Submerged Narrow-Crested Breakwaters
719
variable. Most of the settlement was measured during the
1995. This new fill buried the reef, but the crest quickly be-
first 9 months after installation and ranged between 0.4 m
came exposed, with a scour to the south of the end unit. By
December 1995, the scour zone deepened to 1.22 m ( 4 ft)
(1.3 ft) on the north end to 1.5 m (5 ft) at the south end. The
underlying geology appeared to play a part in this settlement.
below the crest of the reef and extended 12 m (40 ft) landward
These units were placed on a sandy bottom, but a clay layer
of the reef. A geomattress, filled with stone was placed along
some 0.91 m (3 ft) below the surface may have compressed
the landward side of the southern end of the reef to reduce
over time causing the settlement (BRUNO et al., 1996b). To
the undermining and resulting settlement (HERRINGTON and
mitigate for suspected settlement in the Cape May Point and
BRUNO, 1998). At Cape May Point, a scour trough was pres-
Belmar/Spring Lake installations, a filter fabric blanket was
ent even with the filter cloth and geotextile mattress. The
scour trough has reach a depth of between 2.7 to 4.6 m
placed as a foundation over the sand bed. At Cape May Point,
( 9 to 15 ft) and widths ranging from 2.7 to 30.5 m (9 to
the Beachsaver units have only settled a minimal amount
around 15.2 cm (6 inches) or less over the two-year monitor-
100 ft) as of March 2002. The Belmar/Spring Lake site has a
ing. The underlying geology consisted of medium to fine sand
local scour zone of 15 to 18 m (49 to 59 ft) wide and a maxi-
over a mud layer, but the filter cloth and geotextile mattress
mum depth of around 2.1 m (6.5 ft) (HERRINGTON and BRU-
maintained lateral and vertical stability over the monitoring
NO, 1998). The geotextile mattress has settled in some places
period. At the Belmar/Spring Lake site, a thicker geotextile
affected by the scour.
mattress was put under the Beachaver units. The underlying
A scour trench was present along the landward side of all
geology indicated a layer of sand over a layer of clay, which
of the Vero Beach P.E.P. Reef segments, regardless of their
may have accounted for the settlement (BRUNO et al., 1996a).
position inshore or offshore. Some of the reef units became
Average settlement of around 1 m (3.3 ft) was measured as
perched above rock outcrops. The landward scour trench av-
the sand was eroded and the units ended up on the clay layer,
eraging 1.22 m (4 ft) deep and 6.4 m (20.9 ft) wide. A scour
which took about 6 months.
trench was also found seaward of the reef segments on oc-
The staggered placement of the P.E.P. Reef at Vero Beach
casions. Generally, this seaward trench was present after
with an inshore and offshore alignment and the present of
storms and filled in during fair weather.
hardbottom just under the reef units resulted in differential
settlement. All of the units were placed on sand, but the sand
Wave Attenuation
scoured out from under the units until the reef settled on to
the rock surface, resulting in some shifting over the uneven
The narrow-crested design of this class of prefabricated
bottom. Units on a sandy bottom settled further into the bot-
submerged breakwaters appears to have limited their effec-
tom. No filter cloth was used in this installation. The inner
tiveness in wave attenuation. Settlement of the units has also
P.E.P. Reef segments settled averaged around 1 m (3.4 ft)
reduced the freeboard depth and thus the ability to trip the
and the outer segments settled 0.9 m (2.8 ft) with the most
incoming waves. BROWDER (1994) indicated that the free-
settlement occurred during the first four months.
board was the most important variable in submerged break-
water design. The shallower the breakwater, the more wave
attenuation is afforded. If the crest of the structure is close
Scour
to the surface, it may also produce structure-induced cur-
A scour trough formed at all installations just to the land-
rents. Wave energy reduction was found to be a function of
ward side of both the P.E.P. and Beachsaver reef units. This
the configuration and composition of the structure, negative
scour trough appears to be the result of turbulence resulting
freeboard (depth of water over the reef crest) and incident
from the return flow hitting the steeper face of the landward
wave height and period. BRUNO et al. (1996b) observed higher
side of either brand of unit, creating an upward flow that
wave energy reduction when the reef crest was closer to the
interacts with the incoming waves. This flow interaction cre-
water surface and the wave heights were larger. Broad-crest-
ates a vortex on the landward side resulting in scour at the
ed breakwaters dissipate wave energy by wave scattering as
unit base. A substantial trough formed along the entire
the wave travels over the shallow crest. In the case of the
length of both types of breakwaters resulting in undermining
narrow-crested prefabricated concrete breakwaters with crest
the base and enhanced settlement and slumping in some cas-
width of around 0.3 to 0.5 m (1.0 to 1.6 ft), the design is to
es. Some of the projects reported a trough forming on the
induce strong vertical currents on the return flow as the wave
ocean side of the units, but this trough was not as deep as
passes over the unit (HERRINGTON et al., 1997). This upward
the landward side.
deflection of the current is expected to reduce the wave
No mention of the scour trough was given in the Dupont
height.
Property P.E.P. Reef report. The Midtown Palm Beach pro-
Limited wave measurements are available from the Mid-
ject quickly formed a trough along the entire length of the
town Palm Beach, Avalon, Cape May Point and Vero Beach
project. Return flows were redirected along the breakwater
site monitoring, where wave gages were deployed for various
to the ends of the project, causing scour and the measured
lengths of time both seaward and landward of the breakwa-
erosion in the lee of the breakwater was more than twice the
ters and wave attenuation was calculated. At the Midtown
erosion in the control zones (DEAN and CHEN, 1996).
Palm Beach, Avalon and Vero Beach sites control wave gages
At Avalon, a scour zone was measured along the entire
were also deployed over the natural nearshore profile in one
landward side of all of the Beachsaver units. Two years after
of the control zones to compare natural wave attenuation to
placement of the reef a new beach fill was placed in June
that afforded by the breakwaters.
Journal of Coastal Research, Vol. 19, No. 3, 2003
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