PUBLISHED: 08 September 2023

Flood Mitigation: Underground Flood Tunnels

What is an underground flood tunnel?

An underground floodway diverts excess flood or stormwater from the surface into tunnel facilities underground. This type of flood tunnel is built in stages and in areas where river channelization cannot occur, due established urban infrastructure. Flood tunnels can be an effective form of flood mitigation; however, construction timelines for underground floodways are complex and lengthy [i].

How does this option help to mitigate the impacts of a flood?

Underground flood tunnels are effective mitigation methods that limit the surface impacts of flooding. When a river basin reaches capacity, underground flood tunnels are used to divert water underground away from the original basin. Similar to surface diversion channels, the diverted water is returned to the river basin further downstream. For example, Japan has an extensive system of underground floodways that divert water from major cities like Tokyo to prevent extensive flood damage. The city of San Antonio, Texas also has two major flood tunnels that divert water under the central downtown area to locations further downstream [ii].

Have flood tunnels been proposed in Canada?

Underground flood tunnels do not currently exist in Canada. In Alberta, the Glenmore Reservoir Diversion was a tunnel proposed to protect Calgary following the 2013 flood. While considered to be technically possible, the idea did not pass the preliminary benefit:cost analyses and was dropped in favour of the Springbank Off-stream Reservoir [iii].

What are the costs to build and maintain?

Underground flood tunnels are expensive mitigation options that divert river flows around important communities, re-releasing the flow downstream. Costs of flood tunnels vary depending on size, ground type, and materials used. A much larger and complex underground floodway system than the one proposed in Calgary was built in Japan at the cost of nearly $3 billion. In comparison, two underground flood tunnels built in San Antonio, Texas cost $111.4 million for the larger tunnel, which is approximately 16,200 feet long and took 13 years to build, and $38.8 million for the smaller tunnel, which is approximately 6,000 feet long and took 10 years to construct [iv].

Picture of San Antonio River Tunnel tunnel construction and design schematic.
Source: San Antonio River Authority. Accessed 2023-09-08.

How long does a flood tunnel take to build?

Underground flood tunnels typically take a few years to build, depending on size and resources. For example, Japan’s Water Discharge Tunnel was under construction for seven years between 1993 and 2006. Comparatively, the San Antonio River tunnel was built between 1987 and 1999, while its counterpart, the San Pedro Creek tunnel, was built between 1987 and 1997.

What is the operations lifetime expectancy?

Lifetime expectancy depends on the construction design of underground flood tunnel projects. This specific information is not widely available to the public. Nonetheless, underground tunnels are built to withstand certain flood events. For example, the underground flood tunnel in Japan was built to withstand a 1:200 flood [v], while the tunnels in San Antonio were built to carry the flow of a 1:100 flood.

What are the associated risks?

Risks associated with building an underground flood tunnel are largely related to the quality and quantity of geologic formations on the proposed construction route. Information on rock type, density, fault characteristics, weathering, groundwater, and bedding planes is needed to accurately determine the feasibility of building underground. Not understanding the characteristics of an underground area could result in excessive costs and uncertainty [vi]. Concerns surrounding water quality are also considered in the construction phase, due to the possibility of stagnant water. This problem is addressed in San Antonio through the use of ventilation and recirculation systems.

Similar to the risks raised with diversion channels, there are concerns that underground tunnels can increase the speed of water. Also, as water levels continue to rise and storms continue to worsen, current existing infrastructure could become unable to cope with future floods.

What are the environmental and watershed impacts of an underground flood tunnel?

In the event of a flood, underground flood tunnels reduce flood impacts in one area, but can create downstream flooding in another, due to large amounts of water moving quickly through the tunnel. If outflow facilities are not properly built, then downstream bank erosion, flooding, and destruction to the surrounding environment can occur. Another impact to the environment could be disruption to the sedimentation balance, if not incorporated into the construction design, which would further impact the river’s ecology [vii]. The environmental and watershed impacts will vary on a case-by-case basis.

Would an underground floodway help in a drought?

No, underground floodways are developed to handle flood waters and rainfall, not to mitigate droughts.


[i] Zolbert, A., 2012, How giant tunnels protect Tokyo from flood threat.  Accessed 2023-05-01.

[ii] San Antonio River Authority, n.d.,  San Antonio River Tunnel – Protecting Downtown San Antonio.  Accessed 2023-05-01.

[iii] IBI Group, 2015, Benefit/Cost Analysis of Flood Mitigation Projects for the City of Calgary: Glenmore Reservoir Diversion.  Accessed 2023-05-01

[iv] San Antonio River Authority, n.d.,  San Pedro Creek Tunnel.  Accessed 2023-05-01.

[v], n.d., G-Cans Project, Kasukabe, Saitama, Greater Tokyo area, Japan. Accessed 2023-05-01.

[vi] Ioannou, Photios G. “Geological Exploration and Risk Reduction in Underground Construction.” Journal of Construction Engineering and Management. Vol 114, No 4. December 1988. Pg. 532- 547.  Accessed 2023-05-26.

[vii] World Meteorological Organization, 2006, Associated Programme on Flood Management Technical Document No. 3, Flood Management Policy Series – Environmental Aspects of Integrated Flood Management.  Accessed 2023-05-26.