An undersea tunnel is a tunnel which is partly or wholly constructed under a body of water. They are often used where building a bridge or operating a ferry link is impossible, or to provide competition or relief for existing bridges or ferry links.
Compared with bridges
One such advantage would be that a tunnel would still allow shipping to pass. A low bridge would need an opening or swing bridge to allow shipping to pass, which can cause traffic congestion. Conversely, a higher bridge that does allow shipping may be unsightly and opposed by the public. Higher bridges can also be more expensive than lower ones. Bridges can also be closed due to harsh weather such as high winds. Another possible advantage is space: the downward ramp leading to a tunnel may leave a smaller carbon footprint compared to the upward ramps required by most bridges.
- Further information: Tunnel – Choice of tunnels vs. bridges
As with bridges, albeit with more chance, ferry links will also be closed during adverse weather. Strong winds or the tidal limits may also affect the workings of a ferry crossing. Travelling through a tunnel is significantly quicker than travelling using a ferry link, shown by the times for travelling through the Channel Tunnel (75–90 minutes for Ferry and 21 minutes on the Eurostar). Ferries offer much less frequency and capacity and furthermore travel times tend to be longer with a ferry compared to a tunnel.
Compared with bridges
Tunnels require far higher costs of security and construction than bridges. This may mean that over short distances bridges may be preferred rather than tunnels (for example Dartford Crossing). As stated earlier, bridges may not allow shipping to pass, so solutions such as the Øresund Bridge have been constructed.
As with bridges, ferry links are far cheaper to construct than tunnels, but not to operate. Also tunnels don't have the flexibility to be deployed over different routes as transport demand changes over time. Without the cost of a new ferry, the route over which a ferry provides transport can easily be changed.
List of notable examples
|Tunnel||Description||Distance||Depth (from surface)||Constructed in|
|Thames Tunnel||The oldest underwater tunnel in the world, crossing the Thames in London||0.4 km||1825–1843|
|Mersey Tunnel||Oldest and first longest underwater rail tunnels in the world, Crossing the Mersey in Liverpool||1.21 km||1881–1886|
|Severn Tunnel||One of the oldest underwater tunnels in the world||7.01 km||1873–1886|
|Elbe Tunnel (1911)||Pioneering underwater pedestrian and vehicular tunnel crossing the Elbe River in Hamburg, Germany||0.426 km||24 m||1907-1911|
|Holland Tunnel||The first modern underwater road tunnel in the world, crossing the Hudson River between Manhattan and Jersey City||2.6 km||28.3 m||1920-1927|
|Queensway Tunnel||The longest road tunnel of any type in the world when first built, crossing the River Mersey between Liverpool and Birkenhead||3.24 km||1925-1934|
|Bankhead Tunnel||Carries Hwy. 90 in Mobile, AL. Business District, to Blakely Island. The eastern end has large "flood door" that can be closed to prevent water from the Mobile Bay from flooding the tunnel during hurricanes or tropical storms. Two lanes that only allows cars and pick up trucks now to travel through the tunnel.||1.033 km||12.2m||1938-1942|
|Lincoln Tunnel||Set of road tunnels built in three stages, crossing the Hudson River between Manhattan and New Jersey||2.4 km average||30 m||1934-1957|
|Chesapeake Bay Bridge Tunnel||Connecting Virginia Beach with the Eastern Shore of Virginia||1.6 km (tunnel section)||1960–1964|
|Cross Harbour Tunnel||A busy road tunnel in Hong Kong||1.86 km||1969–1972|
|Elbe Tunnel (1975)||8-lane road tunnel crossing the Elbe River in Hamburg, Germany||3.3 km||1968–1975|
|Ahmed Hamdi Tunnel||Passes under Suez Canal connecting the Asian Sinai Peninsula to the town of Suez on the African mainland||1.63 km||1979–1981|
|Vardø Tunnel||Connecting the small island community of Vardø in northern Norway to the mainland||2.9 km||88 m||1979–1982|
|Seikan Tunnel||World's longest undersea railway tunnel when non-undersea portions of the tunnel are also measured||53.9 km||1971–1988|
|Sydney Harbour Tunnel||2.8 km||1988–1992|
|Channel Tunnel||World's longest undersea portion railway tunnel||37.9 km||1988–1994|
|Hitra Tunnel||The deepest in the world at the time of construction||5.6 km||264 m||1992–1994|
|Tokyo Bay Aqua-Line||World's longest undersea portion road tunnel||9.6 km||1988–1997|
|North Cape Tunnel||The tunnel goes under the Magerøysundet strait between the Norwegian mainland to the large island of Magerøya and the North Cape, Norway||6.8 km||212 m||1993–1999|
|Bømlafjord Tunnel||The deepest point of the International E-road network. Connects Stord municipality to the Norwegian mainland.||7.8 km||260.4 m||1997–2000|
|Eiksund Tunnel||World's deepest undersea road tunnel||7.7 km||287 m||2003–2008|
|Xiamen Xiang'an Tunnel||6.05 km||70 m||2005–2010|
|Qingdao Jiaozhou Bay tunnel||7.808 km||84.2 m||2006–2011|
|Xiamen Haicang Tunnel||6.306 km||73.6 m||2016–2020|
|Busan–Geoje Fixed Link||World's deepest immersed road tunnel||3.7 km||48 m||2008–2010|
|Marmaray||Rail tunnel connecting Asia and Europe in Istanbul||13.6 km||2004–2013|
|Marina Coastal Expressway||Singapore's first undersea tunnel||5 km||2008–2013|
|Port of Miami Tunnel||2.1 km||2010–2014|
|Eurasia Tunnel||Road tunnel connecting Asia and Europe in Istanbul||5.4 km||106 m||2011–2016|
|Transbay Tube||Rail tunnel for Bay Area Rapid Transit. Connects Oakland to San Francisco. It is the longest underwater tunnel in North America.||5.8 km||41 m||1965-1969|
- Eysturoyartunnilin, Faroe Islands to be opened in 2020. The tunnel will have three entrances connected by an underwater roundabout. link The road length from Hvítanes to the roundabout will be 7.5 km, whilst the road length from the roundabout to Strendur and to Saltnes will be 1.7 km and 2.2 km, respectively. This results in an overall length of 11.24 km of sub sea tunnel.
- Ryfylke tunnel in Norway – to be opened in 2019, 14.3 km length, 292 m depth.
- Rogfast tunnel in Norway – construction having started in 2018, at 27 km length, 392 m depth, it will be the longest road tunnel and deepest undersea tunnel in the world.
- Bohai Strait tunnel in China between Dalian and Yantai (decided, construction to start 'as soon as possible'.)
- Helsinki to Tallinn Tunnel under the Gulf of Finland (proposed)
- Irish Sea Tunnel (suggested)
- Rio de Janeiro Metro Bay Tunnel (Line 3 – Rio de Janeiro-Niterói) (proposed)
- Fehmarn Belt Fixed Link between Denmark and Germany (decided, construction start 2020)
- East West Metro Tunnel, Kolkata Metro in India (under construction, to be opened in 2021)
- Mumbai–Ahmedabad high-speed rail corridor of India (decided, construction start November 2018)
- Penang Undersea Tunnel in Malaysia – to be opened in 2025
- Sullivan, Walter. Progress In Technology Revives Interest In Great Tunnels, New York Times, June 24, 1986. Retrieved 15 August 2010.
- Dover–Calais Ferry Times, poferries.com website.
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