Editing Jetty (section)

==At entrances to jetty harbors==
[[File:Humboldt Bay and Eureka aerial view.jpg|300px|thumb|Aerial view of jetties, constructed of [[dolos]]se, at [[Humboldt Bay]] harbor entrance. These jetties mitigate unpredictable shifting sands. ]]
The approach channel to some ports situated on [[sand]]y coasts is guided and protected across the beach by parallel jetties. In some cases, these are made solid up to a little above low water of [[tide|neap tide]]s, on which open timber-work is erected, provided with a planked platform at the top raised above the highest tides. In other cases, they consist entirely of solid material without timber-work. 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 [[sluice|sluicing]] basins; but it is now often considerably deepened by sand-pump [[dredging]]. It is protected to some extent by the solid portion of the jetties from the inroad of sand from the adjacent beach, and from the levelling action of the waves; while the upper open portion serves to indicate the channel and to guide the vessels, if necessary (see [[harbor]]).

The bottom part of the older jetties, in such long-established jetty ports as [[Calais]], [[Dunkirk]] and [[Ostend]], was composed of [[clay]] or rubble stone, covered on the top by fascine-work or pitching, but the deepening of the jetty channel by dredging and the need that arose for its enlargement led to the reconstruction of the jetties at these ports. The new jetties at Dunkirk were founded in the sandy beach, by the aid of compressed air, at a depth of {{convert|22.75|ft}} below low water of spring tides; and their solid masonry portion, on a concrete foundation was raised {{convert|50|ft}}. above low water of [[neap tide]]s.<ref name="EB1911" />