Performance of Submerged Prefabricated Structures to Improve Sand Retention at Beach Nourishment Projects

Submerged breakwaters were examined to evaluate their performance in maintaining the natural beach and retaining beach fill. These included sites where prefabricated concrete reefs, sills, and ball-like reef structures had been deployed. Most of these innovative structures have not performed well in exposed, open-coast, settings unless they were mounted on hard bottom. The Cape May Point, New Jersey, Section 227 demonstration site was used to develop a conceptual analysis model. The project monitored the functional, structural, and economic performance of two innovative prefabricated concrete structures placed at the seaward end of a groin compartment to form an enclosed sill. The project was conducted in conjunction with an ecosystem restoration beach fill to assess if a Beachsaver Reef (TM) and a Double-T sill could help to hold fill sediment in place at the entrance to Delaware Bay, which is influenced by high wave energy and strong tidal currents. The Beachsaver Reef (TM) units appear to be retaining the fill sand, while the Double-T sill settled completely into the bed and is no longer functioning as intended. Settlement and scour of these prefabricated units present the most problems in project performance. Wave attenuation performance is poor when the units settle, and they are better suited to act as a sill to hold sediment on a beach. Guidance is provided from the conceptual model at Cape May Point on the components that are needed to monitor a successful project. Criteria are given on how to evaluate project performance.