thumb| Coastal [[lagoons fronted by barrier spits typically have entrances that migrate over time. Here, the entrance has been fixed by jetty construction. Carlsbad, California, April 1998.]]
thumb|Aerial view of a jetty at [[Swakopmund, Namibia (2017)]]
A jetty is a man-made structure that protrudes from land out into water. A jetty may serve as a breakwater, as a walkway, or both; or, in pairs, as a means of constricting a channel. The term derives from the French word ', , signifying something thrown out.
For regulating rivers
Wing dams
Jetties of one form, wing dams, are extended out, opposite one another, from each bank of a river, at intervals, to contract a wide channel, and concentrate the current to deepen the channel.
At the outlet of tideless rivers
thumb|North Jetty on the left and South Jetty on the right at the mouth of [[Sebastian Inlet in Florida from the Indian River to the Atlantic Ocean.]]
Jetties have been constructed on each side of the outlet river of some of the rivers flowing into the Baltic, with the objective of prolonging the scour of the river and protecting the channel from being shoaled by the littoral drift along the shore. Another application of parallel jetties is in lowering the bar in front of one of the mouths of a deltaic river flowing into a tide — a virtual prolongation of its less sea, by extending the scour of the river out to the bar by banks.
Jetties prolonging the Sulina branch of the Danube into the Black Sea, and the south pass of the Mississippi River into the Gulf of Mexico, formed of rubble stone and concrete blocks, and respectively, have enabled the discharge of these rivers to scour away the bars obstructing the access to them; and they have also carried the sediment-bearing waters sufficiently far out to come under the influence of littoral currents, which, by conveying away some of the sediment, postpone the eventual formation of a fresh bar farther out (see river engineering).
See also
- Breakwater
- Dock
- Groyne
- Jettied floors in medieval houses
- Mole
- Pier
- Port
- Spiral Jetty
- Wharf