Journal of Coastal Research
19
3
684722
West Palm Beach, Florida
Summer 2003
The Use of Submerged Narrow-Crested Breakwaters
for Shoreline Erosion Control
Donald K. Stauble† and Jeffery R. Tabar‡
‡PBS&J
†U.S. Army Engineer
Coastal Engineering Division
Research and Development
5300 West Cypress Street,
Center
Suite 300
Coastal and Hydraulics
Tampa, FL 33607, USA
Laboratory
3909 Halls Ferry Road
Vicksburg, MS 39180, USA
ABSTRACT
STAUBLE, D.K. and TABAR, J.R., 2003. The Use of Submerged Narrow-Crested Breakwaters for Shoreline Erosion
Control. Journal of Coastal Research, 19(3), 684722. West Palm Beach (Florida), ISSN 0749-0208.
The performance of six installations of modular narrow-crested submerged breakwaters constructed of prefabricated
concrete has been reviewed as a possible lower cost shoreline erosion prevention device. Two types of breakwaters
that have been deployed since 1988, are reviewed here. Three Prefabricated Erosion Prevention (P.E.P.) Reefs have
been constructed and monitored to assess performance at three sites on the lower central east coast of Florida, two
in Palm Beach County and one in Indian River County. Three Beachsaver Reefs have been installed along the New
Jersey Coast, two in Cape May County and one in Monmouth County. Both types of reef breakwaters have similar
dimensions and are triangular in cross-section. The objective of these relatively low cost reef structures were to reduce
wave heights, maintain a stable shoreline position, retain the existing volume of sand on the beach and protect the
beach from storm waves. All six projects have been monitored for at least two years after installation. The monitoring
evaluated the change in the position of a project defined shoreline, the volume of sand gained or lost behind the
breakwater as well as control areas adjacent to the reef installation, settlement of the structure, scour around the
base of the units and the amount of wave attenuation afforded by the structure. The installation configurations differ
along with coastal morphology, underlying geology, coastal processes and placement relative to other shore protection
structures. All of the installations measured scour at the landward side of the reef. This scour along with turbulence
induced by waves interacting with the return flow deflected upward by the reef shape resulted in settlement of the
reef over a two to six month period after installation. The settlement was mitigated by the use of a geotextile mattress
and filter cloth on two of the projects. With and without settlement of the reef, the wave transmission was around
10% for all of the installations. The three Florida projects placed the P.E.P. Reef some distance off the beach in a
shore parallel configuration. Two of the projects had a single solid reef line resulting in structure induced scour and
erosion of the beach. The third placement was modified to a staggered inshore and offshore placement with gaps
between the segments. All of the projects measured erosion or less accretion behind the reef relative to the control
areas. The three New Jersey projects placed the Beachsaver Reef adjacent to groins, with one project semi-enclosed
adjacent to an inlet terminal groin and the other two completely across the seaward end of groin compartments,
forming a perched beach. Beach fills were also placed as part of the initial installation on two of the projects. The
two locations that were completely enclosed in groin compartments retained the most sand.
ADDITIONAL INDEX WORDS: Prefabricated concrete breakwaters, narrow-crested submerged breakwaters, shoreline
change, beach sand volume change, scour at structures, wave attenuation, settlement of structures, breakwater perfor-
mance evaluation.
INTRODUCTION
day is the shore-parallel rubble mound structure. These types
of structures were usually emergent, constructed out of some
With increasing erosion pressures and limited resources,
type of natural rock material or concrete units, and placed
alternative methods of shore protection are needed to protect
some distance seaward of the shoreline. Some deployments
upland property and provide recreational and environmental
are a single structure and some are segmented with gaps
habitat at a lower cost than conventional techniques. One
between the structures. Design guidance is available on this
such alternative method that has been tried in recent years
type of shore protection structure (CHASTEN et al., 1993). Pro-
has been the submerged prefabricated modular breakwater
ject design and performance data are available in DALLY and
or artificial reef unit. The purpose of this type of structure is
POPE (1986), POPE and DEAN (1986), KRAFFT and HERBICH
to attenuate waves and provide shoreline stabilization. Six
(1989), CHASTEN et al. (1994) and BASCO (2001).
such deployments have been monitored over the past decade
A modification to this traditional concept is a submerged
to assess their usefulness as an erosion prevention tool. This
breakwater. These so-called reef breakwaters are usually
paper will review their performance.
shallow, narrow-crested rubble mounds, with a crest height
The most common type of detached breakwater in use to-
below the still water level, and without a traditional multi-
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