Jetty

[2] Where a river is narrow near its mouth, has a generally feeble discharge and a small tidal range, the sea is liable on an exposed coast to block up its outlet during severe storms.

This system was long ago applied to the shifting outlet of the river Yare to the south of Yarmouth, and has also been successfully employed for fixing the wandering mouth of the Adur near Shoreham, and of the Adour flowing into the Bay of Biscay below Bayonne.

Solid jetties, moreover, lined with quay walls, are sometimes carried out into a wide dock, at right angles to the line of quays at the side, to enlarge the accommodation; and they also serve, when extended on a large scale from the coast of a tideless sea under shelter of an outlying breakwater, to form the basins in which vessels lie when discharging and taking in cargoes in such a port as Marseille.

The channel between the jetties was originally maintained by tidal scour from low-lying areas close to the coast, and subsequently by the current from sluicing basins; but it is now often considerably deepened by sand-pump dredging.

[2] A small tidal rise spreading tidal water over a large expanse of lagoon or inland backwater causes the influx and efflux of the tide to maintain a deep channel through a narrows no longer confined by a bank on each side, becomes dispersed, and owing to the reduction of its scouring force, is no longer able at a moderate distance from the shore effectually to resist the action of tending to form a continuous beach in front of the outlet.

Other examples are provided by the long jetties extended into the sea in front of the entrance to Charleston harbour, formerly constructed of fascines weighed down with stone and logs, but subsequently of rubble stone, and by the two converging rubble jetties carried out from each shore of Dublin Bay for deepening the approach to Dublin harbour.

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.
Aerial view of a jetty at Swakopmund , Namibia (2017)
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.
Aerial view of jetties, constructed of dolosse , at Humboldt Bay harbor entrance. These jetties mitigate unpredictable shifting sands.
Private jetties near the mouth of the Nambucca River , New South Wales, Australia