Bridges, causeways, and underwater tunnels are all passageways that allow travel above, across, and beneath bodies of water such as rivers, bayous, and bays. On the other hand, these artificial structures can create barriers or obstacles to wildlife that live underneath and above these waters. For instance, fish may be hindered or prevented from moving either upstream or downstream.
A bridge is a structure designed to provide passage over an obstacle, oftentimes water. Bridges carry such transportation elements as roadways, railroad tracks, and walkways over waterbodies, along with such utilities as water pipes, support power cables, and telecommunications lines. There are many examples of bridges that have had dramatic impacts on the areas through which they were built: only three are discussed below.
The Brooklyn Bridge was opened for use on May 24, 1883, crossing the East River in New York City between the boroughs of Brooklyn and Manhattan. As the first bridge over the East River, the Brooklyn Bridge had a major impact on the small, independent city of Brooklyn. The newly energized city soon became closely linked to Manhattan, one of the world's leading commercial, cultural, financial, medical, and tourist centers. By opening a new transportation link, the bridge made possible the rapid growth of Brooklyn.
For many years, the only way to cross San Francisco Bay in California was by ferry, and by the early twentieth century the bay was filled with ferries. * When the bridge was first designed, an immense engineering challenge was introduced because the area produces winds of up to 100 kilometers (60 miles) per hour, strong ocean currents, and frequent fogging conditions. In addition, the bay lies within a large earthquake zone.
Taking 10 years to complete, the construction of the Golden Gate Bridge was finished in 1937. Despite the many problems with its construction, it
The 12.9-kilometer (8-mile) Confederation Bridge linked Prince Edward Island to mainland New Brunswick, Canada beginning in June 1997. It is the world's longest bridge over ice-covered waters. The bridge was designed to withstand the harsh waters that flow beneath it.
Researchers will study the bridge's performance using a variety of sophisticated monitoring devices, including 450 thermal sensors, 28 ice-load panels, and 76 vibration sensors. The comprehensive study, which spans a 20-year period that began in 1997, will be the largest data-gathering project ever undertaken in the areas of bridge and marine engineering.
With any structure that is built on or near a waterbody, concerns usually develop about possible impacts on the environment near and around the structure. The Confederation Bridge was no exception. Fishermen were worried that the government was ignoring their livelihood by neglecting the impact on the lobster industry. Environmentalists were worried about the effect the bridge would have on the local ecosystem and, especially, about the increased pollution of the surrounding environment.
On the other hand, such bridges that connect two separated landmasses can positively impact both areas. The total number of tourist parties visiting Prince Edward Island in 1997 was up by 30 percent over 1996. Expenditures by tourists in 1997 were about $245.9 million, an increase of 63 percent from 1996. Annual increases in tourism are anticipated for the period 2000–2015.
A causeway is a raised road or track across low or wet ground. For example, the Lake Pontchartrain Causeway connects the New Orleans coastal area on the southern shore with the northern shore. The causeway is the world's longest highway bridge, a continuous-span beam bridge that is almost 39 kilometers (24 miles) long.
Because the Lake Pontchartrain Causeway travels directly across Lake Pontchartrain, oil, grease, and other pollutants from vehicles using the highway are deposited on the causeway. These contaminants eventually wash off and drain into the lake. Causeways, by their very nature of crossing shallow waterbodies, can generate many types of pollutants that find their way into those waters.
On the other hand, unexpected positive results can occur. Twice a year, about 8 million purple martin birds migrate through southeast Louisiana and use Lake Pontchartrain as a stopover point. During spring and summer, more than 200,000 of these birds have adopted the understructure of the southern end of the Lake Pontchartrain Causeway as their evening roost area. Bird-lovers and spectators gather to watch the birds swoop in and out of the structure. With continuing loss of habitat for such wildlife, the causeway provides an important resting and feeding place for the purple martins as they migrate through the area.
An underwater tunnel is a passage, gallery, or roadway beneath a body of water. Underwater tunnels are used for highway traffic, railroads, and subways; to transport water, sewage, oil, and gas; to divert rivers around dam sites while the dam is being built; and for military and civil defense purposes. A few examples are discussed below.
An underground tunnel is located at the Chesapeake Bay. The 28.2kilometer (17.5-mile) crossing between Norfolk and Cape Charles, Virginia, begins as a bridge, but disappears into the water midway. A combination structure, the Chesapeake Bay Bridge–Tunnel combines two bridges with two tunnels that pass under major shipping channels.
One notable underwater tunnel is the 137-meter (450-foot) Orwigsburg Tunnel (the first tunnel dug in the United States) that was completed in 1821 at Orwigsburg Landing, near Auburn, Pennsylvania. Also impressive are the New York City tunnels: the Holland Tunnel (the world's only threetube tunnel) and the Lincoln Tunnel (the Hudson River's first tunnel) under the Hudson river; the Queens-Midtown Tunnel under the East River; and the Brooklyn–Battery Tunnel under New York Bay.
The Channel Tunnel, frequently called the "Chunnel," is a 50-kilometer (32-mile) tunnel that provides a railroad link under the Strait of Dover in the English Channel between Cheriton (near Kent), England and Coquelles, France. The tunnel, one of the most impressive civil engineering projects of the twentieth century, has an ultimate design capacity of 600 trains per day each way.
Throughout its history, the English Channel has proven to be very hazardous for marine travel. The Chunnel is delivering a large market share of the transportation industry between England and continental Europe, and is doing it in a much safer way than traveling through dangerous ocean waters.
William Arthur Atkins
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Bridges. WGBH Educational Foundation, PBS Online. <http://www.pbs.org/wgbh/buildingbig/bridge/index.html> .
Geologic Framework and Processes of the Lake Pontchartrain Basin. Coastal and Marine Geology Program, U.S. Geological Survey. <http://coastal.er.usgs.gov/pontchartrain/wetland.html> .
McGloin, John Bernard. Symphonies in Steel: Bay Bridge and the Golden Gate. The Museum of the City of San Francisco. <http://www.sfmuseum.org/hist9/mcgloin.html> .
Tunnels. WGBH Educational Foundation, PBS Online. <http://www.pbs.org/wgbh/buildingbig/tunnel/index.html> .
* See "Bays, Gulfs, and Straits" for a photograph of San Francisco Bay.