ERDC/CHL CHETN-II-44
September 2001
The long-term shoreline behavior of central Jones Beach Island is detailed in Figure 6. An active
tidal inlet was located in the center of the island, causing a substantial offset in beach planform
prior to closing in 1900. Substantial downdrift offset in the 1835 shoreline was caused by
interruption of the longshore transport system. The updrift (eastern) side of the inlet is offset
seaward by approximately 1 km. Following inlet closure, the beach responded by reorienting,
and the updrift offset exhibited high rates of erosion as the shoreline naturally straightened.
Shoreline change rates prior to and after inlet closure are given in Figure 7. Long-term shoreline
change rates (1835-1991) illustrate the presumed EHS. However, shoreline behavior after
reorientation (1941-1991) is less erosional. The attenuated erosion is a consequence of the
geomorphic reorientation of the shoreline and beach nourishment during the 1960s and 1970s.
Figure 6. Evolution of Jones Beach Island, Long Island, NY, a relict inlet
Translatory Longshore Sand Waves: Large undulations in the shoreline have been
observed to propagate in the net direction of longshore transport (e.g., Grove, Sonu, and Dykstra
1987; Inman 1987; Thevenot and Kraus 1995). A longshore sand wave (LSW; plural LSWs)
moves as an organized form similar to a solitary water wave and can be preceded by an erosion
wave that may extend alongshore farther than the wave's crest (accretionary portion).
Translatory LSWs have long-shore extent on the order of a kilometer and crest elevation
(distance of crest from trend of shoreline) of tens of meters. A common cause of LSWs is a
sudden injection or impulse of sediment to the beach. Examples are attachment of a small ebb
shoal or bar to shore, a river discharge, or other release of a compact slug of sediment. LSWs
move with speeds on order of 1 km/year; along Southampton beaches on the south shore of Long
Island, NY, (Figure 8) the speed of LSWs was seasonally dependent, in accordance with
expected magnitudes in seasonal longshore sediment transport.
If an erosion wave arrives to a particular site, it may persist a year or more. LSWs can move
through groin fields as well, as calculated by Hanson, Thevenot, and Kraus (1996). Mohr, Pope,
and McClung (1999) document "sand slugs" (possible LSWs) traveling through a field of
detached breakwaters. A temporary fill would mitigate the erosion wave at a vulnerable area;
otherwise, passage of the wave can be estimated.
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