An investigation of the effects of breakwaters on beach sediment characteristics

Abstract

The south-east reach of Castiglione della Pescaia beach, Italy, has been studied to demonstrate the effect of the breakwaters on sediment characteristics as regards as the cuspate shape of the shoreline. Several samples have been collected to investigate the longshore distribution of sediment parameters such as mean size, sorting and skewness. In addition, comparison with data from natural cuspate beaches has been made. It shows an opposite sediment size distribution while sorting and skewness are almost the same for cusps and bays. That is a consequence of different hydrodynamic processes evenly resulting in a similar shoreline morphology. The grain size analysis has also allowed to state that the sediment of the protected beach is substantially of the same quality of that where natural conditions prevail and that the influence of breakwaters can be neglected in terms of skewness sign. Statistical tests, using mean size, confirmed the main longshore direction of sediment transport, calculated theoretically. Core samples have been taken in order to examine the subsurface sediment characteristics in the breakwater protection area. The core sediments show a predominant positive skewness while the grab and the berm samples are more frequently negatively skewed. The analysis of core stratigraphy taken in different periods shows a sea-bed scour of at least 1 m in the aggradational protected area during ordinary sea-storms.