Performance of a submerged breakwater along a hardbottom influenced coast: Vero Beach, Florida

This paper summarizes the results of a 3-year monitoring study on the performance of the Vero Beach Prefabricated Erosion Prevention (P.E.P.)Reef Approximately 2.5 years (August 1996 to June 1999) of findings for this project are presented here. The project consisted of placement of 217 concrete units to form a nearshore submerged reef off the City of Vero Beach, Florida between 20 July and 16 August, 1996. The reef was configured into 11 segments, each containing several individual units. Segments were placed in alternating inshore and offshore positions, which provided gaps for the wave set-up to return offshore. Monitoring of the P.E.P. Reef and north and south control areas included analysis of shoreline movement; beach sediment volume gain and loss; nearshore bathymetric change; wave and current modifications; scour and settlement of the P.E.P units; and interactions with existing seawalls and naturally occurring hardbottom. Findings indicate that the existing natural hardbottom affects the beach response. Three nearshore areas, each with different hard bottom morphology, correspond to areas with accretion, high erosion, and moderate erosion of the beach face sand volume. This high erosion zone coincides with the largest natural hardbottom expanse and is also the area with the most seawall construction. The P.E.P. Reef was placed in this area to mitigate erosion pressures. Within four months of placement, the P.E.P. Reef units settled an average 0.86 m (2.8 ft). Most of the units came to rest on the hardbottom, which limited further settlement. A scour trench formed on both the landward and seaward side of the units. Wave transmission attenuation was initially around 12% until settlement when the transmission was the same as the adjacent natural reef. It now appears that the P.E.P. Reef is producing slightly more wave attenuation (in the order of 8%) than the surrounding natural hardbottom. Immediately after P.E.P. reef placement there appeared to be little difference in the erosion of the seawall and non-seawall profiles. After the first year, seaward movement of the shoreline and profile volume accretion have been measured to the north of the project. Erosion has been measured within the P.E.P. reef area and to the south where the natural hardbottom intersects the shoreline at an orientation of around S20 degrees E. The natural hardbottom appears to be controlling the main coastal processes of the area. The P.E.P. Reef seems to be modifying sediment transport, acting as a groin, trapping sand on its updrift end and enhancing the hot spot erosion. In 1999, a sand wave like feature formed behind the segments and is now progressing south through the P.E.P. Reef zone into the south control zone.