A REFRACTION GROYNE BUILT BY SAND

Abstract

The interaction between a normal groyne and waves mostly is by diffraction, i.e. the structure of the groyne, especially the width, is very small in comparison to the wave length. But when a groyne is built with lateral slopes comparable with normal beach slopes, also refraction processes occur and surf zones are created nearly parallel to the center line of the groyne. It can be shown that, for all directions of wave approach, the longshore currents always are directed shoreward. So, when such a groyne is built by sand, due to the erosion of the groyne an artificial sand spit is formed and a beach nourishment at the adjacent areas of the shore can be expected. Furthermore, sand originating from the normal littoral drift can be caught by such a structure as it happens in case of normal groynes. In a field experiment at the shore of the city of Westerland on the island Sylt/Germany, with 680,000 m of sand such an artificial sand groyne was built in 1972. One year later, this sand masses were grown up to nearly 1,050,000 m . After a sequence of five heavy storm surges in November and December 1973, still 770,000 m could be detected before the shore in 1974. The evaluation of the experiment is continued; the aim of it is find out whether repeated beach nourishment is more economic than protective works. For this reason the knowledge of the "half decay time" of a beach nourishment is necessary.