The incident wave reflecting off the east jetty is wave 1, the reflected wave that arrives at
the west jetty is wave 2, and the wave reflected off the west jetty that can be surfed is
wave 3. Waves 2 and 3 pass over a large tip shoal present at the base of the west jetty.
Formation of Threes is an infrequent occurrence, and surfers on Long Island are on the
alert for favorable wave conditions conducive to its breaking.
This paper examines the necessary conditions for creating Threes and includes
graphics from a numerical simulation demonstrating the phenomenon. The suppression
or the enhancement of Threes depends on a certain window of combined tide and wave
conditions, as well as on the presence of a shoal and reflective structural condition of the
jetties, bringing to play many aspects of coastal processes and engineering. We also
discuss why Threes is important to the local surfing population and how such tidal inlet
processes can be described by means of state-of-the-art numerical simulation technology.
SHINNECOCK INLET AND SURFING
Shinnecock Inlet is located on the south shore of Long Island, New York, 59 km west
of Montauk Point, and it is the eastern-most of six stabilized inlets located along the
barrier island chain (Figure 2). The modern inlet opened in September 1938 during the
passage of the Great New England Hurricane. Local interests constructed a revetment on
the west side of the inlet in 1947, probably to halt westward migration of the inlet. The
revetment was extended to a jetty on the west side from 1953 to 1955, and the east jetty
was constructed from 1952 to 1953. The depth of the inlet as maintained by the local
governments tended to be about 3 m, with considerable sediment shoaling.
Shinnecock Inlet provides access to the Atlantic Ocean for the largest commercial
fishery in New York. After a fatal boating accident of commercial fishermen, in the early
1990s the Federal Government assumed responsibility for inlet maintenance. Although
the authorized depth is about 4 m including advance dredging, construction of a
deposition basin to intercept sediment alongshore to the west (the predominant direction
of transport except during summer), and natural deepening of the inlet channel have
increased the effective depth to some 6 m. As an inlet serving major commercial fishing
and recreational boating, and because the beach to the west experiences chronic erosion
(Figure 3), Shinnecock Inlet and the adjacent beaches have received considerable study
(e.g., Kassner and Black 1983; Williams, Morang, and Lillycrop 1998; Morang 1999;
Batten and Bokuniewicz 2000; Pratt and Stauble 2000; Militello and Kraus 2001a,
2001b; Militello et al. 2001).
Despite care to maintain reliable navigability, similar to all inlets opening directly to
the ocean, Shinnecock Inlet can be treacherous during storms, with waves breaking along
the ebb shoal and propagating into the inlet. Current velocity in the inlet can exceed
2 m/sec, indicating that this inlet has not reached equilibrium cross section for the tidal
prism it maintains (Militello and Kraus 2001a). Westward migration of the ebb jet has
been identified as a mechanism for realigning the entrance channel Militello and Kraus
2001a, 2001b). During the ebb portion of the tidal cycle, the strong current can create
standing waves at the mouth of the inlet, because the incident waves cannot propagate