Kanda River Underground Regulating Reservoir

Kanda River Underground Regulating Reservoir under the Loop Road 7

We have been always aware of the danger of living near a river but Typhoon no. 19 that happened 2 weeks ago triggered my interest to visit this flood prevention system.

Signboard to explain how this flood prevention system works

We did a quick google check and realised that they conduct free study tours every Tuesday and Thursday, with 2 sessions per day (morning and afternoon), so an appointment was reserved via telephone. Unfortunately, the tour is only conducted in Japanese.

Entrance of the facility

Be prepared to spend around 1.5 hours in this facility, but I totally enjoyed myself. Probably it was also because my previous job in Singapore covers such study tours at this kind of government-related facilities too.

Tour Program

Introduction – 5 minutes
Presentation – 20 minutes
Live Demonstration & Control Room Visit – 15-20 minutes
Tunnel Tour – 20-25 minutes
Q&A – 10 minutes

Presentation at the meeting room

The presentation starts with a short video that features the flooding that occurred in 1991 around Nakano area, which causes great damage to the neighbourhood.

Urbanisation is listed as one of the main causes of river flooding and how usual solutions like widening the river banks will not solve the issue easily. Due to the high density of nearby housing, roads and rail tracks, widening the river banks didn’t seem like a feasible option.

I think the below comparisation of the same bridge area really illustrates the dangers of river flooding and how quickly things can change without a good system in place.

The scene of a bridge on a usual day
The scene of the same bridge during heavy flooding

Live Demonstration

After the presentation, we moved to the 2nd floor of the facility where they showed us a live demonstration of how the system is triggered to open its gate to allow floodwater to flow into the underground tunnel and kept in the reservoir.

I was impressed how they made efforts to make sure the way the huge amount of water drop from ground level to underground 43 metres will not cause noise and vibration to the surrounding residential area.

Japan’s new emperor has a high interest in water management too.

Central Control Room

We then moved over to the central control room which is the actual place they will monitor the water levels, and control the water gates.

24-hour real surveillance cameras at 6 points of the different rivers

Depending on the level of warning from the Japan Meteorological Agency, the staff will need to come to this control room and be prepared to manage the situation, but usually, it will be 2-3 people called back regardless of the day and time. The guide made a joke that he doesn’t think he will need to demonstrate the part on how to use the control panel today.

Plastic covers are put over the control buttons to prevent any handling by mistake.

Main highlight : Tunnel Tour!

Moving over to the next building where we can finally descend to the tunnel level via the elevator which can only take 9 people at one time. Apparently, when they have study tours with primary school kids, they need to use the stairs which is equivalent to 11 floors of a building.

43 metres underground…
The water pumps which will return the storage water back to the river when it’s safe again.
Temperature is usually around 20 degrees all year round.

We were gathered at the pump room which already has a faint sewage odour. Those who are sensitive to smells are recommended to wear a mask. We were also warned beforehand that there will be water puddles in the tunnel and also some water may drop from the ceiling, and there will be some areas we need to move around narrow passageways so we should be wearing clothes that you can move around easily.

“Drop shaft” where the flood water will be dropped to the retention pond below.
The retention pond

A brief explanation of the “drop shaft” and “retention pond” was given. It was really easy to understand when we imagine it’s a waterfall from 40 metres coming down, and the bottom of the pond will be damaged without any water to “cushion” the impact, thus the “retention pond”.

After this, we were asked to move across the tunnel for 150 metres before reaching the actual point where we can see the reservoir. There were around 4-5 accompanying staff who were carrying torchlights to guide the way.

Moving across 150 metres before reaching the reservoir
Some artworks by school children as commemoration

The Underground Reservoir

We finally reached the underground reservoir which is of course emptied of water by this time. It was such a vast tunnel of darkness, that I feel as if I am in a sci-fi movie set.

The underground reservoir

Basic facts

Total water storage capacity : 540,000 cubic metres
Phase 1 : 240,000 cubic metres (Finished works in 1998)
Phase 2 : 300,000 cubic metres (Finished works in 2008)
Length of tunnel : 4.5 km (Phase 1 : 2km, Phase 2, 2.5km)
Diameter of tunnel : 12.5m
Water collection points : 3 (Phase 1 : Kanda River, Phase 2 : Zenpukuji River, Myoshoji River)

Well, guess what? It costs 1000 billion JPY to build this system.

The sewage remains after emptying the floodwater.

We asked about the amount of water that was stored in the reservoir from Typhoon no. 19, and apparently it was 90% full. OMG! We were fortunate that our area is not flooded, all thanks to this system.

The cracks on the reservoir walls

It’s actually pretty scary to see that there are a lot of cracks in the reservoir walls, but the staff assured that they are closely monitoring this.

Experience the “Dark”

There was a “fun” moment where the staff switched off all the lights so that we can experience the total darkness of the tunnel. It was like for 2 seconds, and we were a group of 22 “old people” so no screaming around… I can imagine when it was the primary school kids tour, it must be so funny.

Moving back up to ground level and we saw the area where they have the elevator to move vehicles or maintenance trucks to the tunnel level.

The enclosed area is where the elevator for vehicles is at.


Overall, it has been a very insightful and highly educational trip. It’s kind of personal discovery when you visit a facility that is so important to keep you and your property safe from flooding caused by heavy rain or typhoons which have been pretty rampant these days.

The outline of the 12.5-metre tunnel diameter


  1. I didn’t realise it’s you!! Until I saw your sub heading “Extraordinary life of a Singaporean in Japan” XD
    Nice write up!

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