Aspetti geotecnici e marittimi di due interventi di difesa costiera

Abstract

Two case-histories of the design of submerged breakwaters for coast protection are shortly reported in the paper. The first one refers to the stabilisation and protection works of the sand beach of S.Alessio Siculo village, in Sicily, that suffered severe erosion triggered, some forty years ago, by the construction along the shoreline of a vertical seawall in lieu of the pre-existing sand-dune system. As a consequence, long stretches of the seawall were undermined and collapsed, and the beach almost disappeared. A submerged breakwater was built to reduce the inten-sity of wave attack, the seawall was rebuilt where necessary and provided with a rock revetment at the toe, and then a “perched” beach was formed. The submerged breakwater option was necessary to pre-serve the aptitude for bathing activities of the vil-lage. The second case-history refers to the submerged breakwater built in front of a marine cliff in order to protect it against erosion and undercutting. The cliff is located on the eastern narrow promontory of Or-tigia Isle (Syracuse), on which rests the medieval Maniace Castle dating back to the age of Fedrerik II. The cliff as well as the nearby sea bottom consist of yellowish soft calcarenitic layers interbedded with weak or poorly cemented yellow calcarenitic sand lenses. The breakwater consists of two layers of unrein-forced concrete blocks connected to the rocky sea bottom by means of active steel bar anchorages. Blocks are spaced 1 m so to form a network of channels permitting water and fish fauna to pass through the breakwater. During construction some basal concrete blocks, not yet anchored to the rocky sea bottom, were dis-placed or overturned by heavy sea. Therefore it was necessary to modify the design of the damaged stretches of the breakwater and revert to a mixed so-lution composed of the displaced concrete blocks, rockfill and large stones. The behaviour of the breakwater has been fully satisfactory during its 28 years life span. The design of both breakwaters has been assisted by physical model tests carried out at Wallingford and at Delft Hydraulics for S.Alessio Siculo and Or-tigia breakwaters respectively. Finally it must be mentioned that the S. Alessio Siculo project explicity contemplated the observa-tional approach as far as the seaside rock armour of the barrier (i.e.”sacrificial layer”) was concerned. On the contrary, the initial design of the defense system of Ortigia cliff did not foresee the use of the obser-vational approach. Therefore the designer had to think all over again a new solution when heavy sea damaged the uncompleted sections of the breakwa-ter. Luckily, a valid and economically viable solution was found.