exhibit directions seen in a standing wave field in a bounded domain. Model results in
these figures depict a distinct and discernable change occurring to the waves. The change
takes place once waves have intercepted the east jetty. Waves striking the east jetty
appear to have turned to a northwesterly direction, heading toward the end of the left inlet
side bank. The model also indicates that not all waves do so and, in fact, waves
unaffected by the jetties tend to move almost straight through the inlet center or graze the
east jetty.
Figures 9-12 depict the situation of waves arriving at the end of the west side bank,
where surfers enjoy Threes. Contours in in these figures illustrate the progression of
wave fronts through the inlet. Locations where wave height contours change shape
(expanding, coalescing, or disappearing) signify a local increase or decrease in wave
height. Lobes (round or elongated wave height contours) developing near the end of west
side bank and their merger indicate wave reinforcing (amplification) at that location.
This is the instant when surfers can enjoy big waves the most. Interestingly, this increase
in wave height is not sustained as waves move away from one location to another inside
the inlet; wave breaking dissipates energy with distance from the end of the bank. To see
the evolution of this wave field, please see animation files on our website.
CONCLUSIONS
Threes is a historic Long Island surf break produced by a fortuitous combination of
interactions among engineered structures and channels, natural inlet morphology, waves,
tide, and wind. When the break is active, surfers ride energetic plunging breakers along a
bay-side shoal that is attached to the western barrier. Threes has been surfed since at
least the 1960s and was enhanced during the jetty rehabilitation projects of the early
1990s. The U.S. Army Corps of Engineers attempts to accommodate surfing interests in
preserving Threes. The complex diffraction, refraction, and reflection patterns around the
ebb-shoal and inlet throat were successfully modeled with CGWAVE, numerically
reproducing the wave. The model verified the incident wave directions necessary to
activate the break, and illustrated the potential benefits numerical analysis can provide for
engineers and surfers alike.
ACKNOWLEDGEMENTS
The authors would like to acknowledge legendary Long Island surfer Mr. Joe Alber
for his insight and photographic record of Threes. We also appreciate conversations
about Threes with Mr. Aram Terchunian, a Westhampton native and coastal geologist,
and with Ms. Lynn Bocamazo of the New York District, U.S. Army Corps of Engineers.
This work was conducted as an activity of the Coastal Inlets Research Program, U.S.
Army Corps of Engineers (USACE). Permission was granted by Headquarters, USACE,
to publish this information.
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