Submerged Narrow-Crested Breakwaters
685
layer cross section (AHRENS, 1987). They are applied to pro-
individual units were locked together, and placed parallel to
vide partial attenuation of waves to protect a beach. Since
shore in segments of various lengths and configurations, de-
they are below still water, they are not visible from the beach.
pending on the project location. The general triangular shape
As waves encounter the structure, they shoal and break dis-
units have three openings just below the crest, and the flatter
sipating some of their energy as they pass over the crest.
sloping face was oriented seaward. The design purpose of the
Theses structures can also be less costly to build than other
P.E.P. Reef was to a) reduce wave height, b) stabilize the
shore protection options. Performance characteristics of wave
shoreline position, c) limit sediment volume changes in the
transmission, wave reflection and energy dissipation have
vicinity of the breakwater, and d) lower wave energy land-
been determined by laboratory model tests (AHRENS, 1987;
ward of the breakwater during storms (AMERICAN COASTAL
AHRENS and FULFORD, 1988). These tests showed that the
ENGINEERING, 1993).
A second design, called the Beachsaver breakwater unit
submerged breakwater caused premature breaking of waves,
therefore dissipating wave energy more than a natural slop-
was designed by Breakwaters International (CRETER et al.,
ing beach. Submerged breakwaters dissipated between 17
1994) and was installed in three project locations in New Jer-
and 56 percent of the wave energy in tank tests (AHRENS and
sey to test different configurations and site conditions. The
FULFORD, 1988). Reefs with small cross-sections had less in-
first installation of this type of breakwater was in Long Is-
fluence on larger waves.
land Sound at Oakwood, New York, in 1984. A shifting prob-
Several factors control the effectiveness of any submerged
lem resulted in a redesign of the units. The initial open ocean
breakwater configuration including: breakwater dimensions,
deployment of these units was at Sea Isle City, New Jersey,
depth of water and placement distance offshore of the beach,
in 1989. Two 61 m (200 ft) long breakwaters were placed in
the incident wave climate and the nearshore profile (DEAN et
the Atlantic Ocean 76 m (250 ft) from shore and were moni-
al., 1994b; WAMSLEY et al., 2002). The parameters of relative
tored for 9 months by Lehigh and Drexel University research-
crest width and relative depth of submergence of the crest
ers (HERRINGTON, 1988). The shoreline moved seaward some
below the water surface were identified as significant param-
21 m (69 ft) at the structure but uneven settlement of the
eters by DATTATRI et al. (1978). HARRIS (1996) has identified
individual units caused the removal of this installation. The
three types of low crested breakwaters (1) rubble mound with
State of New Jersey sponsored a pilot project to evaluate the
a trapezoidal cross section of rock or concrete, (2) prefabri-
Beachsaver Reef at Avalon, New Jersey adjacent to Town-
cated modular units constructed of concrete, timber or other
sends Inlet in July 1993. A second deployment was construct-
materials and (3) flexible-membrane units constructed of con-
ed at Cape May Point, New Jersey, in May 1994 in an area
crete-, sand- or water-filled containers. Little is written on
influenced with strong tidal currents at the entrance to Del-
the design or performance of prefabricated modules or flexi-
aware Bay. The third placement was at Belmar/Spring Lake,
ble membrane units (HARRIS, 1996).
New Jersey, in August 1994 along an open coast with high
Several different designs have emerged for the prefabri-
wave activity. All of theses projects placed Beachsaver units
cated narrow-crested concrete breakwater or artificial reef.
in shore-parallel locations adjacent to the beach in the Atlan-
These breakwaters consist of various configurations of mod-
tic Ocean. Theses reinforced prefabricated concrete units
ular units placed in the nearshore. GOLDSMITH et al. (1992)
have similar dimensions to the P.E.P. Reef and were 1.83 m
evaluated deployments of theses types of units in the 1970's
(6 ft) high, 3.05 m (10 ft) long and 4.57 m (15 ft) wide, with
and 1980's. They list 29 installations along the Atlantic,
a crest width of around 0.46 m (1.5 ft) and weigh around 19.1
Great Lakes, Gulf of Mexico and Hawaii coasts of the United
metric tons (21 tons) (HERRINGTON and BRUNO, 1998). These
States.
individual units also interlock together to form longer reef
Six projects have been monitored to some extent in the past
structures. These units are also triangular and have a raised
decade using two types of prefabricated concrete breakwater
crest area with openings designed to allow water and sedi-
s
configurations. The first type of unit is called P.E.P. Reef
ment to pass through (Figure 2).
hort for Prefabricated Erosion Prevention concrete break-
Both of theses prefabricated concrete breakwaters are con-
water (AMERICAN COASTAL ENGINEERING, 1993), which was
sidered narrow-crested due to their triangular shape with a
installed in two separate experimental projects in Palm
smallest dimension at the crest width. Both types of units
Beach County, FL at the Dupont property and at Midtown
are placed with the longer flatter slopping face in the sea-
Palm Beach from 1988 through 1995. The third installation
ward direction and the steeper shorter sloping face toward
was in Indian River County, FL at Vero Beach in 1996. The
the beach. The raised crest area on both types is designed to
first configuration of the P.E.P. Reef used at the Dupont
trip the waves as they pass over the units. The steeper slope
property was constructed of reinforced concrete units that
of the shoreward face cause return flow under the breaker to
were 1.52 m (5 ft) high, 7.32 m (24 ft) long and 3.66 m (12 ft)
be forced upward to enhance the wave tripping mechanism
wide (MITCHELL, 1994). Units used in the second Palm Beach
and any sand placed in suspension should be transported
and Vero Beach installation were modified to 1.83 m (6 ft)
back toward the beach. Sand is then supposed to be trapped
high, 3.66 m (12 ft) long and 4.57 m (15 ft) wide at the base
on the shoreward face preventing it from flowing offshore.
with a 0.31 m (1.0 ft) crest width and weights of approxi-
This paper is produced as part of a US Army Corps of En-
mately 22.7 metric tons (25 tons). The units are made of re-
gineers Research and Development program authorized un-
inforced concrete and cast offsite in a mold. They are then
der Section 227 of the Water Resources and Development Act
transported by barge and placed by crane in the nearshore
of 1996 called the National Shoreline Erosion Control Devel-
adjacent to the beach in the Atlantic Ocean (Figure 1). The
opment and Demonstration Program. The focus of Section
Journal of Coastal Research, Vol. 19, No. 3, 2003
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