Thesis study into an alternative design for breakwaters made of granular materials like sand and quarry stone. In this design, the breakwater is constructed of a sand core and a layer of large stones directly on top of it, without the traditional filter layers in-between. The single layer of stones serves as one open filter layer that has to reduce the wave induced flow and turbulence well enough to keep the erosion of sand through the filter layer within an acceptable amount. By accepting a controlled amount of erosion and necessity of maintenance, the alternative design can be very cost-effective over the life-span of the structure. In cooperation with Van Oord, one of the world market leaders in dredging and coastal engineering, a research into the applicability of very open filters on sand cores has been initiated at Delft University of Technology. My thesis study is the second consecutive in this larger research that has the goal of finding a good theoretical description of the occurring erosion processes and developing a practical design tool for breakwaters with a very open filter layer on a sand core. The focus of my study is the relation between the wave height, -period and regularity and the amount of erosion occurring with it. Physical model tests for this study have been performed in the long sediment transport flume of the Fluid Mechanics Laboratory. This flume is equipped with a wave generator that loads the structure with the desired wave spectrum. The results have been analysed for relations between loading and erosion.