The Use of Submerged Narrow-Crested Breakwaters for Shoreline Erosion Control

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), 684‐722. 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.) Reefsy 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 Reefsy 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.