Performance of a Submerged Breakwater for Shore Protection

A summary is presented of the results from a 3-year monitoring study of a submerged breakwater placed alongshore for shore protection. The experimental breakwater project, located in Palm Beach, Florida, USA, was concluded in August, 1995. The monitoring program was instituted to determine changes in wave climate and bathymetry, and to assess the shoreline response relative to pre-project conditions. The Palm Beach breakwater consisted of 330 interlocking concrete units, each 1.8 meters in height, 3.7 m long, and 4.6 m wide, which formed a 1,260 m-long shore parallel barrier. The breakwater, termed the Prefabricated Erosion Prevention (P.E.P.) Reef was placed in approximately 3 m of water roughly 73 m from the shoreline. The project was intended to increase shore protection against storm waves and to create a wider beach in the lee of the Reef. Wave transmission measurements indicated a 5 to 15% reduction in incident wave heights due to the Reef. The results of the monitoring program at the end of two years indicated erosion throughout the project area, primarily in the lee of the Reef, where the annual volumetric erosion rate was measured to be 2.3 times higher than the pre-project rate. The submerged breakwater is interpreted to have increased the longshore currents via ponding of water trapped behind the breakwater, which was then diverted alongshore. Laboratory experiments were conducted to investigate this ponding phenomenon and to test the performance of various segmented and repositioned breakwater arrangements. Shoreline erosion in the lee of the Reef was sufficiently severe to warrant removal of the structure in August, 1995.