Full scale monitoring study of a submerged breakwater, Palm Beach, Florida, USA

In an attempt to reduce beach erosion and wave impact on a protective seawall, an experimental proprietary submerged breakwater (Reef) of 1,260 m length was installed in a water depth of approximately 3 m off the Town of Palm Beach, Florida, To provide a basis for evaluating the effects of this installation, a comprehensive field monitoring program was carried out and included: wave measurements, beach and offshore profiles, settlement of the units, local scour data and information related to the background coastal processes. The system consists of 330 units, 57 of which were installed in the summer of 1992 and the remainder in the summer of 1993, Monitoring surveys, available up to June 1995, are presented herein, The wave height reduction attributable to the Reef ranges from 5% for the lower waves and higher tides to 15% for the higher waves and lower tides. The profile data documented erosion in the entire monitored area with the greatest erosion landward of the Reef, Changes in profile data were consistent with and are interpreted as due to water conveyed over the submerged breakwater as mass transport, a partial pending of this water due to its return flow being impeded by the presence of the breakwater, resulting in a portion being redirected as longshore currents, These longshore currents transport sand from landward of the breakwater with deposition occurring near the south end of the system in the direction of the predominant natural transport. Although these breakwater-induced longshore currents were not documented in the monitoring program, the mechanisms are supported by the patterns of erosion and deposition, by a model study carried out to evaluate installation of a submerged breakwater at a different location and by previous documentation and discussion of the phenomenon in the available literature. Results obtained here are interpreted in terms of design implications.