Knowledge Framework
Use Cases
- Forecast flood risk ahead of events — Use weather forecasts, river data, and hydrological models to predict where and when flooding wil...
- Operate flood defences during events — Decide when to close barriers, activate pumps, open sluices, or divert floodwater through emergen...
Decision Patterns
- Predictive modelling systems — Forecast future system states
- Integrated flood systems — Combine infrastructure and monitoring
System Models
Flood & hydrological models
Tool: Urban flood routing models — Simulates extreme rainfall and underground storage performance
Overview
The Metropolitan Area Outer Underground Discharge Channel (commonly known as G-Cans or the “Underground Temple”) is the world’s largest underground flood diversion facility. Located beneath Kasukabe in Saitama Prefecture, it protects the densely populated lowland areas of northern Tokyo from catastrophic flooding during typhoons and extreme rainfall events.
The system comprises five massive cylindrical shafts (each 30 metres in diameter and up to 70 metres deep) connected by a 6.3 km tunnel (10.6 metres in diameter) running 50 metres underground. Floodwater from five rivers is captured by the shafts, flows through the tunnel, and is discharged into the larger Edogawa River via a cathedral-like pressure-equalising surge tank measuring 177 m long, 78 m wide, and 18 m high, supported by 59 concrete pillars each weighing 500 tonnes.
The system can discharge 200 cubic metres of water per second, equivalent to a 25-metre swimming pool every second.
Timeline & Location
1992: Construction begins after decades of severe flooding in the Nakagawa and Ayase river basins. 1993–2000: Five intake shafts constructed sequentially. 2002: Partial operations begin with the first shafts operational. 2006: Full system completed and fully operational. 2019: Typhoon Hagibis tests the system at near-capacity; the facility successfully prevents catastrophic flooding. The system activates approximately 7–8 times per year during typhoon and heavy rain seasons.
Stakeholders
The facility is owned and operated by the Ministry of Land, Infrastructure, Transport and Tourism (MLIT) through the Kanto Regional Development Bureau. The Edogawa River Office manages day-to-day operations.
Local municipalities in Saitama Prefecture benefit from flood protection. The facility also serves as a tourist attraction and educational facility, with over 100,000 visitors per year touring the surge tank (known as the “Underground Temple” for its cathedral-like architecture). It has become one of Japan’s most famous infrastructure landmarks.
Digitalisation & Data
The G-Cans operates with sophisticated monitoring and control:
Automated Control Centre
A 24/7 control centre monitors river levels, rainfall, and system operations. Intake gates and the four 14,000-horsepower pumps are controlled via SCADA systems that can activate the full system within minutes of detecting dangerous water levels.
Real-Time River Monitoring
An extensive network of rain gauges, river level sensors, and CCTV cameras across the catchment provides real-time data for operational decision-making and flood forecasting.
Hazards
Exogenous Hazards
Climate change intensifying typhoon rainfall and increasing the frequency of extreme precipitation events. Sea level rise affecting the discharge capacity of the Edogawa River outfall. Upstream urbanisation increasing runoff volume and speed.
Endogenous Hazards
System capacity may be insufficient for the most extreme future climate scenarios. Maintenance challenges for underground infrastructure in a corrosive, wet environment. The system’s four massive pumps are single points of potential failure.
Cost & Benefit
Cost: ¥230 billion (approximately $2.2 billion) over 14 years of construction. Annual operating costs include energy for the four massive discharge pumps and ongoing maintenance.
Key Benefits: Flood damage in the protected area has been reduced by approximately 90% since the system became operational. During Typhoon Hagibis (2019), the system operated at near full capacity and is estimated to have prevented billions of dollars in damages. Property values in the protected area have increased. The facility attracts over 100,000 tourists annually.
Resilience Principles Assessment
Assessment of meeting Principles of Resilient Infrastructure
Proactively Protected (P2)
The system was designed to handle extreme flood events, with 200 m³/s discharge capacity and massive underground storage volume. It operates proactively, activating as river levels rise before flooding occurs.
Continuously Learning (P1)
Operational data from each activation informs flood modelling and system optimisation. The 2019 Typhoon Hagibis event provided critical data on near-capacity operations that is being used to assess future augmentation needs.
Shared Responsibility (P5)
National government funds and operates the facility, protecting multiple municipal jurisdictions. The system integrates with local flood management plans and upstream river management by prefectural governments.
Socially Engaged (P4)
The “Underground Temple” has become a landmark civic facility, with over 100,000 visitors annually building public understanding of flood risk and infrastructure investment. It has featured in films, documentaries, and media globally.
Environmentally Integrated (P3) To Do
Details pending.
Adaptively Transforming (P6) To Do
Details pending.
Futures
Climate change projections suggest that extreme rainfall events will intensify, potentially exceeding the system’s current design capacity. MLIT is assessing augmentation options and integration with upstream green infrastructure and retention basins. The G-Cans model is being studied for application in other flood-prone megacities worldwide.