PUBLISHED: 29 November 2013

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 of underground floodways are complex and lengthy[1]

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[2]. The city of San Antonio, Texas also has two major flood tunnels that divert water under the central downtown area to locations further downstream[3].

Are underground floodways already built in Alberta? 

Underground flood tunnels do not currently exist in Alberta or Canada. The Government of Alberta is currently looking at numerous flood mitigation options including an underground flood tunnel for the City of Calgary that would divert 500 cubic meters of water per second[4].  

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. The City of Calgary is currently considering the feasibility of an underground floodway in a $250,000 study. Costs of flood tunnels vary depending on size and materials used. A much larger and complex underground floodway system than would be built in Calgary was built in Japan at the cost of $3 billion[5]. In comparison, underground flood tunnels built in San Antonio, Texas cost $111.4 million for a larger tunnel that is approximately 16,200 feet long and took 13 years to build and $38.8 million for a smaller tunnel that is approximately 6,000 feet long and took 10 years to construct[6].

How long does it 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[7]. Comparatively, the San Antonio River tunnel was built between 1987 and 1999 while its counterpart, the San Antonio Creek tunnel was built between 1987 and 1997[8]

Operations lifetime expectancy?

Lifetime expectancy depends on the construction design of particular 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 200 year flood[9] while the tunnels in San Antonio have been built to carry the flow of a 100 year flood[10]

What are the associated risks? 

Risks associated with building an underground flood tunnel are largely related to the quality and quantity of geologic formation of 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[11]. 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 be 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 the flood impacts in one area, but can create downstream flooding in another area 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 this is not incorporated into the construction design, further impacting the river’s ecology[12]. The environmental and watershed impacts will vary on a case-by-case basis, for example, in San Antonio, Texas there were no major concerns of significant negative impacts on the natural environment due to the location of the tunnel[13]

Would an underground floodway help in a drought?

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

[1] Inomata, Jun. “Multiple use of Flood Prevention Facilities in Japan.” National Institute for Land and Infrastructure Management, Ministry of Land, Infrastructure and Transportation. Accessed November 23, 2013. Slides 26-30. 

[2] Ibid. 

[3] “Flood control in the city of San Antonio, Texas: San Antonio River and San Pedro Creek Tunnels.” San Antonio Channel Improvement Project. (December 1997) Pg. 1. 

[4] “Alberta takes big steps to protect against future flooding.” Government of Alberta. Published November 21, 2013. Accessed February 20, 2014. 

[5] Zolbert, Alex. “How giant tunnels protect Tokyo from flood threat.” CNN. Published November 1, 2012. Accessed January 15, 2014. 

[6] “Flood control in the city of San Antonio, Texas: San Antonio River and San Pedro Creek Tunnels.” San Antonio Channel Improvement Project. (December 1997) Pg. 6 and 7. 

[7] Ibid. 

[8] Ibid. 

[9] “G-Cans Project, Kasukabe, Saitama, Greater Tokyo area, Japan.” accessed February 5, 2014. 

[10] “Flood control in the city of San Antonio, Texas: San Antonio River and San Pedro Creek Tunnels.” San Antonio Channel Improvement Project. (December 1997) Pg. 6. 

[11] 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. 

[12] “Environmental Aspects of Integrated Flood Management.” Global Water Partnership. August 2006; Pg. 43-44. 

[13] “Flood control in the city of San Antonio, Texas: San Antonio River and San Pedro Creek Tunnels.” San Antonio Channel Improvement Project. (December 1997) Pg. 5.