Wave Forces on Solid and Perforated Caisson Breakwaters: Comparison of Field and Laboratory Measurements

Abstract

Following an old Italian tradition of prototype measurements of wave pressures at vertical breakwaters (Franco, 1994), a new twin recording station was set up and operated in 1992-1994 at Porto Torres (Sardinia, Italy) industrial harbour breakwater. In the framework of the MAST3-PROVERBS project an extensive analysis of the available data has been carried out. Within the same project 2D model tests have also been performed in order to investigate the relation between pressure distribution and overall forces on and under both plain and perforated (multichamber) caissons. The results have been compared with the available design formulae like Goda's (1985). Statistical distributions of the horizontal and uplift forces for both structure types have also been derived. The scope of the study has been to assess the reliability of the present design methods and to outline a more physically based approach especially for the perforated structure type. Introduction The prototype structure is a vertical composite breakwater subjected to nonbreaking wave conditions. Two caissons (20.5x13.9 m), one with plain (solid) wall and one perforated, 62 m apart, based at -15 m on rubble footing in 20 m water depth, were instrumented with ultrasonic wave gauges and pressure sensors along the caisson's base, on the vertical face and also in the internal chambers as described by De Girolamo et al. (1995). The operating time of the instrumentation was 1992-1994, in which 10 significant storms (2<HS<3.5 m, 6<TP<9.2 s) have been recorded. A directional wave gauge has been installed 700 m away from the breakwater at the same depth (20 m). Water levels at the wall, front and uplift pressures have been recorded with a sampling frequency of 20 Hz, except for deep sensors (2 Hz). 1 Dept. Civil Engineering 3rd University, via C. Segre 60, 00146 Roma (Italy) 2 Hydraulic Laboratory ENEL-PIS, via Pozzobonelli 6, 20162 Milano (Italy) 3 DIIAR Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano (Italy)