Detached Breakwaters and their Use in Italy

Abstract

Detached (or "offshore") breakwaters of emerging type are a common and effective approach for beach protection, particularly for areas with small tidal oscillations. This kind of breakwaters aims at acting on sediment transport by decreasing the transport capacity of long-shore currents, thus locally causing settlement of sediments. Unlike groins, which act directly on long-shore currents, detached breakwaters act on incoming waves decreasing their height in the protected areas and thus also decreasing the strength of associated currents and causing settlement of sediments. Another functional aspect of detached breakwaters is that they influence not only long-shore transport, but also on-offshore transport of sediments. The use of detached breakwaters is quite common along the coasts of the Mediterranean (particularly in Italy) and in Japan. It is well known that, as for all protective structures of the "rigid" type, the benefit for the protected beach, following the increased deposition of sediment, is usually offset by reduced supply of sediments to and subsequent erosion phenomena of the downdrift beaches. After a brief review of general characteristics and performance of detached breakwaters of emerging and submerged type, in this report some applications of this kind of protective system along the coasts of Italy will he illustrated and discussed.