ground above, they act as a counter weight against the buoyancy of the cistern in the surrounding groundwater.
It's difficult to appreciate in photos just how huge they are in real life, but to give a sense of scale each column is seven metres long and two metres wide, and an adult standing next to one would be about the same height as the buttressed base.
After an explanation of the system and its construction, we were invited to visit the underground surge tank.
Outside and at the far side of the field, an oblique concrete structure, like a sunken cube, serves as the entrance to the access shaft. A small black and gold plaque at the side listed the dimensions: 177.1 metres long, 78.0 metres wide, 25.4 metres high.
A hundred or so metal stairs down, the temperature steadily falling, then into the middle of the massive, dimly-lit cavern, where an array of giant columns towered, unshakeable.
Inside, greeted by staff, then up to the top floor where the control room is situated adjacent to a large display area, the floor of which features a satellite map detailing the path of the system's main tunnel.
The system is designed to prevent flooding of the basin during typhoons and heavy rain events by draining water from four rivers spread across the region. Each of the rivers has a 32 metre diameter, 60-70 metre deep underground shaft into which water can be diverted. These silos are joined 50 metres below the surface by a 6.4 kilometre long, 10 metre diameter tunnel that feeds the final shaft, which in turn connects to one end of the giant pressure control/surge tank. The tank acts as a buffer to regulate the flow of water so that it can be pumped into the Edo River 江戸川 at up to 200 cubic metres per second.
The taxi arrived, white gloves and auto-door, and we cruised across town, driver and interpreter chatting on the way, lamenting the loss of an ancient ginkgo 銀杏 tree in a recent storm.
There were no people to be seen or cars on the roads, but the tidiness of the farms that cover the countryside suggested a large workforce must exist.
At the facility, the large turbine and control building rises conspicuously at the end of an otherwise unremarkable soccer field. Hiding below it, at the end of kilometres of tunnels, is the mammoth “chika shinden” 地下神殿 (“underground temple”) – the surge tank that regulates the floodwater as it's pumped out of the channel.