Causeway

A causeway is an elevated road on elevated ground, usually across a broad body of water or wetland. A roadway that is carried instead on a system of arches or a bridge is a viaduct. In the US a brief stretch of viaduct is called an overpass.

It may be constructed as a 'coffer dam': two parallel (geometry) steel sheet pile or concrete retaining walls, anchored to each other with steel cables or rods. This construction may also serve as a dyke that keeps two bodies of water apart, e.g. with a different water level on both sides, and/or with salt water on one side and fresh water on the other side (this may also be the primary purpose, the road being a side benefit).

Examples are those that connect Marken, Singapore and Venice to the mainland, and the Afsluitdijk, Brouwersdam, Markerwaarddijk. In Louisiana, a very long causeway takes Interstate 10 across Lake Pontchartrain for several miles or kilometers.

Causeways are also common in Florida, where several low bridges connect several manmade islands, often with a much higher bridge (or part of a single bridge) in the middle so that taller boats may pass underneath safely. Causeways are most often used to connect the barrier islands with the mainland.

Causeways are often a problem with an approaching hurricane or strong tropical storm, because the high winds and waves make them dangerous. Along with traffic jams, this is a major reason why evacuations are done so early on islands.

Causeways affect currents and may therefore be involved in beach erosion or changed deposition patterns.

The oldest engineered road yet discovered is the Sweet Track in England, dating from the 3800s BC.