Recent Italian Experience in the Design and Construction of Vertical Breakwaters

Abstract

Italy is often considered as a mother country of vertical breakwaters for harbour protection, since they have been widely used all along our coasts long since. An updated location map is given in fig. 1, which also shows the position of the directional wave recording stations of the existing Italian network. The most common vertical breakwaters are in fact composed by prefabricated monolithic cellular r.c. caissons, which are typically floated and sunk with seawater ballast upon a rubble mound foundation and then filled with sand and/or concrete. They are also called "upright" or "composite" breakwaters. New research work (including large-scale physical models and advanced numerical models) is being addressed to the dynamic behaviour of vertical structures, particularly under breaking wave impact loading, as within the EC-funded MAST G6-S project (1990-92), which already produced many new contributions (see references). Also a new PIANC working group (n.28) is just about to start. This report gives useful information on the most recent caisson breakwaters designed and constructed in Italy, which include particular solutions for the configuration of the caisson walls and superstructure in order to reduce the main "drawbacks" of vertical structures, such as wave reflection and toe scour, wave forces and wave overtopping. This report concludes that vertical breakwaters are still very popular structures in Italy, despite the dramatic failures occurred to a few old breakwaters in the last 60 years. The main reasons for this "success" can be attributed to: - the progress in construction technology of prefabricated monolithic concrete caissons which ensures reduced costs, shorter installation times and better quality and durability of the structure (with low maintenance); - a favourable environmental impact in relation to spatial and visual obstruction, potential removability of infilled caissons and smaller air/water/acoustic pollution during construction compared to a rubble mound; - the greater confidence in the design which takes advantage of the recent advances of knowledge in maritime hydraulics and of the extensive use of laboratory model testing; - the introduction of new alternative caisson geometries (e.g. cylindrical fronts, perforated absorbing chambers, sloping parapet walls) which can reduce the wave forces, wave reflection, overtopping discharge and toe scour effects. The safety against wave overtopping in particular is gaining importance for the increased recreational use of breakwaters which should he easily accessible to the public. Further improvement of knowledge of the complex wave-caisson-foundation interaction are being achieved from new research activity (particularly within the present European MAST G6-S project) and increased practical engineering experience. Useful information is also expected from new prototype measurements, just about to start in two instrumented caissons of the West Breakwater at Porto Torres industrial harbour. A better insight of the effective dynamic response of vertical structures under high impact forces due to breaking waves will undoubtedly promote a wider application of the caisson technology even in shallow waters and will lead to safer and more economic breakwaters.