FEDERAL JETTY AND SAND DIKE AT THE ENTRANCE OF FIRE ISLAND INLET, NEW YORK

Cyril Galvin, Charles J. Rooney, Gilbert K. Nersesian

Abstract


Prior to construction at Fire Island Inlet, Fire Island was moving westward at more than 200 feet per year, the north shore of the inlet was eroding severely, and navigation in the inlet was difficult. The Federal Jetty, completed in 1941, and the sand dike, built in 1959, have halted the westward migration, eliminated the severe erosion, and partially improved navigation, with minimal maintenance or repair to the structures. There has been a large net accretion of sand east of the jetty and west of the dike, an unknown part of which is at the expense of shores to the west of the inlet. At the State Park on the south side of the inlet interior, erosion accelerated, probably because of the dike. The middle and ocean segments of the 4750-foot Federal Jetty are now (1987) in good condition, although the design implies a stability coefficient for the quarrystone jetty head at time of construction that would now be considered risky. Stability has been promoted by a stone blanket under and east of the jetty, a thick stone apron seaward of the jetty, a low (8 feet MLW) crest, and armor stone that has been partially keyed in place. Damage due to scour, common at other single-jetty inlets, is absent here because longshore transport, which easily overtops the low crest, keeps the inlet channel away from the jetty. Although the two seaward segments of the jetty remain in good condition, the inshore segment of the jetty is in poor condition, despite its apparently sheltered location. The cumulative effects of waves, possibly channeled to the site along recurved spits during storms, have damaged 1200 feet, and tidal scour has destroyed about 230 feet. The damaged segment has a design cross section which is onefifth and one-twelfth the cross sections of the jetty trunk and head.

Keywords


Fire Island Inlet; dike; jetty; inlet entrance

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